Coarse-resolution Global Ocean Model Research Articles

Variability of sea surface height and circulation in the North Atlantic: Forcing mechanisms and linkages

Simulations with a coarse-resolution global ocean model during 1958–2004 are analyzed to understand the inter-annual and decadal variability of the North Atlantic. Analyses of Empirical Orthogonal Functions (EOFs) suggest relationships among basin-scale variations of sea surface height (SSH) and depth-integrated circulation, and the winter North Atlantic Oscillation (NAO) or the East Atlantic Pattern (EAP) indices. The linkages between the atmospheric indices and ocean variables are shown to be related to the different roles played by surface momentum and heat fluxes in driving ocean variability. In the subpolar region, variations of the gyre strength, SSH in the central Labrador Sea and the NAO index are highly correlated. Surface heat flux is important in driving variations of SSH and circulation in the upper ocean and decadal variations of the Atlantic Meridional Overturning Circulation (AMOC). Surface momentum flux drives a significant barotropic component of flow and makes a noticeable contribution to the AMOC. In the subtropical region, momentum flux plays a dominant role in driving variations of the gyre circulation and AMOC; there is a strong correlation between gyre strength and SSH at Bermuda.

The CMCC-INGV Global Ocean Data Assimilation System (CIGODAS)

Centro Euro-Mediterraneo per i Cambiamenti Climatici, Bologna, Italy Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy

Relaxing the Boussinesq Approximation in Ocean Circulation Models

There is a growing need for ocean circulation models that conserve mass rather than volume (as in traditional Boussinesq models). One reason is bottom pressure data expected to flow from satellite-mounted gravity-measuring instruments, and another is to provide a complete interpretation of data from satellite altimeters such as TOPEX/Poseidon. In this paper, it is shown that existing, hydrostatic Boussinesq ocean model codes can easily be modified, with only a modest increase in the CPU requirement, to integrate the hydrostatic, non-Boussinesq equations. The method can be used to integrate both coarse-resolution and eddy-resolving non-Boussinesq models. The basic equations can also be used to formulate a fully nonhydrostatic, non-Boussinesq model. The method is illustrated for the case of the Parallel Ocean Program (POP), the parallel version of the Bryan‐Cox‐Semtner code developed at Los Alamos National Laboratory. A comparison of eddy-permitting model solutions under double-gyre wind forcing shows that the error in making the Boussinesq approximation is, reassuringly, only a few percent. The authors also consider a coarse-resolution global ocean model under seasonal forcing. The nonBoussinesq model shows a seasonal variation in global mean sea surface height (SSH) with a range of about 3 cm, attributable mostly to changes in the mass of the ocean due to the freshwater flux forcing, but with a roughly 25% contribution from the steric expansion effect. The seasonal cycles of model-computed SSH are also compared with TOPEX/Poseidon data from the South Pacific and South Atlantic Oceans. It is shown that the seasonal cycle in global mean SSH contributes to the model-computed seasonal cycle, and improves the model performance compared to the data. It is found that the difference between the seasonal cycles in the Boussinesq and nonBoussinesq models is almost entirely accounted for by the seasonal cycle in global mean SSH. On the other hand, on longer timescales the difference field between the non-Boussinesq and Boussinesq models shows spatial variability of several centimeters that is not accounted for by a globally uniform correction to the Boussinesq model.

Meridional Overturning and Dianeutral Transport in az-Coordinate Ocean Model Including Eddy-Induced Advection

The present study examines the marked changes in the patterns of meridional overturning and dianeutral motion that occur upon introduction of the Gent and McWilliams (GM) scheme for eddy-induced transport into a coarse-resolution global ocean model. Results from two versions of the ocean model are compared. The first version does not have the GM scheme and uses a standard background horizontal diffusivity. The second version includes the GM scheme and has zero horizontal diffusivity. Both versions include a weak vertical diffusivity and isoneutral tracer diffusion as implemented by Cox. First, representations of the meridional overturning circulation computed via integration along (i) level, (ii) potential density, and (iii) neutral density (γ) surfaces are compared. Differences between the level surface representations are similar to those noted in previous studies. Differences between the density surface representations (not previously studied) are major at high densities (γ or σθ > 27.0). The residual Deacon cell of the first version has completely vanished in the GM version. The separate direct Antarctic and deep circulation cells of the first version are fully merged in the GM version. In these respects, the solution of the GM version is broadly more consistent with that of the FRAM eddy-permitting simulation. However, introduction of the GM scheme does not aid long-standing model problems of excessive upwelling of deep water into the thermocline in the Pacific and excessive penetration of Antarctic Bottom Water into the North Atlantic. The marked differences between the versions noted above imply marked differences in the dianeutral transport of fluid. A direct calculation of dianeutral transport shows that this transport is much weaker in the Southern Ocean and in the western boundary currents in the GM run. A breakup of the dianeutral transport into components resulting from the individual model mixing processes shows that the sole factor responsible for these qualitative changes is the absence of dianeutral motion induced by horizontal diffusive fluxes in the GM version. The interior dianeutral transport in this version is characterized by widespread and gentle upward motion induced by vertical diffusive fluxes. The results are shown to be insensitive to details of the surface boundary restoration. Consequences for our understanding of (i) the role of the Deacon cell as a tracer transport mechanism and (ii) the nature of the eddy-induced transports as provided by the GM scheme are discussed. In particular, although the total effective transport in the ocean interior is nearly isoneutral in the GM case, the eddy-induced part of that transport is shown to have a large dianeutral component and so cannot be interpreted as an isoneutral “bolus” transport.

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate.