The ENSO Signal in Stratospheric Temperatures from Radiosonde

Observations

Melissa FreeNOAA Air Resources Lab

Silver Spring

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slp

Surface T

Observed surface climate response to ENSO

From Ineson & Scaife 2008

Recent work confirms ENSO effects on winter surface climate in Europe

El Nino Weaker polar vortexWarmer arctic stratosphere

North Atlantic Oscillation-less zonal flow

Colder European winter

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This effect may operate through changes in the Arctic stratosphere

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Weak polar vortex is associated with cooling at surface

Surface T From Thompson et al. 2002

Warm

Cold

Previous work

1. Tropics— Several works note cooling in stratosphere during El Ninos, but others found little or no effect in zonal mean satellite temperature.

2. Arctic—Models find warming T and weakening vortex for El Nino winters

Sassi et al. 2004Taguchi and Hartmann 2005Manzini et al. 2006 (but no signal for La Nina)Ineson and Scaife 2008

Data No significant relation: Hamilton 1993 and other papers Significant only during solar minima: Kryjov and Park 2007 Significant warming at 10 mb: Garfinckel and Hartmann 2007 Significant warming from 10 to 50 mb: Camp and Tung 2007

Depends on phase of Quasi-biennial Oscillation (QBO)?

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Data

Radiosonde datasets used (1958-2004)

Radiosonde Atmospheric TemperatureProducts for Assessing Climate RATPAC (NOAA) 85 stationsHadley Center Atmospheric Temperatures HadAT2 (UKMO) 676Iterative Universal Kriging IUK (Yale) 527Integrated Global Radiosonde Archive IGRA (NCDC) >1000

IndicesENSO index = Nino3.4 SSTsQBO index = 50 mb Singapore winds

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RATPAC HadAT2 IUK IGRA

Methods

Problem = distinguishing QBO and volcanic effects from ENSO effects (limited number of cases)

1. Linear regression using ENSO and QBO indicesRemove QBO first (stepwise multiple r.)Temperatures lagged 4 months behind SSTs

2. Compositing of El Nino versus La Nina years

3. Correlation coefficients

(Skip 3 post-volcano years)

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ENSO temperature signal from regression using all months

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Pre

ssur

e (m

b)

latitude

K

ENSO signal from regression using DJF mean temperatures

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Pre

ss

ure

(m

b)

T (K)

Arctic signal is strongest in February

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Pre

ssu

re (

mb

)

IUK

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Compositing warm and cold ENSO events1. Compositing for all QBO phases K

Pre

ssur

e (m

b)

DJF temperature

Compositing warm and cold ENSO events--2. Compositing for West QBO only

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Pre

ssu

re (

mb)

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Winter mean T at 100 mb for 70-90N with SST3.4 ENSO index

K

RATPAC HadAT IUK IGRA NCEP reanalysis

Max r2 0.20 0.46 0.19 0.24 0.22

Level of max r2

100 mb 30 mb 250 mb 10 mb 50 mb

Mean r at 100 mb

0.41 0.45 0.40 0.39 0.45

Correlations and explained variance—70-90N

Conclusions

1. Significant ENSO temperature signal in the tropical stratosphere around 50 mb.

2. Significant ENSO signal, up to 5 K, in the winter Arctic stratosphere extending into the troposphere, primarily in February. (Consistent with previous work but located lower in atmosphere)

3. Arctic winter signal is strong for West QBO and absent for East QBO (as in reanalysis studies).

4. Level of maximum Arctic warming depends on dataset and method of analysis.

5.Variance in Arctic winter temperature in stratosphere explained by ENSO is ~20% for most datasets.

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Remaining issues:

MechanismWhy QBO effect?Is the effect limited to sudden stratospheric

warmings, or is there a general seasonal warming?

[Why no cold European winter in 1997-98?]

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Regression signal, DJF