Collaborative Research: Extratropical-Triggering of ENSO Events Through-the Trade--Wind Charging Mechanism
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El Nino/Southern Oscillation (ENSO) events affect weather and climate worldwide, with El Nino impacts over the United States including flooding in California, mild winters in Alaska and the northern tier, and drier conditions in the Ohio valley, with largely opposite conditions during La Nina, or cold phase ENSO, events. The predictability of ENSO is a matter of both practical and scientific interest, and there have been many efforts to identify precursor patters in sea surface temperature (SST), sea level pressure (SLP), and other variables so that the onset of ENSO events can be anticipated several months to seasons in advance. The PIs have proposed a novel mechanism through which changes in surface wind over the North Pacific associated with the North Pacific Oscillation (NPO, a prominent mode of Northern Hemisphere climate variability) can lead to ENSO events 9 to 12 months later. The mechanism, dubbed trade-wind charging (TWC), involves the southward transport of relatively warm subsurface waters from the central subtropical North Pacific, driven by the curl of the surface wind stress anomalies associated with the NPO (a Sverdrup transport). The southward heat transport warms the ocean thermocline in the central equatorial Pacific, thus creating favorable conditions for the development of an El Nina event (a similar chain of events can promote a La Nina event starting from the opposite phase of the NPO). The examination of the TWC mechanism involves testing three related hypotheses: 1) Positive and negative trade wind charging (TWC) of the equatorial Pacific heat content in isolation is sufficient to generate warm and cold boreal winter ENSO events, respectively; 2) Positive and negative TWC enhances the efficacy of initial subsurface ocean conditions in generating warm and cold boreal winter ENSO events; and 3) Positive TWC enhances the efficacy of Meridional Mode (MM) dynamics in generating warm boreal winter ENSO events. The research is performed through experiments with a climate model, the Coupled Model version 2.5 developed at the NOAA Geophysical Fluid Dynamics Laboratory. In one set of experiments, the ocean component model is integrated subject to surface atmospheric conditions taken from reanalysis products, so that the ocean state bears the imprint of an observed NPO event, after which the ocean is coupled to the atmospheric component model and the fully coupled climate model is integrated for several additional months to track the development (or lack thereof) of a TWC-induced ENSO event. The work has broader impacts due to the societal value of better long-lead predictions of ENSO events, as noted above. More specifically, the research offers specific targets for the evaluation of operational forecast model used for ENSO prediction, and the PIs plan to develop an index that can be used to monitor TWC behavior relevant to ENSO prediction. In addition, the project supports a graduate student and a postdoctoral research associate, thereby providing future workforce development in this research area.