Talia Tamarin
Weizmann Institute

"The poleward deflection of midlatitude storm tracks and its variation under climate change"

The Atlantic and Pacific storm tracks in the northern hemisphere are characterized by a downstream poleward deflection that has important consequences for the distribution of heat, wind and precipitation in the midlatitudes. In this study, the spatial structure of the storm tracks is examined by tracking transient cyclonic eddies in an idealized GCM with a localized ocean heat flux. The localized atmospheric response is decomposed in terms of a time-zonal mean background flow, a stationary wave and a transient eddy field. The Lagrangian tracks are used to construct cyclone composites and perform a spatially varying PV budget. Three distinct mechanisms that contribute to the poleward deflection emerge: transient nonlinear advection, latent heat release and stationary advection.

The downstream evolution of the PV composites shows the different role played by the stationary wave in each region. In the region where the tilt is maximized, all three mechanisms contribute to the poleward propagation of the low level PV anomaly associated with cyclone. Upstream of that region, the stationary wave is opposing the former two and the poleward tendency is therefore reduced. It is also shown, through repeated experiments with enhanced strength of the heating source, that the poleward deflection of the storms intensifies when the amplitude of the stationary wave increases. Finally, the implications for how this poleward deflection may change under global warming and its relation to the expected poleward shift of storm tracks is examined, by comparing idealized GCMs with increased global mean temperatures to comprehensive climate prediction models forced by enhanced CO2 emissions.