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