Patrick Marchesiello

Open boundary conditions for long-term integration of regional oceanic models

Regional oceanic models can be developed and used efficiently for the investigation of regional and coastal domains, provided a satisfactory prescription for the open boundary conditions (OBCs) is found. We propose in this paper an adaptive algorithm where inward and outward information fluxes are treated separately. Because of the essentially hyperbolic nature of the incompressible, hydrostatic Primitive Equations, external data are required only for inward boundary fluxes. The outward fluxes are treated with a new algorithm for two-dimensional radiation. Special attention is given to the estimation of the radiation phase speed, essential for detecting the direction of boundary fluxes. The boundary conditions are applied and assessed on a US West Coast (USWC) configuration of the Regional Oceanic Modeling System (ROMS). Our guiding principles are that the numerical solution be stable over multiple years, reach a meaningful statistical equilibrium, and be realistic with respect to the available observational data. A sensitivity analysis suggests that the oblique radiation is robust and sufficiently accurate to detect the direction of information fluxes. The adaptive nudging adequately incorporates the external information minimizing over- and under-specification problems. In addition, a volume constraint based on global correction of normal barotropic velocities improves the overall performances of the open boundary conditions.

Equilibrium structure and dynamics of the California Current, System

This paper addresses the structure and dynamical mechanisms of regional and mesoscale physical variability in the subtropical northeast Pacific Ocean using the Regional Oceanic Modeling System (ROMS). The model is configured with a U.S. West Coast domain that spans the California Current System (CCS) with a mesoscale horizontal resolution up to as fine as 3.5 km. Its mean-seasonal forcing is by momentum, heat, and water fluxes at the surface and adaptive nudging to gyre-scale fields at the open water boundaries. Its equilibrium solutions show realistic mean and seasonal states and vigorous mesoscale eddies, fronts, and filaments. The level of eddy kinetic energy (EKE) in the model is comparable to drifter and altimeter estimates in the solutions with sufficiently fine resolution. Because the model lacks nonseasonal transient forcing, the authors conclude that the dominant mesoscale variability in the CCS is intrinsic rather than transiently forced. The primary eddy generation mechanism is the baroclinic instability of upwelling, alongshore currents. There is progressive movement of meanseasonal currents and eddy energy offshore and downward into the oceanic interior in an annually recurrent cycle. The associated offshore eddy heat fluxes provide the principal balance against nearshore cooling by mean Ekman transport and upwelling. The currents are highly nonuniform along the coast, with important influences by capes and ridges in both maintaining mean standing eddies and launching transient filaments and fronts.

Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analyses

A surface thermal boundary condition for a world ocean model is proposed. The formulation is based on previous methods which have used bulk formulas to define a model-dependent correction to the air-sea fluxes applied to the model. An estimate of the flux correction is calculated from a recent 3-year climatology of atmospheric surface fields provided by the 6-hour analyses performed at the European Center for Medium-range Weather Forecasts. The mean correction term and its seasonnal cycle are analysed and compared to similar climatological quantities.

Software data news: Software tools for pre- and post-processing of oceanic regional simulations

ROMSTOOLS, a collection of global data sets and a series of Matlab programs collected in an integrated toolbox, generates the grid, surface forcing, initial condition, open boundary conditions, and tides for climatological and inter-annual ROMS ocean simulations. ROMSTOOLS also generates embedded models, real-time coastal modeling systems, as well as experiments including biology. Tools for visualization, animations and diagnostics are also provided.

About the role of Westerly Wind Events in the possible development of an El Niño in 2014

Similarities between early 1997 and 2014 has prompted climate scientists to wonder if an El Nino matching the 1997 “El Nino of the century” could develop in 2014. Until April 2014, the equatorial Pacific exhibited positive heat content anomalies along with an eastward warm pool displacement similar to those found during the onset of strong El Nino events. Yet in July 2014, the warm pool had retreated back to its climatological positions and equatorial temperature anomalies were much weaker than in mid-1997. Dedicated oceanic simulations reveal that these weak interannual anomalies can be attributed to differences in Westerly Wind Event (WWE) sequences. In contrast with 1997, the lack of WWEs from April to June significantly limited the growth of eastern Pacific anomalies and the eastward warm pool displacement in 2014. With the absence of additional WWE activity, prospects for a mature El Nino in late 2014 are fading.

Generation of cyclonic eddies by the Agulhas Current in the lee of the Agulhas Bank

Anti-cyclonic rings are shed from the Agulhas Current at its retroflection. They subsequently drift off into the South Atlantic. Smaller, cyclonic eddies have also been observed in this region. The origin of these latter eddies has remained unknown. We present model results that indicate that the configuration of the southern Agulhas Current and the poleward termination of the continental shelf of Africa allows shedding of cyclonic lee eddies by a flow detachment process. Hydrographic data, thermal infrared satellite images and altimetric observations are furnished that show that this model simulation is consistent with the characteristics and the behaviour of cyclonic eddies in the region.

Surface circulation in the Gulf of Cadiz: Model and mean flow structure

[1] The mean flow structure of the Gulf of Cadiz is studied using a numerical model. The model consists of a set of one-way nested configurations attaining resolutions on the order of 2.6 km in the region of the Gulf of Cadiz. In the large-scale configuration, the entrainment of the Mediterranean Water is parameterized implicitly through a nudging term. In medium- and small-scale nested configurations, the Mediterranean outflow is introduced explicitly. The model reproduces all the known features of the Azores Current and of the circulation inside the Gulf of Cadiz. A realistic Mediterranean Undercurrent is generated and Meddies develop at proper depths on the southwest tip of the Iberian slope. The hypothesis that the Azores Current may generate in association with the Mediterranean outflow (β-plume theories) is confirmed by the model results. The time-mean flow is dominated by a cyclonic cell generated in the gulf which expands westward and has transports ranging from 4 to 5 Sv. The connection between the cell and the Azores Current is analyzed. At the scale of the Gulf, the time-mean flow cell is composed by the westward Mediterranean Undercurrent, and by a counterflow running eastward over the outer edge of the Mediterranean Undercurrent deeper vein, as the latter is forced downslope. This counterflow feeds the entrainment at the depths of the Mediterranean Undercurrent and the Atlantic inflow at shallower levels. Coastward and upslope of this recirculation cell, a second current running equatorward all the way along the northern part of the gulf is revealed. This current is a very robust model result that promotes continuity between the southwestern Iberian coast and the Strait of Gibraltar, and helps explain many observations and recurrent SST features of the Gulf of Cadiz.

A sigma-coordinate primitive equation model for studying the circulation in the South Atlantic. Part I: Model configuration with error estimates

This paper describes the configuration of a topography-following (sigma) coordinate, numerical ocean model for studying the circulation in the South Atlantic. An analysis is performed (i) to ensure that the model configuration does not introduce a numerical bias in the model solution and (ii) to give estimates of numerical errors. The model is the Semi-spectral Primitive Equation Model (SPEM) from Rutgers University (Haidvogel et al., 1991). Two important issues relating to the sigma-coordinate are investigated: the pressure gradient calculation and the diffusion of tracers. Errors in the pressure gradient calculation are investigated by simulating an ocean at rest, and the choice is made to reduce errors by smoothing the bathymetry. A smoothing criterion is derived that permits a limitation of the errors in the pressure gradient calculation to an acceptable level (i.e. maximum errors on velocities below a millimeter per second). It is applied to define the model bottom topography. Errors in the tracer fields, induced by a diffusion scheme operating along constant sigma surfaces, generates large unrealistic velocities (of the order of 10 cm/s). A rotation of the diffusion tensor into geopotential coordinates is proposed. Tests show that errors are then reduced to an insignificant level. The rotation of the diffusion tensor is therefore retained. The numerical treatment of the open boundaries and the flux conditions that yields the most realistic circulation is also described. Open boundary conditions are based on radiation conditions and relaxation to climatology. They appear to be numerically robust, and to be able to bring into the South Atlantic basin the necessary information from the outer oceans. A configuration of the SPEM model to study the large scale circulation in the South Atlantic is then obtained. Errors due to model configuration are shown to be small compared to the signal one wants to simulate, and their spatial pattern is known, which will facilitate the interpretation of the model simulations presented in following papers.

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.

Upwelling limitation by onshore geostrophic flow

Although coastal upwelling has now been the subject of many studies, the only available dynamical upwelling index is derived from a simple relation between upwelling intensity and alongshore winds stress. In this study, we first present a primitive equation model simulation of the New Caledonia island region to show that upwelling intensity can be limited by onshore geostrophic flow. Then, we propose a new analytical model accounting for the effect of onshore geostrophic flow on the structure of upwelling. The analytical model is shown to match remarkably well the solution of the primitive equation model. Used as an upwelling index, it is also a better fit to SST indices over the worlds four large upwelling regions than the classical Bakun index. We conclude that alongshore pressure gradients (in addition to shelf topography) are crucial to the upwelling problem and its addition to theoretical models can improve their explanatory value and predictive skills.