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