Chris Parkes Rm 455 Kelvin Building Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 7: Water Chris Parkes Rm 455 Kelvin Building
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
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
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
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
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 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
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
Ocean Currents Currents –across pacific
Constant winds “pile-up” water - Pacific Trade winds Pacific trade winds (easterlies) Constant flow EW 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
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 El Niño: Warm water pool approaches South American coast. Absence of cold upwelling increases warming. Occurs every 3-7 years Lasts 1-2 years
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
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