Numerical models of the ocean circulation

2002

The double-gyre circulation induced by a symmetric wind-stress pattern in a quasi-geostrophic model of the mid-latitude ocean is studied analytically and numerically. The model is discretized vertically by projection onto normal modes of the mean stratification.

Journal of Geophysical …, 1990

Numerical approximations to the dynamic equations &re given which allow basin-size ocean circulation models formulated in isopycnic coordinates to accommodate variable bottom topography and irregular coastlines. Emphasis is placed on computational economy through the use of a splitexplicit time integration scheme, on the proper formulation of the advection and Coriolis terms in the momentum equation in case of strongly varying layer thickness, and on the correct estimation of the horizontal pressure gradient force in grid boxes truncatcd by steep bottom slopes. The algorithms are tested in a series of two-and three-layer double-gyre experiments. In cases of steady forcing leading to a steady circulation, we are able to reproduce the expected motionless final state in coordinate layers that are below the direct influence of the wind forcing. This includes layers intersecting topographic obstacles. A long-term (25-30 year) vacillation tentatively associated with the outcropping of isopycnals along the edge of the cyc!mfic gyre in a steadily forced model is documented. The paper forms the basis for a subsequent s• udy in which circulation features obtained in a realistic North Atlantic setting will be discussed.

Journal of Physical Oceanography, 2001

Horizontal momentum flux in a global ocean climate model is formulated as an anisotropic viscosity with two spatially varying coefficients. This friction can be made purely dissipative, does not produce unphysical torques, and satisfies the symmetry conditions required of the Reynolds stress tensor. The two primary design criteria are to have viscosity at values appropriate for the parameterization of missing mesoscale eddies wherever possible and to use other values only where required by the numerics. These other viscosities control numerical noise from advection and generate western boundary currents that are wide enough to be resolved by the coarse grid of the model. Noise on the model gridscale is tolerated provided its amplitude is less than about 0.05 cm s Ϫ1. Parameter tuning is minimized by applying physical and numerical principles. The potential value of this line of model development is demonstrated by comparison with equatorial ocean observations. In particular, the goal of producing model equatorial ocean currents comparable to observations was achieved in the Pacific Ocean. The Equatorial Undercurrent reaches a maximum magnitude of nearly 100 cm s Ϫ1 in the annual mean. Also, the spatial distribution of near-surface currents compares favorably with observations from the Global Drifter Program. The exceptions are off the equator; in the model the North Equatorial Countercurrent is improved, but still too weak, and the northward flow along the coast of South America may be too shallow. Equatorial Pacific upwelling has a realistic pattern and its magnitude is of the same order as diagnostic model estimates. The necessary ingredients to achieve these results are wind forcing based on satellite scatterometry, a background vertical viscosity no greater than about 1 cm 2 s Ϫ1 , and a mesoscale eddy viscosity of order 1000 m 2 s Ϫ1 acting on meridional shear of zonal momentum. Model resolution is not critical, provided these three elements remain unaltered. Thus, if the scatterometer winds are accurate, the model results are consistent with observational estimates of these two coefficients. These winds have larger westward stress than NCEP reanalysis winds, produce a 14% stronger EUC, more upwelling, but a weaker westward surface flow. In the Indian Ocean the seasonal cycle of equatorial currents does not appear to be overly attenuated by the horizontal viscosity, with differences from observations attributable to interannual variability. However, in the Atlantic, the numerics still require too large a meridional viscosity over too much of the basin, and a zonal resolution approaching 1Њ may be necessary to match observations. Because of this viscosity, increasing the background vertical viscosity slowed the westward surface current; opposite to the response in the Pacific.

Meteorology and Atmospheric Physics, 2003

A three-dimensional Ocean General Circulation Model has been developed in stretched coordinate from scratch. The same model has been used to perform some numerical experiments to simulate the basic circulation pattern and the model variability to atmospheric forcing. For numerical simulations 72 Â 25 grid points in the horizontal directions and nine (10, 30, 75, 250, 500, 1000, 1500, 2000 and 3000 m) vertical levels are considered. The lateral boundaries are set at 60 N and 60 S. The basic focus of the paper is on the demonstration of the performance of the model and its assessment by employing appropriate forcing from the outputs of an atmospheric general circulation model. Hence, the model was forced with the forcing (wind and thermodynamic) derived from the ECMWF runs from the AMIP archives. The preliminary results show the realistic simulation of basic pattern of different fields. The model simulations show that the model is able to reproduce some of the general features of the ocean, such as surface currents, surface temperature and salinity, mass transport and meridional heat transport. It is also to be noted that the model is capable to capture the El-Niñ no and La-Niñ na type events.

Journal of Physical Oceanography, 2015

We perform eddy-resolving and high vertical resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of the upper-ocean currents. Two main simulations are discussed: solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform, and solution 2, with realistic coastlines and an annual cycle of wind forcing varying zonally. Somewhat surprisingly, solution 1 produces the more realistic DEC; the large, vertical-scale currents [equatorial intermediate currents (EICs)] are found over a large zonal portion of the basin, and the small, vertical-scale equatorial currents [equatorial deep jets (EDJs)] form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. This study demonstrates that both types of currents arise from the rectification of DEIV, consistent with previous theories. The authors also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, this study finds that the main reason for this weaker DEIV is the use of realistic coastlines in solution 2. It remains to be determined what needs to be modified or included to obtain a realistic DEC in the more realistic configuration.

Marine Geodesy, 2007

Journal of The Geological Society of India, 2010

In this article, the authors examine Sea surface temperature (SST), Sea surface circulation (SSC) and Vertical velocity (VV) fields from simulation of 25 layers coarse resolution Modular ocean model (MOM version 3.0) with prescribed wind forcing for the region 74.25°S to 65°N, 180°W-180°E. It is found that distribution of SST simulated by the model shows its consistency with the observed climatology. However, simulated SST in the areas of Arabian Sea, Bay of Bengal, Indonesian Throughflow (ITF) region and east of North America near equator exhibit slight warming with respect to observation, which may be due to model deficiency and forcing problems. Circulation features suggest that one of the strongest current viz. Antarctic circumpolar current (ACC) along with other major current systems viz. Gulf stream current, North and South Pacific current, Agulhas current, Labrador current, Canary current, etc are captured well by the model. In the Indian Ocean and other ocean basins, current patterns are well captured by the model simulation. Intense upwelling as well as downwelling areas is marked in the horizontal distribution of VV, which is as expected. VV show quasi-stagnant and convergent regions suggesting that floating materials may be accumulated during January/July in the real ocean and wind driven circulation may act as an important contribution for such transport of floating materials in these regions. An attempt has also been made to understand the fluctuations of the SST in NINO 3.4 region during the period of model simulation using SST anomalies.

Journal of Physical Oceanography, 1992

A high-resolution, multilevel, primitive equation model is initialized with climatological data to investigate the combined effects of wind and thermal forcing on the ocean circulation off Western Australia during the austral fall and winter, corresponding to the period of strongest flow for the anomalous Leeuwin Current. This process-oriented study builds on previous modeling studies, which have elucidated the role of thermal forcing in the generation of the Leeuwin Current and eddies, by including the additional effects of wind forcing for the eastern boundary current region off' Western Australia. The ocean circulation is generated by the model using a combination of density forcing from the climatological Indian Ocean thermal structure, the influx of warm low-salinity waters from the North West (NW) Shelf, and the climatological wind stress. In the first experiment (case I), forcing by the Indian Ocean and wind stress are imposed, while in the second experiment (case 2), the additional effects of the North West (NW) Shelf waters are considered. In the absence of the NW Shelf waters (case I), geostrophic flow, driven by the Indian Ocean thermal gradient, dominates the wind forcing at the poleward end of the domain and establishes an equatorward undercurrent and a poleward surface current (the Leeuwin Current), which accelerates poleward into the prevailing wind. Wind-forcing effects are discernible only offshore at the equatorward end of the region. The inclusion of NW Shelf waters (case 2) completely dominates the wind forcing at the equatorward end of the model. The effects of the NW Shelf waters weaken away from the source region but they continue to augment the Indian Ocean forcing, resulting in a stronger flow along the entire coastal boundary. The ocean circulation also has significant mesoscale variability. In the first experiment, both the Indian Ocean thermal structure and wind forcing lead to the dominance of barotropic (horizontal shear) instability over baroclinic (vertical shear) instability. In the second ~xperiment, the NW Shelf waters add baroclinicity, which weakens poleward, to the Leeuwin Current and locally increase the barotropic instability near their source. Away from the source waters, where there is a mixed instability, the combined effect of the Indian Ocean thermal structure and wind forcing is stronger than the NW Shelf waters and leads to a dominance of barotropic over baroclinic instability. Several scales of eddies are found to be dominant. The forcing by the Indian Ocean and wind stress (case I) leads to an eddy wavelength of-330 km. With the inclusion of the NW Shelf waters (case 2), the wavelengths associated with mesoscale variability are-150 and 330 km, consistent with observed. eddy length scales.

Journal of Geophysical Research, 1986

A linear, viscid, continuously stratified model is used to study the response of the equatorial ocean to forcing by a wind patch moving zonally at the velocity U. For simplicity, solutions are found in an unbounded basin, and transient effects are ignored. This problem is mathematically equivalent to one in which the ocean has a uniform background current -U and the forcing is stationary, and it is possible to interpret solutions from either point of view. An important result is that resonant and near-resonant Rossby and Kelvin waves can be preferentially excited over other equatorially trapped waves forced by the wind. For two of the solutions, parameters are set as realistically as the model allows, and the solutions are compared with observations. One of them is forced by an eastward moving wind field like the westerly wind anomaly associated with the 1982-1983 E1 Nifio event. It develops subsurface, anomalously westward flow that is strong enough to eliminate the normal Equatorial Undercurrent. The other is forced by a stationary wind in the presence of a weak, westward background current. It has a current structure in the deep ocean that resembles observations of deep equatorial jets, but the surface currents are not realistic. Near-resonant Kelvin waves contribute significantly to both solutions and are responsible for their interesting properties.

Journal of Geophysical Research, 1999

Satellite wind and wind stress fields at the sea surface, derived from the scatterometers on European Remote Sensing satellites 1 and 2 (ERS-1 and ERS-2) are used to drive the ocean general circulation model (OGCM) "OPA" in the tropical oceans. The results of the impact of ERS winds are discussed in terms of the resulting thermocline, current structures, and sea level anomalies. Their adequacy is evaluated on the one hand by comparison with simulations forced by the Arpege-Climat model and on the other hand by comparison with measurements of the Tropical Atmosphere-Ocean (TAO) buoy network and of the TOPEX/Poseidon altimeter. Regarding annual mean values, the thermal and current responses of the OGCM forced by ERS winds are in good agreement with the TAO buoy observations, especially in the central and eastern Pacific Ocean. In these regions the South Equatorial Current, the Equatorial Undercurrent, and the thermocline features simulated by the OGCM forced by scatterometer wind fields are described. The impact of the ERS-1 winds is particularly significant to the description of the main oceanic variability. Compared to the TAO buoy observations, the highfrequency (a few weeks) and the low-frequency of the thermocline and zonal current, variations are described. The correlation coefficients between the time series of the thermocline simulated by ERS winds and that observed by the TAO buoy network are highly significant; their mean value is 0.73, over the whole basin width, while it is 0.58 between Arpege model simulation and buoy observations. At the equa. tor the time series of the zonal current simulated by the ERS winds, at three locations (110øW, 140øW, and 165øE) and at two depths, are compared to the TAO current meter and acoustic Doppler current profiler (ADCP) measurements. The mean value of the significant correla, tio• coefficients computed with the in situ measurements is 0.72 for ERS, while it is 0.51 for the Arpege-Climat model. Thus ERS wind fields through the OGCM generate more realistic current variations than those obtained with Arpege climate winds, and they are particularly efficient in capturing abrupt cha, nges ("wind bursts") which ma,y be important regarding ocean dynamics. 1. Introduction To increase our knowledge of oceanic general circulation model (OGCM) mechanisms, it is essential, first,

Reviews of Geophysics, 1975

succeeded in modeling seasonal variations, in particular, those of the tropical circulation [Manabe et aL, 1974]. Wetheraid and Manabe [1972] investigated the response of such a model to the seasonal variation of solar radiation, and Houghton et al. [1974], its response to sea surface temperature perturbations. Faegre [1972] and Sellers [1973] have specifically addressed the heat exchange between ocean and atmosphere within these models. Dwyer and Petersen [1973] have addressed the energy transfer through large-scale motion. A statistical dynamic model capable of simulating the seasonal variations in the tropics has been developed by Pike [1972]. The response of the ocean to large-scale surface heat and momentum flux has been investigated by Haney [1974]. Spar [1973a, b] has modeled the effect of ocean surface anomalies on atmospheric circulation, and Haney [1971] has discussed surface thermal boundary conditions of ocean circulation models coupled to the atmosphere. A numerical simulation of the influence of ice conditions on climate was made by Fletcher et aL [1972]. Robinson [1971] presented a general review of climatic models, and Lorenz [1970, 1973] discussed climatic changes as a mathematical problem and their predictability. In this article I will attempt to review some features of the general ocean circulation as they have come into print over the past 4 years. While preparing for this article I became more keenly aware of the truth of the expression 'one man's signal is another man's noise.' There is a clear trend in oceanic studies, not restricted to the past 4 years, to recognize the enormous variability found in the ocean. An important part of this variability, associated with the so-called mesoscale eddies, is discussed in this review series by Robinson. But the 'general ocean circulation,' as

Discrete & Continuous Dynamical Systems - A, 2016

The large-scale, near-surface flow of the mid-latitude oceans is dominated by the presence of a larger, anticyclonic and a smaller, cyclonic gyre. The two gyres share the eastward extension of western boundary currents, such as the Gulf Stream or Kuroshio, and are induced by the shear in the winds that cross the respective ocean basins. This physical phenomenology is described mathematically by a hierarchy of systems of nonlinear partial differential equations (PDEs). We study the low-frequency variability of this wind-driven, double-gyre circulation in mid-latitude ocean basins, subject to time-constant, purely periodic and more general forms of time-dependent wind stress. Both analytical and numerical methods of dynamical systems theory are applied to the PDE systems of interest. Recent work has focused on the application of non-autonomous and random forcing to double-gyre models. We discuss the associated pullback and random attractors and the non-uniqueness of the invariant measures that are obtained. The presentation moves from observations of the geophysical phenomena to modeling them and on to a proper mathematical understanding of the models thus obtained. Connections are made with the highly topical issues of climate change and climate sensitivity.

A comparison between hydrographic observations and output from two realistically forced z-level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12-0.40) and too warm in POCM_4C (by >2°C) suggesting problems with the airsea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C Ocean Modelling 9 (2005) 105-132 www.elsevier.com/locate/ocemod although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.

1994

Journal of Physical Oceanography

This paper examines the factors determining the distribution, length scale, magnitude and structure of mesoscale oceanic eddies in an eddy-resolving primitive equation simulation of the Southern Ocean (MESO). In particular, we investigate the hypothesis that the primary source of mesoscale eddies is baroclinic instability acting locally on the mean state. Using local mean vertical profiles of shear and stratification from the MESO simulation, we integrate the forced-dissipated quasi-geostrophic equations in a doubly periodic domain at various locations. We also perform a linear stability analysis of the profiles. The scales, energy levels and structure of the eddies found in the MESO simulation are compared to those predicted by the linear analysis, as well as to the eddying structure of the quasi-geostrophic simulations,. This allows us to quantitatively estimate the role of local non-linear effects and cascade phenomena in the generation of the eddy field. We find that typically t...