Abstract
Oceanic vortex evolution on the lee side of Taiwan’s Green Island (~7 km in diameter), where the Kuroshio flows at a speed of 1–1.5 m s
−1
, is observationally examined and compared to ...theories and the preceding results of laboratory experiments. In the near wake, recirculation occurs with a relative vorticity of
ζ
~ 20
f
(where
f
is the planetary vorticity) and subsequently sheds at a combination of periods resulting from the tidal oscillations and the intrinsic time scale of eddy evolution. The tidal oscillations are the predominant processes. Our analysis suggests that an island positioned in the Kuroshio with periodic and cross-stream tidal excursions is analogous to a cross-stream oscillating cylinder. Consequently, the shedding period of the vortex is synchronized to a tidal period occurring close to the intrinsic period. The free shear layer, which is characterized by an ~30
f
relative vorticity band (2 km wide) and a wavy thermal front, develops between the Kuroshio and recirculation. The frontal wave occurring over a time period of 0.5–2 h resembles Kelvin–Helmholtz instability corresponding to high Re values. For the far wake, repeated cross-wake surveys suggest that cyclonic and anticyclonic vortices are alternatively present at a period close to the period of M
2
tides in agreement with near-wake measurements. Repeated along-wake surveys reveal a cyclonic eddy shedding downstream at a speed of 0.35 m s
−1
, 1/3 of the upstream current speed, from the near wake. In comparing our observations with the results of previous water tank experiments, an Re value of
O
(10
3
) for the submesoscale wake regime is expected.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In mesoscale eddies, the chemical properties and biological composition are different from those in the surrounding water due to their unique physical processes. The mechanism of physical–biological ...coupling in warm-core eddies is unclear, especially because no studies have examined the effects of environmental factors on bacteria and viruses. The purpose of the present study was to examine the influence of an anticyclonic warm eddy on the relationship between bacterial and viral abundances, as well as viral activity (viral production), at different depths. At the core of the warm eddy, the bacterial abundance (0.48 to 2.82 × 105 cells mL−1) fluctuated less than that outside the eddy (1.12 to 7.03 × 105 cells mL−1). In particular, there was a four-fold higher viral–bacterial abundance ratio (VBR) estimated within the eddy, below the layer of the deep chlorophyll maximum, than outside the eddy. An anticyclonic warm eddy with downwelling at its center may contribute to viruses being transmitted directly into the deep ocean through adsorption on particulate organic matter while sinking. Overall, our findings provide valuable insights into the interaction between bacterial and viral abundances and their ecological mechanisms within a warm eddy.
Two-years of data from an array of pressure-sensor equipped inverted echo sounders (PIESs) deployed across the Kuroshio east of Taiwan are analyzed in this study. Cross sections of temperature, ...absolute, barotropic and baroclinic velocities, as well as time series of volume transports and maximum velocity axis location are analyzed to better understand the response of the Kuroshio to local arrivals of mesoscale eddies and to upstream variations. A measure of Kuroshio position, X
vmax
, defined by the velocity in the upper 500 m, does not always coincide with the position of the Kuroshio core defined by the velocity in the upper 150 m. These respond differently to single eddies, dipole eddies and upstream variations. Single mesoscale eddies affect the Kuroshio baroclinicity via a see-saw like vertical displacement of isopycnals from the surface to at least 1000 m depth. Single anticyclonic (cyclonic) eddies deepen (shoal) the isopycnals on the offshore side of the X
vmax
, and shoal (deepen) the isopycnals on the inshore side. As a result of single anticyclonic (cyclonic) eddy-Kuroshio interactions, only the X
vmax
shifts towards the east (west) while the Kuroshio core location is unchanged. Upstream variations of the Kuroshio in the Luzon Strait can weaken the Kuroshio and favor the eastward migration of both X
vmax
and the Kuroshio core east of Taiwan. Coincident with this, a southward current is observed below 200 m depth off the coast of Taiwan. Dipole eddies were also observed to generate migrations of both the Kuroshio core and X
vmax
.
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and ...the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Typhoon is a major cause of multiple disasters in coastal regions of East Asia. To advance our understanding of typhoon-ocean interactions and thus to improve the typhoon forecast for the disaster ...mitigation, two data buoys were deployed in the western North Pacific, which captured Super Typhoon Nepartak (equivalent to Category 5) in July 2016 at distances <20 km from the typhoon's eye center. Here we demonstrate that the unprecedented dataset combined with the modeling results provide new insights into the rapid temperature drop (~1.5 °C in 4 h) and the dramatic strengthening of velocity shear in the mixed layer and below as the driving mechanism for this rapid cooling during the direct influence period of extremely strong winds. The shear instability and associated strong turbulence mixing further deepened the mixed layer to ~120 m. Our buoys also observed that inertial oscillations appeared before the direct wind influence period.
We carry out the Lagrangian diurnal warm layer (DWL) observations, supplemented by satellite products, along the strong Kuroshio east of Taiwan, where the boundary terrain modifies the winds and the ...advection effect is significant. The synoptic easterly wind bifurcates as it encounters Taiwan's coast, forming the southward‐blowing and the northward‐blowing branches in the southern and northern regions of the bifurcation, respectively. The two branches produce areas with enhanced and weakened apparent wind speed relative to the northeastward flowing Kuroshio. The DWL can reach 5 m in the south of the bifurcation due to the shear instability‐induced mixing. Sea surface temperature (SST) enhancement occurs around and in the north of the wind bifurcation, where the solar heat is only trapped in the upper 2 m due to the absence of instability, forming inhomogeneous SST along the Kuroshio. The synoptic wind variations and Kuroshio advection further modulate the location of SST enhancement.
Plain Language Summary
Sea surface temperature could change in a day due to solar heating over the top few meters, where a thin surface warm layer is created. This layer's thickness and mean temperature are related to solar heating and wind speed. The wind blowing works like a stick stirring the surface water to spread the surface heat from the sun to the deeper ocean and, therefore, affect the thickness of the upper ocean's warm layer. However, the warm layer and its associated surface temperature variation exist only in the weak to moderate wind (0–4 m s−1). In the region of a strong current, such as the western boundary current, the moving speed of surface water (1–2 m s−1) is comparable to the wind speed suitable for warm layer formation. It is essential to use the wind speed that the surface waters “feel” as they move to evaluate the warm layer. We found that the above effect has resulted in the asymmetric sea surface warming along the Kuroshio in response to the daily solar heating.
Key Points
Lagrangian observations and satellite remote sensing reveal inhomogeneous diurnal surface temperature warming along the Kuroshio path
Inhomogeneous diurnal surface warming is due to terrain‐induced bifurcated wind and the strong Kuroshio flow speed
The critical wind speed of ∼2 m s−1 relative to the current determines whether the solar heat could transfer deeper via shear instability
How Palaeolithic maritime transportation originated and developed is one of the key questions to understand the world-wide dispersal of modern humans that began 70,000-50,000 years ago. However, ...although the earliest evidence of maritime migration to Sahul (Australia and New Guinea) has been intensively studied, succeeding development of Paleolithic maritime activity is poorly understood. Here, we show evidence of deliberate crossing of challenging ocean that occurred 35,000-30,000 years ago in another region of the western Pacific, the Ryukyu Islands of southwestern Japan. Our analysis of satellite-tracked buoys drifting in the actual ocean demonstrated that accidental drift does not explain maritime migration to this 1200 km-long chain of islands, where the local ocean flows have kept the same since the late Pleistocene. Migration to the Ryukyus is difficult because it requires navigation across one of the world's strongest current, the Kuroshio, toward an island that lay invisible beyond the horizon. This suggests that the Palaeolithic island colonization occurred in a wide area of the western Pacific was a result of human's active and continued exploration, backed up by technological advancement.
Abstract
The South China Sea (SCS) is a high biodiversity region in the world ocean, supports abundant marine resources to the peripheral nations, and affects weather/climate in southeast Asia. A ...better understanding of its circulation is important to better prediction and management of the SCS. Here we reveal sizable intraseasonal oscillations at period ~ 50 days between May and November 2017 in the acoustic Doppler current profiler observed velocity in the central SCS. Satellite observed wind and sea level data together with a process-oriented numerical experiment suggest that the oscillations were caused by locally-generated and remotely-penetrated westward-propagating Rossby waves. The summer southwesterly monsoon strengthening/weakening and the resultant Ekman pumping velocity and shoreward Ekman transport increase/decrease and consequent coastal sea level rise/fall off the west coast of Palawan create westward-propagating Rossby waves causing velocity oscillations in the central SCS. Besides the local generation, Rossby waves with sea level anomaly > 0.2 m propagating from the Pacific through the Sulu Sea into the SCS could contribute to the intraseasonal velocity oscillations in the central SCS.
A magnetic fluid (MF)-infiltrated antiresonant reflecting optical waveguide has been fabricated for the magnetic field sensing based on resonant leaky modes. In a hollow core photonic crystal fiber, ...the MF is selectively infiltrated into one hollow hole in the air cladding. A Fabry-Perot resonator can be formed between the MF and the silica cladding. Both the resonant condition and the reflectivity of the Fabry-Perot resonator are changed under different magnetic field, which can be measured through the wavelength shift and transmission power at the dip wavelength. The sensitivities of up to 109 pm/Oe and 0.064 dB/Oe are achieved for methods of wavelength shift and transmission power at the dip wavelength. The antiresonant reflecting optical waveguides are widely used for the accurate measurement of magnetic field in the fields of transportation, medicine, smart grids, and so on.
The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. Factors controlling
Cs (half-life, 30.2 year) and
Cs (half-life, 2.06 year) activity ...concentrations in the Kuroshio east of Taiwan and the Taiwan Strait (latitude 20° N-27° N, longitude 116° E-123° E) remain unclear. This study collected seawater samples throughout this region and analyzed
Cs and
Cs activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all
Cs activity concentrations were below the detection limit (0.5 Bq m
). Analyses of water column
Cs profiles revealed a primary concentration peak (2.1-2.2 Bq m
) at a depth range of 200-400 m (potential density σ
25.3 to 26.1 kg m
). The PCA result suggests that this primary peak was related to density layers in the water column. A secondary
Cs peak (1.90 Bq m
) was observed in the near-surface waters (σ
= 18.8 to 21.4 kg m
) and was possibly related to upwelling and river-to-sea mixing on the shelf. In the Taiwan Strait,
Cs activity concentrations in the near-surface waters were higher in the summer than in the winter. We suggest that upwelling facilitates the vertical transport of
Cs at the shelf break of the western NPO.
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