The path of water into the deep ocean
There is no other area in the ocean where the surface water finds its way so quickly into the deep as in the convection areas, and at no other place do changes at the sea surface or in the atmosphere become so rapidly apparent in the ocean’s interior, for example, in the increased carbon dioxide levels in the water as a result of higher carbon dioxide concentrations in the atmosphere. Convection connects two distinctly different components of the ocean: the near-surface layers that are in contact with the variable atmospheric fields of wind, radiation and precipitation, and the deep regions of the ocean. At the surface, currents, temperature and salinity fluctuate on a scale of weeks to months. But at greater depths the environmental conditions change at time scales of decades or centuries.
In the consistently warm oceanic regions of the tropics (the warm regions of the Earth between 23.5 degrees north and 23.5 degrees south latitude) and the subtropics (the regions between 23.5 and 40 degrees in the northern and southern hemispheres), there is no exchange between the surface and deep waters that is comparable to polar convection. This is because, averaged over the year, there is a net radiation excess of the surface-layer waters. The warm water, with a minimum temperature of ten degrees Celsius, has a relatively low density and floats as a warm layer on top of the deeper, colder water masses. The two layers are distinctly separated with no gradual transition between them. At the boundary where they meet there is a sharp temperature jump, and therefore also an abrupt density difference that inhibits penetration of the heat to greater depths. The warm surface layer has an average thickness of several hundred metres, which is relatively thin compared to the total depth of the oceans. In very warm ocean regions such as the western equatorial Pacific, there is hardly any vertical mixing at all. Nearer to the poles, however, there is more vertical mixing of the oceans and layering is less well-defined. Because there is no abrupt temperature and density change there, changes in the sea surface can be transmitted to the interior depths of the ocean. But the convection areas are still the express elevator to the deep.