Climatology and trends of the Indian Ocean surface waves based on 39-year long ERA5 reanalysis data

摘要

In this study, the wind and the surface waves in the Indian Ocean (IO) during 1979-2017 are studied based on the ECMWF ERA5 Reanalysis data. Long-term statistical analysis of extreme waves is carried out based on Generalized Extreme Value distribution using annual maxima and the spatial distribution of return levels for 50 and 100 years are studied. In general, the ERA5 significant wave height (H-s) and maximum wave height (H-max) show a good agreement with measured buoy data in the coastal (bias similar to 0.29 m) and deep waters (bias similar to 0.18 m). During the tropical cyclone, underestimation of H-s and H-max in the ERA5 data compared to buoy data is 2.7 and 1.4%, but in general the bias is large (similar to 0.69 m). Swell domination is observed in larger regions of the IO, whereas wind-seas are comparable to swells in the Southern Ocean. The meridional wind speed largely influences the spatial pattern of H-s in the North IO. The stronger winds over the Southern Ocean play a major role in generating higher waves at higher latitudes. Maximum value of the 100-year return level for H-s in IO is 17.8 m, whereas highest value of the H-s is 16.7 m and H-max is 32.0 m. Severe wave events are common at 50 degrees-60 degrees S and only during 25% of the time in a year, H-max is less than 5 m in this region. Ratio of the H-max to H-s varies from 1.46 to 2.3 with a mean value of 1.87. The 100-year return value of H-s changed by -4 to 5 m, when the length of the dataset is decreased from 39 years (1979-2017) to recent 20 years. On an average, from 1979 to 2017, the annual average H-max increased by 0.73 cm/year. In major areas of the IO, a clear decrease of the H-s is observed during 1991-2017, whereas during 1979-2017, an increase in H-s is found.