Abstract

Subtropical anticyclones and midlatitude storm tracks are key components of the large-scale atmospheric circulation. Focusing on the southern hemisphere, the seasonality of the three dominant subtropical anticyclones, situated over the South Pacific, South Atlantic and South Indian Ocean basins, has a large influence on local weather and climate within South America, Southern Africa and Australasia, respectively. Generally speaking, sea level pressure within the southern hemisphere subtropics reaches its seasonal maximum during the winter season when the southern hemisphere Hadley Cell is at its strongest. One exception to this is the seasonal evolution of the South Pacific subtropical anticyclone. While winter maxima are seen in the South Atlantic and South Indian subtropical anticyclones, the South Pacific subtropical anticyclone reaches its seasonal maximum during local spring with elevated values extending into summer. In this study we investigate the hypothesis that strength of the austral summer South Pacific subtropical anticyclone is largely due to heating over the South Pacific Convergence Zone. Using reanalysis data, and AGCM added cooling and heating experiments to artificially change the strength of diabatic heating over the South Pacific Convergence Zone, our results show that increased heating triggers a Rossby wave train over the Southern Hemisphere mid-latitudes by increasing upper-level divergence. The propagating Rossby wave train creates a high-low sea level pressure pattern that projects onto the center of the South Pacific Subtropical Anticyclone to intensify its area and strength. The southern hemisphere storm tracks also shift poleward due to increased heating over the South Pacific Convergence Zone.