Formation mechanism of the Pacific equatorial thermocline revealed by a general circulation model with a high accuracy tracer advection scheme

Formation processes of the hydrographic structures and meridional overturning circulations in the equatorial Pacific are investigated using an ocean general circulation model where a high accuracy tracer advection scheme, the second-order moment scheme, is implemented. It is also addressed why the equatorial thermocline tends to be diffuse in conventional ocean models. The upper equatorial thermocline water is fed by the subtropical cell of the northern hemisphere, which lies over the lower thermocline water fed by the subtropical cell of the southern hemisphere. Additional experiments reveal that even slight numerical diffusion induced by a third-order tracer advection scheme disturbs the thermodynamic balance of the Pacific equatorial thermocline and makes it significantly diffuse. Magnitude of vertical numerical diffusion is of primary importance in properly reproducing the Pacific equatorial thermocline. Smaller vertical diffusion makes the isopycnals around 25.5σθ shallower and consequently intensifies the equatorward transport by the lower part of the southern hemisphere subtropical cell. This cold water transport from the southern hemisphere is also important in further tightening the equatorial thermocline, and it is significantly reduced by horizontal numerical diffusion under less accurate advection schemes.