Downstream impact of the North Pacific subtropical sea surface temperature front on the North Atlantic westerly jet stream in winter

Abstract

Previous studies have documented the significant influences of the North Pacific subtropical sea surface temperature (SST) front (NPSTF) on the overlying atmospheric circulation. However, whether and how the NPSTF could affect remote circulations has not been well addressed. Here, we demonstrate that the enhanced NPSTF can induce a southward shift of the downstream North Atlantic westerly jet stream (NAWJS) in winter. A strong NPSTF with an increased meridional SST gradient induces a positive (negative) air temperature anomaly in its south (north) flank through the heat flux, leading to a direct thermal wind response and an indirect transient eddy feedback. The direct thermal wind response benefits a negative height anomaly south of Gulf of Alaska below 250 hPa and a positive one above 200 hPa, while the transient eddy activity could locally enhance and tilt northward with height. The kinetic energy significantly grows from the baroclinic energy conversion below 250 hPa over the NPSTF and from the barotropic energy conversion north of the NPSTF at the upper levels. This enhanced transient eddy activity gives rise to a negative height anomaly south of Gulf of Alaska through high-frequency transient eddy feedback forcing at all levels dominated by transient eddy vorticity flux. The anomalous low south of Gulf of Alaska favors an anomalous high over Canada and an anomalous low over the United States through Rossby energy propagation, causing the southward shift of the NAWJS. The numerical model further verifies the downstream influences of the NPSTF on the NAWJS in winter.