The winter western boundary current of the South China Sea:physical structure and volume transport in December 1998

The unique survey in December 1998 mapped the entire western boundary area of the South China Sea（SCS）,which reveals the three-dimensional structure and huge volume transport of the swift and narrow winter western boundary current of the SCS（SCSwwbc） in full scale. The current is found to flow all the way from the shelf edge off Hong Kong to the Sunda Shelf with a width around 100 km and a vertical scale of about 400 m. It appears to be the strongest off the Indo-China Peninsula, where its volume transport reached over 20×10~6 m^3/s. The current is weaker upstream in the northern SCS to the west of Hong Kong. A Kuroshio loop or detached eddy intruded through the Luzon Strait is observed farther east where the SCSwwbc no more exists. The results suggest that during the survey the SCSwwbc was fed primarily by the interior recirculation of the SCS rather than by the＂branching＂ of the Kuroshio from the Luzon Strait as indicated by surface drifters, which is likely a near-surface phenomenon and only contributes a minor part to the total transport of the SCSwwbc. Several topics related to the SCSwwbc are also discussed.

Volume transport in the East Taiwan Channel in response to diff erent tracks of typhoons as revealed by HYCOM data

Northward infl ow through the East Taiwan Channel is vital in modulation of water exchange processes off northeastern Taiwan,China.In addition to the eff ects of the Kuroshio Current and westward-propagating oceanic mesoscale eddies,the seas off eastern Taiwan,China,are frequently infl uenced by typhoons.Focusing on extrema of East Taiwan Channel volume transport(ETCVT)that appear within days of typhoon infl uence,this study investigated 124 historical cyclones including 91 typhoons that passed over the study sea area off eastern Taiwan,China.Based on 25-year HYbrid Coordinate Ocean Model(HYCOM)data,71%of short-term(within 10 d)ETCVT absolute values with>5 Sv occurred under the infl uence of typhoons crossing the study sea area,and the maximum short-term ETCVT extrema induced by typhoons were 12.5 and-10.9 Sv.The ETCVT extrema induced by typhoons showed reasonable positive correlation with typhoon wind speed.More importantly,the ETCVT extrema diff ered in response to diff erent typhoon tracks.Three types of typhoon were identifi ed based on their track and impact on ETCVT.Representative typhoon cases were examined to elucidate the specifi cs of each typical response.Based on historical best track data and HYCOM data,it was established that Type I typhoons caused the ETCVT to exhibit a negative extremum followed by a positive extremum.All Type I,II,and III typhoons tended to result in typical ETCVT fl uctuations in the surface mixing layer above the depth of 50-100 m,while Type II typhoons were more likely to induce ETCVT fl uctuations in the subsurface layer.The fi ndings of this study enhance understanding of ETCVT extrema that occur following typhoon passage,which is valuable for short-term physical-biogeochemical studies both in the study region and in areas downstream owing to the large net volume transport changes induced by typhoons.

The annual mean sketches and climatological variability of the volume and heat transports through the inter-basin passages:A study based on 1 400-year spin up of MOM4p1

The annual mean volume and heat transport sketches through the inter-basin passages and transoceanic sections have been constructed based on 1400-year spin up results of the MOM4p 1. The spin up starts from a state of rest, driven by the monthly climatological mean force from the NOAAWorld Ocean Atlas （1994）. The volume transport sketch reveals the northward transport throughout the Pacific and southward transport at all latitudes in the Atlantic. The annual mean strength of the Pacific-Arctic-Atlantic through flow is 0.63x106 m3/s in the Bering Strait. The majority of the northward volume transport in the southern Pacific turns into the Indonesian through flow （ITF） and joins the Indian Ocean equatorial current, which subse- quently flows out southward from the Mozambique Channel, with its majority superimposed on the Ant- arctic Circumpolar Current （ACC）. This anti-cyclonic circulation around Australia has a strength of 11 x 106 ms /s according to the model-produced result. The atmospheric fresh water transport, known as P-E^R （pre- cipitation minus evaporation plus runoff）, constructs a complement to the horizontal volume transport of the ocean. The annual mean heat transport sketch exhibits a northward heat transport in the Atlantic and poleward heat transport in the global ocean. The surface heat flux acts as a complement to the horizontal heat transport of the ocean. The climatological volume transports describe the most important features through the inter-basin passages and in the associated basins, including： the positive P-E＋R in the Arctic substantially strengthening the East Greenland Current in summer; semiannual variability of the volume transport in the Drake Passage and the southern Atlantic-Indian Ocean passage; and annual transport vari- ability of the ITF intensifying in the boreal summer. The climatological heat transports show heat storage in July and heat deficit in January in the Arctic; heat storage in January and heat deficit in July in the Antarctic circumpolar current regime （ACCR）; and intensified heat transport of the iTF in July. The volume transport of the ITF is synchronous with the volume transport through the southern Indo-Pacific sections, but the year-long southward heat transport of the ITF is out of phase with the heat transport through the equatorial Pacific, which is northward before May and southward after May. This clarifies the majority of the ITF origi- natinR from the southern Pacific Ocean.

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.

The Luzon Strait transport variations during 1997～2000

Based on the output data from 1997 to 2000 obtained by the MITgcm's(general circulation model)adjoint assimilation method, volume, heat and salt transports through the Luzon Strait are calculated. The results indicate that there are obvious different characteristics between 1997 and 1998~2000 on the transports through the Luzon Strait. During 1997, the Luzon Strait had a mean net westward transport of 3.93 ×106 m3/s with a maximum transport of 7.34×106 m3/s in October. During 1998~2000, the Luzon Strait possessed an annual mean eastward transport of 0.93 ×106 , 1.80 ×106 and 1.00 ×106 m3/s respectively with a maximum eastward transport of 4.10 ×106 /3.31 ×106 m3/s in July 1998/ 1999 and 2.06 ×106 m3/s in April 2000, respectively. Moreover, the transports in 1997 indicated a difference from the other years, i.e., that the ranges of westward inflows expanded more obviously to north of the Luzon Strait and downwards exceeding those of the other years. The westward inflows expanded horizontally to the north part of the Luzon Strait until 21o N.

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.

Several characteristics of water exchange in the Luzon Strait

Using the hydrographic data obtained from two sectional observations crossing the Luzon strait in the summer of 1994 and in the winter of 1998, the volume transport through this strait is calculated. It is found that in winter the volume transport (4.45×106 m3/s) is far larger than that in the summer (2.0 ×106 m3/s), respectively being about equal to 15.0% and 6.9% of the Kuroshio.And the paths of water in and out of the section of the strait vary distinctly with the season. In summer, the water flows in and out of the South China Sea (SCS) three times: that is, the inlet passages almost appear on the southern sides of the three deep troughs,the outlet passages are all located on the northern sides of the troughs,and the in-out volume transports through the channel are not lower than 4.0×106 m3/s. The highest velocity (>80 cm/s) and the largest entering water capacity (6.6×106 m3/s) all occur in the Balintang Channel. Except for the north outlet passage in the section, all the higher velocities over 10 cm/s are mainly distributed on the layer above 500 m. In winter,the water flows in and out of the strait two times:the southern sides of the second and third deep troughs are the main passages of the Kuroshio water running into the SCS,while the whole section of the first deep trough and the bottom section of the second deep trough are the outlet passages.The higher velocities over 10 cm/s are almost distributed on the layer above 300 m. Numerical calculation shows that the northern side of the third trough may be the outlet passage.

A quasi-synoptic interpretation of water mass distribution and circulation in the western North Pacific II:Circulation

Using the data of conductivity-temperature-depth （CTD） intensive observations conducted during Oct-Nov. 2005, this study provides the first three-dimension quasi-synoptic description of the circulation in the western North Pacific. Several novel phenomena are revealed, especially in the deep ocean where earlier observations were very sparse. During the observations, the North Equatorial Current （NEC） splits at about 12°N near the sea surface. This bifurcation shifts northward with depth, reaching about 20°N at 1 000 m, and then remains nearly unchanged to as deep as 2 000 m. The Luzon Undercurrent （LUC）, emerging below the Kuroshio from about 21°N, intensifies southward, with its upper boundary surfacing around 12°N. From there, part of the LUC separates from the coast, while the rest continues southward to join the Mindanao Current （MC）. The MC extends to 2 000 m near the coast, and appears to be closely related to the subsurface cyclonic eddies which overlap low-salinity water from the North Pacific. The Mindanao Undercurrent （MUC）, carrying waters from the South Pacific, shifts eastward upon approaching the Mindanao coast and eventually becomes part of the eastward undercurrent between 10°N and 12°N at 130°E. In the upper 2 000 dbar, the total westward transport across 130°E between 7.5°N and 18°N reaches 65.4 Sv （1 Sv = 10-6 m3s^-1）, the northward transport across 18°N from Luzon coast to 130°E is up to 35.0 Sv, and the southward transport across 7.5°N from Mindanao coast to 130°E is 27.9 Sv.

Numerical simulation of the Kuroshio intrusion into the South China Sea by a passive tracer

Owing to lack of observational data and accurate definition,it is difficult to distinguish the Kuroshio intrusion water from the Pacific Ocean into the South China Sea（SCS）.By using a passive tracer to identify the Kuroshio water based on an observation-validated three-dimensional numerical model MITgcm,the spatio-temporal variation of the Kuroshio intrusion water into the SCS has been investigated.Our result shows the Kuroshio intrusion is of distinct seasonal variation in both horizontal and vertical directions.In winter,the intruding Kuroshio water reaches the farthest,almost occupying the area from 18°N to 23°N and 114°E to 121°E,with a small branch flowing towards the Taiwan Strait.The intrusion region of the Kuroshio water decreases with depth gradually.However,in summer,the Kuroshio water is confined to the east of 118°E without any branch reaching the Taiwan Strait; meanwhile the intrusion region of the Kuroshio water increases from the surface to the depth about 205 m,then it decreases with depth.The estimated annual mean of Kuroshio Intrusion Transport（KIT） via the Luzon Strait is westward to the SCS in an amount of –3.86×106 m3/s,which is larger than the annual mean of Luzon Strait Transport（LST） of –3.15×106 m3/s.The KIT above 250 m accounts for 60%–80% of the LST throughout the entire water column.By analyzing interannual variation of the Kuroshio intrusion from the year 2003 to 2012,we find that the Kuroshio branch flowing into the Taiwan Strait is the weaker in winter of La Ni？a years than those in El Ni？o and normal years,which may be attributed to the wind stress curl off the southeast China then.Furthermore,the KIT correlates the Ni？o 3.4 index from 2003 to 2012 with a correlation coefficient of 0.41,which is lower than that of the LST with the Ni？o 3.4 index,i.e.,0.78.

Observation of an anti-cyclonic mesoscale eddy in the subtropical northwestern Pacific Ocean from altimetry and Argo profiling floats

The comprehensive three-dimensional structures of an anti-cyclonic mesoscale eddy(AE)in the subtropical northwestern Pacific Ocean were investigated by combining the Argo floats profiles with enhanced vertical and temporal sampling and satellite altimetry data.The AE originated near the Kuroshio Extension and then propagated westward with mean velocity of 8.9 cm/s.Significant changes and evolutions during the AE’s growing stage(T1)and further growing stage(T2)were revealed through composite analysis.In the composite eddy core,maximum temperature(T)and salinity(S)anomalies were of 1.7(1.9)℃ and 0.04(0.07)psu in T1(T2)period,respectively.The composite T anomalies showed positive in almost whole depth,but the S anomalies exhibited a sandwich-like pattern.The eddy’s intensification and its influence on the intermediate ocean became more significant during its growth.The trapping depth increased from 400×10^4 Pa to 580×10^4 Pa while it was growing up,which means more water volume,heat and salt content in deeper layers can be transported.The AE was strongly nonlinear in upper oceans and can yield a typical mean volume transport of 0.17×10^6 m^3/s and a mean heat and salt transport anomaly of 3.6×10^11 W and–2.1×10^3 kg/s during the observation period.The Energy analysis showed that eddy potential and kinetic energy increased notably as it propagated westward and the baroclinic instability is the major energy source of the eddy growth.The variation of the remained Argo float trapped within the eddy indicated significant water advection during the eddy’s propagation.

Current and Thermohaline Characteristics of the Arabian Sea During January 1998

Based on a ship survey during January 1998, the characteristics of the flow, the thermohaline properties and the volume transport of the Arabian Sea are discussed. A strong westward flow exists between 10.5?N and 11?N, part of which turns to the south as the Somali current near the coast at about 10?N and the rest turns north. At the passage between the African continent and the So- cotra Island, the northern branch separates into two flows: the left one enters the passage and the right one flows eastward along the southern slope of the island. Off the island the flow separates once more, most of it meandering northeast and a small fraction flow- ing southeast. Volume transport calculation suggests that the tidal transport is one or two orders of magnitude smaller than the total transport in this region and it becomes more important near the coast. The average velocity of the flow in the upper layer (0-150 m) is about 20 cm s-1, with a maximum of 53 cm s-1 appearing east of the Socotra Island, and the subsurface layer (200-800 m) has an aver- age velocity of 8.6 cm s-1; the velocity becomes smaller at greater depths. The depth of the seasonal thermocline is about 100 m, above which there is a layer with well mixed temperature and dissolved oxygen. High-salinity and oxygen-rich water appears near the surface of the northern Arabian Sea; a salinity maximum and oxygen minimum at 100 m depth along 8?N testifies the subduction of surface water from the northern Arabian Sea. Waters from the Red Sea and the Persian Gulf also influence the salinity of the area.

NONLINEAR DYNAMICAL ANALYSIS OF BIFURCATION AND CONFLUENCE OF THE PACIFIC WESTERN BOUNDARY CURRENTS

In this paper, we analyze the bifurcation and the confluence of the Pacific western boundary currents by an analytical approach. Applying the conservation law, the geostrophic balance relation and the Bernoulli integral to a reduced gravity model, we get a quantitative relation for the outflow and the inflow, and establish the related formulae for the width and the veering angle of offshore currents under the inflow condition. Furthermore, a comparison between the volume transport based on the observation data and the analytical value for the Pacific western boundary currents is presented, which validates the theoretical analysis.

Role of sea level pressure in variations of the Canadian Arctic Archipelago throughflow

The throughflow in the Canadian Arctic Archipelago(CAA)had a significant impact on the North Atlantic Ocean with the Arctic climate change.The findings of physical mechanisms driving the throughflow in the CAA differed and few studies about the impact of sea level pressure(SLP)on the CAA throughflow were made.A high-resolution ice-ocean coupled Arctic Ocean Finite-Volume Community Ocean Model(AO-FVCOM)was used over the period 1978-2016 to examine the interannual and seasonal variability of the outflows in the CAA and the mechanism of SLP in driving the variation of the CAA throughflow quantitively.The simulated volume transport through Davis Strait,Nares Strait,Lancaster Sound and Jones Sound showed consistent increasing trends over 1978-2016 and the larger flux in winter and spring than in summer and fall.The variation of volume transport through Nares Strait contributed more than Lancaster and Jones Sound to the variation through Davis Strait.Five process-oriented experiments were made to further explore the role of SLP in setting up and controlling the sea surface height(SSH)difference and thus the throughflow transport in the CAA.The SLP was a primary forcing to control the SSH difference and the outflow transport compared with the wind forcing.The memory of the SSH to the SLP was short and an equilibrium state could be reached if the SLP varied with time.The upstream and downstream SLP difference,however,made a slight direct contribution to driving the volume transport of the CAA throughflow.In addition to the external forcing of SLP and wind,the variability of the CAA outflow was also influenced by the variability of the inflow/outflow and SSH on boundaries connected to the Pacific and Atlantic Oceans.The choice of SLP datasets from CORE-v2,ECMWF and NCEP was sensitive to the simulated uncertainty of volume transport.