The Diurnal Cycle of Salinity Kyla Drushka 1, Sarah Gille 2, Janet Sprintall 2 [email protected] 1. Applied Physics Lab, Univ. of Washington 2. Scripps.

The Diurnal Cycle of Salinity

Kyla Drushka1, Sarah Gille2, Janet Sprintall2

[email protected]

1. Applied Physics Lab, Univ. of Washington 2. Scripps Inst. of Oceanography

Aquarius Science Team Meeting12 November 2014

day night

thin, warm mixed layer(traps atmospheric

fluxes) deep, cool mixed layer

surface cooling, overturning,

mixing

Diurnal variations in solar radiation affect SST, mixing, etc.– can improve climate modeling of air-sea processes

…Do diurnal salinity variations matter?e.g. in regions where salinity controls mixed-layer depth

Aquarius mean ascending–descending difference:(V3.0 CAP L2 data, 3-yr average, 2o bins)

0.5 psu

–0.5

Ascending = evening, descending=morningDoes diurnal salinity account for any of this signal?

Diurnal salinity from TAO buoy data at 1 m depth

156E,2N

1-m salinity

Hour of day (local time) *showing two cycles

6 12 18 0–0.005

0

6 12 18

psu

0

+0.005

±0.006 psu

1. One year of hourly data2. high-pass filter3. bin by hour of day

Nighttime salinity increase(max S at 8am)

Daytime freshening

(min S at 3pm)

Diurnal salinity at all TAO moorings: Diurnal salinity amplitude (psu)

0

0.01

60E 120E 180E 120W 60W 0 60E

0

15N

15S

Aquarius ascending–descending salinity >> 1-m diurnal salinity

0.5

–0.5

Diurnal salinity amplitude (psu)

0

0.01

60E 120E 180E 120W 60W 0 60E

0

15N

15S

Diurnal salinity at all TAO moorings:

Diurnal salinity decays with depth. Open question: how much?

depth

Diurnal salinity

precipitation

evaporation entrainment

Salinity at 1-m (buoy) depth ≈

advection

assume hp~3m

assume hE ~1m

negligible h =mixed-layer

depth

Total ∆S: ±0.006 psu

hour (local time)0 6 12 18 24

mm/hr

–0.005

0.005

0

–0.1

0.1

0

4

–4

0

–0.01

0.01

0

6 12 18

psu

mm/hr

m

precipitation–0.004 psu

evaporation+0.001 psu

entrainment+0.003 psu

Estimated contributions:

1-m salinity

precipitation evaporation

mixed-layer depth

deeper

156E,2N

Diurnal salinity amplitude (psu)

0

0.01

0 0.01 0.02 0 0.01 0.02 0 0.01 0.020

0.01

0.005

Obse

rved ∆

S,

psu

Precipitation contribution,

psu

Evaporation contribution,

psu

Entrainment contribution, psu

Precipitation & entrainment consistently dominate diurnal salinity

60E 120E 180E 120W 60W 0 60E

0

15N

15S

Where diurnal salinity is expected to be strong:

60E 120E 180E 120W 60W 0 60E

0

30N

30S

0

30N

30S

Diurnal rain rate amplitude (TRMM 3-hrly data)

Climatological "stratification strength" (∆S/h)from the MIMOC product, an Argo-based climatology

0

0.1

diurnal rain rate,

mm/hr

psu/m x 10-3

0

2

Observed diurnal salinityDiurnal salinity amplitude (psu)

0

0.01

0

15N

15S

Recap

1. Diurnal salinity at 1-m depth is small but significant

2. Rain drives diurnal salinity. Entrainment sets the phase.

3. Ascending–descending Aquarius differences are much bigger than 1-m diurnal salinity

– but: 1cm signal is likely larger than 1m signal, so diurnal salinity could still affect Aquarius

What we need to know to understand if diurnal salinity affects Aquarius:

1. What is the thickness / salinity anomaly of fresh pools?

2. How does salinity decay with depth in the upper few meters?

…1-d modeling

Diurnal salinity

Generalized Ocean Turbulence Model (GOTM)(Burchard & Bolding 2001, www.gotm.net)

1-d model:– 2-parameter k-ε turbulence closure scheme

– forced with hourly TAO observations (shortwave flux, wind, rain)

– T and S profiles initialized once per day (at sunrise)

– COARE bulk formula

– surface wave-breaking (Burchard, 2001) and internal wave parameterizations (Large et al., 1994)

– <5cm resolution within the top 5m (<30cm within the mixed layer)

– 1-min time step

– has been used for diurnal/surface layer studies

(e.g., Jeffery et al., 2008; Pimental et al., 2008)

Validation: GOTM vs TAOForcing: Precipitation

0

10

20

mm

/hr

28

32

oC 30

ModelObservation (TAO)

ModelObservation (TAO)

32

34

psu

33

14 Sept 21 Sept

1-m temperature

1-m salinity

Fresh events captured(R2=0.77 over 8-month run)

Diurnal warming well reproduced

(R2=0.89 over 8-month run)

Lens formation under rain with GOTMIdealised rain (Gaussian pulse) + wind (sinusoid)

0

15

mm

/hr

0

6m

/s

0

20

z, m

-0.05

0.05

psu

wind speed(4±1 m/s)

10rain rate

(peak 5mm/hr)5

salinity anomaly relative to

no-rain case

00:00 06:00 12:00 18:0018:00

psu

salinity anomaly at z=1 m

0

-0.1

-0.2

Local time

Varying the strength of rain

Rain rate affects:– Strength of salinity anomaly– Lens thickness (hp)

Weaker rain (2 mm/hr)

0

15

mm

/hr

0

10

0

20

z, m

0

15

0

10

0

10

00:00 06:00 12:00 18:0018:00

m/s

Stronger rain (10 mm/hr)

00:00 06:00 12:00 18:0018:00

0

-0.2

psu S'1m

0

-0.2

-0.05

0.05

S', psu

rain ratewind speed

Varying the strength of wind

0

15

mm

/hr

0

10

0

20

z, m

0

15

0

10

0

10

00:00 06:00 12:00 18:0018:00

m/s

00:00 06:00 12:00 18:0018:00

0

-0.2

psu S'1m

0

-0.2

-0.05

0.05

S', psu

Weaker wind (5 m/s) Stronger wind (10 m/s)

Wind speed controls:– Strength of salinity anomaly– Duration of salinity anomaly

rain ratewind speed

Strongest salinity anomalies when:

00:0000:00

12:00

06:00

18:00

Hour

of

peak

win

d

0.05

0.05

0.10

0.15Magnitude of

salinity anomaly, psu

– rain coincides with weak winds– surface mixed layer is thin (mid-day)

12:0006:00 18:00Hour of peak rain

SummaryTAO mooring data show a significant, weak diurnal salinity cycle driven by rain + entrainment (Drushka et al., JGR, 2014)

Obse

rved

diu

rnal

∆S

Precipitation contribution

Evaporation contribution

Entrainment contribution

wind

weak

rain

strong

weak

strong

A 1-d turbulence model shows that wind & rain strength significantly affects lens formation & salinity anomaly

References

Bernie, D., E. Guilyardi, G. Madec, J. Slingo, and S. Woolnough. 2007. "Impact of resolving the diurnal cycle in an ocean-atmosphere GCM. Part 1: A diurnally forced OGCM." Clim. Dyn., 29(6), 575–590, doi:10.1007/s00382-007-0249-6.

Burchard, Hans, and Karsten Bolding. 2001. “Comparative Analysis of Four Second-Moment Turbulence Closure Models for the Oceanic Mixed Layer.” Journal of Physical Oceanography 31 (8): 1943–68. doi:10.1175/1520-0485(2001)031<1943:CAOFSM>2.0.CO;2.

Cronin, M. F., and M. J. McPhaden.1999. "Diurnal cycle of rainfall and surface salinity in the western Pacific warm pool." Geophys. Res. Lett., 26(23), 3465–3468.

Fairall, C. W., E. F. Bradley, D. P. Rogers, J. B. Edson, and G. S. Young. 1996. “Bulk Parameterization of Air-Sea Fluxes for Tropical Ocean-Global Atmosphere Coupled-Ocean Atmosphere Response Experiment.” Journal of Geophysical Research: Oceans 101 (C2): 3747–64. doi:10.1029/95JC03205.

Jeffery, C. D., I. S. Robinson, D. K. Woolf, and C. J. Donlon. 2008. “The Response to Phase-Dependent Wind Stress and Cloud Fraction of the Diurnal Cycle of SST and Air–sea CO2 Exchange.” Ocean Modelling 23 (1–2): 33–48. doi:10.1016/j.ocemod.2008.03.003.

Pimentel, S., K. Haines, and N. K. Nichols. 2008. “Modeling the Diurnal Variability of Sea Surface Temperatures.” Journal of Geophysical Research: Oceans 113 (C11): C11004. doi:10.1029/2007JC004607.