Oceanic mixing studies from satellite data Cristóbal López Instituto de Física Interdisciplinar y Sistemas Complejos, IFISC (CSIC-UIB), Palma de Mallorca, Spain. Collaboration: - E. Hernández-García, IFISC, Spain. - F. d'Ovidio, Lab. D'Oceanographie et du Climat, Paris, France. -V. Rossi, J. Sudre, V. GarÇon, LEGOS/CNRS, Tolousse, France. - E. Tew-Kai, IRD, Centre Recherche Halieutique, Sete, France. Articles (avalaible at IFISC webpage): - Geophysical Research Letters, 31 L17203 (2004). - Geophysical Research Letters, 35, L11602 (2008). -Deep Sea Research I, to appear (2008). - Paper to be submitted.

Outline Outline: - Introduction. - Understanding transport and measuring mixing. - Horizontal stirring and marine ecosystem dynamics (many more open questions than answers): - Two eastern upwelling systems: comparing mixing and biological activity. - Conclusions.

Introduction This talk is not about inertial particles, but on some applications of Lagrangian techniques to Oceanic processes. Lagrangian diagnostics needed because: - Able to reveal the dynamical structures in the flow which strongly organises fluid motion (transport barriers, avenues, etc..). - - Simple enough to be applied in a practical way to complex, aperiodic, and huge velocity data sets. - - Give additional information of oceanographic interest: time scales, mixing strength… - Is able to reveal oceanic structures under the resolution of the velocity field. - Natural framework to study the interaction: hydrodynamics/biological tracers.

Introduction. There is a growing avalaibility of (surface) oceanic data from both improved numerical simulations and satellite missions: - Surface velocity field. - Chlorophyll (phytoplankton pigment concentration). - Temperature. - Salinity. - Winds, etc…

ODEs Phase portrait KAM tori, stable/unstable manifolds Chaotic sets..... Velocity field Transport organization. Transport barriers, eddies, fronts and avenues Mixing regions..... => Extracting information from data. First aim: deduction of the phase portrait from the velocity field Introduction Phase portrait of the velocity field Physical meaning in fluid dynamics

Introduction. Second aim: measure surface mixing at meso- ( Kms) and submeso- (1-10 km) scales) and analyse correlation with biological activity. In particular, mesoscale eddies are fundamental structures in the habitat of marine communities: i)Enrichment: primary production occurs. ii)Concentration: areas where food accumulates. iii)Retention: food is trapped. iv)Transport: eddies transport them to oligotrophic zones. v) They are one of the main responsibles of the formation of sub-mesoscale filamental structures. Still a better understanding of the influence of submesoscale structures in marine ecosystems

Transport and mixing Finite Size Lyapunov Exponents (FSLEs)‏ time needed for the perturbation to grow. Aurell et al., Phys. Rev. Lett. 77, 1262 (1996)‏ Boffetta et al., J. of Phys. A, 30, 1 (1997) Measuring horizontal stirring (or dispersion rates): local Lyapunov exponents FSLEs are functions of the initial position and time Similar to FTLEs

Transport and mixing Catching Lagrangian Coherent structures with ridges (maxima) of FSLEs Stable and unstable manifods are also called Lagrangian Coherent Structures

Transport and mixing Velocity fieldFSLEs Very important: features under the resolution of the velocity field submesoscale Mesoscale eddy Submesoscale filament

- Mallorca - Spain Intersection of stable and unstable manifolds: hyperbolic points A measure of mixing strength: counting the number of hyperbolic points in a given area

- Mallorca - Spain APPLICATIONS TO MARINE ECOSYSTEMS

- Mallorca - Spain Primary production (december 1998) Carr et al., DSR (2006)

- Mallorca - Spain Marine ecosystem dynamics and horizontal stirring Eastern upwelling areas: Canary and Benguela Canar y Benguela Importance of upwelling areas: – large contribution in the world ocean productivity and biomasses. – several and intense human activities (about half of the world fisheries). Crucial the vertical velocities  However our study will be mainly bidimensional.

- Mallorca - Spain Upwelling dynamics: Ekman transport

- Mallorca - Spain Surface velocity data (u,v) computed at each grid point (¼°) composed of: Geostrophic currents computed from a composite SSH field, Ekman currents from QuikSCAT wind stress fields. Chlorophyll a surface concentration from monthly SeaWiFS product. Period covered: June 2000 – June Data: surface current and chlorophyll 1 m.s -1 January 1st, 2003 VelocityFSLE

- Mallorca - Spain Geographical subdivision of the upwelling zones attending to their mixing activity Temporal average of FSLE Canary Benguela

- Mallorca - Spain Backward FSLE spatially averaged in the Benguela, per subsystem Northern Southern Stronger mixing activity in the southern part. High seasonal variability in the southern system whereas the northern one is quite stable.

- Mallorca - Spain Biological activity in the upwelling areas Hovmoller diagrams (averaging along lines of constant latitude) of the chlorophyll concentrations for the corresponding time period. time Canary Benguela Northern Southern Northern Southern

- Mallorca - Spain Negative correlation between mixing and biomass concentration: - Areas with lower mixing (low FSLE) correspond to larger phytoplankton concentration, and viceversa. This happens for all eastern boundary upwelling regions: Humboldt and California. more chaos less biomass Similar results (more chaos less biomass) in idealized 2d settings: Tel et al, Phys. Rep. (2005); Birch, Tsand and Young, PRE (2007).

- Mallorca - Spain But in upwelling regions the vertical dimension and Ekman transport are crucial. What’s their role?

- Mallorca - Spain Let us evaluate the horizontal divergence of the surface velocity field: Negative (blue)  Upwelling //// Positive (red)  Downwelling Temporal averages of « distance » from incompressibility assumption Canary Benguela Dominance of negative vertical velocities (upwelling areas) in the less turbulent subsystem (lower FSLE). Dominance of positive vertical velocities (downwelling areas) in the most turbulent subsystem (higher FSLE). POSITIVE CORRELATION

- Mallorca - Spain Spatial averaged westward Ekman transport versus averaged chlorophyll per subsystem Negative (Blue)  Westward transport offshore Positive (red)  Eastward Positive correlation: Westward offshore transporte  Higher chloro content

- Mallorca - Spain How to explain this inverse relationship (more turbulent less biomass)? -In open ocean, eddies tend to enhance biological productivity (particularly in low nutrient environments). This seems not to apply to upwelling regions (rich nutrient). -Most straightforward conclusion: Horizontal turbulent mixing of nutrients in surface waters of the most productive subsystems is second order for biomass enhancement as compared to the vertical mechanisms. -Areas with high FSLE are correlated with intense vertical movements (both up and down). Areas with low FSLE are dominated by upwards vertical velocities. GROWING EVIDENCE OF THE PRESENCE OF STRONG VERTICAL DYNAMICS AT ANY PLACE WHERE SUBMESOSCALE OCCURS (Mahadevan et al 2006,2007). - Areas where Ekman drift dominates over mesoscale activity there is not a large dispersion of particles. -Mixing modifies the 3D flow  weakening of the Ekman transport induced upwelling. - Need of further 3d numerical studies. Other factors influencing production …

- Mallorca - Spain CONCLUSIONS -Lagrangian tools can detect submesoscale structures in the oceanic surface. - The role of submesoscale structures (filaments) in marine ecosystems deserves further efforts.

- Mallorca - Spain Lagrangian Coherent Structures Surface currents in the Mozambique Channel

- Mallorca - Spain cm

- Mallorca - Spain California Humboldt

- Mallorca - Spain Biological signature of previous subdivisions Hovmöller diagram of Chlorophyll a concentration from June 2000 till June 2005 for the 4 EBUS Canary Benguela Humboldt California

- Mallorca - Spain Spatial averaged backward FSLE versus averaged chlorophyll per subsystem Negative correlation Clustering Less turbulent systems are characterized by: LOW FSLE / HIGH CHLOROPHYLL. Most turbulent systems: HIGH FSLE / LOW CHLOROPHYLL.

- Mallorca - Spain The Mozambique Channel Large anticyclonic cell Mesoscale eddies moving southward (De Ruijter et al., 2002; Schouten et al., 2003; Quartly and Srokosz, 2004) Upwelling Bipolar structures (anticyclonic and cyclonic mesoscale lobes) (De Ruijter et al., 2004) Unique zone in the world ocean: topography (channel) Strong mesoscale activity : fisheries resources