Blue Planet – Introduction to Marine Biology: Deep Ocean Currents and Vertical Motion

Understand deep ocean currents driven by density gradients, and recognize their importance in global climate regulation and nutrient distribution across ocean systems.

Define thermohaline circulation, which is essential for understanding how varying temperature and salinity affect water density and contribute to deep ocean currents. Describe upwelling, the process where deep ocean water rises to the surface, and Ekman transport, which influences the movement of surface waters through wind-driven currents and contributes to ecological productivity.

Explain the Atlantic Meridional Overturning Circulation (AMOC), a critical component of the global thermohaline circulation, emphasizing its role in transporting warm water from the tropics towards the North Atlantic and how it impacts weather patterns and climate in various regions.

Ocean Currents

Surface currents vs Deep ocean currents:

• Surface currents are primarily driven by wind patterns, influenced by the Earth's rotation (Coriolis effect), while deep ocean currents are driven by differences in water density created by variations in temperature and salinity.

• Surface currents typically circulate in the upper 400 meters of the ocean and can significantly affect climate by redistributing heat, while deep currents can reach depths of several thousand meters and play a crucial role in global ocean circulation.

Primary Productivity

Primary productivity refers to the synthesis of organic compounds from carbon dioxide (CO₂) through processes such as photosynthesis, primarily by organisms like phytoplankton.

• Nutrient availability, including nitrogen, phosphorus, and trace metals, profoundly impacts productivity levels across different oceanic regions. High productivity is typically found in areas near coasts and at the equator due to nutrient upwelling, whereas ocean gyre centers are characterized by low productivity owing to nutrient depletion and stratification.

Ekman Transport

Ekman transport is a wind-driven vertical circulation effect that causes surface currents to spiral due to the Coriolis effect, which varies with latitude.

• This spiral movement is crucial for the distribution of nutrients in the water column and drives processes such as upwelling and downwelling:

  • Upwelling occurs when cold, nutrient-rich water rises to the surface, significantly enhancing biological productivity in those areas.

  • Downwelling refers to the sinking of surface water, often resulting in lower productivity levels due to reduced nutrient availability in those regions.

Upwelling and Downwelling

• Equatorial Upwelling: At the equator, trade winds create surface water divergence, allowing nutrient-rich waters from below to rise, supporting high fisheries productivity.

• Coastal Upwelling: Winds can push surface waters away from the coast, allowing deeper waters to rise and replace them, bringing nutrients to the surface and sustaining marine life.

• Coastal Downwelling: This process entails the sinking of surface water, typically occurring in subtropical regions, where warm surface waters converge, leading to areas of lower biological productivity.

Thermohaline Circulation

Thermohaline circulation is driven primarily by density differences, which arise from temperature variations (thermal) and salinity levels (haline).

• It plays a key role in global heat transport across the oceans; deep water formations at the poles, such as Antarctic Bottom Water and North Atlantic Deep Water, are fundamental to this process.

• This interconnected oceanic “conveyor belt” significantly affects global climate and nutrient cycling, taking about 1,000 years for water to complete a circumnavigation of the globe.

Atlantic Meridional Overturning Circulation (AMOC)

AMOC is a significant part of global thermohaline circulation that includes a northward flow of warm, salty water in the upper layers of the Atlantic Ocean and a southward flow of cold, dense water at greater depths.

• Variations in AMOC strength can lead to significant shifts in global climate patterns, affecting temperature and precipitation distribution across multiple regions, including Europe and North America.

Effects of Weakened Thermohaline Circulation

A potential decrease in the strength of thermohaline circulation may lead to shifts in ocean heat distribution, impacting climate systems.

• This could result in more extreme weather events, changes in marine biodiversity, and disruptions to global carbon cycling, highlighting the critical nature of ocean currents in maintaining ecological balance.

Review Questions

2. What is Ekman transport, and how does it lead to coastal upwelling?

3. Why does downwelling occur in subtropical ocean gyres?

4. Describe AMOC and provide a diagram illustrating its circulation patterns.

5. Discuss the impacts of weakened thermohaline circulation on ocean heat distribution and potential climate consequences.