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Analysis of two continuous, high‐resolution palaeo‐flood records from southern Norway reveals that the frequency of extreme flood events has changed significantly during the Holocene. During the early and middle Holocene, flood frequency was low; by contrast, it was high over the last 2300 years when the mean flood frequency was about 2.5–3.0 per century. The present regional discharge regime is dominated by spring/summer snowmelt, and our results indicate that the changing flood frequency cannot be explained by local conditions associated with the respective catchments of the two lakes, but rather long‐term variations of solid winter precipitation and related snowmelt. Applying available instrumental winter precipitation data and associated sea‐level pressure re‐analysis data as a modern analogue, we document that atmospheric circulation anomalies, significantly different from the North Atlantic Oscillation (NAO), have some potential in explaining the variability of the two different palaeo‐flood records. Centennial‐scale patterns in shifting flood frequency might be indicative of shifts in atmospheric circulation and shed light on palaeo‐pressure variations in the North Atlantic region, in areas not influenced by the NAO. Major shifts are found at about 2300, 1200 and 200 years ago (cal. a BP). Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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The spatial and temporal distributions of marine cold air outbreaks (MCAOs) over the northern North Atlantic have been investigated using re-analysis data for the period from 1958 to 2007. MCAOs are large-scale outbreaks of cold air over a relatively warm ocean surface. Such conditions are known to increase the severity of particular types of hazardous mesoscale weather phenomena. We used a simple index for identifying MCAOs: the vertical potential temperature gradient between the sea surface and 700 hPa. It was found that atmospheric temperature variability is considerably more important than the sea surface temperature variability in governing both the seasonal and the inter-annual variability of MCAOs. Furthermore, a composite analysis revealed that a few well-defined and robust synoptic patterns are evident during MCAOs in winter. Over the Labrador and Irminger Seas the MCAO index was found to have a correlation of 0.70 with the North Atlantic Oscillation index, while over the Barents Sea a negative correlation of 0.42 was found. 相似文献
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Brian C. O'Neill Paul Crutzen Arnulf Grübler Minh Ha Duong Klaus Keller Charles Kolstad 《Climate Policy》2013,13(5):585-589
Abstract Learning—i.e. the acquisition of new information that leads to changes in our assessment of uncertainty—plays a prominent role in the international climate policy debate. For example, the view that we should postpone actions until we know more continues to be influential. The latest work on learning and climate change includes new theoretical models, better informed simulations of how learning affects the optimal timing of emissions reductions, analyses of how new information could affect the prospects for reaching and maintaining political agreements and for adapting to climate change, and explorations of how learning could lead us astray rather than closer to the truth. Despite the diversity of this new work, a clear consensus on a central point is that the prospect of learning does not support the postponement of emissions reductions today. 相似文献
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Observed and simulated precursors of stratospheric polar vortex anomalies in the Northern Hemisphere
The Northern Hemisphere stratospheric polar vortex is linked to surface weather. After Stratospheric Sudden Warmings in winter, the tropospheric circulation is often nudged towards the negative phase of the Northern Annular Mode (NAM) and the North Atlantic Oscillation (NAO). A strong stratospheric vortex is often associated with subsequent positive NAM/NAO conditions. For stratosphere?Ctroposphere associations to be useful for forecasting purposes it is crucial that changes to the stratospheric vortex can be understood and predicted. Recent studies have proposed that there exist tropospheric precursors to anomalous vortex events in the stratosphere and that these precursors may be understood by considering the relationship between stationary wave patterns and regional variability. Another important factor is the extent to which the inherent variability of the stratosphere in an atmospheric model influences its ability to simulate stratosphere?Ctroposphere links. Here we examine the lower stratosphere variability in 300-year pre-industrial control integrations from 13 coupled climate models. We show that robust precursors to stratospheric polar vortex anomalies are evident across the multi-model ensemble. The most significant tropospheric component of these precursors consists of a height anomaly dipole across northern Eurasia and large anomalies in upward stationary wave fluxes in the lower stratosphere over the continent. The strength of the stratospheric variability in the models was found to depend on the variability of the upward stationary wave fluxes and the amplitude of the stationary waves. 相似文献
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Learning and international environmental agreements 总被引：1，自引：0，他引：1
In this paper we synthesise and extend our earlier analysis (Ulph, J Risk Uncertain 29(1):53–73, 2004; Kolstad, J Environ Econ Manage 53(1):68–79, 2007) of the formation of an International Environmental Agreement (IEA) under uncertainty about the damages that might be caused by climate change and different models of learning, in which better information about these damage costs become available. Our results are generally pessimistic: the possibility of either complete or partial learning generally reduces the level of global welfare that can be achieved from forming an IEA. This suggests that information can have negative value. This may seem strange, since for a single decision-maker information cannot have negative value, because it can always be ignored. However in this case there are strategic interactions between a number of decision-makers responding to information, and it is these strategic interactions which can give information a negative value. 相似文献
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Marine cold-air outbreaks in the future: an assessment of IPCC AR4 model results for the Northern Hemisphere 总被引：1，自引：1，他引：0
For many locations around the globe some of the most severe weather is associated with outbreaks of cold air over relatively
warm oceans, referred to here as marine cold-air outbreaks (MCAOs). Drawing on empirical evidence, an MCAO indicator is defined
here as the difference between the skin potential temperature, which over open ocean is the sea surface potential temperature,
and the potential temperature at 700 hPa. Rare MCAOs are defined as the 95th percentile of this indicator. Climate model data
that have been provided as part of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report Four (AR4) were
used to assess the models’ projections for the twenty-first century and their ability to represent the observed climatology
of MCAOs. The ensemble average of the models broadly captures the observed spatial distribution of the strength of MCAOs.
However, there are some significant differences between the models and observations, which are mainly associated with simulated
biases of the underlying sea ice, such as excessive sea-ice extent over the Barents Sea in most of the models. The future
changes of the strength of MCAOs vary significantly across the Northern Hemisphere. The largest projected weakening of MCAOs
is over the Labrador Sea. Over the Nordic seas the main region of strong MCAOs will move north and weaken slightly as it moves
away from the warm tongue of the Gulf Stream in the Norwegian Sea. Over the Sea of Japan there is projected to be only a small
weakening of MCAOs. The implications of the results for mesoscale weather systems that are associated with MCAOs, namely polar
lows and arctic fronts, are discussed. 相似文献
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