North Atlantic Gyre Research Articles

Fast Physics and Slow Physics in the Nonlinear Dansgaard–Oeschger Relaxation Oscillation

AbstractThe Dansgaard–Oeschger (D-O) relaxation oscillation that governed glacial climate variability during marine isotope stage 3 has been accurately simulated using a high-resolution coupled climate model. Here the authors present additional detailed analyses of both the slow physics transition between warm and cold states and the fast physics transition between cold and warm states of the D-O cycle. First, the authors demonstrate that the mechanisms active during the slow transition from interstadial to stadial conditions involves the continuous flux of thick and old sea ice from the Arctic basin into the North Atlantic subpolar gyre region along the East Greenland Current. During this slow physical process, the freshwater input from sea ice melting as it moves over the surface of the warm ocean restratifies the high-latitude North Atlantic and leads to a significant reduction in the rate of North Atlantic Deep Water formation. A detailed freshwater budget and hydrography analysis is also presented to demonstrate that the D-O cycle is a low-latitude–high-latitude salt oscillator as the authors have previously argued. Second, the authors provide a more detailed analysis than previously of the fast-time-scale processes that govern the extremely rapid transition from cold stadial conditions back to the warm interstadial state. These are associated with the onset of a sub-sea ice thermohaline convective instability, which opens a massive polynya to the north of the southern boundary of the extensive North Atlantic sea ice lid that is characteristic of stadial conditions. This instability is enabled by the continuous increase of salinity above the sub-sea ice pycnocline, which eliminates the vertical salinity gradient that prevents convective destabilization of the water column under full stadial conditions. This reduction in the vertical salinity gradient beneath the sea ice lid results from the continuing northward salt transport by the North Atlantic gyre circulation once the expansion of the stadial sea ice lid has ceased. The onset of instability occurs in the Irminger basin to the south of Denmark Strait, and the authors discuss the reason for this localization of instability onset.

Holocene lowering of the Laurentide ice sheet affects North Atlantic gyre circulation and climate

The Laurentide ice sheet, which covered Canada during glacial periods, had a major influence on atmospheric circulation and surface climate, but its role in climate during the early Holocene (9–7 ka), when it was thinner and confined around Hudson Bay, is unclear. It has been suggested that the demise of the ice sheet played a role in the 8.2 ka event (an abrupt 1–3 °C Northern Hemisphere cooling lasting ~ 160 years) through the influence of changing topography on atmospheric circulation. To test this hypothesis, and to investigate the broader implications of changing ice sheet topography for climate, we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9.5 to 8.0 ka. Between 9.5 and 8.0 ka, our simulations show a 2 °C cooling south of Iceland and a 1 °C warming between 40° and 50°N in the North Atlantic. These surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid-North Atlantic as the ice sheet lowers. The climate response is strongest during the period of peak ice volume change (9.5–8.5 ka), but becomes negligible after 8.5 ka. The climatic effects of the Laurentide ice sheet lowering during the Holocene are restricted to the North Atlantic sector. Thus, topographic forcing is unlikely to have played a major role in the 8.2 ka event and had only a small effect on Holocene climate change compared to the effects of changes in greenhouse gases, insolation and ice sheet meltwater.

Nutrient-specific responses of a phytoplankton community: a case study of the North Atlantic Gyre, Azores

Nutrient-specific responses of a phytoplankton community: a case study of the North Atlantic Gyre, Azores

Carbon sequestration in the deep Atlantic enhanced by Saharan dust

Enhanced atmospheric input of dust-borne nutrients and minerals to the remote surface ocean can potentially increase carbon uptake and sequestration at depth. Nutrients can enhance primary productivity, and mineral particles act as ballast, increasing sinking rates of particulate organic matter. Here we present a two-year time series of sediment trap observations of particulate organic carbon flux to 3,000 m depth, measured directly in two locations: the dust-rich central North Atlantic gyre and the dust-poor South Atlantic gyre. We find that carbon fluxes are twice as high and a higher proportion of primary production is exported to depth in the dust-rich North Atlantic gyre. Low stable nitrogen isotope ratios suggest that high fluxes result from the stimulation of nitrogen fixation and productivity following the deposition of dust-borne nutrients. Sediment traps in the northern gyre also collected intact colonies of nitrogen-fixing Trichodesmium species. Whereas ballast in the southern gyre is predominantly biogenic, dust-derived mineral particles constitute the dominant ballast element during the enhanced carbon fluxes in the northern gyre. We conclude that dust deposition increases carbon sequestration in the North Atlantic gyre through the fertilization of the nitrogen-fixing community in surface waters and mineral ballasting of sinking particles.

A tale of two gyres: Contrasting distributions of dissolved cobalt and iron in the Atlantic Ocean during an Atlantic Meridional Transect (AMT-19)

Cobalt (Co) and iron (Fe) are essential for phytoplankton nutrition, and as such constitute a vital link in the marine biological carbon pump. Atmospheric deposition is an important, and in some places the dominant, source of trace elements (TEs) to the global ocean. Dissolved cobalt (dCo) and iron (dFe) were determined along an Atlantic Meridional Transect (AMT-19; Oct/Nov 2009) between 50°N and 40°S in the upper 150m in order to investigate the behaviour and distribution of these two essential, bioactive TEs. During AMT-19, large differences in the distributions of dCo and dFe were observed. In the North Atlantic gyre provinces, extremely low mixed layer dCo concentrations (23±9pM) were observed, which contrasts with the relatively high mixed layer dFe concentrations (up to 1.0nM) coincident with the band of highest atmospheric deposition (∼5–30°N). In the South Atlantic gyre, the opposite trend was observed, with relatively high dCo (55±18pM) observed throughout the water column, but low dFe concentrations (0.29±0.08nM). Given that annual dust supply is an order of magnitude greater in the North than the South Atlantic, the dCo distribution was somewhat unexpected. However, the distribution of dCo shows similarities with the distribution of phosphate (PO43−) in the euphotic zone of the Atlantic Ocean, where the North Atlantic gyre is characterised by chronically low PO4, and higher concentrations are observed in the South Atlantic gyre (Mather et al., 2008), suggesting the potential for a similar biological control of dCo distributions. Inverse correlations between dCo and Prochlorococcus abundance in the North Atlantic gyre provinces, combined with extremely low dCo where nitrogen fixation rates were highest (∼20–28°N), suggests the dominance of biological controls on dCo distributions. The contrasting dCo and dFe distributions in the North and South Atlantic gyres provides insights into the differences between the dominant controls on the distribution of these two bioactive trace metals in the central Atlantic Ocean.

Atmospheric deposition of soluble trace elements along the Atlantic Meridional Transect (AMT)

We briefly review the role of atmospheric deposition measurements within the Atlantic Meridional Transect (AMT) programme and then go on to present new data on the soluble concentrations of a range of trace metals (Fe, Al, Mn, Ti, Zn, V, Ni and Cu) and major ions in aerosols collected along the AMT transect. The results allow us to identify emission sources of the trace metals particularly in terms of the relative importance of anthropogenic versus crustal sources. We identify strong gradients in concentrations and deposition for both crustal and anthropogenically sourced metals with much higher inputs to the North Atlantic compared to the South Atlantic, reflecting stronger land based emission sources in the Northern Hemisphere. We suggest anthropogenic sources of Ni and V may include an important component from shipping.We consider the extent to which these gradients are reflected in surface water concentrations of these metals based on the GEOTRACES water column trace metal data. We find there is a clear difference in the concentrations of surface water dissolved Al and Fe between the north and south Atlantic gyres reflecting atmospheric inputs. However for Mn, V or Ni, higher inputs to the North Atlantic compared to the South Atlantic are not clearly reflected in their water column concentrations.

The vertical distribution of buoyant plastics at sea: an observational study in the North Atlantic Gyre

Abstract. Millimetre-sized plastics are numerically abundant and widespread across the world's ocean surface. These buoyant macroscopic particles can be mixed within the upper water column by turbulent transport. Models indicate that the largest decrease in their concentration occurs within the first few metres of water, where in situ observations are very scarce. In order to investigate the depth profile and physical properties of buoyant plastic debris, we used a new type of multi-level trawl at 12 sites within the North Atlantic subtropical gyre to sample from the air–seawater interface to a depth of 5 m, at 0.5 m intervals. Our results show that plastic concentrations drop exponentially with water depth, and decay rates decrease with increasing Beaufort number. Furthermore, smaller pieces presented lower rise velocities and were more susceptible to vertical transport. This resulted in higher depth decays of plastic mass concentration (milligrams m−3) than numerical concentration (pieces m−3). Further multi-level sampling of plastics will improve our ability to predict at-sea plastic load, size distribution, drifting pattern, and impact on marine species and habitats.

Higher Laurentide and Greenland ice sheets strengthen the North Atlantic ocean circulation

During the last glacial–interglacial cycle, changes in the large-scale North Atlantic ocean circulation occurred, and at the same time topography of the Laurentide and Greenland ice sheets also varied. In this study, we focus on detecting the changes of the North Atlantic gyres, western boundary current, and the Atlantic meridional overturning circulation (AMOC) corresponding to different Laurentide and Greenland ice sheet topographies. Using an Earth System Model, we conducted simulations for five climate states with different ice sheet topographies: Pre-industrial, Mid Holocene, Last Glacial Maximum, 32 kilo years before present and Eemian interglacial. Our simulation results indicate that higher topographies of the Laurentide and Greenland ice sheets strengthen surface wind stress curl over the North Atlantic ocean, intensifying the subtropical and subpolar gyres and the western boundary currents. The corresponding decrease in sea surface height from subtropical to subpolar favors a stronger AMOC. An offshore shift of the Gulf Stream is also identified during the glacial periods relative to that during the Pre-industrial due to lower sea levels, explaining a weaker glacial Gulf Stream detected in proxy data. Meanwhile, the North Atlantic gyres and AMOC demonstrate a positively correlated relation under each of the climate conditions with higher ice sheets.

Dynamic variability of dissolved Pb and Pb isotope composition from the U.S. North Atlantic GEOTRACES transect

This study presents dissolved Pb concentration and isotopic composition distributions from GEOTRACES GA03, the U.S. North Atlantic Transect. Pb in the ocean is primarily derived from anthropogenic sources and Pb fluxes into the North Atlantic Ocean have been steadily decreasing following the phase-out of alkyl leaded gasoline usage in North America and Europe between 1975 and 1995. A compilation of dissolved Pb profiles from three stations occupied repeatedly during the last three decades reveals a dramatic decrease in concentrations within the surface layers and the thermocline maxima, although elevated concentrations greater than 60pmol/kg are still observed in the center of the North Atlantic gyre where ventilation timescales are longer than at the western boundary. The evolution of stable Pb isotopes at these stations shows a shift from dominantly North American-like composition in surface waters in the early 1980s towards a more European-like composition in later years. The most recent shallow signatures at the Bermuda Atlantic Time Series station (BATS) show an even more recent trend returning to higher 206Pb/207Pb ratios after the completed phase-out of leaded gasoline in Europe, presumably because recently deposited Pb is more strongly influenced by industrial and incineration Pb than by residual alkyl leaded gasoline utilization. In surface waters, trends toward a more prominent European influence are also found in the middle of the basin and toward the European coast, coincident with higher concentrations of surface dissolved Pb. Scavenging of anthropogenic Pb is observed within the TAG hydrothermal plume, and it is unclear if there is any significant contribution to deep water by basaltic Pb leached by hydrothermal fluids. In the upper water column, many stations along the transect show Pb concentration maxima at ~100m depth, coincident with a low 206Pb/207Pb isotopic signature that is typical of European emission sources. Although Pb ores from the United States historically tend to carry 206Pb/207Pb signatures >1.17 (Hurst, 2002), subsurface signatures as low as 1.1563 in 206Pb/207Pb were observed in this feature. This signature appears to be carried westward within saline Subtropical Underwater (STUW), that ventilates from the Central Eastern part of the North Atlantic Subtropical Gyre where the lowest surface isotope 206Pb/207Pb ratios are observed. Along the western boundary, deep water masses of different ages carry distinct isotope ratios corresponding to their respective times of ventilation. Finally, a low 206Pb/207Pb signature in bottom water along the Eastern margin suggests that there may be some mobilization of European-derived anthropogenic Pb from recent surface deposits on the ocean floor.

The distribution of dissolved manganese in the tropical–subtropical North Atlantic during US GEOTRACES 2010 and 2011 cruises

Dissolved manganese (Mn) concentration was determined in seawater samples collected along a section across the North Atlantic between 17°N and 40°N during US GEOTRACES North Atlantic 2010 and 2011 cruises (GT GA03). Dissolved Mn exhibits elevated concentrations in the surface, at the mid-ocean ridge and near oceanic boundaries, and low concentrations in the deep water of the gyre center. At both western and eastern boundaries, dissolved Mn concentration in the surface mixed layer (30–40m) and at depths below 200m decreases with increasing distance from shore, in contrast to the seaward increase of dissolved Mn concentration at depth between 80 and 120m. Simple model calculation suggests that in the North Atlantic gyre center, dissolved Mn concentration is controlled primarily by eolian Mn(II) deposition and in-situ photochemical reduction of MnO2 in the surface mixed layer (0–40m), and by the intensity of sunlight available for in-situ MnO2 photochemical reduction in the euphotic zone below the mixed layer (40–200m). At depth between 200 and 700m, dissolved Mn concentration is determined by regeneration, preformed Mn in the source water and lateral inputs from hydrothermal and sedimentary sources. Below 700m, dissolved Mn concentration is controlled predominantly by lateral inputs from hydrothermal and sedimentary sources. Accounting for the observed dissolved Mn concentration by lateral mixing at this depth range requires low rates of Mn(II) oxidation.

Refining Globigerinoides ruber Mg/Ca paleothermometry in the Atlantic Ocean

The Mg/Ca ratio of the planktonic foraminifera Globigerinoides ruber (white) has become a widely used proxy for reconstructing sea surface temperature (SST), as numerous studies have shown that temperature is the primary control on Mg/Ca ratios in foraminiferal calcite. However, a recent study of core-top sediments across an Atlantic meridional transect suggests that salinity might have a stronger control on foraminiferal Mg/Ca ratios than previously thought. By analyzing Mg/Ca ratios and δO18 values in G. ruber (white), Arbuszewski et al. (2010) found a 27% increase in G. ruber Mg/Ca ratios per 1 salinity unit increase for seawater salinities above 35.5. Here, we use shell weight analyses and SEM images from a subset of the core-tops used in the Arbuszewski et al. (2010) study across a narrow depth range (3197–3733 m) to show that G. ruber shells from the equatorial region are highly dissolved compared to those from the subtropical North and South Atlantic gyres, significantly impacting their Mg/Ca-SSTs. Shell weights from the higher-productivity equatorial region of the Atlantic are on average 20% and 15% lower than those from the oligotrophic North and South Atlantic gyres, respectively. Given the large preservation gradient along the transect studied by Arbuszewski et al. (2010), application of a single dissolution-corrected Mg/Ca:SST calibration equation (Dekens et al., 2002) on cores from the subtropical gyres and the equatorial region is not appropriate. When regional differences in preservation are considered, as well as realistic habitat depths and calcification seasons for G. ruber in temperate latitudes, we find a strong correlation between observational SSTs and calculated G. ruber Mg/Ca-SSTs in core-top samples spanning 43°N to 25°S in the Atlantic. In addition, our re-calibrated Mg/Ca-SSTs are more strongly correlated with isotopic calcification temperatures across the transect than originally reported by Arbuszewski et al. (2010). This study provides evidence that regional differences in preservation, rather than salinity, significantly affects G. ruber Mg/Ca-SSTs in the Atlantic, and supports culturing experiments that found salinity only has a minor influence on foraminiferal Mg/Ca ratios.

Growth and survival of Neoceratium hexacanthum and Neoceratium candelabrum under simulated nutrient-depleted conditions

The dinoflagellate Neoceratium is commonly observed in oceanic waters, depleted in major inorganic nutrients such as nitrogen and phosphorus. Using culture isolates, we investigated whether two Neoceratium species (N. hexacanthum and N. candelabrum) can grow phototrophically at low nutrient concentrations found in surface waters of oligotrophic subtropical gyres (OSGs). No phototrophic growth (indicated by changes in cell numbers, the presence of dividing cells or cellular protein increase) was observed when N. hexacanthum and N. candelabrum were grown in low nutrient seawater. In separate experiments, to determine survival time under oligotrophic nutrient conditions, 68% of N. hexacanthum cells were able to re-establish growth after spending 1–10 days in North Atlantic gyre seawater; 40% recovered after 11–20 days and only 3% recovered after 21–30 days. The longest period any single cell survived, and then went on to divide, was 26 days. These findings demonstrate that Neoceratium cells could remain viable for >3 weeks in surface waters of OSGs, but to sustain their growth nutrients must be obtained periodically from an alternative source: via ephemeral upwelling of nutrient-rich waters, phagotrophy and/or movement to and from the nutricline.

Daily scale wintertime sea surface temperature and IPC-Navidad variability in the southern Bay of Biscay from 1981 to 2010

Abstract. The combination of remotely sensed gappy Sea surface temperature (SST) images with the missing data filling DINEOF (data interpolating empirical orthogonal functions) technique, followed by a principal component analysis of the reconstructed data, has been used to identify the time evolution and the daily scale variability of the wintertime surface signal of the Iberian Poleward Current (IPC), or Navidad, during the 1981–2010 period. An exhaustive comparison with the existing bibliography, and the vertical temperature and salinity profiles related to its extremes over the Bay of Biscay area, show that the obtained time series accurately reflect the IPC-Navidad variability. Once a time series for the evolution of the SST signal of the current over the last decades is well established, this time series is used to propose a physical mechanism in relation to the variability of the IPC-Navidad, involving both atmospheric and oceanic variables. According to the proposed mechanism, an atmospheric circulation anomaly observed in both the 500 hPa and the surface levels generates atmospheric surface level pressure, wind-stress and heat-flux anomalies. In turn, those surface level atmospheric anomalies induce mutually coherent SST and sea level anomalies over the North Atlantic area, and locally, in the Bay of Biscay area. These anomalies, both locally over the Bay of Biscay area and over the North Atlantic, are in agreement with several mechanisms that have separately been related to the variability of the IPC-Navidad, i.e. the south-westerly winds, the joint effect of baroclinicity and relief (JEBAR) effect, the topographic β effect and a weakened North Atlantic gyre.

Uncertainties of SeaWiFS and MODIS remote sensing reflectance: Implications from clear water measurements

A fundamental parameter derived from satellite ocean color measurements is the spectral remote sensing reflectance, Rrs(λ) (sr−1), which is used as the input to all inversion algorithms to derive bio-optical properties (e.g., chlorophyll-a concentration or Chl in mgm−3) and water's inherent optical properties (IOPs). The accuracy and uncertainties of the satellite-derived Rrs have only been assessed through comparisons with in situ measurements that were often limited in both space and time. Here, a novel approach was developed and used to estimate Rrs uncertainties from SeaWiFS and MODIS/Aqua (MODISA) measurements over clear waters. The study focused on two oligotrophic ocean gyres in the North Atlantic and South Pacific, and used a recently developed new Chl algorithm to provide a constraint to determine the highest-quality Rrs data with minimal errors. These data were used as surrogates of “ground truth” or references (termed as Rrs,true) to estimate the Rrs error in each data point, with uncertainty estimates (in both relative and absolute forms) generated from statistical analyses. The study led to several findings: One, both SeaWiFS and MODISA have met their mission goals of achieving Rrs uncertainties and absolute accuracy (assuming that the Rrs,true values can represent the truth) to within 5% for blue bands and blue waters. As a comparison, nearly all previous in situ-based validation efforts reported mean (or median) percentage differences exceeding 10% between in situ and satellite Rrs in the blue bands. Two, for the green bands, Rrs uncertainties are significantly higher, often in the range of 10–15% for oligotrophic waters. Three, SeaWiFS Rrs uncertainties are generally higher than those of MODISA, possibly due to its lower signal-to-noise ratio (SNR). Four, all Rrs errors are spectrally related in a monotonous way from the blue to the red wavelengths, suggesting that these errors are resulted primarily from the imperfect atmospheric correction algorithms as opposed to sensor noise or vicarious calibration. Such empirical relationships are shown to be useful in reducing the Rrs uncertainties for the North Atlantic Gyre and may also be useful for most of the ocean waters. Finally, the tabulated results provide lower bounds of Rrs(λ) uncertainties for more productive waters. The findings may serve as references for future ocean color missions, and they have also significant implications for uncertainty estimates of other ocean color data products.

QUANTIFYING REGIONAL SEA LEVEL RISE CONTRIBUTIONS FROM THE GREENLAND ICE SHEET

This study projects the sea level contribution from the Greenland ice sheet (GrIS) through to 2100, using a recently developed ice dynamics model forced by atmospheric parameters derived from three different climate models (CGCMs). The geographical pattern of the near-surface ice warming imposes a divergent flow field favoring mass loss through enhanced ice flow. The calculated average mass loss rate during the latter half of the 21 st century is ~0.64±0.06 mm/year eustatic sea level rise, which is significantly larger than the IPCC AR4 estimate from surface mass balance. The difference is due largely to the positive feedbacks from reduced ice viscosity and the basal sliding mechanism present in the ice dynamics model. This inter-model, inter-scenario spread adds approximately a 20% uncertainty to the IPCC ice model estimates. The sea level rise is geographically non-uniform and reaches 1.69±0.24 mm/year by 2100 for the northeast coastal region of the United States, amplified by the expected weakening of the Atlantic meridional overturning circulation (AMOC). In contrast to previous estimates, which neglected the GrIS fresh water input, both sides of the North Atlantic Gyre are projected to experience sea level rises. The impacts on a selection of major cities on both sides of the Atlantic and in the Pacific and southern oceans also are assessed. The other ocean basins are found to be less affected than the Atlantic Ocean.

The effect of Stokes drift on Ekman transport in the open sea

By introducing the wave-induced Coriolis-Stokes forcing into ageostrophic motion equation, the Eulerian transport is modified by the wave-induced Stokes drift. The long-term mean contributions of the Stokes transport with remotely generated swells being included to the ageostrophic transport are analyzed using the ECMWF (European Centre for Medium-RangeWeather Forecasts) reanalysis data. The ratio of Stokes transport to Ekman transport in north-south (N-S) direction can reach a maximum of over 50% in the subtropical region. The preliminary influence of the Stokes transport on the North Pacific gyre is all year persistent, while the effect on the North Atlantic gyre is only obvious in boreal winter and early spring.

Subtropical gyre variability as seen from satellites

A satellite multi-sensor approach is used to analyse the biological response of open ocean regions of the subtropical gyres to changes in physical forcing. Thirteen years (1998–2010) of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-a (chl-a) data, combined with concurrent satellite records of sea-surface temperature (SST) and sea level height, were analysed to investigate the seasonal and inter-annual variability of chl-a concentration within these immense so-called ocean deserts. The seasonal variability of chl-a within the gyres is driven mostly by the warming/cooling of surface waters. Summer warming promotes shallower mixed layers and lower chl-a due to a reduction of vertical mixing and consequently a decrease in nutrient supply. The opposite happens during the winter cooling period. Therefore, long-term trends in SST have the potential to cause an impact on the inter-annual variability of chl-a. Our analyses show that, during the 13 whole years of SeaWiFS data record, the North Pacific, Indian Ocean and North Atlantic gyres experienced a decrease in chl-a of 9%, 12% and 11%, respectively, with corresponding SST increases of 0.27°C, 0.42°C and 0.32°C, respectively. The South Pacific and South Atlantic gyres also showed warming trends but with weak positive trends in chl-a that are not statistically significant. We hypothesize that the warming of surface waters in these two gyres is counterbalanced by other interacting physical- and biological-driving mechanisms, as indicated in previous studies.

Spatial and temporal patterns of coral black band disease in relation to a major sewage outfall

Spatial and temporal patterns of coral black band disease (BBD) prevalence were examined during the summers of 2004 to 2008 at 10 reef sites located along a sewage gradient on either side of a major marine outfall on Bermuda's south shore. The gradient was identified by cur- rent meter and drogue deployments and confirmed by a water quality monitoring using fecal indi- cator bacteria (gastrointestinal enterococci) as a sewage marker. BBD prevalence was also exam- ined at 22 locations across the Bermuda platform in different physiographic reef zones, identified by reef survey techniques and analysis of community composition. BBD prevalence was generally low and was recorded in Diploria strigosa > Montastraea franksi > M. cavernosa = D. labyrinthi- formis > Porites astreoides and the hydrocoral Millepora alcicornis. Most occurrences were in D. strigosa, and BBD prevalence was highest on the outer rim reef (range: 0.3 to 1.9%), followed by the outer lagoonal patch reefs (range: 0.05 to 0.8%) and the deeper terrace reefs (range: 0.1 to 0.2%). BBD prevalence levels decreased over the study period, and BBD was only rarely observed in D. labyrinthiformis, which appears to be immune to infection in Bermuda. The BBD prevalence in D. strigosa was lower on reefs regularly exposed to sewage than on the near pristine outer rim reef sites, which experience the exceptional water quality characteristics of the oligotrophic North Atlantic gyre.

High resolution determination of nanomolar concentrations of dissolved reactive phosphate in ocean surface waters using long path liquid waveguide capillary cells (LWCC) and spectrometric detection

In the last decade, long path length, low volume, liquid waveguide capillary cells (LWCC) in conjunction with conventional nutrient auto‐analyzers have been applied to determinations of nanomolar levels of phosphate, nitrate, and nitrite in oligotrophic waters. This article reports a high resolution, real‐time, continuous method for nanomolar dissolved reactive phosphate measurements in ocean surface waters with data logging every 30 seconds for up to 16 consecutive hours. Surface seawater is pumped continuously from a shipboard underway tow‐fish unit to a helium gas‐segmented, continuous‐flow, nutrient auto‐analyzer modified with a 250 cm LWCC. To circumvent baseline instability due to reagents, a parallel channel with deionized water (DI) and reagents is run and later subtracted from the sample absorbances. The detection limit is 0.8 nmol/L. The precision (as relative standard deviation) at 5 nmol/L phosphate is 6.1% (n = 5) and 0.8% (n = 5) at 50 nmol/L. We also report an optimized method for discrete samples using a 200 cm LWCC. To minimize any background phosphate concentration in low nutrient seawater used as wash water solution, we use DI water, but increase sample and wash times to achieve plateau‐shaped peaks after the transient wash/sample mixing period. The detection limit is 0.5 nmol/L. The precision at 10 nmol/L phosphate is 1.8% (n = 8) and 0.9% (n = 9) at 60 nmol/L. The two systems have successfully been deployed on the U.S. GEOTRACES 2010 cruise, transecting the upwelling area northwest of Africa and the highly stratified, oligotrophic, subtropical North Atlantic gyre.

The Western Mediterranean summer variability and its feedbacks

The anomalous climatic variability of the Western Mediterranean in summer, its relationships with the large scale climatic teleconnection modes and its feedbacks from some of these modes are the targets of this study. The most important trait of this variability is the recurrence of warm and cold episodes, that take place at 2–4 year intervals, and which are monitored in the Western Mediterranean Index. We find that the Western Mediterranean events are part of a basin scale mode, and are related to the previous spring atmospheric anomalies. These anomalies are related mainly to the Pacific North America teleconnection pattern and the North Atlantic Oscillation, but also to a number of other climatic modes, connected with the previous two, as the Southern Oscillation, the Indian Core Monsoon and the Scandinavian teleconnection pattern. We identify the main spatial and temporal traits of the Western Mediterranean summer variability, the physical mechanisms at play in the generation of the events and their impacts. Considering the Atlantic Ocean, the Mediterranean events influence the sea surface temperature in the southeastern part of the North Atlantic Gyre. Additionally, they are significantly related to summer precipitation anomalies of the opposite sign in the Baltic basin (Central Germany and Poland) and near the Black Sea. We then estimate the mutual influence that the anomalous previous state of the Western Mediterranean, of the Pacific North America teleconnection pattern and of the North Atlantic Oscillation have on their summer conditions using a simple stochastic model. As the summer Western Mediterranean events have an influence on a part of the Baltic basin, we propose a second stochastic model in order to investigate if thereafter the Baltic basin variability will feedback on the Western Mediterranean sea surface temperature anomalies. Among the variables included in the second model are, in addition to the Western Mediterranean previous state, that of the Baltic Sea and of the Scandinavian teleconnection pattern. From each of the feedback matrices, a linear statistical analysis extracts spatial patterns whose evolution in time exhibits predictive capabilities for the Western Mediterranean evolution in summer and autumn that are above those of persistence, and that could be improved.