Variation Of Ocean Circulation Under The Changing Climate

Previous observation and model studies show that the upper ocean stratificationis enhanced under global warming. It is of great significance to make a discussion onthe response of the circulation, especially the Western Boundary Current system, tointensified stratification, in order to deepen our understanding of the variation ofocean circulation under the changing climate.A2-layer Quasi-geostrophic model is used to investigate the response of thedouble-gyre circulation to enhanced stratification (g′), forced by a steady wind forcing.It is found that the barotropic transport of the circulation first increases withstratification but then decreases due to saturation of the surface layer circulationintensity when the stratification exceeds a threshold. PV budget analysis indicates thatthe saturation is caused by the increased inter-gyre transport of relative potentialvorticity resulting from the intensified variability of the jet location.Both the barotropic instability and bifurcation mechanisms contribute to theintensified variability of the jet location. Due to barotropic instability, eddies aregenerated in the confluence region of the WBCs and advected eastward, causing thevariability of the jet location. With increased stratification, the surface layercirculation is strengthened and the barotropic instability is intensified. As a result, thesurface flow becomes more variable with excessive eddies and intense variability ofthe jet. With the increasing stratification, three regimes, each marked by its own variation of the jet location, emerge due to the successive system bifurcations. In thelast two regimes, variability of the jet location is further enhanced by frequentswitches among the different dynamic states on multi-decadal time scales.The Empirical Orthogonal Function (EOF) analysis is applied to study thelow-frequency variability of the circulation, and the double-mode structure isconfirmed. With increasing stratification, noteworthy evolutions happen to the spatialstructure of Mode2. By spectrum analysis, it is presented that the spectrum of theprincipal component (PC) for different cases and modes share the same pattern, whichis interpreted in this article using a linear vorticity model. In addition, it is shown that,for larger g′, the low-frequency part of the spectrum is intensified significantly, withthe high-frequency part seldom changed.