Wind-driven Surface Currents Research Articles

The Maldives, a key location of carbonate drifts

Wind-driven surface currents and deeper water-mass circulation shape carbonate sediment drifts in shallow- to deep-water depositional settings of the Maldives. Drift sediments are located in the platform interior, on top of drowned banks, and around the atolls. They are composed of an admixture of pelagic and shallow-water components. Drift bodies vary in dimension, architecture, and genesis. Small patch drifts and sheeted drifts are present in the platform interior and on top of drowned atolls; large patch drifts preferably form in the current-shadow of large atolls and banks. Small inter-atoll channels and large gateways are dominated by channel-related drifts. The Inner Sea, a large perched basin surrounded by atolls comprises a complex contourite depositional system of giant elongated mounded and sheeted drifts, mixed drifts, as well as delta drifts. This succession of different drift types is organized in sequences, delimited by unconformities caused by changes in the current regime. These may be triggered by a re-configuration of the inter-atoll gateways or by allo-cyclic processes such as climate variability. Effects based on to changes in the global water-mass circulation could not be proven. With exception of the delta drift, carbonate drift types in the Maldives are analogous to their siliciclastic counterparts. By contrast, however, these carbonate drifts are emplaced at a short distance to the shallow-water carbonate factory, which acts as main sediment source. Variability and abundance of drifts in the Maldives emphasize the significant role of current-controlled sediment transport and redistribution in carbonate platform development.

Asymmetric abundances of submarine sediment waves around the Azores volcanic islands

Sediment waves are common on the submarine flanks of volcanic islands, but the processes that form them are difficult to decipher from geophysical data alone. Here, we identify downslope-trending trains of sediment waves in multibeam sonar data from five islands of the Azores. Train abundances are derived by counting the train intersections with depth contours around each island, revealing that they are typically twice as common on the north slopes of the islands compared with their south slopes. To provide context, we also characterise the Holocene mobility of sediments on the shelves, ocean density structure (for evaluating internal wave critical gradients) and sediment lithologies in four cores from beneath the slopes, which contain shelf carbonates in some volcaniclastic turbidites. Holocene mobility is assessed using sidescan sonar data, which reveal shelf-crossing sand streams. Using results of a high-resolution wave model, wave-imposed shear stresses are found to have exceeded the stresses at threshold of motion for most of the modern shelf sediments during recent extreme wave conditions.Sediment waves could potentially arise from sedimentary flows, gravity mass movements (shallow slumping) or ocean internal waves. Sedimentary flow origins are favoured by streamwise changes of wavelengths and upslope bedform migrations suggested by seismic reflections. The asymmetry of abundances north and south of each island is also an important clue. For example, it is unlikely that systematically different sediment geotechnical properties occur on the two sides of the islands, hence no reason to expect greater mass movements on northern slopes. Internal waves originate mainly from the south and hence also do not explain the asymmetry. Primary volcaniclastic flows are likely to be equally important on both sides. Sedimentary flows originating from shelf sediment agitated during storms or from failed slope sediment appear to explain best the asymmetry. In that case, prevailing wind and waves approaching the islands from the northwest lead to more frequent sediment gravity flows because they lead to greater flux of sediment from coastal erosion towards northerly outer shelves and upper slopes, where it is prone to slope failure. Offshore-directed near-bed currents originating from northwesterly wind-driven surface currents also transport shelf sediment efficiently to the north, further aided by greater wave agitation on that side. The high abundances of wave trains on the north sides of the islands therefore favours origins involving sedimentary gravity flows associated with wind and wave effects.

Assessments of Nitrate Budgets in the Yellow Sea Based on a 3D Physical-Biogeochemical Coupled Model

Nitrate (NO3–) plays an important role in ecosystems and aquaculture in the Yellow Sea (YS). Sparse observational data suggest that ocean currents and nitrification are crucial to NO3– flux in the YS; however, a quantitative assessment of these fluxes has not yet been performed. This study investigates seasonal and spatial variations in NO3– flux via currents and biological processes in the YS from 2006 to 2019 using a physical-biogeochemical coupled model. The model results show that the current-driven fluxes exceeded biological processes in the eastern and central regions of the YS, unlike in the western and northern regions. Advection of NO3– in the YS is mainly driven by cyclonic circulation in summer and fall, and anticyclonic circulation in spring and winter. The Subei Coastal Current along the coast of China plays a primary role in net advective influx of NO3– to the YS year round. The NO3– influx by the Yellow Sea Warm Current along the lower layer of the southcentral YS is offset by outflux through wind-driven surface currents in winter. The southward movements of the Yellow Sea Bottom Cold Water in summer and the Korean Coastal Current in winter are major NO3– outfluxes to the East China Sea. In terms of biological processes, NO3– is mainly consumed by phytoplankton during the spring bloom and supplied through organic matter decomposition and nitrification. Net supply of NO3– by biological processes was the greatest in the southcentral YS where the Yellow Sea Bottom Cold Water is present.

Frictional effects in wind-driven ocean currents

Surface ocean currents have a significant influence on the climate and their dynamics depend to a large extent on the behaviour of the vertical eddy viscosity. We present an analytic study of wind-driven surface currents for general depth-dependent vertical eddy viscosities. A novel formulation for Ekman-type flows, that relies of a transformation to polar coordinates, enables us to show that in the Northern Hemisphere the horizontal current profile decays in magnitude and turns clockwise with increasing depth, irrespective of the vertical variations in diffusivity. Using a perturbation approach, we also derive a formula for the deflection angle of the current at the surface from the wind direction and discuss its implications.

Wind Waves in the Strait of Georgia

ABSTRACT The Strait of Georgia, British Columbia, Canada, is an important ocean region in which wave and weather conditions can vary rapidly in time and space because of the complex mountain topography that surrounds it. Here we analyze existing observational data and a newly developed near real-time numerical wave model, forced by modelled local winds and ocean currents, to characterize the surface wave conditions of the Strait under a variety of wind and weather conditions. Wave heights are generally largest in the northern Strait. However, we find that there are some deficiencies in the existing observational data, with buoy measurements in the northern Strait overestimating wave heights by as much as 0.4 m. The wave modelling also shows that strong tidal flows near Discovery Passage and Boundary Pass lead to increased wave heights, but current-related increases are not predicted near Sand Heads because of deficiencies in the numerical ocean current model. Outflow wind conditions in the Fraser Valley result in large waves south of Point Roberts but Howe Sound outflows do not noticeably affect significant wave heights offshore of Howe Sound. Stokes drift associated with surface waves can cause surface drifts as large as 20% of the directly wind-driven surface currents but is minimal for winds less than about , which characterize the Strait most of the time. Whitecap fractions throughout the Strait are similar to open-ocean conditions (i.e., 1–2% at moderate wind speeds) but about twice as high in the southern region of the Strait during strong outflow conditions.

Vertical structure of ocean surface currents under high winds from massive arrays of drifters

Abstract. Very-near-surface ocean currents are dominated by wind and wave forcing and have large impacts on the transport of buoyant materials in the ocean. Surface currents, however, are under-resolved in most operational ocean models due to the difficultly of measuring ocean currents close to, or directly at, the air–sea interface with many modern instrumentations. Here, observations of ocean currents at two depths within the first meter of the surface are made utilizing trajectory data from both drogued and undrogued Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters, which have draft depths of 60 and 5 cm, respectively. Trajectory data of dense, colocated drogued and undrogued drifters were collected during the Lagrangian Submesoscale Experiment (LASER) that took place from January to March of 2016 in the northern Gulf of Mexico. Examination of the drifter data reveals that the drifter velocities become strongly wind- and wave-driven during periods of high wind, with the pre-existing regional circulation having a smaller, but non-negligible, influence on the total drifter velocities. During these high wind events, we deconstruct the total drifter velocities of each drifter type into their wind- and wave-driven components after subtracting an estimate for the regional circulation, which pre-exists each wind event. In order to capture the regional circulation in the absence of strong wind and wave forcing, a Lagrangian variational method is used to create hourly velocity field estimates for both drifter types separately, during the hours preceding each high wind event. Synoptic wind and wave output data from the Unified Wave INterface-Coupled Model (UWIN-CM), a fully coupled atmosphere, wave and ocean circulation model, are used for analysis. The wind-driven component of the drifter velocities exhibits a rotation to the right with depth between the velocities measured by undrogued and drogued drifters. We find that the average wind-driven velocity of undrogued drifters (drogued drifters) is ∼3.4 %–6.0 % (∼2.3 %–4.1 %) of the wind speed and is deflected ∼5–55∘ (∼30–85∘) to the right of the wind, reaching higher deflection angles at higher wind speeds. Results provide new insight on the vertical shear present in wind-driven surface currents under high winds, which have vital implications for any surface transport problem.

On the seasonal and inter-annual variability of the equatorial Indian Ocean surface winds

Variability of the Equatorial Indian Ocean (EIO) winds play crucial roles in driving the upper EIO dynamics and modifying the ocean–atmosphere interactions in the Indian Ocean (IO). This study, using a satellite-sensed high-resolution (0.25° × 0.25°) monthly winds, has revealed many salient features of the seasonal and inter-annual variability of winds at various dynamically significant regions of the EIO. Though annual mean wind pattern of the EIO shows westerlies east of 60°E, the different local areas in this ocean exhibit significant seasonal wind variations. Migrations and fluctuations of southerlies and westerlies primarily determine the EIO wind variability. In the western (eastern) EIO, SW and NE monsoon winds are stronger (weaker) and of longer (shorter) duration. Near to the equator (1°S–1°N), in the central EIO, weaker winds occur during SW and NE monsoons, whereas stronger winds during transition periods. Near the equator, meridional winds show a significant annual period, especially in the western EIO. But zonal winds exhibit semi-annual period east of 55°E with a peak in the central ocean associated with the active westerlies during spring and fall, and annual period west of 55°E in the western EIO due to monsoon reversals. Westerlies are stronger during the fall compared to spring. Zonal wind variability in the central EIO is the essential deciding factor for the zonal wind variability occurring in the whole EIO. Both Zonal Sea Level Pressure Gradient (ZSLPG) and Momentum Advection (MA) fields determine the dynamics of zonal winds in the EIO. Zonal winds are weak in the western EIO because of the semi-annual harmonics of both ZSLPG and MA, though with substantial amplitude, exhibiting opposite accelerations there. But stronger zonal winds occur in the central EIO (60°E–80°E) because of the same accelerations set by the relatively weaker ZSLPG and MA fields. The zonal winds show substantial inter-annual variability compared to the meridional winds. The inter-annual variability of zonal winds shows the predominance of two modes, with EOF1 50% and EOF2 20% of the total variance. Zonal wind pattern of EOF1 resembles the monsoon transition seasons, while that of EOF2 describes the SW monsoon. In the EIO, Indian Ocean Dipole (IOD) influences the patterns of winds and SST anomalies better than that of El Nino–Southern Oscillation (ENSO). Spatiotemporal variability of the EIO winds during ENSO and IOD indicates the prevalence of stronger zonal wind anomalies in the eastern and central EIO and weaker anomalies in the western EIO. Although spatial patterns of wind anomalies are similar during El Nino and Positive IOD (PIOD) periods, the monthly variations, spatial extensions, and intensities of anomalies are much different while comparing. Prospectus for understanding the EIO winds variations can help to understand the changes occurring in the wind-driven surface and subsurface zonal currents.

A State-of-the-Art Compact Surface Drifter Reveals Pathways of Floating Marine Litter in the German Bight

Lagrangian observations are important for the understanding of complex transport patterns of floating macroscopic litter items at the ocean surface. Satellite-tracked drifters and numerical models are an important source of information relevant to transport processes as well as distribution patterns of floating marine litter (FML) on a regional to global scale. Sub-mesoscale processes in coastal and estuarine systems have an enormous impact on pathways and accumulation zones of FML and are yet to be fully understood. Here we present a state-of-the-art, low-cost and robust design of a satellite-tracked low-cost drifter applicable in studying complex pathways and sub-mesoscale dynamics of floating litter in tidally influenced coastal and estuarine systems. It is compact, lightweight less than 5 kg, capable of refloating, easily recovered and modified. The drifter motion resolves currents of the ocean surface layer (top 0.5 m layer) taking into account wind induced motions. We further showcase findings from seven of our custom-made drifters deployed from RV Heincke and RV Senckenberg in the German Bight during spring and autumn 2017. Drifter velocities were computed from high resolved drifter position data and compared to local wind field observations. It was noted that the net transport of the drifters in areas far away from the coast was dominated by wind-driven surface currents, 1 % of the wind speed, whereas the transport pattern in coastal areas was mainly overshadowed by local small-scale processes like tidal jet currents, interactions with a complex shoreline and fronts generated by riverine freshwater plumes.

Numerical simulations of onshore transport of larvae and detritus to a steep pocket beach

Larvae of intertidal invertebrates need to cross the surf zone to settle in their adult habitat. Onshore transport of invertebrate larvae and detritus at a steep beach was simulated with a biophysical larval tracking model. Hydrodynamic model calculations were performed for 24 h after a 24 h spin-up stage with bathymetry and averaged wave data obtained during the summer of 2011 at Carmel River State Beach, California, and with and without onshore wind. The physical model output was then transferred to a Lagrangian larval tracking model using several types of particles representing larvae. A southward alongshore current controlled particle distribution in the middle and north of the domain. At the southern shore, negatively buoyant particles were trapped by eddies generated between the alongshore current and shore, while positively buoyant particles were carried onshore by wind-driven surface currents. The concentration of modeled detritus in the surf zone was positively correlated with that of negatively buoyant larvae. Additionally, the concentrations of detritus and competent larvae within the surf zone were negatively correlated with wave height, consistent with the observations of the accompanying field study. Some eddies contributed to forming high particle concentration patches by trapping them in the surf zone. More small eddies were generated closer to the shore with smaller waves, leading to high larval and detrital concentration in the surf zone. As waves increased in size, fewer and larger eddies formed, predominantly outside the surf zone, and consequently fewer larvae and detritus particles entered or stayed in the surf zone.

Influence of Sediment Composition, Total Organic Carbon on Benthic Organisms at Pulicat Lagoon: A Case Study

Benthic environments are dynamic habitats where the sediments, the functional building blocks of the habitat, are continually structured by both the organisms living in and on the seabed as well as by the local physical regime. Benthic animals are an important component of the marine ecological realm and play a large part in marine food chains, organic production and the turnover of sediments and nutrients on the ocean floor. Because the sub-littoral seabed is less subject to variations in tides, swell and wind driven surface currents, the soft sediments of this environment are relatively stable (i.e., unchanging) compared to intertidal and shallow near shore substrates. Similarly, their populations are also more stable and diverse. These biological processes are also inextricably linked with physical variables, thus benthic communities are very susceptible to both natural and anthropogenic changes in the topography and sediment matrix. Inorder to understand the association between environmental variables and benthic communities, the distribution of benthic organisms in the dynamic mixing zone of Pulicat and the surrounding coastal waters has been studied. The sediment samples obtained from Pulicat were analyzed for sediment composition and total organic carbon. Further the benthic organisms were sorted, enumerated and grouped into different communities. Based on the analysis; it was found that the distribution of benthic organisms varied with the differences in sediment composition and total organic carbon. The mixing of wastewaters from Buckingham canal with the waters of the dynamic zone of Pulicat could be a significant factor influencing the distribution of benthic communities.

The Occurrence, Drivers, and Implications of Submesoscale Eddies on the Martha’s Vineyard Inner Shelf

AbstractThe occurrence, drivers, and implications of small-scale O(2–5) km diameter coherent vortices, referred to as submesoscale eddies, over the inner shelf south of Martha’s Vineyard, Massachusetts, are examined using high-frequency (HF), radar-based, high-resolution (400 m) observations of surface currents. Within the 300 km2 study area, eddies occurred at rates of 1 and 4 day−1 in winter and summer, respectively. Most were less than 5 h in duration, smaller than 4 km in diameter, and rotated less than once over their lifespan; 60% of the eddies formed along the eastern edge of study area, adjacent to Wasque Shoal, and moved westward into the interior, often with relative vorticity greater than f. Eddy generation was linked to vortex stretching on the ebb and flood tide as well as the interaction of the spatially variable tide and the wind-driven currents; however, these features had complex patterns of surface divergence and stretching. Eddies located away from Wasque Shoal were related to the movement of wind-driven surface currents, as wind direction controlled where eddies formed as well as density effects. Using an analysis of particles advected within the radar-based surface currents, the observed eddies were found to be generally leaky, losing 60%–80% of particles over their lifespan, but still more retentive than the background flow. As a result, the combined translation and rotational effects of the observed eddies were an important source of lateral exchange for surface waters over the inner shelf.

Isle of Man, Galapagos and sunspot data show net cooling hid double exponential ocean warming danger: +3°C in 2014, +4°C likely by 2016.

Anthropogenic global warming (AGW) heat is trapped by the greenhouse gas (GHG) blanket, and the ocean surface layer. It is 93% in the ocean and drives atmospheric warming. The 111-year mean daily surface temperatures are 10.5±0.5°C at Port Erin (PE) Isle of Man compared with 9.6±4.8°C in Central England (CET) air. The Port Erin 5½-year max-min heat cycle synchronizes to the 11-year solar heat pump sunspot cycle. Tropical heat arrives 2 years after a solar maximum on wind-driven currents in the stratified sea surface. Runoff from bottom-up melted Arctic icesheets arrives 3½ year later at solar minimum. These warm and cold waters are the biodiversity source. PE is unique with seasonal meltwaters of Pacific and Atlantic origin. The North Pacific warms twice as fast as other oceans. All ocean near-surface gyre currents harmonize with sunspot cycles. Net cooling by polar icemelt masks catastrophic exponential ocean warming and icemelt. Eleven counter-rotating surface gyres carry heat and nutrients globally in verified ocean surface circulation system.Exponential growth is unsustainable in a finite system. It trends to infinity. Double-exponential gets there twice as quickly. The GHG blanket, grown double-exponentially for 250 years, is now in control. Ocean heat absorption takes 150-250 years. Arctic icemelt increases double-exponentially. The Arctic long-term annual freeze-melt volume cycle is 16.8±1.3 thousand cubic km per year. Polar land icemelt adds ~500 km3 per year. Freeze-brine of salinity >40‰ and temperature –1°C, sinks to the bottom. Equatorial evaporative-brine of salinity >36.4‰ and >28°C floats subsurface under fresh warm layers thickening westwards in tropical meridional cells to ~75m depth. This is consistent with observed extreme weather.Heat imbalance forced Pacific Ocean temperatures above proposed limits of +2°C in 1993, to +3°C in 2014, and is on track for +4°C for 2016. Century-long daily records confirm processes ongoing for 300 years. Coast locations are where impacts are felt and real-time data collected. Corporate governance degraded physics teaching in only 60 years. Individual discovery and data collection was lost. Big science is unnecessary. Satellites cannot do plankton tows. Computer models are governed by the rule of ‘garbage-in garbage-out’. They must be verified by in situ data that cannot be collected retrospectively. Continuous timeseries surface profile data from fixed ocean station locations on a global variable-boundary network are essential. Scientists, if well-trained in ocean experimental physics, can do the hard work.Time-poor scientists, stripped of their intellectual property rights, under rewarded, poorly educated, and ruthlessly exploited by growth-obsessed commercial interests, missed catastrophic global warming and multiple extreme consequences. Climate scientists abandoned classical physics and Newton-Hooke field verification in favor of unverified beliefs, models, and apps. Climate studies confuse heat with temperature, do not include basal icemelt, density temperature-salinity function, Clausius-Clapeyron evaporation exponential skin temperature function, asymmetric brine-heat sequestration, solar and tidal pumping, infra-red GHG heat trap, vertical tropical cells, freshwater warm pools; or wind-driven surface currents at 3 percent of windspeed. Climate model mistaken assumptions lead to the absurd conclusion that evaporation in the Labrador Sea at midnight in midwinter is greater than at the midday Equator.The Isle of Man provides an ideal location for continued monitoring and mitigation research, teaching and public service in a dedicated non-commercial independent multidisciplinary university-type setting. Quality teaching is the major priority. Commercial monopoly rights need replacement with free, fully open discussions and publications. Quality not quantity should be paramount. Internationally competitive academics should control subservient lower paid support staff.Every day without ocean surface data means vital scientific truth lost of interest and concern to all populations. Predictions are groundless without accurate continuous ocean surface data. Skeptics, politicians, statisticians, those with stakes in the status quo, and established research censors obstructing scientific progress squabble in ignorance while the globe burns.

Numerical simulations of larval transport into a rip‐channeled surf zone

Competent larvae of intertidal invertebrates have to migrate toward shore for settlement; however, their migration through the surf zone is not understood. We investigated larval transport mechanisms at a rip‐channeled beach. Because tracking larvae in the surf zone is infeasible, we used a three‐dimensional biophysical model to simulate the processes. The coupled model consists of a physical module for currents and waves, and a biological module for adding larval traits and behaviors as well as Stokes drift to Lagrangian particles. Model calculations were performed with and without onshore wind forcing. Without wind, wave‐driven onshore streaming occurs in the bottom boundary layer outside the surf zone. With onshore wind, onshore currents occur near the surface. In the surf zone, offshore‐directed rip currents and compensating onshore‐directed currents over shoals are formed in both no‐wind and wind cases. In the biological module, neutral, negative, and positive buoyant particles were released offshore. Additionally, particles either sank in the presence of turbulence or not. Two scenarios achieved successful onshore migration: Negatively buoyant larvae without wind forcing sink in the turbulent bottom boundary layer and are carried onshore by streaming; positively buoyant larvae drift toward shore in wind‐driven surface currents to the surf zone, then sink in the turbulent surf zone and remain near the bottom while transported shoreward. In both cases, the larval concentration is highest in the rip channel, consistent with field data. This successful result is only obtained if turbulence‐dependent sinking behavior and Stokes drift are included in the transport of larvae.

Detection and variability of the Congo River plume from satellite derived sea surface temperature, salinity, ocean colour and sea level

Despite its size and importance the dispersal, variability and dynamics of the Congo River plume have until recently been minimally documented. In this paper we analyse remotely sensed sea surface temperature, sea level anomaly, salinity and chlorophyll concentration from 2010 to describe and quantify the extent, strength and seasonality of the plume and to explain its behaviour in relation to winds, ocean currents and fresh water discharge. Empirical Orthogonal Function (EOF) analysis reveals strong seasonal and coastal upwelling signals, potential bimodal seasonality and responses to fresh water discharge peaks in all data sets. Salinity and chlorophyll are significantly negatively correlated (−0.4 to −0.9) across a 500km2 zone west of the river mouth. Lagged correlations with river discharge show the westward translation, dispersal and strengthening of a similar sized region of positive chlorophyll-discharge and negative salinity-discharge correlations. Peaks in chlorophyll lag the discharge by 1–4months, although it is not possible from ocean colour alone to distinguish increases in chlorophyll resulting from the dispersal of nutrient rich river discharge from other physical and biological mechanisms that promote changes in surface chlorophyll, or indeed have confidence that the signal has not been contaminated by other optically active substances. The strongest plume-like signatures from EOF analysis are found in the salinity and colour data sets, which are then also analysed using a statistically based water mass detection technique to isolate the behaviour of the plume. The validity of the technique is checked against ARGO profiles. Throughout the year the main axis of the fresh water plume extends between 400 and 1000km, northwest along the coastline, or west–southwest out into the open south Atlantic. Changes in the magnitude and direction of wind stress and hence wind driven surface currents are the principal driving force behind this far-field variability, although the strength and direction of the Angola and Benguela Coastal currents may also play a role. The mean surface salinity within the plume and its surface area have qualitative links to the seasonal cycle in river discharge and wind stress.

Controls of Caribbean surface hydrology during the mid- to late Holocene: insights from monthly resolved coral records

Abstract. Several proxy-based and modeling studies have investigated long-term changes in Caribbean climate during the Holocene, however, very little is known on its variability on short timescales. Here we reconstruct seasonality and interannual to multidecadal variability of sea surface hydrology of the southern Caribbean Sea by applying paired coral Sr/Ca and δ18O measurements on fossil annually banded Diploria strigosa corals from Bonaire. This allows for better understanding of seasonal to multidecadal variability of the Caribbean hydrological cycle during the mid- to late Holocene. The monthly resolved coral Δδ18O records are used as a proxy for the oxygen isotopic composition of seawater (δ18Osw) of the southern Caribbean Sea. Consistent with modern day conditions, annual δ18Osw cycles reconstructed from three modern corals reveal that freshwater budget at the study site is influenced by both net precipitation and advection of tropical freshwater brought by wind-driven surface currents. In contrast, the annual δ18Osw cycle reconstructed from a mid-Holocene coral indicates a sharp peak towards more negative values in summer, suggesting intense summer precipitation at 6 ka BP (before present). In line with this, our model simulations indicate that increased seasonality of the hydrological cycle at 6 ka BP results from enhanced precipitation in summertime. On interannual to multidecadal timescales, the systematic positive correlation observed between reconstructed sea surface temperature and salinity suggests that freshwater discharged from the Orinoco and Amazon rivers and transported into the Caribbean by wind-driven surface currents is a critical component influencing sea surface hydrology on these timescales.

Recruitment processes in Baltic sprat – A re-evaluation of GLOBEC Germany hypotheses

The GLOBEC Germany program (2002–2007) had the ambitious goal to resolve the processes impacting the recruitment dynamics of Baltic sprat (Sprattus sprattus L.) by examining various factors affecting early life history stages. At the start of the research program, a number of general recruitment hypotheses were formulated, i.e. focusing on (1) predation, (2) food availability, (3) physical parameters, (4) the impact of current systems, and finally (5) the importance of top-down vs bottom-up effects. The present study synthesizes the results of field sampling (2002 and 2003), laboratory experiments, and modeling studies to re-evaluate these hypotheses for the Baltic sprat stock. Recruitment success was quite different in the 2years investigated. Despite a lower spawning stock biomass in 2003, the total number of recruits was almost 2-fold higher that year compared to 2002. The higher recruitment success in 2003 could be attributed to enhanced survival success during the post-larval/juvenile stage, a life phase that appears to be critical for recruitment dynamics. In the state of the Baltic ecosystem during the period of investigation, we consider bottom-up control (e.g. temperature, prey abundance) to be more important than top-down control (predation mortality). This ranking in importance does not vary seasonally. Prevailing water circulation patterns and the transport dynamics of larval cohorts have a strong influence on sprat recruitment success. Pronounced transport to coastal areas is detrimental for year-class strength particularly at high sprat stock sizes. A suggested mechanism is density-dependant regulation of survival via intra- and inter-specific competition for prey in coastal areas. A documented change in larval vertical migration behavior between the early 1990s and early 2000s increased the transport potential to the coast, strengthening the coupling between inter-annual differences in the magnitude and direction of wind-driven surface currents and year-to-year changes in reproductive success. However, due to the strong linkages and feed-back loops in the Baltic Sea food web, the most robust projections of the future strength of the Baltic sprat stock will need to take into account climate-driven changes in both abiotic (e.g., drift trajectories) and biotic (trophodynamic) factors. Although our understanding of processes affecting pre-recruit (larval) growth and survival has been advanced by the integrated research conducted within the GLOBEC Germany program, key mechanisms potentially affecting life stages outside of the spawning basins remain to be explored including the dynamics of coastal habitats of juveniles and the feeding and overwintering grounds of adults.

二次循環流を伴う吹送流の三次元構造に関する現地観測

Detailed investigation of three-dimensional structure of wind-driven currents due to secondary circulations is needed to an accurate estimation of material transport on lakes and oceans. In this study, the convenient method to observe this structure by using the web camera, H-ADCP and Workhorse is proposed and the spatially-periodic profiles of horizontal wind-driven currents were obtained by using the method. Furthermore, surface velocities in the high-speed zones were estimated by applying the logarithmic law and the friction coefficient. From the results, the maximum surface velocity is about twice of 3.5% of U10, which is used in the general estimation of wind-driven surface currents.

Tidal and wind-driven surface currents in the German Bight: HFR observations versus model simulations

Tidal and wind-driven surface currents in the German Bight between shallow mudflats of the North Frisian islands and the island of Helgoland are studied using coastal high-frequency radar (HFR) observations and hindcasts from a primitive equation numerical model. The setup of the observational system is described, and estimates of expected measurement errors are given. A quantitative comparison of numerical model results and observations is performed. The dominant tidal components are extracted from the two data sources using tidal harmonic analysis and the corresponding tidal ellipses are defined. Results show that the spatial patterns of different tidal ellipse parameters are consistent in the two data sets. Model sensitivity studies with constant and variable salinity and temperature distributions are used to study density-related mechanisms of circulation. Furthermore, the role of the surface wind field in driving the German Bight circulation is investigated using the complex correlation between wind and surface current vectors. The observed change of the respective correlation patterns from the coastal to open ocean is shown to be due to a combination of density effects, the coastline and topography. The overall conclusion is that HFR observations resolve the small-scale and rapidly evolving characteristics of coastal currents well in the studied area and could present an important component for regional operational oceanography when combined with numerical modelling. Some unresolved issues associated with the complex circulation and large instability of circulation in front of the Elbe River Estuary justify further considerations of this area using dedicated surveys and modelling efforts.

Survival probability of larval sprat in response to decadal changes in diel vertical migration behavior and prey abundance in the Baltic Sea

We employed a coupled three-dimensional biophysical model to explore long-term inter- and intra-annual variability in the survival of sprat larvae in the Bornholm Basin, a major sprat spawning area in the Baltic Sea. Model scenarios incorporated observed decadal changes in larval diel vertical distribution and climate-driven abiotic and biotic environmental factors including variability in the abundance of different, key prey species (calanoid copepods) as well as seasonal changes, long-term trends, and spatial differences in water temperature. Climate forcing affected Baltic sprat larval survival both directly (via changes in temperature) and indirectly (via changes in prey populations). By incorporating observed changes in larval diel vertical migration, decadal changes in modeled and observed survival of Baltic sprat agreed well. Higher larval survival (spawning stock biomass) was predicted in the 1990s compared to the 1980s. After changing their foraging strategy by shifting from mid-depth, low prey environment to near-surface waters, first-feeding larvae encountered much higher rates of prey encounter and almost optimal feeding conditions and had a much higher growth potential. Consequently, larvae were predicted to experience optimal conditions to ensure higher survival throughout the later larval and early juvenile stages. However, this behavioral shift also increased the susceptibility of larvae to unfavorable winddriven surface currents, contributing to the marked increase in interannual variability in recruitment observed during the past decade.

Spatiotemporal variability in recruitment around Iriomote Island, Ryukyu Archipelago, Japan: implications for dispersal of spawning corals

Spatiotemporal recruitment patterns of scleractinian corals were investigated around Iriomote Island, Ryukyu Archipelago, Japan, in relation to adult coral cover in 2005 and 2006. Although almost all corals were broadcasting spawners, the relationship between recruitment and adult coral cover differed among coral families (Acroporidae, Poritidae, and Pocilloporidae), likely due to differences in embryonic development time. For spawning pocilloporid corals, whose larvae develop relatively more rapidly, recruitment was higher at sites where adult coral cover was higher. In contrast, recruitment was not related to adult coral cover in acroporid and poritid corals, whose embryonic development times were relatively slow. Moreover, recruitment of acroporid corals varied between years, and recruitment was greater at leeward compared to windward reefs for a few days after spawning. These results suggest that embryonic development time and wind-driven surface currents affect larval dispersal and subsequent recruitment patterns at a local scale. Based on embryonic development time, some spawning corals are more likely to have higher rates of self-seeding than others. Our results predict that among spawning corals, local populations of acroporid and poritid corals, whose larvae potentially disperse over long distances and recruit in neighboring reefs, are more resilient to local disturbances than those of pocilloporid corals, whose recruitment relies upon local stock.