The joint impact of Tropical North Atlantic and Equatorial Atlantic SSTA on ENSO evolution

Abstract:

In this study, the joint impact of Tropical North Atlantic (TNA) and Equatorial Atlantic (EA) Sea Surface Temperature Anomaly (SSTA) on ENSO is investigated. Empirical Orthogonal Function (EOF) analysis towards the boreal spring-summer SSTA over tropical Atlantic reveals that the first (second) EOF mode exhibits a basin-warming (meridional dipole) pattern, which reflects the same (opposite) sign between TNA and EA pattern. The observational partial-regression analysis, idealized coupled model (CESM1.2) experiments with prescribed Atlantic forcing and CMIP6 multi-model ensemble analysis suggest that the basin-warming (meridional dipole) forcing is (is not) sufficient to induce a La Niña in subsequent winter, which we primarily attribute to their distinct Kelvin wave response.

For basin-warming forcing, the uniform Atlantic positive heating generates cyclonic Rossby (easterly Kelvin) wave to its west (east). During May to August, the easterly Kelvin wave could interact with the mean westerly monsoon circulation over Northern Indian Ocean (NIO) through wind-evaporation-SST feedback to induce warm SSTA and positive heating in suit, which can induce easterly anomaly over Equatorial Western Pacific (EWP). This easterly anomaly is crucial in generating La Niña by inducing oceanic upwelling Kelvin wave propagating eastward. In contrast, for meridional dipole forcing, even the cyclonic Rossby wave response still presents, the Kelvin wave response to the Atlantic dipole heating is too weak to induce air-sea interaction over NIO and the easterly anomaly over EWP. Due to the strong (weak) Kelvin wave-monsoon interaction over NIO, the easterly anomaly over EWP and the La Niña condition for basin-warming (meridional dipole) forcing are significant (insignificant).

Furthermore, the role of Atlantic-induced Kelvin wave-monsoon interaction over NIO in conveying the impact from Atlantic to Pacific is further verified through sensitivity experiments with prescribed TNA-only and EA-only forcing.

In summary, it is concluded that the sign between TNA and EA SSTA is crucial in influencing ENSO and the Atlantic-induced Kelvin wave-monsoon interaction over NIO is important. How the current mechanism is relevant to previous hypotheses and how ENSO-type vary in response to TNA/EA/basin-wide forcing are discussed.