Surface Wave Climatology and Its Variability in the North Indian Ocean Based on ERA-Interim Reanalysis

The climate over the north Indian Ocean (NIO) is one of the most dynamic in the world because of seasonally reversing monsoon winds. In this study, the climate of the NIO and the variability of its surface waves using the European Centre for Medium-Range Weather Forecasts (ECMWF) global atmospheric reanalysis product (ERA-Interim) for the period 1979-2012 are analyzed. Annual average significant wave height (SWH) of the NIO ranges from 1.5 to 2.5 m and the seasonal average is highest (3-3.5 m) during the monsoon period [June-September (JJAS)]. Swells propagating from the Southern Hemisphere are present in the NIO during the premonsoon [February-May (FMAM)] and postmonsoon [October-January (ONDJ)] periods. The waves are separated into wind seas and swells based on the wave energy statistical method. The results show that the NIO is swell dominated and that wind sea heights are lower compared to the swell heights. Higher wind sea and swell heights are observed during the monsoon in the western NIO because of strong cross-equatorial winds of the Somali (Findlater) jet. In the postmonsoon period, the eastern NIO shows a higher swell height than the western NIO shows. SWH shows an annual increasing trend in the western NIO. On a seasonal scale, the trends are increasing significantly in the monsoon compared to the postmonsoon period in a major part of the NIO, whereas the premonsoon period shows a decline in SWH. In the NIO, the monsoon is the dominant mode of variability and it covers 92% of the total variability. Wave climate is also influenced by the annual and interannual variability in monsoon wind and rainfall.