A New Extratropical Tracer Describing the Role of the Western Pacific in the Onset of El Nino: Implications for ENSO Understanding and Forecasting

A complex empirical orthogonal function analysis was applied to sea surface temperature data in the southern high-latitude Pacific to identify and isolate primary processes related to the onset of El Nino (EN) events. Results were compared to those of a lead-lag composite analysis of a new tracer of EN events in the southern high-latitude Pacific, the Ross-Bellingshausen (RB) dipole. Both techniques successfully isolate the main low-frequency features in the interaction among the tropical and southern extratropical Pacific during the onset of recent eastward-propagating EN events. Particularly, positive RB peaks were followed by EN events around 9 months later, on average. In turn, RB maxima were anticipated by local warm anomalies in the western tropical Pacific a year in advance, which enhance local convection and upper-troposphere divergence and generate an anomalous wave train extending eastward and poleward in the southern extra-tropics. In addition, circulation changes lead to a warm SST region in the central tropical Pacific, which is then strengthened by suppressed equatorial easterlies. Convection thus starts to move to the central Pacific and so the Walker circulation weakens, activating the positive Bjerknes feedback that ultimately leads to the development of an EN event. These results highlight the enormous potential of the interaction between the tropics and this high-latitude region in the Southern Hemisphere to increase El Nino-Southern Oscillation understanding and to improve the long-lead prediction skill of EN phenomenon.