The different stratospheric influence on cold-extremes in Eurasia and North America

The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes

Parameter optimisation of a marine ecosystem model at two contrasting stations in the Sub-Antarctic Zone

SeaWiFS surface chlorophyll estimates in the Sub-Antarctic Zone show low concentrations south west of Tasmania and high concentrations south east of Tasmania. Data assimilation experiments were performed using simulated annealing to obtain parameter estimates of a simple nitrogen based mixed-layer marine ecosystem model at two locations in this region (station P1 at 140°E, 46.5°S and station P3 at 152°E, 45.5°S). The assimilation methods and parameter sensitivities are assessed in a twin experiment. This assessment determined that inversion method was successful at estimating the correct parameters but that only a sub-set of the model parameters can be uniquely determined using chlorophyll a observations. An analysis of parameter uncertainties shows at both stations accurate parameterisations of phytoplankton growth and zooplankton mortality, and the biological recycling processes are required to realistically model chlorophyll.Applying the inversion method to the climatological SeaWiFS chlorophyll a observations from the two sites we estimate model parameters at these two sites. The most significant differences in parameters between the two stations are the parameters relating to phytoplankton growth and zooplankton mortality. The difference in growth parameters results in spring time productivity estimates of 659mgCm-2d-1 at P1 and 203mgCm-2d-1 at P3. In situ estimates from the SAZ-Sense cruise do not support such dramatic differences in primary production between the two stations. We conclude that the same ecosystem model structure is not applicable at both stations and we need additional processes at P3 to reproduce the observed seasonality of phytoplankton and the observed primary productivity. We hypothesise that the missing processes in the ecosystem model at P3 are iron limitation of phytoplankton and the seasonal variations in atmospheric deposition of iron. © 2011

Phytoplankton growth in the Australian sector of the Southern Ocean, examined by optimising ecosystem model parameters

SeaWiFS surface chlorophyll estimates and Levitus nitrate estimates in the Southern Ocean south of Australia (140°E) show that this region is characterised by a high-nitrate low-chlorophyll (HNLC) regime typical of Southern Ocean waters. The HNLC conditions become more prominent moving south from the Sub-Antarctic Zone, with surface chlorophyll generally decreasing and nitrate increasing with latitude. Parameter optimisation experiments were performed using simulated annealing to fit a zero-dimensional nitrogen-based four-component ecosystem model to SeaWiFS surface chlorophyll data in the Sub-Antarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ). We hypothesise that bioavailability of iron limits phytoplankton growth in this region. A physiological indicator of iron availability was investigated by optimising three of the model parameters defining maximum photosynthetic growth and maximum photosynthetic efficiency of phytoplankton. The effect of zooplankton grazing and light, mixed layer depth and temperature forcing data on the optimisation results was investigated in further optimisation experiments. An error analysis of the optimised parameter estimates was performed by analysing the Hessian matrix of the cost function. The parameter optimisations indicate that phytoplankton growth rates in the Polar Frontal Zone and Antarctic Zone are limited by some process not explicitly included in this model, with iron availability being the most likely candidate. Based on these optimisations we support the theory that micronutrient availability is the primary cause of the HNLC conditions in the Australian sector of the Southern Ocean. © 2013 Elsevier B.V

Osmunda pulchella sp. nov. from the Jurassic of Sweden--reconciling molecular and fossil evidence in the phylogeny of modern royal ferns (Osmundaceae)

Background: The classification of royal ferns (Osmundaceae) has long remained controversial. Recent molecular phylogenies indicate that Osmunda is paraphyletic and needs to be separated into Osmundastrum and Osmunda s.str. Here, however, we describe an exquisitely preserved Jurassic Osmunda rhizome (O. pulchella sp. nov.) that combines diagnostic features of both Osmundastrum and Osmunda, calling molecular evidence for paraphyly into question. We assembled a new morphological matrix based on rhizome anatomy, and used network analyses to establish phylogenetic relationships between fossil and extant members of modern Osmundaceae. We re-analysed the original molecular data to evaluate root-placement support. Finally, we integrated morphological and molecular data-sets using the evolutionary placement algorithm. Results: Osmunda pulchella and five additional Jurassic rhizome species show anatomical character suites intermediate between Osmundastrum and Osmunda. Molecular evidence for paraphyly is ambiguous: a previously unrecognized signal from spacer sequences favours an alternative root placement that would resolve Osmunda s.l. as monophyletic. Our evolutionary placement analysis identifies fossil species as probable ancestral members of modern genera and subgenera, which accords with recent evidence from Bayesian dating. Conclusions: Osmunda pulchella is likely a precursor of the Osmundastrum lineage. The recently proposed root placement in Osmundaceae—based solely on molecular data—stems from possibly misinformative outgroup signals in rbcL and atpA genes. We conclude that the seemingly conflicting evidence from morphological, anatomical, molecular, and palaeontological data can instead be elegantly reconciled under the assumption that Osmunda is indeed monophyletic.Exceptional permineralized biotas - windows into the evolution and functional diversity of terrestrial ecosystems through tim

Charophyceae (charales)

The charophytes, or stoneworts, are a group of green algae with six extant genera in one family, distributed worldwide in freshwater ponds and lakes. They are among the green algal groups most closely related to land plants and exhibit a complex thallus, with multinucleate internodal cells joined at nodes comprising smaller, uninucleate cells giving rise to whorled branchlets. Two genera (Chara, Nitella) contain most of the described species, with a third (Tolypella) containing several dozen taxa. The remaining genera have one or a few species. Reproduction is oogamous, with sperm and eggs produced in separate multicellular structures. The thallus is haploid; the zygote is the only diploid cell in the life cycle, and meiosis is followed by the development of a resistant spore. Thalli and spores are often encrusted with calcium carbonate. Such spores are abundant in the fossil record of the Charales, which extends to the Upper Silurian, and many genera and families have become extinct. These algae provide important ecosystem services, for example, as colonizing species, as biological agents for producing water clarity, or as the base of the food web. Charophytes are important for the study of evolution of embryophyte development, growth meristems, and cell biophysics. As one of the green algal groups most closely related to land plants, the rich charophyte fossil record may reveal clues regarding the earliest algae that invaded the land