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  • JAMSTEC

    Kuroshio/Oyashio Watch 2015/05/25

    Contents

    1. Sea surface temperature and sea surface height anomaly
    2. Kuroshio path south of Japan
    3. Oyashio
    4. M2 baroclinic tide variability modulated by the ocean circulation south of Japan
    5. Upcoming events
    6. Recent papers

    1. Sea surface temperature and Sea surface height anomaly

    Figure 1.1 shows the sea surface temperature (SST) anomalies of the JCOPE2 from April 21 to 20 (Fig. 1.1a), from May 1 to 10 (Fig. 1.1b), and May 11 to 20 (Fig. 1.1c). The negative SST anomalies (indicated by the red arrows in Fig.1.1c) in the Japan Sea and the region east of the northern Japan is fading. Instead, the SST anomaly further offshore of the northern Japan (indicated by the red arrow) became strong positive. This positive anomaly was due to the northward propagation of warm anticyclonic eddies. See Section 3 in the previous issue and this issue. See also the corresponding positive sea surface height anomaly in Fig. 1.2. As discussed in Section 3 of the 2015/02/9 issue, the positive (negative) SST anomaly south (north) of the Subarctic Front in the Pacific Ocean was found (indicated by the green arrow). The SST anomaly of the Chishima Islands remained positive (indicated by the blue arrow). The SST anomaly in the subtropics south of Japan (indicated by the black arrow) has been positive since the 2015/4/20 issue. The SST anomaly in the East China Sea (indicated by the purple arrow) was negative.

    Fig1.1
    Anomaly of SST (K) of the JCOPE2 analysis relative to the 1993-2012 climatology averaged (a) between April 21 and 30, (b) between May 1 and 10, and (c) between May 11 and 20. Data for (c) includes the predictions from May 16.

    Fig. 1.1


    Following the figure for March (Fig. 1.2 in the 2015/04/20 issue), Figure 1.2a shows the sea surface height (SSH) and its anomaly in April. The SSH anomalies averaged in the region 31-36ºN, 140-165ºE (the box in Fig. 1.2), which was defined by Qiu et al. (2014) as a good indicator of the dynamics state of the Kuroshio Extension, was near the values in March and in last April (Fig. 1.2b,c and Fig. 1.3c). Thus, the recirculation of the Kuroshio Extension remained strong. 

    The thick black curve in Fig 1.3a is the time series of the path length of the upstream Kuroshio-Extension path integrated from 141 to 153ºE, which is an index of the stability of the Kuroshio Extension. The values in March were low. The short path length means that the upstream Kuroshio Extension was stable. The stability can be also seen in the daily Kuroshio paths in Fig. 1.2a (the green curves).

    The mean latitude of the Kuroshio Extension in March (the red curve in Fig. 1.2) was near the climatological location (the black curve in Fig. 1.2). See also the thick black curve in Fig 1.3b, which is the time series of the latitude of the upstream Kuroshio-Extension path between 141 and 153ºE. The values in March were near or slightly north of the climatology (the thin black curve) while the values were high throughout 2014.

    Fig1.2
    (a) The sea-surface-height contour lines of 5 cm in the JCOPE2 are regarded as the Kuroshio axes. The thin green lines are daily Kuroshio paths from April 1 to April 30, 2015. The red line is the Kuroshio path averaged in April 2015. The black line is the climatological (1993-2012) Kuroshio path in April. Color shade shows the sea surface height anomaly in April 2015 relative to the 1993-2012 climatology (m). The dashed box is used for the calculation of Fig. 1.3c. (b) Change of the SSH in April from the previous month (m). (c) Change of the SSH in April from the previous year (m).  

    Fig. 1.2

    Fig1.3

    (a) Time series of the upstream Kuroshio-Extension path length integrated from 141 to 153ºE using JCOPE2 reanalysis.
    (b) Time series of the mean latitude of the Kuroshio-Extension path between 141 and 153ºE using JCOPE2 reanalysis.
    (c) Time series of the SSH anomalies averaged in the region 31-36ºN, 140-165ºE (the box in Fig. 1.2) using JCOPE2 reanalysis.
    Thick black line: the reanalysis in 2014 and 2015. Thin black line: the daily 1993-2012 climatology with the range of the daily standard deviation (gray shade).

    Fig. 1.3

    Other information:


    2. Kuroshio path south of Japan

    Figure 2.1 shows the analysis and predictions starting from May 16, 2015 of JCOPE2. The offshore path of the Kuroshio continues (Fig. 2.1a,b).

    Figure 2.2 shows the recent analysis and prediction of the sea level height (SSH) near Hachijo Island, which is an index of the Kuroshio nearshore/offshore path around the Izu Islands. All recent predictions in Fig 2.2 show that the offshore path (low SSH) will continue, except temporal fluctuations (See eddies in Fig. 2.3). See also Fig 2.1c,d.

    The left panels in Fig. 2.3 show the sequence of SSH in the latest prediction. In order to highlight short-term fluctuations, the right panels of the Fig 2.3 show the sequence of the SSH anomaly relative to the 41-day running mean. The predictions show that a small meander of the Kuroshio (indicated by the red arrows) will develop southeast of Kyushu and will propagate downstream from June to July. See also Fig 2.1b,c,d (pointed by the black arrows). The development of the meander seems to occur through the interaction with westward propagating eddies: an anticyclonic eddy (indicated by the purple arrows) and a cyclonic eddy (indicated by the green arrows). See also the also the same eddies in Fig 2.1a,b pointed by the red arrow. The importance of mesoscale eddies in the development of small meanders has been suggested by some studies (Miyazawa et al., 2008, Tsujino et al. 2013).  

    Please note that the Japanese version of the Kuroshio/Oyashio Watch is updated every week.

    Fig2.1
    The latest analyses and predictions of temperature and velocity at 200m depth from JCOPE2. Red star marks are the location of Hachijo Island.

    Fig. 2.1

    Fig2.2
    Sea level height (m) near Hachijo Island (33.1N, 139.7E) from the JCOPE2 analysis and prediction from May 2  (black curve), May 9 (blue curve), and May 16 (red curve).

    Fig 2.2


    Fig2.3

    (Left) Sequence of SSH (m, c.i.=0.1 m). (Right) Sequence of the SSH anomaly (m, c.i.=0.05 m) relative to the 41-day running mean (the mean of available data if all 41-day data are not available).  .

    Fig. 2.3


    Other information


    3. Oyashio

    As an index of the area influenced by the Oyashio Current, Figure 3.1 shows the time series of the Oyashio area defined as an area less than 5ºC in 141-148E, 35-43N at 100 m depth (104 km2) using JCOPE2. The Oyashio area calculated by the JCOPE2 reanalysis (the thick black curve) became far below the climatological value (the thin black curve). The value was near the prediction in the last issue (the green curve). 

    Following Fig. 3.2 in the previous issue, Figure 3.2 shows the animation of the temperature and velocity at 100 m depth from April 1 to May 31. Warm anticyclonic eddies propagated northward. These warm eddy pushed northward the boundary of the Oyashio area (5ºC). Such a northward propagating warm core ring is often found in this region (Ito and Yasuda, 2014).

    Affected by this warn-core eddy, SST off the northern Japan (indicated by the orange arrow in Fig. 1.1) is extremely warm. The SST averaged from May 1 to 15 over the region 40-42ºN, 145-148ºE was the warmest since 1993 (Fig. 3.3). 

    The latest prediction (the red curve) in Fig. 3.1 shows that the values of the Oyashio area will remain below the climatology. See more in the JCOPE web page for the analysis and the prediction of the temperature distribution at 100 m depth.


    Fig3.1

    Time series of the Oyashio area defined as an area less than 5deg.C in 141-148E, 35-43N at 100 m depth (104km2) using JCOPE2. Thick black line: the reanalysis in 2014-2015. Thin black line: the 1993-2012 climatology with the range of the daily standard deviation (gray shade). Green, blue, and red thick lines: the predictions by the JCOPE from May 2, May 9, and May 16, respectively.

    Fig. 3.1


    Animation of temperature (ºC, shade) and velocity (vector) at 100 m depth in the JCOPE2 reanalysis from April 1 to May 31. Data after May 16 are predictions. The thick black contour indicates the 5ºC line used in the definition of the Oyashio area.

    Fig 3.2

    Fig3.3
    Time series of the SST of each year averaged from May 1 to 15 over the region 40-42ºN, 145-148ºE using JCOPE2 reanalysis.

    Fig. 3.3



    Other information:


    4. M2 baroclinic tide variability modulated by the ocean circulation south of Japan

    by Sergey Varlamov (APL JAMSTEC)

    In paper of Varlamov et al. (2015) authors analyze results of concurrent annual simulation of ocean circulation and tidal currents using a data assimilative ocean general circulation model covering the Western North Pacific with horizontal resolution of 1/36 degree to investigate possible interactions between them.  To confirm the reproducibility of tidal processes by given model, model data were compared with the tidal gauge data and, for validation of the surface manifestation of the baroclinic tide, with the space high-pass filtered satellite SSHA data. The intense variability of the M2 baroclinic tide harmonics around Tokara Strait, Izu Ridge, Luzon Strait, and Ogasawara Ridge (the hot spots, Fig. 4.1) are represented by the model nearly as observed.

    Special experiment with M2 only tidal forcing was used for analyzing variability of M2 internal tides with the primary attention to the barotropic to baroclinic energy conversion rate (BEC).  Special forcing allows the calculation of M2 harmonics for a short-term (3-day) period. Energy cycle analysis of the simulated M2 baroclinic tide indicates two types of the hot spots: dissipation (Tokara Strait and Izu Ridge) and radiation (Luzon Strait and Ogasawara Ridge) dominant sites (Fig. 4.2). BEC mainly occurs over the bottom slope at the relatively shallow depth at Tokara Strait and Izu Ridge, and thus most of the baroclinic tidal energy is locally dissipated and the stratification in the relatively shallow layer is crucial for the modulation of the baroclinic tide generation. At the other two spots, in contrast, the radiation of the baroclinic tidal energy is more active than at the former spots because of the relatively deep bottom slope where the active energy conversion occurs, and the stratification in the relatively deep layer is influential in the modulation of the baroclinic tide generation.

    Energy conversion from barotropic to baroclinic M2 tides at the hot spots is modulated considerably by the lower-frequency changes in the density field. The EOF analysis of the buoyancy frequency averaged at the spots indicates that the temporal variations of some EOF modes correlate with the variation of BEC averaged there. Characteristics of the most influential mode at each spot imply the lower-frequency oceanic phenomena that are mainly responsible for modulating the baroclinic tide variability there through changes in intensity of the local stratification. At Tokara Strait (Fig. 4.3) and Izu Ridge, the enhanced stratification in the upper thermocline layer caused by the development of the Kuroshio small meander together with the seasonal thermocline variation considerably affects the temporal variation of BEC . At Luzon Strait and Ogasawara Ridge, the energy conversion is modulated by the variations in the stratification around the bottom of the main thermocline governed by the Kuroshio intrusion into the South China Sea (Luzon Strait) and the mesoscale eddy activity (Ogasawara Ridge). The remote effect due to the temporal variation of the depth-integrated tidal energy supply, which is caused by BEC occurring over the all baroclinic tide generation sites, is not negligible especially at Luzon Strait, as suggested also by the previous studies [Kerry et al., 2013; 2014].

    The multiple regression using the stratification intensity and depth-integrated tidal energy supply does not completely explain the variance of the found energy conversion; in particular, the explained ratio is relatively low at the radiation dominant hot spots: Luzon Strait and Ogasawara Ridge. Detailed BEC processes could be affected by the additional contribution from the local dissipation and advection [Jan et al., 2012]. Future studies await further exploration in these directions.


     Reference:
    Varlamov, S.M., X. Guo, T. Miyama, K. Ichikawa, T. Waseda and Y. Miyazawa, “M2 baroclinic tide variability modulated by the ocean circulation south of Japan”, Journal of Geophysical Research: Oceans (2015), doi: 10.1002/2015JC010739



    Fig4.2
    Depth-integrated M2 baroclinic energy radiation flux (in KWm-1) evaluated from the ‘M2-Tide’ simulation. Boxes indicate the regions of the active generation sites of the baroclinic tides (hot spots): Tokara Strait, Izu Ridge, Luzon Strait, and Ogasawara Ridge. Vectors indicate the direction and magnitude.

    Fig. 4.1


    Fig4.2
    M2 baroclinic tidal energy budgets calculated at the hot spots (See Figure 4.2 for their locations and areas, respectively) for the sub-periods with 3-day duration. Red closed circles: BEC, blue open circles: the divergence of the energy radiation fluxes, black open squares: the dissipation and advection.

    Fig. 4.2


    fig4.3
    Time mean buoyancy frequency (shade; in s-1) at 200m depth for the sub-periods showing the typical locally weakened (a) and enhanced (b) BEC states among the all sub-periods. Contours denote the time mean sea surface height with interval of 0.05m for the sub-periods. A box indicates the region of the Tokara Strait spot. Dates denote the beginning date of the sub-periods. (c) and (d): Zoom up view of the Tokara Strait spot showing time mean buoyancy frequency (shade; in s-1) at 200m depth for the sub-periods showing the typical locally weakened (c) and enhanced (d) BEC states among the all sub-periods. Contours denote the root mean square variability (0.02 ms-1 to 0.10 ms-1) of the band-pass filtered current with the target period from 11 to 14 hours. Contour interval is 0.005 ms-1 and thick contours denote the values upper 0.05 ms-1. (e) As in (c) except for showing a vertical section along 29.5ºN. (f) As in (d) except for showing a vertical section along 29.5ºN.

    Fig. 4.3



    5. Upcoming events

    Events


    6. Recent papers


    Papers by APL authors/coauthors

    Papers by
    JAMSTEC authors/coauthors
    Others

    Contact: jcope at_mark jamstec.go.jp