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[AOS17-P01] Deoxygenation and shallow shift of oxygen minimum layer in the Japan Sea Proper Water
Keywords:deoxygenation, Japan Sea Proper Water, long term change
The Japan Sea is a semi-closed deep sea located in the northwestern margin of the North Pacific. Its deep layers below about 300 m is filled with a quite homogeneous water called Japan Sea Proper Water (JSPW). From vertical profiles of water properties, the JSPW is classified into three distinctive layers such as the Upper JSPW (UJSPW), the Deep Water and the Bottom Water. A gradual warming and deoxygenation in the JSPW have been observed since 1950s. These changes have been considered to be associated with the weakened cooling and ventilation in winter due to climate change in the northwestern region of the Japan Sea where the JSPW is formed. We have reported decrease trend of dissolved oxygen (O2) in the Japan Basin (JB) and in the Yamato Basin (YB) using time-series hydrographic observations in 2010–2016 acquired by the Japan Meteorological Agency. We also reported that the depth of oxygen minimum layer was shifted from 2000m to 1000m in the period. However, the cause has not been clarified. In this study, we evaluate the long-term change in the JSPW using time series data from 2010 to 2020 and elucidate the mechanism of shallower shift of oxygen minimum layer.
In the Bottom Water in JB below 2500 m where the water is vertically uniform, increasing trend in potential temperature and decreasing trend in dissolved O2 were determined at almost the same rate vertically with a mean rate of +0.0016 °C/yr and –0.55 μmol/kg/yr, respectively. The decrease of O2 accompanied the increase of nitrate, thereby they are attributed to temporal accumulation of remineralization in this water. It suggests that the supply of “fresh” bottom water has been decreased. The similar trends were also determined in the Bottom Water in YB.
The largest O2 decreases in the JB was observed in the UJSPW at 600 m (–2.99 μmol/kg/yr) where the vertical gradients of O2 are large. This depth corresponds to a High Salinity Intermediate Water (HSIW) observed in the lower UJSPW (S > 34.07, σθ=27.32 kg/m3). Based on the observation in the past 11 years, salinity of HSIW has been freshening. This suggest that the formation of HSIW was weakened and then concentration of O2 in HSIW was decreased. On the other hand, the largest O2 decrease in the YB was also observed in the UJSPW although the rate was much smaller than that in the JB (–1.32 μmol/kg/yr). Because the HSIW is mainly distributed in the eastern JB, the difference in the decreasing rate would be attributed to the distribution of HSIW.
These results suggest that the controlling factors of decreasing dissolved O2 are different in the Bottom Water and UJSPW. Because of difference in their changing rates, the oxygen minimum layer appears to be shifted to shallower layer in the period.
Figure caption
Linear rates of changes in (a) potential temperature, (b) dissolved O2, (c) nitrate, and (d) silicate in the Japan Sea. Blue and red lines show the trends in the Japan Basin and the Yamato Basin, respectively. Thin lines show confidence intervals of 95%.
In the Bottom Water in JB below 2500 m where the water is vertically uniform, increasing trend in potential temperature and decreasing trend in dissolved O2 were determined at almost the same rate vertically with a mean rate of +0.0016 °C/yr and –0.55 μmol/kg/yr, respectively. The decrease of O2 accompanied the increase of nitrate, thereby they are attributed to temporal accumulation of remineralization in this water. It suggests that the supply of “fresh” bottom water has been decreased. The similar trends were also determined in the Bottom Water in YB.
The largest O2 decreases in the JB was observed in the UJSPW at 600 m (–2.99 μmol/kg/yr) where the vertical gradients of O2 are large. This depth corresponds to a High Salinity Intermediate Water (HSIW) observed in the lower UJSPW (S > 34.07, σθ=27.32 kg/m3). Based on the observation in the past 11 years, salinity of HSIW has been freshening. This suggest that the formation of HSIW was weakened and then concentration of O2 in HSIW was decreased. On the other hand, the largest O2 decrease in the YB was also observed in the UJSPW although the rate was much smaller than that in the JB (–1.32 μmol/kg/yr). Because the HSIW is mainly distributed in the eastern JB, the difference in the decreasing rate would be attributed to the distribution of HSIW.
These results suggest that the controlling factors of decreasing dissolved O2 are different in the Bottom Water and UJSPW. Because of difference in their changing rates, the oxygen minimum layer appears to be shifted to shallower layer in the period.
Figure caption
Linear rates of changes in (a) potential temperature, (b) dissolved O2, (c) nitrate, and (d) silicate in the Japan Sea. Blue and red lines show the trends in the Japan Basin and the Yamato Basin, respectively. Thin lines show confidence intervals of 95%.