Investigating Tropical Pacific Variability from models and (future) observations

Variations in the eastern Pacific cold tongue dominate variability of the zonal sea surface temperature (SST) gradient that drives ENSO, which provides basin-wide subseasonal and global seasonal prediction skill in weather, ocean, and climate patterns. The cold tongue is supplied with cold water by the equatorial undercurrent that follows the upward sloping thermocline to the east, transporting cold water towards the surface. As the thermocline shoals, its water is exposed to the strong current shear between the eastward equatorial undercurrent and the westward surface current. In this regime prone to shear-driven mixing, thermocline water undergoes the diabatic processes of water mass transformation (WMT) allowing for heat uptake from the surface into the ocean. Here, we examine WMT in the cold tongue region from a global high resolution ocean simulation with saved budget terms and from a regional ocean state estimate. We quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, solar penetration), and find that vertical mixing is the single most important contribution in the thermocline, while solar heating dominates close to the surface. We investigate how WMT changes from (sub)-seasonal to interannual timescales. During El Niño events vertical mixing, and hence WMT as a whole, is much reduced, while during La Niña periods strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances surface cooling in the cold tongue region.