Regional climate research

Impact of wind resolution on simulation of a major coastal upwelling event

In March 2002 a major upwelling event was observed at a mooring off the Southern California coast, marked around the middle of the month by precipitous cooling of at least 4 deg C within a few km of the coast, and slow recovery to pre-event temperatures by month's end. Imposing winds of a 2km-resolution regional atmospheric model (MM5) constrained by larger-scale observed conditions on a 1km-resolution regional ocean model (ROMS), we reproduce this event qualitatively. The simulation exhibits dramatic coastal cooling of about 3 deg C at the same time as observations, followed by realistic relaxation to normal conditions. However, when atmospheric model resolution is reduced to 18km and then 54km and these coarser winds are imposed on the same ocean model configuration, the simulation's realism degrades significantly, with the cooling associated with the upwelling event being cut in half. These results are summarized in the top panel of the accompanying figure, which shows time series of sea surface temperature recorded at the mooring (location shown in the bottom panel), and simulated by the ROMS model forced by winds of varying resolution. The bottom panel shows a snapshot of simulated sea surface temperature at the event's peak on March 18 when ROMS is forced by 2km MM5 winds. The simulated upwelling zone is narrowly confined to the coast, and protruding filaments in the front separating the upwelling waters from the waters of the rest of the Southern California Bight are also seen in this submesoscale-resolving ocean model.

The figure's top panel shows that the oceanic response to 6km resolution winds is very similar to the 2km wind response, suggesting little realism would be gained if wind forcing resolution were made finer than 2km. Our results therefore demonstrate wind resolution between 2km and 6km is probably required to simulate major upwelling events along the Southern California coast, and point to the importance of interactions between large-scale atmospheric flows and nearshore topography in generating wind stress anomalies responsible for coastal upwelling. Comparably fine wind resolution is likely also necessary to simulate upwelling faithfully in coastal areas where adjacent topography is comparably complex, such as along the west coast of South America.

Download the publication (Dong et al. 2006) describing these results in more detail.

Charles Dong (Center for Earth System Research, UCLA), Alex Hall, Mimi Hughes, and Jim McWilliams (Center for Earth System Research, UCLA) make up the team that performed this research.