On the secular trends in sea ice extent over the antarctic region based on OCEANSAT-1 MSMR observations

Space-based passive microwave radiometers have been used to monitor southern ocean sea ice characteristics over the Antarctic region for the last 25 years. Decade-long observations from SMMR onboard Nimbus-7 during 1978-1987 indicated significant long-term trends in sea ice cover over different regions of the southern polar ocean, in addition to the strong seasonal and inter-annual variability in various sea ice characteristics. In the post-SMMR era, passive microwave monitoring of the polar region has continued with the availability of SSM/I onboard DMSP series of satellites. With the launch of MSMR onboard India's OCEANSAT-1 in May 1999, an independent set of multi channel passive microwave observations of the Antarctic and Arctic sea ice have become available. In this paper, we present the analysis of the first two years of MSMR observed brightness temperatures to infer sea ice extent over the Antarctic region. The analysis brings out the characteristic seasonal variability of sea ice extents over different sectors of the Southern Polar Ocean. The annual average sea ice extent values esimated from MSMR for 1999-2001 for important regions, e.g. the Weddell Sea and Ross Sea sectors, are found to be significantly different compared to the average values observed through SMMR. The MSMR estimates suggest a continuation of the secular trends observed during the SMMR period. The trend in the sea ice extent over the Antarctic region as a whole was observed to be slightly positive (0.043 million km 2 per year). The MSMR estimates of the sea ice extent over the entire Southern Ocean region, of 12.634 million km 2 during 1999-2001, support the continuation of the secular increase observed during the SMMR as well as SSM/I periods. The observed increasing trend in sea ice extent derived from SMMR and MSMR data is discussed along with some more recent estimates derived from SSM/I measurements, with possible indications of an acceleration in time. In the current greenhouse induced global warming scenario, with amplified warming over the polar region, this represents an intriguing result requiring continued investigations