Filling the Gap:The Structure of Near Coastal

WindsJeroen Molemaker, Francois Colas and Xavier Capet

University of California Los Angeles

Forcing our ocean models: wind

Global atmospheric model (reanalysis) Observed winds (scatterometer,

climatology, stations) Regional atmospheric model (early stage

of coupling)

Wind forcing Global atmospheric models are too course to

resolve any structure near the coast.

Coupling with regional atmospheric models may be the future but is there an intermediate option?

quikscat leaves an 25-50 km gap

Why should we care?

Wind stress curl determines upwelling. Small changes in curl result ‘big’ changes in

upwelling. (Capet, 2004) and, again, this talk.

Near-shore wind drop-off

• Absent in coarse reanalysis wind products• Still questionable in (uncoupled) atmospheric

regional models

Near-shore wind drop-off

• Land-sea changes in surface drag and boundary layer.

• In-situ observations : Very scarce, but give indication of a drop-off

Point Reyes

Summer: /4 within ~ 25km

Winter: /2 within ~ 25km

Dever et al. (2006), Dorman et al. (2006)

A relation between wind and SST gradients(Chelton using QSCAT)

(Chelton et al. 2001, 2007)

Wind / SST gradient Empirical Relation

Wind / SST Empirical Relation implementation in ROMS

“online”: Correct Wind stress during simulation using actual computed SST’s .

Application: wind feedback on eddies and fronts.

“offline”: Correct QuikCOW wind stress climatology before using in ROMS Objective : impact on quasi-equilibrium climatological solutions.

Try to fill wind ‘gap’ using this relation.

Online wind/sst coupling

Test wind/SST relation at climatologic time scalesCurl() Div ()

Crosswind SST gradient Downwind SST gradient

Test wind/SST relation at climatologic time scales

-- crosswind grad(SST)-- curl (t)

Along-shore

Surprisingly good already!!

So what is missing?

• Land-sea changes in surface drag and boundary layer.

Summer: /4 within ~ 25km

Winter: /2 within ~ 25km

Dever et al. (2006), Dorman et al. (2006)

Orographic effects

Orography: zeroth order approach Reduce near coastal wind by 50% corr = [1- 0.5exp(-D) ]

D = distance to coast = 1/(20 km)

Coast wind reduction

Wind / SST Empirical Relation in ROMS

2 ROMS configurations – climatological conditions

Peru/Chile (4 km)(VOCALS region)California (5 km)

(Capet et al. 2008) (Colas et al. 2008)

Near-shore SST sensitivity (CCS)

Difference with Pathfinder Climatology

quikCOWquikCOW plus ‘Orography’

Alongshore current difference

Average structure of undercurrent

quikCOW quikCOW plus ‘Orography’

Lagrangian estimate of undercurrent flow

Garfield et al. (2001)

Surface Eddy Kinetic Energy original quikCOW winds corrected winds altimetry (DUACS)

[cm2.s-2]

Along-shore averaged EKE

-- quikCOW-- With

‘Orography-- Altimetry

original QSCAT winds corrected winds

Near-shore SST sensitivity (PCS)

Difference

Why did we care: Oceanic heat balance in the PCS - role of eddies

Systematic errors in CGCMs in the South East Pacific: Difficult to reproduce the stratus cloud deck and to simulate the upwelling and its

effects.Role of eddies in the transport of heat (VOCALS project).

(Large and Danabasoglu, 2006)

SST warm bias in CGCM

Potential upscaling effects

from EBUS

- Wind/SST empirical coupling in regional model: near-shore wind drop-off, significant changes on upwelling structure and consequent related

eddy activity (heat balance, offshore transport).

- Wind/SST relation and near-shore structure need to be tested using an

coupled atmospheric model

- Possible upscaling effects at regional scale and even larger scale are

important.