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Oceanic fronts and jets around Japan: a review

  • Special Section: Review
  • “Hot Spots” in the Climate System: New Developments in the Extratropical Ocean-Atmosphere Interaction Research
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Abstract

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

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Notes

  1. This name is also used by Sugimoto and Hanawa (2011), but many other names have been used to indicate this region, for example, the Perturbed Area (e.g., Kawai 1972), the Mixed Water Region (e.g., Talley et al. 1995), the Kuroshio-Oyashio interfrontal zone (e.g., Yasuda et al. 1996), and the Kuroshio-Oyashio Extension region (e.g., Schneider et al. 2002), etc.

  2. The SAFZ almost coincides with the region conventionally termed the Transition Domain in the oceanographic community (Favorite et al. 1976), which is bounded by the SAC and the subarctic boundary, which are described later in this section (see also Fig. 6).

  3. The northern branch of the KE (Mizuno and White 1983) is also called the Kuroshio Bifurcation front (Yasuda 2003) or the Kuroshio northern branch (Hurlburt and Metzger 1998).

  4. Roden et al. (1982) and Belkin et al. (2002) termed this front “the Polar Front.”

  5. Roden et al. (1982) termed this boundary “the Subarctic Front.”

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Acknowledgments

We thank the editor, Dr. Shoshiro Minobe, and three anonymous reviewers for their constructive comments on the manuscript. The altimeter products are produced by Ssalto/Duacs and distributed by AVISO with support from CNES (http://www.aviso.oceanobs.com/duacs/). APDRC at the University of Hawaii at Manoa (http://apdrc.soest.hawaii.edu/) was also used to obtain some of the data sets. This work is supported by the Japan Society for Promotion of Science through a Grant-in-Aid for Scientific Research on Innovative Areas 2205.

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Correspondence to Shinichiro Kida.

Appendix

Appendix

The methods and data sets used for estimating the metrics shown in Tables 1, 2, 3, 4, and 5 are described here. Common data sets used for the SST and its gradient are monthly means of AMSR-E SST (Gentemann et al. 2010), MGDSST (Kurihara et al. 2000), and OISST (Reynolds et al. 2007) from 2003 to 2008. Monthly climatology of WOA05 (Antonov et al. 2006), WOA09 (Antonov et al. 2010), and WOA13 (Zweng et al. 2013) is used for the SSS and its gradient. Monthly means of AVISO (http://www.aviso.oceanobs.com/duacs/) from 1993 to 2007 are used for the SSH gradient and EKE. The J-OFURO2 (Tomita et al. 2010) from 1993 to 2007 is used for the surface heat fluxes. Specific data sets used for each front are shown in Table 6, and detailed differences are described below. Other metrics are primarily based on past observational studies.

Table 6 Data sets and references used for estimating the metrics associated with the SST, SSS, and their gradients, SSH gradient, and heat fluxes in Tables 1, 2, 3, 4, and 5

1.1 Kuroshio Extension

The location of the KE is based on the regions defined in Qiu and Chen (2005). Since the behavior of the frontal axis is significantly different between the upstream and downstream, two metrics are provided. One is the average over the whole frontal axis and the other is that limited to the upstream. The frontal axis is determined from the maximum flow speed based on the methods described in Ambe et al. (2004). FRA-JCOPE2 utilizes the flow speed at 10 m depth. SST and its gradient are estimated from MGDSST (1993–2007). SST is estimated by averaging the SST 50 km north and south of the frontal axis, and the SST gradient is estimated by taking the difference between the two. However, we exclude the months when the KE axis is obviously obscured by the presence of a meso-scale eddy or the method detects the KENB instead. SSS and its gradient are estimated from WOA13 using similar methods as the SST and its gradient, respectively. However, only the upstream values are presented since WOA13 is a climatology and thus unable to capture the transient variability of the frontal axis downstream. Only the upstream values, where variability is less, are presented. The SSH gradient is estimated from the monthly means of AVISO, but the annual mean is presented since the magnitude of the SSH gradient does not change significantly with seasons. Other dynamical properties are based on observational studies. Transport and flow speed are both based on Howe et al. (2009), which are observations from 146°E. Note that the transport decreases significantly to about 75 Sv at 148.5°E. Depth is determined from WOCE hydrography (http://www-pord.ucsd.edu/whp_atlas/pacific_index.html). The EKE is based on the downstream and upstream values estimated from satellite altimetry in Qiu and Chen (2005). Heat fluxes are estimated from the J-OFURO2 by taking a spatial average between 141°–163°E and 34°–38°N to represent the whole KE region and 141°–153°E and 34°–38°N to represent the upstream KE region.

1.2 Kuroshio south of Japan

The location of the Kuroshio axis is estimated from the monthly mean SSH derived from AVISO. Using the SSH gradient, the latitudinal position of the Kuroshio axis is determined at each longitude with an interval of 0.25° between 131° and 140°E. The SSH gradient is then calculated by taking a zonal average of the gradient at this Kuroshio axis. SST and its gradient are estimated from monthly means of MGDSST using a similar method. Since the SST front associated with the Kuroshio is not closely matched with the current axis as mentioned in Sect. 3.2, the position of the SST front is determined from the maximum SST gradient within the region between 1° south and north of the current axis defined from the SSH. The SSS and its gradient are estimated from monthly climatology of WOA09 and are determined as the maxima around the mean Kuroshio axis. The length of the K-SOJ is estimated between 131° and 140°E using the Kuroshio axis data set of Ambe et al. (2004) from 1993 to 2007, which includes periods of both straight and large meander paths. Flow speed and transport are from observations by Ambe et al. (2004) and Imawaki et al. (2001), respectively. While the estimates of the Kuroshio transport tend to be contaminated by local phenomena such as the Kuroshio path variations and mesoscale eddies, the transport value presented in Imawaki et al. (2001) was observed off-shore of Shikoku where the Kuroshio is relatively stable. The depth of the Kuroshio is based on observations by Book et al. (2002). The EKE is estimated from the monthly mean geostrophic velocity anomaly of AVISO. Heat fluxes are estimated from the J-OFURO2 by taking a spatial average within 131°–140°E and 30°–35°N. The dynamical properties, such as the SSH gradient, flow speed, transport, and EKE, do not change significantly with seasons and thus are presented by their annual means.

1.3 Kuroshio along the shelf break of the East China Sea

The location of the K-ECS follows the current field given by Qiu et al. (1990) with the exclusion of the Kuroshio northeast of Taiwan because there is no apparent thermal front there (HicKox et al. 2000). SST, SSS, and their gradients are based on quarterly hydrographic data along the PN line in January, April, July, and October from the period of 1980–2012, provided by the Japan Meteorological Agency (http://www.data.jma.go.jp/kaiyou/db/vessel_obs/data-report/html/ship/ship.php). The SSH gradient is estimated from the monthly means of AVISO with the Kuroshio axis defined by the location of the maximum SSH gradient between 124° and 127°E. Heat fluxes are also estimated along this Kuroshio axis from the monthly means of the J-OFURO2. Flow speed, transport, and depth are based on observations by Oka and Kawabe (1998). The EKE is based on numerical simulations by Miyazawa et al. (2004).

1.4 Gulf Stream

All values are based on the GS after the separation at Cape Hatteras, where the location of the GS frontal axis is based on Sasaki and Schneider (2011): The jet latitude is defined as a contour of −10 cm from absolute dynamic topography data by Niiler et al. (2003b). SST and its gradient are estimated from the monthly means of AMSR-E SST, where the gradient is based on values 2° north of the frontal axis averaged from 60°W to 70°W, and only the maximum month is presented. SSS and its gradient are estimated from the annual mean WOA09 averaged along the frontal axis and along 2° north of the frontal axis, respectively, from 60°W to 70°W. The SSH gradient and EKE are estimated from AVISO and are also annual means. The SSH gradient is at 70°W, and the EKE is estimated averaged along the frontal axis from 60°W to 70°W. The transport is based on the maximum value from in situ observations near 55°W by Hogg (1992). The depth is based on observations by Johns et al. (1995). Heat fluxes are estimated from the J-OFURO2 averaged along the frontal axis from 60°W to 70°W.

1.5 Agulhas Current and Agulhas Return Current

The location of the AC and ARC is based on Gordon (1985) and Lutjeharms and Ansorge (2001), respectively. Metrics related to SST, SSS, and SSH are estimated from AMSR-E SST, WOA05, and AVISO, respectively. SST and SSS are those found along the frontal axis and are estimated from the monthly means for SST, but the annual mean for SSS. The SST, SSS, and SSH gradients are maxima found along the frontal axis in the annual means. Heat fluxes are estimated from the J-OFURO2 along the frontal axis for the AC but maximum values within 15°E–60°E and 45°S–30°S for the ARC. Flow speed, transport, and depth are all based on observational studies by Bryden et al. (2005) for the AC and Lutjeharms and Ansorge (2001) for the ARC. The EKE is estimated from the monthly mean geostrophic velocity anomaly of AVISO.

1.6 Antarctic Circumpolar Current-Subantarctic front (ACC-SAF)

The location of the front is based on Orsi et al. (1995). SST and its gradients are estimated from the monthly mean OISST data from 1982 to 2011, with a spatial resolution of 1° by 1° (Reynolds et al. 2002). SSS and its gradient are estimated from WOA09 and are annual means. These estimates are the circumpolar averages along the fixed frontal location of Orsi et al. (1995). The dynamical properties are based on observational studies; the SSH gradient is from Sallee et al. (2008), flow speed is from Hofmann (1985), transport is from Rintoul and Sokolov (2001) and Cunningham et al. (2003), depth is from Tomczak and Godfrey (2003), and EKE is from Patterson (1985). Heat fluxes are estimated from the J-OFURO2 and are the circumpolar averages along the frontal axis.

1.7 Kuroshio-Oyashio confluence region

Metrics for KENB, SAC, J1, and J2 are estimated by averaging the respective parameters along the frontal axes associated with the location metrics in Table 3. The KENB axis is indexed by a straight line that connects its location metrics, which lies between 6 and 8 °C at a depth of 300 m, so as to be consistent with Mizuno and White (1983). The metrics are estimated within 153°–170°E, where the axis is distinctly separated from KE (Fig. 4a). The SAC axis is defined by 4 °C isotherms at 100 m depth (Favorite et al. 1976; Belkin et al. 2002). The metrics of SAC are estimated within 150°–155°E along the 4 °C isotherm as a typical value where the SST gradient is maximum [typically >3 °C (100 km)−1 in winter], although the front extends farther eastward. J1 and J2 are indexed by straight lines that connect their location metrics; these lines are located immediately southward of the maximum SST gradients [typically >3 °C (100 km)−1 in winter] in the SAFZ north of 40°N (Fig. 4c). The SST and its gradient for KENB, SAC, J1, and J2 are estimated from the monthly means of AMSR-E SST (Figs. 4c, 3d). SSS and its gradient for KENB, SAC, J1, and J2 are estimated from the monthly climatology of WOA13. The SSH gradients and surface currents are estimated from the absolute SSH of AVISO (e.g., Rio et al. 2011). The EKE is based on the monthly mean surface geostrophic anomaly. Depths for KENB, J1, and J2 are based on Isoguchi et al. (2006). Transport for SAC is based on Ohtani (1970). Estimates of transport for other fronts are estimated from WOA13 based on geostrophy referenced to 1,500 m depth. Eastward flows that are parallel to the frontal axis are used. We are currently not aware of past studies to compare these values with, but the geostrophic velocity estimates near the surface match reasonably well with those of Isoguchi et al. (2006), as well as with those calculated from the AVISO SSH, and thus consider the transports reasonable to the first order. The difference between SAC and J1 is unclear in observational data sets where spatial smoothing is applied. Heat fluxes are estimated from the J-OFURO2.

For the metrics of SAFZ, SST is estimated along the line that connects (145°E, 40°N) and (180°, 45°N). SSS and its gradient are based on WOA13, and the heat fluxes are based on the J-OFURO2 along the same line. The SST gradient within the SAFZ is characterized by those of J1 and J2 and thus is not presented here.

1.8 Northern, Southern, and Eastern Subtropical fronts (NSTF, SSTF, and ESTF)

The locations of the STFs are determined from the AVISO reference series of the delayed-time Maps of Absolute Dynamic Topography (MADT) product from 1993 to 2007. STFs are defined as a continuous band of eastward surface geostrophic velocity in the monthly climatology. The climatology is still a bit patchy, probably because of high eddy activity, but the positions of the STFs show no clear seasonal variations. The eastern end of the SSTF is determined based on the climatology map of subsurface temperature fronts (Kobashi et al. 2006) because of the presence of the HLCC there. SST is estimated from the monthly means from an optimally interpolated SST data set produced from the blend of infrared and microwave satellite observations and in situ measurements from 2003 to 2008 (Reynolds et al. 2007). SSS is estimated from the monthly climatology of WOA09. SST, SSS, and their gradient are estimated at the current axis of the STCCs after calculating the zonal averages.

The flow speed is estimated from the surface eastward geostrophic velocity since the meridional velocity blurs the characteristic of the STFs. The meridional velocity is generally related to the basin-scale subtropical gyre circulation and has the same order as the zonal one at STFs. To estimate the metrics, zonal averages of zonal velocity are first calculated from the monthly mean each year between 145°E and 165°E for the N- and S-STFs and 175°W–155°W for the ESTF, and then the monthly climatology is estimated. We extracted a meridional peak of the velocity and its associated SSH gradient for each month. Since the ESTF shows no significant seasonal cycle, its metrics are estimated from the annual mean. The transport and depth are left blank because these metrics need information about the vertical extent of the fronts, which may not necessarily be easily accessible from observations. Synoptic ship observations (e.g., Aoki et al. 2002) appear to be inconsistent with altimeter-derived climatology when compared at the surface (see a review paper by Kobashi and Kubokawa 2012), probably because of observation sparsity and eddy contamination. The EKE is calculated from the monthly means of the geostrophic velocity anomaly. We first computed the monthly spatial averages of the EKE each year within 23°–25°N and 145°–165°E for the NSTF, 18°–20°N and 145°–165°E for the SSTF, and 25°–27°N and 175°–155°W for the ESTF, and then estimated the monthly climatology. Heat fluxes are assessed in the same way as the EKE using the monthly means of the J-OFURO2.

1.9 Hawaiian Lee Countercurrent

The location of the HLCC front is determined by examining the surface zonal geostrophic velocity relative to 400 db in climatological hydrographic observations (Kobashi et al. 2006). The western end changes interannually from the east of the international dateline to about 165°E from altimeter observations (Sasaki et al. 2010; Abe et al. 2013). When the HLCC and SSTF influence each other, the western extent of the HLCC cannot be determined clearly. SST is estimated from the monthly means of the OISST data set from 2003 to 2008 (Reynolds et al. 2007). The SST gradient is not provided since the SST shows a maximum along the HLCC. The SSS and its gradient are based on the monthly climatology of WOA05. The SSH gradient and surface flow speed are estimated from AVISO reference series of delayed-time MADT from 1993 to 2007 assuming geostrophy between 180 and 160°W at 19.5°N. The depth and transport are determined between 180° and 160°W with the transport estimated from the numerical simulations of the OFES Quikscat run (2001–2008) (Masumoto et al. 2004; Sasaki et al. 2008). The EKE is determined from Yoshida et al. (2011) between 170.0°E–160.0°W and 17.0°–21.7°N, which is based on AVISO SSH. Annual mean is presented with its variability estimated based on 1 year running weakly means. The seasonal EKE level peaks in June (0.025 m2 s−2) and drops to a minimum in January (0.015 m2 s−2). Heat fluxes are estimated from the monthly J-OFURO2 from 2002 to 2007.

1.10 Seto Inland Sea tidal front

The location, SST, SSS, and their gradients are based on observations during April by Yanagi and Koike (1987). The SSH gradient, flow speed, transport, depth, and EKE are based on the general knowledge of tidal fronts found in the Seto Inland Sea from observations and numerical models (e.g., Takeoka 2002; Chang et al. 2009). Heat fluxes are based on numerical simulations (Chang et al. 2009; Shi et al. 2011) for a limited period; thus, their interannual variability is not provided.

1.11 Japan Sea subpolar front

The location of the front is based on Park et al. (2004) where the JS-SPF does not include major bifurcations. The frontal axis is determined from the maximum SST gradient near 40°N from the monthly means of AMSR-E SST. SST and its gradient are the averages along this frontal axis. SSS and its gradient are estimated from the monthly climatology of WOA13 based on the monthly climatological position of the frontal axis. The SSS gradient is presented with a sign since the sign changes during the year. Positive values represent higher salinity toward the north. The SSH gradient is also estimated along the frontal axis using the monthly means of AVISO (2003–2008). Flow speed is based on direct observations by Isobe and Isoda (1997), and transport is based on inverse calculation using the observational data set by Chu et al. (2001). Depth is based on the observational analysis of Isoda (1994) and Minobe et al. (2004). The EKE does not show a clear maximum along the front (Jacobs et al. 1999) and thus is not presented. Larger values are found towards the south of the front, where the Tsushima Current exists, and the subpolar front appears to be much of a transition area from high (south) to low (north) EKE areas. Heat fluxes are estimated along the frontal axis using the monthly means of the J-OFURO2 (2003–2008).

1.12 FRA-JCOPE2

The Japanese Fishery Research Agency (FRA)-Japan Coastal Ocean Prediction Experiment (JCOPE) (FRA-JCOPE2, Miyazawa et al. 2009) uses the JCOPE2 ocean model, which is based on the Princeton Ocean Model with a generalized coordinate of sigma (Mellor et al. 2002) and provides daily mean ocean data covering the western North Pacific (10.5–62°N, 108–180°E) with a horizontal resolution of 1°/12°. The model assimilates the remote-sensing data of altimetry and surface temperature and in situ data of temperature and salinity profiles including the data from the FRA using the 3D-VAR method. Surface momentum and heat fluxes are calculated using the bulk formulae (Kagimoto et al. 2008) with atmospheric variables obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis (Kalnay et al. 1996). SSS is relaxed to monthly climatological data (Conkright et al. 2002). The FRA-JCOPE2 data set is provided by the Japan Agency for Marine-Earth Science and Technology (http://www.jamstec.go.jp/jcope/).

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Kida, S., Mitsudera, H., Aoki, S. et al. Oceanic fronts and jets around Japan: a review. J Oceanogr 71, 469–497 (2015). https://doi.org/10.1007/s10872-015-0283-7

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  • DOI: https://doi.org/10.1007/s10872-015-0283-7

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