Ocean (surface) salinity an in situ perspective

G. ReverdinLOCEAN, UMR CNRS/UPMC/IRD, Paris, France

(indebted to numerous colleagues in France, US, Spain…)

Salinity

• Water with anions and cations in fixed’ ratios (Dittmar, 40 samples from the Challenger Expedition1874-1877)

• Thus, to 0-order S=fn(C,T), and density=fn(T,S,P)(small increase 0(0.003 psu) due to increase

DIC; small overall decrease due to glacial melt/change of ocean mass)

S evolves with E-P+R, ocean circulation, mixing

What do we measure

• Measured with respect to a standard water (more or less since 1900)- first by measuring Chlorinity; - more recently, conductivity

Lost in Fathom: London

How do we measure it

• Discrete samples 110 y (0.1 psu)

• Thermosalinographs 40 y (ships, drifters) (0.02 psu)

• Argo floats (0.01 psu) (and other profilers)

Different networks

SO SSSG. Alory, T. DelcroixSOCAT, GOSUD, SAMOS

(all require careful validation)

Surface drifters (SPURS)L. Centurioni, V. HormannJ. Font, G. Reverdin

Requires also careful validation)

Mapping of climatology

Sufficient data: Last 40 yearsReverdin et al, 2007; Delcroix et al., 2005

Gordon and Giulivi, 2014

Examples of variability

Subpolar gyre : TSGs (20 years)earlier sampling 100 years

NAC

LC

Binning in boxes or along tracks…

Subpolar gyre

Binning data 1°x1 month :scales resolved a few degrees and a few monthsLarge spatial coherence : modulation of gyre

Larger signal : circulation

West Greenland / Nfld shelves

Larger seasonal modulation on shelves; harder to interpret(for example, expectation of huge melt in 2011-2012, and no low S)Possible phase opposition West Greenland/ Newfoundland shelf

Spatial mapping of individual fields (~10 years)

April-June 1997

300-500 km scales can be retrieved (away from fronts),even with the spare Argo sampling (and surface TSG sampling)ISAS (Gaillard, 2009); Reverdin et al., 2002; Reverdin et al., 2007

Displacement of the fresh poolwestern equatorial Pacific

(binning Delcroix et al., 2011; scales 1000 km*100km*3 months)

Singh et al., 2012, but also Delcroix and Picaut, 1998, Maes et al, 2006

Displacements advected by zoal currents + P influence

ENSO/El Niño

Singh et al., 2011Different flavors of ENSO (EP, CP)With SSS patterns that relate to currents/P

Warm pool

(Cravatte et al, 2007) Trends in PDO and SSS

Trends– natural variabilityA climate change perspective

Trend SSS/century in climate models(compared to obs 1970-2002)Terray et al 2012; Delcroix et al., 2011

Pacific seems to be robust, but N. Atlantic within natural variability

Longer time series(feasible in NE subpolar gyre with some data adjustments over the last 120 years)

Reverdin et al., 2010

T and S correlated, but present also differencesLow-frequency S presents weak seasonal dependencyrelated to modulation of westerlies (NAO) with 0-4 years lag (0.63)Studies on the 1990s transition indicates that in winter it is related mostly to changes in ocean circulation/inputs to the gyre (and E-P)

Multi-decadal filter

Fresh

Salty

Fresh

Further southThe worse sampling

Better sampling further northBut gaps remain

In all cases: issues of corrections/qualification of data(particularly in the 1920s) (requires adjustment of data)There is often a need to bin in ‘big’ boxes (and interannual smoothing+ averaging different seasons)

Trends

No relation (in phase) between NATLAnd NASG (possible delayed phase)45-50°N, already signal characteristicof NASG (2 years earlier)

NA

TA

IG

Meso-scales need to be resolved to study higher

frequency variability (seasonal or less)even on the large scales

Examples from SPURS (1-year survey of NA subtropical gyre)

Feature associated with transport of fresh/warm anomaly from south(Busecke et al., 2014)

R. SchmittA. Gordon

The largest scales haveRms =0.14 (cor=0.5 with SMOS)

The smaller meso-scalescan have 0.2 psu signals asMuch as large scales over 1000 km…Thus possible strong fronts and vertical circulations…(but in some areas/seasons T and S Compensated, whichis scale-dependent)Kolodziejczyk et al., 2014

Meso-scaleSSS variability

100 km

20 km

SPURS SSS seasonal signalinfluence of the eddy and meso-scale structures to d(V’S’)/dy

80 0.10

Gordon et al, 2014(SODA reanalysis)

Local SPURS budget evaluated100 km x 100 km box

Estimates of dSSS/dt from driftersNear SPURS moring (red)Compared to Aquarius+Aviso estimateCenturioni et al., 2015

Farrar

Near surface stratification

The low wind; high SW case (Asher et al., 2014)

Gradients day-time, a few percents of the surface of the oceans

Rainfall-induced stratificationS(15-cm) – S(45-cm)

17 events SVP-BS / Surplas (ITCZ/SPCZ)

Individual rain events : 25% more at 15-cm, but for less than an hour(also, T decrease and stratification); wind estimated ‘SSMI’, not measured

Extreme rain event!

No wind! (means no wave energy in 20 cm – 1m wave length domain)Little S gradient between 4cm and 50 cm (~1 unit) thus at least 12 cmRain in 90 minutes, but should be more as decrease below 50 cm...Strong T-gradient (implies probably ~100 W/m2 cooling); secondary drops?

1 h

Conclusions & Perspectives- In situ data can be used to have long time series, but issues on some old data remain

and cast doubts on some results

- No identification of Atlantic basin-scale SSS trends over the last 118 years

- When data density high enough, spatial patterns of low frequency modes of variability can be investigated (from 2-D to 3-D in the last 10y)

- Dedicated surveys/instrumentation to test certain balances on smaller spatial scales: example of SPURS1 (but also COARE)

V’S’ ~ 1 E-3 from SSS (smoothed 200 km/1 month) + Aviso current V’S’ ~ 3 E-3 from drifter SSS + velocitiesThus 22-30 cm of equivalent rainfall (compared to 130 cm for E-P)

- Complementarity with satellite data to be developed both in SSS and improved surface current products