Diagnosing projected storm track changes under global warming

With increasing greenhouse gases, it is generally accepted that global mean temperature will increase in much of the 21st century. However, how that impacts regional climate is still not entirely clear. Regional climate impacts depend critically on how the storm tracks change. Several recent studies have suggested that the mid-latitude storm tracks will shift poleward based on analyses of the IPCC AR4 experiments. At the same time, the Hadley circulation is expected to expand poleward as well. However, there is still no consensus on what drives these poleward expansions.

Several studies have suggested that the poleward shift of the storm tracks may be explained by factors such as increase in static stability, increase in the tropopause height, changes in the meridional temperature gradients, or changes in the stratospheric circulation, including changes due to varying amounts of stratospheric ozone. Several idealized model studies have suggested that a number of these candidates may be able to produce changes with amplitudes close to those predicted by GCM simulations. Nevertheless, up to the present moment, it is still not clear which of these mechanisms is most important, or whether all of them are operating simultaneously.

One weakness of most of these idealized modeling studies is that the mean climates of these model experiments are more or less arbitrarily imposed. A recent study has suggested that the forced response of the westerly jet may be very sensitive to the state of the model climate. Hence in this study, an idealized storm track model with a climate closely resembling the observed climate is used to diagnose the forcings that are required to give rise to the jet and storm track shifts predicted by the GCM experiments.

Our preliminary results suggest that changes in upper tropospheric/lower stratospheric temperature gradient appears to be the main forcing giving rise to the poleward shift of the storm tracks, while increase in tropospheric static stability appears to be ineffective. However, part of the increase in temperature gradient could be due to increase in the tropopause height. Currently, additional sets of experiments are being conducted to isolate the changes due to increase in tropopause height, versus the changes brought about by heating and cooling due to changes in atmospheric composition.