Abstract
Abstract. An ocean buoy, UBIM (Ulleung Basin Integrated Mooring), deployed during the spring transition from February to May 2010 reveals for the first time highly resolved temporal variation of biochemical properties of the upper layer of the Ulleung Basin in the southwestern East Sea/Sea of Japan. The time-series measurement captured the onset of subsurface spring bloom at 30 m, and collocated temperature and current data gives an insight into a mechanism that triggers the onset of the spring bloom not documented so far. Low-frequency modulation of the mixed layer depth ranging from 10 m to 53 m during the entire mooring period is mainly determined by shoaling and deepening of isothermal depths depending on the placement of UBIM on the cold or warm side of the frontal jet. The occurrence of the spring bloom at 30 m is concomitant with the appearance of colder East Sea Intermediate Water at buoy UBIM, which results in subsurface cooling and shoaling of isotherms to the shallower depth levels during the bloom period than those that occurred during the pre-bloom period. Isolines of temperature-based NO3 are also shown to be uplifted during the bloom period. It is suggested that the springtime spreading of the East Sea Intermediate Water is one of the important factors that triggers the subsurface spring bloom below the mixed layer.
Highlights
The East Sea/Sea of Japan (East Sea hereafter) is a semienclosed deep marginal sea in the northwestern Pacific with mean water depth of about 1800 m (Fig. 1)
Mean chlorophyll fluorescence 3 (CF) values became doubled from 0.9 μg L−1 during the pre-bloom period to 1.9 μg L−1 during the bloom period
The fluctuations of isotherms at buoy UBIM arise from the placement of the mooring location between the cold side and warm side of the front due to the short-term frontal variability set by the northeastward flowing East Korea Warm Current (EKWC)
Summary
The East Sea/Sea of Japan (East Sea hereafter) is a semienclosed deep marginal sea in the northwestern Pacific with mean water depth of about 1800 m (Fig. 1). East Sea using the critical depth hypothesis (Kim et al, 2000; Yamada et al, 2004; Yamada and Ishizaka, 2006), where the mixed layer depth (MLD) is the crucial physical factor for the onset of spring bloom (Sverdrup, 1953). (Yoo and Kim, 2004) This argument, is contrary to model results which suggest the nutrient supply from the Tsushima Current through Korea Strait (Kim et al, 2013) The East Sea Intermediate Water (ESIW) characterized by a 23 is the main source of primary production in the UB (Onitlayer of shallow salinity minimum with a potential temper- suka et al, 2007). Son et al.: Physical cause of the onset of subsurface spring phytoplankton bloom bloom below the surface mixed layer, which, so far to our knowledge, has not been documented
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