Extended Abstract (104K)6.5
The atmospheric response to ocean perturbations in the Kuroshio Oyashio extension
Elena Yulaeva, SIO/Univ. of California, La Jolla, CA; and N. Schneider
There is significant evidence of low frequency climate variability over North Pacific / North American Continent regions. The air-sea surface variables such as SST and heat fluxes are controlled by both atmospheric and ocean processes. However, the extended memory of the ocean-atmosphere system and, therefore,its predictable part, is determined by the ocean component. Thus predictable part of low-frequency climate changes is linked to oceanicly induced disturbances of the surface variables. Recent experiments with the coupled model showed that there exist two primary regions of low-frequency variability of sea surface temperature (SST) in the mid-latitude Pacific. In the first region, located in the Central Pacific, the SST variability is set predominantly by the atmospheric forcing. In the second zone, located in Kuroshio Oyashio Extension (KOE) region, low-frequency component of SST pattern is set by oceanic dynamics. The role of the atmosphere in this region resides in damping anomalies of air-sea flux perturbations produced by the ocean. Therefore, physically meaningful boundary forcing for the simulation of the atmospheric component should be expressed in terms of perturbations of the oceanic mixed layer heat budget induced by the ocean. In the framework of this approach, the SST changes can be viewed as the byproduct of ocean-atmosphere interaction. This methodology allows for eliminating of unrealistic damping of the atmospheric response implied by the prescription of SST anomalies and for better understanding of the physical mechanism of low-frequency variability in mid-latitudes. We examine the wintertime response of an atmospheric general circulation model (CCM3) coupled to mixed-layer ocean model (SOM) to mixed layer heat-budget perturbations of up to 40 W/M2 in the KOE region. The statistically significant wintertime atmospheric response is seen in the sea level pressure, precipitation, and 500 mb height anomalies over mid-latitude Pacific. Response in 500 mb height field resembles Western Pacific pattern, the second mode of the simulated internal variability of the atmosphere. Examination of the vorticity and thermodynamic budgets reveals the crucial role of submonthly transient eddies in maintaining the anomalous circulation of the free atmosphere. The corresponding response in the wind stress field alters Ekman Pumping in such a way that the expected change of the oceanic gyre, as measured by Sverdrup transport, would counteract the prescribed forcing in KOE region, thus causing a negative feedback. The experiments also revealed statistically significant winter-time responses in 2m temperature and precipitation over North America suggesting that low-frequency fluctuations over the continent can be predicted based on conditions in the KOE region.
Supplementary URL: http://meteora.ucsd.edu/~yulaeva/PAPER/paper.htm
Session 6, Extratropical Air-Sea Interaction
Tuesday, 15 May 2001, 8:30 AM-1:15 PM
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