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Are the Subtropical-Tropical Cells responsible for decadal variability in the tropical Pacific?

Antonietta Capotondi

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One of the mechanisms proposed for tropical decadal variability relies upon changes in the upper-ocean wind-driven circulation. From a zonally averaged perspective, this circulation can be viewed as a shallow overturning circulation, the Subtropical-Tropical Cell (STC): water subducts into the main thermocline in the subtropics, flows equatorward, and upwells at the equator. The loop is closed by the poleward return flow in the surface Ekman layer. Since upwelling has a controlling influence on equatorial sea surface temperatures (SSTs), changes in the strength of the STCs can be expected to affect upwelling and SSTs. Observational studies show that the slow-down of the STCs after the mid-seventies is indeed concurrent with an increase in tropical SSTs. However, due to the sparsity of subsurface observations, the time resolution of the transport estimates is very coarse (~10 years) preventing a clear assessment of phase relationships between transport and SST. In this talk we use the output from an ocean general circulation model (the NCAR ocean model, NCOM) to examine the evolution and relative phase of the different branches of the STCs (equatorward flow, upwelling, surface poleward flow), and the relationship of these different components with the SST changes. To understand phase relationships it is very important to consider the 3-dimensional evolution of the STCs under changing wind forcing. We will examine the adjustment of the STCs, and will relate the transport changes to the passage of baroclinic Rossby waves, a fundamental component of the oceanic baroclinic adjustment. When the adjustment process is taken into account, the changes in the strength of the STCs are largely correlated and lead the changes in SST, supporting the idea that the STCs can have a controlling influence on SST.

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5 May, 2004
2 PM/ DSRC 1D 403
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