5.1
Significantly reduced mid-Holocene ENSO variance in a coupled model and links to the North Pacific climate
John C.H. Chiang, University of California, Berkeley, Berkeley, CA; and Y. Fang and P. Chang
A number of paleoclimate records collectively show dramatically reduced El Nino-Southern Oscillation (ENSO) variance during the mid Holocene. Fully coupled models, while able to simulate reduction to ENSO activity, generally struggle to achieve it at the level suggested by the paleodata. We report results of coupled model that achieves significant reduction to ENSO variance (~45 percent) under mid-Holocene orbital conditions. The amplitude of the cold tongue seasonal cycle is also significantly reduced in the mid Holocene simulation. This model has recently been shown to reproduce present-day ENSO with fidelity, and in particular substantiating a hypothesis for North Pacific extratropical forcing on ENSO through via a ‘meridional mode' conduit. Our analysis - including several idealized simulations designed to isolate the origins the tropical Pacific climate changes - indicates that the reduction to the model ENSO variance is linked to reduction in the north Pacific wintertime extratropical activity; the latter originating from global climate changes wrought by the mid-Holocene insolation changes. We elucidate the mechanisms for changes to both the variability and mean climate in the mid-Holocene tropical Pacific.
Session 5, Air-sea interaction in paleo-climates
Tuesday, 21 August 2007, 8:30 AM-9:45 AM, Broadway-Weidler-Halsey
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