Subsurface Cooling in the Tropical Pacific Under a Warming Climate

Abstract

AbstractThe present study investigates the subsurface temperature trend in the tropical Pacific in historical and SSP2‐4.5 simulation by 14 models from Coupled Model Intercomparison Project Phase 6. Robust cooling trends exist between 100 and 200 m in the off‐equatorial region (2°–8°N/S) of both the North and South Pacific during 1950–2100. Both spiciness variation and thermocline heave are important in driving this off‐equatorial subsurface cooling at a fixed level. The cooling spiciness trends in the off‐equatorial region originates from the eastern subtropical Pacific along isopycnal streamlines. Besides, the contribution of the thermocline heave to the cooling is governed by the large wind stress curl trend. Furthermore, the maximum negative spiciness trends occur in the eastern subtropics of both the North and South Pacific. The consistently poleward migration of outcropping lines makes cooler subducting water at higher latitudes directly flow into the subtropical subsurface, which induces the negative spiciness trends here. The generation of the negative spiciness trends is also partly attributed to the anomalous advection process on distinct isopycnal surfaces. These two processes both contribute to the generation of spiciness variation in both the North and South Pacific. The generation region is partly covered by the outcropping region, thus, the strong vertical mixing within the mixed layer may weaken the effect of the subduction and advection. This study mainly highlights the significance of the spiciness variation in the projection of the subsurface temperature trend on a longer time scale under global warming.