Understanding GCM predicted 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.

Several suites of experiments are currently being conducted, with the model climate forced to resemble either the 20th or 21st century climate simulated in GCM experiments to diagnose the forcings required to generate the changes predicted by those GCM experiments. Results of these experiments will be discussed at the conference.