Hemispheric Asymmetry in Future Wave Power Changes: Seasonality and Physical Mechanisms

Abstract

This study inspects the global changes in ocean wave power (WP) by the end of the 21st century, relative to the 1979-2005 period, focusing on seasonality and physical mechanisms. The multimodel ocean wave simulations from WAVEWATCH-III, forced with surface winds simulated by seven different CMIP5 global climate models, are particularly examined. These models robustly show a hemispheric asymmetry in WP changes, that is, a decrease in the Northern Hemisphere but an increase in the tropics and Southern Hemisphere, with large seasonal and regional variations. Individual terms of differential WP, which include wave height and wave period changes, are analyzed to isolate the role of each process. Although wave height changes dominate the overall WP changes, wave period changes also play a comparable role in the Southern Hemisphere extratropics especially in the Indian Ocean sector during austral winter (12%). Wave period increase is strikingly higher in austral winter than summer, resembling swell height changes in the Southern Ocean. Intermodel correlation and composite analyses further indicate that the WP increase in the Southern Hemisphere is closely associated with positive Southern Annular Mode (SAM) trend. On the other hand, the WP decrease in the Northern Hemisphere is robustly related with weakened meridional surface temperature gradient during winter, a consequence of Arctic amplification. As a result, models with higher positive SAM trend and larger Arctic warming project stronger hemispheric asymmetries in WP changes.