Coupling of the intertropical convergence zone and the Hadley cells to the ocean's circulation

Abstract

Patterns of tropical precipitation are sensitive to the atmosphere's energy balance and shift, for example, into the hemisphere heated most strongly by radiation and surface heat fluxes. By redistributing heat around the globe, the ocean circulation plays an important role in the atmosphere's energy balance and is a potentially strong control on the region of intense tropical rainfall known as the intertropical convergence zone, or ITCZ. This thesis explores several aspects of the coupling of the ocean's heat transport to the ITCZ and atmospheric circulation. First, I study connections between Atlantic Ocean heat transport variability and the position of the ITCZ in the 20th Century. Using atmospheric reanalyses and reconstructions of tropical precipitation, I find correlations between sea surface temperatures in the North Atlantic, the ITCZ position, and tropospheric temperatures that are consistent with Atlantic Ocean-forced ITCZ shifts. The rest of the thesis focuses on aspects of the coupling of the ocean's subtropical cells (STCs) to the ITCZ and the atmosphere's Hadley cells. By forcing an idealized atmosphere-ocean global climate model with an inter-hemispheric heating contrast, I find the STCs act to strongly damp the resulting ITCZ shift through their cross-equatorial heat transport, which partially compensates the imposed heating contrast. Coupled to the Hadley cells and ITCZ by the trade winds, heat transport by the STCs always acts to weaken ITCZ shifts and is a powerful control on the ITCZ position, keeping it "stuck" to latitudes near the equator. Applying the results from the idealized experiments, I estimate the STCs act to damp ITCZ shifts on Earth by a factor of two. In the case of a hemispherically symmetric climate with the ITCZ on the equator, I study the influence of the STCs on the strength of the Hadley cells by performing a range of global warming and cooling experiments on the same idealized model. Compared to the case without any ocean heat transport, the STCs act to strongly weaken the Hadley cells, particularly in cold climates, by reducing the meridional heating contrast across the cells. Using a new energy balance framework to quantify this cross-cell heating contrast, I show that part of the impact of the STCs' poleward heat transport is offset by anomalous equatorward energy transport by atmospheric eddies. My results suggest the STCs act to weaken the Hadley cells further than previously thought.