The impact of salt water inflows on the distribution of polycyclic aromatic hydrocarbons in the deep water of the Baltic Sea

Salt-water inflows into the Baltic Sea are important events for renewing the deep and bottom waters of the deep basins of the Baltic Sea. These events occur only at irregular intervals. The last strong event was in January 1993 followed by minor inflows in winter 1993/1994. As a result of these inflows, the deep water of the central Baltic basins was completely renewed.Based on extensive observations of polycyclic aromatic hydrocarbons (PAHs) in water, fluffy layer material and surface sediments between 1992 and 1998, the transformation of PAHs and the modification of their distribution in the Baltic deep water is discussed in connection with the spreading of the inflowing highly saline and oxygen-rich water along its pathway from the sills into the central basins. In the course of the inflows in 1993/1994, the PAH concentration in the deep water of the different basins increased significantly. The concentrations were elevated, at least by a factor of 2 and as much as seven to eight times (for the four-ring PAHs) compared to the previous and the following years. Two hypotheses for the causes were discussed: the inflowing salt water may have entrained more highly polluted surface water in the western Baltic Sea, or it may have entrained contaminated fluffy layer material or sediment particles along the route of transport.     

Physical processes in the transition zone between North Sea and Baltic Sea. Numerical simulations and observations

The dynamics in the transition zone between the North Sea and Baltic Sea are analyzed here using data from a 22-year-long climatic simulation with a focus on the periods 1992–1994 and 2001–2003 when two recent major inflow events occurred. Observations from gauges and in situ measurements are used to validate the model. Parameters, which cannot be easily measured, such as water and salt transports through straits, have been compared against similar previous estimates. The good performance of simulations is attributed to the finer resolution of the model compared to earlier set ups. The outflow in the Kattegat, which is an analogue of the tidal outflows, tends to propagate to the North over the shallows without showing a substantial deflection to the right due to the Earth's rotation. The inflow follows the topography. The different inflow and outflow pathways are explained as a consequence of the specific combination of bathymetry, axial and lateral processes. The circulation in Kattegat is persistently clockwise with an eastern intensification during inflow and a western one during outflow regimes. The tidal wave there propagates as Kelvin wave, keeping the coast on its right. The flows in the two main straits reveal very different responses to tides, which are also highly asymmetric during inflow and outflow conditions. The circulation has a typical two-layer structure, the correlation between salinity and velocity tends to increase the salt transport in the salinity conveyor belt. The transversal circulation in the entrance of the Sound enhances the vertical mixing of the saltier North Sea water. The long-term averaged ratio of the water transports through the Great Belt and the Sound is ∼2.6-2.7 but this number changes reaching lower values during the major inflow in 1993. The transports in the straits are asymmetric. During inflow events the repartition of water penetrating the Baltic Sea is strongly in favor of the pathway through the Sound, which provides a shorter connection between the Kattegat and Baltic proper. The wider Great Belt has a relatively larger role in exporting water from the Baltic into the North Sea. A demonstration is given that the ventilation of the Baltic Sea deep water is not only governed by the dynamics in the straits and the strong westerly winds enhancing the eastward propagation of North Sea water (a case in 1993), but also by the clockwise circulation in the Kattegat acting as a preconditioning factor for the flow-partitioning.     

Ventilation of the Black Sea pycnocline. Parameterization of convection, numerical simulations and validations against observed chlorofluorocarbon data

Data from field observations and numerical model simulations are used to understand and quantify the pathways by which passive tracers penetrate into the Black Sea intermediate and deep layers. Chlorofluorocarbon (CFC) concentrations measured during the1988 R.V. Knorr cruise show strong decrease with increasing density in the Black Sea and illustrate the very slow rate of ventilation of deep water in this basin. We develop a 3D numerical model based on the Modular Ocean Model (MOM), and calibrate it in a way to produce consistent simulations of observed temperature, salinity and CFCs. One important feature is the implementation of a special parameterization for convection, which is an alternative of the convective adjustment in MOM and handles the penetration of the Bosporus plume into the halocline. The model forcing includes interannually variable wind, heat and water fluxes constructed from Comprehensive Ocean–Atmosphere Data Set and ECMWF atmospheric analysis data and river runoff data. The analysis of observations and simulated data are focused on correlations between thermohaline and tracer fields, dynamic control of ventilation, and the relative contributions of sources at the sea surface and outflow from the Bosporus Strait in the formation of intermediate and deep waters. A simple theory is developed which incorporates the outflow from the strait along with the vertical circulation (vertical turbulent mixing and Ekman upwelling) and reveals their mutual adjustment. The analyses of simulated and observed CFCs demonstrate that most of the CFC penetrating the deep layers has its source at the sea surface within the Black Sea rather than from the Marmara Sea via the Bosporus undercurrent. Under present-day conditions, the surface CFC signals have reached only the upper halocline. Intrusions below 600 m are not simulated. The major pathways of penetration of CFCs are associated with cold-water mass formation sites, Bosporus effluent, as well as with the diapycnal mixing in the area of Rim Current. Future CFC sampling strategies coherent with the unique conditions in the Black Sea are discussed.     

Modeling the pathways and ages of inflowing salt- and freshwater in the Baltic Sea

A three-dimensional, eddy-permitting ocean circulation model with implemented bottom boundary layer model and flux-corrected transport scheme is used to calculate the pathways and ages of various water masses in the Baltic Sea. The agreement between simulated and observed temperature and salinity profiles of the period 1980–2004 is satisfactory. Especially the renewal of the deep water in the Baltic proper by gravity-driven dense bottom flows is better simulated than in previous versions of the model. Based upon these model results details of the mean circulation are analyzed. For instance, it is found that after the major Baltic inflow in January 2003 saline water passing the Słupsk Furrow flows directly towards northeast along the eastern slope of the Hoburg Channel. However, after the baroclinic summer inflow in August/September 2002 the deep water flow spreads along the southwestern slope of the Gdansk Basin. Further, the model results show that the patterns of mean vertical advective fluxes across the halocline that close the large-scale vertical circulation are rather patchy. Mainly within distinct areas are particles of the saline inflow water advected vertically from the deep water into the surface layer. To analyze the time scales of the circulation mean ages of various water masses are calculated. It is found that at the sea surface of the Bornholm Basin, Gotland Basin, Bothnian Sea, and Bothnian Bay the mean ages associated to inflowing water from Kattegat amount to 26–30, 28–34, 34–38, and 38–42 years, respectively. Largest mean sea surface ages of more than 30 years associated to the freshwater of the rivers are found in the central Gotland Basin and Belt Sea. At the bottom the mean ages are largest in the western Gotland Basin and amount to more than 36 years. In the Baltic proper vertical gradients of ages associated to the freshwater inflow are smaller than in the case of inflowing saltwater from Kattegat indicating an efficient recirculation of freshwater in the Baltic Sea.     

North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects

We survey the recent progress in studies of North Pacific Intermediate Water (NPIW) in SAGE (SubArctic Gyre Experiment), including important results obtained from related projects. Intensive observations have provided the transport distributions relating to NPIW and revealed the existence of the cross-wind-driven gyre Oyashio water transport that flows directly from the subarctic to subtropical gyres through the western boundary current as well as the diffusive contribution across the subarctic front. The anthropogenic CO₂ transport into NPIW has been estimated. The northern part of NPIW in the Transition Domain east of Japan is transported to the Gulf of Alaska, feeding the mesothermal (intermediate temperature maximum) structure in the North Pacific subarctic region where deep convection is restricted by the strong halocline maintained by the warm and salty water transport originating from NPIW. This heat and salt transport is mostly balanced by the cooling and freshening in the formation of dense shelf water accompanied by sea-ice formation and convection in the Okhotsk Sea. Intensive observational and modeling studies have substantially altered our view of the intermediate-depth circulation in the North Pacific. NPIW circulations are related to diapycnal-meridional overturning, generated around the Okhotsk Sea due to tide-induced diapycnal mixing and dense shelf water formation accompanied by sea-ice formation in the Okhotsk Sea. This overturning circulation may possibly explain the direct cross-gyre transport through the Oyashio along the western boundary from the subarctic to subtropical gyres. This revised version was published online in July 2006 with corrections to the Cover Date.     

A numerical study of the South China Sea deep circulation and its relation to the Luzon Strait transport

A fine-resolution MOM code is used to study the South China Sea basin-scale circulationand its relation to the mass transport through the Luzon Strait. The model domain includes the South China Sea, part of the East China Sea, and part of the Philippine Sea so that the currents in the vicinity of the Luzon Strait are free to evolve. In addition, all channels between the South China Sea and the Indonesian seas are closed so that the focus is on the Luzon Strait transport. The model is driven by specified Philippine Sea currents and by surface heat and salt flux conditions. For simplicity, no wind-stress is applied at the surface.The simulated Luzon Strait transport and the South China Sea circulation feature a sandwich vertical structure from the surface to the bottom. The Philippine Sea water is simulated to enter the South China Sea at the surface and in the deep ocean and is carried to the southern basin by western boundary currents. At the intermediate depth, the net Luzon Strait transport is out of t     

Combined tidal velocity and duration asymmetries as a determinant of water transport and residual flow in Paranaguá Bay estuary

Net transports of water, salt and suspended particulate matter (SPM) for a cross-section in front of the Paranaguá Harbour (Paranaguá Bay, Brazil) are presented for eight distinct tidal cycles. Data include measurements over single spring and neap tidal cycles, during both wet and dry seasons. The main drive forces of circulation and SPM dynamics are identified. Advective transport dominated under moderate to high vertical salinity stratification and weak currents, while mixing processes dominated under well-mixed conditions generated by high currents and low freshwater input. Under partially mixed conditions, both advective and mixing processes were important. The tide-induced residual circulation dictated the magnitude and direction of residual currents and net transports of water and salt, but not of SPM transport. The SPM dynamics was intrinsically related to cyclical processes of erosion, resuspension and deposition driven by tidal currents. The turbulent mixing intensity conditioned the vertical mixing of SPM. Resuspension and vertical mixing were conspicuous in spring cycles, while the horizontal advection preponderated in the neaps. Lags between maximum currents and SPM peak concentrations occurred, with more pronounced hysteresis during ebb periods.     

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean

A numerical study using a 3-D nonhydrostatic model has been applied to baroclinic processes generated by the K ₁ tidal flow in and around the Kuril Straits. The result shows that large-amplitude unsteady lee waves are generated and cause intense diapycnal mixing all along the Kuril Island Chain to levels of a maximum diapycnal diffusivity exceeding 10³ cm²s⁻¹. Significant water transformation by the vigorous mixing in shallow regions produces the distinct density and potential vorticity (PV) fronts along the Island Chain. The pinched-off eddies that arise and move away from the fronts have the ability to transport a large amount of mixed water (∼14 Sv) to the offshore regions, roughly half being directed to the North Pacific. These features are consistent with recent satellite imagery and in-situ observations, suggesting that diapycnal mixing within the vicinity of the Kuril Islands has a greater impact than was previously supposed on the Okhotsk Sea and the North Pacific. To examine this influence of tidal processes at the Kurils on circulations in the neighboring two basins, another numerical experiment was conducted using an ocean general circulation model with inclusion of tidal mixing along the islands, which gives a better representation of the Okhotsk Sea Mode Water than in the case without the tidal mixing. This is mainly attributed to the added effect of a significant upward salt flux into the surface layer due to tidal mixing in the Kuril Straits, which is subsequently transported to the interior region of the Okhotsk Sea. With a saline flux into the surface layer, cooling in winter in the northern part of the Okhotsk Sea can produce heavier water and thus enhance subduction, which is capable of reproducing a realistic Okhotsk Sea Mode Water. The associated low PV flux from the Kuril Straits to the open North Pacific excites the 2nd baroclinic-mode Kelvin and Rossby waves in addition to the 1st mode. Interestingly, the meridional overturning in the North Pacific is strengthened as a result of the dynamical adjustment caused by these waves, leading to a more realistic reproduction of the North Pacific Intermediate Water (NPIW) than in the case without tidal mixing. Accordingly, the joint effect of tidally-induced transport and transformation dominating in the Kuril Straits and subsequent eddy-transport is considered to play an important role in the ventilation of both the Okhotsk Sea and the North Pacific Ocean. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oceanic vertical mixing of the lower halocline water in the Chukchi Borderland and Mendeleyev Ridge

Oceanic vertical mixing of the lower halocline water (LHW) in the Chukchi Borderland and Mendeleyev Ridge was studied based on in situ hydrographic and turbulent observations. The depth-averaged turbulent dissipation rate of LHW demonstrates a clear topographic dependence, with a mean value of 1.2×10–9 W/kg in the southwest of Canada Basin, 1.5×10–9 W/kg in the Mendeleyev Abyssal Plain, 2.4×10–9 W/kg on the Mendeleyev Ridge, and 2.7×10–9 W/kg on the Chukchi Cap. Correspondingly, the mean depth-averaged vertical heat flux of the LHW is 0.21 W/m2 in the southwest Canada Basin, 0.30 W/m2 in the Mendeleyev Abyssal Plain, 0.39 W/m2 on the Mendeleyev Ridge, and 0.46 W/m2 on the Chukchi Cap. However, in the presence of Pacific Winter Water, the upward heat released from Atlantic Water through the lower halocline can hardly contribute to the surface ocean. Further, the underlying mechanisms of diapycnal mixing in LHW—double diffusion and shear instability—was investigated. The mixing in LHW where double diffusion were observed is always relatively weaker, with corresponding dissipation rate ranging from 1.01×10–9 W/kg to 1.57×10–9 W/kg. The results also show a strong correlation between the depth-average dissipation rate and strain variance in the LHW, which indicates a close physical linkage between the turbulent mixing and internal wave activities. In addition, both surface wind forcing and semidiurnal tides significantly contribute to the turbulent mixing in the LHW.     

Advantages of vertically adaptive coordinates in numerical models of stratified shelf seas

Shelf seas such as the North Sea and the Baltic Sea are characterised by spatially and temporally varying stratification that is highly relevant for their physical dynamics and the evolution of their ecosystems. Stratification may vary from unstably stratified (e.g., due to convective surface cooling) to strongly stratified with density jumps of up to 10 kg/m³ per m (e.g., in overflows into the Baltic Sea). Stratification has a direct impact on vertical turbulent transports (e.g., of nutrients) and influences the entrainment rate of ambient water into dense bottom currents which in turn determine the stratification of and oxygen supply to, e.g., the central Baltic Sea. Moreover, the suppression of the vertical diffusivity at the summer thermocline is one of the limiting factors for the vertical exchange of nutrients in the North Sea. Due to limitations of computational resources and since the locations of such density jumps (either by salinity or temperature) are predicted by the model simulation itself, predefined vertical coordinates cannot always reliably resolve these features. Thus, all shelf sea models with a predefined vertical coordinate distribution are inherently subject to under-resolution of the density structure.To solve this problem, Burchard and Beckers (2004) and Hofmeister et al. (2010) developed the concept of vertically adaptive coordinates for ocean models, where zooming of vertical coordinates at locations of strong stratification (and shear) is imposed. This is achieved by solving a diffusion equation for the position of the coordinates (with the diffusivity being proportional to the stratification or shear frequencies). We will show for a coupled model system of the North Sea and the Baltic Sea (resolution ∼ 1.8 km) how numerical mixing is substantially reduced and model results become significantly more realistic when vertically adaptive coordinates are applied. We additionally demonstrate that vertically adaptive coordinates perform well in simulating the two dynamically different regions North Sea and Baltic Sea with a single parameter set.An analysis of the computational overhead of the adaptive coordinates indicates an increase of 5–8% in runtime. This is still less expensive than adding more sigma-layers to reduce spurious numerical mixing.     

Spring thermocline formation in the coastal zone of the southeastern Baltic Sea based on field data in 2010–2013

The transition from winter vertical mixing to the formation of the spring thermocline in the southeastern Baltic Sea is studied based on data from the hydrophysical measurements program (11 expeditions) in the Russian part of Gdansk Bay in March–June 2010, 2011, and 2013. CTD measurements were taken along the standard 18-km transect across the isobaths with a 500-m step abeam the city of Baltiysk. A set of frequently measured data was collected in a 1–2 week interval from the end of March to the beginning of May, which made it possible to analyze the transformation of the vertical thermal structure of water from inverse winter type to the summer stratification with the transition of temperature over the temperature of the density maximum. Series of repeated measurements at the deep and coastal stations as well as surface and subsurface towed measurements were carried out. The fact that lenses of freshened warmer water appear at the surface almost simultaneously with intensification of cold intrusions in intermediate (10–40 m) layers makes it possible not only to confirm the advective nature of the formation of the spring thermocline in the Baltic Sea, but also to hypothesize about the intensification of intrabasin exchange when winter-time vertical mixing ceases: the potential energy excess supported by vertical mixing in the 60-m upper quasi-homogeneous layer (UQL) of the Baltic Proper, in which the horizontal estuarine salinity gradient is significant, is converted to kinetic energy of exchange currents as the mixing process terminates. Such water dynamics makes it possible to explain the intensification of intrusions in the Baltic in spring and the formation of the cold intermediate layer due to the fast propagation of late-winter UQL water from the Bornholm Basin to the Baltic Proper. The results agree well with earlier published studies of other authors.     

Transport Through the Contraction Area in the Little Belt

The Great Belt, the Øresund and the Little Belt connect the central Baltic Sea and the Kattegat. A fixed station was moored in the contraction area in the Little Belt during the period 18–28 July 1995, measuring temperature, salinity and current in two levels, while discharge was measured by the RVDana. The composite Froude number calculated at the fixed station shows that the two layer flow through this area was most often supercritical. The discharges were satisfactorily related to the currents measured at the fixed station, and time-series of transports through the Little Belt were established. When compared to the transports through the Øresund the water transport ratio (Øresund:Little Belt) was found to be 4·4, while the salt transport ratio was found to be 3·0. The resistance of the Little Belt, when considering the differences in sea level from Gedser to Hornbæk, was 1839×10⁻¹² s² m⁻⁵. On the basis of water level and surface salinity measurements made during the period 1931–76, a net discharge of 2300 m³ s⁻¹and a net salt transport of 36 tonnes s⁻¹through the Little Belt from the central Baltic Sea were found.     

Numerical Study of Formation of Dichothermal Water in the Bering Sea

A one-dimensional numerical model with a level-2.5 turbulent closure scheme to provide vertical mixing coefficients has been used to investigate the process by which the dichothermal water is formed in the Bering Sea, the density of which is about 26.6 sigma-theta. The water column to be simulated is assumed to move along a predetermined path. That is, the present model is of the Lagrangian-type. Surface boundary conditions are given using the climatologies of heat, freshwater and momentum fluxes. In order to obtain a plausible moving speed of the water column along the path, pre-liminary experiments were done using the surface fluxes in the central part of the Bering Sea for the initial temperature and salinity profiles at the entrance of the Sea. As a result, it was found that the temperature minimum layer, i.e., the dichothermal water with temperature similar to the climatology at the exit of the Bering Sea, was formed after about two years of integration. Based on the result, the movement speed of the water column along the path was set as 4.5 cm/s in the standard run. It was found that this model could plausibly reproduce the subsurface temperature minimum layer. That is, the dichothermal water was formed in the winter mixed layer process in the Bering Sea. The existence of the subsurface halocline (pycnocline) prohibited the deeper penetration of the winter mixed layer, and therefore water with a temperature colder than that under the mixed layer was formed in the mixed layer due to wintertime surface cooling. In the warming season this water remains as the subsurface temperature minimum layer between the upper seasonal thermocline and the lower halocline. This revised version was published online in August 2006 with corrections to the Cover Date.     

The variation of turbulent diapycnal mixing at 18°N in the South China Sea stirred by wind stress

The spatial and temporal variations of turbulent diapycnal mixing along 18°N in the South China Sea(SCS) are estimated by a fine-scale parameterization method based on strain, which is obtained from CTD measurements in yearly September from 2004 to 2010. The section mean diffusivity can reach ~10~(–4)m~2/s, which is an order of magnitude larger than the value in the open ocean. Both internal tides and wind-generated near-inertial internal waves play an important role in furnishing the diapycnal mixing here. The former dominates the diapycnal mixing in the deep ocean and makes nonnegligible contribution in the upper ocean, leading to enhanced diapycnal mixing throughout the water column over rough topography. In contrast, the influence of the wind-induced nearinertial internal wave is mainly confined to the upper ocean. Over both flat and rough bathymetries, the diapycnal diffusivity has a growth trend from 2005 to 2010 in the upper 700 m, which results from the increase of wind work on the near-inertial motions.     

Influence of the dynamics of currents on the hydrophysical structure of the waters and the vertical exchange in the active layer of the Black Sea

An analysis of the data of measurements of the fine structure and microstructure fluctuations of hydrophysical fields in the upper 200-m layer of the Black Sea carried out using CTD profilers and a Baklan free falling microstructure and turbulence profiler revealed the existence of a positive correlation between the intensity of the fine structure and microstructure fluctuations and the dynamics of the currents. On the other hand, the level of the fine structure and microstructure fluctuations reflects the rate of the vertical turbulent exchange. It was shown that, in the case of the absence of the Black Sea Rim Current (BSRC) jet or clearly manifested mesoscale eddy structures, the vertical turbulent exchange in the pycnocline is weak, while, in the opposite case, it is stronger. The results obtained support the supposition that the interbasin dynamics play an important role in the maintenance of the rate of small-scale mixing in the pycnocline and halocline and provide the vertical transport of dissolved oxygen from the cold intermediate layer into the deeper layers of the sea.     

Vertical transport in the Black Sea

Investigations of the vertical exchange coefficientK _z , considering turbulent and advective transport, are summarized. The values ofK _z are determined from the climaticT, S characteristics, heat and salinity fluxes, and the rate of the low-Bosporus water transformation over the entire Black Sea water column; namely, in the upper mixed layer, the active layer, the cold intermediate layer, the halocline, and in the deep and near-bottom waters. A characteristic for annual fluctuations ofK _z in the active layer is given; it is noted that the coefficient tends to grow in transit from the centre of the cyclonic gyre toward its periphery.Translated by V. Puchkin.     

夏季黄东海硝酸盐垂向扩散通量的分析

湍流扩散过程导致的硝酸盐垂向输运对海水表层的浮游植物生长和初级生产力的大小有着重要影响。本文基于2018年夏季黄、东海水文环境、硝酸盐浓度和湍动能耗散率的同步、原位数据,分析了海域温度、盐度和硝酸盐的空间分布特征,结果表明营养盐含量丰富的黄海冷水团、长江冲淡水、东海北部底层混合水与黑潮次表层水是影响研究海域硝酸盐分布的主要水团。利用垂向湍扩散硝酸盐通量公式,计算了三个选定断面上的硝酸盐垂向扩散通量,其高值区与湍流扩散系数的高值区的位置基本一致。针对存在明显硝酸盐跃层的站位,计算得到跨硝酸盐跃层的垂向通量F_ND的范围在-9.78—36.60mmol/（m²·d）之间,在黄海冷水团区,夏季温跃层限制了该区营养盐向近表层的湍流垂向扩散;东海北部底层混合水区,湍流垂向扩散向上层补充了大量硝酸盐,促进了跃层之上浮游植物的生长;黑潮次表层水影响海区,夏季中层水体混合较弱,跨跃层的垂向通量也普遍偏低。开展硝酸盐垂向扩散通量的计算与分析,对进一步明确营养盐的输运机制有着重要研究意义。     

Parameterization of vertical mixing in the Weddell Sea

A series of vertical mixing schemes implemented in a circumpolar coupled ice–ocean model of the BRIOS family is validated against observations of hydrography and sea ice coverage in the Weddell Sea. Assessed parameterizations include the Richardson number-dependent Pacanowski–Philander scheme, the Mellor–Yamada turbulent closure scheme, the K-profile parameterization, a bulk mixed layer model and the ocean penetrative plume scheme (OPPS). Combinations of the Pacanowski–Philander parameterization or the OPPS with a simple diagnostic model depending on the Monin–Obukhov length yield particularly good results. In contrast, experiments using a constant diffusivity and the traditional convective adjustment cannot reproduce the observations. An underestimation of wind-driven mixing in summer leads to an accumulation of salt in the winter water layer, inducing deep convection in the central Weddell Sea and a homogenization of the water column. Large upward heat fluxes in these simulations lead to the formation of unrealistic, large polynyas in the central Weddell Sea after only a few years of integration. Furthermore, spurious open-ocean convection affects the basin-scale circulation and leads to a significant overestimation of meridional overturning rates. We conclude that an adequate parameterization of both wind-induced mixing and buoyancy-driven convection is crucial for realistic simulations of processes in seasonally ice-covered seas.     

The effects on water mass formation of surface mixed layer timedependence and entrainment fluxes

An air-sea buoyancy flux out of the ocean between the surface outcroppings of different isopycnals must be balanced by a convergence of advective and diffusive fluxes of buoyancy across those isopycnals (Walin, 1982; Tziperman, 1986; Garrett et al., 1995). For steady conditions, the diapycnal diffusive flux due to vertical mixing in the surface mixed layer is very small, so that the advective buoyancy flux dominates (Speer, 1993; Garrett et al., 1995). The associated advective buoyancy flux can then be used to estimate the volume flux of water out of the base of the surface mixed layer. The resulting thermodynamic algorithm provides a valuable estimate of water mass formation in the ocean.In contrast, for the time-dependent real ocean with horizontal and vertical gradients of the horizontal buoyancy gradient, diurnal and seasonal mixed layer deepening and entrainment in the presence of a buoyancy jump at the base of the mixed layer contributes to the annual volume flux out of the base of the deepest (wintertime) mixed layer. The mismatch between the predictions of the ideal algorithm and measured rates of water mass formation (Speer, 1997) may thus be partly due to mixed layer processes rather than diapycnal mixing in the thermocline.     

苏禄海的“通风”过程：历史数据分析

Based on historical observations, ventilation of the Sulu Sea （SS） is investigated and, its interbasin exchange is also partly discussed. The results suggest that near the surface the water renewal process not only occurs through the Mindoro Strait （MS） and the Sibutu Passage, but also depends on the inflows through the Surigao Strait and the Bohol Sea from the Pacific and through the Balabac Strait from the South China Sea （SCS）. Both inflows are likely persistent year round and their transports might not be negligible. Below the surface, the core layer of the Subtropical LowerWater （SLW） lies at about 200 m, which enters the SS through the Mindoro Strait not hampered by topography. Moreover, there is no indication of SLW inflow through the Sibutu Passage even though the channel is deep enough to allow its passage. The most significant ventilation process of the SS takes place in depths from 20a m to about 1200 m where intermediate convection driven by quasi-steady inflows through the Mindoro and Panay straits （MS-PS） dominates. Since the invaded water is drawn from the upper part of the North Pacific Intermediate Water （NPIW） of the SCS, it is normally not dense enough to sink to the bottom. Hence, the convective process generally can only reach some intermediate depths resulting in a layer of weak salinity minimum （about 34.45）. Below that layer, there is the Sulu Sea Deep Water （SSDW） homogeneously distributed from 1200 m down to the sea floor, of which the salinity is only a bit higher （about 34.46） above the minimum. Observational evidence shows that hydrographic conditions near the entrance of the MS in the SCS vary significantly from season to season, which make it possi- ble to provide the MS-PS overflowwith denser water of higher salinity sporadically. It is hence proposed that the SSDW is derived from intermittent deed convection resulted from DroDertv changes of the MS-PS inflow.