Viral Dynamics in the Tropical Pacific Ocean: A Comparison between Within and Outside a Warm Eddy.

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.

Observational evidence of overlooked downwelling induced by tropical cyclones in the open ocean.

Tropical cyclones (TCs) cause severe natural hazards and drive intense upper ocean cooling through a series of oceanic and atmospheric physical processes, including vertical mixing and upwelling. Among these processes, TC-induced warming of near-surface waters in the open ocean has rarely been noted. This study provides a detailed analysis of upper ocean responses to 30 TC events observed by two buoys in the western North Pacific between 2016 and 2021. Supplemented with numerical experiments, we suggest that downwelling frequently occurs at the periphery of upwelling regions (around the radius of the 34 knot wind speed) following the passage of a TC. Downwelling is identified via pronounced warm anomalies under a shallow mixed layer depth, and its dynamics are attributed to negative wind stress curl and current-induced convergence. These findings highlight the important role played by TC-induced downwelling and offer insights for reconsidering the influence of TCs on biogeochemical processes.

Dataset on Cs-137 in waters surrounding Taiwan.

The Fukushima accident released short-lived Cs-134 and longer-lived Cs-137 to the ocean. The amount, although substantial, is much less than that produced during the atomic bomb tests 60 yrs ago. Cs-134 and Cs-137 are anthropogenic radionuclides and soluble in seawater; hence, the radioactivity can be used as a tracer for special events or currents. Samples of Cs-134 and Cs-137 in seawater were collected around Taiwan, including the Kuroshio, the northern South China Sea, the Taiwan Strait, and the southern East China Sea from 2018 to 2021. The average surface Cs-137 activity was 1.18±0.25 Bq m - 3, and the activities of Cs-134 samples were all under the detection limit. Complete data are archived, including sampling date, location, water depth, temperature, salinity, and Cs-137 activity; the total sample amount is 577.

Plankton Community Respiration and Particulate Organic Carbon in the Kuroshio East of Taiwan.

Biological organic carbon production and consumption play a fundamental role in the understanding of organic carbon cycling in oceans. However, studies on them in the Kuroshio, the western boundary current in the North Pacific Ocean, are scarce. To better understand the variations of plankton community respiration (CR) and particulate organic carbon (POC), eight cruises. which covered four seasons over a 2-year period, were surveyed across the Kuroshio at the KTV1 transect east of Taiwan. Spatially, a coastal uplift of isotherms (i.e., onshore lifting and offshore deepening) was observed along the KTV1 transect. During the uplift, the cold and nutrient-rich deep waters shoal to shallow water and enhance phytoplankton growth, resulting in higher values of phytoplankton, POC, and plankton CR on the onshore side. In this study, phytoplankton was dominated by picophytoplankton including Prochlorococcus, Synechococcus, and picoeukaryotes. Plankton CR was low, and its mean depth-normalized integrated rate (the upper 100 m water depth) ranged from 7.07 to 22.27 mg C m-3 d-1, to which the picophytoplankton and heterotrophic bacteria contributed the most. The mean depth-normalized integrated value of POC ranged from 12.7 to 21.6 µg C L-1. POC is mainly associated with phytoplankton biomass with a mean carbon ratio of chlorophyll a/POC ≈ 1.03. All results suggest that plankton CR and POC variations may be associated with picoplankton dynamics in the Kuroshio.

The Kuroshio radiocesium stream.

The Fukushima accident released short-lived Cs-134 and longer-lived Cs-137 to the ocean. The amount, although substantial, is much less than that produced during the atomic bomb tests 60 years ago. But, the latter has received little attention. Here we found only Cs-137 in waters near the origin of the Kuroshio. The speed of the Kuroshio current generally decreases with water depth, yet, the Cs-137 activity increases with depth to reach a subsurface maximum of 2.4 Bq m-3. As a result, a core of high Cs-137 flux, or a radiocesium stream, exists at approximately 200-400 m in depth. In total, the Kuroshio transports about 1 PBq year-1 Cs-137 northward between 121 and 123°E, 1000 times more than the 0.73-1.0 TBq year-1 discharged to the ocean at Fukushima in 2016-2018.

Radiocesium in the Taiwan Strait and the Kuroshio east of Taiwan from 2018 to 2019.

The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. Factors controlling 137Cs (half-life, 30.2 year) and 134Cs (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 134Cs and 137Cs activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all 134Cs activity concentrations were below the detection limit (0.5 Bq m-3). Analyses of water column 137Cs profiles revealed a primary concentration peak (2.1-2.2 Bq m-3) at a depth range of 200-400 m (potential density σθ: 25.3 to 26.1 kg m-3). The PCA result suggests that this primary peak was related to density layers in the water column. A secondary 137Cs peak (1.90 Bq m-3) was observed in the near-surface waters (σθ = 18.8 to 21.4 kg m-3) and was possibly related to upwelling and river-to-sea mixing on the shelf. In the Taiwan Strait, 137Cs 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 137Cs at the shelf break of the western NPO.

Mooring observed intraseasonal oscillations in the central South China Sea during summer monsoon season.

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.

Palaeolithic voyage for invisible islands beyond the horizon.

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.

Vertical Beta-Diversity of Bacterial Communities Depending on Water Stratification.

Many studies indicate that variation of marine bacterial beta diversity in the horizontal dimension is mainly attributable to environmental and spatial effects. However, whether and how these two effects drive bacterial beta diversity in the vertical dimension remains unclear, especially when considering seasonal variation in the strength of water stratification. Here, we used 78 paired bacterioplankton community samples from surface and deep chlorophyll maximum (DCM) layers along a transect in the Kuroshio region east of Taiwan across multiple seasons. Variance partitioning was used to evaluate the mechanisms driving the vertical beta diversity between surface-DCM bacterioplankton communities during weak stratification periods (i.e., spring and fall) versus strong stratification periods (i.e., summer). During strong periods of stratification, vertical beta diversity was shaped by both environmental and spatial effects; notably, the strength of stratification played an important role in enhancing environmental dissimilarity and creating a barrier to dispersal. In contrast, during periods of weak stratification, environmental effects dominate, with a non-significant spatial effect due to mixing. Variation of vertical beta diversity for bacterioplankton communities in the Kuroshio region east of Taiwan was structured by different mechanisms across seasons, and was further dependent on stratification strength of the water column.

Glider observations of interleaving layers beneath the Kuroshio primary velocity core east of Taiwan and analyses of underlying dynamics.

Submesoscale interleaving layers are caused by lateral intrusions of dissimilar water masses in frontal zones, which are significant processes in shaping physical, biogeochemical, and ecological parameters in the ocean. Possible interleaving layers were sometimes observed by ship-based conductivity-temperature-depth (CTD) surveys with coarse spacing between adjacent stations in the Kuroshio region east of Taiwan but have never been examined dynamically. Here we show the characteristics of interleaving layers observed by a Seaglider with two repeated hydrographic surveys along a triangle track east of Taiwan from December 2016 to March 2017. Salinity profiles indicate that prominent interleaving layers appeared in the intermediate layer (approximately 500-800 m) with vertical and horizontal length scales of O(50) m and O(10-100) km, respectively, during our observations. A dipole eddy pair and a relatively large anticyclonic eddy impinged on the Kuroshio during the first and second surveys, respectively, which brought certain impacts on the interleaving motion as the eddy potentially altered the density slope across the Kuroshio. The associated instability analysis and the Turner angle suggest that the double diffusive instability is the primary driving mechanism for the development of interleaving layers.

The role of enhanced velocity shears in rapid ocean cooling during Super Typhoon Nepartak 2016.

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.

The formation and fate of internal waves in the South China Sea.

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.