Tropospheric Circulations Modulating the Boreal Winter Lower Stratospheric Polar Vortex

Abstract. Here we present an observational study of the variations in the lower stratospheric polar vortex (LSPV) associated with the main tropospheric circulation patterns in the Northern Hemisphere in winter (November to March). The LSPV is based on daily geopotential height at 100 hPa, and the circulation patterns are based on the empirical orthogonal function analysis of geopotential height anomalies at 500 hPa. The LSPV is found to strengthen with the Ural trough and the negative phase of the Pacific-North America (PNA-) pattern, and to weaken with the Ural ridge and the PNA+ pattern. These relations result first from the anomalous poleward, isentropic transport of oceanic air masses on the western flank of the positive tropospheric geopotential height anomalies, and of continental air masses on the eastern flank of the negative anomalies. Secondly, they result from troposphere-stratosphere propagation of anomalies, which may intensify or flatten the stationary anomalies in the stratosphere. In particular, the northeast Asia trough is deepened during the Ural ridge, and the Alaska ridge is enhanced during PNA+ regimes, which increase meridional fluxes of heat and cause a weakening of the polar vortex. This work thus supports wave-propagation-and-interferences based theory for polar vortex variations in previous studies, while highlighting the role of certain tropospheric circulation patterns. The large-scale anomalous advection of airmasses explain coherently tropospheric and stratospheric changes associated with the circulation patterns, and suggest covariations of the LSPV with clouds, latent heating, precipitation, and radiative feedbacks in the Arctic atmosphere.