Coccolithophores in surface sediments of the Arabian Sea (TAble 1)

One hundred surface sediment samples of the Arabian Sea (Indian Ocean) were investigated and relative abundances of coccoliths were compared to mean annual gradients of temperature, salinity, chlorophyll, PO4 and mixed layer depth. Total coccolith concentrations ranged from 42*10**6/g sediment in coastal areas to more than 19000*10**6/g sediment in oceanic regions. The general distribution does not seem to be dependent on coccolithophore productivity in surface waters alone, but also on the diluting input of terrigenous material. A total of 27 taxa were identified. The main species dominating the assemblages were Gephyrocapsa oceanica, Emiliania huxleyi and Florisphaera profunda with a combined average abundance of more than 70%. Several species and species groups reflect with their distribution the environmental parameters of the overlying water masses and may be successfully used to improve palaeoclimatic reconstructions, e.g. (a) F. profunda exhibits a high similarity or even positive correlation to the mean annual mixed layer depth, (b) calciosolenids can be described as coastal or shelf species. While temperature and salinity gradients do not seem to be crucial for coccolithophores in this region, the mean mixed layer depth as well as the PO4 concentration (representative for total nutrient availability) may control in part the coccolithophore assemblages. According to the results of a cluster analysis and the distribution pattern of all species, it was possible to differentiate three main coccolithophore assemblages. A G. oceanica dominated assemblage mainly occurs in the northern part of the study area and can be described as 'high nutrient assemblage'. The second assemblage, dominated by F. profunda, may be typical for oligotrophic and stable conditions in open ocean waters. A third assemblage, with high amounts of 'coastal species', characterises coastal conditions on the shelves.

Planktonic foraminiferal flux and shell geochemistry data in the sediment trap time series JAM 1-3 off south Java

Results from sediment trap experiments conducted in the seasonal upwelling area off south Java from November 2000 until July 2003 revealed significant monsoon-, El Niño-Southern Oscillation-, and Indian Ocean Dipole-induced seasonal and interannual variations in flux and shell geochemistry of planktonic foraminifera. Surface net primary production rates together with total and species-specific planktonic foraminiferal flux rates were highest during the SE monsoon-induced coastal upwelling period from July to October, with three species Globigerina bulloides, Neogloboquadrina pachyderma dex., and Globigerinita glutinata contributing to 40% of the total foraminiferal flux. Shell stable oxygen isotopes (d18O) and Mg/Ca data of Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii in the sediment trap time series recorded surface and subsurface conditions. We infer habitats of 0-30 m for G. ruber at the mixed layer depth, 60-80 m (60-90 m) for P. obliquiloculata (N. dutertrei) at the upper thermocline depth, and 90-110 m (100-150 m) for G. menardii in the 355-500 mm (>500 µm) size fraction corresponding to the (lower) thermocline depth in the study area. Shell Mg/Ca ratio of G. ruber (s.l. and s.s.) reveals an exponential relationship with temperature that agrees with published relationships particularly with the Anand et al. (2003) equations. Flux-weighted foraminiferal data in sediment trap are consistent with average values in surface sediment samples off SW Indonesia. This consistency confirms the excellent potential of these proxies for reconstructing past environmental conditions in this part of the ocean realm.

The significance of nitrogen regeneration for new production within a filament of the Mauritanian upwelling system

The Lagrangian progression of a biological community was followed in a filament of the Mauritanian upwelling system, north-west Africa, during offshore advection. The inert dual tracers sulfur hexafluoride and helium-3 labelled a freshly upwelled patch of water that was mapped for 8 days. Changes in biological, physical, and chemical characteristics were measured, including phytoplankton productivity, nitrogen assimilation, and regeneration. Freshly upwelled water contained high nutrient concentrations but was depleted in N compared to Redfield stoichiometry. The highest rate of primary productivity was measured on the continental shelf, associated with high rates of nitrogen assimilation and a phytoplankton community dominated by diatoms and flagellates. Indicators of phytoplankton abundance and activity decreased as the labelled water mass transited the continental shelf slope into deeper water, possibly linked to the mixed layer depth exceeding the light penetration depth. By the end of the study, the primary productivity rate decreased and was associated with lower rates of nitrogen assimilation and lower nutrient concentrations. Nitrogen regeneration and assimilation took place simultaneously. Results highlighted the importance of regenerated NHC 4 in sustaining phytoplankton productivity and indicate that the upwelled NO3 pool contained an increasing fraction of regenerated NO3 as it advected offshore. By calculating this fraction and incorporating it into an f ratio formulation, we estimated that of the 12:38Tg C of annual regional production, 4:73Tg C was exportable.

The significance of nitrogen regeneration for new production within a filament of the Mauritanian upwelling system

The Lagrangian progression of a biological community was followed in a filament of the Mauritanian upwelling system, north-west Africa, during offshore advection. The inert dual tracers sulfur hexafluoride and helium-3 labelled a freshly upwelled patch of water that was mapped for 8 days. Changes in biological, physical, and chemical characteristics were measured, including phytoplankton productivity, nitrogen assimilation, and regeneration. Freshly upwelled water contained high nutrient concentrations but was depleted in N compared to Redfield stoichiometry. The highest rate of primary productivity was measured on the continental shelf, associated with high rates of nitrogen assimilation and a phytoplankton community dominated by diatoms and flagellates. Indicators of phytoplankton abundance and activity decreased as the labelled water mass transited the continental shelf slope into deeper water, possibly linked to the mixed layer depth exceeding the light penetration depth. By the end of the study, the primary productivity rate decreased and was associated with lower rates of nitrogen assimilation and lower nutrient concentrations. Nitrogen regeneration and assimilation took place simultaneously. Results highlighted the importance of regenerated NHC 4 in sustaining phytoplankton productivity and indicate that the upwelled NO3 pool contained an increasing fraction of regenerated NO3 as it advected offshore. By calculating this fraction and incorporating it into an f ratio formulation, we estimated that of the 12:38Tg C of annual regional production, 4:73Tg C was exportable.

Optimizing PAT data transmission: assessing the accuracy of temperature summary data to estimate environmental conditions

Pop-up archival tags (PAT) provide summary and high-resolution time series data at predefined temporal intervals. The limited battery capabilities of PATs often restrict the transmission success and thus temporal coverage of both data products. While summary data are usually less affected by this problem, as a result of its lower size, it might be less informative. We here investigate the accuracy and feasibility of using temperature at depth summary data provided by PATs to describe encountered oceanographic conditions. Interpolated temperature at depth summary data was found to provide accurate estimates of three major thermal water column structure indicators: thermocline depth, stratification and ocean heat content. Such indicators are useful for the interpretation of the tagged animal's horizontal and vertical behaviour. The accuracy of these indicators was found to be particularly sensitive to the number of data points available in the first 100 m, which in turn depends on the vertical behaviour of the tagged animal. Based on our results, we recommend the use of temperature at depth summary data as opposed to temperature time series data for PAT studies; doing so during the tag programming will help to maximize the amount of transmitted time series data for other key data types such as light levels and depth.

Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies.

Submesoscale features in regions with different baroclinic deformation radius

In questo studio, un modello nidificato "child" ad alta risoluzione (risoluzione 1/48°) è ottenuto attraverso la piattaforma SURF. Il modello "child" è ottenuto tramite "downscaling" dei campi medi giornalieri dal modello globale "parent" (1/12°). Questo permette di osservare le caratteristiche della sottomesoscala in due regioni dell'oceano Atlantico settentrionale, Azzorre e Bermuda, dal 4 al 12 gennaio 2021. Questa tesi si propone di condurre un'analisi preliminare della relazione fra il raggio di deformazione baroclino e l'esordio dell'attività di sottomesoscala, nelle regioni di interesse. A questo scopo, vengono effettuati molti confronti fra i campi risultanti dal "parent" e dal "child". In particolare, ci concentriamo sulla Mixed-Layer Instability (MLI) studiata attraverso variabili come la Mixed-Layer Depth (MLD), la vorticità relativa, le velocità orizzontali e verticali, l'energia cinetica e la frequenza di Brunt-Vaisala. Dai risultati, mentre il modello "parent" sembra inadeguato, quello "child" è in grado di rilevare la MLI e la presenza di filamenti e scie topografiche in entrambe le regioni, riproducendo meglio le correnti orizzontali e verticali alla sottomesoscala. Ciò fornisce una connessione tra mesoscala e sottomesoscala: mentre nelle Azzorre la MLI sembra svolgere un ruolo chiave nella ristratificazione della colonna d'acqua, lo stesso non sembra nelle Bermuda dove i vortici di mesoscala sono più ampi, influendo maggiormente sulla stratificazione verticale. Qui, i processi di ristratificazione sembrerebbero correlati alle mesoscale piuttosto che alle sottomesoscale. La MLI giocherebbe comunque un ruolo nella generazione di correnti alla sottomesoscala, insieme ad altri processi come la frontogenesi e le scie topografiche. In conclusione, il valore del raggio di deformazione baroclino non sembra influenzare l'attivazione dei processi di sottomesoscala, mentre sembrerebbe determinare l'importanza della MLI nel processo di ristratificazione.

STUDY OF THE EQUATORIAL ATLANTIC OCEAN MIXING LAYER USING A ONE-DIMENSIONAL TURBULENCE MODEL

The General Ocean Turbulence Model (GOTM) is applied to the diagnostic turbulence field of the mixing layer (ML) over the equatorial region of the Atlantic Ocean. Two situations were investigated: rainy and dry seasons, defined, respectively, by the presence of the intertropical convergence zone and by its northward displacement. Simulations were carried out using data from a PIRATA buoy located on the equator at 23 degrees W to compute surface turbulent fluxes and from the NASA/GEWEX Surface Radiation Budget Project to close the surface radiation balance. A data assimilation scheme was used as a surrogate for the physical effects not present in the one-dimensional model. In the rainy season, results show that the ML is shallower due to the weaker surface stress and stronger stable stratification; the maximum ML depth reached during this season is around 15 m, with an averaged diurnal variation of 7 m depth. In the dry season, the stronger surface stress and the enhanced surface heat balance components enable higher mechanical production of turbulent kinetic energy and, at night, the buoyancy acts also enhancing turbulence in the first meters of depth, characterizing a deeper ML, reaching around 60 m and presenting an average diurnal variation of 30 m.

Coupling between upper ocean layer variability and size-fractionated phytoplankton in a non-nutrient-limited environment

[EN] We describe the coupling between upper ocean layer variability and size-fractionated phytoplankton distribution in the non-nutrient-limited Bransfield Strait region (BS) of Antarctica. For this purpose we use hydrographic and size-fractionated chlorophyll a data from a transect that crossed 2 fronts and an eddy, together with data from 3 stations located in a deeply mixed region, the Antarctic Sound (AS). In the BS transect, small phytoplankton (<20 μm equivalent spherical diameter [ESD]) accounted for 80% of total chl a and their distribution appeared to be linked to cross-frontal variability. On the deepening upper mixed layer (UML) sides of both fronts we observed a deep subducting column-like structure of small phytoplankton biomass. On the shoaling UML sides of both fronts, where there were signs of restratification, we observed a local shallow maximum of small phytoplankton biomass. We propose that this observed phytoplankton distribution may be a response to the development of frontal vertical circulation cells. In the deep, turbulent environment of the AS, larger phytoplankton (>20 μm ESD) accounted for 80% of total chl a. The proportion of large phytoplankton increases as the depth of the upper mixed layer (ZUML), and the corresponding rate of vertical mixing, increases. We hypothesize that this change in phytoplankton composition with varying ZUML is related to the competition for light, and results from modification of the light regime caused by vertical mixing.

Characterization of late Campanian and Maastrichtian planktonic foraminiferal depth habitats and vital activities based on stable isotopes

Depth habitats of 56 late Cretaceous planktonic foraminiferal species from cool and warm climate modes were determined based on stable isotope analyses of deep-sea samples from the equatorial Pacific DSDP Sites 577A and 463, and South Atlantic DSDP Site 525A. The following conclusions can be reached: Planoglobulina multicamerata (De Klasz) and Heterohelix rajagopalani (Govindan) occupied the deepest plankton habitats, followed by Abathomphalus mayaroensis (Bolli), Globotruncanella havanensis (Voorwijk), Gublerina cuvillieri Kikoine, and Laeviheterohelix glabrans (Cushman) also at subthermocline depth. Most keeled globotruncanids, and possibly Globigerinelliodes and Racemiguembelina species, lived at or within the thermocline layer. Heterohelix globulosa (Ehrenberg) and Rugoglobigerina, Pseudotextularia and Planoglobulina occupied the subsurface depth of the mixed layer, and Pseudoguembelina species inhabited the surface mixed layer. However, depth ranking of some species varied depending on warm or cool climate modes, and late Campanian or Maastrichtian age. For example, most keeled globotruncanids occupied similar shallow subsurface habitats as Rugoglobigerina during the warm late Campanian, but occupied the deeper thermocline layer during cool climatic intervals. Two distinct types of "vital effect" mechanisms reflecting photosymbiosis and respiration effects can be recognized by the exceptional delta13C signals of some species. (1) Photosymbiosis is implied by the repetitive pattern of relatively enriched delta13C values of Racemiguembelina (strongest), Planoglobulina, Rosita and Rugoglobigerina species, Pseudoguembelina excolata (weakest). (2) Enriched respiration 12C products are recognized in A. mayaroensis, Gublerina acuta De Klasz, and Heterohelix planata (Cushman). Isotopic trends between samples suggest that photosymbiotic activities varied between localities or during different climate modes, and may have ceased under certain environmental conditions. The appearance of most photosymbiotic species in the late Maastrichtian suggests oligotrophic conditions associated with increased water-mass stratification.

Water chemistry and fluffy layer geochemistry and mineral content at four sites in the western Baltic Sea

The fluffy layer was sampled repeatedly during nine expeditions between October 1996 and December 1998 at four stations situated along a S-N-transect from the Oder Estuary to the Arkona Basin. Geochemical and mineralogical analyses of the fluff show regional differences (trends) in composition, attributed to provenance and to hydrographical conditions along their transport pathways. Temporal variability is very high at the shallow water station of the estuary, and decreases towards the deeper stations in the north. In the shallow water area, intensive resuspension of the fluff due to wind-driven waves and currents leads to an average residence time of only one to two days. Near-bottom lateral transport of the fluff is the main process that transfers the fine grained material, containing both nutrients and contaminants, from the coastal zone into the deeper basins of the Baltic Sea. Seasonal effects (e.g. biogenic production in relation to trace metal variation) are observed at the Tromper Wiek station, where the residence time of the fluffy material is in the scale of seasons. Thus, the fluffy layer offers suitable material for environmental monitoring programs.

Studies on ocean surface layer responses to atmospheric forcing in the North Indian Ocean

The present study on upper ocean responses to atmospheric forcing (associated with cyclone passage) in North Indian Ocean revealed significant variability between AS and BoB. The analysis of cyclone frequency during 1947 to 2006 exhibited lesser frequency of cyclones in AS than that of BoB. The analysis also revealed significant reduction in cyclone frequency after the year 1976 with substantial reduction during monsoon season. The long term SST data at selected points in AS and BoB could not reveal any relation with reduction in cyclone frequency. However the SLP at same locations exhibited considerable increase during mid 1970’s, which could have contributed to the observed reduction in cyclone frequency after the year 1976.The response in waves during cyclone passage exhibited significant asymmetry on either side of the track in AS and BoB and the response is observed at 100’s of kilometers away from the track. The significant clockwise rotation in wave direction is observed on the right side of the track starting from near the track to far away locations, which existed for a longer duration. However, the anticlockwise rotation in wave direction is observed over a shorter distance on the left side of the track and dissipated immediately.Inertial oscillation is observed in surface current and in the mixed layer temperature associated with cyclone passage, which revealed the role of relative location(s) on either side of the track. The inertial peak closer to the local inertial period indicates maximum transfer of energy during the cyclone passage in both AS and BoB. The absence of strong inertial oscillation even with clockwise rotation in surface current and wind indicates the dominant role of duration of strong wind in generating inertial oscillation.The oceanic response associated with cyclone passage reveal the variable response(s) which depends on cyclone intensity, the proximity to track and cyclone translation speed. It is observed that resonance with wind generates higher response in surface current, wave and SST on the right side of the track and it lasts for a longer duration. The maximum oceanic response is observed at a few kilometers away on right side of the track. However lesser rightward bias in the location of maximum cooling is observed for cyclones with low cyclone translation speed. The response on the left side of the track is less and is limited over a shorter distance and dissipates immediately. It is observed that the ocean response, in general, increases with intensity of cyclones. However the differential cooling produced by the same intensity cyclones in AS and in BoB indicates the dominant role of low cyclone translation speed in oceanic response.The surface cooling exhibited strikingly differential responses between AS and BoB. The TMI-SST and buoy observations exhibited significant cooling for a longer duration in AS compared to that of BoB. The spatial extent of cooling is also much higher in AS than that of BoB. The wide spread cooling associated with cyclone passage in AS indicates the dominant role of thermal structure in oceanic response in AS than that of BoB.

Thermohaline variability in the upper layers of the Arabian Sea

The Arabian Sea is an area of complex air-sea interaction processes with seasonal reversing monsoons. The associated thermohaline variability in the upper layers appears to control the large scale monsoon flow which is not yet completely understood. The variability in the thermohaline fields is known to occur in temporal domain ranging from intra-diurnal to inter-annual time scales and on spatial domains of few tens of kilometers to few thousands of kilometers. In the Arabian Sea though the surface temperature was routinely measured by both conventional measurements and satellites, the corresponding information on the subsurface thermohaline field is very sparse due to the lack cw adequate measurements. In such cases the numerical models offer promise in providing information on the subsurface features given an initial thermohaline field and surface heat flux boundary conditions. This thesis is an outcome of investigations carried out on the various aspects of the thermohaline variability on different time scales. In addition to the description of the mean annual cycle. the one dimensional numerical models of Miller (1976) and Price et a1 (1986) are utilised to simulate the observed mixed layer characteristics at selected locations in the Arabian Sea on time scales ranging from intra-diurnal to synoptic scales under variable atmospheric forcing.

Reactive Halogens in the Marine Boundary Layer (RHaMBLe): the tropical North Atlantic experiments

The NERC UK SOLAS-funded Reactive Halogens in the Marine Boundary Layer (RHaMBLe) programme comprised three field experiments. This manuscript presents an overview of the measurements made within the two simultaneous remote experiments conducted in the tropical North Atlantic in May and June 2007. Measurements were made from two mobile and one ground-based platforms. The heavily instrumented cruise D319 on the RRS Discovery from Lisbon, Portugal to São Vicente, Cape Verde and back to Falmouth, UK was used to characterise the spatial distribution of boundary layer components likely to play a role in reactive halogen chemistry. Measurements onboard the ARSF Dornier aircraft were used to allow the observations to be interpreted in the context of their vertical distribution and to confirm the interpretation of atmospheric structure in the vicinity of the Cape Verde islands. Long-term ground-based measurements at the Cape Verde Atmospheric Observatory (CVAO) on São Vicente were supplemented by long-term measurements of reactive halogen species and characterisation of additional trace gas and aerosol species during the intensive experimental period. This paper presents a summary of the measurements made within the RHaMBLe remote experiments and discusses them in their meteorological and chemical context as determined from these three platforms and from additional meteorological analyses. Air always arrived at the CVAO from the North East with a range of air mass origins (European, Atlantic and North American continental). Trace gases were present at stable and fairly low concentrations with the exception of a slight increase in some anthropogenic components in air of North American origin, though NOx mixing ratios during this period remained below 20 pptv. Consistency with these air mass classifications is observed in the time series of soluble gas and aerosol composition measurements, with additional identification of periods of slightly elevated dust concentrations consistent with the trajectories passing over the African continent. The CVAO is shown to be broadly representative of the wider North Atlantic marine boundary layer; measurements of NO, O3 and black carbon from the ship are consistent with a clean Northern Hemisphere marine background. Aerosol composition measurements do not indicate elevated organic material associated with clean marine air. Closer to the African coast, black carbon and NO levels start to increase, indicating greater anthropogenic influence. Lower ozone in this region is possibly associated with the increased levels of measured halocarbons, associated with the nutrient rich waters of the Mauritanian upwelling. Bromide and chloride deficits in coarse mode aerosol at both the CVAO and on D319 and the continuous abundance of inorganic gaseous halogen species at CVAO indicate significant reactive cycling of halogens. Aircraft measurements of O3 and CO show that surface measurements are representative of the entire boundary layer in the vicinity both in diurnal variability and absolute levels. Above the inversion layer similar diurnal behaviour in O3 and CO is observed at lower mixing ratios in the air that had originated from south of Cape Verde, possibly from within the ITCZ. ECMWF calculations on two days indicate very different boundary layer depths and aircraft flights over the ship replicate this, giving confidence in the calculated boundary layer depth.