The South Atlantic Subtropical High: Climatology and Interannual Variability

The South Atlantic Subtropical High (SASH) is the dominant circulation feature over the South Atlantic basin, and understanding variations in this high pressure system is essential for understanding regional climate variations. The purpose of this study is to detail the annual cycle of the SASH and examine how its interannual variability relates to both regional and large-scale climate variability. To accomplish this, two reanalysis datasets, ERA-Interim and JRA-55, are analyzed.

From a climatological perspective, the annual cycle of the SASH has two distinct peaks in both intensity and size. The SASH is strongest and largest during the solstitial months when its center is either closest to the equator and on the western side of the South Atlantic basin as in austral winter, or farthest from the equator in the center of the South Atlantic basin as in austral summer. Although year-to-year variations in the SASH’s position are larger in the west-east (up to ~40 degrees of longitude from May to August) than in the south-north direction, the intensity of the high decreases when it is positioned to the north. This relationship is statistically significant in every month.

Seasonal composites for austral summer and winter are formulated from detrended time series to develop a better understanding of the connections between the SASH and the large-scale climate. During austral summer, north/south changes in the position of the SASH dominate interannual variations. In particular, the anticyclone tends to be displaced poleward in El Niño years when the southern annular mode (SAM) is in its positive phase. Wave activity flux vectors suggest that the variability of the summertime SASH position is enhanced when ENSO anomalies are located in the central Pacific. In southern winter, when ENSO and the SAM are less influential, east/west variations become more pronounced as Atlantic-basin forcing mechanisms (e.g., tropical convection and land surface heating over southern Africa) play a more prominent role.