4.1E Thermohaline circulation (HL) Notes

Upwelling in Oceans and Freshwater Bodies

1. Upwelling is the vertical movement of cold, dense, nutrient-rich water from deeper ocean or lake layers to the surface.
2. It occurs when wind-driven surface currents displace surface water, allowing deeper water to rise and replace it.
3. This process brings essential nutrients such as nitrates, phosphates, and silicates to the surface, supporting phytoplankton growth and fueling marine food webs.
4. Upwelling zones are among the most biologically productive ecosystems on Earth, supporting nearly 25% of global fish catches despite occupying only ~5% of the ocean’s surface area.

Causes of Upwelling

1. Wind-Driven Movement

1. When surface winds blow parallel to the coastline, they push warm water away.
2. Cold, nutrient-rich deep water rises to replace it.
3. Common along western coasts of continents (e.g., California, Peru, Namibia).

2. Coriolis Effect

1. Due to Earth’s rotation, moving water is deflected:
   1. To the right in the Northern Hemisphere.
   2. To the left in the Southern Hemisphere.
2. This deflection helps move surface waters away from the coast.

3. Ekman Transport

1. Describes how surface water moves at an angle (≈45°) to the wind direction due to friction and Coriolis forces.
2. This creates a spiral movement where deeper layers move further from the coast, facilitating upward water replacement.

4. Seasonal and Lake Upwellings

1. In stratified lakes, seasonal winds or temperature shifts break stratification.
2. Cold bottom water rises during spring or autumn turnover, redistributing nutrients and oxygen.

Process of Coastal Upwelling

1. Winds blowing parallel to the coast push warm surface water offshore.
2. Cold, nutrient-rich water rises from depths (100–300 m) to replace it.
3. This cold water cools coastal climates and supports dense phytoplankton blooms, which are visible as greenish swirls in satellite images.
4. These blooms feed zooplankton, small fish, and ultimately top predators such as tuna, seabirds, and whales.

Ecological and Economic Importance

1. Upwelling zones are among the most productive marine ecosystems.
2. Cold, nutrient-rich waters support phytoplankton blooms, forming the base of the food web.
3. This supports vast populations of zooplankton, fish (anchovies, sardines), and marine mammals.

Upwelling and ENSO (El Niño Southern Oscillation)

1. During normal conditions, trade winds blow from east to west across the Pacific, pushing warm surface water westward.
2. This allows cold, nutrient-rich water to rise off the coast of South America (Peru and Ecuador).
3. During El Niño, these trade winds weaken or reverse, reducing upwelling.
   1. Surface water remains warm.
   2. Nutrient input declines.
   3. Phytoplankton and fish populations drop dramatically.
4. During La Niña, trade winds strengthen, increasing upwelling and boosting productivity.

Risks and Threats to Upwelling Systems

1. Overfishing: Overexploitation of nutrient-rich waters can lead to ecosystem collapse.