Seasonal variations in a linear barotropic model of the North Pacific driven by the Hellerman and Rosenstein wind stress field

Abstract

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.

Barotropic variability in the presence of an ocean gyre

“…This is within the monthly mean time scale of the sea level data and suggests that the advection mechanism deserves further investigation. It is also possible that the absence of a mean flow in the models of Anderson and Corry (1985b) and Greatbatch and Goulding (1989a) could explain the failure of these models to properly account for the amplitude of the observed seasonal variation of transport through the Florida Straits. Both models underestimate this signal by roughly a factor of 2.…”

“…[48, I model of Greatbatch and Goulding (1989a) was particularly successful along the southeastern seaboard of the United States where they showed that the coastal sea level in the model is largely determined by the model predicted transport variations offshore. This lends support to Blaha's (1984) conclusion that coastal sea level in the region is strongly influenced by transport variations in the Gulf Stream and, indeed, it is well known (Maul et al, 1985) that at the annual period or less, sea level fluctuations at Miami are a good indicator of transport variations in the Florida Straits.…”

Vortex stretching and bottom torques in the Bryan‐Cox ocean circulation model

“…Attention has been concentrated on diagnostics for integrations of only 3 months duration for three reasons in addition to those mentioned in the introduction. First, the barotropic flow adjusts to a given density field within less than a month of integration [Greatbatch and Goulding 1989]. Second, given a realistic density field, a model suitable for data assimilation or climate simulations should produce a realistic barotropic flow, which, for example, does not result in rapidly developing density errors.…”

Mechanism of the seasonal transport variation through the Tokara Strait

“…Blaha and Reed (1982) suggested Ekman pumping, particular between 11°N and 19°N determines the seasonal transport fluctuations of the Kuroshio. On the other hand, Greatbatch and Goulding (1989) recently demonstrated that the seasonal transport variations predicted by a barotropic model are very similar to those of the observed annual cycle of sea level difference across the Tokara Strait. Furthermore, Greatbatch and Goulding (1990) examined in detail the seasonal variation of the transport through the Tokara Strait by using a limited-area barotropic model of the North Pacific with a fine-resolution, 1/3° × 1/3°.…”