Tropical Thermocline Helps Power Pacific Equatorial Upwelling

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 “pull” water up toward the surface via Ekman suction. In contrast, studies of decadal variability suggest that the subtropical cells “push” equatorial upwelling from below. Therefore, it is unclear whether upwelling is “pulled from above” by Ekman divergence or “pushed from below” by geostrophic convergence. Here, we use a framework of local available energetics to study the Pacific shallow overturning circulation and find that at least 20%–50% of equatorial upwelling cannot be powered directly by winds along the equator, as commonly understood. Instead, this fraction of upwelling is powered by potential energy that is transferred to the thermocline via off-equatorial downwelling and diabatic processes. Water parcels holding excess potential energy in the equatorial thermocline are then able to upwell without additional energy input such that equatorial upwelling can in fact be pushed from below. The strength of this push is not only largely set by the trade winds but may also be influenced by energy sources across the subtropical ocean. Unlike previous available energetics analyses of the equatorial region, our study uses complete local conservation laws that allow us to trace all energy sources and pathways. This makes our dynamical formulation particularly useful to explain variations in equatorial Pacific upwelling at interannual and decadal time scales alike.