Overview
This lecture explains how differences in water temperature and salinity create density layers in the ocean, focusing on the concept of the pycnocline and its effect on mixing, waves, and ocean biology.
Density Differences in Ocean Water
- Salty water is denser (weighs more) than fresh water of equal volume because salt adds mass.
- Cold water is denser than warm water of equal volume.
- Cold, salty water tends to sink, while warm, fresh water stays near the surface.
Ocean Stratification and the Pycnocline
- The ocean is layered: warm, fresh water at the surface; cold, salty water at depth.
- The pycnocline is the region of strong vertical density gradient separating these layers.
- The pycnocline is important for both physical and biological ocean properties.
Internal Waves and Density Layers
- Internal waves form within the ocean at the pycnocline and move slowly but have large vertical amplitudes (tens of meters).
- Surface waves move rapidly and are much smaller due to the large density difference between water and air.
- The restoring force for internal waves is weaker due to the small density differences between layers.
Demonstration Insights
- Mixing water of intermediate density results in it settling between less dense and more dense layers, forming a distinct middle layer.
- Internal waves can move slowly across large vertical distances inside the ocean.
Mixing and Biological Implications
- The pycnocline acts as a barrier to mixing between surface and deep waters.
- It requires significant energy (e.g., from winds) to mix nutrients up through the pycnocline to the surface.
- Phytoplankton need both sunlight (at surface) and nutrients (at depth), relying on wind-mixing to bring nutrients upward.
Summary of Key Concepts
- The ocean is stratified with light (warm, fresh) water on top and dense (cold, salty) water below.
- Internal waves in the ocean are much slower and larger than surface waves.
- The pycnocline resists mixing, limiting nutrient transport to surface waters.
Key Terms & Definitions
- Density — mass per unit volume; affects buoyancy in water.
- Pycnocline — the ocean layer with a rapid vertical change in density.
- Stratification — formation of water layers based on temperature and salinity differences.
- Internal wave — a wave that travels along a density interface within the ocean.
Action Items / Next Steps
- Review the role of the pycnocline in nutrient mixing for ocean biology.
- Prepare to discuss how winds affect nutrient transport across the pycnocline.