Rapid freshening of Japan Sea Intermediate Water in the 2010s

Japan Sea Intermediate Water (JSIW) is in the salinity-minimum layer present between the thermocline and deep water in the Japan Sea. Annual shipboard measurements over the past 20 years revealed that JSIW rapidly freshened in the 2010s, particularly from 2010 to 2011, from 2012 to 2013, and from 2015 to 2016. The mean rate of salinity change at σ θ = 27.16 kg m −3 ( σ θ indicates potential density minus 1000 kg m -3 ) in JSIW was − 0.078 ± 0.013 per decade (mean ± standard error) from 2009 to 2016. This rate is more than twice of those observed in intermediate waters in open oceans. The Polar Front in the Japan Sea, the boundary between the subtropical and subpolar waters, considered the JSIW formation region. Using a biogeochemical tracer PO = [O 2 ] + [PO 4 ] × 135, it was determined that the contribution of less saline water from the south of the Polar Front increased in JSIW in the 2010s. The atmospheric and oceanic reanalysis data indicated that a large freshwater influx from the East China Sea to the Tsushima Strait in the summers contributed to the freshening in the south of the Polar Front in 2010 and 2012, whereas, a large local precipitation in the autumn freshened in the region in 2015.