Guillaume Maze

The Climode Field Campaign: Observing the Cycle of Convection and Restratification over the Gulf Stream

Abstract A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as “mode water” in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air–sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.

Using Transformation and Formation Maps to Study the Role of Air-Sea Heat Fluxes in North Atlantic Eighteen Degree Water Formation

Abstract The Walin water mass framework quantifies the rate at which water is transformed from one temperature class to another by air–sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air–sea heat fluxes (formation). Walin’s framework provides a precise integral statement at the expense of losing spatial information. In this study the integrand of Walin’s expression to yield transformation and formation maps is plotted and used to study the role of air–sea heat fluxes in the cycle of formation–destruction of the 18° ± 1°C layer in the North Atlantic. Using remotely sensed sea surface temperatures and air–sea heat flux estimates based on both analyzed meteorological fields and ocean data–model syntheses for the 3-yr period from 2004 to 2006, the authors find that Eighteen Degree Water (EDW) is formed by air–sea heat fluxes in the western part of the subtropical gyre, just south of the Gulf Stream. The f...

Subantarctic Mode Water Formation, Destruction, and Export in the Eddy-Permitting Southern Ocean State Estimate

Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air-sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air-sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air-sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air-sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air-sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air-sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.

Diagnosing the Observed Seasonal Cycle of Atlantic Subtropical Mode Water Using Potential Vorticity and Its Attendant Theorems

Analyzed fields of ocean circulation and the flux form of the potential vorticity equation are used to map the creation and subsequentcirculation of lowpotential vorticity waters known as subtropical mode water (STMW) in the North Atlantic. Novel mapping techniques are applied to (i) render the seasonal cycle and annual-mean mixed layer vertical flux of potential vorticity (PV) through outcrops and (ii) visualize the extraction ofPV from the mode water layer in winter, over and to the south of the Gulf Stream. Both buoyancy loss and wind forcing contributetotheextractionofPV,buttheauthorsfindthattheformergreatlyexceedsthelatter.Thesubsequent path of STMW is also mapped using Bernoulli contours on isopycnal surfaces.

Mixed Layer Lateral Eddy Fluxes Mediated by Air-Sea Interaction

Abstract The modulation of air–sea heat fluxes by geostrophic eddies due to the stirring of temperature at the sea surface is discussed and quantified. It is argued that the damping of eddy temperature variance by such air–sea fluxes enhances the dissipation of surface temperature fields. Depending on the time scale of damping relative to that of the eddying motions, surface eddy diffusivities can be significantly enhanced over interior values. The issues are explored and quantified in a controlled setting by driving a tracer field, a proxy for sea surface temperature, with surface altimetric observations in the Antarctic Circumpolar Current (ACC) of the Southern Ocean. A new, tracer-based diagnostic of eddy diffusivity is introduced, which is related to the Nakamura effective diffusivity. Using this, the mixed layer lateral eddy diffusivities associated with (i) eddy stirring and small-scale mixing and (ii) surface damping by air–sea interaction is quantified. In the ACC, a diffusivity associated with su...

Global interior eddy available potential energy diagnosed from Argo floats

By combining all Argo profiles for the period 2002 to present, a cumulative density function is constructed on a 3-D grid of the global ocean. This function quantifies the statistics of isopycnals: time-averaged density, root-mean square of isopycnal displacement, and eddy available potential energy (EAPE). EAPE is the analogue of the eddy kinetic energy, but for the potential energy reservoir. Because it is essentially tied to the spatial structure and magnitude of mesoscale activity, EAPE is an important quantity that should be useful to evaluate eddy resolving/permitting model turbulence and circulation. Among other striking features are the turbulent behavior of Pacific and southern Atlantic Tsuchiya jets and subsurface EAPE maxima in some parts of the ocean, particularly in the Southern Ocean.

A two-dimensional vortex merger in an external strain field.

In a two-dimensional incompressible fluid, the merger of two Rankine vortices in an external strain field is studied analytically and numerically. In the absence of merger, the trajectory of the vortex centres is elliptical near the centre of the plane and hyperbolic at larger distances. When merger occurs, it can be accelerated or decelerated by the external strain field depending on the orientation of the strain axes, but in all the cases studied here, external strain decreases merger efficiency. This article was chosen from Selected Proceedings of the 4th International Workshop on Vortex Flows and Related Numerical Methods (UC Santa-Barbara, 17-20 March 2002) ed E Meiburg, G H Cottet, A Ghoniem and P Koumoutsakos.

Low‐frequency variability in the Southern Ocean region in a simplified coupled model

[1] Patterns of interannual variability of the ocean-atmosphere coupled system in the Southern Hemisphere extratropics are studied with a simple dynamical model in order to determine the basic physical processes of interaction independently of tropical forcing. The model used is an atmospheric quasi-geostrophic model coupled to a ‘‘slab’’ oceanic mixed layer, which includes mean geostrophic advection by the Antarctic Circumpolar Current (ACC). The ocean-atmosphere coupling occurs through surface heat fluxes and Ekman current heat advection. In a fully coupled simulation, the atmospheric part of the model, which includes high-frequency transient eddies at midlatitudes, exhibits a strong Southern Annular Mode (SAM) as the first mode of variability at interannual timescales. The SAM-related wind anomalies induce Ekman currents in the mixed layer which produce sea surface temperature anomalies. These are then advected along by the ACC. A forced mechanism where the ocean role is reduced to advect the sea surface temperature (SST) appears sufficient to reproduce the main features of the variability. Nevertheless, a positive feedback of the ocean was also found. It operates through anomalous Ekman currents heat advection and contributes to the maintenance of the SST anomaly.

Intensification of Upper-Ocean Submesoscale Turbulence through Charney Baroclinic Instability

AbstractThis study focuses on the description of an oceanic variant of the Charney baroclinic instability, arising from the joint presence of (i) an equatorward buoyancy gradient that extends from the surface into the ocean interior and (ii) reduced subsurface stratification, for example, as produced by wintertime convection or subduction. This study analyzes forced dissipative simulations with and without Charney baroclinic instability (C-BCI). In the former, C-BCI strengthens near-surface frontal activity with important consequences in terms of turbulent statistics: increased variance of vertical vorticity and velocity and increased vertical turbulent fluxes. Energetic consequences are explored. Despite the atypical enhancement of submesoscale activity in the simulation subjected to C-BCI, and contrary to several recent studies, the downscale energy flux at the submesoscale en route to dissipation remains modest in the flow energetic equilibration. In particular, it is modest vis a vis the global energy...

“Deep-Arvor”: A New Profiling Float to Extend the Argo Observations Down to 4000-m Depth

AbstractThe international Argo program, consisting of a global array of more than 3000 free-drifting profiling floats, has now been monitoring the upper 2000 m of the ocean for several years. One of its main proposed evolutions is to be able to reach the deeper ocean in order to better observe and understand the key role of the deep ocean in the climate system. For this purpose, Ifremer has designed the new “Deep-Arvor” profiling float: it extends the current operational depth down to 4000 m, and measures temperature and salinity for up to 150 cycles with CTD pumping continuously and 200 cycles in spot sampling mode. High-resolution profiles (up to 2000 points) can be transmitted and data are delivered in near–real time according to Argo requirements. Deep-Arvor can be deployed everywhere at sea without any preballasting operation and its light weight (~26 kg) makes its launching easy. Its design was done to target a cost-effective solution. Predefined spots have been allocated to add an optional oxygen s...