Jean-Marc Molines

Circulation characteristics in three eddy-permitting models of the North Atlantic

A systematic intercomparison of three realistic eddy-permitting models of the North Atlantic circulation has been performed. The models use different concepts for the discretization of the vertical coordinate, namely geopotential levels, isopycnal layers, terrain-following (sigma) coordinates, respectively. Although these models were integrated under nearly identical conditions, the resulting large-scale model circulations show substantial differences. The results demonstrate that the large-scale thermohaline circulation is very sensitive to the model representation of certain localised processes, in particular to the amount and water mass properties of the overflow across the Greenland–Scotland region, to the amount of mixing within a few hundred kilometers south of the sills, and to several other processes at small or sub-grid scales. The different behaviour of the three models can to a large extent be explained as a consequence of the different model representation of these processes.

A hydrodynamic ocean tide model improved by assimilating a satellite altimeter‐derived data set

An upgraded version of the tidal solutions (FES94.1) is presented, obtained by assimilating an altimeter-derived data set in the finite element hydrodynamic model, following the representer approach. The assimilated data are drawn from the CSR2.0 Texas solutions sampled on a 5° × 5° grid. The assimilation is applied over the Atlantic, Indian, and Pacific Oceans. The standard release of the new FES95.2 solutions is a 0.5° × 0.5° gridded version of the full finite element solutions. The associated tidal prediction model includes 26 constituents. The eight major constituents are drawn directly from the hydrodynamic model: K1, O1, Q1, M2, S2, N2, K2, and 2N2, corrected by assimilation except K2 and 2N2. The other 18 constituents are derived by admittance. Among them are μ2, ν2, L2, T2, M1, P1, J1, and OO1. The quality of these solutions is evaluated by reference to a standard sea truth data set of 95 stations. This quality is significantly improved after the assimilation process is applied: the root-sum-square (RSS) of the differences between solutions and observations, for the eight major constituents, is reduced from 3.8 cm for FES94.1 to 2.8 cm for FES95.2, i.e., a gain of 1 cm. The performances of the prediction model are evaluated by comparing tidal predictions with observations at 59 sites distributed over the world ocean and by looking at the level of variance of the sea surface variability observed by the T/P altimeter at its cross-over track points after tidal correction. These evaluations lead to the same conclusion: this new prediction model performs much better than the one based on FES94.1, because of correction of the major constituents by T/P data assimilation and because of the increase in the number of constituents from 13 to 26. The tidal predictions are at the level of accuracy of those produced by the best recent T/P empirical models.

Causes of Interannual–Decadal Variability in the Meridional Overturning Circulation of the Midlatitude North Atlantic Ocean

The causes and characteristics of interannual to decadal variability of the meridional overturning circulation (MOC) in the North Atlantic are investigated with a suite of basin-scale ocean models (FLAME) and global ocean-ice models (ORCA) varying in resolution from medium- to eddy-resolving (1/2° – 1/12°), using various forcing configurations built on bulk formulations invoking atmospheric reanalysis products. Comparison of the model hindcasts indicates similar MOC variability characteristics on time scales up to a decade; both model architectures also simulate an upward trend in MOC strength between the early 1970s and mid-1990s. The causes of the MOC changes are examined by perturbation experiments aimed selectively at the response to individual forcing components. The solutions emphasize an inherently linear character of the mid-latitude MOC variability by demonstrating that the anomalies of a (non eddy-resolving) hind-cast simulation can be understood as a superposition of decadal and longer-term signals originating from thermohaline forcing variability, and a higher-frequency wind-driven variability. The thermohaline MOC signal is linked to the variability in subarctic deep water formation, and rapidly progressing to the tropical Atlantic. However, throughout the subtropical and mid-latitude North Atlantic this signal is effectively masked by stronger MOC variability related to wind forcing and, especially north of 30–35° N, by internally-induced (eddy) fluctuations.

Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales

AbstractThis paper evaluates in a realistic context the local contributions of direct atmospheric forcing and intrinsic oceanic processes on interannual sea level anomalies (SLAs). A ¼° global ocean–sea ice general circulation model, driven over 47 yr by the full range of atmospheric time scales, is quantitatively assessed against altimetry and shown to reproduce most observed features of the interannual SLA variability from 1993 to 2004. Comparing this simulation with a second driven only by the climatological annual cycle reveals that the intrinsic part of the total interannual SLA variance exceeds 40% over half of the open-ocean area and exceeds 80% over one-fifth of it. This intrinsic contribution is particularly strong in eddy-active regions (more than 70%–80% in the Southern Ocean and western boundary current extensions) as predicted by idealized studies, as well as within the 20°–35° latitude bands. The atmosphere directly forces most of the interannual SLA variance at low latitudes and in most mid...

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.

On the origin and pathway of the saline inflow to the Nordic Seas: insights from models

The behaviours of three high-resolution ocean circulation models of the North Atlantic, differing chiefly in their description of the vertical coordinate, are investigated in order to elucidate the routes and mechanisms by which saline water masses of southern origin provide inflows to the Nordic Seas. An existing hypothesis is that Mediterranean Overflow Water (MOW) is carried polewards in an eastern boundary undercurrent, and provides a deep source for these inflows. This study, however, provides an alternative view that the inflows are derived from shallow sources, and are comprised of water masses of western origin, carried by branches of the North Atlantic Current (NAC), and also more saline Eastern North Atlantic Water (ENAW), transported northwards from the Bay of Biscay region via a ‘Shelf Edge Current’ (SEC) flowing around the continental margins. In two of the models, the MOW flows northwards, but reaches only as far as the Porcupine Bank (53°N). In third model, the MOW also invades the Rockall Trough (extending to 60°N). However, none of the models allows the MOW to flow northwards into the Nordic Seas. Instead, they all support the hyporthesis of there being shallow pathways, and that the saline inflows to the Nordic Seas result from NAC-derived and ENAW water masses, which meet and partially mix in the Rockall Trough. Volume and salinity transports into the southern Rockall Trough via the SEC are, in the various models, between 25 and 100% of those imported by the NAC, and are also a similarly significant proportion (20–75%) of the transports into the Nordic Seas. Moreover, the highest salinities are carried northwards by the SEC (these being between 0.13 and 0.19 psu more saline at the southern entrance to the Trough than those in the NAC-derived waters). This reveals for the first time the importance of the SEC in carrying saline water masses through the Rockall Trough and into the Nordic Seas. Furthermore, the high salinities found on density surfaces appropriate to the MOW in the Nordic Seas are shown to result from the wintertime mixing of the saline near-surface waters advected northwards by the SEC/NAC system. Throughout, we have attempted to demonstrate the extent to which the models agree or disagree with interpretations derived from observations, so that the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models.

Water Mass Transformation in the North Atlantic and Its Impact on the Meridional Circulation: Insights from an Ocean Model Forced by NCEP–NCAR Reanalysis Surface Fluxes

Decadal-scale climate variability in the North Atlantic thermohaline circulation is simulated using a sigma-coordinate primitive equation model, forced by NCEP–NCAR reanalysis surface forcing fields for the period from 1958 to 1997. Surface heat and freshwater flux are expressed in terms of surface thermal and haline density inputs, diagnosed by the model. Variability in surface density fluxes is closely correlated with the North Atlantic Oscillation and demonstrates differences with the original surface heat and freshwater fluxes. Leading modes of surface water mass transformation are considered in the T–S plane. They identify decadal-scale variability associated with the transformation of the Labrador Sea Waters and Subtropical Mode Waters. Analysis of the model responses to the surface forcing shows an immediate reaction of meridional heat transport to the wind stress curl, resulting in a decrease of meridional heat transport at 48°N and an increase in the subtropics. Delayed baroclinic responses to the surface heat forcing are identified at time lags of 3 and 7 yr. The 3-yr response is represented by an increase in the total meridional heat transport in subpolar latitudes and its simultaneous increase in the Tropics and midlatitudes. The 7-yr delayed response to the surface heat forcing is associated with the strengthening of meridional heat transport at all latitudes. However, 7-yr responses may be influenced by the self-correlation in the meridional heat transport and forcing function. Meridional overturning is largely responsible for the variability observed, demonstrating high correlation with meridional heat transport.

Assimilation of TOPEX/POSEIDON altimeter data into a circulation model of the North Atlantic

Assimilation experiments were conducted using the first 12 months of TOPEX/POSEIDON (T/P) altimeter measurements in a multilayered quasi-geostrophic model of the North Atlantic between 20°N and 60°N. These experiments demonstrate the feasibility of using T/P data to control a basin-scale circulation model by means of an assimilation procedure. Moreover, they allow us to recreate the four-dimensional behavior of the North Atlantic Ocean during the year October 1992–September 1993 and to improve our knowledge and understanding of such circulation patterns. For this study we used a four-layer quasigeostrophic model of high horizontal resolution (1/6° in latitude and longitude). The assimilation procedure used is an along-track, sequential, nudging technique. The evolution of the model general circulation is described and analyzed from a deterministic and statistical point of view, with special emphasis on the Gulf Stream area. The gross features of the North Atlantic circulation in terms of mean transport and circulation are reproduced, such as the path, penetration and recirculation of the Gulf Stream, and its meandering throughout the eastern basin. The North Atlantic Drift is, however, noticeably underestimated. A northern meander of the north wall of the Gulf Stream above the New England Seamount Chain is present for most of the year, while, just downstream, the southern part of the jet is subject to a 100-km southeastward deflection. The Azores current is shown to remain stable and to shift southward with time from the beginning of December 1992 to the end of April 1993, the amplitude of the shift being about 2°. The computation of the mean latitude of the Gulf Stream as a function of time shows an abrupt shift from a northern position to a southern position in January, and a reverse shift, from a southern position to a northern position, in July. Finally, some issues are addressed concerning the comparison of assimilation experiments using T/P data and Geosat data. The first results show that the T/P simulations are more energetic than the Geosat simulations, especially east of the Mid-Atlantic Ridge, for every wavelength from 50 km to 500 km. This property is also verified in the deep ocean. The predicted abyssal circulation is indeed more energetic in the T/P case, which is more in accordance with what we know of the real ocean. Moreover, the good T/P altimeter coverage near the coasts greatly improves the model eddy kinetic energy levels in these areas, especially east of 25°W.

Seasonal heat balance in the upper 100 m of the equatorial Atlantic Ocean

The variability of sea surface temperature (SST) in the equatorial Atlantic is characterized by strong cooling in May-June and a secondary cooling in November-December. A numerical simulation of the tropical Atlantic is used to diagnose the different contributions to the temperature tendencies in the upper ocean. Right at the equator, the coolest temperatures are observed between 20°W and 10°W due to enhanced turbulent heat flux in the center of the basin. This results from a strong vertical shear at the upper bound of the Equatorial Undercurrent (EUC). Cooling through vertical mixing exhibits a semiannual cycle with two peaks of comparable intensity. During the first peak, in May-June, vertical mixing drives the SST while during the second peak, in November-December, the strong heating due to air-sea fluxes leads to much weaker effective cooling than during boreal summer. Seasonal cooling events are closely linked to the enhancement of the vertical shear just above the core of the EUC, which appears to be not driven directly by the strength of the EUC but by the strength and the direction of the surface current. The vertical shear is maximum when the northern branch of the South Equatorial Current is intense. The surface cooling in the eastern equatorial Atlantic is not as marked as in the center of the basin. Mean thermocline and EUC rise eastward, but a strong stratification, caused by the presence of warm and low-saline surface waters, limits the vertical mixing to the upper 20 m and disconnects the surface from subsurface dynamics.

On the seasonal variability and eddies in the North Brazil Current: insights from model intercomparison experiments

The time dependent circulation of the North Brazil Current is studied with three numerical ocean circulation models, which differ by the vertical coordinate used to formulate the primitive equations. The models are driven with the same surface boundary conditions and their horizontal grid-resolution (isotropic, 1/3° at the equator) is in principle fine enough to permit the generation of mesoscale eddies. Our analysis of the mean seasonal currents concludes that the volume transport of the North Brazil Current (NBC) at the equator is principally determined by the strength of the meridional overturning, and suggests that the return path of the global thermohaline circulation is concentrated in the NBC. Models which simulate a realistic overturning at 24°N of the order of 16–18 Sv also simulate a realistic NBC transport of nearly 35 Sv comparable to estimates deduced from the most recent observations. In all models, the major part of this inflow of warm waters from the South Atlantic recirculates in the zonal equatorial current system, but the models also agree on the existence of a permanent coastal mean flow to the north-west, from the equator into the Carribean Sea, in the form of a continuous current or a succession of eddies. Important differences are found between models in their representation of the eddy field. The reasons invoked are the use of different subgrid-scale parameterisations, and differences in stability of the NBC retroflection loop because of differences in the representation of the effect of bottom friction according to the vertical coordinate that is used. Finally, even if differences noticed between models in the details of the seasonal mean circulation and water mass properties could be explained by differences in the eddy field, nonetheless the major characteristics (mean seasonal currents, volume and heat transports) appears to be at first order driven by the strength of the thermohaline circulation.