Local energetic constraints on Walker circulation strength

Abstract

he weakening of tropical overturning circulations is a robust response to global warming in climate models and observations. However, there remain open questions on the causes of this change and the extent to which this weakening affects individual circulation features such as the Walker circulation. We present idealized GCM simulations of a Walker circulation forced by prescribed ocean heat-flux convergence in a slab ocean, where the longwave opacity of the atmosphere is varied to simulate a wide range of climates. The weakening of the Walker circulation with warming results from an increase in gross moist stability (GMS), a measure of the tropospheric moist-static-energy (MSE) stratification, which provides an effective static stability for tropical circulations. Baroclinic-mode theory is used to determine changes in GMS in terms of the tropical-mean profiles of temperature and MSE. The GMS increases with warming, due primarily to the rise in tropopause height, decreasing the sensitivity of the Walker circulation to zonally anomalous net energy input. Without large changes in net energy input, this results in a rapid weakening of the Walker circulation with global warming.