Controls of Caribbean Surface Hydrology During the Mid- to Late Holocene

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icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Clim. Past Discuss., 8, 3901–3948, 2012 www.clim-past-discuss.net/8/3901/2012/ Climate doi:10.5194/cpd-8-3901-2012 of the Past CPD © Author(s) 2012. CC Attribution 3.0 License. Discussions 8, 3901–3948, 2012

This discussion paper is/has been under review for the journal Climate of the Past (CP). Controls of Please refer to the corresponding ﬁnal paper in CP if available. Caribbean surface hydrology during the mid- to late Holocene

Controls of Caribbean surface hydrology C. Giry et al. during the mid- to late Holocene: insights from monthly resolved coral records Title Page Abstract Introduction 1 1 1 2 2 3 C. Giry , T. Felis , M. Kolling¨ , W. Wei , G. Lohmann , and S. Scheﬀers Conclusions References

1 MARUM – Center for Marine Environmental Sciences, University of Bremen, Tables Figures 28359 Bremen, Germany 2Alfred Wegener Institute for Polar and Marine Research, 27570 Bremerhaven, Germany 3Marine Ecology Research Centre, Southern Cross University, Lismore, NSW 2480, Australia J I

Received: 29 July 2012 – Accepted: 3 August 2012 – Published: 21 August 2012 J I Correspondence to: C. Giry ([email protected]) Back Close

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3901 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Abstract CPD Here we reconstruct seasonality and interannual to multidecadal variability of sea surface hydrology of the southern Caribbean Sea by applying paired coral Sr/Ca and δ18O 8, 3901–3948, 2012 measurements on fossil annually-banded Diploria strigosa corals from Bonaire. This 5 allows for better understanding short-term (i.e., seasonal to multidecadal) variability Controls of of the Caribbean hydrological cycle during the mid- to late Holocene. The monthly- Caribbean surface 18 resolved coral ∆δ O records are used as a proxy for the oxygen isotopic composition hydrology during the 18 of seawater (δ Osw) of the southern Caribbean Sea. Consistent with modern day con- mid- to late Holocene 18 ditions, annual δ Osw cycles reconstructed from three modern corals reveal that fresh- C. Giry et al. 10 water budget at the study site is inﬂuenced by both the evaporation/precipitation ratio and the seasonal advection of tropical freshwater brought by wind-driven surface cur- 18 rents. In contrast, the annual δ Osw cycle reconstructed from a mid-Holocene coral indicates sharp peaks towards more negative values in summer suggesting intense Title Page summer precipitation at 6 ka before present (BP). In line with this our model simulations Abstract Introduction 15 indicate that increased seasonality of the hydrological cycle at 6 ka BP results from enhanced precipitation in summertime. On interannual to multidecadal timescales, the Conclusions References systematic positive correlation observed between reconstructed sea surface temper- Tables Figures ature and salinity suggests that freshwater discharged from the Orinoco and Amazon rivers and transported into the Caribbean by wind-driven surface currents is a critical J I 20 component inﬂuencing sea surface hydrology on these timescales. J I

1 Introduction Back Close

Proxy records and model simulations indicate that hydrological conditions in the Full Screen / Esc Caribbean have experienced significant changes on glacial/interglacial timescales. Changes in the tropical hydrological cycle have been associated with both the mean Printer-friendly Version 25 latitudinal position of the Intertropical Convergence Zone (ITCZ) and strength of Interactive Discussion the Atlantic Meridional Overturning Circulation (AMOC) (Lohmann, 2003; Zhang and 3902 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Delworth, 2005). The AMOC is characterised by a net northward surface flow of warm and salty water from low-latitudes to the North Atlantic. The associated heat trans- CPD port to higher latitudes affects North Atlantic climate, which in turn can influence hy- 8, 3901–3948, 2012 drological conditions in the tropics by shifting the mean position of the ITCZ. High 5 salinity anomalies were reported in the Caribbean during cold periods associated with a reduced AMOC and a southward shift of the ITCZ (Carlson et al., 2008; Wan et Controls of al., 2010; Leduc et al., 2007; Schmidt and Spero, 2011; Schmidt et al., 2004; Sepul- Caribbean surface cre et al., 2011). Moreover, there is evidence for a reduced surface flow through the hydrology during the Caribbean during these cold periods (Lynch-Stieglitz et al., 1999). The observed pat- mid- to late Holocene 10 terns of orbital-scale variability indicate that cold periods are characterised by more saline conditions in the western tropical Atlantic and Caribbean combined with a re- C. Giry et al. duced surface flow through the Caribbean. It is clear from both proxy records and model studies that changes in Caribbean freshwater budget resulting from tropical and Title Page extratropical atmospheric circulation changes (i.e., Hadley circulation) have a profound 15 effect on the salinity and density structure of the AMOC, all of which point to the critical Abstract Introduction role of the tropical Atlantic in mediating global climate changes (Leduc et al., 2007; Jaeschke et al., 2007). Conclusions References

Recent literature indicates that the surface branch of the AMOC, bringing warm and Tables Figures salty waters from the tropical to the North Atlantic, is characterised by energetic short- 20 term variability (Cunningham et al., 2007; Cunningham and Marsh, 2010). Despite the J I relevance of such studies, which reveal pronounced annual cycles, the observational period is too short to fully understand the nature of both the short- and long-term vari- J I ations of the surface flow of the AMOC. Nevertheless, recent model simulations found Back Close significant interannual variability of the AMOC that reflects the influence of ocean- 25 atmosphere modes of interannual climate variability on the strength of the meridional Full Screen / Esc overturning circulation (Vellinga and Wu, 2004). Furthermore, a recent observational study identified low-frequency variability of the North Brazilian Current transport as a Printer-friendly Version potential indicator of the multidecadal variability of the surface return flow of the AMOC (Zhang et al., 2011). Interactive Discussion

3903 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Low-latitude climate reconstructions have designated the net precipitation (P − E, precipitation minus evaporation) associated with latitudinal movement of the ITCZ as CPD the main driver of the Caribbean freshwater budget on glacial/interglacial timescales 8, 3901–3948, 2012 (Leduc et al., 2007; Schmidt et al., 2004; Sepulcre et al., 2011). For example, dur- 5 ing the early and mid-Holocene, there are indications for a more northward position of the ITCZ which intensiﬁed precipitation over the northern tropical Atlantic and the Controls of Caribbean (Haug et al., 2001; Hodell et al., 1991), whereas today, with the position of Caribbean surface the Atlantic ITCZ in a more southerly position, net evaporation due to enhanced subsi- hydrology during the dence contributes to the negative freshwater budget of the Caribbean (Etter et al., 1987 mid- to late Holocene 10 and references therein). Decadally-resolved titanium concentration data from Cariaco Basin sediments (Haug et al., 2001), a proxy for riverine input of terrigeneous materi- C. Giry et al. als from local rivers and the Orinoco River, revealed long-term changes in river runoﬀ associated with the latitudinal migration of the ITCZ following orbitally-driven seasonal Title Page changes of insolation. This record provides additional evidence for rapid changes (i.e., 15 decadal to centennial scales) of hydrological conditions over northern South America. Abstract Introduction Mechanisms responsible for this short-term variability remain, however, poorly under- stood. Due to the lack of suitable high-resolution proxy data that clearly resolve the Conclusions References

annual cycle, the seasonal dynamics of the Atlantic ITCZ under diﬀerent climate back- Tables Figures ground states have yet to be investigated in order to reconcile the recent debate on the 20 dynamics of the tropical rain belt over the course of the Holocene (e.g., Collins et al., J I 2011). In order to investigate seasonal to multidecadal variability patterns of surface hy- J I drology in the Caribbean Sea during the mid- to late Holocene, we use decades-long Back Close monthly-resolved paired Sr/Ca and δ18O records of fossil corals from Bonaire (south- 25 ern Caribbean Sea) to assess changes in the oxygen isotopic composition of seawater Full Screen / Esc 18 (δ Osw) over the last 6 ka. The Sr/Ca ratio in coral skeletons is a proxy for sea sur- 18 face temperature (SST) (Beck et al., 1992; Smith et al., 1979), whereas the coral δ O Printer-friendly Version 18 18 reﬂects changes of both temperature and seawater δ O(δ Osw) which is linearly related to sea surface salinity (SSS) (Carriquiry et al., 1994; Urey, 1947; Watanabe et al., Interactive Discussion

3904 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 2001; Wellington et al., 1996). Therefore, paired measurements of δ18O and Sr/Ca in coral skeletons provide a unique opportunity to reconstruct SST and SSS changes at CPD subseasonal to interannual resolution (Felis et al., 2009; Gagan et al., 1998; Hendy et 8, 3901–3948, 2012 al., 2002; Linsley et al., 2006; McCulloch et al., 1994). 5 This study aims at better understanding the natural range of surface hydrology variability in the Caribbean. The paper is organised as follows. In Sect. 2, the regional Controls of setting of the study is presented. Materials and methods used are brieﬂy described Caribbean surface in Sect. 3. In Sect. 4, changes in the isotopic composition of seawater as inferred hydrology during the from coral records are assessed for a wide range of timescales (i.e., seasonal to multi- mid- to late Holocene 10 decadal) throughout the mid- to late Holocene. Forcing mechanisms responsible for the C. Giry et al. reconstructed variability are discussed in Sect. 5 and conclusions are given in Sect. 6.

2 Regional setting and hydrography of the study area Title Page

Abstract Introduction Coral colonies used in this study were collected on Bonaire (Giry et al., 2012), an open- ocean island in the southern Caribbean Sea, located ∼ 100 km North off Venezuela Conclusions References ◦ 0 ◦ 0 15 (∼ 12 10 N, 68 18 W). Caribbean climate is primarily forced by a co-varying pattern of SST and trade winds associated with the seasonal migration of the ITCZ (Hastenrath, Tables Figures 1984). Bonaire has a tropical-arid climate characterised by low-annual mean rainfall (∼ 500 mm yr−1) and high evaporation rate. In contrast to other less-arid Caribbean Islands J I which receive most of their yearly rain in summer (Giannini et al., 2000), Bonaire’s J I 20 main rainy season lasts from October through January and is followed by a dry season between February to May with a transition period called “small rains” season (Martis et Back Close al., 2002) from June to September. The seasonal displacement of the ITCZ affects the Full Screen / Esc strength of the northeast trade winds blowing over the western tropical Atlantic and the Caribbean. Trade winds weaken in boreal summer and strengthen in boreal winter and Printer-friendly Version 25 thus, affect the patterns of ocean circulation and precipitation in the Caribbean. Freshwater budget in the Caribbean suggests the influence of two major processes: Interactive Discussion (1) the local freshwater flux through evaporation (E) and precipitation (P ) (Etter et al., 3905 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 1987) over the basin and (2) freshwater input from the Amazon and Orinoco rivers by advection of low salinity water masses by surface currents (Hellweger and Gordon, CPD 2002). Net water loss due to yearlong easterly trade winds results in excessive E over 8, 3901–3948, 2012 P , which is strongest over the basin in wintertime (Etter et al., 1987; Giannini et al., 5 2000; Hastenrath and Lamb, 1978). Caribbean freshwater budget indicates that river discharge in the western Caribbean (west of 70◦ W) does not compensate for net water Controls of loss due to net evaporation (Etter et al., 1987), whereas it does in the western tropical Caribbean surface Atlantic and eastern Caribbean (Hellweger and Gordon, 2002). Hence, the freshwa- hydrology during the ter budget in the eastern Caribbean and at the study site might reflect both the local mid- to late Holocene 10 freshwater flux and freshwater supply from tropical rivers. Caribbean hydrography is characterised by a stratified upper 500 m of the water C. Giry et al. column due to the presence of two dominant water masses with contrasting physical properties (Wust,¨ 1964). The North Atlantic Subtropical Under Water (SUW) formed in Title Page the subtropical north Atlantic where E exceeds P is characterised by a salinity maxi- 15 mum (> 36.5) that sinks below fresher surface water referred to as Caribbean Surface Abstract Introduction Water (CSW) at about 120 m depth (Wust,¨ 1964). The latter water mass is thought to be a mixture of North Atlantic surface water and freshwater from Amazon and Orinoco Conclusions References

rivers as well as from other South American rivers that is transported north-westward Tables Figures by the Guyana and Caribbean Currents (Fig. 1). As the CSW is transported westward 20 by the Caribbean Current, freshwater masses are evaporated and mixed with the more J I saline SUW (Gordon, 1967). At interannual timescales, unlike the Pacific where the influence of the El Nino-˜ J I Southern Oscillation (ENSO) dominates, the tropical Atlantic is linked to the competing Back Close influence of local and remote forcing emanating from tropical and subtropical oceans. 25 On these timescales, ENSO-related anomalous atmospheric circulation is one of the Full Screen / Esc dominant factors suppressing rainfall over the Caribbean (Chiang et al., 2002) suggested that while the teleconnection of ENSO warm events to the tropical Atlantic re- Printer-friendly Version sults in an anomalous warm north/cool south SST gradient that would shift the ITCZ anomalously north; warm ENSO also induces a strong anomalous Walker circulation Interactive Discussion

3906 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion that suppresses precipitation over the western tropical Atlantic and Caribbean by strengthening subsidence. Such a pattern of ENSO teleconnection to the tropical At- CPD lantic is consistent with other studies (Alexander and Scott, 2002; Giannini et al., 2001). 8, 3901–3948, 2012 At inter- to multidecadal timescales, the Atlantic Multidecadal Oscillation (AMO) plays 5 a critical role in controlling both SST and rainfall in the Caribbean (Sutton and Hodson, 2005). Forcing mechanisms responsible for the leading large-scale pattern of multi- Controls of decadal climate variability are partially related to variability in the oceanic thermohaline Caribbean surface circulation on these (Delworth and Mann, 2000; Dima and Lohmann, 2007; Knight et hydrology during the al., 2005) as recently identiﬁed along the north-eastern coast of Brazil in the surface mid- to late Holocene 10 return ﬂow of the AMOC (Zhang et al., 2011). C. Giry et al.

3 Material and methods Title Page U-series dating, screening for diagenesis, and Sr/Ca analyses were performed on fossil Diploria strigosa corals from Bonaire as recently described (Giry et al., 2012). Based Abstract Introduction

on the annual density bands inferred from X-radiographs, a sampling resolution of ∼ 12 Conclusions References 15 samples per year was targeted along the growth axis of pristine coral skeletons. For each sample, skeletal powder was carefully drilled using a 0.6 mm diameter drill bit Tables Figures along the centre of the dense thecal walls which is the skeletal element that provides the best environmental seasonal signal (Giry et al., 2010a). This careful sampling strat- J I egy allows for evaluating coral-based climate variability at near-monthly resolution. 18 J I 20 Each powdered sample was split for both stable oxygen isotope (δ O) and Sr/Ca analyses performed at MARUM and the Department of Geosciences of the University Back Close of Bremen, respectively. Further details on stable isotope and Sr/Ca analyses have been described (Giry et al., 2010a, 2012). Full Screen / Esc The internal chronology of individual coral records is based on annual density- Printer-friendly Version 25 banding patterns inferred from both X-radiographs and annual Sr/Ca cycles. In a previous study, we demonstrated that the timing of annual SST cycle has not significantly Interactive Discussion changed during the investigated period (Giry et al., 2012). Therefore, maximum and 3907 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion minimum Sr/Ca values in any given year were assigned to the on average coldest (February/March) and warmest months (September/October) of present-day SST, re- CPD spectively. This method introduces a non-cumulative error of ±1 month in any given 8, 3901–3948, 2012 year. The same anchor points as for Sr/Ca were used for the age model construction 18 18 5 of the corresponding δ O record. To obtain monthly time series, coral Sr/Ca and δ O records were linearly interpolated to monthly resolution following established proce- Controls of dures (e.g., Felis et al., 2004, 2009; Giry et al., 2010b). As both Sr/Ca and δ18O were Caribbean surface measured on the same powder samples, the variation of Sr/Ca relative to that of δ18O hydrology during the has no age uncertainty. mid- to late Holocene 18 18 10 Subseasonal reconstruction of seawater δ O(δ Osw) variations is achieved by C. Giry et al. paired coral Sr/Ca and δ18O measurements. The most commonly used method to cal- 18 18 culate δ Osw exploits proxy-SST regressions to convert δ O and Sr/Ca to temperature (Gagan et al., 1998; McCulloch et al., 1994). However, the regressions have gen- Title Page erally different intercepts due to different mean values between different coral colonies 15 (e.g., Abram et al., 2009; Felis et al., 2003, 2004). Therefore, omitting the intercept val- Abstract Introduction 18 ues in order to assess instantaneous changes of δ Osw is critical (Ren et al., 2003). To do so, the centering method and the combined analytical error are used (Cahyarini Conclusions References et al., 2008). Multitaper Method (MTM) spectral analysis (Ghil et al., 2002), with sig- Tables Figures nificance relative to a red noise null hypothesis determined with the robust method of 20 noise background estimation (Mann and Lees, 1996), is applied to the detrended and 18 J I normalised δ Osw and instrumental records with the mean annual cycle removed. Gaussian band-pass filtering is performed with AnalySeries software (Paillard et al., J I 1996). In correspondence with previous work (Felis et al., 2009; Gagan et al., 1998; 18 Back Close Hendy et al., 2002), we term the δ Osw variations reconstructed from paired coral 18 18 25 Sr/Ca and δ O measurements coral ∆δ O. Full Screen / Esc The numerical experiments were performed with the coupled general circulation model COSMOS consisting of the atmospheric model ECHAM5 (Roeckner et al., Printer-friendly Version 2003), ocean model MPIOM (Marsland et al., 2003), and dynamical vegetation model JSBACH (Raddatz et al., 2007). The atmospheric model has a resolution of T31 Interactive Discussion 3908 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion (3.75◦ × 3.75◦) in horizontal and 19 vertical hybrid sigma pressure levels. The ocean model has a 3◦ × 1.8◦ averaged horizontal grid with 40 unevenly spaced vertical levels CPD with higher resolution around Greenland and Antarctica (Marsland et al., 2003). We 8, 3901–3948, 2012 carried out two experiments: a pre-industrial (CTL) experiment and a mid-Holocene 5 one (6 ka), by prescribing the appropriate orbital parameters and greenhouse gases. Details of the model experiments are described in Wei et al. (2012) and Wei and Controls of Lohmann (2012). Caribbean surface hydrology during the mid- to late Holocene 4 Results C. Giry et al. 4.1 Modern coral δ18 O-SST relationship

18 10 δ The coral O-SST relationship of the D. strigosa colony collected live in Bonaire Title Page (BON-0-A) was assessed for the period 1993–2009. The regression equation in reference to gridded monthly SST data (Smith et al., 2008) is: Abstract Introduction 18 2 δ O(‰) = −0.106(±0.010) × SST − 1.432(±0.259)(r = 0.41,p < 0.01,N = 188) (1) Conclusions References 18 Moreover, coral δ O is investigated in reference to local SST data from a nearby is- Tables Figures 15 land, Curacao (distance ∼ 80 km), for a period of 18 months spanning April 1999 to September 2000 (M. J. A. Vermeij, personal communication, 2010). The linear regression for this data is: J I δ18O(‰) = −0.124(±0.016) × SST − 0.951(±0.437)(r2 = 0.79,p < 0.0001,N = 18) (2) J I We note that the coral δ18O-SST regression slopes are lower than the published val- Back Close 20 ues for D. strigosa from Guadeloupe in the eastern Caribbean (Hetzinger et al., 2006). Full Screen / Esc This discrepancy might arise from distinct annual temperature and hydrological cycles between Bonaire and Guadeloupe that in turn affects the amplitude of the oxygen Printer-friendly Version isotopic ratio of coral skeleton at both sites. Consequently, in order to circumvent un- 18 certainties associated with calibrating proxy data, the reconstructed δ Osw is derived Interactive Discussion 25 from a range of proxy-SST calibrations rather than from fixed calibration values. 3909 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.2 Surface hydrology changes during the mid- to late Holocene CPD Monthly-resolved coral Sr/Ca and coral δ18O of three modern and six fossil corals are shown in Fig. 2. Each coral record shows clear annual cycles in both Sr/Ca and δ18O 8, 3901–3948, 2012 which correlate with the corresponding annual-density bands. Therefore, robust coral 5 internal chronologies could be established enabling accurate assessment of seasonal Controls of to multidecadal variability of coral proxies for well-distributed time windows throughout Caribbean surface the mid- to late Holocene. hydrology during the 18 Mean coral ∆δ O values for individual colonies were calculated by averaging data mid- to late Holocene of each year and subsequently averaging the resulting annual means of all years con- 18 10 stituting the coral record. The mean ∆δ O values for individual Bonaire corals are C. Giry et al. shown in Fig. 3. This has been calculated using the coral Sr/Ca-SST and δ18O-SST relationships of −0.061 mmol/mol ◦C−1 (Correge,` 2006) and −0.180 ‰ ◦C−1 (Gagan et al., 1998), respectively, following established procedures (Cahyarini et al., 2008). The Title Page 18 three modern corals exhibit between colony offsets in mean ∆δ O. In this case, the Abstract Introduction 15 standard deviation of such intercolony variability is 0.161 ‰ (1 σ), but this is depen- dent on the proxy-SST relationships used. The mean ∆δ18O values of all nine corals Conclusions References suggest a trend towards more positive values since the mid-Holocene (Fig. 3). The ten- Tables Figures dency in the coral data was assessed using a range of proxy-SST relationships (Table 1 in the Supplement). This experiment indicates that no trend reversal is observed when J I 20 using different calibrations. Consequently, Bonaire coral data indicate a trend toward 18 more positive mean ∆δ O over the last 6.2 ka. Since modern intercolony variability in J I 18 18 mean coral ∆δ O is large, and equivalent to ∼ 1.6 psu (2 σ) using the δ Osw-salinity relationship of Watanabe et al. (2001) for modern Caribbean surface waters, quantifi- Back Close 18 cation of mean ∆δ O in terms of SSS for past time intervals is not further considered. Full Screen / Esc

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3910 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.3 Seasonal changes in coral ∆δ18O CPD 4.3.1 Insights from raw data 8, 3901–3948, 2012 The seasonality is defined as the difference between maximum and minimum monthly values of a given year that is then averaged for all years constituting the record. Controls of 18 18 5 The coral δ O records of three modern corals indicate a mean δ O seasonality of Caribbean surface 0.440 ± 0.047 ‰ (1 σ) ranging from 0.400 to 0.491 ‰ (Fig. 4). It is assumed that the 18 hydrology during the between-colony differences in coral δ O seasonality do not result from changing en- mid- to late Holocene vironmental conditions over the last century, but rather reflect coral growth processes (e.g., Gagan et al., 1998). Therefore, the combined error (Abram et al., 2009) that C. Giry et al. 18 10 takes into account modern between-colony offsets in coral δ O seasonality (Giry et al., 2012) is considered for our estimates of Holocene coral δ18O seasonality. Consid- ering this uncertainty, most of the fossil coral records show δ18O seasonality that is Title Page not significantly different from that given by three modern corals. However, significantly Abstract Introduction increased δ18O seasonality (0.622 ± 0.098 ‰) is observed in the 6.22 ka coral (Fig. 4). 18 15 Composite annual Sr/Ca and δ O cycles derived from monthly-resolved records Conclusions References of individual corals reveal clear annual cycles for both modern and fossil colonies Tables Figures (Fig. 5a). Therefore, changes in the timing between both proxies are assessed for the annual cycle (Fig. 5b). Cross-spectral analyses between measured coral proxies reveal that coral δ18O annual cycles lead corresponding coral Sr/Ca cycles by about J I 20 a month in modern colonies. The phase angle for modern coral records indicates very J I similar values showing good reproducibility among modern colonies. Annual Sr/Ca and δ18O cycles from fossil corals indicate different patterns in the past. For instance, most Back Close of the fossil corals reveal that coral Sr/Ca either leads or is in phase with coral δ18O Full Screen / Esc (Fig. 5b). However, the 6.22 ka coral indicates that δ18O leads Sr/Ca changes by only 25 0.3 month (Fig. 5b). All of which provide evidence for changes in the seasonality of the Printer-friendly Version hydrological cycle throughout the mid- to late Holocene. Interactive Discussion

3911 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.3.2 Insights from calculated coral ∆δ18 O CPD Composite annual coral ∆δ18O cycles were calculated for individual coral records using a range of proxy-SST calibrations. This is used in order to test the impact of varying 8, 3901–3948, 2012 calibrations on the reconstructed ∆δ18O (Fig. 1 in the Supplement). Annual ∆δ18O 5 cycles calculated using calibrations from Hetzinger et al. (2006) are very different in Controls of amplitude and timing from that given by a set of four other calibrations, including local Caribbean surface 18 calibrations (Giry et al., 2012, this study). Since the amplitude of the annual ∆δ O hydrology during the cycle given by our local calibrations provide reasonably good estimates of the real mid- to late Holocene 18 Caribbean annual δ Osw cycle (Watanabe et al., 2001), we rely on well-established C. Giry et al. 10 coral proxy-SST calibrations (Correge,` 2006; Gagan et al., 1998) for the interpretation of our fossil coral records. The combined analytical uncertainties for reconstructing ∆δ18O using the Sr/Ca-SST relationship of −0.061 mmol/mol per ◦C (Correge,` 2006) and the δ18O-SST relationship of −0.18 ‰ per ◦C (Gagan et al., 1998) is ±0.077 ‰ Title Page 18 (1 σ). Consequently, coral ∆δ O variability with amplitude greater than 0.154 ‰ were Abstract Introduction 15 considered significant with respect to analytical uncertainty (Cahyarini et al., 2008). Assuming that the above calibrations (Correge,` 2006; Gagan et al., 1998) are the Conclusions References δ18 most representative, the composite annual Osw cycles from three modern corals Tables Figures indicate lower values in spring and summer and higher values in fall and winter with average amplitude of 0.157 ‰ (Fig. 6). Since the composite annual cycles are derived J I 20 from multiple years constituting the record, their timing is reliable, although the reconstructed amplitude appears insignificant in some records. The reconstructions of coral J I 18 18 ∆δ O indicate a similar seasonal hydrological regime (e.g., low/high coral ∆δ Osw values in summer/winter) at 6.22 ka, 4.27 ka and 3.79 ka, whereas distinct regimes are Back Close observed at 3.83 ka, 2.35 ka and 1.84 ka. The 6.22 ka coral record reveals increased Full Screen / Esc 18 25 coral ∆δ O seasonality with amplitude of 0.193 ‰ that is 23 % greater than the am- 18 plitude of the modern annual ∆δ O cycle given by three modern corals. Moreover, a Printer-friendly Version period of reversed seasonality of the hydrological cycle is detected at 2.35 ka BP. This record indicates more positive values in summer and more negative values in winter. Interactive Discussion

3912 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion The deviation from the mean ∆δ18O value is investigated for individual months in each coral record (Fig. 6). It is found that greatest deviation from the mean occurs in fall and CPD winter in the three modern corals. Although this pattern is fuzzy in the Holocene corals, 8, 3901–3948, 2012 the 6.22 ka coral record indicates that the greatest negative and positive deviation from 5 the mean value occurs during both summer and winter seasons, respectively. Controls of 4.4 Interannual to multidecadal coral ∆δ18O variability Caribbean surface hydrology during the 18 The MTM spectral analysis of monthly coral ∆δ O records indicates significant quasi- mid- to late Holocene biennial and interannual variability that is often superimposed on inter- to multidecadal scale variability (Fig. 7). Significant spectral peaks in the quasi-biennial band (defined C. Giry et al. 10 here as < 2.3-yr) are detected in most Bonaire coral records. Interannual (2.3- to 7-yr), near-decadal (7- to 15-yr) as well as inter- to multidecadal (15- to 40-yr) variability of coral ∆δ18O are identified in most of the coral records. Instrumental rainfall data (Hulme Title Page

et al., 1998) reveal that precipitation over Bonaire is also characterised by signiﬁcant Abstract Introduction quasi-biennial and interannual spectral peaks (Fig. 7), whereas inter- to multidecadal 15 variability of Bonaire precipitation is not signiﬁcant over the period 1930–1999. Further- Conclusions References more, while interannual variability seems to be a prominent feature of the coral ∆δ18O Tables Figures records, the coral record at 2.35 ka indicates enhanced spectral power at interannual periodicities centred at 6.1-yr and the 6.22 ka coral reveals pronounced variability at 7.8-yr. Moreover, inter- to multidecadal variability seems to be a prominent feature in J I 18 20 the Holocene coral ∆δ O records. These periodicities are, however, at the limit of J I detection with respect to the length of the considered time series, where oscillatory behaviour becomes indistinguishable from a secular trend. Nevertheless, the 6.22 ka Back Close

coral record indicates enhanced spectral power in this band that is unprecedented in Full Screen / Esc any other coral records. 18 25 The magnitude of ∆δ O changes throughout the mid- to late Holocene is further Printer-friendly Version investigated for different timescales. Individual coral ∆δ18O records have been filtered with a Gaussian bandpass filter (Paillard et al., 1996) to isolate quasi-biennial Interactive Discussion

3913 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion (< 2.3-yr), interannual (2.3- to 7-yr), near-decadal (7- to 15-yr) and inter- to multidecadal (15- to 40-yr) components of variability (Fig. 8). The amplitude of the quasi- CPD biennial, interannual, and near-decadal coral ∆δ18O variability has not notably changed 8, 3901–3948, 2012 throughout the mid- to late Holocene compared to that of the inter- to multidecadal 5 band. This experiment reveals that the standard deviation of the 15- to 40-yr bandpass- filtered time series of the 6.22 ka coral ∆δ18O record is twice as large as the average Controls of variance given by all coral records (Fig. 8). Caribbean surface hydrology during the 18 4.5 Correlation between coral ∆δ O and Sr/Ca-SST mid- to late Holocene 18 The correlation between Sr/Ca-derived SST and ∆δ O-related salinity is investigated C. Giry et al. 10 for quasi-biennial, interannual, near-decadal, and inter- to multidecadal timescales (Fig. 9) by using the corresponding filtered time series. Coral data indicate that warmer than average SSTs are characterised by more saline conditions and vice versa. The Title Page correlation coefficient is positive for all investigated timescales. The correlation coefficients between filtered time series are greater than 0.5 in most of the records suggest- Abstract Introduction 18 15 ing that the observed relationship between SST and ∆δ O was prominent in the south- Conclusions References ern Caribbean Sea over the last 6.2 ka. To investigate the effect of different calibration values on the sign of the correlation between ∆δ18O and Sr/Ca-SST records, similar Tables Figures experiments were performed using ∆δ18O records derived from a range of proxy-SST calibrations (i.e., Hetzinger et al., 2006). Results show that different calibration values J I 18 20 do not affect the sign of the correlation between Sr/Ca-derived SST and ∆δ O-related salinity (not shown). J I Back Close

5 Discussion Full Screen / Esc

18 5.1 Mean δ Osw changes over the last 6.2 ka Printer-friendly Version

In the tropical Atlantic as well as other tropical locations, orbitally-driven changes in in- Interactive Discussion 25 solation received on top of the atmosphere have been shown to influence the latitudinal 3914 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion mean position of the zonal band of maximum precipitation also referred to as the In- tertropical Convergence Zone (ITCZ) (e.g., Haug et al., 2001; Fleitmann et al., 2003). CPD For instance, during the early and mid-Holocene, mid-latitudes of the northern Hemi- 8, 3901–3948, 2012 sphere were characterized by greater insolation in summer compared to today and 5 proxy reconstructions showed that the ITCZ was located further to the north at that time thus, enhancing precipitation in more northern tropical latitudes (Haug et al., 2001; Controls of Hodell et al., 1991). In line with this, our coral ∆δ18O records indicate a trend from the Caribbean surface mid- toward the late Holocene (Fig. 3) suggesting that the hydrological balance of the hydrology during the Southern Caribbean shifted from less saline in the mid-Holocene towards more saline mid- to late Holocene 10 conditions today. However, this trend is not significant, possibly due to the combined effect of different vital effects between coral colonies (Giry et al., 2012) and rapid shifts of C. Giry et al. the local hydrological conditions linked to tropical and extratropical forcing on the mean position of the ITCZ (Haug et al., 2001). Although accurate quantification of changes 18 Title Page in mean δ Osw across the mid- to late Holocene is difficult to infer from our coral 18 15 records, the decrease of δ Osw over the last 6.2 ka did not exceed 0.322 ‰ (2 σ given Abstract Introduction by three modern corals) which would correspond to a salinity increase of < 1.6 psu according to Watanabe et al. (2001). In line with this, numerical simulations integrating Conclusions References 18 an isotopic module (Oppo et al., 2007) indicated lighter Caribbean δ Osw during the Tables Figures mid-Holocene that is linked to a reduced Atlantic to Pacific water transport.

J I 20 5.2 Todays hydrological cycle and advection of fresh water J I The composite annual ∆δ18O cycles derived from modern corals revealed lower values in spring and summer and higher values in fall and winter (Figs. 6 and 10). Cli- Back Close matological data from the World Ocean Atlas 09 (Antonov et al., 2010) indicate sea Full Screen / Esc surface salinity (SSS) seasonality near Bonaire of ∼ 1.2 psu with maximum and mini- 25 mum SSS values occurring in May and September, respectively. However, according Printer-friendly Version to the SODA salinity dataset (Simple Ocean Data Assimilation Reanalysis) (Carton and Giese, 2008), the seasonal cycle for the Bonaire gridbox is ∼ 0.6 psu in amplitude Interactive Discussion with maximum and minimum SSS values occurring in April and November, respectively 3915 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion (Fig. 10). Moreover, together with a lack of local salinity data, these datasets indicate that there is no consensus on the real amplitude and timing of the annual salinity cycle CPD around Bonaire. Since Bonaire’s climate is characterized by high evaporation rate and 8, 3901–3948, 2012 low annual rainfall with the rainy season lagging maximum SST values by a month, the 18 5 local rainfall amount cannot explain the phase lag between coral δ O and Sr/Ca as inferred from the three modern corals (Fig. 6). Consequently, since local rainfall and Controls of 18 salinity data cannot explain the modern annual δ Osw cycles, alternate mechanisms Caribbean surface are proposed for explaining modern freshwater budget of the southern Caribbean as hydrology during the documented in Bonaire corals. mid- to late Holocene 10 The freshwater budget in the Caribbean is inﬂuenced by both the evaporation and precipitation over the basin (Etter et al., 1987) and transported freshwater from the C. Giry et al. Amazon and Orinoco rivers by surface currents to the Caribbean (Hellweger and Gor- don, 2002). The period of strongest evaporation occurs in wintertime (Etter et al., 1987; Title Page Giannini et al., 2000; Hastenrath and Lamb, 1978) and the lowest in summertime. 18 15 Therefore, the annual ∆δ O cycle inferred from modern corals could be partially ex- Abstract Introduction plained by seasonal changes in the the local freshwater ﬂux over the Caribbean. The Caribbean Current (CC) is the dominant surface current in the Caribbean Conclusions References

Sea transporting warm and fresh surface water northwestward from the southeast- Tables Figures ern Caribbean to the Gulf of Mexico (Wust,¨ 1964) (Fig. 1). The outﬂow of this warm 20 water mass from the Caribbean concentrates in the Florida Strait between the Florida J I Keys and Cuba contributing to the Gulf Stream and the Western Boundary Current. −1 The highest surface velocity in the Caribbean Sea (exceeding 100 cm s ) was found J I near the southern boundary along the coast of Venezuela and the Netherlands Antilles Back Close (Fratantoni, 2001; Hernandez´ et al., 2000) suggesting that the southern Caribbean 25 Sea and the study site are highly sensitive to changes in surface water transport by Full Screen / Esc the CC. Water masses transported through the Caribbean originate from the North Brazilian Current (NBC) and the Guyana Current (GC) which transport South Atlantic Printer-friendly Version waters and fresh, isotopically light, waters discharged from the Amazon River, into the Caribbean (Cherubin´ and Richardson, 2007; LeGrande and Schmidt, 2006; Hellweger Interactive Discussion

3916 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion and Gordon, 2002) (Fig. 1). The total flow through the Caribbean displays a seasonal cycle that is strongest in the far southeastern Caribbean through the Windward Islands CPD passages (Johns et al., 2002). Moreover, the inflow in the far southern Caribbean is 8, 3901–3948, 2012 intimately linked to the outflow from the Caribbean to the subtropical North Atlantic 5 through the Florida Current (Johns et al., 2002). Extensive studies on surface water transport by the Florida Current indicate strong seasonality in water transport through Controls of the Florida Strait (Johns et al., 2002; Larsen, 1992; Molinari et al., 1990; Schott et al., Caribbean surface 1988) (Fig. 10). The outflow from the Caribbean shows maximum transport in spring hydrology during the and summer and minimum in fall. From spring to summer, maximum transport appears mid- to late Holocene 10 to result from a strengthened NBC and GC primarily driven by changes in seasonal winds (Johns et al., 2002; Muller-Karger¨ et al., 1989). The reduction of Atlantic inflow C. Giry et al. into the Caribbean in the second half of the year is thought to be linked to a weakened NBC and GC that redirect fresher tropical Atlantic water eastward at this time (Johns et Title Page al., 2002). Consequently, the annual ∆δ18O cycle of our three modern Bonaire corals 15 can also reflect the annual cycle of fresher and isotopically light water transported by Abstract Introduction the western boundary flow into the Caribbean. Conclusions References 5.3 Mid-Holocene: enhanced precipitation in summer Tables Figures The mid-Holocene coral indicates significantly increased seasonality of the hydrologi- 18 cal cycle at 6.2 ka as inferred from both the measured coral δ O (Figs. 4 and 5) and J I 18 18 20 the coral ∆δ O records (Fig. 6). The composite annual ∆δ O cycle from this mid- J I Holocene coral indicates lowest and highest values in August and January, respectively. 18 The large uncertainties for estimating mean ∆δ O conditions (cf. Sect. 4.2) along with Back Close differences in the offset between mean coral Sr/Ca and δ18O among individual colonies Full Screen / Esc do not allow us to investigate whether increased seasonality is due to enhanced precip- 25 itation in summer and/or increased evaporation in winter. Nevertheless, the magnitude and timing of this annual ∆δ18O cycle are further investigated. Periods of the year Printer-friendly Version characterised by deviation from the mean greater than 0.4 ‰ (standard choice) are Interactive Discussion displayed in Fig. 6. The annual ∆δ18O cycle documented in the mid-Holocene coral 3917 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion indicates that significant deviations occur during both summer and winter seasons. However, the negative deviation from the mean during summer months is unprece- CPD dented in this record. The sharp transition from more positive to more negative values 8, 3901–3948, 2012 seems to occur within less than two months. This sharp summer peak occurring in 5 June–July is typical for a transition from more to less saline conditions that could be linked to enhanced convective activity at that time. Controls of Proxy reconstructions of mid-Holocene hydrological conditions in the Caribbean Caribbean surface showed that the ITCZ was located further to the north at that time thus, enhancing hydrology during the precipitation in more northern tropical latitudes (Haug et al., 2001; Hodell et al., 1991). mid- to late Holocene 10 These records document terrestrial climate and have a temporal resolution that is sub- decadal at best. Our sub-seasonally resolved 6.22 ka coral record indicates that the C. Giry et al. 18 large deviation towards more negative δ Osw values documented in summer provides strong support for intense summer precipitation in the southern Caribbean Sea, which Title Page is possibly brought by a more northern position of the ITCZ as the main driver of mid- 15 Holocene annual sea surface hydrology. In line with this, our numerical simulations Abstract Introduction forced by insolation changes, using the coupled general circulation model COSMOS (Wei et al., 2012), indicate more local humid conditions during the mid-Holocene due Conclusions References

to more precipitation in the Caribbean (Fig. 11b, blue colour). The annual hydrological Tables Figures cycle inferred from the model suggests that more summer precipitation contributes to 20 an increased annual hydrological cycle over the Caribbean at 6 ka BP (Fig. 11a). J I The local salinity is affected by both the ocean currents and local P − E. In this coastal area, the model is not suitable to resolve regional details and shows non- J I coherent structures. However, one can detect two main features. At the coast where the Back Close freshwater enters the sea (Fig. 12), salinity is reduced for 6 ka whereas offshore salinity 25 is increased, mainly for the boreal spring season (not shown). The hydrology is largely Full Screen / Esc affected by ocean currents transporting relatively fresh water from the southward locations which are under the influence of the Amazon and Orinoco rivers. Easterlies Printer-friendly Version get weakened at 6 ka (∼ 10 %) resulting in slower ocean current and less import of Interactive Discussion

3918 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion freshwater from the south. Consequently, the water becomes more under the influence of the subtropical regions with negative P − E. CPD These simulations indicate that increased precipitation in the summer months is the 8, 3901–3948, 2012 prevailing factor controlling an increased seasonality of the hydrological cycle during 18 5 the mid-Holocene (Fig. 11). Therefore, our monthly-resolved mid-Holocene δ Osw record is consistent with both the increased mean precipitation over the southern Controls of Caribbean (Haug et al., 2001) and the enhanced precipitation in Northern Hemisphere Caribbean surface summer as observed over the North African continent at that time (Collins et al., 2011). hydrology during the mid- to late Holocene 5.4 Mid- to late Holocene transition C. Giry et al. 10 The 6.22 ka coral suggests a dominant effect through enhanced summer rainfall relative to advection of freshwater, whereas the three modern corals indicate less P − E relative to the advection of freshwater as the prevailing factor controlling the annual cycle of sea Title Page surface hydrology. These differences suggest a transition since the mid-Holocene. Abstract Introduction The 4.27 and 3.79 ka corals show high δ18O seasonality having no phase lag with 18 15 the corresponding annual Sr/Ca record suggesting that more negative (positive) δ O Conclusions References values occurred in phase with warmer (colder) SST. This suggests wet summers and/or 18 Tables Figures dry winters. As for the 6.22 ka coral record, “sharp” δ Osw summer peaks towards more negative values are observed in the composite annual cycle of these two coral records suggesting that wet summers were potentially characteristic in the southern J I 20 Caribbean climate at these time intervals of the distant past. However, this “sharp” J I peak towards lower values in summer is less pronounced in more recent coral records and especially in modern corals suggesting that either loss of summer precipitation or Back Close reduced advection of fresher water through the Caribbean Current in summer occurred Full Screen / Esc throughout the mid- to late Holocene, thus damping this “sharp” summer peak in more 25 recent coral records (Fig. 6). Moreover, the 3.83 and 1.84 ka coral records indicate that annual coral Sr/Ca record leads corresponding δ18O records by about half a month Printer-friendly Version suggesting that less (more) saline conditions occurred after maximum (minimum) SST Interactive Discussion values. One can infer from the measured coral Sr/Ca and δ18O data that changes in 3919 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 18 the timing between annual SST and δ Osw cycles occurred in the southern Caribbean Sea throughout the mid- to late Holocene. This gives rise to the possible competing CPD influence of both the hydrological cycle and oceanic advection of freshwater on the 8, 3901–3948, 2012 annual hydrology at the study site. 5 An exceptionally large increased SST seasonality compared to both today and the mid-Holocene is indicated by the 2.35 ka coral (Giry et al., 2012, 2010b). This is ac- Controls of companied by a dampening of coral δ18O seasonality despite of an enhanced coral Caribbean surface 18 hydrology during the Sr/Ca seasonality resulting in a reversal of the δ Osw annual cycle. This reversal in 18 mid- to late Holocene the seasonality of the hydrological cycle around 2.35 ka BP indicates low (high) δ Osw 10 values in winter (summer). Considering local rainfall as the main driver of seasonal 18 C. Giry et al. δ Osw variations, enhanced precipitation/reduced evaporation during winter and/or reduced precipitation/enhanced evaporation during summer/fall relative to today could 18 explain the observed annual ∆δ O cycle at 2.35 ka. However, reduced advection of Title Page freshwater during the warm season and/or enhanced advection during the cold season Abstract Introduction 15 could be an alternate mechanism. To validate the latter, surface winds would modulate both temperature (i.e., wind-induced heat loss) and hydrological conditions (i.e., wind- Conclusions References induced oceanic advection of freshwater from tropical Atlantic) at the sea surface. In this sense, weakened (strengthened) surface winds in summer (winter) could generate Tables Figures positive (negative) SST anomalies and reduced (intensified) transport of fresh tropical 20 water to the study site, thus explaining the large SST seasonality and reversal of the J I annual hydrological cycle at 2.35 ka. To summarise, summer precipitation dominated the annual hydrological cycles in the J I mid-Holocene. In contrast, enhanced evaporation in winter and oceanic advection of Back Close freshwater discharged from the Orinoco and the Amazon Rivers by wind-driven surface 18 Full Screen / Esc 25 currents dominate the annual δ Osw cycle at Bonaire today. This is consistent with the strengthening of easterly trade-winds associated with the southward migration of the ITCZ throughout the Holocene. Printer-friendly Version

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3920 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 5.5 Control of surface winds on surface temperature and hydrology CPD Holocene coral Sr/Ca-SST and coral ∆δ18O records reveal that warmer than average SST around Bonaire were characterised by more saline conditions (Fig. 9). This is true 8, 3901–3948, 2012 for all investigated timescales including quasi-biennial and interannual to multidecadal 5 timescales. On a quasi-biennial timescale, the oscillation of zonal winds in the tropical Controls of troposphere has shown to be associated with sea level pressure anomalies and hurri- Caribbean surface cane activity in the Atlantic (Gray, 1984). On interannual timescales, ENSO is known to hydrology during the be a major player in controlling both tropical Atlantic SST and precipitation (Alexander mid- to late Holocene and Scott, 2002; Chiang et al., 2002; Giannini et al., 2000). An observational study by 10 Chiang et al. (2002) demonstrated that an El Nino˜ teleconnection to the tropical Atlantic C. Giry et al. results in an anomalous warm tropical North Atlantic, mainly through a warming of the tropical troposphere, owing to anomalous heating in the eastern equatorial Paciﬁc. In addition, warming of the troposphere in the Atlantic reduces convection and thus pre- Title Page

cipitation over the tropical Atlantic. Moreover, intense convection in the tropical Paciﬁc Abstract Introduction 15 associated with warm ENSO leads to suppression of rainfall over northern South Amer- ica and the Amazon Basin (Garreaud et al., 2009). Although there are arguments that Conclusions References ENSO activity was reduced in the mid-Holocene (Clement et al., 1999; Tudhope et Tables Figures al., 2001), we ﬁnd that the 6.22 ka coral record also shows a similar relationship between reconstructed salinity and SST at interannual timescale suggesting that a unique J I 20 physical mechanism was responsible for such relationship over the last 6.2 ka around

Bonaire. On multidecadal timescales, the Atlantic Multidecadal Oscillation (AMO) is J I known to play a critical role in controlling tropical Atlantic precipitation anomalies (Sut- ton and Hodson, 2005). For instance, during the warm phase of the AMO the tropical Back Close

Atlantic and Caribbean experience positive SST anomalies and positive summer pre- Full Screen / Esc 25 cipitation anomalies, whereas summer precipitation over northern South America and the Amazon Basin seems to show an inverse relationship with the AMO (Sutton and Printer-friendly Version Hodson, 2005). Interactive Discussion

3921 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Given the positive correlation between SST and salinity as documented in Bonaire corals (i.e., warmer/colder conditions go along with more/less saline conditions) for a CPD wide range of timescales, it is hypothesised that a single physical mechanism controlled 8, 3901–3948, 2012 this correlation over the last 6.2 ka. We propose that changes in the strength of zonal 5 surface winds modulate both the SST and the advection of tropical Atlantic freshwater into the southern Caribbean Sea as illustrated in Fig. 13. In a previous study (Giry et Controls of al., 2012), we reported quasi-biennial to multidecadal variability of southern Caribbean Caribbean surface SST as documented in our Bonaire coral Sr/Ca records. Since SST and surface winds hydrology during the are strongly correlated around Bonaire (Fig. 9 from Wang, 2007), we assume that inter- mid- to late Holocene 10 annual to multidecadal changes of surface winds drive southern Caribbean SST variations on these timescales. Moreover, as modern Caribbean throughflow shows strong C. Giry et al. wind-driven variations (e.g., Johns et al., 2002), we suggest that zonal surface winds force the inflow of fresh tropical water into the Caribbean via Ekman transport through Title Page the North Brazilian-Guyana-Caribbean current system. This is further supported by 15 the high correlation coefficient found between annual mean SST and zonal surface Abstract Introduction wind and current at Bonaire (Fig. 2 in the Supplement). Moreover, this is consistent with a coral-based study from Puerto Rico (Kilbourne et al., 2007) which indicated that Conclusions References

equatorial water is transported to the Caribbean by Ekman transport and is related to Tables Figures trade-winds variability. At Bonaire, strengthened easterlies contribute to stronger heat 20 loss that cool down SST, while it forces Ekman transport and oceanic surface circula- J I tion that transport fresher tropical Atlantic water into the Caribbean and thus contribute to colder and fresher conditions on interannual timescales in the southern Caribbean. J I

5.6 Enhanced inter- to multidecadal variability of surface hydrology at 6.2 ka Back Close

Full Screen / Esc Coral ∆δ18O records from Bonaire indicate prominent inter- to multidecadal period- 25 icities (Fig. 7) suggesting a well-marked feature of southern Caribbean Sea surface hydrological conditions over the last 6.2 ka. The 6.22 ka coral ∆δ18O record shows en- Printer-friendly Version hanced variability at inter- to multidecadal timescales (Figs. 7 and 8). Since Caribbean Interactive Discussion hydrology at 6 ka was influenced by increased precipitation and more river runoff, the 3922 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 18 δ Osw-salinity linear regression known in today’s oceans might not be valid for the mid-Holocene time slice (Craig and Gordon, 1965; Delaygue et al., 2000; Rohling and CPD 18 Bigg, 1998; Schmidt, 1999), thus hindering accurate quantification of ∆δ O-related 8, 3901–3948, 2012 salinity changes for the distant past. Nevertheless, the amplitude of this multidecadal 18 5 ∆δ O signal (∼ 0.3 ‰) is larger than the corresponding annual cycle, thus providing evidence for enhanced multidecadal variability of southern Caribbean climate during Controls of the mid-Holocene. Caribbean surface A previous coral δ18O-based study from the Dominican Republic suggested inter- hydrology during the decadal variability of tropical Atlantic precipitation in the mid-Holocene (Greer and mid- to late Holocene 10 Swart, 2006). It was suggested that the latitudinal migration of ITCZ or increased storm C. Giry et al. activity have modulated interdecadal changes of northern Caribbean precipitation cap- tured in fossil corals of the Enriquillo Valley. In line with this, Knudsen et al. (2011)

showed that precipitation over northern South America as inferred from the Cariaco Title Page Titanium record (Haug et al., 2001) has experienced multidecadal ﬂuctuations linked 15 to the AMO over the last 8 ka. A recent study showed that the southern Caribbean Sea Abstract Introduction experienced inter- to multidecadal variability of SST during the mid-Holocene (Giry et Conclusions References al., 2012). All of which points to the role of the atmospheric circulation in controlling inter- to multidecadal variability as documented in Bonaire corals records. Tables Figures Other mechanisms reported in the literature could explain the positive correlation ob-

20 served between SSS and SST and possibly the enhanced inter- to multidecadal vari- J I ability as documented in the mid-Holocene coral records. On multidecadal timescales, the AMO can be assumed to be partly linked to the strength of the AMOC (Delworth J I

and Mann, 2000; Knight et al., 2005). During periods of stronger AMOC, meridional Back Close heat transport through ocean circulation from low- to high-latitude influences SST in 25 the North Atlantic realm which is then characterised by a warmer than average SST Full Screen / Esc characteristic of the positive phase of the AMO (Sutton and Hodson, 2003). There is a general agreement from water hosing experiments that a slowdown of the AMOC Printer-friendly Version cools down most of the entire North Atlantic (Stouffer et al., 2006). However, similar experiments using a high-resolution coupled ocean-atmosphere model (Wan et Interactive Discussion 3923 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion al., 2009) revealed that atmospheric processes in response to a weakened AMOC in- deed produce surface cooling in most of the North Atlantic realm, whereas a narrow CPD strip of warmer surface water appears along the northern coast of South America. 8, 3901–3948, 2012 Wan et al. (2009) suggested that a weakening of the AMOC produces a weakening 5 of the western boundary current which in turn creates a strong subsurface temperature warming that extends to the surface mixed layer (such feature is actually seen Controls of also in coarse-resolution models, e.g. in Lohmann (2003) and some of the models Caribbean surface mentioned in Stouffer et al. (2006). In addition, there is a general consensus based hydrology during the on water hosing experiments in climate model simulations that the mean position of mid- to late Holocene 10 the ITCZ is shifted southward during a weakened AMOC resulting in negative precipitation anomalies over the northern tropical Atlantic and northern South America C. Giry et al. (e.g., Stouffer et al., 2006; Zhang and Delworth, 2005; Lohmann, 2003; Vellinga and Wood, 2002). This is consistent with the model-based study of Wan et al. (2010) in- Title Page dicating that atmospheric responses to reduced AMOC would increase salinity in the 15 Caribbean. Given that changes from warm and saline to cold and fresh conditions in Abstract Introduction the southern Caribbean Sea were observed during periods of weakened large-scale ocean circulation (e.g., Younger Dryas) (Ruhlemann¨ et al., 1999; Schmidt et al., 2004), Conclusions References

our study indicates that on inter- to multidecadal timescales, a scale relevant for ocean Tables Figures circulation, changes in the strength of the AMOC could partially explain the positive 20 relationship between SST and SSS derived from Bonaire coral records. Moreover, J I if this were proven true, enhanced climate variability observed in the mid-Holocene Caribbean climate (e.g., Greer and Swart, 2006; Giry et al., 2012) would reﬂect mul- J I tidecadal variability potentially linked to changes in the strength of the AMOC. Finally, Back Close this would suggest that in-phase variations between the ocean and the atmosphere 25 would create strong inter- to multidecadal climate variability. Full Screen / Esc

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3924 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 6 Conclusions CPD Seasonality and interannual to multidecadal variability of surface hydrology in the Caribbean is investigated for well-distributed time windows throughout the mid- to late 8, 3901–3948, 2012 Holocene by using decades-long monthly-resolved paired Sr/Ca and δ18O measure- 18 5 ments of Bonaire corals (southern Caribbean Sea). Derived coral ∆δ O is used as a Controls of 18 proxy of the oxygen isotopic composition of the surface seawater (δ Osw). While this Caribbean surface study indicate less saline conditions during the mid-Holocene compared to today, sea- hydrology during the sonality and interannual to multidecadal variability of the hydrological cycle have been mid- to late Holocene investigated and can be summarised as follows: C. Giry et al. 10 1. Consistent with modern Caribbean surface-water hydrology, the large positive deviation of the coral ∆δ18O annual cycle in the late part of the year indicates that enhanced evaporation and reduced advection of fresh surface water in winter- 18 Title Page time are the dominant factors controlling the annual δ Osw cycle as inferred from three modern corals. Abstract Introduction

15 2. The 6.22 ka coral indicates increased seasonality of the annual hydrological cy- Conclusions References cle during the mid-Holocene. This increased seasonality is very likely induced by enhanced precipitation in summertime as inferred from both climate model simu- Tables Figures lations and the composite annual ∆δ18O cycle at 6.22 ka. J I 3. On interannual to multidecadal timescales, coral Sr/Ca and ∆δ18O reveal that 20 warmer than average conditions were characterised by more saline condi- J I

tions and vice versa. Since this systematic relationship is found for very short Back Close timescales relevant for atmospheric circulation, we propose that the surface wind- driven advection of fresher surface waters from the Amazon and Orinoco rivers Full Screen / Esc to the Caribbean explains the positive correlation between SST and salinity doc- 25 umented in the Bonaire coral records. Printer-friendly Version

4. Our mid-Holocene coral record provides evidence for enhanced inter- to mul- Interactive Discussion tidecadal variability of the sea surface hydrology during the mid-Holocene. 3925 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Although an accurate quantiﬁcation of the relative salinity changes is diﬃcult to infer for this time interval of the distant past, the record indicates that the amplitude CPD of the inter- to multidecadal variability exceeds the corresponding annual cycle at 8, 3901–3948, 2012 6.22 ka BP.

5 P E This study identified both the freshwater flux − and oceanic advection of fresher Controls of surface waters by wind-driven surface currents as the prevailing factors influencing the Caribbean surface freshwater budget of the southern Caribbean during the mid- to late Holocene. Since hydrology during the the density structure of North Atlantic water can have drastic impacts on ocean cir- mid- to late Holocene culation patterns and heat transport from low to high latitudes, we propose to further 10 investigate interannual to millennial variability of western tropical Atlantic climate and C. Giry et al. riverine discharge from South American tropical rivers as a potential feedback mechanism affecting the strength of the AMOC. Title Page

Supplementary material related to this article is available online at: Abstract Introduction http://www.clim-past-discuss.net/8/3901/2012/cpd-8-3901-2012-supplement. Conclusions References 15 pdf. Tables Figures

Acknowledgements. We thank the Government of the Island Territory of Bonaire (former Netherlands Antilles) for research and ﬁeldwork permission, E. Beukenboom (STINAPA J I Bonaire National Parks Foundation) for support, J. Patzold¨ for support with coral drilling and J I discussion, M. Segl and her team for stable isotope analysis, and S. Pape for operating and 20 maintaining the ICP-OES. This work was funded by Deutsche Forschungsgemeinschaft (DFG) Back Close under the Special Priority Programme INTERDYNAMIC, through grants to T.F. (FE 615/3-1, 3-2; CaribClim). Full Screen / Esc

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Fig. 1. Modern oceanographic setting of the Caribbean and western North Atlantic. Schematic of major surface currents constituting the upper branch of the Atlantic meridional overturning J I circulation (AMOC) are superimposed on annual mean sea surface salinity (Antonov et al., J I 2006). Major surface currents: North Brazilian Current (NBC), Guyana Current (GC), Caribbean Current (CC), Florida Current (FC), Gulf Stream (GS) and North Equatorial Current (NEC). Back Close White circle indicates the study site (Bonaire). Full Screen / Esc

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1 Abstract Introduction 0.0 d D l a

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1.0 Tables Figures 10 a Internal chronology (a) Growth direction

18 Fig. 2. Monthly-resolved coral Sr/Ca (red) (Giry et al., 2012), coral δ O (blue) records and de- J I rived coral ∆δ18O (black) records of modern (yellow shading) and fossil (grey shading) Diploria strigosa corals from Bonaire and corresponding 5-yr running mean. Coral ∆δ18O records were J I 18 calculated using the centering method (Cahyarini et al., 2008) and Sr/Ca-SST and δ O-SST Back Close slopes of −0.061 mmol/mol per ◦C and −0.180 ‰ per ◦C (Correge,` 2006; Gagan et al., 1998), respectively. The U-series ages of the individual coral colonies (Giry et al., 2012) are indicated. Full Screen / Esc

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CPD 8, 3901–3948, 2012

0.4 Ti content (%), Controls of

) Wet 0.3 Haug et al., (2001) Caribbean surface % ( hydrology during the m

u 0.2 i Dry mid- to late Holocene n a t i )

T 0.1 Less

n C. Giry et al. -0.4 ‰ a

saline ( e

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More u saline 0.2 n Title Page 18 n

Coral Dd O 0.4 A Coral Abstract Introduction

0 1 2 3 4 5 6 Age ka BP Conclusions References

Fig. 3. Comparison between mean coral ∆δ18O and Cariaco Basin Ti record (Haug et al., Tables Figures 2001). Coral ∆δ18O has been calculated for individual records using established procedures (Cahyarini et al., 2008; Correge,` 2006; Gagan et al., 1998). Error bars are the combined error J I (root of the sum of the squares) of the standard deviation (2 σ) from mean coral ∆δ18O of three modern Diploria strigosa and the standard error (2SE) of the mean coral ∆δ18O value for indi- J I vidual corals, following established procedure (Abram et al., 2009) but applied to reconstructed Back Close coral ∆δ18O. Note that coral ∆δ18O records indicate trend toward more saline conditions during the mid- to late Holocene that might be linked to more precipitation at 6 ka. Full Screen / Esc

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CPD 8, 3901–3948, 2012

) 0.8 Controls of ‰

( Caribbean surface y

t hydrology during the i l 0.6 Modern a mid- to late Holocene n o

s C. Giry et al. a 0.4 e s O 8

1 Title Page

d 0.2 AD AD AD l a

r Abstract Introduction o 1957 4.27 ka 3.83 ka 3.79 ka 2.35 ka 1.84 ka 1912 2009 6.22 ka C 0 Conclusions References

18 Fig. 4. Coral δ O seasonality calculated for individual Bonaire coral records. Dashed line Tables Figures represents the mean modern δ18O seasonality (0.440 ‰) calculated from the mean seasonality of the three modern corals. Error bars for fossil corals are the combined error (root of the sum J I of the squares) of the standard deviation (2 σ) from mean δ18O seasonality of three modern 18 Diploria strigosa and the standard error (2SE) of the averaged δ O seasonality measurements J I for individual coral records, following established procedures (Abram et al., 2009) but applied to reconstructed seasonality. Error bars for modern corals are 2SE. Note that the 6.22 ka coral Back Close shows signiﬁcantly increased δ18O seasonality compared to that given by three modern corals. Full Screen / Esc

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CPD 8, 3901–3948, 2012

a) 6.22 ka 4.27 ka 3.83 ka 3.79 ka 2.35 ka 1.84 ka 1912 AD 1957 AD 2009 AD Controls of 18

e 1.5 ) Coral d O DJF MAM JJA SON l C c Caribbean surface ° y (

c 1.0 T l S a Less hydrology during the u S 0.5 Warm n saline d n e a

v mid- to late Holocene

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o -0.5 y p x m o C. Giry et al.

r -1.0 More o

P Cold C Coral Sr/Ca saline -1.5

J MM J S N J MM J S N J MM J S N J MM J S N J MM J S N J MM J S N J MM J S N J MM J S N J MM J S N Months b) Title Page 1.0 18

e Coral d O leads l ) 0.5 g s

n Abstract Introduction h t a n

e 0 o s a m ( h -0.5

P Conclusions References Coral Sr/Ca leads -1.0

6 5 4 3 2 1 0 Tables Figures Age ka

18 Fig. 5. (a) composite annual cycles of coral Sr/Ca and δ O for each coral record used in J I this study converted in degree Celsius by using Sr/Ca-SST and δ18O-SST relationships of −0.061 mmol/mol per ◦C and −0.180 ‰ per ◦C (Correge,` 2006; Gagan et al., 1998). Error bars J I indicate the standard error of the mean calculated for individual months. (b) phase angles of Back Close the annual cycles estimated by cross-spectral analyses between monthly coral Sr/Ca and coral 18 δ O time series. Error bars indicate the 80 % conﬁdence interval. Full Screen / Esc

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1912 AD 1957 AD 2009 AD e

l -0.15 CPD

c 18

y Dd O analytical

c -0.10 uncertainty l 8, 3901–3948, 2012 ) a

u -0.05 ‰ n (

n ±0.077‰ (1s) O a 0.00 8 1 e 0.147‰ 0.162‰ d t Controls of i 0.162‰ D 0.05 s

o Caribbean surface p 0.10 m

o hydrology during the 0.15 DJF MAM JJA SON C mid- to late Holocene J A J O J A J A J O J A J A J O J A Months C. Giry et al. 6.22 ka 4.27 ka 3.83 ka 3.79 ka 2.35 ka 1.84 ka e

l -0.15 c y

c -0.10 l ) a Title Page

u -0.05 ‰ n ( n O a 0.00 8

1 Abstract Introduction e 0.193‰ d t i

D 0.05 s o

p 0.10 Conclusions References m

o 0.15 C Tables Figures J A J O J A J A J O J A J A J O J A J A J O J A J A J O J A J A J O J A Months J I Fig. 6. Composite annual coral ∆δ18O cycles for three modern (top) and six fossil (bottom) corals calculated using the centering method (Cahyarini et al., 2008) and Sr/Ca-SST and J I 18 ◦ ◦ δ O-SST relationships of −0.061 mmol/mol per C and −0.180 ‰ per C (Correge,` 2006; Back Close Gagan et al., 1998). Combined analytical uncertainty is 0.154 ‰ (2 σ). Composite annual cycles represented in black (gray) are characterised by an amplitude of the annual ∆δ18O cycle Full Screen / Esc higher (lower) than 2 σ. Vertical gray shading indicates seasons: winter (December–February, DJF), spring (March–May, MAM), summer (June–August, JJA) and fall (September–November, Printer-friendly Version SON). Negative and positive deviations from the mean value greater than 0.4 ‰ are indicated by blue and red colours, respectively. Interactive Discussion

a) b) 400 e )

c 100 n

h Bonaire rainfall CPD n o t 300 i 5.3 1.5 a t n i r a

o 3.6 1.9

t 10 a i 200 99 % m v p / 95 % i

e 90 % c m v

e 1 100 i r m t

( 8, 3901–3948, 2012 a P

l UEA CRU 1930-1999

0 e 0.1 1930 1940 1950 1960 1970 1980 1990 2000 R 0 0.2 0.4 0.6 Years AD Frequency (cycles yr-1)

-1.0 1912 ± 8 AD 1957 ± 5 AD

O 2009 AD 1837 ± 18 a BP 8

1 -0.5 d ) D Controls of

l 0.0 ‰ ( a r

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c 100

2347 ± 49 a BP n O 34* 6.1 hydrology during the a 8 i

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l 0.0 ‰ 95 % ( a e 90 % r

v mid- to late Holocene 1 i o

0.5 t C a 2.35 ka BP l

1.0 e 0.1 R

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3793 ± 29 a BP c 100 O C. Giry et al. 8 n 19*

1 -0.5 a d i

) 3.9 r D 10 1.9 a

l 0.0 ‰ (

v 99 % a 1.5

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l 0.0 ‰ ( a 99 % e r 95 % v 1 o 0.5 i 90 % t C a 6.22 ka BP l

1.0 e 0.1 R 0 0.2 0.4 0.6 Growth direction -1 10 years Frequency (cycles yr ) J I

Fig. 7. (a) monthly rainfall record from gridbox centred on Bonaire (12.5◦ N; 67.5◦ W) (top) (Hulme et al., 1998) and J I monthly reconstructed coral ∆δ18O records from Bonaire corals calculated using the centering method (Cahyarini et Back Close al., 2008). Thick blue lines represent the 5-yr running average. Sr/Ca-SST and δ18O-SST regression coefficients used are −0.061 mmol/mol per ◦C and −0.180 mmol/mol per ◦C, respectively. U-series ages of fossil corals (Giry et al., 2012) are indicated. (b) multitaper method (MTM) spectral analysis (significance relative to a red noise null hypothesis Full Screen / Esc determined with the robust method of noise background estimation (Ghil et al., 2002; Mann and Lees, 1996); number of tapers 3; bandwidth parameter 2) of detrended and normalised monthly precipitation and coral ∆δ18O records with Printer-friendly Version length > 20 yr where the mean annual cycle has been removed. 90 %, 95 %, and 99 % confidence levels are indicated. Significant spectral peaks are labelled. Asterisks indicate periodicities at the limit of detection with respect to the length of the time series, where oscillatory behaviour becomes indistinguishable from a secular trend. Interactive Discussion

CPD 8, 3901–3948, 2012 6.22 ka 4.27 ka 3.83 ka 3.79 ka 2.35 ka 0.3 4.27 ka 3.83 ka 3.79 ka 2.35 ka 6.22 ka 0.2 0.15 )

Quasi-biennial O )

Quasi-biennial 8 1

(<2.3-year) (<2.3-yr) d

‰ 0.1 0.10 D ( Controls of

0.0 ‰ O ( 8 0.05 1 D

d -0.1

T Caribbean surface D 0.3 S -0.2 Interannual 0.00 0.2 hydrology during the

0.15 ) -0.3 (2.3- to 7-year) )

Interannual O 8 ‰ 0.1 (2.3- to 7-yr) 1 ( 0.10 d mid- to late Holocene D

0.0 O 8 ‰ 1 (

-0.1 d 0.05 D D

T C. Giry et al.

-0.2 S 0.3 0.00 -0.3

0.2 Near-decadal 0.15 ) )

Near-decadal O (7- to 15-year) 8 (7- to 15-yr) 1 ‰ 0.1 d

( 0.10 D 0.0 O ‰ 8 ( Title Page 1 0.05 d -0.1 D T D -0.2 0.3 0.00 S Multidecadal Abstract Introduction -0.3 0.2 0.15 ) (15- to 40-year) )

Multidecadal O 8 ‰ 0.1 (15- to 40-yr) 1 ( 0.10 d D

0.0 O Conclusions References 8 ‰ 1 ( -0.1 d 0.05 D D T

-0.2 0.00 S Tables Figures -0.3 4.27 ka 3.83 ka 3.79 ka 2.35 ka 10 years 6.22 ka J I Fig. 8. Quasi-biennial, interannual, near-decadal and multidecadal Gaussian bandpass filtered time series for individual coral ∆δ18O records (from Fig. 7) with length > 20 yr (left panel) and J I the corresponding standard deviations of filtered time series (right panels). Horizontal dashed Back Close lines and grey areas indicate the mean standard deviations (STD) of the filtered time series and the corresponding 2 σ, respectively. Black arrows indicate that the variance of the multidecadal Full Screen / Esc bandpass filtered time series of the 6.2 ka coral ∆δ18O record is twice as large as given by the corresponding mean standard deviation of all coral records. Printer-friendly Version

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) 1.0 C °

( CPD y l 0.5 8, 3901–3948, 2012 a m

o 0.0 n

a Controls of

T -0.5 Caribbean surface S

S Quasi-biennial - Interannual hydrology during the a -1.0 (<2.3-year) (2.3- to 7-year) mid- to late Holocene C / r

S C. Giry et al.

) 1.0 C ° ( y

l 0.5 Title Page a

m Abstract Introduction o 0.0 n a Conclusions References T -0.5 S S

- Tables Figures Near-decadal Multidecadal a -1.0 (7- to 15-year) (15- to 40-year) C / r

S J I SST + -0.2 -0.1 0.0 0.1 0.2 -0.2 -0.1 0.0 0.1 0.2 Dd18O anomaly (‰) Dd18O anomaly (‰) J I Salinity + Back Close

Full Screen / Esc Fig. 9. Linear regressions between Gaussian bandpass ﬁltered coral ∆δ18O and Sr/Ca-SST time series for Bonaire coral records with length > 20 yr as calculated after Correge` (2006) Printer-friendly Version and Gagan et al. (1998). Note that for all investigated timescales, most coral records indicate positive correlation coeﬃcients which suggest that periods of warmer than average sea surface Interactive Discussion temperatures (SST) are characterised by more saline conditions and vice versa. 3944 34.8 a) SODA, SSS 35.2 ) u

WOA09, SSS s

35.6 p ( S

36.0 S S 36.4

100 ) b) h 28.5 t ERSST, SST Wet n 80 Warm o Rainfall 28.0 m / 60 27.5 ) m C ° ( m

40 T l l S a f 26.5 S

Dry n 20 Cold i

a 26.0 R 0 25.5 CPD J F M A M J J A S O N D Months 8, 3901–3948, 2012

18 d Osw

18 e 34.8 Modern Coral Dd O -0.10 l c) c

More y ) a) c ) SODA, SSS 35.2 negative -0.05 l Controls of ‰ u a ( s

WOA09, SSS u p O 35.6 n (

0.00 8

n 1 Caribbean surface d a S D 36.0 S 0.05 e l t S i

a hydrology during the s r

36.4 o More 0.10 o p C

positive m mid- to late Holocene

100 o ) b) 0.15 h 28.5 C t ERSST, SST 35 d) Larsen (1992) Wet n 80 Warm Model o Rainfall 28.0 C. Giry et al. m / 60 27.5 ) Strong m C ° ( m ( 27.0 30 40 T l Schott et al., (1988) l S a f 26.5 S

Dry n 20 Cold Weak i ransport (Sv) a 26.0 T Molinari et al., (1990)

R Title Page 0 25.5 25 Caribbean water transport J F M A M J J A S O N D J F M A M J J A S O N D Months Months Abstract Introduction

18 d Osw Conclusions References 18 e Fig. 10. Left: monthlyModern mean Coral climatology Dd O -0.10 ofl environmental parameters at the study site, Bonaire.

c) c

More y ) c

(a) compositenegative annual sea surface salinity-0.05 l (SSS) cycles from World Ocean Atlas 09 (Antonov et ‰ a ( Tables Figures ◦ ◦ u O n al., 2010) in a 1 × 1 gridbox (blue) and0.00 from8 CARTON-GIESE SODA v2p0p2-4 (Carton and n 1 d ◦ ◦ a D

0.05 e l Giese, 2008) in a 0.5 × 0.5 gridbox (black);t (b) sea surface temperature (SST) monthly clima- i a s r ◦ ◦ o More 0.10 o J I tology for 1970–2000 from ERSSTv3b (Smithp et al., 2008) in a 2 × 2 gridbox (red line) and C

positive m climatology of monthly mean precipitation0.15 o for the same period from UEA CRU Hulme Global 35 d) Larsen (1992) ◦ C ◦ prcp in a gridboxModel centred at 67.5 and 12.5 (Hulme et al., 1998) (vertical bars). Right: (c) J I 18 Strong composite annual coral ∆δ O cycle of three modern corals, 1912 AD (dashed line), 1957 AD Back Close 30 (dotted line) and 2009Schott et al., AD (1988) (solid line) calculated as previously described (cf. Fig. 6); (d) mod- Weak elled (blackransport (Sv) dashed line) and observed annual cycle of the Florida Current (modiﬁed from Johns Full Screen / Esc T Molinari et al., (1990) et al., 2002).25 Caribbean water transport J F M A M J J A S O N D Months Printer-friendly Version

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Controls of Caribbean surface hydrology during the mid- to late Holocene

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b) Title Page

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Full Screen / Esc Fig. 11. Precipitation minus Evaporation (P − E) anomaly (mm day−1) at 6 ka BP inferred from numerical simulations with the coupled general circulation model COSMOS. (a) normalised Printer-friendly Version annual cycle of P − E (mm day−1) for Pre-Industrial (PI) times (black), 6 ka (red) and their diﬀer- ◦ ◦ ence (blue dotted) averaged over the area of 65–83 W, 10–20 N. (b) annual mean P − E for Interactive Discussion PI (contour) and for 6 ka anomalies relative to PI (shaded). 3946 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion

CPD 8, 3901–3948, 2012

Controls of Caribbean surface hydrology during the mid- to late Holocene

C. Giry et al.

Title Page

Abstract Introduction

Conclusions References

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J I

Back Close Fig. 12. Annual mean salinity anomaly (psu) at 6 ka relative to Pre-Industrial (PI) inferred from numerical simulations from the coupled general circulation model COSMOS. Full Screen / Esc

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80°W 70°W 60°W 50°W

Transect a) Currents CPD

CC GC 8, 3901–3948, 2012

Bonaire

Orinoco Amazon Controls of b) Bonaire Dominant Orinoco wind direction Amazon Caribbean surface 37 hydrology during the 36 35 mid- to late Holocene Caribbean Current Guyana Current 34

33 Salinity (psu) Main surface circulation 32 C. Giry et al.

Less heat loss Weak wind c)

Title Page

Abstract Introduction Caribbean Current Guyana Current

Conclusions References d) More heat loss Strong wind Tables Figures

Caribbean Current Guyana Current J I

Longitude J I (a) map showing the transect of interest (red line) and the dominant surface currents (Guyana Current, Fig. 13. Back Close GC; Caribbean Current, CC). (b) observed sea surface Salinity (SSS) in the upper 60 m along a transect from the western tropical Atlantic to the southeastern Caribbean Sea. Freshwater discharged by the Amazon and Orinoco rivers contribute to freshening of the western tropical Atlantic that extend northwestward due to transport by Guyana and Full Screen / Esc Caribbean Currents. (c) and (d) response of sea surface conditions under two diﬀerent surface wind strength scenarios. (c) weak wind scenario: reduced extent of fresh water (contour color lines) transport northwestward due to reduced Guyana/Caribbean current system, and reduced heat loss and vertical mixing at sea surface that in turn contribute to Printer-friendly Version warmer sea surface temperature (SST). (d) strong wind scenario: intensiﬁcation of surface water transport from the western tropical Atlantic to the Caribbean by surface current system and enhanced heat loss and vertical mixing at sea Interactive Discussion surface which in turn reduce SST thus, contributing to colder and less saline conditions around Bonaire. 3948