Satellite based study on the surface currents in the Indian Ocean

Based on the methods developed in his PhD studies The circulation of the Norwegian Sea - an investigation from space and ocean (2014) Dr. Roshin P. Raj has calculated the ocean surface current velocities in the Northern parts of the Indian Ocean. His study Surface velocity estimates of the North Indian Ocean from satellite gravity and altimeter missions was recently published in the International Journals of Remote Sensing. 

Monthly climatologies of surface geostrophic velocity (speed in colours) for the Indian Ocean for (a) January, (b) February, (c) March, (d) April, (e) May, (f) June, (g) July, (h) August, (i) September, (j) October, (k) November and (l) December.Monthly climatologies of surface geostrophic velocity (speed in colours) for the Indian Ocean for (a) January, (b) February, (c) March, (d) April, (e) May, (f) June, (g) July, (h) August, (i) September, (j) October, (k) November and (l) December.The circulation in the North Indian Ocean (NIO) is one of the most complex systems compared with other regions of global oceans, mostly due to its interactions with the monsoon winds. In recent years, the ability to measure the ocean’s mean dynamic topography (MDT) from space has improved immensely with the availability of satellite gravity measurements from Gravity Recovery and Climate Experiment (GRACE) and Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) missions. In the published study only satellite data - from GOCE and GRACE satellite gravity missions together with satellite radar altimeter data in retrieving the geoid, satellite-only MDT, and surface velocities has been calculated to the North Indian Ocean. The study estimates geoid heights of the NIO from all five recalculations and releases of the direct approach and the time-wise GOCE gravity field data. The formal error associated with geoid heights at different resolutions is found to be the lowest for the latest release of direct approach GOCE data. In addition, a new satellite-only MDT is estimated from the direct approach using the GOCE geoid and the CNES_CLS11 mean sea surface estimates. This MDT corrected to a 20-year time reference is used together with the newly reprocessed sea level anomaly data to estimate absolute dynamic topography and surface geostrophic velocities in the NIO. The total surface velocities computed from the Ekman and geostrophic velocity fields reproduce all major observed surface currents in the NIO, along with their seasonal variability. Furthermore, total surface velocity estimates computed in this study are validated using surface drifters and are found to be highly comparable (difference within ± 10 cm s-1) with more than 170,000 individual surface drifter observations. The total surface velocities estimated in this study are are used to examine the variability of the East Indian Coastal Current. The temporal evolution of the total speed of EICC during the past two decades (1993– 2014) revealed annual and inter-annual variability to dominate, but with no significant long-term trend. The published paper demonstrates the potential of a satellite-only MDT in retrieving realistic surface velocities in the India Ocean.

Citation: Roshin P. Raj (2017) Surface velocity estimates of the North Indian Ocean from satellite gravity and altimeter missions, International Journal of Remote Sensing, 38:1, 296-313. http://dx.doi.org/10.1080/01431161.2016.1266106  

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