1. UNDERSTANDING OCEAN SALINITY
Stephen Riser
Jessica Anderson
University of Washington
NASA SST Science Team Meeting
October 29, 2013
[ Acknowledgements to NASA, NOAA, NSF, ONR ]

2. Ocean salinity: what is it, and why are we interested?
(Hartmann, 1994)
(Peixoto and Oort, 1992)

3. Ocean salinity: classical and modern
salinity is tied to the hydrological cycle
classical definition of salinity
[ / = parts per thousand ]
the chemical composition of seawater
Perhaps surprisingly, the proportions of the major constituents of seawater are approximately constant nearly everywhere in the deep ocean.

4. the modern equation of state for seawater
modern definition of salinity; the 1978 Practical Salinity Scale
the modern equation of state for seawater
the density (mass) distribution determines the flow, via Newton’s Laws
1.023
1.024
Data from UW float 6889 (WMO ) in the subtropical N. Atlantic
1.025
1.026
1.027
1.028
1.029

5. The annual mean global sea surface salinity (PSU)

6. classical definition of salinity
average this over a surface mixed layer of thickness h :
“ocean rain gauge”
advection of vertically-averaged salinity by the average flow
advection of salinity by the sheared part of the horizontal flow
entrainment/detrainment and/or obduction/subduction of salinity through the base of the mixed layer
change of salinity at the sea surface through evaporation and/or precipitation
small scale mixing

7. canonical Argo mission: 0-2000 m; T, S, p t = 10 days
The Argo Float Array
canonical Argo mission:
m; T, S, p
(0.005 C; 0.01 PSU ; 2.5 dbar)
t = 10 days
6-7 years profiles
32 countries, 800 floats/year deployed
UW: 120 floats/year
All data are publicly available with 24 hours of collection

8. SBE41: T  0.002C; S  0.005 PSU; p  2.4 dbars
SBE CTD performance in Argo: generally excellent
SBE41: T  0.002C; S  PSU; p  2.4 dbars
Indian Ocean, 32S: 142 floats/shipboard CTD
(1000 m)
[UW laboratory check, N=380]
Results from recovered floats
DMQC results:
 10 % adjusted

9. WOCE section P1 (dash/orange) 1994

10. Integrals along 24N: the upper ocean has freshened
Argo minus WOCE
Argo, 2007
WOCE, 1994
Integrals along 24N: the upper ocean has freshened
[Ren and Riser, 2010]

11. T S  Upper ocean (250 m), changes in the past 25 yr
[Roemmich and Gilson, 2009]

12. Float with auxiliary STS sensor and PAL hydrophone
Argo float with CTD sensor (cuts off at 3 m)
Float with auxiliary STS sensor and PAL hydrophone

13. T and S profiles (0-30 m) from the same float
T and S profiles ( m) from UW float 6117, in the western equatorial Pacific T S S T
T and S profiles (0-30 m) from the same float

14. Barrier Layer Thickness =
Barrier Layers – Definition and Importance
Mixed Layer
MLDσ
Barrier Layer
MLDt
Barrier Layer Thickness =
MLDt – MLDσ
MLDσ : 0.01 kg/m4
MLDT : 0.05 °C / m
MLDS : 0.02 PSU/m
Lukas and Lindstrom (1991)

15. Locations of 69 UW floats equipped with STS sensors

16. T and S from the upper 10 m of the ocean from float 6117 during a 3 week period when it was profiling m at intervals of 2 hours

17. Float 6117 7/3/2009 Rainfall: 25 mm ∆S : 0.15 PSU ∆T : 0.18 °C
TAO mooring
2°N, 147°E
TRMM
(3 hourly)
STS
Temperature
7/3/2009
Rainfall: 25 mm
∆S : PSU
∆T : °C
STS
Salinity
Hour (Local)

18. Comparisons of T and S (sea surface minus 15 m value) for the fast cycle period of float 6117 in the western equatorial Pacific

19. T and S differences between the sea surface and a depth of 4 m from float 6117, plotted as a function of wind speed measured at a nearby TAO mooring

20. T (C)
S (PSU)
Binned T and S anomalies as a function of local time, shown with binned colocated rainfall from TRMM. Diurnal cycles in T and S are clearly present.

21. Global variability in the amplitude of T and S diurnal cycles from all STS floats

22. Summary
Ocean salinity is an indicator of the ocean’s role in the global hydrological cycle as well as a state variable for the ocean circulation. The salinity distribution is more than simply a measure of precipitation and evaporation.
Argo data, and data from auxiliary STS sensors on some floats, has proven to be useful in understanding the global distribution of upper ocean salinity, especially very near the sea surface. The data can also be used in Aquarius/SAC-D validation exercises.
Many storm events can be seen in the data, lasting several hours, changing the upper ocean temperature and salinity.
There are measureable diurnal cycles in both T and S in the observations; T variability is clearly tied to diurnal heating in the atmosphere; S variability is related to diurnal signals in rainfall.
Estimates of decadal-scale variability are only beginning; the longer that programs like Argo and Aquarius continue, the more we will understand long-term variability in the hydrological cycle and the ocean’s role in climate.

24. N = 16625
Global salinity difference (Aquarius minus Argo) for the period 9/2011-9/2013

25. Aquarius SSS October 2013