current and turbulence observations of north pacific … · 34.0 34.0 34.0 34.2 34.2 34.2 large...
TRANSCRIPT
Current and TurbulenceObservations of North Pacific
Intermediate Water in the Kuroshio-Oyashio Confluence Region
Hitoshi KANEKO and Ichiro YASUDAOcean Research Institute of University of Tokyo
IntroductionNorth Pacific Intermediate Water(NPIW)is characterized by salinity minimum inthe mid depth of the subtropical gyre(e.g. Sverdrup et al., 1942; Reid, 1965)
Yasuda et al (1996) :Subarctic Oyashio waterflows into the intermediate depthof the Kuroshio Extension east of Japan
Fig. Schematic illustration of water massdistribution (Yasuda, 2004)
However, formation processes in the confluence of theOyashio/Kuroshio Extension just after the separation ofthe Kuroshio have not been investigated in detail.
• Question:• How do the Oyashio and the Kuroshio merge?• How is the salinity minimum developed and modified just after the confluence?
• Objective:Clarifying hydrography, current structure, andvertical mixing in the region just after theconfluence
Purpose of the present study
We performed observations in the confluenceregion just east of Japan.
Observation & Data
Oyashio
Observation lines superposed on week CompositeSST (NOAA/HRPT SST, May 09-15 2007)
Kuroshio Extension
SST(oC)
Confluence Region
BosoPeninsula
turbulenceobservation
XCTD
ShipboardADCP
Tansei-Maru:May 8-15, 2007
• Estimate absolute geostrophic velocity from ADCP and baroclinic geostrophic velocity from CTD (Masujima et al., 2003)
• Calculate Oyashio mixing ratio assuming isopycnal mixing of Oyashio & Kuroshio reference waters (Shimizu et al., 2001)
• Calculate Oyashio component volume transport ( = Oyashio mixing ratio x total volume transport)
• Directly observed turbulence dissipation rate (ε) from vertical microstructure profiler
• Estimate Vertical diffusivity coefficient (Kρ)Kρ = 0.2 ε / N2 (Osborn, 1980; N: buoyancy frequency)
• Indirect estimated ε (parameterization)εprm=7x10-10<N2/N0
2><S104/S4
GM> (Gregg, 1989)using 10m scale velocity shear (S10) from ADCP
Method
Along the line2, line3, around26~27σθ, low salinityOyashio water (S<34PSU)is found south of the KuroshioExtension axis(14oC @200m)
Result:Oyashio water intrusion
Line1
Line2
Salinity
Line3
S NK.E. Axis(14oC@200m)
25.5σθ
26.7σθ
25.5σθ
26.7σθ
25.5σθ
26.7σθ
Line3Line2
Line1
Bosopeninsula
Oyashio intrusion into the intermediate layer
25.5σθ
In the upper layer (25.5σθ),relatively high salinity waterflows northeastward.
In the intermediate layer(26.7σθ), low salinity waterflows eastward.(the direction is different)
36.5N
26.7σθ
34.633.8 34.434.2Salinity
Fig. Horizontal distribution of current &salinity at each isopycnal surface
140E 143E35N
35N
It suggests that…remarkable salinity minimumis formed.
Oyashio water intrudes intothe mid depth of the KuroshioExtension across the K.E.axis.
1.0 / 3.6(total)
2.3 / 5.8(total)
2.0 / 5.1(total)
Volume transport (26.6~27.4σθ)
Oyashio component volume transport into the intermediate layer of K.E.
1Sv (106ms-1) Oyashiowater flows eastwardacross line1 (2Sv; line2, 3)
This result shows that…BosoPeninsula
Oyashio
Kuroshio Extension(K.E.)
!" #$ %&'()* !"+
,-./0 012 314 514 3617
,-./3 315 518 917 8219
,-./5 312 513 910 5:13
26. 6~27.4σθ(intermediate layer)
Fig. Oyashio component volumetransport across each section
1.0
2.3
2.0
Volume transport (26.6~27.4σθ)
Oyashio component volume transport into the intermediate layer of K.E.
1Sv (106ms-1) Oyashiowater flows eastwardacross line1 (2Sv; line2, 3)
This result shows that…Oyashio water does notflow inshore but offshorein the confluence region.
BosoPeninsula
Oyashio
Kuroshio Extension(K.E.)
!" #$ %&'()* !"+
,-./0 012 314 514 3617
,-./3 315 518 917 8219
,-./5 312 513 910 5:13
26. 6~27.4σθ(intermediate layer)
Across the red line (between line1,2)1Sv Oyashio water joins the K.E.
~1
Fig. Oyashio component volumetransport across each section
~1
Structure of Oyashio intrusion
Where and how much does vertical mixing occur?
Intrudes intermediate layer
Oyashio
Kuroshio Extension (surface)
2Sv
Based on the results, we infer that…
• Southward Oyashio intrudes into the intermediate layer of K.E. off the Boso Peninsula • About 1Sv Oyashio Water flows into the K.E. west of line1 • Across the line between Line1&2, about 1Sv Oyashio water joins • Salinity mininum associated with the Oyashio intrusion becomes prominent.
1Sv
1Sv
In the confluence region where the Oyashio intrudes, vertical salinity gradient and shear of velocity are so large.
Salinity (PSU)
34.0 34.5
Direct observation of verticaldiffusivity Kρ
Fig. Vertical profile of observed diffusivity coefficient,and Salinity at Stn.27 located on the K.E. SST front.
Low salinity water is found at200-300m, 300-400m.Kρ is relatively large at100~200m, 300~400m.
Stn.27
Kρ (cm2 s-1)
10.110-2 10
Implying that…Relatively large verticalmixing occurs aroundsalinity minimum.
At each station, 100m-averageddissipation rate is calculated.
Indirectly parameterizeddissipation rate (εprm) is roughlyproportional to the directlyobserved one (εobs )
εobs= 2.22εprm(r ~ 0.5: statistically significant)
Using this relation, distribution ofvertical diffusivity Kρ is mappedfor each section.
Comparison of dissipation rate, εbetween direct and indirect methods
Indirectly parameterized
Fig. Scatter plot of dissipation rate between directobservation and Gregg’s parameterization
10
Kρ (cm2s-1)0.001 10.01
Distribution of vertical diffusivity Kρ
Line1
Line2
Line3
S N
34.0
34.0
34.034.2
34.2
34.2
Large vertical diffusivity(~10cm2s-1) distributes aroundthe upper part of low salinitywater intrusion.
Diffusivity is particularly largealong line 2, corresponding tolarge velocity shear.
Intruded Oyashio water may betransformed by this strongvertical diffusion.
Fig. cross-sections of vertical diffusivityestimated from indirect methodWhite contours indicate salinity(CI=0.2PSU)0.1
Intrudes Intermediate layer
Strong vertical Mixing by large shear
Oyashio
Kuroshio Extension (surface)
2Sv
• Oyashio water intrudes into the intermediate layer of the Kuroshio Extension just after the confluence east of Boso peninsula.
• Intruded Oyashio water transport is 2 Sv in the intermediate layer(26.4~27.4σθ)
Summary
• Strong vertical diffusion occurs around the uppermost of the salinity minimum just after the confluence.
• This implies erosion of the uppermost of the salinityminimum and this could contribute to the NPIW formation.
BosoPeninsula
Fig. structure of Oyashio intrusion
Section of velocity from ADCP(V)Line1
Line2
stn.17
stn.18stn.23
stn.27stn.24
Line3
S N
stn.11
V(cm s-1)(Nortward Positive)
Line1
Line2
Salinity
Line3
S NK.E. Axis(14oC@200m)
25.5σθ
26.7σθ
25.5σθ
26.7σθ
25.5σθ
26.7σθ
34.0
34.2
34.234.0
34.0
34.2