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.

Based on the experiments with the Coupled Forecast System version 2 (CFSv2), the mechanism by which the Madden-Julian Oscillation (MJO) modulates the North Atlantic Oscillation (NAO) is investigated. To isolate the cyclic MJO heating with an eastward propagation over the Indian Ocean and western Pacific, three sets of experiments are conducted with spatio-temporal varying heating added to the model’s internally generated heating. The results suggest that the anomalous MJO heating over the Indian Ocean, rather than the western Pacific, dominates the formation of the NAO anomaly in the following 10-20 days. The MJO heating triggers a westward propagation of the storm track that influences Europe, complementing the eastward pathway of influence via the North Pacific. Both pathways contribute to an enhanced storm track over the North Atlantic and the positive NAO anomaly. The Eurasian pathway is less important for the formation of the negative NAO anomaly.