Contrasting upper and lower atmospheric metrics of tropical expansion in the Southern Hemisphere

Observational studies suggest that the tropics are expanding, but a wide range of expansion rates have been reported (from 0.2∘ to 2∘ of latitude per decade). This is due, in part, to the great variety of metrics used to define the tropical width. Here we ask whether these metrics are measuring tropical width consistently, focusing on the Southern Hemisphere, where the circulation is better approximated by the zonally symmetric component. Analyzing output from the Community Earth System Model (CESM) Large Ensemble Project, we show that tropical expansion robustly occurs in the model in response to forcings. In addition, we find that whereas the width of the Hadley circulation is strongly correlated with the midlatitude jet, from interannual to decadal time scales, it is essentially uncorrelated with upper atmospheric metrics, e.g., the maximum gradient of tropopause height, both in terms of variability and of response to forcings.