Simulated Global Swell and Wind-Sea Climate and Their Responses to Anthropogenic Climate Change at the End of the Twenty-First Century

Abstract

he seasonal structure of the wind sea and swell is analyzed from the existing 29-yr surface gravity wave climatology produced using a coupled atmosphere?wave model. The swell energy fraction analysis shows that swell dominates most of the World Ocean basins for all four seasons, and the Southern Ocean swells dominate swell in the global ocean. The swells are loosely correlated with the surface wind in the midlatitude storm region in both hemispheres, while their energy distribution and propagation direction do not show any relation with local winds and vary significantly with season because of nonlinear interactions. The same coupled system is then used to investigate the projected future change in wind-sea and swell climate through a time-slice simulation. Forcing of the coupled model was obtained by perturbing the model sea surface temperatures and sea ice with anomalies generated by representative Working Group on Coupled Modelling (WGCM) phase 3 of the Coupled Model Intercomparison Project (CMIP3) coupled models that use the IPCC Fourth Assessment Report (AR4) A1B scenario late in the twenty-first century. Robust responses found in the wind seas are associated with modified climate indices. A dipole pattern in the North Atlantic during the boreal winter is associated with more frequent occurrence of the positive North Atlantic Oscillation (NAO) phases under global warming, and the wind-sea energy increase in the Southern Ocean is associated with the continuous shift of the southern annular mode (SAM) toward its positive phase. Swell responses are less robust because of nonlinearity. The only consistent response in swells is the strong energy increase in the western Pacific and Indian Ocean sector of the Southern Ocean during the austral winter and autumn.