Submesoscale variability of the Peruvian upwelling system as observed from glider data
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Transcript of Submesoscale variability of the Peruvian upwelling system as observed from glider data
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