Response of the Inertial Recirculation to Intensified Stratification in a Two-Layer Quasigeostrophic Ocean Circulation Model

Abstract

Abstract Previous observation and model studies show that the upper-ocean stratification is enhanced under global warming (Capotondi et al.; Cravatte et al.; Deser et al., etc.). The response of the recirculation, which is associated with the western boundary current (WBC) jet extension and significantly increases its transport, to the intensified stratification, is studied in a two-layer quasigeostrophic ocean circulation model. It is found that the barotropic transport of the circulation first increases with stratification but then decreases as a result of saturation of the surface-layer circulation intensity when the stratification exceeds a threshold. PV budget analysis indicates that the saturation is caused by the increased intergyre transport of relative potential vorticity resulting from the intensified variability of the jet location. Both the barotropic instability and bifurcation mechanisms contribute to the intensified variability of the jet location. Because of barotropic instability, eddies are generated in the confluence region of the WBCs and advected eastward, causing the variability of the jet location. With increased stratification, the surface-layer circulation is strengthened and the barotropic instability is intensified. As a result, the surface flow becomes more variable with excessive eddies and intense variability of the jet. With the increasing stratification, three regimes, each marked by its own variation of the jet location, emerge owing to the successive system bifurcations. In the last two regimes, variability of the jet location is further enhanced by frequent switches among the different dynamic states on multidecadal time scales.