Transport and modification processes of dense shelf water revealed by long‐term moorings off Sakhalin in the Sea of Okhotsk

Abstract

[1] The region off the east coast of Sakhalin is thought of as an important pathway of dense shelf water (DSW) from its production region in the northwestern Okhotsk Sea to the southern Okhotsk Sea. From July 1998 to June 2000, the first long-term mooring experiment was carried out in this region to observe the southward flowing East Sakhalin Current (ESC) and DSW. Moored and associated hydrographic data show considerable modification of cold dense water via mixing with warm offshore water in the slope region off northern Sakhalin. Significant onshore eddy heat flux was observed at the northernmost mooring (54.9°N), which suggests the occurrence of baroclinic instability. The eddy heat flux was not significant farther south. At moorings along 53°N, cold anticyclonic eddies were identified that were consistent with isolated eddies seen in the hydrographic data. The three years of hydrographic data also showed large differences in extent and properties of DSW. Furthermore, the mooring data show that seasonal variability of DSW was quite different in the two years. The average DSW transport for s q > 26.7 evaluated using the moored data at 53°N for 1 year (1998)(1999) was $0.21 Sv (= 10 6 m 3 s À1 ). This value is at the lower end of the previous indirect estimates. Along with the DSW modification, this transport estimate indicates that DSW was not only carried southward by the ESC but was spread offshore by eddies off northern Sakhalin.

Tracing dense shelf water in the Sea of Okhotsk with an ocean general circulation model

“…Simizu and Ohshima, 2006). The concentration is rapidly decreasing as the tracer flows southward along Sakhalin, consistent with observational results (Yamamoto et al, 2002;Fukamachi et al, 2004). The ESC turns eastward around 47°N and therefore the tracer extends eastward along 47°N.…”

“…Associated with the ice formation, cold and saline dense shelf water (DSW) is formed there in winter as extremely saline water, called brine, is rejected from sea ice (Kitani, 1973;Shcherbina et al, 2003). Then, DSW outflows from the shelf, and moves southward along Sakhalin to the Kuril Basin, mixing with surrounding water (Yamamoto et al, 2002;Fukamachi et al, 2004;YamamotoKawai et al, 2004). In the Kuril Basin, it mixes further with other waters originating from the Japan Sea and the Pacific.…”

Direct observations of sea-ice thickness and brine rejection off Sakhalin in the Sea of Okhotsk

“…and M3 over the slope (Fukamachi et al, 2004). The current estimates over the shelf west of the midpoint are ∼13 and 5% of the previous estimates over the slope during January-March and annually, respectively.…”

“…In the Sea of Okhotsk, there have been direct timeseries observations of currents and water properties in the northwest shelf polynya (Shcherbina et al, 2003(Shcherbina et al, , 2004a, which is considered to be a main DSW formation site (Martin et al, 1998;Gladyshev et al, 2000;Ohshima et al, 2003), and across the DSW pathway off northern Sakhalin (Fukamachi et al, 2004). However, there is no available dataset of ice thickness in the northern part, where ice forms most actively.…”

Causes of the Multidecadal-Scale Warming of the Intermediate Water in the Okhotsk Sea and Western Subarctic North Pacific

“…A large amount of sea ice within the Okhotsk Sea is produced along the northwestern coastal polynyas by severe wintertime winds blowing from the Eurasian continent (Martin et al 1998;Ohshima et al 2003;Nihashi et al 2009;Kawaguchi et al 2010), and the brine rejection associated with sea ice production leads to vertical convection of up to 27.0s u [s u is defined as potential density 2 1000 (kg m 23 ); thus, the density of seawater 1027.0 kg m 23 means 27.0s u ], resulting in the formation of cold, oxygen-rich, dense shelf water (DSW) (Kitani 1973;Gladyshev et al 2000;Shcherbina et al 2003;Nakamura et al 2006;Matsuda et al 2009). DSW is transported southward by the cyclonic ocean circulation in the Okhotsk Sea Fukamachi et al 2004), and it then mixes with the warm, saline water originating from the North Pacific. The resulting mixed water is called Okhotsk Sea Mode Water (Yasuda 1997) or Okhotsk Sea Intermediate Water (OSIW; Itoh et al 2003).…”