Phase speed spectra and the latitude of the Southern Hemisphere surface westerlies: ENSO-induced variability and long-term trend

We compare the ENSO-induced variability and long-term trend in the latitude of the Southern Hemisphere surface westerlies, by examining the space-time spectra of the upper tropospheric momentum fluxes in the reanalysis data and in the output of coupled atmosphere-ocean and uncoupled atmospheric models.

It is found that the subtropical critical latitude dynamics plays an important role in controlling the eddy momentum fluxes in the upper troposphere. In the El Nino years, the increased subtropical zonal winds allow the midlatitude eddies to penetrate further into the deep tropics, and result in the equatorward shift in the subtropical breaking region. In the long-term trend, the key to this displacement of the circulation is an increase in the eastward phase speed of the tropospheric eddies, which, in turn, results in a poleward displacement of the region of wave breaking in the subtropics. This enhancement in the eddy phase speeds is thought to be due to the direct influence on tropospheric eddy propagation of the positive zonal wind anomalies in the lower stratosphere, which is attributed to the stratospheric ozone depletion or increasing greenhouse gases.