Statistical descriptions of extreme met-ocean conditions are essential for the safe and reliable design and operation of structures in marine environments. The significant wave height ( \({H}_{S}\) ) is one of the most essential wave parameters for coastal and offshore structural design. Recent studies have reported that a time-varying component exists globally in the \({H}_{S}\) . Therefore, the non-stationary behavior of an annual maximum series of \({H}_{S}\) is important for various ocean engineering applications. This study aims to analyze the frequency of \({H}_{S}\) over the northern Indian Ocean by modeling the non-stationarity in the \({H}_{S}\) series using a non-stationary Generalized Extreme Value (GEV) distribution. The hourly maximum \({H}_{S}\) data (with a spatial resolution of 0.5° longitude × 0.5° latitude) collected from the global atmospheric reanalysis dataset of the European Centre for Medium-Range Weather Forecasts (ECMWF) is used for the study. To model the annual maximum series of \({H}_{S}\) using a non-stationary GEV distribution, two physical covariates (El-Ni \(\widetilde{n}\) o Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD)) and time covariates are introduced into the location and scale parameters of the GEV distribution. The return levels of various frequencies of \({H}_{S}\) are estimated under non-stationary conditions. From the results, average increases of 13.46%, 13.66%, 13.85%, and 14.02% are observed over the study area for the 25-year, 50-year, 100-year, and 200-year return periods, respectively. A maximum percentage decrease of 33.3% and a percentage increase of 167% are observed in the return levels of various return periods. The changes in the non-stationary return levels over time highlight the importance of modeling the non-stationarity in \({H}_{S}\) .
