Southern annular mode impacts on global ocean surface waves

Abstract

We assess the impact of the Southern Annular Mode (SAM) on global ocean wind waves using 30 years of wave data from a wave model hindcast that is forced with high resolution surface winds from the NCEP-CFSR reanalysis. Our primary focus is on the wave field and swell influence in the Southern Hemisphere, and we apply our analysis to each of the four temperate-zone seasons comprising March–May (MAM), June–August (JJA), September-November (SON), and December–February (DJF). Statistically significant anomalies in significant wave height (Hs), peak wave period (Tp) and zonal wave energy flux (CgE) are found to covary with local variations in surface zonal wind induced by the low and high polarity phases of the SAM.The signature of the SAM in ocean surface waves extends beyond local wind-generated forcing in the Southern Hemisphere extratropics to remote forcing in the Northern Hemisphere extratropics during DJF, with the associated atmospheric signal resembling a tropically-forced Rossby wave train. This has a significant impact on surface waves in the central North Pacific and North Atlantic oceans. The El Niño Southern Oscillation (ENSO) acts to weaken the link between the SAM and Northern Hemisphere winter climate, with the zonal wind / divergence anomalies amplifying over the tropical Pacific Ocean when we exclude El Niño and La Niña years from the analysis. This remote link to the SAM thus occurs independent of ENSO via SAM-driven changes to the tropical atmospheric circulation, suggesting that the SAM may provide a useful predictive signal for the Northern Hemisphere during DJF. The SAM also influences the occurrence of daily low (below the 5th percentile) and high (above the 95th percentile) wave conditions, emphasizing that the SAM may be a valuable source of predictable wave variability from sub-weekly to seasonal timescales. Future work should explore wave data suitable for trend analysis, considering the positive trend in the SAM over recent decades.