Upwellings & Ocean Circulation (HL) (DP IB Environmental Systems & Societies (ESS))

Upwellings

What are upwellings?

• Upwellings are areas in oceans and freshwater bodies where cold, nutrient-rich water rises from deep below to the surface

  • Surface waters move vertically as a result of currents or winds pushing them away

  • This causes deeper water to rise and replace it

  • Upwelling is important because it brings nutrients like nitrates and phosphates from deep waters to the surface

  • These nutrients are essential for phytoplankton growth, which forms the base of marine food webs

Causes of upwellings

• Wind-driven upwellings occur when strong winds blow across the surface of the ocean, particularly along coastlines

  • Winds push warm surface water away from the coast

  • Cold water from the deep ocean is pulled up to replace it

  • This process often occurs along the west coasts of continents, such as in California, Peru, and Namibia

• Seasonal upwellings can occur in stratified lakes and oceans

  • This occurs where layers of water are separated by temperature

  • As seasonal winds blow across these bodies of water, they disrupt the layers and bring up nutrients

• ENSO (El Niño Southern Oscillation) is a phenomenon that can also trigger or disrupt upwellings

  • During La Niña, stronger trade winds increase upwelling, while El Niño reduces it, affecting global fish stocks

Benefits of upwellings

• Nutrient supply:

  • The nutrient-rich water supports large populations of phytoplankton

  • This in turn attracts fish, marine mammals, and seabirds

  • Upwellings are some of the most productive marine ecosystems in the world

    • For example, the Peru upwelling system supports a major fishery for anchovies

    • This is a very important species for both marine food webs and commercial fishing

• Fisheries:

  • The enhanced nutrient availability supports fish stocks and marine biodiversity

  • This is crucial for fisheries and local economies

Limitations and risks of upwellings

• Overfishing can exploit these nutrient-rich waters

  • This can lead to the depletion of fish populations and the collapse of ecosystems if not managed sustainably

• Climate change could affect wind patterns

  • This could disrupt upwelling cycles and alter nutrient availability in these regions

  • This could potentially cause the breakdown of marine food webs or fisheries in these regions

Thermohaline Circulation Systems

What is thermohaline circulation?

• The global ocean current system known as the thermohaline circulation is due to variations in temperature and salinity, which affect water density

  • This system is also known as the ocean conveyor belt

  • This system plays a critical role in regulating the Earth's climate by distributing heat across the globe

How thermohaline circulation works

• Cold, salty water is denser than warm, fresh water

• In polar regions, surface waters are cooled and become more saline due to evaporation and sea ice formation

  • Evaporation increases salinity because, as water evaporates, it leaves salt behind, concentrating the remaining salt in the water

  • Similarly, when sea ice forms, only the water freezes while the salt is left in the surrounding water, increasing the salinity of the remaining liquid water

• The circulation starts in the North Atlantic, where cold, salty water sinks and flows southward towards the equator

  • This is known as the North Atlantic conveyor belt

• As the water sinks, it pulls surface water along with it, creating a continuous flow

• This process repeats, forming deep currents 

• These currents then make their way around the world, into areas where the water will heat up again

• This warmer (less dense) water returns to the surface, moving further around the world and eventually reaching the point where the process started

  • Warm water from the tropics moves northward to replace the sinking water, bringing heat with it and warming coastal regions

• The cycle repeats

• One full loop of the Oceanic Conveyor Belt could take anywhere between 100 and 1000 years

• The process helps to regulate the climate by transferring heat from the tropics to the poles

Impacts of thermohaline circulation

• The Gulf Stream is part of the thermohaline system

  • It brings warm water from the Gulf of Mexico up the eastern coast of the US and across to Western Europe

  • This is why regions like Western Europe have a milder climate than other areas at the same latitude

• The sinking of cold water in the North Atlantic drives the entire conveyor belt, but this process is being threatened by climate change

  • Melting ice caps in Greenland and Antarctica are adding large amounts of freshwater to the oceans

    • This is reducing the salinity and density of surface waters

    • This could potentially slow down or even stop the conveyor belt

  • If the thermohaline circulation slows significantly, it could lead to major climate shifts

    • E.g. cooling Northern Europe while warming tropical regions and disrupting global weather patterns