Where moist baroclinic waves terminate the Hadley circulation: an idealized GCM study

Observational studies suggest that the mean meridional overturning circulation in the tropics has widened the past 30 years. This widening has been qualitatively reproduced in general circulation model (GCM) simulations of a warming climate, in which it is also accompanied by a poleward shift of the subtropical dry zones. Comprehensive GCM studies suggest that the widening of the Hadley circulation under global warming may be caused by a poleward shift in baroclinic wave activity. Yet the conclusions of these studies are limited in scope because the complexity of the GCMs used and the limited amplitude of the climate change signals analyzed effectively preclude an in-depth quantitive comparison with theory.

Here we use 2 idealized GCMs, one with and one without an active hydrologic cycle, to investigate changes in the terminus of the Hadley circulation over a wide range of climates; both dry and moist GCM simulations span a wide range of global-mean temperatures and pole-to-equator thermal contrasts. To characterize the degree of control exerted by baroclinic waves in the storm track regions, we use a supercriticality criterion that compares the depth of baroclinic waves to that of the troposphere. We modify this criterion, following O'Gorman (2011), to account for the interaction of diabatic heating by vertical convection with the large-scale dynamics. This modification is found to be critical for providing a quantitative agreement in between moist and dry simulations.

We find that the location of the terminus is always below a threshold latitude, regardless of the thermal forcing. This suggests a maximum bound for the extent of the Hadley circulation on a planet with Earth-like orbital and thermal parameters. We confirm that the terminus of the Hadley circulation defines the equatorward edge of a region of deep baroclinic waves for climates with relatively strong differential heating or relatively mild global-mean surface temperatures. As the differential heating weakens or the global-mean surface temperature rises, we find that convective adjustment progressively exerts a greater degree of control on the thermal stratification of the troposphere.