Analyzing the 2010-2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations and a numerical simulation

Audrey Hasson(1, *), Thierry Delcroix(1), Jacqueline Boutin(2),

Raphael Dussin(3), Joaquim Ballabrera-Poy(4)

The tropical Pacific Ocean remained in a La Niña phase from mid 2010 to mid-2012. The near-surface salinity signature of this cold El Niño-Southern Oscillation (ENSO) phase is shown in the figure below and analysed in Hasson et. al (2014) using a combination of numerical model output, in situ data and SMOS satellite salinity products.

boutin-la nina

Figure: Sea Surface Salinity anomalies relative to each product 2010-2011 monthly climatology (pss) in July 2010 (left panels) and July 2011 (right panels) for (a, d) ISAS in situ product (b, e) SMOS and (c, f) the model. Blue lines represent the Voluntary Observing Ship routes and the 170°E-180° hatched areas computation zones. (Figure from Hasson et al., 2014)

Comparisons of all salinity products show a good agreement between them, with a RMS error of 0.2-0.3 between the thermosalinograph (TSG) and SMOS data and between the TSG and model data. The last 6 months of 2010 (La Niña) are characterized by an unusually strong tri-polar anomaly captured by the three salinity products in the western half of the tropical Pacific. A positive SSS anomaly sits north of 10ºS (>0.5), a negative tilted anomaly lies between 10ºS and 20ºS and a positive one south of 20ºS. In 2011, anomalies shift south and amplify up to 0.8, except for the one south of 20ºS. Equatorial SSS changes are mainly the result of anomalous zonal advection, resulting in negative anomalies during El Niño (early 2010), and positive ones thereafter during La Niña. The mean seasonal and interannual poleward drift then exports those anomalies toward the south in the southern hemisphere, resulting in the aforementioned tripolar anomaly. The vertical salinity flux at the bottom of the mixed layer tends to resist the surface salinity changes. The observed basin-scale La Niña SSS signal is then compared in Hasson et al. (2014) with the historical 1998-1999 La Niña event using both observations and modelling.

for more details see Hasson, A., T. Delcroix, J. Boutin, R. Dussin, and J. Ballabrera-Poy (2014), Analyzing the 2010–2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations, and a numerical simulation, Journal of Geophysical Research: Oceans, 119(6), 3855-3867, doi:10.1002/2013JC009388.

(1) LEGOS, UMR 5566, CNES, CNRS, IRD, Université de Toulouse 14 avenue Edouard Belin, 31400 Toulouse, France

(2) LOCEAN, UMR7159, CNRS, UPMC, IRD, MNHN, Paris, France

(3) LEGI, Grenoble, France

(4) ICM/CSIC, Barcelona, Spain

(*) Corresponding author, Currently at the Jet Propulsion Laboratory,California Institute of Technology, Pasadena, California, USA

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