Link of the Warm Arctic Cold Eurasian pattern to the Southern Annular Mode variability

Abstract

The Warm Arctic–Cold Eurasian pattern (WACE) is a pair of opposing surface air temperature anomalies over the polar region and the midlatitude Eurasian continent, which is usually associated with Arctic Sea ice melt and midlatitude extreme weather. The Southern Annular Mode (SAM) is the leading mode of atmospheric variability in the Southern Hemisphere. Here we show evidence of an inter-hemispheric connection between SAM and WACE during boreal winter. A positive SAM leads to a negative WACE (and vice versa) 30–40 days later. This lagged remote influence of SAM is transmitted via a Rossby wave that crosses the equator in the eastern Pacific and tropical Atlantic. The ensuing WACE temperature pattern is then amplified by the greenhouse effect associated with the local humidity response. SAM also leads to a convection anomaly in the equatorial Atlantic which generates a Northern Hemisphere Rossby wave that reinforces WACE. The link between SAM and WACE revealed here indicates a dynamical origin for WACE. This long-distance teleconnection, and the lagged association between SAM and WACE may be relevant for subseasonal-to-seasonal predictions.