ALBERT

Description: The variation in life-history patterns that allow closely related species to co-exist has been an important theme in ecology for decades. We examined intra- and interspecific variation in a key life-history trait - diapause - for two congeneric copepods (Eudiaptomus gracilis and Eudiaptomus graciloides) inhabiting three lakes in Northern Germany. Diapause has been hypothesized in both theoretical and empirical studies to be important in the coexistence of competing species. We found no diapause for E. gracilis whereas we documented two distinct periods of diapause in the life cycle of E. graciloides. In the latter species, diapausing eggs were produced in all three lakes during autumn, however, relative investment in diapausing eggs differed between lakes. Diapausing egg production was delayed or fewer females switched to making diapausing eggs in the more productive lakes relative to the less productive systems. In the sediments of all three lakes, viable diapausing eggs were found buried from the sediment-water interface down several centimeters in each lake, suggesting the presence of a long-lived egg bank. In addition, detailed population studies carried out on both species in one lake (Schöhsee) revealed a second period of diapause in E. graciloides. After the diapausing eggs are produced in autumn, females accumulate lipids, reduce feeding and apparently over-winter in a state of active diapause. The prevalence of diapause in E. graciloides and lack of it in E. gracilis raises questions regarding the factors that shape the life-histories of these congeners, and the role that the life-history differences observed may play in their coexistence.

Description: A three-dimensional fully coupled high resolution atmosphere-ocean model for the BALTEX (Baltic Sea Experiment) region has been developed from two independent models, the atmospheric regional model REMO and the Kiel Baltic Sea model. The coupled model was set up in the framework of the BALTEX program to contribute to one of its major objectives, the investigation and quantification of the energy and water cycle in the Baltic Sea and its catchment area. As a first step towards the fully coupled system, sensitivity studies with different forcings for its uncoupled components, the atmosphere and ocean models. were performed. These sensitivity studies demonstrated that both models are able to produce rea onable results which in turn can act as forcing for the respective other model. In the first simulation of the fu lly coupled system the modeled sea surface temperatures (SST) agree well with satellite observations. Thus they are at least as good as the previously used SSTs from operational analyses and in some cases even better. The detailed evaluation of the coupled model results reveals that often the coupling effects are superimposed by advective influences and that only under specific conditions the atmospheric variables show a remarkable response to different fluxes. The atmosphere-ocean model is coupled directly via the corresponding fluxes across the interface between atmosphere and ocean. For the here presented simulation no flux corrections were necessary. Thus a consistent model system has been developed which can be utilized for further studies of the energy and water cycle in the BALTEX area.

Description: Bottom water temperatures in the central Greenland Sea have been increasing for the last two decades. The warming is most likely related to the absence of deep convective mixing, which cools and freshens the deep water. However, recent observations confirm a slow and steady increase of anthropogenic tracers such as chlorofluorocarbons (CFCs). This points to some amount of bottom water “ventilation” in the absence of deep convective mixing and poses a challenge to our understanding of deep water renewal. One explanation for the observed trends in both temperature and CFCs is significant vertical mixing. The basin-averaged diapycnal diffusivity, required to explain both trends, kυ,av 2–3 (×10−3 m2 s−1), is very unlikely to occur in the interior of the ocean. However, a diffusivity of kυ,bbl 10−2 m2 s−1 within a 150-m thick bottom boundary layer would be sufficient to explain the deep tracer increase. The implications of a secondary circulation driven by such large boundary layer mixing are discussed.

Description: The understanding of the tectonic processes shaping the Pacific margin off Costa Rica has undergone a dramatic evolution during the past 25 years. The margin, initially interpreted to be built by accretion of sediment from the ocean plate, is now interpreted as made of ophiolitic rocks that are exposed onshore, with no net accretion currently active. New seismic images indicate that upper plate tectonic erosion might be the dominant process. Erosion is accomplished in some cases through transport of large bodies from upper to lower plate by plate boundary readjustment. Subduction of seamounts locally accelerates tectonic erosion.

Description: The World Ocean Circulation Experiment has established Lagrangian observations with neutrally buoyant floats as a routine tool in the study of deep-sea currents. Here a novel variant of the well-proven RAFOS concept for seeding floats at locations where they can be triggered on a timed basis is introduced. This cost-effective method obviates the need to revisit sites with a high-priced research vessel each time floats are to be deployed. It enables multiple Lagrangian time series, for example, for the observation of intermediate point sources of water masses, which are independent but have identical start points. This can be done even in environmentally challenging regions such as below the ice. The successfully tested autonomous float park concept does not rely on a release carousel moored on the seafloor. Instead, a second release was added to the standard RAFOS float for optional delay of regular drift missions. A float park can easily be installed by a conductivity–temperature–depth recorder system with a slightly modified rosette sampler.

Description: A new approach based on statistical estimation is proposed for the analysis of tomographic traveltime data in cases of significant nonlinear dependence of the traveltimes on the sound-speed variations. Traditional tomography schemes based on linear perturbative inversions about a single, a priori fixed background state cannot properly handle such cases since the linearized model relations will lead to considerable inversion errors, depending on the extent of nonlinearity. In contrast, the background state is considered here as a variable unknown quantity to be estimated from the traveltime data, simultaneously with the peak identification function and the sound-speed perturbation. Using the maximum likelihood approach and the Gaussian assumption, the statistical estimation problem reduces to a weighted least squares problem to be solved simultaneously for the three unknown quantities. A posteriori inversion-error estimates are derived accounting also for uncertainties in the background selection and the peak identification. The proposed method is applied to nine-month-long traveltime data from the Thetis-2 experiment, conducted from January to October 1994 in the Western Mediterranean Sea, where the variability of the ocean environment gives rise to significant nonlinear dependencies between sound-speed and traveltime variations. The recovered temporal variability and stratification compare well with independent XBT observations.

Description: Numerical experiments are performed to examine the causes of variability of Atlantic Ocean SST during the period covered by the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis (1958-98). Three ocean models are used. Two are mixed layer models: one with a 75-m-deep mixed layer and the other with a variable depth mixed layer. For both mixed layer models the ocean heat transports are assumed to remain at their diagnosed climatological values. The third model is a full dynamical ocean general circulation model (GCM). All models are coupled to a model of the subcloud atmospheric mixed layer (AML). The AML model computes the air temperature and humidity by balancing surface fluxes, radiative cooling, entrainment at cloud base, advection and eddy heat, and moisture transports. The models are forced with NCEP-NCAR monthly mean winds from 1958 to 1998. The ocean mixed layer models adequately reproduce the dominant pattern of Atlantic Ocean climate variability in both its spatial pattern and time dependence. This pattern is the familiar tripole of alternating zonal bands of SST anomalies stretching between the subpolar gyre and the subtropics. This SST pattern goes along with a wind pattern that corresponds to the North Atlantic Oscillation (NAO). Analysis of the results reveals that changes in wind speed create the subtropical SST anomalies while at higher latitudes changes in advection of temperature and humidity and changes in atmospheric eddy fluxes are important. An observational analysis of the boundary layer energy balance is also performed. Anomalous atmospheric eddy heat fluxes are very closely tied to the SST anomalies. Anomalous horizontal eddy fluxes damp the SST anomalies while anomalous vertical eddy fluxes tend to cool the entire midlatitude North Atlantic during the NAO's high-index phase with the maximum cooling exactly where the SST gradient is strengthened the most. The SSTs simulated by the ocean mixed layer model are compared with those simulated by the dynamic ocean GCM. In the far North Atlantic Ocean anomalous ocean heat transports are equally important as surface fluxes in generating SST anomalies and they act constructively. The anomalous heat transports are associated with anomalous Ekman drifts and are consequently in phase with the changing surface fluxes. Elsewhere changes in surface fluxes dominate over changes in ocean heat transport. These results suggest that almost all of the variability of the North Atlantic SST in the last four decades can be explained as a response to changes in surface fluxes caused by changes in the atmospheric circulation. Changes in the mean atmospheric circulation force the SST while atmospheric eddy fluxes dampen the SST. Both the interannual variability and the longer timescale changes can be explained in this way. While the authors were unable to find evidence for changes in ocean heat transport systematically leading or lagging development of SST anomalies, this leaves open the problem of explaining the causes of the low-frequency variability. Possible causes are discussed with reference to the modeling results.

Description: The horizontal and vertical structure of large-amplitude internal solitary waves propagating in stratified waters on a continental shelf is investigated by analyzing the results of numerical simulations and in situ measurements. Numerical simulations aimed at obtaining stationary, solitary wave solutions of different amplitudes were carried out using a nonstationary model based on the incompressible two-dimensional Euler equations in the frame of the Boussinesq approximation. The numerical solutions, which refer to different density stratifications typical for midlatitude continental shelves, were obtained by letting an initial disturbance evolve according to the numerical model. Several intriguing characteristics of the structure of the simulated large-amplitude internal solitary waves like, for example, wavelength–amplitude and phase speed–amplitude relationship as well as form of the locus of zero horizontal velocity emerge, consistent with those obtained previously using stationary Euler models. The authors’ approach, which tends to exclude unstable oceanic internal solitary waves as they are filtered out during the evolution process, was also employed to perform a detailed comparison between model results and characteristics of large-amplitude internal solitary waves found in high-resolution in situ data acquired north and south of the Strait of Messina, in the Mediterranean Sea. From this comparison the importance of using higher-order theoretical models for a detailed description of large-amplitude internal solitary waves observed in the real ocean emerge. Implications of the results showing the complexity related to a possible inversion of sea surface manifestations of oceanic internal solitary waves into characteristics of the interior ocean dynamics are finally discussed.

Description: In the Ordovician, Gondwana in the area of northwestern Argentina and northern Chile had a west-facing active margin. The evolution of this margin culminated in the Oclóyic orogeny at the end of the Ordovician. An inspection of the available stratigraphical and geochronological data on sedimentary, volcanic and plutonic units of the southern Central Andes of northern Chile and NW Argentina reveals a lull in magmatic and metamorphic activity lasting for c. 100 Ma from the Early Silurian to the early Late Carboniferous. This is interpreted as corresponding to a tectonic scenario in which the present Andean margin was a passive margin of Gondwana. This passive margin developed after the Oclóyic orogeny due to an hitherto unexplained change in plate kinematics. The Late Carboniferous marks the renewed onset of subduction, initiating the Andean plate tectonic setting still prevailant today. The Early Paleozoic evolution of northwestern Argentina and northern Chile and the absence of allochthonous or even exotic terranes contrasts markedly with the accretionary history of central Argentina and central Chile where the Precordillera and Chilenia Terranes docked in the Late Ordovician and Late Devonian, respectively. Models explaining the Late Ordovician Oclóyic orogeny by the collision of Laurentia with western South America during Laurentia's clockwise motion around South America and away from its position in the Neoproterozoic supercontinent are difficult to reconcile with the Paleozoic tectonostratigraphic evolution of the southern Central Andean region.