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 4 th 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

Profondeur (m)  CCW : Cold Coastal Water  STSW : SubTropical Surface Water  ESPIW : Eastern South Pacific Intermediate Water 01 novembre 2008 Water masses and alongshore circulation 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) Depth (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 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 Submesoscale structures

Section 5 3 – 5 salinity intrusions observed on every section:  km width  m depth Distance (km) isopycnal Submesoscale structures

Section 5 dz dx 3 – 5 salinity intrusions observed on every section:  km width  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:  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 (~ ) Kelvin Helmholtz / Double diffusion Kelvin-Helmholtz instability: ☒ Richardson number: > ¼ (except at the very surface and using geostrophic velocities) ☒ Scale of the layering: O(10 m)

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 14 th to 18 th chlorophylle composite Nov 2008 Submesoscale structures: Horizontal extension

Negative PV located below the surface density fronts:  Strong vertical shear  Horizontal density gradient  qg=qg= 2D potential vorticity: ~ 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  Coherence between the theoretical and the observed scale  Process potentially able to generate the observed layering ☒ Can cells reach depths below the mixed layer? L 0 ~ [ 20 – 40 ] km w Enl ~ 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 wind Pietri et al., 2013: Finescale Vertical Structure of the Upwelling System off Southern Peru as Observed from Glider Data. J. Phy Oceanogr., 43, 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