Observed low-frequency linkage between Northern Hemisphere tropical expansion and polar vortex weakening from 1979 to 2012

Abstract

In recent decades, the northern hemisphere (NH) atmospheric meridional circulation has experienced unprecedented changes. The NH tropical belt is significantly expanding toward high latitudes, and the Arctic polar vortex is continually weakening. Both phenomena have led to severe consequences on the Earth's surface, such as more frequent droughts in the subtropics and accelerated sea ice loss in the Arctic. However, the potential linkage between these phenomena and the underlying mechanisms have rarely been discussed. In this study, we report strong observational evidence that the NH tropical boundary and tropospheric polar vortex are synchronously changing, based on analysis of long-term satellite records from 1979 to 2012. Our investigation suggests that both the variance of the NH tropical boundary and tropospheric polar vortex from 1979 to 2012 are associated to the natural anomaly of NH mid-latitude sea surface temperature. The low-frequency sea surface temperature anomalies over the North Pacific (i.e., the Pacific Decadal Oscillation, PDO) and Atlantic (i.e., the Atlantic Multidecadal Oscillation, AMO) could explain 61% of the NH tropical boundary variance and 56% of the polar vortex variance from 1979 to 2012.