Salinity Boundary Conditions and the Atlantic Meridional Overturning Circulation in Depth and Quasi-Isopycnic Coordinate Global Ocean Models

This paper compares the Atlantic Meridional Overturning Circulation (AMOC) in global simulations performed with the depth coordinate Parallel Ocean Program (POP) ocean model and with the HYbrid Coordinate Ocean Model (HYCOM) under different surface salinity boundary conditions. When forced by the Coordinated Ocean-ice Reference Experiment (CORE) repeat Normal Year Forcing, HYCOM develops internal inter-annual variability during the model spin-up, while POP does not and HYCOM is more sensitive to the salinity boundary condition in the Southern Ocean. Otherwise the AMOC and related fields in the two models are qualitatively similar, but neither is able to maintain a non-trivial AMOC, because of a positive feedback that continually freshens the high-latitude surface waters. However, with salinity restoring at the ocean surface the AMOC becomes progressively stronger as the piston velocity is increased. The different restoring strategies in POP and HYCOM cause differences in the AMOC simulation. The components of the AMOC and closely related fields, including the oceanic deep convection, thermohaline fluxes, three dimensional currents, water mass distribution and overflows, are compared between the models with different salinity boundary conditions. The comparison provides insights on the models' response and increases our understanding of ocean climate simulations. It also provides motivation for the future development of ocean climate models capable of simulating the AMOC more realistically. The Atlantic Meridional Overturning Circulation (AMOC) is one of the most important components of the global ocean circulation. The AMOC here refers to the circulation associated with the oceanic deep convection and North Atlantic Deep Water (NADW) formation in the high latitudes of the North Atlantic. A vigorous AMOC in climate models transports a large amount of heat and salt northward in its upper branch, thereby maintaining the high surface temperature and salinity in the northern North Atlantic compared to other oceans at the same latitudes. Failure to represent a strong AMOC would lead to a simulated climate state very different from reality, both in terms of the mean state and in climate variability It is challenging to obtain a realistic AMOC in ocean climate numerical models because the modeled circulation is highly sensitive to the thermohaline forcing at the ocean surface and its equilibration is extremely slow (Rahmstorf, 1997; Griffies et al., 2009). A very small bias in the thermohaline forcing, especially in the freshwater flux, can cause a persistent drift and eventual collapse of the AMOC. One factor is the lack of direct feedback between …