Distribution and transport variations of source waters for North Pacific Intermediate Water formation revealed by multiple tracer analysis

Transcription

1 Distribution and transport variations of source waters for North Pacific Intermediate Water formation revealed by multiple tracer analysis Yugo Shimizu 1, Lynne D. Talley 2, Shin-ichi Ito 1, and Miyuki Tatesawa 1 ( 1 Tohoku National Fisheries Research Institute, 2 Scripps Institution of Oceanography) Background *North Pacific Intermediate Water = NPIW NPIW is characterized by a salinity minimum layer in the subtropical gyre. The Mixed Water Region (MWR) is a formation site of NPIW (Talley 1993). The ventilation source of NPIW is considered as the Okhotsk Sea (Yasuda 1997). However, the time variation of NPIW source waters are still unknown. Subtropical gyre Subarctic gyre Russia Okhotsk Sea Depth (m) 1 NPIW Kuroshio JAPAN old NPIW Oyashio new NPIW Eq. 55N Fig. Schematic representation of Fig. Vertical section of salinity along 17 E NPIW formation (Yasuda 1997). (World Ocean Atlas 98).

2 Our goal Methods To reveal the time and spatial variation of NPIW source waters. Calculating the source water components (Kuroshio, Okhotsk Sea and East Kamchatka waters) in the mixed water by multiple tracer analysis. Examining the distribution of these source water components. Examining the time variation of the source water transports. The analyzed density range is from 26.7 σ θ to 27.3 σ θ. Data CTDO data of a repeat observing section called OICE Oyashio Intensive observation line off Cape ErimoOICE Four seasonal cruises in 21: WK11/ Jan.-Feb. 21 WK14/ Apr. 21 TR18/ Aug.-Sep. 21 WK111/ Nov. 21 *WK=R/V Wakataka-maru TR= R/V Torishima Japan ÃŒyŠC 26.7σ θ e ª Oyashio Sea ƒiƒz[ƒcƒnšc of Okhotsk Kuril Islands ƒ uƒ bƒ \ƒ Š C warm g ò eddy Œƒ Jƒ ƒ `ƒ ƒ cƒ JŠ C East Kamchatka current Kuroshio warm water ªŒn g (38N,14545 E) ª ± Kuroshio Extension 27.3σ θ Analyzed density range

3 Multiple tracer analysis Assumption Equations Condition Solution Non-negative least square method for over-determined equations The mixed water is formed by an isopycnal mixture among three source waters with ratio of : Okhotsk Sea: E. Kamchatka: Kuroshio waters= r 1 :r 2 :r 3 When temperature, salinity, dissolved oxygen and potential vorticity are denoted as (θ (ρ ), S (ρ), X (ρ), Q (ρ)) for the observed mixed water and (θ i (ρ), S i (ρ), X i (ρ), Q i (ρ)) for the source waters at density ρ (i=1:okhotsk Sea water, 2E. Kamchatka, 3: Kuroshio components) Mixture of temperature : r 1 θ 1 + r 2 θ 2 + r 3 θ 3 = θ (1) of salinity : r 1 S 1 + r 2 S 2 + r 3 S 3 = S (2) of dissolved oxygen: r 1 X 1 + r 2 X 2 + r 3 X 3 = X (3) of potential vorticity: r 1 Q 1 + r 2 Q 2 + r 3 Q 3 = Q (4) (Danielsen 199, Haynes and McIntyre 199) Summation of mixing ratios =1 : r 1 + r 2 + r 3 = 1 (5) Mixing ratio cannot be negative: r 1 >, r 2 >, r 3 > (6) We normalize eqs. (1)-(4) by observation errors in each tracer and standard deviations in the source water variance, and then numerically solve the equations with a least square method on conditions (5) and (6).

4 PotentialTem perature Salinity Source water profiles composed from the WOCE data archives etc. Okhotsk sea water: Composed from the data near Bussol s Strait in the Okhotsk Sea. East Kamchatka w.: Data where the water flows southward east of Kamchatka Pen. Kuroshio w. : Data on 137 E and off Boso Pen. These are isopycnal average PotentialDensity y ote ta o tc ty θ: Potential temp. O khotsk Q : Potential vorticity 3 E.Kam chatka S : Salinity Kuroshio w. E. Kamchatka water Kuroshio Okhotsk Sea water Okhotsk E.Kam chatka Potential Vorticity ( 1-11 m -1 s -1 ) Dissolved Oxygen (ml/ L) 2 1 Kuroshio E.Kam chatka Okhotsk PotentialDensity X : Oxygen Kuroshio Okhotsk E.Kam chatka 33.2 Kuroshio PotentialDensity PotentialDensity

5 Calculation example: OICE data in TR18/ Vertical sections of T, S and DO T S DO 26.7σ θ 26.7σ θ 27.σ θ 27.σ θ 27.3σ θ 27.3σ θ Calculation results of mixing ratio Okhotsk component: r 1 (%) E. Kamchatka comp.: r 2 (%) Kuroshio comp.: r 3 (%) 26.7σ θ 26.7σ θ 27.σ θ 27.σ θ 27.3σ θ 27.3σ θ

6 Mixing ratio (%) Mixing ratio (%) Results: average mixing ratio north of 38 N on OICE M ixing ratio averaged north of38n / Okhotsk Com ponent Okhotsk comp PotentialDensity Mixing ratio averaged north of38n / Kuroshio Component Kuroshio comp. W K11 W K14 TR18 W K111 Average W K11 W K14 TR18 W K111 Average Mixing ratio (%) Mixing ratio averaged north of38n / E.Kam chatska Com ponent W K11 W K14 TR18 W K111 Average E. Kamchatka comp PotentialDensity Table. Annual mean of the mixing ratios Layers Okhotsk E.. Kamch Kuroshio Average PotentialDensity Okhotsk Sea water component is seen mainly Fig. Average mixing ratios for density at σ θ, while E. Kamchatka comp. in each cruise is seen more at densities denser than 27. σ θ. Kuroshio comp. seem less density-dependent.

7 Transport(Sv) Transport components north of 38N across OICE (15dbar reference, positive northeastward).1σ θ -thickness transport north of38n / E.Kam.Com ponent W K11 W K14 Okhotsk comp. E. Kamchatka comp. 1. TR18 1. Okhotsk comp.: 1.1 Sv, E. Kamchatka comp.:.4 Sv southwestward, W K111 Average Transport(Sv).. Layers Total O khotsk E.Kam ch.kuroshio W K11 W K14 TR18 W K111 Average PotentialDensity PotentialDensity.1σ θ -thickness transport north of38n / Kuroshio Com ponent 1.5 W K W K14 TR18 1. W K111 Average Transport(Sv).1σ θ -thickness transport north of38n / O khotsk Com ponent The annual mean of the transport components in an isopycnal layer σ θ :.5 Kuroshio comp.: 1.6 Sv northeastward. Table. Annual mean of each transport component Kuroshio comp. Layers Total O khotsk E.Kam ch.kuroshio Total Total PotentialDensity

8 Annual mean transports in σ θ across OICE *15 dbar reference *1 Sv = 1 6 m 3 /s 45 Okhotsk Sea Kuril Islands 45 Hokkaido Okhotsk comp. 1.1 Sv 4 E. Kamchatka comp..4 Sv => What do these values mean?? 35 Honshu OICE Kuroshio comp. 1.6 Sv

9 Schematic circulation in intermediate layers We have to know the oceanographic location of OICE 45 Okhotsk Sea Kuril Islands 45 We want to know this transport! 4 Including: Okhotsk/E. Kamchatka components and their seasonal variation Honshu Hokkaido Oyashio OICE Mixed water region Oyashio front 4 35 Kuroshio

10 How can we calculate the transport components contributing to NPIW formation? If we integrate only in the Oyashio area; 45 we can obtain the transport components which contribute to the new NPIW formation: This transport 4 Including: Okhotsk/E. Kamchatka components and their seasonal variation 35 Honshu Hokkaido Okhotsk Sea Kuril Islands Oyashio Mixed water region 45 Oyashio front

11 Oyashio area definition: Temperature at 1m depth < 5 C (the blue area in lower panels). Jan. Apr. Aug. Nov. Figure. Temp. at 1 m depth in Jan., Apr., Aug. and Nov. 21.

12 Transport(Sv) Transport(Sv) Okhotsk and East Kamchatka transport components integrated in the Oyashio area E. Kamchatka E.Kam chatska comp. ponentwhere T 1 T1 < 5 C < 5C (positive southwestward) Okhotsk comp. where T 1 < 5 C (positive southwestward) <Okhotsk (Ok) component> Lighter layer ( ) Annual mean transport in Denser layer ( ) Total( ) Ave σ θ = 3.5 Sv. 3.5 There is a clear seasonal signal in a denser layer σ θ. 1.8 The transport in a lighter layer σ θ is almost constant. winter spring summer autumn => The Okhotsk component is larger in winter to spring. O khotsk com ponent where T1 < 5C (positive southwesteward) W K11 W K14 TR18 W K111 Lighter layer ( ) Denser layer ( ) Total( ) Ave W K11 W K14 TR18 W K111 <E. Kamchatka (EK) comp.> Annual mean transport is smaller than Okhotsk comp. (1.5 Sv, Ratio of Ok: EK=7:3) The summer EK transport in a lighter σ θ is quite small (.3 Sv). => The E. Kamchatka comp. is larger in winter to spring.

13 Results We applied a multiple tracer analysis to see the variation of NPIW source waters: <Spatial distribution of each mixing ratio> (1) The Okhotsk comp. is seen at lighter densities (especially σ θ ), while E. Kamchatka comp. increases with increasing density. (2) It seems that the Kuroshio component distribution isn t related to density so much (it seems to depend on whether a warm eddy exists or not). <Subarctic transport components in the Oyashio area> (1) The Okhotsk comp. is relatively larger than E. Kamchatka comp. in the net southwestward Oyashio transport in annual average (ratio of Ok: EK =7:3). (2) Both Okhotsk and E. Kamchatka components have a seasonal signal: Their southwestward transports have a maximum in winter and minimum in summer. => The outflow from the Okhotsk Sea has a seasonal variation!

14 Itoh et al. (23, JGR): The ratio of the Okhotsk and E. Kamchatka component in the Oyashio southward flow (opposite to this study). Japan ÃŒyŠC e ª Oyashio Sea ƒiƒz[ƒcƒnšc of Okhotsk Kuril Islands ƒ uƒbƒ \ƒ Š C Œƒ Jƒ ƒ `ƒ ƒ cƒ JŠ C East Kamchatka current Kuroshio warm water ªŒn g warm g ò eddy OICE ª ± Kuroshio Extension The subarctic main stream might turn eastward in the east of OICE?

15 The method to obtain the least-square solutions in these equations Matrices Expressing Eqs. (1)(4) as matrices: θ θ 2 θ 3 θ r S 1 S 2 S 1 3 r X 1 X 2 X 2 = S 3 X r Q 1 Q 2 Q 3 3 Q ~~~~~~~~~ ~~~ =A =x ~~~ =B Normalize W= ν(θ) i.e. Ax-B = O (7) (O: Zero matrix) Defining a diagonal matrix W with RMS ν of each tracer s observation error and variation in the used station data for the source waters in order to lessen the difference among the tracers error and variation. ν(s) O O ν(x) ν(q) Multiply eq. (7) by W in left side W(Ax-B) =O (8) Fitting Searching the best solutions x numerically by minimizing the error of (8): L= W(Ax-B) 2 on condition of (5) and (6)

16 Integrate the subarctic transport components only in the Oyashio area OICE Oyashio Oyashio Oyashio Front New NPIW In order to find the net transport components contributing to the new NPIW formation because the observation line OICE cuts through a part of new NPIW formation area. We ll examine the seasonal variation in the two subarctic components (Okhotsk and E. Kamchatka Kuroshio SP components) integrated in the Oyashio area. Fig. Schematic representation of intermediate circulation near Japan Jan. Apr. Aug. Nov. The Oyashio area is defined as the area with T < 5 C at 1m depth (the blue area in left panels). Figure. Temp. at 1 m depth in Jan., Apr., Aug. and Nov. 21.