11

Opposite phases of the Antarctic Oscillation and Opposite phases of the Antarctic Oscillation and Relationships with Intraseasonal to Interannual Relationships with Intraseasonal to Interannual

Activity in the Tropics during the Austral Summer Activity in the Tropics during the Austral Summer

((submitted to J. Climatesubmitted to J. Climate))

Leila M. V. CarvalhoLeila M. V. Carvalho1,21,2,Charles Jones,Charles Jones11, and Tercio Ambrizzi, and Tercio Ambrizzi22

11ICESS, University of California, Santa BarbaraICESS, University of California, Santa Barbara22Dept. Atmospheric Science, Universidade de São PauloDept. Atmospheric Science, Universidade de São Paulo

22

MOTIVATION:MOTIVATION:

• The Antarctic Oscillation (AAO) is known as the leading mode of variability of extratropical circulation in the Southern Hemisphere

• This mode of variability is characterized by deep and zonally symmetric or “annular” structures (Thompson and Wallace 2000) with opposite geopotential height perturbations over the pole and approximately in a zonal ring with center around 45o S.

• The annular and zonally symmetric structure involves exchanges of mass between mid and high latitudes.

33

FOCUS:FOCUS:

• Observational investigation of the variability of the AAO during Observational investigation of the variability of the AAO during the austral summer (DJF) the austral summer (DJF)

ADDRESSED ISSUESADDRESSED ISSUES ::

1)1) Are distinct AAO phases related to the variability of convection Are distinct AAO phases related to the variability of convection and circulation in the tropics such as El Niño/La Niña? and circulation in the tropics such as El Niño/La Niña?

2)2) Similarly, given the depth of the annular mode, can phases of the Similarly, given the depth of the annular mode, can phases of the Quasi Biennial Oscillation (QBO) be related to phases of AAO? Quasi Biennial Oscillation (QBO) be related to phases of AAO?

3) On intraseasonal timescales, do tropical anomalies such as the 3) On intraseasonal timescales, do tropical anomalies such as the Madden-Julian Oscillation modulate the phases of the AAO?Madden-Julian Oscillation modulate the phases of the AAO?

4) What are the key teleconnection mechanisms linking the tropical 4) What are the key teleconnection mechanisms linking the tropical activity to extratropics and, therefore, AAO? activity to extratropics and, therefore, AAO?

44

1.1. The Daily AAO IndexThe Daily AAO Index1979-20001979-2000

• The daily AAO index is computed as the leading mode of the EOF analysis of daily anomalies of the 700hPa geopotential height (H700) from Reanalysis (20-90 S).

• Positive (negative) phases

of the AAO : defined when the time coefficient of the first EOF is greater (less) than 1 standard deviation of the DJF time series.

POSITIVE PHASE

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POSITIVE AAO PHASES: 349 DAYS IN 70 INDEPENDENT EVENTS NEGATIVE AAO PHASES: 326 DAYS IN 40 INDEPENDENT EVENTS. PERIOD: DJF 1979-2000

PERSISTENCE: Number of

consecutive days during positive (negative) phases

Negative Positive-2

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Pe

rsis

ten

ce

of D

JF

AA

O P

ha

se

s (d

ay

s)

Median 25%-75% Non-Outlier Range Outliers

75%

25%25%

75%

Maximum

Maximum

Median

Median

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2. The Role of Low-Frequency Anomalies2. The Role of Low-Frequency Anomalies

• ENSOENSOSeveral previous Several previous studies have shown the studies have shown the impacts of ENSO in impacts of ENSO in modifying convection modifying convection and the circulation and the circulation patterns from tropics to patterns from tropics to extratropics extratropics

• QBOQBOOther studies have Other studies have suggested the existence of suggested the existence of links between QBO and links between QBO and ENSO (e.g. Gray et al. 1992) ENSO (e.g. Gray et al. 1992)

Easterly phases -> El NinoEasterly phases -> El Nino

Westerly phases -> La NinaWesterly phases -> La Nina

• Are AAO phases related to both low-frequency forcing ?Are AAO phases related to both low-frequency forcing ?How?How?

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Composites of SST and 200 hPa zonal wind (U200Composites of SST and 200 hPa zonal wind (U200LFLF):):Low-Frequency ( Retained Periods: above 365 days)Low-Frequency ( Retained Periods: above 365 days)

Negative AAO Phase Positive AAO Phase

U200U200LFLF U200U200LFLF

SSTSSTLFLF SSTSSTLFLF

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The Lower Stratosphere : Composites for Low-Frequency Zonal Winds in 50hPa (U50LF)

Negative AAO

Positive Positive AAOAAO

EASTERLYQBO PHASE

WESTERLYQBO PHASE

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Relationships between ENSO patterns and AAO for Relationships between ENSO patterns and AAO for DJFDJF : : Combined EOF was computed with: H700, U200 Combined EOF was computed with: H700, U200 ((daily daily

anomaliesanomalies) ) and OLR and OLR low frequencylow frequency (OLR (OLRLFLF))

H700

OLRLF

U200

1010

Intraseasonal Activity and AAO PhasesIntraseasonal Activity and AAO Phases

• Tropical intraseasonal convective anomalies (TICA) Tropical intraseasonal convective anomalies (TICA) such as MJO are known as playing an important role such as MJO are known as playing an important role in modulating convection patterns in the tropics and in modulating convection patterns in the tropics and circulation patterns in both tropics and extratropics circulation patterns in both tropics and extratropics (e.g. Hendon and Salby 1994)(e.g. Hendon and Salby 1994)..

• Some previous works have suggested that MJO may Some previous works have suggested that MJO may interact with El Nino episodesinteract with El Nino episodes

• Nonetheless, there is significant interannual Nonetheless, there is significant interannual variability in the frequency of TICA (e.g. Jones et al. variability in the frequency of TICA (e.g. Jones et al. 2003)2003)

• Therefore TICA events may be an important Therefore TICA events may be an important intraseasonal forcing for circulation in the tropics intraseasonal forcing for circulation in the tropics and extratropics : and extratropics : IMPORTANCE FOR AAO PHASES IMPORTANCE FOR AAO PHASES

1111

Lag composites (Days) OLR anomalies (20-70 daysLag composites (Days) OLR anomalies (20-70 days)

NEGATIVE AAO POSITIVE AAOPOSITIVE AAO

LAG 0

LAG +5

LAG +10

LAG +15

LAG +20

LAG +25

LAG 0

LAG +5

LAG +10

LAG +15

LAG +20

LAG +25

1212

AAO ANDAAO AND TELECONECTIONS WITH THE TELECONECTIONS WITH THE TROPICSTROPICS

Teleconnection patterns were examined by applying the Teleconnection patterns were examined by applying the Wallace and Gutzler(1981) methodology:Wallace and Gutzler(1981) methodology:

1.1. Variable investigated : Variable investigated : Zonal Wind 200hPaZonal Wind 200hPa (daily anomalies ) (daily anomalies )

2.2. PositivePositive and and negativenegative AAO events were analyzed AAO events were analyzed separatelyseparately

3.3. A correlation matrix A correlation matrix C is obtained by calculating the temporal is obtained by calculating the temporal (simultaneous) correlation coefficients between anomalies at any given (simultaneous) correlation coefficients between anomalies at any given point point uiui ( (from 90S to 40S and all longitudes-from 90S to 40S and all longitudes- annular modes have annular modes have large amplitudes) and anomalies at every gridpoint large amplitudes) and anomalies at every gridpoint ujuj ( (from 90S to from 90S to 90N and all longitudes90N and all longitudes).).

4.4. The element The element ccijij of the matrix is the correlation of anomalies at a of the matrix is the correlation of anomalies at a

gridpoint gridpoint uuii with anomalies at the gridpoint with anomalies at the gridpoint uujj. The teleconnectivity . The teleconnectivity TTii

of the gridpoint of the gridpoint uuii is defined as the strongest negative element is defined as the strongest negative element ccijij of the of the

matrix matrix CC, for all gridpoints , for all gridpoints uujj: :

TTii= minimum [c= minimum [cijij] for all u] for all ujj

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Teleconnection Patterns obtained for U200 (40oS-90oS)

NEGATIVENEGATIVE

POSITIVE POSITIVE

DIFFERENCE DIFFERENCE

•Equatward shift of the Subtropical Jet

•Weakening Polar Jet

• Strengthening

of the Polar Jet

• Poleward shift subtropical jetWesterly jets

can act as Rossby waveguides for propagation from tropics to midlatitudes

1414

QBOQBO

Deep Tropical Deep Tropical Convection Convection sea level pressuresea level pressure

Equatorial west Equatorial west wind anomalieswind anomalies Kelvin wavesKelvin waves MJO MJO

Possible warm Possible warm pool build uppool build up

SST =>SST => ENSOENSO

Intensification and Intensification and Equatorward shift of Equatorward shift of the subtropical Jetthe subtropical Jet

Poleward shift of the Poleward shift of the subtropical Jetsubtropical JetIntensification of the Intensification of the Polar jetPolar jet

Cold SST Warm SST

Dominant Dominant Negative AAONegative AAO

Dominant Dominant Positive AAOPositive AAO

Gray et al (1992)Gray et al (1992)MechanismMechanism

CONCLUSIONS:

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Storm minimum pressure in opposite AAO phases

Negative Positive930

940

950

960

970

980

990

1000

1010

1020

Sto

rm M

inim

um

Pre

ss

ure

(h

Pa

)

Median 25%-75% Non-Outlier Min-Max Outliers Extremes

Life cycle duration in opposite AAO phases

Negative Positive0

5

10

15

20

25

30

35

40

45

50

Lif

e c

yc

le (

*12

ho

urs

)

Median 25%-75% Non-Outlier Min-Max

Outliers Extremes

Storms maximum latitude in opposite AAO phases

Negative Positive-90

-80

-70

-60

-50

-40

-30

Sto

rm M

ax

imu

m L

ati

tud

e

Median 25%-75% Non-Outlier Min-Max

Outliers

Extratropical Cyclones properties – Murray and Simmonds (1991)tracking scheme