Research papers

The current filters are: Starting year = 2019, Ending year = 2020
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Al-Nassar A.R., Pelegrí J.L., Sangrà P., Alarcon M., Jansa A. (2020)
International Journal of Climatology, 40, 2, 908-926. DOI: 10.1002/joc.6247. (BibTeX: alnassar.etal.2020)
Abstract: See
We combine daily in situ precipitation data with meteorological reanalysis data inorder to explore the contribution of cut-off low systems to the seasonal and inter-annual rainfall variations over Baghdad from 2005 to 2016. During these 12 years(average rainfall of 131 ± 67 mm/year), 38 rainy cut-off lows brought 43% of thetotal precipitation, with extreme inter-annual variations. Indeed, precipitation asso-ciated with autumn cut-off lows was the principal factor that turned an arid into awet year: during the three most arid years cut-off lows contributed about 25% ofthe average rainfall (10 out of 40 mm/year) while during the three wettest yearsthey contributed near 67% (171 out of 254 mm/year). The extreme-rain cut-off lowsystems displayed analogous synoptic characteristics: upper-atmosphere diver-gence, upwards vertical motions in the middle atmosphere, and lower-atmospherewinds into central Iraq at times when the surface Red Sea and Persian Gulf waterswere warmer than the surface air. During those days previous to an extreme event,the surface waters cooled substantially and the amount of precipitable waterincreased largely, suggesting high latent heat transfer. In order to characterize thoseconditions that favour rainfall, we focus on the November 18–20, 2013 cut-off lowsystem, which led to the largest flooding and wettest year in Baghdad between2005 and 2016. The distribution of properties in the middle (500 hPa) and upper(250 hPa) troposphere shows that the region was affected by intense horizontaldivergence and upwards motions, coinciding with a surface low over the ArabianPeninsula that caused intense northwards winds over the Persian Gulf and broughtsubstantial moisture to central Iraq. The analysis of several stability indexes indi-cates that convective instability played a secondary role during the episode.
Keywords: cut-off low, extreme precipitation, Middle East, moisture source, rainfall records, synoptic conditions
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Ceccaroni L., Piera J., Wernand M.R., Zielinski O., Busch J.A., Jan Van Der Woerd H., Bardaji R., Friedrichs A., Novoa S., Thijsse P., Velickovski F., Blaas M., Dubsky K. (2020)
Plos One. Open acces, 15, 3, e023008. DOI: 10.1371/journal.pone.0230084. (BibTeX: ceccaroni.etal.2020a)
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The European-Commission—funded project ‘Citclops’ (Citizens’ observatory for coast and ocean optical monitoring) developed methods, tools and sensors, which can be used by citizens to monitor natural waters, with a strong focus on long-term data series related to environmental sciences. The new sensors, based on optical technologies, respond to a number of scientific, technical and societal objectives, ranging from more precise monitoring of key environmental descriptors of the aquatic environment (water colour, transparency and fluorescence) to an improved management of data collected with citizen participation. The sensors were tested, calibrated, integrated on several platforms, scientifically validated and demonstrated in the field. The new methods and tools were tested in a citizen-science context. The general conclusion is that citizens are valuable contributors in quality and quantity to the objective of collecting, integrating and analysing fragmented and diverse environmental data. An integration of these data into data-analysis tools has a large potential to support authoritative monitoring and decision-making. In this paper, the project’s objectives, results, technical achievements and lessons learned are presented.
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Grieco G., Stoffelen A., Portabella M. (2020)
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 13, 3-13. DOI: 10.1109/JSTARS.2019.2938327. (BibTeX: grieco.etal.2020)
Abstract: See
Global navigation satellite system reflectometry (GNSS-R)-derived winds from the cyclone GNSS (CYGNSS) satellite constellation are expected to significantly improve weather forecasts in the tropical region. Delay–Doppler maps (DDMs) acquired by the TechDemosat-1 (TDS-1) GNSS-R satellite mission suffer from distortions that are highly correlated to on-board specular point estimation inaccuracies. Such distortions may affect wind retrievals, especially when multilook approaches aiming at exploiting the ambiguity-free area of the DDM are applied. This article demonstrates: that CYGNSS DDMs are also affected by such distortions; the rationale ofDDMshape asymmetries induced by specular point location inaccuracies; and a simple strategy for reducing such induced distortions.Two different datasets have been used, consisting of both regular and raw intermediate frequency CYGNSS measurements. The results show that, similar to TDS-1, the CYGNSS DDM distortions are correlated to specular point location inaccuracies. Furthermore, such inaccuracies are significantly reduced if more accurate specular point related parameters are used to recompress the raw GNSS-R echo, highlighting some sampling issues that are common to both TDS-1 and CYGNSS missions. These results suggest that multilook wind retrieval approaches aiming at exploiting also the peripheral parts of theDDM may be seriously compromised by such distortions. The latter may be substantially reduced by oversampling the outcomingDDMand by a posteriori choosing the proper DDM subsample. For future upcoming GNSS-R missions, it is strongly recommended to store the raw data for eventual reprocessing in case of miscalibration or processing issues such as those shown in this article.
Keywords: Delay–Doppler map (DDM) distortions; Dlobal navigation satellite system (GNSS); Reflectometry
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Isern-Fontanet J., García-Ladona E., Jiménez-Madrid J.A., Olmedo E., García-Sotillo M., Orfila A., Turiel A. (2020)
Remote Sensing, 12, 724-741. DOI: 10.3390/rs12040724. (BibTeX: isernfontanet.etal.2020a)
Abstract: See
Surface currents in the Alboran Sea are characterized by a very fast evolution that is not well captured by altimetric maps due to sampling limitations. On the contrary, satellite infrared measurements provide high resolution synoptic images of the ocean at high temporal rate, allowing to capture the evolution of the flow. The capability of Surface Quasi-Geostrophic (SQG) dynamics to retrieve surface currents from thermal images was evaluated by comparing resulting velocities with in situ observations provided by surface drifters. A difficulty encountered comes from the lack of information about ocean salinity. We propose to exploit the strong relationship between salinity and temperature to identify water masses with distinctive salinity in satellite images and use this information to correct buoyancy. Once corrected, our results show that the SQG approach can retrieve ocean currents slightly better to that of near-real-time currents derived from altimetry in general, but much better in areas badly sampled by altimeters such as the area to the east of the Strait of Gibraltar. Although this area is far from the geostrophic equilibrium, the results show that the good sampling of infrared radiometers allows at least retrieving the direction of ocean currents in this area. The proposed approach can be used in other areas of the ocean for which water masses with distinctive salinity can be identified from satellite observations.
Keywords: sea surface temperature; altimetry; surface quasi-geostrophic equations; surface currents
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Portal G., Jagdhuber T., Vall-llossera M., Camps A., Pablos M., Entekhabi D., Piles M. (2020)
Remote Sensing, 12(3), 570 DOI: 10.3390/rs12030570. (BibTeX: portal.etal.2020)
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In the last decade, technological advances led to the launch of two satellite missions dedicated to measure the Earth’s surface soil moisture (SSM): the ESA’s Soil Moisture and Ocean Salinity (SMOS) launched in 2009, and the NASA’s Soil Moisture Active Passive (SMAP) launched in 2015. The two satellites have an L-band microwave radiometer on-board to measure the Earth’s surface emission. These measurements (brightness temperatures TB) are then used to generate global maps of SSM every three days with a spatial resolution of about 30–40 km and a target accuracy of 0.04 m3/m3. To meet local applications needs, different approaches have been proposed to spatially disaggregate SMOS and SMAP TB or their SSM products. They rely on synergies between multi-sensor observations and are built upon di erent physical assumptions. In this study, temporal and spatial characteristics of six operational SSM products derived from SMOS and SMAP are assessed in order to diagnose their distinct features, and the rationale behind them. The study is focused on the Iberian Peninsula and covers the period from April 2015 to December 2017. A temporal inter-comparison analysis is carried out using in situ SSM data from the Soil Moisture Measurements Station Network of the University of Salamanca (REMEDHUS) to evaluate the impact of the spatial scale of the dfferent products (1, 3, 9, 25, and 36 km), and their correspondence in terms of temporal dynamics. A spatial analysis is conducted for the whole Iberian Peninsula with emphasis on the added-value that the enhanced resolution products provide based on the microwave-optical (SMOS/ERA5/MODIS) or the active–passive microwave (SMAP/Sentinel-1) sensor fusion. Our results show overall agreement among time series of the products regardless their spatial scale when compared to in situ measurements. Still, higher spatial resolutions would be needed to capture local features such as small irrigated areas that are not dominant at the 1-km pixel scale. The degree to which spatial features are resolved by the enhanced resolution products depend on the multi-sensor synergies employed (at TB or soil moisture level), and on the nature of the fine-scale information used. The largest disparities between these products occur in forested areas, which may be related to the reduced sensitivity of high-resolution active microwave and optical data to soil properties under dense vegetation.
Keywords: soil moisture; moisture variability; temporal dynamics; moisture patterns; spatial disaggregation; Soil Moisture Active Passive (SMAP); Soil Moisture and Ocean Salinity (SMOS); REMEDHUS
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Ruiz‐González C., Mestre M., Estrada M., Sebastián M., Salazar G., Agustí S., Moreno‐Ostos E., Reche I., Álvarez‐Salgado X.A., G. Morán X.A., Duarte C.M., Sala M.M., Gasol J.M. (2020)
Molecular ecology DOI: 10.1111/mec.15454. (BibTeX: ruizgonzalez.etal.2020)
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Deep ocean microbial communities rely on the organic carbon produced in the sunlit ocean, yet it remains unknown whether surface processes determine the assembly and function of bathypelagic prokaryotes to a larger extent than deep‐sea physico‐chemical conditions. Here, we explored whether variations in surface phytoplankton assemblages across Atlantic, Pacific and Indian ocean stations can explain structural changes in bathypelagic (ca. 4000 m) free‐living and particle‐attached prokaryotic communities (characterized through 16S rRNA gene sequencing), as well as in prokaryotic activity and dissolved organic matter (DOM) quality. We show that the spatial structuring of prokaryotic communities in the bathypelagic strongly followed variations in the abundances of surface dinoflagellates and ciliates, as well as gradients in surface primary productivity, but were less influenced by bathypelagic physico‐chemical conditions. Amino acid‐like DOM components in the bathypelagic reflected variations of those components in surface waters, and seemed to control bathypelagic prokaryotic activity. The imprint of surface conditions was more evident in bathypelagic than in shallower mesopelagic (200‐1000 m) communities, suggesting a direct connectivity through fast‐sinking particles that escapes mesopelagic transformations. Finally, we identified a pool of endemic deep‐sea prokaryotic taxa (including potential chemoautotrophic groups) that appear less connected to surface processes than those bathypelagic taxa with a widespread vertical distribution. Our results suggest that surface planktonic communities shape the spatial structure of the bathypelagic microbiome to a larger extent than the local physico‐chemical environment, likely through determining the nature of the sinking particles and the associated prokaryotes reaching bathypelagic waters.
Keywords: Deep ocean; Marine prokaryotic communities; Particle sinking; Carbon export; Surface phytoplankton; Microbial dispersal, Particle-attached; Fluorescent dissolved organic matter; Bacterial activity
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Sánchez-Gámez P., Gabarro C., Turiel A., Portabella M. (2020)
Remote Sensing, 12(4), 650 DOI: 10.3390/rs12040650. (BibTeX: sanchezgamez.etal.2020)
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The European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) and the National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) missions are providing brightness temperature measurements at 1.4 GHz (L-band) for about 10 and 4 years respectively. One of the new areas of geophysical exploitation of L-band radiometry is on thin (i.e., less than 1 m) Sea Ice Thickness (SIT), for which theoretical and empirical retrieval methods have been proposed. However, a comprehensive validation of SIT products has been hindered by the lack of suitable ground truth. The in-situ SIT datasets most commonly used for validation are affected by one important limitation: They are available mainly during late winter and spring months, when sea ice is fully developed and the thickness probability density function is wider than for autumn ice and less representative at the satellite spatial resolution. Using Upward Looking Sonar (ULS) data from theWoods Hole Oceanographic Institution (WHOI), acquired all year round, permits overcoming the mentioned limitation, thus improving the characterization of the L-band brightness temperature response to changes in thin SIT. State-of-the-art satellite SIT products and the Cumulative Freezing Degree Days (CFDD) model are verified against the ULS ground truth. The results show that the L-band SIT can be meaningfully retrieved up to 0.6 m, although the signal starts to saturate at 0.3 m. In contrast, despite the simplicity of the CFDD model, its predicted SIT values correlate very well with the ULS in-situ data during the sea ice growth season. The comparison between the CFDD SIT and the current L-band SIT products shows that both the sea ice concentration and the season are fundamental factors influencing the quality of the thickness retrieval from L-band satellites.
Keywords: L-band radiometry; Soil Moisture and Ocean Salinity (SMOS) mission; Soil Moisture Active Passive (SMAP); sea ice thickness; retrieval model validation; upward looking sonar; Arctic
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Sohn D.H., Park K.D., Davis J.L., Nettles M., Elosegui P. (2020)
Advances in Space Research, 65, 1673-1684. DOI: 10.1016/j.asr.2020.01.022. (BibTeX: sohn.etal.2020)
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The Helheim glacier, located in southeast Greenland, has more than ten campaign-type Global Positioning System (GPS) sites; data processing led to the observation of a very rapid change in the ionospheric delay. To identify the cause of these sporadic disturbances, we analyzed the slant total electron content (STEC), single-differenced STEC (SD-STEC) and scintillation proxy index called the delta phase rate (DPR). From this analysis, the abrupt change of those ionospheric indicators was attributed to the line-of-sight direction to the satellite and the temporal sequence of the event was found to be highly correlated with the geometry of the GPS sites. In addition, the disturbance based on the result of SD-STEC occurred mostly during the night, from 17 UTC through 7 UTC, and across a band spanning the east-west direction. Based on the DPR indices obtained from GPS stations distributed across all of Greenland, Iceland, and northeastern Canada, the rapid ionospheric variation was found to be correlated with the time of the day and the geomagnetic latitude of the station. The disturbance was larger at the relatively low geomagnetic latitudes at night but was more significant at higher latitudes in the daytime. These rapid ionospheric variations tended to appear in band shapes parallel to the geomagnetic field. These results allow us to attribute such disturbance observed at the Helheim glacier to aurora-related phenomena.
Keywords: GPS; Rapid ionospheric variation; Slant total electron content (STEC); Delta phase rate (DPR); High latitude
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Tang W., Yueh S.H., Yang D., Mcleod E., Fore A., Hayashi A., Olmedo E., Martínez J., Gabarró C. (2020)
Remote Sensing, 12, 873 DOI: 10.3390/rs12050873. (BibTeX: tang.etal.2020)
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Hudson Bay (HB) is the largest semi-inland sea in the Northern Hemisphere, connecting with the Arctic Ocean through the Foxe Basin and the northern Atlantic Ocean through the Hudson Strait. HB is covered by ice and snow in winter, which completely melts in summer. For about six months each year, satellite remote sensing of sea surface salinity (SSS) is possible over open water. SSS links freshwater contributions from river discharge, sea ice melt/freeze, and surface precipitation/evaporation. Given the strategic importance of HB, SSS has great potential in monitoring the HB freshwater cycle and studying its relationship with climate change. However, SSS retrieved in polar regions (poleward of 50º) fromcurrently operational space-based L-band microwave instruments has large uncertainty (~ 1 psu) mainly due to sensitivity degradation in cold water (<5ºC) and sea ice contamination. This study analyzes SSS from NASA Soil Moisture Active and Passive (SMAP) and European Space Agency (ESA) Soil Moisture and Ocean Salinity(SMOS) missions in the context of HB freshwater contents. We found that the main source of the year-to-year SSS variability is sea ice melting, in particular, the onset time and places of ice melt in the first couple of months of open water season. The freshwater contribution from surface forcing P-E is smaller in magnitude comparing with sea ice contribution but lasts on longer time scale through the whole open water season. River discharge is comparable with P-E in magnitude but peaks before ice melt. The spatial and temporal variations of freshwater contents largely exceed the remote sensed SSS uncertainty. This fact justifies the use of remote sensed SSS for monitoring the HB freshwater cycle.
Keywords: sea surface salinity; Hudson Bay; freshwater contents; sea ice; river discharge
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Trindade A., Portabella M., Stoffelen A., Lin W., Verhoef A. (2020)
IEEE Transactions on Geoscience and Remote Sensing, 58, 2, 1337-1347. DOI: 10.1109/TGRS.2019.2946019. (BibTeX: trindade.etal.2020a)
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To address the growing demand for accurate high-resolution ocean wind forcing from the ocean modeling community, we develop a new forcing product, ERA*, by means of a geolocated scatterometer-based correction applied to the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis or ERA-interim (hereafter referred to as ERAi). This method successfully corrects for local wind vector biases present in the ERAi output globally. Several configurations of the ERA* are tested using complementary scatterometer data [advanced scatterometer (ASCAT)-A/B and oceansat-2 scatterometer (OSCAT)] accumulated over different temporal windows, verified against independent scatterometer data [HY-2A scatterometer (HSCAT)], and evaluated through spectral analysis to assess the geophysical consistency of the new stress equivalent wind fields (U10S). Due to the high quality of the scatterometer U10S, ERA* contains some of the physical processes missing or misrepresented in ERAi. Although the method is highly dependent on sampling, it shows potential, notably in the tropics. Short temporal windows are preferred, to avoid oversmoothing of the U10S fields. Thus, corrections based on increased scatterometer sampling (use of multiple scatterometers) are required to capture the detailed forcing errors. When verified against HSCAT, the ERA* configurations based on multiple scatterometers reduce the vector root-mean-square difference about 10% with respect to that of ERAi. ERA* also shows a significant increase in small-scale true wind variability, observed in the U10S spectral slopes. In particular, the ERA* spectral slopes consistently lay between those of HSCAT and ERAi, but closer to HSCAT, suggesting that ERA* effectively adds spatial scales of about 50 km, substantially smaller than those resolved by global numerical weather prediction (NWP) output over the open ocean (about 150 km).
Keywords: ERA*; Numerical weather prediction (NWP); Ocean wind forcing; Oceanic mesoscale; Scatterometer correction (SC); Scatterometer wind
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Vallès‐Casanova I., Lee S.‐K., Foltz G.R., Pelegrí J.L. (2020)
Geophysical Research Letters, 47, 8, e2020GL087108. DOI: 10.1029/2020GL087108. (BibTeX: vallescasanova.etal.2020a)
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The spatiotemporal evolutions of equatorial Atlantic sea surface temperature anomalies (SSTAs) during Atlantic Niño events and the associated climate impacts on the surrounding continents are extremely diverse. In this study, we construct longitude‐time maps of equatorial Atlantic SSTAs for each observed Atlantic Niño event during 1948–2019 and perform a spatiotemporal empirical orthogonal function analysis to identify the four most frequently recurring Atlantic Niño varieties. The first two contrast the timing of dissipation (early terminating vs. persistent) and the other two the timing of onset (early onset vs. late onset). Largely consistent with the differences in the timings of onset and dissipation, these four varieties display remarkable differences in rainfall response over West Africa and South America. Most of the varieties are subject to onset mechanisms that involve preconditioning in boreal spring by either the Atlantic meridional mode or Pacific El Niño, while for the late onset variability there is no clear source of external forcing.
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Viúdez A. (2020)
Journal of Fluid Mechanics, 890 DOI: 10.1017/jfm.2020.130. (BibTeX: viudez.2020a)
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The permanent precession of a baroclinic geophysical vortex is reproduced, under he quasi-geostrophic approximation, using three potential vorticity anomaly modes in spherical geometry. The potential vorticity modes involve the spherical Bessel functions of the first kind jl(ρ)and the spherical harmonics Yml(θ, φ), where l is the degree, m is the order, and(ρ, θ, φ)are the spherical coordinates. The vortex precession is interpreted as the horizontal and circular advection by a large-amplitude background flow associated with the spherical modec0j0(ρ)of the small-amplitude zonal mode c2,0j2(ρ)Y02(θ) tilted by a small-amplitude mode c2,1j2(ρ)Y12(θ,φ), where {c0,c2,0,c2,1} are constant potential vorticity modal amplitudes. An approximate time-dependent, closed-form solution for the potential vorticity anomaly is given. In this solution the motion of the potential vorticity field is periodic but not rigid. The vortex precession frequencyω0depends linearly on the amplitudes c0 and c2,0 of the modal components of order 0, while the slope of the precessing axis depends on the ratio between the modal amplitude c2,1 and ω0.
Keywords: Baroclinic flows; Quasi-geostrophic flows
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Aulicino G., Cotroneo Y., Olmedo E., Cesarano C., Fusco G., Budillon G. (2019)
Remote Sensing, 11, 1361 DOI: 10.3390/rs11111361. (BibTeX: aulicino.etal.2019)
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The Algerian Basin is a key area for the general circulation in the western Mediterranean Sea. The basin has an intense inflow/outflow regime with complex circulation patterns, involving both fresh Atlantic water and more saline Mediterranean water. Several studies have demonstrated the advantages of the combined use of autonomous underwater vehicles, such as gliders, with remotely sensed products (e.g., altimetry, MUR SST) to observe meso- and submesoscale structures and their properties. An important contribution could come from a new generation of enhanced satellite sea surface salinity (SSS) products, e.g., those provided by the Soil Moisture and Ocean Salinity (SMOS) mission. In this paper, we assess the advantages of using Barcelona Expert Center (BEC) SMOS SSS products, obtained through a combination of debiased non-Bayesian retrieval, DINEOF (data interpolating empirical orthogonal functions) and multifractal fusion with high resolution sea surface temperature (OSTIA SST) maps. Such an aim was reached by comparing SMOS Level-3 (L3) and Level-4 (L4) SSS products with in situ high resolution glider measurements collected in the framework of the Algerian Basin Circulation Unmanned Survey (ABACUS) observational program conducted in the Algerian Basin during falls 2014–2016. Results show that different levels of confidence between in situ and satellite measurements can be achieved according to the spatial scales of variability. Although SMOS values slightly underestimate in situ observations (mean dfference is -0.14 (-0.11)), with a standard deviation of 0.25 (0.26) for L3 (L4) products), at basin scale, the enhanced SMOS products well represent the salinity patterns described by the ABACUS data.
Keywords: sea surface salinity; BEC SMOS products; Mediterranean Sea; Algerian Basin; ABACUS gliders
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Bourassa M.A., Meissner T., Cerovecki I., Chang P.S., Dong X., De Chiara G., Donlon C., Dukhovskoy D.S., Elya J., Fore A., Fewings M.R., Foster R.C., Guille S.T., Haus B.K., Hristova-Veleva S., Holbach H.M., Jelenak Z., Knaff J.A., Kranz S.A., Manaster A., Mazloff M., Mears C., Mouche A., Portabella M., Reul N., Ricciardulli L., Rodríguez E., Sampson C., Solis D., Stoffelen A., Stukel M.R., Stiles B., Weissman D., Wentz F. (2019)
Frontiers in Marine Science, 6, 443 DOI: https://doi.org/10.3389/fmars.2019.00443. (BibTeX: bourassa.etal.2019)
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Strengths and weakness of remotely sensed winds are discussed, along with the current capabilities for remotely sensing winds and stress. Future missions are briefly mentioned. The observational needs for a wide range of wind and stress applications are provided. These needs strongly support a short list of desired capabilities of future missions and constellations.
Keywords: satellite, wind, stress, ocean, requirements
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Cabré A., Pelegrí J.L., Vallès-Casanova I. (2019)
Journal of Geophysical Research: Oceans, 124, 7, 4820-4837. DOI: 10.1029/2019JC015160. (BibTeX: cabre.etal.2019)
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Here we explore the water transfer between the subtropical and tropical gyres of the South Atlantic Ocean to better understand its unique equatorward heat delivery. A Lagrangian technique is applied to the reanalysis product GLORYS2V4 in order to trace back the western boundary flow in the tropical (North Brazil Undercurrent, NBUC) and subtropical (Brazil Current) gyres. Most of the northward NBUC core transport (14.9 Sv at 8°S) arrives from the eastern boundary subtropical current (Benguela Current) via the zonal South Equatorial Current. This subtropical‐tropical transfer represents the core of the returning limb of the Atlantic meridional overturning circulation and accounts for most of the observed increase in heat and salt‐volume transports (0.18 PW and 0.19 Sv from 30°S to 8°S, respectively) across the South Atlantic. The NBUC also includes Antarctic Intermediate Water below 400 m (7.4 Sv at 8°S) coming from the interior subtropical gyre, as well as water from the current\'s surface and peripheral components coming from the tropical gyre (13.3 Sv at 8°S). The Brazil Current (9.9 Sv at 29°S) is mostly composed of subtropical water originating in the upper 800mwest of the eastern boundary current at 30°S (8.5 Sv), with a minor contribution of surface tropical water that transfers to the subtropics (1.4 Sv).
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Carnicer O., De La Fuente P., Canepa A., Keith I., Rebolledo-Monsalve E., Diogène J., Fernández-Tejedor M. (2019)
, 6, 235 DOI: 10.3389/fmars.2019.00235. (BibTeX: carnicer.etal.2019)
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It is likely that harmful algal blooms have increased in frequency, intensity, and geographic distribution in the last decades in response to anthropogenic activities. The Galápagos Islands are renowned for their exceptional biological diversity; however, marine dinoflagellate communities have not been represented in biodiversity assessments. Therefore, this study aims to provide key information about dinoflagellate diversity and abundances, with special attention to harmful species, during a weak La Niña event in the Galápagos Marine Reserve (GMR). The study was performed during March–April 2017 and four transects were conducted at four Islands (Santa Cruz, Santa Fé, Seymour, and Pinzón) representing the southern region of the GMR. Water net samples were collected at 2, 5, and 10 nautical miles (nm) from the coast, at a total of 48 sampling sites. The presence of toxic species, and their cell abundance was estimated in seven transects at 0, 15, and 30 m of depth. A total of 152 taxa belonging to 7 orders, 22 families, and 38 genera were registered. The number of taxa found is almost three times higher than the maximum observed in previous studies. Dinoflagellate species richness among stations ranged between 53 and 23 taxa and was higher in northern sites. From the applied cluster analysis, five dinoflagellate assemblages were identified as a discrete community structure, one was found only in Santa Fé Island, which is probably related to the presence of the Equatorial Undercurrent (EUC). Regarding cell abundance estimations, low abundances were registered throughout the sampling sites and no blooms were detected. Higher abundances were registered in the northern transects coinciding with one of the most productive areas of the archipelago, situated north of Santa Cruz. Among the identified taxa, 19 of them were potentially toxic, including epiphytic species, allowing the possibility of blooms in benthic areas. This study presents the first record of several dinoflagellate species in the area (both nontoxic and harmful species) and thus, emphasizing the need for the implementation of phytoplankton monitoring programs by the government to prevent potential ecological, sanitary and economic impacts in the GMR.
Keywords: harmful algal blooms, dinoflagellate assemblages, richness, spatial variability, CCA, Generalized Additive Models, environmental parameters
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Castellanos P., Olmedo E., Pelegrí J.L., Turiel A., Campos E.J.D. (2019)
Remote Sensing, 11, 7, 802. DOI: 10.3390/rs11070802. (BibTeX: castellanos.etal.2019a)
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Three of the world’s most energetic regions are in the tropical and South Atlantic: the North Brazil Current Retroflection, the Brazil-Malvinas Confluence, and the Agulhas Current Retroflection. All three regions display offshore diversions of major boundary currents, which define the intensity of the returning limb of the Atlantic meridional overturning circulation. In this work, we use a sea-surface salinity (SSS) satellite product, combined with a high-resolution numerical model and in situ measurements, in order to explore the seasonal variation of the surface currents and transports in these three regions. The analysis of the model output shows that the SSS patterns reflect the surface velocity structure, with the largest horizontal SSS gradients coinciding with those areas of highest velocity and the most predominant velocity vector being 90º anticlockwise (clockwise) from the horizontal SSS gradient in the northern (southern) hemisphere. This information is then applied to the SSS satellite product to obtain maps of water velocity and salt transports, leading to a quantitative tool to estimate both water and salt transports in key regions of the world ocean.
Keywords: Sea surface salinity; SMOS; Retroflections; Surface velocity; Water transport; Salt transport
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Corbella I., Torres F., Duffo N., Duran I., González-Gambau V., Martín-Neira M. (2019)
Remote Sensing, 11, 682 DOI: 10.3390/rs11060682. (BibTeX: corbella.etal.2019a)
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In microwave interferometric radiometers with a large field of view, as for example the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) onboard the Soil Moisture and Ocean Salinity (SMOS) satellite, one of the major causes of reconstruction error is the contribution to the visibility of the brightness temperature outside the fundamental period, defined on the basis of reciprocal grids. A mitigation method consisting of estimating this contribution through the application of a brightness temperature model outside the fundamental period is proposed. The main advantage is that it does not require any a posteriori addition of artificial scenes to the reconstructed image. Additionally, a method to avoid the sophisticated matrix regularization and inversion techniques usually applied in microwave interferometry is presented. Image reconstruction algorithms are implemented on a minimum grid size in order to maximize their numerical efficiency. An improved method to apply an apodization window to the reconstructed image for reducing Gibbs oscillations is also proposed. All procedures are generally described considering the single polarization case and successively implemented applying the MIRAS layout in both its single polarization and full polarimetric modes. Results show similar performance of the proposed algorithm with respect to the nominal one applied by SMOS. All algorithms are implemented in the MIRAS Testing Software and have been successfully used for scientific studies by other teams.
Keywords: interferometric radiometry; image reconstruction; error correction
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Corbella I., Torres F., Duffo N., Durán I., González-Gambau V., Oliva R., Closa J., Martín-Neira M. (2019)
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 12, 6, 1633-1646. DOI: 10.1109/JSTARS.2018.2885576. (BibTeX: corbella.etal.2019b)
Abstract: See
The Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) is formed by 69 total power radiometers, of which 3 are the noise-injection type. Their calibration is reviewed on the basis of the data gathered during more than eight years of operation. Internally calibrated gain and offset corrections with improved temporal stability are presented. New front-end loss characterization with lower seasonal dependence originated from external temperature swings is also proposed. Finally, a methodology to validate external calibrations, with the instrument pointing to the cold sky, is developed. It seems to indicate that the change of orientation of the instrument, with associated thermal variations, may induce small changes in the radiometer front-end losses, thus introducing calibration errors.
Keywords: SMOS, L-band radiometry, Interferometric synthetic aperture radiometry, Radiometer calibration
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Corcione V., Grieco G., Portabella M., Nunziata F., Migliaccio M. (2019)
IEEE Transactions on Geoscience and Remote Sensing, 57, 6, 3331-3340. DOI: 10.1109/TGRS.2018.2883364. (BibTeX: corcione.etal.2019a)
Abstract: See
In this paper, the synthetic aperture radar (SAR) azimuth cutoff method is thoroughly revised and a new and general implementation is proposed. The key roles of the pixel spacing, the size of the image box, and the texture of the SAR scene are analyzed and optimized in terms of azimuth cutoff estimation. The reliability of the azimuth cutoff estimation is analyzed by measuring the distance between the measured and fitted autocorrelation functions. This analysis shows that it is of paramount importance to ilter unfeasible/unreliable azimuth cutoff values. To identify those values in an objective way, a criterion that is based on the \"Chi-squared\" test performed over a large data set of Sentinel-1 SAR imagery is defined and proven to be effective. The new robust implementation of the azimuth cutoff estimation at about 1-km grid spacing is then used to produce averaged azimuth cutoff at about 10-km grid spacing. The performance of the new estimation procedure, analyzed using a azimuth cutoff-to-wind-speed forward model, is shown to provide improved wind speed retrievals, with a root-mean-square error of 1.8–2 m/s when verified against independent numerical weather prediction model output and scatterometer winds.
Keywords: Cutoff frequency; Sea surface; Spectral analysis; Synthetic aperture radar
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González-Haro C., Ponte A., Autret E. (2019)
Remote Sensing, 11, 19, 2313. DOI: 10.3390/rs11192313. (BibTeX: gonzalezharo.etal.2019)
Abstract: See
The expected amplitude of fixed-point sea surface temperature (SST) fluctuations induced by barotropic and baroclinic tidal flows is estimated from tidal current atlases and SST observations. The fluctuations considered are the result of the advection of pre-existing SST fronts by tidal currents. They are thus confined to front locations and exhibit fine-scale spatial structures. The amplitude of these tidally induced SST fluctuations is proportional to the scalar product of SST frontal gradients and tidal currents. Regional and global estimations of these expected amplitudes are presented. We predict barotropic tidal motions produce SST fluctuations that may reach amplitudes of 0.3 K. Baroclinic (internal) tides produce SST fluctuations that may reach values that are weaker than 0.1 K. The amplitudes and the detectability of tidally induced fluctuations of SST are discussed in the light of expected SST fluctuations due to other geophysical processes and instrumental (pixel) noise. We conclude that actual observations of tidally induced SST fluctuations are a challenge with present-day observing systems.
Keywords: sea surface temperature; satellite observations; tidal currents; internal tides
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Grieco G., Stoffelen A., Portabella M., Belmonte M., Lin W., Fabra F. (2019)
IEEE Transactions on Geoscience and Remote Sensing, 57, 5, 2990-3000. DOI: 10.1109/TGRS.2018.2879059. (BibTeX: grieco.etal.2019c)
Abstract: See
A quality control scheme for TechDemoSat 1 (TDS-1) and Cyclone Global Navigation Satellite System (GNSS) delay-Doppler maps (DDMs) is presented and the results of its application to a data set of more than 700 000 DDMs are discussed. This scheme is proven to be effective for such purpose and its output indices can be successfully used as quality indicators of the DDM. This paper shows that most of the TDS-1 DDMs are affected by some distortions that are attributable to an insufficiently accurate estimation of the specular point location. The errors, moreover, can severely alter the symmetry of the isodelay lines with respect to the iso-Doppler lines leading to an asymmetry in the arrival time of the waveforms. Furthermore, these errors may affect the convolution of the GNSS reflected signal with the Woodward ambiguity function, leading to an unwanted redistribution of the incoming echo energy among the DDM bins. Such distortions may, in turn, affect the accuracy of the wind field retrieval using either the stare processing approach or the more consolidated methods of inverting a Geophysical Model Function based on the DDM peak and/or leading edge slope.
Keywords: Accuracy; Error; Quality Control (QC); Reflected Global Navigation Satellite System (GNSS-R); Stare processing; Wind
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Guallar C., Flos J. (2019)
Progress in Oceanography, 176 (BibTeX: guallar.flos.2019)
Abstract: See
The link between phytoplankton primary production (PPP) and chromophoric dissolved organic matter (CDOM) has been demonstrated indirectly in the laboratory, but not directly in the sea. Here, for the first time, we report a strong link between PPP and CDOM in a study carried out in coastal marine waters around Barcelona (NW Mediterranean) in contrasting seasons. We measured relevant correlations between a254 CDOM concentration and both PPP (r=0.89, p < 0.001, n=16; r=0.76, p < 0.01, n=10; and r=0.68, p < 0.001, n=26; for summer, winter–spring and both seasons together, respectively) and the specific production rate at optimal light intensity (Pm B; r=0.77, p < 0.001, n=16; r=0.86, p < 0.01, n=10; and r=0.85, p < 0.001, n=26; for the same seasons). Based on our findings, we design a model that predicts PPP very well, using a254 CDOM, water temperature and photosynthetically active radiation (PAR) (R2=0.84, p < 0.001, n=26). Relationships between CDOM concentration parameters (measured at wavelengths of 280, 300, 340, 355, 375, 412 and 460 nm) and variables associated to CDOM processes (chlorophyll-a, bacterial abundance, turbidity related to sediment resuspension and PAR) were also evaluated. Bacterial abundance is significantly correlated with CDOM concentration measured at higher wavelengths when all samples are considered (e.g., r=0.82, p < 0.001, n=26, for a355 CDOM). However, in seasonal data analysis, the correlations decrease slightly (summer season: e.g., r=0.79, p < 0.001, n=16, for a355 CDOM) or become non-significant (winter– spring season: e.g., r=-0.02, p=0.95, n=10, for a355 CDOM). Sunlight photobleaching and sediment resuspension processes significantly influence CDOM dynamics in the summer (for a300 CDOM: r=-0.72, p < 0.001, n=16; and r=0.67, p < 0.01, n=16, respectively).
Keywords: Phytoplankton primary production Carbon uptake CDOM production Bacteria Photodegradation Sediment resuspension
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Gupta M., Gabarro C., Turiel A., Portabella M., Martinez J. (2019)
Journal of Glaciology, 65, (251), 481-493. DOI: 10.1017/jog.2019.26. (BibTeX: gupta.etal.2019a)
Abstract: See
Arctic sea ice is going through a dramatic change in its extent and volume at an unprecedented rate. Sea-ice thickness (SIT) is a controlling geophysical variable that needs to be understood with greater accuracy. For the first time, a SIT-retrieval method that exclusively uses only airborne SIT data for training the empirical algorithm to retrieve SIT from Soil Moisture Ocean Salinity (SMOS) brightness temperature (TB) at different polarization is presented. A large amount of airborne SIT data has been used from various field campaigns in the Arctic conducted by different countries during 2011–15. The algorithm attempts to circumvent the issue related to discrimination between TB signatures of thin SIT versus low sea-ice concentration. The computed SIT has a rms error of 0.10 m, which seems reasonably good (as compared to the existing algorithms) for analysis at the used 25 km grid. This new SIT retrieval product is designed for direct operational application in ice prediction/climate models.
Keywords: Remote sensing; Sea ice; Laser altimetry; Electromagnetic induction; Ice thickness measurements
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Hackert E.C., Kovach R.M., Busalacchi A.J., Ballabrera-Poy J. (2019)
Journal of Geophysical Research: Oceans, 124, 7, 4546-4556. DOI: http://dx.doi.org/10.1029/2019JC015130. (BibTeX: hackert.etal.2019)
Abstract: See
This study demonstrates the positive impact of including gridded Aquarius and Soil Moisture, Active/Passive (SMAP) sea surface salinity (SSS) into initialization of intermediate complexity coupled model forecasts for the tropical Indo‐Pacific. An experiment that assimilates conventional ocean observations serves as the control. In a separate experiment, Aquarius and SMAP satellite SSS are additionally assimilated into the coupled model initialization. Analysis of the initialization differences with the control indicates that SSS assimilation causes a freshening and shallowing of the mixed layer depth near the equator and enhanced Kelvin wave amplitude. For each month from September 2011 to September 2017, 12‐month‐coupled ENSO forecasts are initialized from both the control and satellite SSS assimilation experiments. The experiment assimilating Aquarius and SMAP SSS significantly outperforms the control relative to observed NINO3.4 sea surface temperature anomalies. This work highlights the potential importance of inclusion of satellite SSS for improving the initialization of operational ENSO coupled forecasts.
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Hernández-Guerra A., Talley L.D., Pelegrí J.L., Vélez-Belchí P., Baringer M.O., Macdonald A.M., McDonagh E.L. (2019)
Progress in Oceanography, 176 DOI: 10.1016/j.pocean.2019.102136. (BibTeX: hernandezguerra.etal.2019)
Abstract: See
Mass transports for the thermocline, intermediate, deep and abyssal layers in the Atlantic Ocean, at 30°S and for 2003 and 2011, have been estimated using data from GO-SHIP hydrographic transoceanic sections and applying three inverse models with different constraints. The uppermost layers comprise South Atlantic Central Water (SACW) and Antarctic Intermediate Water (AAIW), with a net northward transport in the range of 12.1–14.7 Sv in 2003 and 11.7–17.7 Sv in 2011, which can be considered as the northward returning limb of the Meridional Overturning Circulation (MOC). The western boundary Brazil Current transports twice as much SACW in 2003 (−20.2 ± 0.7 Sv) than in 2011 (−9.7 ± 0.7 Sv). A poleward current consisting of AAIW and Upper Circumpolar Deep Water (UCDW) flows beneath the Brazil Current. The eastern boundary Benguela Current, characterized by a high mesoscale eddy activity, transports 15.6 ± 0.9 Sv in 2003 and 11.2 ± 0.8 Sv in 2011, east of the Walvis Ridge. In the ocean interior, the northward flow is mainly located east of the Mid Atlantic Ridge (MAR) where Agulhas Rings (ARs), observed in both 2003 and 2011, transport warm and salty water from the Indian to the Atlantic Ocean. For the deep layers, the southward transport of North Atlantic Deep Water (NADW) occurs as the Deep Western Boundary Current and also in the eastern basin. The western and eastern basins transport similar amounts of NADW to the south during both years, although the eastern pathway changes substantially between both years. The total NADW transport, which is also considered the MOC, is in the range 16.3–24.5 Sv in 2003 and 17.1–29.6 Sv in 2011, hence with no significant change.
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Lavergne T., Sorensen A.M., Kern S., Tonboe R., Notz D., Aaboe S., Bell L., Dybkjaer G., Eastwood S., Gabarró C., Heygster G., Killie M.A., Kreiner M.B., Lavelle J., Saldo R., Sandven S., Pedersen LT. (2019)
The Cryosphere, 13, 49-78. DOI: 10.5194/tc-13-49-2019. (BibTeX: lavergne.etal.2019a)
Abstract: See
We introduce the OSI-450, the SICCI-25km and the SICCI-50km climate data records of gridded global seaice concentration. These three records are derived from passive microwave satellite data and offer three distinct advantages compared to existing records: first, all three records provide quantitative information on uncertainty and possibly applied filtering at every grid point and every time step. Second, they are based on dynamic tie points, which capture the time evolution of surface characteristics of the ice cover and accommodate potential calibration differences between satellite missions. Third, they are produced in the context of sustained services offering committed extension, documentation, traceability, and user support. The three records differ in the underlying satellite data (SMMR & SSM/I & SSMIS or AMSR-E & AMSR2), in the imaging frequency channels (37 GHz and either 6 or 19 GHz), in their horizontal resolution (25 or 50 km), and in the time period they cover. We introduce the underlying algorithms and provide an evaluation. We find that all three records compare well with independent estimates of sea-ice concentration both in regions with very high sea-ice concentration and in regions with very low sea-ice concentration. We hence trust that these records will prove helpful for a better understanding of the evolution of the Earth’s sea-ice cover.
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Lin W., Portabella M., Foti G., Stoffelen A., Gommenginger C., He Y. (2019)
IEEE Transactions on Geoscience and Remote Sensing, 57, 2, 655-666. DOI: 10.1109/TGRS.2018.2859191. (BibTeX: lin.etal.2019b)
Abstract: See
This paper presents a comprehensive procedure to improve the wind geophysical model function (GMF) for the Global Navigation Satellite System Reflectometry (GNSS-R) instrument onboard the TechDemoSat-1 satellite. The observable used to define the GMF is extracted from the measured delay- Doppler maps (DDMs) by correcting for the nongeophysical effects within the measurements. Besides the instrument and the geometric effects as provided in the bistatic radar equation, a calibration term that accounts for the uncalibrated receiver antenna gain and the unknown transmitter antenna gain is proposed to optimize the calculation of GNSS-R observables. Such calibration term is presented as a function of observing elevation and azimuth angles and is shown to remarkably reduce the measurement uncertainties. First, an empirical wind-only GMF is developed using the collocated Advanced Scatterometer (ASCAT) winds and European Centre for Medium-Range Weather Forecasts (ECMWF) model wind output. This empirical GMF agrees well with the model output. Then, the sensitivity of the observable to waves is analyzed using the collocated ECMWF wave parameters. The results show that it is difficult to include mean square slope (MSS) in the development of an empirical GMF, since the difference between ECMWF MSS and the MSS sensed by GNSS-R varies with incidence angle and wind speed. However, it is relevant to take significant wave height (Hs) in account, particularly for low wind conditions. Consequently, a wind/Hs approach is proposed for improved wind retrievals.
Keywords: Advanced Scatterometer (ASCAT); Calibration; Global Navigation Satellite System Reflectometry (GNSS-R); Wave; Winds
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Lin W., Dong X., Portabella M., Lang S., He Y., Yun R., Wang Z., Xu X., Zhu D., Liu J. (2019)
IEEE Transactions on Geoscience and Remote Sensing, 57, 2, 627-639. DOI: 10.1109/TGRS.2018.2858852. (BibTeX: lin.etal.2019c)
Abstract: See
The China-France Oceanography Satellite (CFOSAT) to be launched in October 2018 will carry two innovative payloads, i.e., the surface wave investigation and monitoring instrument and the rotating fan-beam scatterometer [CFOSAT scatterometer (CFOSCAT)]. Both instruments, operated in Ku-band microwave frequency, are dedicated to the measurement of sea surface wave spectra and wind vectors, respectively. This paper provides an overview of the system definition and characteristics of the CFOSCAT instrument. A prelaunch analysis is carried out to estimate the scatterometer backscatter and wind quality based on the developed CFOSCAT simulator prototype. The overall simulation includes two parts: first, a forward model is developed to simulate the ocean backscatter signals, accounting for both instrument and geophysical noise. Second, a wind inversion processor is used to retrieve wind vectors from the outputs of the forward model. The benefits and challenges of the novel observing geometries are addressed in terms of the CFOSCAT wind retrieval. The simulations show that the backscatter accuracy and the retrieved wind quality of CFOSCAT are quite promising and meet the CFOSAT mission requirements.
Keywords: Backscatter; Inversion; Measurement errors; Remote sensing; Rotating fan-beam scatterometer; Simulation; Wind.
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Olmedo E., González-Gambau V., Turiel A., Martínez J., Gabarró C., Portabella M., Ballabrera-Poy J., Arias M., Sabia R., Oliva R. (2019)
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 12, 7, 2486 - 2503. DOI: 10.1109/JSTARS.2019.2904947. (BibTeX: olmedo.etal.2019c)
Abstract: See
After more than eight years of the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) acquisitions, an exhaustive, empirical characterization of the biases and uncertainties affecting SMOS brightness temperatures over the ocean is possible. We show that both parameters strongly depend not only on the position in the field of view, but also on the geographical location of the acquisition. Metrics based on the differences between expected and theoretical values of the bias and the uncertainty are developed and used for quantitatively assessing the locations where SMOS errors are currently not accurately characterized. This characterization can be used for the definition of a new empirical SMOS sea surface salinity (SSS) bias correction, a better cost function retrieval, and more accurate filtering criteria, which are expected to lead to a better SMOS SSS Level 2 product. We present a new L2 SMOSSSS product based on the described investigation. The performance of this preliminary product is similar to that of the version v662 of the official L2 SMOS SSS product at medium and low latitudes. However, it provides a better coverage at high latitudes and coastal regions affected by radio frequency interference (RFI), which correspond to those regions where the SMOS errors are currently poorly estimated.
Keywords: Brightness temperature (TB) biases, brightness temperature uncertainty, coastal areas, radio frequency interference (RFI), sea surface salinity (SSS), Soil Moisture and Ocean Salinity (SMOS).
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Orué-Echevarría D., Pelegrí J.L., Alonso-González I.J., Benítez-Barrios V.M., De La Fuente P., Emelianov M., Gasser M., Herrero C., Isern-Fontanet J., Peña-Izquierdo J., Ramírez-Garrido S., Rosell-Fieschi M., Salvador J., Saraceno M., Valla D., Vidal M. (2019)
Data in Brief, 22, 185–194. DOI: 10.1016/j.dib.2018.12.004. (BibTeX: orueechevarria.etal.2019b)
Abstract: See
This oceanographic dataset was gathered during the TIC-MOC cruise, which was designed to characterize the dynamics of the Brazil-Malvinas Confluence. The cruise was carried on board the R/V Hespérides ,with departure from Ushuaia and arrival to Salvador de Bahía. A total of 66 conductivity-temperature-depth (CTD) stations were completed between 8 and 22 March 2015, offshore from the continental platform and within 45°S-35°S and 61°W-50°W. At each station, water samples were collected, which were used to calibrate the CTD salinity-oxygen sensors and to determine inorganic nutrient concentrations, and the horizontal current was measured. Along its track, the vessel recorded surface temperature and salinity, as well as the horizontal flow down to about 700 m. Lastly, eight position-transmitting drifters were launched and two profiling
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Orué-Echevarría D., Pelegrí J.L., Machín F., Hernández-Guerra A., Emelianov M. (2019)
Journal of Geophysical Research: Oceans, 124, 1, 527-554. DOI: 10.1029/2018JC014733. (BibTeX: orueechevarria.etal.2019d)
Abstract: See
The Brazil‐Malvinas Confluence arises from the frontal encountering of the subtropical Brazil Current and subantarctic Malvinas Current. It displays a complex regional circulation that is accompanied by mesoscale features and thermohaline intrusions. Here we combine altimetry and cruise data to describe the circulation pattern in the upper 2,000 m at two spatial scales encircling the frontal system. The major regional features appear south of the confluence latitude at 39–40°S: (a) a relatively weak Malvinas Current near 41°S, 56°W (28.3 ± 1.4 Sv), followed by its cyclonic retroflection; (b) an intense subtropical anticyclone (59.3 ± 10.7 Sv) that replaces the Brazil Current overshoot; and (c) a very intense subantarctic inflow (78.9 ± 13.7 Sv) near 53°W that is maintained through both an upstream (near 42°S) earlier diversion of the Malvinas Current and the cyclonic recirculation of the flow exiting east along the confluence. North of the confluence, the Brazil Current provides a net input of 30.8 ± 12.0 Sv (29.1 ± 8.3 Sv along the slope). The southern inflow splits nearly equal between barotropic and baroclinic contributions while the entire northern flow is essentially baroclinic. These northern and southern inputs add to an eastward along‐front transport of 109.7 ± 15.1 Sv, with significant contribution of highly oxygenated, relatively fresh Subantarctic Mode and Antarctic Intermediate Waters (58.7 ± 5.6 Sv). The regional circulation experiences substantial temporal variability, with southern waters flowing into the Brazil‐Malvinas Confluence through along‐slope and interior pathways and partly recirculating within the subtropical South Atlantic gyre.
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Orué-Echevarría D., Castellanos P., Sans J., Emelianov M., Vallès-Casanova I., Pelegrí J.L. (2019)
Geophysical Research Letters, 46, 22, 13234-13243. DOI: 10.1029/2019GL084246. (BibTeX: orueechevarria.etal.2019e)
Abstract: See
Ocean frontal systems may act both as barriers and mixers between different water masses, the latter thanks to very energetic structures with relatively short temporal and spatial scales. Here, we explore the high‐frequency temperature variability in the Brazil‐Malvinas Confluence through the joint analysis of novel high‐resolution SeaSoar measurements and sea surface temperature imagery. Surface spatiotemporal correlation scales range between 1.5 and 6 days and between 20 and 50 km, with the shortest scales along the shelf‐break path of the Brazil Current and over the confluence and the longest ones along the Malvinas Current. The spatial scales display minima along the front, at the surface because of the presence of brackish shelf waters and at the subsurface due to both mesoscale and submesoscale thermohaline intrusions. The smallest cross‐frontal vertical correlations, in the 5‐ to 10‐m range, are associated with submesoscale processes. Overall, temperature variability is enhanced at depth in the frontal system.
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Piles M., Ballabrera-Poy J., Muñoz-Sabater J. (2019)
Remote Sensing, 11, 95 DOI: 10.3390/rs11010095. (BibTeX: piles.etal.2019a)
Abstract: See
Soil moisture observations are expected to play an important role in monitoring global climate trends. However, measuring soil moisture is challenging because of its high spatial and temporal variability. Point-scale in-situ measurements are scarce and, excluding model-based estimates, remote sensing remains the only practical way to observe soil moisture at a global scale. The ESA-led Soil Moisture and Ocean Salinity (SMOS) mission, launched in 2009, measures the Earth’s surface natural emissivity at L-band and provides highly accurate soil moisture information with a 3-day revisiting time. Using the first six full annual cycles of SMOS measurements (June 2010–June 2016), this study investigates the temporal variability of global surface soil moisture. The soil moisture time series are decomposed into a linear trend, interannual, seasonal, and high-frequency residual (i.e., subseasonal) components. The relative distribution of soil moisture variance among its temporal components is first illustrated at selected target sites representative of terrestrial biomes with distinct vegetation type and seasonality. A comparison with GLDAS-Noah and ERA5 modeled soil moisture at these sites shows general agreement in terms of temporal phase except in areas with limited temporal coverage in winter season due to snow. A comparison with ground-based estimates at one of the sites shows good agreement of both temporal phase and absolute magnitude. A global asseSMent of the dominant features and spatial distribution of soil moisture variability is then provided. Results show that, despite still being a relatively short data set, SMOS data provides coherent and reliable variability patterns at both seasonal and interannual scales. Subseasonal components are characterized as white noise. The observed linear trends, based upon one strong El Niño event in 2016, are consistent with the known El Niño Southern Oscillation (ENSO) teleconnections. This work provides new insight into recent changes in surface soil moisture and can help further our understanding of the terrestrial branch of the water cycle and of global patterns of climate anomalies. Also, it is an important support to multi-decadal soil moisture observational data records, hydrological studies and land data assimilation projects using remotely sensed observations.
Keywords: SMOS; Soil moisture; Climatology; Trends; Signal decomposition
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Raj R.P., Chatterjee S., Bertino L., Turiel A., Portabella M. (2019)
Ocean Science, 15, 1729-1744. DOI: 10.5194/os-15-1729-2019. (BibTeX: raj.etal.2019a)
Abstract: See
The Arctic Front (AF) in the Norwegian Sea is an important biologically productive region which is wellknown for its large feeding schools of pelagic fish. A suite of satellite data, a regional coupled ocean–sea ice data assimilation system (the TOPAZ reanalysis) and atmospheric reanalysis data are used to investigate the variability in the lateral and vertical structure of the AF. A method, known as “singularity analysis”, is applied on the satellite and reanalysis data for 2-D spatial analysis of the front, whereas for the vertical structure, a horizontal gradient method is used. We present new evidence of active air–sea interaction along the AF due to enhanced momentum mixing near the frontal region. The frontal structure of the AF is found to be most distinct near the Faroe Current in the south-west Norwegian Sea and along the Mohn Ridge. Coincidentally, these are the two locations along the AF where the air–sea interactions are most intense. This study investigates in particular the frontal structure and its variability along the Mohn Ridge. The seasonal variability in the strength of the AF is found to be limited to the surface. The study also provides new insights into the influence of the three dominant modes of the Norwegian Sea atmospheric circulation on the AF along the Mohn Ridge. The analyses show a weakened AF during the negative phase of the North Atlantic Oscillation (NAO-), even though the geographical location of the front does not vary. The weakening of AF during NAO- is attributed to the variability in the strength of the Norwegian Atlantic Front Current over the Mohn Ridge associated with the changes in the wind field.
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Romera-Castillo C., Álvarez M., Pelegrí J.L., Hansell D.A., Álvarez-Salgado X.A. (2019)
Global Biogeochemical Cycles, 33, 9, 1162-1173. DOI: 10.1029/2018GB006162. (BibTeX: romeracastillo.etal.2019)
Abstract: See
Most dissolved organic carbon (DOC) sequestered in the deep ocean has residence times of decades to thousands of years, with clear implications for climate regulation, though some net removal is typically observed with increasing water mass age. Here, a high‐quality‐high‐resolution data set has allowed us to identify net additions of recalcitrant DOC in specific water masses of the deep South Atlantic. Overall, the South Atlantic is a net source of recalcitrant DOC, adding 0.027 ± 0.019 Pg C/year, while the North Atlantic is a net sink that removes 0.298 ± 0.141 Pg C/year. We find that the balance of addition/removal of recalcitrant DOC depends not only on the origin but also on the temperature, age, and depth of the water masses that circulate and mix in the Atlantic Ocean. Future changes in the water mass composition and circulation patterns due to climate change would eventually affect that balance, altering the carbon cycle.
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Salat J., Pascual J., Flexas M., Chin T.M., Vazquez-Cuervo J. (2019)
Ocean Dynamics, 69, 1067–1084. DOI: 0.1007/s10236-019-01285-z. (BibTeX: salat.etal.2019)
Abstract: See
Marine and atmospheric parameters, including temperature observations from surface to 80 m (at 6 depths) are measured sinceSeptember 1973 on a higher-than-weekly frequency, at a coastal station 4 km offshore L’Estartit (Costa Brava; NWMediterranean). This constitutes the longest available uninterrupted oceanographic time series in the Mediterranean Sea. Thepresent contribution focuses on observed climatic trends in temperature (°C/year) of air (AT; 0.05), sea surface (SST; 0.03), sea at80 m depth (S80T; 0.02) and sea level (SL; 3.1 mm/year) as well as comparison with trends estimated from coincident high-resolution satellite data. The trending evolution is not uniform across seasons, being significantly higher in spring for both ATandSST, while in autumn for S80T. Other climatological results are a stratification increase (0.02 °C/year in summer temperaturedifference between 20 m (S20T) and S80T), trends in summer conditions at sea (when S20T > 18 °C), estimated as 0.5 and0.9 days/year for the starting day and period respectively, and a decreasing trend of nearly 2 days/year in the period of conditionsfavourable for marine evaporation (when AT < SST). This last trend may be related to the observed decrease of coastalprecipitation in spring. The long-term consistency in the in situ SST measurements presents an opportunity to validate themulti-decadal trends. The good agreement for 2013–2018 (RMS 0.5–0.6, bias−0.1 to−0.2; trends of 0.09 °C/year in situ vs.0.06 to 0.08 °C/year from satellite) allows considering this observational site as ground truth for satellite observations and amonitoring site for climate change
Keywords: Oceanographic time series.Climate trends.Satellite ground truth.Stratification.Seasonality.Mediterranean Sea
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Torres G., Carnicer O., Canepa A., De La Fuente P., Recalde S., Narea R., Pinto E., Borbor-Córdova M.J. (2019)
Frontiers in Marine Science, 6, 145 DOI: 10.3389/fmars.2019.00145. (BibTeX: torres.etal.2019)
Abstract: See
Among marine phytoplankton, dinoflagellates are a key component in marine ecosystems as primary producers. Some species synthesize toxins, associated with human seafood poisoning, and mortality in marine organisms. Thus, there is a large necessity to understand the role of environmental variables in dinoflagellates spatialtemporal patterns in response to future climate scenarios. In that sense, a monthly four-year (2013–2017) monitoring was taken to evaluate dinoflagellates abundances and physical-chemical parameters in the water column at different depths. Sampling sites were established at 10 miles in four locations within the Ecuadorian coast. A total of 102 taxa were identified, corresponding to 8 orders, 22 families, and 31 genera. Eight potentially harmful genera were registered but no massive blooms were detected. The most frequent dinoflagellates were Gymnodinium sp. and Gyrodinium sp. Environmental variables showed different mixing layer thickness and a conspicuous and deepening thermocline/oxycline/halocline and nutricline depending on annual and seasonal oceanographic fluctuations. This study confirms that seasonal and spatial distribution of the environmental variables are linked to the main current systems on the Eastern Tropical Pacific, thus the warm Panama current lead to a less dinoflagellates abundance in the north of Ecuador (Esmeraldas), while the Equatorial Upwelling and the cold nutrient-rich Humboldt Current influence dinoflagellates abundance at the central (Manta, La Libertad) and South of Ecuador (Puerto Bolivar), respectively. Interannual variability of dinoflagellates abundance is associated with ENSO and upwelling conditions. Climate change scenarios predict an increase in water surface temperature and extreme events frequency in tropical areas, so it is crucial to involve policy-makers and stakeholders in the implementation of future laws involving long-term monitoring and sanitary programs, not covered at present.
Keywords: dinoflagellates, HABs, ENSO, tropical Eastern Pacific, nutrients, upwelling, humboldt current
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Viúdez A. (2019)
Journal of Fluid Mechanics, 859, R1 DOI: 10.1017/jfm.2018.892. (BibTeX: viudez.2019a)
Abstract: See
Exact solutions for multipolar azimuthal-mode vortices in two-dimensional Euler flows are presented. Flow solutions with non-vanishing far-field velocity are provided for any set of azimuthal wavenumbers m and arbitrary number n of vorticity shells. For azimuthal wavenumbers mD0 and mD1, the far-field velocity is a rigid motion and unsteady flow solutions with vanishing far-field velocity are obtained by means of a time-dependent change of reference frame. Addition of these first two modes, in the case of n = 1, results in a particular Chaplygin–Lamb (C–L) dipole, with continuous and vanishing vorticity at the vortex boundary. Numerical simulations suggest that this particular C–L dipole is stable.
Keywords: Vortex dynamics; Vortex flows
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742 Kb
Viúdez A. (2019)
Journal of Fluid Mechanics, 868, R1, 1-13. DOI: 10.1017/jfm.2019.234. (BibTeX: viudez.2019b)
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An exact solution of a baroclinic three-dimensional vortex dipole in geophysical flows with constant background rotation and constant background stratification is provided under the quasi-geostrophic (QG) approximation. The motion of the dipole is unsteady but the potential vorticity contours move rigidly. The vortex comprises three potential vorticity anomaly modes, with a radial dependence given by the spherical Bessel functions and with azimuthal and polar dependences given by the spherical harmonics. The first mode, or spherical mode, accounts for the horizontal asymmetry of the vortex dipole and curvature of the dipole’s horizontal trajectory. The second mode, or dipolar mode, accounts for the speed of displacement of the vortex dipole. A third mode, or vertical tilting mode, accounts for the dipole’s vertical asymmetry. The QG vertical velocity field has two contributions: the first one is octupolar and depends entirely on the dipolar mode, and the second one is dipolar and depends on the nonlinear interaction between dipolar and vertical tilting modes.
Keywords: baroclinic flows, quasi-geostrophic flows, vortex dynamics
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830 Kb
Viúdez A. (2019)
Journal of Fluid Mechanics, 878, R5, 1-11. DOI: 10.1017/jfm.2019.730. (BibTeX: viudez.2019c)
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An exact solution of a stable vortex tripole in two-dimensional (2-D) Euler flows is provided. The stable tripole is composed of an inner elliptical vortex and two small-amplitude lateral vortices. The non-vanishing vorticity field of this tripole, referred to as here as an embedded tripole because of the closeness of its vortices, is given in elliptical coordinates .; / by the even radial and angular order-0 Mathieu functions Je0./ce0./ truncated at the external branch of the vorticity isoline passing through the two critical points closest to the vortex centre. This tripole mode has a rigid vorticity field which rotates with constant angular velocity equal to 0Je0.1/ce0.0/=2, where 1 is the first zero of Je0 0./ and 0 is a constant modal amplitude. It is argued that embedded 2-D tripoles may be conceptually regarded as the superposition of two asymmetric Chaplygin–Lamb dipoles, separated a distance equal to 2R, as long as their individual trajectory curvature radius R is much shorter than their dipole extent radius.
Keywords: vortex dynamics, vortex instability, vortex interactions
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649 Kb
Zamanillo M., Ortega-Retuerta E., Nunes S., Estrada M., Sala M.M., Royer S.-J., López-Sandoval D.C., Emelianov M., Vaqué D., Marrasé C., Simó R. (2019)
Science of The Total Environment, 691, 736-748. DOI: 10.1016/j.scitotenv.2019.06.524. (BibTeX: zamanillo.etal.2019f)
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Transparent exopolymer particles (TEP) are an abundant class of suspended organic particles, mainly formed by polysaccharides, which play important roles in biogeochemical and ecological processes in the ocean. In this study we investigated horizontal and vertical TEP distributions (within the euphotic layer, including the upper surface) and their short-term variability along with a suite of environmental and biological variables in four distinct regions of the Southern Ocean. TEP concentrations in the surface (4 m) averaged 102.3 ± 40.4 μg XG eq. L−1 and typically decreased with depth. Chlorophyll a (Chl a) concentration was a better predictor of TEP variability across the horizontal (R2 = 0.66, p < 0.001) and vertical (R2 = 0.74, p < 0.001) scales than prokaryotic heterotrophic abundance and production. Incubation experiments further confirmed the main role of phytoplankton as TEP producers. The highest surface TEP concentrations were found north of the South Orkney Islands (144.4 ± 21.7 μg XG eq. L−1), where the phytoplankton was dominated by cryptophytes and haptophytes; however, the highest TEP:Chl a ratios were found south of these islands (153.4 ± 29.8 μg XG eq (μg Chl a)−1, compared to a mean of 79.3 ± 54.9 μg XG eq (μg Chl a)−1 in the whole cruise, in association with haptophyte dominance, proximity of sea ice and high exposure to solar radiation. TEP were generally enriched in the upper surface (10 cm) respect to 4 m, despite a lack of biomass enrichment, suggesting either upward transport by positive buoyancy or bubble scavenging, or higher production at the upper surface by light stress or aggregation. TEP concentrations did not present any significant cyclic diel pattern. Altogether, our results suggest that photobiological stress, sea ice melt and turbulence add to phytoplankton productivity in driving TEP distribution across the Antarctic Peninsula area and Atlantic sector of the Southern Ocean.
Keywords: Transparent exopolymer particles; Phytoplankton; Prokaryotes; Solar radiation dose; Southern Ocean