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The Diurnal Cycle of Salinity
Kyla Drushka1, Sarah Gille2, Janet Sprintall2
1. Applied Physics Lab, Univ. of Washington 2. Scripps Inst. of Oceanography
Aquarius Science Team Meeting12 November 2014
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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
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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?
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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)
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Diurnal salinity at all TAO moorings: Diurnal salinity amplitude (psu)
0
0.01
60E 120E 180E 120W 60W 0 60E
0
15N
15S
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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:
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Diurnal salinity decays with depth. Open question: how much?
depth
Diurnal salinity
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precipitation
evaporation entrainment
Salinity at 1-m (buoy) depth ≈
advection
assume hp~3m
assume hE ~1m
negligible h =mixed-layer
depth
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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
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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
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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
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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
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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
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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)
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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)
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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
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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
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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
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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
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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
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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.