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Abstract Submission No. | ABS-2022-04-0109 |
Title of Abstract | Generation, Evaluation and Inter Annual Variability (1993-2020) Analysis of Satellite Derived Total (Geostrophic + Ekman) Ocean Surface Currents in the Tropical Indian Ocean |
Authors | Rajesh Sikhakolli*, Siva Shankar Manche, Shashank Misra, R. K. Naik, Seshasai M. V. R |
Organisation | NRSC, ISRO |
Address | Earth and Climate Sciences Area, National Remote Sensing Center, ISRO, Balanagar Hyderabad, TELANGANA, India Pincode: 500036 Mobile: 9227760711 E-mail: rajesh_s@nrsc.gov.in |
Country | India |
Presentation | Oral |
Abstract | Conventionally, Ocean Surface Current (OSC) is measured using moored as well as drifting buoys. Even though these conventional observations are proved to be invaluable and have improved our understanding of ocean circulation quite significantly, their coverage is very limited. As far as the data-sparse Indian Ocean is concerned, the surface current observations are very limited and are mainly restricted to the near equatorial region covered by the RAMA buoy network. On contrary, measuring any physical parameters of Ocean through satellites provides the advantage of synoptic repetitive coverage which is not possible by any other means, thus becomes a very important tool for long-term monitoring of global Ocean circulation. Even though there is no operational satellite service available for direct measurement of ocean surface currents from space, scientists have devised mechanisms to estimate this important variable from other satellite derived parameters. In general the absolute Ocean surface current comprises of two parts based on their forcing mechanisms. The first part is the Geostrophic Current (Proportional to the slope of the dynamic topography) arising due to the balance between the pressure gradient force and the Coriolis force (pseudo force due to earthâ¿¿s rotation). Since the beginning of the era of satellite altimetry the geostrophic current is regularly mapped using altimeter derived sea surface height (SSH). The second part is the ageostrophic or Ekman current mainly arising by the frictional force due to wind stress on the Ocean surface. The satellite scatterometers which measures the ocean surface roughness, in turn the vector winds (OVW) provides this information. By synergic use of these two satellite derived parameters the absolute OSC is derived and used widely to understand the intraseasonal and interannual variability of mesoscale Ocean general circulation. However, the equatorial current features could not be reproduced with reasonable accuracy due to equatorial singularity problem. To overcome this Bonjean and Lagerloef, (2002), developed a new diagnostic model using quasi-linear and steady physics. They used three surface variables, namely SSH, OVW and sea surface temperature (SST), all available from satellite measurements, in order to derive the surface current product, known as Ocean Surface Current Analysis Real time (OSCAR). This model formulation combines geostrophic, Ekman, and Stommel shear dynamics, and a complementary term from surface buoyancy gradient through SST data. Since local acceleration is neglected, this diagnostic model is able to calculate the low-frequency motions in near equatorial region with improved accuracies compared to previous studies. This OSCAR OSC product is currently available at 0.33o x 0.33o grid and at five day interval only. Recently, AVISO has started providing daily merged altimeter products on an experimental basis by combining the various satellite altimeter data. Taking advantage of the availability of high resolution daily gridded SSH, OVW and SST data, in the present study we have generated a daily OSC product at 0.25o x 0.25o grid for a long period of more than 28 years (From 1993 to 2020). In the present study we have evaluated this long term high resolution OSC product using RAMA buoy currents and also presented the results of Interannual variability of major current systems in the TIO using this long term data set generated using satellite data. The validation results shows very good agreement with RAMA buoy and HF Radar data with RMSE and correlation ranging from 10 to 30 cm/s and 0.5 to 0.9 respectively. In general excellent match is observed for off equatorial regions and relatively poor results are observed for near equatorial regions especially for meridional currents. The Interannual variability of EICC, WICC, and Monsoon Current shows interesting results associated with basin scale events such as Indian Ocean Dipole. |