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Preliminary Simulation of the Regional Coupled Atmosphere-Ocean Model in the

Southern California Coastal Regions(Santa Ana Winds and Air-Sea Interaction)

Hyodae Seo, Art Miller, John RoadsScripps Institution of Oceanography

UCSD, 0224

La Jolla, CA 92093

5th International RSM workshopYonsei University, Seoul, Korea

July 12-16, 2004

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Outline

• What is the Santa Ana?

: Observational evidence• Numerical Simulations:

: Oceanic and atmospheric response

(coupled and uncoupled simulations)• Conclusions/Ongoing work

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The Santa Ana: Devil’s breath

• Interaction with topography

• Dry and warm due to adiabatic heating

• Wildfires • Clearing of marine stratiform clouds• Dust/Aerosols transport to the ocean

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RSM simulation: Feb. 7 ~ 16, 2002

In San DiegoU10m(m/s) V10m(m/s)

RH2m(%)

FWI(%)

3 hourly Daily averaged

Santa Ana Episode

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925mb winds(m/s) and FWI(%)

Increased likelihood of wildland firesFeb 8 3Z ~ Feb 13Z 2002

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850mb winds(m/s) and Low Level Cloudiness(%)

• Large scale clearing of low-level marine layer cloudsFeb 8 3Z ~ Feb 13Z 2002

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Forcing from RSM(1): zonal windstress(N/m2)

SCBFAR OFFSHORE

Santa Ana Forcing

Climatology forcing

difference

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Forcing from RSM(2): surface net heatflux (W/m2)

SCBFAR OFFSHORE

Santa Ana Forcing

difference

Climatology forcing

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Atmosphere and Ocean Model and their Exchange of Forcing

ATMOSPHEREECPC’ RSM: 20km resolution

OCEAN

The Regional Ocean Modeling System (ROMS)

Primitive equation ocean model (Song and Haidvogel, 1994)

Sigma vertical coordinate (enhanced res. near top and bathymetry)

Curvilinear horizontal grid (~12km and ~5km resolution)

Momentum flux,

Net surface heat, freshwater flux

: 3 hourly

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Forcing from RSM: SHFLUX(W/m2)

Shflux(W/m2) =swrad+lwrad-lhtfl-shtfl

Heat-flux Correction:

Q’=Q+dQdSST*(T_mod-SST)

dQdSST~-40(W/m2/C)

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Oceanic Response(1): SST(C)

FAR OFFSHORE SCB

response to Santa Ana

response to climatology

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Cross-shore Component Ekman Mass Transport(kg/s/m)Measure of Upwelling intensity

FAR OFFSHORESCB

response to Santa Ana

response to climatology

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Oceanic Response(2): Meridional surface current(V, m/s)

FAR OFFSHORE

SCB

INLAND

response to climatology

response to Santa Ana

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Atmosphere and Ocean Model and their Exchange of Forcing

Atmosphere: ECPC RSM: 20km resolution

Ocean:

The Regional Ocean Modeling System(ROMS)

Primitive equation ocean model (Song and Haidvogel, 1994)

Sigma vertical coordinate

Curvilinear horizontal grid (~12km and ~5km resolution)

Momentum flux,

Net surface heat, freshwater flux

: 3 hourly

SST: daily

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Atmospheric Response(1): Zonal winds(U,m/s) @ 10m

forced w/ ROMS SST

forced w/ OBS SST

difference

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Atmospheric Response(2): meridional winds(V, m/s) @ 10m

forced w/ ROMS SST

forced w/ OBS SST difference

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Atmospheric Response(3): surface net heatflux (W/m2)

forced w/ OBS SST

forced w/ ROMS SST

difference

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FWI(%): RSM’s Response to ROMS SST vs Reanalysis SST

forced w/ OBS SST

forced w/ ROMS SST

San Diego

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Conclusion/Ongoing work(1)

• Both RSM and ROMS reproduce qualitatively well the atmospheric extreme and typical oceanic response in terms of spatial intensity and temporal evolution.

• Oceanic response indicates that Santa Ana winds forcing modifies the Ekman dynamics of the ocean surface layer by decreasing upwelling, and thereby increasing SST.

• ROMS could not resolve any small scale cold tongues and chlorophyll blooms adjacent to the coastlines.

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Conclusion/Ongoing Work(2)

• Coupled RSM run with ROMS shows less intense Santa Ana event, mainly because the ocean surface from ROMS is warmer.

• Higher Surface Heatflux (Stronger Winds) from RSM Warmer SST from ROMS lower Heatflux(weaker winds) from RSM ( and presumably colder SST from ROMS?): possibility of stable integration.

• Coupled model requires higher resolution (up to 5km) and better description of coastlines in order to resolve the small scale and short-lived features.

• Sensitivity of Southern California Current System to atmospheric forcing: Use of bulk parameterizations to provide feedback, implementation of 3D NPZD ecosystem model.