Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 7: Water Chris Parkes Rm 455 Kelvin...
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Transcript of Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 7: Water Chris Parkes Rm 455 Kelvin...
Planetary Atmospheres, the Environment and Life (ExCos2Y)
Topic 7: Water
Chris Parkes
Rm 455 Kelvin Building
6. Wind
WindForces:
Pressure gradient
CoriolisCentripetalFriction
Global wind beltsAir masses & FrontsJet Streams
trade winds
westerlies
easterlies
jet streams
ITCZ, Doldrums
Polar Front
Revision
The Effects of Waters
* Heat Capacity * Wind – Water interaction
* Ocean Circulation * 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)
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
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
6840
4814
sea
land
T
T
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 temperatethan 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
The speed of deep water waves
is independent of the depth
- determined by wavelength &
period of waves
Wave continues after wind ceases
The speed of shallow water
waves is independent of
wavelength or wave period
- determined by the depth of water
Wind-water interaction: waves
Ekman spiral
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
Wind
Coriolis
•Spiral of currents or winds near a boundary•Results from Coriolis Force
•Opposite direction in North/South hemisphere
Ocean Layers
• Ocean layers:• Mixed Layer
– 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
Thermocline
Deep water
Mixed layer
Temperature
Depth
Currents –across pacific
Ocean Currents
Pacific trade winds (easterlies)
Constant flow EW
Peru (East Pacific) – Indonesia (West Pacific)
Δh of surface height ~ 50cm
Drives ocean currents
Upwelling
Easterly trade windWest East
Thermocline
Constant winds “pile-up” water - Pacific Trade winds
• 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 (east-
west) 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.
Ocean Temperature & Height Anomalies
El Nino – Effects on Climate
Surface height key:Purple < -18cm; Green – normal; red +10 cm; white +14 to 32 cm
Height Temperatures & Winds
El Nino – Effects on Climate
Causes changes in many parts of the world
Dry AreasWet AreasWarm AreasCool Areas
Monsoon behaviour changes
Hurricane Formation changes
Poorly understood
Rate of El Nino has increasedRelated 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