KAWAI, K., TERADA, R. and KUWAHARA, S. (eds): The Islands of Kagoshima Kagoshima University Research Center for the Pacific Islands, 15 March 2013 Chapter 20 The Kuroshio - Its Physical Aspect and Roles in Kagoshima’s Nature and Culture - Hirohiko NAKAMURA 1. Introduction The author wrote this article to provide general he Kuroshio is one of the largest currents in and useful information about the Kuroshio to per- Tthe oceans of the world, and flows northward sons without any previous knowledge of the Kuro- along the western boundary of the North Pacific shio, such as visitors from foreign countries. This Ocean. Along its path, the Kuroshio flows through first article provides general explanations of the the coastal seas of Kagoshima Prefecture, which Kuroshio from the physical oceanographic point are located at the most upstream stretch of the cur- of view, and then describes regional features of the rent as it passes through the waters south of the current in the seas of Kagoshima, such as the spa- main islands of Japan. As a result, numerous as- tial patterns, temporal variations, water properties pects of Kagoshima’s culture and natural environ- and water colors, before finally focusing on some ment are closely related to the Kuroshio. For ex- effects of the Kuroshio on Kagoshima’s natural ample, coral reef habitats extend northward along environment, such as the region’s ecosystem, fish- the Kuroshio’s path as far as the small islands south ing industries, and regional climate. A number of of Kyushu, where the subtropical seawater it trans- historical events that connect the region with the ports from the southern latitudes is warm enough Kuroshio are added as topics in each section. to nurture such reefs, even in the wintertime. The Kuroshio also sustains Kagoshima’s ac- 2. Topographic features of Kagoshima’s seas tive commercial fishing industry. This is because agoshima Prefecture is composed of a main- the warm current transports larvae and fry of nu- Kland area corresponding to the southernmost merous fish species (e.g., Japanese mackerel, Jap- part of Kyushu, Japan’s third largest island, and anese spiny lobster and Japanese eel) from their several other islands that make up the northern upstream spawning areas into the hospitable nurs- part of the Nanseishoto Island chain (Fig. 1a). The ery and fishing grounds that are associated with western coast of the mainland area faces the north- complex bottom topography present in the coastal ernmost stretch of the Okinawa Trough, which is seas of Kagoshima. As for the cultural aspects of a deep basin (in some cases deeper than 2,000 m) Kagoshima, the Kuroshio is responsible for guid- that follows the continental shelf in the East China ing a multitude of foreigners to the shores of the Sea. The Okinawa Trough and the North Pacific region in previous eras, some of which changed Ocean (with depths more than 5,000 m) are sepa- history by planting foreign cultural influences in rated by the Nanseishoto Island chain, which rides Japanese soil. a north-south extending ridge with three deep fis- To give just one example, it is well known that sures (Fig. 1b). These are the Tokara Strait south of firearms were introduced to Japan in 1543 by Por- Kyushu, the Kerama Gap south of Okinawa Island, tuguese traders whose ship had drifted off course and the Yonaguni-Taiwan Strait (this name is used while en route to China - eventually arriving at only for convenience in this text). Tanegashima Is. south of Kyushu because of the Waters within the upper 1,000 m from the sur- influence of the Kuroshio. There are many other face flow freely between the two basins through instances in which the Kuroshio plays an impor- those three channels. Therefore, the currents and tant role in Kagoshima’s nature and culture. water properties that mark the coastal seas of Ka- 118 The Islands of Kagoshima goshima are a regional element of the large-scale 3. The Kuroshio as part of global ocean cir- oceanic circulation system that makes up the North culations Pacific Ocean. Because of this, the upper-ocean ig. 2 shows an overview of the major surface circulation system that governs all of the World Fcurrents over the World Ocean, which is co- Ocean, not only the North Pacific Ocean, will be herent to a depth of about 1,000 m. You may be considered in the next section. The World Ocean surprised to learn that the circulation patterns for is used in this text as a concise word to express the the Pacific, Atlantic, and Indian Oceans are very oceans of the world, such as the Atlantic, Pacific similar to each other. For example similar circula- and Indian Oceans. tions are present over the mid-latitude oceans in (a) 33 East China Sea Ohsumi St. Kyushu Tokara St. (N) 29 R Okinawa Trough latitude Taiwan-Yonaguni St. Q a 0P00 25 4 Philippine Se Kerama Gap Taiwan O 120 124 128 132 longitude (E) Nanseishoto Islands O: Yonagunijima Is. P: Okinawa Is. Q: Amami-Oshima Is. R: Tanegashima Is. (b) Ohsumi St. Tokara St. Taiwan-Yonaguni St. Kerama Gap Fig. 1. (a) Bathymetry of the seas around the Nanseishoto Island chain. The bold gray line denotes the Kuroshio path. (b) Cross section of the Nanseishoto Island chain from Taiwan (on the left) to Kyushu, Japan (on the right) (Image courtesy of Jae-Hun PARK). 119 Kagoshima University Research Center for the Pacific Islands both the Northern and Southern Hemispheres - section over the central Pacific Ocean, provides travelling clockwise in the north and counterclock- us with information on oceanic motions under the wise in the south. In oceanographic terms, these sea surface. Most of the ocean floor is deeper than mid-latitude, large-scale circulations are known 5,000 m (the mean depth of the World Ocean is as subtropical gyres. The Kuroshio is a northward about 3,800 m), and most such deep-sea ocean component of the North Pacific subtropical gyre, water is 5 ˚C or less. The warmer water extend- while the Gulf Stream is a North Atlantic counter- ing from the surface to a depth of about 1,000 m, part of the Kuroshio. The westward intensification known as the thermocline in oceanographic terms, of the currents is a distinctive characteristic of all rides above the colder deep water in the low- and subtropical gyres; this feature is due to the fact that mid-latitude oceans. The subtropical gyre, includ- the “center” of this type of circulation is actually ing the strong western boundary current, is the located at its western edge. Thus, the Kuroshio result of thermocline water being pushed in a hori- and Gulf Stream, which are located at the western zontal direction by the force of prevailing winds all boundaries of their respective regions, are much over the earth. In contrast, the colder deep-water stronger than the California Current and Cannery motion is governed by another mechanism, re- Current, which are located at the eastern bound- ferred to as thermohaline circulation. aries of their respective regions. In oceanographic The winds driving the upper-ocean circula- terms, the former are generally called western tions prevailing over the mid-latitudes are known boundary currents and latter are known as east- as the Westerlies, while the Trade winds - also re- ern boundary currents. In both the Northern and ferred to as Easterlies - prevail over the low-lati- Southern Hemispheres, the western boundary cur- tudes (Fig. 4a). The wind patterns over the Pacific, rents carry warm water poleward while cold high- Atlantic and Indian Oceans are the roughly similar, latitude waters flow eqatorward along the eastern although directions are reversed for the Northern boundary currents. and Southern Hemispheres. Winds, therefore, Fig. 3, which shows a meridional temperature drive the surface water with the same manner in Fig. 2. A schematic view of the major surface currents over the World Ocean extracted from the homep- age of the Science Education through Earth Observation for High Schools (SEOS) Project (http://www. seos-project.eu/home.html). 120 The Islands of Kagoshima Fig. 3. A meridional temperature section over the central Pacific Ocean. This data is based on an objec- tively analyzed (1˚ grid) climatological field of in situ temperatures from the World Ocean Atlas 2001 (WOA01). each ocean. This is the reason why the upper-ocean which is recognized as a strip about 100 km wide, circulation patterns are very similar for all oceans. originates off the eastern coast of Luzon Island in In both the Northern and Southern Hemi- the Philippines (15˚-18˚ N). From there, it flows spheres, as a result of wind-forced motion, sur- northeastward along the continental slopes east face water accumulates in the central area of the of Taiwan and in the East China Sea, then along subtropical gyres, causing the sea level to become the Japanese main islands of Kyushu and Shikoku, higher in those areas than the surrounding seas before arriving near the Bōsō Peninsula in mid- (Fig. 4b). Here, a scientific question arises; Why Honshu (~35˚ N) where it finally leaves the coastal does the surface water rotate clockwise in the area and veers east toward the International Date subtropical gyre of the Northern Hemisphere and Line in the central North Pacific. counterclockwise in the Southern Hemisphere? Some aspects of the Kuroshio are inherently This is initially counter-intuitive because the water unstable. For example, the current itself tends to normally flows from a high to a low sea-level area. meander over its entire length; emanating meso- The answer is found in a form of motion called a scale eddies with horizontal scales from several 10 “geostrophic current,” in which the pressure gradi- to several 100 km as it travels.