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Sea ice in both polar regions is an important indicator of the expression of global climate change and its polar amplification. Consequently, broad interest exists on sea ice coverage, variability and long-term change. However, its predictability is complex and it depends strongly on different atmospheric and oceanic parameters. In order to provide insights into the potential development of a monthly/seasonal signal of sea ice evolution, we applied a robust statistical model based on different oceanic and atmospheric parameters to calculate an estimate of the September sea ice extent (SSIE) on a monthly timescale. Although previous statistical attempts of monthly/seasonal SSIE forecasts show a relatively reduced skill, when the trend is removed, we show here that the September sea ice extent has a high predictive skill, up to 4 months ahead, based on previous months' oceanic and atmospheric conditions. Our statistical model skillfully captures the interannual variability of the SSIE and could provide a valuable tool for identifying relevant regions and oceanic and atmospheric parameters that are important for the sea ice development in the Arctic and for detecting sensitive/critical regions in global coupled climate models with a focus on sea ice formation. [ABSTRACT FROM AUTHOR]

Sea ice in both Polar Regions is an important indicator for the expression of global climate change and its polar amplification. Consequently, a broad interest exists on sea ice coverage, variability and long term change. However, its predictability is complex and it depends on various atmospheric and oceanic parameters. In order to provide insights into the potential development of a monthly/seasonal signal of sea ice evolution, we developed a robust statistical model based on oceanic and different atmospheric variables to calculate an estimate of the September sea ice extent (SSIE) on monthly time scale. Although previous statistical attempts of monthly/seasonal SSIE forecasts show a relatively reduced skill, when the trend is removed, we show here that the September sea ice extent has a high predictive skill, up to 4 months ahead, based on previous months' atmospheric and oceanic conditions. Our statistical model skillfully captures the interannual variability of the SSIE and could provide a valuable tool for identifying relevant regions and atmospheric parameters that are important for the sea ice development in the Arctic and for detecting sensitive and critical regions in global coupled climate models with focus on sea ice formation. [ABSTRACT FROM AUTHOR]

In austral spring 2016 the Antarctic region experienced anomalous sea ice retreat in all sectors, with sea ice extent in October and November 2016 being the lowest in the Southern Hemisphere over the observational period (1979-present). The extreme sea ice retreat was accompanied by widespread warming along the coastal areas as well as in the interior of the Antarctic continent. This exceptional event occurred along with a strong negative phase of the Southern Annular Mode (SAM) and the moistest and warmest spring on record, over large areas covering the Indian Ocean, the Ross Sea and the Weddell Sea. In October 2016, the positive anomalies of the totally integrated water vapor (IWV) and 2m air temperature (T2m) over the Indian Ocean, western Pacific, Bellingshausen Sea and southern part of Ross Sea were unprecedented in the last 39 years. In October and November 2016, when the largest magnitude of negative daily sea ice concentration anomalies was observed, repeated episodes of poleward advection of warm and moist air took place. These results suggest the importance of moist and warm air intrusions into the Antarctic region as one of the main contributors to this exceptional sea ice retreat event. [ABSTRACT FROM AUTHOR]

In austral spring 2016 the Antarctic region experienced anomalous sea ice retreat in all sectors, with sea ice extent in October and November 2016 being the lowest in the Southern Hemisphere over the observational record (1979-present). The extreme sea ice retreat was accompanied by widespread warming along the coastal areas as well as in the interior of the Antarctic continent. This exceptional event occurred along with a strong negative phase of the Southern Annular Mode (SAM) and the wettest and warmest spring on record, over large areas covering the Indian ocean, the Ross Sea and the Weddell Sea. In October 2016, the positive anomalies of the totally integrated water vapor (IWV) and 2 m air temperature (TT) over the Indian Ocean, Western Pacific, Bellingshausen Sea and southern part of Ross Sea were unprecedented in the last 39 years. In October and November 2016, when the lowest daily sea ice concentration anomalies were observed, repeated episodes of poleward advection of warm and moist air took place, with the most intense episode occurring at the end of November 2016. These results suggest the importance of moist and warm air intrusions into the Antarctic region as one of the main contributors to this exceptional sea ice retreat event. [ABSTRACT FROM AUTHOR]

Global cloud cover represents a critical component of the climate system, with a considerable impact on the Earth's radiation budget. Small changes in clouds properties have a significant climatological impact because of the feedbacks that they generate, thus it is difficult to simulate the global cloud cover evolution in general circulation models. Observational investigations of cloud processes are constrained either by limited temporal and spatial extension of ground-based measurements or by imperfections in satellite data, like changes in geostationary satellite zenith angle, equatorial crossing time, or calibration. In this study, we used the Empirical Orthogonal Functions method to separate global patterns of total cloud cover variability in two satellite datasets from the International Satellite Cloud Climatology Project and the Pathfinder Atmospheres-Extended projects, each corrected for specific errors, and in the ERA5 Reanalysis. The first two modes explain most of the variance from what could be considered “signal” in both satellite data. Through Canonical Correlation Analysis, they are associated in a physically consistent manner with two different types of El Niño-Southern Oscillation (ENSO), namely the canonical ENSO which manifests itself in the eastern tropical Pacific and the El-Niño Modiki which manifest itself in the central Pacific. This work provides a comprehensive picture of the relationship between global total cloud cover and the tropical Pacific processes and indicates that cloud cover in the Indo-Pacific sector plays a significant role in the Earth radiative budget at interannual to decadal time scales. The similarity of the results across satellite and reanalysis data indicate that the both the observed and reanalysis cloud data sets contain consistent and valuable information related to global climate variability. [ABSTRACT FROM AUTHOR]

In Europe, several extreme summer drought events have occurred the last two decades, with the 2018 Northern European drought as the most recent example. A better understanding of the combined characteristics and the large-scale atmospheric circulation driving such events is of high importance to enhance preparation and mitigation planning of drought. Initial analysis based on the E-OBS daily gridded observational datasets, showed record-breaking May and July temperature in large parts of northern Europe caused by blocking systems centred over Fennoscandia. Resulting high evapotranspiration combined with precipitation deficit caused dry conditions over large parts of northern Europe. The objective of this study is to investigate the hydrological aspect (streamflow and groundwater) of the 2018 drought, in a historical perspective, in the Nordic countries (Norway, Sweden, Finland and Denmark). The analysis is based on high quality long records (at least 30 years) daily streamflow data for near-natural catchments in the Nordic countries. For groundwater, daily (or weekly) reference station data for Norway will be analysed. Further, the study will investigate potential co-occurrence of, and changes in, driving mechanisms of northern Europe drought events using gridded historical datasets (e.g. E-OBS and NCEP reanalysis data). This study will highlight the historical distinctiveness of the combined drivers and hydrological characteristics of the 2018 drought in the Nordic countries. [ABSTRACT FROM AUTHOR]

The response of evapotranspiration to anthropogenic warming is of critical importance to the water and carbon cycle. Conflicting observations about changes of evapotranspiration stem mostly from the brevity of observations in time and space as well as a high degree of internal variability. Here we present the first gridded reconstruction of the European summer vapour pressure deficit (VPD) for the past four centuries. The gridded reconstruction is based on 26 European tree-ring oxygen isotope records and is performed using a Random Forest approach. Based on our reconstruction, we show that from the mid- 1700s a trend towards higher VPD occurred in Central Europe and the Mediterranean region which is based on the simultaneous increase in temperature and decrease in precipitation. This increasing VPD trend continues throughout the observational period and recent times. Climate model projections show this increase in VPD for the Mediterranean region continuing until the end of the 21st century, whereby the extent depends on the amount of greenhouse gas emissions. In contrast, projected VPD in North and Central Europe shows a tendency towards higher VPD only in the highest emission scenario. The reconstructed, observed and modelled VPD for past present and future is available here: https://doi.org/10.5281/zenodo.5958837 (Balting, D. F. et al., 2022). [ABSTRACT FROM AUTHOR]

Glaciers worldwide are shrinking at an accelerated rate as the climate changes in response to anthropogenic influence. While increasing air temperature is the main factor behind glacier mass and volume loss, variable patterns of precipitation distribution also play a role, though these are not as well understood. Furthermore, while the response of surface glaciers (from large polar ice sheets to small alpine glaciers) to climatic changes is well documented and continuously monitored, little to nothing is known about how cave glaciers (perennial ice accumulations in rock-hosted caves) react to atmospheric warming. In this context, we present here the response of cave and surface glaciers in SE Europe to the extreme precipitation events occurring between May and July 2019 in SE Europe. Surface glaciers in the northern Balkan Peninsula lost between 17 % and 19 % of their total area, while cave glaciers in Croatia, Greece, Romania and Slovenia lost ice at levels higher than any recorded by instrumental observations during the past decades. The melting was likely the result of large amounts of warm water delivered directly to the surface of the glaciers, leading to rapid reduction in the area of surface glaciers and the thickness of cave glaciers. As climate models predict that such extreme precipitation events are set to increase in frequency and intensity, the presence of cave glaciers in SE Europe and the paleoclimatic information they host may be lost in the near future. Moreover, the same projected continuous warming and increase in precipitation extremes could pose an additional threat to the alpine glaciers in southern Europe, resulting in faster-than-predicted melting. [ABSTRACT FROM AUTHOR]

The Carpathian Mountains host several ice caves that preserve a large variety of geochemical information on past climate and environmental changes. In such caves, ice forms in winter by the freezing on infiltrating water, the later accumulating during the rainy season, between early summer and late autumn (depending on site-specific conditions). Here, we present the first reconstruction of summer temperature during the last 1000 years based on the d18O and d2H values measured in Focul Viu Ice Cave (Western Carpathian Mountains, Romania).Focul Viu (FV) is short (107 m long) descending cave, located at ca. 1165 m above sea level on the windward (western) side of the mountains. A stratified ice block, with a volume of ~25000 m3 and ~20 m thickness, occupies the floor of the cave. In May 2016 we have extracted a 4.86 m long ice core from the cave glacier and measured the stable isotope composition of ice in order to derive information on past climate variability. The chronology of the core is based on six 14C measurements on organic matter embedded in the ice. Monitoring of the stable isotope composition of precipitation in the vicinity of the cave shows that most of the summer precipitation is originating in the Atlantic Ocean, ca. 30 % percent of it being locally recycled during heavy summer storms.We have found a near-perfect correlation between d18O (and d2H) of the ice core and instrumental summer temperatures measurements at three local stations, located west, north and east of the cave's location and have used this correlation to reconstruct summer temperature variability during the past ~1100 years. Our results show little centennial-scale variability during this period, suggesting that summer temperatures were likely similar between the Medieval Warm Period (MWP) and the Little Ice Age (LIA). In contrast, winter temperatures reconstructed from the nearby Scărișoara Ice Cave clearly show lower temperatures during the LIA, compared to the MWP, suggesting that the well-known temperature contrast between the two periods was a the result of winter climatic conditions. On a decadal scale, both during the MWP and the LIA, summer temperatures fluctuated with a periodicity of ~20/30 years, the minima and the maxima matching the fluctuations recorded by the Atlantic Ocean sea surface temperatures. Both the periodicity and the observed variability are similar to those of the Atlantic Multidecadal Oscillation, suggesting the d18O and d2H values in FV cave ice are recording climatic changes at hemispheric scale, thus being an important new proxy for both past temperatures and large-scale circulation changes. [ABSTRACT FROM AUTHOR]

The variability of harsh winters is considered to be a sensitive indicator for climate change. As such, in this study we present a reconstruction of 2000 years of harsh winters in the Eastern Europe based on historical documents. In total, 360 harsh winters were extracted for the period from AD 0 to 2000, and it's the longest one, on record, over this area. Our investigations spans over five distinct periods: the Roman Warm Period (RWP), the Dark Age (DA), the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the Modern Period (MD), respectively. The results obtained show that the number years of harsh winters has decreased considerably during the RWP (AD 0 – 550) and have an occurrence rate < 0.1/year. During the Dark Age is observed a small increase in the number of harsh winter and the occurrence rate is found to be > 0.15/year. The period which covers the MWP (AD 850 – 1250) shows very small variation in number of severe winters and a slight decline in occurrence rate (~ 0.05 to 0.08/year). Overall, the biggest changes have been observed during the LIA (AD 1400 – 1850), when the occurrence rate of harsh winters show an increasing trend of ~ 0.75/year, due to a decreasing in the winter mean temperature over the eastern part of Europe. During the MD (AD 1900 – 2000) the number of harsh winters is decreasing (0.4 – 0.3/year) due to an increasing the winter mean temperature. Based on these information we can reconstruct 2000 years of winter air temperature variability over the Eastern Europe, with warming winters during the RWP, cool during the Dark Age, warm and dry during the MWP, and very cold winters during the LIA. In conclusion the harsh winters reconstruction can be used as a proxy indicator for the air temperature variability in winter, over the eastern part of Europe. [ABSTRACT FROM AUTHOR]

Clouds represent a critical component of the climate system, with a profound influence on the Earth's radiation budget through their associated feedbacks. Investigations of cloud processes are constrained by the limited temporal extension of measurements and by the imperfections of available data. Furthermore, global climate sensitivity is essentially linked to Pacific sea surface temperature and clouds variations. Here, we apply a variety of statistical methods on two corrected versions of observed total cloud cover datasets, the International Satellite Cloud Climatology Project and the Pathfinder Atmospheres–Extended projects, in order to 1) identify dominant modes of total cloud cover variability, 2) identify and remove the anthropogenic footprint on total cloud cover, and 3) identify the impact of Eastern Pacific El Niño – Southern Oscillation, Central Pacific El Niño – Southern Oscillation and Pacific decadal sea surface temperature variability on total cloud cover.We find that the two flavors of El-Niño explain over 30 % of the global total cloud cover variability on interannual time scales. The associated total cloud cover footprints are found to be robust across the choice of dataset and statistical method. The positive correlation between sea surface temperature and total cloud cover in the Pacific basin indicate that convection plays a dominant role in mediating the influence of the associated sea surface anomalies on the total cloud cover field.The physical relevance of the identified total cloud cover footprints is supported by the consistency of the associated total precipitation rate, sea level pressure/wind patterns and indicates that the satellite cloud data contain valuable information related to Pacific climate variability. [ABSTRACT FROM AUTHOR]

A number of emerging studies use stable oxygen and carbon isotopes from tree rings to reconstruct climate variables such as temperature and precipitation. In the mid latitudes, these key variables are essentially determined by the prevailing large scale atmospheric circulation which can be expressed via indices such as the North Atlantic Oscillation (NAO). However, only a few studies have investigated the relation between stable isotopes from trees and large-scale atmospheric circulation. Here we present ongoing research on the link between large scale atmospheric circulation and stable isotopes based on the European tree ring isotope network ISONET. This network contains stable oxygen and carbon isotope from 26 different sites in Europe over the last 400 years. In this study, empirical orthogonal functions (EOFs) were used to extract the key patterns and the climate information in this dataset. We focus on the first two EOFs of carbon and oxygen stable isotopes. Finally we use correlation and composite analysis in combination with gridded climate datasets, such as climate reanalysis and reconstructions, to assess the large scale summer climate conditions during the last 400 years.Strong links are identified between the investigated stable isotopes and the leading EOF of North Atlantic–European summer mean sea level pressure with its corresponding climate extremes. The frequency of extremes shows an increase from the 17th to the 20th century which may be attributed to the footprint of the Maunder Minimum and the Little Ice Age. In addition to the identified summer signal, a significant relation between oxygen isotopes and winter NAO is found. We propose that this mechanism works via precipitation in winter which subsequently modulates the soil moisture and water storage at the beginning of the growth season. Overall, the results highlight the potential of stable isotopes as an indicator for large-scale atmospheric conditions over Europe. [ABSTRACT FROM AUTHOR]