Tropical Convective Influence: A 25-year Climatology of UTLS Anticyclones in the Southern Hemisphere

A 25 year (1985-2009) climatology of convectively driven quasi-stationary anticyclones in the Southern Hemisphere (SH) is presented to provide insight into different dynamical regimes in the SH Upper Troposphere - Lower Stratosphere (UTLS) region during August-October months. We have found that different convective patterns do introduce profound changes in the structure of SH anticyclones and notable differences in the structure of the Australian Subtropical Westerly Jet (ASWJ), power spectra and the wave energy propagation in UTLS region as seen in E-P fluxes. The ECMWF data containing southern winter to spring transition periods was divided into temporal composites according to the relative strength of the Australian High (AH), the South African High (SAH) and the Indian Ocean High (IOH) near 23°S to differentiate the convective forcing and dynamical regimes. Three temporal subsets were extracted to span periods of AH dominance (Type I, 650 days), IOH dominance ( Type II, 1203 days) and the relatively high magnitude for both the AH and SAH anticyclones (Type III, 447 days). All days in the 2300 day August to October subsets over 25 years were binned into one of the three types. These temporal composites were analyzed in terms of meteorological variables like geopotential heights, OLR and winds, as well as power spectra and Eliassen-Palm fluxes. It was found that as the winter to spring transition progresses, the relative strength of the AH generally diminishes, while the SAH and IOH grow. The apparent two-core structure of the ASWJ was also attributed to the type I and III geopotential height structures. Type III events show greater activity of the South American High (SAmH), which also undergoes enhancement during the late transition period in October. The spectral analysis revealed interesting differences between the types of events, namely: stronger k=1 and 2 waves during type III events around 60°S in August. The k=3 and 4 waves for type I show doubling of the amplitude at 50°S in August, while a similar feature is exhibited by k=3 of type III at 65°S in September. This is seen in differences of E-P flux convergence showing a relative enhancement of the tropospheric divergence maximum in the 500-250 hPa region in August and a similar enhancement during September for type III, shifted poleward with respect to the monthly mean and other event types. Both type I and III are shown to be stronger than average during October. The k=1 and 2 waves maximize at higher latitudes during type I events (68°S versus 60°S). The strong signal coming from around 20°S latitude for both wavenumbers reflects the duo of the AH and SAH projected into wave-space. One of the most important hallmarks of the winter to spring transition is the descent of the Polar Night Jet (PNJ) together with its poleward shift that was observed when comparing three months of the analyzed period. This is also reflected in the downward and equatorward shift of the E-P flux divergence region in the stratosphere around 60°S, with corresponding diminution of the upward wave energy propagation.