Diabatic Ocean Processes Drive La Niña Events

Abstract:

The zonal temperature gradient in the tropical Pacific plays an important role in the global climate system. This gradient is most strongly influenced by sea surface temperature (SST) variability in the eastern equatorial cold tongue region (CT), which is at the heart of the El Niño Southern Oscillation (ENSO). CT SSTs are impacted by the ocean circulation in the tropical Pacific. Here, we focus on the vertical branch of the circulation, which connects subsurface eastward flow with westward surface flow. We utilize a novel formulation of the water mass transformation (WMT) framework, retaining spatial information about where water masses are being transformed. This method allows us to partition vertical velocities in the thermocline into diabatic and adiabatic components, to locate areas of high WMT activity, and attribute the WMT to specific physical processes. We find that in the mean as well as during ENSO phases, diabatic velocities make up to a third of the total Eulerian velocity. The bulk of WMT below the CT occurs during La Niña and neutral years, with diabatic processes mostly inactive during El Niño. Thermocline diabatic motion is driven by vertical mixing, whereas in the shallow surface layer it is dominated by solar penetration.