292 research outputs found
The Holocene thermal maximum in the Nordic Seas: the impact of Greenland Ice Sheet melt and other forcings in a coupled atmosphere-sea ice-ocean model
The relatively warm early Holocene climate in the Nordic Seas, known as the Holocene Thermal Maximum (HTM), is often associated with an orbitally forced summer insolation maximum at 10 ka BP. The spatial and temporal response recorded in proxy data in the North Atlantic and the Nordic Seas reveal a complex interaction of mechanisms active in the HTM. Previous studies have investigated the impact of the Laurentide Ice Sheet (LIS), as a remnant from a previous glacial period, altering climate conditions with a continuous supply of melt water to the Labrador Sea and adjacent seas and with a downwind cooling effect from the remnant LIS. In our present work we extend this approach by investigating the impact of the Greenland Ice Sheet (GIS) on the early Holocene climate and the HTM. Reconstructions suggest melt rates of 13 mSv for 9 ka BP, which result in our model in a ocean surface cooling of up to 2 K near Greenland. Reconstructed summer SST gradients agree best with our simulation including GIS melt, confirming that the impact of early Holocene GIS is crucial for understanding the HTM characteristics in the Nordic Seas area. This implies that the modern and near-future GIS melt can be expected to play an active role in the climate system in the centuries to come
The impact of early Holocene Arctic Shelf flooding on climate in an atmosphereâoceanâseaâice model
Glacial terminations are characterized by a strong rise in sea level related
to melting ice sheets. This rise in sea level is not uniform all over the
world, because regional effects (uplift and subsidence of coastal zones) are
superimposed on global trends. During the early Holocene the Siberian Shelf
became flooded before 7.5 ka BP and the coastline reached modern-day high
stands at 5 ka BP. This area is currently known as a sea-ice production area
and contributes significantly to the sea-ice exported from the Arctic through
the Fram Strait. This leads to the following hypothesis: during times of
rising sea levels, shelves become flooded, increasing sea-ice production on
these shelves, increasing sea-ice volume and export through the Fram Strait and
causing the sea-ice extent to advance in the Nordic Seas, yielding cooler and
fresher sea surface conditions. We have tested this hypothesis in an
atmosphereâoceanâseaâice coupled model of intermediate complexity (LOVECLIM).
Our experiment on early Holocene Siberian Shelf flooding shows that in our
model sea-ice production in the Northern Hemisphere increases (15%) and
that sea-ice extent in the Northern Hemisphere increases (14%) but sea-ice
export decreases (â15%) contrary to our hypothesis. The reason of this
unexpected behaviour has its origin in a weakened polar vortex, induced by
the landâocean changes due to the shelf flooding, and a resulting decrease of
zonality in the Nordic Seas pressure regime. Hence the winter Greenland high
and the Icelandic low strengthen, yielding stronger winds on both sides of
the Nordic Seas. Increased winds along the East Greenland Current support
local sea-ice production and transport towards the South, resulting in a
wider sea-ice cover and a southward shift of convection areas. The overall
strength of the Atlantic meridional overturning circulation is reduced by 4%
and the heat transport in the Atlantic basin by 7%, resulting in an annual
cooling pattern over the Nordic Seas by up to â4 °C. We find
that the flooding of the Siberian shelf resulting from an orbitally induced
warming and related glacioeustatic sea level rise causes a Nordic Seas
cooling feedback opposed to this warming
Temperatures recorded by cosmogenic noble gases since the last glacial maximum in the Maritime Alps
While proxy records have been used to reconstruct late Quaternary climate parameters throughout the European Alps, our knowledge of deglacial climate conditions in the Maritime Alps is limited. Here, we report temperatures recorded by a new and independent geochemical techniqueâcosmogenic noble gas paleothermometryâin the Maritime Alps since the last glacial maximum. We measured cosmogenic 3He in quartz from boulders in nested moraines in the Gesso Valley, Italy. Paired with cosmogenic 10Be measurements and 3He diffusion experiments on quartz from the same boulders, the cosmogenic 3He abundances record the temperatures these boulders experienced during their exposure. We calculate effective diffusion temperatures (EDTs) over the last ~22 ka ranging from 8°C to 25°C. These EDTs, which are functionally related to, but greater than, mean ambient temperatures, are consistent with temperatures inferred from other proxies in nearby Alpine regions and those predicted by a transient general circulation model. In detail, however, we also find different EDTs for boulders from the same moraines, thus limiting our ability to interpret these temperatures. We explore possible causes for these intra-moraine discrepancies, including variations in radiative heating, our treatment of complex helium diffusion, uncertainties in our grain size analyses, and unaccounted-for erosion or cosmogenic inheritance
Universal Reconfiguration of Facet-Connected Modular Robots by Pivots: The O(1) Musketeers
We present the first universal reconfiguration algorithm for transforming a modular robot between any two facet-connected square-grid configurations using pivot moves. More precisely, we show that five extra "helper" modules ("musketeers") suffice to reconfigure the remaining n modules between any two given configurations. Our algorithm uses O(n^2) pivot moves, which is worst-case optimal. Previous reconfiguration algorithms either require less restrictive "sliding" moves, do not preserve facet-connectivity, or for the setting we consider, could only handle a small subset of configurations defined by a local forbidden pattern. Configurations with the forbidden pattern do have disconnected reconfiguration graphs (discrete configuration spaces), and indeed we show that they can have an exponential number of connected components. But forbidding the local pattern throughout the configuration is far from necessary, as we show that just a constant number of added modules (placed to be freely reconfigurable) suffice for universal reconfigurability. We also classify three different models of natural pivot moves that preserve facet-connectivity, and show separations between these models
Amplification of holocene multicentennial climate forcing by mode transitions in North Atlantic overturning circulation.
Using a three-dimensional global climate model, we show that mode-transitions in North Atlantic deep-water production can provide an amplifying mechanism of relatively weak climate perturbations during the Holocene. Under pre-industrial boundary conditions, a freshwater forcing in the Labrador Sea pushes the North Atlantic overturning circulation into a deterministically bistable regime, characterized by stochastic "on" and "off" switches in Labrador Sea convection. On a multicentennial time-scale these stochastic mode-transitions can be phase-locked by a small (subthreshold) periodic freshwater forcing. The local small periodic forcing is effectively amplified with the assistance of noise, to have a large-scale impact on North Atlantic overturning circulation and climate. These results suggest a stochastic resonance mechanism that can operate under Holocene boundary conditions and indicate that changes in the three-dimensional configuration of North Atlantic deep-water formation can be an important component of multicentennial climate variability during interglacials. Copyright 2007 by the American Geophysical Union
The origin of the "European Medieval Warm Period"
Proxy records and results of a three dimensional climate model show that European summer temperatures roughly a millennium ago were comparable to those of the last 25 years of the 20th century, supporting the existence of a summer "Medieval Warm Period" in Europe. Those two relatively mild periods were separated by a rather cold era, often referred to as the "Little Ice Age". Our modelling results suggest that the warm summer conditions during the early second millennium compared to the climate background state of the 13thâ18th century are due to a large extent to the long term cooling induced by changes in land-use in Europe. During the last 200 years, the effect of increasing greenhouse gas concentrations, which was partly levelled off by that of sulphate aerosols, has dominated the climate history over Europe in summer. This induces a clear warming during the last 200 years, allowing summer temperature during the last 25 years to reach back the values simulated for the early second millennium. Volcanic and solar forcing plays a weaker role in this comparison between the last 25 years of the 20th century and the early second millennium. Our hypothesis appears consistent with proxy records but modelling results have to be weighted against the existing uncertainties in the external forcing factors, in particular related to land-use changes, and against the uncertainty of the regional climate sensitivity. Evidence for winter is more equivocal than for summer. The forced response in the model displays a clear temperature maximum at the end of the 20th century. However, the uncertainties are too large to state that this period is the warmest of the past millennium in Europe during winter
Long-term variations in IcelandâScotland overflow strength during the Holocene
The overflow of deep water from the Nordic seas into the North Atlantic plays a critical role in global ocean circulation and climate. Approximately half of this overflow occurs via the IcelandâScotland (IâS) overflow, yet the history of its strength throughout the Holocene (~ 0â11 700 yr ago, ka) is poorly constrained, with previous studies presenting apparently contradictory evidence regarding its long-term variability. Here, we provide a comprehensive reconstruction of IâS overflow strength throughout the Holocene using sediment grain size data from a depth transect of 13 cores from the Iceland Basin. Our data are consistent with the hypothesis that the main axis of the IâS overflow on the Iceland slope was shallower during the early Holocene, deepening to its present depth by ~ 7 ka. Our results also reveal weaker IâS overflow during the early and late Holocene, with maximum overflow strength occurring at ~ 7 ka, the time of a regional climate thermal maximum. Climate model simulations suggest a shoaling of deep convection in the Nordic seas during the early and late Holocene, consistent with our evidence for weaker IâS overflow during these intervals. Whereas the reduction in IâS overflow strength during the early Holocene likely resulted from melting remnant glacial ice sheets, the decline throughout the last 7000 yr was caused by an orbitally induced increase in the amount of Arctic sea ice entering the Nordic seas. Although the flux of Arctic sea ice to the Nordic seas is expected to decrease throughout the next century, model simulations predict that under high emissions scenarios, competing effects, such as warmer sea surface temperatures in the Nordic seas, will result in reduced deep convection, likely driving a weaker IâS overflow
A late medieval warm period in the Southern Ocean as a delayed response to external forcing?
International audienceOn the basis of long simulations performed with a threeâdimensional climate model, we propose an interhemispheric climate lag mechanism, involving the longâterm memory of deepwater masses. Warm anomalies, formed in the North Atlantic when warm conditions prevail at surface, are transported by the deep ocean circulation towards the Southern Ocean. There, the heat is released because of large scale upwelling, maintaining warm conditions and inducing a lagged response of about 150 years compared to the Northern Hemisphere. Model results and observations covering the first half of the second millenium suggest a delay between the temperature evolution in the Northern Hemisphere and in the Southern Ocean. The mechanism described here provides a reasonable hypothesis to explain such an interhemipsheric lag
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