The Kuroshio flows along the southern coast of Japan, and the Kuroshio Extension eastward past Japan. These ocean currents never remain stationary, due to the ocean-atmosphere interactions such as El Niño - La Niña phenomena. This report addresses the effect of bottom topography on variations of these ocean currents, revealed by ADEOS.
The Kuroshio Current System generated by global winds
Ocean currents are driven by the distribution of the global wind systems, which are in turn affected by the ocean currents. In the Northern Pacific Ocean the trade winds blow easterly along the equator, and the westerlies blow over the latitudes of Japanese archipelago. These wind systems cause sea water to circulate clockwise. Since the deflecting force (Coriolis force) varies with latitude due to the Earths rotation, an ocean current is intensified at the western boundary along the continental slope, and concentrated within about 100 kilometers in width. This is called a western boundary current, the best known of which is the Kuroshio Current System in the North Pacific. Another North Pacific current, the Oyashio Current System, flows counterclockwise to the north of the westerlies.
Unusual behavior of the Kuroshio/Kuroshio Extension observed in April 1997
We observed an unusual behavior of the path of the Kuroshio and the Kuroshio Extension (which is the extension of the Kuroshio after separating from the mainland of Japan), from December 1996 to the summer of 1997, while ADEOS was operating effectively. This particular behavior included (1) a long-lasting very southerly path of the Kuroshio Extension; and (2) a Kuroshio path very distant from Japan with the following alternating-jet-like north-south flow pattern of the Kuroshio Extension at this event. This event occurred in February and April 1997. The event in April was particularly remarkable.
The above two aspects seemed independent of each other. It was recently revealed, however, that these phenomena were indicative of large-scale ocean-atmosphere interactions, and that the occurrence of the April 1997 event was related also to the local bottom topography with depths of several thousand meters.
Overlaying the Kuroshio paths on a bathymetric chart
Since the sea-surface temperature distributions obtained by ADEOS-OCTS indicate the very sea-surface skin temperatures within several tens of µm, which is extremely thin, the data doesnt necessarily correspond well to the flow pattern, because of short-term changes caused by solar radiation and the like. However, images of ocean color (chlorophyll-a) indicate the phytoplankton concentration of the upper ocean of the order of 30-meter thickness, thereby enabling us to see the state of the core (flow axis) of the Kuroshio Current System.
Figure 1 shows a chlorophyll-a image obtained by ADEOS-OCTS to the southeast of Japans mainland on April 26, 1997, overlaid by a detailed bottom topography chart. Deep blue portions in the image show the core of the Kuroshio Current System. From the image, it can be seen that the Kuroshio flows up north along the western slope of the north-south Izu-Ogasawara Ridge located at the south of Sagami Bay, and flows over the Ridge at 34°N around the 1,000-meter contour line. The Kuroshio then goes down south along the eastern slope of the Ridge, and turns north-northeast at approximately 32°N. After the Kuroshio Extension reaches the eastern flank of the Izu-Ogasawara Trench, it proceeds directly beyond 36°N along the eastern flank of the Trench. A bold pale-pinkish line indicates an 800-meter contour line.
The image also shows the existence of waters containing large amount of phytoplankton along the coast (red areas). The region is clearly portrayed along a 2,000-meter contour line (bold white line) to the south of the Boso Peninsula.
Figure 2 shows an image overlaid by the bottom contours at 100-meter intervals without any distinction of the depth. The image slightly magnifies a central portion of Figure 1 and changes the tone of the color. Ripple structures created by concentrated bottom contours indicate sea floor valleys or locations of larger bottom slopes.
In this image, it can be seen that the Kuroshio core turns north and northeast at approximately 33°N and 141.5°E, and then proceeds to north and further northeast at 34°N. We can see that the Kuroshio Extension is observed to flow along the ripple structures, that is, where there is a pronounced gradient in the seafloor.
Relationship between El Niño, La Niña, the Kuroshio and the Kuroshio Extension
It is known that the North Equatorial Current is weak during the period when La Niña is observed in western equatorial Pacific, and that the Kuroshio Extension shifts to the south with a time lag of about 1.5 years. The large-scale southern shift of the Kuroshio Extension in 1997 corresponded to the La Niña tendency, which was generated in the equatorial Pacific dating as far back as 1.5 years before 1997. In addition, the April 1997 event occurred 1.5 years after the extreme conditions of the La Niña tendency.
By using TOPEX/Poseidon altimeter data, which show the large-scale current distribution, we have found that the sea-surface gradient across the Kuroshio /Kuroshio Extension had diminished greatly from the west to the east sides of the Izu-Ogasawara Ridge, as a very temporary phenomenon that occurred just prior to the April 1997 event. From Figure 1, it is seen that the effect of bottom topography was disclosed at this weaker current conditions in the upper ocean.
It has recently been discovered that deep currents always exist along the Izu-Ogasawara Trench and the Japan Trench at depths of several thousand meters. These currents flow south and north along the western and the eastern flanks, respectively. The Kuroshio Extension flows usually independent of these deep currents (unaffected by the bottom topography), and take the eastward path at approximately 35°N, to the east of the Cape Inubosaki. However, as described above, it is thought that the April 1997 event was caused by disclosure of the effects of the deep currents and bottom topography when the effects of the previous La Niña tendency reached near the southern coast of Japan, thereby weakening the upper ocean current.
The detail of this article is to be published in the Journal of Oceanography as an original research paper by Dr. Yoshiaki Toba, Invited Eminent Scientist of EORC of NASDA and Professor Emeritus of Tohoku University, with five co-authors. Figures 1 and 2 are citations from this paper.