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[AOS18-05] The Distribution Process of North Pacific Subtropical Mode Water revealed from the Biogeochemical Variability
Keywords:Subtropical Mode Water, Dissolved inorganic carbon, Nutrient, Apparent Oxygen Utilization, Isopycnal surface, Decadal variability
1. Introduction
The North Pacific Subtropical Mode Water (STMW) is formed by convective mixing in winter and subsequent subduction in the region to the south of the Kuroshio extension (KE) and is distributed in the Kuroshio Recirculation. Based on the measurement data along the 137°E repeat line of Japan Meteorological Agency, recent studies have shown that the STMW formation volume and biogeochemical parameters such as dissolved inorganic carbon (DIC) and Apparent Oxygen Utilization (AOU) in the STMW varies on a decadal timescale in association with the KE state variability (Oka et al., 2015, 2019). In this study, we examine whether such biogeochemical variabilities are also seen throughout the STMW distribution region in the northwestern part of subtropical gyre including the 137°E line, and particularly focus on the difference in the variabilities between the east and the west in the region.
2. Data & Methods
We used physical and biogeochemical data such as potential temperature, salinity, DIC, phosphate, sum of nitrate and nitrite and AOU at 20°–30°N, 120°–180°E during 1992–2020 taken from GLODAP v2. These data were interpolated vertically at intervals of 0.1σθ for each station using Akima spline method (Akima, 1991) and were averaged yearly at 20°–30°N excluding the data in winter. Additionally, we divided the STMW distribution region at 120°E –180° into 4 regions from the west to the east.
3. Result & Discussion
In the entire regions of longitudinal bands at 120°–180°E, the physical and biogeochemical parameters on the isopycnal surface at σθ = 25.4 varied on a decadal timescale corresponding to the KE state variability (Figure). In addition, biogeochemical parameters were higher in the western region (e.g., AOU and sum of nitrate and nitrite in the region west of 130°E were higher than those in the region east of 160°E by approximately 20 μmol/kg and 2 μmol/kg, respectively). Moreover, we also found the time lag which the variabilities in potential temperature and salinity in the east region are earlier by 1 to 2 years. From the aspect of biogeochemical variabilities in STMW, considering that DIC, AOU and nutrients are relatively low in the newly formed STMW, our results demonstrate the process that STMW formed in the region to the south of KE is transported westward with the flow of Kuroshio recirculation over 1 to 2 years, while being influenced by biological oxygen consumption and remineralization.
Figure caption
Time-series of yearly averages and standard deviations of (a) nDIC, (b) phosphate, (c) sum of nitrate and nitrite, (d) AOU, (e) potential temperature and (f) salinity on the isopycnal surface at σθ = 25.4 at 20°–30°N. Colors represent longitudinal bands at 120°–180°E. The KE unstable periods are colored in gray and the stable periods are uncolored on the basis of Qiu et al. (2020).
The North Pacific Subtropical Mode Water (STMW) is formed by convective mixing in winter and subsequent subduction in the region to the south of the Kuroshio extension (KE) and is distributed in the Kuroshio Recirculation. Based on the measurement data along the 137°E repeat line of Japan Meteorological Agency, recent studies have shown that the STMW formation volume and biogeochemical parameters such as dissolved inorganic carbon (DIC) and Apparent Oxygen Utilization (AOU) in the STMW varies on a decadal timescale in association with the KE state variability (Oka et al., 2015, 2019). In this study, we examine whether such biogeochemical variabilities are also seen throughout the STMW distribution region in the northwestern part of subtropical gyre including the 137°E line, and particularly focus on the difference in the variabilities between the east and the west in the region.
2. Data & Methods
We used physical and biogeochemical data such as potential temperature, salinity, DIC, phosphate, sum of nitrate and nitrite and AOU at 20°–30°N, 120°–180°E during 1992–2020 taken from GLODAP v2. These data were interpolated vertically at intervals of 0.1σθ for each station using Akima spline method (Akima, 1991) and were averaged yearly at 20°–30°N excluding the data in winter. Additionally, we divided the STMW distribution region at 120°E –180° into 4 regions from the west to the east.
3. Result & Discussion
In the entire regions of longitudinal bands at 120°–180°E, the physical and biogeochemical parameters on the isopycnal surface at σθ = 25.4 varied on a decadal timescale corresponding to the KE state variability (Figure). In addition, biogeochemical parameters were higher in the western region (e.g., AOU and sum of nitrate and nitrite in the region west of 130°E were higher than those in the region east of 160°E by approximately 20 μmol/kg and 2 μmol/kg, respectively). Moreover, we also found the time lag which the variabilities in potential temperature and salinity in the east region are earlier by 1 to 2 years. From the aspect of biogeochemical variabilities in STMW, considering that DIC, AOU and nutrients are relatively low in the newly formed STMW, our results demonstrate the process that STMW formed in the region to the south of KE is transported westward with the flow of Kuroshio recirculation over 1 to 2 years, while being influenced by biological oxygen consumption and remineralization.
Figure caption
Time-series of yearly averages and standard deviations of (a) nDIC, (b) phosphate, (c) sum of nitrate and nitrite, (d) AOU, (e) potential temperature and (f) salinity on the isopycnal surface at σθ = 25.4 at 20°–30°N. Colors represent longitudinal bands at 120°–180°E. The KE unstable periods are colored in gray and the stable periods are uncolored on the basis of Qiu et al. (2020).