Global Ocean Circulation Variability Induced by Southern Ocean Winds

Due to its relevance to global climate, understanding the response of the global ocean circulation to perturbations of its steady state is an important task. Previous studies indicate that Southern Ocean wind stress may drive global oceanic overturning variability on decadal timescales. Using a global ocean general circulation model with simplified geometries we investigate the transient and steady state response of the global ocean to a switched-on perturbation to the westerlies over the Southern Ocean. Experiments are designed to tests for the relative strengths of the Atlantic and Indo-Pacific overturning anomaly responses. The sensitivity to varying Pacific basin width and north Pacific density stratification is also investigated. Results reveal that Pacific overturning anomaly is roughly proportional to basin width and that north Pacific density stratification strongly affects the steady state response. Furthermore, results indicate that response timescales are proportional to global basin width consistent with shallow water theory.;Additional experiments are performed to understand the spatial structure and evolution of sea surface temperature (SST), salinity (SSS), density (SSD) and meridional overturning anomalies as a result of a switched-on Southern Ocean wind perturbation under weak and strong restoring boundary conditions. In ocean-only models restoring boundary conditions provide a representation of an atmosphere-ocean coupled system, through temperature feedback, and using weak restoring allows a better representation of such coupling. Results show that sea surface anomalies have magnitudes and spatial structures that vary significantly over time under weak and strong restoring boundary conditions. We find that the evolution of the overturning anomaly is quite distinct between the transient and equilibrium time scales so that the response on multi-decadal time scales is bigger than at equilibrium. Thus, allowing temperature feedback yields two distinct time scales that characterize the evolution of the wind-driven overturning response. Results also suggest that the time evolution of the wind-driven overturning differences between strong and weak restoring are largely due to density effects.