Other Deep Water Masses
There are numerous other deep water masses, especially at intermediate depths, for example, North Pacific Intermediate water. As deep-water masses travel through the ocean they gradually mix with surrounding water masses. For example, NADW mixes with AABW and AIW.
Downwelling supplies oxygen to the deep ocean and therefore ventilates this body of water. It does not bring nutrients. Deep water currents generally move very slowly with a velocity of several cm per second. Typically, surface currents move 10-100 times faster than this. At these rates, deep water currents take thousands of years to encircle the globe. In fact, the oldest deep water in the ocean (in the North Pacific) is about 1500 years old. As deep waters circle the globe their properties change. They mix with waters around them, and their chemistry changes as they acquire nutrients such as phosphate and CO2 from decaying organic matter and lose oxygen.
The opposite process of downwelling is upwelling. Upwelling is where a deep-water mass that is lighter than waters around it rises to the level where it is no longer buoyant. This situation generally results when surface winds move the surface water masses away from a location, resulting in the upward movement of water from depth to fill the void. Upwelling is frequent in coastal regions, especially those in subtropical regions where high pressure results in a dominant offshore wind flow. In addition, the ocean divergences where winds move surface current by Ekman transport are frequented by upwelling. Upwelling is crucial to the supply of nutrients to surface water masses, fueling high levels of productivity in the surface ocean. The most prolific fisheries of the world in coastal regions occur in nutrient-rich waters such as Peru and California and are supplied by upwelling.
As we have seen, the circulation of the deep ocean is driven by density differences that arise as a result of temperature and salinity of the different water masses. This type of circulation is known as thermohaline (temperature=thermo; haline=salt or salinity). Strictly speaking, since surface ocean currents are not driven by thermohaline mechanics but by winds and to a much lesser degree, tides, the circulation of the ocean as a whole is often called the meridional overturning circulation. However, we will continue to use the term thermohaline when addressing deep-water circulation.