The Role of Water Vapor in Midlatitude Stormtracks and the Global Circulation

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A recent study of the isentropic circulations by the principal investigator and his collaborators has shown that the poleward mass transport in the mid-latitudes is equally split between a "dry branch" in the upper troposphere, and a "moist branch", associated with a low level poleward flow of warm, moist subtropical air that ascends in the upper troposphere within the stormtracks.This project will develop a new framework to better assess the relationship between precipitation and mid-latitude weather systems on both the local and global scales, and will evaluate how the global circulation is affected by different climatic conditions. It seeks a better understanding of (i) what controls the partitioning of the circulation between these dry and moist branches, (ii) how does this partitioning affect the energy and momentum transport by the circulation, (iii) what are the implications at the level of the individual storm, and (iv) how this might change in the future either as a result of natural climate variability or as a consequence of anthropogenic greenhouse gas emissions. The approach here is based on analyzing the zonal mean circulation averaged on either dry isentropes, i.e., surfaces of constant potential temperature, or moist isentropes, i.e., surfaces of constant equivalent potential temperature. The research includes theoretical development to assess the role of moist processes on the dynamics of mid-latitude weather systems, numerical simulations of baroclinic life-cycle and of the global atmosphere, and a comprehensive study of the current circulation based on reanalysis datasets. A set of diagnostic tools to assess the isentropic circulation in numerical models will also be developed and made available to the broader community.Broader impacts of the proposed research are in the potential to improve our ability to predict how climate change can affect subtropical and mid-latitude weather, and modify the global distribution of rainfall. Different global climate models used by the Intergovernmental Panel on Climate Change (IPCC) will be carefully evaluated and compared to reanalysis data. This should both indicate possible ways to improve these models, as well as increase our confidence in their ability to accurately simulate the climate system and the hydrological cycle.