The transition from mesoscale to submesoscale in the California Current System

X. Capet, J. McWilliams, J. Molemaker, A. Shchepetkin(IGPP/UCLA), feb. 2006

Offline coupling: boundary

conditions updated every 5 days.

set of 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)

1- set up: downscaling

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

ICC3 ICC0

2- Submesoscale outbreak in the mixed layer: visual evidence

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3- Submesoscale in the mixed layer: connection to the mesoscale

Upwelling and gyre return flow instability generate mesoscale activity. Submesoscale outbreak occurs in the mixed layer at the edge of the eddies. => interior mesoscale exerts some control over tracer dynamics but ultimately, tracer cascade depends on energetic submesoscale frontal features.

vertical vorticity

Convergence toward a -2 slope

4- Submesoscale outbreak in the mixed layer: statistical evidence

KE spectral slope gets shallower with increased resolution which suggests an increasingly effective forward cascade. Convergence toward -2. Shallower than 2D in the submesoscale range

Density variance slope also around -2. Steeper than 2D in the submesoscale range.

5- Submesoscale outbreak in the mixed layer: KE spectral fluxes

Forward cascade inverse cascade

KE injection

In the submesoscale range we have coexisting PE -> KE energy transfer and KE forward cascade.

PE -> KE energy transfer is the consequence of an intense frontogenetic activity in the mixed layer.

6- A surface dynamics submesoscale activity ?

1- Velocity and tracer spectra 2- Vertical buoyancy fluxes in the upper ocean connected to frontogenetic activity. 3- Structural similarity

(active surface tracer being stirred by an homogeneous balanced interior)

vorticity pdf

mixed layer

thermocline

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PV

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6- Outline of the talk

The system we study as a SQG-like character. It is also more complex:

1- on short time scales, ie on the lifetime of the submesoscale, because frontal instability and centrifugal instability modulate frontogenesis.

2- on longer scales because the “surface” tracer evolves under diabatic constraints.

6- Complications: frontal instability

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7- Complications: centrifugal instability due to PV destruction by downfront winds (Thomas, 2005)

Drives an ASC that is associated with vertical PV fluxes.

PV

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8- The mixed layer equilibrium

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. Active wind-induced mixing prevents significant restratification.

ICC0

ICC6

KPP6-∂zHF6

KPP0-∂zHF0

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

With diffusion coefficient equal 10m2/s, horizontal diffusion is significantly smaller than vertical diffusion

restratification vertical mixing

Clear illustration of the limits of a 1D vision (KPP) of vertical mixing: beyond the resolution dependency, it is strongly dependant on velocity/density horizontal gradients.

As resolution is increasing, submesoscale restratifying action is turning KPP into an important two-step front damping mechanism.

versus

The end

What are those wiggles and how important are they for the system in terms of dissipation ?

On short time scales, the dynamics is dominated by SQG-like frontogenetic activity.Frontogenesis is the key process involved in PE -> KE conversion.

7- The CCS as a SQG or SQG+ system (2) ?

Frontogenesis versus centrifugal instability Frontogenesis versus frontal instability

1- set up: mean circulation (ICC1)

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v

w

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