submesoscale secondary instability in an upwelling system: mechanisms and implications for upper...

Submesoscale secondary instability in an upwelling system: mechanisms and

implications for upper ocean dynamics

X. Capet, J. McWilliams, J. Molemaker, A. Shchepetkin(IGPP/UCLA)

Oct. 2005

Plan of the talk1- Set-up2- basic evidences for intense submesoscale activity in the mixed layer3- Mechanisms4- Mean effect of the submesoscale activity5- Implications for mixing parameterization

1-Set up: offline nesting

a- Parent Grid:

12km resolution ROMS with steady forcings corresponding to July (peak of the

upwelling in the CC region). Idealized flat topography, 40 vertical levels. Idealized

straight coastline where the downscaling is going to take place.

Mean SST and surface currents Mean SSH and barotropic velocities

Offline coupling: boundary

conditions updated every 5 days.

b- set of 5 ICC grids at various horizontal resolutions (12km, 6km, 3km, 1.5km, 750m). Vertical resolution, topography and coastline are unchanged from the parent. Boundary conditions are provided by the parent 12km solution available every 5 days from avg files.

1- set up: downscaling

Mesoscale is both generated locally and passed on through the boundary conditions. Submesoscale is generated locally only.

1- set up: mean circulation (ICC1)

u

v

w

T

2- Submesoscale in the mixed layer: visual evidences for its outbreak

ICC12 ICC3

2- Submesoscale in the mixed layer: visual evidences for its outbreak

ICC3 ICC1

2- Submesoscale in the mixed layer: visual evidence for its outbreak

ICC1 ICC0

2- Submesoscale in the mixed layer: statistical evidence for its outbreak

Convergence toward a -5/3 slope in the mixed layer ???

In the mixed layer, the slope gets shallower with increased resolution which suggests an increasingly effective forward cascade.

Mixed layer Interior

3- Mechanisms involved in submesoscale outbreak:frontogenesis (ICC0)

Q (frontogenetic tendency) is mostly positive along active fronts => submesoscale destabilization occurs under frontogenesis conditions. There will be an ageostrophic secondary circulation acting to restore thermal wind balance

3- Mechanisms: centrifugal instability (ICC0)

Presence of negative PV stripes along the front axis : we expect centrifugal instability (also called slantwise convection).

3- Mechanisms involved in restratification by the submesoscale

3- Mechanisms: shear instability (ICC0)

Rayleigh criterion is also met => barotropic/baroclinic instability could explain the existence of the along-front structures. There should be a limitations of their amplitude by the ongoing frontogenesis (Spall, 97)

3- Mechanisms: time evolution (ICC0)

3- Mechanisms: origin of negative PV ?

Buoyancy forcing: heat flux is stabilizing here1- atmospheric heat flux will be a source of positive PV 2- there can only be rearrangements inside the fluid

Friction forces: the wind can be responsible for significant PV destruction at fronts (Thomas and Lee, 05) when the wind is blowing downfront.

3- Mechanisms: origin of negative PV

The wind is responsible for PV destruction in a direct relationship with the gradient of density in the direction of the wind.

PVe as a function of wind/alongfront angle and front magnitude at 5m depth

Both mesoscale eddy stirring (inducing frontogenesis) and wind action (inducing negative PV) seem to be the important exterior ingredients for SSI.

4- Mean effect in the mixed layer

Cross-shore distance(km)

dept

h

Depth of the 16 degree isotherm Tendency to reduce hbl, ie, to restratify. We investigate this using a double decomposition of turbulent fields and fluxes into submesocale ('') and mesoscale (').

x x x' x ' '

long-term

average4 days

running

average

remainder

4- Mean effect in the mixed layer: vertical structure of ' and '' velocities variance (ICC0)

|u''| |u'| |w'| |w''|

w variance close to 10m/day on average over the whole domain

4- Mean effect in the mixed layer: eddy heat fluxes (horizontal)

Noisy but both sub-mesoscale and mesoscale contribute to heat redistribution in favor of cold regions (on average) in the nearshore. Farther offshore it is mostly the mesoscale.

4- Mean effect in the mixed layer: vertical eddy heat fluxes

The submesoscale is responsible for intense vertical heat fluxes mostly confined in the mixed layer, acting to unmix it with a strength equal to 60W/m2 300km offshore.

ICC0 ICC1

4- Mean effect in the mixed layer: resolution dependency

ICC3

Tendency terms in the T equation: one order of magnitude increase from ICC3 to ICC0: in ICC3, the submesoscale -''- is very small but there is a mesoscale -'- activityhaving the same effect => restratification is going on at all resolutions.

5- So what ? Where does all this heat flux go ?

temp ICC0temp ICC12

Limited differences in temperature: with K=10^(-2), all this restratification is still under control. Yes but the submesoscale should be an extremely efficient restratification mechanism in variables winds (to be confirmed).

Submesoscale restratifying action is turning KPP into an important indirect source of “horizontal mixing”.

5- So what ? Where does all this heat flux go ?

with diffusion coefficient equal 2m2/s

ie, equivalent to

restratification vertical mixing

5- So what do we do with KPP ?

We expect the wind-driven convective regime described by Thomas (05) to be relevant for upwelling systems (nonlocal flux accounting for the wind-driven buoyancy flux + modified velocity scale w*).

clear illustration of the limits of a 1D vision (KPP) of mixing: beyond the resolution dependency, mixing is strongly dependant on the 3D properties of the flow.

3- Mechanisms: origin of negative PV

The wind is responsible for PV destruction in a direct relationship with the gradient of density in the direction of the wind.

PVe as a function of wind/alongfront angle and front magnitude at 5m depth

The full 2D vertical PV budget in the mixed layer involves the buoyancy term, ie, negative PV source is generated by having the vertical mixing undo the restratification work done by secondary circulation (work in progress).

3- Mechanisms: frontogenesis versus the role of negative PV

PV

Frontogenesis depends on the deformation field associated with the mesoscale and not wind sensitive. This should give a way to disentangle the role of PV versus frontogenesis once a quantity characterizing the submesoscale has been identified.

Q

3- Mechanisms: frontogenesis versus negative PV