Izumo Takeshi, Vialard Jérôme, Dayan H., Lengaigne Matthieu, Suresh I. (2016). A simple estimation of equatorial Pacific response from windstress to untangle Indian Ocean Dipole and Basin influences on El Nino. Climate Dynamics, 46 (7-8), p. 2247-2268. ISSN 0930-7575.
Titre du document
A simple estimation of equatorial Pacific response from windstress to untangle Indian Ocean Dipole and Basin influences on El Nino
Izumo Takeshi, Vialard Jérôme, Dayan H., Lengaigne Matthieu, Suresh I.
Source
Climate Dynamics, 2016,
46 (7-8), p. 2247-2268 ISSN 0930-7575
Sea Surface Temperature (SST) anomalies that develop in spring in the central Pacific are crucial to the El Nio Southern Oscillation (ENSO) development. Here we use a linear, continuously stratified, ocean model, and its impulse response to a typical ENSO wind pattern, to derive a simple equation that relates those SST anomalies to the low frequency evolution of zonal wind stress anomalies tau (x) over the preceding months. We show that SST anomalies can be approximated as a "causal" filter of tau (x) , tau (x) (t - t (1)) - c tau (x) (t - t (2)), where t(1) is similar to 1-2 months, t(2) - t(1) is similar to 6 months and c ranges between 0 and 1 depending on tau (x) location (i.e. SST anomalies are approximately proportional to the wind stress anomalies 1-2 months earlier minus a fraction of the wind stress anomalies 7-8 months earlier). The first term represents the fast oceanic response, while the second one represents the delayed negative feedback associated with wave reflection at both boundaries. This simple approach is then applied to assess the relative influence of the Indian Ocean Dipole (IOD) and of the Indian Ocean Basin-wide warming/cooling (IOB) in favouring the phase transition of ENSO. In agreement with previous studies, Atmospheric General Circulation Model experiments indicate that the equatorial Pacific wind responses to the IOD eastern and (IOB-related) western poles tend to cancel out during autumn. The abrupt demise of the IOD eastern pole thus favours an abrupt development of the IOB-cooling-forced westerly wind anomalies in the western Pacific in winter-spring (vice versa for an IOB warming). As expected from the simple SST equation above, the faster wind change fostered by the IOD enhances the central Pacific SST response as compared to the sole IOB influence. The IOD thereby enhances the IOB tendency to favour ENSO phase transition. As the IOD is more independent of ENSO than the IOB, this external influence could contribute to enhanced ENSO predictability.
