Generation of cyclonic eddies in the Eastern Gotland Basin of the Baltic Sea following dense water inflows: numerical experiments

A sigma (σ)-coordinate ocean model by Blumberg and Mellor (POM) is applied to study the formation processes of mesoscale cyclones observed in the Eastern Gotland Basin following the dense water inflows. The initial conditions simulate a situation when the Arkona and Bornholm basins and partially the Slupsk Furrow are already filled with the inflow water of the North Sea origin, while the Eastern Gotland and Gdansk basins still contain the old water of pre-inflow stratification. Model runs with constant and time-dependent winds, changing the buoyancy forcing, grid geometry and bottom topography display the following. Entering the Eastern Gotland Basin from the Slupsk Furrow, the bottom intrusion of saline inflow water splits in two: one goes northeast towards the Gotland Deep, and second moves southeast towards the Gulf of Gdansk. An intensive mesoscale cyclonic eddy carrying the inflow water is generated just east of the Slupsk Furrow with the inflow pulse. A number of smaller cyclones with boluses of the inflow water are formed in the permanent halocline along the saline intrusion pathway to the Gotland Deep. Following Spall and Price [J. Phys. Oceanogr. 28 (1998) 1598], the cyclones are suggested to form by the adjustment of the high potential vorticity inflow water column to a low potential vorticity environment.     

Resolving the impact of short-term variations in physical processes impacting on the spawning environment of eastern Baltic cod: application of a 3-D hydrodynamic model

Variations in oxygen conditions below the permanent halocline influence the ecosystem of the Baltic Sea through a number of mechanisms. In this study, we examine the effects of physical forcing on variations in the volume of deep oxygenated water suitable for reproductive success of central Baltic cod. Recent research has identified the importance of inflows of saline and oxygenated North Sea water into the Baltic Sea for the recruitment of Baltic cod. However, other processes have been suggested to modify this reproduction volume including variations in timing and volume of terrestrial runoff, variability of the solubility of oxygen due to variations in sea surface temperature as well as the influence of variations in wind stress. In order to examine the latter three mechanisms, we have performed simulations utilizing the Kiel Baltic Sea model for a period of a weak to moderate inflow of North Sea water into the Baltic, modifying wind stress, freshwater runoff and thermal inputs. The model is started from three-dimensional fields of temperature, salinity and oxygen obtained from a previous model run and forced by realistic atmospheric conditions. Results of this realistic reference run were compared to runs with modified meteorological forcing conditions and river runoff.From these simulations, it is apparent that processes other than major Baltic inflows have the potential to alter the reproduction volume of Baltic cod. Low near-surface air temperatures in the North Sea, the Skagerrak/Kattegat area and in the western Baltic influence the water mass properties (high oxygen solubility). Eastward oriented transports of these well-oxygenated highly saline water masses may have a significant positive impact on the Baltic cod reproduction volume in the Bornholm Basin.Finally, we analysed how large scale and local atmospheric forcing conditions are related to the identified major processes affecting the reproduction volume.     

Chemical and physical fronts in the Bohai, Yellow and East China seas

Associated with strong mixing and stirring, as well as enhanced bioproductivity and ecotones, oceanic fronts have garnered worldwide attention in recent years. Research into oceanic fronts, especially thermal fronts, has gained momentum since the advent of satellites and their increased accessibility. Yet, studies of salinity and nutrient fronts —particularly those that are subsurface are few and far between. This study reviews the most widely accepted facts about surface and subsurface temperature and salinity fronts in the Bohai, Yellow and East China seas and their seasonal variations. The distribution of nutrients in the surface and bottom waters are mapped and nutrient fronts, for the first time, are identified systematically.These fronts are generally strongest in winter when southward flowing coastal currents are influenced most by winter monsoons, and the contrasts between these cold, fresh, nutrient-rich currents and the northward flowing warm, saline but nutrient-poor Kuroshio are strongest. Surface fronts are generally weakest in summer when coastal currents may be weaker and temperature, salinity and nutrient contrasts are diminished. The existence of fronts and why some are disconnected are mainly related to oceanic features such as topography, boundaries between water masses and current flow patterns. Three latitudinal temperature and nutrient fronts in the southern East China Sea in winter may suggest eastward flowing currents. These currents have not been described previously.     

Intrusions of eastern North Atlantic central waters and phytoplankton in the north and northwestern Iberian shelf during spring

The influence of intrusions of eastern North Atlantic central water (ENACW) in the north and northwestern Iberian shelf on phytoplankton composition and abundance and on particle-size distributions of seston was analyzed using data collected on three extensive cruises during spring 1991 and 1992. Water with temperature and salinity values between 12.20 and 13.86 °C and between 35.66 and 35.98 psu, respectively, characteristics of the subtropical type of ENACW (ENACW_t), was detected in the upper 100 m of the water-column in all cruises, but particularly in the western coast in 1992. The highest salinity values of this water were found near the surface (0–100-m depth) and in early spring 1992, while minimum salinity values, and also minimum geographical extension, were found in late spring in both years. Phytoplankton blooms concentrated in frontal areas between different water types, with maximum intensity and extension in early spring.Using temperature and salinity characteristics, samples were classified in four groups corresponding to the major water types found in the region: Bay of Biscay central water (BBCW), two segments of ENACW of different salinity and surface water influenced by continental runoff. This classification was significantly confirmed by three independent discriminant analyses using hydrographic and chemical (dissolved nutrients and chlorophyll) variables, phytoplankton species abundance variables and particle-size concentration of seston variables. Phytoplankton blooms related to the presence of saline waters were characterized by the dominance of either chain-forming diatoms or a mixture of diatoms and phytoflagellates and high concentrations of seston. The diatom species dominating in saline waters were typical of upwelling-induced blooms occurring generally during summer. Blooms occurring in waters influenced by runoff also contained diatoms but in lower numbers than those of saline waters. Nutrients were not exhausted in the region, suggesting that phytoplankton populations were still in active growth. These results are interpreted taking into account the known variability in water-mass formation and in the poleward current driving ENACW_t along the shelf, and indicate that saline intrusions are a major feature affecting the distribution and composition of plankton in the spring in the southern Bay of Biscay, thus enlarging to a wider spatial scale their reported influence on the pelagic ecosystem.     

Seasonal variability in sea surface oceanographic conditions in the Aegean Sea (Eastern Mediterranean): an overview

Seasonal variability and the spatial distribution of sea surface temperatures (SST) and salinities (SSS) are reviewed, in relation to the prevailing climatological conditions, heat fluxes, water budget and general water circulation patterns. Within this context, consideration is given to: sea surface temperatures; air temperatures; precipitation; evaporation; wind speeds and directions; freshwater (mainly riverine) discharges throughout the Aegean; and the exchange of water masses with the Black Sea and eastern Mediterranean Sea. The investigation of satellite images, covering a 6-yr period (1988–1994), has enabled a synthesis of the monthly sea surface thermal distribution to be established.The climate of the Aegean Sea is characterised by annual air temperatures of 16–19.5°C, precipitation of about 500 mm yr⁻¹ and evaporation of some 4 mm d⁻¹. The Aegean has a negative heat budget (approximately −25 W m⁻²) and positive water balance (+ 1.0 m yr⁻¹), when inflow from the Black Sea is considered. During the summer, the (northerly) Etesians are the dominant winds over the Sea.Mean monthly sea surface temperatures (SST) vary from 8°C in the north during winter, up to 26°C in the south during summer. SST depends mainly upon air temperature; there is a month's delay between the former and latter maxima. The sea surface salinity (SSS) varies also spatially and seasonally, ranging from less than 31 psu, in the north, to more than 39 psu, in the southeast; lower values (< 25 psu) occur adjacent to the river mouths. SSSs present their maximum differences during summer, whilst during winter and autumn the distribution of SSS is more uniform. The overall spatial SST and SSS distribution pattern is controlled by: distribution of the (colder) Black Sea Waters; advection of the (warmer) Levantine Waters, from the southeastern part of the Aegean; upwelling and downwelling; and, to a lesser extent, but locally important, freshwater riverine inflows.     

Interannual variation of the Polar Front in the Japan/East Sea from summertime hydrography and sea level data

The Polar Front in the Japan/East Sea separates the southern warm water region from the northern cold water region. A merged TOPEX/POSEIDON and ERS-1/2 altimeter dataset and upper water temperature data were used to determine the frontal location and to examine the structure of its interannual variability from 1993 to 2001. The identified frontal location, where sea surface height gradient has a maximum about 10–20 cm over the horizontal distance of 100 km, corresponds well to the maximum subsurface horizontal temperature gradient. The front migrates more widely (36°N–41°N) in the western part of the sea than in the eastern part. The interannual migration induces large variability in upper water temperatures and sea surface height in the western region. Responsible physical mechanisms were studied using a reduced-gravity model. Differences between inflow and outflow change the total volume of warm water, and total warm water volume change in the warm water region uniformly pushes the front in the meridional direction across its mean position in the model simulation. Interannual variation of wind stress causes relatively wide migration of the modeled front in the western part.     

Inorganic carbon fluxes through the boundaries of the Greenland Sea Basin based on in situ observations and water transport estimates

A carbon budget for the exchange of total dissolved inorganic carbon C_T between the Greenland Sea and the surrounding seas has been constructed for winter and summer situations. An extensive data set of C_T collected over the years 1994–1997 within the European Sub-polar Ocean Programmes (ESOP1 and ESOP2) are used for the budget calculation. Based on these data, mean values of C_T in eight different boxes representing the inflow and outflow of water through the boundaries of the Greenland Sea Basin are estimated. The obtained values are then combined with simulated water transports taken from the ESOP2 version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The fluxes of inorganic carbon are presented for three layers; a surface mixed layer, an intermediate layer and a deep layer, and the imbalance in the fluxes are attributed to air–sea exchange, biological fixation of inorganic carbon, and sedimentation. The main influx of carbon is found in the surface and the deep layers in the Fram Strait, and in the surface waters of direct Atlantic origin, whereas the main outflux is found in the surface layer over the Jan Mayen Fracture Zone and the Knipovich Ridge, transporting carbon into the Atlantic Ocean via the Denmark Strait and towards the Arctic Ocean via the Norwegian Sea, respectively. The flux calculation indicates that there is a net transport of carbon out of the Greenland Sea during wintertime. In the absence of biological activity, this imbalance is attributed to air sea exchange, and requires an oceanic uptake of CO₂ of 0.024±0.006 Gt C yr⁻¹. The flux calculations from the summer period are complicated by biological fixation of inorganic carbon, and show that data on organic carbon is required in order to estimate the air–sea exchange in the area.     

Estimation of TKE dissipation rates in dense bottom plumes using a Pulse Coherent Acoustic Doppler Profiler (PC-ADP) — Structure function approach

During a hydrographic survey in January 2006 the spreading of inflowing saline water was observed in the Arkona Basin (Western Baltic Sea). Two bottom mounted ‘pulse coherent’ acoustic Doppler profilers (PC-ADP) were used to measure the near-bottom current field of the dense plume with a high temporal (1 s) and spatial resolution (5 cm). In order to estimate the dissipation rate of turbulent kinetic energy () a structure function approach was applied to the beam velocity data. Simultaneous measurements with a microstructure shear profiler (MSS) and an acoustic Doppler velocimeter (ADV) supplied independent data for the verification of the structure function method. Additional measurements with standard CTD, near-bottom towed and vessel mounted acoustic Doppler current profilers (ADCP) completed the data set.The estimated dissipation rates from the structure function approach fit well with the values derived from the ADV and the MSS probe. It is shown that the structure function approach is a reliable and easily applicable method to derive estimates of TKE dissipation rates from PC-ADP beam velocities. The observed dissipation rates ranged between 5 · 10^− 6 and 1 · 10^− 8 W kg^− 1 depending on the hydrographic conditions. Inside the plume the dissipation rates exceeded that of the overlaying brackish water by two orders of magnitude. Since the noise level of velocity data in pulse coherent mode is considerably lower than in the Doppler mode the PC-ADP can also be used for estimates in marine environments with low turbulence level. Reynolds stresses estimated from the PC-ADP and the ADV agreed well at the same depth level. TKE production derived from PC-ADP measurements compared reasonably well with the dissipation rate of TKE in a varying environment.     

Seasonal and interannual variation of physical and biological processes during 1994–2001 in the Sea of Japan/East Sea: A three-dimensional physical–biogeochemical modeling study

A Pacific basin-wide physical–biogeochemical model has been used to investigate the seasonal and interannual variation of physical and biological fields with analyses focusing on the Sea of Japan/East Sea (JES). The physical model is based on the Regional Ocean Model System (ROMS), and the biogeochemical model is based on the Carbon, Si(OH)₄, Nitrogen Ecosystem (CoSiNE) model. The coupled ROMS–CoSiNE model is forced with the daily air–sea fluxes derived from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis for the period of 1994 to 2001, and the model results are used to evaluate climate impact on nutrient transport in Mixed Layer Depth (MLD) and phytoplankton spring bloom dynamics in the JES.The model reproduces several key features of sea surface temperature (SST) and surface currents, which are consistent with the previous modeling and observational results in the JES. The calculated volume transports through the three major straits show that the Korea Strait (KS) dominates the inflow to the JES with 2.46 Sv annually, and the Tsugaru Strait (TS) and the Soya Strait (SS) are major outflows with 1.85 Sv and 0.64 Sv, respectively. Domain-averaged phytoplankton biomass in the JES reaches its spring peak 1.8 mmol N m^− 3 in May and shows a relatively weak autumn increase in November. Strong summer stratification and intense consumption of nitrate by phytoplankton during the spring result in very low nitrate concentration at the upper layer, which limits phytoplankton growth in the JES during the summer. On the other hand, the higher grazer abundance likely contributes to the strong suppression of phytoplankton biomass after the spring bloom in the JES. The model results show strong interannual variability of SST, nutrients, and phytoplankton biomass with sudden changes in 1998, which correspond to large-scale changes of the Pacific Decadal Oscillation (PDO). Regional comparisons of interannual variations in springtime were made for the southern and northern JES. Variations of nutrients and phytoplankton biomass related to the PDO warm/cold phase changes were detected in both the southern and northern JES, and there were regional differences with respect to the mechanisms and timing. During the warm PDO, the nutrients integrated in the MLD increased in the south and decreased in the north in winter. Conversely, during the cold PDO, the nutrients integrated in the MLD decreased in the south and increased in the north. Wind divergence/convergence likely drives the differences in the southern and northern regions when northerly and northwesterly monsoon dominates in winter in the JES. Subjected to the nutrient change, the growth of phytoplankton biomass appears to be limited neither by nutrient nor by light consistently both in the southern and northern regions. Namely, the JES is at the transition zone of the lower trophic-level ecosystem between light-limited and nutrient-limited zones.     

Comparison between a reanalyzed product by 3-dimensional variational assimilation technique and observations in the Ulleung Basin of the East/Japan Sea

Reanalyzed products from a MOM3-based East Sea Regional Ocean Model with a 3-dimentional variational data assimilation module (DA-ESROM), have been compared with the observed hydrographic and current datasets in the Ulleung Basin (UB) of the East/Japan Sea (EJS). Satellite-borne sea surface temperature and sea surface height data, and in-situ temperature profiles have been assimilated into the DA-ESROM. The performance of the DA-ESROM appears to be efficient enough to be used in an operational ocean forecast system.Comparing with the results from Mitchell et al. [Mitchell, D. A., Watts, D. R., Wimbush, M., Teague, W.J., Tracey, K. L., Book, J. W., Chang, K.-I., Suk, M.-S., Yoon, J.-H., 2005a. Upper circulation patterns in the Ulleung Basin. Deep-Sea Res. II, 52, 1617-1638.], the DA-ESROM fairly well simulates the high variability of the Ulleung Warm Eddy and Dok Cold Eddy as well as the branching of the Tsushima Warm Current in the UB. The overall root-mean-square error between 100 m temperature field reproduced by the DA-ESROM and the observed 100-dbar temperature field is 2.1 °C, and the spatially averaged grid-to-grid correlation between the two temperature fields is high with a mean value of 0.79 for the inter-comparison period.The DA-ESROM reproduces the development of strong southward North Korean Cold Current (NKCC) in summer consistent with the observational results, which is thought to be an improvement of the previous numerical models in the EJS. The reanalyzed products show that the NKCC is about 35 km wide, and flows southward along the Korean coast from spring to summer with maximum monthly mean volume transport of about 0.8 Sv in August–September.     

Winter phytoplankton blooms in the shallow mixed layer of the South China Sea enhanced by upwelling

The South China Sea (SCS) is the largest marginal sea in the world. Previous studies, including recent intensive paleo-oceanographic studies, suggest that the SCS is sensitive to many types of physical forcing on the short-term (e.g., internal waves and tides, mesoscale eddies, typhoons, etc.), annual (e.g., monsoon), inter-annual (e.g., El Niño), and very long-term (e.g., climate change) time scales. To better understand how various types of physical forcing influence biogeochemical cycles in the water column, a time-series study was initiated. Bimonthly hydrographic surveys occupied stations in the subtropical–tropic SCS at 19°N, 118.5°E. Results suggest that the Southeast Asian monsoons, northeasterly from October to April and southwesterly from May to September, have important effects on biogeochemical cycles in the upper water column. Hydrographic data showed that the mixed layer depth was much shallower in winter than in other seasons. During the winter monsoon period, the nitricline became shallower and upwelling sustained an elevated phytoplankton standing stock. Mean chlorophyll concentrations (0.65 mg Chl m^− 3) in winter were 8 times higher than in summer, and the integrated primary productivity over the euphotic zone reached as high as ca. 684 mg C m^− 2 day^− 1 in winter. The upwelling is produced by convergence of currents in the cyclonic gyre near the Luzon Strait, where the Kuroshio intrudes. In summer the current reverses following the wind change. The nitricline is depressed as downwelling occurs off northwest Luzon, resulting in strong nutrient limitation and very low chlorophyll concentrations.     

Influence of the Mackenzie River plume on the sinking export of particulate material on the shelf

We examined the influence of the Mackenzie River plume on sinking fluxes of particulate organic and inorganic material on the Mackenzie Shelf, Canadian Arctic. Short-term particle interceptor traps were deployed under the halocline at 3 stations across the shelf during fall 2002 and at 3 stations along the shelf edge during summer 2004. During the two sampling periods, the horizontal patterns in sinking fluxes of particulate organic carbon (POC) and chlorophyll a (chl a) paralleled those in chl a biomass within the plume. Highest sinking fluxes of particulate organic material occurred at stations strongly influenced by the river plume (maximum POC sinking fluxes at 25 m of 98 mg C m^− 2 d^− 1 and 197 mg C m^− 2 d^− 1 in 2002 and 2004, respectively). The biogeochemical composition of the sinking material varied seasonally with phytoplankton and fecal pellets contributing considerably to the sinking flux in summer, while amorphous detritus dominated in the fall. Also, the sinking phytoplankton assemblage showed a seasonal succession from a dominance of diatoms in summer to flagellates and dinoflagellates in the fall. The presence of the freshwater diatom Eunotia sp. in the sinking assemblage directly underneath the river plume indicates the contribution of a phytoplankton community carried by the plume to the sinking export of organic material. Yet, increasing chl a and BioSi sinking fluxes with depth indicated an export of phytoplankton from the water column below the river plume during summer and fall. Grazing activity, mostly by copepods, and to a lesser extent by appendicularians, appeared to occur in a well-defined stratum underneath the river plume, particularly during summer. These results show that the Mackenzie River influences the magnitude and composition of the sinking material on the shelf in summer and fall, but does not constitute the only source of material sinking to depth at stations influenced by the river plume.     

Material transport from the nearshore to the basinal environment in the southern Baltic Sea: I. Processes and mass estimates

Processes involved in erosion, transport and deposition of cohesive materials are studied in a transect from shallow (16 m) to deep (47 m) water of the SW Baltic Sea. The wave- and current-induced energy input to the seabed in shallow water is high with strong variability and suspended matter concentrations may double within a few hours. Primary settling fluxes (from sedimentation traps) are less than 10 g m⁻² day⁻¹, whereas resuspension fluxes (evaluated from sedimentation flux gradients) are 15–20 times higher and the residence time for suspended matter in the water column is 1–2 days. Settling velocities of aggregates are on average six times higher than for individual particles resulting in an enhanced downward transport of organic matter. Wave-induced resuspension (four to six times per month) takes place with higher shear stresses on the bottom than current-induced resuspension (three to five times per month). The short residence time in the water column and the frequent resuspension events provide a fast operating benthic–pelagic coupling. Due to the high-energy input, the shallow water areas are nondepositional on time scales longer than 1–2 weeks. The sediment is sand partly covered by a thin fluff layer during low-energy periods. The presence of the fluff layer keeps the resuspension threshold very low (<0.023 N m⁻²) throughout the year. Evaluated from 3-D sediment transport modeling, transport from shallow to deep water is episodic. The net main directions are towards the Arkona Basin (5.5×10⁵ t per year) and the Bornholm Basin (3.7×10⁵ t per year). Energy input to the bottom in deep water is low and takes place much less frequently. Wave-induced resuspension occurs on average once per month. Residence time of particles (based on radioactive isotopes) in the water column is half a year and the sediment accumulation rate is 2.2 mm year⁻¹ in the Arkona Basin.     

Circulation features in the Japan/East Sea related to statistically obtained wind patterns in the warm season

A response of the circulation in the Japan/East Sea (JES) to different kinds of wind forcing is studied, with the emphasis on the warm season, using a primitive equation oceanic model. Wind forcing is based on typical patterns obtained from complex empirical orthogonal functions of 1°-gridded NCEP/NCAR 6 h winds for 1998–2005. These patterns are distinguished by a prevailing wind direction. Northwestern wind and strong cyclonic (C) curl prevail in winter, while a variety of patterns occur in the warm season, differing in the wind direction and curl. Three model runs are performed to examine the circulation in response to a prevailing C wind stress curl or an alternating C and anticyclonic (AC) curl or a strong C curl in the warm season. The simulated features are consistent with the observational evidence, in particular with thermal fronts and frequent eddy locations derived from multi-year infrared satellite imagery. The simulated C circulation intensifies and the subarctic region extends southward with the strengthening of a summer C wind stress curl over the JES. Variability of Subarctic Front (SF) in the western JES (between 130°E and 133°E) is strongly affected by summer wind stress curl. Forcing by an AC curl tends to shift SF northward, while SF shifts to the south under the forcing by a C curl, reaching the southern Ulleung Basin in the case of the strong C curl. In the northwestern JES (off Peter the Great Bay, Russia, and North Korea), the SF northwestern branch (NWSF) is simulated. It is a known feature in autumn and early winter and can also occur in the warm season. The simulation results suggest an AC wind stress curl as the forcing of the formation of the NWSF in the warm season. The Siberia Seamount and sharply bending coastline near Peter the Great Bay facilitate partial separation of the Primorye (Liman) Current from the coast. The wind stress curl can be an additional forcing of the Tsushima Warm Current (TWC) branching off the Korea Strait to the East Korea Warm Current (EKWC) and the offshore branch (OB). In the warm season, the simulated TWC bifurcation occurs farther north, the EKWC is strong, and the OB is weak under the forcing of the AC wind stress curl. The EKWC is weak and the OB is strong under the forcing of the strong C wind stress curl.     

Circulation, hydrographic and nutrient characteristics of the Cilician Basin, Northeastern Mediterranean Sea

The water mass, circulation and chemical properties of the Cilician Basin, the northeastern Levantine Sea, are described on the basis of three hydrographic cruises performed during May 1997 (spring), July 1998 (summer) and October 2003 (autumn). The hydrographic data reveal the presence of Levantine Surface Water (LSW) and Modified Atlantic Water (MAW) within the upper 90 m layer, Levantine Intermediate Water (LIW) between 90 and 250 m, and Transitional Mediterranean Water (TMW) further below. The temporal variability of the circulation system is manifested by a change in shape, size and intensity of eddies as well as the pathways of the Lattakia Basin coastal current system. The nutrient concentrations varied between nitrate + nitrite = 0.16–0.31 μM, phosphate = 0.02–0.03 μM and silicate = 0.95–1.2 μM for the surface layer during sampling periods. Dissolved nutrient concentrations in the Transitional Mediterranean Water were: 2.1–5.3 μM for NO₃ + NO₂, 0.10–0.21 μM for PO₄ and 5.7–10 μM for Si. The molar ratios of nitrate to phosphate in the water column range between 5 and 20 in the surface layer and reach up to a value of 45 at the top of the nutricline at the depths of 29.05 kg/m³ isopycnal surface for most of the year. Below the nutricline the N / P ratios retain the values around 24–28.     

Variability of mesozooplankton spatial distribution in the North Aegean Sea, as influenced by the Black Sea waters outflow

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m^− 3 and 14,157 ind m^− 3) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi, Centropages typicus, Paracalanus parvus. The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera, Oithona copepodites, Oncaea media, Ctenocalanus vanus, Farranula rostrata. During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.     

Recent observations in the straits of the East/Japan Sea: A review of hydrography, currents and volume transports

Recent observations of hydrography, currents and volume transports in the straits of the East/Japan Sea are reviewed. It is newly found that bottom cold water in the Korea/Tsushima Strait originating from the northern region of the East/Japan Sea appears not only in summer and autumn but also in winter. Intensive observations in the Korea/Tsushima Strait revealed two distinct cores of northeastward currents in the upper layer of the western and eastern channels. Mean volume transport through the Korea/Tsushima Strait is calculated as 2.5 ± 0.5 Sv from four-year direct and indirect measurements. As continuous monitoring has started in the Tsugaru and Soya Straits, understanding of temporal variability of currents and volume transports through the straits is in progress. For the first time, simultaneous time series of volume transports are available in the Korea/Tsushima and Tsugaru Straits during the winter of 1999–2000. Ouflow through the Tsugaru Strait accounts for about 70% of inflow through the Korea/Tsushima Strait for this period.     

Bathymetry impacts on water exchange modelling through the Danish Straits

Deep and narrow channels in Danish Straits are one of the governing factors for the Baltic–North Sea water and salt exchange. The channels have a depth up to 50 m and a horizontal scale of a few hundred meters. The typical horizontal resolution used in current operational three dimensional Baltic–North Sea models is 1 nautical mile (nm) which can not well resolve these deep channels. In this paper, an alternative method is used to generate the 1 nm resolution bathymetry so that the deep channel is well resolved and at the same time the total water volume is roughly conserved. The impact of the new bathymetry on modelling water and salt transports as well as temperature and salinity structure is assessed by comparing a 3-year model run with the adjusted bathymetry and a control run with the averaged bathymetry. Volume and salt transports through the Great Belt are examined in the two runs. The results show that the model ocean is dominated by a typical two-layer transport (i.e., upper brackish Baltic outflow and lower saltier inflow), and the new bathymetry significantly enhances the two-layer transport. The lighter Baltic outflow is increased by 18% in the upper 10 m and saltier deep inflow is increased by 300% (in comparison with the old bathymetry) below 10 m. The total net transport into the Baltic Sea is increased by 13%. The temperature and salinity structure is also significantly influenced by the bathymetry, especially during inflow events. The stratification is strengthened and the bottom salinity is increased in Danish Straits and adjacent waters. The bathymetry impact is found significant through the entire 3-year model run period, and the signal is propagated to a large area covering the Stopple Channel. Comparison with observations show that such changes are positive improvements to the models. The results suggest that the deep channels in the Danish Straits have to be carefully resolved in order to correctly simulate the Baltic–North Sea water exchange.     

Large-scale physical controls on phytoplankton growth in the Irminger Sea Part I: Hydrographic zones, mixing and stratification

Hydrographic surveys in three consecutive seasons in the Irminger Sea in 2001/2002 have revealed six physical regimes (zones) in which different surface mixing and spring re-stratification processes dominate. They are the South Irminger Current, the North Irminger Current, the Central Irminger Sea, the Polar-origin East Greenland Current, the Atlantic-origin East Greenland Current and the Reykjanes Ridge. The variations in restratification processes in particular have significant implications for the timing of shallow spring mixed layer development and therefore the timing and strength of the spring bloom. The relative roles of heat and freshwater in controlling re-stratification are examined for each hydrographic zone, and it is shown that the simplest concept of solar warming generating spring stratification is appropriate for the Irminger Current and the central Irminger Sea. However in the East Greenland Current and the Reykjanes Ridge zones, the springtime arrival of fresh or saline water at the surface dominates re-stratification and generates the earliest and strongest spring blooms of the region. In the cool fresh centre of the Irminger Sea the relatively low chlorophyll-a throughout the year cannot be wholly explained by stratification or nutrient concentrations. Details of the annual cycle in temperature, salinity, chlorophyll-a and nutrients are presented for each hydrographic zone.     

Seasonal upwelling underneath the Tsushima Warm Current along the Japanese shelf slope

Seasonal upwelling variations are examined in the eastward coastal boundary branches of the Tsushima Warm Current (TWC). The climatological pattern and the fundamental hydrographic structure of the seasonal appearance of cold water are revealed by analyzing the temperature profile data around the Japanese shelf area. Seasonal maps of temperature at the intermediate layer around 200 m depth show the rise of the main pycnocline along the Japanese coast due to seasonal subsurface cooling from May to September. The cold water areas appear around the strong curvature in the continental shelf break. These areas are confined to the south of the TWC thermal front, i.e., to the coastal boundary region. The seasonal appearance of the localized cooling areas implies that the seasonal upwelling is induced by horizontal variations in shelf topography and the intensifying TWC from May to September.