Influence of Natural Climate Variability on the Extreme Ocean Surface Wave Heights Over the Indian Ocean

Abstract

Extreme ocean waves are part of the climate system but responsible for significant impacts on coastal and offshore environments, structures, and populations. In the Indian Ocean (IO), the wind and wave climate can be significantly influenced by natural climate variability, such as the El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM), yet our understanding on their regional influence is limited, particularly for seasonal extremes. Here, seasonal extreme significant wave heights (SWHs) and winds in the IO are examined over the period 1957-2010 utilizing ERA-20C reanalysis data and the nonstationary generalized extreme value distribution to understand climatic extremes, by considering climate indices as covariates. ENSO influence on extreme SWHs include increases in the Bay of Bengal, southwest tropical IO (TIO), southern IO (SIO; a broad extension south of Australia), and South China and Philippine (SCP) Seas, and decreases in the Arabian Sea in boreal summer during El Nino. Extreme SWH responses to the IOD include increases in the eastern TIO, southwest TIO, and SIO in boreal autumn during its positive phase. Lastly, Southern Annular Mode not only significantly affects the SIO year round but has a weak influence in the northern and tropical IO. Composite analysis of ENSO and IOD events further highlight in phase combinations display less significant influence than out of phase combinations during summer, but not autumn. Mean and extreme wind responses are consistent with SWH responses to natural climate variability, and climate mode teleconnection patterns help explain the seasonal variations. Plain Language Summary Extreme ocean wave heights, produced mainly by large increases in surface wind speeds, can impact many vulnerable coastal populations and cause destruction to coastal or offshore structures. Therefore, better understanding of ocean wave height variability plays a vital role in potentially reducing such risk. This study focuses on the relationship between naturally occurring climate modes that drive wide spread atmospheric changes, namely the El Nino-Southern Oscillation, Indian Ocean Dipole, and Southern Annular Mode, and the wave climate of the Indian Ocean (IO). Climate mode influences on extreme wind and wave climates in the IO are analyzed using the nonstationary generalized extreme value distribution in combination with reanalysis data for 1957-2010. Composite analysis of different El Nino-Southern Oscillation and Indian Ocean Dipole phase combinations also highlights compounding impacts when they occur in (or out) of phase. Response patterns in the wind speeds, mean sea level pressure, and sea surface temperature also help to link the physical mechanisms responsible for driving the variations in the IO extreme wave heights. Patterns in the seasonal extreme wave and wind responses are found to be consistent with corresponding seasonal mean responses, suggesting similar teleconnection mechanisms are at work for both mean and extremes.