A comparative study of the atypical 1998-2000 La Ni

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A comparative study of the atypical 1998-2000 La
Niña with previous La Niña events

• Amir Shabbar and Bin Yu
• Science and Technology Branch
• Environment Canada, Toronto

Canada
Environment Environnement Canada
Canada
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Outline

• Temperature regime in relation to recent warming
  trend
• Anomalous circulation features over the
  Pacific-North American sector. Vertical
  temperature profile
• Sea Surface Temperature regime warm in western
  Pacific and strong north-south gradient over
  North Pacific
• Anomalous Walker and Hadley circulations
• Role of Non-ENSO coupled mode (CCA mode) between
  GZ500 and Tropical Pacific
• Anomalous nature of the stationary wave activity

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Evolution of SSTs in the Tropical Pacific
From N.C. Lau et al. J. Climate 2006
Western Pacific SSTs are low pass filtered
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Oceanic Nino3.4 Index DJFM Average
L7 L2
1950-51 1998-2001
1955-56
1970-71
1973-74
1975-76
1984-85
1988-89
82-83
97-98
91-92
57-58
72-73
87-88
65-66
02-03
55-56
84-85
75-76
50-51
70-71
98-01
73-74
88-89
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L7 L2 Temperature Composite (DJFM) - Detrended
Temp Anom _L7
Temp Anom _L2
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Ratio of Difference (L2-L7) to Standard Deviation
(L7)
Precipitation
Temperature
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GZ500 and U200 Composites
GZ500 L2
GZ500 L7
GZ500 (L2-L7)
U200 (L2-L7)
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MSLP Composites
MSLP _ L7
MSLP _ L2
MSLP _ (L2 L7)
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Tropical Precipitation Composites
Precip_ L2
Precip_ L7
Precip_(L2-L7)
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SST Composites
SSTA_ L7
SSTA_ L2
SSTA_ (L2-L7)
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Vertical Cross Section - Temperatures
55oN 75oN
PNA Path
A
B
C
D
A
B
C
D
C
B
D
Wallace and Gutlzer, MWR, 1981
A
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Walker Circulation Changes (5S-5N)
L7
L2
L2-L7
8 m/s
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Hadley Circulation Changes (150-120W)
L7
L2
L2-L7
8 m/s
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850 hPa Wind Anomaly
L7
C
A
A
C
L2
C
8 m/s
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Lau Wang SVD modes between 850 hPa winds and
Precip and SST anomalies (CLIVAR 2000)
1st mode
2nd mode
Patterns in low-frequency variability are
primarily zonally orientated corresponding to
covariability at widely separated longitudinal
locations.
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GFDL Model Simulation (June 1998 May 2002)
GZ 200 mb
From Lau et al. 2006
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Coupled mode of Variability (CF3) between GZ500
and Precip (CCA)
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Comparison of Temperature Pattern
High Low index of CF3
Pattern Correlation 0.76
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Summary

• Contrary to historical ENSO cold events (La
  Niña), the protracted 1998-2000 event produced
  abnormally warm and dry conditions over North
  America
• Stronger influence of western Pacific variability
  as shown by enhanced convection and changes in
  Walker circulation. Weaker PNA teleconnection
  pattern as revealed by Hadley circulation
• Wave pattern over N. America appears to be a part
  of a much larger and well-organized wave train
  emanating from the tropical western Pacific
• The presence of 3rd CCA coupled mode between
  Tropical Precip and 500 hPa anomalies is
  attributed to warm North American temperatures
  during L2
• Western Pacific warm SST anomalies locally
  strengthened zonal flow which altered transient
  eddies downstream. Eddy momentum transport
  feedback on the local flow, extending its zonal
  elongation

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Thank You
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850 hPa Temperature Changes
E7
E2
E2 E7
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Ratio of Temperature Trend to Standard Deviation
(1951-2002)
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Second Coupled Mode of Variability (CF2) between
U200 and SST (CCA)
U200
SST
U200
SST
Variance Explained by 10 EOFu200 82Variance
Explained by 10 EOFsst 86
Canonical Correlation 0.89
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Comparison of Temperature Pattern
Pattern Correlation 0.77