Eddy activity in the California Current System:
patterns and variability of the
potential to kinetic energy conversion,
underlying processes. Comparison with the
Humboldt system
X. Capet, J. McWilliams, F. Colas
ROMS meeting 2007, Los Angeles
Potential to eddy kinetic energy conversion term: <w'b'>
<.> is some appropriate averaging operator. Unless otherwise stated it is a
long term (10 years) time average with the seasonal cycle removed.
An important quantity !!!
2 ways to compute <w'b'> from model fields.
or (2)
(1)
and then multiply by b
Both are roughly equivalent after all (ie there are no bugs in computation
of w from (1). Forget my abstract !!!
Configurations:
more idealized
5 ICC grids at various horizontal resolutions (12km, 6km, 3km,
1.5km, 750m). 720 x 720 km. 40 vertical levels, flat bottom, straight
coastline. Boundary conditions are provided from 12km idealized
USWC outputs (5 days averages). Atmospheric forcing spatially
smooth and fixed in time (July COADS climatology)
more realistic
USWC 5km resolution, 32 levels with
realistic bathymetry and coastline.
Climatological forcings (QuikSCAT +
COADS heat and freshwater fluxes).
BCs from SODA climatology.
EKE patterns in the CCS
ROMS USWC 5km
Altimetry
Clear offshore EKE maximum along California, ie., away
from the unstable coastal jet region.
Explanation for the offshore maximum ? <w'b'> patterns in the CCS
Offshore propagation of eddy activity with inverse cascade


offshore EKE sources (already in Marchesiello et al,2003)
annual mean <w'b'> averaged from 0 to 200m
The nearshore does not stand out
in terms of energy conversion.
same as left panel longshore averaged between 34.5 and 41.5o N
-400
-200
cross-shore distance [km]
0
<w'b'> patterns in the CCS
There is also offshore propagation of the EKE sources
(<w'b'>), which further highlights the intricate relationship
between (seasonal) mean circulation and eddy activity.
depth-integrated (0 to 200m) and longshore-averaged (34.5 to 41.5o N) <w'b'>
summer
spring
fall
-400
-200
cross-shore distance [km]
winter
0
-400
-200
cross-shore distance [km]
0
Alongshore-averaged structure of <w'b'>
Annual mean longshore-averaged (34.5 to
41.5o N) <w'b'>
x-z section of <w'b'> reveals
three types of processes that
release available potential
energy:
Baroclinic instability of the coastal
jet
Baroclinic instability of the CCS
Boundary layer (BL) buoyancy flux
(BL restratification tendency)

Near surface <w'b'> : ageostrophic secondary
circulations associated with submesoscale fronts
buoyancy and associated w in a CCS 750m horizontal resolution
solution. w is downward on the cold side, upward on the warm side
b
W
x [km]
Submesoscale is not resolved at 5km horizontal resolution. However the
same mechanism of restratification by ASCs around fronts is present.
Because of larger dissipation/diffusion, front intensification is much
reduced and so are wrms and <w'b'>
Near surface <w'b'> : Mixed-layer baroclinic instability
ualong
uacross
w
u'along
u'across
w'
Submesoscale instability growth is overwhelmingly due to w'b'. Its length
scale is coherent with a “mixed layer deformation radius (Boccaletti et al,
2007). In the CCS submesoscale instability starts showing up around
2.5km horizontal resolution.
Near surface <w'b'> spatial/temporal
modulations.
<w'b'> averaged from 0 to 50m
spring
summer
Signal tends to follow the offshore
propagation of the mesoscale (the
mesoscale strain is important and so is the
mean position of the density front)
vertical vorticity
fall
winter
max and widespread in winter when air-sea
fluxes are least stabilizing (or even
destabilizing in southern part of the domain)
and hbl is deepest
(consistent with Fox-Kemper et al (2007),
ie., a restratification by secondary
circulations scaling as h2bl)
Near surface <w'b'> : where does it go ?
Ten-fold increase of near surface <w'b'> between 10 and 1km horizontal
resolution. However eddy kinetic energy itself is only moderately affected
because the injection takes place in the submesoscale range, ie., within or
close to the dissipation range. At resolution around 2-3km a forward cascade of
KE shows up to connect the KE source (<w'b'> at submesoscale) and the sink
(dissipation at somewhat higher wavenumber).
eke injection
eke dissipation
dx=750m
cascade
dx=1.5km
dx=3km
600km
60km
6km
EKE patterns in the Humboldt system
ROMS 5km
Altimetry
ROMS patterns ok although 20 to 30% overestimate.
Offshore EKE maximum as in California, ie., away from the
unstable coastal jet region.
<w'b'> patterns in the Humboldt Current System
annual mean <w'b'> averaged from 0 to 200m
Clear maximum at the coast, ie.,
baroclinic instability of the
coastal jet.
Offshore patches of positive
<w'b'> consistent with regions of
max EKE
EKE ROMS 5km
Coastal jet instability in the Humboldt versus CCS
Annual mean longshore-averaged (34.5 to
41.5o N) <w'b'>
Annual mean longshore-averaged (-32 to
-26o N) <w'b'>
Humboldt
CCS
Annual mean longshore velocity
Annual mean longshore velocity (with minus sign)
The vertical shear is stronger and extends deeper in the Humboldt.
Conclusions:
Potential to kinetic energy conversion term was computed for the California
and Humboldt systems.
Time scales for replenishment of upper ocean (say, 0 to 500m) EKE
deduced from <w'b'> are less than 100 days, ie., <w'b'> is a key term in the
EKE budget (and <w'b'> distribution does correlate with that of the EKE).
Several processes interacting with each other explain the <w'b'>: baroclinic
instability of the coastal jet, the offshore mean currents and ageostrophic
circulations near fronts (possibly related to submesoscale instabilities).
Important differences between California and Humboldt a priori related to
the velocity structure of the coastal jet.
Sorry for not being with you. Enjoy the wine for me !!
A ROMS meeting in Brazil (eg., Arraial de
Cabo, 2 hours drive from Rio) ??
US citizens may need a visa !!!