Optimal estimation of zonal velocity and transport through Luzon Strait using variational data assimilation technique

A P-vector method was optimized using variational data assimilation technique, with which the vertical structures and seasonal variations of zonal velocities and transports were investigated. The results showed that westward and eastward flowes occur in the Luzon Strait in the same period in a year. However the net volume transport is westward. In the upper level (0m -500m),the westward flow exits in the middle and south of the Luzon Strait, and the eastward flow exits in the north. There are two centers of westward flow and one center of eastward flow. In the middle of the Luzon Strait, westward and eastward flowes appear alternately in vertical direction. The westward flow strengthens in winter and weakens in summer. The net volume transport is strong in winter (5.53 Sv) but weak in summer (0.29 Sv). Except in summer, the volume transport in the upper level accounts for more than half of the total volume transport (0m bottom). In summer, the net volume transport in the upper level is eastward (1.01 Sv), but westward underneath.

Characteristics of water exchange in the Luzon Strait during September 2006

The Luzon Strait is the only deep channel that connects the South China Sea(SCS) with the Pacific.The transport through the Luzon Strait is an important process influencing the circulation,heat and water budgets of the SCS.Early observations have suggested that water enters the SCS in winter but water inflow or outflow in summer is quite controversial.On the basis of hydrographic measurements from CTD along 120° E in the Luzon Strait during the period from September 18 to 20 in 2006,the characteristics of temperature,salinity and density distributions are analyzed.The velocity and volume transport through the Luzon Strait are calculated using the method of dynamic calculation.The major observed results show that water exchanges are mainly from the Pacific to the South China Sea in the upper layer,and the flow is relatively weak and eastward in the deeper layer.The net volume transport of the Luzon Strait during the observation period is westward,amounts to about 3.25 Sv.This result is consistent with historical observations.

Response of upper ocean currents to typhoons at two ADCP moorings west of the Luzon Strait

We deployed two ADCP mooring systems west of the Luzon Strait in August 2008,and measured the upper ocean currents at high frequency.Two typhoons passed over the moorings during approximately one-month observation period.Using ADCP observations,satellite wind and heat flux measurements,and high-resolution model assimilation products,we studied the response of the upper ocean to typhoons.The first typhoon,Nuri,passed over one of the moorings,resulting in strong Ekman divergence and significant surface cooling.The cooling of surface water lagged the typhoon wind forcing about one day and lasted about five days.The second typhoon,Sinlaku,moved northward east of the Luzon Strait,and did not directly impact currents near the observation regions.Sinlaku increased anomalous surface water transport exchange across the Luzon Strait,which modulated the surface layer current of the Kuroshio.

Surface inflow into the South China Sea through the Luzon Strait in winter

We studied the driving force of the Kuroshio intrusion into the South China Sea （SCS） during the winter monsoon, using satellite-tracked drifters entering the Luzon Strait （LS） through the Balintany and Babuyan Channels from the Philippine Sea. Most drifters passing through the Babuyan Channel in winter entered the interior SCS without a significant change in velocity. However, half of the drifters passing through the Balintany Channel entered the SCS at -30 cn/s, which was faster than when they entered the LS. The other half continued moving northwestward into the Kuroshio and returned to the North Pacific. Quantitative analyses, using surface climatological wind and sea surface height anomaly （SSHa） data explained both the difference in velocity of drifters between the two channels and their acceleration through the Balintany Channel. The results suggest that the positive meridional gradient of sea surface height in the Luzon Strait, caused by the pileup of seawater driven by the Northeast monsoon, as well as Ekman flow, contribute to the Kuroshio intrusion into the SCS through the Babuyan and Balintany Channels. The former may be the main driving force.

Water transports through the four main straits around the South China Sea

A quasi-global high-resolution HYbrid Coordinate Ocean Model (HYCOM) is used to investigate seasonal variations of water transports through the four main straits in the South China Sea. The results show that the annual transports through the four straits Luzon Strait, Taiwan Strait, Sunda Shelf and Mindoro Strait are -4.5, 2.3, 0.5 and 1.7 Sv (1 Sv=106 m3s-1), respectively. The Mindoro Strait has an important outflow that accounts for over one third of the total inflow through the Luzon Strait. Furthermore, it indicates that there are strong seasonal variations of water transport in the four straits. The water transport through the Luzon Strait (Taiwan Strait, Sunda Shelf, Mindoro Strait) has a maximum value of -7.6 Sv in December (3.1 Sv in July, 2.1S v in January, 4.5Sv in November), a minimum value of -2.1 Sv in June (1.5 Sv in October, -1.0 Sv in June, -0.2 Sv in May), respectively.

Water exchange and circulation structure near the Luzon Strait in early summer

Using hydrographic data covering large areas of ocean for the period from June 21 to July 5 in 2009, we studied the circulation structure in the Luzon Strait area, examined the routes of water exchange between the South China Sea （SCS） and the Philippine Sea, and estimated the volume transport through Luzon Strait. We found that the Kuroshio axis follows a e-shaped path slightly east of 121°E in the upper layer. With an increase in depth, the Kuroshio axis became gradually farther from the island of Luzon. To study the water exchange between the Philippine Sea and the SCS, identification of inflows and outflows is necessary. We first identified which flows contributed to the water exchange through Luzon Strait, which differs from the approach taken in previous studies. We determined that the obvious water exchange is in the section of 121°E. The westward inflow from the Philippine Sea into the SCS is 6.39 Sv in volume, and mainly in the 100-500 m layer at 19.5°-20°N （accounting for 4.40 Sv）, while the outflow from the SCS into the Philippine Sea is concentrated in the upper 100 m at 19°-20°N and upper 400 m at 21°-21.5°N, and below 240 m at 19°-19.5°N, accounting for 1.07, 3.02 and 3.43 Sv in volume transport, respectively.

Observation of material fluxes through the Luzon Strait

Based on field observations carried out in August, 2008, we obtained a set of data on velocity, hydrography, and hydroehemistry in the Luzon Strait, with which the velocity structure of the area, especially in deep channels, was analyzed, and the material fluxes, including water, dissolved oxygen, and nutrients were calculated. The results indicate that a net eastward water flux of 7.0 Sv occurred through the Luzon Strait. The deep layer flux in the southern part, through the deep channel, was westward with a value of 1.9 Sv, which confirms that deep Pacific water flows into the South China Sea via the deep passage in the Luzon Strait. Accordingly, the net flux of dissolved oxygen was 13.2× 10 5 mol/s, and the values for dissolved inorganic nitrogen, phosphate and silicate were 4.6× 10 4 mol/s, 2.4× 10 3 mol/s, and 8.9×10 4 mol/s, respectively. Detailed descriptions of these material fluxes in the upper layer, the upper-intermediate layer, the lower-intermediate layer, and the deep layer through the Luzon Strait are discussed. These results and interpretations highlight the importance of material exchanges between the South China Sea and the Pacific Ocean.

Energetics and temporal variability of internal tides in Luzon Strait:a nonhydrostatic numerical simulation

A fully nonlinear,three-dimensional nonhydrostatic model driven by four principal tidal constituents(M2,S2,K1,and O1) is used to investigate the spatial-temporal characteristics and energetics of internal tides in Luzon Strait(LS).The model results show that,during spring(neap) tides,about 64(47) GW(1 GW=109 W) of barotropic tidal energy is consumed in LS,of which 59.0%(50.5%) is converted to baroclinic tides.About 22(11) GW of the derived baroclinic energy flux subsequently passes from LS,among which 50.9%(54.3%) flows westward into the South China Sea(SCS) and 45.0%(39.7%) eastward into the Pacific Ocean,and the remaining 16(13) GW is lost locally owing to dissipation and convection.It is revealed that generation areas of internal tides vary with the spring and neap tide,indicating different source areas for internal solitary waves in the northern SCS.The region around the Batan Islands is the most important generation region of internal tides during both spring and neap tides.In addition,the baroclinic tidal energy has pronounced seasonal variability.Both the total energy transferred from barotropic tides to baroclinic tides and the baroclinic energy flux flowing out of LS are the highest in summer and lowest in winter.

The upper-ocean response to typhoons as measured at a moored acoustic Doppler current profiler

A moored acoustic Doppler current profiler(ADCP) data,satellite-derived sea surface wind data,and the chlorophyll-a concentration were used to examine the influence of typhoon events on the upper ocean in the central Luzon Strait. The data were collected between August 27 and October 6,2011. Large changes in ocean dynamics and marine life were recorded in the upper layers over the short term during the transit of each of the three violent typhoons that passed over the region during the study period. The geostrophic flow during the period of ADCP monitoring was comparable to the Ekman flow,recently shown to be prominent in the upper layer. Based on the influence of the three typhoon events that swept the Luzon Strait or traversed Luzon Island on their way to the South China Sea,we postulated a typhoon-induced upwelling around the ADCP and found that upward isothermal displacements reached 11.8–39.0 m,which was confirmed by the sea-level anomaly data recorded at the same time. This variability in the upper ocean may play an important role in biological activity,especially in offshore deep-sea regions.

Eddy formation and surface flow field in the Luzon Strait area during the summer of 2009

The formation of mesoscale eddies and the structure of the surface flow field in the Luzon Strait area were examined using in-situ CTD data, Argo float data, and multi-satellite remote sensing data collected from May to August 2009. The results show that vigorous water exchange between Kuroshio water and South China Sea （SCS） water began to emerge over the 200 m water column throughout the strait. Based on an objective definition of surface currents, float A69 tracked an anti-cyclonic eddy southwest of Taiwan Island under a Lagrangian current measurement. The salinity inside the anti-cyclonic eddy was higher than in typical SCS water but lower than in Kuroshio mainstream water, indicating that this eddy was induced by Kuroshio frontal intrusion through the Luzun Strait and into the SCS. From hydrographic data, we propose that continuous horizontal diffusion with high-salinity characteristics in the subsurface layer could extend to 119°E or even further west. The high-temperature filament, large positive sea level anomaly and clockwise geostrophic current all confirmed the existence of this warm eddy in May and June. A strongly negative wind stress curl maintained the eddy until it died. The surface flow field during July and August was rather complicated. Float A83 described an east-west orientated shuttle run in the 20°N section that was not reported by previous studies. At the same time, float A80 indicated a Kuroshio bend into the north-central region of Luzon Strait but it did not cross 120.5°E. The water mass rejoining the Kuroshio mainstream from the southern tip of Taiwan Island was less saline, indicating an entrainment of water from SCS by the Kuroshio bend.

Three-dimensional numerical simulation of internal tides that radiated from the Luzon Strait into the Western Pacific

Recent satellite altimeter observations have indicated that internal tides （ITs） from the Luzon Strait （LS） propagate more than 2 500 km into the Western Pacific （WP）. This study used a high-resolution three-dimensional numerical model to reproduce and examine the ITs radiation process. The propagation of diurnal and semidiurnal ITs showed different patterns and variations. Diurnal ITs with lower frequency were affected more by the earth＇s rotation and they were bent more toward the equator than semidiurnal ITs. ITs phase speeds are functions of latitude and diurnal ITs showed greater distinctions of phase speeds during propagation. For M2 ITs, the wavelength remained nearly unchanged but the beam width increased significantly during propagation away from the LS. For diurnal ITs （K1 and O0, the wavelength decreased noticeably with latitude, while the beam width varied little during propagation because of blocking by land. Baroclinic energy was also examined as a complement to satellite results reported by Zhao （2014）. The magnitude of the generated baroclinic energy flux reduced remarkably within 300 km from the generation site but it then decayed slowly when propagating into abyssal sea. Baroclinic energy of diurnal ITs was found to dissipate at a slower rate than semidiurnal ITs along the main propagation path in the WP.

上海商人与晚清疆吏的“一两重银元”之争

光绪三十年(1904年),吕海寰、盛宣怀电告外务部,说英国希望中国遵守《商约》[1]铸造'一两重银元',以便杜绝胥吏利用现存的银两制度'高下其手',并许诺'将来中英两国人民在中国各处,即用(所铸一两重银元)以完纳各项税'。电文中还引用英国人的话鼓励道'同律度量衡,自古经国善法'。[2]政府是否应该统一度量衡,是否应该用一两重银元来统一度量衡,外国人建议的背后是否有阴谋,一两重银元一旦作为新币流通,它所牵动的全局,究竟是有利的还是有害的,比如新币的铸造成本,与现行的货币之间怎么公正兑换等,围绕这些问题,清廷内部出现了长久而激烈的电报战。

Progress on deep circulation and meridional overturning circulation in the South China Sea

The deep overflow through the Luzon Strait drives the cyclonic deep circulation in the South China Sea(SCS). In the mean time, the intruding Pacific deep water transforms and upwells due to enhanced diapycnal mixing in the SCS. Both processes greatly contribute to the SCS meridional overturning circulation(SCSMOC). At the same time, both the deep circulation and meridional overturning circulation are modulated by rough topography in the SCS. Furthermore, the spatial structure of the SCSMOC infers a link between the upper-layer circulation and deep circulation in the SCS. This paper reviews recent advances in the SCS deep circulation and meridional overturning circulation, including the driving mechanism of the SCS deep circulation and its modulation by topography, as well as the spatial structure of the SCSMOC and its dynamical mechanism.

Nonlinear dynamics of Kuroshio intrusion in the Luzon Strait

This study used a 1.5-layer reduced-gravity numerical model to investigate the nonlinear dynamics of Kuroshio intrusion into the Luzon Strait.The model results suggested that both basin-scale wind curl and lateral friction are the primary factors that control the transformation of the flow,although inertia also plays an important role.Using an idealized model,both the mechanism via which the flow pattern changes depending on the two primary factors and the occurrence of hysteresis were investigated.It was established that the transformation of the Kuroshio flow field between the four previously reported flow patterns(i.e.,leaping across,current looping,eddy shedding,and branch intruding) can be explained under a unified theoretical framework.A diagram is proposed to explain how the flow field transforms between the four patterns from a certain prior state when varying the values of the controlling factors.