Coupling ROMS with RSM: Regional Ocean-Atmosphere Sensitivities

Hyodae Seo, Arthur J. Miller, John O. Roads, Emanuele Di Lorenzo, Masao Kanamitsu
Scripps Institution of Oceanography, La Jolla CA

A regional coupled atmosphere-ocean model is being developed to better understand air-sea interactions in the eastern North Pacific Ocean and California coastal region. The atmospheric part of the coupled model is the Experimental Climate Prediction Center (ECPC) Regional Spectral Model (RSM) and the oceanic component is the eddy-resolving Regional Ocean Modeling System (ROMS). A previous ROMS study for this region forced by RSM winds (at 25 km resolution and monthly time scales) clearly showed that the ocean model captured not only the statistics of observed circulation of the Southern California Current System but also the timing and spatial evolution of annually recurrent westward propagating depth anomalies during active upwelling (Di Lorenzo, 2003, Deep-Sea Res.). The goal here is to develop an operational regional joint air-sea modeling system over the California coastal ocean that could be imbedded within a coarser resolution, or global coupled atmospheric-oceanic forecasting system.

We have already started one-way coupling experiments in order to better understand a characteristic response of each model to the different atmospheric and ocean forcings. Our current atmospheric model uses higher spatial resolution (20km) and higher frequency forcings (6 hourly) than the Di Lorenzo (2003) study. The buoyancy forcing used to force the ocean model at the upper boundary was initially calculated directly from RSM output. Preliminary simulations revealed that simulated area-averaged SST with flux correction was warmer than observed SST by around 0.4 degree C for the year 2003. Rather than directly using the RSM fluxes, we believe, however, that the model will be more stable if we use instead a bulk parameterization that computes surface wind stress and net surface turbulent heat fluxes given the atmospheric boundary layer variables from RSM with ROMS SST. The radiation fluxes may also have to be modified to take into account different assumptions about the ocean boundary conditions. Preliminary multi-year simulations of ROMS with RSM forcing will be discussed.