A numerical study on the winter mixed layer on the shelf ... · Snapshot of Mixed Layer Depth 20...
Transcript of A numerical study on the winter mixed layer on the shelf ... · Snapshot of Mixed Layer Depth 20...
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A numerical study on the winter mixed layer on the shelf-slope region south of Japan
Hiroshi Kuroda*Takashi Setou* Yuichi Hirota*
Manabu Shimizu* Kazuhiro Aoki*
*: National Research Institute of Fisheries Science, Fisheries Research Agency, Japan
Email: [email protected]
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Motivation of this study
Distribution of sardine eggs in February 1991(Watanabe et al., 1997)
+January to MarchSpawning season of sardine
+Main spawning groundsouthwest of Japan
Inshore side of the Kuroshio
+Oceanographic conditionDeveloped winter mixed layer
Kuroshio Axis
Missing Link
The aim of this study isto examine properties and dynamics of mixed layer variation
on the basis of high-resolution model
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Climatological seasonal cycle of mixed layer depth ( )de Boyer Montégut et al. (2004, JGR)
(m)
oo 22 ×
m
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Data Analysis of monthly temperature profileson the region southwest of Japan
R/V Kotaka-maru
Analysis DataStation : Stn.A in Tosa BayPeriod :1991~2004Frequency :monthly after 1995Instrument : CTD&STD&XBTVertical resolution :1m
Definition of Mixed Layer Depth
The same criterion is used inde Boyer Montégut et al. (2004)
Stn.Awater depth: 650m C2.0 o=ΔT
Tosa Bay
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Tem
pera
ture
(d
egC
)M
L D
epth
(m
)
ML TemperatureMonthly ML temperature & depthat Stn.A in Tosa Bay
●:monthly meanvertical bar:±1 S.D.
◎:monthly climatologyde Boyer Montégut et al. (2004, JGR)
Time (month)Time (month)
Net Heat Flux at the sea surface
Hea
t flu
x(W
/m2 )
estimated via bulk formulation (COARE)using JRA25+GHRSST ML Depth
Time (month)
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Tem
pera
ture
(d
egC
)M
L D
epth
(m
)
ML TemperatureMonthly ML temperature & depthat Stn.A in Tosa Bay
●:monthly meanvertical bar:±1 S.D.
◎:climatologyde Boyer Montégut et al. (2004, JGR)
Time (month)Time (month)
Net Heat Flux at the sea surface
Hea
t flu
x(W
/m2 )
estimated via bulk formulation (COARE)using JRA25+GHRSST ML Depth
Time (month)
Within the ML, what can balance with the sea surface cooling
from Jan. to Mar.?
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Tem
pera
ture
(d
egC
)M
L D
epth
(m
)
ML TemperatureMonthly ML temperature & depthat Stn.A in Tosa Bay
●:monthly meanvertical bar:±1 S.D.
◎:climatologyde Boyer Montégut et al. (2004, JGR)
Time (month)Time (month)
Net Heat Flux at the sea surface
Hea
t flu
x(W
/m2 )
estimated via bulk formulation (COARE)using JRA25+GHRSST ML Depth
Time (month)
Can intra-monthly variations affect the monthly variation of
ML from Jan. to Mar.?
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Forcings1. Climatological monthly
mean flux at the sea surface 2. Lateral boundary forcings
based on daily mean outputfrom each parent model
one-way nesting system
ROMSRegionalOceanModelingSystem
1/2 degreeBasin-scale(O(103) km)
1/10 degree Mesoscale
(O(102) km)
1/50 degreeSubmesoscale(O(101) km)
Dynamical downscaling system
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Spinup Schedule of three models
1/2-degree model
1/10-degree model
9th
1 Jan.
1/50-degree model
15th
1 Jan.20th
1 Jan.
Initial valuewoa2001 Jan.
(momentum=0)
Initial valuefrom 1/10-degree
model output
Initial valuefrom 1/2-degree
model output
Total: 5-year simulationAnalysis: 2nd year to 5th year
External ForcingsSea surface flux : climatological monthly mean
Lateral Boundary : daily mean from parent model
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Kuroshio axis positionmodel
observation1992-2003
50%
o
101
Kuroshio axis distance vs.Surface current in Tosa Bay
Kuroshio axis distance vs.Subsurface temperature
in Tosa Bay
o
501
o
501
model
obs.
model
obs.
Kuroda et al. (ready for submit)
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Time (month) Time (month)
(m)
(deg
C)
Monthly mean time series of mixed layer at Stn.A in Tosa Bay
12-hourly time series of mixed layer at Stn.A in Tosa Bayfrom the 2nd to 5th year
ML
temperature
(deg.C)
ML
dep
th (m
)
1 Jan.1 Jan. 1 Jan.
gray area: September to March
1 Jan.
ML depth ML temperature
2nd 3rd 4th 5th
ML depthML temperature
data analysis
simulation
climatology
de Boyer Montégutet al. (2004)
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12-hourly time series of mixed layer at Stn.A in Tosa Bayfrom the 2nd to 5th year
ML
temperature
(deg.C)
ML
dep
th (m
)
1 Jan.1 Jan. 1 Jan.
gray area: September to March
1 Jan.
ML depth ML temperature
2nd 3rd 4th 5th
9 10 11 12 1 2 3 4Time (month)
ML
dep
th (m
)
ML
temperature
(deg.C)
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Snapshot of Mixed Layer Depth20 January in the 3rd year
9 10 11 12 1 2 3 4Time (month)
ML
dep
th (m
)
ML
temperature
(deg.C)
Stn.A
Kuroshio
Spatial features
Patch-like deeper MLStreak-like shallower MLalong the Kuroshio front
m
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Daily snapshots ofSST (upper panels) and ML depth (lower panels)
20 Jan. 21 Jan. 22 Jan. 23 Jan.
m
Co
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Correlation coefficient of mixed layer depth for Stn.A from January to March
dotted circlearound Stn.A
small: radius = 10kmlarge:radius = 20km
e-folding scale=10-20km
0.3
0.20.1
20km0
0.2
0.2>
scale
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Heat balance of mixed layer temperature at a fixed station (Stn.A)
( ) ( )
( ) ( )( ) Tth
hT
hdzT
h
Th
Thz
qCt
T
h
hzz
Δ∂∂
−∇∇+⋅∇−
∇−∇+⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
=∂
∂
∫−
−==
111
111
HHH
0
zz0zzp
ML
κ
κκρ
u
shortwavepenetration
longwavesensible and
latent heat fluxes
advection entrainment
vertical diffusion
at ML bottom
horizontaldiffusion
Each term is diagnostically estimated from model output by a method of Kim et al. (2006, JPO)
Heat flux through the sea surface
dzTh
Th∫−
=0
ML1
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Term estimation of the mixed-layer-temperature equation on the slope of Tosa BayPart 1: Monthly Mean
( ) ( ) ( ) ( )( ) Tth
hT
hdzT
hT
hT
hzq
CtT
hhzz Δ
∂∂
−∇∇+⋅∇−∇−∇+⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
=∂
∂∫−
−==
111111HHH
0
zz0zzp
ML κκκρ
u
2. Heat flux through
the sea surface
4.Advection
5.Horizontal
diffusion
1. Tendency
6.Entrainment
3Verticaldiffusion
Oct.
Nov.
Dec.
Jan.
Feb.
Mar.
( )610C −×so
( )610C −×so
( )610C −×so
( )610C −×so
( )610C −×so
( )610C −×so
MLTemp.
ML Depth
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Term estimation of the mixed-layer-temperature equation on the slope of Tosa BayPart 2: 12-hour mean
( ) ( ) Tth
hdzT
hT
hzq
CtT
hz Δ
∂∂
−⋅∇−∇+⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
≈∂
∂∫−
=
1111 0
0zzp
ML uκρHeat flux through
the sea surfaceAdvectionTendency Entrainment
( )610C −×so
12-hour time series from January to March in the 3rd year
Jan. Feb. Mar.
The sign of tendency isreversed.
minus TendencySurface heat fluxAdvectionEntrainment
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( )
),,(
1 0
wvu
dzTh h
=
⋅∇− ∫−
u
u
HL uuuu ′+′+=
HL TTTT ′+′+=
( )( )
HHLHH
HLLLL
HL
HLHL
'
'
TTT
TTT
TTT
TTTT
′′⋅∇+′⋅∇+′⋅∇+
′′⋅∇+′⋅∇+′⋅∇+
′⋅∇+′⋅∇+⋅∇=
′+′+′+′+⋅∇=⋅∇
uuu
uuu
uuu
uuuu
( )
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )∫∫∫
∫∫∫
∫∫∫
∫
−−−
−−−
−−−
−
′′⋅∇−′′⋅∇−′⋅∇−
′′⋅∇−′′⋅∇−′⋅∇−
′⋅∇−′⋅∇−⋅∇−=
⋅∇−
0
HH
0
LH
0
H
0
HL
0
LL
0
L
0
H
0
L
0
0
111
111
111
1
hhh
hhh
hhh
h
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
uuu
uuu
uuu
u
Decompositionof monthly mean advection
Original term
AverageLow Frequency(>30-day period)
High Frequency(Intra-monthly)
If h is constant, Orange Terms=0
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( )
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )∫∫∫
∫∫∫
∫∫∫
∫
−−−
−−−
−−−
−
′′⋅∇−′′⋅∇−′⋅∇−
′′⋅∇−′′⋅∇−′⋅∇−
′⋅∇−′⋅∇−⋅∇−=
⋅∇−
0
HH
0
LH
0
H
0
HL
0
LL
0
L
0
H
0
L
0
0
111
111
111
1
hhh
hhh
hhh
h
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
dzTh
uuu
uuu
uuu
u ( )( ) ( ) ( )( ) ( ) ( )( ) ( ) ( )HHLHH
HLLLL
HL
,,,
,,,
,,,
,
TfTfTf
TfTfTf
TfTfTf
Tf
′′+′′+′+
′′+′′+′+
′+′+=
uuu
uuu
uuu
u
Contribution of decomposed components (February)
( )610C −×so
4-ye
ar m
ean
deco
mpo
sed
valu
eIn
Feb
ruar
y
( )Tf ,u
( )H,Tf ′u
( )L,Tf ′u
( )Tf ,Hu′
( )Hu Tf ′′ ,H
( )LH ,Tf ′′u
( )Tf ,Lu′
( )Hu Tf ′′ ,L
( )LL ,Tf ′′u
3rd-order upstream
2rd-order centered
Intra-monthly eddy heat advection
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ConclusionsSeasonal cycle of ML depth and temperature exhibits two regimes
1. September to NovemberMonthly mean ML depth increases & ML temperature decreases
2. January to MarchMonthly mean ML depth and temperature become constant
For the monthly mean heat balance in the ML Heat flux through the sea surface + entrainment ~
advection
There is dominant intra-monthly submesoscale variability associated with the Kuroshio or its frontal disturbances
eddy heat advection
Intra-monthly variation contributes significantly to the monthly/seasonal variation of heat balance within the ML
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Thank you for your attention