Intra-seasonal variability in Oceansat-2 scatterometer sea-surface winds over the Indian summer monsoon region

Abstract

In September 2009, the Indian Space Research Organisation launched a Ku-band microwave scatterometer (OSCAT) onboard the polar orbiting satellite ‘Oceansat-2’. In this article, the capabilities of the newly available OSCAT sea-surface winds are demonstrated by studying the monsoon intra-seasonal variabilities during the 2010 summer monsoon season. A preliminary validation of OSCAT surface winds with European Centre for Medium Range Weather Forecasting (ECMWF) analysis surface winds carried out during June to August 2010 suggests that the quality of the OSCAT winds are able to meet the mission specifications. The observed mean monthly features of the Indian summer monsoon in July and August 2010 from OSCAT match well with those of ECMWF reanalysis winds. The OSCAT winds capture the known characteristics of the Indian summer monsoon, such as the northward propagation of a low level jet, and its preferred locations during active and break monsoon conditions, reasonably well. The Morlet wavelet transform is used for time series analysis. The OSCAT measured sea-surface winds were found to possess two dominant modes of variability during the 2010 monsoon season: one with a periodicity between 32 and 64 days, and another with a periodicity between 8 and 16 days. Rainfall activity over the Indian summer monsoon region is closely associated with the phases of the two above-mentioned dominant intra-seasonal variabilities. This study demonstrates that the OSCAT winds can be used very well and with confidence for meteorological studies.