A Mesoscale Decomposition of the Tropical Hadley Cell

The global Hadley circulation (HC) is traditionally depicted as a zonal average, yet significant variability exists in the longitudinal distribution of clouds and precipitation. Further uncertainties remain regarding the mechanisms controlling HC interannual variability, with previous studies differing over the relative importance of forcing by tropical sea surface temperature anomalies (e.g., ENSO cycles) or influences from higher latitudes including subtropical stability and extratropical wave activity.

This study presents a regional and phenomenological decomposition of the HC by matching independent observations of mesoscale cloud regimes from the 25-year International Satellite Cloud Climatology Project (ISCCP) dataset to reanalysis-derived quantities describing the zonal mean circulation. Interannual variability and HC extremes are explained through changes in the relative frequency of occurrence of different cloud regimes or “weather states”, demonstrating the importance of both rigorous deep convection (e.g., mesoscale convective systems) and less robust, more frequent tropical convection in sustaining the HC.

Time permitting, a modified version of the Community Atmosphere Model (CAM4) will be used to help elucidate potential upscale feedbacks of each weather state. Variations in the spatially-dependent heating (corresponding to fluctuations in the ISCCP cloud regimes) are used to drive the model with the resulting large-scale dynamical response compared to HC variability in observations and reanalyses. Additional simulations will address which regions and cloud regimes (e.g., tropical or subtropical, shallow or deep convection) are most important in comprising the global circulation.