The Regime Shift of North Atlantic Sea Surface Temperature in the 1990s and Its Impact on Atmospheric Rivers

The authors investigates the variation characteristics of the leading mode of sea surface temperature (SST) anomalies in the North Atlantic during 1980—2001 and analyzes its influence on the North Atlantic Oscillation (NAO) and Atmospheric Rivers (ARs). The results indicate that the leading pattern of North Atlantic SST anomalies predominantly exhibits a tripolar structure (North Atlantic Tripole, NAT) during 1980—2001. However, in the early winter of the 1990s, the leading mode shifts to a horseshoe-like pattern (North Atlantic Horseshoe, NAH), which exerts a significant impact on the atmospheric circulation in late winter. Analysis of the physical mechanism reveals that the NAH-like SST anomalies, through the transport of water vapor and heat at the sea-air interface exchange, alter the direction of atmospheric wind fields and moisture transport, which lead to a northward shift in the tracks of ARs. When ARs made landfall in northern Europe, enhanced precipitation-induced diabatic heating over Norway intensified baroclinic instability and storm track activity in this region, further triggering an eastward-shifted NAO anomaly pattern. In contrast, during the early winters of the 1980s, SST anomalies exhibited the NAT-type pattern, and under the influence of abnormally warm SSTs in the Gulf Stream region, the NAO phase rapidly reversed and AR pathways shifted southward.