Ocean Overturning and Deep Circulation

In a handful of polar regions, surface water grows cold and salty enough to sink kilometres deep, setting in motion a slow global overturning that ventilates the abyss and shapes climate over centuries.

Definition

The overturning circulation is the deep, density-driven (thermohaline) flow in which surface water sinks in polar regions, spreads through the deep ocean, and slowly returns to the surface, ventilating and connecting the global ocean.

Scope

This topic covers the formation of deep and bottom water in polar seas, the structure and transport of the meridional overturning circulation, the distribution and tracing of water masses, the role of mixing in closing the overturning, and the possibility of multiple stable circulation states.

Core questions

• Where and how does deep and bottom water form, and what sets its properties?
• What is the structure and strength of the meridional overturning circulation?
• How do water masses spread and mix to close the overturning loop?
• Could the overturning circulation have more than one stable state and shift abruptly?

Key theories

Thermohaline overturning and water-mass formation

Cooling and salinification at high latitudes make surface water dense enough to sink, forming deep water masses that spread along density surfaces and define the deep circulation.

Multiple equilibria of overturning

Stommel showed that the competing effects of temperature and salinity on density can give the overturning more than one stable state, raising the possibility of abrupt transitions in climate history.

Mechanisms

In the North Atlantic and around Antarctica, intense cooling and brine rejection during ice formation increase surface density until water sinks to form deep and bottom water; this water spreads through the abyss and is gradually returned upward by mixing and wind-driven upwelling, completing a global overturning that takes roughly a thousand years.

Clinical relevance

The overturning circulation transports heat that warms regional climates, ventilates the deep ocean with oxygen, and sequesters carbon; concern that warming and freshening could weaken it has made the Atlantic overturning a focus of climate monitoring and abrupt-change research.

History

Water-mass analysis by early-twentieth-century oceanographers mapped the deep circulation; Stommel's 1961 model revealed its multiple equilibria, and Broecker's conveyor-belt synthesis linked deep circulation to abrupt climate changes recorded in ice and sediment, spurring sustained observation of the overturning.

Key figures

• Henry Stommel
• Wallace Broecker
• Carl Wunsch

Seminal works

• talley2011
• stommel1961

Frequently asked questions

How long does deep water stay below the surface?

Water that sinks into the deep ocean can remain there for several centuries to about a thousand years before mixing and upwelling return it to the surface, making the deep circulation very slow.

Could the Atlantic overturning circulation collapse?

Models and past records suggest the overturning can weaken or shift between states if surface waters become too warm or fresh to sink; whether and how fast this might happen is an active area of research.