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.
2. Climate Change: Altered wind patterns and warming surface layers may reduce upwelling intensity.
3. Pollution: Runoff and eutrophication can disrupt the delicate nutrient balance.
4. Ocean Acidification: Increased CO₂ reduces calcium carbonate availability, affecting shell-forming organisms in upwelling zones.

Thermohaline Circulation and the Ocean Conveyor Belt

1. The thermohaline circulation (THC) is the global system of ocean currents driven by differences in temperature (thermo-) and salinity (haline), which together determine water density.
2. It is also called the global ocean conveyor belt because it circulates water across all major oceans, redistributing heat, nutrients, and gases around the planet.

Mechanisms of the Thermohaline Circulation

1. Temperature and Salinity Gradients

1. Cold, salty water is denser and tends to sink, while warm, less saline water is less dense and remains near the surface.
2. These density differences create a vertical movement of water masses, establishing global current systems.

2. Deep Water Formation

1. Occurs mainly in polar regions (North Atlantic and around Antarctica).
2. When seawater freezes, freshwater ice forms, leaving behind saltier, denser brine, which sinks to form deep ocean currents.

3. Surface Currents

1. Warm tropical water flows poleward via surface currents such as the Gulf Stream, carrying heat from the equator toward higher latitudes.
2. As it cools and increases in salinity, it becomes dense enough to sink, creating a continuous circulation loop.

Global Conveyor Belt

1. The sinking cold water flows through deep ocean basins toward the equator.
2. Eventually, it rises (upwells) in the Indian and Pacific Oceans, completing the circulation that can take up to 1000 years for one full cycle.

Importance of the Thermohaline Circulation

1. Global Heat Distribution

1. Moves heat from equatorial to polar regions, moderating global climate.
2. Western Europe’s mild climate results from warm North Atlantic currents.

2. Carbon and Oxygen Transport

1. Deep circulation stores and redistributes CO₂ and O₂ throughout the oceans.
2. Helps regulate atmospheric CO₂ levels and influences carbon sequestration.

3. Nutrient Cycling

1. Deep currents bring nutrient-rich waters from the ocean floor to the surface.
2. Supports marine productivity similar to upwelling zones.

Role in Climate Regulation

1. The THC transports vast amounts of heat, maintaining a balance between tropical and polar regions.
2. It acts as a climate regulator, moderating temperature extremes across continents.
3. Any change in this system affects global weather, precipitation patterns, and marine ecosystems.

Consequences of Thermohaline Circulation Changes

1. Climate Disruption: Weaker currents mean less heat transport—potentially leading to cooler northern climates and more extreme weather patterns.
2. Reduced Oxygen and Nutrient Mixing: Stagnant deep water limits nutrient upwelling, reducing marine productivity.
3. Carbon Cycle Effects: Less carbon is transported to the deep ocean, weakening a major carbon sink.
4. Regional Climate Effects: Possible cooling in North Atlantic regions and warming in tropical waters.

Interconnection Between Upwelling and Thermohaline Circulation

1. Both processes are linked through vertical water movement.
2. Upwelling occurs locally, bringing nutrients and CO₂ to the surface.
3. Thermohaline circulation operates globally, moving heat and salt over centuries.
4. Regions of deep-water formation are balanced by upwelling zones, maintaining equilibrium in ocean chemistry and energy transfer.