Observational Changes in Cloud Regimes Associated with the Interannual Variability of Hadley Cell Width

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 examines the interannual variability of HC width as a function of the observed frequency of tropical and subtropical cloud regimes using the International Satellite Cloud Climatology Project (ISCCP) dataset. HC metrics were derived from an ensemble of eight reanalyses and the corresponding widest and narrowest overturning events were identified for each hemisphere during the summer and winter seasons. Differences in the relative frequency of occurrence (RFO) for matching cloud regimes suggest that the northern hemispheric winter cell is dominated by an El Niño response with deep convective clouds shifting eastward over the central Pacific. Other variations associated with HC intensity and width include the magnitude and position of the Pacific ITCZ, though the monsoons may play a minor role in altering HC strength. We demonstrate the importance of changes in both rigorous deep convection (i.e., mesoscale convective systems) and less robust, more frequent tropical convection associated with changes in HC width.

The anomalous ISCCP RFOs during HC extremes were scaled by the regime-average vertical heating profiles based on a sounding-derived look-up table in order to determine which cloud mixtures contribute the most towards the anomalous diabatic heating associated with variations in HC width. Although the RFO changes for the three deep convective weather states were similar, the regime describing the most vigorous and organized convection appears to be most important for HC width extremes, despite an overall low frequency of occurrence. Time permitting, we present a similar phenomenological decomposition of the HC using cloud classifications available from CloudSat data.