Transcript Title here - University of Manchester

Planetary Atmospheres, the Environment and Life
(ExCos2Y)
Topic 7: Water
Chris Parkes
Rm 455 Kelvin Building
Revision
6. Wind
easterlies
Wind
jet streams
Forces:
Pressure gradient
Coriolis
Centripetal
Friction
Global wind belts
Air masses & Fronts
Jet Streams
westerlies
trade winds
ITCZ, Doldrums
Polar Front
The Effects of Waters
* Heat Capacity
* Ocean Circulation
* Wind – Water interaction
* El Nino
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)
Substance
Specific Heat (J/kg.K)
Water
4814
Wet mud
2512
Sand
840
Dry air
795
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)
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
Tland 4814

6
Tsea
840
Also, release of latent heat due to condensation of water vapour drives
further convection
• Driven by:
Ocean Currents
– 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
Upper ocean layer interactions
Surface current wind driven
- clockwise spirals in the northern hemisphere
- counter-clockwise rotation in the southern
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
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
Ekman spiral
•Spiral of currents or winds near a boundary
•Results from Coriolis Force
•Opposite direction in North/South hemisphere
Wind
Coriolis
1. Wind blows on ocean surface
force shown in red
2. Coriolis Force perpendicular
force shown in yellow
3. Net force in pink on layer
below
4. Coriolis force at right angles
to new force
- hence
causes spiral
Ocean Layers
• Ocean layers:
• Mixed Layer
Temperature
Mixed layer
Thermocline
–
–
–
–
mixed by wind, turbulence, convection;
Sunlight absorbed in first few cm
Temperture warm, mixed
Temp. varies day/night
• Thermocline
Deep water
Depth
– Region where temperature reduces
– Boundary layer
– Typically 100m deep
• Deep Water
– colder, not mixed
– stable temperature
Ocean Currents
Currents –across pacific
Constant winds “pile-up” water
- Pacific Trade winds
Pacific trade winds (easterlies)
Easterly trade wind
West
East
Upwelling
Thermocline
Constant flow EW
Peru (East Pacific) –
Indonesia (West Pacific)
Δh of surface height ~ 50cm
Drives ocean currents
• 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
Walker Circulation
• A model of zonal (eastwest) air flow
Anomalies
– El Nino
– Southern Oscillation
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
•Occurs every 3-7 years
•Lasts 1-2 years
El Niño: Warm water pool
approaches South American
coast.
Absence of cold upwelling
increases warming.
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
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?
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
El Nino – Walker Circulation