Wintertime temperature maximum at the subtropical stratopause in a T213L256 GCM

Abstract

A wintertime temperature maximum can be observed at the subtropical stratopause but does not appear in the radiative equilibrium temperature distribution. This structure was well simulated using a high‐resolution general circulation model and examined in detail. The stratopause temperature is latitudinally maximized in the winter subtropics because of a strong downwelling of the meridional circulation from summer tropics to winter subtropics passing above the stratopause. Its strong meridional flow consists of two parts: a strong poleward flow in the winter subtropics with nearly vertically aligned contours of absolute angular momentum (M) and a strong cross‐equatorial flow toward winter hemisphere above the tropical stratopause with nearly horizontally aligned M contours. The strong poleward flow in the winter subtropics is driven by a large Eliassen‐Palm (E‐P) flux convergence in a small absolute vorticity region. The large E‐P flux convergence is due to extratropical planetary waves and asymmetric inertial instability. On the other hand, the strong cross‐equatorial flow is induced along the M contour to satisfy mass continuity with the strong poleward flow in the winter subtropics and exists in the easterly phase of S‐SAO with a small latitudinal gradient of M. The easterly of S‐SAO during its mature phase acts as a corridor for cross‐equatorial meridional circulation, while cross‐equatorial meridional circulation contributes to driving the easterly of S‐SAO in its development phase.