409 research outputs found
Dansgaard-Oeschger events: tipping points in the climate system
Dansgaard-Oeschger events are a prominent mode of variability in the records
of the last glacial cycle. Various prototype models have been proposed to
explain these rapid climate fluctuations, and no agreement has emerged on which
may be the more correct for describing the paleoclimatic signal. In this work,
we assess the bimodality of the system reconstructing the topology of the
multi--dimensional attractor over which the climate system evolves. We use
high-resolution ice core isotope data to investigate the statistical properties
of the climate fluctuations in the period before the onset of the abrupt
change. We show that Dansgaard-Oeschger events have weak early warning signals
if the ensemble of events is considered. We find that the statistics are
consistent with the switches between two different climate equilibrium states
in response to a changing external forcing (e.g. solar, ice sheets...), either
forcing directly the transition or pacing it through stochastic resonance.
These findings are most consistent with a model that associates
Dansgaard-Oeschger with changing boundary conditions, and with the presence of
a bifurcation point.Comment: Final typeset version freely available at: Clim. Past, 9, 323-333,
2013 www.clim-past.net/9/323/2013/ doi:10.5194/cp-9-323-201
Climate scenarios of sea level rise for the northeast Atlantic Ocean: a study including the effects of ocean dynamics and gravity changes induced by ice melt
Here we present a set of regional climate scenarios of sea level rise for the northeast Atlantic Ocean. In this study, the latest observations and results obtained with state-of-the-art climate models are combined. In addition, regional effects due to ocean dynamics and changes in the Earth’s gravity field induced by melting of land-based ice masses have been taken into account. The climate scenarios are constructed for the target years 2050 and 2100, for both a moderate and a large rise in global mean atmospheric temperature (2 °C and 4 °C in 2100 respectively). The climate scenarios contain contributions from changes in ocean density (global thermal expansion and local steric changes related to changing ocean dynamics) and changes in ocean mass (melting of mountain glaciers and ice caps, changes in the Greenland and Antarctic ice sheets, and (minor) terrestrial water-storage contributions). All major components depend on the global temperature rise achieved in the target periods considered. The resulting set of climate scenarios represents our best estimate of twenty-first century sea level rise in the northeast Atlantic Ocean, given the current understanding of the various contributions. For 2100, they yield a local rise of 30 to 55 cm and 40 to 80 cm for the moderate and large rise in global mean atmospheric temperature, respectively. <br/
Is the salivary gland associated with honey bee recognition compounds in worker honey bees (Apis mellifera)?
Cuticular hydrocarbons (CHCs) function as recognition compounds with the best evidence coming from social insects such as ants and honey bees. The major exocrine gland involved in hydrocarbon storage in ants is the post-pharyngeal gland (PPG) in the head. It is still not clearly understood where CHCs are stored in the honey bee. The aim of this study was to investigate the hydrocarbons and esters found in five major worker honey bee (Apis mellifera) exocrine glands, at three different developmental stages (newly emerged, nurse, and forager) using a high temperature GC analysis. We found the hypopharyngeal gland contained no hydrocarbons nor esters, and the thoracic salivary and mandibular glands only contained trace amounts of n-alkanes. However, the cephalic salivary gland (CSG) contained the greatest number and highest quantity of hydrocarbons relative to the five other glands with many of the hydrocarbons also found in the Dufour's gland, but at much lower levels. We discovered a series of oleic acid wax esters that lay beyond the detection of standard GC columns. As a bee's activities changed, as it ages, the types of compounds detected in the CSG also changed. For example, newly emerged bees have predominately C19-C23 n-alkanes, alkenes and methyl-branched compounds, whereas the nurses' CSG had predominately C31:1 and C33:1 alkene isomers, which are replaced by a series of oleic acid wax esters in foragers. These changes in the CSG were mirrored by corresponding changes in the adults' CHCs profile. This indicates that the CSG may have a parallel function to the PPG found in ants acting as a major storage gland of CHCs. As the CSG duct opens into the buccal cavity the hydrocarbons can be worked into the comb wax and could help explain the role of comb wax in nestmate recognition experiments
Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy
The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage
is found to influence the Atlantic overturning circulation in a low-resolution
Ocean General Circulation Model. The model used is the Hamburg Large-Scale
Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas
Leakage is parameterized by sources of heat and salt in the upper South
Atlantic Ocean, that extend well into the intermediate layers.
It is shown that the models overturning circulation is sensitive to the applied
sources of heat and salt. The response of the overturning strength to changes in
the source amplitudes is mainly linear, interrupted once by a stepwise change.
The South Atlantic buoyancy sources influence the Atlantic overturning strength
by modifying the basin-scale meridional density and pressure gradients. The nonlinear,
stepwise response is caused by abrupt changes in the convective activity
in the northern North Atlantic.
Two additional experiments illustrate the adjustment of the overturning circulation
upon sudden introduction of heat and salt sources in the South Atlantic.
The North Atlantic overturning circulation responds within a few years after the
sources are switched on. This is the time it takes for barotropic and baroclinic
Kelvin waves to reach the northern North Atlantic. The advection of the anomalies
takes 3 decades to reach the northern North Atlantic.
The model results give support to the hypothesis that the re-opening of the
Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological
data, may have stimulated the coincident strengthening of the Atlantic overturning
circulation
Dynamical attribution of oceanic prediction uncertainty in the North Atlantic: application to the design of optimal monitoring systems
In this study, the relation between two approaches to assess the ocean predictability on interannual to decadal time scales is investigated. The first pragmatic approach consists of sampling the initial condition uncertainty and assess the predictability through the divergence of this ensemble in time. The second approach is provided by a theoretical framework to determine error growth by estimating optimal linear growing modes. In this paper, it is shown that under the assumption of linearized dynamics and normal distributions of the uncertainty, the exact quantitative spread of ensemble can be determined from the theoretical framework. This spread is at least an order of magnitude less expensive to compute than the approximate solution given by the pragmatic approach. This result is applied to a state-of-the-art Ocean General Circulation Model to assess the predictability in the North Atlantic of four typical oceanic metrics: the strength of the Atlantic Meridional Overturning Circulation (AMOC), the intensity of its heat transport, the two-dimensional spatially-averaged Sea Surface Temperature (SST) over the North Atlantic, and the three-dimensional spatially-averaged temperature in the North Atlantic. For all tested metrics, except for SST
Revealing chlorinated ethene transformation hotspots in a nitrate-impacted hyporheic zone
Hyporheic zones are increasingly thought of as natural bioreactors, capable of transforming and attenuating groundwater pollutants present in diffuse baseflow. An underappreciated scenario in the understanding of contaminant fate hyporheic zones is the interaction between point-source trichloroethene (TCE) plumes and ubiquitous, non-point source pollutants such as nitrate. This study aims to conceptualise critical biogeochemical gradients in the hyporheic zone which govern the export potential of these redox-sensitive pollutants from carbon-poor, oxic aquifers. Within the TCE plume discharge zone, discrete vertical profiling of the upper 100 cm of sediment pore water chemistry revealed an 80% increase in dissolved organic carbon (DOC) concentrations and 20–60 cm thick hypoxic zones (50 mg L−1) create a large stoichiometric demand for bioavailable DOC in discharging groundwater. With the benefit of a high-resolution grid of pore water samplers investigating the shallowest 30 cm of hypoxic groundwater flow paths, we identified DOC-rich hotspots associated with submerged vegetation (Ranunculus spp.), where low-energy metabolic processes such as mineral dissolution/reduction, methanogenesis and ammonification dominate. Using a chlorine index metric, we show that enhanced TCE to cDCE transformation takes place within these biogeochemical hotspots, highlighting their relevance for natural plume attenuation
The decrease in ocean heat transport in response to global warming
The ocean is taking up additional heat but how this affects ocean circulation and heat transport is unclear. Here, using coupled model intercomparison project phase 5/6 (CMIP5/6) climate projections, we show a future decrease in poleward ocean heat transport (OHT) across all Northern Hemisphere latitudes and south of 10° S. Most notably, the CMIP5/6 multimodel mean reduction in poleward OHT for the Atlantic at 26.5° N and Indo-Pacific at 20° S is 0.093–0.304 PW and 0.097–0.194 PW, respectively, dependent on scenario and CMIP phase. These changes in OHT are driven by decline in overturning circulation dampened by upper ocean warming. In the Southern Ocean, the reduction in poleward OHT at 55° S is 0.071–0.268 PW. The projected changes are stronger in CMIP6, even when corrected for its larger climate sensitivity. This is especially noticable in the Atlantic Ocean for the weaker forcing scenarios (shared socioeconomic pathway SSP 1-2.6/representative concentration pathways RCP 2.6), where the decrease is 2.5 times larger at 26.5° N due to a stronger decline in the Atlantic meridional overturning circulation
The role of atmospheric rivers in compound events consisting of heavy precipitation and high storm surges along the Dutch coast
Atmospheric river (AR) systems play a significant role in the simultaneous
occurrence of high coastal water levels and heavy precipitation in the
Netherlands. Based on observed precipitation values (E-OBS) and the output of
a numerical storm surge model (WAQUA/DSCMv5) forced with ERA-Interim sea
level pressure and wind fields, we find that the majority of compound events
(CEs) between 1979 and 2015 have been accompanied by the presence of an AR
over the Netherlands. In detail, we show that CEs have a 3 to 4 times higher
chance of occurrence on days with an AR over the Netherlands compared to any
random day (i.e. days without knowledge on presence of an AR). In contrast,
the occurrence of a CE on a day without AR is 3 times less likely than on any
random day. Additionally, by isolating and assessing the prevailing sea level
pressure (SLP) and sea surface temperature (SST) conditions with and without
AR involvement up to 7 days before the events, we show that the presence of
ARs constitutes a specific type of forcing conditions that (i) resemble the
SLP anomaly patterns during the positive phase of the North Atlantic
Oscillation (NAO+) with a north–south pressure dipole over the North
Atlantic and (ii) cause a cooling of the North Atlantic subpolar gyre and
eastern boundary upwelling zone while warming the western boundary of the
North Atlantic. These conditions are clearly distinguishable from those
during compound events without the influence of an AR which occur under SLP
conditions resembling the East Atlantic (EA) pattern with a west–east
pressure dipole over northern Europe and are accompanied by a cooling of the
West Atlantic. Thus, this study shows that ARs are a useful tool for the
early identification of possible harmful meteorological conditions over the
Netherlands and supports an effort for the establishment of an early warning
system.</p
Evidence for passive chemical camouflage in the parasitic mite Varroa destructor
Social insect colonies provide a stable and safe environment for their members. Despite colonies been heavily guarded, parasites have evolved numerous strategies to invade and inhabit these hostile places. Two common strategies are chemical mimicry via biosynthesis of the hosts' odour or chemical camouflage were compounds are acquired straight from the host. The ectoparasitic mite Varroa destructor feeds on the heamolymph of its honeybee host Apis mellifera and uses chemical mimicry to remain undetected as it lives on the adult host during its phoretic phase or while reproducing on the honeybee brood. During the mite life cycle it switches between host adults and brood, which requires it to adjust its profile to mimic the very different odours of honeybee brood and adults. In a series of transfer experiments using adult bees and pupae, we tested whether V. destructor does this by synthesising compounds or using chemical camouflage. We show that V. destructor required direct access to the host cuticle to mimic its odour and was unable to synthesise host-specific compounds itself. Mites use chemical camouflage to mimic the host odour, even when dead, indicating a passive physico-chemical mechanism of the parasite cuticle. The chemical profile of V. destructor was adjusted within three to nine hours after switching hosts, demonstrating that passive camouflage is a highly efficient, fast and flexible way for the mite’s to adapt to a new host's profile when moving between different host life stages, or host colonies
The introduction history of invasive garden ants in Europe: Integrating genetic, chemical and behavioural approaches
<p>Abstract</p> <p>Background</p> <p>The invasive garden ant, <it>Lasius neglectus</it>, is the most recently detected pest ant and the first known invasive ant able to become established and thrive in the temperate regions of Eurasia. In this study, we aim to reconstruct the invasion history of this ant in Europe analysing 14 populations with three complementary approaches: genetic microsatellite analysis, chemical analysis of cuticular hydrocarbon profiles and behavioural observations of aggression behaviour. We evaluate the relative informative power of the three methodological approaches and estimate both the number of independent introduction events from a yet unknown native range somewhere in the Black Sea area, and the invasive potential of the existing introduced populations.</p> <p>Results</p> <p>Three clusters of genetically similar populations were detected, and all but one population had a similar chemical profile. Aggression between populations could be predicted from their genetic and chemical distance, and two major clusters of non-aggressive groups of populations were found. However, populations of <it>L. neglectus </it>did not separate into clear supercolonial associations, as is typical for other invasive ants.</p> <p>Conclusion</p> <p>The three methodological approaches gave consistent and complementary results. All joint evidence supports the inference that the 14 introduced populations of <it>L. neglectus </it>in Europe likely arose from only very few independent introductions from the native range, and that new infestations were typically started through introductions from other invasive populations. This indicates that existing introduced populations have a very high invasive potential when the ants are inadvertently spread by human transport.</p
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