Submesoscale variability of the Peruvian upwelling

system as observed from glider data

Alice PIETRI

Pierre Testor, Vincent Echevin, Laurent Mortier, Gerd Krahmann, Johannes Karstensen

Trieste, Italy, June 4th 2013

PCC

PCUC(Penven et al., 2005)

The Peruvian upwelling system

Section of alongshore velocity at 15°S mars-mai 1977 (Brink et al., 1980)

Peru Coastal Current (PCC)

Peru-Chile Under Current(PCUC)

Coastal upwelling: Offshore Ekman transport Ekman pumping Upwelling of cold, nutrient-rich water along the coast

Pisco (14°S, Peru) : Year long Equatorward coastal winds (Trade winds) Strong upwelling cell

October-November 2008 (VOCALS Rex): R/V Olaya (119 profiles, ~30 km horizontal res. , 3D sampling) Glider Pytheas (1400 profiles, ~800 m horizontal res, 2D sampling)

The Peruvian upwelling system

9 sections

~1400 profils

200m

DENSITY

SALINIT

Y

OXYGEN

TURBIDITY

FLUO: ChlA

TEMPERATURE

100km ~ 5 days

Depth averaged velocities measured

by the glider

Gliders: Pytheas, Oct-Nov 2008 (Austral Spring)

horizontal resolution:~ 800m

Pro

fond

eur

(m)

CCW : Cold Coastal Water STSW : SubTropical Surface Water ESPIW : Eastern South Pacific Intermediate Water

01 novembre 2008

Water masses and alongshore circulation

0

100

200

35.1

35

34.9

34.8

34.7

110 100 90 80 70 60 50 40 30 20 10 0

0

50

100

150

200

Peru-Chile Current (PCC): Equatorward Maximum speed: 30 cm/s

Peru Chile Undercurrent: Situated above the continental slope Poleward Maximum speed on the section: 15 cm/s

Distance (km)

Dep

th (

m)

De

pth

(m

)

Salinity :- ESPIW below the thermocline- Layering on every sections

Fluorescence :- High concentration in the surface layer- Subsurface patches

Temperature :- Warming of the surface- upwelling

3 regions: 1) Upwelling 2) Transition zone 3) Offshore

3 2 1 3 2 1 3 2 1

Submesoscale structures

Salinity :- ESPIW below the thermocline- Layering on every sections

Fluorescence :- High concentration in the surface layer- Subsurface patches

Temperature :- Warming of the surface- upwelling

3 regions: 1) Upwelling 2) Transition zone 3) Offshore

3 2 1 3 2 1 3 2 1

Submesoscale structures

Section 5

3 – 5 salinity intrusions observed on every section: 20-40 km width 100-150 m depth

Distance (km)

isopycnal

Submesoscale structures

Section 5

dz

dx

3 – 5 salinity intrusions observed on every section: 20-40 km width 100-150 m depth Cross-isopycnal structures: slopes = 0,2 - 1,5 %

Distance (km)

Submesoscale structures

Section 5

Which dynamical processes could be responsible of this cross-isopycnal signal?

Submesoscale structures

• Divergence of Q-vector:

• Estimates of w through the Omega equation:

€

w = ±2 m. jour−1

Vertical velocities driven by frontogenesis

☒ Horizontal scale >> layering observed by the glider

☒ Relatively weak vertical velocities

W at 100 m estimated from the Ω-equation

Frontogenesis

3D process driven by the meandering of the front

Cruise with R/V Olvaya (IMARPE)Mesoscale survey

Double diffusion:

☒ No « staircases » on salinity/temp

Turner angle:

→ flow susceptible to salt fingering

☒ Baroclinicity of the flow

→ Maximum slope of the interleaving (May and

Kelley, 1997):

Much smaller than the observed slopes (~5.10-3)

Kelvin Helmholtz / Double diffusion

€

ΔH ~ρ 0(U1 −U2)

2(ρ 2 − ρ1)g

Kelvin-Helmholtz instability:

☒ Richardson number: > ¼

(except at the very surface and using geostrophic velocities)

☒ Scale of the layering: O(10 m)

€

s* (∂x S__

,∂z S__

,∂x ρ__

,∂z ρ__

) =1.10−3

€

∂xS∂zS

=f

N

⇔ log(∂xS) = log(∂zS) + log(f

N)

Smith and Ferrari (2009)

Process potentially able to generate the observed layering

Submesoscale structures: Mesoscale Stirring

s ~ 0.2% to 1.5%

f /N ~0.3% to 1.2%

Large scale gradients and isohalines inclined to isopycnals

Mesoscale activity

Generation of intrusions with a slope close to the value of f/N (Smith and Ferrari, 2009)

C

A

F

Presence of 2 eddies (A et C) ~ 50 km diameter

Filament (F) ~ 150 km long

Glider section from November 14th to 18th chlorophylle composite 15-19 Nov 2008

Submesoscale structures: Horizontal extension

Negative PV located below the

surface density fronts:

Strong vertical shear

Horizontal density gradient

qg=

2D potential vorticity:

41414 3.101.10 sqml 41414 3.101.10 sqml~ S-4

Submesoscale structures: Wind forced symmetric instability

Down-front winds (wind blowing along a frontal jet) drive: strengthening of the density contrast across the front symmetric instability (negative PV) ageostrophic secondary circulations (Thomas and Lee, 2005)

30 km

€

L0 =4H −q

f 2

Coherence between the theoretical and the observed scale

Process potentially able to generate the observed layering

Can cells reach depths below the mixed layer?

L0 ~ [ 20 – 40 ] km

wEnl ~ 85 m/j

30 km

Submesoscale structures: Wind forced symmetric instability

Conclusions and prospects

First measurements at such a fin scale in that area: a single glider repeat-section (1.5 months) physical and biogeochemical observations, estimates of the alongshore velocity.

Evidence of subsurface submesoscale structures in salinity and fluorescence in the transition zone of the upwelling.

The observed submesoscale features (key to explain the biological activity) are likely a combination of 1) frontogenesis, 2) stirring by mesoscale turbulence, 3) symmetric instability forced by the windPietri et al., 2013: Finescale Vertical Structure of the Upwelling System off Southern Peru as Observed from Glider Data. J. Phy Oceanogr., 43,631-646.

• Are the submesoscale features a persistent phenomena? Longer deployments, rotations of gliders. Ex: CalCOFI survey line off California

• What is the relative contribution of each processes? A fleet of gliders would be required (3D view). Ex: deployment of 7 gliders along parallel cross-shore tracks off Peru carried out in January 2013 by GEOMAR scientists.

Questions remaining:

January-February 2013: shelf exchange processes in the OMZ (GEOMAR)

7 shallow and deep Slocum gliders deployed in parallel 3D survey of the coastal area pattern optimized for observation of submeso to meso spatial scales

Conclusions and prospects

Large scale temperature and salinity gradients

Turbulent mesoscale flow

Stirring of properties whose isolines are inclined to isopycnals

Generation of intrusions with a slope close to the value of f/N (Smith and Ferrari, 2009)

→ Region rich with mesoscale processes

→ Isolines of salinity cross- isopycnals

Klein et al. (1998)

Submesoscale structures: Mesoscale Stirring

Down-front winds (wind blowing along a frontal jet) drives: vertical mixing reduction of the stratification strengthening of the density contrast across the front

Lee et al. (2006)

Thomas and Lee (2005)

Apparition of an ageostrophique secondary circulation:

→ Downwelling on the dense side of the front

→ Upwelling on the frontal interface

A geostrophic flow is symmetrically unstable when its

potential vorticity is negative

Vertical circulation

warm

cold

Submesoscale structures: Wind forced symmetric instability