Oliver Jahn

The ground state of three quarks

We measure the static three-quark potential in SU(3) lattice gauge theory with improved accuracy, by using all available technical refinements, including Luscher-Weisz exponential variance reduction. Together with insight gained from 3-state Potts model simulations, our results allow us to sort out the merits of the Delta- and Y-ansaetze.

Environmental characteristics of Agulhas rings affect interocean plankton transport

Agulhas rings provide the principal route for ocean waters to circulate from the Indo-Pacific to the Atlantic basin. Their influence on global ocean circulation is well known, but their role in plankton transport is largely unexplored. We show that, although the coarse taxonomic structure of plankton communities is continuous across the Agulhas choke point, South Atlantic plankton diversity is altered compared with Indian Ocean source populations. Modeling and in situ sampling of a young Agulhas ring indicate that strong vertical mixing drives complex nitrogen cycling, shaping community metabolism and biogeochemical signatures as the ring and associated plankton transit westward. The peculiar local environment inside Agulhas rings may provide a selective mechanism contributing to the limited dispersal of Indian Ocean plankton populations into the Atlantic.

Polyakov loop and its relation to static quark potentials and free energies

It appears well accepted in the literature that the correlator of Polyakov loops in a finite temperature system decays with the average free energy of the static quark-antiquark system and can be decomposed into singlet and adjoint (or octet for QCD) contributions. By fixing a gauge respecting the transfer matrix, attempts have been made to extract those contributions separately. In this paper we point out that the average and adjoint channels of Polyakov loop correlators are misconceptions. We show analytically that all channels receive contributions from singlet states only, and give a corrected definition of the singlet free energy. We verify this finding by simulations of the 3D SU(2) pure gauge theory in the zero-temperature limit, which allows one to cleanly extract the ground state exponents and the nontrivial matrix elements. The latter account for the difference between the channels observed in previous simulations.

Direct estimate of lateral eddy diffusivity upstream of Drake Passage

AbstractThe first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy ph...

Dispersal, eddies, and the diversity of marine phytoplankton

We examined the role of physical dispersal in regulating patterns of diversity of marine phytoplankton in the context of global ocean simulations at eddy-permitting and coarse resolutions. Swifter current speeds, faster dispersal, and increased environmental variability in the higher-resolution model enhanced local diversity almost everywhere. In the numerical simulations, each resolved phytoplankton type was characterized as “locally adapted” at any geographical location (i.e., having net local biological production and physical export) or “immigrant” (i.e., net local biological loss but a population sustained by immigration via physical transport). Immigrants accounted for a higher fraction of the total diversity in the equatorial and subtropical regions, where the exclusion timescale is long relative to the physical transport between “provinces.” Hotspots of diversity were associated with western boundary currents and coastal upwelling regions. The former had high locally adapted diversity within the core of the current system, maintained by confluence of upstream populations and the induction of nutrient resources, as well as environmental variability associated with mesoscale eddies. Downstream of strong nutrient sources, convergence of populations led to immigrant-dominated diversity. The numerical simulations provide testable predictions of patterns in diversity and hypotheses regarding the mechanisms that control them. Molecular approaches to characterizing diversity in microbial populations will provide a means to test these hypotheses.

Structure and dynamics of monopoles in axial gauge QCD

An investigation of singular fields emerging in the process of transforming QCD to the axial gauge is presented. The structure of the singularities is analyzed. It is shown that apart from well known neutral magnetic monopole singularities, the field configurations also exhibit singularities in their charged and transverse components. This complex singularity structure guarantees finite non-Abelian field strength and thus finite action if expressed in terms of gauge fixed fields. A relation between the monopole charges of singular field configurations and their topological charge is derived. Qualitative dynamical aspects of the role of the monopoles are discussed. It is argued that the entropy associated with monopoles increases with decreasing temperature and that the coupling to quantum fluctuations favors monopole-antimonopole binding.

The dynamical landscape of marine phytoplankton diversity

Observations suggest that the landscape of marine phytoplankton assemblage might be strongly heterogeneous at the dynamical mesoscale and submesoscale (10–100 km, days to months), with potential consequences in terms of global diversity and carbon export. But these variations are not well documented as synoptic taxonomic data are difficult to acquire. Here, we examine how phytoplankton assemblage and diversity vary between mesoscale eddies and submesoscale fronts. We use a multi-phytoplankton numerical model embedded in a mesoscale flow representative of the North Atlantic. Our model results suggest that the mesoscale flow dynamically distorts the niches predefined by environmental contrasts at the basin scale and that the phytoplankton diversity landscape varies over temporal and spatial scales that are one order of magnitude smaller than those of the basin-scale environmental conditions. We find that any assemblage and any level of diversity can occur in eddies and fronts. However, on a statistical level, the results suggest a tendency for larger diversity and more fast-growing types at fronts, where nutrient supplies are larger and where populations of adjacent water masses are constantly brought into contact; and lower diversity in the core of eddies, where water masses are kept isolated long enough to enable competitive exclusion.

The baryon static potential from lattice QCD

Lattice QCD simulations offer the possibility of determining the potential between three static quarks from first principles. We review the status of such simulations, and the relative standing of the two theoretical proposals for the baryonic potential: the Delta law (sum of two-body terms) and the Y law (length of three flux strings joined together at a junction). We also present new results on the leading Luscher-like corrections to the asymptotic linear potential.

Phytoplankton diversity and community structure affected by oceanic dispersal and mesoscale turbulence

We explore the role of oceanic dispersal in setting patterns of phytoplankton diversity, with emphasis on the role of mesoscale turbulence, using numerical simulations that resolve mesoscale eddies and a diverse set of phytoplankton types. The model suggests that dispersal of phytoplankton by oceanic transport processes increases phytoplankton diversity at the local scale of O(10–100) km (α-diversity), extends the range of many phytoplankton types, and decreases the ability of rare types to persist in isolated areas. As a consequence, phytoplanktonic assemblages are modified and diversity decreases at the regional scale of O(1000) km (γ-diversity). By progressively accounting for different classes of motion, we show that the increase of α-diversity ensues from vertical mixing of the organisms, dispersal by mean lateral currents, and in slightly larger proportion, dispersal due to eddies. With the progressive inclusion of mechanisms of dispersal, the community becomes dominated by a smaller number of types but with larger degree of coexistence, in larger home range areas. From a resource competition perspective, physical transport can reduce the effective concentration R* of a limiting resource R, thus allowing more types to become equally fit. In addition, mixing of nearby populations allows coexistence of types with unequal fitness. The simulations suggest that mesoscale turbulence plays a particular role, concomitantly providing a means for different phytoplankton types to achieve comparable fitness and extending the exclusion time scale for less competitive types.

Modeling selective pressures on phytoplankton in the global ocean.

Our view of marine microbes is transforming, as culture-independent methods facilitate rapid characterization of microbial diversity. It is difficult to assimilate this information into our understanding of marine microbe ecology and evolution, because their distributions, traits, and genomes are shaped by forces that are complex and dynamic. Here we incorporate diverse forces—physical, biogeochemical, ecological, and mutational—into a global ocean model to study selective pressures on a simple trait in a widely distributed lineage of picophytoplankton: the nitrogen use abilities of Synechococcus and Prochlorococcus cyanobacteria. Some Prochlorococcus ecotypes have lost the ability to use nitrate, whereas their close relatives, marine Synechococcus, typically retain it. We impose mutations for the loss of nitrogen use abilities in modeled picophytoplankton, and ask: in which parts of the ocean are mutants most disadvantaged by losing the ability to use nitrate, and in which parts are they least disadvantaged? Our model predicts that this selective disadvantage is smallest for picophytoplankton that live in tropical regions where Prochlorococcus are abundant in the real ocean. Conversely, the selective disadvantage of losing the ability to use nitrate is larger for modeled picophytoplankton that live at higher latitudes, where Synechococcus are abundant. In regions where we expect Prochlorococcus and Synechococcus populations to cycle seasonally in the real ocean, we find that model ecotypes with seasonal population dynamics similar to Prochlorococcus are less disadvantaged by losing the ability to use nitrate than model ecotypes with seasonal population dynamics similar to Synechococcus. The model predictions for the selective advantage associated with nitrate use are broadly consistent with the distribution of this ability among marine picocyanobacteria, and at finer scales, can provide insights into interactions between temporally varying ocean processes and selective pressures that may be difficult or impossible to study by other means. More generally, and perhaps more importantly, this study introduces an approach for testing hypotheses about the processes that underlie genetic variation among marine microbes, embedded in the dynamic physical, chemical, and biological forces that generate and shape this diversity.