Download - ENSO Variability in SODA: 1871-2008
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ENSO Variability in SODA: 1871-2008
SULAGNA RAYBENJAMIN GIESE
TEXAS A&M UNIVERSITY
WCRP 2010, Paris, 17-19 Nov. 2010
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Compo et al., BAMS, 2006
Historical Winds
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Comparison of NINO 4 zonal wind stress between 20CRv2 and ERA-40
Black : 20CRv2 Red : ERA-40
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Model
− Parallel Ocean Program 2.0.1
Domain
− Global (including Arctic)
Resolution
− 0.4° × 0.25° average resolution
− 40 levels, 10m spacing from surface to 450m
Winds
− 20CRv2 daily stress 1871-2008
Heat and Salt fluxes
− Bulk formulae using 20CRv2 daily variables
SODA Data Assimilation
− WOD09 hydrographic and ICOADS 2.5 SST data
SODA 2.2.4
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HADISST SODA 2.2.4
1877
1997
DJF SST anomaly of two strong El Niños from HadISST & SODA 2.2.4
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Standard Measure of El Niño : NINO-3.4 SST anomaly
NINO-3.4 SST anomaly from SODA 2.2.4 (Red) and HADISST (Black)
Stronger El Niños in SODA compared to HADISST
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NINO-3.4 Index
DJF SST anomaly of 1997-98 El Niño
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First moment of SST anomaly - Like the center of mass
SST anomaly must be greater than 0.5°C
Area must be greater or equal to the NINO-3.4 region
CHI Longitude = center of El Niño warming
CHI Amplitude = strength of El Niño
Same for La Niña
Center of Heat Index : CHI
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Am
pli
tud
e (C
)
Years
CHI- Amplitude showing strength of El Niños
El Niño in the late 19th century as strong as those in late 20th century
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Am
pli
tud
e (C
)
Years
CHI- Amplitude showing strength of La Niña
La Niña in the last century do not show much variation
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Lo
ng
itu
de
Years
Circle radius proportional to the strength of CHI-amplitude
CHI-Longitude showing Location of El Niños
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An Analysis of the position of El Niño in SODA
• Histogram of the position overlayed by a Gaussian with same mean and standard deviation
• Null hypothesis: Position of El Niño randomly distributed about 140W
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Obstacles in Ocean reanalysis
• Ocean observations are inhomogenous in space and time- Data thinning experiment ✔
• Model bias - Simulation vs. Assimilation ✔
• Errors in surface forcing
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WOD09 Hydrographic Temperature Observations
1920s
1990s1960s
1940s
Per decade
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ICOADS 2.5 Number of SST Observations
Per decade
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Data Thinning Experiment
Sample the 1990s as though sampled in different periods
5 Experiments : 1.) No Assimilation
As though sampled in the
2.) 1920s
3.) 1940s
4.) 1960s
5.) 1990s – Control run
All other elements of the run are identical
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SST RMS Difference in the Tropical Pacific
Value in assimilating even sparse observations
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Model Bias in CHI-Amplitude and Longitude
SODA 2.2.4 CHI-Amplitude
SO
DA
2.2
.0 C
HI-
Am
pli
tud
e
SO
DA
2.2
.0 C
HI-
Lo
ng
itu
de
SODA 2.2.4 CHI-Longitude
• Bias in the model does not seem to affect the amplitude of El Niño events
• There is a slight westward bias in the position of the El Niño
Red− Before 1950 Blue− After 1950
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Comparison of CHI from SODA and HadISSTH
AD
ISS
T C
HI-
Am
plitu
de
SODA 2.2.4 CHI-Amplitude SODA 2.2.4 CHI-Longitude
HA
DIS
ST
CH
I-Lo
ngitu
de
• El Niños are warmer in SODA compared to HadISST before 1950
• El Niños in HadISST are east of those in SODA for the post-1950 period
• No correlation before1950 in terms of location
Red− Before 1950 Blue− After 1950
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• A 138-yr reanalysis is used to explore ENSO variability
• First moment of temp. anomaly (CHI) is used to describe El Niño
• Prominent decadal variability of El Niño strength, but little trend
• Location of El Niño varies considerably… But the distribution cannot
be distinguished from Gaussian
• Model bias in SODA does not significantly affect the strength of El Niño but
does introduce a slight westward bias in location
• Assimilation of sparse data adds value to the reanalysis
Conclusions