Sensitivity of the Near‐Shore Oceanic Circulation Off Central Chile to Coastal Wind Profiles Characteristics

• Published in: Journal of geophysical research. Oceans 2019-07, Vol.124 (7), p.4644-4676
• Main Authors: Astudillo, O., Dewitte, B., Mallet, M., Rutllant, J. A., Goubanova, K., Frappart, F., Ramos, M., Bravo, L.
• Format: Article
• Language: English
• Subjects: air sea land interactions; Altimeters; altimetry‐derived surface wind; Atmospheric correction; Atmospheric forcing; Atmospheric models; Bias; Coastal dynamics; Coastal jets; Coastal upwelling; coastal wind drop‐off; Coastal winds; Coasts; Computer simulation; Drift; Dynamics; Earth Sciences; Eastern Boundary Upwelling System; Ekman pumping; Ekman transport; Geophysics; Heat budget; Heat flux; Heat transfer; mesoscale processes; Mixed layer depth; Nonlinear response; numerical modeling; Ocean circulation; Ocean models; Oceans; Offshore; Reduction; Sciences of the Universe; Sensitivity; Suction; Undercurrents; Upwelling; Upwelling dynamics; Wind; Wind profiles; Wind speed; Winds
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2018JC014051

In Eastern Boundary Upwelling Systems (EBUS), the upwelling favorable wind speeds decrease toward the coast in the so‐called wind drop‐off coastal strip, which has been shown to be influential on the coastal upwelling dynamics, particularly in terms of the relative contributions of Ekman drift and Ekman suction to coastal upwelling. Currently, the wind drop‐off length scale is not properly resolved by the atmospheric forcing of regional ocean models in EBUS, featuring a smoother cross‐shore wind profile that results in stronger near‐shore speeds that could partly explain the coastal cold bias often found in those model simulations. Here, as a case study for the upwelling system off Central Chile, the sensitivity of upwelling dynamics to the coastal wind reduction is investigated using a Regional Ocean Modeling System (ROMS). Coastal wind profiles at different resolutions are first generated using a regional atmospheric model, validated from altimeter data, and then used to correct the coarse atmospheric wind forcing used for sensitivity experiments with ROMS. It is shown that the wind drop‐off correction induces a reduction in the oceanic coastal jet intensity, a stronger poleward undercurrent and a coherent offshore Ekman drift. It also yields a significant reduction of the cold bias along the coast compared to the simulation with “uncorrected” winds. Such reduction cannot be solely explained by the reduced Ekman transport only partially compensated by increase in Ekman suction. The analysis of the surface heat budget reveals in fact that an important contributor to the cooling reduction along the coast in the presence of coastal wind drop‐off is the heat flux term mediated by the reduction in the mixed‐layer depth. Overall, our results illustrate the nonlinear response of the upwelling dynamics to the coastal wind profiles in this region. Key Points The characteristics of coastal wind profiles simulated by a regional atmospheric model are sensitive to horizontal resolution The onshore wind reduction influences the oceanic circulation in the coastal domain and at regional scale off Central Chile The onshore wind reduction influences strongly the balance of processes associated to the maintenance of cool SST along the coast