On the ratio between shifts in the eddy-driven jet and the Hadley cell edge

This study explores the relationship between latitudinal shifts in the eddy-driven jet and in the Hadley cell edge as depicted in models and reanalyses. We calculate an interannual shift ratio of approximately 1.5:1 between the eddy-driven jet and the Hadley cell edge over the Southern Hemisphere during austral summer in model data. We further find that the ratio varies from season to season, with similarities between corresponding seasons over each hemisphere. Ratios are broadly consistent between models in this study, and appear to be realistic when compared to those from reanalyses. Mean tropical SSTs and the strength of zonal winds in the tropics appear to be critical to determining the ratio, while sea surface temperature variability is not. We argue that conditions in the tropics act to modulate the effect of midlatitude eddies on the Hadley cell, and the action of eddies in turn explains most of the correlated shifts from year to year. In contrast, the mean state of the tropics is a poor P. W. Staten Department of Atmospheric Sciences University of Utah 135 South 1460 East Room 819 Salt Lake City, UT 84112-0110 Tel.: +1-818-354-4179 Fax: +1-818-354-3233 E-mail: [email protected] Present address: Jet Propulsion Laboratory California Institute of Technology 4800 Oak Grove Rd. M/S 233-304 Pasadena, CA 91109 T. Reichler Department of Atmospheric Sciences University of Utah 135 South 1460 East Room 819 Salt Lake City, UT 84112-0110 Tel.: +1-801-585-0040 Fax: +1-801-581-4362 E-mail: [email protected] predictor of both the ratio of observed trends in reanalyses and the ratio of modeled externally forced shifts. We show that the ratios of modeled shifts are dependent on the type of external forcing.