A Theory for the Prevailing Easterlies and Eastward-Shoaling Sloped Theromocline over Equatorial Pacific and the Accompanying ENSO Variability

A simple fully coupled model is used to study the most essential dynamic and thermodynamic processes that can spontaneously lead to both the climatology and ENSO of the equatorial Pacific ocean-atmosphere system. The coupled model consists of three components: (i) a shallow water equatorial b-plane model, (ii) a stripped-down equatorial SST equation governed by Ekman-layer and thermocline feedbacks, and (iii) a conceptual basin-wide Walker circulation model driven by a basin-wide SST gradient. The only nonlinearity of this system is in the SST equation.

It is shown that the equatorial Pacific basin atmosphere-ocean system is operated at a coupling strength such that the zonally windless atmosphere and basin-wide leveled thermoline with a uniform SST is unstable. This primary coupled instability together with nonlinearity leads to one of two possible zonally asymmetric mean states via a Hopf bifurcation. The direction of zonal asymmetry that can be observable is dictated by the equatorial Ekman dynamics. As a result, only one of the two possible mean states is observed, namely, the atmosphere has a prevailing easterly and the ocean basin has a deep-in-west/shallow-in-east thermocline with a warm-in-west/cold-in-east surface temperature. The ENSO variability is just a by-product of the coupled process. The feedback processes in the SST equation act to amplify the oceanic response at the eastern basin because the coupled mean thermocline is shallower there. As a result, the delayed oscillator mechanism is naturally evoked so that the otherwise damped oscillator becomes self-sustainable.

Under the coupling strength that is independently determined from the observation, this fully coupled model can produce a realistic mean state in which the basin-wide SST (thermocline depth) difference is 4.2°C (116 meters) and the westward wind stress at the central Pacific basin is 0.54 dyne/cm2. The corresponding self-sustained oscillation has a primary period of 3.7 years. The basin-wide SST difference can be as small (large) as 2.5°C (6.0°C) during a warm (cold) episode.