Marine Science (0697) | Upwelling

🌊 Upwelling: Bringing the Deep Ocean to Life

Hello Marine Scientists! This chapter introduces one of the most vital processes in the ocean: Upwelling.
It might sound like a simple movement of water, but upwelling is responsible for creating some of the richest fishing grounds on Earth. It is the ocean's natural way of recycling nutrients, directly linking the chemistry of the deep sea to the productivity of the surface.

Let's dive into how this process works, why it matters, and what happens when it stops (hello, El Niño!).

1. The Mechanics of Upwelling (2.6.1)

Upwelling is defined as the process where deep, cold, nutrient-rich water rises to replace warm, nutrient-poor surface water that has been moved away.

How Winds Cause Upwelling

The primary driver for most major coastal upwelling systems is wind.

Step-by-Step: Wind-Driven Upwelling

1. Wind Blows Surface Water: Steady, strong winds (often the prevailing winds) blow parallel to the coast, or slightly offshore.
2. Water is Moved Away: Due to the spinning of the Earth (the Coriolis effect), the surface water is pushed away from the shore (or offshore). Think of the wind acting like a giant broom sweeping the top layer of water.
3. Creating a Vacuum: As the surface water is dragged away, a 'gap' or void is created near the coast. Nature doesn't like a void!
4. Deep Water Rises: To fill this space, water from deeper down is pulled up towards the surface. This is the upwelling.

Analogy Tip: Imagine taking a straw and quickly sucking a layer of oil off the top of a glass of water. The cold water immediately rushes up to replace the layer you removed.

The Crucial Difference: Nutrients

The water at the surface is usually warm and contains few nutrients because photosynthetic organisms (like phytoplankton) rapidly use them up.

The water sinking deep down, however, carries with it dead organisms and waste products (called marine snow). As this material decays, the deep water becomes incredibly rich in essential elements like nitrate and phosphate.

When upwelling brings this cold, deep water to the sunlit surface (the photic zone), it provides a massive fertilizer boost.

2. Upwelling and Productivity (4.3.8)

Upwelling areas are the most biologically productive regions in the entire ocean.

The sudden availability of nutrient-rich water at the surface allows producers (mainly phytoplankton, like diatoms and dinoflagellates) to grow and reproduce rapidly in huge numbers. This explosion of growth is called an algal bloom.

This vast population of producers forms the base of the food chain, supporting large populations of zooplankton, fish, squid, marine mammals, and seabirds.

Example: Areas off the coasts of Peru and California, where strong persistent winds drive upwelling, are famous for their enormous fisheries.

Key Takeaway: Upwelling is vital because it replenishes the nutrients necessary for primary productivity, fueling the marine food web.

3. The Disrupter: El Niño (2.6.2 & 2.6.3)

Upwelling is usually a steady process, but sometimes, global climate patterns disrupt it. The most famous disruption is called El Niño (officially part of the El Niño-Southern Oscillation, ENSO).

What is El Niño?

El Niño is a natural climate pattern that occurs every few years, characterized by unusual warming of the sea surface temperature in the central and eastern Pacific Ocean.

The Formation of El Niño (2.6.2)

Normally, strong trade winds blow warm surface water from the eastern Pacific (near South America) towards the western Pacific (near Australia/Asia). This causes upwelling in the eastern Pacific, bringing up cold water.

During an El Niño event, two things happen:

1. Reduction or Reversal of Trade Winds: The usual strong easterly trade winds weaken significantly or even reverse direction.
2. Increased Surface Water Temperature: Since the winds aren't pushing the warm water west anymore, the warm surface water pools and builds up in the eastern Pacific (e.g., Peru, Ecuador).
3. Reduced Upwelling: The lack of strong offshore wind means the nutrient-rich cold water cannot rise to the surface along the eastern Pacific coast.

Local Effects of El Niño (2.6.3)

The effects of El Niño are localized but often devastating to coastal economies and ecosystems:

A. Effects on the Eastern Pacific (e.g., Peru/Ecuador)

• Water Temperature (2.6.3a): Surface temperatures increase significantly (warmer than normal).
• Nutrient Availability (2.6.3b): Nutrient availability plummets due to the stop in upwelling.
• Ecosystem Impact: The sudden lack of nutrients causes a collapse in the phytoplankton population, leading to the collapse of the entire food chain, impacting commercially important fish like anchovies.
• Rainfall (2.6.3c): Increased warm water leads to high evaporation and heavy rainfall/flooding along the coasts of South America.

B. Effects on the Western Pacific (e.g., Australia/Asia)

While the Eastern Pacific gets warm and wet, the Western Pacific experiences the opposite due to the shift in atmospheric circulation:

• Rainfall (2.6.3c): This region experiences reduced rainfall, often leading to severe droughts and fires in places like Australia and Indonesia.

Summary of Upwelling and El Niño

Upwelling is a vital physical process driven by wind that introduces cold, dense, nutrient-rich water to the surface, supporting high productivity.
The El Niño climate event is characterized by weakened trade winds, which stops upwelling in the eastern Pacific, resulting in warmer, nutrient-poor water and reduced productivity.