The Influence of Air-Sea Interaction in the North Pacific on ENSO

Vimont et al. (2001, 2003, J. Climate) proposed an extratropical-to-tropical connection termed the “seasonal footprinting mechanism” (SFM), where internal fluctuations in the atmospheric circulation over the North Pacific during winter imparts a “footprint” on the ocean through changes in the surface heat fluxes. The SST anomalies in the subtropical Pacific, subsequently impact the atmospheric circulation including zonal wind stress and heat flux anomalies that extend into the central and western tropical Pacific in spring and summer. These anomalies can excite an ENSO response in the subsequent fall and winter. The heat content in the western equatorial Pacific also serves as a precursor to ENSO but is most effective in combination with SFM-like anomalies (Anderson 2007, GRL). Observations indicate that an anomalously deep thermocline in the western equatorial Pacific in summer, followed by anomalous westerlies over the subtropical North Pacific during winter, favors the development of El Niño events during the subsequent winter, 12-15 month after the initial thermocline anomalies.

Here, we examine the joint impact of these precursors on ENSO using observations and CCSM4, NCAR's general circulation model. We also explore the processes through which the SFM signal propagates from the subtropics towards the equator and its interactions with the local thermocline anomalies.