Local Energetic Constraints on Walker Circulation Strength
The 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. The study presents 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, owing primarily to the rise in tropopause height, decreasing the sensitivity of the Walker circulation to zonally anomalous net energy input. In the absence of large changes in net energy input, this results in a rapid weakening of the Walker circulation with global warming.
© 2017 American Meteorological Society. Manuscript received 25 July 2016, in final form 20 January 2017, published online 24 March 2017. The idealized GCM simulations for this study were performed on ETH Zürich's EULER computing cluster. We thank Simona Bordoni, Jess Adkins, Andy Thompson, Michael Byrne, and Chris Bretherton for useful comments and discussion during the development of this manuscript.
Published - jas-d-16-0219.1.pdf
Erratum - jas-d-19-0246.1.pdf