Subtropical modulation of the equatorial undercurrent: a mechanism of Pacific variability

Abstract

Combining data from Argo and the TAO buoy array we present new observations of variability in the Pacific Ocean. Argo profiles reveal the development of a thickness anomaly in the lower levels of the ventilated thermocline of the South Pacific in 2010. Data through 2017 show this anomaly propagating as a baroclinic wave westward and towards the equator. Theory suggests that this wave will reduce the velocity of the equatorial undercurrent (EUC) when it reaches the equator, transitioning the equatorial Pacific to a warm state. This is supported by TAO array observations that show a past decadal shift in EUC strength around 2000, as well as radiocarbon coral measurements which suggest a similar change in the 1970s, both of which align with phase changes in Pacific decadal variability. Using model simulations with enhanced vertical resolution in the thermocline, we affirm this link between the subtropical south Pacific thermocline and the EUC, which also manifests in eastern Pacific sea surface temperatures. We combine these results to hypothesize a mechanism that may explain some of the decadal variability observed in the Pacific. This mechanism relies on the propagation of anomalies in the structure of the ventilated thermocline from the southeastern Pacific to the equatorial Pacific, modulating the strength of the EUC. The cycle is enhanced by atmospheric teleconnections between the equatorial Pacific and the southeast Pacific that periodically reverse the anomaly in thermocline thickness. If correct, our hypothesis predicts a return to a warm state of the equatorial Pacific when the Kelvin wave reaches the equator and the thermocline adjustment slows the EUC.