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The climate around the world including Saudi Arabia has been fluctuating from cold to warm during different periods. The climate of the earlier period of the 650 ka BP was warmer than the present time in Saudi Arabia due to greenhouse gases in the atmosphere. The current climate of Saudi Arabia is arid to semi-arid with different climate classes. The seasonal surface air temperatures (SATs) are high in the central and northern regions compared to the southern region. The summer of Saudi Arabia is the warmest around the globe with the exception of the coastal region. Due to different air masses that invade the regions of Saudi Arabia, there are different SATs in different seasons. Depending upon seasonal and annual basis, the frequency of the extreme cold SAT is less than the extreme warm SAT. The circulation pattern of high and low pressures plays an important role in the climatic SAT of Saudi Arabia. The coldest year is associated with the Siberian high-pressure during winter and early spring, especially in the central and northern areas, while the warmest year is related to the Indian monsoon low-pressure during summer and early autumn especially on the northeastern parts, majority of the east coast, and central regions of Saudi Arabia. On the other hand, the Icelandic low pressure extended to the southern region causes cooling air over the area, especially, the northern part of Saudi Arabia, while the Sudan low-pressure causes warming and moisture from the southern and southwestern regions in the winter season. The synoptic situation in the spring season is almost similar to the autumn season. During the spring and autumn seasons, the synoptic circulation over Saudi Arabia is Siberian high-pressure from the east, subtropical high-pressure from the west, Mediterranean depression from the north, and Sudan low and/or Asian monsoon low from south. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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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]

This paper analyzes interannual variations of the blocking high over the Ural Mountains in the boreal winter and their association with the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In January, the relationship between the Ural blocking high (UR) and the AO index is statistically significant. The UR tends to occur more frequently and with greater strength during negative AO periods. Some strong URs also occur during positive AO phases (positive UR-AO events), as in January 2008. This paper discusses the characteristics of atmospheric circulation in the cases of positive UR-AO events and contrast cases (negative UR-AO events). The eastward extending of the Icelandic Low (IL) center and the associated NAO dipole anomaly pattern in the upstream region may play a more important role for the UR-AO events. When the center of the IL shifts eastward to 30°W, the amplitude of zonal wavenumber 2 (wavenumber 3) is intensified in the positive (negative) UR-AO events, which favors positive (negative) height anomalies over the Urals. Further analyses indicate that the intensified zonal wind in high latitudes and weakened zonal wind in midlatitudes over the North Atlantic Ocean render the eastward shift of the IL and the NAO dipole anomaly pattern. The Ural blocking in January 2008 bears similar characteristics to the positive UR-AO events. [ABSTRACT FROM AUTHOR]