It is shown that subtle changes in the velocity profile across the seaward extension of midlatitude jets, such as the Gulf Stream, can lead to dramatic changes in the zonal-penetration scale. In particular, if α = dq/dψ > 0, where q is the absolute vorticity and ψ is a streamfunction for the geostrophic flow, then the jet tends to penetrate across to the eastern boundary; conversely if α < 0, the jet turns back on itself creating a tight recirculation on the scale of order |α|−frac12;. This behavior is demonstrated in a quasigeostrophic ocean model in which a jet profile is prescribed as an inflow condition at the western margin of a half-basin, and radiation conditions along the remainder of the western boundary allow the injected fluid to escape. Jet inflows with both vertical and horizontal structure are considered in one and one-half-, two-, and three-layer models.
Finally, the implications of our study for numerical simulations of ocean gyres, which frequently show sensitivity of jet penetration to horizontal and vertical resolution and to choice of boundary conditions, are discussed. In particular, it is demonstrated that poor resolution of the horizontal jet structure may lead to a dramatic reduction in penetration.