Natalie Perlin, Eric Skyllingstad, and Roger Samelson

1. 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

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

3. 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

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

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

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

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

8. Numerical study design for a coupled model coastal bend • 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

9. Wind stress: control case

10. Surface currents and SST

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

12. Potential temperature and meridional wind component cross-sections control case

13. 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

14. Sea surface temperatures control case case 1

15. Sea surface temperatures case 2 case 3

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

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

18. SST and wind stress: case 1

19. 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)