Troposphere-Stratosphere Dynamical Coupling in the Southern High Latitudes and its Linkage to the Amundsen Sea

Abstract:

As in the Northern Hemisphere, extremes in the distribution of stratospheric heat flux are connected instantaneously to anomalous high latitude tropospheric weather patterns in reanalysis. Coupling to the stratosphere via planetary waves has significant impacts on the tropospheric circulation of both hemispheres and it is important to understand which coupled climate models can reproduce this phenomenon. A dynamical metric based on extreme stratospheric planetary-scale wave heat flux events, defined as the 10th and 90th percentiles of the daily high-latitude wave-1 heat flux distribution at 50hPa, developed for the Northern Hemisphere is applied to the Southern Hemisphere. Extreme negative (positive) heat flux events occur with a strengthening (weakening) of the Southern Ocean jet and anomalously warm (cool) temperatures over the Amundsen Sea. The metric is used to evaluate troposphere-stratosphere coupling in models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and compare their performance across hemispheres. Some models, mostly low-top models, are found to have a degraded representation of stratospheric extremes. A large bias in the wave-2 and -3 tropospheric geopotential height eddy climatology of the CMIP5 models masks the effect of stratosphere-troposphere planetary wave coupling on the climatology. After removing the climatalogy, it is shown that planetary wave coupling is linked to tropospheric temperature variability over the Amundsen Sea. Models which fail to capture the extremes in stratospheric heat flux, significantly under-estimate the magnitude of temperature anomalies over the Amundsen Sea Low.