ENSO Variability in SODA: 1871-2008

SULAGNA RAYBENJAMIN GIESE

TEXAS A&M UNIVERSITY

WCRP 2010, Paris, 17-19 Nov. 2010

Compo et al., BAMS, 2006

Historical Winds

Comparison of NINO 4 zonal wind stress between 20CRv2 and ERA-40

Black : 20CRv2 Red : ERA-40

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

HADISST SODA 2.2.4

1877

1997

DJF SST anomaly of two strong El Niños from HadISST & SODA 2.2.4

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

hhhhh

NINO-3.4 Index

DJF SST anomaly of 1997-98 El Niño

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

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

Am

pli

tud

e (C

)

Years

CHI- Amplitude showing strength of La Niña

La Niña in the last century do not show much variation

Lo

ng

itu

de

Years

Circle radius proportional to the strength of CHI-amplitude

CHI-Longitude showing Location of El Niños

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

Obstacles in Ocean reanalysis

• Ocean observations are inhomogenous in space and time- Data thinning experiment ✔

• Model bias - Simulation vs. Assimilation ✔

• Errors in surface forcing

WOD09 Hydrographic Temperature Observations

1920s

1990s1960s

1940s

Per decade

ICOADS 2.5 Number of SST Observations

Per decade

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

SST RMS Difference in the Tropical Pacific

Value in assimilating even sparse observations

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

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

• 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