The Northern Hemispheric Circumpolar Vortex under Climate Change

Abstract:

The Circumpolar Vortex (CPV) in the troposphere is a main feature of weather and climate steering phenomena on both hemispheres. Its extent, position and strength are strongly linked to respective anomalies not only in the mid- to upper-troposphere but also to surface conditions. Thus, the CPV has fundamental importance for the positioning of e.g., the polar frontal structure, the inherent temperature gradients, the baroclinic jet and related baroclinic wave disturbances. For example, in winter 2013/14 an extended southward extension combined with a deep geopotential trough over North America led to extreme cold conditions and upstream surface high pressure anomalies were responsible for a significant reduction of cyclones over the North Pacific.

This study aims at the anthropogenic impact on the structure of the winter CPV on the northern hemisphere (NH, DJF) as a main driver of subsequent changes also for extreme event occurrence. We analyse the transient change up to 2100 under the RCP8.5 scenario using 8 state-of-the-art CMIP5 AOGCMs with in total 62 individual simulations for the 20C- and 32 members for the RCP8.5-period. Our results show a good validation of the models’ performance (ensemble mean) against ERA-Interim reanalyses for circularity, a slightly slower velocity, but an underestimation for the extent of the CPV, accompanied by a high model-to-model variability.

The climate change signal is characterized by a reduction of the extent of the CPV from 33% to 26% of the NH area (a relative change of about 20%), a reduction from 32 m/s to 28m/s (a relative change of about 13%), and a moderate decrease in circularity. Impacts on specific regions will be discussed.