An Energetic Perspective on the Tropical Atmosphere and its Response to Climate Warming

Abstract

This thesis evaluates the dynamics of the tropical atmosphere and its response to warming using energetic approaches. We focus on three features of the atmosphere over tropical oceans: the response of precipitation to warming, the gross moist stability (GMS), and the response of the Pacific Walker circulation (WC) to warming. There have been a number of mechanisms proposed to explain the response of precipitation to warming. The ``wet-get-wetter'' mechanism describes an amplification of the pattern of precipitation in a moister atmosphere, and the ``warmer-get-wetter'' mechanism describes enhanced upward motion and precipitation in regions where the increase in sea surface temperature (SST) exceeds its tropical-mean increase. Studies of the current climate have shown that surface convergence (SC) over the tropical oceans is largely driven by horizontal gradients of low-level temperature. Chapter 2 finds that a ``Laplacian-of-warming'' mechanism is of comparable importance to wet get wetter and warmer get wetter for the response of precipitation to climate change over tropical oceans. The GMS quantifies the energy import or export of a circulation but, despite its importance, is a difficult quantity to understand and to observe. Chapter 3 approximates the vertical GMS using SST and the Laplacian of SST and finds that the approximation works well in the mean and seasonal cycle. There is uncertainty about the sign and magnitude of the response of the WC to warming. Chapter 4 finds a strong relationship between GMS response and WC strength response across a hierarchy of GCMs. Further, Chapter 4 finds that WC strength and GMS responses are sensitive to the degree of parameterized convective entrainment, but that the spread in GMS responses due to differing entrainment rates is smaller than the spread in GMS response across CMIP5 models. Taken together, this thesis progresses our understanding of the tropical circulation and precipitation pattern and its response to warming.