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This paper summarizes the formulation of the ocean component to the Geophysical Fluid Dynamics Laboratory's (GFDL) coupled climate model used for the 4th IPCC Assessment (AR4) of global climate change. In particular, it reviews elements of ocean climate models and how they are pieced together for use in a state-of-the-art coupled model. Novel issues are also highlighted, with particular attention given to sensitivity of the coupled simulation to physical parameterizations and numerical methods. Features of the model described here include the following: (1) tripolar grid to resolve the Arctic Ocean without polar filtering, (2) partial bottom step representation of topography to better represent topographically influenced advective and wave processes, (3) more accurate equation of state, (4) three-dimensional flux limited tracer advection to reduce overshoots and undershoots, (5) incorporation of regional climatological variability in shortwave penetration, (6) neutral physics parameterization for representation of the pathways of tracer transport, (7) staggered time stepping for tracer conservation and numerical efficiency, (8) anisotropic horizontal viscosities for representation of equatorial currents, (9) parameterization of exchange with marginal seas, (10) incorporation of a free surface that accomodates a dynamic ice model and wave propagation, (11) transport of water across the ocean free surface to eliminate unphysical "virtual tracer flux" methods, (12) parameterization of tidal mixing on continental shelves. [ABSTRACT FROM AUTHOR]

In order to investigate the causes of sea ice volume variations, two simulations have been performed using two different sea ice–ocean models driven by NCEP–NCAR reanalysis data over the period 1955–97. In both models, the largest changes in ice thickness occur in the central Arctic. The anomalous exchanges of sea ice, air, and water between the North Atlantic and the Arctic play a large role in explaining ice volume variations. In particular, the anomaly of oceanic heat transport between Iceland and Norway could alone explain a large part of the simulated ice volume changes. The time evolution of the ice volume appears well correlated with the low-frequency variations of the North Atlantic Oscillation (NAO). Nevertheless, the link between NAO and ice volume changes could operate through different mechanisms between the first and second half of the simulation. The influence of the NAO on the meridional heat transport in the North Atlantic and in the Nordic Seas hold during the whole simulation. Besides, the link between NAO and Fram Strait ice export, which is another important source of ice volume variability, is valid only for the second half of the simulation. [ABSTRACT FROM AUTHOR]

We investigate the dependence of surface fresh water fluxes in the Gulf Stream and North Atlantic Current (NAC) area on the position of the stream axis which is not well represented in most ocean models. To correct this shortcoming, strong unrealistic surface fresh water fluxes have to be applied that lead to an incorrect salt balance of the current system. The unrealistic surface fluxes required by the oceanic component may force flux adjustments and may cause fictitious long-term variability in coupled climate models. To identify the important points in the correct representation of the salt balance of the Gulf Stream a regional model of the northwestern part of the subtropical gyre has been set up. Sensitivity studies are made where the westward flow north of the Gulf Stream and its properties are varied. Increasing westward volume transport leads to a southward migration of the Gulf Stream separation point along the American coast. The salinity of the inflow is essential for realistic surface fresh water fluxes and the water mass distribution. The subpolar–subtropical connection is important in two ways: The deep dense flow from the deep water mass formation areas sets up the cyclonic circulation cell north of the Gulf Stream. The surface and mid depth flow of fresh water collected at high northern latitudes is mixed into the Gulf Stream and compensates for the net evaporation at the surface. [ABSTRACT FROM AUTHOR]