5/4/10 MET 61 topic 04 1

MET 61

Topic 4

Climate & Climate Dynamics

5/4/10 MET 61 topic 04 2

Climate

– A time average of weather– Typically over 30 years

Climate Variability

– Variations year-to-year– Decade-to-decade

5/4/10 MET 61 topic 04 3

Climate is complicated! We must study:

– External factors (oceans, sun etc.)– How the atmosphere behaves…

• Day-to-day• Month-to-month• season-to-season• Year-to-year• Decade-to-decade

– Need to fully understand this variability to best understand anthropogenic changes

5/4/10 MET 61 topic 04 4

Source of Climate Variability?

External? – SSTs– Biosphere– Cryosphere– Solar variations (e.g., sunspots)– Volcanic activity– Anthropogenic activity

5/4/10 MET 61 topic 04 5

Source of Variability?

Internal?– Internal to the basic dynamics of the earth-

atmosphere system?

• With fixed forcing, we would still year year-to-year variability etc.

– This is internal variability

5/4/10 MET 61 topic 04 6

Present climateAnnual surface pressures

5/4/10 MET 61 topic 04 7

World climate map

5/4/10 MET 61 topic 04 8

Present climate

– Read §10.1

– The result of net radiative forcing

– Solar heating (positive) – infrared cooling (negative)

– Recall Ex. 4.6 (p.119)

5/4/10 MET 61 topic 04 9

– Solar constant (So) is 1368 W m-2

– Intercepted by area Re2

– And assuming an albedo of A=0.3

– Incoming radiation =• (1-A).So.(Re

2)

– Outgoing is emitted by the earth’s surface (area 4Re

2) according to Stefan-Boltzman

– Outgoing radiation = .Te

4.(4Re2)

5/4/10 MET 61 topic 04 10

– Equate and solve for Te.... Te = 255 K

– This is the radiative equilibrium temperature

– The observed global mean temperature is 288 K … 33 K warmer

– Difference is due to the greenhouse effect of the atmosphere with greenhouse gases

– Above is modified by emissivity factors etc.

5/4/10 MET 61 topic 04 11

– Fig. 10.2 → annual average net radiative driving of the earth-atmosphere system

– High latitudes: deficit– Low latitudes: surplus

Equator → pole transport(s) required

– Baroclinic eddies in mid-latitudes– Hadley cells in tropical latitudes– Oceans

5/4/10 MET 61 topic 04 12

– Fig. 10.5 → annual variation of incident radiation @ top of atmosphere

→ winter-summer driving of climate

– Fig. 10.6 → net radiation @ top of atmosphere