The seasonal cycle of tropical Pacific sea surface temperature in a coupled GCM |
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Authors: | J Thuburn R T Sutton |
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Institution: | (1) Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading RG6 6BB, UK E-mail: swsthubn@met.rdg.ac.uk, GB |
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Abstract: | The mechanisms responsible for the seasonal cycle in the tropical central and eastern Pacific sea surface temperature (SST)
are investigated using a coupled general circulation model. We find that the annual westward propagation of SST anomalies
along the equator is explained by a two-stage process. The first stage sets the phase of the variation at the eastern boundary.
The strengthening of the local Hadley Circulation in boreal summer leads to a strengthening of the northward winds that blow
across the equator. These stronger winds drive enhanced evaporation and entrainment cooling of the oceanic mixed layer. The
resulting change in SST is greatest in the east because the mixed layer is at its shallowest there. As the east Pacific SST
cools the zonal SST gradient in the central Pacific becomes more negative. This development signals the onset of the second
stage in the seasonal variation of equatorial SST. In response to the anomalous SST gradient the local westward wind stress
increases. This increase drives cooling of the oceanic mixed layer in which no single mechanism dominates: enhanced evaporation,
wind-driven entrainment, and westward advection all contribute. We discuss the role that equatorial upwelling plays in modulating
mixed layer depth and hence the entrainment cooling, and we highlight the importance of seasonal variations in mixed layer
depth. In sum these processes act to propagate the SST anomaly westward.
Received: 22 February 1999 / Accepted: 20 March 2000 |
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