Precipitation changes in the Mediterranean basin

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Clim. Past, 13, 249265, 2017 www.clim-past.net/13/249/2017/ doi:10.5194/cp-13-249-2017 Author(s) 2017. CC Attribution 3.0 License.

Odile Peyron1, Nathalie Combourieu-Nebout2, David Brayshaw3, Simon Goring4, Valrie Andrieu-Ponel5, Stphanie Desprat6,7, Will Fletcher8, Belinda Gambin9, Chryssanthi Ioakim10, Sbastien Joannin1, Ulrich Kotthoff11, Katerina Kouli12, Vincent Montade1, Jrg Pross13, Laura Sadori14, and Michel Magny15

1Institut des Sciences de lEvolution (ISEM), Universit de Montpellier, France

2UMR 7194 MNHN, Institut de Palontologie Humaine, Paris, France

3Department of Meteorology, University of Reading, Reading, UK

4Department of Geography, University of Wisconsin-Madison, Wisconsin, USA

5Institut Mditerranen de Biodiversit et dEcologie marine et continentale (IMBE), Aix Marseille Universit, Aix-en-Provence, France

6EPHE, PSL Research University, Laboratoire Paloclimatologie et Paloenvironnements Marins, Pessac, France

7University of Bordeaux, EPOC UMR5805, Pessac, France

8Geography, School of Environment, Education and Development, University of Manchester, Manchester, UK

9Institute of Earth Systems, University of Malta, Msida, Malta

10Institute of Geology and Mineral Exploration, Athens, Greece

11Center for Natural History and Institute of Geology, Hamburg University, Hamburg, Germany

12Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Athens, Greece

13Paleoenvironmental Dynamics Group, Institute of Earth Sciences, Heidelberg University, Heidelberg, Germany

14Dipartimento di Biologia Ambientale, Universit di Roma La Sapienza, Rome, Italy

15Universit de Franche-Comt, UMR6249 Chrono-Environnement, Besanon, France

Correspondence to: Odile Peyron ([email protected])

Received: 16 June 2016 Discussion started: 28 June 2016

Revised: 20 January 2017 Accepted: 25 January 2017 Published: 24 March 2017

Abstract. Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability, which is notoriously difcult for models to reproduce. We propose here a new proxy-based climate synthesis synthesis and its comparison at a regional ( 100 km)

level with a regional climate model to examine (i) opposing northern and southern precipitation regimes and (ii) an east-to-west precipitation dipole during the Holocene across the Mediterranean basin. Using precipitation estimates inferred from marine and terrestrial pollen archives, we focus on the early to mid-Holocene (8000 to 6000 cal yr BP) and the late Holocene (4000 to 2000 cal yr BP), to test these hypotheses on a Mediterranean-wide scale. Special attention was given to the reconstruction of season-specic climate in-

Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a modeldata comparison

formation, notably summer and winter precipitation. The reconstructed climatic trends corroborate the northsouth partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the southcentral and eastern Mediterranean regions, while drier conditions prevailed from 45 N northwards. These patterns then reverse during the late Holocene. With regard to the existence of a westeast precipitation dipole during theHolocene, our results show that the strength of this dipole is strongly linked to the reconstructed seasonal parameter; early-Holocene summers show a clear eastwest division, with summer precipitation having been highest in Greece and the eastern Mediterranean and lowest over Italy and the western Mediterranean. Summer precipitation in the east re-

Published by Copernicus Publications on behalf of the European Geosciences Union.

250 O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene

mained above modern values, even during the late-Holocene interval. In contrast, winter precipitation signals are less spatially coherent during the early Holocene but low precipitation is evidenced during the late Holocene. A general drying trend occurred from the early to late Holocene, particularly in the central and eastern Mediterranean.

For the same time intervals, pollen-inferred precipitation estimates were compared with model outputs, based on a regional-scale downscaling (HadRM3) of a set of global climate-model simulations (HadAM3). The high-resolution detail achieved through the downscaling is intended to enable a better comparison between site-based paleo-reconstructions and gridded model data in the complex terrain of the Mediterranean; the model outputs and pollen-inferred precipitation estimates show some overall correspondence, though modeled changes are small and at the absolute margins of statistical signicance. There are suggestions that the eastern Mediterranean experienced wetter summer conditions than present during the early and late Holocene; the drying trend in winter from the early to the late Holocene also appears to be simulated. The use of this high-resolution regional climate model highlights how the inherently patchy nature of climate signals and paleo-records in the Mediterranean basin may lead to local signals that are much stronger than the large-scale pattern would suggest.Nevertheless, the east-to-west division in summer precipitation seems more marked in the pollen reconstruction than in the model outputs. The footprint of the anomalies (like today, or dry winters and wet summers) has some similarities to modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive Arctic OscillationNorth Atlantic Oscillation (AONAO)) and a weak westerly circulation in summer associated with anticyclonic blocking; however, there also remain important differences between the paleo-simulations and these analogues.The regional climate model, consistent with other global models, does not suggest an extension of the African summer monsoon into the Mediterranean. Therefore, the extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question.

1 Introduction

The Mediterranean region is particularly sensitive to climate change due to its position within the conuence of arid northern African (i.e., subtropically inuenced) and temperate and/or humid European (i.e., midlatitudinal) climates (Lionello, 2012). Palaeoclimatic proxies, including stable isotopes, lipid biomarkers, palynological data, and lake levels, have shown that the Mediterranean region experienced climatic conditions that varied spatially and temporally throughout the Holocene (e.g., Bar-Matthews and Ayalon, 2011; Luterbacher et al., 2012; Lionello, 2012; Trianta-

phyllou et al., 2014, 2016; Mauri et al., 2015; De Santis and Caldara, 2015; Sadori et al., 2016a; Cheddadi and Khater, 2016) and well before (eg. Sadori et al., 2016b). Clear spatial climate patterns have been identied from east to west and from north to south within the basin (e.g., Zanchetta et al., 2007; Magny et al., 2009b, 2011, 2013; Zhornyak et al., 2011; Sadori et al., 2013; Fletcher et al., 2013). Lake-level reconstructions from Italy thus suggest contrasting patterns of paleo-hydrological changes for the central Mediterranean during the Holocene (Magny et al., 2012, 2013). Specically, lake-level maxima occurred south of approximately 40 N in the early to mid-Holocene, while lakes north of 40 N recorded minima. This pattern was reversed at around 4500 cal yr BP (Magny et al., 2013). Quantitative pollen-based precipitation reconstructions from sites in northern Italy indicate humid winters and dry summers during the early to mid-Holocene, whereas southern Italy was characterized by humid winters and summers; the NS pattern reverses in the late Holocene, with drier conditions at southern sites and wet conditions at northern sites (Peyron et al., 2011, 2013). These ndings support a northsouth partition for the central Mediterranean with regards to precipitation, and they also conrm that precipitation seasonality is a key parameter in the evolution of Mediterranean climates. The pattern of shifting NS precipitation regimes has also been identied for the Aegean Sea (Peyron et al., 2013). Taken together, the evidence from pollen data and from other proxies covering the Mediterranean region suggest a climate response that can be linked to a combination of orbital, ice-sheet, and solar forcings (Magny et al., 2013).

An eastwest pattern of climatic change during the Holocene is also suggested in the Mediterranean region (e.g., Combourieu Nebout et al., 1998; Geraga et al., 2010;Colmenero-Hildago et al., 2002; Kotthoff et al., 2008; Dormoy et al., 2009; Finn et al., 2011; Roberts et al., 2011, 2012; Luterbacher et al., 2012; Guiot and Kaniewski, 2015).An eastwest division during the Holocene is observed from marine and terrestrial pollen records (Dormoy et al., 2009; Guiot and Kaniewski, 2015), lake-level reconstructions (Magny et al., 2013), and speleothem isotopes (Roberts et al., 2011).

This study aims to reconstruct and evaluate NS and EW precipitation patterns for the Mediterranean basin over two key periods in the Holocene, the early Holocene at 8000 6000 cal yr BP, corresponding to the Holocene climate optimum, and the late Holocene at 40002000 cal yr BP, corresponding to a trend towards drier conditions. Precipitation reconstructions are particularly important for the Mediterranean region given that precipitation rather than temperature represents the dominant controlling factor on the Mediterranean environmental system during the early to mid-Holocene (Renssen et al., 2012). Moreover, the reconstruction of precipitation parameters seems robust for the Mediterranean area (Combourieu-Nebout et al., 2009; Mauri et al., 2015; Peyron et al., 2011, 2013; Magny et al., 2013).

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O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene 251

Precipitation is estimated for ve pollen records from Greece, Italy, and Malta and for eight marine pollen records along a longitudinal gradient from the Alboran Sea to the Aegean Sea. Because precipitation seasonality is a key parameter of change during the Holocene in the Mediterranean (Rohling et al., 2002; Peyron et al., 2011; Mauri et al., 2015), the quantitative climate estimates focus on reconstructing changes in summer and winter precipitation.

Paleoclimate proxy data are essential benchmarks for model intercomparison and validation (e.g., Morrill et al., 2012; Heiri et al., 2014). This holds particularly true considering that previous modeldata intercomparisons have revealed substantial difculties for general circulation models (GCMs) in simulating key aspects of mid-Holocene climate (Hargreaves et al., 2013) for Europe and notably for southern Europe (Davis and Brewer, 2009; Mauri et al., 2014). We also aim to identify and quantify the spatiotemporal climate patterns in the Mediterranean basin for the two key intervals of the Holocene (80006000 and 4000 2000 cal yr BP) based on regional-scale climate model simulations (Brayshaw et al., 2011a). Finally, we compare our pollen-inferred climate patterns with regional-scale climate model simulations in order to critically assess the consistency of the climate reconstructions revealed by these two complimentary routes.

The rst originality of our approach is that we estimate the magnitude of precipitation changes and reconstruct climatic trends across the Mediterranean using both terrestrial and marine high-resolution pollen records. The reconstructed signal is then more regional than in the studies based on terrestrial records alone. Moreover, this study aims to reconstruct precipitation patterns for the Mediterranean basin over two key periods in the Holocene, while the existing large-scale quantitative paleoclimate reconstructions for the Holocene are often limited to the mid-Holocene (6000 yr BP; Cheddadi et al., 1997; Bartlein et al., 2011; Mauri et al., 2014), except for the climate reconstruction for Europe proposed by the study of Mauri et al. (2015).

The second originality of our approach is that we propose a datamodel comparison based on (1) two time slices instead of just the mid-Holocene, a standard benchmark time period for this kind of datamodel comparison; (2) a high resolution regional model (RCM), which provides a better representation of local and regional processes and helps to better simulate the localized patchy impacts of Holocene climate change when compared to coarser global GCMs (e.g., Mauri et al., 2014); and (3) changes in seasonality, particularly changes in summer atmospheric circulation that have not been widely investigated (Brayshaw et al., 2011).

2 Sites, pollen records, and models

The Mediterranean region is at the conuence of continental and tropical air masses. Specically, the central and eastern

Mediterranean is inuenced by monsoonal systems, while the northwestern Mediterranean is under stronger inuence from midlatitude climate regimes (Lionello et al., 2006).Mediterranean winter climates are strongly affected by storm systems originating over the Atlantic. In the western Mediterranean, precipitation is predominantly affected by the North Atlantic Oscillation (NAO), while several systems interact to control precipitation over the northern and eastern Mediterranean (Giorgi and Lionello, 2008). Mediterranean summer climates are dominated by descending high-pressure systems that lead to dry and hot conditions, particularly over the southern Mediterranean where climate variability is strongly inuenced by African and Asian monsoons (Alpert et al., 2006), with strong geopotential blocking anomalies over central Europe (Giorgi and Lionello, 2008; Trigo et al., 2006).

The palynological component of our study combines results from ve terrestrial and eight marine pollen records to provide broad coverage of the Mediterranean basin (Fig. 1, Table 1). The terrestrial sequences comprise pollen records from lakes along a latitudinal gradient from northern Italy (Ledro and Accesa lakes) to Sicily (Lake Pergusa), one pollen record from Malta (Burmarrad), and one pollen record from Greece (Tenaghi Philippon). The marine pollen sequences are situated along a longitudinal gradient across the Mediterranean Sea: from the Alboran Sea (ODP Site 976 and core MD95-2043), SiculoTunisian strait (core MD04-2797), Adriatic Sea (core MD90-917), and Aegean Sea (cores SL152, MNB-3, NS14, HCM2/22). For each record we used the chronologies as reported in the original publications (see Table 1 for references).

Climate reconstructions for summer and winter precipitation (Figs. 2 and 3), inferred from the terrestrial sequences and marine pollen records, were performed for two key intervals of the Holocene: 80006000 and 40002000 cal yr BP; the climate values available during each period have been averaged. We use here the modern analogue technique (MAT;Guiot, 1990), a method that compares fossil pollen assemblages to modern pollen assemblages with known climate parameters. The MAT is calibrated using an expanded surface pollen dataset with more than 3600 surface pollen samples from various European ecosystems (Peyron et al., 2013).In this dataset, 2200 samples are from the Mediterranean region, and the results show that the analogues selected here are limited to the Mediterranean basin. Since the MAT uses the distance structure of the data and essentially performs local tting of the climate parameter (as the mean of n closest sites), it may be less susceptible to increased noise in the dataset and less likely to report spurious values than others methods (for more details on the method, see Peyron et al., 2011). Pinus is overrepresented in marine pollen samples (Heusser and Balsam, 1977; Naughton et al., 2007), and as such Pinus pollen was removed from the assemblages (both modern and fossil) for the calibration of marine records using MAT. The reliability of quantitative climate reconstructions from marine pollen records has been tested using marine

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252 O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene

Ledro

(978m)

P mm

127 - 971

100

0 J D

P mm

MD 90-917 159 - 616

100

0 J D

Tenaghi PhiliSpon (40 m)

147 - 568

P mm

100

0 J D

P mm

SL 152 165 - 612

100

0 J D

P mm

100

0 J D

P mm

100

P mm

NS 14

100

0 J D

P mm

100

P mm

100

P mm

HCM2-22 187 - 649

100

0 J D

P mm

MD04-2797 185 - 464

100

0 J D

P mm

Burmarrad

(0.5 m)

186 - 512

100

0 J D

Pergusa

(667 m)

172 - 599

P mm

100

0 J D

used to perform a series of time slice experiments. Each time slice simulation corresponds to 20 model years after spin-up (40 model years for preindustrial). The time slices correspond to present day (19601990), 2000, 4000, 6000, and 8000 cal BP conditions and are forced with appropriate insolation (associated with changes in the Earths orbit) and atmospheric CO2 and CH4 concentrations. The heat uxes in the ocean are held xed using values taken from a preindustrial control run (i.e., the ocean circulation is assumed to be invariant over the time slices) and there is no sea-level change, but sea-surface temperatures are allowed to evolve freely. The coarse global output from the model for each time slice is downscaled over the Mediterranean region using HadRM3 (i.e., a limited-area version of the same atmospheric model; resolution 0.44 [notdef] 0.44 , with 19 vertical lev-

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Figure 1. Locations of terrestrial and marine pollen records along a longitudinal gradient from west to east and along a latitudinal gradient from northern Italy to Malta. Ombrothermic diagrams are shown for each site, calculated with the New LocClim software program and database, which provides estimates of average climatic conditions at locations for which no observations are available (marine pollen cores, for example).

core-top samples from the Mediterranean in Combourieu-Nebout et al. (2009), which shows an adequate consistency between the present day observed and MAT estimations for annual and summer precipitation values; however, the MAT seems to overestimate the winter precipitation reconstructions in comparison with the observed values. More top cores are needed to validate these results at the scale of the Mediterranean basin, particularly in the eastern part where only one marine top core was available (Combourieu-Nebout et al., 2009).

The climate model simulations used in the modeldata comparison are taken from Brayshaw et al. (2010, 2011a, b).The HadAM3 global atmospheric model (resolution 2.5 latitude [notdef] 3.75 longitude, 19 vertical levels; Pope et al., 2000)

is coupled to a slab ocean (HadSM3, Hewitt et al., 2001) and

O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene 253

Table 1. Metadata for the terrestrial and marine pollen records evaluated. The temporal resolution is calculated for the two periods (8000 6000 and 40002000) and for the entire record.

Terrestrial pollen records

Longitude Latitude Elev. Temporal References(m a.s.l) resolution (non-exhaustive)

Ledro (Northern Italy)

10 76[prime] E 45 87[prime] N 652 80006000: 71 40002000: 60 10966-10: 66

Joannin et al. (2013), Magny et al. (2009, 2012a), Vannire et al. (2013), Peyron et al. (2013)

Accesa (Central Italy)

10 53[prime] E 42 59[prime] N 157 80006000: 90 40002000 : 133 11029-100: 97

Drescher-Schneider et al. (2007), Magny et al. (2007, 2013), Colombaroli et al. (2008), Sadori et al. (2011),Vannire et al. (2011),Peyron et al. (2011, 2013)

Trifoglietti (Southern Italy)

16 01[prime] E 39 33[prime] N 1048 80006000: 95 40002000: 86 9967-14: 73

Joannin et al. (2012), Peyron et al. (2013)

Pergusa (Sicily)

14 18[prime] E 37 31[prime] N 667 80006000: 166 40002000: 90 12749-53: 154

Sadori and Narcisi (2001),Sadori et al. (2008, 2011, 2013, 2016b), Magny et al. (2011, 2013)

Tenaghi Philippon (Greece)

24 13.4[prime] E 40 58.4[prime] N 40 80006000: 64 40002000: no 10369-6371:53

Pross et al. (2009, 2015), Peyron et al. (2011), Schemmel et al. (2016)

Burmarrad (Malta)

14 25[prime] E 35 56[prime] N 0.5 80006000: 400 40002000: 285 6904-1730: 110

Djamali et al. (2013), Gambin et al. (2016)

Marine pollen records

Longitude Latitude Water Temporal References depth resolution

ODP 976 (Alboran Sea)

4 18[prime] W 36 12[prime] N 1108 80006000: 142 40002000: 181 10903-132: 129

Combourieu-Nebout et al. (1999, 2002, 2009), Dormoy et al., (2009)

MD95-2043 (Alboran Sea)

2 37[prime] W 36 9[prime] N 1841 80006000: 111

40002000: 142 10952-1279: 106

Fletcher and Snchez Goi (2008), Fletcher et al. (2010)

MD90-917 (Adriatic Sea)

17 37[prime] E 41 97[prime] N 845 80006000: 90

40002000: 333 10495-2641: 122

Combourieu-Nebout et al. (2013)

MD04-2797 (SiculoTunisian strait)

11 40[prime] E 36 57[prime] N 771 80006000: 111

40002000: 666 10985-2215: 127

Desprat et al. (2013)

SL152(Northern Aegean Sea)

24 36[prime] E 40 19[prime] N 978 80006000: 60

40002000: 95 9999-0: 76

Kotthoff et al. (2008, 2011), Dormoy et al. (2009)

NS14(Southern Aegean Sea)

27 02[prime] E 36 38[prime] N 505 80006000: 80

40002000: 333 9988-2570: 107

Kouli et al. (2012), Gogou et al. (2007), Triantaphyllou et al. (2009a, b)

HCM2/22 (Southern Crete)

24 53[prime] E 34 34[prime] N 2211 80006000: 181

40002000: 333 8091-2390: 247

Ioakim et al. (2009), Kouli et al. (2012), Triantaphyllou et al. (2014)

MNB-3(Northern Aegean Sea)

25 00[prime] E 39 15[prime] N 800 80006000: 153

40002000: 166 8209-2273: 138

Geraga et al. (2010), Kouli et al. (2012), Triantaphyllou et al. (2014)

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254 O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene

els). Unlike the global model, HadRM3 is not coupled to an ocean model; instead, sea-surface temperatures are derived directly from the HadSM3 output.

Following Brayshaw et al. (2011a), time slice experiments are grouped into mid-Holocene (80006000 cal yr BP) and late Holocene (40002000 cal yr BP) experiments because(1) these two periods are sufciently distant in the past to be substantially different from the present but close enough that the model boundary conditions are well known and (2) these two periods are rich in high-resolution and well-dated paleoecological sequences, providing a good spatial coverage suitable for large-scale modeldata comparison. The combination of the simulations into two experiments (mid- and late Holocene) rather than assessing the two extreme time slices (2000 and 8000 cal yr BP) is intended to increase the signal-to-noise ratio by doubling the number of data in each experiment. This is necessary and possible since the change in forcing between adjacent time slices is relatively small, making it difcult to detect differences between each individual simulation. To aid comparison with proxies, changes in climate are expressed as differences with respect to the present day (roughly 19601990) rather than the preindustrial control run. Therefore, the climate anomalies shown include a component that is attributable to anthropogenic increases in greenhouse gases in the industrial period, as well as longer-term natural changes (e.g., orbital forcing). We suggest it may be better to use present day to be in closer agreement with the pollen data (modern samples), which use the late 20th century long-term averages (19611990). However, there are some quite substantial differences between model runs under present-day and preindustrial forcings (Fig. 4).Statistical signicance is assessed with the WilcoxonMannWhitney signicance test (Wilks, 1995).

The details of the climate model simulations are discussed at length in Brayshaw et al. (2010, 2011a, b). These include a detailed discussion of verication under the present climate, the models physical and/or dynamical climate responses to Holocene period forcings, and comparison to other palaeo-climate modeling approaches (e.g., PMIP projects) and paleoclimate syntheses. The GCM used (HadAM3 with a slab ocean) is comparable to the climate models in PMIP2, but key advantages of the present dataset are (a) the inclusion of multiple time slices across the Holocene period and (b) that the additional high-resolution regional climate model down-scaling enables the impact of local climatic effects within larger-scale patterns of change to be distinguished (e.g., the impact of complex topography or coastlines; Brayshaw et al., 2011a), potentially allowing clearer comparisons between site-based proxy data and model output.

3 Results and discussion

3.1 A northsouth precipitation pattern?

Pollen evidence shows contrasting patterns of paleohydrological changes in the central Mediterranean. The early to mid-Holocene was characterized by precipitation maxima south of around 40 N, while at the same time, northern Italy experienced precipitation minima; this pattern reverses after 4500 cal yr BP (Magny et al., 2012b; Peyron et al., 2013).Other proxies suggest contrasting northsouth hydrological patterns not only in the central Mediterranean but also across the Mediterranean (Magny et al., 2013), suggesting a more regional climate signal. We focus here on two time periods (early to mid-Holocene and late Holocene) in order to test this hypothesis across the Mediterranean and to compare the results with regional climate simulations for the same time periods.

3.1.1 Early to mid-Holocene (8000 to 6000 cal yr BP)

Climatic patterns reconstructed from both marine and terrestrial pollen records seem to corroborate the hypothesis of a northsouth division in precipitation regimes during the Holocene (Fig. 2a). Our results conrm that northern Italy was characterized by drier conditions (relative to modern), while the southcentral Mediterranean experienced more annual, winter, and summer precipitation during the early to mid-Holocene (Fig. 2a). Only Burmarrad (Malta) shows drier conditions in the early to mid-Holocene (Fig. 2a), although summer precipitation reconstructions are marginally higher than modern precipitation levels at the site. Wetter summer conditions in the Aegean Sea suggest a wetter regional climate signal over the central and eastern Mediterranean. Winter precipitation in the Aegean Sea is less spatially coherent than summer signal, with dry conditions in the northern Aegean Sea and/or near-modern conditions in the southern Aegean Sea (Figs. 2a and 3).

Non-pollen proxies, including marine and terrestrial biomarkers (terrestrial n alkanes), indicate humid mid-Holocene conditions in the Aegean Sea (Triantaphyllou et al., 2014, 2016). Results within the Aegean support the pollen-based reconstructions, but non-pollen proxy data are still lacking at the basin scale in the Mediterranean, limiting our ability to undertake independent evaluation of precipitation reconstructions.

Very few large-scale climate reconstruction of precipitation exist for the whole Holocene (Guiot and Kaniewski, 2015; Tarroso et al., 2016) and, even at local scales, pollen-inferred reconstructions of seasonal precipitation are very rare (e.g., Peyron et al., 2011, 2013; Combourieu-Nebout et al., 2013; Nourelbait et al., 2016). Several large-scale studies focused on the 6000 cal yr BP period (Cheddadi et al., 1997;Wu et al., 2007; Bartlein et al., 2011; Mauri et al., 2014).Wu et al. (2007) reconstructed regional seasonal and annual

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O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene 255

Figure 2. Pollen-inferred climate estimates as performed with the modern analogues technique (MAT): annual precipitation, winter precipitation (winter is the sum of December, January, and February precipitation), and summer precipitation (summer is the sum of June, July, and August precipitation). Changes in climate are expressed as differences with respect to the modern values (anomalies, mm day1). The modern values are derived from the ombrothermic diagrams (see Fig. 1). Two key intervals of the Holocene corresponding to the two time slice experiments (Fig. 3) have been chosen: 80006000 (a) and 40002000 (b) cal yr BP. The climate values available during these periods have been averaged (stars).

precipitation and suggested that precipitation did not differ signicantly from modern conditions across the Mediterranean; however, scaling issues render it difcult to compare their results with the reconstructions presented here. Cheddadi et al. (1997) reconstruct wetter-than-modern conditions at 6000 cal yr BP in southern Europe; however, their study uses only one record from Italy and measures the moisture availability index, which is not directly comparable to precipitation sensu stricto since it integrates temperature and precipitation. At 6000 cal yr BP, Bartlein et al. (2011) reconstruct Mediterranean precipitation at values between 100 and 500 mm higher than modern. Mauri et al. (2015), in an updated version of Davis et al. (2003), provide a quantitative climate reconstruction comparable to the seasonal precipitation reconstructions presented here. Compared to Davis et al. (2003), which focused on reconstruction of temperatures, Mauri et al. (2015) reconstructed seasonal precipitation for Europe and analyzed their evolution throughout

the Holocene. The results from Mauri et al. (2015) differ from the current study in using MAT with plant functional type scores and in producing gridded climate maps. Mauri et al. (2015) show wet summers in southern Europe (Greece and Italy), with a precipitation maximum between 8000 and 6000 cal yr BP, where precipitation was 20 mm month1

higher than modern. As in our reconstruction, precipitation changes in the winter were small and not signicantly different from present-day conditions. Our reconstructions are in agreement with Mauri et al. (2015), with summer conditions above 45 N similar to present day during the early

Holocene and summer conditions over much of the south central Mediterranean south of 45 N wetter than today, while winter conditions appear to be similar to modern values. The results from Mauri et al. (2015) inferred from terrestrial pollen records and the climatic trends reconstructed here from marine and terrestrial pollen records seem to corroborate the hypothesis of a northsouth division in precipi-

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256 O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene

Mid-Holocene: 8000 to 6000 cal BP

Late Holocene: 4000 to 2000 cal BP

(a) Annual precipitation (anomaly mm day ) (b) Winter precipitation (anomaly mm day ) (c) Summer precipitation (anomaly mm day )

-1 -0.5 -0.25 -0.125 -0.05 0 0.05 0.125 0.25 0.5 1

-1 -1 -1

0 15 E 30 E 45 E 0 15 E 30 E 45 E 0 15 E 30 E 45 E

0 15 E 30 E 45 E

50 N

40 N

30 N

50 N

40 N

30 N

0 15 E 30 E 45 E

-1 -0.5 -0.25 -0.125 -0.05 0 0.05 0.125 0.25 0.5 1

-2 -1 -0.5 -0.25 -0.1 0 0.1 0.25 0.5 1 2

0 15 E 30 E 45 E

Figure 3. Datamodel comparison for mid- and late-Holocene precipitation, expressed in anomaly compared to the present day (mm day1). Simulations are based on a regional model (Brayshaw et al., 2010): standard model HadAM3 coupled to HadSM3 (dynamical model) and

HadRM3 (high-resolution regional model). The hatching representing statistical signicance refers to the anomalies shown on the same plot,i.e., the difference between the experiment (either 80006000 or 40002000) and the present-day control run. The hatched areas indicate areas where the changes are not signicant (signicance level of 0.30). Pollen-inferred climate estimates (stars) are the same as in Fig. 2: annual precipitation, winter precipitation (winter is the sum of December, January, and February precipitation), and summer precipitation (summer is the sum of June, July, and August precipitation).

tation regimes during the early to mid-Holocene in the central Mediterranean. However, higher resolution above 45 N is still needed to validate this hypothesis.

3.1.2 Late Holocene (4000 to 2000 cal yr BP)

Late-Holocene reconstructions of winter and summer precipitation indicate that the pattern established during the early Holocene was reversed by 4000 cal yr BP, with precipitation in southern Italy, Malta, and SiculoTunisian strait that was similar to present day or lower than present day (Figs. 2b and3). Annual precipitation reconstructions suggest drying relative to the early Holocene, with modern conditions in northern Italy and modern conditions or drier than modern conditions in central and southern Italy during most of the late Holocene. Reconstructions for the Aegean Sea still indicate summer and annual precipitation that was higher than modern values (Fig. 2b). Winter conditions reverse the early-tomid-Holocene trend, with modern conditions in the northern Aegean Sea and wetter-than-modern conditions in the southern Aegean Sea (Fig. 3). Our reconstructions from all sites show a good t with Mauri et al. (2015), except for the Alboran Sea, where we reconstruct relatively high annual pre-

cipitations. Conversely, Mauri et al. (2015) reconstruct dry conditions, but here too, more sites are needed to conrm or refute this pattern in Spain. Our reconstruction of summer precipitation for the eastern Mediterranean is very similar to Mauri et al. (2015), where wet conditions are reported for Greece and the Aegean Sea.

3.2 An eastwest precipitation pattern?

A precipitation gradient, or an eastwest division during the Holocene, has been suggested for the Mediterranean from pollen data and lake isotopes (e.g., Dormoy et al., 2009;Roberts et al., 2011; Guiot and Kaniewski, 2015). However, lake levels and other hydrological proxies around the Mediterranean basin do not clearly support this hypothesis and rather show contrasting hydrological patterns south and north of 40 N, particularly during the Holocene climatic optimum (Magny et al., 2013).

3.2.1 Early to mid-Holocene (8000 to 6000 cal yr BP)

The pollen-inferred annual precipitation indicates conditions south of 42 N that are unambiguously wetter than today in

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Tunisian strait appear to have experienced higher precipitation than present day, while drier conditions exist in the east and in northern Italy (Fig. 2a).

Our reconstruction shows a good match to Guiot and Kaniewski (2015), who also discussed a possible east-to-west division in the Mediterranean with regard to precipitation (summer and annual) during the Holocene. They reported wet centennial-scale spells in the eastern Mediterranean during the early Holocene (until 6000 yr BP), with dry spells in the western Mediterranean. Mid-Holocene reconstructions show continued wet conditions, with drying through the late Holocene (Guiot and Kaniewski, 2015). This pattern indicates a see-saw effect over the last 10 000 years, particularly during dry episodes in the Near and Middle East.Similar to our ndings, Mauri et al. (2015) also reconstructed high annual precipitation values over much of the southern Mediterranean and a weak winter precipitation signal.Mauri et al. (2015) conrm an eastwest dipole for summer precipitation, with conditions drier or close to present in southwestern Europe and wetter in the central and eastern Mediterranean (Fig. 2b). These studies corroborate the hypothesis of an east-to-west division in precipitation during the early to mid-Holocene in the Mediterranean as proposed by Roberts et al. (2011). Roberts et al. (2011) suggested that the eastern Mediterranean (mainly Turkey and more eastern regions) experienced higher winter precipitation during the early Holocene, followed by an oscillatory decline after 6000 yr BP. Our ndings reveal wetter annual and summer conditions in the eastern Mediterranean, although the winter precipitation signal is less clear. However, the highest precipitation values reported by Roberts et al. (2011) were from sites located in westerncentral Turkey; these sites are absent in the current study. Climate variability in the eastern Mediterranean during the last 6000 years is also documented in a number of studies based on multiple proxies (Finn et al., 2011). Most palaeoclimate proxies indicate wet mid-Holocene conditions (Bar-Matthews et al., 2003; Stevens et al., 2006; Eastwood et al., 2007; Kuhnt et al., 2008; Verheyden et al., 2008), which agrees well with our results; however, most of these proxies are not seasonally resolved.

Roberts et al. (2011) and Guiot and Kaniewski (2015) suggest that changes in precipitation in the western Mediterranean were smaller in magnitude during the early Holocene, while the largest increases occurred during the mid-Holocene, around 60003000 cal BP, before declining to modern values. Speleothems from southern Iberia suggest a humid early Holocene (90007300 cal BP) in southern Iberia, with equitable rainfall throughout the year (Walczak et al., 2015), whereas our reconstructions for the Alboran Sea clearly show an amplied precipitation seasonality (with higher annualwinter rainfall and summer rainfall that is similar to present day) for the Alboran sites. It is likely that seasonal patterns dening the Mediterranean climate must have been even stronger than present in the early Holocene to sup-

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-1

Figure 4. Model simulation showing present day minus preindus-trial precipitation anomalies (hatching at 70 % statistical signicance over the insignicant regions).

the western, central, and eastern Mediterranean, except for Malta (Fig. 3). A prominent feature of the summer precipitation signal is an eastwest dipole, with increasing precipitation in the eastern Mediterranean (as for annual precipitation). In contrast, winter conditions show less spatial coherence, although the western basin, Sicily, and the Siculo

icance hatched). Encouragingly, there is a good overall correspondence between patterns and trends in pollen-inferred precipitation and model outputs. Caution is required when interpreting climate model results, however, as many of the changes depicted in Fig. 3 are very small and of marginal statistical signicance, suggesting a high degree of uncertainty around their robustness.

For the early to mid-Holocene, both the model and data indicate wet annual and summer conditions in Greece and in the eastern Mediterranean and conditions that were drier than today in northern Italy. There are indications of an east-to-west division in summer precipitation simulated by the climate model (e.g., between the ocean to the south of Italy and over Greece and Turkey), although the changes are extremely small (not signicant with a p<0.30). Furthermore, in the Aegean Sea, the model shows a good match with pollen-based reconstructions, suggesting that the increased spatial resolution of the regional climate model may help to simulate the localized, patchy, impacts of Holocene climate change when compared to coarser global GCMs (Fig. 3).In Italy, the model shows a good match with pollen-based reconstructions with regards to the contrasting northsouth precipitation regimes, but there is little agreement between model output and climate reconstruction with regard to winter and annual precipitation in southern Italy. The climate model suggests wetter winter and annual conditions in the far western Mediterranean (i.e., France, western Iberia, and the northwestern coast of Africa) similar to pollen-based reconstructions and near-modern summer conditions during summers (except for in France and northern Africa).A prominent feature of winter precipitation simulated by the model and partly supported by the pollen estimates is the reduced early-Holocene precipitation everywhere in the Mediterranean basin except for in the southeast.

Model and pollen-based reconstructions for the late Holocene indicate declining winter precipitation in the eastern Mediterranean and southern Italy (Sicily and Malta) relative to the early Holocene. In contrast, late-Holocene summer precipitation is higher than today in Greece and the eastern Mediterranean, near modern levels in the central and western Mediterranean, and relatively lower than today in southern Spain and northern Africa. The eastwest division in summer precipitation is strongest during the late Holocene in the proxy data and there are suggestions that it appears to be consistently simulated in the climate model; the signal is reasonably clear in the eastern Mediterranean (Greece and Turkey) but non-signicant in the central and western Mediterranean (Fig. 3).

Our ndings can be compared with previous datamodel comparisons based on the same set of climate model experiments; although here we take our reference period as present day (19601990) rather than preindustrial and thus include an additional signal from recent anthropogenic greenhouse gas emissions. Previous comparisons nevertheless suggested that the winter precipitation signal was strongest in the

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258 O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene

port the wider development of sclerophyll forests in southern Spain (Fletcher et al., 2013).

3.2.2 Late Holocene (4000 to 2000 cal yr BP)

Annual precipitation reconstructions suggest drier or near-modern conditions in central Italy, the Adriatic Sea, the SiculoTunisian strait, and Malta (Figs. 2b and 3). In contrast, the Alboran and Aegean seas remain wetter. Winter and summer precipitation produce opposing patterns; a clear eastwest division still exists for summer precipitation, with a maximum in the eastern and a minimum over the western and central Mediterranean (Fig. 2b). Winter precipitation shows the opposite trend, with a minimum in the central Mediterranean (Sicily, SiculoTunisian strait, and Malta) and eastern Mediterranean and a maximum in the western Mediterranean (Figs. 2b and 3). Our results are also in agreement with lakes and speleothem isotope records over the Mediterranean for the late Holocene (Roberts et al., 2011) and the Finn et al. (2011) palaeoclimate synthesis for the eastern Mediterranean. There is a good overall correspondence between trends and patterns in our reconstruction and that of Mauri et al. (2015), except for the Alboran Sea.High-resolution speleothem data from southern Iberia show Mediterranean climate conditions in southern Iberia between 4800 and 3000 cal BP (Walczak et al., 2015) that are in agreement with our reconstruction. The Mediterranean climate conditions reconstructed here for the Alboran Sea during the late Holocene are consistent with a climate reconstruction available from the Middle Atlas (Morocco), which shows a trend over the last 6000 years towards arid conditions as well as higher precipitation seasonality between 4000 and 2000 cal yr BP (Nourelbait et al., 2016). There is also good evidence from many records to support late-Holocene arid-ication in southern Iberia. Paleoclimatic studies document a progressive aridication trend since 7000 cal yr BP (e.g.,

Carrion et al., 2010; Jimenez-Moreno et al., 2015; Ramos-Roman et al., 2016), although a reconstruction of the annual precipitation inferred from pollen data with the probability density function method indicates stable and dry conditions in south of the Iberian Peninsula between 9000 and 3000 cal BP (Tarroso et al., 2016).

The current study shows that a prominent feature of late-Holocene climate is the eastwest division in summer precipitation: summers were overall dry or near-modern in the central and western Mediterranean and clearly wetter in the eastern Mediterranean. In contrast, winters were drier or near modern levels in the central and eastern Mediterranean (Fig. 3), while they were only wetter in the Alboran Sea.

3.3 Datamodel comparison

Figure 3 shows the datamodel comparisons for the early to mid-Holocene (a) and late Holocene (b) compared to the present-day control run (in anomalies, with statistical signif-

O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene 259

northeastern Mediterranean (near Turkey) during the early Holocene and that there was a drying trend in the Mediterranean from the early Holocene to the late Holocene, particularly in the east (Brayshaw et al., 2011a; Roberts et al., 2011). This is coupled with a gradually weakening seasonal cycle of surface air temperatures towards the present.

It is clear that most global climate models (PMIP2, PMIP3) simulate only very small changes in summer precipitation in the Mediterranean during the Holocene (Braconnot et al., 2007a, b, 2012; Mauri et al., 2014). The lack of a summer precipitation signal is consistent with the failure of the northeastern extension of the west African monsoon to reach the southeastern Mediterranean, even in the early to mid-Holocene (Brayshaw et al., 2011a). The regional climate model simulates a small change in precipitation compared to the proxy results, and it can be robustly identied as statistically signicant. This is to some extent unsurprising, insofar as the regional climate simulations presented here are themselves driven by data derived from a coarse global model (which, like its PMIP2/3 peers, does not simulate an extension of the African monsoon into the Mediterranean during this time period). Therefore, questions remain about summer precipitation in the eastern Mediterranean during the Holocene. The underlying climate dynamics therefore need to be better understood in order to condently reconcile proxy data (which suggest increased summer precipitation during the early Holocene in the eastern Mediterranean) with climate model results (Mauri et al., 2014). Based on the high-resolution coupled climate model EC-Earth, Bosmans et al. (2015) show how the seasonality of Mediterranean precipitation should vary from minimum to maximum precession, indicating a reduction in precipitation seasonality due to changes in storm tracks and local cyclogenesis (i.e., no direct monsoon required). Such high-resolution climate modeling studies (both global and regional) may prove to be key in simulating the relevant atmospheric processes (both local and remote) and providing ne-grain spatial detail necessary for comparing results to paleo-proxy observations.

Another explanation proposed by Mauri et al. (2014) is linked to the changes in atmospheric circulation. Our reconstructed climate, characterized by dry winters and wet summers, shows a spatial pattern that is somewhat consistent with modern day variability in atmospheric circulation rather than simple direct radiative forcing by insolation. In particular, the gross NWSE dipole of reconstructed winter precipitation anomalies is perhaps similar to that associated with a modern-day positive AONAO. The west coast of Spain is, however, also wetter in our early-Holocene simulations, which would seem to somewhat confound this simple picture of a shift to a positive NAO-like state compared to present. In summer, an anticyclonic blocking close to Scandinavia may have caused a more meridional circulation, which brought dry conditions to northern Europe but relatively cooler and somewhat wetter conditions to many parts of southern Europe. It is of note that some climate models

that have been used for studying palaeoclimate have difculty reproducing this aspect of modern climate (Mauri et al., 2014). Future work based on transient Holocene model simulations is important; nevertheless, transient-model simulations have also shown mid-Holocene datamodel discrepancies (Fischer and Jungclaus, 2011; Renssen et al., 2012). It is, however, suggested that further work is required to fully understand changes in winter and summer circulation patterns over the Mediterranean (Bosmans et al., 2015).

3.4 Data limitations

Classic ecological works for the Mediterranean (e.g., Ozenda, 1975) highlight how precipitation limits vegetation type in plains and lowland areas, but temperature gradients take primary importance in mountain systems. Also, temperature and precipitation changes are not independent but interact through bioclimatic moisture availability and growing season length (Prentice et al., 1996). This may be one reason why certain sites may diverge from model outputs; the Alboran sites, for example, integrate pollen from the coastal plains through to mountain (+1500 m) elevations. At high el

evations within the source area, temperature effects become more important than precipitation in determining the forest cover type. Therefore, it is not possible to fully isolate precipitation signals from temperature changes. Particularly for the semiarid areas of the Mediterranean, the reconstruction approach probably cannot distinguish between a reduction in precipitation and an increase in temperature and potential evapotranspiration (PET) or vice versa.

Similarly, while the concept of reconstructing winter and summer precipitation separately is very attractive, it may be worth commenting on some limitations. Although different levels of the severity or length of summer drought are an important ecological limitation for vegetation, reconstructing absolute summer precipitation can be difcult because the severity and length of bioclimatic drought is determined by both temperature and precipitation. We are dealing with a season that has, by denition, small amounts of precipitation that drop below the requirements for vegetation growth.Elevation is also of concern, as lowland systems tend to be recharged by winter rainfall, but high mountain systems may receive a signicant part of precipitation as snowfall, which is not directly available to plant life. This may be important in the long term for improving the interpretation of long-term Holocene changes and contrasts between different proxies, such as lake levels and speleothems. Although these issues may initially appear to be of marginal importance, they may nevertheless have a real inuence leading to problems and mismatches between different proxies (e.g., Davis et al., 2003; Mauri et al., 2015).

Another important point is the question of human impact on the Mediterranean vegetation during the Holocene. Since human activity has inuenced natural vegetation, distinguishing between vegetation change induced by humans and

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260 O. Peyron et al.: Precipitation changes in the Mediterranean basin during the Holocene

climatic change in the Mediterranean is a challenge requiring independent proxies and approaches. Therefore, links and processes behind societal change and climate change in the Mediterranean region are being increasingly investigated (e.g., Holmgren et al., 2016; Gogou et al., 2016; Sadori et al., 2016a). Here, the behavior of the reconstructed climatic variables between 4000 and 2000 cal yr BP is likely to be inuenced by unnatural ecosystem changes due to human activities such as the forest degradation that began in the lowlands and progressed to mountainous areas (Carrin et al., 2010). These human impacts add confounding effects for fossil pollen records and may lead to slightly biased temperature reconstructions during the late Holocene, likely biased towards warmer temperatures and lower precipitation. However, if human activities become more marked at 3000 cal yr BP, they increase signicantly over the last millennia (Sadori et al., 2016), which is not within the timescale studied here. Moreover, there is strong agreement between summer precipitation and independently reconstructed lake-level curves (Magny et al., 2013). For the marine pollen cores, human inuence is much more difcult to interpret given that the source area is so large and that, in general, anthropic taxa are not found in marine pollen assemblages.

4 Conclusions

The Mediterranean is particularly sensitive to climate change but the extent of future change relative to changes during the Holocene remains uncertain. Here, we present a reconstruction of Holocene precipitation in the Mediterranean using an approach based on both terrestrial and marine pollen records, along with a modeldata comparison based on a high-resolution regional model. We investigate climatic trends across the Mediterranean during the Holocene to test the hypothesis of an alternating northsouth precipitation regime and/or an eastwest precipitation dipole. We give particular emphasis to the reconstruction of seasonal precipitation, considering the important role it plays in this system.

Climatic trends reconstructed in this study seem to corroborate the northsouth division of precipitation regimes during the Holocene, with wet conditions in the southcentral and eastern Mediterranean and dry conditions above 45 N during the early Holocene, while the opposite pattern dominates during the late Holocene. This study also shows that a prominent feature of Holocene climate in the Mediterranean is the east-to-west division in precipitation, which is strongly linked to the seasonal parameter reconstructed. During the early Holocene, we observe an east-to-west division with high summer precipitation in Greece and the eastern Mediterranean and a minimum over Italy and the western Mediterranean. There was a drying trend in the Mediterranean from the early Holocene to the late Holocene, particularly in central and eastern regions, but summers in the east remained wetter than today. In contrast, the signal for winter precipi-

tation is less spatially consistent during the early Holocene, but it clearly shows conditions similar to present day or drier everywhere in the Mediterranean except for in the western basin during the late Holocene.

The regional climate model outputs show a remarkable qualitative agreement with our pollen-based reconstructions, although it must be emphasized that the changes simulated are typically very small or are of questionable statistical signicance. Nevertheless, there are indications that the east-to-west division in summer precipitation reconstructed from the pollen records does appear to be simulated by the climate model. The model results also suggest that parts of the eastern Mediterranean experienced conditions similar to present day or drier in winter during the early and late Holocene and wetter annual and summer conditions during the early and late Holocene (both consistent with the paleo-records).

Although this study has used regional climate model data, it must always be recalled that the regional models high-resolution output is strongly constrained by a coarser-resolution global climate model, and the ability of global models to correctly reproduce large-scale patterns of change in the Mediterranean over the Holocene remains unclear (e.g., Mauri et al., 2015). The generally positive comparison between model and data presented here may therefore simply be fortuitous and not necessarily replicated if the output from other global climate model simulations was downscaled in a similar way. However, it is noted that the use of higher-resolution regional climate models can offer signicant advantages for datamodel comparison insofar as they assist in resolving the inherently patchy nature of climate signals and paleo-records. Notwithstanding the difculties of correctly modeling large-scale climate change over the Holocene (with GCMs), we believe that regional downscaling may still be valuable in facilitating modeldata comparison in regions and/or locations known to be strongly inuenced by local effects (e.g., complex topography).

Data availability. Data for this paper are available in the Supplement.

The Supplement related to this article is available online at http://dx.doi.org/10.5194/cp-13-249-2017-supplement

Web End =doi:10.5194/cp-13-249-2017-supplement .

Competing interests. The authors declare that they have no conict of interest.

Acknowledgements. This study is a part of the LAMA ANR Project (MSHE Ledoux, USR 3124, CNRS) nancially supported by the French CNRS (National Centre for Scientic Research).Simon Goring is currently supported by NSF MacroSystems grant 144-PRJ45LP. This is an ISEM contribution no. 2017-042.

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Edited by: J. GuiotReviewed by: two anonymous referees

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Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability, which is notoriously difficult for models to reproduce. We propose here a new proxy-based climate synthesis synthesis and its comparison - at a regional (∼ 100km) level - with a regional climate model to examine (i) opposing northern and southern precipitation regimes and (ii) an east-to-west precipitation dipole during the Holocene across the Mediterranean basin. Using precipitation estimates inferred from marine and terrestrial pollen archives, we focus on the early to mid-Holocene (8000 to 6000calyrBP) and the late Holocene (4000 to 2000calyrBP), to test these hypotheses on a Mediterranean-wide scale. Special attention was given to the reconstruction of season-specific climate information, notably summer and winter precipitation. The reconstructed climatic trends corroborate the north-south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south-central and eastern Mediterranean regions, while drier conditions prevailed from 45°N northwards. These patterns then reverse during the late Holocene. With regard to the existence of a west-east precipitation dipole during the Holocene, our results show that the strength of this dipole is strongly linked to the reconstructed seasonal parameter; early-Holocene summers show a clear east-west division, with summer precipitation having been highest in Greece and the eastern Mediterranean and lowest over Italy and the western Mediterranean. Summer precipitation in the east remained above modern values, even during the late-Holocene interval. In contrast, winter precipitation signals are less spatially coherent during the early Holocene but low precipitation is evidenced during the late Holocene. A general drying trend occurred from the early to late Holocene, particularly in the central and eastern Mediterranean. For the same time intervals, pollen-inferred precipitation estimates were compared with model outputs, based on a regional-scale downscaling (HadRM3) of a set of global climate-model simulations (HadAM3). The high-resolution detail achieved through the downscaling is intended to enable a better comparison between site-based paleo-reconstructions and gridded model data in the complex terrain of the Mediterranean; the model outputs and pollen-inferred precipitation estimates show some overall correspondence, though modeled changes are small and at the absolute margins of statistical significance. There are suggestions that the eastern Mediterranean experienced wetter summer conditions than present during the early and late Holocene; the drying trend in winter from the early to the late Holocene also appears to be simulated. The use of this high-resolution regional climate model highlights how the inherently patchy nature of climate signals and paleo-records in the Mediterranean basin may lead to local signals that are much stronger than the large-scale pattern would suggest. Nevertheless, the east-to-west division in summer precipitation seems more marked in the pollen reconstruction than in the model outputs. The footprint of the anomalies (like today, or dry winters and wet summers) has some similarities to modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive Arctic Oscillation-North Atlantic Oscillation (AO-NAO)) and a weak westerly circulation in summer associated with anticyclonic blocking; however, there also remain important differences between the paleo-simulations and these analogues. The regional climate model, consistent with other global models, does not suggest an extension of the African summer monsoon into the Mediterranean. Therefore, the extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question.

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Title

Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a model-data comparison

Author

Peyron, Odile; Combourieu-Nebout, Nathalie; Brayshaw, David; Goring, Simon; Andrieu-Ponel, Valérie; Desprat, Stéphanie; Fletcher, Will; Gambin, Belinda; Ioakim, Chryssanthi; Joannin, Sébastien; Kotthoff, Ulrich; Kouli, Katerina; Montade, Vincent; Pross, Jörg; Sadori, Laura; Magny, Michel

Pages

249-265

Publication year

2017

Publication date

2017

Publisher

Copernicus GmbH

ISSN

18149324

e-ISSN

18149332

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.5194/cp-13-249-2017

ProQuest document ID

1880395837

Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a model-data comparison

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