Collaborative Research: Understanding the Links Between Tropical Cyclones and Tropical Circulation Under Climate Change Through Idealized Coupled Climate Modeling

Tropical cyclones (TCs), including the hurricanes of the Atlantic and Caribbean, derive their destructive power from the heat of the ocean beneath them. The fueling of TCs by warm sea surface temperatures (SSTs) thus suggests that the warming of the oceans by greenhouse gas emissions should bring more frequent and more damaging TCs. One part of this expectation holds true: the strongest TCs have become even stronger in recent decades, in agreement with model simulations and theoretical arguments. But there is no satisfactory theory for how TC frequency will change in a warming world and model-based studies have shown mixed results. The recent study of Chand et al. (2022) shows a decline in TC numbers which the authors attribute to a reduction in the strength of the the north-south oriented Hadley cell and the east-west oriented Walker cell. The former features rising air over the warm SSTs of the deep tropics and subsidence over the cooler subtropics, while the latter features convection over the "warm pool" of the tropical western Pacific and subsidence over eastern Pacific "cold tongue". But other studies suggests that reductions in overturning strength should result in an increase in the number of TCs rather than a decrease. Research under this award considers the role of the overturning cells in setting TC frequency and other aggregate properties and the consequences for TC behavior of changes in overturning due to greenhouse warming. The work is conducted using two idealized configurations of the Community Earth System Model (CESM) which are designed to isolate the mechanisms of interest and study them in their simplest form. The first configuration is an "aquaplanet", with an atmosphere coupled to a dynamic ocean model but no land surface. In this configuration there is no equivalent of the Pacific warm pool and cold tongue and hence no analog for the Walker cell, although the SST contrast between lower and higher latitudes still produces a Hadley cell. The second is a "ridge" configuration, in which a narrow oceanic ridge stands in for the American landmass that separates the tropical Atlantic and Pacific Oceans. The ocean dynamics created by the imposition of a north-south ridge creates a contrast between cooler surface water to the west of the ridge and warmer SST to the east, which in turn induces a Walker circulation. These configurations are augmented by simulations at higher resolution (0.25 degree grid spacing) which are better able to capture TC behavior. For reasons of computational cost the high-resolution simulations use a motionless "slab" ocean in which the SST contrasts generated by the aquaplanet and ridge configurations are imposed indirectly so as not to compromise heat transfer from the ocean to the TCs. The work is of societal as well as scientific interest given the tremendous destructive power of TCs and the need to assess changes in TC risk due to greenhouse warming. The work also has scientific broader impacts through the addition of new configurations to the CESM model hierarchy. The model versions developed in this project will be made available to the research community through the Simpler Models web portal (www.cesm.ucar.edu/models/simple) and will be supported with documentation and reference simulations. In addition, the project provides support and training to two graduate students, thereby providing for the future scientific workforce in this research area. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.