1. Chris Parkes Rm 455 Kelvin Building
Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 7: Water
Chris Parkes
Rm 455 Kelvin Building

2. 6. Wind Wind Forces: Global wind belts Air masses & Fronts Jet Streams
Revision
6. Wind
trade winds
westerlies
easterlies
jet streams
ITCZ, Doldrums
Polar Front
Wind
Forces:
Pressure gradient
Coriolis
Centripetal
Friction
Global wind belts
Air masses & Fronts
Jet Streams 2

3. The Effects of Waters * Heat Capacity * Wind – Water interaction
* Ocean Circulation * El Nino

4. Heat Capacity
Heat Capacity: ratio of heat absorbed by substance to rise in temperature C = ΔQ / ΔT
Specific Heat: heat needed to raise given amount of substance by 1ºC (or 1K)
Top 2.5m of water holds as much energy as all atmosphere
Latent Heat: energy required to change from solid to liquid, and liquid to gas (latent heat of evaporation of water = 2.5×106 J/kg)
Substance
Specific Heat (J/kg.K)
Water
4814
Wet mud
2512
Sand
840
Dry air
795

5. Smaller scale convection – Sea Breezes (again)
Land heats up quicker than sea
Air above land begins to rise
Sea air moves inland since rising air above
land produces lower pressure
For equal amount of heating of land & sea (assume same mass)
relative change in T is ratio of specific heats
Also, release of latent heat due to condensation of water vapour drives further convection

6. Additional Strength & Direction factors:
Ocean Currents
Driven by:
Wind; Coriolis Force;
temperature & Salinity differences;
tides caused by gravitational pull of moon & sun
Additional Strength & Direction factors:
Depth contours; shoreline; other currents
Important role
in determining climate
e.g. Gulf Stream:
Northwest Europe
more temperate
than other regions
at same latitude

7. Thermohaline current “global conveyor belt”
Color key is NOT temperature!
Salinity and temperature driving currents (as well as wind)
Low temperature more dense – sinks
High salinity more dense sinks

8. Upper ocean layer interactions
Surface current wind driven
- clockwise spirals in the northern hemisphere
- counter-clockwise rotation in the southern

9. Wind-water interaction: waves
Waves caused by frictional force of water on the bottom layer of wind
Growth of surface wave depends on wind speed and duration

10. Wind-water interaction: waves
The speed of shallow water
waves is independent of
wavelength or wave period
- determined by the depth of water
The speed of deep water waves
is independent of the depth
- determined by wavelength &
period of waves
Wave continues after wind ceases

11. Ekman spiral Spiral of currents or winds near a boundary
Results from Coriolis Force
Opposite direction in North/South hemisphere
Wind blows on ocean surface force shown in red
Coriolis Force perpendicular force shown in yellow
Net force in pink on layer below
Coriolis force at right angles to new force
- hence causes spiral
Wind
Coriolis

12. Ocean Layers Ocean layers: Mixed Layer Thermocline Deep Water
mixed by wind, turbulence, convection;
Sunlight absorbed in first few cm
Temperture warm, mixed
Temp. varies day/night
Thermocline
Region where temperature reduces
Boundary layer
Typically 100m deep
Deep Water
colder, not mixed
stable temperature
Temperature
Mixed layer
Thermocline
Deep water
Depth

13. Ocean Currents
Currents –across pacific

14. Constant winds “pile-up” water - Pacific Trade winds
Pacific trade winds (easterlies)
Constant flow EW
Peru (East Pacific) – Indonesia (West Pacific)
Δh of surface height ~ 50cm
Drives ocean currents
Upwelling
Easterly trade wind
West
East
Thermocline
Coast of Peru: Upwelling
Cooler water driven to ocean surface ~23ºC
nutrients rich for fish
Mixed layer typically 50 m deep
Coast of Indonesia:
Warmer water ~27ºC
Mixed layer typically 200 m deep

15. Walker Circulation A model of zonal (east-west) air flow Anomalies
El Nino
Southern Oscillation

16. El Niño-Southern Oscillation (ENSO)
El Niño: effect in water – temperature changes
Breakdown of Walker circulation
Southern Oscillation: effect in atmosphere
- air pressure changes
ENSO is associated with floods, droughts
Normal pattern
El Niño: Warm water pool approaches South American coast.
Absence of cold upwelling increases warming.
Occurs every 3-7 years
Lasts 1-2 years

17. El Nino – Effects on Climate
Ocean Temperature & Height Anomalies
Height
Temperatures & Winds
Surface height key:
Purple < -18cm; Green – normal; red +10 cm; white +14 to 32 cm 17

18. El Nino – Effects on Climate
Causes changes in many parts of the world
Dry Areas
Wet Areas
Warm Areas
Cool Areas
Monsoon behaviour changes
Hurricane Formation changes
Poorly understood
Rate of El Nino has increased
Related to climate change?

19. Example exam questions
Q1. Explain the effect of El Nino on the temperature and rainfall over the coast of Peru.
Q2. Name three factors affecting the surface current of the ocean besides wind.
Q3. Draw a diagram to explain the Walker circulation.
Q4. What is the Ekman spiral ?
Next topic – Storm systems

21. El Nino – Walker Circulation