The Annual Cycle of the Japan

The mechanism responsible for the annual cycle of the flow through the straits of the Japan Sea is investigated using a two-layer model. Observations show maximum throughflow from summer to fall and minimum in winter, occurring synchronously at the three major straits: Tsushima, Tsugaru, and Soya Straits. This study finds the subpolar winds located to the north of Japan as the leading forcing agent, which first affects the Soya Strait rather than the Tsushima or Tsugaru Straits. The subpolar winds generate baroclinic Kelvin waves along the coastlines of the subpolar gyre, affect the sea surface height at the Soya Strait, and modify the flow through the strait. This causes barotropic adjustment to occur inside the Japan Sea and thus affect the flow at the Tsugaru and Tsushima Straits almost synchronously. The barotropic adjustment mechanism explains well why the observations show a similar annual cycle at the three straits. The annual cycle at the Tsugaru Strait is further shown to be weaker than that in the other two straits based on frictional balance around islands, that is, frictional stresses exerted around an island integrate to zero. In the Tsugaru Strait, the flows induced by the frictional integrals around the northern (Hokkaido) and southern (Honshu) islands are in opposite directions and tend to cancel out. Frictional balance also suggests that the annual cycle at the Tsugaru Strait is likely in phase with that at the Soya Strait because the length scale of the northern island is much shorter than that of the southern island. 1. The Japan Sea and the North Pacific The Japan Sea is a marginal sea located in the western North Pacific. It is deep (about 1500m) but semienclosed from the North Pacific by the Japanese Archipelago. What makes this sea unique among the various marginal seas located in the western North Pacific is that the four straits that connect the Japan Sea to its surrounding seas, the Tsushima, Tsugaru, Soya, and Tartar Straits (Fig. 1), are only 150m deep or shallower. The Japan Sea is therefore influenced by the in/outflows through these straits near the surface, while the deeper part of the basin is separated from other seas. Observations show that about 2–3 Sverdrups (Sv; 1Sv[ 10m s) of watermass exchange occurs across the straits on annual average [see Talley et al. (2006) and reference therein]. Although this transport magnitude is significantly less than that of the Corresponding author address: Shinichiro Kida, Application Laboratory, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan. E-mail: kidas@jamstec.go. jp JANUARY 2016 K IDA ET AL . 23 DOI: 10.1175/JPO-D-15-0075.1 2016 American Meteorological Society Kuroshio (about 40–50Sv along the southern coast of Japan; e.g., Imawaki et al. 2001), the exchange flows play a critical role in determining the upper oceanic circulation and sea surface temperature (SST) in the Japan Sea. Recent studies show that the SST variability affects not only the local atmospheric conditions (Hirose and Fukudome 2006) but also the East Asian monsoonal climate (Yamamoto and Hirose 2011; Seo et al. 2014). The upper oceanic circulation in the Japan Sea consists of an inflow from the south (Tsushima Strait) and outflows to the northeast (Tsugaru and Soya Straits). The inflow is known as the Tsushima Warm Current, and its annual-mean transport, according to long-term measurements from bottom-mounted ADCP (Teague et al. 2002) and ship-mounted ADCP (Isobe et al. 2002; Takikawa et al. 2005), is about 2.4–2.7 Sv. The Tsushima Warm Current bifurcates to a flow along the coast of Japan and a flow along the Korean Peninsula, which then crosses the Japan Sea zonally at about 408N (Talley et al. 2006). The outflows are known as the Tsugaru trait Warm Current and Soya Strait Warm Current. Geostrophic transport estimates from hydrography suggest the annual-mean transport of the Tsugaru Strait Warm Current to be about 1.5 Sv (Onishi and Ohtani 1997), which is supported by ADCP measurements (Shikama 1994; Nishida et al. 2003). Observations at the Soya Strait have been limited, but recent year-long bottommounted ADCP measurements reveal an annual-mean transport of about 1.0Sv (Fukamachi et al. 2008). The flow through the Tartar Strait is considered negligible because of its narrowwidth (about 7km) and shallow depth (about 4m) (Fukamachi et al. 2008). We will refer to this system of flows that occur across the straits as the Japan Sea Throughflow or simply Throughflow hereinafter (Fig. 1). a. The observed annual cycle Thebasicmechanism responsible for driving the annualmean transport of the Throughflow has been intensively studied (e.g., Minato and Kimura 1980; Ohshima 1994; Tsujino et al. 2008) and most of these studies suggest the annual-mean wind stress over the North Pacific to be responsible. However, the mechanism responsible for driving the annual cycle of the Throughflow transport remained an open question partly because observations have been insufficient to describe seasonal changes at all three straits, especially at the Soya Strait (Fukamachi et al. 2008). Among the three straits, the Tsushima Strait is the most frequently observed [see Teague et al. (2006) and reference therein]. Past direct measurements reveal an annual cycle at the Tsushima Strait with a maximum FIG. 1. The topography and straits of the JapanSea.Theblack solid arrows show thedirection of the annual-meanThroughflowat the straits. The dashed arrows indicate the pathways of thewaves that enter the Tsushima Strait from the subpolar region (blue) and the subtropical region (red). 24 JOURNAL OF PHYS ICAL OCEANOGRAPHY VOLUME 46