Two stable equilibria have been obtained from a global model of the coupled ocean-atmosphere system developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The model used for this study consists of general circulation models of the atmosphere and the world oceans and a simple model of land surface. Starting from two different initial conditions, “asynchronous” time integrations of the coupled model, under identical boundary conditions, lead to two stable equilibria. In one equilibrium, the North Atlantic Oman has a vigorous thermohaline circulation and relatively saline and warm surface water. In the other equilibrium, there is no thermohaline circulation, and an intense halocline exists in the surface layer at high latitudes. In both integration the, air-sea exchange of water is adjusted to remove a systematic bias of the model that surpresses the thermohaline circulation in the North Atlantic. Nevertheless these results raise the intriguing possibility that the coupled system may have a...

Two Stable Equilibria of a Coupled Ocean-Atmosphere Model

The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high-quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments show that deep ocean circulation patterns were significantly different during the most recent glacial maximum. In the western North Atlantic Ocean, the nutrient content of waters from 2500 to 3500 m was twice as high during glacials as during interglacial periods, signifying an increase in the proportion of waters of Antarctic origin in this depth interval and a decrease (but not a cessation) in the flux of North Atlantic Deep Waters through this depth range. Below 3500 m, the nutrient increase was greater, indicating a higher proportion of Antarctic Bottom Water. Above 2500 m, the nutrient content was lower during glacial periods than it has been during interglacial times. The Cd content of high-latitude North Atlantic and southern ocean surface waters does not show a significant glacial/interglacial change, which argues against theories for changes atmospheric carbon dioxide that require changes in high-latitude nutrient concentrations.

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Cadmium: Chemical tracer of deepwater paleoceanography

From oceanographic measurements of currents and temperature used in a direct calculation, ocean heat transport at 25°N in the Atlantic Ocean was estimated to be 1.1 × 1015W. The method used is discussed and the estimate revised to 1.2 × 1015W by using more complete data. An analysis indicates that the results have an error of at most 0.3 × 1015W and that contributions due to the eddy heat flux and seasonal variation are relatively small. The variation of mean meridional volume transport with depth at 25°N has been computed; the profile leads to the estimate of transport in various temperature classes and to some conclusions concerning the mechanisms of heat flux. The freshwater flux at 25°N has been computed; within error it is zero, a result that agrees with the literature. One direct and two indirect methods of calculating oceanic heat fluxes are compared. Indirect computations of heat flux based on air-sea interaction data provide good global coverage. Computations using satellite data are subject to large errors, but improved technology could make satellites an excellent means for monitoring global radiation. The direct method appears to be the most accurate: various independent, oceanographic estimates agree well with results from the air-sea exchanges. Ocean heat transport can be determined simply and accurately given appropriate measurements.

Direct estimates and mechanisms of ocean heat transport

A brief discussion of, and a little speculation about, the relevance of the polar regions on climate is given. The main body of the paper gives a survey of the known deep convection areas of the world ocean. There are two distinct types of convection. The first is the classic sinking occurring on continental shelf slope systems, as typified by various locations around the Antarctic coast. The freezing of sea ice, and resulting brine ejection, creates dense salty water on the shelf which descends the slope under a balance of Coriolis, gravity, and frictional forces, entraining the surrounding warm deep water as it goes. The second process is the more recently observed open-ocean convection, occurring in locations such as the Mediterranean, the Labrador Sea, and two locations in the Weddell gyre, and is hypothesized to occur in the Greenland Sea. Open-ocean convection has many overall similarities in all these areas: it occurs in narrow (20–50 km) areas; it forms about 10 m³ s−l of deep water; it occurs only in regions of cyclonic mean circulation; more than one water mass in the mean circulation is involved; a preconditioning seems to be required; some surface forcing (cooling or sea ice formation) is necessary; a violent breakup of the water mass frequently occurs on time scales of 2 weeks.

Deep convection in the World Ocean

This is a review about the Atlantic Meridional Overturning Circulation (AMOC), its mean structure, temporal variability, controlling mechanisms, and role in the coupled climate system. The AMOC plays a central role in climate through its heat and freshwater transports. Northward ocean heat transport achieved by the AMOC is responsible for the relative warmth of the Northern Hemisphere compared to the Southern Hemisphere and is thought to play a role in setting the mean position of the Intertropical Convergence Zone north of the equator. The AMOC is a key means by which heat anomalies are sequestered into the ocean's interior and thus modulates the trajectory of climate change. Fluctuations in the AMOC have been linked to low-frequency variability of Atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses. On intra-annual timescales, variability in AMOC is large and primarily reflects the response to local wind forcing; meridional coherence of anomalies is limited to that of the wind field. On interannual to decadal timescales, AMOC changes are primarily geostrophic and related to buoyancy anomalies on the western boundary. A pacemaker region for decadal AMOC changes is located in a western “transition zone” along the boundary between the subtropical and subpolar gyres. Decadal AMOC anomalies are communicated meridionally from this region. AMOC observations, as well as the expanded ocean observational network provided by the Argo array and satellite altimetry, are inspiring efforts to develop decadal predictability systems using coupled atmosphere-ocean models initialized by ocean data.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015RG000493

Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review

The heat transported by currents across 25°N in the Atlantic Ocean is estimated from oceanographic measurements to be 1.1 x 1015watts northward. This figure agrees, within estimated error, with the value obtained from charts of energy exchange between ocean and atmosphere but is smaller by a factor of 2 than the recent value derived from satellite radiation measurements.

Mindy M. Hall

Papers

Heat Transport by Currents Across 25°N Latitude in the Atlantic Ocean