Suppose one were given the task of mapping the general circulation in an unfamiliar ocean. The ocean, like our own, is subdivided into basins and marginal seas interconnected by sea straits. Assuming a limited budget for this undertaking, one would do well to choose the straits as observational starting points. To begin with, the currents flowing from one basin to the next, over possibly wide and time-varying paths, are confined to narrow and stable routes within the straits. Mass, heat and chemical budgets for individual basins can be formulated in terms of the fluxes measured across the...