Interpreting Low-Frequency Modes of Southern Hemisphere Atmospheric Variability as the Rotational Response to Divergent Forcing

The principal modes of Southern Hemisphere low-frequency variability have recently been calculated using a 39-yr record of 300-hPa streamfunction fields from the NCEP-NCAR reanalysis dataset. The authors attempt to interpret these modes as the rotational response to some divergent forcing. For a range of mean states the linearized barotropic vorticity equation (BVE) is used to solve for the divergent wind that would generate (or at least be consistent with) the observed vorticity modes. Several of these low-frequency modes can be generated by forcing the BVE with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection. In particular this is found to be true for streamfunction anomalies with El Nino-Southern Oscillation (ENSO), high-latitude mode, South Pacific wave, and Madden-Julian oscillation structure. The authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed OLR fields and hence explain some of the variance of the next month's 300-hPa streamfunction by solving the inverse problem. These results are further evidence that linear Rossby wave propagation provides an important link between anomalous convection in the Tropics and the occurrence of circulation anomalies in higher latitudes.