The Efficiency of the Hadley Cell Response to Wide Variations in Ocean Heat Transport

The Hadley cell (HC) plays a key role in the climate response to variations in ocean heat transport (OHT). Increased OHT is characterized by both a robust slowdown of this overturning circulation, with consequent changes in cloudiness driving the climate response, and a compensating reduction in the atmospheric heat transport (AHT). Here a suite of slab-ocean aquaplanet GCM simulations is used to study the robustness of mechanisms driving changes in HC mass and energy transport across a wide range of idealized spatial patterns of OHT. The HC response is intrinsically related to both the spatial pattern of OHT and the dynamical mechanisms driving the slowdown of the cell. The reduced energy flux of the HC is associated with reductions in both the mass flux and the gross moist stability (GMS) of the cell in all cases. However, when OHT convergence patterns are confined to the subtropics and equatorward thereof (i.e., subtropical overturning cells), the circulation response is largely momentum-conserving in nature when compared to OHT convergence patterns that extend into the midlatitudes, resulting in a deformation of the anomalous streamfunction following angular momentum contours. The effects of this deformation are quantified through a simple, yet novel approach of splitting the streamfunction anomalies into their speed and shape components. The tilt of the outer branch of the streamfunction anomaly dampens the direct climate effects of the slowdown of the cell while enhancing the change in GMS, effectively decoupling the change in the energy flux from the slowdown.