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Understanding the SAM influence on the South
Pacific ENSO Teleconnection
Understanding the SAM influence on the South
Pacific ENSO Teleconnection
Ryan L. FogtDepartment of Geography
Ohio University, Athens, OH
David H. Bromwich & Keith M. HinesPolar Meteorology Group, Byrd Polar Research Center
The Ohio State University, Columbus, OH
Ryan L. FogtDepartment of Geography
Ohio University, Athens, OH
David H. Bromwich & Keith M. HinesPolar Meteorology Group, Byrd Polar Research Center
The Ohio State University, Columbus, OH
The Southern Annular Mode (SAM)
The Southern Annular Mode (SAM)
L
H
H
H
SAM positive phase
Increased meridional pressure gradient
Stronger circumpolar westerly winds
Compositing ENSO / SAM Events
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
• 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
Compositing ENSO / SAM Events
Individual Months #
In-Phase Events
Out-of-Phase Events
1. SAM + 88
7. La Niña / SAM - 53
6. El Niño / SAM - 675. La Niña / SAM + 65
4. El Niño 753. La Niña 592. SAM - 69
8. El Niño / SAM + 47
Climatology 103
NCEP MSLP CompositesNCEP MSLP CompositesSAM +SAM +
El NiñoEl NiñoLa NiñaLa Niña
SAM -SAM -
Combined NCEP MSLP Composites
Combined NCEP MSLP Composites
La Niña & SAM +La Niña & SAM +
La Niña & SAM -La Niña & SAM -
El Niño & SAM -El Niño & SAM -
El Niño & SAM +El Niño & SAM +
Rossby Wavetrains: 500 hPa Streamfunction zonal
anomalies
Rossby Wavetrains: 500 hPa Streamfunction zonal
anomaliesSAM +SAM +
Shading denotes zonal anomalies outside 2 stdev of zonal mean
SAM -SAM -
La NiñaLa Niña El NiñoEl Niño
Rossby Wavetrains: 500 hPa Streamfunction zonal
anomalies
Rossby Wavetrains: 500 hPa Streamfunction zonal
anomaliesLa Niña & SAM +La Niña & SAM +
La Niña & SAM -La Niña & SAM -
El Niño & SAM -El Niño & SAM -
El Niño & SAM +El Niño & SAM +
Detailing the SAM ImpactDetailing the SAM Impact
500 hPa Height Anomalies500 hPa Height Anomalies
El NiñoEl NiñoLa NiñaLa Niña
Determining the Causality for the
Differences
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
• 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 CaseZonal Mean Zonal Wind
Composites: La Niña Case
[u]t
fo[v] [ u v ]
y
Shading: zonal mean zonal wind anomalies
Shading: zonal mean zonal wind anomalies
Thin contours: meridional transient eddy momentum flux
Thin contours: meridional transient eddy momentum flux
Thick contours: meridional transient eddy momentum flux convergence
Thick contours: meridional transient eddy momentum flux convergence
Zonal Mean Zonal Wind Composites: La Niña CaseZonal Mean Zonal Wind
Composites: La Niña Case
Spatial Perspective of Forcing
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
• 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 CaseEu Vectors: La Niña Case
ConclusionsConclusions• 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
• 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