Hemispheric Asymmetry in the Ventilated Thermocline of the Tropical Pacific

To understand sources of variability in the eastern equatorial Pacific, a region integral to modulating global temperatures, the waters upwelling from the Equatorial Undercurrent (EUC) are characterized. Past work is updated using temperature and salinity measurements from the Argo array and current measurements from Tropical Atmosphere Ocean (TAO) buoys. A larger hemispheric asymmetry is found in the water mass contribution through the ventilated thermocline to the EUC than previously reported, with 80%–90% of waters in the western Pacific originating from the Southern Hemisphere. South Pacific subtropical waters are the dominant source feeding the EUC, although in the central equatorial Pacific upper layers of the EUC experience freshening due to the addition of North Pacific waters. Anomalous volume transport, advection of anomalous waters, and shifts in hemispheric contributions contribute to variability in the EUC. These results suggest that variability in the EUC caused by anomalies in the South Pacific ventilated thermocline can explain variability in the eastern equatorial Pacific.