Laboratory and numerical experiments are used to study flow of a uniform-density
fluid on the β-plane around a thin zonally elongated island (or ridge segment in
the abyss). This orientation is chosen specifically to highlight the roles of the zonal
boundary layer dynamics in controlling the circulation around the island. There are
examples of deep ocean topography that fall into this category which make the work
directly applicable to oceanic flows. Linear theory for the transport around the island
and the flow structure is based on a modification of the Island Rule (Pedlosky et
al. 1997; Pratt & Pedlosky 1999). The linear solution gives a north–south symmetric
flow around the island with novel features, including stagnation points which divide
the zonal boundary layers into eastward and westward flowing zones, and a western
boundary layer of vanishing length, and zonal jets. Laboratory experiments agree
with the linear theory for small degrees of nonlinearity, as measured by the ratio of
the inertial to Munk boundary layer scales. With increasing nonlinearity the north–south
symmetry is broken. The southern stagnation point (for anticyclonic forcing)
moves to the eastern tip of the island. The flow rounding the eastern tip from the
northern side of the island now separates from the island. Time-dependence emerges
and recirculation cells develop on the northern side of the island. Mean transport
around the island is relatively unaffected by nonlinearity and given to within 20% by
the modified Island Rule. Numerical solutions of the shallow water equations are in
close agreement with the laboratory results. The transition from zonal to meridional
island orientation occurs for island inclinations from zonal greater than about 20°.
