2.4.1 The transitional climatic zone (Romania) - present day
The Carpathian mountains are located in a transitional climatic zone between central Europe, western Eurasia and the Mediterranean. By their position and the main mor-phological features, they divide the country into two major regions of different climatic characteristics: with colder and wetter climatic conditions in the north, whereas the south is dominated by a drier and warmer climate [Micu et al., 2016]. Winter climate in Romania is strongly affected by the North Atlantic Oscillation (NAO) [Bojariu and Paliu, 2001; Micu et al., 2016], which is characterized by a meridional displacement in the atmospheric pressure between the Icelandic Low and the Azores High [Wanner et al., 2001] and associated with changes in the westerlies across the Atlantic onto Europe [Hurrell, 1995]. For central and northern Europe, NAO + years are associated with relatively humid and mild winters, while NAO − conditions lead to less precipita-tion and colder temperatures during European winter (posiprecipita-tion of atmospheric patterns see figure A.4) [Dai et al., 1997; Trouet et al., 2009]. For the Mediterranean region another teleconnection pattern affects the weather process, the East Atlantic West Russia (EAWR) pattern [Krichak and Alpert, 2005]. Positive trends in the NAO and the EAWR are associated with drier than normal weather conditions in the Mediter-ranean region [Bojariu and Paliu, 2001].
For an inland continental site, like the location of the cave, the dominant control on the isotopic composition of the precipitation is the Rayleigh rainout effect. The air masses follow an eastward progressing rainout trajectory over the European land mass with an isotopic composition of the precipitation, which becomes more depleted if regional temperatures are decreased and vice versa [Rozanski et al., 1993]. The di-rect link of fluid inclusions to drip water and therefore to paleoprecipitations, enables the reconstruction of temperatures based on the relationship between δ18O and ac-cordingly δ2H in precipitation and air temperature for continental climate [Rozanski
2.4 Climatological setting of the three different study sites concerning the time scale of the speleothem records
et al., 1992]. For Hungary, Demény et al. [2017] showed that the application of the δ2H/T relationship on stable isotope measurements of fluid inclusions led to reliable temperature reconstructions for the last interglacial. At present, there are indications of progressive climate change from global observation data sets, which suggests visible changes in the average values or the overall variability of climate characteristics. In general, Europe has shown a greater warming trend since 1979 compared to the global mean and the related climate trends in mountainous regions are even more pronounced [Böhm et al., 2010].
2.4.2 Central Europe (Germany) - Holocene
The weather system of central Germany (location of the Bunker Cave) is, like most of the climate in Europe, determined by the NAO (see section 2.4.1), which dominates the mid - latitude westerly wind systems and thus influences the precipitation and temper-ature pattern [Hurrell, 1995]. The present interglacial, the Holocene, covers the period of the last 11 700 years and marks the rapid transition from the cold period (Younger Dryas) to a subsequent, generally warmer period with relatively small temperature variations [Dansgaard et al., 1989; Mayewski et al., 2004]. For the Holocene on the multidecadal to multicentury timescale, periods of more stable and warmer climate were interrupted by several cold relapses. These cold interruptions were most likely favoured by decreasing solar insolation combined with a possible slowdown of the ther-mohaline circulation and in some cases also combined with a series of tropical volcanic eruptions [Wanner et al., 2011]. One pronounced cooling event during the Holocene is the 8.2 ka event, which was triggered by cooler conditions in the North Atlantic due to a slowdown of the thermohaline circulation [Fohlmeister et al., 2012]. For the Atlantic episode of the Holocene (9.6 - 5.5 ka), [Niggemann et al., 2003b] found an increased stalagmite growth for a cave in central Germany, which is associated with variations in precipitation. The mainly wet and warmer period of the Holocene was interrupted by colder and drier conditions. Stalagmites offer the possibility to study variation in precipitation with the analysis of stable isotopes in fluid inclusions, but are biased towards autumn and winter in Central Europe, as the drip sites are mainly active in winter due to evapotranspiration in summer [Wackerbarth et al., 2010].
2.4.3 The tropical west Atlantic (Puerto Rico) - millennial timescale Puerto Rico, as the easternmost island of the Greater Antilles, is a tropical island in the north - eastern Caribbean and lies between the high pressure cell over the At-lantic and the Inter - Tropical Convergence Zone (ITCZ). The surface temperature of the sea, which averages around 27 °C has a direct effect on the climate through large amounts of latent and sensible heat transfer throughout the year [Granger , 1985]. This ocean - atmosphere coupling is of great importance for the climate in the Caribbean since the area is dominated by sea surface and the numerous islands are not large
enough to cause strong climatic fluctuations. The climate is, therefore, predomi-nantly maritime, which nowadays leads to comparatively small temperature variations [Schellekens et al., 2004]. The seasonal weather patterns for the Caribbean region are broadly divided into dry winter (December March) and rainy summer (June -November) seasons [Grist, 2002] with the climate strongly dominated by easterly trade winds especially in the winter months [Scholl et al., 2009]. Thereby, the rain isotopic signature of precipitation differ between rain and dry season due to a varying source or different climate patterns. The dry season in the Caribbean is characterized by orographic precipitation from frontal system rain (cold fronts from North America, trade winds and sea breeze showers) which occasionally pass over the island. In con-trast, during the rainy season convective rainfall, which originates from the passage of tropical easterly wave and is formed by low pressure system (tropical storms and hurricanes) accounts for half of the total precipitation [Scholl and Murphy, 2014].
On millennial timescale, rainfall variability in the tropical regions of the western At-lantic dependence on the latitudinal position of the ITCZ [Lachniet et al., 2009], which effects the strength and track of easterly trade winds. For the tropics, the increased input of freshwater into the North Atlantic and the weakening of the AMOC (Atlantic Meridional Overturning Circulation) during Heinrich events led to a southward shift of the ITCZ. The consequences are colder and drier climate conditions for westerly Atlantic tropics during stadials as has been shown in various paleostudies of stalag-mites, ocean sediment or lake sediment cores [Lachniet et al., 2009; Hodell et al., 2012;
Grauel et al., 2016; Escobar et al., 2012; Deplazes et al., 2013; Arienzo et al., 2015].
In contrast, a northward shift of the ITCZ is expected during the interstadials, which will be accompanied by warmer and wetter climatic conditions [Deplazes et al., 2013].
It is questionable how pronounced these rapid climatic fluctuations from cold (stadial) to warmer (interstadial) conditions appear in the Caribbean tropics, since in some stalagmite records no characteristic of D/O events can be found, although HS events are very clearly represented [Carolin et al., 2013; Arienzo et al., 2017].