Zonal Asymmetry of the Belt-like Dynamic Sea Level Change in the Southern Ocean

Abstract

Satellite altimetry (1993–2024) reveals that the Southern Ocean dynamic sea level (DSL) exhibits a distinct belt-like pattern, with smaller rises south of 50°S and larger rises to the north. This zonal belt is not uniform but strongly asymmetric, with the largest DSL increase in the Pacific sector and weaker changes in the Atlantic and Indian sectors. These pronounced Pacific anomalies are primarily driven by internal climate variability, particularly the phase transition of the Interdecadal Pacific Oscillation (IPO). The IPO during 1993–2024 induces a zonal wind stress curl dipole in the Pacific sector, forcing a distinct belt-like sea level pattern that dominates the observed zonal asymmetry. However, this observed pattern starkly diverges from the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model mean. By filtering out internal climate variability, the models project a diametrically opposite zonal asymmetry, with the most robust DSL changes in the Indian and Atlantic sectors, while the Pacific response remains comparatively weak. Single forcing experiments from CMIP6-FAFMIP and an ocean general circulation model indicate that surface heat flux leads this forced zonal asymmetry, while wind stress forcing mainly contributes to zonally symmetric changes. Furthermore, freshwater hosing experiments suggest that buoyancy-forced weakening of the Atlantic Meridional Overturning Circulation (AMOC) enhances the belt-like DSL signal predominantly in the Indian and Atlantic sectors, thereby reinforcing the zonal asymmetry in the forced response.