Fast Wind-Induced Migration of Leddies in the South China Sea

Abstract

Abstract Eddies off the Strait of Luzon (termed here as “Leddies,” analogous to “Teddies” originating from the Indonesian Throughflow) are formed rapidly and migrate swiftly. Their migration rate (~10–20 cm s−1) is an order of magnitude faster than that of most eddies of the same scale (~1 cm s−1). On the basis of observations, it has been suggested earlier that the rapid generation process is due to the southeast monsoon. Here, the authors place this earlier suggestion on a more solid ground by developing both analytical and process-oriented numerical models. Because the eddies are formed by the injection of foreign, lighter Kuroshio water into the South China Sea (SCS), the eddies are modeled as lenses: that is, “bullets” that completely encapsulate the mass anomaly associated with them. It turns out that the rings migrate at an angle α (between 0° and 90°) to the right of the wind direction {i.e., tan−1[(2 − γ)f2R/8g′CD, where in conventional notation γ is the vorticity, R the eddy radius, and CD the interfacial friction coefficient along the lower interface of the lens}. Their fast migration speed is given by 2(τS/ρW)(sinα)/fH, where τS is the wind stress on the surface, ρw the water density, and H is the maximum eddy depth. With high interfacial drag (i.e., large CD), the rings move relatively slowly (but still a lot faster than Rossby waves) in the wind direction, whereas with low drag they move fast at 90° to the right. These analytically predicted values are in good agreement with isopycnic numerical simulations.