Storm track response to climate change: how the Hadley circulation influences shifts of the midlatitude storm tracks

Midlatitude storm tracks are an important medium for energy and angular momentum redistribution within Earth's atmosphere, and they will change as the climate changes. Storm tracks are projected to shift poleward under global-warming scenarios, even in extremely idealized simulations. It is important to understand the storm track response to perturbations in climate to appreciate how midlatitude weather and climate will change as a result. But the dominant mechanisms controlling the storm track response to climate change remain uncertain.

In an earlier study using an idealized dry GCM, we showed that near-surface meridional temperature gradients were important for understanding storm track shifts. We noted, consistent with similar studies, the tandem shifts of the storm tracks and the Hadley cell terminus. In the present study, we test the idea that expansions of the Hadley circulation contribute to shifts in the storm tracks. We use a diffusive energy-balance model forced by Newtonian relaxation towards radiative equilibrium. The model contains a simple Hadley cell parameterization that strongly reduces temperature gradients within the Hadley cell. The termination latitude of the Hadley cell is determined based on a criterion that quantifies where baroclinic eddies become deep enough to reach the tropopause.

This work demonstrates a clear link between Hadley circulation dynamics and the storm track position. The energy balance model is very simple and contains few parameters; however, they are all physically motivated. The results show that expansions of the Hadley cells, induced, for example, by increases in tropical static stability, can shift the storm tracks poleward. This explains the majority of the shift in the storm tracks seen in an idealized dry GCM. Storm track sensitivity to several other quantities in dry climates is also elucidated.

With the aid of a hierarchy of models, we obtain a closed theory of storm track shifts in dry climates. One can begin to generalize the results contained herein to moist atmospheres, to first order, by interpreting moisture transports as acting to increase the efficiency of dry eddy energy transports and as modifying the static stability of the atmosphere.