Changes%20in%20circulation%20patterns%20due%20to%20air%20pollution

1
Changes in circulation patterns due to air
pollution
IIASA, Vienna Jan 27, 2003
Surabi Menon, Jim Hansen and Larissa Nazarenko
NASA Goddard Institute for Space
Studies/Columbia University

• What do we know about anthropogenic influences on
  climate?
• Greenhouse gases Warmer, wetter climate
• Sulfate aerosols Cooler, dryer climate
• Carbonaceous aerosols Organic and Black Carbon??

2
Dynamical ResponseRainfall anomalies and 500 hPa
Geopotential height

• Strong correlation between summer rainfall and F
  500 anomaly from 1957 to 1999. From 1979,
  corresponding to positive rainfall anomalies,
  geopotential height also shows a jump. Gong and
  Ho, 2002, GRL
• Variation of the F 500 over western Pacific ---
  indicative of subtropical northwestern Pacific
  high (SNPH) activity.
• Stronger and equator-ward shift of SNPH gt
  anti-cyclonic circulation anomaly over south
  China Sea and to western Pacific gt causes
  prevailing southerlies over Southern China gt
  flooding along Yangtze.
• High SST conditions gt strengthening of
  sub-tropical high and movement southwards.
• Time lag of 1-2 seasons for circulation response.

3
Radiative Forcings (Hansen et al. 2001)
4 3 2 1 0 -1 -2 -3 -4
Greenhouse gases
Solar Irradiance
Ozone
F (W m-2)
Trop. aerosols
Stratos. aerosols
1850 1900 1950 2000
2050
4
Greenhouse gases, Aerosols and Summer Monsoon

• In a transient climate simulation with greenhouse
  gases and sulfate aerosols (Roeckner et
  al.,1999)
• Poleward shift of midlatitude westerlies and
  sea-level pressure patterns (mainly due to
  greenhouse gases).
• Over Pacific and Indian ocean, precipitation
  increases along equator.
• No weakening of Asian summer monsoon (smaller
  direct effect over land).
• Analysis from Douville et al. (2000) on effects
  of doubled CO2 on Asian summer monsoon indicate
• a general weakening of the monsoon circulation
  and
• factors other than large-scale dynamics play a
  role in the monsoon response.
• Increased rainfall during greenhouse gas induced
  warming was related to increased water content
  (horizontal transport and precipitation
  efficiency) rather than to changes in
  circulation.

5
Asian summer monsoon

• Surface radiation primary energy source of summer
  monsoon
• Reduction in surface radiation results in
  reduction of SH and LH, suppresses continental
  heat and modifies moisture transport through
  change in low-level pressure field.
• Aerosols and land-cloud enhancement significantly
  delay the northward migration of typical
  precipitation bands around east Asia (Iwasaki and
  Kitagawa, J. Metorol. Soc. Jap, 1998)
• reduction of water vapor transport due to
  southwesterlies over southeast Asia and adjacent
  oceans.

6
Black Carbon Inventories BC emissions 1996 in GgT
per 1x1 cell
(T.C. Bond et al. 2002)
7
Aerosol Optical Depth
Aerosol optical depths used in GCM experiments to
identify changes in climate relative to a control
run with 1950 aerosol optical depths. Over China
data is from Luo et al. (2001, JGR). Over India
data is from Collins et al. (2001, JGR).
2 scenarios Sulfate and Black carbon Single
scatter albedo 0.85 EXP A Sulfate only
Single scatter albedo 1.0 EXP B
8
Jun-Jul-Aug Change in GCM temperature and
precipitation
DTs (K) Exp. A 0.08
DPrec (mm/d) Exp. A -.02
DTs (K) Exp. B -0.03
DPrec (mm/d) Exp. B 0.0
9
Jun-Jul-Aug Change in GCM Net Radiation
Top of Atmosphere
Surface
10
Pressure vs. Latitude plot of change in GCM
vertical velocity winds for the region between
90E to 130E and from 2 S to 62N. Reds indicate
rising air and blues indicate subsidence.
Jun-Jul-Aug Vertical Velocity
Exp. A
Exp. A
P (hPa)
Latitude
11
JJA Change in GCM sea-level pressure
Jun-Jul-Aug Change in GCM Sea-level Pressure
Exp. A
Exp. B
12
Jun-Jul-Aug Change in GCM Surface wind
Southwesterly winds bring in moist air from the
South China Sea and Indian Ocean region
13
Jun-Jul-Aug Change in GCM upper level wind
Anticyclone centered over the Tibetan plateau
upper level westerlies and easterlies to north
and south of anticyclone
14
Summary

• BC effects on surface and TOA radiation budget
  are unlike other aerosols and greenhouse gas
  effects.
• Absorption by BC ? lower level heating, change in
  vertical motions, large-scale circulation, and
  thus cloud cover and rainfall.
• Similar findings have been reported by Chung,
  Ramanathan and Kiehl (J. Climate, 2002) for the
  INDOEX region.
• Aerosol vertical distribution is important and
  can affect the climate features we observe.
• Besides these climate effects, effects of BC on
  human health are also quite detrimental.

15
Future trends in aerosols (IPCC, 2001)
16
REDUCTIONS ??