The Diurnal Cycle
of Salinity
Kyla Drushka1,
Aquarius Science Team Meeting
12 November 2014
Sarah Gille2, Janet Sprintall2
kdrushka@apl.uw.edu
1. Applied Physics Lab, Univ. of Washington
2. Scripps Inst. of Oceanography
Diurnal variations in solar radiation affect SST, mixing, etc.
– can improve climate modeling of air-sea processes
night
day
surface cooling,
overturning, mixing
thin, warm mixed layer
(traps atmospheric fluxes)
deep, cool mixed layer
…Do diurnal salinity variations matter?
e.g. in regions where salinity controls mixed-layer
Aquarius mean ascending–descending difference:
(V3.0 CAP L2 data, 3-yr average, 2o bins)
0.5 psu
–0.5
Ascending = evening, descending=morning
Does diurnal salinity account for any of this signal?
Diurnal salinity from TAO buoy data at 1 m depth
1. One year of hourly data
2. high-pass filter
3. bin by hour of day
156E,2N
1-m salinity
+0.005
psu
±0.006 psu
0
–0.005
0
6
18
12
0
12
6
18
Hour of day (local time)
*showing two cycles
Daytime freshening
(min S at 3pm)
Nighttime
salinity increase
(max S at 8am)
Diurnal salinity at all TAO moorings:
Diurnal salinity
amplitude (psu)
15N
0.01
0
15S
60E
0
120E
180E
120W
60W
0
60E
Diurnal salinity at all TAO moorings:
Diurnal salinity
amplitude (psu)
15N
0.01
0
15S
60E
0
120E
180E
120W
60W
0
60E
Aquarius ascending–descending salinity >> 1-m diurnal
salinity
0.5
–0.5
Diurnal salinity decays with depth.
Open question: how much?
depth
Diurnal salinity
Salinity at 1-m (buoy) depth ≈
precipitation
assume
hp~3m
evaporation
assume
hE ~1m
advection
negligible
entrainment
h=
mixed-layer depth
1-m salinity
156E,2N
0.005
psu
Total ∆S:
±0.006 psu
0
–0.005
0.1
mm/hr
0.01
0 mm/hr
0
precipitation
–0.1
–4 mixed-layer depth
m
0
deeper
4
Estimated
contributions:
–0.01
evaporation
precipitation
–0.004 psu
evaporation
+0.001 psu
0
6
12
18
24
6
hour (local time)
12
18
entrainment
+0.003 psu
Precipitation & entrainment consistently
dominate diurnal salinity
Diurnal salinity
amplitude (psu)
15N
0.01
0
0
15S
60E
120E
180E
60W
120W
60E
0
Observed ∆S, psu
0.01
0.005
0
0
0.01
0.02
Precipitation
contribution, psu
0
0.01
0.02
Evaporation
contribution, psu
0
0.01
0.02
Entrainment
contribution, psu
Where diurnal salinity is expected to be strong:
30N
Diurnal rain rate amplitude (TRMM 3-hrly data)
0.1
0
0
30S
diurnal
rain rate,
mm/hr
Climatological "stratification strength" (∆S/h)
30N
from the MIMOC product, an Argo-based climatology
2
0
psu/m x 10-3
30S
60E
15N
120E
180E
120W
Observed diurnal salinity
60W
0
60E
0
0.01
Diurnal salinity
amplitude (psu)
0
15S
0
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?
Diurnal salinity
…1-d modeling
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)
mm/hr
Validation: GOTM vs TAO
20
Forcing: Precipitation
10
0
oC
32
Model
Observation (TAO)
30
28 1-m
Diurnal warming well
reproduced
(R2=0.89 over 8-month run)
temperature
1-m salinity
psu
34
33
32
Model
Observation (TAO)
14 Sept
21 Sept
Fresh events captured
(R2=0.77 over 8-month run)
Lens formation under rain with GOTM
Idealised rain (Gaussian pulse) + wind (sinusoid)
wind speed
(4±1 m/s)
10
rain rate
(peak 5mm/hr)
5
0
0
-0.1
0
salinity anomaly relative
to
no-rain case
z, m
20
0
psu
6
m/s
mm/hr
15
0.05
psu
-0.05
salinity anomaly at
z=1 m
-0.2
18:00
00:00
06:00
Local time
12:00
18:00
Varying the strength of rain
z, m
psu
15
rain rate
wind speed
0
0
20
0
Stronger rain (10 mm/hr)
10
15
10
0
0
0
0
10
0
S'1m
-0.2
18:00
00:00
06:00
12:00
18:00
18:00
00:00
0.05
S', psu
-0.05
m/s
mm/hr
Weaker rain (2 mm/hr)
Rain rate affects:
– Strength of salinity anomaly
– Lens thickness (hp)
06:00
12:00
-0.2
18:00
Varying the strength of wind
z, m
psu
15
rain rate
wind speed
0
0
20
0
Stronger wind (10 m/s)
10
15
10
0
0
0
0
10
0
S'1m
-0.2
18:00
00:00
0.05
S', psu
-0.05
m/s
mm/hr
Weaker wind (5 m/s)
06:00
12:00
18:00
18:00
00:00
Wind speed controls:
– Strength of salinity anomaly
– Duration of salinity anomaly
06:00
12:00
-0.2
18:00
Strongest salinity anomalies when:
– rain coincides with weak winds
– surface mixed layer is thin (mid-day)
0.05
Hour of peak wind
18:00
0.15
Magnitude of
salinity anomaly,
psu
12:00
0.10
06:00
00:00
00:00
18:00
06:00
12:00
Hour of peak rain
0.05
Summary
TAO mooring data show a significant, weak diurnal salinity cycle driven
by rain + entrainment (Drushka et al., JGR, 2014)
Evaporation
contribution
Entrainment
contribution
Observed
diurnal ∆S
Precipitation
contribution
A 1-d turbulence model shows that wind & rain strength significantly
affects lens formation & salinity anomaly
rain
wind
weak
weak
strong
strong
References
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Cronin, M. F., and M. J. McPhaden.1999. "Diurnal cycle of rainfall and surface salinity in the western Pacific warm
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