Transcript Importance of Winds for Climate and Stratospheric Processes

Importance of Winds for Climate and
Stratospheric Processes
Mike Hardesty
NOAA Earth System Research Lab
Boulder, CO 80305
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
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
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
Example: Observing System Change in
1978/1979
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 => Need a long
term strategy
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
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
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
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
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
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
Dynamical processes and clouds (Tony Del
Genio, GISS discussion)
• Issue: Improving single column models and parameterizations
for representing clouds in GCM’s
• 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 aren’t 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
Transport in the stratosphere
From Ted Shepherd presentation at ADM workshop
http://esamultimedia.esa.int/docs/3-11_Shepherd.pdf
Stratospheric Issues
From Shepherd (2)
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 => need for a long
term observational strategy