weather regimes and european heat waves. summer 2003: a case study jpl ose meeting, february...

Weather regimes and European heat waves.

Summer 2003: a case study

JPL OSE Meeting, February 2006Christophe Cassou, Laurent Terray & Adam Phillips

Outline of the OSE talk

1. The extreme events of 2003

2. The weather regime paradigm

3. Summertime North Atlantic weather regimes

4. Suggestion for tropical Atlantic forcing in summer 2003

JPL OSE Meeting, February 2006

The extreme heat events of summer 2003

Part 1

1. The extreme heat events of 2003

Source : Météo France Several heat events

NASA Earth Observatory, based on data from the MODIS land team

20 July-20 AugSurf. Temp. anomaly

SeaWiFS project

8th August

Cloud free zone

Paris

A large scale European pattern

Fail the qualitycontrol of the data

1. The heat wave of August 2003

Anomalous Daily [(Min+Max)/2] temperature in Paris 27oF

11oF

0oF

-11oF

10%

100% Soil moisture

Portugal


Source : Météo France Several heat events

Anomalous Daily [(Min+Max)/2] temperature in Paris 27oF

11oF

0oF

-11oF

2. Persistent extreme temperature from May 15 to August 25

~16,000

~20,000

2005 estimate

15 aug. 2003

Mortality excess due to heat

20oC 69oF

Anomalous JJA temperature in Paris

Source : Météo France

Linear Trend = 0.39oC/dec.

1. The extreme heat events of 2003 3. An extreme event on top of a pronounced trend

~ 5

Need for an integrated approach to understand BOTHthe mean changes but also their associated daily changesthat can have tremendous impacts (extreme events etc.)

The weather regime approach

JJA 2003 Temp. @850hPa Surface wind JJA 2003 500 hPa Geop. JJA 2003

5oK

4. The statistico-dynamical approach

What does ‘mean’ mean in the extratropics?Temporal integration over a given period of the

occurrence of daily or quasi-daily events named “weather regimes”


Means and associated statistics mask the high frequency of the observed weather especially in the extratropics.

Weather regimesWeather regimes : elementary bricks of the large scale atmosphericcirculation that are spatially well defined, with a 5-10 day lifetime (persistent)and recurrent (e.g. Lorenz 1963, Vautard et al 1988)

Examples of weather regimesExamples of weather regimes: blocking events, persistent zonal flow etc.i.e. synoptic-type atmospheric circulation whose occurrence or recurrence has a significant influence in terms of impacts (temperature, precipitation, extremes etc.)

Predicting means and associated statistics masks what the daily weather could be.

5. Weather/Climate

Daily variability (weather) Seasonal-to-decadal (climate)

Spatio-temporal Scale Interaction/Downscaling

transitions between régimes modification of the frequencyof occurrence of regimes

Example: T850anom(season) = T850anom (regime)∫day

1. The extreme heat events of 2003: Introduction

Application of the weather regime paradigm to the case of summer 2003

Determination ofWeather regimes

Part 2

1. Attractors in the EOF space

EOF11pt=1day (e.g. Z500 daily map) [JJA 1950-2003 i.e. 92x54 maps]Max

Max

Determination of the regimes: Determination of the maxima of density in the EOF space, or determination of the most probable i.e. recurrent atmospheric states (e.g. MSLP, Z500 patterns etc.)

Regimes can be considered as attractors in the climate phase space

EOF1

EOF3

Probability

Of occurrence

2. Determination of weather regimes

2. Classification

Weather regimes obtained by classification methods (no linearity constraint)

Ex of classified variable: 500 hPa Geopotential maps over the North Atlantic-Europe domain for a given season over a given period

1. Predetermined choice of the k number of regimes (nb. of attractors)2. Aggregation of the 2 most similar maps (choice of a criterion of similarity )

Optimal classification : Maximization of the variance inter-regimes

Optimal classification : Minimization of the variance intra-regimes

Optimization of the k number (Michelangeli et al 1995)


Day 1 Day N

3. Attractors in the EOF space after classification

Max

Max

1pt=1day (e.g. Z500 daily map) [JJA 1950-2003 i.e. 92x54 maps]

After classif.(here k=4)


4. Movement in the EOF phase space

Typical path of the atmosphereduring a given summer

2nd June

1st June

The weather we experience can be explained by the alternance/transition between the different regimes


Summertime NorthAtlantic weather regimes

Part 3

3. Summertime North Atlantic regimes 1. Z500 summertime weather regimes

Classification from geopotential height @ 500hPa for JJA NCEP-NCAR Reanalyses over 1950-2005.

2. Relationship between regimes and mean daily temperature

Classified Z500

3. Summertime North Atlantic regimes

Anomalous Surface

Temperature(daily

composites)

Atl. Low

Blocking

Atl. Ridge

NAO-

3. Summer 2003

Blocking

NAO-Atl.Ridge

Atl.Low

Decomposition in weather regimes leads to a better interpretation of the interannual variability and build a bridge between impacts and large scale atmospheric fluctuations (Importance of scale interaction)


~ +( )JJA 2003

4. Interannual variability

2003:WarmestSummer

TypicalSummer


5. Weather regime and low frequency variability

Positive trend

NCEP-NCAR Reanalyses (JJA) [1950-2004]

Number of daysIn JJA


Positive trend

No trend

Negative trend

Changes in regime occurrence are consistent with the

observed TS trend :High frequencies dynamical entities explains part of the

very low-frequency fluctuations

15%5%30°C15°C

5%

Nu

mb

er o

f d

ays

(no

rmal

ized

)

TMAXMean

Relative change of extreme occurrence = (%)

+300%

-90%

TMAX ClimatologicalDistribution (Gaussian)

for a givenStation-data (all days)

TMAX distributionsper regime

(days where regimesare excited) : 4 distributions

TMAX

6. Relationship between regimes and extremes3. Summertime North Atlantic regimes

Extremedefinition

7. % of chances for heat wave occurrence

SQR MétéoFrance Data [1950-2002]


Atl. LowBlocking

Atl. RidgeNAO-

Change of extreme occurrence =0% 5% 10% 15%

x2 x3Clim

8. Link between mean and regimes3. Summertime North Atlantic regimes

+( )JJA 2003 Z500

+( )Anomalous Daily temperature in Paris (2003)

( )+Anomalous JJA temperature[50-03]

Time scale interaction : day-decade

Tropical Atlantic forcingOn European heat waves

Part 4

4. Tropical Atlantic forcing on European heat waves 1. Impact of the forcing

Chaos (not predictive) + External forcing (ocean, Greenhouse gazes etc.)

The low frequency variability (seasonal to decadal) can be explained by changes in amplitude of the probability density function or in preferentialtransitions between regimes.

Change in the regime occurrence ratherthan change in regimes by themselves

Anomalous obs. OLR (proxy for convection) [satellite data]Wet Dry

Question: Could the anomalous ITCZ position/strength have had a role in theoccurrence of the 2003 heat events?

Model experiments

Displacement/Reinforcement of the ITCZIncreased convection over the western part of the Tropical Atlantic

4. Tropical Atlantic forcing on European heat waves 2. Tropical Atlantic ITCZ in 2003

3. Experimental setup

Model = Community Atmospheric Model (CAM2+) coupled to anOceanic mixed layer (MLM) (NCAR-Cerfacs collaboration)

-120 year of control simulation-40 members of 7 months long starting April 1st and perturbedby diabatic heating anomalies anomalies limited to the tropical Atlantic domain and estimated from observations

The 40 members differ by their 1st April i.e. 1st day atmospheric initial conditions(random selection from the control integration) and the coupling between theOcean and atmosphere is activated only in the Atlantic (north of 40S).The 40 members have the same 3D initial oceanic conditions (average of the120 April 1st from the control integration) No oceanic anomalies are imposed

degrees Celcius/day

Anomalous diabatic heating

4. Tropical Atlantic forcing on European heat waves

z

Td’

500mb

4. Summertime weather regimes in CAM4. Tropical Atlantic forcing on European heat waves

CAM

NECP

The model is able tocorrectly representthe summertime weather regimesAtl. Low

Blocking

Atl. Ridge

NAO-

Favo

r

inh

ibit

Atl.Low ++Atl.Ridge --

Blocking ++Atl.Ridge --

5. Regime response to the tropical forcing4. Tropical Atlantic forcing on European heat waves

Change in the position/strength of the Atlantic ITCZ in 2003 favors (inhibits) the occurrence of the warm regimes

(cold regimes).

Thanks to the links between extremes and regimes from observations, assessing the changes of regime occurrence in response to a forcing is promising in a seasonal forecast context (complementary information to the traditional ensemble mean).

6. Mean response to the tropical forcing4. Tropical Atlantic forcing on European heat waves

JJAT850 response (ensemble mean)

JJA NCEP T850 Model

7. Mechanisms4. Tropical Atlantic forcing on European heat waves

PluvieuxSec

Low High

Rossby wave(PLN)

Sahel-Mediterraneanconnection

Anomalous convectionin the Caribbean favors

Atl.Low regimes (via forced Rossby Waves)

Anomalous convectionin the Sahel favors

Blocking regimes (viadirect cell circulation)

Conclusions

The weather regime approach is powerful to investigatethe day-to-decade variability

Scale interaction from extremes to trends

Suggestions of tropical Atlantic forcing in summer 2003

New challenge for seasonal-to-interannual forecast for the extratropics

JJA TMAX France

Corr. TMAX June/August = 0.18

PluvieuxSec

Precip Precip

Low High

Rossby Wave(Carïbbean)

Summer NAO(direct cell?)

Z500

Z500

7. Monthly dependence of the tropical-extratropical connection4. Tropical Atlantic ….

JJA TMAX France

Corr. TMAX June/August = 0.18

PluvieuxSec

Precip

Low High

Summer NAO(direct cell?)

Z500

4. Tropical Atlantic forcing on European heat waves

Model anomalous Aug. Meridional Stream function [45oW-30oE]

8. Rossby waves + direct cell

8. Link between mean and regimes

% of regime occurrence for the 5 warmest year in France (JJA 1950-2003)

Decomposition in regime builds a bridge between a large blend ofSpatio-temporal scales

Low frequency, seasonal characteristics and extremes


8. Link between mean and regimes

Info ici sur la nature des regimes et sur leur modification

+ recurrent mais moins persistentet mois creuse.


weather regimes and european heat waves. summer 2003: a case study jpl ose meeting, february...

Documents