Wind-SST Coupling in the Coastal Upwelling
--- An Empirical Numerical Simulation
X. Jin, C. Dong, and J. C. McWilliams
(IGPP/UCLA)
D. B. Chelton (COAS/OSU)
Thanks: Z. Li (JPL)
OUTLINE
• Introduction
• Methods
• Results
• Conclusions and Outlook
Introduction
• ObservationsSignificant coupling
between SST and wind stress:
– CCS, (Chelton et al., 2007)
– Southern Ocean, (O’Neill et al., 2003)
– East Tropical Pacific (Wallace et al., 1989;
Chelton et al., 2001)
• Coupling: SST---- Wind stress--SST
• SST and wind stress fields for an example 29-day averages of
QuikSCAT wind stress : (left) AMSR-E estimates of SST with wind
stress vectors overlaid, (middle) wind stress curl with contours of
crosswind SST gradient, and (right) wind stress divergence with
contours of downwind SST gradient. the contours are the magnitude
of the vector-average wind stress with a contour interval of 0.03 N
m−2, and the heavy contour corresponds to 0.12 N m−2. The contour
interval for the SST gradient components in the middle and right
panels is 0.5°C (100 km)−1 (Chelton et al., 2007)
• Maps of the correlations (left) between the wind stress curl and the
crosswind SST gradient and (right) between the wind stress
divergence and the downwind SST gradient computed from
summertime 29-day averages (Chelton et al., 2007).
Chelton et al, 2007
Mechanisms for the coupling
• SST-- Wind stress:
– Cold—decrease, and Warm—increase
(Wallace et al. 1989; Chelton et al. 2001)
• Wind stress ----SST:
– SST was reduced (Coupled modeling, Perlin
et al., 2007)
Objective
• We take the advantage of empirical
relationship between the SST gradient and
wind stress in place of a dynamical
atmosphere and couple it to a regional
oceanic numerical model to examine the
coupling effect.
Empirical Coupling Model
Wind Stress
  F ( SST )
,
SST
ROMS Model
Empirical Relationship (Chelton et al.)
Basis for Empirical Coupling Model
Helmholtz decomposition
  ẑ    
Poisson equations
  zˆ       
2
      
2

Within a closed domain, is uniquely determined by
solving the Poisson equations (Li et al. (2006)
ROMS Model
• Domain: 120 km west-east, 240 km north-south, and depth
500 m
• Uniform wind stress: 0.1 N/m2, equatorward constantly
• Open boundaries: Flather condition
• Temp: A typical vertical temperature profile, uniform
• Salinity: constant (35.00 PSU)
• Grid size: horizontal 1 km, vertical 30 levels with theta_s
=5.0 and theta_b=0
Experiments
• Uncoupled simulation: initial condition:rest
• Coupled simulation:
– Initial condition :rest
• SST in the uncoupled simulation at day 20
• Cross-shore vertical section averaged
along shore: SST; along-shore current
(cm/s); u (cm/s). Uncoupled
• SST time series: from day 50 to day 57 with
interval of 30 hours
• Along-shore
averaged wind
stress in
coupled
simulations on
different days.
The
background
wind stress is 0.1 N/m2
• The distributions of wind stress changes
simulated in the coupled model on day 20
(unit: N/m2)
• Simulated SST distributions. Left, uncoupled; middle,
coupled; right, the differences. Note: the colorbar for
uncoupled and coupled simulations are different.
• Cross-shore vertical section of T on day 20, averaged
alongshore.
• Along-shore velocity (cm/s) on day 20
• Cross-shore velocity (cm/s) on day 20
• Time variations of the kinetic energy averaged over the
model surface (u^2+v^2)/2
• Time variations of the
surface.
 u
averaged over the model
Conclusions and Further work
•SST gradients induce substantial wind stress
changes
•Large impacts on ocean circulations by
increasing SST near the coast and moving SST
front westenly.
•Improving uncoupled model
•Further analyses
•Impacts on meso- and submeso- scale
dynamics