Diagnostic calculations of the circulation in the North Atlantic are described. Three basic cases are considered: the climatological mean state and the circulation in the pentads 1955–1959 and 1970–1974. Density data from Levitus (1982, 1989) are used as input together with the annual mean wind stress field of Hellerman and Rosenstein (1983) for the climatological case and wind stress data derived from the Comprehensive Ocean‐Atmosphere Data Set for the 1950s and 1970s. The results suggest that the Gulf Stream was some 30 Sv weaker in the 1970–1974 pentad than in the pentad 1955–1959. About 20 Sv of this is due to a dramatic weakening of the circulation of the subtropical gyre. This is traced to a change in bottom pressure torque associated with the bottom topography on the western side of the Mid‐Atlantic Ridge. This same general area is also shown to be important for enhancing the transport of the climatological mean subtropical gyre above that predicted by the flat‐bottomed Sverdrup relation. The remaining 10 Sv is due to a weakening of the cyclonic gyre in the continental slope region south of Atlantic Canada and north of the Gulf Stream. This too is associated with a change in bottom pressure torque. We find that changes in the density field above 1500 m depth contribute about half of the transport change. It is not clear how reliable is the estimate of the remaining half. This is because it is dependent on changes in the analyzed density field at depths greater than 1500 m, and these could be a result of insufficient or unreliable data. No significant change in the total transport of the subpolar gyre is diagnosed by our calculations. In order to interpret the results we have split the joint effect of baroclinicity and relief (JEBAR) term into two parts: a part associated with bottom pressure torque and a part associated with compensation by the density stratification for the effect of variable bottom topography. This leads to a natural division of the volume transport stream function Ψ into three parts; Ψ = Ψ W + Ψ C + Ψ B. Ψ W is calculated using wind forcing alone and assumes a uniform density ocean. Ψ C is the difference between this and the stream function, Ψ S, calculated using the flat‐bottomed Sverdrup relation. It is driven by that part of JEBAR associated with density compensation. Finally, Ψ B is the difference between Ψ and Ψ S and is that part of Ψ driven by bottom pressure torque. (Ψ C + Ψ B) then gives the total contribution to Ψ from the JEBAR term. We find that for the climatological mean subpolar gyre, density compensation is particularly important, with bottom pressure torque displacing the gyre southward rather then enhancing its transport. For the subtropical gyre, density compensation plays less of a role. Almost all the difference between the two pentads occurs in the bottom pressure torque part.

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Sea level data from St. John's, Newfoundland, and from Nain, Labrador, are compared with results from a 1/4 ø x 1/4 ø resolution numerical model of the Newfoundland and Labrador shelf and the neighboring Labrador Sea. The model is barotropic, employs dynamics linearized about a state of rest, and uses linear bottom friction. The model is driven by the seasonal part of the Hellerman and Rosenstein wind stress field and by inflow from the rest of the North Atlantic specified along the eastern boundary. The latter is taken from a 1 ø x 1 ø version of the model applied to the North Atlantic between 10øS and 80øN. The results indicate that our simple model can account for that part of the annual cycle of sea level on the shelf directly attributable to wind forcing. In particular, the model can account for the difference between observed monthly mean sea level at St. John's, corrected for atmospheric pressure variations, and the seasonal steric height anomalies above 150 dbar on the neighboring shelf. We also compare our model results with the linear regression analysis carried out by Thompson et al. on monthly mean sea level data from Nain for the period 1964 to 1972. The model confirms the gains calculated for both the local wind driven signal and also that part of the signal due to wind forcing over the North Atlantic beyond the Labrador Sea. Comparing St. John's with Nain, we find that at St. John's, the model response is accounted for primarily by wind set-up on the shelf, whereas at Nain, the influence of North Atlantic wind forcing is a larger part of the total signal. These results suggest that data from the tide gauge at Nain could be useful for verifying model-predicted changes in the large-scale ocean circulation. The results also seem to be confirming the presence of the offshore transport variations predicted by the model.

An efficient numerical model is used to solve the linear barotropic equations of motion with North Pacific bottom topography and seasonal wind forcing (that is, monthly mean wind stresses with the annual mean removed). The model domain extends from 10°S to 60°N and from 120°E to 100°W with 1°×1° resolution. The model‐predicted seasonal transport and sea level variations are described and compared with the available data. It is found that at the Tokara Strait, to the south of Japan, the model‐predicted seasonal transport variations are very similar to those of the observed annual cycle of sea level difference across the Strait, although the amplitude of the model response is less than that suggested by the data. Both model and data indicate enhanced flow of the Kuroshio through the Strait in summer with a minimum in that flow in the winter. This is 6 months out of phase with the seasonal transport variations predicted by the flat‐bottomed Sverdrup relation. The model response is very sensitive, however, to the smoothing of the topography, with additional peaks in transport being found in the spring and the fall when the smoothing is increased. This suggests that care in handling the topography is required if a general circulation model is to reproduce this seasonal cycle. Away from the western boundary, the model‐predicted transport variations are quite similar to those in a flatbottomed ocean. The model also shows some success at reproducing features of the observed annual cycle of sea level corrected for both atmospheric pressure variations and the steric density effect. An interesting feature of the results is that the model‐calculated sea level at the offshore edge of the coastal waveguide is remarkably similar all the way along the North American coast from the Aleutian Islands to California. This shows a peak in model sea level in the summer and a minimum in the winter and indicates that it is the seasonal fluctuations of the subtropical gyre which dominate the model response even in the northern latitudes. However, the amplitude of this signal is somewhat less than that observed at the coast which, as previous studies have shown, is strongly influenced by coastal effects. The importance of wind forcing of the coastal waveguide is also apparent in the simple model described here, as is demonstrated by comparing model‐calculated sea level at the coast with that at points offshore.

Abstract:Results are described from a limited area barotropic model of the North Pacific with 1/3°xl/3 ° resolution and bounded by latitudes 10°N and 50°N and longitudes 120°E and 160°E. The model employs dynamics linearised about a state of rest and incorporates realistic bottom topography. It is driven by the seasonally varying part of the Hellerman and Rosenstein wind stress field and by inflow along the northern and eastern boundaries specified from a 1 ° x 1 ° version of the model applied to the whole North Pacific. The model-calculated transport variations through the Tokara Strait are similar to those of the observed seasonal sea level differences across the Strait, although the model appears to underestimate the amplitude of the signal by a factor of at least 2. The inclusion of realistic bottom topography is shown to be crucial in determining the model response.

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Sea, J. Phys. Oceanoor., 5, 361-368, 1975. Okubo, A., and C. C. Ebbesmeyer, Determination of vorticity, divergence, and deformation rates from analysis of drogue observations, Deep Sea Res., 23, 349-352, 1976.