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The role of water vapor in midlatitude storm track dynamics

Final Report Summary - CIGSTYK2011 (The role of water vapor in midlatitude storm track dynamics)

In the midlatitudes, atmospheric variability is dominated by large-scale baroclinic eddies. These eddies carry out the bulk of the transport of momentum, heat, and moisture in the extratropics and therefore play a vital role in Earth’s climate. Eddies are generated by baroclinic processes, preferentially in regions of strong meridional temperature gradients and vertical wind shear near the entrance of the storm tracks, the regions of strong eddy activity. Even small shifts in the location of the storm tracks can have significant effects on regional climate, for example, through changes in hydrologic balances, especially in regions that in the present climate are on the flanks of the storm tracks.
Therefore studying midlatitude storm tracks is important for understanding the dynamics governing these climate processes. In this research project we investigate the mechanisms that set the latitudinal position and longitudinal
extent of storm tracks, and control the formation and intensity of baroclinic eddies in them. We will focus on dynamical effects of water vapor and latent heat release, over the seasonal cycle that play a central role in storm
track dynamics. In the series of studies resulting from this grant we analyzed how eddy-mean flow interaction controls the midlatitude storm track. This was done mainly using an idealized aquaplanet GCM, with a variety of methods, including, inserting a localized zonal asymmetry such as surface heating to create a storm track, variations to the vertical structure of baroclinicity and PV cyclone tracking. The idealized GCM provides a platform which is easy and cheap to run numerically, yet provides a realistic setting with which the governing physics can be analyzed. This is done by controlling parameters such as the mean surface temperature and the strength and location of the heating. We find that localized surface heating produces a storm track that resembles the observed features, including the downstream poleward tilt of the storm track, which increases as the heating strength is increased. Analysis of the time mean vorticity budget in both the simulations and observations shows that barotropic momentum fluxes produce upgradient fluxes that tend to strengthen the mean jet and storm track in a tilted way on its poleward flank, while the baroclinic heat fluxes tend to produce mean vorticity forcing that will weaken the jet. Analysis of the temporal variations and evolution of the storms allows us to isolate three mechanisms that control the poleward tilt of the storm tracks: nonlinear advection by upper level PV, mid-atmosphere latent heat release due to water vapor condensation and stationary waves. Each is analyzed separately, with a quantitative comparison to observational data. The storm track response to global warming is studied by analyzing how the vertical structure of baroclinicity affects the intensity of eddies in the atmosphere, and how the shape of the storm track, and its poleward deflection, is affected by change in the mean surface temperature.
This grant resulted in 11 publications in peer-reviewed journals (8 published and 3 in stages of publication). It contributed to understanding the physics governing the dynamics of midlatitude storm tracks, with a focus on the role of water vapor. Implications of the results for global warming were studies as well.