Interannual variation of the Polar Front in the Japan/East Sea from summertime hydrography and sea level data Byoung-Ju Choi a, ⁎, Dale B. Haidvogel a , Yang-Ki Cho b a Institute of Marine and Coastal Sciences, Rutgers—the State University of New Jersey, New Brunswick, New Jersey 08901-8521, USA b Department of Oceanography, Chonnam National University, Kwangju, 500-757, Republic of Korea abstract article info Article history: Received 17 October 2006 Received in revised form 23 May 2008 Accepted 7 November 2008 Available online 20 February 2009 Regional index terms: Northwestern Pacific Ocean Japan/East Sea 126°E–142°E 34°N–52°N Keywords: Polar Front Hydrographic data Altimeter data Numerical model Interannual variation Japan/East Sea The Polar Front in the Japan/East Sea separates the southern warm water region from the northern cold water region. A merged TOPEX/POSEIDON and ERS-1/2 altimeter dataset and upper water temperature data were used to determine the frontal location and to examine the structure of its interannual variability from 1993 to 2001. The identified frontal location, where sea surface height gradient has a maximum about 10–20 cm over the horizontal distance of 100 km, corresponds well to the maximum subsurface horizontal temperature gradient. The front migrates more widely (36°N–41°N) in the western part of the sea than in the eastern part. The interannual migration induces large variability in upper water temperatures and sea surface height in the western region. Responsible physical mechanisms were studied using a reduced-gravity model. Differences between inflow and outflow change the total volume of warm water, and total warm water volume change in the warm water region uniformly pushes the front in the meridional direction across its mean position in the model simulation. Interannual variation of wind stress causes relatively wide migration of the modeled front in the western part. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The Polar Front marks an important climate boundary in terms of both water mass formation and air–sea fluxes. Accordingly, variation of the Polar Front in the Southern Ocean and the North Pacific Ocean has been studied extensively (Roden et al., 1982; Moore et al., 1999). The Polar Front of the North Pacific extends from 57°N in the Gulf of Alaska to 40°N off Japan. The front is relatively stable, except along 170°E, where it shifts north–south by 400 km every 6 years (Belkin et al., 2002). The western end of the Polar Front in the North Pacific extends into the Japan/East Sea, and is located between 38°N and 40°N (Tomczak and Godfrey, 1994; Belkin and Cornillon, 2003). The Polar Front in the Japan/East Sea is often referred to as the subpolar front in other recent studies (Park et al., 2004; Talley et al.; 2006). The Japan/East Sea is a marginal sea of the North Pacific Ocean (Fig. 1a). Its dimensions are about 1600 × 900 km and the mean depth is about 1350 m. It communicates with the East China and Yellow Seas to the south, with the Pacific Ocean to the east, and with the Sea of Okhotsk to the north. Water is exchanged through narrow channels with sill depths not exceeding 135 m. Warm and saline water enters from the Kuroshio, flows through the southern part of the sea, and exits to the subpolar gyre with a throughflow transport of about 2.5 × 10 6 m 3 /s (Sv). The warm water occupies the depth range of 0– 200 m and is characterized by a shallow salinity maximum at about 50 m depth in the warm southern Japan/East Sea (Talley et al., 2006). Below 200 m, the water is remarkably uniform with temperature of 0– 1 °C and salinity of 34.1 (Tomczak and Godfrey, 1994; Preller and Hogan, 1998). The Polar Front separates the southern warm water from the northern cold water in the Japan/East Sea. In Fig. 1a, solid lines with arrows indicate surface currents in the southern warm water region and dashed lines denote those in the northern cold water region. The dotted line is an approximate position of the Polar Front. A line of open circles represents hydrographic stations where upper layer tempera- ture data were obtained to identify vertical structure of the upper water across the Polar Front in July 1993. The vertical section of upper ocean temperature along the north–south transect shows the structure of the Polar Front (Fig. 1b). Due to summer heating, the upper 50-m layer is highly stratified and the front submerges below the surface. There is a strong subsurface horizontal temperature gradient at depths of 50–150 m across 39.2°N. Horizontal maps of the Journal of Marine Systems 78 (2009) 351–362 ⁎ Corresponding author. Present address: Department of Oceanography, Kunsan National University, San 58, Miryong-dong, Gunsan, Jeollabuk-do 573-701, Republic of Korea. Tel.: +82 63 469 4607; fax: +82 63 469 4990. E-mail address: bjchoi@kunsan.ac.kr (B.-J. Choi). 0924-7963/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2008.11.021 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys