IB Environmental systems and societies 4.1E Thermohaline Circulation Notes

Ocean upwelling and thermohaline circulation: wind and density move water, nutrients and heat around the planet

1. Upwelling is the rise of cold, dense, nutrient-rich water from depth to the surface when wind pushes surface water away.
2. The rising water carries nitrates, phosphates and silicates that fuel phytoplankton growth at the base of the food web.
3. Upwelling zones are among the most productive ecosystems on Earth, supplying roughly 25% of the global fish catch from about 5% of the ocean surface.

What drives coastal upwelling

1. Wind-driven surface flow: winds blowing parallel to a coast push surface water offshore, so deep water rises to replace it.
2. Coriolis effect: Earth's rotation deflects the moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
3. Ekman transport: combining wind and Coriolis deflection, the net movement of the surface layer is about 90° to the wind direction, driving water away from the coast.
4. Seasonal and lake turnover: in stratified lakes, seasonal winds and cooling break stratification so cold, nutrient-rich bottom water rises.

Coastal upwelling is strongest along western continental margins, such as the Peru (Humboldt) and Benguela systems.

Why upwelling matters

1. Nutrients drive phytoplankton blooms that feed zooplankton, small fish, and top predators such as tuna, seabirds and whales.
2. The Peruvian anchovy fishery is the classic example: one of the largest single-species fisheries in the world, built entirely on the Humboldt upwelling.

Thermohaline circulation: the global conveyor belt

1. Thermohaline circulation is the global movement of ocean water driven by density differences that arise from BOTH temperature (thermo-) and salinity (haline).
2. Cold water and salty water are both denser (the temperature-density link is covered in 4.1B), so cold, salty water sinks while warm, fresher water stays at the surface.
3. This links all the ocean basins into one conveyor belt that redistributes heat, carbon, oxygen and nutrients, with a full cycle taking up to about 1000 years.

How the conveyor works

1. Temperature and salinity gradients: density contrasts between water masses set up the vertical movement that powers the whole system.
2. Deep-water formation: in the North Atlantic and around Antarctica, cold, salty surface water becomes dense enough to sink and form deep currents.
3. Surface return currents: warm currents such as the Gulf Stream carry heat poleward, cool and sink, closing the loop.

What the conveyor does for the planet

1. Heat distribution: it moves heat from the tropics toward the poles, and its warm North Atlantic currents keep north-west Europe's climate mild.
2. Carbon and oxygen transport: deep circulation stores and redistributes CO₂ and O₂, making the ocean a major carbon sink.
3. Nutrient cycling: deep currents return nutrients to the surface where they upwell, sustaining marine productivity.

Disruption risk: an AMOC tipping point

1. Freshwater from melting ice lowers the salinity, and so the density, of North Atlantic surface water.
2. If the water no longer sinks, deep-water formation weakens and the AMOC (the Atlantic limb of the conveyor) can slow or shut down, a recognised climate tipping point.
3. A slowdown would cool north-west Europe, shift rainfall belts, and weaken the ocean carbon sink.

How upwelling and the conveyor connect

1. The two processes are one system: the deep water formed by the thermohaline circulation is the same water that upwells elsewhere, so sinking in the North Atlantic feeds nutrient-rich upwelling in the Indian and Pacific Oceans.
2. Deep-water formation and upwelling therefore balance each other, keeping ocean heat, carbon and nutrients in circulation.