In physical ocean sciences, with little opportunity to experiment on the natural system, numerical circulation models provide estimates of ocean currents that are increasingly important in leading this field. Probably the most common application of these tools is to study the large-scale, lowfrequency currents of the global ocean using a general circulation model (GCM). Ocean GCMs are used in coupled climate models, familiar from the Intergovernmental Panel on Climate Change, for instance. Ocean models are also widely used for operational purposes, including seasonal forecasts (especially for El NiñoSouthern Oscillation), storm surges, tides, and tsunamis. Dozens of basic studies of ocean fluid dynamics and biogeochemical cycles also exist.