12th Conference on Interactions of the Sea and Atmosphere


Pacific Decadal Air Sea Interaction

Niklas Schneider, SIO/Univ. of California, La Jolla, CA

In this talk, hypothesis and relevant studies for decadal variability in the North Pacific will be reviewed. We focus on the atmospheric response to oceanic perturbations of the surface heat budget in two areas, the equatorial upwelling regions, and the mid-latitude North Pacific.

The coupled response to the equatorial emergence of water mass anomalies has been implicated in the decadal climate variability and decadal modulation of El Nino. This conjecture is tested with a coupled ocean-atmosphere model. Water mass anomalies are artificially inserted into the western Pacific thermocline, and the coupled feedbacks in response to their emergence at the surface of the eastern Pacific upwelling region are investigated. The largest feedback is a strong reduction of the water mass anomalies by a venting of heat at the surface, and dilution of the signals by waters of the shallow equatorial cell. The atmospheric response to warm water mass anomalies includes zonal wind stress that converge on the emergence region and depresses the thermocline there. This thermocline feedback enhances the subsurface temperature anomalies and represents a weak positive feedback. The models El Nino exhibits a strong sensitivity: Cold water mass anomalies lead to an increased interannual variability.

Recent analyses of ocean hindcasts, coupled models, and observations of North Pacific decadal variability indicate advection by Ekman currents to be important. This raises the question of the possible role of this perturbation of the oceanic mixed layer heat budget on atmospheric variability.

Anomalous Ekman advection is implemented in the extratropics of an oceanic mixed-layer atmospheric general circulation model. Comparison of integrations with and without this effect Ekman advection reveals that Ekman advection shifts the leading mode of variability, with maximum loadings in the north-west Pacific, in a south-easterly direction, in accordance with observed variability. The perturbation of the heat budget by the Ekman advection is balance primarily by the latent heat flux, with some effect on atmospheric precipitation.

Session 2, extratropical atmosphere-ocean interaction
Monday, 10 February 2003, 10:45 AM-2:15 PM

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