coupled Ocean-Atmosphere

General/Global Circulation Models

~100km horizontal res’n;Finer (~2) for ocean than atmosphere.

~20 layers atmosphere;~20 layers ocean.

Geography inevitably coarse.

“Regionalisation”: redo simulation of one region only (e.g. Europe) matched onto cruder global simulation

Basic Physics …

Conservation of fluid momentum:

Navier-Stokes eqns (-> fluids modules) in rotating frame (Coriolis effects)

Vertically: acceln negligible & balance forces;

Conservation of material:

air, water

salinity

Thermodynamics (conservation of energy);

Equation of State: (pressure, temperature,

salinity(ocean), water vapour(atmosphere))

Features and processes

Moist processes: Evaporation & condensation: clouds and latent heat

Radiation processes, light and IR

Sub-resolution convection -> effective viscosity & dispersion

Surface changes, e.g. of albedo and roughness

Atmosphere-Surface exchange: heat, vapour, liquid, momentum

Turbulence

<<10km:three

dimensional

>>10km:two

dimensional

Sub-resolution convection -> effective viscosity & dispersion

State of art ca 1997

Major Volcanos

-> Major volcanic eruptions are a visible and predictable perturbation on the climate.

Figure TS.23

IPC 2007

“Greenhouse gas forcing has very likely [90%]caused mostof the observed global warming over the last 50 years.”

IPCC 2007

….likely [67%]..IPCC 2001

IPCC 2007

Previous predictions … correct to show wide error bars

=Temperature rise from doubling CO2 (equivalent)

IPCC2007

PredictabilityGeneral claim:

Weather chaotic (beyond ~two weeks)

but wider/longer scale climate changes relatively predicable.

Evidence: Correlation of ice ages with Milankovitch cycles (of earth orbit) Successful replication of response to largest sudden terrestrial perturbations: Volcanoes El Nino changes in ocean circulation Replication of 20th climate trends

Exploit:

Expensive AOGCM calculations

-> calibrate simpler climate simulations

-> mass produce climate predictions for different future scenarios

SpecialReport on Emissions Scenarios

1. Population peaks mid century.A1: technology-led economy,

F fossil fuels vs ( B “balanced” ) vs T non-fossil fuelled.

not predictions, but a range of plausible assumptions

2. Population continues to increase. A2: very heterogeneous world (“business as usual”)

B2: lower growth rate; emphasis on local solutions (smart but laissez-faire)

B1: info & service economy; sustainability & global sol’ns.

B2

The global climate of the 21st century

IPCC S/PM 2001

Figure TS.28

IPCC 2007: Scenario -> OAGCM -> Climate prediction

Figure 10.4

IPCC 2007

IPCC 2007

Observed and projected changes in extreme weather and climate events.

IPCC S/PM 2001

OAGCM predictions (sample)

B2

annual mean change of the temperature (colour shading) and its range (isolines) (Unit: °C)

for SRES A2 and B2 .

period 2071 to 2100relative to 1961 to 1990

IPCC2001

Projected temperatures