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A Modeling Study of Summertime East Pacific Wind-Induced Ocean–Atmosphere Exchange in the Intraseasonal Oscillation

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Abstract
  • Intraseasonal precipitation variability over the northeast Pacific warm pool during June–October in the National Center for Atmospheric Research 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 intraseasonal oscillation (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 the forcing of west Pacific ISO-related convection. The surface exchange mechanism is apparently active within regions of mean westerly low-level flow. In contrast, summertime eastern North Pacific intraseasonal wind variance and spatial structure does not differ significantly between the control and fixed-evaporation simulations. A strong coupling between the east Pacific flow and precipitation over Central America may be responsible for the relatively small changes in wind variability between the simulations. Interactions among the coarsely resolved Central American orography, the large-scale flow, and the convection parameterization in the model likely contribute to this anomalous coupling.
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  • Maloney, Eric D., Steven K. Esbensen, 2005: A Modeling Study of Summertime East Pacific Wind-Induced Ocean–Atmosphere Exchange in the Intraseasonal Oscillation. Journal of Climate, 18, 568–584.
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  • 18
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  • 4
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  • This work was supported by the Climate Dynamics Program of the National Science Foundation under Grants ATM- 0327460 (EDM) and ATM-0002322 (SKE).
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