Diabatic upwelling in the tropical Pacific -- the role of water mass transformation processes in sea surface temperature variability

Speaker: Anna-Lena Deppenmeier

Sea surface temperature variability in the eastern tropical Pacific cold tongue plays a major role in the global climate system. The strength of the cold tongue sets the zonal temperature gradient in the Pacific, coupling the ocean with the atmospheric Walker circulation. This coupling is an essential component of the El Niño Southern Oscillation (ENSO). 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 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. 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 changesfrom (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.