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Paasche O.,Bjerknes Center for Climate Research | Lovlie R.,University of Bergen
Geology | Year: 2011

The primary succession history of magnetotactic bacteria (MTB) is reconstructed in postglacial lake sediments using a rock magnetic approach that discriminates biologically produced magnetite from nonorganic magnetic carriers. MTB are among the oldest prokaryotes found in the fossil record, but little is known about how they have colonized and recolonized habitats around the world. Here we observe how MTB synchronously colonized 4 freshwater lakes 9760 ± 160 yr ago. The lakes are more than 1400 km apart, representing both coastal and inland regions, and have altitudinal differences of almost 800 m. The synchronous colonization of Norway (and possibly Sweden) suggests that the pathways were extremely efficient, and that the sources must have been wide ranging. We propose that birds could have carried and spread the bacteria as northward migration routes were reestablished following the onset of the current interglacial. This unique data set underscores the tenacity of MTB as evolutionary survivors, and also demonstrates their response to large-scale environmental changes in ways not previously anticipated. © 2011 Geological Society of America.

Kolstad E.W.,Bjerknes Center for Climate Research | Kolstad E.W.,University of Bergen | Charlton-Perez A.J.,University of Reading
Climate Dynamics | Year: 2011

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-troposphere 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-troposphere 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. © 2010 The Author(s).

Kolstad E.W.,University of Bergen | Kolstad E.W.,Bjerknes Center for Climate Research | Johansson K.A.,University of Bergen
Environmental Health Perspectives | Year: 2011

Background: Climate change is expected to have large impacts on health at low latitudes where droughts and malnutrition, diarrhea, and malaria are projected to increase. Objectives: The main objective of this study was to indicate a method to assess a range of plausible health impacts of climate change while handling uncertainties in a unambiguous manner. We illustrate this method by quantifying the impacts of projected regional warming on diarrhea in this century. Methods: We combined a range of linear regression coefficients to compute projections of future climate change-induced increases in diarrhea using the results from five empirical studies and a 19-member climate model ensemble for which future greenhouse gas emissions were prescribed. Six geographical regions were analyzed. Results: The model ensemble projected temperature increases of up to 4°C over land in the tropics and subtropics by the end of this century. The associated mean projected increases of relative risk of diarrhea in the six study regions were 8-11% (with SDs of 3-5%) by 2010-2039 and 22-29% (SDs of 9-12%) by 2070-2099. Conclusions: Even our most conservative estimates indicate substantial impacts from climate change on the incidence of diarrhea. Nevertheless, our main conclusion is that large uncertainties are associated with future projections of diarrhea and climate change. We believe that these uncertainties can be attributed primarily to the sparsity of empirical climate-health data. Our results therefore highlight the need for empirical data in the cross section between climate and human health.

Langehaug H.R.,Nansen Environmental and Remote Sensing Center | Langehaug H.R.,Bjerknes Center for Climate Research | Falck E.,University of Bergen
Progress in Oceanography | Year: 2012

The Fram Strait is the only deep connection between the Arctic Ocean and the Nordic Seas (Greenland, Iceland, and Norwegian Seas), with a sill depth of approximately 2600. m. Consequently, observations from this area reflect changes in the deep waters of the Arctic Mediterranean. Possible changes in the properties and distribution of the intermediate and deep water masses passing through the Fram Strait have been studied for the period from 1982 to 2008 at a zonal section at 79°N, with special emphasis on the period from 1997 to 2008. The temperature of the Arctic Intermediate Water shows large interannual variability, where the period from 2002 to 2004 was especially cold, indicating a strong winter convection in the Nordic Seas prior to those years. The deep water originating from the Norwegian Sea has become warmer (by about 0.10°C) during the study period. The changes in the properties of the Norwegian Sea Deep Water are caused by changes in its composition, i.e. an increasing fraction of the Eurasian Basin Deep Water and a decreasing fraction of the Greenland Sea Deep Water each year. If this trend continues, the Norwegian Sea Deep Water will have similar characteristics as the Eurasian Basin Deep Water in about 15. years. A prominent feature seen in the ΘS-diagrams from the deep Fram Strait is the disappearance of water with the characteristics of Greenland Sea Deep Water. This very cold water (temperature below -1.1°C) was not observed after 1997, and the minimum temperature in the strait increased by 0.24°C between 1982 and 2008. In the Fram Strait the fraction of intermediate waters has increased, while the fraction of deep waters from the Nordic Seas has decreased. The fraction of deep waters originating from the Arctic Ocean shows no trend. © 2011 Elsevier Ltd.

Eldevik T.,University of Bergen | Eldevik T.,Bjerknes Center for Climate Research | Nilsen J.E.O.,Bjerknes Center for Climate Research | Nilsen J.E.O.,Nansen Environmental and Remote Sensing Center
Journal of Climate | Year: 2013

The Atlantic Ocean's thermohaline circulation is an important modulator of global climate. Its northern branch extends through the Nordic Seas to the cold Arctic, a region that appears to be particularly influenced by climate change. Athermohaline circulation is fundamentally concerned with two degrees of freedom. This is in particular the case for the inflow of warm and saline Atlantic Water through the Nordic Seas toward the Arctic that is balanced by two branches of outflow. The authors present an analytical model, rooted in observations, that constrains the strength and structure of this Arctic-Atlantic thermohaline circulation. It is found, maybe surprisingly, that the strength of Atlanticinflow is relatively insensitive to anomalous freshwater input; it mainly reflects changes in northern heat loss. Freshwater anomalies are predominantly balanced by the inflow's partition into estuarineand overturning circulation with southward polar outflow in the surface and dense overflow at depth, respectively. More quantitatively, the approach presented herein provides a relatively simple framework for making closed and consistent inference on the thermohaline circulation's response to observed or estimated past and future change in the northern seas. © 2013 American Meteorological Society.

Li C.,University of Bergen | Wettstein J.J.,Bjerknes Center for Climate Research | Wettstein J.J.,U.S. National Center for Atmospheric Research
Journal of Climate | Year: 2012

Two important dynamical processes influence the extratropical zonal wind field: angular momentum transport by the thermally direct Hadley circulation (thermal-driving T) and momentum flux convergence by atmospheric waves (eddies) that develop in regions of enhanced baroclinicity (eddy-driving E). The relationship between extratropical zonal wind variability and these driving processes is investigated using 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) data. Indices representing the processes (iT and iE) are defined based on vertically integrated diabatic heating and meridional convergence of the meridional flux of zonal momentumby eddies, respectively. Zonal wind signatures associated with these indices are identified via composite analysis. In the Atlantic sector, zonal wind variability is mainly associated with momentum flux convergence by baroclinic eddies, supporting the established view that the Atlantic jet is primarily eddy driven. In the Pacific sector, zonal wind variability is associated with both driving processes, evidence that the Pacific jet is both thermally driven and eddy driven. The thermally driven Pacific signature reflects changes in jet strength (intensity and longitudinal extent) with some resemblance to the zonal wind anomalies of the Pacific-North America (PNA) pattern. The eddy-driven signature reflects a latitudinal shift of the jet exit region in both sectors that resembles the zonal wind anomalies of the North Atlantic Oscillation (NAO) or West Pacific (WP) patterns. © 2012 American Meteorological Society.

Wettstein J.J.,Bjerknes Center for Climate Research | Wallace J.M.,University of Washington
Journal of the Atmospheric Sciences | Year: 2010

Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthlyaveraged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as "pulsing" and "latitudinal shifting" of the climatologicalmean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent. Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)-Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of stormtrack and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific-North America (PNA) patterns. Wave-mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet. © 2010 American Meteorological Society.

Kolstad E.W.,Bjerknes Center for Climate Research
Quarterly Journal of the Royal Meteorological Society | Year: 2011

Polar lows (PLs) are small-scale and intense low-pressure systems that form at high latitudes in both hemispheres. Due to their limited spatial scale and brief lifetimes, weather and climate models are often unable to resolve these systems. One way to overcome this problem is to define a suitable proxy for PLs, with which the likelihood of PL formation can be assessed even in coarse-resolution datasets. This study draws on previous studies and an empirical database of 63 PLs to quantify the respective influences of low-level static stability and upper-level forcing on PL formation, as both of these factors are known to favour PL development. Little redundancy between the two parameters is found. After defining threshold values for the two parameters, climatological properties of favourable conditions for PLs are computed for the North Atlantic, the North-West Pacific and the Southern Hemisphere. The low-level static stability, which is strongly modified during marine cold-air outbreaks, puts important constraints on where PLs can form, while the upper-level forcing determines whether or not they will form. As a result of the climatologically lower tropopause in the Labrador Sea region, favourable conditions for PLs occur more often there than in the Nordic Seas, which has long been believed to be the main PL region in the Northern Hemisphere (NH). In the Southern Hemisphere, favourable conditions for PLs occur substantially less often than in the NH. The PL index defined here is suitable for other climatological studies and PL forecasting. © 2011 Royal Meteorological Society.

Adakudlu M.,Bjerknes Center for Climate Research | Barstad I.,Bjerknes Center for Climate Research
Quarterly Journal of the Royal Meteorological Society | Year: 2011

Extreme mesoscale weather in the Arctic region consists mainly of cases with shallow fronts that often form in the vicinity of the ice-edge and intense storms called polar lows. This article describes high-resolution numerical simulations of a severe weather event that occurred on 1 March 2008 over the Barents Sea. The event was recorded during the IPY-THORPEX field experiments carried out during February and March 2008. The numerical simulations indicated the formation of a low-pressure system over the Barents Sea on 29 February 2008 due to baroclinic instability. On 1 March, the surface low moved onto the sea-ice around Spitsbergen and decayed later on. The conditions that prevailed before the dissipation of the surface low were favourable for the formation of a polar low. Two experiments were performed to test the possibilities of triggering a polar low through certain modifications to the surface conditions. In the first experiment, the sea-ice around Spitsbergen was removed. No polar low developed in this case, since the static stability was too high. In the second experiment, an attempt to reduce the static stability was made by raising the sea-surface temperature by 5 K. The surface low persisted over the Barents Sea area due to the increased surface heating and led to a strong outbreak of Arctic air over the Norwegian Sea on 2 March. The Arctic-air outbreak formed a sharp baroclinic zone which was absent in the control simulation. A secondary mesoscale low was triggered near the baroclinic zone over the Norwegian Sea, which grew into an intense polar low with surface winds reaching hurricane force. Formation of the polar low was due to baroclinic instability, whereas convective instability was important during the growth of the low. © 2011 Royal Meteorological Society.

Sobolowski S.,University of North Carolina at Chapel Hill | Sobolowski S.,Bjerknes Center for Climate Research | Pavelsky T.,University of North Carolina at Chapel Hill
Journal of Geophysical Research: Atmospheres | Year: 2012

In order to make well-informed decisions in response to future climate change, officials and the public require reliable climate projections at the scale of tens of kilometers, rather than the hundreds of kilometers that the current atmosphere-ocean general circulation models provide. Recent efforts such as the North American Regional Climate Change Assessment Program (NARCCAP) aim to address this need. This study has two principal aims: (1) evaluate the seasonal performance of the NARCCAP simulations over the southeast United States for both present (1971-2000) and future (2041-2070) periods and (2) assess the impact of a performance-based weighting scheme on bias and uncertainty. Application of the weighting scheme results in a substantial reduction in magnitude and percent area exhibiting significant bias in all seasons for both temperature and precipitation. The weighting scheme is then expanded to evaluate future change. Temperature changes are universally positive and outside the bounds of natural variability over the entire region and in all seasons. Application of the weighting scheme tightens confidence intervals by as much as 1.6°C. Future precipitation changes are modest, are of mixed sign, and vary by season and location. Though uncertainty is reduced by as much as 50%, the projected changes are generally not outside the bounds of natural background variability. Thus, under the NARCCAP simulations, stress on water resources is most likely to come from increased temperatures and not changes in mean seasonal precipitation. For energy use, the implication is that the ∼3°C temperature increase during the peak use summer season may place additional strain on power grids. Copyright 2012 by the American Geophysical Union.

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