a numerical study on the winter mixed layer on the shelf ... · snapshot of mixed layer depth 20...
TRANSCRIPT
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]
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
Climatological seasonal cycle of mixed layer depth ( )de Boyer Montégut et al. (2004, JGR)
(m)
oo 22 ×
m
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
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)
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.?
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.?
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
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
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)
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)
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)
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
Daily snapshots ofSST (upper panels) and ML depth (lower panels)
20 Jan. 21 Jan. 22 Jan. 23 Jan.
m
Co
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
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
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
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
( )
),,(
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
( )
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )∫∫∫
∫∫∫
∫∫∫
∫
−−−
−−−
−−−
−
′′⋅∇−′′⋅∇−′⋅∇−
′′⋅∇−′′⋅∇−′⋅∇−
′⋅∇−′⋅∇−⋅∇−=
⋅∇−
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
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
Thank you for your attention