Stability of Near-Coastal Stratifi cation and Upwelling Source Depth to Gradual Climate Warming and Cooling

Abstract:

Oceanic proxy records from mid-latitude eastern boundary zones spanning the mid-Pliocene Warm Period to present indicate gradual distinct shifts in upwelling-related variables, such as sea surface temperature, productivity and mixed layer depth. These trends have been inferred to record changes in coastal upwelling strength and source depth, due to shifts in the magnitude or position of upwelling-favorable surface wind stress and wind stress curl or due to change in global ocean stratification and thermocline depth. Modern studies of ocean fronts and near-coastal regions demonstrate that inclusion of submesoscale mixed-layer re-stratification is essential to characterization of upwelling and other near-surface processes. Using submesoscale-eddy-permitting ocean GCM experiments, we examine the adjustment of an idealized eastern boundary upwelling front to gradual changes in upwelling-favorable winds and large- (basin-) scale ocean stratification by considering variables such as frontal position, near-coastal stratification and upwelling source depth. We compare the stability of a warm (Pliocene-like) climate state in response to slow cooling to that of a cool (modern-like) climate state to slow warming. Further, we develop a simplified nonlinear layer model, with and without explicit parameterization of submesoscale eddy mixing, and evaluate its ability to capture the essential steady-state behavior of a coastal upwelling front.