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.

This study uses ERA5 reanalysis data from 1940 to 2022 to investigate the decadal variation characteristics and causes of the correlation between sea surface temperature (SST) anomalies in North Atlantic and surface air temperature in northwestern Eurasia (NWESAT). The research reveals that the correlation between North Atlantic SST and NWESAT is significant during two high epochs, 1940—1981 and 2001—2008. During these epochs, the SST anomalies associated with NWESAT exhibit a horseshoe pattern. In contrast, during low epochs, the SST anomalies display a tripole pattern. Further analysis of the reasons behind the changes in correlation shows that, in the high epoch, SST anomalies lead NWESAT, with higher sea surface temperatures north of the Gulf Stream, creating better moisture conditions, more precipitation, and stronger upper-level diabatic heating. Additionally, there is more active eddy-mediated processes above the Gulf Stream, with stronger wave sources in the upper atmosphere to the north of the current, promoting the development of upper-level wave trains. In low epoch, the SST anomalies mainly exhibit a tripole pattern, with lower sea surface temperatures north of the Gulf Stream, weakening the ocean influence on the atmosphere and causing a lag in SST anomalies relative to NWESAT. Therefore, the decadal variations in the correlation between North Atlantic SST anomalies and NWESAT are primarily driven by differences in SST anomaly patterns and their modulation of atmospheric wave trains over time.

Using the numerical results of community atmosphere model Version 5 (CAM5), the influence mechanism of the oceanic front in the gulf stream (GS) and its extension region (referred to as GS front) on the upward airflow in the extratropical cyclone is explored. In the control experiment, composite results of cyclonic turbulences appearing in the GS region show that when the cyclonic turbulence is located over the oceanic front, it develops rapidly, and two strong upward airflows appear in the south of the oceanic front and the north of the cyclonic turbulence center (near 50°N, north of the oceanic front). Through the energy budget analysis, it is found that when the center of the cyclonic turbulence reaches the oceanic front, eddy available potential energy (EAPE) and eddy kinetic energy (EKE) in the center and north of the turbulence are significantly enhanced, which is closely related to the heat and water vapor provided by the oceanic front. Then, the results of the comparative experiments of the strength of the oceanic front show that the influences of the oceanic front on EAPE and EKE equations are different before and when the cyclonic turbulence center reaches the oceanic front. Before the turbulence reaches the oceanic front, the oceanic front mainly enhances baroclinic generation term (a source term in the EAPE equation) in the north of the front by affecting eddy heat and water vapor transport to high latitudes, which provides energy for the development of EKE in the north side of the cyclonic turbulence center (also in the north side of GS front). When the cyclonic turbulence center reaches the oceanic front, the ascending motion and precipitation in the north of the turbulence center are greatly enhanced, which mainly enhances diabatic heating term (another source term in the EAPE equation) in the north of the turbulence and continues to promote the development of the turbulence.