Importance of Winds for Climate and Stratospheric Processes

Mike HardestyNOAA 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