Tropical convection and preferred modes of Southern Hemisphere circulation at intraseasonal time scales

We investigate the extent that forcing from the tropics is responsible for circulation modes over the Southern Hemisphere (SH) on subseasonal time scales. Outgoing Longwave Radiation (OLR) was used as a proxy for tropical convection, and the NCEP/NCAR reanalysis data set was used to represent the SH circulation. The starting point was an EOF analysis of the circulation data. Data were filtered to retain only fluctuations at subseasonal time scales, by using high pass filters with 120 or 30 day cutoffs. Results are for the two seasons November-March and May-October.

The primary SH circulation modes on the 30-120 day time scale involve hemispheric wide, low wavenumber fluctuations affecting the jet streams and storm track circulations, particularly over the Pacific sector. These perturbations are highly correlated with tropical convection associated with the Madden-Julian Oscillation (MJO). During southern winter, the MJO primarily affects the subtropical jet to the north of New Zealand, and can be associated with Rossby wave energy dispersion from the region of Australia into the midlatitude South Pacific. A link to blocking over the preferred region of the southeast Pacific is found when MJO convection is over southeast Asia is found.

During southern summer, the primary association with MJO convection occurs in the region of the South Pacific Convergence Zone (SPCZ). SPCZ convection is modulated to a large extent by the MJO, and this convective heating leads to large perturbations over the South Pacific, with Rossby wave dispersion leading to perturbations extending southward then eastward from Australasia, then northeastward into the South America.

Hemispheric-wide circulation modes on the less-than-30 day time scales were identified in the same manner as above. These modes are dominated by zonal waves 4-6 along the SH storm tracks, that tend to be more zonally-oriented than those at lower intraseasonal frequencies. Overall, these circulations show less of a statistical relationship to tropical convection. The strongest relationships are during southern summer, when it is found that convection in the SPCZ and SACZ are forced by the wave activity, rather than vice-versa.

Nevertheless, it is found that more localized SH circulation can be forced by tropical convection on submonthly time scales. The extent of this forcing is being established through an extended EOF analysis combining circulation in the upper troposphere and tropical OLR at submonthly time scales. As on the 30-120 day time scale, the strongest links with the tropics occur in the Pacific sector, although statistically significant links are also found over South America and Africa.

The consistency between the induced mass circulations and the flow over the SH has been verified for several modes through the use of the "chi-problem" approach, whereby the divergent circulation in balance with a given rotational flow field over the SH is derived using the barotropic vorticity equation. Results point to a tropical mass source, corresponding to the observed OLR field, to fully explain the derived SH circulation modes found on subseasonal time scales.