Sensitivity of Southern Hemisphere Circulation to Tropical Convective Forcing: Modulation of Polar Regions via Planetary Wave Trains in the UTLS

The high latitude Southern Hemisphere (SH) plays crucial roles in the climate system, including the thermohaline circulation, the distribution of sea and land ice, the anthropogenic ozone hole, and jet migration. Centers of deep tropical convection can affect higher latitudes by mass outflow surges in the upper troposphere / lower stratosphere (UTLS) and radiation of planetary wave trains through the connecting westerly waveguide. Depending on the season and on the phase of the El Nino Southern Oscillation (ENSO), these wave trains will alter high latitude patterns of temperature, geopotential height, and wind, hence of the Southern Annular Mode (SAM) and distribution of column ozone.

The sensitivity of SH atmospheric structures to tropical forcing is investigated through observational analysis and a new modeling strategy. We investigate the hypothesis that SH atmospheric structures are sensitive to the location and timing of outflow from deep convection in the UTLS, and that this modulation depends on season and phase of ENSO. Our approach avoids the complexity of modeling tropical convection by representing tropical convective anomalies with meridional outflow perturbations in the SH subtropical UTLS. Monthly mean and ENSO mean basic states from global analyses are used as boundary conditions for ensembles of multi-week SH simulations with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS).

Changes in the location of convective centers and changes in the UTLS wind structure (waveguide) modulate the propagation of planetary wave trains, thereby influencing the distribution of climate anomalies from the surface of Antarctica into the polar stratosphere. Column ozone anomalies correlate highly with barotropic temperature anomalies in the UTLS, and are linked to distinctive patterns of Antarctic surface temperature and wind anomalies for El Nino versus La Nina. During the SH winter to spring transition a westward shift in convection during La Nina of 30-50° corresponds to a 30-50° westward shift of planetary wave patterns from the tropics to Antarctica and of the zonal asymmetry in column ozone.