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Tropical thermocline helps power Pacific equatorial upwelling

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Abstract

Upwelling in the equatorial Pacific Ocean exerts a primary influence on Earth's climate, but there is great uncertainty on whether this influence will intensify or weaken under global warming. The dominant dynamical theory of equatorial upwelling argues that the easterly trade winds induce meridional divergence throughout the ocean mixed layer and thus 'pull' thermocline water up towards the surface (Ekman suction). Here, we use an available energetics framework to study the Pacific shallow overturning circulation and show that the majority of upwelling in the equatorial thermocline west of ~210E cannot be powered directly by winds. Rather, we demonstrate that upwelling in this region is powered by the equatorward advection of potential energy transferred to the thermocline via off-equatorial downwelling and mixing. This energy source is primarily linked to shallow overturning in the near-equatorial cells; therefore, it is highly correlated with the trade winds but allows for interaction with the subtropical cells. Ongoing efforts are dedicated to quantifying the subtropical influence on equatorial upwelling. The view of equatorial upwelling described here has thermocline waters 'pushed from below' rather than 'pulled from above' and provides a new dynamical basis for oceanic theories that explain multidecadal variability in equatorial sea surface temperatures.

Category
Modes of Variability and Teleconnections, Trends
Water Cycle and Hydroclimate
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