8 Spatial Patterns and Intensity of the Winter Surface Storm Tracks in the CESM, GFDL and GISS CMIP5 Models

Monday, 15 August 2016
Grand Terrace (Monona Terrace Community and Convention Center)
James Booth, City University of New York, City College, New York, NY; and Y. O. Kwon, R. J. Small, S. Ko, and R. Msadek

The western boundary currents and their extensions (WBCs) coincide with the regions of the atmospheric storm track entrance and strong air-sea interaction, making the surface winds in these regions an important feature for general circulation models to capture. Therefore, the surface storm tracks in WBCs from three CMIP5 models: NCAR CESM1-LE, NOAA GFDL-CM3, and NASA GISS-ModelE2 are examined against the ERA-Interim. GFDL-CM3 and GISS-ModelE2 reproduce storm track location and intensity with moderate skill, while CESM1-LE has too strong surface winds at almost every location. In the reanalysis, the atmospheric wave packets that propagate from upstream drive temporal variability of surface storm tracks in WBCs. This top-down control and the speed of wave propagation are both reproduced realistically by all the models. In terms of spatial patterns, the models capture the westward shift (towards the WBCs) in the maximum surface storm track locations as compared to the free-tropospheric storm tracks. This shift is likely driven by differential momentum mixing within the boundary layer over warm versus cold waters, as shown by estimated storm track location using only the free tropospheric storm track and a bulk measure of low-level vertical stability. In two out of the three models, surface storm track intensity appears to be primarily dictated by the strength of the free tropospheric storm track; in the third model parameterizations appear to affect this relationship. However, spatial biases in the surface storm tracks that occur over the regions of SST biases in the models suggest the ocean partly controls the location of the surface storm tracks.
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