Changes of the Shallow Pacific Meridional Overturning Circulation Under Global Warming

The equatorial thermocline in the Pacific ocean is known to be ventilated by the subtropical surface waters via the shallow meridional overturning circulation (aka. the subtropical cells, STCs). Both observational and modeling studies have shown such a tropical-subtropical linkage and suggested the variations of both the thermohaline properties of the source waters and the strength of the STCs that carry them can affect the equatorial thermocline, the sea surface temperature in the equatorial upwelling regions, and consequently the tropical climate and its variability. Under global warming the variations of both kinds are expected to happen and to impact the tropical climate in response to the changing wind stress and the surface density patterns. In this study, we diagnose the model output from the IPCC AR4 simulations and identify some robust changes of the STCs under global warming. The surface branch of the STCs (namely the poleward Ekman transport) is weakened, in accord with the weakening of the local easterlies that drive the STCs. The subsurface branch of the STCs (namely the equatorward pycnocline transport) is also weakened, but not so much as the surface branch. This weakening mainly takes place in the Northern Hemisphere, while the STCs transport in the Southern Hemisphere remains stable. In the equatorial regions, both the equatorial undercurrent (EUC) and the equatorial upwelling become shallower, consistent with the shoaling and flattening of the equatorial pycnocline. The EUC transport, however, is nearly invariant. It is found that there is also a weakening of the Indonesian Throughflow (ITF) which compensates the reduction of the equatorward pycnocline transport of the STCs, and leaves the pycnocline transport that actually ventilates the EUC insensitive to global warming. Despite this, the thermohaline properties of the ventilated water do change. A suite of adjoint passive tracer and backtracking Lagrangian trajectory experiments are carried out with an ocean general circulation model to study the changes of the subduction regions and the hydrographic properties of the source waters of the equatorial thermocline under global warming.