Assessment of Sea Surface Salinity obtain from SMOS and Aquarius Satellites over Indian Ocean
Calla, OPN¹; Dadhich, Harendra Kumar²; Singhal, Shruti^1
1International Centre for Radio Science, INDIA; ²International Center for Radio Science, INDIA

Historically, measurement of Sea Surface Salinity (SSS) have been sparse, being limited to ship based measurements, moorings and drifters. However, conditions have improved remarkably in last few years with the launch of the two key satellite missions; Soil Moisture and Ocean Salinity (SMOS) and Aquarius. This study has been stimulated so as to investigate the performance of SSS obtained from the both. SSS is one key parameter in oceanic and climate studies. Together with the sea surface temperature, SSS influences the density of the water masses, participates in the earth's water cycle and drives the thermohaline circulation. Frequent knowledge at a global scale with higher accuracy of SSS is important in climate predications and modeling. It even acts as indicator of amount of evaporation and precipitation.

The first ever satellite to measure SSS from space is SMOS which was launched by European Space Agency (ESA) on November 2, 2009 with a global earth coverage period of 3 days. It carries a passive microwave radiometer MIRAS (Microwave Imaging Radiometer using aperture Synthesis) that estimates SSS from the brightness temperature (BT) of the sea surface using L-band (1.4 GHz) frequency. After SMOS, the Aquarius Mission was launched by National Aeronautics and Space Administration (NASA) on June 10, 2011 which also focused on measuring SSS from space with a 7 day global earth coverage period. It too carries a microwave integrated 1.413 GHz Polarimetric radiometer along with a 1.26 GHz Scatterometer for winds measurement and for surface roughness correction. Both the mission aims in providing answer to the primal questions about how our oceans act in response to the changing climate and the water cycle.

In this paper, assessment is done of SSS obtained from both SMOS and Aquarius for couple of months over Indian Ocean (IO). The SSS values of the Southern Indian Ocean (SIO) are being investigated as the North Indian Ocean (NIO) is found much corrupted with the Radio Frequency Interference and even due to large variability of SSS in IO; the study area has been divided into different sub regions. The data of both the satellites at same location and of same processing level that is Level-2 have been procured and evaluated. Even the in-situ data sets available from the Research Moored Array for African-Asian-Australian Monsoon Analysis and prediction (RAMA) buoys were acquired and compared with the two satellite SSS on hand. The resolution factor is also being taken care for both onboard sensors. The resolution of SMOS L2 data products is 15 X 15 Km and for Aquarius there are three different resolutions according to the BEAM’s. BEAM 1 has a resolution of 76 X 94 Km, BEAM 2 has 84 X 120 Km and BEAM 3 has 96 X 156 Km. The data have been averaged of SMOS in the same way so as to match up with Aquarius resolution.

By this paper we want to convince the readers that measuring SSS from space is a practical idea. SSS remote sensing now bears no more scientific perils than other remote sensing techniques did in their formative years. Advancing technology with proper resources has significantly reduced the errors. There are some dispute in error estimation but the counter argument is that comparison made at 4 buoys locations suggests that the RAMA_SSS (In-situ) always lies between the SMOS_SSS (SSS measured by SMOS) and Aquarius_SSS (SSS measured by Aquarius). By this analysis one finds difference of about 0 - 2 p.s.u when SMOS_SSS is compared to RAMA_SSS where as when Aquarius_SSS is compared with RAMA_SSS an error range of 0 - 1.5 p.s.u is obtained. However, the difference between SMOS_SSS and Aquarius_SSS lies between 0 - 2.5 p.s.u. Thus one needs an assessment for a large period of time to find the unseen reality. This comparison will provide a new insight into Earth's water cycle and climate.