North-South Shifts of the Gulf Stream: Ocean-Atmosphere Interactions in the North Atlantic

Year-to-year changes in the latitude of the north wall of the Gulf Stream are very similar to those seen in the abundances of zooplankton observed by the Continuous Plankton Recorder Survey around the British Isles and also to those in the abundance of zooplankton in Lake Windermere. These connections must reflect changing weather patterns across the North Atlantic. The index of Gulf Stream position was constructed from the north wall data by principal components analysis. The first principal component, the index used, has eigenvector coefficients that all have the same sign, and is a measure of the latitude of the whole of the north wall. However, the component may represent the occurrences of meanders that are extensive in space and time rather than displacements of the Gulf Stream as a whole. This principal component has been used to calculate weighted averages of monthly mean sea-level pressure and of monthly mean numbers of cyclone tracks in order to show the changes in weather patterns associated with displacements of the north wall. Northward displacements of the north wall were accompanied by significantly reduced cyclone numbers in the northernmost regions of the Atlantic (annually and in the autumn) and, in spring, summer, and autumn, a region of reduced atmospheric pressure in the central Atlantic area 40°–60°N, 30°–50°W (locally significant). There was some tendency (not significant) for storm tracks to be deflected around the south side of this region. The pattern in winter is less clear and shows no statistical significance. Changes in the vicinity of the British Isles were generally too small to be statistically significant but were generally consistent with a lower frequency of storms in spring and autumn. As the biological changes appear to be caused by variations in the onset of thermal stratification during the spring they may be the result of relatively small changes in the atmospheric forcing. The atmospheric changes show no indications of the sources of the Gulf Stream displacements, the anomaly winds opposing the displacements. This may be because meanders of the Gulf Stream are not simply related to any single atmospheric variable. The clearest and most statistically significant meteorological signals were all well downstream from the north wall. Although the displacements of the north wall are caused by changing weather patterns over the North Atlantic, the Gulf Stream is also a region of strong heat transfers from the ocean to the atmosphere. Sawyer's criterion indicates that distortions of this heat source could cause noticeable disturbances to the atmospheric circulation over the North Atlantic. A numerical model based on the analytical model of Smagorinsky is used to investigate the perturbations of the zonal circulation that might be caused by displacements of this local heat source. The predictions are in agreement with the changes seen in the central Atlantic during summer, spring, and autumn (but not those during winter). In the region where the model predicts atmospheric pressure reductions should occur, there are no positive correlation coefficients between the position of the north wall and the surface atmospheric pressure but a significant excess of negative correlation coefficients compared with chance, and northward shifts of the Gulf Stream were accompanied by significant reductions in atmospheric pressure. It is therefore possible that displacements of the north wall could influence weather patterns further east. The model predicts that any changes over the European continental shelf will be weak. An accurate description of the dynamics of the Gulf Stream may be an important requirement of coupled ocean–atmosphere models.