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We investigate the uncertainty (i.e., inter-model spread) in future projections of the boreal winter climate, based on the forced response of ten models from the CMIP5 following the RCP8.5 scenario. The uncertainty in the forced response of sea level pressure (SLP) is large in the North Pacific, the North Atlantic, and the Arctic. A major part of these uncertainties (31%) is marked by a pattern with a center in the northeastern Pacific and a dipole over the northeastern Atlantic that we label as the Pacific–Atlantic SLP uncertainty pattern (PA∆SLP). The PA∆SLP is associated with distinct global sea surface temperature (SST) and Arctic sea ice cover (SIC) perturbation patterns. To better understand the nature of the PA∆SLP, these SST and SIC perturbation patterns are prescribed in experiments with two atmospheric models (AGCMs): CAM4 and IFS. The AGCM responses suggest that the SST uncertainty contributes to the North Pacific SLP uncertainty in CMIP5 models, through tropical–midlatitude interactions and a forced Rossby wavetrain. The North Atlantic SLP uncertainty in CMIP5 models is better explained by the combined effect of SST and SIC uncertainties, partly related to a Rossby wavetrain from the Pacific and air-sea interaction over the North Atlantic. Major discrepancies between the CMIP5 and AGCM forced responses over northern high-latitudes and continental regions are indicative of uncertainties arising from the AGCMs. We analyze the possible dynamic mechanisms of these responses, and discuss the limitations of this work. [ABSTRACT FROM AUTHOR]

Applying the daily ERA-interim reanalysis data from 1979 to 2016, we found that widespread cold (warm) wintertime extreme events in Northern Europe occurred most frequently in winter 1984–1985 (2006–2007). These events often persisted for multiple days, and their primary drivers were the pattern of atmospheric large-scale circulation, the direction of surface wind and the downward longwave radiation. Widespread cold extremes were favoured by the Scandinavian Pattern and Ural Blocking, associated with advection of continental air-masses from the east, clear skies and negative anomalies in downward longwave radiation. In the case of widespread warm extremes, a centre of low pressure was typically located over the Barents Sea and a centre of high pressure over Central Europe, which caused south-westerly winds to dominate over Northern Europe, bringing warm, cloudy air masses to Northern Europe. Applying Self-Organizing Maps, we found out that thermodynamic processes explained 80% (64%) of the decreasing (increasing) trend in the occurrence of extreme cold (warm) events. The trends were due to a combined effect of climate warming and internal variability of the system. Changes in cases with a high-pressure centre over Iceland were important for the decreased occurrence of cold extremes over Northern Europe, with contribution from increasing downward long-wave radiation and south-westerly winds. The largest contribution to the increased occurrence of widespread warm extremes originated from warming and increased occurrence of the Icelandic low. [ABSTRACT FROM AUTHOR]

Correlations of the North Atlantic Oscillation (NAO) with climate variables in continental Europe are strong but weak over Britain. This article presents evidence that interannual variations of British climate are explained more successfully by variations of the Icelandic Low (IL), the northern component of the NAO. The IL changes continuously in intensity as well as position, and these variations are not represented in the NAO. Monthly values of intensity and latitude and longitude positions for the IL are estimated from gridded sea level pressure data. These are used to analyse their impact on interannual variations of winter precipitation and temperature over Britain for the period 1981–2014. It is found that the primary influence on the interannual variations of winter precipitation over Britain are the pressure and the longitude position of the IL. When the IL is deeper or situated east of its mean location, there is above normal precipitation over Britain. The primary influence on air temperature is the meridional migration of the IL; it is warmer when the IL is situated north of its mean position. The impact of the IL on precipitation and air temperature are evaluated also for Scotland, North England and South England separately. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) exerts a profound influence on global climate change. In the boreal winter, the TP has a notable impact on the Asian winter monsoon. Nevertheless, the relationship between the TP and the climate of the remote Arctic region is not yet fully understood. The North Pacific and North Atlantic pressure systems exhibit dynamic linkages through the Aleutian Low–Icelandic Low seesaw in the boreal winter season. However, the extent to which large mountains, such as those in the TP, influence this coupling remains unclear. In this study, we conducted an orographically sensitive experiment using the Whole Atmosphere Community Climate Model Version 6. The results obtained from the model indicate that the TP influences Arctic pressure anomalies specifically via the modulation of the Aleutian Low–Icelandic Low seesaw. Specifically, the TP favors the eastward extension of a pattern resembling the Pacific–North American pattern, achieved by enhancing the horizontally propagating wave trains originating from the eastern Pacific, thus establishing a clear Aleutian Low–Icelandic Low seesaw. In addition, the TP was a critical factor in the stratosphere–troposphere coupling as it facilitated the shift of the stratospheric polar vortex towards Eurasia in a poleward direction, potentially leading to the extension of the wave train emanating from the North Pacific across North America. These results suggest that the TP significantly contributes to Arctic climate variability. This influence is observed via the TP-mediated modulation of the teleconnection between the North Pacific and North Atlantic regions and stratosphere–troposphere coupling. • An orographically sensitive experiment was conducted using the WACCM Version 6. • TP modulates the Aleutian–Icelandic Low seesaw, affecting Arctic pressure anomalies. • TP favors eastward extension of a pattern like the Pacific–North American pattern. [ABSTRACT FROM AUTHOR]

We identify that the projected uncertainty of the pan-Arctic sea-ice concentration (SIC) is strongly coupled with the Eurasian circulation in the boreal winter (December-March; DJFM), based on a singular value decomposition (SVD) analysis of the forced response of 11 CMIP5 models. In the models showing a stronger sea-ice decline, the Polar cell becomes weaker and there is an anomalous increase in the sea level pressure (SLP) along 60°N, including the Urals-Siberia region and the Iceland low region. There is an accompanying weakening of both the midlatitude westerly winds and the Ferrell cell, where the SVD signals are also related to anomalous sea surface temperature warming in the midlatitude North Atlantic. In the Mediterranean region, the anomalous circulation response shows a decreasing SLP and increasing precipitation. The anomalous SLP responses over the Euro-Atlantic region project on to the negative North Atlantic Oscillation-like pattern. Altogether, pan-Arctic SIC decline could strongly impact the winter Eurasian climate, but we should be cautious about the causality of their linkage. [ABSTRACT FROM AUTHOR]

In this article, the spatiotemporal relationship between winter (DJF) precipitation over Spain and sea level pressure fields is studied using the Center of Action (COA) approach. The COAs which make up the North Atlantic Oscillation (NAO), i.e., the Azores High (AH) and the Icelandic Low (IL), are considered separately, and we find that interannual variations in the strength and position of the AH explain a very high fraction of the variance of Spain winter precipitation. The correlation of AH properties with winter precipitation is strongest in the northwest of the country. A regression model was developed that explains 83 percent of the variance of the Spain precipitation over 1981–2011. We found that the positive extreme values of AH indices are related to droughts, and negative extremes to heavy rain. The westward and southward shifts of the AH encourage the presence of low pressure system over the Iberian Peninsula and high precipitation in Spain occurs due to the moisture transport off the North Atlantic impinging upon Peninsula Spain and France. • Centre of Action approach is used to determine relationship between winter rainfall over Spain and sea level pressure fields. • Azores high have a significant influence on the variation in Spain's winter rainfall. • Spain's droughts are related to the high positive extreme values in the Azores. [ABSTRACT FROM AUTHOR]

A 60 year minimum temperature record of 11 stations in inner Eurasia enabled the characterization of the Siberian High (SH) intensity. The decline in the SH intensity is observed in tandem with the positive mode of the Arctic Oscillation (AO), both increasing in recent years. The coldest 1968-1969 winter in the 60 year period corresponds with the lowest AO annual index value. Spatial correlation analyses indicate that enhanced cyclogenetic conditions over the eastern flank of the Icelandic Low (IL) are associated with a milder SH. Seasonal composite analyses of the circulation pattern during the coldest 1968-1969 winter are characterized by a retreat of the IL, allowing a westward expansion of the SH cold core. A robust methodology, assuring an adequate representation of extremely cold spells in both their extent and duration, was developed. This methodology yielded three exceptional events, the most severe one lasting 10 days and affecting all stations in the SH domain. Analysing this event on a fine temporal resolution enabled the detection of short-term synoptic scale processes, such as the polar air mass penetration, resulting in a mean minimum temperature of − 40 °C over the whole domain. This short-term polar air incursion and its termination featuring this spell, as modulated by the location of the IL, point at its important role in modifying the SH. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Abstract: We measured cosmogenic 3He production rates in olivine phenocrysts from four radiocarbon-dated postglacial basaltic lava flows in the Western Volcanic Zone of Iceland. These measurements provide important new calibrations of cosmogenic nuclide production rates near sea level at high latitudes. Calibration sites from Lambahraun (4040±250cal yr BP; n =4), Leitahraun (5210±110cal yr BP; n = 5), Búrfellshraun (8060±120cal yr BP; n = 6), and Þingvallahraun (10,330±80cal yr BP; n = 4) yield a mean production rate of 132±5atoms 3He g−1 yr−1 (±1σ; normalized to sea level at high latitudes with the standard atmosphere). Mean production rates from the four flows agree within uncertainty, indicating that these calibrations establish a reproducible local 3He production rate that will significantly increase the accuracy of exposure dating in Iceland. The 3He production rate in Iceland is ∼17% higher than the mean of normalized Holocene values previously determined in the western USA. The high production rates in Iceland are attributed to the influence of persistent low atmospheric pressure over Iceland (the Icelandic Low) through the Holocene, which yielded higher cosmic ray fluxes. The Icelandic calibrations thus support previous suggestions that cosmogenic isotope production rates should be adjusted for regional variations in long-term atmospheric pressure. By extending the latitudinal extent of previous calibration studies, these results are also useful for evaluating scaling models. [Copyright &y& Elsevier]