ENSO Variability in SODA: SULAGNA RAY BENJAMIN GIESE TEXAS A&M UNIVERSITY WCRP 2010, Paris, 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  Domain − Global (including Arctic)  Resolution − 0.4° × 0.25° average resolution − 40 levels, 10m spacing from surface to 450m  Winds − 20CRv2 daily stress  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 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 (Red) and HADISST (Black) Stronger El Niños in SODA compared to HADISST hhhhh

NINO-3.4 Index DJF SST anomaly of 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

Amplitude (  C) Years CHI- Amplitude showing strength of El Niños El Niño in the late 19th century as strong as those in late 20th century

Amplitude (  C) Years CHI- Amplitude showing strength of La Niña La Niña in the last century do not show much variation

Longitude 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 CHI-Amplitude SODA CHI-Amplitude SODA CHI-Longitude SODA 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 HadISST HADISST CHI-Amplitude SODA CHI-Amplitude SODA CHI-Longitude HADISST CHI-Longitude 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