A simple model of interannual displacements of the Gulf Stream

Interannual variations in the latitude of the north wall of the Gulf Stream 1966-1999 can be hindcast by a simple model forced solely by monthly value s of the North Atlantic Oscillation (NAO) index. The model is a modified ve rsion of the one-dimensional model developed by D. Behringer et al. and uti lizes the observed tendency for the centers of the Azores high and the Icel and low to move north or south as the NAO strengthens or weakens. Only one parameter, the active upper layer thickness, is changed from the original v alues. Although the model incorporates the thermal feedback on the wind str ess used by Behringer et al., the results are almost unchanged without it. In generating the Gulf Stream position the model averages the NAO index ove r several months and introduces a time lag of at least a year. These delays are associated with the time taken for the lateral advection of heat. The model results are also compared with estimates of the latitude of the Gulf Stream made by A. Gangopadhyay et al. 1977-1988 and by T.M. Joyce et al. 19 55-1997, and with the series of Gulf Stream transports 1954- 1997 construct ed by R.G. Curry and M.S. McCartney. The model reproduces the observed weak ness of the seasonal variations in comparison to interannual variations. Th e NAO data are used to hindcast the annual latitude of the Gulf Stream for the period 1825-1999. The results show that since 1970 the Gulf Stream may have been more consistently farther south than during any period of the pas t 170 years.