Island Mass Effect

Hasegawa, Daisuke

doi:10.1002/9781119428428.ch10

“…Enhanced biological production associated with flow disturbance due to island or shallow seamounts is known as the “Island Mass Effect” (IME) (Doty & Oguri, 1956). Because ridges topped by multiple small islands and seamounts are a consistent feature along the Kuroshio flow path (Hasegawa, 2019), several studies since the 1950s have reported that the Kuroshio induces IMEs (e.g., Furuya et al., 1986; Uda & Ishino, 1958, and others). However, in these earlier studies, the detailed physical processes of the IMEs were not thoroughly studied due to the complexities of the flow disturbance associated with the abrupt topographic features.…”

Section: Introductionmentioning

confidence: 99%

“…

Enhanced biological production associated with flow disturbance due to island or shallow seamounts is known as the "Island Mass Effect" (IME) (Doty & Oguri, 1956). Because ridges topped by multiple small islands and seamounts are a consistent feature along the Kuroshio flow path (Hasegawa, 2019), several studies since the 1950s have reported that the Kuroshio induces IMEs (e.g., Furuya et al, 1986; Uda & Ishino, 1958, and others). However, in these earlier studies, the detailed physical processes of the IMEs were not thoroughly studied due to the complexities of the flow disturbance associated with the abrupt topographic features.Since the reporting of extremely large upwelling velocities of O(0.01 m s −1 ) and turbulence kinetic energy dissipation rates of O(10 −4 W kg −1 ) in lees of small islands located along the Kuroshio main axis (Hasegawa et al, 2008(Hasegawa et al, , 2004, the ridges (I-Lan, Tokara, and Izu) over which the Kuroshio passes have been recognized as key locations for open-ocean upwelling and mixing.

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mentioning

confidence: 99%

How a Small Reef in the Kuroshio Cultivates the Ocean

Hasegawa

¹

,

Matsuno

²

,

Tsutsumi

³

et al. 2021

Geophysical Research Letters

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Vertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin‐Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo‐sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10−1 m2 s−1), resulting in strong nitrate fluxes of O(1–103 mmol m−2 day−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10−3 m s−1), as well as an upwelling nitrate flux of O(102 mmol m−2 day−1) in the entire subsurface layer.

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“…However, along strong northward flows of the Kuroshio current, nutrient inputs to the euphotic zone are enhanced due to boundary exchange and diapycnal mixing (Nagai et al, 2019;Pelegrí et al, 2019), which leads to an increase in phytoplankton biomass and a shift in cell size composition toward larger phytoplankton, depending on season and location (Kobari et al, 2019;Nagai et al, 2019). Furthermore, since the Kuroshio current passes ridges with many small islands and seamounts, surface nutrification caused by a topographic flow disturbance (Island Mass Effect) appears to contribute to enhancing productivity in the region (Hasegawa, 2019;Nagai et al, 2021). Despite dynamic and unique hydrographic and ecological features of the Kuroshio region, which are distinct from those of the central North Pacific Subtropical Gyre, data are limited regarding export flux in the region (Oguri et al, 2003;Hung and Gong, 2007;Qu et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

Geographic Variation of Particle Size Distribution in the Kuroshio Region: Possible Causes in the Upper Water Column

Yamada

¹

,

Fukuda

²

,

Umezawa

³

et al. 2021

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Particle size distribution (PSD) in the ocean is a fundamental property that influences carbon export and food webs; however, PSD variation and its causes in oligotrophic oceans are not entirely clear. Here, we used Laser In-Situ Scattering and Transmissometry to investigate PSD (size range 5.2–119 μm) and related variables at 11 stations in the surface layer (0–20 m) of the Kuroshio region of the western North Pacific, where strong current causes dynamic hydrographic and ecological conditions. PSD slopes (range –3.2 to –4.2), derived from the power law model, were steeper at onshore stations and flatter at oligotrophic stations located offshore and at lower latitudes. Notably, slopes tended to become steeper with increasing chlorophyll a concentration, opposing the generally observed relationship between the two variables, whereas they became flatter with increasing transparent exopolymer particle (TEP) concentration. Possible explanations of the above results are localized occurrence of nanophytoplankton and TEP facilitation of particle aggregation. The results support the hypothesis that PSD slopes are controlled by a multitude of factors, including phytoplankton community dynamics and aggregation processes. To determine whether TEP-induced particle aggregation enhances or suppresses carbon export, we need a better understanding of the nature (porosity, density, and sinking velocity) of aggregates in oligotrophic oceans.

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“…The Kuroshio, a warm western boundary current in the North Pacific that flows along the south coast of Japan, is one of the strongest currents in the world ocean. It is a very distinctive current as it encounters many rough topographic features (Hasegawa et al 2019). In the upstream regions, the Kuroshio hits Green Island and flows over the I-Lan Ridge east of Taiwan.…”

Section: Introductionmentioning

confidence: 99%

Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension

Nagai

¹

,

Quintana

²

,

Gómez

³

et al. 2021

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While the Kuroshio is known to be a nutrient stream, as these nutrients are in dark subsurface layers, they are not immediately available for photosynthesis unless they are supplied to the sunlit surface layers. Recent microstructure observations have revealed that strong diapycnal mixing caused by the Kuroshio flowing over topographic features and double diffusion in the subsurface layers of the Kuroshio. However, it is still unclear how much nutrient flux can be provided by these microscale mixing processes. In this study, using an autonomous microstructure float and nutrient samplings, nutrient flux caused by the Kuroshio over the Izu Ridge, and that caused by double diffusion in the Kuroshio Extension are quantified. The nitrate diffusive flux is estimated to be $$>1 \,\hbox {mmol} \,\hbox {N}\,\hbox {m}^{-2}\hbox {day}^{-1}$$ > 1 mmol N m - 2 day - 1 over a distance, 20–30 km near the Izu Ridge and $$>0.1 \,\hbox {mmol} \,\hbox {N}\, \hbox {m}^{-2}\hbox {day}^{-1}$$ > 0.1 mmol N m - 2 day - 1 , which persists further downstream direction over 100 km along the Kuroshio, increasing the subsurface chlorophyll-a concentration in the region 200 km downstream. The double-diffusion-induced nitrate flux is estimated to be 1-$$10 \,\hbox {mmol} \,\hbox {N} \,\hbox {m}^{-2}\hbox {day}^{-1}$$ 10 mmol N m - 2 day - 1 in the pycnostad 26–$$26.5\,\hbox {kgm}^{-3}$$ 26.5 kgm - 3 of the Kuroshio Extension, suggesting that whether this double-diffusion-induced nutrient flux in the subsurface layers can ultimately contribute to surface primary production depends on additional eddy up- and northward fluxes.

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Island Mass Effect

Cited by 15 publications

References 35 publications

How a Small Reef in the Kuroshio Cultivates the Ocean

How a Small Reef in the Kuroshio Cultivates the Ocean

Geographic Variation of Particle Size Distribution in the Kuroshio Region: Possible Causes in the Upper Water Column

Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension

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