effects of ocean-atmosphere coupling in a modeling study of coastal upwelling in the area of...

Effects of Ocean-Atmosphere Coupling in Effects of Ocean-Atmosphere Coupling in

a Modeling Study of Coastal Upwelling in a Modeling Study of Coastal Upwelling in

the Area of Orographically-Intensified Flowthe Area of Orographically-Intensified Flow

Natalie Perlin, Eric Skyllingstad, and Roger Samelson

College of Oceanic and Atmospheric Sciences, Oregon State University

2007 ROMS/TOMS Workshop2007 ROMS/TOMS WorkshopOctober 1-3, UCLAOctober 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)Enriquez and Friehe (1995)

Wind intensification downwind of major capes off the U.S. West coast

Perlin et al., 2004

Enriquez and Friehe, 1995Enriquez and Friehe, 1995

Wind-driven coastal upwelling

Satellite SST

Huyer et al., 2005

Coastal Ekman transport at Coastal Ekman transport at the ocean boundary:the ocean boundary:

fM ys

cE

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 experimentOregon coast, COAST experiment Duck, North CarolinaDuck, 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 temperaturescontrol case case 1

Sea surface temperaturescase 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

s

s

s

k

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)