Storm Tracks and Barotropic Deformation in Climate Models

Abstract

The relationship between the time-mean planetary-scale deformation field and the structure of midlatitude storm tracks is studied in wintertime simulations of the National Center for Atmospheric Research (NCAR) Community Climate Model and the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System model. Model biases are determined by contrasting model simulations (forced by observed SSTs) with parallel analyses of NCEP?NCAR reanalyses. Barotropic diagnostics are employed to identify potential dynamical linkages between regional biases in the midlatitude storm tracks and the horizontal deformation field. Initial observational analyses confirm that synoptic eddies are optimally configured to transfer kinetic energy to the mean flow in the jet exit regions, where strong stretching deformation exists. In these regions, the major axes of the synoptic eddies are aligned along the dilatation axes of the mean flow. Consequently, mean flow advection stretches synoptic eddies along their major axes, thereby increasing their anisotropy and weakening their kinetic energy. A strong link is identified between model biases in the horizontal structure of the midlatitude storm tracks and the representation of upper-tropospheric barotropic deformation. In particular, model-simulated storm tracks extend too far downstream in regions where the zonal stretching deformation (associated with horizontal diffluence in jet exit regions) is either too weak in magnitude or displaced westward in comparison with observations. These biases are associated with anomalously weak or westward-displaced patterns of negative barotropic energy conversions, which normally act as a sink of synoptic eddy activity in the jet exit. The anomalous energy conversion patterns are primarily due to model biases in the winter-mean flow rather than the simulated horizontal eddy structures, which closely resemble observations. The results indicate that the horizontal structure of midlatitude storm tracks in climate models is strongly controlled by the large-scale patterns of barotropic deformation in the upper troposphere. It is suggested that barotropic deformation analyses may provide a useful diagnostic measure for assessing climate simulation errors in atmospheric general circulation models.