Surface Salinity Gradient Research Articles

Change in the intensity of low-salinity water inflow from the Bay of Bengal into the Eastern Arabian Sea from the Last Glacial Maximum to the Holocene: Implications for monsoon variations

A 100–400km wide region of the coastal Eastern Arabian Sea (EAS), off the west-coast of India, is characterized by a low-salinity tongue formed by the inflow of low-salinity surface water from the Bay of Bengal (BoB). This low-salinity tongue is largely driven by the sea level higher in BoB than in the Arabian Sea and by alongshore pressure gradient between southern- and northern-EAS, and is expected to respond to summer monsoon freshwater flux to the bay. Here, we report past variation in the relative intensity of summer- and winter-monsoons based on changes in the north–south salinity gradient within this low-salinity tongue. The salinity gradient is estimated from paired measurement of δ18O and Mg/Ca in Globigerinoides sacculifer extracted from sediment cores collected at northern high-salinity and southern low-salinity locations within this tongue.The Last Glacial Maximum (LGM) to peak-Holocene δ18O and sea surface temperature gradients at both locations are ~−2‰ and +2°C respectively, while the sea surface salinity gradient at northern-EAS is 0.5psu higher than in the southern-EAS, suggesting distinctly different SSS structure in the LGM-EAS. The north–south surface salinity gradient was also larger by ~0.5psu during the LGM (1.2psu) as compared to the gradient during the Holocene (0.7psu). Increased north–south surface salinity gradient during the LGM suggests diminished flow of low-salinity water into the coastal EAS caused by combined effect of decreased freshening of the BoB and reduced seasonal mountain-river discharge into the EAS. Such surface hydrographic conditions in the coastal EAS clearly indicate significantly weakened summer monsoons and strengthened winter monsoons during the LGM.

Spring phytoplankton communities shaped by interannual weather variability and dispersal limitation: Mechanisms of climate change effects on key coastal primary producers

Spring bloom composition in the Baltic Sea, a partially ice‐covered brackish coastal waterbody, is shaped by winter‐spring weather conditions affecting the relative dominance of diatoms and a heterogeneous assemblage of cold‐water dinoflagellates, dominated by the chain‐forming Peridiniella catenata and a complex of at least three medium‐sized, single‐celled species: Biecheleria baltica, Gymnodinium corollarium, and Scrippsiella hangoei. During the last decades, the bloom community has dramatically changed in several basins. We analyze here a 30 yr time series of quantitative phytoplankton data, as predicted by hindcast modeled ice thickness and storminess for three distinct Baltic Sea localities, to verify climate‐driven mechanisms affecting the spring bloom composition. Thick (> 30 cm) and long‐lasting ice cover favored diatom‐dominated spring blooms, and mild winters, with storms and thin ice cover (10 to 20 cm), supported blooms of the B. baltica complex. Dispersal limitation plays an important role in the spatial extent of blooms of the B. baltica complex, caused by intricate interplay of local hydrodynamics and the dinoflagellate life cycle. Proportion peaks of key phytoplankton groups have shifted about 10 d earlier in the northwestern Baltic Sea (P. catenata and diatoms) and in the Gulf of Riga (P. catenata). The significant weather effects imply future shifts in spring bloom composition and consequent biogeochemical cycles, driven by the predicted changes in winter storminess and decrease in ice cover extent and duration in climate change models.

Field-based validation of a diagenetic effect onG. ruberMg/Ca paleothermometry: Core top results from the Aegean Sea (eastern Mediterranean)

Recent work across the Mediterranean Sea has illustrated the salinity and overgrowth effects on planktonic foraminiferal Mg/Ca, which potentially confound the use of this as a temperature proxy for paleoceanographic reconstructions. To test and verify these effects, we present new Aegean Sea results which reveal Mg/Ca values that were unreasonably high to be explained by temperature or salinity variations alone, confirming that foraminiferal Mg/Ca is affected by diagenesis. We have specifically targeted Globigerinoides ruber (w, sensu stricto), from a series of modern core tops spanning a strong sea surface salinity gradient and a minor sea surface temperature range, along a north-south Aegean Sea transect. Scanning Electron Microscopy analyses show that G. ruber specimens were covered by microscale euhedral crystallites of inorganic precipitates. This secondary calcite phase seems to be responsible for the anomalously high Mg/Ca ratios and likely formed near the sediment/water interface from CaCO3 supersaturated interstitial seawater. We also have clear evidence of diagenetic alteration in a north-south direction along the Aegean Sea, possibly depending on salinity and calcite saturation state gradients. These observations illustrate the necessity of alternative techniques (e.g., flow-through time resolved analysis or laser ablation inductively coupled plasma mass spectrometry) to potentially overcome these diagenetic issues and develop a more reliable and sensitive temperature proxy in similar subtropical settings characterized by high salinity, excessive evaporation, and restricted circulation.

Application of a 3D Lagrangian model to explain the decline of a Dinophysis acuminata bloom in the Bay of Biscay

During July 2006, a cruise was carried out in the Northern Bay of Biscay (off Brittany, France) to study meso- and microscale patterns of phytoplankton distribution. Moderate concentrations (10 2–10 3 cells L − 1) of Dinophysis acuminata were constrained to specific depths (upper layers above the pycnocline) at stations with lower surface salinity (34.5) and steep temperature gradients (18–13.5 °C between 4 and 7 m depth) within the Loire and Vilaine river plumes. On board incubations revealed a healthy D. acuminata population at the biomass maxima with 89% of viable (FDA-treated) cells and moderate division rates (up to 0.10 d − 1). Despite its good physiological condition, the population of D. acuminata declined rapidly to undetectable levels during the second leg of the cruise. A 3D Lagrangian Particle-Tracking Model (3D LPTM) was used to simulate the transport of D. acuminata cells in the Loire and Vilaine river plumes from 15 June to 30 July. This 3D LPTM model was coupled to a MARS3D (Model for Applications at Regional Scale) model previously calibrated and validated for the region. Model results suggest that physical processes alone could explain the dispersion of the D. acuminata population within the area. The application of a 3D-LPTM model shows how individual based model approaches are valuable to identify Dinophysis spp. transport pathways and reproduce retention/dispersion patterns observed in the field.

Implementation of a wetting-and-drying model in simulating the Kennebec–Androscoggin plume and the circulation in Casco Bay

A high-resolution coastal ocean model was developed to simulate the temporal/spatial variability of the Kennebec–Androscoggin (K–A) river plume and the circulation in Casco Bay. The model results agree favorably with the moored and shipboard observations of velocity, temperature, and salinity. The surface salinity gradient was used to distinguish the plume from the ambient coastal water. The calculated plume thickness suggests that the K–A plume is surface trapped. Its horizontal scales correlate well with Q 0.25, where Q is the volume discharge of the rivers. Directional spreading is affected by the wind with the upwelling favorable wind transporting the plume water offshore. Both the wind and the tide also enhance mixing in the plume. The inclusion of a wetting-and-drying (WAD) scheme appears to enhance the mixing and entrainment processes near the estuary. The plume becomes thicker near the mouth of the estuary, the outflow velocity of the plume is weaker, and the radius of the river plume shrinks. The flow field in the model run with the WAD is noisier, not only in shallow areas of Casco Bay but also in the plume and even on the shelf. We speculate that the WAD processes can affect much larger areas than the intertidal zones, especially via a river plume that feeds into a coastal current.

Late Quaternary moisture export across Central America and to Greenland: evidence for tropical rainfall variability from Costa Rican stalagmites

We present a high-resolution terrestrial archive of Central American rainfall over the period 100–24 and 8.1–6.5 ka, based on δ 18O time series from U-series dated stalagmites collected from a cave on the Pacific Coast of Costa Rica. Our results indicate substantial δ 18O variability on millennial to orbital time scales that is interpreted to reflect rainfall variations over the cave site. Correlations with other paleoclimate proxy records suggest that the rainfall variations are forced by sea surface temperatures (SST) in the Atlantic and Pacific Oceans in a fashion analogous to the modern climate cycle. Higher rainfall is associated with periods of a warm tropical North Atlantic Ocean and large SST gradients between the Atlantic and Pacific Oceans. Rainfall variability is likely linked to the intensity and/or latitudinal position of the intertropical convergence zone (ITCZ). Periods of higher rainfall in Costa Rica are also associated with an enhanced sea surface salinity gradient on either side of the isthmus, suggesting greater freshwater export from the Atlantic Basin when the ITCZ is stronger and/or in a more northerly position. Further, wet periods in Central America coincide with high deuterium excess values in Greenland ice, suggesting a direct link between low latitude SSTs, tropical rainfall, and moisture delivery to Greenland. Our results indicate that a stronger tropical hydrological cycle during warm periods and large inter-ocean SST gradients enhanced the delivery of low latitude moisture to Greenland.

Barrier layer variability in the western Pacific warm pool from 2000 to 2007

Major features of the equatorial portion of the western Pacific warm pool (WP) were brought to light through the analysis of an unprecedented collection of temperature and salinity profiles derived from Argo floats from 2000 to 2007. A region of thick (>15–25 m) and quasi‐permanent barrier layers (BLs) was found to occur in a band from 10° to 20° longitude to the west of the maximum zonal sea surface salinity gradient (∂S/∂x), which occurs at the eastern edge of the WP. In this region, thick BLs and associated maxima (∂S/∂x) were displaced eastward (westward) during El Niño (La Niña) over a distance of more than 6000 km. The thickness of the BL in this region is, to the first order, proportional to ∂S/∂x and quasi‐permanently associated with the occurrence of sea surface temperatures warmer than 28–29°C, which are a good proxy for maximum atmospheric convection for the current Pacific climate. Statistics indicated that a thick BL forms preferentially under low wind conditions, heavy precipitation, eastward advection of low sea surface salinity, zonal current vertical shear, and/or in conjunction with equatorial downwelling Kelvin and Rossby waves (favoring the vertical stretching of the upper water column). None of these processes seemed to dominate the others, indicating that the formation of a thick BL results from a combination of different and complex mechanisms. The fact that a thick BL represents a quasi‐permanent feature in the WP signifies that its specific stratification and likely impact on the sea surface temperature balance should be accounted for in coupled models.

A three-dimensional model study on processes of stratification and de-stratification in the Limfjord

Investigations on stratifying and de-stratifying processes in the Limfjord in Denmark, stretching from the North Sea at the western end to the Kattegat at the eastern end, were carried out. The forcing of the Limfjord is dominated by mean sea surface elevation and salinity gradients, diffuse freshwater supply, as well as episodic saline inflow events from the North Sea boundary. The physical regime of the Limfjord switches periodically from stratified to mixed conditions. In order to investigate the complexity of the system, a realistic three-dimensional high-resolution numerical model study was conducted for the year 2003. The results are based on, and compared to, available field data. A central subject of this study is the anomaly of potential energy as a measure for stratification. The stratifying and de-stratifying processes are identified as contributing terms in a dynamic equation for the potential energy anomaly. The comparison of model results and observations shows that the model has the capability of reproducing the observed physical regimes. Changes in the simulated stratification turn out to be a complex interaction of differential advection, heating and turbulent mixing in the central Limfjord.

Forcing of Low-Frequency Ocean Variability in the Northeast Pacific*

Abstract An ocean model is used to examine and compare the forcing mechanisms and underlying ocean dynamics of two dominant modes of ocean variability in the northeast Pacific (NEP). The first mode is identified with the Pacific decadal oscillation (PDO) and accounts for the most variance in model sea surface temperatures (SSTs) and sea surface heights (SSHs). It is characterized by a monopole structure with a strong coherent signature along the coast. The second mode of variability is termed the North Pacific Gyre Oscillation (NPGO). This mode accounts for the most variance in sea surface salinities (SSSs) in the model and in long-term observations. While the NPGO is related to the second EOF of the North Pacific SST anomalies (the Victoria mode), it is defined here in terms of SSH anomalies. The NPGO is characterized by a pronounced dipole structure corresponding to variations in the strengths of the eastern and central branches of the subpolar and subtropical gyres in the North Pacific. It is found that the PDO and NPGO modes are each tied to a specific atmospheric forcing pattern. The PDO is related to the overlying Aleutian low, while the NPGO is forced by the North Pacific Oscillation. The above-mentioned climate modes captured in the model hindcast are reflected in satellite altimeter data. A budget reconstruction is used to study how the atmospheric forcing drives the SST and SSH anomalies. Results show that the basinwide SST and SSS anomaly patterns associated with each mode are shaped primarily by anomalous horizontal advection of mean surface temperature and salinity gradients (∇ Tand ∇ S) via anomalous surface Ekman currents. This suggests a direct link of these modes with atmospheric forcing and the mean ocean circulation. Smaller-scale patterns in various locations along the coast and in the Gulf of Alaska are, however, not resolved with the budget reconstructions. Vertical profiles of the PDO and NPGO indicate that the modes are strongest mainly in the upper ocean down to 250 m. The shallowness of the modes, the depth of the mean mixed layer, and wintertime temperature profile inversions contribute to the sensitivity of the budget analysis in the regions of reduced reconstruction skill.

Red Sea during the Last Glacial Maximum: Implications for sea level reconstruction

The Red Sea is connected to the Indian Ocean via a narrow and shallow strait and exhibits a high sensitivity to atmospheric changes and a reduced sea level. We used an ocean general circulation model to investigate the hydrography and circulation in the Red Sea in response to reduced sea level and modified atmospheric conditions occurring during the Last Glacial Maximum (LGM). The model salinity shows high sensitivity to sea level reduction together with a mild atmospheric impact. Sea level reduction affects the stratification and alters the circulation pattern at the Strait of Bab el Mandab, which experiences a transition from a submaximal flow to a maximal flow. The best correlation to reconstructed conditions during LGM exists when the water depth of the Hanish Sill (the shallowest part in the Strait of Bab el Mandab) is 33 ± 10.75 m, which would be affected by a sea level lowering of approximately 105 m. Our results support the reconstructed maximum salinity of around 57 practical salinity units because of a simple model (that takes into account mixing processes along the strait) and comparison of the surface salinity gradient to reconstructions based on isotopic records from sedimentary cores. The salinity and δ18O are sensitive to the mixing process at the strait, and the sensitivity increases as the sea level is further reduced. A local relative sea level reduction of approximately 105 m is also in close agreement with the inference of the LGM low stand of the sea at the location of the sill based on the ICE‐5G (VM2) model.

Distribution of organic-walled dinoflagellate cysts in shelf surface sediments of the Benguela upwelling system in relationship to environmental conditions

To obtain insight in the relationship between the spatial distribution of organic-walled dinoflagellate cysts (dinocysts) and local environmental conditions, fifty-eight surface sediment samples from the coastal shelf off SW Africa were investigated on their dinocyst content with special focus on the two main river systems and the active upwelling that characterise this region. To avoid possible overprint by species-selective preservation, samples have been selected mainly from shelf sites where high sedimentation rates and/or low bottom water oxygen concentrations prevail. Multivariate ordination analyses have been carried out to investigate the relationship between the distribution patterns of individual species to environmental parameters of the upper water column and sediment transport processes. The main oceanographical variables at the surface (temperature, salinity, nutrients chlorophyll- a) in the region show onshore–offshore gradients. This pattern is reflected in the dinocyst associations with high relative abundances of heterotrophic dinocyst species in neritic regions characterised by high chlorophyll- aand low salinity conditions in surface waters. Phototrophic dinocyst species, notably Operculodinium centrocarpum, dominate in the more oceanic area. Differences in the distribution of phototrophic dinocyst species can be related to sea surface salinity and sea surface temperature gradients and to a lesser extent to chlorophyll- a concentrations. Apart from longitudinal gradients the dinocyst distribution clearly reflects regional environmental features. Six groups of species can be distinguished, characteristic for (1) coastal regions (cysts of Polykrikos kofoidii and Selenopemphix quanta), (2) the vicinity of active upwelling ( Brigantedinium spp., Echinidinium aculeatum, Echinidinium spp. and Echinidinium transparantum), (3) river mouths ( Lejeunecysta oliva, cysts of Protoperidinium americanum, Selenopemphix nephroides and Votadinium calvum), (4) slope and open ocean sediments ( Dalella chathamense, Impagidinium patulum and Operculodinium centrocarpum, (5) the southern Benguela region (south of 24°S) ( Spiniferites ramosus) and (6) the northern Benguela region (north of 24°S) ( Nematosphaeropsis labyrinthus and Pyxidinopsis reticulata). No indication of overprint of the palaeo-ecological signal by lateral transport of allochthonous species could be observed.

Records of the Nd isotope composition of seawater from the Bay of Bengal: Implications for the impact of Northern Hemisphere cooling on ITCZ movement

This study presents a record of planktonic foraminiferal neodymium isotopic gradients along a north–south transect in the Bay of Bengal during time slices of late Holocene and last glacial maximum (LGM) age, together with a record of planktonic foraminiferal variation in the northern Bay of Bengal (15°N) over the last 195 ky. In late Holocene core top planktonic foraminifera, the north–south ε Nd gradient rises from nonradiogenic values of − 12 at 20°N to − 10 at 5°N, in parallel with the modern surface salinity gradient controlled by discharge of Himalayan rivers in the northern Bay. During the LGM, ε Nd increased throughout the Bay, the contrast between northernmost and southernmost sites decreased, and maximum ε Nd values of − 6.5 occurred between 12 and 15°N. A small part of the shift to higher mean ε Nd throughout the Bay during the glacial may arise from a uniform increase in deposition of far-field dust from Arabian and Persian Gulf regions. However, the spatial pattern of ε Nd variation between LGM and late Holocene also suggests a shift from modern dominance of nonradiogenic Nd sources from the Ganges–Brahmaputra basin to LGM dominance of more radiogenic Nd sources from Arakan coastal rivers. Over the last 195 ky at 15°N, the most radiogenic ε Nd values of − 7 occur at glacial maxima and the most nonradiogenic values of − 11 occur during interglacials. ε Nd values are highly correlated with glacial interglacial variations in planktonic foraminiferal δ 18O. In this record, shifts in river sources from the more northerly Ganges–Brahmaputra watershed to the more southerly Arakan coastal river systems respond dominantly to Inter-Tropical Convergence Zone (ITCZ) movement driven by Northern Hemisphere cooling during 100 ky glacial/interglacial cycles, with a small component of variation on precessional timescales. The nonlinear correlation of ε Nd with ice volume suggests that ITCZ movement responds to aerial coverage of ice sheets and snow rather than to ice thickness and volume. These data add support to recent general circulation models of ITCZ response to Northern Hemisphere ice sheets, which simulate decreased glacial precipitation in the Ganges–Brahmaputra basin and increased glacial precipitation in the southern Arakan coastal basin.

Morphological variation of Emiliania huxleyi and sea surface salinity

Morphometric analysis of coccoliths of various coccolithophore species from core top and culture samples has revealed that the morphology of single placoliths can be affected by environmental parameters such as temperature, salinity, and productivity. In this study, we have tested the morphological variations of placoliths of Emiliania huxleyi, the most common coccolithophore species in the modern ocean, with respect to environmental gradients. We have investigated especially the potential effect of salinity on the morphology, as it is known from culture studies that the growth of E. huxleyi placoliths is strongly influenced by varying salinities. Morphometric analysis of about 2150 placoliths from 37 globally distributed core top and plankton samples revealed that the morphology of E. huxleyi varies significantly along a sea surface salinity gradient from 33 to 38. Morphological parameters such as the mean size of placoliths are used as explaining variables in multiple regression analysis to reconstruct sea surface salinities (SSS). The best model for mean SSS yields an R 2 of 0.88 with a standard error of 0.49 whereas statistical models describing surface water temperature or productivity are less reliable and accurate. Application of the best model to plankton samples allows the prediction of in-situ salinities within given error estimates. The application of the best model to fossil assemblages from the LGM, however, is more puzzling. Paleosalinity reconstructions in the South Atlantic, the Caribbean Sea, and the subtropical Atlantic are in good agreement with the published values.

Influence of physical environmental factors on the composition and horizontal distribution of summer larval fish assemblages off Mallorca island (Balearic archipelago, western Mediterranean)

Two ichthyoplankton surveys were carried out in two areas off Mallorca island (Balearic archipelago, western Mediterranean) in June 1996 and August 1996, respectively. The aim of both surveys was to assess the influence of physical environmental factors on the horizontal distribution of larval fish assemblages, focusing on larvae of large migratory pelagic fish species. Canonical correspondence analysis (CCA) indicated that in the June survey, the patterns of horizontal distribution offish larvae were mainly conditioned by depth and by the distribution of two surface water masses: the Modified Atlantic Waters (MA W), of recent Atlantic origin, and the older Surface Mediterranean Waters (SMW). The effect of depth gradient was clear in mesopelagic and neritic species, but it was not so evident in the larvae of large pelagic species which presented a highly patchy distribution. Contrastingly, the August patterns of horizontal fish larvae distribution were significantly correlated with the surface salinity gradient resulting from successive MAW inflows, whereas depth did not show any significant effect, probably linked to the bottom topography with a very narrow shelf area. The data obtained highlight that the Balearic Islands constitute an important spawning ground for most of the large pelagic fishes inhabiting Mediterranean waters, both highly migratory and resident species.

Eddies and the Seesaw

CLIMATE SCIENCE A series of warm episodes, each lasting several thousand years, occurred in Antarctica between 90,000 and 30,000 years ago. These events correlated with rapid climate oscillations in the Arctic, with Antarctica warming while the Arctic was cooling or already cold. This bipolar seesaw is thought to have been driven by changes in the strength of the deep overturning circulation in the North Atlantic Ocean, but some have questioned how completely that process can account for the fine details of Antarctic warming events. Keeling and Visbeck offer an explanation that builds upon earlier suggestions that include the effects of shallow-water processes as well as deep ones. They suggest that changes in the surface salinity gradient across the Antarctic Circumpolar Current were caused by the melting of icebergs discharged from the Arctic, which allowed increased heat transport to Antarctica by ocean eddies. This mechanism produces Antarctic warming of the magnitude observed in ice core records. — HJS Quat. Sci. Rev. 10.1016/j/quascirev.2005.04.005 (2005).

Sensitivity of the northern extratropics hydrological cycle to the changing insolation forcing at 126 and 115 ky BP

The orbital configuration at the end of the last interglacial, 115,000 years BP (115 ky BP), was such that the Northern Hemisphere seasonal contrast was decreased when compared to the last interglacial maximum, 126 ky BP. Climatic reconstructions argue for increased latitudinal surface temperature and salinity gradients in the North Atlantic at 115 ky BP compared to 126 ky BP. According to proxy measurements the high-latitude ocean freshening may be explained by enhanced northward atmospheric moisture advection which would have then led to decreased deep convection activity in the northern seas. To evaluate such re-adjustments of the atmospheric circulation to the insolation forcing changes, we have explored the changes in atmospheric energy balance and transport with two AGCM experiments, one for each climate. We show that the northward increase in static heat transport at 115 ky BP to 126 ky BP constitutes a first order response to the changing insolation. It tends to equalise the heat balance of the atmosphere. Despite sea surface temperatures fixed (SSTs) to present-day this feature is strongly amplified by the air–sea heat flux exchanges. By comparing with OAGCM experiments for the same periods, we find that the simulated surface ocean heat flux responses to insolation forcing are similar whether the ocean is allowed to vary or not. The latent heat transport does not undergo the same changes as the dry static one. On an annual basis, it decreases over the high northern latitudes. This is the result of summer modification of moisture sources and transient activity. The latter appears to affect latent heat transport much more than the dry static one. The winter response, however, differs from the summer response which dominates the annual mean. There is an enhanced northward atmospheric moisture advection during winter at 115 ky BP, which is responsible for the freshening of high-latitude ocean during this season. This result seems to confirm the hypothesis inferred from marine data.

Impacts of salinity on the eastern Indian Ocean during the termination of the fall Wyrtki Jet

The impact of salinity and the barrier layer on eastern Indian Ocean dynamics is investigated using the OPA ocean general circulation model for 1979–1993. The study focuses on processes involved in the eastern Indian Ocean circulation during the period of the fall Wyrtki Jet. An exploration of interactions between salinity and ocean dynamics is made using a set of salinity sensitivity experiments. The inclusion of salinity favors a stronger Wyrtki Jet that extends further eastward in two steps. First, a sporadic barrier layer along the jet increases the jet speed by trapping wind momentum in a thinner mixed layer. Second, zonal advection transports the current maximum eastward and participates in the creation of a strong current front at the eastern edge of the jet during the following month. Off Sumatra, the sea surface salinity gradient, bordering the Indian Ocean fresh pool, strengthens when the Wyrtki Jet reaches this region. The pressure gradient associated with the enhanced salinity front then becomes larger than the eastward wind stress forcing in the zonal momentum equation. This initiates a westward surface current opposed to the wind stress. The later penetration of the Wyrtki Jet into the Indian warm and fresh pool modifies the sea level and favors geostrophic westward currents around 4°N. A combination of these mechanisms additionally favors a northward shift of the Wyrtki Jet. All these phenomena affect the zonal displacement of the Indian fresh pool and create SST anomalies of 0.5°C, in a region of high ocean‐atmosphere coupling.

Deep flow variability under apparently stable North Atlantic deep water production during the Last Interglacial of the subtropical NW Atlantic

The environmental history of the last interglacial period is equivocal, since its climate has been reported to be anything from unstable, with at least one significant cold event, to generally stable and comparable to the Holocene. Here the activity of the southerly Deep Western Boundary Current (DWBC) in the ∼130–110 ka interval is studied at two sites (3481 and 4760 m) in the subtropical NW Atlantic presently bathed by Lower North Atlantic Deep Water (LNADW). Proxies for both bottom water chemistry (δ13C) and near‐bottom flow vigor ( , the mean size of the “sortable silt” fraction) are employed. The δ13C data indicate the interval of peak interglaciation when LNADW bathed both sites studied is followed by the development of a vertical water mass gradient between them, with nutrient‐rich waters of southern origin flooding the deeper parts of the area. After ∼113 ka this gradient disappears as the deeper, low δ13C waters homogeneously invade the whole of the depth range investigated. The data imply these events are likely to be associated with the vertical migration of the flow axis of the DWBC. Furthermore, they also reveal unexpected fluctuations in near‐bottom flow activity during the period of minimum ice volume which are tentatively ascribed to small changes in the convective activity of the Nordic Seas. Alternatively, DWBC flow fluctuations may be controlled by processes occurring farther downstream from the areas of deep water formation, therefore not requiring changes in the production rates of LNADW. In either case, this series of small fluctuations during the peak of the last interglaciation may ultimately be linked to the insolation‐related evolution of latitudinal sea surface temperature and salinity gradients in the North Atlantic.

An evaluation of a steerable sidescan sonar for surveys of near-surface fish

A towed 330 kHz sector scanning sonar was evaluated for surveys of near-surface migratory salmonids ( Oncorhynchus sp.) and other fish in the Strait of Georgia near the mouth of the Fraser River near Vancouver, BC (Canada). Detection of large near-surface fish presents a serious challenge to conventional net trawls or echo-sounder surveys due to vessel avoidance behaviour by the fish. The sonar was towed beside a research vessel on a custom-built towfish at a nominal depth of 18 m. The optimum configuration was determined to be with the sonar scanning a 30° vertical sector oriented perpendicular to the tow direction, at a ping rate of 5 Hz and a sector scan period of 5 s. Fish were detected up to a range of 100 m, limited by systemic noise and backscattered reverberation from the sea surface. Some evidence of vessel-avoidance behaviour by the fish was observed. Near surface temperature and salinity gradients are shown to induce downwards refraction of the sonar beam, resulting in biases in apparent depth and target strength of the fish. A simple model of the sonar performance in this scanning mode, including beam-pattern effects, is used to generate curves of fish target detectability and beam-pattern induced bias as a function of range and fish target strength. After correction for refraction effects, beam-pattern bias, and detectability, the survey results show reasonable agreement with net trawl and riverine escapement data.

Impact of estuarine fronts on the dispersal of piscivorous birds in the German Bight

ABSTRACT: Survey data on seabird distribution at sea, hydrographic data and optical satellite datacollected for the German Bight were used to analyse the variability of the distribution of wintering red-throated diver and black-throated diver Gavia stellata / arctica in relation to oscillations of the JutlandCoastal Current (JCC) and associated surface fronts. Data collected from hydrographic stations weresummarised by means of principal component analysis; the first component, reflecting characteristicsof the JCC, provided a satisfactory quantitative measurement of the average meso-scale habitat usedby both species. The pelagic range of divers clearly followed the outer estuarine front between surfaceNorth Sea water and the JCC, which was located between the 20 and 30 m depth contours. Despite ahighly transient trailing edge of the JCC, no divers were ever observed in North Sea water. Hydro-graphic as well as composited satellite CZCS (Coastal Zone Color Scanner) data indicated the pres-ence of an inner front, spanning a stronger surface salinity gradient than the outer estuarine front. Theinner estuarine front was located between mixed estuarine water, with salinities between 32 and34 psu, and the core of the estuarine water mass from the river Elbe, with salinities below 32 psu.Seven-year composite images, produced from all available CZCS-Chl (chlorophyll) data taken overthe German Bight during the Nimbus-7 mission, revealed the mean distribution of the inner estuarinefront as a sharp gradient between the 15 and 20 m depth contours, extending less than 10 km inlongitude. The variability of the inner estuarine front was measured over a series of cruises, and itindicated a quasi-stable structure; the prevailing position of the surface front was in a zone stretchingfrom 07°30’E at Horns Reef (55°45’N) to 07°50’E south of Amrum Bank (54°30’N), except duringeasterly winds when the front was advected offshore as far as 6°50’E. The recorded patches of highdensities of divers were almost confined to areas within 5 km distance from the mean frontal zone. Thelink between divers and the inner estuarine front seemed rather persistent, as peak densities coincidedwith the location of this front during all cruises, while lower densities were normally observed in thecore Elbe water and in the mixed water outside the front. The authors suggest that the quasi-stabilityand strength of the inner estuarine front give rise to a predictable location of food resources (i.e. smallfish) for divers in the German Bight. Our study highlights the potential for frontal structures of the JCCto influence the marine ecosystem of the southeastern North Sea.KEY WORDS: Estuarine fronts · German Bight · Seabird foraging · Red-throated/black-throated diver ·