Thermohaline circulation (THC)

Introduction

Thermohaline circulation (THC), often referred to as the “global ocean conveyor belt,” is a fundamental component of Earth’s climate system. It describes the large-scale movement of ocean water driven by differences in temperature (thermo) and salinity (haline), which together determine seawater density. This circulation plays a crucial role in regulating global climate, distributing heat and nutrients, and supporting marine ecosystems.

Meaning and Origin

• Definition: Thermohaline circulation is a density-driven process in which variations in seawater temperature and salinity create global density gradients, setting vast volumes of water in motion across the world’s oceans.
• Etymology: The term “thermohaline” combines thermo (temperature) and haline (salinity), highlighting the two primary factors influencing seawater density and, consequently, deep ocean currents.

Mechanism of Thermohaline Circulation

Surface and Deep Water Interaction

• Surface currents, primarily wind-driven, move warm, less dense water from the equator towards the poles.
• As this water reaches higher latitudes, it cools, and processes such as evaporation and sea ice formation increase its salinity.
• The colder, saltier water becomes denser and sinks, particularly in the North Atlantic, forming North Atlantic Deep Water (NADW).

Global Conveyor Belt

• The sinking water initiates a deep, slow-moving current that flows southward, eventually reaching the Southern Ocean.
• Here, the deep water is split: some moves into the Indian Ocean, while the rest continues into the Pacific.
• In these regions, upwelling occurs, bringing nutrient-rich deep water back to the surface, where it warms, becomes less dense, and the cycle repeats.

Timescales

• The entire circulation is extremely slow, with water taking approximately 500 to 1000 years to complete a full cycle.

Factors Influencing Thermohaline Circulation

Temperature: 

• Colder water is denser and more likely to sink, especially in polar regions where surface cooling is intense.

Salinity: 

• Higher salinity increases water density, promoting sinking.
• Processes such as evaporation and ice formation increase salinity, while freshwater input from melting ice or precipitation decreases it.

Density Gradients: 

• The interplay of temperature and salinity creates global density gradients, which are the primary drivers of thermohaline circulation.

Importance of Thermohaline Circulation

• Climate Regulation: THC transports heat from equatorial regions to the poles, moderating global temperatures. It is responsible for the relatively mild climate of Western Europe compared to other regions at similar latitudes.
• Nutrient Distribution: Upwelling of deep, nutrient-rich water supports marine life, fueling the growth of plankton and sustaining oceanic food webs.
• Carbon Sequestration: The circulation helps lock away carbon dioxide in the deep ocean, mitigating the effects of global warming by absorbing greenhouse gases from the atmosphere.

Threats and Disruptions

Impact of Global Warming

• Rising global temperatures and melting polar ice caps reduce salinity and disrupt the formation of dense, sinking water in the North Atlantic.
• This leads to a slowdown in the thermohaline circulation, with projections suggesting a potential 40% reduction in the coming decades if current carbon emissions persist.

Consequences of Disruption

• A weakened or collapsed THC could result in:

• Increased climate and weather instability

• Changes in sea levels and regional flooding

• Reduction in marine biodiversity and fish stocks

• Altered global heat distribution, potentially triggering extreme climate events similar to the Younger Dryas period.

Download as PDF