Abstract
A numerical model of the Atlantic Ocean was used to study the low-frequency variability of meridional transports in the North Atlantic. The model shows a behaviour similar to those used in previous studies, and the temporal variability of certain variables compares favourably to observed time series. By changing the depth and width of the sills between the subpolar North Atlantic and the Nordic Seas, the mean horizontal and overturning circulation as well as some water mass properties are modified significantly. The reaction of meridional oceanic transports to atmospheric forcing fluctuations remains, however, unchanged. The critical role of the surface heat flux retroaction term for the meridional heat transport in stand-alone ocean models is discussed. The experiments underline the role of atmospheric variability for fluctuations of the large-scale ocean circulation on time scales from years to decades, and they support the hypothesis that the mean overturning strength is controlled by the model representation of the density of the overflow water masses.
Highlights
The stability and variability of the meridional overturning circulation (MOC) in the North Atlantic are of interest in climate research for at least two reasons: first, the subpolar North Atlantic and the adjacent Arctic Ocean are important elements in the global conveyor belt circulation because here the only locations of deep-water formation in the Northern Hemisphere can be found
Several recent studies have shown that much of the North Atlantic variability during the past decades is related to the North Atlantic Oscillation (NAO)
Our experiments suggest that two modes of deep circulation might exist in the subpolar North Atlantic and that the relative strength of the overflows to west and east of Iceland controls in which state the circulation is
Summary
The stability and variability of the meridional overturning circulation (MOC) in the North Atlantic are of interest in climate research for at least two reasons: first, the subpolar North Atlantic and the adjacent Arctic Ocean are important elements in the global conveyor belt circulation because here the only locations of deep-water formation in the Northern Hemisphere can be found. The objective of the present study is to investigate the robustness of the simulated response of the MOC in the North Atlantic to atmospheric forcing fluctuations as observed during the past decades under different circulation regimes characterized by different representations of the northern overflows To this end, the model bathymetry in the region of the Greenland–Iceland– Scotland ridge is modified to allow for stronger overflows of dense water. The model bathymetry in the region of the Greenland–Iceland– Scotland ridge is modified to allow for stronger overflows of dense water These sensitivity studies are similar to the experiments by Roberts and Wood (1997), but the emphasis of the analysis is more on the variability of the large-scale circulation and meridional transports and on a possible modification of the atmospheric influence on the ocean. The overflows of dense water being rather weak in this experiment, we slightly deepened the Denmark Strait and introduced a deep wide connection from the Norwegian Sea to the North Atlantic
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