Climates of the 20th and 21st Centuries Simulated by NCAR
Climate System Model
Dai, A., T.M.L Wigley, B.A. Boville, J.T. Kiehl, and L.E.
Buja
J. Climate, submitted in Oct.1999, revised
in March 2000
The Climate System Model (CSM), a coupled global climate model without ``flux
adjustments'' recently developed at NCAR, was used to simulate the 20th century
climate using historical greenhouse gas and sulfate aerosol forcing. This
simulation was extended through the 21st century under two newly developed
scenarios, a business-as-usual case (ACACIA-BAU, CO2 ~710 ppmv in 2100) and a
CO2 stabilization case (STA550, CO2 ~540 ppmv in 2100). Here we compare the
simulated and observed 20th century climate, and then describe the simulated
climates for the 21st century. The model simulates the spatial and temporal
variations of the 20th century climate reasonably well. These include the rapid
rise in global-and zonal-mean surface temperatures since the late 1970s, the
precipitation increases over northern middle- and high-latitude land areas,
ENSO-induced precipitation anomalies, and pole-midlatitude oscillations (such
as the NAO) in sea-level pressure fields. The model has a cold bias (2-6°C)
in surface air temperature over land, overestimates of cloudiness (by 10-30%)
over land, and underestimates of marine stratus clouds to the west of North and
South America and Africa. The projected global surface warming from the 1990s to
the 2090s is ~1.9°C under the BAU scenario and ~1.5°C under the STA550
scenario. In both cases, the middle stratosphere cools due to the increase
in CO2, whereas the lower stratosphere warms in response to recovery of the
ozone layer. As in other coupled models, the surface warming is largest at
winter high latitudes (>5.0°C from the 1990s to the 2090s) and smallest
(~1.0°C) over the southern oceans, and is larger over land areas than ocean
areas. Globally averaged precipitation increases by ~3.5% (3.0%) from the 1990s
to the 2090s in the BAU (STA) case. In the BAU case, large precipitation
increases (up to 50%) occur over northern mid- and high-latitudes and over
India and the Aradian Peninsula. Marked differences occur between the BAU and
STA regional precipitation changes resulting from inter-decadal variability.
Surface evaporation increases at all latitudes except for 60-90°S. Water
vapor from increased tropical evaporation is transported into mid- and
high-latitudes and returned to the surface through increased precipitation
there. Changes in soil moisture content are small (within ±3%). Total
cloud cover changes little, although there is a upward shift of middle-level
clouds. Surface diurnal temperature range decreases by about 0.2-0.5°C
over most land areas. 2-8 day synoptic storm activity decreases (by up to 10%)
at low latitudes and over midlatitude oceans, but increases over Eurasia and
Canada. The cores of subtropical jets move slightly up- and equator-ward.
Associated with reduced latitudinal temperature gradients over mid-and
high-latitudes, the wintertime Ferrel cell weakens (by 10-15%). The Hadley
circulation also weakens (by ~10%), partly due to the upward shift of
cloudiness which produces enhanced warming in the upper troposphere.
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Hongjun Zhang:
zhangho@ucar.edu