ESRL/PSD Seminar Series

The Role of Sub-Seasonal Tropical Convective Variability for the Midlatitude Response to ENSO<

Erik Swenson
George Mason University, Fairfax, Virginia


The boreal winter midlatitude response to a significant ENSO event has been understood primarily in terms of an average across the season. However, its development and variability in terms of a highly sub-seasonally variable atmospheric forcing is still unclear. For instance, what convective variability matters for the response: short-lived strong events or more persistent moderate events? Using CAM4.0, the response to ENSO is examined using large ensembles of seasonal integrations forced with observed SST from multiple ENSO events. We decompose the diabatic heating rate (Q) across the tropical Indo-Pacific in terms of temporal and intra-ensemble variability. We then repeat the integrations by prescribing subsets of Q in an effort to diagnose the impact of sub-seasonal and intra-ensemble Q variability. Neglecting sub-seasonal Q variability has a systematic impact on the response, generally extending the Pacific jet and deepening upper-level heights across the North Pacific. Much of this appears to be forced by enhanced tropical upper-level divergence, a direct result of averaging together vertical profiles associated with moderate and deep convection. The effects of persistent convection and intra-ensemble Q variability are also investigated, but implications for the response are less clear.

Monday, Jan 9 2012

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