Ocean-atmosphere interactions over the east Pacific warm pool associated with the boreal summer intraseasonal oscillation

The tropical intraseasonal oscillation (ISO) forces precipitation anomalies over the eastern north Pacific warm pool during June-September that are associated with wind-induced variations in surface latent heat flux. Latent heat fluxes at the 12^oN, 95^oW and 10^oN, 95^oW TAO buoys are significantly correlated with TRMM precipitation on intraseasonal timescales during 2000-2003. These buoys were part of the enhanced monitoring phase of EPIC2001. Further observational results related to intraseasonal east Pacific warm pool ocean-atmosphere interactions will be presented.

Modeling work is used to test the sensitivity of intraseasonal convection to wind-induced surface heat exchange. Intraseasonal precipitation variability over the northeast Pacific warm pool during boreal summer in the NCAR Community Atmosphere Model 2.0.1 with a relaxed Arakawa-Schubert convection parameterization is found to be strongly sensitive to wind-induced variations in surface latent heat flux. A control simulation with interactive surface fluxes produces northeast Pacific warm pool intraseasonal wind and precipitation variations that are of similar magnitude and structure to those associated with the observed ISO. Periods of low-level westerly intraseasonal wind anomalies are associated with enhanced surface latent heat fluxes and enhanced precipitation, as in observations. Variations in surface wind speed primarily control the surface flux anomalies.

A simulation in which eastern north Pacific oceanic latent heat fluxes are fixed produces intraseasonal precipitation variations that are significantly weaker than those in the control simulation and in observations. These results support the observational findings of Maloney and Esbensen, who suggested that wind-induced latent heat flux variability is a significant driver of ISO-related convective variability over the northeast Pacific warm pool during Northern Hemisphere summer. East Pacific ISO-related convection in this model thus appears to be forced by an analogous wind-induced surface heat exchange mechanism to that proposed by Maloney and Sobel to explain forcing of west Pacific ISO-related convection. The surface exchange mechanism is apparently active within regions of mean westerly low-level flow.