Pascal Rivière

North Pacific Gyre Oscillation links ocean climate and ecosystem change

Decadal fluctuations in salinity, nutrients, chlorophyll, a variety of zooplankton taxa, and fish stocks in the Northeast Pacific are often poorly correlated with the most widely-used index of large-scale climate variability in the region - the Pacific Decadal Oscillation (PDO). We define a new pattern of climate change, the North Pacific Gyre Oscillation (NPGO) and show that its variability is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are driven by regional and basin-scale variations in wind-driven upwelling and horizontal advection - the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. The NPGO pattern extends beyond the North Pacific and is part of a global-scale mode of climate variability that is evident in global sea level trends and sea surface temperature. Therefore the amplification of the NPGO variance found in observations and in global warming simulations implies that the NPGO may play an increasingly important role in forcing global-scale decadal changes in marine ecosystems.

Bringing physics to life at the submesoscale

A common dynamical paradigm is that turbulence in the upper ocean is dominated by three classes of motion: mesoscale geostrophic eddies, internal waves and microscale three-dimensional turbulence. Close to the ocean surface, however, a fourth class of turbulent motion is important: submesoscale frontal dynamics. These have a horizontal scale of O(1–10) km, a vertical scale of O(100) m, and a time scale of O(1) day. Here we review the physical-chemical-biological dynamics of submesoscale features, and discuss strategies for sampling them. Submesoscale fronts arise dynamically through nonlinear instabilities of the mesoscale currents. They are ephemeral, lasting only a few days after they are formed. Strong submesoscale vertical velocities can drive episodic nutrient pulses to the euphotic zone, and subduct organic carbon into the oceans interior. The reduction of vertical mixing at submesoscale fronts can locally increase the mean time that photosynthetic organisms spend in the well-lit euphotic layer and promote primary production. Horizontal stirring can create intense patchiness in planktonic species. Submesoscale dynamics therefore can change not only primary and export production, but also the structure and the functioning of the planktonic ecosystem. Because of their short time and space scales, sampling of submesoscale features requires new technologies and approaches. This paper presents a critical overview of current knowledge to focus attention and hopefully interest on the pressing scientific questions concerning these dynamics.

Propagation of Wind Energy into the Deep Ocean through a Fully Turbulent Mesoscale Eddy Field

Abstract The authors analyze the 3D propagation of wind-forced near-inertial motions in a fully turbulent mesoscale eddy field with a primitive equation numerical model. Although the wind stress is uniform, the near-inertial motion field quickly becomes spatially heterogeneous, involving horizontal scales much smaller than the eddy scales. Analysis confirms that refraction by the eddy relative vorticity is the main mechanism responsible for the horizontal distortion of the near-inertial motions, which subsequently triggers their vertical propagation. An important result is the appearance of two maxima of near-inertial vertical velocity (both with rms values reaching 40 m day−1): one at a depth of 100 m and another unexpected one much below the main thermocline around 1700 m. The shallow maximum, captured by the highest vertical normal modes, involves near-inertial motions with a spatial heterogeneity close to the eddy vorticity gradient field. These characteristics match analytical results obtained with Y...

Effects of Bottom Friction on Nonlinear Equilibration of an Oceanic Baroclinic Jet

Abstract Bottom friction is an important sink of energy in the ocean. Indeed, high-resolution ocean models need bottom friction to achieve a satisfactory kinetic energy level at equilibrium. However, bottom friction has also subtle and discriminating effects on the different energy transfers and therefore on the 3D structure of the flow, some of which have to be clarified. In this study, those effects on an unstable baroclinic jet are reexamined using a primitive equation model. As in previous studies using quasigeostrophic models, it was found that bottom friction strongly affects the barotropic mode whereas the baroclinic modes are weakly changed. The new result is that bottom friction yields a significant space-scale selection. Analysis of the dynamics reveals strong agreement with previous quasigeostrophic studies at the mesoscale in the interior but differences in the eddy field at small scales close to the surface. A rationalization of these results is proposed by a comparison with preceding atmosph...

Effects of surface quasi-geostrophic turbulence on phytoplankton competition and coexistence

This paper aims at studying numerically the competition between two mutually exclusive phytoplankton species in a fully-turbulent field consisting of interacting mesoscale and submesoscale structures. A simple NPPZD ecosystem model is embedded in a Surface Quasi-Geostrophic model which is able to reproduce frontogenesis and the associated nutrient vertical pump. The two simulated phytoplankton species differ by their size and their affinity for nutrients. In this study, we rationalize the role played by eddies and filaments in the distribution of the two phytoplankton species. We show that the SQG dynamics are responsible for the coexistence of the two phytoplankton species on a single limiting resource at statistical steady state. In addition, we show that as a result of strong vertical injections, filaments contain 64% of the phytoplankton biomass. The two phytoplankton species coexist in filaments but the large phytoplankton is predominant. By contrast, this latter is completely excluded from eddy cores where only the small phytoplankton develops. Since eddies are coherent structures (unlike filaments) and since their edges are almost impermeable to horizontal transport, the large phytoplankton can barely enter eddies. Therefore, eddies are ecological niches which shelter the small phytoplankton. Finally we show that interactions between eddies such as eddy merger can favor the survival of phytoplankton species within eddies on long time scales in the ocean.

Correction to “North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system”

[1] In the paper “North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system, ” by F. Chenillat et al. (Geophysical Research Letters, 39, L01606, doi:10.1029/ 2011GL049966, 2012), Figure 4b is modified without change to the caption: black and white dots (averaged Chl-a CalCOFI data) were erroneous. The second sentence of paragraph 17 is changed in consequence to “General patterns of the twin simulations are in agreement with the main characteristics of the dynamics of the CCS (Figure 4b), albeit with a high Chl-a bias nearshore.”

Plankton dynamics in a cyclonic eddy in the Southern California Current System

The California Current System is an eastern boundary upwelling system (EBUS) with high biological production along the coast. Oligotrophic offshore waters create cross-shore gradients of biological and physical properties, which are affected by intense mesoscale eddy activity. The influence of eddies on ecosystem dynamics in EBUS is still in debate. To elucidate the mechanisms that influence the dynamics of ecosystems trapped in eddies, and the relative contribution of horizontal and vertical advection in determining local production, we analyze a particular cyclonic eddy using Lagrangian particle-tracking analyses of numerical Eulerian. The eddy formed in a coastal upwelling system; coastal waters trapped in the eddy enabled it to leave the upwelling region with high concentrations of plankton and nutrients. The ecosystem was initially driven mainly by recycling of biological material. As the eddy moved offshore, production in its core was enhanced compared to eddy exterior waters through Ekman pumping of nitrate from below the euphotic zone; this Ekman pumping was particularly effective due to the shallow nitracline in the eddy compared to eddy exterior waters. Both eddy trapping and Ekman pumping helped to isolate and maintain the ecosystem productivity in the eddy core. This study shows the importance of cyclonic eddies for biological production in EBUS: they contribute both to the redistribution of the coastal upwelling ecosystem and are local regions of enhanced new production. Together, these processes impact cross-shore gradients of important biological properties.

California Coastal Upwelling Onset Variability: Cross- Shore and Bottom-Up Propagation in the Planktonic Ecosystem

The variability of the California Current System (CCS) is primarily driven by variability in regional wind forcing. In particular, the timing of the spring transition, i.e., the onset of upwelling-favorable winds, varies considerably in the CCS with changes in the North Pacific Gyre Oscillation. Using a coupled physical-biogeochemical model, this study examines the sensitivity of the ecosystem functioning in the CCS to a lead or lag in the spring transition. An early spring transition results in an increased vertical nutrient flux at the coast, with the largest ecosystem consequences, both in relative amplitude and persistence, hundreds of kilometers offshore and at the highest trophic level of the modeled food web. A budget analysis reveals that the propagation of the perturbation offshore and up the food web is driven by remineralization and grazing/predation involving both large and small plankton species.

Quantifying tracer dynamics in moving fluids: a combined Eulerian-Lagrangian approach

Eulerian models coupling physics and biology provide a powerful tool for the study of marine systems, complementing and synthesizing in situ observations and in vitro experiments. With the monotonic improvements in computing resources, models can now resolve increasingly complex biophysical interactions. Quantifying complex mechanisms of interaction produces massive amounts of numerical data that often require specialized tools for analysis. Here we present an Eulerian-Lagrangian approach to analyzing tracer dynamics in moving fluids. As an example of its utility, we apply this tool to quantifying plankton dynamics in oceanic mesoscale coherent structures. In contrast to Eulerian frameworks, Lagrangian approaches are particularly useful for revealing physical pathways, and the dynamics and distributions of tracers along these trajectories. Using a well-referenced Lagrangian tool, we develop a method to assess the variability of biogeochemical properties (computed using an Eulerian model) along particle trajectories. We discuss the limitations of this new method, given the different biogeochemical and physical timescales at work in the Eulerian framework. We also use Lagrangian trajectories to track coherent structures such as eddies, and we analyze the dynamics of the local ecosystem using two techniques: (i) estimating biogeochemical properties along trajectories, and (ii) averaging biogeochemical properties over dynamic regions (e.g., the eddy core) defined by ensembles of similar trajectories. This hybrid approach, combining Eulerian and Lagrangian model analyses, enhances the quantification and understanding of the complex planktonic ecosystem responses to environmental forcings; it can be easily applied to the dynamics of any tracer in a moving fluid.

Effects of an Asymmetric Friction on the Nonlinear Equilibration of a Baroclinic System

Following some recent linear and nonlinear studies the authors examine, using numerical simulations of a classical two-layer model, the effect of an asymmetric friction on the nonlinear equilibrium of moderately unstable baroclinic systems. The results show that the presence of an asymmetric friction leads to a significant wave scale selection: ‘‘long’’ waves (in terms of their zonal wavelengths) emerge with a traditional asymmetric friction (with the upper layer less viscous than the lower layer), while only ‘‘short’’ waves dominate with a nontraditional asymmetric friction (with the lower layer less viscous than the upper layer). The role of the nonlinear interactions and, more precisely, the effects of an asymmetric friction on the wave‐mean flow and wave‐wave interactions and their consequences on the wave scale selection are examined.