Influence of tropical cyclones on the 100-year return period wave height- A study based on 39-year long ERA5 reanalysis data

Abstract

Various engineering activities in a coastal and offshore environment require detailed assessment about the probable extreme wave characteristics in its lifetime to prevent human as well as property loss. The accurate estimation of wave height with a 100-year return period not only contribute to the level of protection but also the scale of investments. In this study, significant wave height return levels for a northwestern Bay of Bengal location are estimated using extreme value models; generalized extreme value (GEV) and generalized Pareto distribution (GPD). Both block maxima and initial distribution methods are used in the GEV and GPD incorporated with peak over threshold (POT) series. The study is based on the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis hourly data at 0.25ox 0.25o spatial resolution from 1979 to 2017. The GPD estimates show that the return levels do not follow prevailing seasonality over the location since the return levels greatly depend upon the extreme events and most of the extreme events are during the northeast monsoon season (October-January). The POT with October-January data gives 9.08 m as 100-year return level, and corresponding Indian southwest monsoon (June-September) return level is only 5.41 m. When different block periods ranging from 15 to 39 years are considered, the 50-year return levels range from 5.12 to 8.67 m for GEV with annual maxima. The influence of tropical cyclones (TCs) on return levels are examined considering different case studies, and the results show that the strength of a TC alters the return levels more significantly than the number of TCs in the observation. The 100-year return level decreases by ~30% when wave height during the two cyclones; Phailin and Hudhud are removed from the data set.