J. zyxwvutsrqp Fluid zyxwvutsrqponm Mech. zyxwvutsrqpo (1994), uol. 280, zyxwvuts pp. zyxwvutsrq 349-368 Copyright zyxwvutsrqp @ 1994 Cambridge University Press 349 Topographic Hadley cells By S. A. CONDIET AND P. B. RHINES School of Oceanography WB-10, University of Washington, Seattle, WA 98195, USA (Received 7 July 1993 and in revised form 9 June) When a rotating fluid over sloping topography is heated from below and/or cooled from above, horizontal temperature gradients develop which drive convection cells aligned with isobaths. We refer to these cells as topographic Hadley celZs. Laboratory experiments reveal that sinking occurs in small cyclonic vortices situated in relatively shallow regions. This is balanced by slower upwelling in adjacent deeper regions. The cross-isobath motions which connect the upwelling and downwelling are accelerated by Coriolis forces, resulting in strong jets which follow isobathic contours. For anti- clockwise rotation, the surface jets keep the shallows to their left when looking in the direction of flow, which is opposite to both Kelvin and Rossby wave propagation. The width of the jets scales with the Rossby deformation radius and if this is much less than the width of the slope region then a number of parallel jets form. Motions on the deeper side of the jets where the flow is accelerating are adequately described by linear inviscid theory. However, the strong shears generated by this acceleration lead to baroclinic instability. The resulting cross-stream momentum fluxes broaden and flatten the velocity profile, allowing the flow on the shallow side of the jet to decelerate smoothly before sinking. Topographic Hadley cells are dynamically similar to terrestrial atmospheric Hadley cells and may also be relevant to the zonal jet motions observed on Jupiter and Saturn. It is also suggested that in coastal seas they may represent an important mode of heat (or salt) transfer where surface cooling (or evaporation) drives convection. 1. Introduction If a fluid of varying depth is uniformly cooled from above, the coldest fluid will be produced in shallow regions owing to the smaller heat capacity per unit area. The flow responds to the cross-isobath temperature gradient by establishing convection cells, characterized by downwelling in shallows balanced by upwelling in deeper regions. In an inertial reference frame there is no along-isobath component. However, if the fluid is also rotating, the horizontal motions within the cell will be accelerated in the along-isobathic direction by Coriolis forces. This is illustrated schematically in figure 1. As relatively warm fluid nears the free surface and begins to move toward the shallows, it is accelerated to the right (anti-clockwise rotation), thus forming an along-isobath jet which keeps the shallows to its left. The bottom flow will similarly form a jet in the opposite direction, although this may be significantly weakened by dissipation in the bottom Ekman layer. Because of a close dynamical analogy between the formation of zonal jets over topography and the formation of the jet t Present address: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. zyxwvu 12 FLM 280