Midlatitude Excitation of Tropical Variability in the Pacific: The Role of Thermodynamic Coupling and Seasonality

A set of ensemble model experiments using the National Center for Atmospheric Research Community Atmospheric Model version 3.0 (CAM3) is run to investigate the tropical Pacific response to midlatitude atmospheric variability associated with the atmospheric North Pacific Oscillation (NPO). Heat flux anomalies associated with the NPO are used to force a set of model simulations during boreal winter (when the NPO is most energetic), after which the forcing is switched off and the coupled model evolves on its own. Sea surface temperature (SST) and wind anomalies continue to amplify in the tropical Pacific after the imposed forcing has been shut off, indicating that coupled ocean-atmosphere interactions in the tropical Pacific alter the spatial and temporal structure of variability associated with midlatitude forcing. The tropical circulation evolves through feedbacks between the surface wind, evaporation, and SST (the WES feedback), as well as through changes in the shortwave radiative heat flux (caused by changes in convection). Sensitivity experiments are run to investigate how thermodynamic coupling and seasonality affect the tropical response to NPO-related forcing. Seasonality is found to affect the WES feedback through (i) altering the sensitivity of surface evaporation to changes in the low-level wind field and (ii) altering the structure and strength of the lower-level wind response to SST anomalies. Thermodynamic coupling causes an equatorward and westward development of SST anomalies and an associated equatorward shift in the lower-level zonal wind anomalies.