Global distribution of the density of sea water is maintained primarily by polar cooling and tropical heating together with polar precipitation and subtropical evaporation at the sea surface. This thermohaline forcing drives a global convection called the deep ocean general circulation. Carried by the circulation, sea water goes around the world oceans on a time scale as long as 1000 years, the longest time scale of the fluid earth. The deep circulation therefore is supposed to dominate the long-term variability of the climate of our aqua-planet.
Long history of oceanographic study has accumulated a vast amount of hydrographic data and established rather reliable though rough global maps of water properties in deep oceans, such as temperature, salinity, dissolved oxygen, silica, nutrients, and so on. Details have been known only little, however, on how water actually circulates in the world oceans from the surface to the bottom, from polar to tropical regions, and from west to east or vice versa, though recent hydrographic explorations have gradually revealed finer structure of circulation. Much less has been understood about what dynamics is responsible for the present state of deep circulation and distribution of water properties.
