Seasonal Sensitivity of the Hadley Cell and Cross-Hemispheric Responses to Diabatic Heating in an Idealized GCM

The seasonal sensitivity of the Hadley cell to localized diabatic forcing is studied using a dry idealized atmospheric general circulation model. Sensitivities are broadly consistent with Hadley cell responses in observations and climate models to El Nino-Southern Oscillation and global warming-like forcings. However, the exact seasonal sensitivity patterns highlight the importance of reducing the uncertainty in the size and position of expected anthropogenic forcings to understand how the atmospheric circulation will respond. The sensitivities reveal cross-hemispheric Hadley cell responses that project onto the eddy-driven jets and storm tracks. For summer hemisphere heating, the winter Hadley cell extent and jet latitude responses are highly correlated. For winter hemisphere heating, the summer Hadley cell extent and jet speed responses are highly correlated. These seasonal differences arise due to the contrast between the dominant winter Hadley cell and weaker summer Hadley cell. Plain Language Summary This study analyzes changes in the Hadley cell circulation to locally applied heating in the atmosphere in a simple model. The aim is to further understanding about how circulation responds to changes in the climate, for example, global warming. We find differences between the circulation changes in summer and winter, which highlight how important it is to understand exactly how global warming will heat the atmosphere so that we can predict how weather and climate will change. We also observe changes in the Hadley cell, and consequently the jet stream, in one hemisphere from heating in the other hemisphere. The relationship between the Hadley cell and jet stream depends on the season when these cross-hemisphere responses are observed. The implications of this are that we must ensure that studies looking at circulation changes do so using a seasonal cycle, instead of just considering annual average changes. The results also point to specific areas in the atmosphere (e.g., high in the troposphere over the tropics) where we need to better understand how heating due to global warming will affect the atmosphere. This will give us greater confidence in predicting how climates will change in the future.