Seasonal dynamics of the near-surface alongshore flow off central Chile

Abstract

The seasonal cycle of the near-surface circulation off central Chile was analyzed using satellite altimetry and an oceanic model. To evaluate the role of the wind stress curl on the circulation we performed two identical simulations except for the wind-forcing: the "control run" used long-term monthly mean wind stress and the "no-curl run" used a similar wind stress field, but without curl. The observed and modeled (control run) surface currents showed a strong seasonal cycle and a well-defined equatorward flow with a jet like-structure. This jet develops during spring and summer, consistent with the presence of a low-level wind jet. South of Punta Lavapie cape (similar to 37 degrees S), the equatorward surface current remains close to the coast. After the flow-passes this cape, however, it separates to become an offshore jet. In contrast, in the no-curl simulation the separation at Punta Lavapie is not observed and the offshore jet farther north is not present, demonstrating the importance of the wind stress curl on the dynamics of this flow. Although the offshore integrated Sverdrup transport was similar to the model transport, the offshore jet was not located where the wind stress curl was maximum. Instead, the position of the jet followed approximately the zero wind stress curl, which corresponds to the climatological location of the low-level wind jet axis. These results illustrate the importance of the offshore upwelling/downwelling associated with curl-driven Ekman pumping, which tilts isopycnals upward (downward) toward the east (west) of the wind jet, forcing a northward flow through thermal wind balance.