Favorable connections between seasonal footprinting mechanism and El Nino

Previous studies suggested that the wintertime SST in the North Pacific that are generated by the concurrent North Pacific Oscillation (NPO) are able to force El Nio during subsequent winter via the so-called 'seasonal footprinting mechanism' (SFM). We examine how the NPO effectively generates the El Nio via the SFM in the observations and models. The occurrence ratio for El Nio under conditions of NPO forcing during the previous winters is about 41 % for the period of 61 years (1949-2009), indicating that the atmospheric forcing from the mid-latitudes through the SFM does not always trigger an El Nio. We observed certain favorable conditions under which the SFM may effectively induce El Nio. We directly compared these observations with two cases: when the wintertime NPO leads to El Nio during the following winter through the SFM, and when the wintertime NPO is not followed by El Nio. Our analysis demonstrates that the spatial structures of the NPO, associated wind speed and net heat flux in the northeast Pacific, differ between the two cases. Such differences determine the existence of a footprint SST in the northeastern Pacific during the late spring and summer, which plays a key role in initiating the El Nio via the projection of westerly wind stress anomalies onto the equatorial Pacific during the same seasons. By conducting linear baroclinic model experiments, it is found that the positions of La Nia SST forcing during the previous winter are able to modify the spatial structures of the NPO, which produces favorable conditions for the El Nio during subsequent winter via the SFM.