1. Introduction
Climate change-related drought events are becoming increasingly frequent and extended [
1]. The influence of climate change on drought variability is particularly significant in drought-prone regions, such as the Mediterranean Basin, a biodiversity hotspot where warming and increasing droughts frequency are expected over the 21st century [
2]. The Mediterranean mountains have a complex climatology due to the situation of transition between arid climates in the south to temperate climates in northern forests [
3,
4,
5,
6]. Thus, the predicted changes in drought trends are linked to shifting atmospheric circulation patterns with contrasting impacts at the regional scale, such as the North Atlantic Oscillation (NAO), one of the main atmospheric circulation patterns affecting the surface climate of continental Europe and North Africa [
3,
4,
5,
6].
Present and future paleoclimate research should therefore focus more on regional climatic and environmental responses to global or hemispherical changes, as well as on changes specific to the past hydrological cycle [
7]. The calibration of future climate scenarios for the Mediterranean region is challenging because local and regional climate fluctuations exhibit a high degree of spatial and temporal variability due to the complex relationships of synoptic nodes [
8] and the contrasting seasonal patterns along an east–west Mediterranean dipole [
9,
10,
11]. Large-scale climate indices can help to explain temporal climate variability (e.g., monthly temperature and precipitation) and its associations with local ecological processes [
12]. The influence of large-scale atmospheric circulation patterns were discussed in depth for Mediterranean forests [
13,
14,
15]; however, the connection among forest growth, drought variability, and the circulation patterns across the Mediterranean forests is rarely considered [
10,
11].
In order to provide a more accurate measurement of the local influence of the atmospheric mechanisms driving drought variability across Europe [
16], here, we analyze the impact of NAO, the oscillation of an air mass between the Artic and the subtropical Atlantic, that modulates winter and spring precipitation and temperature across the Western Mediterranean [
17,
18,
19] and the Westerly Index (WI), which measures the frequency of the days with westerly winds over the English Channel with records back to the 17th century [
20], via the Standardized Precipitation–Evapotranspiration Index (SPEI), which is a composed drought index obtained from different time scales [
21]. Previous studies showed that the WI shows a strong signal on local precipitation and temperature and that NAO and SPEI are complementary to large spatial patterns of drought [
16,
20,
22]. However, the heterogeneous climatology of the Mediterranean Basin reveals contrasting eastern and western drought patterns and their influence on regional hydrological cycles [
9,
11]. For this reason, a better understanding of the variability of large and local mechanisms of drought variability in the Mediterranean is needed in order to improve the resolution of forecasts of the climate change impacts on the Mediterranean Basin [
23].
However, to define more accurate future scenarios of drought (SPEI), NAO, WI, and local climate data are needed for calibration to account for low-frequency (multidecadal) variability (cf. [
16]). Tree-ring width (TRW) series are used as a proxy to extend back in time the influence of local climate conditions, NAO signal, and drought variability at a high resolution [
19,
24,
25,
26]. In fact, recent dendroclimatological studies in the Mediterranean basin have demonstrated a strong coherence between TRW series and large-scale atmospheric circulation patterns, although these studies are largely restricted to the last few centuries and some regions [
7,
9,
27,
28]. In this sense, disentangling the controls of instrumental atmospheric indices (e.g., NAO, WI) on long historical drought variability in the Mediterranean area and their associated climate variables is highly desirable to obtain high-resolution and annually resolved climate reconstructions based on long-term dendroclimatological proxies [
29].
In this study, we evaluated for the first time the potential of TRW for dendroclimatic reconstructions across the Mediterranean Basin of the interconnected large-scale atmospheric circulation indices (NAO, WI) and drought (SPEI) [
5]. Our aims are: (1) to compare the regional climate, NAO, WI, and SPEI signals contained in tree-ring width series across the Mediterranean area, (2) to analyze the temporal stability and changes in the spatial pattern of these relationships over the 20th century, and (3) to explore the potential of these TRW series for dendroclimatic reconstructions of WI, NAO, and drought (SPEI). We hypothesized that western forests subjected to the increasingly drier conditions [
10] will be the most responsive to NAO and WI indices, whereas eastern forests will be most sensitive to drought variability and spring precipitation. To test these hypotheses, we examined climate–growth associations and their variability across the Mediterranean Basin.
4. Discussion
Regional climatic changes affect societies and natural ecosystems, endorsing the basis for efficient adaptation measures [
1]. Several studies support that regional climates are the result of the interaction of large-scale dynamics with the orography and physical properties at the regional and local scales. Notwithstanding, improving our knowledge about regional and local climate linkages, including extremes, remains highly relevant [
7,
51]. Here, we obtained reliable information on the amplitude and changes of past climate dynamics at regional scales using tree rings in relation with drought variability (SPEI) and atmospheric circulation patterns (NAO, WI). Despite significant recent research efforts, detailed interactions among regional drought variability and atmospheric patterns are not sufficiently understood yet [
8]. Furthermore, the analysis of long-term climate series at large spatial scales provides relevant information on climate variability and trends in relation to climate change [
1,
52,
53]. Since the climate of the Mediterranean Basin is characterized by seasonal drought during the dry and warm summer conditions [
54,
55], this region is potentially sensitive to climate change, with periods of water shortage negatively affecting the tree growth and dynamics of forest ecosystems, as well as to several socio-economic implications within and beyond the region [
56,
57]. Our results showed higher drought sensitivity in the western areas, compared to the eastern part, according to significant tree growth sensitivity due to siring drought intensity caused by recent changes on large atmospheric oscillations patterns [
11].
Since the Mediterranean Basin’s climate is modulated by atmospheric patterns [
7], their changes influence the local climate and drought conditions [
58]. In terms of the modulation of the temperature, positive phases of the winter NAO show a high correlation with cold air temperatures in the Southwestern Mediterranean area, while the opposite response is observed during negative NAO phases [
3,
4]. Similarly, the Central-eastern European and Northern Turkish climates showed positive associations with NAO, while the NAO influenced the winter temperatures in Northern Italy (
Figure 2) [
20,
24,
58]. Several studies showed positive relationships between winter NAO and lower precipitation in the Western and Northern Mediterranean areas [
13,
59]. Conversely, the NAO presented weak relationships during other seasons, except for the NAO changes which translate into variations in precipitation in the Southern Mediterranean [
7,
11,
13,
23,
58].
The influence of NAO on drought severity (SPEI) and its relationships with the WI was confirmed in the Mediterranean Basin [
23]. The WI index accounts for climate variability during the summer season and higher temperatures during winter in Western Europe (
Figure 2). A positive WI was related with lower temperatures in Northern Europe and warmer conditions in the Eastern Mediterranean area [
20]. However, high frequency of westerly winds (positive WI) increases cloudiness and humidity in Northern Europe, with decreasing precipitation and humidity towards the Mediterranean area. Winter and summer seasons show an extended decrease of precipitation, covering the west and east of the Mediterranean Basin, while decreasing spring precipitation prevails mainly in the west of the Mediterranean [
20,
23]. Many previous studies showed the relationships between NAO and drought in Southern Europe and North Africa [
23], although the WI index captures a similar signal to the NAO during summer and winter seasons, especially during dry spring–summer seasons in Western and Central Europe [
23]. While the NAO is a pattern that considers wind strength, the WI is an index that only considers the prevalence of westerly winds [
20]. This difference is important since it has been proven that the WI, unlike the NAO, is a better indicator of precipitation with robustness throughout the year in Northern and Central Europe. However, the WI presents difficulty to capture temperature anomalies that are determined by the advection and intensity of air masses, whereas NAO can capture it more easily. Nevertheless, our study is the first attempt to understand the combined influence of these patterns in the effects of drought across Mediterranean ecosystems. As [
20] showed, NAO and WI have many similarities in behavior and effects on the European continent and can complement each other to improve our knowledge on both short- and long-term drought effects across European forests [
23].
Based on our findings about the relationships among tree-ring data (TRWi) and NAO, WI, and SPEI, we conclude that drought, as reflected by the SPEI, is the main limiting factor of tree growth in agreement with previous regional studies [
60]. This pattern has been already observed in different studies showing higher correlation between SPEI and tree growth variation than NAO or other atmospheric patterns [
11,
12,
16]. This result might be related to the temporal scale of drought. Hence, while SPEI is able to capture short-term weather conditions and extreme weather events more accurately when seasonal droughts occur, NAO and WI reflect large spatial and temporal atmospheric patterns, mainly sensitive to intra-annual periods, such as winter, where tree growth stops and drought conditions remain low [
10,
13,
15].
An important issue raised by our study is the lack of stationary relationships among NAO, WI, and the climate sensitivity observed in the different tree species (TRWi) (
Figure 7). It is important to note that, although the correlations might be significant for the entire time period, decreasing correlations may be present at some shorter time spans, where the relationship is weakened. Here, sequential coupling and de-coupling of climate sensitivity could be related to contrasting and sometimes opposite local environmental responses over time and also among nearby forest ecosystems during a given time period [
11]. Non-stationary climate sensitivity relies on ecosystem development and its complex response to climate changes, even in the same region, supporting that these responses are highly influenced by the landscape and the environmental evolution of the area [
58,
61,
62]. In addition, these shifting environmental responses might be amplified in our tree species during the last centuries, suggesting a different resilience and sensitivity to climate variations between the studied mountain forest ecosystems [
2,
11,
53]. Anthropogenic influences should be taken into account, as they also appear to modulate the effects of solar and atmospheric cycles in the tree rings’ environmental signals [
62]. The obtained climate sensitivity of the Mediterranean forests studied here provides paleoenvironmental records likely influenced by several drivers, including climate variations but also human-induced environmental changes.
However, our results showed that depending on the Mediterranean zone, the inter-annual NAO variations can explain large-scale growth variability (
Figure 5). For the western area, we found higher associations with TRWi for autumn and winter NAO for
A. pinsapo and
P. nigra (
Figure 6). These results are consistent with other studies that associate an increase in the temperatures during the previous winter with an increase in the growth rates due to an early bud break [
63]. This relationship has been proven in studies that show how the NAO strongly modulates the inter-annual variability of precipitation in the Western Mediterranean [
62]. This situation explains the negative impact of spring NAO in
C. atlantica that translates into a lower soil water reserve and availability, which acts as a limiting factor in the tree radial growth during the growing season. In contrast, a positive NAO phase in summer increases growth rates in Central Europe [
64]. Regarding the Eastern Mediterranean region, the summer NAO conditions influence water availability during the growing season [
10,
16,
23], as does the autumn NAO, consequently affecting tree-ring growth of eastern forests [
10,
62].
Regarding the relationship between tree growth and WI, we observed higher correlations with Western Mediterranean forests (
Figure 6). For instance,
P. nigra and
A. pinsapo showed positive and negative WI signals in tree growth during winter, summer, and spring seasons, respectively (
Figure 6)—periods in which there was an increase in temperature and a decrease in precipitation in western regions [
11]. In this case, the increase in temperature is related to the onset of growing season of
P. nigra and
C. atlantica (
Figure 3). However, autumn WI negatively affected the tree growth of
C. atlantica in relation with the influence of WI on the precipitation and temperature patterns (
Figure 2 and
Figure 6). The autumn WI was related with drought that consequently affected the growth [
5,
13]. On the other hand, the relationships between WI and growth in Eastern Mediterranean forests were higher with previous winters (
A. cilicica and
C. libani) and March and July in
P. nigra (
Figure 6). This relationship with winter WI was consistent with drought–growth relationships in this area. Our results showed the ability of the WI to explain how the tree growth in the Central Mediterranean area is mainly limited by spring conditions; with a positive effect on growth during autumn. This finding is consistent with those obtained in other studies on the WI, where we can see how its potential is located in Western and Northern Europe, presenting difficulties in explaining the behavior of droughts in Southern Europe [
23].
The combination of SPEI, WI, and NAO allows us to study the effects of large-scale atmospheric patterns in the regional growth of tree species in drought-prone Mediterranean areas. While the NAO is better related with large-scale drought periods, the WI allows us to study shorter drought effects, having a stable seasonal influence on drought severity [
23]. This combination of characteristics between both atmospheric patterns seems promising for a better understanding and reconstruction of drought periods in the Western Mediterranean area (
Table 5). The potential of using TRWi as a proxy in annual climate reconstructions has been proven in numerous studies [
65,
66]. However, to our knowledge, this is the first study using TRWi information to understand and reconstruct long-term regional climate patterns in the Mediterranean Basin related with the WI. Our results evidence a strong connection between drought and large atmospherics patterns, especially the interconnected WI and NAO, which is reflected in annual tree-ring chronologies (
Table 5). Although tree-ring information from the Mediterranean basin is not scarce, many of the currently available chronologies do not cover centuries to address long-term climate reconstructions, due to the scarce long-lived trees [
67]. The oldest trees in our study (
C. atlantica) were found in the Atlas Mountains in Morocco [
24,
67]. This species showed the highest correlations between TRWi and summer precipitation in the Western Mediterranean [
68], while negative correlations between TRWi chronologies and summer precipitation were obtained from some Eastern Mediterranean mountains [
11], which have been demonstrated to reflect a distinct temperature signal, most likely reflecting the inverse relationship between summer temperature and precipitation (
Figure 5). Nonetheless, Eastern Mediterranean drought responses are also available, for instance, in Southern Turkey [
33,
69]. Hence, the use of multiple species with different intra-annual periods of climate–growth sensitivity provides complementary information. Even with these shortcomings, TRWi data in our study proved to reconstruct local climatic variables and to detect significant relationships with large-scale atmospheric oscillations such as NAO or WI [
23].
Overall, our results support significant relationships among Mediterranean tree-ring chronologies and the SPEI3, in relation with atmospheric oscillations (
Table 5), suggesting that TRWi datasets may be widely suitable proxies to perform reliable reconstructions of the spatial and temporal patterns of drought frequency and intensity in relation with NAO and WI oscillations in the Western Mediterranean area. Thus, despite the NAO and WI presenting periods related to tree growth, the reliability seems much lower than that of the SPEI. Then, although we obtained limited adjustment, the use of drought-sensitive tree-ring chronologies would improve the suitability of TRWi to reconstruct millennial WI patterns. In summary, our understanding of variations in short-duration and extensive drought across the Mediterranean basin is still limited, as a small number of well-dated high-temporal-resolution proxies are available [
7] and due to the non-stationary relationships among TRWi, NAO, WI, and SPEI (
Figure 7). In this context, in order to improve climate reconstructions, further studies could identify new potential long-term tree-ring series with higher sensitivity to regional-scale circulation patterns, such as WI and NAO indices [
70]. Despite the limitations of our results using TRWi, this is a first attempt to understand the spatiotemporal associations among drought patterns and the interconnected NAO and WI oscillations over the Mediterranean Basin, although these relationships should be considered carefully given the relative level of non-stationarity (
Figure 7). Further research is needed to understand past and future spatiotemporal variability of the tree growth associations with NAO and WI.