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MOTIVATION:

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Opposite phases of the Antarctic Oscillation and Relationships with Intraseasonal to Interannual Activity in the Tropics during the Austral Summer

(submitted to J. Climate)

Leila M. V. Carvalho1,2,Charles Jones1, and Tercio Ambrizzi2

1ICESS, University of California, Santa Barbara

2Dept. Atmospheric Science, Universidade de São Paulo

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.

FOCUS:

- Observational investigation of the variability of the AAO during the austral summer (DJF)
ADDRESSED ISSUES:

- Are distinct AAO phases related to the variability of convection and circulation in the tropics such as El Niño/La Niña?
- Similarly, given the depth of the annular mode, can phases of the Quasi Biennial Oscillation (QBO) be related to phases of AAO?
3) On intraseasonal timescales, do tropical anomalies such as the Madden-Julian Oscillation modulate the phases of the AAO?

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

- 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

Maximum

Maximum

75%

75%

Median

Median

25%

25%

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

ENSO

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

QBO

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

Easterly phases -> El Nino

Westerly phases -> La Nina

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

Negative AAO Phase

Positive AAO Phase

SSTLF

SSTLF

U200LF

U200LF

Negative

AAO

EASTERLY

QBO PHASE

WESTERLY

QBO PHASE

Positive

AAO

Relationships between ENSO patterns and AAO for DJF: Combined EOF was computed with: H700, U200 (daily anomalies) and OLR low frequency (OLRLF)

U200

H700

OLRLF

- Tropical intraseasonal convective anomalies (TICA) such as MJO are known as playing an important role in modulating convection patterns in the tropics and circulation patterns in both tropics and extratropics (e.g. Hendon and Salby 1994).
- Some previous works have suggested that MJO may interact with El Nino episodes
- Nonetheless, there is significant interannual variability in the frequency of TICA (e.g. Jones et al. 2003)
- Therefore TICA events may be an important intraseasonal forcing for circulation in the tropics and extratropics : IMPORTANCE FOR AAO PHASES

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

NEGATIVE AAO

POSITIVE AAO

LAG 0

LAG 0

LAG +5

LAG +5

LAG +10

LAG +10

LAG +15

LAG +15

LAG +20

LAG +20

LAG +25

LAG +25

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

Variable investigated : Zonal Wind 200hPa (daily anomalies )

Positive and negative AAO events were analyzed separately

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

The element cij of the matrix is the correlation of anomalies at a gridpoint ui with anomalies at the gridpoint uj. The teleconnectivity Tiof the gridpoint uiis defined as the strongest negative element cijof the matrix C, for all gridpoints uj:

Ti= minimum [cij] for all uj

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

- Equatward shift of the Subtropical Jet
- Weakening Polar Jet

NEGATIVE

- Strengthening
of the Polar Jet

- Poleward shift subtropical jet

POSITIVE

Westerly jets can act as Rossby waveguides for propagation from tropics to midlatitudes

DIFFERENCE

QBO

CONCLUSIONS:

Gray et al (1992)

Mechanism

Deep Tropical Convection sea level pressure

Equatorial west wind anomalies

Kelvin waves

MJO

Possible warm pool build up

Poleward shift of the subtropical Jet

Intensification of the Polar jet

Intensification and Equatorward shift of the subtropical Jet

Warm SST

Cold SST

SST =>

ENSO

Dominant Positive AAO

Dominant Negative AAO

Extratropical Cyclones properties – Murray and Simmonds (1991)

tracking scheme