The currents in the low-latitude western North Pacific are critical to the general circulation. The North Equatorial Current flows westward, bifurcating off the coast of the Philippines to form the ...northward-flowing Kuroshio and the southward-flowing Mindanao Current. Below the thermocline, undercurrents flow opposite to the currents above and include the southward-flowing Luzon Undercurrent, the northward-flowing Mindanao Undercurrent, and an eastward-flowing series of North Equatorial Undercurrents. Two complementary programs, Origins of the Kuroshio and Mindanao Current (OKMC) and Observations of Kuroshio Transport and Variability (OKTV), focused on observations and modeling of these currents. Results address the region's circulation, water masses, and eddies. The connectivity between the currents as determined by water masses is of special interest.
The South China Sea (SCS) Two-Island Monsoon Experiment (SCSTIMX) is the observational task of an integrated project aimed to better understand interactions of convection over the SCS and Maritime ...Continent (MC) with large-scale flow. The project consists of sub-projects in three research groups: convective processes, large-scale dynamics, observations. SCSTIMX is one of the field campaigns of the Years of the Maritime Continent (2017 - 2020). Special observations made or derived for the SCSTIMX are documented in this article that include field campaigns at Taiping and Dongsha islands during intensive observation periods (December 2017, March and May 2018), cruises by the National Taiwan University ocean research vessel (OR1) in December 2016 and December 2017, and satellite observations in the SCSMC for the project years. We show the spatial-temporal evolution of the La Niña event from December 2016 to June 2018 and embedding interactions of convectively coupled equatorial waves (CCEW) with diurnal cycles and intraseasonal oscillations (ISO) in the SCS-MC. The La Niña-ISO-CCEWs together resulted in the late SCS summer monsoon onset in May 2018. In addition to the La Niña influence, the strong Indian Ocean dipole signal in December 2017 further weakened CCEWs in eastern Indian Ocean. We also show research highlights from participating projects published in the current special issue of TAO.
Internal solitary waves are ubiquitous in coastal regions and marginal seas of the world’s oceans. As the waves shoal shoreward, they lose the energy obtained from ocean tides through globally ...significant turbulent mixing and dissipation and consequently pump nutrient-rich water to nourish coastal ecosystem. Here we present fine-scale, direct measurements of shoaling internal solitary waves in the South China Sea, which allow for an examination of the physical processes triggering the intensive turbulent mixing in their interior. These are convective breaking in the wave core and the collapse of Kelvin–Helmholtz billows in the wave rear and lower periphery of the core, often occurring simultaneously. The former takes place when the particle velocity exceeds the wave’s propagating velocity. The latter is caused by the instability induced by the strong velocity shear overcoming the stratification. The instabilities generate turbulence levels four orders of magnitude larger than that in the open ocean.
Fine-scale, direct measurements of shoaling internal solitary waves reveal that convective breaking and collapse of Kelvin–Helmholtz billows often occur simultaneously and generate turbulence four orders of magnitude larger than in the open ocean.
Vigorous internal tides excited in the Luzon Strait are the putative cause of regular temperature drops of 2–10°C within only 12–24 h at the southernmost coast of Taiwan. Significant amounts of ...nutrient pumping, i.e., 2.4–22.9 mmol N m−2 h−1 nitrate (NO3−) and 0.4–1.75 mmol P m−2 h−1 phosphate (PO43−), occur during the upward phase in an internal tide. The upwelling, which is potentially induced by internal tides, not only helps cap the summertime mean temperature at ∼27.6°C but also supplies nutrients to the euphotic layer, which may be beneficial to the growth of coral reefs. A baroclinic tide model with 1°/12 (∼9.2 km) resolution, which is sufficient to resolve topography around southern Taiwan, was used to evaluate the internal tide generation in the Luzon Strait and the physical and biogeochemical influences of these tides in this coastal region. The timing and amplitude, as well as fortnightly and seasonal variations of the modeled temperatures, agreed with the observations, suggesting that observed temperature decreases and nutrient enrichment are primarily caused by internal tide‐induced upwelling. This investigation further indicated that the internal tides propagating to the coast are remotely generated on the eastern flank of Heng‐Chun Ridge, rather than locally, near the southernmost coast of Taiwan. The model also evaluated that the seasonal variation of the pycnocline in the receiving waters, which is stronger and shallower during summer than winter, results in relatively larger temperature decreases in the summer.
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.
In the subtropical western North Pacific Ocean, the Kuroshio delivers heat, salt, and momentum poleward, much like its North Atlantic analog, the Gulf Stream. Though the Kuroshio generally flows ...along the western boundary from Taiwan to southeastern Japan as an "attached" current, the Kuroshio's strength, vertical structure, and horizontal position undergo significant temporal and spatial variability along this entire route. Ubiquitous mesoscale eddies and complicated topography associated with a string of marginal seas combine to make the western North Pacific a region with complex circulation. Here, we synthesize results from the recent US Origins of the Kuroshio and Mindanao Currents and Taiwan Observations of Kuroshio Transport Variability observational programs with previous findings to build a comprehensive picture of the Kuroshio on its route from northeastern Taiwan to southeastern Japan, where the current finally transitions from a western boundary current into the Kuroshio Extension, a vigorously meandering free jet.
The sustainable development of society depends on the reliable supply of electricity while keeping impacts on the environment to a minimum. A 951 MWe nuclear power plant in the semi-enclosed Nanwan ...Bay at the southern tip of Taiwan began operating in May 1984. Part of the bay is in Kenting National Park, which is known for its coral reefs and abundant marine life; thus, thermal pollution from the cooling water discharge is a great concern. Fortunately, the bay opens south to face the Luzon Strait, where the world’s strongest internal tides are generated. Because the bay is deep enough, internal waves bring up cold deep water and reduce the surface temperature by as much as 10 °C for a few hours every day. These internal waves and topographically generated upwelling also bring nutrients to the euphotic layer from the depths, but the upwelled waters quickly leave the bay along with the cooling water. As a result, a thermal plume with a temperature of 1 °C or higher than the ambient temperature only covers 1 km. By way of comparison, El Niño—Southern Oscillation- or Pacific Decadal Oscillation-related interannual variations in temperature are as high as 5 °C. The rapid turnover of the upwelled waters also helps to prevent heat released by the power plant from accumulating and diminishes the thermal stress, thus sustaining corals and other marine life forms. Typhoons, even hundreds of kilometers away, could also induce the upwelling of cold subsurface water. Consecutive typhoons have been observed to reduce the water surface temperature by up to 10 °C for two weeks or longer. Furthermore, the currents are such that the thermal plume flows out of the bay most of the time. All of these factors make the surface waters in the bay about 0.5 °C cooler than the waters outside of the bay, despite the operation of a nearby nuclear power plant.
This study examines the evolution of the Kuroshio Tropical Water (KTW) from the Luzon Strait to the I-Lan Ridge northeast of Taiwan. Historical conductivity temperature depth (CTD) profiles are ...analyzed using a method based on the calculation of the root mean square (rms) difference of the salinity along isopycnals. In combination with analysis of the distribution of the salinity maximum, this method enables water masses in the Kuroshio and the vicinity, to be tracked and distinguished as well as the detection of the areas where water masses are modified. Vertical and horizontal eddy diffusivities are then calculated from hydrographic and current velocity data to elucidate the dynamics underlying the KTW interactions with the surrounding water masses. Changes in KTW properties mainly occur in the southern half of the Luzon Strait, while moderate variations are observed east of Taiwan on the right flank of the Kuroshio. In spite of a front dividing the KTW from the South China Sea Tropical Water (SCSTW) on Kuroshio׳s western side, mixing between these two water masses seemingly occurs in the Luzon Strait. These water masses׳ interaction is not evident east of Taiwan. The estimation of eddy diffusivities yields high horizontal diffusivities (Kh~102m2s−1) all along the Kuroshio path, due to the high current shear along the Kuroshio׳s flanks. The vertical diffusivity approaches 10−3m2s−1, with the highest values in the southern Luzon Strait. Instabilities generated when the Kuroshio encounters the rough topography of this region may enhance both vertical and horizontal diffusivities there.
•The Kuroshio Tropical Water (KTW) is tracked and its evolution is defined.•Vertical and horizontal diffusivities are calculated along the Kuroshio path.•Enhanced mixing in the southern Luzon Strait causes the main changes in the KTW.•Advection of a saltier water mass balances the effects of the vertical diffusion east of Taiwan.
Using a three‐dimensional model with idealized settings, this study evaluated the effect of the Kuroshio on the generation and propagation of semidiurnal M2 and diurnal K1 baroclinic tides (BCTs) in ...the Luzon Strait (LS). The influences of the varied Kuroshio transport, the location of the Kuroshio, and the separation distance of the two meridional ridges were investigated. The absence of the Kuroshio in the model leads to 30% decreases in the westward baroclinic energy flux (Fbcx) emanating from the west slope of the east ridge compared to the Fbcx with the presence of the Kuroshio. Compared to the Fbcx without the Kuroshio's influence, the westward Fbcx is increased by 20–40% when the Kuroshio is located in either the middle or the west portions of the LS, whereas both the westward and eastward Fbcx are 25–40% decreased as the Kuroshio is moved to the east of the east ridge. The wavelength of the mode 1 M2 BCT is similar to the separation distance of the two ridges, favoring a resonance generation for M2 rather than K1 BCTs. The increase of the Kuroshio transport from 20 to 30 Sv (1 Sv = 106 m3 s−1) enhances the density front but weakens the westward Fbcx owing to the effects of energy advection and lateral friction from the Kuroshio. The influence of the Kuroshio on the westward Fbcx of K1 is opposite of that on M2. The characteristic slope of K1 is closer to the isopycnal slope across the Kuroshio than M2, thereby causing the discrepancy.
Key Points
The presence of Kuroshio increases spatial decay rate of westward internal tides
The increase of Kuroshio transport weakens westward baroclinic tide energy flux
The Kuroshio's modification to westward K1 energy is opposite to that in M2
The Kuroshio is the western boundary current of the North Pacific Ocean. In the subtropical region of eastern Taiwan, a coastal uplift of the isotherms has occurred. To explore its impact on this ...oligotrophic ecosystem, hydrographic data along the transect line at 23.75°N were measured between September 2012 and September 2014. Results show that the intensity of coastal uplift was positively correlated to the flow volume transport of the Kuroshio. Significant dissolved inorganic nutrients were uplifted to the sunlit zone, especially in the onshore. For example, compared to the offshore, nitrate concentration increased 0.49 μM (or ∼178%) in the upper 100 m of the onshore. The increased nutrients thereafter enhanced the growth of phytoplankton; for instance, the Chl
a
concentration increased 88.3%, compared to offshore, in the upper 100 m of the onshore. Phytoplankton community was mostly composed of picophytoplankton (<2 μm in size), which were dominated, in terms of relative abundance, by
Prochlorococcus
(83.6%), followed by
Synechococcus
(13.8%) and picoeukaryotes (2.7%). The relative abundance of
Prochlorococcus
increased from onshore toward offshore, but the trend was reversed for
Synechococcus
and picoeukaryotes, which may be affected by the coastal uplift. The results also suggest that the coastal uplift may support more energy transferred to higher tropical levels in this oligotrophic Kuroshio ecosystem.
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