The Impact of Extratropical Atmospheric Variability on El Niño: Contrasting Thermodynamic versus Dynamic Coupling

Abstract:

During the positive phase of the North Pacific Oscillation (NPO), wintertime westerly wind anomalies over the subtropical North Pacific influence the tropical Pacific in two ways: 1) by inducing the seasonal footprinting mechanism (SFM) through altered surface heat fluxes; and 2) by inducing the trade-wind charging (TWC) mechanism through altered ocean dynamics. These two mechanisms can influence the atmosphere–ocean system over the tropics and lead to an El Niño in the following winter. Previous studies connecting the extratropical precursors to El Niño generation mainly focused on the heat flux-driven thermodynamic coupling and largely ignored the wind-driven dynamical coupling. Here for the first time, using a coupled-model, we show the relative importance of thermodynamic and dynamical coupling associated with NPO on El Niño generation. We conduct a set of carefully constructed ensemble experiments where we force an ENSO neutral ocean in winter and spring with either TWC-favorable wind stress or SFM-favorable heat flux forcing. We then allow the model to evolve in a fully-coupled configuration through the following winter. The distinct model response to the dynamic versus thermodynamic coupling on El Niño amplitude, duration, and diversity will be discussed.