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New topics Ocean Currents around Hokkaido posted on 27 August 2025
New topics Ocean Currents around Hokkaido posted on 6 October 2023
New topics Pacific Ocean posted on 1 September 2023
Site opening on 29 July 2023
Fine Line
Earth's Co-Evolutionary Ladder: This term describes the fact that both the environment and life have been continuously influencing each other since the formation of the Earth and have been involved in the evolution of life on Earth.
While FoM has focused on introducing marine organisms, we now focus on the oceans themselves, the place where they live. Since the ocean is recognized as the birthplace of life, studies of the physical and chemical characteristics of the ocean will also provide various clues to help us understand the characteristics of marine life.
Associate Professor Yutaka Isoda, Faculty of Fisheries Sciences, Hokkaido University, an expert in ocean physics, will give an easy-to-understand presentation on the ocean currents of the Japan Sea, including the basics necessary to read the currents and the results of his advanced research. We believe that you will find that you can acquire the ability to read the vast ocean currents using mathematics and physics. In the future, he plans to add explanations of other ocean currents around the Japanese archipelago .
Professor Isoda always explains physical phenomena of the oceans in a logical manner, which clarifies everything for me when I hear him speak at seminars and lectures. His laboratory has fostered many talented people who have been influenced by him. In fact, we have seen them frequently in the media and they are surprisingly familiar to me. All of them are experts in their respective fields, and they are all following the most up-to-date discoveries and trends.
FoM Editorial
29 Jul 2023 posted
Photo by Dr. ONISHI Hiroji, Hokkaido Univ.
Seasonal Variations of Ocean Currents Around the Japanese Islands
In this issue, we would like to introduce not pictures (or stories) of “beautiful and fascinating creatures” as introduced in previous FoMs, but rather the "ocean currents around the Japanese islands" that pulsate like living creatures.
The image you have of the ocean currents is probably the schematic arrows in high school geography textbook that show the direction of the Kuroshio and Oyashio (on the Pacific Ocean side) and the Tsushima Warm Current (on the Sea of Japan side). In other words, it is a kind of conveyor belt that carries seawater, nutrients, fish, and more recently, marine debris in the direction of the arrows. While this may be sufficient in the social textbooks, it is an unacceptable and unsatisfactory arrow in the natural science, which pursues the question “why and how”. River water is surrounded by soil and rock walls and flows down from the mountains to the sea according to the gravity. However, even though seawater has no such walls, it can circulate horizontally in currents several tens of kilometers wide, and it also pulsates (including seasonal variation), similar to the breathing of a living creature. Our field of expertise, geophysics (or geophysical fluid dynamics) is an attempt to understand this phenomenon.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
29 July 2023 posted
Photo by Dr. ONISHI Hiroji, Hokkaido Univ.
Currents in the Japan Sea-1
Figure 1-1 represents the pulsating Japan Sea surface current as a spatial distribution of colored flow vectors (modified from Fig. 6b in Asahi and Isoda, 2016). Although we omit the method used to create this figure, the original data is spatial-temporal data of sea-surface elevation (convexity and concavity of the sea surface) measured from satellites orbiting the earth. The figure shows the direction and magnitude of the maximum anomaly velocity during the six-month period from July to December (strictly speaking, one of the two major axis directions of the annual cycle velocity ellipse) as an arrow vector, and the maximum month is color-coded. In other words, the flow pattern is summarized as a single picture of how the timing of flow enhancement varies from place to place. The reason why the currents appear to be interrupted in the middle is that two currents exist in the interrupted area with different seasons, and when they are represented in a single picture, the one with the higher current velocity is emphasized. Can you imagine the pulsating currents in this picture? To us, it looks like three overlapping currents.
The first is the northeastward red arrow vector along the Japanese islands from the southern Tsushima Strait (stronger during the summer months of July and August), which is called the "coastal branch current" of the Tsushima Warm Current. The second is the green to purple arrow vector that flows northward from the northern part of the Tsushima Strait off the coast of Korea, then meanders north-south while flowing eastward (strengthened during the fall and winter months of September to December). Many researchers call this current the "offshore branch current" of the Tsushima Warm Current, but we call it the "offshore meandering current" because of its characteristic flow pattern. The third is the red arrow vector of two closed eddy currents, E1 and E2, off North Korea, the existence of which has never been pointed out before. Therefore, they are important eddy currents that dominate the flow field off North Korea, but they have not yet been given a current name. Note that the strengthening of these vortex currents is not in summer (July-August, indicated by the red arrows), but in winter (January-February), when the vortex flow anomalies are counter-rotating, as described below.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
朝日啓二郎・磯田 豊・方 曉蓉（2016）日本海における海面高度偏差と海面地衡流偏差の季節変化．海の研究. 25(3): 43-61.
29 July 2023 posted
Currents in the Japan Sea-2
So, this is where our original work, "geophysical fluid dynamics," comes into play. In the field of fluid dynamics research, waves with gravity as the restoring force, such as wind waves, tsunamis, and tidal waves, are called "gravity mode waves," while vortex-like waves with a large spatial-temporal scale, such as warm and cold ocean eddies, oceanic horizontal circulation currents, weather-related high and low atmospheric pressure, and westerly winds around the earth, are called "eddy mode waves. In fact, the seasonally changing ocean currents shown in Figure 1-1 belong to the latter (eddy-mode) waves. However, because the behavior of these waves is very specific (as it is difficult to compare them with fluid phenomena that we can observe in our daily lives), their physical interpretation requires us to use abstract ideas from mathematics.
If you are not interested, you may skip the passages marked with ※ below. It remains an inadequate and unsatisfactory description, but it explains the mathematics of it (the gray area on the right side of Figure 1-2) in order to give you some background. The words in [ ] that are paraphrased are technical descriptions and terms.
※ When studying the behavior of vortex currents that are aligned in an east-west direction and are attached to the rotating earth, the strength and direction of rotation of the vortex currents are expressed as a physical quantity called "relative vorticity: ζ (zeta)" (Figure 1-2a). Equation (1) [vorticity equation for non-divergent Rossby wave] is the equation governing the vortex flow in a rotating earth system, although its derivation is omitted here. Mathematically, this equation has a form similar to the advection equation, and thus the magnitude of advection depends on the value of β. There are two physical interpretations of β in equation (2): vortices sensing the curvature of the earth move in a westward direction [planetary β effect, "planetary Rossby waves"], and vortices sensing changes in the bottom topography of the land shelf (in the Northern Hemisphere) move left side of shallow water area [topographic Rossby waves" due to topographic β effect]. Next, substituting the wave solution of equation (3) into equation (1), we obtain equation (4). Since the wave solution assumes an arbitrary wavelength (inverse is the wavenumber k) and period (inverse is the frequency σ), equation (4) becomes an equation relating σ(k) and k [dispersion relation]. Using equation (4) (the mathematical notation and its physical interpretation are omitted here), we can analytically obtain equations (5) and (6) for the velocity of movement of the uneven shape of the vortex flow [phase velocity: C = σ/k ] (5) and the energy of the vortex flow [group velocity: Cg = ∂σ/∂k ] (6). When the vertical axis is the frequency σ(k) and the horizontal axis is the wavenumber k, the blue thick line in Figure 1-2b is a curve representation of equation (4) [dispersion curve diagram]. The blue thick line in Fig. 1-2b is the dispersion curve diagram [dispersion curve diagram]. The low wavenumber vortex on the left side of the "へ" has a horizontal longitudinal shape with the same sign for C and Cg, and the high wavenumber vortex on the right side has a vertical longitudinal shape with a different symbol for C and Cg. Both vortexes cannot maintain their initial shape over time, and equation (4) can be said to be an expression for the dispersion of the vortexes.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
Gill A.E. (1982) Atmosphere-Ocean Dynamics, Academic press.INC. (London).
29 July 2023 posted
Currents in the Japan Sea-3
Here, using a numerical model experiment (i.e., solving equation (1) numerically by computer programming), we show how the vortex flow (localized vorticity ζ) initially set as the initial value shows the degree of disparity [dispersion] by comparing (a) a long transverse vortex [long wave Rossby] and (b) a longitudinal vortex [short wave Rossby] in Figs 1-3. (modified from Fig. 1 in Isoda, 1997). The horizontal axis of the figure is the dimensionless horizontal distance (space) divided by the vertical width of the initial vortex, with (a) 10,000 folds level (represented by an extremely long transverse vortex) and (b) the same orders (represented by a longitudinal vortex with similar vertical and horizontal scales).
A long transverse vortex [long-wave Rossby] with C and Cg of the same sign is moving in the left direction of the figure (Figure 1-3a), keeping almost the same shape of the vortex flow. The "coastal branching current" seen in Figure 1-1 is explained by the direction of movement and fast-moving speed (July-August) indicated by this long wave Rossby, and is interpreted as an ocean current due to the "topographic beta effect" with the Japanese Islands on the shallow water side to the right.
In a longitudinal vortex [short wave Rossby] where C and Cg have opposite signs, the shape (phase) of the concavity moves to the left in the figure (C < 0), while the large energy portion (amplitude of the flapping concavity) moves in the opposite direction to the right (Cg > 0), and a new concavity (vortex flow) is generated on its right (Figure 1-3b). This strange time variation cannot be compared to the waves we see in ordinal life, which is why we call it a special behavior. If we were to use a non-wave analogy, it might be similar to the moonwalk of the late American singer (dancer) Michael Jackson (who appears to be walking to the left, but his body is actually moving in the opposite direction, to the right). The "offshore meandering current" shown in Figure 1-1 can be seen as a series of three longitudinal eddies in an east-west direction. If we consider this as an ocean current caused by the "planetary beta effect," the slow eastward strengthening of the current over the four month period from September to December can be explained by the group velocity of the longitudinal eddies [short wave Rossby]. (Strictly speaking, we must discuss "standing Rossby waves" with eastward advection and westward phase velocity balanced (Morie et al., 2015).
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
磯田 豊（1997）ロスビー波の分散性．沿岸海洋研究. 34: 173-181.
森江亮介・磯田 豊・藤原将平・方 曉蓉（2015）対馬暖流の蛇行発達に対する定在ロスビー波の寄与．海の研究. 24(1): 29-47.
29 July 2023 posted
Currents in the Japan Sea-4
The last part of this chapter is about a third current (or eddy current) which is currently unnamed and is located off the coast of North Korea. Figure 1-4 (a) shows the monsoon vector prevailing over the Japan Sea in February in winter, and (b) is the result of a numerical model experiment of the flow field driven by wind stress (modified from Fig. 10a,b in Asahi and Isoda, 2016). The horizontal shear (vorticity supply from the atmosphere to the sea) of the winter monsoon coming from the Asian continent into the Japan Sea is strongest off the North Korean coast (Fig. 1-4a). The numerical experiments predicted the occurrence of clockwise vortex currents at two locations (E1 and E2) off the North Korean coast due to the supplied vorticity (Figure 1-4b). The location and shape of both vortexes are very similar to E1 and E2 in Figure 1-1, although (due to the different seasonal notation) the direction of rotation is exactly opposite (counterclockwise vortex deviation). Based on the experimental results, it is suggested that currents with the vortex shapes of E1 and E2 are dominant only in winter, when the monsoon is blowing continuously, and that they affect a part of the offshore meandering current (around E3). We believe in the existence of both eddy currents and dream of a day in the future when we can prove their existence by conducting oceanographic observations together with oceanographers in North Korea (recently, it has also been suggested that these eddy currents may be driving the deep currents in the Japan Sea, and we consider this phenomenon to be very important in that sense).
We hope that the correct (not "new") picture of the ocean currents shown in Figure 1-1 will present a new perspective not only when describing the marine environment of migratory fish in fisheries science, but also when discussing environmental and international issues (relations among Russia, China, North Korea, South Korea, and Japan surrounding the Japan Sea) in social geography. From the next issue onward, we would like to introduce "2. ocean currents in the Pacific Ocean" and "3. ocean currents around Hokkaido" and their respective seasonal changes based on data analysis of field data similar to that in this issue.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
朝日啓二郎・磯田 豊・方 曉蓉（2016）日本海における海面高度偏差と海面地衡流偏差の季節変化．海の研究. 25(3): 43-61.
29 July 2023 posted
Currents in the Pacific Ocean -1
First, let’s look at the bottom topography in the (North) Pacific Ocean, which extends to the east from the Japanese archipelago (Figure 2-1a). Compared to the Japan Sea, the Pacific Ocean has about 10 times wider in the eastward direction and more than twice in a depth. Therefore, the large-scale horizontal circulations in the Pacific Ocean are subject to the effects of the curvature of the Earth (planetary β effect), i.e., the characteristics of "planetary Rossby waves" (Figure 1-2, equation (2), βp = df/dy). Also, the Pacific Ocean has two distinctive shallow depth topographic features that run in an almost north-south direction. One is the Izu-Ogasawara Ridge (abbreviated as IOR) in the southern part of the Japanese Islands, and the other is the Emperor's Seamounts (abbreviated as ESs), which curves into a “く” shape in the center. These shallows have major influence on the seasonal flow variations in the Pacific Ocean.
The Kuroshio and Oyashio are known as the strengthening "Western Boundary Currents" of the "wind-driven circulations", i.e., horizontal circulations driven by wind stress blowing over the ocean surface. On the stationary field of wind-driven circulation, its physical mechanism is widely recognized and well described in textbooks. However, the actual situation of its seasonal response introduced in this FoM has not yet been described in textbooks. The difficulty of research on wind-driven circulation lies in the fact that planetary Rossby waves must be distinguished into two waves due to the barotropic and baroclinic responses. Fujiwara et al. (2014) attempted to distinguish these responses by combining several observational data, resulting in a somewhat flawed picture of the actual seasonal variations. Our goal is to present this information, but it is necessary to confirm that the responses were correctly distinguished. Tateno et al. (2016) conducted an experiment in which a numerical model that approximates the vertical density field with two layers (light upper layer and heavy lower layer of seawater) was forced by seasonally varying zonal east-west winds, taking into account the shallows of the IOR and ESs (Figure 2-1b: wind-driven circulation model). Here, we use the results of this experiment to divide the dynamical response into a stationary field (long-term average field) and a seasonally variable field, and further break down the latter into barotropic and baroclinic responses, and then compare them with both responses of Fujiwara et al. (2014).
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
藤原将平・磯田豊・舘野愛実（2014）��部北太平洋における海面高度偏差の季節変化. 海の研究, 23(6), 197-216.
舘野愛実・藤原将平・磯田豊・朝日啓二郎(2016)：北太平洋風成循環流の季節変化に関する数値モデル実験．北大水産彙報, 66(3),87-97.
1 September 2023 posted
Currents in the Pacific Ocean-2
In the case of a two-layer fluid in the Earth's rotating system, two planetary Rossby waves, i.e., barotropic and baroclinic waves, can exist simultaneously. Although I omitted an explanation in "Currents in the Japan Sea", the offshore meandering current in the Japan Sea can be understood as the baroclinic wave with the lower layer assumed to be infinite, and the coastal branching currents as the shelf-trapped (topographic β) barotropic waves. Generally, the barotropic wave is accompanied by a pressure change in sea surface displacement, and the currents in both upper and lower layers fluctuate periodically with the same amplitude in phase (Figure 2-2b). Baroclinic wave is accompanied by pressure changes in the internal boundary surface displacement, and the currents upper and lower layers fluctuate in the out of phases (Figure 2-2c). The dispersion curves of both waves qualitatively show the same "へ" shape, and the one-year period variation is on the lower wavenumber side of the "へ" peak, so that the phase velocity C (and group velocity Cg) are both negative (westward propagation; C < 0) (Figure 2-2a). However, the magnitude of the phase velocity C differs between the two. In term of propagation time (response time) from the U.S. side to the Japanese side, it is several weeks for the barotropic wave and about 10 years for the baroclinic wave.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
1 September 2023 posted
Currents in the Pacific Ocean-3
The zonal easterly and westerly winds (wind stress τ) that drive the wind-driven circulation in the Pacific Ocean is represented by the mid-latitude eastward “westerly winds” and low-latitude easterly “trade winds”, which are enhanced in winter (τmax around February) when the Aleutian Low develops (Figure 2-3a: modified from Fig. 3b in Tateno et al., 2016). The Japan Sea is also located in the “westerly winds” latitudinal zone, but the Japan Sea is more influenced by the “northwesterly monsoon” than by the “westerly winds” (see Fig. 1-4a) because it is located at the boundary between the Aleutian Low (Pacific side) and the Siberian High (continental side) in winter. Therefore, the currents in the Japan Sea cannot be explained only by wind-driven circulations but can be interpreted by (1) inflow-outflow currents forcing through the straits and (2) a personal interpretation of a kind of thermohaline circulation called "Cooling Induced Current (CIC)” (Isoda, 1999; Fang and Isoda, 2020).
Let us begin with a textbook explanation by looking at the steady-state field of wind-driven circulation model (Figure 2-3b: modified from Fig. 6 in Tateno et al., 2016). South of the “westerly winds” maximum latitude including “trade winds”, wind stress has a clockwise rotational component (also called negative vorticity supply), which drives the clockwise subtropical circulation associated with the Kuroshio. North of the “westerly winds” maximum latitude, wind stress has a counterclockwise rotational component (positive vorticity supply), which drives the counterclockwise subarctic circulation associated with the Oyashio. Both circulations propagate westward due to the nature of planetary Rossby waves, and the center of these circulations are pushed toward the western coastal boundary (Japanese Islands). Therefore, given a closed horizontal circulation flow (or water-mass conservation), the flow near the western coastal boundary is always stronger. This is why the Kuroshio and Oyashio are known as the strengthening “Western Boundary Current”.
As a result, it appears that the eastward current is driven in response to the eastward “westerly winds” and the westward current is driven in response to the westward “trade winds”. Since this appears to be the case, it is incorrect to understand that the wind-driven circulation is a flow in which sea surface water is dragged in a downwind direction. The total volume transport of both circulations is more than 30 Sv (Sv is read as Sverdrup, and 1 Sv is a unit of transport defined below), whereas the sea water that the wind can drag (Ekman flow) is sallower than a few ten meters below the sea surface (Ekman boundary layer), and hence its transport is only a few Sv (Ekman transport). Although the details of the dynamic process named "Ekman" are omitted here, the "rotational component of wind stress" only promotes the convergence and divergence of sea surface water (Ekman pumping).
In the subtropical circulation region, the clockwise component of wind stress causes the convergence of surface water. Therefore, the sea surface rises and the internal boundary surface descends. The upper convex part of the sea surface is under high pressure and the lower convex part of the internal boundary is under low pressure, so that both pressures work in opposite ways to the lower layers. Therefore, in this lower layer, the horizontal pressure difference (horizontal pressure gradient) becomes smaller. The subarctic circulation region is a divergent field, so it has exactly the opposite concavity-convexity relationship, but the horizontal pressure gradient in the lower layers is reduced in the same way. When the horizontal pressure gradient in the lower layer becomes exactly zero, the lower current disappears, and this condition is called "isostasy”. Looking at the long-term mean field in Figure 2-3b, the effects of IORs and ESs, which should exist in the lower layer, are not seen at all. In fact, the long-term mean field is in a state of “isostasy”, in which the lower layer flow is completely zero.
The process of isostasy formation was discussed in detail in Isobe and Imawaki (2002). In the early stages of wind forcing (several weeks), both currents in the upper and lower layers are present as a westward propagating barotropic wave together with an upper convex sea surface displacement (the condition shown in Figure 2-2b). When the wind forcing period approaches 10 years (response time of the baroclinic wave), the westward propagating baroclinic wave with a lower convex internal boundary displacement (Figure 2-2c) begins to slowly overlap the previously formed barotropic wave. Since the flow direction in the lower layer of the baroclinic wave is opposite to that in the lower layer of the barotropic wave, the superposition of these two waves causes the lower layer flow to gradually dissipate from east to west (superposition of (b) and (c) in Figure 2-2). This is a good interpretation for the long-term mean field in a state of isostasy. However, isostasy cannot be established because the annual period of a seasonal fluctuation field (1 year) is longer than a few weeks of the barotropic response and shorter than 10 years of the baroclinic response.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
Isoda, Y.（1999）Cooling-induced current in the upper ocean of the Japan Sea. J.Oceanogr., 55, 585-596.
Fang, X. and Isoda, Y.（2020）Dynamic equilibrium state of a Cooling Induced Current in the Japan Sea. 北大水産彙報, 70(1), 25-40.
舘野愛実・藤原将平・磯田豊・朝日啓二郎 (2016) 北太平洋風成循環流の季節変化に関する数値モデル実験．北大水産彙報, 66(3), 87-97.
Isobe, A., and S. Imawaki（2002）: Annual variation of the Kuroshio transport in a two-layer numerical model with a ridge, J. Phys. Oceanogr., 32, 994-1009.
1 September 2023 posted
Currents in the Pacific Ocean-4
Let us finally look at the seasonal variation field of the ocean currents in the Pacific Ocean. Fujiwara et al. (2014) combined the satellite sea surface height anomalies (abbreviated as SSHA) and the dynamic sea surface height anomalies from Argo float buoy observations in the western North Pacific Ocean (analysis region is within the red line in Figure 2-1b), and calculated the barotropic response component as ΔDBT = SSHA－ΔD2000 (ΔD2000 is the dynamic depth anomaly of the sea surface referred to a depth of 2000db), and the baroclinic response component is ΔDBC = ΔD2000－ΔDML (ΔDML is the dynamic depth anomaly of the sea surface referred to a depth of the surface mixed layer). The seasonal variation fields of the barotropic response (ΔDBT) and baroclinic response (ΔDBC) are represented by the results of a one-year period harmonic analysis using 12 years of data. The respective responses are shown on the left side (a) of Figures 2-4 and 2-5 as the spatial distribution of phase θ (upper panel) and amplitude Amp (lower panel) (Fig.4a in Fujiwara et al. 2014 and Fig. 8 in Fujiwara et al.) The phase θ is indicated as the month in which the water level anomaly reaches its upper convex maximum, so that the barotropic response is interpreted as the month in which the clockwise circulating flow reaches its maximum. To help understand this data more clearly, the wind-driven circulation model results of Tateno et al. (2016) are also displayed on the right side (b) of both figures as spatial distributions of phase θ and amplitude Amp (flow stream function for the barotropic response and internal boundary surface displacement for the baroclinic response) for a similar one-year period (modified from Fig. 7a of Tateno et al., 2016).
Since isostasy is not established in seasonal variations, it is expected that the barotropic wave excited by the annual cycle wind forcing should be affected by the shallow depth topography. So, look at the barotropic response in Figure 2-4. Based on the results of the analysis on the left side (a), the phase θ of the subarctic circulation region (northern half) is in August to September (colored red) and that of the subtropical circulation region (southern half) is in January to March (colored blue). These are almost the same as the phases of wind forcing (not shown). The amplitude of the seasonal variation tends to be larger in the subarctic circulation region than in the subtropical circulation region. As an expected, the August-September phase θ (red system) in the subarctic circulation region appears to extend southward along the IORs and ESs in the subtropical circulation region. Since the wind-driven circulation model on the right side (b) explicitly represents the barotropic response, it also reproduces the barotropic waves ① and ② propagating southward along the IOR and ESs (upper panel), and the stream function over their shallows is highly distorted (lower panel). The other important result is that the amplitude values west of the IOR, where the Kuroshio is present, are extremely small. The seasonal variation of the barotropic response can be summarized as follows: a part of the counterclockwise subarctic circulation, which is enhanced in winter, migrates southward along the shallow depth topography, affecting the subtropical circulation, e.g., weakening the seasonal variation of the Kuroshio etc.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
Refrences
藤原将平・磯田豊・舘野愛実（2014）西部北太平洋における海面高度偏差の季節変化. 海の研究, 23(6), 197-216.
舘野愛実・藤原将平・磯田豊・朝日啓二郎 (2016) 太平洋風成循環流の季節変化に関する数値モデル実験．北大水産彙報, 66(3), 87-97.
1 September 2023 posted
Currents in the Pacific Ocean-5
As the barotropic wave can respond immediately to the 1-year period of wind forcing, the current with 1-year period in the lower layer will not be zero because isostasy is not established. Therefore, when the lower current of a barotropic wave crosses over the shallows of IORs and ESs, “impinging (vertical flow)” occurs there, which changes the internal boundary surface up and down (see the schematic diagram near the seafloor slope in Figure 2-2b). Numerical experiments by Isobe and Imawaki (2002) with IOR and Wagawa et al. (2010) with ESs showed that this impinging process excites baroclinic waves with a 1-year period over each slope of shallows. The numerical experiments of Tateno et al. (2016) are more realistic than these, considering both subtropical and subarctic circulations and both IOR and ESs.
In the baroclinic response shown in Figure 2-5, let us look first at the model result on the right side (b), where the baroclinic wave due to the impinging process is reproduced. Clean stripe structures of A1-A3 are seen in the amplitude west of the ESs, and the change in phase θ indicates that A1 was excited over the ESs three years ago, A2 two years ago, and A3 one year ago, and that it is a baroclinic wave that propagated slowly westward. Amplitude west of the IOR is high at the region B, where the baroclinic wave excited on the IOR reaches the western boundary (the Japanese Islands) in about half a year. Note that phase θ in the subarctic circulation region (northern half) is dominant around November (purple), and phase θ in the subtropical circulation region (southern half) is dominant from April to May (yellowish green). Such phase distribution is caused by internal boundary surface displacement (opposite phase in the two circulation regions) due to the “Ekman pumping” in the annual cycle, which actually overlaps with the baroclinic wave propagation and creates spatial amplitude strength and weakness (stripe structures) (Tateno et al., 2016). Refer to this model, looking at the results on the left side (a), it seems that it can be partially explained by the seasonal variation of wind-driven circulation. First, looking at the spatial distribution of phase θ roughly, there are a few purple November spots in the north, and light blue to yellow-green February to May spots in the south, which are characteristic of “Ekman pumping”. Due to the spatial lower resolution, the phase θ variation over the one-year cycle is not clear, but we can find A1-A3 stripe-like structures with almost the same phase θ (yellow-green in February-March) among the scattered noise in the amplitude west of the ESs.
However, in order to understand the baroclinic response in the Pacific Ocean, we must take into account not only the wind-driven circulation, but also the thermohaline circulation caused by seasonal heating and cooling through the sea surface and the advection of mode water (uniform mixed water) formed in the winter. Thus, the interpretation of seasonal dynamical response in the Pacific Ocean has only just begun. Although this is the level of our research, I would be happy if I could convey to you the image of the Kuroshio and Oyashio that fluctuate seasonally, unlike the stable arrow currents in the geography textbooks.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
舘野愛実・藤原将平・磯田豊・朝日啓二郎 (2016) 北太平洋風成循環流の季節変化に関する数値モデル実験．北大水産彙報, 66(3), 87-97.
Isobe, A., and S. Imawaki（2002） Annual variation of the Kuroshio transport in a two-layer numerical model with a ridge, J. Phys. Oceanogr., 32, 994-1009.
Wagawa, T., Y. Yoshikawa, and A. Masuda（2010）Bathymetric influences of the Emperor Seamounts upon the subarctic gyre of the North Pacific: Examining Boundary Current Dynamics along the Eastern Side of the Mountain Ridge with an Idealized Numerical Model, J. Oceanogr., 66(2), 259–271.
1 September 2023 posted
Above photograph shows a group of waves that from a series of stripes in the center of the Tsugaru Strait, despite the sea conditions with little or no wind. In fact, large-amplitude internal waves exit below the surface of those waves, and just at the convergence zone of the internal waves, the sea surface is violently surging.
Ocean Currents around Hokkaido
In this last section, let’s introduce the ocean currents around Hokkaido and their seasonal variations. So far, we have described the seasonal changes of the ocean currents in the Japan Sea and the Pacific Ocean, i.e., the Tsushima Current, the Kuroshio, and the Oyashio, as "pulsations". This is because the propagation of seasonal disturbances (mainly, Rossby waves) locally changes the strength of currents but does not significantly change the location of each current or the direction of its flow, so that "pulsation" was an appropriate description. However, when the ocean currents around Hokkaido were examined, subtropical (high-temperature, high-salinity water) and subarctic (low-temperature, low-salinity water) currents switched seasonally in certain areas, reversing the direction of the currents and even disappearing. These are not gentle "pulsations" but dramatic seasonal changes. There are two possible reasons for these “dramatic changes”.
As described in "Currents in the Pacific Ocean" the boundary between the subarctic circulation (including the southward Oyashio) and the subtropical circulation (including the northward Kuroshio) is located off the Boso. Sea area off the east coast of Hokkaido is about several hundred kilometers farther north than its boundary. That is, it is completely located in the cold subarctic region. The first reason is as follows; warm subtropical water (Tsushima Current water) flows northward in the Japan Sea, and outlets directly from the Tsugaru-Soya Straits to the subarctic region. In other words, the subtropical waters are transported near the center of the subarctic region via the Japan Sea. The second reason is as follows; warm currents with subtropical water are more easily weakened than cold currents with subarctic water when the cold, dry northwest monsoon blows over the sea in winter. This is because subtropical water has a high-salinity and thus tends to become heavy (low-temperature, high-salinity water) by cooling, while subarctic water remains light and low-salinity even at slightly lower temperature.
Knowing the reasons mentioned above, let us try to explain the seasonal changes in the ocean currents depicted in the picture in Figure 3-1. The solid arrows represent surface currents in summer, with black schematically depicting warm currents (flows with high-salinity subtropical water) and gray schematically depicting cold currents (flows with low-salinity subarctic water). Change in ocean currents under the influence of the winter northwest monsoon (schematic purple arrow drawn in the upper left) is represented by the dashed arrows or ellipses.
First, let us look at the summer ocean currents (see Figure 3-2). Most of the Tsushima Current flows into the Tsugaru Strait, and its remainder flows northward off the west coast of Hokkaido. Warm water outflowing from the Tsugaru Strait into the North Pacific becomes known as the "Tsugaru Warm Current" and extends offshore, forming a large clockwise eddy current (Gyre mode) in Hidaka Bay. Part of Gyre separates on the shelf slope off Hidaka as a small patchiness, which moves along the slope to the mouth of Funka Bay. This warm patchiness flows into Funka Bay through the bottom layer, and conversely, low-salinity water in the surface layer (river water) flows out of the bay, forming a clockwise surface circulation that extends over the entire bay. On the other hand, the Tsushima Current off the west coast of Hokkaido meanders due to the occurrence of vortex off the Shakotan Peninsula and its westward movement. To the further north, the current splits into two branches around the Musashi Rise, one is the offshore current and the other is the coastal current. Warm water discharging from the Soya Strait into the Okhotsk Sea becomes known as the "Soya Warm Current", which reaches the Shiretoko Peninsula as a shelf-slope or coastal trapped current. Since oceanographic data is not available for Russian area, the behavior of warm water north of Shiretoko is still unknown. However, part of warm water enters the Nemuro Strait from the Kunashiri Island side (speculation). In addition, Soya Warm Water entering the Okhotsk Sea flows out to the North Pacific somewhere in the Kuril Islands, and then appears offshore of Douto (East Hokkaido) due to the westward shelf-trapped current.
Thus, in summer, the coastal areas around Hokkaido are generally surrounded by warm currents with subtropical waters. Now, how do these summer currents change when the winter monsoon blows? The strong sea surface cooling caused by the winter monsoon changes the high-temperature, high-salinity water that makes up the eddy currents into low-temperature, high-salinity heavy water. That is, the vortex currents in Hidaka Bay and off the Shakotan Peninsula weaken or disappear. As a result, the Tsugaru Warm Current in Hidaka Bay changes from an eddy current (Gyre mode) to a coastal current (coastal mode). In return, a portion of the Oyashio begins to intrude into the Hidaka Bay. The vortex off the west coast of Hokkaido current also disappears and the northward Tsushima Current almost disappears. Furthermore, deep vertical mixing due to sea surface cooling forms the “Japan Sea Intermediate Water” in this area. In the Okhotsk Sea in winter, the northwesterly monsoon generates a basin-scale wind-driven circulations, and its western boundary current (or wind-forced coastal trapped current) becomes known as the "East Sakhalin Current" and moves southward along the shelf-slope area off Abashiri. Since the East Sakhalin Current has light water consisting of low salinity water of Amur River origin, the water level is higher on the Okhotsk Sea side than on the Japan Sea side across the Soya Strait. This causes the weakening or disappearance of the Soya Warm Current. After that, the southward East Sakhalin Current, together with the northwesterly winds, transports sea ice to the coast of Hokkaido. In early spring, when sea ice melts, a density current originating from the melting water is generated and discharges to the North Pacific somewhere in the Kuril Islands. At this time, the density current with low salinity becomes known as the "coastal Oyashio". Off the east coast of Hokkaido, it changes from the warm Soya Warm Current water in summer to the cold coastal Oyashio water (2°C or lower). This coastal Oyashio, as a westward shelf trapped current, together with the Oyashio intrusion, reaches from offshore of Douto into Funka Bay via Cape Erimo. Although there are many unknowns because half of the Nemuro Straits are in Russian area, the following seasonal change is clear in the Goyoumae Channel at the tip of the Nemuro Peninsula. In summer, water flows into the straits from the North Pacific side (inflow of Soya Warm Current water), and in winter, the opposite is true: water flows out of the straits to the North Pacific side, probably connecting with the coastal Oyashio.
Although the above is an explanation of the ocean currents, these findings were obtained by combining the research studies for each of the ocean regions indicated by ① through ⑧ shown in the figure. Because of the small spatial scale of oceanic phenomenon, they cannot be captured by coarse spatial resolution sea-surface height anomaly data from satellites like as the Japan Sea and Pacific Ocean studies. They can only be obtained by collecting observational data (current velocity, temperature, salinity, etc.) accumulated in the past and analyzing the data season by season. Therefore, those who are interested in more detailed oceanic phenomenon or analysis methods, please refer to the explanations for each ocean area and individual references cited in the subsections from ① to ⑧ below.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
6 October 2023 posted
① Ocean currents in the Tsugaru Strait (Figure 4-1)
The ocean current in the Tsugaru Strait is an eastward passage-flow through the strait from the Japan Sea to the Pacific Ocean. The annual variation of its volume transport is very small. The approximately 70% of its seawater originates from the Tsushima Warm Current (red arrow), while the remainder originates from the cold, low-salinity intermediate water of the Japan Sea (black-green arrow), see upper right schematic diagram. Looking at the horizontal distribution of water temperature near the sea bottom (Figure A, top), a colder intermediate water intrudes into the strait from the Hokkaido side, and this phenomenon can also be observed from surface drift buoys (Figure A, bottom). Within the strait, the horizontal velocity amplitudes of diurnal tidal current (approximately 24-hour tidal cycles) has the same order velocity of passage-flow. Then, a huge internal tidal wave with a vertical amplitude of about 150 m occurs at downstream side of the sill topography, when the eastward tidal current has the maximum (Figure B, bottom). Behind the cape at the eastern entrance, when the eastward tidal current weakens, a pair of clockwise and anti-clockwise horizontal vortices is generated (Figure C below). At the eastern entrance of the strait, an oceanic front is formed between the warm, high-salinity water of passage-flow (red arrow) and the cold, low-salinity water of Oyashio (blue arrow), while the generated vortex pair also contributes to the horizontal mixing of these two water masses.
伊田智喜・山下慎司・磯田 豊・��林直人（2016）日本海中層水を起源とした低渦位水の津軽海峡への流入過程．海の研究，25(4)，101-122.
② Ocean currents around the Hidaka Bay and off Doto areas (Figure 4-2)
The origin of seawater can be estimated from the spatiotemporal changes in the combination of water temperature T and salinity S (TS diagram: contour lines indicate density), and based on this, we can infer the appearance/disappearance of seawaters or route of ocean flow. In the Hidaka Bay, the main seawater types are the Tsugaru Warm Water (TW), originating from the Tsugaru Strait; the Oyashio Water (OW), originating from the Bering Sea; and the Coastal Oyashio Water (CO), originating from the melted ice water of the Okhotsk Sea. During the latter half of the sea surface cooling period (January to March), TW, OW, and CO have different T and S values, respectively, but are arranged in nearly the same density band (Figure A left). In this time, both of bottom trapped OW and surface trapped CO can enter into Hidaka Bay from the eastern side of Hokkaido, i.e., off Doto (Figure B top). On the other hand, during the latter half of the sea surface heating period (July–September), TW becomes the warmest (Figure A, right), and thus the lighter TW rides on top of the heavier OW, forming a large clockwise vortex, known as the “Tsugaru Gyre” (Figure B, bottom). When this Gyre connects to the shelf slope in the northern part of the bay, part of it becomes a small meander (Figure C). In model experiments, this small meander propagates as a vortex mode wave (topographic Rossby wave) with looking at shallow coastal side on the right, and the appearance of TW from the north is predicted at the mouth of the Funka Bay (Figure D).
Rosa, A.L., Y. Isoda, K. Uehara and T. Aiki（2007）Seasonal variations of water system distribution and flow patterns in the southern sea area of Hokkaido, Japan. J.Oceanogr.,63:573-588.
小林直人・太田紗生・磯田 豊・荘司堅也・工藤 勲・荒巻能史（2014）日高湾陸棚斜面上における津軽Gyreの分岐．海と空，90(1)，1-10.
小林直人・磯田 豊・朝日啓二郎（2015）日高湾陸棚斜面に沿って西方へ引き延ばされる津軽Gyreの数値実験．海の研究，24(5)，171-187．
③ Ocean currents in the Funka Bay (Figure 4-3)
Since the Funka Bay is located at the innermost part of the Hidaka Bay, its seasonal changes are significantly influenced by the three seawaters described in “② Ocean current around the Hidaka Bay and off Doto areas” During the latter half of sea surface cooling (spring), OW or CO flows into the bay from the upper layer on the Muroran side, moves anti-clockwise within the bay while cooling, and then heavier seawater flows out from the lower layer on the Sawara side (Figure A left). During the latter half of the sea surface heating period (summer), TW reaching the bay entrance from the northern side of the shelf flows into the bay from the bottom layer. At this time, a clockwise vortex is just formed at the surface layer, and part of it flows out of the bay from the Sawara side (Figure A, right). These flows through the bay mouth contribute to positive horizontal heat transport, i.e., sense of warming the bay (Figure B). The surface clockwise eddy current formed in summer appear to occur in conjunction with the TW (Bottom Intensify: BI flow) invading the bottom layer in an anti-clockwise direction (Figure C). When investigating the factors contributing to the formation of surface eddy current through model experiments, it was inferred that summer sea surface heating (Only Heat) is the most important forcing factor (Figure D).
磯田 豊･長谷川伸彦（1997）噴火湾の熱収支.海と空，72，13-21.
長谷川伸彦･磯田 豊（1997）噴火湾の水収支.海と空，73，113-121.
磯田 豊･長谷川伸彦･清水 学（1998）噴火湾の塩収支と海水交換.海と空，74，27-37.
柴田 遥・小林直人・磯田 豊・奥村裕弥・工藤 勲・宮園 章（2013）初夏の噴火湾表層時計回り循環流形成に同期した海底捕捉流．沿岸海洋研究，51(1)，65-78.
小林直人・磯田 豊・堀尾一樹（2019）初夏の噴火湾表層時計回り水平循環流の数値実験. 海の研究，28(4,5,6)，51-74.
④ Ocean currents off the west coast of Hokkaido (Figure 4-4)
Comparing the TS diagrams off the west coast of Hokkaido (Japan Sea side) (Figure A above) with those of the Hidaka Bay off the east coast of Hokkaido (Pacific Ocean side) (Figure A in “② Ocean current around the Hidaka Bay and off Doto areas”), it can be seen that the change in salinity of the Japan Sea side is relatively small. Therefore, the seasonal changes in sea surface temperature due to heating and cooling directly affect the density of the seawater in the Japan Sea. Here, we have distinguished between the low-salinity side of the Tsushima Warm Current T as TL, the high-salinity side as TH, and the slightly cooler seawater than TL as JIL (meaning the Japan Sea Intermediate Water) (Figure A below). The seasonal appearance of the three types of seawater—TH (black), TL (gray), and JIL (gray dashed line)—is shown in Figure B as a vertical cross-sectional distribution (J3 offshore measurement line) and in Figure C as a horizontal distribution at a depth of 50 m. Figure D depicts the schematic flow during the sea surface cooling/heating periods. During the cooling period, the Tsushima Warm Current (TL and TH) nearly disappears. As the heating period begins, TL appears from the south, followed by TH, and the Tsushima Warm Current gradually forms, moving northward while greatly bypassing the clockwise eddy off the Shakotan.
檜垣直幸・磯田 豊・磯貝安洋・矢幅 寛（2008）北海道西岸沖における水系分布と流れパターンの季節変化.海の研究，17(4)，223-240.
檜垣直幸・磯田 豊・本田 聡（2009）北海道西方の武蔵堆周辺海域で観測されたモード水．海の研究，18(6)，1-16.
⑤ Ocean currents at the Soya Strait (Figure 4-5)
The northern half of the Soya Strait is Russian territorial area, so hydrographic data is not available. Therefore, although the spatial resolution is low data, we performed harmonic analysis of satellite-derived sea surface height data (SSHA) on an annual cycle to investigate the seasonal changes in sea level anomalies around the Soya Strait (Figure A). In the northern hemisphere, a current with a maximum positive anomaly on the right side is predicted (physical reasons omitted). It is inferred that the Soya Warm Current (SWC) flowing from the west coast of Sakhalin in the Japan Sea into the strait will strengthen from September to October, and the East Sakhalin Current (ESC) flowing southward along the east coast of Sakhalin in the Okhotsk Sea will strengthen from December to January. It is clear that this ESC has a northward flow that weakens SWC along the coast of Hokkaido. The disappearance of SWC in winter is supported by ADCP current velocity distributions, despite the limited data available for the southern half of the strait (Figure B). In seasons other than winter, both of eastward passage-flow and diurnal tidal current dominate. Along line X (see Figure C), approximately 24-hour repeated observations were conducted by ship. At the surface, the Cold-Water Belt CWB was found (Figure D left). A boundary front between Japan Sea-origin water JSW and Okhotsk Sea-origin water OSW immediately below CWB were found to have a vortex structure that fluctuates on a diurnal cycle (Figure D right).
阿部祥子・磯田 豊・矢幅 寛（2009）春季の宗谷暖流沖合域に形成される亜表層反流．海の研究，18(4)，265-286.
有田 駿・磯田 豊・工藤 勲・宮園 章・伊田智喜（2015）宗谷暖流域における日周潮流と順圧不安定波の相互作用．沿岸海洋研究，52(2)，183-195.
飯田博之・磯田豊・小林直人・堀尾一樹（2018）宗谷暖流沖合域の冷水帯を伴った日周期渦流の観測とモデル実験．海の研究，27(4)，155－174.
堀尾一樹・飯田博之・磯田 豊（2019）サハリン島南部Aniva湾における夏季の表層時計回り循環流. 北大水産彙報，69(2)，57-69．
⑥ Ocean currents in the shelf area on the Okhotsk Sea side (Figure 4-6)
The Cold-Water Belt (CWB) observed on the sea surface near the Soya Strait extends in a long strip along the isobaths on the shelf in the Okhotsk Sea side (Figure A), and the offshore position of this strip-shaped CWB almost coincides with the offshore boundary of the Soya Warm Current (SWC). The seasonal variation in the SWC's strength resembles that of “⑦ Ocean currents at the Soya Strait” with the SWC weakening in winter and another weak southward current, possibly the East Sakhalin Current (ESC), appearing offshore (Figure B). To investigate the origin and formation factors of the CWB, we set the strait at the center of the model domain and conducted a tracer experiment that drove both the SWC and diurnal tides using the temporal changes in sea level differences at the east-west open boundaries. In this model, Japan Sea-origin water (mJSW) is highlighted in red, Okhotsk Sea-origin water (mOSW) in blue, and Japan Sea Intermediate-origin water (mJSIW) in green. The SWC (Figure C left) captured in the shelf area and moving southward is reproduced, accompanied by several anti-clockwise eddies driven by diurnal tides (the shape of rearward-breaking mJSW) on its offshore side (Figure C right). The central part of each eddy incorporates mJSIW (Figure D right) upwelled from the intermediate water of the Japan Sea, which is then advected by the southward SWC in the same eddy shape. We think that the distribution of mJSIW in Figure D left resembles the band-like distribution of CWB in Figure A. Although there is insufficient observational evidence, this model result suggests the possibility that CWB originates from the Japan Sea Intermediate Water.
阿部祥子・磯田 豊・矢幅 寛（2009）春季の宗谷暖流沖合域に形成される亜表層反流．海の研究，18(4)，265-286.
飯田博之・磯田豊・小林直人・堀尾一樹（2018）宗谷暖流沖合域の冷水帯を伴った日周期渦流の観測とモデル実験．海の研究，27(4)，155－174.
⑦ Ocean currents in the shelf-basin boundary region on the Okhotsk Sea side (Fig. 4-7)
The Soya Warm Current (SWC) has the property of being trapped on the shelf slope, but its shelf suddenly brokens off the Abashiri, and then connects to a deep basin (see the bottom topography in Figure C). Based on the seasonal changes in seawater classified in the TS diagram (Figure A), we can estimate the ocean currents in the shelf-basin boundary region. The seawater associated with the two representative ocean currents was the warm, high-salinity Soya Warm Current SCW (orange) and the cold, low-salinity East Sakhalin Current ESCW (green). In addition to these two, ICW (blue), which is melt-water below 1°C; DSCW (pink), which is slightly heavier than SCW and originates from the same Japan Sea Intermediate Water as CWB. Looking at the horizontal distribution in Figure B, ESCW dominates from November to January, ICW appears during the sea ice period from February to April, and remains offshore after May. At this time, DSCW, which is heavier than SCW, begins to appear (therefore, it is also called the “preformed water of Soya Warm Current”). The remaining period from May to October is the season when SCW dominates. Therefore, when conducting the ship observations in summer (Figure C), in the shelf area ICW is located in the offshore middle layer, and DSCW is located in the offshore bottom layer at a depth of about 150 m (Figure D upper). However, in the basin area, heavy DSCW sinks rapidly to a depth of 500 m or more, and then DSCW is located below SCW. (Figure D, lower panel). In model experiments (not shown here), this boundary region reproduces the transition of the SWC from a shelf slope trapped flow (physically described as a barotropic topographic Rossby wave) to a coastal tapped density flow (as a baroclinic internal Kelvin wave).
千葉 彩・堀尾一樹・磯田 豊・小林直人（2021） 網走沖の陸棚-海盆境界域におけるdense Soya Current Waterの輸送と変質過程．海の研究，30(2)，15－46.
⑧ Ocean currents in the Nemuro Strait (Figure 4-8)
Similar to the Soya Strait, half of the Nemuro Strait is Russian territorial area, so there are no hydrographic observation points. The only data available is the flow velocity measured by the Japan Coast Guard (probably from surveillance vessels) using GEK or ADCP. However, the amount of accumulated data is not enough. Therefore, we divided the area from the Shiretoko Peninsula via the Nemuro Strait to the exit of the Goyoumae Channel into 18 small grids (Figure A left) and calculated the monthly averaged velocities within each grid (Figure A center). Based on these results and some previous studies, we attempted to create a schematic diagram of the current flow (Figure A right). Although the reliability is very low, I believe it is possible to point out that (1) in the Goyoumae Channel, water flows out to the Pacific Ocean in winter and flows into the strait in summer, and (2) there are the bifurcated currents on the south side of the Shiretoko Peninsula in summer (① and ② in the schematic diagram). For confirmation, we conducted hydrographic observation in summer around the Shiretoko Peninsula by ship. A very strong northward flow was detected on the northern side of the peninsula, and a weak northward flow, believed to be one of the two bifurcated flows, was detected on the southern side (left two images in Figure B). When examining the cross-section across the tip of the peninsula, the high-temperature, high-salinity Soya Warm Current (SWCW) is only located in the northern surface layer, and the strong northward flow is confirmed to be the Soya Warm Current (SWC) (right two images in Figure B).
Research on the Nemuro Strait is always complicated by the Northern Territories issue. In the future, the regular observations within the Japanese territorial area will be conducted, and we hope that more information than what is shown in the schematic diagram (Figure A, right) can be obtained.
森 文洋・磯田 豊・阿部祥子・小林直人・矢幅 寛・磯貝安洋（2010）根室海峡における表層流の季節変化．海の研究，19(2)，89-110.
ISODA Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
27 August 2025 posted
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