Upwelling circulation during downwelling-favorable wind
Ekman transport is one of the most fundamental concepts in oceanography. As shown by Ekman’s seminal work in 1905, when forced by surface wind stress, the vertical integral of upper ocean transport is to the right (left) of the wind in northern (southern) hemisphere. The transport magnitude depends solely on the strength of wind stress and latitude, independent of the turbulence characteristics of the upper oceans. This means that, in the northern hemisphere, shore-parallel wind with a coast on its right (left) will drive onshore (offshore) surface current and induce downward (upward) vertical velocity (termed downwelling and upwelling, respectively) near the coast.
The coastal upwelling and downwelling have been observed globally and shown to be a very robust phenomenon. Yet, a study (Moffat and Lentz 2012) recently found that, in the presence of strong horizontal density gradients (like those near river-ocean boundary), downwelling-favorable wind can in fact drive nearshore upwelling, opposite to what is expected from the conventional Ekman dynamics.
In Chen and Chen (2017), we offered a mechanistic explanation for the reversed circulation described above. We showed that the wind-intensified horizontal density gradients can induce strong geostrophic shear, causing the vertical stress divergence to change sign. The reversed circulation occurs when a controlling parameter, the ratio of geostrophic shear to wind-supported shear, is greater than one. This condition represents that, near the surface, the wind-intensified pressure gradient associated with the geostrophic shear exceeds the maximal Coriolis force of wind-driven current. The imbalance drives upwelling circulation, acting to restore geostrophic balance. Using a theoretical model, Chen and Chen (2017) also proposed a regime diagram that helps to classify wind-driven downwelling and upwelling for the coastal oceans.
Reference: Chen, S. Y. and S. N. Chen, 2017: Generation of upwelling circulation under downwelling-favorable wind within bottom-attached, buoyant coastal currents. Journal of Physical Oceanography, 48, 2499-2519