Modelling of the air-sea interaction off central Chile: sensitivity to the resolution


Coastal upwelling regions are of special interest as they are places of intense biological activity, mesoscale and submesoscale circulations which play a key role in the exchange of water properties between the open and coastal oceans. Our interest is on the upwelling region off central Chile (26°S-36°S), which is under the influence of persistent low-level along-shore southerly winds. Satellite data, marine reports, and coastal in situ observations off central Chile indicate that those along-shore winds intensify at intraseasonal timescales leading to so-called Costal Jet events. Off central Chile, the southerly jet events occur year-round but are more frequent during the summer upwelling season. The jet is characterized by an elongated maximum of surface wind speed (10 ms-1) with its axis at about 150 km off the coast and a cross-shore scale of about 500 km. It is associated with a significant oceanic mesoscale variability that contributes to cross shore exchanges of heat, salt and biogeochemical material between open and coastal oceans.
Using the high-resolution ocean (ROMS) and atmosphere (WRF) regional models, one year of atmosphere and ocean variability is simulated to investigate the sensitivity of the upwelling variability to the spatial resolution of the atmospheric forcing. The sensitivity of surface winds and air-sea heat fluxes to the horizontal resolution of the atmospheric model is evaluated. Near the coast, increased resolution improves the realism of the cross-shore variability of the wind stress. The simulated atmospheric fields at various resolutions (40km, 10km) are then used as surface boundary conditions for the regional high-resolution ocean model. The results indicate a large sensitivity of the oceanic turbulent flow to the characteristics of the atmospheric forcing and they suggest that air-sea coupling at mesoscale is a significant contributor to the upwelling variability in this region. The well defined Coastal Jet event of October 2000 is also examined in more details, focusing on its impact on the mixed-layer dynamics. A significant sensitivity of the heat-budget to the resolution is found.


Renault Lionel (LEGOS), Dewitte Boris(LEGOS,IRD), Echevin Vincent(LOCEAN,IRD), Yves du Penhoat (LEGOS,IRD)