Effects of Ocean-Atmosphere Coupling in a Modeling Study of Coastal Upwelling in the Area of Orographically-Intensified Flow Natalie Perlin, Eric Skyllingstad, and Roger Samelson College of Oceanic and Atmospheric Sciences, Oregon State University 2007 ROMS/TOMS Workshop October 1-3, UCLA

Outline of the talk Background : observations, theory, modeling Recent modeling efforts: study design, test cases Modeling results Conclusions, discussion, future work

Background Three phenomena/processes involved: Flow intensification downwind of major capes along the Oregon-California coastline – satellite, in-situ observations, atm. modeling Wind-driven coastal upwelling in the summertime – observations, theories, ocean and coupled ocean-atmosphere modeling Mesoscale air-sea interaction affecting boundary layers in both ocean and atmosphere – observations, theories, coupled modeling

Enriquez and Friehe (1995) Wind intensification downwind of major capes off the U.S. West coast Perlin et al., 2004 Enriquez and Friehe, 1995

Wind-driven coastal upwelling Satellite SST Huyer et al., 2005 Coastal Ekman transport at the ocean boundary:

Air-sea interaction in the marine boundary layer : airborne observations Courtesy of John Bane, UNC Vickers et al., 2001 Potential temp. (K) and v-wind (m/s) Oregon coast, COAST experiment Duck, North Carolina Momentum flux and wind speed

First results from a coupled model Perlin et al., 2007 Courtesy of John Bane, UNC

Numerical study design for a coupled model Coupled ocean-atmosphere model, COAMPS (atm.) and ROMS (ocean) Horiz. domain 160 x 210, 3-km grid Vertical: 47 lvs. (atm.) and 40 (ocean) Time step: 5 s (atm) and 100 s (ocean) Atm. model is driven by 15 m/s geostrophic wind in the atm. boundary layer; 5 m/s above 2000 m. Ocean model: initially at rest, stratified in temp. and salinity 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 coastal bend

Wind stress: control case

Surface currents and SST

Marine boundary layer height 1.Atmospheric boundary layer grows over most of the domain 2.The localized region of low boundary layer height (<200m) is sustained throughout the run

Potential temperature and meridional wind component cross-sections control case

Three more study cases considered Case 1: a) Run a coupled model for 36 hours, save the output for restart b) Use 36-h wind stress to re-start ocean model and run for 108 h (4.5 days) c) Re-couple the models and run them for 36 h (total of 72 hours for the atmosphere, or 180 h for the ocean) Case 2: a) Use a coupled 36-h run to determine wind stress 100 km offshore b) Force the ocean model with spatially and temporarily invariable wind stress, run for 72 hours Case 3 a) Use a 36-h forecast of the wind stress from the coupled model b) Force the ocean model with spatially variable, but constant in time wind forcing; run for 72 hours

Sea surface temperatures control case case 1

Sea surface temperatures case 2 case 3

SST: Case 1 extension to 22 days 1.Further widening of cold water area near the coast 2.SST front remains relatively sharp 3.Beginning of eddy formation, more robust in the offshore region downstream of an initial coastal bend

Wind stress-SST coupling (Figure courtesy of Dudley Chelton, COAS) H. Hashizume et al., J. Climate. 15, 3379 (2002).

SST and wind stress: case 1

Conclusions Marine boundary layer structure in the area of wind intensification was simulated well in the case study Onset of upwelling circulation occurred sooner in the area of wind acceleration, downstream of the first coastal bend Coastal jet develops instabilities with time, more pronounced in the area of wind acceleration No definite relationship between wind stress curl and SST gradient has been found in the coastal region (on meso-alpha scale)