Global warming seems leveling off somewhat during 1993–2013 despite increasing atmospheric greenhouse gases. What has happened to the Kuroshio Current system concurrently? Available independent data ...sets from 1993 to 2013 point to a single answer. Here we show a systemwide weakened Kuroshio during the period despite enhanced warming along its path. The Pacific warm pool upstream of the Kuroshio is still becoming warmer during the period. It injects more heat into the Current despite the weakened Kuroshio, which is associated with weakened westerlies and cyclonic trends of basin‐scale wind stress curl. The weakened Kuroshio will modulate heat and mass exchanges between the tropics and extratropics, impacting the energy balance of climate system. It will also significantly influence mass, heat, salinity, and nutrient exchanges between the Pacific and adjacent marginal seas, which in turn impacts the regional weather, fisheries, and environments.
Key Points
A systemwide weakened Kuroshio is found during 1993–2013, capable of redistributing mass and energy between the marginal seas and Pacific
Weakened westerlies and cyclonic wind stress curl are the most responsible for the weakened Kuroshio
The basin wind stress curl may be a potent indicator for the Kuroshio strength in the future
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
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DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Despite numerous previous estimates of Luzon Strait transport (LST), we attempt an update using a fine‐resolution model. With these improvements, the circulation in and around Luzon Strait shows up ...rather realistically. Intrusion of a Kuroshio meander into the South China Sea (SCS) is seasonally varying. The LST, especially in the upper ocean, caused by a small difference between the large meander inflow and outflow, is also seasonally varying and subject to large standard deviation. The annual mean LST is estimated to be westward (−4.0 ± 5.1 Sv) along 120.75°E. We have also conducted process of elimination experiments to assess the relative importance of open ocean inflow/outflow, wind stress, and surface heat flux in regulating LST and its seasonality. The East Asian monsoon winds stand out as the predominant forcing. Without it, the upper ocean LST changes from westward to eastward (ranging up to 4 Sv) and, with misaligned seasonality, triggering an inflow from the Mindoro Strait to the SCS to replenish the water mass loss. Discounting monsoon winds, sea level in the Sulu Sea is generally higher because it receives the Indonesian Throughflow before the SCS, which causes an inflow from the Sulu Sea to the SCS. On the other hand, the annual mean wind from the northeast invites outflow from the SCS to the Sulu Sea (or inflow from the Luzon Strait). Weighing the two competing factors together, we see the cessation of northeast monsoon as a condition favorable for the Luzon Strait outflow or the Mindoro Strait inflow.
Key Points
Process experiments and relative importance of driving forcing
Predominant forcing and monsoon winds
The LST from an inflow to an outflow of up to 4 Sv without wind forcing
The northward-flowing Kuroshio often intrudes westward and modulates the water masses of the South and East China Seas. These intrusions transcend multiple scales in time and space, which we ...demonstrate here using various independent data sets. There are two hot spots of intrusion, one in the Luzon Strait and the other off northeast Taiwan, which occur synchronously when the upstream Kuroshio weakens during winter. Beyond seasonal time scales, the two intrusions were not synchronous during 1993-2013. While intrusions into the South China Sea echoed the Pacific Decadal Oscillation, the intrusion northeast of Taiwan decreased markedly before 2002 but regularly reached the shelf thereafter. This change was due to the influence of westward impingements of cyclonic eddies from the open ocean on the Kuroshio main stream in place of anticyclonic eddies. During 1993-2001, decreasing cyclonic eddy impingements moved the Kuroshio away from northeast Taiwan, weakening the Kuroshio intrusion onto the East China Sea shelf. Thereafter, enhanced cyclonic eddy impingement during 2002-2013 weakened the Kuroshio transport, moving it closer to the shelf and enhancing its intrusion into the East China Sea.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The predominance of diurnal surface tides over semidiurnal surface tides in the South China Sea (SCS) has been attributed to the near‐resonance response of the former in the SCS. Recent observations ...further revealed vigorous internal tides in the northern SCS. Conceivably, internal tides generated in the Luzon Strait could modify the surface tide in the SCS. We use a three‐dimensional tide model of the East Asian seas to address this issue. With a typical summertime stratification of the SCS as the initial condition, energy budget indicates that one third of the incident K1 surface tide energy is converted to the baroclinic energy over topographic ridges in the Luzon Strait. In comparison with a global tidal model Matsumoto et al., 2000, our numerical experiments that annihilated or reduced the K1 internal tide in the Luzon Strait led to up to 50% amplification of the simulated K1 surface tide in the SCS. This suggests that the baroclinic energy conversion substantially reduces the amplitude of K1 surface tide in the SCS. The simulated phases in the SCS differ little from those calculated from Matsumoto’s tide model, suggesting that the modification is primarily on the amplitude. Two‐dimensional surface tidal models lack baroclinic energy conversion in the Luzon Strait; the consequent overestimation of surface K1 tide can be reduced only through precise prescription of sea levels in the Luzon Strait or assimilation of sea level data.
We examined the spatial and temporal variations of upwelling off northeast Taiwan, using a fine‐resolution numerical model with realistic bathymetry. The zonally running shelf break in the area ...deflects the Kuroshio seaward and produces upwelling on its on‐shelf edge. The upwelling, in turn, manifests a cold dome or a cyclonic eddy. In depths below 150 m or so, the upwelling and hence the cyclonic eddy exist year‐round. Above this depth, the eddy waxes and wanes as the upper portion of the Kuroshio migrates seaward and shoreward, respectively. The eddy event fluctuates in a wide range of timescales. Seasonally, the occurrence heavily favors summer rather than winter, because the mean Kuroshio axis migrates seaward in summer. Intraseasonally, the fluctuation contains two dominant periods centered at 70 days and 30 days. Local wind forcing and channeling by two local canyons do not affect the eddy statistics significantly.
The generation of internal solitary waves by barotropic tides over a ridge is studied in a nonhydrostatic numerical model under idealized oceanographic settings. The experiments examine the effects ...of ridge width, barotropic tidal strength, and stratification on wave generation. The barotropic tidal flow produces internal wave beams emitting from the ridge top if the slope of the ridge exceeds a critical value equal to the slope of the wave beam. Reflection and refraction of a wave beam in an upper ocean waveguide associated with a strong shallow thermocline produce horizontally propagating internal tides. When the local Froude number over a ridge is not small, lee waves generated on the ridge convert enough energy from the barotropic tides to the internal tides to form tidal bores and solitary waves. Increasing stratification at ridge depths enhances the generation of internal waves, particularly at the diurnal periods. In the Luzon Strait, the slope of the wave beam decreases in spring and summer as stratification at the ridge depths increases, favoring the generation of internal tides. Without the presence of a strong shallow thermocline, internal solitary waves are not observed east of the Luzon Strait. In the northern South China Sea, internal solitary waves are likely observed from April to July when a strong shallow thermocline is present. A deep mixed layer in winter suppresses the production of internal solitary waves.
In recent years, benthic hypoxia has been observed in the outflow region of the Changjiang River in the East China Sea. Because the nitrogen input to the Changjiang watershed, mainly from human ...activities, has increased by 3 fold in the last four decades and the nitrogen load had grown exponentially, it is speculated that anthropogenic nutrients may be responsible for the hypoxia in the East China Sea shelf. We employ a coupled 3-D physical–biogeochemical model of the East China Sea to investigate how the changing Changjiang nutrient loads from 1970 to the end of 2002 may have impacted on primary production in the water column and the seafloor oxygen demand (SOD) on the seafloor. The model predicts an average value of 437mgCm−2d−1 for primary production and 10.0mmol O2m−2d−1 for SOD for the ECS shelf over the entire modeling period. The model results compare reasonably with observations during the period from December 1997 to October 1998. Responding to the increase of the Changjiang DIN loading by a factor of ~2.4, the modeled primary production in the East China Sea shelf has increased by 17%, and the modeled SOD by 22%. In the inner shelf, where the impact is the strongest, the SOD increases by 30%. We are able to identify areas of potential hypoxia using two criteria: SOD>30mmol O2m−2d−1 and water depth >25m. The maximum area of potential hypoxic region in any month of a year has increased dramatically after 1991; the change appears related to the Changjiang DIN loads from May to July that showed a sudden increase after 1990. The responses in potential hypoxic area are more pronounced than the increases in DIN (dissolved inorganic nitrogen) loads, suggesting strong nonlinear effect in the development of hypoxia, which warrants further investigation. It is cautioned that the SOD calculation was based on the Redfield C/N ratio, but the actual C/N ratio may deviate from it. Direct observations of the sediment oxygen consumption are needed to validate our modeling approach. We also assessed the potential impacts of particulate organic matter from Changjiang by introducing a load of reactive particulate nitrogen (PN), which was assumed proportional to DIN based on estimated yields in the watershed. The modeled impacts on primary productivity and SOD are significant, but more accurate quantification of the monthly PN load and better characterization of its reactivity are required for better assessment.
•The nitrogen load of Changjiang increased by 140% from 1970 to 2002.•Modeled primary production in the East China Sea shelf increases by 17%.•Modeled seafloor oxygen demand increases by 30% in the inner shelf.•The area of modeled potential hypoxia increases significantly after 1991.•Strong non-linear effect is apparent in the development of potential hypoxic area.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The predominance of diurnal surface tides over semidiurnal surface tides in the South China Sea (SCS) has been attributed to the near‐resonance response of the former in the SCS. Recent observations ...further revealed vigorous internal tides in the northern SCS. Conceivably, internal tides generated in the Luzon Strait could modify the surface tide in the SCS. We use a three‐dimensional tide model of the East Asian seas to address this issue. With a typical summertime stratification of the SCS as the initial condition, energy budget indicates that one third of the incident K
1
surface tide energy is converted to the baroclinic energy over topographic ridges in the Luzon Strait. In comparison with a global tidal model Matsumoto et al., 2000, our numerical experiments that annihilated or reduced the K
1
internal tide in the Luzon Strait led to up to 50% amplification of the simulated K
1
surface tide in the SCS. This suggests that the baroclinic energy conversion substantially reduces the amplitude of K
1
surface tide in the SCS. The simulated phases in the SCS differ little from those calculated from Matsumoto’s tide model, suggesting that the modification is primarily on the amplitude. Two‐dimensional surface tidal models lack baroclinic energy conversion in the Luzon Strait; the consequent overestimation of surface K
1
tide can be reduced only through precise prescription of sea levels in the Luzon Strait or assimilation of sea level data.
The wake of Green Island was investigated in the alongshore flowing Kuroshio east of Taiwan, where current speeds range from 1 to 1.5 ms–1. Vertical profiling with shipboard lowered acoustic Doppler ...current profiler and conductivity‐temperature‐depth profiler revealed sizable anomalies in flow and water characteristics in the lee of Green Island. Two different stages of wake evolution were observed from two shipboard surveys. In the first stage, a recirculation developed leeward of the island, followed by a wavy (wave‐like) tail that resembled a weak vortex street. In the second stage, a cold eddy probably originating from the leeward side of the island showed up 14 km downstream of the island. The wake water was colder, saltier, higher in chlorophyll‐a concentration, and produced isopycnal doming up to 60 m. In the recirculation area, the relative vorticity, either positive or negative, was 10 times of planetary vorticity, and the horizontal divergence or convergence was O (10–4 s–1) on average. Flow divergence and convergence in the wake were expected to form upwelling and downwelling zones, producing a vertical circulation that vertically displaces isotherms. High inverse Richardson number, produced by strong vertical shear of horizontal currents, was associated with intense overturning events in the wake. High vertical shear of horizontal currents drove the mixing. The dissipation rate of turbulent kinetic energy in the overturn regions is O (10–7–10–5) W Kg–1; the corresponding eddy diffusivity is O (10–3–10–1) m2 s–1. The wake water properties are vertically diffused via upwelling and turbulence and can be delivered downstream through eddy shedding or advection. The extent of downstream influence remains to be investigated.
Key PointsVortex‐Street like structure and shedding eddy are observed.Wake water is colder, saltier, and higher chlorophyll, and produces ~60 m domeTurbulence mixing occurred in the island wake
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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