03 Thermohaline Circulation There are two major systems of ocean currents: Currents driven by density differences are THERMOHALINE currents. They are mostly deep, slow moving and massive. Currents may also be WIND-DRIVEN. These are shallow, and faster moving. Both systems of currents are linked throughout the world ocean. Read: Segar, Douglas A., 2007. Introduction to Ocean Sciences, Second Edition Ch 10, Ocean Circulation. We begin with 10.13 Thermohaline circulation and 10.14 Ocean circulation and climate Read also: Joyce, T and l Keigwin, 2005. WHOI Ocean and Climate Change Institute. http://www.whoi.edu/institutes/occi/currenttopics/abruptclimate_joyce_keigwin.html

Density-driven currents:. Deep circulation of the world ocean -- and also estuarine circulation Key fact: dense water (high salinity, low temperature) will always lie under less dense water (low salinity, high temperature) Ocean waters are arranged in a series of horizontal layers of increasing density from the surface to the ocean floor. Throughout most of the oceans, the layers form three principle zones: the surface zone ( the mixed layer), the pycnocline zone, and the deep zone.

Depth zones of the ocean The pycnocline separates the upper water mass and the surface layer from the deep water mass. Where density is increasing rapidly with depth, mixing is suppressed; the upper and lower current systems are independent. A quantitative measure of “density increasing rapidly with depth” is stability, E ≈ (1/ρ)*(Δσt/Δz). Deep and shallow waters respond to different forces, and have different circulation patterns.

Plot of salinity, temperature and density off Beqa, 1997 Density, σt

The Atlantic has an excess of evaporation; the Pacific has an excess of heat. As a result, of this imbalance, density differences occur, which aids thermohaline circulation Global salinity and temperature differences create and maintain a deep flow from the Atlantic to the Pacific, and a surface flow from the Pacific to the Atlantic. .

NADW and ABW Ocean water has an average salinity of 34.72%. When the surface water evaporates, it leaves behind dissolved salts which give the remaining surface water a salinity, and thus a density, which is higher than this. Consequently, this water sinks to the ocean floor, taking oxygen with it. Salinity also differs geographically. Some of the water evaporated from the Atlantic precipitates into the Pacific, either directly or by way of rivers and streams. As a result, the Atlantic loses pure water and the Pacific gains it, making the Atlantic more saline. It is this difference in salinity which drives deep ocean circulation, thus creating water masses. The Atlantic is more saline at the surface. The Pacific is less saline at the surface. When the Atlantic ocean is cooled the deep water that is formed is also more saline. These conditions form most of the densest water – the North Atlantic Deep Water (NADW). The other mechanism by which extremely dense water is formed is by exclusion of salt by freezing out. This occurs where ice is formed directly from seawater. This occurs under the ice shelves of Antarctica. This source is the densest water on earth, the Antarctic Bottom Water (ABW). ABW it is not so abundant as NADW

Formation of NADW Cold air from Canadian Arctic Warm surface water of the conveyor moves north in the Atlantic until it hits a region near Iceland where it is cooled by cold arctic air. Effective transfer of heat to the atmosphere occurs, thus keeping northern Europe usually warm for it’s latitude. The surface water cools due to this heat transfer and, as a result becomes more dense and sinks. This sinking is aided by the extra salinity of the Arctic Cold air from Canadian Arctic Warm air to Europe Shallow, warm, salty northward current Deep, cold southward current

The second method of deepwater formation is by freezing out When ice freezes, salt is excluded. This raises the salinity of the underlying water, making it more dense. The densest water on the ocean bottom is formed by freezing out in Antartica – but this water is not a plentiful source, compared to North Atlantic Deep Water.

Deep waters are formed only in limited areas

Example 1a: Atlantic Ocean

Example 1b: Pacific Ocean

The Great Ocean Conveyor Belt .. ..is driven by the sinking of cold, saline water in the waters off Iceland. Warm shallow currents Two counteracting forces drive the thermohaline circulation: 1, thermal forcing, when water is cooled and sinks, and 2, haline forcing, when excess precipitation makes water less dense, and thus resistant to sinking. Cold, saline deep currents Some areas of deep water formation The balance is in favor of thermal forcing – for now

Deep water is formed off Iceland and Greenland

In the formation areas for NADW, salinity has declined over the last 40 years

How do oceanographers study the thermohaline circulation? By T-S diagrans By tracer experiments Oxygen Tritium By direct measurements Swallow floats Argos system

T-S Diagrams – A useful tool for understanding ocean circulation

Deep water is formed in the North Atlantic As it moves southward in the Atlantic, and into the Pacific, The water masses decline in salinity. In the North Pacific, there is further decline in salinity in the upper layers, but the bottom water is nearly constant.

Oxygen profiles. Deep oxygen is replenished in the NADW, but declines with isolation from a surface source.

Tritium (3H) as a tracer. Tritium, mostly from Russian nuclear bomb tests, has spread throughout the North Atlantic between 1972 and 1981. It is slowly spreading from a source in the Russian Arctic into deep water.

New Toys: ARGO deployment

Example 2: Estuarine circulation

End Thermohaline Circulation