Kuroshio Recirculation Research Articles

黑潮延伸体区域海洋涡旋研究进展

黑潮延伸体区域是中纬度海洋–大气相互作用的关键区域，也是海洋涡旋最活跃的区域之一，该区域海洋涡旋不仅对黑潮延伸体区域海洋急流及锋面变化起着重要影响，还是北太平洋副热带模态水生成的重要因素，并在中纬度大气-海洋相互作用过程中起着关键性的作用。本文对近年来有关黑潮延伸体区域海洋涡旋的研究进行了综述，探讨了海洋涡旋的生成及分布特征、移动及时间变化特征及其物理机制等方面的研究进展，并讨论了海洋涡旋在年代际和季节时间尺度上变化机制的异同，以及局地海面风应力对海洋涡旋的影响，为今后黑潮延伸体区域海洋涡旋方面的研究提供背景资料。 The Kuroshio Extension (KE) region is the key region in mid-latitude ocean-atmosphere interaction where mesoscale eddies achieve their largest magnitude. Mesoscale eddies’ interaction with the mean flow plays a critical role in Kuroshio meander dynamics. Anticyclonic eddies prevailing in Kuroshio recirculation region are crucial to North Pacific subtropical mode water formation. This paper summarizes works on spatial-temporal characteristics of mesoscale eddies and their interaction with the local wind in the KE region. The characteristics of these eddies, such as spatial distribution, migration speed, water mass structure were intensively studied. The mechanism of the eddy formation was known as the baroclinic instability. The seasonal cycle of the eddy kinetic energy, which has a maximum in summer, is proved. The three dimensional structure, the nonlinearity, the temporal variation, the available gravitational potential energy of mesoscale eddies in the KE region will be the focus of near future study. The interaction between nonlinear eddies and local wind will be investigated to advance the understanding of the role of eddies in the mid-latitude ocean-atmosphere interaction.

Surface Heat Flux Variations across the Kuroshio Extension as Observed by Surface Flux Buoys

Abstract Wintertime sea surface heat flux variability across the Kuroshio Extension (KE) front is analyzed using data from the Kuroshio Extension Observatory (KEO) buoy in the Kuroshio recirculation gyre south of the KE front and from the Japan Agency for Marine–Earth Science and Technology KEO (JKEO) buoy in the north of the front. The coincident data used are from periods during two winters (2007 and 2008), when both buoys had a complete suite of meteorological data. In these two winter periods, the focus of this research is on three types of typical weather patterns referred to here as the northerly wind condition, the monsoon wind condition, and the normal condition. During the northerly wind condition, latent and sensible heat fluxes were large and often varied simultaneously at both sites, whereas during the monsoon wind condition the latent heat flux at the KEO site was significantly larger than that at the JKEO site. The difference between these heat flux patterns is attributed to the different airmass transformations that occur when prevailing winds blow across the KE front versus along the front. Reanalysis products appear to reproduce these heat flux spatial patterns at synoptic scales. It is suggested that the relative frequencies of these different types of weather conditions result in anomalous spatial patterns in the heat fluxes on monthly time scales.

Impact of Aleutian Low activity on the STMW formation in the Kuroshio recirculation gyre region

To understand the formation of North Pacific subtropical mode water (STMW) in the Kuroshio recirculation gyre region, the cause of STMW thickness variation is investigated using temperature profiles in a historically archived data set. The thickness variation is predominantly controlled by the main thermocline depth (MTD). When the main thermocline deepens (shoals), the wintertime mixed layer depth can develop (not develop), and consequently, thicker (thinner) STMW is observed in summer. The large‐scale atmospheric forcing controlling the MTD is explored using a wind‐driven hindcast ocean model. The MTD variation stems primarily from a baroclinic response in the ocean to the Aleutian Low (AL) activity; especially, the meridional movement of the AL exerts a remarkable influence.

Southward recirculation of the East China Sea Kuroshio west of Okinawa Island

The surface current of the Kuroshio recirculation region in the East China Sea is investigated. The surface currents in the west of Okinawa Island observed by HF radars and the assimilation results of the Japan Coastal Ocean Predictability Experiment (JCOPE) are analyzed. It appeared that currents in the easternmost edge of the Kuroshio bifurcate southward and flow into the west of Okinawa Island from JCOPE data. The currents are called Kuroshio southward recirculation currents. The currents are controlled by the bottom topography. The wind stress curl in the area where the bifurcated current can be seen in JCOPE data is negative, while the curl is positive in the eastern portion of the Kuroshio. The Kuroshio southward recirculation currents are sometimes enhanced, and their enhancement coincides with that of the positive and negative wind stress curls.

Decadal variability of the Subtropical Front of the western North Pacific in an eddy‐resolving ocean general circulation model

We examined decadal variability of the Subtropical Front (STF) of the western North Pacific by using a North Pacific ocean general circulation model (OGCM), comparing the results of three simulations with different horizontal resolutions. In the long‐term mean fields, the eddy‐resolving model (10 km) was able to simulate the distributions of the STF and the associated Subtropical Countercurrent (STCC) between 20°N and 30°N better than either the non‐eddy‐resolving model (100 km) or the eddy‐permitting model (20 km) because it simulated the Kuroshio recirculation gyre more realistically. The simulated STF intensity exhibited significant decadal‐scale variations: it was stronger in the late 1970s and weaker in the early 1990s. During these two periods, the simulated mode waters showed corresponding differences in their potential vorticity minima density and paths. We also investigated the relationship between the decadal‐scale STF variability and atmospheric forcing. The results suggest that the decadal‐scale STF variability can be largely explained by changes in the mode waters formed in the western North Pacific and advected to north of the STF by the subtropical gyre in response to a change in surface westerly winds that occurred in the mid‐1970s.

Regional Weather Patterns during Anomalous Air–Sea Fluxes at the Kuroshio Extension Observatory (KEO)*

Abstract The weather patterns during periods of anomalous surface fluxes in the Kuroshio recirculation gyre of the western North Pacific are documented. Separate analyses are carried out for the cold season (October– March) when the net surface heat flux is controlled by the combination of the turbulent sensible and latent heat fluxes (Qturb), and for the warm season (May–August) when the net heating is dominated by the net radiative fluxes (Qrad). For analysis of high-frequency (daily to weekly) variations in the fluxes, direct measurements from the Kuroshio Extension Observatory (KEO) for the period June 2004–November 2005 are used to specify flux events. For analysis of interannual variations, these events are selected using NCEP–NCAR reanalysis estimates for Qturb in the cold season, and International Comprehensive Ocean–Atmosphere Data Set (ICOADS) data for cloud fraction, as a proxy for Qrad, in the warm season. During the cold season, episodic high-frequency flux events are associated with significant anomalies in the east–west sea level pressure gradients, and hence meridional winds and lower-tropospheric air temperature, reflecting the dominance of the atmospheric forcing of the flux variability. On the other hand, interannual variations in Qturb are associated with relatively weak atmospheric circulation anomalies, implying a relatively important role for the ocean. During the warm season, high-frequency fluctuations in the net surface fluxes occur due to a mix of anomalies in Qturb and Qrad. Enhanced cloudiness in the vicinity of KEO, and hence reduced Qrad, tends to occur in association with weak cyclonic disturbances of extratropical origin. A regional atmospheric circulation favoring these types of events also was found for warm seasons that were cloudier on the whole. Results suggest that the ocean’s influence on air–sea fluxes at KEO is manifested mostly on interannual time scales during the cold season.

Generation of a trigger meander for the 2004 Kuroshio large meander

A generation process of a small meander southeast of Kyushu, which triggered the subsequent Kuroshio large meander in 2004, has been investigated using an ocean data assimilation and prediction system. The assimilation result reveals that the small meander is the most developed one since 1993. A cyclonic eddy propagating westward from the Kuroshio recirculation (KR) and a disturbance from the East China Sea (ESC) that accompanies a high potential vorticity (high‐PV) anomaly play important roles for the generation of the small meander. The cyclonic eddy triggers an initial cyclonic anomaly southeast of Kyushu. When the high‐PV anomaly passes through the Tokara Strait, a large positive vorticity is generated because the stretching of the water column is enhanced. The positive vorticity is brought in the cyclonic anomaly and accumulates there. As a result, the small meander is generated. From two sensitivity experiments an occurrence condition is determined by the cyclonic eddy from the KR by triggering the initial small‐scale meander, and the disturbance from the ECS contributes to the horizontal scale of the small meander. Tracing back the disturbance further to the upstream region of the Kuroshio, it is revealed that the disturbance is a frontal wave with a large horizontal scale generated by collision between a strong anticyclonic eddy and the Kuroshio east of Taiwan. The frontal wave propagates along the Kuroshio accompanying high‐PV generated around the continental shelf edge by viscosity. Thus the large frontal wave could contribute to the generation of the developed trigger meander by transporting a larger amount of high‐PV anomaly to the Tokara Strait.

Three subtropical fronts in the North Pacific: Observational evidence for mode water‐induced subsurface frontogenesis

Subsurface fronts in the subtropical North Pacific were investigated by constructing a high‐resolution temperature climatology. Three distinct subtropical fronts (STFs) are identified, which are the southern, northern and eastern STFs according to their relative geographical locations. The southern STF extends along 19°–21°N west of the dateline, while the northern and eastern STFs appear along 21°–24°N and 26°N in the western and central subtropical gyre, respectively. Our analysis showed that each of these STFs is associated with large negative meridional potential vorticity (PV) gradient in the thermocline below the front. The northern STF is located on the southern edge of the Kuroshio recirculation, where the negative PV gradient occurs within a narrow density range and is maintained by the low PV core of the subtropical mode water (STMW). On the other hand, the negative PV gradient at the eastern and southern STFs spans over a wide density range. The eastern STF forms near the center axis of the subtropical gyre, north of which the upper and lower portions of the central mode water (CMW) converge, forming a thick low PV pool that maintains the negative PV gradient on the front. Similarly, at the southern STF, the negative PV gradient is due to a low PV pool to the north, with the southernmost portion of the STMW stacked above the upper CMW advected from the central subtropical gyre. It is concluded that the mode waters are essential for the North Pacific STFs.

Velocity structures and transports of the Kuroshio and the Ryukyu current during fall of 2000 estimated by an inverse technique

An inverse calculation using hydrographic section data collected from October to December 2000 yields velocity structure and transports of the Kuroshio in the Okinawa Trough region of the East China Sea (ECS) and south of central Japan, and of the Ryukyu Current (RC) southeast of the Ryukyu Islands. The results show the Kuroshio flowing from the ECS, through the Tokara Strait (TK), with a subsurface maximum velocity of 89 cm s−1 at 460 dbar. In a section (TI) southeast of Kyushu, a subsurface maximum velocity of 92 cm s−1 at 250 dbar is found. The results also show the RC flowing over the continental slope from the region southeast of Okinawa (OS) to the region east of Amami-Ohshima (AE) with a subsurface maximum velocity of 67 cm s−1 at 400 dbar, before joining the Kuroshio southeast of Kyushu (TI). The volume transport around the subsurface velocity maximum southeast of Kyushu (TI) balances well with the sum of those in TK and AE. The temperature-salinity relationships found around these velocity cores are very similar, indicating that the same water mass is involved. These results help demonstrate the joining of the RC with the Kuroshio southeast of Kyushu. The net volume transport of the Kuroshio south of central Japan is estimated to be 64∼79 Sv (1 Sv ≡ 106 m3s−1), of which 27 Sv are supplied by the Kuroshio from the ECS and 13 Sv are supplied by the RC from OS. The balance (about 24∼39 Sv) is presumably supplied by the Kuroshio recirculation south of Shikoku, Japan.

Responses of the Kuroshio and the Kuroshio Extension to global warming in a high‐resolution climate model

Using a high‐resolution atmosphere–ocean coupled climate model, responses of the Kuroshio and the Kuroshio Extension (KE) to global warming are investigated. In a climate change experiment with atmospheric CO2 concentration ideally increased by 1% year−1, the current velocity of the Kuroshio and KE increases, while the latitude of the Kuroshio separation to the east of Japan does not change significantly. The increase of the current velocity is up to 0.3 m s−1 at 150°E. This acceleration of the Kuroshio and KE is due to changes in wind stress over the North Pacific and consequent spin‐up of the Kuroshio recirculation gyre. The acceleration of the currents may affect sea level along the southern coast of Japan and northward heat transport under global warming.

Mesoscale eddy effects on the wintertime vertical mixing in the formation region of the North Pacific Subtropical Mode Water

Mesoscale eddy effects on the wintertime vertical mixing in the formation region of the North Pacific Subtropical Mode Water (NPSTMW) are studied using hydrographic data from Argo profiling floats deployed in the Kuroshio recirculation region in February and March of 2001. Anticyclonic (warm) eddy enhances the wintertime vertical mixing and results in the deep mixed layer and the deep thermocline. Consequently, a large volumetric water mass with low potential vorticity corresponding to the prototype of NPSTMW tends to be formed. By contrast, cyclonic (cold) eddy is unfavorable for the vertical mixing process and halts the deepening of the mixed layer and thus the formation of mode water. Further analysis shows that cyclonic eddies prevail in the late 1990s in the formation region of NPSTMW, which lead to significant suppression of the wintertime vertical mixing (96–98) and thus are unfavorable for the formation of NPSTMW; while the situation is completely reversed in the early 1990s (93–95).

Roles of Mesoscale Eddies in the Kuroshio Paths

A high-resolution ocean general circulation model is developed to simulate connections between the Kuroshio path variations and mesoscale eddy activities as realistically as possible. The climatological mean of the modeled Kuroshio takes a nearshore nonlarge meander path. It is found that the model is capable of simulating two types of nonlarge meander state and a possible version of the large meander state. The offshore nonlarge meander is generated through interaction between the Kuroshio and an anticyclonic eddy. The large meander occurs just after significant intensification of the anticyclonic Kuroshio recirculation; successive intrusion of anticyclonic eddies from the upstream region is responsible for this process. Those anticyclonic eddies are advected by the Kuroshio from the region northeast of Luzon Island and increase the upstream Kuroshio volume transport on an interannual time scale. The cyclonic eddies propagating from the Kuroshio Extension region, on the other hand, weaken the Kuroshio meander after the merger. The Kuroshio path variations south of Japan thus seem to be closely related to eddy activities in the subtropical gyre system.

Hydrographic Features off the Southeast Coast of Kyushu during the Kuroshio Small Meanders: A Case Study for Small Meanders that Occurred in 1994 and 1995 Spring

Three Kuroshio small meanders off the southeast coast of Kyushu that occurred during 1994 to 1995 were investigated by using satellite-derived sea surface temperature (SST) and sea surface height (SSH) maps, World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP) repeat section and Japan Meteorological Agency (JMA) hydrographic observations. Based on the satellite data, we observed that the three small meanders are formed by different processes: the triggering and growth of these meanders are caused by a cyclonic eddy propagating from the Kuroshio recirculation region or Kuroshio front meanders traveling from the East China Sea. Investigation of the two small meanders in 1994 and 1995 spring that are captured by the WHP observation showed quite consistent hydrographic features. On the nearshore side of the meandering Kuroshio, a countercurrent appears, associated with vertically uniform upward lifts of the isopycnals from sea surface to bottom at the boundary between the countercurrent and the Kuroshio. In the countercurrent region, the waters in the density ranges of the North Pacific subtropical mode water (NPSTMW) and the North Pacific Intermediate Water (NPIW) are more saline and less saline than typical waters that the Kuroshio carries in a non-small meander state, respectively. There are indications that high-salinity NPSTMW and low-salinity NPIW distributed off the Kuroshio was supplied to the countercurrent region. In the meandering Kuroshio flow, while there is no notable change in properties around the NPSTMW density range, salinity of the NPIW is significantly higher than that carried by the Kuroshio in a non-small meander state, but not higher than that in the Kuroshio at the Tokara Strait, which suggests that saline NPIW from the Tokara Strait, less mixed with low-salinity NPIW off the Kuroshio, may be carried by the meandering Kuroshio.

Anticyclonic Eddy Revealing Low Sea Surface Temperature in the Sea South of Japan: Case Study of the Eddy Observed in 1999–2000

Various kinds of datasets, such as satellite-derived sea surface temperature (SST), sea surface height, surface velocity produced by combining surface drifter and satellite altimeter data, and hydrographic data, led to the discovery of an anticyclonic eddy with lower SST than those of surrounding waters in the Kuroshio recirculation region south of Shikoku, as if the eddy were cyclonic. This anticyclonic eddy was formed east of Kyushu in late August to early September 1999 from the merger of two anticyclonic eddies which had migrated in the recirculation region to the sea south of Japan from the east. After the merger, the anticyclonic eddy strengthened abruptly and began to exhibit the low SST. In October, this eddy coalesced with the Kuroshio and moved swiftly eastward, accompanied by an amplitude growth of the Kuroshio meander. In mid November, off the Kii Peninsula, the eddy detached from the meandering Kuroshio. It then moved southwestward and again slowly propagated westward along the 30°N line. During this period, at least from late October 1999 to January 2000, SSTs over the anticyclonic eddy were found to be continuously lower than those of surrounding waters. This case tells us that we have to pay careful attention to the interpretation of mesoscale SST distributions.

A role of eddies in formation and transport of North Pacific Subtropical Mode Water

Hydrographic data acquired during 2001 by Argo profiling floats deployed in the Kuroshio recirculation region are used to verify the inference that mesoscale eddies prevailing in the recirculation region play an important role in the formation and transport of North Pacific Subtropical Mode Water (NPSTMW). That is, the deeper winter mixed layer is formed preferentially where the thermocline is deeper in association with anticyclonic eddies. In the succeeding seasons, mesoscale eddies retain NPSTMW during their southwestward movement from the NPSTMW formation region, so that anticylonic eddies contribute substantially to the NPSTMW transportation. The spatial distributions of the mixed layer depth in winter and the NPSTMW thickness in the succeeding seasons, based on the float data, suggest that they are affected by mesoscale eddies. The float data also provide the statistical relations of these quantities against the thermocline depth, which are consistent with our inference.

Influence of mesoscale eddies on variations of the Kuroshio path south of Japan

The influences of mesoscale eddies on variations of the Kuroshio path south of Japan have been investigated using time series of the Kuroshio axis location and altimeter-derived sea surface height maps for a period of seven years from 1993 to 1999, when the Kuroshio followed its non-large meander path. It was found that both the cyclonic and anticyclonic eddies may interact with the Kuroshio and trigger short-term meanders of the Kuroshio path, although not all eddies that approached or collided with the Kuroshio formed meanders. An anticyclonic eddy that revolves clockwise in a region south of Shikoku and Cape Shionomisaki with a period of about 5–6 months was found to propagate westward along about 30°N and collide with the Kuroshio in the east of Kyushu or south of Shikoku. This collision sometimes triggers meanders which propagate over the whole region south of Japan. The eddy was advected downstream, generating a meander on the downstream side to the east of Cape Shionomisaki. After the eddy passed Cape Shionomisaki, it detached from the Kuroshio and started to move westward again. Sometimes the eddy merges with other anticyclonic eddies traveling from the east. Coalescence of cyclonic eddies, which are also generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region south of Japan, into the Kuroshio in the east of Kyushu, also triggers meanders which mainly propagate only in a region west of Cape Shionomisaki.

Repeat meridional survey of the Western North Pacific Subtropical Gyre by a VOS ADCP during 1997 to 1998

During 1997 to 1998, near‐surface currents in the western North Pacific subtropical gyre have been measured repeatedly by an ADCP mounted on the hull of the volunteer observing ship (VOS) operated mainly between Japan and Australia. The Kuroshio Current, the Kuroshio Recirculation and the North Equatorial Current are clearly identified as two‐year mean currents in the latitude belt 10–34.5° N. Current variability was dominated by baroclinic Rossby waves or mesoscale eddies over the 10–30° N belt except in the latitude belt 13–17° N where the north‐to‐south shifts of the North Equatorial Current stream axis may dominate current variability. The energy of the mesoscale variability was concentrated to form large peaks around 18.5°N and 23°N.

Relations between Sea Surface Temperature and Air-Sea Heat Flux at Periods from 1 Day to 1 Year Observed at Ocean Buoy Stations around Japan

Relations between sea-surface temperature (T s) and heat flux at the sea surface (F) have been investigated using data from ocean observation buoys located off Shikoku in the Sea of Japan and in the East China Sea. Wavelet transformation decomposed F and T s to wavelet coefficients (WLC) in the period-time domain. Assuming one-dimensional heat transfer by eddy diffusion in the upper ocean, the phase difference (δθ) defined as the difference between the phase of the temporal change rate of T s, and the phase of F ranges statistically from 0 to +π/4 when F changes T s, and is around −π/2 when heat convergence in the sea (Av) forces T s. The δθ values are distributed from 0 to +π/4 at one-day and one-year periods at all buoys. WLC amplitude (WLCA) of F at periods from 16 to 32 day periods, which may be caused by the atmospheric ridge-trough systems, maintains energy longer than WLCA at periods from 2 to 16 days, which may be caused by monsoonal surges. At periods from 2 to 64 days, δθ values distribute from 0 to +π/4 or around −π/2 at each event, reflecting the surroundings of each ocean, i.e., Kuroshio recirculation in the off-Shikoku area, water-temperature front in the Sea of Japan, and water exchange in the continental shelf edge in the East China Sea. We demonstrate that the wavelet analysis can characterize the correspondence between irregular signals of F and T s in various time scales and locations.

Trajectory of mesoscale eddies in the Kuroshio recirculation region

Trajectories of mesoscale eddies in the Kuroshio recirculation region were investigated by using sea surface height (SSH) anomaly observed by the TOPEX/POSEIDON and ERS altimeters. Cyclonic and anticyclonic eddies have been traced on maps of the filtered SSH anomaly fields composed from the altimeter observations every ten days. Both the cyclonic and anticyclonic eddies propagate westward in the Kuroshio recirculation region from a region south of the Kuroshio Extension. The propagation speed of these eddies has been estimated as about 7 cm s−1, which is much faster than the phase speed theoretically estimated for the baroclinic first-mode Rossby wave in the study area. It was also found that in the Izu-Ogasawara Ridge region, most of eddies pass through the gap between the Hachijojima Island and Ogasawara (Bonin) Islands, and some of the eddies decay around the Izu-Ogasawara Ridge. It seems that the trajectory of the eddies is crucially affected by the bottom topography. In the region south of Shikoku and east of Kyushu, some of the eddies coalesce with the Kuroshio. It is also suggested that this coalescence may trigger the path variation of the Kuroshio in the sea south of Japan.

Subtropical mode water variation in the northwestern part of the North Pacific subtropical gyre

Temporal variations of the North Pacific subtropical mode water (NPSTMW) are studied with the proper temporal resolution for describing seasonal variation using monthly hydrographic data taken near the Ogasawara Islands. NPSTMW is formed south of the Kuroshio and the Kuroshio Extension in late winter and advected to the Ogasawara site by the southwestward/westward return flow of the Kuroshio recirculation. While NPSTMW is seen at the Ogasawara site in almost all seasons, NPSTMW with a higher core temperature is advected to this site in April, and one with a colder core follows in and after June until January or February. Furthermore, the NPSTMW core temperature shows considerable interannual variation. This variation is very similar to that found in other regions away from the Ogasawara site, suggesting that it reflects the larger‐scale variation of NPSTMW and that this site would be suitable for monitoring the NPSTMW core temperature. A significant correlation between the NPSTMW core temperature and the monsoon index, which is defined as the difference in the sea level pressure between Nemuro, Japan, and Irkutsk, Russia, as an indicator of wintertime cooling, indicates that NPSTMW contains a memory of wintertime cooling in the preceding winter.