Density-driven circulation

Question 1

What does seawater density depend on?

Answer 1

• Temperature (T)
• Salinity (S)
• Pressure (p) (although this is usually negligible)

Question 2

What is true of temperature and salinity?

Answer 2

It is controlled by SURFACE processes
→ heating and cooling
→ evaporation and precipitation

depends on climate (i.e. LATITUDE)

Question 3

What is true for 75% of ocean waters?

Answer 3

T = 0-5°C
S = 34-35 ppt

Cold → Bulk of ocean water originates in polar regions
Uniform → Ocean basins are not closed

Question 4

What is true of T-S characteristics of seawater?

Answer 4

• Seawater changes T-S characteristics when at surface
• Once water sinks its T-S values become ‘fixed’

Question 5

What are the different types of water masses?

Answer 5

• Deep (>~2km)
• Intermediate (~1-2km)
• Central (~0-1km)

From different source regions
With different T-S characteristics

Question 6

What is true of water masses?

Answer 6

• They can be used to trace water flows
• Mixing occurs but is usually slow (gradually modifies T-S values)

Question 7

What is density-driven flow?

Answer 7

Deep water motion driven by density differences between water masses due to:
- Temperature differences → THERMAL effect
- Salinity differences → HALINE effect

→ THERMOHALINE circulation

Question 8

What is the process of thermohaline circulation?

Answer 8

1. Temperature decreases with latitude and depth
2. Cold air cools surface, ice formation rises salinity
3. Dense water sinks and spreads throughout ocean basins (sinks and moves under less dense water)
4. Mixing raises deep water upwards to surface
5. Solar heating offsets high evaporation → thin lens of warm salty water

Question 9

Where do deep water masses form?

Answer 9

• Polar seas
• Relatively shallow basins