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

Doron Nof, Yinglai Jia, Eric Chassignet, Alexandra Bozec
2011 Journal of Physical Oceanography  
Eddies off the Strait of Luzon (which we term here "Leddies" in analogy to Teddies originating from the Indonesian Throughflow) are formed rapidly and migrate swiftly. Their migratory rate (~ 10-20 cm/s) is an order of magnitude faster than that of most eddies of the same scale (~ 1 cm/s). On the basis of observations, it has been suggested earlier that the rapid generation process is due to the Southeast Monsoon. Here, we place this earlier suggestion on a more solid ground by developing both
more » ... 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), we model them as lenses, i.e., "bullets" that completely en-capsule the mass anomaly associated with them. It turns out that the rings migrate at an angle α (between zero and 90°) to the right of the wind direction [i.e., Tan -1 (2 ! " ) f 2 R / 8g 'C D , where, in the conventional notation, γ is the vorticity, R the eddy radius and C D the interfacial friction coefficient along the lens' lower interface]. Their fast migration speed is given by 2 (! S / " W ) (Sin # ) / fH , where τ S is the wind stress on the surface, ! w the water density and H is the maximum eddy depth. With high interfacial drag (i.e., large C D ) 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 our isopynic numerical simulations.
doi:10.1175/2011jpo4530.1 fatcat:khse4rfqo5ectif4bidyzgtiha