The oceans are not static but wind their path around the Earth both at the surface and at depth. Wind-driven surface currents head polewards from equatorial regions, cooling all the while and eventually sinking at high latitudes. This dense water then flows downhill into the deep water basins, only resurfacing in the NE Pacific 1200 years later. In this sense, Earth's ocean truly is global, and distinctions between the different basins should be taken with a pinch of salt ¹. On their journey, the water masses piggyback both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe - an effective circulatory system for a Gaian Earth system. As such, the state of the ciculation has a large impact on the climate of our planet. Part 1 will cover the system itself, and how and why it happens. Part 2 explores the effect of this redistribution on the state of our planet, and its links with past and possible future climate shifts.

The System

It is quite intuitive to suppose that surface currents are caused by the wind grabbing hold of the water and pulling them along: we have all seen wind ripples on the surface of a pond. So you could reasonably expect the deep ocean - devoid of wind - to be perfectly static, as was assumed by many of the first oceanographers. But it is not - more and more sophisticated instruments dangled or dropped from research vessels show that the deep water is on the move ². So what lies behind deep currents?

Water Masses and their Density

Ocean water is not homogeneous - its properties vary significantly throughout the global ocean. Surprsingly, it does not vary gradually with depth but takes huge leaps in temperature, salinity over small depth intervals. This leads us to the concept of water masses, which retain their own identity within the ocean ³, and position themselves one above or below each other largely according to their density. Lighter water masses will float over denser ones just as a piece of wood, or ice will float on water. In order to take up their most stable positions they must flow, providing a driving force for deep currents. But what makes these water masses dense or light?

Water density depends on both temperature and salinity ⁴. Separately its all quite simple. Hot water expands and is thus less dense than colder water. Fresh water has less mass than salty water per unit volume, so it also has the least density. Both will float over saltier, colder water. Things get interesting when we try to look at both together. What happens, for example when hot, salty water meets cold, fresh water? The precise relationship between the two is so complicated that it takes up reams of paper, and requires a powerful computer to calculate.

Formation of the Deep Water Masses

The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean⁵. Here the water is intensively cooled by the wind, then becomes salty as sea ice forms and excludes the salt fraction of the water. The increasing salinity pushes the freezing temperature of the brine down, so cold liquid brine is formed in inclusions within a honeycomb of ice. Being extremely dense, it slowly drips out of the ice matrix and sinks to the sea bottom. These deep water masses are so dense they flow downhill, like a stream within the surrounding less dense fluid, and fill up the basins of the polar seas. Just as river valleys direct streams and rivers on the continents, the bottom topography steers the bottom water masses.

In the Norwegian Sea - north of Britain and Iceland, and west of Scandinavia - wind cooling is predominant, and the sinking water mass (the North Atlantic Deep Water, or NADW) fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Britain. Flow into the Pacific, however is blocked. It then flows very slowly⁶ into the deep abyssal plains of the Atlantic, always in a southerly direction.

By contrast in the Weddell Sea - north of Antarctica, but near the edge of the ice pack - the effect of wind cooling is intensified by brine exclusion. The resulting Antarctic Bottom Water (AABW) sinks and flows north into the Atlantic Basin, but is so dense it actually underflows the NADW. Again, flow into the Pacific is blocked, this time by the Drake Passage between the Antarctic Peninsula and the southernmost tip of South America.

The Journey of the Deep Water Masses

-route of dw from atlantic to indian to pacific oceans

-aging of the water masses and their chemical signatures

Upwelling

All these dense water masses sinking into the ocean basins displace the water above them, so that elsewhere water must be rising in order to maintain a balance. However, because this thermohaline upwelling is so widespread and diffuse, its speeds are very slow even compared to the movement of the bottom water masses ⁷. It is therefore fiendishly difficult to measure where upwelling occurs using current speeds, given all the other wind-driven processes going on in the surface ocean. Deep waters do however have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth ⁸. This signature can be found in surface waters in the North Pacific, indicating that this is where most upwelling happens.

[ Surface water masses sink in the Arctic and Southern Oceans only to resurface in the Pacific Ocean around 1200yrs later. As it travels, the water bodies are continually accumulating material falling from the surface, so they emerge quite different from how they started out in the Arctic.]

Impacts on Climate

-redistribution of heat from equator to poles

-meltwater disruption of dw formation (Younger Dryas)

Sources

Apel, JR. Principles of Ocean Physics

Knauss, JA. Introduction to Physical Oceanography