Illustration of the High Performance of SARAL Ka-Band Altimeter in Observing the Mesoscale and Coastal Oceanic Features - Example of the Central Mediterranean Sea
1Université de Toulon, CNRS/INSU, Aix Marseille Université, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 83957 La Garde, France; 2Institut National des Sciences et Technologies de la Mer (INSTM), 28 rue 2 Mars 1934, 2035 Carthage Salammbô, Tunisia; 3Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS), OMP, 14 avenue Edouard Belin, 31400 Toulouse, France; 4RHEA for European Space Agency, Earth Observation Directorate, ESRIN/EOP GMQ, Italy
In this study, the performance of along track SARAL/AltiKa data with regard to standard altimetry is analyzed for the first time over the Central Mediterranean Sea. Such a key coastal region connects the eastern and the western sub-basins of the Mediterranean Sea and holds a number of signiﬁcant dynamical processes covering the full spectrum of temporal and spatial scales. The relative performance of SARAL/AltiKa and Jason-2 data is assessed using comparisons with tide gauge measurements, and surface current observations from a ship Acoustic Doppler Current Profiler. The SARAL/AltiKa altimeter-derived geostrophic velocities are also compared to those derived from the research regional altimetric X-TRACK data set (already validated and analyzed in details by Jebri et al., 2016 over the study area). Results indicate the good potential of Ka-band satellite altimetry to capture higher resolution oceanic features. The analysis is further extended to the study of mesoscale processes associated to the surface circulation observed between April 2013 and September 2015. The evolution of the surface oceanic features is also analyzed by using remotely sensed sea surface temperature observations. The synergy of these combined data sets (i.e. SARAL/AltiKa, X-TRACK and sea surface temperature) allows a relatively good spatial coverage and clearly reveals mesoscale features never observed before with standard altimetry solely (in addition to the known Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre).
Utilizing SAR Imagery, Ocean Color, SST, and Radar Altimetry to Study Upwelling and Ocean Circulation in the Coastal Arabian Sea
Geospatial Research and Education Lab, Institute of Space Technology, Pakistan
The surface circulation of the Arabian Sea (AS) is primarily driven by ocean surface winds, which follow the Monsoon seasonal pattern. During the summer months, winds blow northward towards Pakistan, carrying moisture from the Arabian Sea, and also cause significant upwelling along the Oman and Eastern Arab coast. There are not many regular in-situ observations performed in the AS, and no long-term archive of in-situ observations of physical oceanography parameters exists. Present methods of observing coastal upwelling in the AS with SST and OC have limitations due to cloud coverage and dust storms. Microwave sensors offer the advantage of day-night coverage in nearly all weather conditions at high resolution; SAR imaging sensors can offer the resolution of a few meters, while the upcoming delay-Doppler / SAR altimeter missions can provide resolutions on the scale of 100 meters.
The interation of microwaves with the ocean surface at moderate incident angles results in Bragg scattering from short-scale waves, modulated by long-scale waves. Monomolecular biogenic slicks, byproducts of photosynthesis (enhanced due to upwelling), show low-backscatter in SAR intensity images due to Bragg wave damping. The same slicks can be observed in altimeter backscatter data too, however their wave damping causes the normal-incident altimeter backscatter brighter. Exploration of coastal upwelling using altimetry has been problematic because of the problems of coastal altimetry, along with lower resolution. The upcoming higher resolution delay-Doppler / SAR altimeters can open new vistas to explore coastal upwelling. We will present our current research results on detecting coastal upwelling and biogenic slicks in the coastal AS during summer monsoon months using L-band ALOS PALSAR SAR intensity imagery, and will discuss how SAR altimetery can be useful in this work also.
In the Arabian Sea, the significant variability of ocean surface currents has largely been missing in trying to understand the physical oceanography here. One established method for ocean surface currents generation from sequential SST and OC imagery is the Maximum Cross Correlation (MCC) method (Crocker et al., 2007) and the application of this method over the AS can generate a long-term record of ocean currents. Qazi et al. (2014) have also shown that advection of biogenic slicks can be tracked in sequential SAR imagery to determine ocean surface currents. This can be supported by deriving geostrophic current fields from radar altimetry and ocean surface winds from satellite scatterometry. Work to derive ocean surface current fields in the AS has just been started by our research group, and we will discuss the implications; we also expect to learn more about eddies in this region by deriving surface circulation (Qazi et al., 2014).
Crocker, R. I., Matthews, D. K., Emery, W. J., & Baldwin, D. G. (2007). Computing Coastal Ocean Surface Currents From Infrared and Ocean Color Satellite Imagery. IEEE Trans. Geosc. & Rem. Sens., 45(2), 435–447.
Qazi, W. A., Emery, W. J., & Fox-Kemper, B. (2014). Computing Ocean Surface Currents Over the Coastal California Current System Using 30-Min-Lag Sequential SAR Images. IEEE Trans. Geosc. & Rem. Sens., 52(12), 7559–7580.
Use of Coastal Altimetry Data in Submesoscale Process Studies
Korea Advanced Institute of Science and Technology (KAIST), Korea, Republic of (South Korea)
This work evaluates feasibility and capability of the use of coastal altimetry data in submesoscale process (hourly and km-scale) studies with comparisons among independent mesoscale and submesoscale observations including sea surface heights (SSHs, or sea surface elevations) obtained from coastal altimetry, tide gauges, and coastal radar-derived surface currents, and passive tracer maps obtained from geostationary ocean color imagery. The coastal surface currents are decomposed into current components associated with stream functions and velocity potentials, and their stream functions are comparable with mesoscale SSHs and contain finer scale features, i.e., submesoscale fronts and eddies, which are supported by Chlorophyll maps having hourly and 500-m resolution. Some of coastal altimeter data exhibit consistent mesoscale and submesoscale features and have a reasonable agreement with passive tracer maps as well.
Evaluation of Operational Altimeter-Derived Ocean Currents for Shelf Sea Applications - a Case Study in the NW Atlantic
1University of New Hampshire, United States of America; 2Rutgers University
It is now possible to examine long-term coastal ocean circulation dynamics as measured using in situ and space-based platforms by using a decade plus of data collected along the shelf of the NW Atlantic. Here we seek to evaluate the strengths and weaknesses of direct application of gridded altimeter-based ocean surface current products (OSCAR and GlobCurrent) in coastal process studies even though these products are expressly developed with open-ocean assumptions and applications in mind. In situ data come from platforms within the US Integrated Ocean Observing System (IOOS) and include upper ocean currents and hydrography. Several recent studies of the region suggest strong connections between variable geostrophic currents and water mass advection along the shelf and shelf-slope fronts. This investigation will intercompare currents derived from along-track altimeter data with those from OSCAR and GlobCurrent in their application to several regional process evaluations including mean and time-variable circulation in the Scotian-Gulf of Maine system, cross-correlation analysis with subsurface hydrography and currents, and cross-shelf gradients in derived currents.
Seasonal Circulation in the Northern Bay of Bengal with Special Reference to Shelf-Slope Region
University of Dhaka, Bangladesh, People's Republic of
The Bay of Bengal is very dynamic. One of the reasons is the huge quantities of fresh water and sediment that Bay receives annually from its adjoining river systems. Also the coastal as well as open Bay bathymetry is different and complex. The seasonal monsoonal rains and the seasonal cyclones are important attributes of this region which adds to the complexities of the regional oceanography. Though there are a large body of research work on the basin-wide hydrographic characteristics in the Bay of Bengal, especially on the seasonal time-scale, the several aspects of circulation is largely unexplored. The most explored circulation in the Bay of Bengal is the East India Coastal Current (EICC). In this study, to determine the current pattern, altimetry data of 1999-2014 have been used. The data has been analysed using a variety of tools including band-pass filtering, chi-square test, mean, standard deviation and linear trend analysis. In addition, to infer the circulation pattern geostrophic current and Ekman currents were computed. Also, sea surface temperature and salinity were examined in conjuncture with current pattern. The domain-averaged monthly mean sea surface height anomaly (SSHA) for the study period showed large inter-annual variability. The monthly mean climatology showed a distinct seasonality. However, the shelf-slope circulation in the northern Bay of Bengal remains largely unknown. It is in this context that the present study investigates the seasonal cycle of circulation with special focus on shelf-slope region in the northern Bay of Bengal.
The Norwegian Coastal Current Observed by CryoSat-2 SARIn Altimetry
1Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, Ås, Norway; 2DTU Space, Technical University of Denmark, Kgs. Lyngby, Denmark
The Norwegian Coastal Current (NCC) transports warm and relatively fresh water along the Norwegian coast and into the Barents Sea, with its origin in Baltic water entering Skagerrak. Along its way northward it is fed by additional freshwater discharge. The NCC is important for the regional marine ecosystem and contributes to the poleward transport of warm Atlantic Water, maintaining the relatively mild climate in northwest Europe.
Although satellite altimetry is a mature technique, globally observing the sea surface height with an accuracy of a few centimeters, numerous effects degrade the observations in the coastal zone. For example, the radar footprint is contaminated by land and bright targets, and the range and geophysical corrections become difficult to model. The rugged Norwegian coast presents a further challenge, and the NCC, at times only a few tens of kilometers wide, typically falls into a zone where conventional altimeters do not deliver reliable observations.
The European Space Agency's CryoSat-2 (CS2) satellite is the first to carry a SAR altimeter instead of the conventional pulse-limited system, resulting in higher range precision and along-track resolution. This allows for tracking finer structures of the sea surface and get closer to the coast. We use CS2 low resolution and SARIn observations, for the period 2012-2015, along the Norwegian coast and determine a mean dynamic topography (CS2MDT) that is validated using tide gauges. In turn, geostrophic surface currents are derived from both the CS2MDT and the operational coastal numerical ocean model of MET Norway and compared. For the first time, the NCC is revealed by space-geodetic techniques, giving confidence in the new-generation SAR altimeters for coastal sea-level recovery.