Rare northward flow in the Taiwan Strait in winter: a note

Abstract

Conventional wisdom is that, in winter, the China Coastal Current flows southward on the western part of the Taiwan Strait, but on the eastern part of the Strait, a branch of the Kuroshio flows northward. The net flux is subject to debate, but it is believed to be small. This author opted to choose a northward flux of 0.2 Sv. Others believe that there is zero net flux, or the net flow may even be towards the south. Thus, it is very surprising to find a net northward flux as much as 2.74 Sv in early March 1997 (reported). Historical data are presented here to show that the salinity, temperature and nutrient concentrations are relatively homogeneous in the Taiwan Strait, in summer. The high salinity and temperature, but low nutrients, suggest that the water originates from the South China Sea (SCS) or the Kuroshio. In winter, on the other hand, steady and strong NE winds push the fresh, cold, nutrient-rich China coastal water southward, along the western part of the Taiwan Strait. Some salty and warm, but still nutrient-poor, SCS or Kuroshio water flows northward on the eastern part of the Strait. The present author concludes that the observed northward flux in early 1997 was caused by a sudden relaxation of the steady NE wind. As a result, any seawater that would have moved northward, but had been halted for several months by the NE wind, moved suddenly northward. Indeed, rarely occurring southerly wind in winter was observed briefly during the research cruise. Other physicochemical parameters of the waters suggest also that the observed northward-flowing water had originated mainly from the southward-flowing China Coastal Current, rather than the SCS or the Kuroshio. The physicochemical data support also the concept that the western part of Taiwan Strait was occupied by the China Coastal Current, which normally flows southward in winter.