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The variability of stable oxygen isotope ratios (δ18O) from Greenland ice cores is commonly linked to changes in local climate and associated teleconnection patterns. In this respect, in this study we investigate ice core δ18O variability from a synoptic scale perspective to assess the potential of such records as proxies for extreme climate variability and associated weather patterns. We show that positive (negative) δ18O anomalies in three southern and central Greenland ice cores are associated with relatively high (low) Rossby Wave Breaking (RWB) activity in the North Atlantic region. Both cyclonic and anticyclonic RWB patterns associated with high δ18O show filaments of strong moisture transport from the Atlantic Ocean towards Greenland. During such events, warm and wet conditions are recorded over southern, western and central part of Greenland. In the same time the cyclonic and anticyclonic RWB patterns show enhanced southward advection of cold polar air masses on their eastern side, leading to extreme cold conditions over Europe. The association between high δ18O winters in Greenland ice cores and extremely cold winters over Europe is partly explained by the modulation of the RWB frequency by the tropical Atlantic sea surface temperature forcing, as shown in recent modeling studies. We argue that δ18O from Greenland ice cores can be used as a proxy for RWB activity in the Atlantic European region and associated extreme weather and climate anomalies. [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]

• We provide fundamentals of climate seasonality and break it down into external and internal forcing, and regional and local and modulating factors. • We present a transdisciplinary overview on how seasonality is recorded in, and extracted from, different palaeonvironmental archives. • We propose a framework for transparent communication of seasonality-related research across different communities. The role of seasonality is indisputable in climate and ecosystem dynamics. Seasonal temperature and precipitation variability are of vital importance for the availability of food, water, shelter, migration routes, and raw materials. Thus, understanding past climatic and environmental changes at seasonal scale is equally important for unearthing the history and for predicting the future of human societies under global warming scenarios. Alas, in palaeoenvironmental research, the term ' seasonality change ' is often used liberally without scrutiny or explanation as to which seasonal parameter has changed and how. Here we provide fundamentals of climate seasonality and break it down into external (insolation changes) and internal (atmospheric CO 2 concentration) forcing, and regional and local and modulating factors (continentality, altitude, large-scale atmospheric circulation patterns). Further, we present a brief overview of the archives with potentially annual/seasonal resolution (historical and instrumental records, marine invertebrate growth increments, stalagmites, tree rings, lake sediments, permafrost, cave ice, and ice cores) and discuss archive-specific challenges and opportunities, and how these limit or foster the use of specific archives in archaeological research. Next, we address the need for adequate data-quality checks, involving both archive-specific nature (e.g., limited sampling resolution or seasonal sampling bias) and analytical uncertainties. To this end, we present a broad spectrum of carefully selected statistical methods which can be applied to analyze annually- and seasonally-resolved time series. We close the manuscript by proposing a framework for transparent communication of seasonality-related research across different communities. [ABSTRACT FROM AUTHOR]