On the symmetry of upwelling in the tropical stratosphere

Upwelling in the tropical stratosphere driven by planetary waves originating in the high latitudes is explored in idealized atmospheric General Circulation Models. Tropical upwelling in the stratosphere is remarkably symmetric across the equator, despite the fact that the wave forcing is confined to the winter hemisphere. We explain the symmetry by drawing insight from the weak temperature gradient approximation, which has been successfully applied to the tropical troposphere to understand the balance between vertical motion and diabatic warming or cooling. Asymmetric upwelling is possible in the tropical troposphere because it can be balanced by latent heating and radiative cooling in the summer and winter hemispheres, respectively. In the stratosphere, however, the absence of latent heating makes it impossible to sustain asymmetric vertical motion. While most of the waves entering the high latitude stratosphere break along the vortex edge, the small amount that leaks into the tropics is sufficient to drive significant upwelling due to the vanishing impact of rotation. Upwelling in one hemisphere, however, leads to unstable temperature gradients across the equator, and the atmosphere adjusts rapidly to spread the ascent across the equator, maintaining weak temperature gradients. The impact of model numerics is explored by contrasting the behavior of pseudospectral and finite-volume models. The nature of the fast adjustment varies substantially between the two models, suggesting that the dynamics are not fully captured even at high numerical resolution, but the net result of the adjustment is the same as it is governed by large scale temperature constraints.