Characteristics of water mass under the surface mixed layer in Tsushima Straits and the southwestern Japan Sea in autumn

Abstract. Two Argo floats equipped with oxygen, chlorophyll (Chl), backscatter, and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. Float backscatter observations were converted to particulate organic carbon (POC) concentrations using an empirical relationship derived from contemporaneous float profiles of backscatter and shipboard observations of suspended organic carbon particles. During the float deployment periods, repeated meteorological disturbances (storms) passed over the study area and caused the mixed layer to deepen. During these events, nitrate was entrained from deeper layers into the surface mixed layer, while Chl and POC in the surface mixed layer were redistributed into deeper layers. After the storms, the upper layer gradually restratified, nitrate concentrations in the surface layer decreased, and Chl and POC concentrations increased. When the floats observed the same water mass, the net community production within the euphotic layer (0–70 m), determined from the increases in POC, was 126–664 mg C m−2 d−1 (10.5–55.3 mmol C m−2 d−1) close to the values reported from a nearby area. The C/N ratio of the increase in POC and the decrease in nitrate was closed to the Redfield ratio, which indicates that the sensors were able to observe the net biochemical processes in this area despite the relatively low concentrations of nitrate and POC. To determine the fate of particles transported from the surface ocean to the twilight layer, the ratio of oxygen consumption and nitrate regeneration rates were compared. This O2/N ratio approached the Redfield ratio when the floats followed the same water mass continuously, but the consumption rate of POC was significantly lower than what would be expected based on the oxygen consumption and nitrate release rates. This suggests that dissolved organic carbon was the main substrate for the respiration in the twilight layer.

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Nonseasonal Variations in Near-Inertial Kinetic Energy Observed Far Below the Surface Mixed Layer in the Southwestern East Sea (Japan Sea)

Journal of Marine Science and Engineering ◽

10.3390/jmse10010009 ◽

2021 ◽

Vol 10 (1) ◽

pp. 9

Author(s):

Suyun Noh ◽

SungHyun Nam

Keyword(s):

Kinetic Energy ◽

Mixed Layer ◽

Surface Wind ◽

Japan Sea ◽

Relative Vorticity ◽

East Sea ◽

Total Strain ◽

Surface Mixed Layer ◽

Capture Process ◽

Long Time

Near-inertial internal waves (NIWs) generated by surface wind forcing are intermittently enhanced below and within the surface mixed layer. The NIW kinetic energy below the surface mixed layer varies over intraseasonal, interannual, and decadal timescales; however, these variations remain unexplored, due to a lack of long-term, in situ observations. We present statistical results on the nonseasonal variability of the NIW kinetic energy 400 m below the surface mixed layer in the southwestern East Sea, using moored current measurements from 21 years. We used long time series of the near-inertial band (0.85–1.15 f) kinetic energy to define nine periods of relatively high (period high) and seven periods of relatively low (period low) NIW kinetic energy. The NIW kinetic energy average at period high was about 24 times higher than that at period low and those in specific years (2003, 2012–2013, 2016, and 2020) and decade (2010s) were significantly higher than those in other years and decade (2000s). Composite analysis revealed that negative relative vorticity and strong total strain significantly enhance NIW kinetic energy at 400 m. The relative vorticity was negative (total strain was positively enhanced) during seven (six) out of nine events of period high. NIW trapping in a region of negative relative vorticity and the wave capture process induce nonseasonal variations in NIW kinetic energy below the surface mixed layer. Our study reveals that, over intraseasonal, interannual, and decadal timescales, mesoscale flow fields significantly influence NIWs.

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Chalk-Ex: Transport of Optically Active Particles from the Surface Mixed Layer

10.21236/ada444169 ◽

2005 ◽

Author(s):

Albert J. Plueddemann

Keyword(s):

Mixed Layer ◽

Optically Active ◽

Surface Mixed Layer ◽

Active Particles

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Mixed Layer Models and Their Application to Water Quality Problems

Water Quality Research Journal ◽

10.2166/wqrj.1994.015 ◽

1994 ◽

Vol 29 (2-3) ◽

pp. 221-232

Author(s):

M.J. McCormick

Keyword(s):

Water Quality ◽

Mixed Layer ◽

Thermal Structure ◽

Mass Conservation ◽

Eddy Diffusion ◽

Rate Of Change ◽

Surface Mixed Layer ◽

Storm Events ◽

Quality Model ◽

Mixing Processes

Abstract Four one-dimensional models which have been used to characterize surface mixed layer (ML) processes and the thermal structure are described. Although most any model can be calibrated to mimic surface water temperatures, it does not imply that the corresponding mixing processes are well described. Eddy diffusion or "K" models can exhibit this problem. If a ML model is to be useful for water quality applications, then it must be able to resolve storm events and, therefore, be able to simulate the ML depth, h, and its time rate of change, dh/dt. A general water quality model is derived from mass conservation principles to demonstrate how ML models can be used in a physically meaningful way to address water quality issues.

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Simulation of diurnal variability in vertical density structure using a coupled model

Scientific Reports ◽

10.1038/s41598-021-90426-w ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

B. Yadidya ◽

A. D. Rao ◽

Sachiko Mohanty

Keyword(s):

Mixed Layer ◽

3D Model ◽

Coupled Model ◽

Wave Model ◽

Andaman Sea ◽

Surface Mixed Layer ◽

Diurnal Variability ◽

Rama Buoy ◽

Vertical Displacements ◽

Primary Advantage

AbstractThe changes in the physical properties of the ocean on a diurnal scale primarily occur in the surface mixed layer and the pycnocline. Price–Weller–Pinkel model, which modifies the surface mixed layer, and the internal wave model based on Garrett–Munk spectra that calculates the vertical displacements due to internal waves are coupled to simulate the diurnal variability in temperature and salinity, and thereby density profiles. The coupled model is used to simulate the hourly variations in density at RAMA buoy (15° N, 90° E), in the central Bay of Bengal, and at BD12 (10.5° N, 94° E), in the Andaman Sea. The simulations are validated with the in-situ observations from December 2013 to November 2014. The primary advantage of this model is that it could simulate spatial variability as well. An integrated model is also tested and validated by using the output of the 3D model to initialize the coupled model during January, April, July, and October. The 3D model can be used to initialize the coupled model at any given location within the model domain to simulate the diurnal variability of density. The simulations showed promising results which could be further used in simulating the acoustic fields and propagation losses which are crucial for Navy operations.

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Niche partitioning by photosynthetic plankton as a driver of CO2-fixation across the oligotrophic South Pacific Subtropical Ocean

The ISME Journal ◽

10.1038/s41396-021-01072-z ◽

2021 ◽

Author(s):

Julia Duerschlag ◽

Wiebke Mohr ◽

Timothy G. Ferdelman ◽

Julie LaRoche ◽

Dhwani Desai ◽

...

Keyword(s):

Mixed Layer ◽

Niche Partitioning ◽

Euphotic Zone ◽

South Pacific ◽

Niche Differentiation ◽

Surface Mixed Layer ◽

The South ◽

The Core ◽

Photosynthetic Eukaryotes ◽

Phytoplankton Communities

AbstractOligotrophic ocean gyre ecosystems may be expanding due to rising global temperatures [1–5]. Models predicting carbon flow through these changing ecosystems require accurate descriptions of phytoplankton communities and their metabolic activities [6]. We therefore measured distributions and activities of cyanobacteria and small photosynthetic eukaryotes throughout the euphotic zone on a zonal transect through the South Pacific Ocean, focusing on the ultraoligotrophic waters of the South Pacific Gyre (SPG). Bulk rates of CO2 fixation were low (0.1 µmol C l−1 d−1) but pervasive throughout both the surface mixed-layer (upper 150 m), as well as the deep chlorophyll a maximum of the core SPG. Chloroplast 16S rRNA metabarcoding, and single-cell 13CO2 uptake experiments demonstrated niche differentiation among the small eukaryotes and picocyanobacteria. Prochlorococcus abundances, activity, and growth were more closely associated with the rims of the gyre. Small, fast-growing, photosynthetic eukaryotes, likely related to the Pelagophyceae, characterized the deep chlorophyll a maximum. In contrast, a slower growing population of photosynthetic eukaryotes, likely comprised of Dictyochophyceae and Chrysophyceae, dominated the mixed layer that contributed 65–88% of the areal CO2 fixation within the core SPG. Small photosynthetic eukaryotes may thus play an underappreciated role in CO2 fixation in the surface mixed-layer waters of ultraoligotrophic ecosystems.

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