Transcript Understanding the SAM influence on the South Pacific ENSO

Understanding the SAM influence
on the South Pacific ENSO
Teleconnection
Ryan L. Fogt
Department of Geography
Ohio University, Athens, OH
David H. Bromwich & Keith M. Hines
Polar Meteorology Group, Byrd Polar Research Center
The Ohio State University, Columbus, OH
The Southern Annular Mode
(SAM)
H
H
Increased
meridional
pressure gradient
L
H
Stronger circumpolar
westerly winds
SAM positive phase
Compositing ENSO / SAM
Events
• Marshall (2003) SAM index and
inverted Niño 3.4 SST Anomalies
• Data from 1957-February 2009
• Detrended data
• Standardized by month, 1971-2000 base
period
• Used a 0.5 standard deviation
threshold to define ENSO and SAM
events
Compositing ENSO / SAM
Events
Individual Months
#
1. SAM +
2. SAM 3. La Niña
4. El Niño
In-Phase Events
88
69
59
75
5. La Niña / SAM +65
6. El Niño / SAM - 67
Out-of-Phase Events
7. La Niña / SAM - 53
8. El Niño / SAM + 47
Climatology
103
NCEP MSLP Composites
SAM +
SAM -
La Niña
El Niño
Combined NCEP MSLP
Composites
La Niña & SAM +
El Niño & SAM -
La Niña & SAM -
El Niño & SAM +
Rossby Wavetrains: 500 hPa
Streamfunction zonal anomalies
SAM +
SAM -
Shading denotes zonal anomalies outside 2 stdev of zonal mean
La Niña
El Niño
Rossby Wavetrains: 500 hPa
Streamfunction zonal anomalies
La Niña & SAM +
El Niño & SAM -
La Niña & SAM -
El Niño & SAM +
Detailing the SAM Impact
500 hPa Height Anomalies
La Niña
El Niño
Determining the Causality
for the Differences
• We examined the transient eddy mean flow interactions for each case
• Only show the La Nina composites,
since the interaction is more clearly
observed
• Based on data from NCEP-NCAR
reanalysis 1979-2009
Zonal Mean Zonal Wind
Composites: La Niña Case
Shading: zonal mean zonal wind
anomalies
Thin contours: meridional transient
eddy momentum flux
Thick contours: meridional transient
eddy momentum flux convergence
 [u]
 [uv ]
 fo [v] 
t
y
Zonal Mean Zonal Wind
Composites: La Niña Case
Spatial Perspective of
Forcing
• The ENSO forcing is most marked in
the Pacific Sector
• Use the Eu vectors to see local forcing
• Similar to E-P fluxes, but for transient
wave propagation
• Divergence of Eu strengthens local zonal
wind, and vice-versa
Eu Vectors: La Niña Case
Conclusions
• ENSO response in South Pacific dependent
on SAM phase / strength
• Transient eddy interactions in the Pacific
play a key mechanistic role in modifying
the circulation during each case
• In Phase cases: reinforce each other,
maintaining or amplifying the teleconnection
• Out of Phase cases: oppose each other,
drastically reducing the teleconnection
magnitude
• Supported by NSF ATM-0751291