Energizing the Ocean's Large-Scale Circulation for Climate Change
J. R. Toggweiler and B. Samuel (GFDL/NOAA, Princeton University, P.O. Box 308, Princeton, NJ 08542)
The most frequently invoked mechanism for abrupt climate change is a disruption of the ocean's thermohaline circulation. A weakened and/or shallower thermohaline circulation carrying less heat into the North Atlantic helps explain sharply lower temperatures and expanded sea ice in the glacial North Atlantic. However, turning on and turning off the thermohaline circulation in the North Atlantic does not explain observed changes far afield from the North Atlantic. We would argue that a thermohaline model does not work very well outside the North Atlantic because it does not provide a very good description of the large-scale circulation in the modern ocean.
In this paper, a new description of the large-scale circulation is offered in which the ocean's meridional overturning is characterized mainly by its links to the Antarctic Circumpolar Current (ACC) and the gap between South America and Antarctica (Drake Passage). We illustrate this effect with an idealized ocean GCM which is coupled to a simple energy-balance atmosphere with imposed zonal winds. The model describes a nearly water covered planet with two Antarctica-like polar islands at each pole and a long thin continent which extends from one polar island to the other. Sections of the thin continent can be removed to illustrate the effect of gaps between continents in the real world. All atmospheric and oceanic temperatures in the water-planet model are free to vary according to the atmospheric radiation balance and ocean heat transport. The model has no salinity forcing, no sea ice, and it does not have a separate Atlantic basin.
Simply putting in a gap like Drake Passage near the south polar island produces a strikingly modern looking circulation in the water-planet model. A Drake Passage-like gap produces an interhemispheric "conveyor" circulation which warms the entire northern hemisphere by several degrees over the base case without a gap. It also produces a realistic interhemispheric circulation of bottom water which is not present in the base case. The gap generates a thermal structure in the ocean's interior which is very similar to the Levitus climatology. Importantly, the energy which propels this circulation comes into the system because of the gap itself, not because of vertical mixing.
An observer viewing the ocean's circulation through thermohaline lenses inevitably sees the glacial ocean as being less energetic than the modern ocean. From the perspective of the water-planet model, the relative energy states come out the other way around. It is the modern ocean with its strong deep-water formation in the North Atlantic which sits closest to a state of minimum energy. Lower northern densities and reduced sinking should raise the ocean's overall energy level and intensify the large-scale circulation everywhere else. A system close to its minimum energy state should be stable against changes in forcing; a system at a higher energy level is more susceptible to abrupt change.