Natalie Perlin, Eric Skyllingstad, Roger Samelson, Philip Barbour nperlin@coas.oregonstate

1. Coastal Upwelling Studies Using a Coupled Ocean-Atmosphere Model Natalie Perlin, Eric Skyllingstad, Roger Samelson, Philip Barbour nperlin@coas.oregonstate.edu skylling@coas.oregonstate.edu rsamelson@coasoregonstate.edu barboup@engr.orst.edu College of Oceanic and Atmospheric Sciences, Oregon State University, USA

2. Outline • Why do we need the ocean-atmosphere coupled modeling system? • System main components and model coupling strategy • Numerical study of upwelling and simulations description • Ocean response to the coupled system • Atmospheric response to the coupled system • Summary and future plans

3. The need for the ocean-atmosphere coupled system • Atmospheric models – - narrow coastal upwelling zone of 10-50 km offshore is still a sub-grid scale for most operational numerical weather prediction systems Trouble: the effect of cooler sea surface temperatures (SST) near the coast is not resolved directly in the atmospheric models! • Ocean models - - usually have neither adequate surface wind and wind stress data, nor the surface heat fluxes Trouble: ocean models do not include the effects of spatially and temporary inhomogeneous atmospheric forcing that potentially affect ocean surface boundary layer development • Satellite and in-situ observations – - show the internal boundary layer development over the cold water - indicate that cooler SST-s tend to reduce the surface wind stress - turbulence collapse over the cold water restricts downward momentum transfer to the ocean surface

4. Regional Ocean-Atmosphere Modeling (ROAM) system main components • Atmospheric component: COAMPS(Coupled Ocean- Atmosphere Mesoscale Prediction System; Hodur, 1997), based on non-hydrostatic fully compressible dynamics • Ocean component: ROMS (Regional Ocean Modeling System), hydrostatic free-surface primitive-equation model • Flux coupler: MCT (Model Coupling Tookit; Larson et al., 2004), a software tool consisting of Fortran 90 modules for data exchange between parallel earth system models to create a parallel coupled model Fig.1. ROAM chart, a single-executable modeling system with its components running in concurrent mode

5. Simulation domain for the numerical study of upwelling • Study summertime wind-forced upwelling off Oregon coast using a fully coupled ocean-atmosphere model • Horizontal domain is 50 x 20 km, 1-km grid boxes • Linear shelf slope from 10 m to 300 m at 50 km offshore • Ocean vertical grid has 40 layers, surface to bottom • Atm. vertical grid has 42 layers, surface to over 9 km • Atm. model time step is 3 s, ocean time step is 300 s • Ocean initialization from the profiles of T and S that are typical for that time and location; ocean starts at rest • Atmospheric initialization: horiz.-homogeneous, from chosen temperature and moisture profiles • Atm. pressure gradient is maintained constant during the simulation, and is computed from 15 m/s northerly winds • Periodic N-S boundary conditions in both atm. and ocean models; the domain becomes a periodic channel • Open W-E boundary conditions; eastern wall in ROMS • LMD mixing in ROMS, with surface and bottom KPP Fig. 2. Atmospheric potential temp., water vapor mixing ratio, ocean temp. and salinity profiles that are used for models initialization Fig. 3. Ocean bathymetry W-E

6. Effects of incomplete data exchange between ocean and atmosphere models • Fully coupled model system provides the following data exchange between its components every 300 s (every ocean model time step): • wind stress from the atmospheric model to the ocean model • ocean SST update from the ocean to the atmospheric model • latent, sensible heat fluxes, longwave and shortwave/solar radiation from the atmospheric to the ocean model Table 1. Summary of the differences of four simulation to study the effects of incomplete data exchange between the models

7. Figure 4. SST in cross-shore direction in the end of 72-h simulations 4 km offshore (i=47) 40km offshore (i=10) last inshore pt. (i=50) Figure 5. SST time evolution at different offshore locations, from the beginning till the end of a forecast Ocean model results: Sea Surface Temperature response • Fully coupled case (red) has its lowest SST extending to the inshore points • Non-coupled case (black) has greatest difference between its lowest SST and that at the inshore point • When solar radiation is included (red), gradual heating of the surface is well noticed at offshore point • Upwelling effect starts sooner at the coastal point when heat flux exchange is included • In non-coupled case (black) upwelling at the coast starts later, with rapid drop of SST

8. Ocean model results: temperature and velocity cross-sections W-ths WT-hs WTH-s WTHS Fig. 6. Ocean temperature (left), v-velocity (center), and u-velocity (right) cross-sections in the end of the simulation, 72-h forecast. • Upslope propagation of the upwelling front extends further inshore in fully coupled case (bottom panel) • Stronger temperature and momentum vertical gradients yield in the fully coupled case in the shallow regions (5 km offshore) • Southward surface jet in non-coupled case extends deeper, but more horizontally limited • Southward sfc. jet in fully coupled case is more shallow, broadening horizontally • Ekman layer is thinner in the fully coupled case, with higher offshore velocities

9. Atmospheric model results: wind stress, wind stress curl, and Ekman transport Fig. 7. Atm. model wind stress, wind stress curl, and computed cumulative offshore Ekman transport Ekman pumping at the offshore location: Coastal Ekman transport at the ocean boundary: Cumulative Ekman transport at the offshore location: • Two-fold decrease of the meridional wind stress near the coast as compared to its offshore values lead to strong wind stress curl in the nearshore in the coupled cases • Offshore Ekman transport computed from the atmospheric wind stress differs nearly two-fold at the coast as a result of this wind stress decrease • Wind stress curl has such an effect that cumulative offshore Ekman transport at the western boundary of the domain becomes of the similar value for all the simulations

10. Atmospheric model: internal boundary layer development Figure 9. Cross-section of the potential temp. (contours) and v-wind (color) in the end of 72- h run, fully coupled case Figure 8. Vertical profiles of potential temp. at three offshore locations, 72-h forecast for four simulations. Open circles show the surface temperatures in the atmospheric model • Well mixed marine boundary layer forms in all cases, with the capping inversion at the reasonable height (close to the observed) • Internal boundary layer forms only in the coupled cases, and is defined best in the fully coupled case • Region of stronger winds forms below the inversion and over the cooler coastal waters Figure 10. Similar variables as above, observed off Oregon coast during the COAST experiment on July 24-25, 2001. Courtesy of John Bane, UNC.

11. Summary • Coupled ocean-atmosphere simulations demonstrated qualitative improvement of the model results in a study of coastal upwelling • Cooler upwelled water penetrates further inshore in the fully coupled simulations; lower SST are then found in the nearshore shallow regions • Fully coupled case produces oceanic southward surface jet that is more shallow but horizontally broader, more shallow Ekman layer • Internal boundary layer develops in the atmospheric marine boundary layer over the colder waters during the coupled simulation, in response to ocean upwelling • Increase of southward flow below the marine layer inversion and above the cold water supports the conception of turbulent collapse that restricts downward momentum transfer Future plans • Westward extension of the ocean domain, include shelf break • Include coastal land and coastal topography • Include alongshore variations in coastline, topography, and bathymetry • Use different model resolutions for the ocean and atmospheric models • Include realistic topography and bathymetry of the US West coast • Include fresh water sources, as Columbia river plume