Importance of Winds for Climate and Stratospheric Processes

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Importance of Winds for Climate and Stratospheric
Processes

• Mike Hardesty
• NOAA Earth System Research Lab
• Boulder, CO 80305

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Some Climate Topics where winds are important

• Change in intensity and positions of jet streams,
  polar vortices, storm tracks
• Changes in meridional moisture transport
• CO2 sources and sinks in the ocean
• Impact of synoptic scale transport on cloud
  formation and properties

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IPCC 2007 Winds

• Most findings based on reanalyses (ECMWF and
  NCEP)
• Hemispheric teleconnections strongly influenced
  by jet streams
• Some trends from reanalysis data
• weakening of the North Pacific winter jet since
  1987 (Nakamura et al, 2002) allowing efficient
  coupling of upper level disturbances with surface
  temperature gradients
• Deeper polar vortex and Iceland low
• Intensification and poleward displacement of the
  Atlantic polar front jet
• Associated enhancement of Atlantic storm track
  activity
• Strengthening tropospheric Antarctic vortex

4
Bengtsson Utility of reanalysis data for climate
trends

• Reanalyzed data sets overcome deficiencies
  associated with analysis of global observational
  records
• Reanalysis incorporating advanced data
  assimilation techniques best way to interpolate
  data in time and space and obtain dynamical
  consistancy
• Usefulness depends on quality and distribution of
  observations
• Fictitious trends can be generated by introducing
  new types of observations

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Example Observing System Change in 1978/1979
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Reanalysis Issues

• Atmospheric models used for reanalysis are prone
  to biases
• If observations are abundant and unbiased, model
  biases can be corrected
• Biases in observations can introduce long term
  trends
• Important to identify and correct both model and
  observational biases
• Emphasizes importance of unbiased wind
  measurements from a GWOS
• Wind data over data-sparse regions such as oceans
  and SH will likely improve reanalysis, however
  data need to be unbiased and long-term gt Need a
  long term strategy

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Poleward Moisture Transport Across the Southern
Ocean (Zou and Van Woert, 2001)

• Precipitation over Antarctica is an important
  climate variable relating to mass budget of
  Antarctic ice sheets
• Precipitation is difficult to measure due to lack
  of direct observations in the region
• Net precipitation estimated indirectly using
  moisture transport estimates as input into water
  vapor budget equation

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Wind data is source of error in moisture
transport estimates (Zou and Van Woert)

• Satellite derived moisture fluxes mean moisture
  fluxes show good agreement with radiosondes at
  Macquarie Island
• However, eddy fluxes are significantly
  underestimated, due to averaging of satellite
  wind data (derived from temperature soundings)
  and lack of ageostrophic components in the wind
  derivations
• Improvements in the moisture and wind
  observations plus incorporation of higher order
  dynamics are critical

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Arctic Tropospheric Winds (Francis et al)

• Winds from reanalyses exhibit large biases
  relative to rawinsonde winds
• Computed wind fields from 22 years of TOVS
  satellite-retrieved thermal wind profiles using a
  mass conservation scheme.
• Trends indicate polar vortex has strengthened and
  shifted toward central Siberia

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Relationship between midlatitude westerlies,
atmospheric CO2 and climate change (Toggweiler et
al, 2006)

• Idealized GCM of ocean deep circulation and CO2
  system to explain glacial-interglacial CO2 cycles
• Tight correlation between CO2 and Antarctic temps
• Lead of Antarctic temps over CO2
• Changes occur during on/off transitions of the
  southern overturning circulation
• Occur through positive feedback with midlatitude
  westerly winds
• Cold glacial climates have equatorward shifted
  westerlies
• Volcanism and weathering drive system toward
  mean, which is in stability threshold region

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Southern Ocean Winds and the CO2 Sink

• Southern Ocean sink has weakened between 1981 and
  2004 relative to the trend expected from the
  large increase in atmospheric CO2
• Weakening attributed to the increase in Southern
  Ocean winds resulting from human activities
• Consequences reduction in the efficiency of the
  Southern Ocean sink of CO2, possibly higher level
  of stabilization of atmospheric CO2

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Circumpolar flow and Southern Hemisphere climate
change (Thompson and Solomon, 2002)

• Climate variability in the high latitude Southern
  Hemisphere is dominated by the SH annular mode,
  variability characterized by fluctuations in the
  strength of the circumpolar vortex
• Recent trends indicate stronger westerly flow
  encircling the polar cap
• Largest and most significant trends coupled to
  trends in the lower stratosphere polar vortex due
  largely t ozone losses
• This has contributed to observed warming over
  Antarctic peninsula and Patagonia, cooling over
  E. Antarctica and the Antarctic plateau

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Dynamical processes and clouds (Tony Del Genio,
GISS discussion)

• Issue Improving single column models and
  parameterizations for representing clouds in
  GCMs
• Global reanalysis products are adequate on
  monthly or longer time scales to characterize
  large scale dynamical aspects of variability
• Problem How to diagnose reasons for observed
  errors in model clouds (based on e.g., monthly
  averages of precipitation or radiative fluxes)
• Pull out single column model and run with
  observed horizontal large scale advective fluxes
  of heat and moisture and vertical velocity fields
• Need realistic horizontal advective fluxes on
  synoptic time scales
• Outside northern midlatitudes reanalyses arent
  good enough on short time scales to force the
  single climate models
• Wind data in tropics, SH, and polar regions
  either as a dataset or assimlated, would likely
  be very useful for improving SGMs

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Transport in the stratosphere
From Ted Shepherd presentation at ADM workshop
http//esamultimedia.esa.int/docs/3-11_Shepherd.pd
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Stratospheric Issues
From Shepherd (2)
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Summary

• Winds are important both as drivers and
  indicators of climate change
• Most climate-related applications of global wind
  observations sets will involve reanalysis data
• To improve reanalysis data sets, data should be
  unbiased and continuous gt need for a long term
  observational strategy