Ryukyu Currents, Gaps, & Winds

From Maritime Ryukyu, 1050–1650, by Gregory Smits (University of Hawaii Press, 2018), Kindle pp. 34-35:

Stressing the interconnectedness of exchange networks within the East Asian region during approximately the fourteenth century, historian Angela Schottenhammer points out: “The centers of this exchange doubtlessly lay in China, Japan, and Korea. But also smaller countries and regions in the north and south of the China Sea, such as the Ryūkyūs or even traders from an island as small as Tsushima, participated and were integrated into this supra-regional system. Its initiators were often private organizations and merchants who sought to maintain and cherish their contacts even under politically unfavorable conditions.” Maritime routes connected the nodes within the network. Several factors influenced movement around the East China Sea, including currents, winds, and landforms.

The Kuroshio is a strong current flowing northward between Taiwan and Yonaguni and continuing northward to the west of the Ryukyu islands. Northwest of Amami-Ōshima, the Kuroshio turns eastward and flows through the sea between the Tokara islands and Amami-Ōshima, an area known as the Shichitō-nada ['seven-island rough-sea']. In other words, the Kuroshio forms a natural barrier between the Tokara and the northern Ryukyu islands. Its flow created dangerous conditions for shipping, and it defines a biological barrier with substantially different flora and fauna on either side of it. The Shichitō-nada also marks a cultural boundary, albeit a permeable one that people could and did cross. This boundary divides Ryukyuan languages and the Kyushu dialects of Japanese. In 1893, when Sasamori Gisuke sailed from Kagoshima to visit the Ryukyu islands, he was struck by the terrifying power of the current in the Shichitō-nada. A sailor explained to Sasamori that if a typical Japanese-style sailing vessel encountered the current, it might be swept far off course into the Pacific. The Kuroshio surging through the seas around the southern Tokara islands is one reason mariners from that area became especially prized as pilots throughout the network. Likewise, the Kuroshio serves as a marine barrier between Yonaguni and both Taiwan and the southeast coast of China.

Between Okinawa and Miyako, the expanse of sea known as the Kerama gap functioned as a barrier to travel because crossing it required navigation without reference to visible landforms. Despite the significance of these natural obstacles, properly equipped and piloted vessels regularly overcame them. Sailing from Kikai to the port of Naha in Okinawa could be done entirely within sight of land on clear days. Therefore, relatively small ships could travel this route without advanced navigational skills. To sail from Amami-Ōshima to Tokara, or from Okinawa to Sakishima, or to the China coast, on the other hand, required superior ships, knowledge, and skill. The wind was the main driving force for vessels plying routes around the East China Sea. Storms, of course, could be disruptive, but generally wind patterns were predictable. The winds in the region changed during approximately the third and ninth lunar months.