Analyzing the Relationships Between Southern Hemisphere Westerly Wind Strength and Latitude, SAM, ENSO, and Pacific Climate Conditions

Abstract

Southern Hemisphere westerly winds (SHW) form a broad band across the midlatitudes, steering weather systems toward southern continents and driving circulation in the Southern Ocean. These winds have intensified and shifted poleward in recent decades, but the factors associated with their year-to-year changes across the Pacific remain uncertain. This study examined Pacific-sector SHW strength and latitude from 1979 to 2018 in relation to the Southern Annular Mode (SAM) and El Niño–Southern Oscillation (ENSO). Full-period correlations, nine-year moving correlations, and a lagged comparison between December–February ENSO and the following June–August winds were combined with maps of mean sea-level pressure (MSLP) and sea-surface temperature (SST). Spatial relationships were examined for the full record and separately during El Niño and La Niña years. Positive SAM conditions were associated with stronger winds (r=0.73 p<0.001) and a more poleward wind belt (r=-0.64, p<0.001). ENSO had a weaker lagged relationship with following-winter wind strength (r=0.36, p=0.023) and no significant relationship with latitude (r=0.16, p=0.326). Pressure and temperature patterns also differed between El Niño and La Niña years, suggesting that ENSO may modify how the SAM–SHW relationship appears across time and location. Years with stronger winds tended to have higher MSLP across approximately 20°S–45°S, lower MSLP across approximately 50°S–70°S, and cooler SSTs near 50°S–60°S in the central South Pacific. Changes in SHW latitude were associated with a concentrated MSLP pattern near 40°S–55°S and weaker, more scattered SST relationships. These findings identify SAM as the climate mode most consistently associated with SHW variability, while ENSO may alter its regional expression. Future work should directly test the SAM–ENSO interaction and evaluate whether these relationships persist in longer, independent climate records