Eastern Intensification of Ocean Spin-Down: Application to El Niño

Abstract

The El Niño studies carried out by Bjerknes indicate that the anomalous warming of the eastern boundary tropical ocean in 1939 and 1958 extended 1000 km into the interior ocean, corresponding with a diversion of the Humboldt Current offshore south of Peru. Furthermore, these features were associated with a large-scale weakening of the Southern Hemisphere wind systems. This suggests that major changes occurred in the wind-driven eastern boundary circulation during this time. To see if this is plausible from a theoretical viewpoint, we consider a mathematical model of large-scale ocean spin-down, induced by a reduction in the mean strength of the large-scale wind systems. This model shows that spin-down of the interior ocean is intensified along the eastern boundary with de-intensification propagating in time toward the west and extending out to 1000 km after one year. The interior portions of the ocean circulation are only weakly affected. Moreover, the spin-down is asymmetric, with greater de-intensification in the equatorial eastern boundary than in the poleward regions, making it appear as though the Humboldt Current is directed off-shore just south of Peru. This asymmetric aspect of gyre spin-down can be explained in terms of non-dispersive Rossby waves propagating energy [C_g=−β/(f²/g′H] to the west at a faster rate near the equator than near the poles. The results of this study have direct application to El Niño. First, the general decrease in equatorward transport of the Humboldt Current off the coast of Peru allows for the anomalous increase in temperature simply by reducing the advection of cold subtropical waters. Moreover, the asymmetric intensification of eastern boundary spin-down has the effect of lowering the dynamic height of the sea surface in the eastern equatorial region. This situation is conducive for the cross-equatorial transport of warm water to the south; possible mechanisms for the anomalous warming have been proposed by Bjerknes and Wyrtki. The advection of warm interior water to the east does not seem to be allowed by the spin-down process and therefore, cannot be considered as a source of warm El Niño waters.