The horizontal circulation in ocean gyres contributes to the meridional transports of heat and fresh water in the climate system (see Section 7.6.2), and therefore is of immediate relevance for climate change. Much of that transport occurs through Western Boundary Currents (WBCs) such as the Gulf Stream, Kuroshio, Agulhas Current and others which are prominent elements of the ocean circulation. These currents are mainly driven by the wind, and have a typical width of 50 km. Once a WBC has left the continental slope, it is characterised by recirculation regimes and strong mesoscale variability. The dynamical influence of the WBCs on the less vigorous interior circulation is not fully understood. Their pathways are, however, crucial in determining the location of the sub-polar front which separates the warm waters of the sub-tropics from the colder sub-polar waters. The lower branch of the THC is also dominated by deep WBCs. In the Atlantic, these are concentrated within 30 km of the continental slope, and are accompanied by substantial recirculation and variability (Lee et al., 1996).
The width and strength of boundary currents in climate models are very sensitive to resolution, they become stronger and narrower as the resolution is increased, provided that at the same time the sub-grid scale transports are also reduced. For example, a recent simulation for the North Atlantic using a grid resolution of 1/10° (equivalent to 11 km or better) indicates that features such as the width, location and variability of boundary currents, the eddy field and its statistics, as well as regional current features are in rather good agreement with observations (Smith et al., 2000). On the other hand, in non-eddy-resolving climate models, boundary currents are quite unrealistic, they lack the observed sharp fronts and recirculation regimes and hence miss the associated air-sea heat fluxes and their dynamical influence on mid-ocean circulation. The effect of having weaker and wider boundary currents for the model's climate and climate sensitivity has so far not been systematically evaluated.
In the Atlantic Ocean, the THC is responsible for the relatively mild climate in Western Europe (see Section 7.6.1). While palaeo-oceanographic analyses suggest that the Atlantic THC has been relatively stable for the last 8,000 years, a series of large and rapid climatic changes during the last Ice Age has been reconstructed from numerous palaeoclimatic archives (see Chapter 2, Section 2.4). Based on the presently available evidence, these changes are best explained by major reorganisations involving the THC (Broecker, 1997; Stocker and Marchal, 2000). Changes of the THC, due to natural variability or slowly changing surface forcing, thus have an important effect on the climate on a regional to hemispheric scale, and numerous model studies since the SAR have investigated potential changes in THC and elucidated the underlying mechanisms and their impact on the climate system.
Generally, in high latitudes the ocean loses heat and gains fresh water (precipitation and continental runoff) which has opposite effects on the density of ocean water. In addition, the density of sea water is influenced by the supply of warm and salty water from the low latitudes which constitutes the positive feedback maintaining the Atlantic THC (Figure 7.4). This balance is influenced by the surface fluxes of heat and fresh water, i.e., precipitation, evaporation, continental runoff and sea-ice formation, all processes that are likely to change in the future. Some models also suggest an influence of Southern Ocean wind on the Atlantic THC (Toggweiler and Samuels, 1995).
The response of the THC to a perturbation depends on the relative strength of further feedbacks. Reduced convection and advective heat transport into a region lead to colder SSTs which counteracts the effect of salinity on density and thus limits the strength of the destabilising oceanic feedbacks (Figure 7.4). The atmospheric heat transport compensates for parts of the changes in ocean heat transport and is a key factor in determining the stability of the THC (Zhang et al., 1993; Mikolajewicz and Maier-Reimer, 1994). Atmospheric moisture transport among basins provides another feedback (Schiller et al., 1997). Model results suggest that in the presence of overflow between Greenland and Iceland the THC is not very sensitive to changes in the atmosphere-ocean fluxes (Lohmann and Gerdes, 1998).
It is likely that sea ice also has an effect on the stability of the THC:
decreased THC and hence oceanic heat transport leads to more sea ice formation
(Schiller et al., 1997). Formation of sea-ice tends to increase the density
of sea water both through brine rejection and cooling of the overlying air via
increased surface albedo and enhanced insulation. Increased export of sea ice
from these areas represents a significant fresh water transport over long distances
which can decrease deep water formation, thus representing a positive feedback.
A reliable estimate of the net effect of sea ice on the stability of the THC
is hampered by the still crude representation of sea ice in most current climate
models and the resulting unrealistic simulation of sea-ice distributions. A
negative feedback contributing to a stabilisation of the THC was recently proposed
by Latif et al. (2000). They suggested that during El Niño conditions
fresh water export from the tropical Atlantic via the atmosphere is enhanced;
this tendency is also found in observations (Schmittner et al., 2000). This
implies an increased supply of saltier waters towards the northern North Atlantic
which facilitates deep water formation, hence a stabilising process for the
THC; the opposite effect occurs for La Niña conditions.
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