On the Computation of the Barotropic Mode of a Free-surface World Ocean Model

The free-surface formulation of the equations of our world ocean model is briefly described. The barotropic mode equations are solved according to the split-explicit method, using different time steps for the external and internal modes. Because the numerical algorithm is implemented on the B-grid, a spurious, free-surface, two-grid interval mode may develop. This mode must be filtered out. The properties of two filters are theoretically investigated and their actual performance is tested in a series of numerical experiments. It is seen that one of these filters may severely perturb the local mass conservation, rendering it impossible to enforce the impermeability of the surface or the bottom of the ocean. The dynamics of the external mode is also examined, by studying the depth-integrated momentum equations. The depth-integral of the pressure force due to the slope of the ocean surface is approximately balanced by the depth-integral of the force ensuing from the horizontal variations of the density. The depth-integral of the Coriolis force is an order of magnitude smaller, except in the Southern Ocean. Two variational principles are resorted to for computing the fictitious ocean surface elevation corresponding to the approximate equilibrium between the dominant forces of the barotropic momentum equations

The effects of the water flow through the Canadian Archipelago in a global ice-ocean model

Numerical experiments are conducted with a global ice-ocean model in order to evaluate the influence of the water flow from the Arctic Ocean to Baffin Bay through the Canadian Archipelago on the water-mass properties of the Arctic Ocean and adjacent seas and, more generally, on the global ocean circulation. The results indicate that this flow plays a significant role in controlling the freshwater budget of the Arctic Ocean. When the Canadian Archipelago passage is open in the model, the Arctic pycnocline experiences a noticeable increase in salinity. Furthermore, the flow of relatively fresh Arctic waters through the passage yields a pronounced decrease of surface salinity and density in the Labrador Sea, which leads to a diminution of convective activity there. As a result, the North Atlantic Deep Water outflow in the model is reduced by about 5%. Deep convection in the Norwegian Sea exhibits almost no change, and this despite a weakening of the inflow of relatively fresh Arctic waters through Fram Strait

The 8.2 kyr BP event simulated by a global atmosphere-sea-ice-ocean model

Seven freshwater perturbation experiments were performed with a global atmosphere-sea-ice-ocean model to study the mechanism behind the 8.2 kyr BP Holocene cooling event. These experiments differed in initial state and duration of the applied freshwater pulse, while the amount of freshwater was kept constant (4.67x10(14) m(3)). One of the scenarios, with freshwater added to the Labrador Sea at a rate of 0.75 Sv during 20 years, resulted in weakening of the North Atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5 degreesC over adjacent continents. This result is consistent with proxy data, suggesting that a meltwater-induced weakening of the thermohaline circulation caused the event. Moreover, our results indicate that the time-scale of the meltwater release and the initial state are important, as both have a strong effect on the magnitude and duration of the produced model response

Sensitivity of a global ice-ocean model to the Bering Strait throughflow

To understand the influence of the Bering Strait on the World Ocean's circulation, a model sensitivity analysis is conducted. The numerical experiments are carried out with a global, coupled ice-ocean model. The water transport through the Bering Strait is parametrized according to the geostrophic control theory. The model is driven by surface fluxes derived from bulk formulae assuming a prescribed atmospheric seasonal cycle. In addition, a weak restoring to observed surface salinities is applied to compensate for the global imbalance of the imposed surface freshwater fluxes. The freshwater flux from the North Pacific to the North Atlantic associated with the Bering Strait throughflow seems to be an important element in the freshwater budget of the Greenland and Norwegian seas and of the Atlantic. This flux induces a freshening of the North Atlantic surface waters, which reduces the convective activity and leads to a noticeable (6%) weakening of the thermohaline conveyor belt. It is argued that the contrasting results obtained by Reason and Power are due to the type of surface boundary conditions they used

Multiple sea-ice states and abrupt MOC transitions in a general circulation ocean model

Sea ice has been suggested, based on simple models, to play an important role in past glacial–interglacial oscillations via the so-called “sea-ice switch” mechanism. An important requirement for this mechanism is that multiple sea-ice extents exist under the same land ice configuration. This hypothesis of multiple sea-ice extents is tested with a state-of-the-art ocean general circulation model coupled to an atmospheric energy–moisture-balance model. The model includes a dynamic-thermodynamic sea-ice module, has a realistic ocean configuration and bathymetry, and is forced by annual mean forcing. Several runs with two different land ice distributions represent present-day and cold-climate conditions. In each case the ocean model is initiated with both ice-free and fully ice-covered states. We find that the present-day runs converge approximately to the same sea-ice state for the northern hemisphere while for the southern hemisphere a difference in sea-ice extent of about three degrees in latitude between the different runs is observed. The cold climate runs lead to meridional sea-ice extents that are different by up to four degrees in latitude in both hemispheres. While approaching the final states, the model exhibits abrupt transitions from extended sea-ice states and weak meridional overturning circulation, to less extended sea ice and stronger meridional overturning circulation, and vice versa. These transitions are linked to temperature changes in the North Atlantic high-latitude deep water. Such abrupt changes may be associated with Dansgaard–Oeschger events, as proposed by previous studies. Although multiple sea ice states have been observed, the difference between these states is not large enough to provide a strong support for the sea-ice-switch mechanism

Sea Ice Modelling

Peer reviewe

The ocean’s role in setting the mean position of the Inter-Tropical Convergence Zone

Through study of observations and coupled climate simulations, it is argued that the mean position of the Inter-Tropical Convergence Zone (ITCZ) north of the equator is a consequence of a northwards heat transport across the equator by ocean circulation. Observations suggest that the hemispheric net radiative forcing of climate at the top of the atmosphere is almost perfectly symmetric about the equator, and so the total (atmosphere plus ocean) heat transport across the equator is small (order 0.2 PW northwards). Due to the Atlantic ocean’s meridional overturning circulation, however, the ocean carries significantly more heat northwards across the equator (order 0.4 PW) than does the coupled system. There are two primary consequences. First, atmospheric heat transport is southwards across the equator to compensate (0.2 PW southwards), resulting in the ITCZ being displaced north of the equator. Second, the atmosphere, and indeed the ocean, is slightly warmer (by perhaps 2 °C) in the northern hemisphere than in the southern hemisphere. This leads to the northern hemisphere emitting slightly more outgoing longwave radiation than the southern hemisphere by virtue of its relative warmth, supporting the small northward heat transport by the coupled system across the equator. To conclude, the coupled nature of the problem is illustrated through study of atmosphere–ocean–ice simulations in the idealized setting of an aquaplanet, resolving the key processes at work