The Influence of Tropical-Extratropical and Atmosphere-Ocean by yurtgc548

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									         The Influence of
Tropical-Extratropical Interactions
      on ENSO Variability


         Michael Alexander
   NOAA/Earth System Research Lab
  Pacific Ocean-Atmosphere Variability Nov-Mar
EOF 1 SST
                                                                  Z500




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                                                             CI = 5 m

Deser and Blackmon, 1995, J. Climate
            Response to tropical SSTAs


Z 200 (m)
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  SSTA                                       Quic kTime™ and a




   °C
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   Blackmon, Geisler and Pitcher 1983
        Response to North Pacific SSTAs


Z 200 (m)




 SSTA
  °C
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Pitcher, Blackmon, Bates, and Muñoz, 1988
Response to Tropical and
  North Pacific SSTs



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       Pacific SST Structure and
     Atmospheric Forcing/Feedback
Pitcher et al.:
• Noted that past work indicated significant correlation
  between tropical and midlatitude SSTs and that the
  two might be related
• “For example, a tropical Pacific SST could produce
  an atmospheric anomaly which could produce a
  midlatitude SST anomaly”
• “Another possibility is that an enhanced atmospheric
  anomaly might occur if both a tropical SST anomaly
  and a favored midlatitude SST anomaly were to
  occur simultaneously”
• “Other permutations of possible cause and effects
  can be imagined.”
     Oceanic Response to ENSO:
      the “Atmospheric Bridge”

                          Mixed Layer
                          Ocean L
                                Model




                     Prescribed Climatological SSTs



Alexander 1990 Climate Dyn.; Alexander 1992 J Climate
   ENSO’s impact on the North Pacific


  Qnet
  W m-2
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 °C
    Response to Pacific SST Anomalies
              z200 (m) DJF
Tropical


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 North


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  Shading: > 95% significance by t-test
 Response to ENSO:
Role of Ekman transport
   El Niño – La Niña SST (˚C) JFM




Alexander et al. 2002 J. Climate; Alexander and Scott 2007
      Z500 (m) Niño - Niña Composite
Obs                  EKM




MLM                   ∆
Niño - Niña SST, SLP,  JFM
Extratropical => Tropical
  Air Sea Interactions
 Seasonal Foot Printing Mechanism (SFM)


                                           • NPO in NDJ (-1)
                                                    
                                           • Winds & Heat Flux
                                                    
                                           • SST in MAM (0)
                                                    
                                           • Tropical Winds
                                                    
                                           • Bjerknes Feedback
                                                    
                                           • El Nino in NDJ(0)




(Vimont et al. 2001, 2003a&b J. Climate)
               Experiment Design
• Model (Chang et al. 2007, GRL):
   •AGCM: CCM3
   •Reduced Gravity Ocean (Cane-Zebiak) Model 30S-30N in Pacific.
   •Slab model over remainder of the ocean
   •Models are anomaly coupled
•100-year Control run
•SFM Experiment
   •Add additional heat flux forcing associated with the NPO
        •20°S-60°N; similar results when forcing > 10°N
   •Initiate 60 heat flux anomaly runs from Nov in control run.
   •Apply Heat flux anomaly during first Nov-Mar
   •Then let model evolve with unperturbed fluxes for 12 more months.
• Compare ENSO evolution in perturbation and control runs.
• Note: model already includes SFM
                  Additional SFM Forcing
• NPO from AGCM                              2nd EOF SLP & Qnet Nov-Mar
    – With Climatological SST
        • Isolates intrinsic variability
    – 2nd EOF of SLP EOF in
      North Pacific in Winter
    – Regress Sfc Heat flux on PC
    – double flux values
        • Max values of ~30 Wm-2
                                                 SLP
• Add identical/constant forcing in
   each of the experiments
                                                           Qnet

                              Exp n+1            Exp n+2
            Exp n
                                                                  Control
     Nov      Mar         Nov      Mar     Nov     Mar
           Experiment - Control

SST (°C)




                                  MJJ(0)


 Winds
 &
 SST
                 Experiment - Control
Thermocline depth (m)




 SST (°C)                         NDJ(1)
   Experiment - Control Nino 3.4 SST (°C)
                  NDJ(1)




                             Run number

In 43 of the 60 cases (~72%) SSTs warmed in the Nino 3.4 region in the
subsequent winter after the forcing was applied. The Mean difference
between Exp and Control is 0.47°C (significant at the 99% level).
Forcing added 11 more warm events - Nino 3.4 NDJ > 1  control.
                          Summary
• Connection Between tropical Pacific and extratropics:
   • Atmospheric Bridge
      • Global, including N. Pacific & N Atlantic
      • Ekman transport important in generating SSTs
   • Feedback of bridge-related SST anomalies on the atmosphere:
      •   ~1/3 of response to ENSO SSTs (signal/noise issues)
      •   May involve multiple bridges
      •   Nature of feedback depends on region/seasonal cycle
      •   Model dependence?
• Extratropics => Tropics
   • Atmosphere
      • Seasonal Footprinting Mechanisms
   • Ocean Pathways
      • Rossby waves
      • Subtropical Cells
Additional Slides
Model Configration
Net Surface Heat FLux
Ekman
U200 (m/sec)
  Composite Niño - Niña JFM
Precipitation, 200 mb streamlines
              Air-Sea Feedback N. Atlantic
 N. Atl: Climatological SST               N. Atl: 50 m slab ocean



   Atmospheric Response
   500 mb January


                                       N. Atl: 75 m slab + Ekman Trans
                              Africa



 South
 America

           SSTA


Peng, Robinson, Li, Hoerling, Alexander
   Optimal
   Structure
SST pattern that undergoes
maximum growth - defined
here as the domain
integrated SSTA variance
When is Optimal Structure most
        likely to occur?
                  Evolution of Air-Sea
                  System based on OS:

                  Difference maps between
                  composite averages when
                  the correlation between
                  the OS and the observed
                  SSTA field > 0.4 and    <
                  -0.4 during MAM.
                  Starting in the winter
                  before through the
                  following winter
Composite Based on Optimal Structure in MAM
Composite Based on Optimal Structure II

								
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