5:15 PM - 6:45 PM
[AOS16-P14] Horizontal finestructure and turbulent vertical mixing in the Kuroshio front over the summer continental shelf off Choshi/Boso area
Keywords:continental shelf, turbulent mixing, horizontal diffusion, subsurface chlorophyll maximum
Nutrient circulation that supports production in coastal seas is driven by (i) influx from external sources such as rivers, sediment layers and the open ocean, (ii) biological consumption and regeneration, and (iii) advective and diffusive transports. In the continental shelf off Choshi/Boso, with the Tone River mouth on the inshore side and the Kuroshio on the offshore side, exchange and mixing between coastal and offshore waters are expected to be active, which makes this area suitable for examining nutrient circulation processes. However, comprehensive field surveys have yet to be conducted. In this study, CTD, ADCP, VMP and Underway CTD (UCTD) measurements were carried out in the continental shelf off Choshi/Boso in September 2023 to (i) clarify the characteristics of the flow field and water mass distribution, and (ii) explore the impact of advection and diffusion processes on biological production.
Horizontal high-resolution UCTD and Shipboard ADCP profiles along the shelf slope revealed (i) low-salinity coastal water extending for at least 30 km in the upper 20-m layer, presumably due to summer precipitation, (ii) the Kuroshio flowing over the shelf, and (iii) low-salinity, high-turbidity and high-chlorophyll water intruding along the coastal/Kuroshio front. VMP profiles captured enhanced turbulent kinetic energy dissipation rates ε~ 10-6 W/kg in the surface 20 m and near the shelf break, and ε~ 10-7 W/kg in the pycnocline of the coastal/Kuroshio front.
Subsurface chlorophyll maximum (SCM) was observed at the base of pycnocline which was particularly enhanced at the coastal/Kuroshio front. The SCM depth coincided with the euphotic layer depth and was also consistent with the horizontal and vertical nitrate gradients maxima obtained from SUNA attached to the CTD. These results suggest that surface nutrients are depleted, and the SCM is maintained by advection and/or diffusion of abundant nutrients in the deeper layer.
Nitrate fluxes by vertical and horizontal diffusion are quantified. VMP-derived turbulent vertical diffusion Kv can provide nitrate concentration with a rate d(NO3)/dt ~ 1 (μmol/l)/day at the base of SCM. Applying the mixing length theory (e.g., Naveira Garabato et al. 2011) to the UCTD profiles, submesoscale horizontal diffusion is estimated to be small in the coastal/Kuroshio front with KH ~ 1 m2/s and associate nitrate supply d(NO3)/dt < 10-2 (μmol/l)/day. These results suggest that turbulent vertical mixing plays a significant role in nutrient supply to the summer SCM in the continental shelf off Choshi/Boso, whereas the contribution of submesoscale horizontal diffusion is minor.
Horizontal high-resolution UCTD and Shipboard ADCP profiles along the shelf slope revealed (i) low-salinity coastal water extending for at least 30 km in the upper 20-m layer, presumably due to summer precipitation, (ii) the Kuroshio flowing over the shelf, and (iii) low-salinity, high-turbidity and high-chlorophyll water intruding along the coastal/Kuroshio front. VMP profiles captured enhanced turbulent kinetic energy dissipation rates ε~ 10-6 W/kg in the surface 20 m and near the shelf break, and ε~ 10-7 W/kg in the pycnocline of the coastal/Kuroshio front.
Subsurface chlorophyll maximum (SCM) was observed at the base of pycnocline which was particularly enhanced at the coastal/Kuroshio front. The SCM depth coincided with the euphotic layer depth and was also consistent with the horizontal and vertical nitrate gradients maxima obtained from SUNA attached to the CTD. These results suggest that surface nutrients are depleted, and the SCM is maintained by advection and/or diffusion of abundant nutrients in the deeper layer.
Nitrate fluxes by vertical and horizontal diffusion are quantified. VMP-derived turbulent vertical diffusion Kv can provide nitrate concentration with a rate d(NO3)/dt ~ 1 (μmol/l)/day at the base of SCM. Applying the mixing length theory (e.g., Naveira Garabato et al. 2011) to the UCTD profiles, submesoscale horizontal diffusion is estimated to be small in the coastal/Kuroshio front with KH ~ 1 m2/s and associate nitrate supply d(NO3)/dt < 10-2 (μmol/l)/day. These results suggest that turbulent vertical mixing plays a significant role in nutrient supply to the summer SCM in the continental shelf off Choshi/Boso, whereas the contribution of submesoscale horizontal diffusion is minor.