Sea floor relief (or bathymetry) affects the sea surface topography, by creating 'hills' or inducing friction by bottom roughness (see Image of the Month January 1999: A glassy sea... of ridges and valleys , and June 2002: Undersea mountains slow down Earth rotation). It also steers ocean circulation as currents tend to pass through deep gaps in ridges. Oceanographers often imagine that flow should follow contours of constant depth at a given latitude. In reality, we've learned from altimetry and other data that this simple model does not work perfectly, because surface flow is stronger than deep ocean flow. Nonetheless, ocean currents clearly stay in deep channels, following the gaps in the ridges. This steering of ocean currents is particularly apparent in the Southern Ocean because the current is relatively coherent from top to bottom. This coherence occurs because the upper ocean is cold and dense, and therefore not much denser than the deep water on the sea floor. Closer to the tropics, stratification between the surface ocean and the deep ocean is greater and upper ocean currents can thus be very different from deep ocean currents.