Natalie Perlin, Eric Skyllingstad, Roger Samelson, Philip Barbour

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Coastal Upwelling Studies Using a Coupled
Ocean-Atmosphere Model
Natalie Perlin, Eric Skyllingstad, Roger
Samelson, Philip Barbour nperlin_at_coas.oregonstate
.edu skylling_at_coas.oregonstate.edu rsamelson_at_coas
oregonstate.edu barboup_at_engr.orst.edu
College of Oceanic and Atmospheric Sciences,
Oregon State University, USA
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Outline

1. Why do we need the ocean-atmosphere coupled
   modeling system?
2. System main components and model coupling
   strategy
3. Numerical study of upwelling and simulations
   description
4. Ocean response to the coupled system
5. Atmospheric response to the coupled system
6. Summary and future plans

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The need for the ocean-atmosphere coupled system

• Atmospheric models
• - narrow coastal upwelling zone of 10-50 km
  offshore is still a sub-grid scale for most
  operational numerical weather prediction systems
• Trouble the effect of cooler sea surface
  temperatures (SST) near the coast is not resolved
  directly in the atmospheric models!
• Ocean models -
• - usually have neither adequate surface wind
  and wind stress data, nor the surface heat fluxes
  
• Trouble ocean models do not include the
  effects of spatially and temporary inhomogeneous
  atmospheric forcing that potentially affect ocean
  surface boundary layer development
• Satellite and in-situ observations
• - show the internal boundary layer
  development over the cold water
• - indicate that cooler SST-s tend to reduce
  the surface wind stress
• - turbulence collapse over the cold water
  restricts downward momentum transfer to the ocean
  surface

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Regional Ocean-Atmosphere Modeling (ROAM) system
main components

• Ocean component ROMS (Regional Ocean
  Modeling System), hydrostatic free-surface
  primitive-equation model

• Atmospheric component COAMPS (Coupled Ocean-
  Atmosphere Mesoscale Prediction System Hodur,
  1997), based on non-hydrostatic fully
  compressible dynamics

• Flux coupler MCT (Model Coupling Tookit
  Larson et al., 2004), a software tool consisting
  of Fortran 90 modules for data exchange between
  parallel earth system models to create a parallel
  coupled model

Fig.1. ROAM chart, a single-executable modeling
system with its components running in concurrent
mode
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Simulation domain for the numerical study of
upwelling

• Study summertime wind-forced upwelling off
  Oregon coast using a fully coupled
  ocean-atmosphere model
• Horizontal domain is 50 x 20 km, 1-km grid
  boxes
• Linear shelf slope from 10 m to 300 m at 50 km
  offshore
• Ocean vertical grid has 40 layers, surface to
  bottom
• Atm. vertical grid has 42 layers, surface to
  over 9 km
• Atm. model time step is 3 s, ocean time step is
  300 s
• Ocean initialization from the profiles of T and
  S that are typical for that time and location
  ocean starts at rest
• Atmospheric initialization horiz.-homogeneous,
  from chosen temperature and moisture profiles
• Atm. pressure gradient is maintained constant
  during the simulation, and is computed from 15
  m/s northerly winds
• 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
• LMD mixing in ROMS, with surface and bottom KPP

Fig. 2. Atmospheric potential temp., water vapor
mixing ratio, ocean temp. and salinity profiles
that are used for models initialization
Fig. 3. Ocean bathymetry W-E
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Effects of incomplete data exchange between ocean
and atmosphere models

• Fully coupled model system provides the following
  data exchange between its components every 300 s
  (every ocean model time step)
• wind stress from the atmospheric model to the
  ocean model
• ocean SST update from the ocean to the
  atmospheric model
• latent, sensible heat fluxes, longwave and
  shortwave/solar radiation from the atmospheric to
  the ocean model

simulation simulation atm. wind stress passed ocean SST passed to the atm. heat flxs. exchange, solar radiation passed to the
name to the ocean atmosphere except solar rad. ocean
1 W-ths yes no no no
2 WT-hs yes yes no no
3 WTH-s yes yes yes no
4 WTHS yes yes yes yes
Table 1. Summary of the differences of four
simulation to study the effects of incomplete
data exchange between the models
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Ocean model results Sea Surface Temperature
response

• Fully coupled case (red) has its lowest SST
  extending to the inshore points
• Non-coupled case (black) has greatest
  difference between its lowest SST and that at the
  inshore point
• When solar radiation is included (red), gradual
  heating of the surface is well noticed at
  offshore point
• Upwelling effect starts sooner at the coastal
  point when heat flux exchange is included
• In non-coupled case (black) upwelling at the
  coast starts later, with rapid drop of SST

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Ocean model results temperature and velocity
cross-sections
W-ths
WT-hs
WTH-s
WTHS
Fig. 6. Ocean temperature (left), v-velocity
(center), and u-velocity (right) cross-sections
in the end of the simulation, 72-h forecast.

• Upslope propagation of the upwelling front
  extends further inshore in fully coupled case
  (bottom panel)
• Stronger temperature and momentum vertical
  gradients yield in the fully coupled case in the
  shallow regions (5 km offshore)
• Southward surface jet in non-coupled case
  extends deeper, but more horizontally limited
• Southward sfc. jet in fully coupled case is more
  shallow, broadening horizontally
• Ekman layer is thinner in the fully coupled
  case, with higher offshore velocities

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Atmospheric model results wind stress, wind
stress curl, and Ekman transport
Fig. 7. Atm. model wind stress, wind stress
curl, and computed cumulative offshore Ekman
transport
Ekman pumping at the offshore location
Coastal Ekman transport at the ocean boundary
Cumulative Ekman transport at the offshore
location

• Two-fold decrease of the meridional wind stress
  near the coast as compared to its offshore values
  lead to strong wind stress curl in the nearshore
  in the coupled cases
• Offshore Ekman transport computed from the
  atmospheric wind stress differs nearly two-fold
  at the coast as a result of this wind stress
  decrease
• Wind stress curl has such an effect that
  cumulative offshore Ekman transport at the
  western boundary of the domain becomes of the
  similar value for all the simulations

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Atmospheric model internal boundary layer
development
Figure 9. Cross-section of the potential temp.
(contours) and v-wind (color) in the end of 72- h
run, fully coupled case
Figure 8. Vertical profiles of potential temp. at
three offshore locations, 72-h forecast for four
simulations. Open circles show the surface
temperatures in the atmospheric model

• Well mixed marine boundary layer forms in all
  cases, with the capping inversion at the
  reasonable height (close to the observed)
• Internal boundary layer forms only in the
  coupled cases, and is defined best in the fully
  coupled case
• Region of stronger winds forms below the
  inversion and over the cooler coastal waters

Figure 10. Similar variables as above, observed
off Oregon coast during the COAST experiment on
July 24-25, 2001. Courtesy of John Bane, UNC.
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Summary

1. Coupled ocean-atmosphere simulations demonstrated
   qualitative improvement of the model results in a
   study of coastal upwelling
2. Cooler upwelled water penetrates further inshore
   in the fully coupled simulations lower SST are
   then found in the nearshore shallow regions
3. Fully coupled case produces oceanic southward
   surface jet that is more shallow but horizontally
   broader, more shallow Ekman layer
4. Internal boundary layer develops in the
   atmospheric marine boundary layer over the colder
   waters during the coupled simulation, in response
   to ocean upwelling
5. Increase of southward flow below the marine layer
   inversion and above the cold water supports the
   conception of turbulent collapse that restricts
   downward momentum transfer

Future plans

• Westward extension of the ocean domain, include
  shelf break
• Include coastal land and coastal topography
• Include alongshore variations in coastline,
  topography, and bathymetry
• Use different model resolutions for the ocean
  and atmospheric models
• Include realistic topography and bathymetry of
  the US West coast
• Include fresh water sources, as Columbia river
  plume