François Colas

On the pathways of the equatorial subsurface currents in the eastern equatorial Pacific and their contributions to the Peru-Chile Undercurrent

[1] Theconnections between theEquatorial CurrentSystem andthePeru CurrentSystem in the eastern tropical Pacific (ETP) are examined with a primitive equations eddy‐resolving regional model. The quasi‐equilibrium solutions reproduce three eastward equatorial subsurface currents of interest: the Equatorial Undercurrent (EUC, located between 1°N and 1°S), the primary Southern Subsurface Countercurrent (pSSCC, between 3° and 4°S) and, farther south the secondary Southern Subsurface Countercurrent (sSSCC, between 7° and 8 °S). Using a Lagrangian tracking procedure, the fate of these currents in the ETP and their contribution to the Peru‐Chile Undercurrent (PCUC) are studied. Lagrangian diagnostics show that for the most part the EUC water contributes to westward flows, including the South Equatorial Current and deeper flows below it, and strikingly only a very little fraction feeds the PCUC, while a significant part of both SSCCs contribute substantially. Mesoscale eddies are shown to exert an effect on these connections. In addition, about 30% of the PCUC is fed by the three subsurface equatorial flows (EUC, pSSCC, sSSCC). The remaining part of the PCUC comes from an alongshore recirculation associated with flows below it, and from the southern part of the domain (south of ∼9°S).

Eddies in Eastern Boundary Subtropical Upwelling Systems

Over the last decade, mesoscale-resolving ocean models of eastern boundary upwelling systems (EBS) have helped improve our understanding of the functioning of EBS and, in particular, assess the role of eddy activity in these systems. We review the main achievements in this regard and highlight remaining issues and challenges. In EBS, eddy activity arises from baroclinic/barotropic instability of the inshore and also offshore currents. Mesoscale eddies play a significant (although not leading) role in shaping the EBS dynamical structure, both directly and through associated submesoscale activity (i.e., primarily frontal). They do so by modifying both momentum and tracer balances in ways that cannot simply be understood in terms of diffusion. The relative degree to which these assertions about eddy activity and eddy role apply to each of the four major EBS (Canary, Benguela, Peru–Chile, and California Current Systems) remains to be established. Besides resolving the eddies, benefits from EBS high-resolution modeling include the possibility of accounting for the fine-scale structures of the nearshore wind, a better representation of the Ekman-driven coastal divergence, and (at resolution  (1 km) or lower) inclusion of submesoscale (i.e., mainly frontal) processes. Recent numerical experiments suggest that accounting for these various processes in climate models, through resolution increase (possibly locally) or parameterization, would lead to significant basin-scale bias reduction. The mechanisms involved in upscaling from EBS toward the larger scale remain to be fully elucidated.

Mesoscale Eddy Buoyancy Flux and Eddy-Induced Circulation in Eastern Boundary Currents

AbstractA dynamical interpretation is made of the mesoscale eddy buoyancy fluxes in the Eastern Boundary Currents off California and Peru–Chile, based on regional equilibrium simulations. The eddy fluxes are primarily shoreward and upward across a swath several hundred kilometers wide in the upper ocean; as such they serve to balance mean offshore air–sea heating and coastal upwelling. In the stratified interior the eddy fluxes are consistent with the adiabatic hypothesis associated with a mean eddy-induced velocity advecting mean buoyancy and tracers. Furthermore, with a suitable gauge choice, the horizontal fluxes are almost entirely aligned with the mean horizontal buoyancy gradient, consistent with the advective parameterization scheme of Gent and McWilliams. The associated diffusivity κ is surface intensified, matching the vertical stratification profile. The fluxes span the across-shore band of high eddy energy, but their alongshore structure is unresolved because of sampling limitations. In the sur...

Broad impacts of fine-scale dynamics on seascape structure from zooplankton to seabirds

In marine ecosystems, like most natural systems, patchiness is the rule. A characteristic of pelagic ecosystems is that their ‘substrate’ consists of constantly moving water masses, where ocean surface turbulence creates ephemeral oases. Identifying where and when hotspots occur and how predators manage those vagaries in their preyscape is challenging because wide-ranging observations are lacking. Here we use a unique data set, gathering high-resolution and wide-range acoustic and GPS-tracking data. We show that the upper ocean dynamics at scales less than 10 km play the foremost role in shaping the seascape from zooplankton to seabirds. Short internal waves (100 m–1 km) play a major role, while submesoscale (~1–20 km) and mesoscale (~20–100 km) turbulence have a comparatively modest effect. Predicted changes in surface stratification due to global change are expected to have an impact on the number and intensity of physical structures and thus biological interactions from plankton to top predators.

Sensitivity of the Northern Humboldt Current System nearshore modeled circulation to initial and boundary conditions

[1] The influence of the eastern Pacific equatorial circulation on the dynamics of the Northern Humboldt Current System is studied using an eddy‐resolving regional circulation model forced by boundary conditions from three distinct ocean general circulation models. The seasonal variability of the modeled nearshore circulation and the mesoscale activity are contrasted in order to evaluate the role of the density forcing. The seasonal variability of the surface and subsurface alongshore currents strongly depends on the amplitude and timing of the seasonal eastward propagating equatorial waves. The equatorward flow and upwelling intensity are also impacted by nonlinear processes, such as the seasonal generation of nearshore mesoscale eddies, which create alongshore pressure gradients modulating the surface current. Boundary conditions affect differently the intensity and phase of the eddy kinetic energy, as baroclinic instability is triggered by coastal waves during austral summer and fall, whereas it is sustained by the wind‐ driven upwelling during austral winter.

Lagrangian study of the Panama Bight and surrounding regions

Near-surface circulation of the Panama Bight and surrounding regions [0-9°N; 73°W-90°W] was studied using satellite-tracked drifter trajectories from 1979-2004. This region encompasses three major currents showing typical velocities of ∼30 cm s−1: (1) the eastward North Equatorial Counter Current (NECC), (2) the near-circular Panama Bight Cyclonic Gyre (PBCG), and (3) the westward South Equatorial Current (SEC). We do not observe significant modification of the mean surface circulation during El Nino Southern Oscillation events, even if the SEC is slightly reinforced during relatively warm El Nino periods. At seasonal scales, the circulation is strongly controlled by the activity of the Panama wind-jet: in boreal winter, the currents are stronger and an anticyclonic cell is present west of the PBCG. This dipole leads to a strong ∼200 km wide southward current which then disappears during the rest of the year. In summer, the three major currents have reduced intensity by 30%-40%. Large-scale current vorticity shows that the upwelling associated with the PBCG is also 3-4 times stronger in winter than during summer months. The kinetic energy is largely dominated by eddy activity and its intensity is double in winter than during summer. Ageostrophic motions and eddy activity appear to have a substantial impact on the energy spatial distribution. In the NECC and SEC regions, Lagrangian scales are anisotropic and zonally enhanced in the direction of the mean currents. The typical integral time and length scales of these regions are 2.5 days and 50-60 km in the zonal direction and 1.5 days and 25-30 km in the meridional direction. Lateral eddy diffusivity coefficients are on the order of 11-14 107 cm2 s−1 zonally and 5-6 107 cm2 s−1 meridionally. In contrast, in the PBCG region, the Lagrangian characteristics are isotropic with typical timescales of 1.7 days, space scales of 30 km and eddy diffusivity coefficients of 6 107 cm2 s−1 in both directions.

An Ocean Observing and Prediction Experiment in Prince William Sound, Alaska

The observing and forecasting conditions of coastal oceans in Alaska is technically challenging because of the mountainous terrain, the notoriously stormy seas, and a complex hydrological system of freshwater from rivers and glaciers. The Alaska Ocean Observing System and their partners developed a demonstration project over a 5-yr period in Prince William Sound. This location was chosen because of historical efforts to monitor ocean circulation following the Exxon Valdez oil spill of 1989. The primary goal is to develop a quasi-operational system that delivers weather and ocean information in near–real time to diverse user communities. This observing system now consists of a spatial array of atmospheric and oceanic sensors and a new generation of computer models to numerically simulate and forecast weather, waves, and ocean circulation. The project culminated in a 2009 field experiment that evaluated the performance of the model forecasts. Three ships, 44 surface drifters, an underwater glider, and an au...