Week 4: Atmosphere and Ocean Circulation

Where does the energy in Earth’s atmosphere come from?

the sun (solar irradiance)

Is energy in the Earth’s atmosphere spread equally?

• No

• differential heating by latitude

What causes seasons?

• Elliptical orbit

• Varying Earth-Sun distance

Difference between weather and climate

• Time period

• Climate = 30 years or more

Vertical pressure gradient force

the difference between the force of the air molecules pushing downward and the force of the air molecules pushing upward.

Hydrostatic equilibrium

When the vertical pressure gradient force, (directed upward), and gravity, (directed downward), are in balance, the parcel moves neither up nor down

Hydrostatic Equation

• Change in pressure with height is proportional to air density and the gravitational acceleration:

  d𝑃/ d𝑧 = −𝜌g

What is density of air a function of?

• function of temperature and moisture content → increasing water vapor or air temperature will cause the density to decrease

• Thus, d𝑃/ d𝑧 is large in cold air (the change in pressure with height is large →the pressure decreases rapidly in cold air and the thickness of a cold air mass is small).

• Warm air expands and takes up larger volume.

What is geopotential height?

Height of a surface of constant pressure

Where was geopotential height the highest?

• At equator

• warmer

A meridional (north-south) temperature gradient

• = meridional pressure gradient

• pressure gradients drives winds, which transports atmospheric energy poleward in both hemisphere

Movement of air high in the atmosphere

• air from the region of high pressure moves toward the lower pressure

• warm air at the equator creates higher pressure that moves air towards the polar regions

Movement of air at sea level

• air is moving from the high pressure in the cooler column toward the lower pressure in the warmer column

• Notice: pressure at the bottom column change as molecules leave the warm column at upper levels and are added to the cold colum

Single Cell Circulation

• If Earth had no oceans or mountains and did not rotate

• Differential heating by latitude

• Transport of heat energy equator to pole

Circulation Cells (Equator→ Poles)

1. Hadley

2. Ferrel

3. Polar

Effect of Coriolis Force

• Deflects direction of motion to the right in the N. Hemisphere

• Deflects direction of motion to the left in the S. Hemisphere

Coriolis Paramtere

f = 2ω sin ϕ

ω the angular velocity of Earth’s rotation

ϕ the latitude, ρ the air density (mass per unit volume)

Coriolis Equation

where:

u is the zonal wind speed (+ eastward)

v the meridional wind speed (+ northward)

How does the coriolis parameter (f) differ from equator to the poles?

sin(0)=0 → no effect at latitude 0

sin(90)=1

Coriolis effect is at its maximum at the poles.

Conservation of Angular Momentum

relative speed of air increases as it moves away from the equator

Jet Streams

• Fast-flowing air currents > 300 km/h

• 9 – 16 km above the surface

• Ribbon-like: hundreds of km wide, a few km deep

• Stronger in winter than summer

• Polar + subtropical jet streams

• driver of polar jet stream = temperature contrast

Net radiation/ Net Flux

the balance between incoming and outgoing energy at the top-of-the-atmosphere (TOA)

𝑅_TOA = S𝑊 𝑖𝑛 ( 1 − 𝛼) − 𝐿𝑊_out

radiation at the top of the atmosphere = short wave in (1-albedo) - long wave out

Poleward energy flux

The time rate change of energy content of the climate system is the balance of the net incoming radiation at the TOA and the divergence of the horizontal energy flux in the atmosphere and ocean

How is heat transported to the poles?

• sensible heat

• latent heat

• geopotential

• kinetic energy

What is an adiabatic process?

a process which involves no transfer of heat to or from a system

Dry adiabatic lapse rate

The rate at which the temperature of such a dry air parcel falls with altitude

Specific Heat

The amount of heat required to warm something:

• For water this is 4.2 kJ kg^-1 K^-1

• For air it is 1.0 kJ kg^-1 K^-1

Latent Heat

The heat exchanged when the phase of matter changes

• takes 336 kJ kg^-1 to melt ice

• takes 2230 kJ kg^-1 to evaporate water

Water changing phases

• Water carries lots of energy when it changes phases

• The energy released by condensing water drives many of the extremes of weather

• Steam burns because water vapour condenses on you, releasing heat

The moist adiabatic lapse rate

• The warmer an air parcel is, the more water vapour it can hold.

• As a moist air parcel rises it cools and its saturation water vapour content drops

• As it cools water vapour condenses, releasing energy and offsetting some of the cooling

• The moist adiabatic lapse rate is therefore lower than the dry one

What happens when a warm air parcel rises through the atmospheric column?

• As the warm air parcel from the surface rises it will warm the air throughout the atmospheric column.

• After some time the atmospheric column will reach a new equilibrium temperature profile

The Troposphere

• the lower atmosphere

• most of the atmosphere by mass

• temperature drops with altitude→ reaches a minimum which marks the boundary with the stratosphere

The Stratosphere

• Layer above the troposphere

• Contains the ozone layer

• Temperature rises with altitude

Absorption of light by the atmosphere

• Water vapour absorbs some bands of the near-infrared

• Ozone absorbs UV strongly

• Ozone forms in the stratosphere and absorbs so much UV it warms it substantially

What causes ocean currents

wind, density differences in water masses caused by temperature and salinity variations, gravity, and events such as earthquakes or storms

Role of Salt in Seawater

• salt content of water dictates how dense it is

• dense, salty water more dense than cold water (in example)

Thermohaline Circulation

driven by temperature and salinity gradients

Sea ice vs. icebergs

sea ice= frozen sea water→ not icebergs

iceberg = fresh-water → form on land (break off glaciers)

Why is sea important for the climate

freezing temp of salt water = -1.8 C

salinity of sea ice is lower than salinity of sea-water → salt ejected→ sea water sinks and floats downwards → deep ocean circulation

sea ice = higher albedo

El Niño:

• ~Every 2-7 years

• Weaker trade winds

• Less warm water moved West

• Less upwelling and even downwelling in the East

• Altered thermocline

• Changes to precipitation, drought, warming

• Global effects (e.g. can increase likelihood cold winter in UK)

→ La Niña can follow El Niño

North Atlantic Oscillation (NAO)

• Defined by two pressure centers in the North Atlantic:

  • Low pressure located near Iceland

  • High pressure over the Azores

• Fluctuations in the strength of these pressures significantly influences the jet stream and therefore impacts temperature and precipitation.

What does a positive phase of (NAO) mean?

• Icelandic low is stronger and the Azores high is higher.

• This results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength.

• Increased westerlies bring more moisture from the Atlantic over Europe

Effect of positive NAO on 500 mb geopotential height

• Below average heights over the icelandic region

• above average heights over the western Atlantic

Effect of negative NAO on 500 mb geopotential height

• above average heights located over the icelandic region

• below average heights are located in the western Atlantic