Changing Position and Strength of Southern Hemisphere Westerlies in Response to Precessional Forcing

Abstract

The Southern Hemisphere westerlies (SHW) is a key component of the climate system, influencing ocean circulation and the global carbon cycle. Modern observations show a poleward intensification of the SHW under warming, while paleoclimate records reveal strong orbital‐scale variability. However, the mechanisms linking Earth's orbit to SHW variability remain unclear. In particular, precession affects the seasonal distribution of solar radiation, while leaving the annual‐mean insolation largely unchanged, making its seasonal climatic effects difficult to resolve from geological records alone. Here, using AWI‐ESM simulations spanning a full precession cycle, we explore how precession affects the seasonal variations in the intensity and position of the SHW. The results indicate that perihelion (aphelion) drives a poleward‐intensification (equatorward‐weakening) of the SHW, with maximum response occurring 1–3 months after the timing of perihelion (aphelion). These changes in position and intensity are dynamically consistent with the changes in the position of the meridional temperature gradient and the intensity of the meridional heat imbalance, respectively. Furthermore, the seasonally asymmetric SHW response does not fully cancel over the annual cycle, producing persistent annual‐mean wind anomalies that are transmitted to the Southern Ocean and induce associated adjustments in the Antarctic Circumpolar Current and wind‐driven upwelling. These results provide a physical mechanism by which seasonally redistributed precessional forcing can influence Southern Ocean circulation and potentially affect ocean‐atmosphere exchange, even when annual‐mean solar radiation remains nearly unchanged.