Top PDF Linking Northern High-Latitude Cryospheric Changes to Large-Scale Atmospheric Circulation

Linking Northern High-Latitude Cryospheric Changes to Large-Scale Atmospheric Circulation

Linking Northern High-Latitude Cryospheric Changes to Large-Scale Atmospheric Circulation

While no changes in sea level result from sea ice melt, glacial melt from Greenland and other Arctic glaciers constituted over 40 % of the observed total 3 .1 mm global sea level rise each year over 2003–2008 (AMAP , 2011). The GrIS SMB melting found in Papers I and III are linked to changes in the North At- lantic storm track. Moreover, the projected increase in cyclonic activity over the Labrador Sea and Baffin Bay in September is accompanied by significant precipitation increments along the west coast of Greenland in Paper II. Alterations in cyclone numbers and inten- sities can have significant consequences in continental regions at the landfall of the North Atlantic and North Pacific storm tracks (AMAP , 2011). Nevertheless, these regions are generally accustomed to stormy weather, leaving the largest impacts related to changes in cyclone statistics likely to result from storm track displacements (Stocker et al., 2013, and references therein). As shown in Paper II, there is a tendency of poleward-shifted and intensified storm tracks. Although the high-latitudes are home to fewer people than the midlatitudes, the consequences might cause more harm to the lifestyle in the former region (AMAP , 2011, and references therein).
Show more

47 Read more

Interannual variability of heat waves in South Korea and their connection with large-scale atmospheric circulation patterns

Interannual variability of heat waves in South Korea and their connection with large-scale atmospheric circulation patterns

mid-latitude dynamics (e.g. Teng et al., 2013), how- ever, do not entirely exclude the possible influences from the changes in large-scale diabatic heat source either in tropics or high-latitudes. This must be particularly relevant in explaining the mechanisms of the interannual variability in the occurrence frequency and intensity changes of severe heat waves, although the development and maintenance of severe heat waves are largely affected by the quasi-stationary Rossby waves in short-term sub- seasonal variability. A large heat source in the Indo-Pacific warm pool region can be a source of large-scale Rossby waves that emanate from the tropics and propagate into the extratropics, which is also subject to the variability in inter- annual time scale of ENSO and Asian summer monsoon (Ding and Wang, 2005; Wang et al., 2009; Ding et al., 2011; Lee et al., 2013; Moon et al., 2013). The studies of Ding and Wang (2005) and Moon et al. (2013) showed that the atmospheric teleconnection patterns are quite dis- tinctive between El Nino and La Nina. Nitta (1987) found out that a linkage between anomalous convection over the tropical western North Pacific and the large-scale atmo- spheric circulation over East Asia in summer, so-called the Pacific-Japan (PJ) teleconnection pattern. Wang et al. (2001) demonstrated that the circulation changes asso- ciated with an enhanced western North Pacific summer monsoon (WNPSM) can lead to deficient rainfall over East Asia and the Great Plains of the United States. On the other hands, the weak WNPSM suppresses the convection over western North Pacific and transports moisture to the East Asia region, thus forming a dipole-like anomalous rainfall pattern. This dipole pattern is characterized by zonally elongated anticyclone pattern along 35∘N and the cyclone along 20∘N in the subtropical WNP. This pattern is also known to be closely linked to the tropical Indian Ocean Sea surface temperature (SST) (Yang et al., 2007; Li et al., 2008; Wu et al., 2009). Through these mechanisms, the changes in South Asian summer monsoon circula- tion can cause either extremely hot or wet condition in East Asia.
Show more

16 Read more

Coupled High-Latitude Climate Feedbacks and Their Impact on Atmospheric Heat Transport

Coupled High-Latitude Climate Feedbacks and Their Impact on Atmospheric Heat Transport

Perturbations to the energy balance, caused directly and indirectly by enhanced greenhouse gas concentrations, af- fect the meridional energy flux and may manifest as changes in the strength or position of the tropical Hadley circulation or in large-scale extratropical eddies. In a previous study, we related changes in energy transport by the circulation to meridional gradients in climate feedbacks, radiative forcing, and ocean heat uptake, which provides the basis for a di- agnostic decomposition of the response of the tropical mean circulation in CMIP5 simulations ( Feldl and Bordoni 2016 ). For the most part these effects are large and compensating. At the level of individual feedbacks, it becomes difficult to grasp intuitively how any single feedback affects the circu- lation response given their interactive nature. For example, compensation between lapse rate and water vapor feed- backs is well known ( Bony et al. 2006 ; Cess 1975 ). Assuming the feedbacks act in isolation, we can quantify the contri- bution of the temperature feedback as a 5%–10% K 21 weakening of the Hadley cell and of the water vapor feedback as a 5%–10% K 21 strengthening. But does that level of specificity matter if the net result is no change at all? Naturally, such results are still useful for un- derstanding uncertainty among climate change pro- jections, and moreover any particular coupled model is not required to cancel so neatly. Herein we present a series of idealized modeling experiments designed to reveal the coupling between regional climate feedbacks at a mechanistic level. This is conceptually similar to a perturbed physics ensemble (e.g., Sanderson et al. 2008 ). In contrast to studies such as Kang et al. (2009) , which forces the high latitudes by applying an ocean heat source and sink, or Graversen and Wang (2009) , which suppresses the surface albedo feedback, we modify the ‘‘sensitivity’’ of sea ice to warming. Specifically, we manipulate the strength of the surface albedo feedback in order to 1) identify compensating behavior in other feedbacks and 2) assess the impact of the net high- latitude feedback on the remote climate response.
Show more

13 Read more

Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks

Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks

In this study, we highlight the potential of evolving func- tional paleoclimate networks for investigating climate vari- ability during the Common Era (last 2 kyr) in the European North Atlantic region. Climate dynamics within this region is of crucial importance not only at regional scales (Scaife et al., 2008; Trigo et al., 2002), but also as a pacemaker for the whole Northern Hemisphere (Delworth et al., 2016). Inter-annual to multi-decadal climate variability in the North Atlantic sector is strongly influenced by large-scale variabil- ity patterns like the North Atlantic Oscillation (NAO; Hurrell et al., 2003). The NAO is related to the persistent redistribu- tion of air masses between the Arctic and the central Atlantic (Hurrell and Deser, 2010) and is commonly defined as a pres- sure dipole over the North Atlantic, consisting of a predom- inant low-pressure system over Iceland and a high-pressure system close to the Azores. The strength of the gradient be- tween both varies in time and provides a basis for the quanti- tative description of the NAO based on an index where high (low) values correspond to a strong (weak) gradient. This pressure gradient has severe consequences for climate vari- ability in Europe, Greenland, and North America. A positive phase of the NAO is commonly associated with more moder- ate temperatures and higher precipitation sums during winter in northern Europe and the eastern United States, whereas Greenland, Canada, and southern Europe often exhibit oppo- site characteristics. While the influence of the NAO phase is strongest during boreal winter, it also affects summer condi- tions (Ogi et al., 2003; Folland et al., 2009).
Show more

16 Read more

Evidence for elevated emissions from high-latitude wetlands contributing to high atmospheric CH4 concentration in the early Holocene

Evidence for elevated emissions from high-latitude wetlands contributing to high atmospheric CH4 concentration in the early Holocene

high-latitude regions. The elevation-corrected temperature reconstruction from Greenland ice cores show an abrupt warming at the onset of the Holocene [Figure 2b; Vinther et al., 2009]. Also, a recent synthesis of global proxy temper- ature records shows that a rapid warming occurred at the onset of the Holocene across the northern hemisphere, espe- cially at high latitudes above 60 [Figure 2b; Shakun et al., 2012], where many peatlands are distributed. Many peat- land-dominated regions experienced the Holocene thermal maximum in the early Holocene, including Arctic Canada (e.g., Agassiz ice melt record; Figure 2b; Fisher et al., 1995), Alaska [Kaufman et al., 2004], and other high-latitude regions [IPCC, 2007]. Summer temperatures during the Holocene thermal maximum were up to about 2 C warmer than recent pre-industrial period [Kaufman et al., 2004]. High peat carbon accumulation has been documented during the early Holocene from recent large-scale data synthesis [Figure 2c; Yu et al., 2009] as well as at individual sites in Alaska [Jones and Yu, 2010] and elsewhere. The high accu- mulation rates were most likely caused by high primary productivity under warmer summer conditions during the Holocene thermal maximum under maximum summer insola- tion (Figure 2a). This high productivity overcompensated the increased decomposition expected under warmer summer conditions — which likely contributed to the high CH 4
Show more

12 Read more

Interdependence of the growth of the Northern Hemisphere ice sheets during the last glaciation: the role of atmospheric circulation

Interdependence of the growth of the Northern Hemisphere ice sheets during the last glaciation: the role of atmospheric circulation

Figure 1 shows the azonal sea-level pressure for the north- ern winter for NCEP reanalysis (Fig. 1a) and for the three parameterizations B, C and D (Fig. 1b–d). The most strik- ing feature is that the patterns of azonal sea-level pressure simulated by CLIMBER are smoother than the NCEP ones due to the spatial resolution. The thermal effect (Fig. 1b) al- lows to account for the main structures linked to the land– sea temperature difference. Low pressure over North Pacific and North Atlantic and high pressure over the continental re- gions are clearly represented. However, their meridional ex- tent is too large. This leads to a negative SLP anomaly over Greenland and a positive one over the Scandinavian and the Barents–Kara regions, in contradiction with NCEP reanal- ysis. Moreover, the amplitudes of the anomalies over the Northern Hemisphere are weaker than those of the NCEP database. This implies that the anti-cyclonic structure over the North American continent is almost absent. The am- plitudes of sea-level pressure anomalies are smaller in the Southern Hemisphere (Fig. 1a) than in the Northern Hemi- sphere and occur over a smaller spatial scale. Due to the coarse horizontal resolution of CLIMBER, these structures are poorly resolved whatever the azonal sea-level parame- terization is. With the thermal effect (Fig. 1b) this translates into a large anticyclone centered over South Atlantic that ex- pands over the Indian Ocean, western Pacific and the Antarc- tic ice sheet. As a result, the bipolar structure over Antarctica observed in NCEP is not represented in the simulations car- ried out with the thermal forcing. Nevertheless, the Southern Hemisphere regions are beyond the regions of interest for the purpose of the present study.
Show more

14 Read more

Improving the dynamics of Northern Hemisphere high-latitude vegetation in the ORCHIDEE ecosystem model

Improving the dynamics of Northern Hemisphere high-latitude vegetation in the ORCHIDEE ecosystem model

Abstract. Processes that describe the distribution of veg- etation and ecosystem succession after disturbance are an important component of dynamic global vegetation mod- els (DGVMs). The vegetation dynamics module (ORC-VD) within the process-based ecosystem model ORCHIDEE (Or- ganizing Carbon and Hydrology in Dynamic Ecosystems) has not been updated and evaluated since many years and is known to produce unrealistic results. This study presents a new parameterization of ORC-VD for mid- to high-latitude regions in the Northern Hemisphere, including processes that influence the existence, mortality and competition between tree functional types. A new set of metrics is also proposed to quantify the performance of ORC-VD, using up to five dif- ferent data sets of satellite land cover, forest biomass from remote sensing and inventories, a data-driven estimate of gross primary productivity (GPP) and two gridded data sets of soil organic carbon content. The scoring of ORC-VD de- rived from these metrics integrates uncertainties in the obser- vational data sets. This multi-data set evaluation framework is a generic method that could be applied to the evaluation of other DGVM models. The results of the original ORC-VD published in 2005 for mid- to high-latitudes and of the new parameterization are evaluated against the above-described data sets. Significant improvements were found in the model- ing of the distribution of tree functional types north of 40 ◦ N. Three additional sensitivity runs were carried out to separate the impact of different processes or drivers on simulated veg-
Show more

21 Read more

Trends in Mediterranean gridded temperature extremes and large-scale circulation influences

Trends in Mediterranean gridded temperature extremes and large-scale circulation influences

For winter, the influence of the MO pattern is the most geographically wide-spread, with the MOI-1 index exhibit- ing stronger relationships than the MOI-2 index (Table 7). The NAO/AO patterns, which are partly correlated with the MO (e.g., D¨unkeloh and Jacobeit, 2003, Table 4), exhibit their strongest impact in south-eastern case-study domains. Across the Mediterranean, the SLP-based NAO-CRU in- dex is selected somewhat more frequently than the 500-mb geopotential height-based NAO-CPC index (Table 9). For the other mid-tropospheric indices, the EA/WR pattern af- fects cold extremes in Eastern Mediterranean domains, and less strongly warm extremes in Western Mediterranean do- mains. Some indications are also found of EA pattern in- fluence in the Central Mediterranean (Italy). The SCAND pattern is related weakly with cold extremes in the southwest Mediterranean (Oran). As expected (Kutiel et al., 2002), the NCP appears as probably the most influential pattern for the Eastern Mediterranean region and has preferably high skill in explaining cold extremes there. Previous studies focused on the impact of the EMP in the Eastern Mediterranean (Hatzaki et al., 2009). Here it is found that the influence of the EMP is considerably more extensive and is actually stronger in the Central and Western than in the Eastern Mediterranean (see also discussion of Fig. 8 in Sect. 5). Cold extremes are most strongly affected by the EMP in the Gulf of Gab`es and warm extremes in Valencia. All case-study domains appear to be affected by the major Euro-Atlantic atmospheric circulation patterns to a considerable degree. Nevertheless, the diver- sity of this influence among the various domains and also the indices of extremes show a geographical and/or climatic preference. In particular, cold extremes (especially TN5n) in the Southern Mediterranean domains (i.e., Gulf of Oran, Gulf of Gab`es, Alexandria, Judean Foothills, and Beirut) ex- hibit the closest relationship with the atmospheric patterns (also confirmed by a preliminary multiple regression analy- sis – not shown here – using the circulation indices appear- ing in Table 7). With regard to duration-related indices of extremes, the duration of cold events (CWDI) appears more prone to the influence of large-scale patterns than the dura- tion of warm events (WSDI and HWDI).
Show more

16 Read more

Five hundred years of gridded high-resolution precipitation reconstructions over Europe and the connection to large-scale circulation

Five hundred years of gridded high-resolution precipitation reconstructions over Europe and the connection to large-scale circulation

Abstract We present seasonal precipitation reconstruc- tions for European land areas (30W to 40E/30–71N; given on a 0.5·0.5 resolved grid) covering the period 1500–1900 together with gridded reanalysis from 1901 to 2000 (Mitchell and Jones 2005 ). Principal component regression techniques were applied to develop this dataset. A large variety of long instrumental precipitation series, precipitation indices based on documentary evidence and natural proxies (tree-ring chronologies, ice cores, corals and a speleothem) that are sensitive to precipitation sig- nals were used as predictors. Transfer functions were de- rived over the 1901–1983 calibration period and applied to 1500–1900 in order to reconstruct the large-scale pre- cipitation fields over Europe. The performance (quality estimation based on unresolved variance within the cali- bration period) of the reconstructions varies over centu- ries, seasons and space. Highest reconstructive skill was found for winter over central Europe and the Iberian Peninsula. Precipitation variability over the last half millennium reveals both large interannual and decadal fluctuations. Applying running correlations, we found major non-stationarities in the relation between large- scale circulation and regional precipitation. For several periods during the last 500 years, we identified key atmospheric modes for southern Spain/northern Mor-
Show more

19 Read more

Ammonia large scale atmospheric dispersion experiments in industrial configurations

Ammonia large scale atmospheric dispersion experiments in industrial configurations

It should be noted that pellistors EEV VQ41 and electrochemical cells SENSORIC NH3 3rd 1000, have been tested at INERIS in laboratory and during middle-scale releases (releases through 0,5 mm to 8 mm nozzle diameters). In order to simplify the installation of the electronic boxes before a release, the test-field was beforehand equipped of more than 150 masts of 3 meters height. These masts could receive 4 sensors at different heights, namely and 3 meters. They were laid out on 7 circle arcs of centred on the release flagstone and whose rays were 20, 50, 100, 200, 500, 800 and 1,700 meters (see On the first two arcs of circles located at 20 and 50 meters, 41 masts were installed, spaced each other with an angle of 4.5°, and on all the other arcs of circles, masts were installed with 9° spacing.
Show more

18 Read more

Understanding and Predicting Changes in Precipitation and Water Availability Under the Influence of Large-Scale Circulation Patterns: Rio Grande and Texas

Understanding and Predicting Changes in Precipitation and Water Availability Under the Influence of Large-Scale Circulation Patterns: Rio Grande and Texas

Water availability plays an important role in the socio-economic development of a region. It is however, subject to the influence of large-scale circulation patterns, resulting in periodic excesses and deficits. An assessment of the degree of correlation between climate indices and water availability, and the quantification of changes with respect to major climate events is important for long-term water resources planning and management, especially in transboundary basins as it can help in conflict avoidance. In this study, following the assessment of the correlation of the Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO) with gauged precipitation in RG discussed in section 2, the changes in water availability using runoff generated from the Noah land surface model are quantified. Both spatial and temporal variations are noted, with winter and spring being most influenced by conditions in the Pacific Ocean. Negative correlation is observed at the headwaters and positive correlation across the rest of the basin. The influence of individual ENSO events, classified using four different criteria, is also examined. El Niños (La Niñas) generally cause an increase (decrease) in runoff, but the pattern is not consistent; the percentage change in water availability varies across events. Further, positive PDO enhances the effect of El Niño and dampens that of La Niña, but during neutral/transitioning PDO, La Niña dominates meteorological conditions. Long El Niños have more influence on water availability than short duration high intensity events. We also note that the percentage increase during El Niños significantly offsets the drought-causing effect of La Niñas.
Show more

197 Read more

Large- and small-scale turbulent spectra in MHD and atmospheric flows

Large- and small-scale turbulent spectra in MHD and atmospheric flows

One of the results of the phenomenological approach sug- gested by Kolmogorov in his fundamental papers more than 60 years ago was the dependence of a two-point correlator of turbulent fluctuations of a locally homogeneous and isotropic velocity field of an incompressible fluid (and its spectral density) on the distance between these two point in the in- ertial interval. According to Kolmogorov, this dependence should be universal for any developed turbulent flow, where the mean turbulent energy flux from larger to smaller scales is presumed to be constant. Until today, despite a consider- able progress in the study of turbulent flows, this result forms the basis of various applications of semi-empirical turbulence theories. As a matter of fact, it reflects the energy conserva- tion law in a dynamically equilibrium system, where all tur- bulent energy getting into the large scales is transformed into heat in the small scales after passing all intermediate scales.
Show more

8 Read more

Effects of high latitude atmospheric gravity wave disturbances on artificial HF radar backscatter

Effects of high latitude atmospheric gravity wave disturbances on artificial HF radar backscatter

the use of HF coherent scatter radars and incoherent scat- ter radars. TIDs leave spatial and temporal signatures in the power returned by backscattering of an HF radar sig- nal from the ground after reflection from the disturbed iono- sphere. These periodic power variations are produced by the focussing and defocussing of the radar beam by the electron density perturbations in the ionosphere produced by the TID. Samson et al. (1990) presented observations of such power variations which they interpreted as being due to equator- ward propagating earth-reflected gravity waves. They were able to determine the horizontal wavelength, direction of propagation and phase speed of the waves from their ob- servations. Bristow and Greenwald (1995) developed an approach to allow the estimation of the TID amplitude in terms of the electron density perturbation from the ground backscatter power variations. They found that the electron density perturbations were on the order of 20–35%. Arnold et al. (1998) adopted a different approach to interpreting the ground backscatter observations. Instead of using the backscatter power variations, they examined the variation in the range to the edge of the skip zone, the region of the Earth’s surface from which ground backscatter cannot be obtained because the radar wave takeoff elevation angle re- quired to reach inside the zone is so high that the wave pen- etrates the ionosphere (Davies, 1965). This range is modu- lated by the varying ionospheric electron density. They com- pared their measurements, made with the HF radar at Han- kasalmi, Finland with electron density measurements from the EISCAT UHF radar near Tromsø, Norway and found good correlation between the skip distance variation and the electron density variation at a height of 235 km. We use the same instrumentation in the present study.
Show more

15 Read more

Influence of high-resolution surface databases on the modeling of local atmospheric circulation systems

Influence of high-resolution surface databases on the modeling of local atmospheric circulation systems

In order to show the spatial discrepancies between the HR and the CTL runs on the G5 domain, we present the horizon- tal cross section of the difference of potential temperature between the two runs, defined as θ(HR) – θ(CTL), for the period from 14:00 to 17:00 UTC, 7 September 2007, as illus- trated in Fig. 15a–d. This figure highlights the areas where there are visible discrepancies between both runs. These re- sults also indicate that changes alone on the vegetation type and not merely on the soil type provide meaningful differ- ences on the air flow, as we can see from the contrast between the continent and the sea. We inserted a dashed line in each figure to indicate the vertical cross section which we con- sidered in the analysis of the sea-breeze front based on the TKE distribution that we present in Sect. 4.3. At 14:00 UTC (11:00 local time – LT), we can clearly observe that the ma- jor discrepancies are on the western side and over the water bodies, mainly in the vicinity of a water reservoir located in the continent. Overall, the potential temperature values of the HR run tend to be higher than those of the CTL, except around the entrance of Sepetiba Bay, where a colder air par- cel due to the HR run appears (Fig. 15a). In this case, the re- sults for the HR run present an accentuated differential heat- ing between the sea and the continent, which is able to turn on an efficient thermodynamic trigger to start the sea–land breeze mechanism. Thus, it is possible that the vegetation- type database changes the heat and water-vapor fluxes in order to represent more adequately the local circulation. At 15:00 UTC (Fig. 15b) the discrepancy increases slightly and the cool air parcel moves from the southeast to the northwest, indicating that the sea-breeze front penetrates perpendicu- larly into the continent with respect to the grid’s east-side shoreline (approximately between − 43.60 and − 43.40 arc- deg west longitude). During this motion, the breeze does not feel the change in direction that the Sepetiba Bay shoreline presents after − 43.60 arc-deg west longitude, approximately, which the dashed line crosses. Likewise, this behavior indi- cates that the soil-type database that represents this change in direction of the Sepetiba Bay shoreline is not influenc- ing the flow enough to capture the wind direction suitably in this area. At 16:00 and 17:00 UTC (Fig. 15c, d) we note the same discrepancy areas between the runs as seen in the previous hour, but the potential temperature difference val- ues are smaller. The cool air parcel moves towards the grid’s west side at 16:00 UTC (Fig. 15c), and practically disappears at 17:00 UTC (Fig. 15d). This behavior shows the impor- tance of increasing the density of surface-weather observa- tion stations at MARJ in order to evaluate whether the phys- ical trend captured with the high-resolution ARPS modeling is in agreement with the sea-breeze front advance analyzed.
Show more

19 Read more

Reconstructing past atmospheric circulation changes using oxygen isotopes in lake sediments from Sweden

Reconstructing past atmospheric circulation changes using oxygen isotopes in lake sediments from Sweden

56 C. E. Jonsson et al.: Reconstructing past atmospheric circulation changes 4.1 Holocene long-term trend Numerous paleoclimate studies, a few including oxygen iso- topes, have been undertaken in northern Fennoscandia, es- pecially in the Abisko area (Fig. 1). A comparison of car- bonate and diatom silica δ 18 O records from two small non- evaporative lakes in Abisko show similar decreasing trends over the Holocene (Fig. 4). Both the δ 18 O carbonate record from the groundwater fed Lake Tibetanus (Hammarlund et al. 2002) and the δ 18 O diatom record from the open through flow Lake 850 (Shemesh et al., 2001) are assumed to reflect annual mean δ 18 O p . These records show high δ 18 O in the early Holocene (ca. 10 000–8000 cal yr BP) and lower values in the mid – (ca. 8000–4000 cal yr BP) and late Holocene (ca. 4000–100 cal yr BP). A similar depletion trend is also shown in a speleothem δ 18 O record (SG93) from Mo i Rana in Northern Norway (Fig. 4) (Lauritzen and Lundberg, 1999). The long-term Holocene trend in the records follows the pat- tern of orbital forcing of summer insolation at 65 ◦ N, consid- erably higher insolation values during early Holocene than today (Berger and Loutre, 1991), and therefore Holocene decreasing summer temperatures would be expected and consequently lower δ 18 O p . However, quantitative recon- structions of Fennoscandia summer and annual mean tem- peratures based on biological proxies from lake sediments (Barnekow 1999; Sepp¨a and Birks, 2001; Hammarlund et al., 2002; Sepp¨a and Birks, 2002; Bjune et al., 2005; Sepp¨a et al., 2005; Bigler et al., 2006) suggest a Holocene climate with a cool early Holocene, a mid-Holocene temperature maxi- mum and decreasing temperature in late Holocene. Sepp¨a and Birks (2002) suggest that the effect of high summer so- lar radiation on summer temperatures in Fennoscandia in the early Holocene was subdued by a cooling effect of a stronger than present zonal circulation (Sepp¨a and Birks, 2001), with enhanced Atlantic airflow across the Scandes mountains, and as a result a cool and moist climate.
Show more

14 Read more

Reconstructing past atmospheric circulation changes using oxygen isotopes in lake sediments from Sweden

Reconstructing past atmospheric circulation changes using oxygen isotopes in lake sediments from Sweden

The climate in Sweden is strongly influenced by oceanic and atmospheric circulations over the North Atlantic and the Scandinavian mountain range (Fig. 1). Precipitation is closely related to the passage of cyclones that nor- mally follow the westerly-easterly track across Scandinavia ( ˚ Angstr¨om, 1974). The highest amount of precipitation in Sweden is found along the west coast and in the Scandina- vian mountain range (1961–1990; Alexandersson and An- dersson, 2004) (Fig. 2). Precipitation distribution is strongly influenced by topography (Johansson and Chen, 2003). On the windward west side of the Scandes, forced lifting of ap- proaching air masses over the mountain causes the release of rainfall and an increase in precipitation with elevation (orographic rain). The eastern leeward side of the Scan- des is often drier with a more continental climate (Smith, 1979). Strong zonal airflow with strong westerly winds (pos- itive North Atlantic Oscillation (NAO) index) and increased cyclonic frequency across the North Atlantic leads to high amounts of winter precipitation and higher winter air tem- peratures in Fennoscandia (Chen and Hellstr¨om, 1999; Chen, 2000; Jacobeit et al., 2001; Marshall et al., 2001). In contrast, cold and dry winters occur when westerly winds in the North Atlantic region are weak (negative NAO index) and merid- ional airflow brings cold polar air south over Fennoscandia. Additionally, during negative NAO winters southeast airflow could bring moisture to Sweden from the Baltic Sea (Uvo, 2003). Based on instrumental temperature data with informa- tion on cloud amount, meridional geostrophic wind and air pressure, Moberg et al. (2003) showed that low summer tem- peratures over Scandinavia are associated with dominance of cyclonic circulation (cool and wet conditions), and con- versely, high summer temperatures with dominance of anti- cyclonic circulation (warm and dry conditions).
Show more

14 Read more

Numerical simulations of composition changes in the high latitude thermosphere during disturbed conditions

Numerical simulations of composition changes in the high latitude thermosphere during disturbed conditions

( ≈ 8 km/sec) of the satellite and the large temperature difference that frequently existed between the ambient atmosphere and the ion source region (temperatures in the ion source region were normally 300-350 K [von Zahn, 1974] while temperatures in the thermosphere could range from 500-1500 K). Atomic oxygen and other reactive species also had a tendency to react with any satellite surface they collided with, making the determination of these number densities particularly difficult as high speed gas-surface interactions were not very well understood during this time period. Closed ion source mass spectrometers, e.g. the gas analyzers on OV3-6 [Philbrick, 1974], OGO-6 [Carignan and Pinkus, 1968], ESRO-4 [Trinks and von Zahn, 1975], S3-1 [Philbrick, 1976], Dynamics Explorer 2 [Carignan et al., 1981] and the Neutral Atmosphere Composition Experiment on the Atmosphere Explorer series of satellites [ Pelz et al. , 1973], addressed the calibration issue by allowing the ambient species to be thermally accommodated in an antechamber before entering the ionization region (Figure 1.15). This ensured that any non-reactive ambient species entering the ionization region would have been at the same temperature as the accommodation chamber surfaces, eliminating any uncertainties about incomplete accommodation in these measurements. It also allowed one to relate the number densities in the ion source to the ambient number densities through the expression
Show more

201 Read more

Developmental changes in large-scale network connectivity in autism

Developmental changes in large-scale network connectivity in autism

Resting state fMRI studies in neurotypical individuals have identi fied several major intrinsically connected networks related to visual, motor, auditory, memory and executive processes ( Damoiseaux et al., 2006 ). Re- search examining individuals with ASD has recently focused on hypo- and hyper-connectivity differences observed in two major large-scale brain networks. The default mode network (DMN) consists of key nodes in the posterior cingulate cortex (PCC), the medial temporal lobes (MTL) and the medial prefrontal cortex (MPFC) and is active in self-related tasks such as autobiographical memories or social tasks such as theory of mind ( Fox and Raichle, 2007 ; Spreng et al., 2009 ). The salience network (SN) involves the anterior insula (AI) and the anterior cingulate cortex (ACC) and is thought to regulate switching of endogenous and exogenous attention to relevant stimuli that helps in guiding behavior ( Uddin, 2015 ). Studies using rsfMRI have found both hypo- and hyper-connectivity of these and other functional networks when comparing individuals with ASD with neurotypical (NT) individuals. Hypo-connectivity in ASD com- pared with TD individuals has been identi fied in connections between the insula and amygdala ( Ebisch et al., 2011 ) and most consistently be- tween connections of nodes within the DMN( Assaf et al., 2010 ; Ebisch et al., 2011 ; Kennedy and Courchesne, 2008a ; Monk et al., 2009 ; Weng et al., 2010 ). Hyper-connectivity in ASD compared with TD individuals has been identi fied in motor and visual networks, as well as the DMN and SN( Uddin et al., 2013 ; Washington et al., 2014 ), and between striatal areas and the insula ( Di Martino et al., 2011 ). Finally, one study has even demonstrated extremely small to no differences in functional connectivi- ty in ASD compared with neurotypical adults ( Tyszka et al., 2014 ), pro- ducing additional con flicting evidence.
Show more

10 Read more

Wind-driven changes in Southern Ocean residual circulation, ocean carbon reservoirs and atmospheric CO2

Wind-driven changes in Southern Ocean residual circulation, ocean carbon reservoirs and atmospheric CO2

argues that during glacial-interglacial transitions, the ven- tilation of the abyssal ocean was achieved by the formation and erosion of a low-salinity surface water layer around Antarctica. Our model experiments with ‘‘mixed’’ surface buoyancy boundary conditions only include limited salinity changes of O(0.1g kg -1 ), compared with the O(1g kg -1 ) changes invoked by Toggweiler et al. (2006). While our experiments do reveal some buoyancy-driven changes in abyssal circulation, we also find that there are compen- sating carbon changes: carbon component anomalies of saturation and disequilibrium associated with model cir- culation changes are largely compensated by opposing anomalies in the soft tissue reservoir. If climatic pertur- bations to the Southern Hemisphere westerly winds are to induce greater changes in atmospheric CO 2 , then larger
Show more

20 Read more

The signatures of large-scale patterns of atmospheric variability in Antarctic surface temperatures

The signatures of large-scale patterns of atmospheric variability in Antarctic surface temperatures

Analogous results for the SAM are shown in Figures 3b and S2. Overall, the SAM has a much greater impact on Antarctic SATs than the BAM. The pattern of anomalously warm SATs over the northern Peninsula and cool SATs across both West and East Antarctica during the positive polarity of the SAM has been widely estab- lished in previous work based on monthly mean data [Thompson and Solomon, 2002; Kwok and Comiso, 2002; Marshall, 2007]. Here we demonstrate that the structure of the SAM in Antarctic SAT is readily repro- duced in daily SAT anomaly data from ERA-Interim, from the station-based observations, and in all four seasons (Figure S3). The only regions of Antarctica in which signi ficant differences in SAT anomaly do not exist between the positive and negative polarities of the SAM are the southern part of the Peninsula and an area of the Filchner-Ronne Ice Shelf. The greatest negative differences in daily SAT anomalies between the two polarites of the SAM occur in the Marie Byrd Land region of West Antarctica that includes Byrd (where the station-based difference is a substantial –5.3°C) and in parts of East Antarctica (Figure 3b). In both ERA-Interim and the observations there is a positive SAT difference across most of the Peninsula, extending further south on the west coast. The largest SAT differences are found at Esperanza (located on the east side of the Peninsula, cf. Figure 1), where the station-based anomaly difference is 4.2°C. The increases in Esperanza SAT have been linked to the advection of warm air from west to east over the Peninsula and the related föhn effect on the lee side where the station is located [Marshall et al., 2006]. In addition, Marshall et al. [2006] demonstrated that regression coef ficients between SAT and the SAM are 3 times greater on the east side of the Peninsula than the west. Analysis of the seasonal data (Figure S3) suggests that the longitudinal differences in SAT anomalies across the Peninsula are robust across the year, as expected given the föhn effect is prevalent during all seasons [Cape et al., 2015] but that the amplitude of the results varies between ERA-Interim and the station-based observations, particularly during summer.
Show more

14 Read more

Show all 10000 documents...

Related subjects