365 Days of Climate Awareness 44: Ocean currents 2 - thermohaline circulation

The second major driver of ocean currents is density, dubbed the "thermohaline circulation", from the Greek "thermos", or heat, and "hals", or salt. Temperature and salinity are the main factors which determine the density of seawater, and they drive the bulk of vertical water motion in the ocean worldwide. When combined with the wind-driven surface currents, these density currents help form a rough, slow, conveyor belt of circulation around the world.

One of the main thermohaline processes is evaporation. When a large amount of ocean water in a given location evaporates, it leaves a much cooler body of water behind, denser than the surrounding water. The colder, denser water sinks either to the bottom, or to a depth where the surrounding water is equally dense. In the Weddell Sea off Antarctica, these convective events produce Antarctic Bottom Water, which spreads northward through the Atlantic. In the North Atlantic, east of Greenland, the last of the North Atlantic Current water evaporates, leaving behind a colder parcel which sinks and becomes North Atlantic Deep Water, and then spreads south.

This North Atlantic convection combines with the momentum of the Gulf Stream to produce the North Atlantic Current. Fluid dynamics depends on a principle called "conservation of mass", which means mass is neither created nor destroyed (in conventional Newtonian physics--not dealing with nuclear reactions here!). When water evaporates upward and sinks downward convectively, the loss of mass in that area of the ocean requires that more water replace it. In the North Atlantic, the source of that replacement mass is the northeastward-moving Gulf Stream water. When wind stress is no longer driving the current, it continues, in a slower and broader current, to the northeast. The liquid must remain continuous--no gaps--so the loss of water mass by evaporation and sinking must be balanced by incoming water on the surface. In this way the thermohaline circulation helps drive surface currents.

It also drives deeper currents. By the same principle of the conservation of mass, when water moves north in the Gulf Stream and North Atlantic Current, it must move back south again, to balance the loss of water from the tropical region. This occurs in part by means of the Deep Western Boundary Current, a cold, slower, southward ocean current which runs beneath the northward Gulf Stream. Processes like this occur all over the world, with deep currents balancing the wind-driven surface currents. In this way the entire ocean around the globe is stirred roughly once a millennium.

Tomorrow: El Nino-Southern Oscillation.

Be well!