Trends in the surface wind-wave heights in the North Indian Ocean based on 50-year NCEP/NCAR reanalysis wind-wave data

Analysis of 50 years of hourly significant wave heights of the Indian Ocean between 30 degrees N and 60 degrees S is carried out and linear trends of mean significant wave heights and maximum significant wave heights are obtained on annual, seasonal and decadal scales. The significant wave heights (SWH) are obtained from a third-generation spectral wind-wave model forced with six-hourly NCEP-NCAR winds over the Indian Ocean. These SWHs are validated with measurements available in the north Indian Ocean and the correlation coefficient is more than 0.87. The climatology obtained from the spectral wind wave model matches well with the climatology reported in the literature using other models and satellite-based datasets. The maximum SWH ranged from 0.25 m to 22.5 m with the max-SWH exceeding 7 m South of 30 degrees S, and greater than 13 m between 45 degrees S-55 degrees S and 50 degrees E95 degrees E. In the Northern Indian Ocean (NIO), max SWH reached 5.5 m in the central Bay of Bengal and over 8 m in the western Arabian Sea. The annual trend of the mean SWH showed an overall positive trend in the Indian Ocean, except in the west BoB, northwest Arabian Sea, and a part of the central Indian Ocean. The maximum SWH also followed an overall similar positive trend as that of mean SWH albeit with a prominent negative trend in the BoB, southeastern and northwestern Arabian Sea. There was a significant difference in the SWH trend between the seasons both spatially and temporally for all other decades. However, the maximum SWH showed highly varying trends for all the seasons and decades. The variation in the mean SWH decadal trend showed greater variation in the lower latitudes compared to the higher latitudes.