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Pliocene aridification of Australia caused by tectonically induced weakening of the Indonesian throughflow
Palaeogeography, Palaeoclimatology, Palaeoecology 309 (2011) 111–117
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Pliocene aridification of Australia caused by tectonically induced weakening of the Indonesian throughflow Uta Krebs a,⁎, W. Park b, B. Schneider a a b
Institute for Geosciences, Kiel University, Ludewig-Meyn-Str. 10, D-24118 Kiel, Germany Leibniz Institute of Marine Sciences, IFM-GEOMAR, Duesternbrooker Weg 20, D-24105 Kiel, Germany
a r t i c l e
i n f o
Article history: Received 12 November 2010 Received in revised form 1 June 2011 Accepted 3 June 2011 Available online 16 June 2011 Keywords: Coupled modelling Indonesian throughflow Pliocene Neogene Australian vegetation Aridification Tectonic Gateway Indian Ocean Africa
a b s t r a c t Tectonic changes of the Early to Mid-Pliocene largely modified the Indonesian Passages by constricting and uplifting the passages between today's New Guinea and Sulawesi. The associated changes in strength and water mass properties of the Indonesian throughflow (ITF) might have influenced the amount of heat transported from the Pacific to the Indian Ocean and thus contributed to Pliocene climate change of the IndoPacific. We study the climate response to changes in the geometry of the Indonesian Passages in an atmosphere–ocean general circulation model (AOGCM). We compare climate simulations with present-day topography and with a topography resembling the Early Pliocene situation in the Indo-Pacific, i.e. passages East of Sulawesi deepened and widened to the South. We find that transport through the Indonesian Archipelago is weakened in the constricted passage by 1.7 Sv and in the unchanged Makassar Strait West of Sulawesi by 3.5 Sv, while transport weighted temperature of the outflow into the Indian Ocean increases by 1 °C. Consistent with recent proxy evidence the reduction in ITF transport causes a decrease in subsurface temperatures in the Indian Ocean while surface waters of the equatorial Pacific exhibit an increase by up to 0.9 °C centred in the warm pool. As a local response to the sea surface temperature anomalies, we observe an anomalous precipitation dipole across the Indonesian passages with increased rainfall over the Pacific warm pool and decreased precipitation in the eastern Indian Ocean. The Australian continent experiences a pronounced aridification with mean annual precipitation rates dropping by 30% over most parts of the continent. Using an uncoupled vegetation model, we demonstrate that the simulated climate change might partly explain the observed Late Pliocene desertification of Australia. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Pliocene atmospheric CO2 concentrations are estimated to have been higher than at preindustrial times (Kurschner et al., 1996; Raymo et al., 1996). For peak temperatures during the Pliocene, Pagani et al. (2010) provide estimates ranging from 365 ppm to 415 ppm which cover the range of projections for CO2 concentrations in the next decades (Friedlingstein et al., 2006; IPCC, 2007). It is however essential to assess the influence of those Pliocene boundary conditions that differ from today, if we want to use Pliocene climate as an analogue for future climate or as benchmark for global climate models. The present climate seems to represent the transition from an ‘icehouse’ period, with Greenland still being ice covered, to a warmer greenhouse world. In contrast, the Pliocene is transitioning in the opposite direction with sea level estimated to have been about 25 m higher and continental ice volume strongly reduced. In particular northern hemispheric ice sheets were almost absent (Miller et al.,
⁎ Corresponding author. E-mail address: [email protected] (U. Krebs). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.06.002
2010). Tectonic changes in oceanic key regions, such as the closing of the Isthmus of Panama (Coates et al., 1992; Haug and Tiedemann, 1998; Klocker et al., 2005; Lunt et al., 2008) and the constriction of the Indonesian Passages by northward drift of New Guinea and Australia (Daly et al., 1991; Hall, 2009), are considered as possible causes for the transition into the Pleistocene (around 1.8 million years before present) with its repeated glaciations. Cane and Molnar (2001) propose that the northward displacement of New Guinea during the Early- and Mid-Pliocene reduced the inflow of warm South equatorial Pacific waters into the Indian Ocean via the passages between New Guinea and Sulawesi and strengthened the influence of relative cool North equatorial Pacific waters primarily passing through the Makassar Strait. Speculatively, the authors link the tectonic change to the observed Pliocene aridification over East Africa — which may have triggered human evolution by enforcing adaption to the expansion of grassland habitats and faunal turnover (Bobe and Behrensmeyer, 2004; Wynn, 2004). Cane and Molnar (2001) suggest that changes in the Indonesian throughflow (ITF) might have reduced Indian Ocean sea surface temperature and resulted in reduced precipitation over East Africa. Indeed Karas et al. (2009) find evidence for a 4 °C Late-Pliocene cooling of subsurface water in the tropical East
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Indian Ocean and suggest a link between cooler throughflow waters and the observed global shoaling of the thermocline. Marine records of the Pliocene Pacific suggest a meridionally expanded warm pool (Brierley et al., 2009) and a decreased zonal sea surface temperature gradient (Dowsett and Robinson, 2009) when compared to today. These changes are likely to primarily reflect global climate trends (Lunt et al., 2010) or the influence of the closure of the Central American Seaway (Steph et al., 2010). Though marine data coverage is somewhat sparse, land climate changes are recorded around the IndoPacific for the last 10 million years before present. During Pliocene several regional shifts towards drier conditions are recorded not only in East Africa but also on the Indian sub-continent (Quade and Cerling, 1995; Barry et al., 2002) and in Australia (Dodson and Macphail, 2004; Martin, 2006; Fujioka et al., 2009). Reviewing numerous records Martin (2006) finds evidence for a Pliocene trend to a drier Australian climate with the exception of a brief period of wetter climates in the Early Pliocene. Southeastern Australian records however indicate that the fundamental transition towards the modern climate with pronounced dry seasons was taking place less than 1.5 million years ago (Sniderman et al., 2009; McLaren and Wallace, 2010). Tectonically induced climate change might not exclusively be caused by changes in the properties of the throughflow waters but also by changes in the strength of the throughflow. Idealised modelling studies with an ocean general circulation model (OGCM) and AOGCMs (Hirst and Godfrey, 1993; Schneider, 1998; Song et al., 2007) consistently demonstrate that a complete closure of the Indonesian passages and suppression of the ITF produces a negative temperature anomaly in and above the thermocline downstream of the passage. In the coupled GCMs (Schneider, 1998; Song et al., 2007) an Eastern Indian Ocean/equatorial Pacific sea surface temperatures decrease/ increase is accompanied by negative/positive precipitation anomalies respectively. Model experiments using a Pliocene inspired geometry of the Indonesian Passages in comparison to experiments with unchanged, modern geometry (Rodgers et al., 2000; Jochum et al., 2009) yield inconsistent results and indicate a strong dependence on experimental design and/or model. The experiments of Rodgers et al. (2000) support the cooling of the subsurface Indian Ocean with less influence of south equatorial waters, but do not include the effect of atmospheric feedbacks. In contrast, the coupled GCM used in Jochum et al. (2009) exhibits a weak warming in the subsurface Indian Ocean due to an increased ITF transport for the constricted ‘modern’ setting. Neither model experiments resolve the Makassar Strait and therefore may be unsuitable to test the hypothesis of Cane and Molnar (2001). In this study we analyse the response of a coupled global climate model to a constriction of the Indonesian passages by two equilibrated simulations using different land sea distributions. The geometry of the passages is designed to resemble modern geographic conditions in the first experiment and the Early Pliocene bathymetry in the second one where the passages East of Sulawesi are deepened and expanded to the South. The central question of this study is whether tectonic changes may explain the aridification observed on various locations around the Indian Ocean. For this purpose, we make use of an uncoupled vegetation model forced by climatologies from the two experiments. In the next section we will describe the AOGCM, the decoupled vegetation model and the setting of the two presented experiments. In Section 3 we evaluate the model's ITF with respect to modern observations and analyse oceanic and atmospheric response to the tectonic forcing. In Section 4 we study the vegetation changes as simulated by the decoupled vegetation model. Results are summarised and discussed in Section 5. 2. Model and experiment We use the KCM (Kiel Climate Model), a state-of-the-art coupled atmosphere–ocean–sea ice general circulation model. KCM consists of the ECHAM5 (Roeckner et al., 2006) atmosphere general circulation
model run at T31 (3.75° × 3.75°) horizontal resolution and the NEMO (Madec, 2008) ocean–sea ice general circulation model with horizontal resolution based on a 2° Mercator mesh. Meridional resolution increases towards the low latitudes with a maximum resolution of 0.5° at the equator (see detailed description by (Park et al., 2009)). The meridional refinement improves the representation of tropical climate and also of the area of interest in the present study. Neither flux correction nor anomaly coupling was used. (Schneider et al., 2010) use the model for a model data comparison of Holocene temperature trends. We consider the two experiments MODERN and PLIO, which only differ in the geometry of the Indonesian Passages. Both experiments are initialised with an ocean state that has been spun-up for 1000 years under preindustrial boundary conditions (i.e. preindustrial atmospheric composition with 286 ppm CO2 concentration, modern topography and modern orbital parameters). MODERN is a 500 year control experiment using a representation of today's bathymetry, and PLIO is a 1000 year experiment, where the geometry of the Indonesian Passages is designed to resemble the Early Pliocene setting as it was assumed by (Cane and Molnar, 2001): the region of Halmahera is 1000 m deeper than today, the northern part of New Guinea has been removed so that its coast is located 2° farther south, the exit passage between Timor and Australia has been deepened and widened to the North by removing parts of the island of Timor. It has to be noted, that the design is highly idealised and that in particular the movement of the Australian tectonic plate is not represented. A consideration of limitations of the experiment design with respect to tectonic reconstructions can be found in the Discussion section. Fig. 6 shows modern coastlines, geographic expressions and differences in land sea distribution between the two experiments. For practical reasons we refer to the deepened and widened passages between Sulawesi and New Guinea as “Halmahera Strait”, which in reality is a complicated system of several passages, which cannot be resolved by the model. In order to estimate the effect of the tectonic constriction on terrestrial vegetation we use the output of KCM to force an uncoupled (offline) version of the vegetation model BIOME4 (Kaplan et al., 2003). We use monthly mean precipitation and temperature fields of the last 100 years of both experiments interpolated onto a 0.5° × 0.5° grid. For the remaining boundary conditions we apply the default boundary conditions as documented in Kaplan et al. (2003), i.e. modern values of incoming solar radiation, soil water holding capacity, soil water percolation index and annual absolute minimum temperature. The use of modern values for annual absolute minimum temperature may somewhat limit the value of our results in high latitudes while sensitivity of vegetation to changes in the minimum temperature is marginal in low latitudes (Nemani et al., 2003). 3. Climate response In the following we focus on those oceanic and atmospheric responses to ITF changes which are capable of triggering environmental change. We use the years 501 to 1000 of experiment PLIO and year 1 to 500 of experiment MODERN for analysis. The first 500 years of experiment PLIO are not considered as they are not sufficiently equilibrated to the setting with widened and deepened Indonesian Passages. Note that MODERN already has been spun-up for 1000 years before reaching the analysis period. The vertical structure of temperature anomalies and cross section velocities in the main ITF passages are shown in Fig. 1. Specifically, we consider sections InW located in Makassar Strait, InE in “Halmahera Strait” and section Out at the exit of the Australasian Mediterranean basin (Fig. 6). The associated cross section transports, transport weighted temperature and temperature in the centre of the ITF are given in Table 1. In agreement with observations the ITF in the MODERN setting transports Pacific waters, primarily through Makassar Strait, into the subsurface Indian Ocean. The Makassar Strait
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Fig. 1. The Indonesian Passages and their representation in the model: black contours depict modern coast lines, dark grey shaded areas represent land in both experiments, light grey areas represent land only in experiment MODERN. Black lines indicate the position of the three vertical sections used in Fig. 2.
transport amounts to 9.3 Sv with a transport weighted temperature of 14.6 °C. Export into the Indian Ocean via the Timor Sea is 16.0 Sv. This is in good agreement with observations both in terms of absolute ITF transport and relative contribution of Makassar Strait: Wijffels et al. (2008) estimate 20 years of averaged modern ITF transports of 8.9 ± 1.7 Sv using geostrophic measurements and Ekman transports from satellite and reanalysis derived wind fields. For the years 2004 to 2006 Gordon et al. (2010) provide an Indonesian throughflow transport estimate of 15 Sv dominated by 11 Sv through Makassar Strait with a transport weighted temperature of 15.6 °C (Gordon et al., 2008). In contrast to the flow through Makassar Strait flow through Lifamatola passage (a deep passage in the region of our “Halmahera Strait”) is not surface intensified in the observations, which is also true for the simulated transport through our Halmahera Strait (Fig. 1). Compared to experiment PLIO transport into the Australasian Mediterranean decreases in experiment MODERN in (constricted) Halmahera Strait at section InE by 1.7 Sv and by 3.5 Sv at section it InW in Makassar Strait, even though the geometry of Makassar Strait was not changed in experiment PLIO. Outflow through the Timor Sea decreases by 4.6 Sv at section Out. The transport weighted temperature of water entering through “Halmahera Strait” increases due to the reduced depth of the inflow (which is not surface intensified, Fig. 2, upper right panel) while Makassar Strait throughflow cools on average due to reduced feeding of South Pacific Waters. Both passages contribute to the Indonesian throughflow water that enters the Indian Ocean through the Timor Sea. Even though temperature at the location of maximal velocity across Out is reduced, transport weighted
Table 1 Transport (integrated over the whole water column) and temperature of inflowing waters from the Pacific into the Australasian Mediterranean (AAM) (first two row) and outflowing waters from AAM into the Indian Ocean through the Timor Sea (third row). Southward transport is integrated across a zonal section through Makassar Strait (InW) and across a zonal section through “Halmahera Strait” (InE). Westward transport is integrated across a meridional section off the Timor Sea (Out) (Fig. 1 for position of sections). Temperature T@ max is the temperature at the maximum of southward/ westward velocities respectively, and Tweighted is the mean temperature of southward/ westward waters weighted with their respective transports. Section
InW InE Out
Transport (Sv)
Tweighted (°C)
T@max (°C)
Modern
Plio
Modern
Plio
Modern
Plio
9.3 4.9 16.0
12.8 6.6 20.6
25.7 27.8 27.4
26.1 26.6 27.7
14.6 20.4 18.0
15.9 16.4 17.0
temperature is increased in the experiment MODERN which can be related to a shallower and wider ITF core (Fig. 2, lower panel). Despite warmer Indonesian throughflow the Indian Ocean exhibits a pronounced negative subsurface temperature anomaly in experiment MODERN compared to PLIO (Fig. 3). This can be explained by the reduced ITF transport in both inflow passages as idealised modelling studies consistently show a cooling in the Indian Ocean warm water sphere when ITF transport is artificially suppressed (Schneider, 1998; Wajsowicz and Schneider, 2001; Song et al., 2007). The cold water anomaly follows the topography along the western coast of Indonesia towards the Bay of Bengal. Temperature anomalies in the centre of the basin are relatively weak in the annual mean but exhibit a strong interannual variability (not shown). At 88.5° E and 11.3° S we only find negative anomalies of less than 1 °C where (Karas et al., 2009) reconstruct a 4 °C cooling. The negative temperature anomaly surfaces in the eastern tropical Indian Ocean and manifests itself as a −0.5 °C sea surface temperature (SST) anomaly. On the Pacific side of the passages we observe a surface warming spanning across the entire equatorial Pacific. The SST dipole across the Indonesian archipelago can be associated with the reduced heat transported from Pacific into the upper Indian Ocean. In the warm pool the anomaly takes values of up to 1 °C. The increased warm pool SST may provide changes of ENSO characteristics, given the importance of the mean state in the tropical Pacific. This will be addressed in a forthcoming paper. In addition to the reduction of ITF heat transport the Indo-Pacific SST dipole anomaly is intensified by enhanced upwelling/downwelling due to anomalous divergence/convergence of Ekman transports. The anomalies of annual mean wind at 10 m height in Fig. 3 can be associated with divergent Ekman transports in the ITF outflow region and the equatorial Indian Ocean and anomalous convergence of Ekman transports in the Pacific warm pool. In the region of the Indonesian Passages the anomalous wind direction is opposed to that of the ITF. The reduction in Makassar Strait transport might thus be due to a reduction of its wind driven component. The anomalous upwelling and subsurface cooling in the eastern Indian Ocean is associated with a shoaling of the thermocline while we observe a deepening of the thermocline in the eastern equatorial Pacific (not shown). In the tropics the great contribution of convective rain results in a high sensitivity of precipitation to changes in SST. The sea surface temperature dipole across the Indonesian Passages thus results in strong local precipitation dipole with reduced rainfall in the eastern Indian Ocean and increased precipitation over the warm pool (Fig. 4). Over Australia changes in precipitation are smaller than the marine precipitation anomalies but with a reduction of more than 0.2 mm/day
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Fig. 2. Temperature anomalies for experiment MODERN with respect to PLIO at vertical sections InW InE and Out located in Makassar Strait, Halmahera Strait and in the ITF outflow and velocity across these sections; black contours for experiment MODERN and green for experiment PLIO. Contour interval is 0.1 m/s, negative velocities are indicated by dashed, positive by solid contours. Negative velocities are directed to the South in the Makassar Strait and Halmahera Strait and to the West in the Timor Sea.
in most parts of the continent represent a strong relative change (N 30%, not shown) towards a drier climate. Likewise, we observe a considerably drier climate over the Indian subcontinent with respect to precipitation in experiment PLIO. In contrast, changes over Africa are comparably small. Environmental change in Australia is additionally favoured as reduced rainfall in experiment MODERN is accompanied by warmer surface temperatures (Fig. 6). 4. Vegetation response Next we investigate whether the tectonic closing of the Indonesian Passages may have contributed to the observed Pliocene aridification and desertification over Africa. To estimate the vegetation response to a constriction of the Indonesian Passages, we force the uncoupled vegetation model BIOME4 with simulated monthly temperatures and
precipitation. Climate change due to the constricted Indonesian Passages reduces net primary production (NPP) over extensive areas around the Indian Ocean: We observe the strongest vegetation response in Australia, where we obtain a reduction in terrestrial so NPP that almost everywhere exceeds 10% for experiment MODERN relative to experiment PLIO. The NPP reduction exceeds 30% in extensive areas of central Australia and along the northern and eastern coast which corresponds to absolute changes of 50–150 g C/(m 2 yr) in the desert and more than 150 g C/(m 2 yr) in the coastal regions (Fig. 5). Fundamental environmental changes manifest in changes in so called biomes which are classifying the type of vegetation dominating a certain habitat. Biome 4 distinguishes between 28 biomes. The climate change from experiment PLIO to MODERN leads to a expansion of the central Australian desert at the expense of xerophytic shrubland in the East and North West. In the South East of the continent temperate
Fig. 3. Upper panel: anomalies of surface temperature and wind at 10 m above surface for experiments MODERN–PLIO. A 1 m/s wind vector is given in the top left corner. Lower panel: anomalous temperature (MODERN–PLIO) at 129 m depth.
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Fig. 4. Annual net precipitation (precipitation minus evaporation) difference between experiment MODERN and PLIO. Contours are marine net precipitation anomalies with a contour interval of 1 mm/day. Over land, net precipitation is given in higher resolution and in filled contours. Blue colours and contours indicate drier climate conditions for experiment MODERN than for experiment PLIO.
sclerophyll woodland recedes towards the coast (Fig. 6). Another considerable response in vegetation is found in the western Indian subcontinent where NPP reduces by more than 20%. A reduction in NPP of more than 10% is also found in southern Africa, in the Arabian Desert and south of the Sahara. However, the corresponding absolute changes exceed 50 g C/m 2 only in southern Africa, since the affected regions are generally low in NPP and it is questionable whether this response is robust, as it depends only on small changes in precipitation. 5. Discussion We have studied the response of Indo-Pacific climate to the Early Pliocene geometry change of the Indonesian passages by using a stateof-the-art climate model in order to test the Cane and Molnar (2001) hypothesis that these tectonic changes caused a detectable environmental change in East Africa. In qualitative agreement with reconstructions at Deep Sea Drilling Project site 214 (Karas et al., 2009) we find decreased subsurface temperatures in the eastern Indian Ocean in response to a constriction of the passages between New Guinea and Sulawesi. At the exact site 214, however, simulated anomalies are weaker than reconstructed, which may be related to the rather poor
representation of Indonesian exit passages as a consequence of the coarse resolution of the model. We find that the simulated subsurface cooling in the Indian Ocean in the present day configuration is primarily related to the reduced volume transport through the passages and increased Ekman upwelling and associated thermocline shoaling. Changes in the water mass properties are counteracting the subsurface cooling as transport weighted temperature of the throughflow increases by about 1 °C. If volume transport change was indeed the dominant driver of subsurface temperatures, Indonesian throughflow response might be dependent only on depth and width of the passages rather than on a shift from South to North equatorial Pacific waters feeding the ITF as hypothesised by Cane and Molnar (2001). Indeed, the simulated climate response of the Indian Ocean to a complete closure of the passages (Schneider, 1998; Wajsowicz and Schneider, 2001; Song et al., 2007) qualitatively resembles this study's response to a change from Pliocene (deeper and wider) to modern (constricted) passage geometry. In these studies no-ITF scenarios are compared to their respective control experiments which yield mean ITF transport rates of 11.1 Sv to 13.8 Sv. Suppressing the throughflow results in a reduction in SST by 0.6 °C to 1.8 °C and in precipitation of 2.0–3.5 mm in the eastern Indian Ocean. Sensitivities
Fig. 5. Difference in annual NPP for experiment MODERN–PLIO: absolute change in g C/(m2 yr) (upper panel) and relative change with respect to experiment PLIO in % (lower panel).
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Fig. 6. Australian vegetation for climate forcing taken from experiment PLIO (left panel) and experiment MODERN (right panel). Biomes indicated by numbers are 1: tropical deciduous forest/woodland, 2: tropical savanna 3: warm mixed forest, 4: temperate grassland, 5: temperate conifer forest.
are within the same range in our experiments (here an ITF reduction by 4.6 Sv translates into up to 0.6 °C SST reduction and into a reduction of up to 3 mm/day in precipitation, the latter two also being centred in the tropical eastern Indian Ocean). In the equatorial Pacific the ITF blockage experiments also exhibit an upper ocean warming and positive SST anomalies but the spatial patterns and amplitudes are less coherent between the individual experiments than in the Indian Ocean. By using a vegetation model we demonstrate that the simulated climate change is capable of causing a substantial (and detectable) vegetation response in Australia and on the Indian subcontinent which qualitatively agrees with environmental reconstructions (Quade and Cerling, 1995; Barry et al., 2002; Dodson and Macphail, 2004; Martin, 2006; McLaren and Wallace, 2010). Our experiments particularly suggest that the constriction of Indonesian passages contributed to the observed Australian aridification. This climate response to tectonic changes in the Pliocene Indonesian passage might reconcile model-data inconsistencies that are pointed out in Salzmann et al. (2009): A coupled climate simulation using Pliocene boundary conditions but neglecting tectonic changes is found to particularly underestimate the relatively humid Australian climate as it is indicated in vegetation reconstructions of the Pliocene. The Australian vegetation response might be even stronger in a vegetation model which is interactively coupled to the atmosphere due to positive biogeophysical feedbacks associated with desertification. Our experiments are designed to investigate the climatic impact of a reducing south equatorial Pacific contribution to the Indonesian throughflow. The greater south equatorial influence of the Pliocene scenario is implemented by deepening the passage between New Guinea and Sulawesi and by removing the Northern parts of New Guinea. Due to its highly simplified nature, our experiment does not reflect a specific time period in the Late Neogene evolution of the Indonesian Passages. According to tectonic reconstructions the northward moving Australian plate did not result in a simple progressive restriction of the gateway between Australia and Indonesia during Late Neogene. It is likely that the Indonesian Passages were considerably shallower and/or more restricted in the Middle Miocene (around 12 million years ago) than the Early Pliocene (around 5 million years ago), since deep basins opened between Sulawesi and New Guinea (Hall, 2009; Spakman and Hall, 2010) during this period. Thus, it seems likely that the Pliocene Indonesian throughflow not only had a higher fraction of south equatorial waters than at present, but also a greater volume transport than during the Late Miocene. Even after the Pliocene, the south equatorial influence might have been subject to strong temporal changes due to small scale changes in the geometry of the passages as the northward movement of individual islands such as Halmahera (R. Hall, personal communication, 2011). It is therefore impossible to link specific changes in the Australian vegetation of the Pliocene and Pleistocene to tectonic processes. In order to explain Australian climate variations, however,
local tectonic changes should be taken into account as well as global climate shifts related to variations in ice-volume and greenhouse gas concentration. 6. Conclusion Our simulations should be understood as an idealised process study and as a test of the hypothesised link between tectonic changes of the Indonesian Passages and East African climate change. A major limitation of our results arises from the limited representation of the complex Indonesian archipelago. In particular the narrow Lombok and Ombai exit passages of the ITF which are supposed to contribute more than 50% to the ITF export (Gordon et al., 2008) are not represented due to the relative coarse zonal resolution of 2°. However, unlike most other paleo-climate models, our simulation captures important characteristics of the Indonesian throughflow: the Makassar Strait is resolved and Makassar Strait transport, its transport weighted temperature and the total Indonesian throughflow are in good agreement with observations (Gordon et al., 2008). Future studies, possibly also from earlier periods in Earth's history, might concentrate on sensitivity to the experiment design with respect to the depth and width of the passage. Modification of the whole Australian continental plate might significantly impact on ocean circulation. The effect of atmospheric feedbacks and teleconnections might be identified with partially decoupled experiments as in Krebs and Timmermann (2007). The presented experiments simulate the isolated effect of a constriction of the Indonesian Passages. In experiments with full Pliocene boundary conditions synergy effects might arise from fundamental changes in the Indo-Pacific region due to an open Central American Seaway and changed meridional and zonal temperature gradients. This study demonstrates the need for coupled modelling studies in order to link tectonic changes to changes in the complex Indo-Pacific climate. From the present coupled modelling study (note that Cane and Molnar (2001) rely on data from an ocean only model, while the only other coupled modelling study modifying the Indonesian passages by Jochum et al. (2009) finds only a very weak climate response in general), neither a strong impact on African climate nor a contribution to a global thermocline shoaling as envisioned in Karas et al. (2009) are supported in response to the Pliocene closure of the Indonesian Passages. In contrast, local tectonic changes should be considered to be a potential origin of Australian climate variations. Acknowledgements We would like in particular to thank R. Hall for helping us to interpret the tectonic reconstructions. We also thank M. Prange and K. Sniderman who provided very constructive comments that helped to
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Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Pliocene aridification of Australia caused by tectonically induced weakening of the Indonesian throughflow Uta Krebs a,⁎, W. Park b, B. Schneider a a b
Institute for Geosciences, Kiel University, Ludewig-Meyn-Str. 10, D-24118 Kiel, Germany Leibniz Institute of Marine Sciences, IFM-GEOMAR, Duesternbrooker Weg 20, D-24105 Kiel, Germany
a r t i c l e
i n f o
Article history: Received 12 November 2010 Received in revised form 1 June 2011 Accepted 3 June 2011 Available online 16 June 2011 Keywords: Coupled modelling Indonesian throughflow Pliocene Neogene Australian vegetation Aridification Tectonic Gateway Indian Ocean Africa
a b s t r a c t Tectonic changes of the Early to Mid-Pliocene largely modified the Indonesian Passages by constricting and uplifting the passages between today's New Guinea and Sulawesi. The associated changes in strength and water mass properties of the Indonesian throughflow (ITF) might have influenced the amount of heat transported from the Pacific to the Indian Ocean and thus contributed to Pliocene climate change of the IndoPacific. We study the climate response to changes in the geometry of the Indonesian Passages in an atmosphere–ocean general circulation model (AOGCM). We compare climate simulations with present-day topography and with a topography resembling the Early Pliocene situation in the Indo-Pacific, i.e. passages East of Sulawesi deepened and widened to the South. We find that transport through the Indonesian Archipelago is weakened in the constricted passage by 1.7 Sv and in the unchanged Makassar Strait West of Sulawesi by 3.5 Sv, while transport weighted temperature of the outflow into the Indian Ocean increases by 1 °C. Consistent with recent proxy evidence the reduction in ITF transport causes a decrease in subsurface temperatures in the Indian Ocean while surface waters of the equatorial Pacific exhibit an increase by up to 0.9 °C centred in the warm pool. As a local response to the sea surface temperature anomalies, we observe an anomalous precipitation dipole across the Indonesian passages with increased rainfall over the Pacific warm pool and decreased precipitation in the eastern Indian Ocean. The Australian continent experiences a pronounced aridification with mean annual precipitation rates dropping by 30% over most parts of the continent. Using an uncoupled vegetation model, we demonstrate that the simulated climate change might partly explain the observed Late Pliocene desertification of Australia. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Pliocene atmospheric CO2 concentrations are estimated to have been higher than at preindustrial times (Kurschner et al., 1996; Raymo et al., 1996). For peak temperatures during the Pliocene, Pagani et al. (2010) provide estimates ranging from 365 ppm to 415 ppm which cover the range of projections for CO2 concentrations in the next decades (Friedlingstein et al., 2006; IPCC, 2007). It is however essential to assess the influence of those Pliocene boundary conditions that differ from today, if we want to use Pliocene climate as an analogue for future climate or as benchmark for global climate models. The present climate seems to represent the transition from an ‘icehouse’ period, with Greenland still being ice covered, to a warmer greenhouse world. In contrast, the Pliocene is transitioning in the opposite direction with sea level estimated to have been about 25 m higher and continental ice volume strongly reduced. In particular northern hemispheric ice sheets were almost absent (Miller et al.,
⁎ Corresponding author. E-mail address: [email protected] (U. Krebs). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.06.002
2010). Tectonic changes in oceanic key regions, such as the closing of the Isthmus of Panama (Coates et al., 1992; Haug and Tiedemann, 1998; Klocker et al., 2005; Lunt et al., 2008) and the constriction of the Indonesian Passages by northward drift of New Guinea and Australia (Daly et al., 1991; Hall, 2009), are considered as possible causes for the transition into the Pleistocene (around 1.8 million years before present) with its repeated glaciations. Cane and Molnar (2001) propose that the northward displacement of New Guinea during the Early- and Mid-Pliocene reduced the inflow of warm South equatorial Pacific waters into the Indian Ocean via the passages between New Guinea and Sulawesi and strengthened the influence of relative cool North equatorial Pacific waters primarily passing through the Makassar Strait. Speculatively, the authors link the tectonic change to the observed Pliocene aridification over East Africa — which may have triggered human evolution by enforcing adaption to the expansion of grassland habitats and faunal turnover (Bobe and Behrensmeyer, 2004; Wynn, 2004). Cane and Molnar (2001) suggest that changes in the Indonesian throughflow (ITF) might have reduced Indian Ocean sea surface temperature and resulted in reduced precipitation over East Africa. Indeed Karas et al. (2009) find evidence for a 4 °C Late-Pliocene cooling of subsurface water in the tropical East
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Indian Ocean and suggest a link between cooler throughflow waters and the observed global shoaling of the thermocline. Marine records of the Pliocene Pacific suggest a meridionally expanded warm pool (Brierley et al., 2009) and a decreased zonal sea surface temperature gradient (Dowsett and Robinson, 2009) when compared to today. These changes are likely to primarily reflect global climate trends (Lunt et al., 2010) or the influence of the closure of the Central American Seaway (Steph et al., 2010). Though marine data coverage is somewhat sparse, land climate changes are recorded around the IndoPacific for the last 10 million years before present. During Pliocene several regional shifts towards drier conditions are recorded not only in East Africa but also on the Indian sub-continent (Quade and Cerling, 1995; Barry et al., 2002) and in Australia (Dodson and Macphail, 2004; Martin, 2006; Fujioka et al., 2009). Reviewing numerous records Martin (2006) finds evidence for a Pliocene trend to a drier Australian climate with the exception of a brief period of wetter climates in the Early Pliocene. Southeastern Australian records however indicate that the fundamental transition towards the modern climate with pronounced dry seasons was taking place less than 1.5 million years ago (Sniderman et al., 2009; McLaren and Wallace, 2010). Tectonically induced climate change might not exclusively be caused by changes in the properties of the throughflow waters but also by changes in the strength of the throughflow. Idealised modelling studies with an ocean general circulation model (OGCM) and AOGCMs (Hirst and Godfrey, 1993; Schneider, 1998; Song et al., 2007) consistently demonstrate that a complete closure of the Indonesian passages and suppression of the ITF produces a negative temperature anomaly in and above the thermocline downstream of the passage. In the coupled GCMs (Schneider, 1998; Song et al., 2007) an Eastern Indian Ocean/equatorial Pacific sea surface temperatures decrease/ increase is accompanied by negative/positive precipitation anomalies respectively. Model experiments using a Pliocene inspired geometry of the Indonesian Passages in comparison to experiments with unchanged, modern geometry (Rodgers et al., 2000; Jochum et al., 2009) yield inconsistent results and indicate a strong dependence on experimental design and/or model. The experiments of Rodgers et al. (2000) support the cooling of the subsurface Indian Ocean with less influence of south equatorial waters, but do not include the effect of atmospheric feedbacks. In contrast, the coupled GCM used in Jochum et al. (2009) exhibits a weak warming in the subsurface Indian Ocean due to an increased ITF transport for the constricted ‘modern’ setting. Neither model experiments resolve the Makassar Strait and therefore may be unsuitable to test the hypothesis of Cane and Molnar (2001). In this study we analyse the response of a coupled global climate model to a constriction of the Indonesian passages by two equilibrated simulations using different land sea distributions. The geometry of the passages is designed to resemble modern geographic conditions in the first experiment and the Early Pliocene bathymetry in the second one where the passages East of Sulawesi are deepened and expanded to the South. The central question of this study is whether tectonic changes may explain the aridification observed on various locations around the Indian Ocean. For this purpose, we make use of an uncoupled vegetation model forced by climatologies from the two experiments. In the next section we will describe the AOGCM, the decoupled vegetation model and the setting of the two presented experiments. In Section 3 we evaluate the model's ITF with respect to modern observations and analyse oceanic and atmospheric response to the tectonic forcing. In Section 4 we study the vegetation changes as simulated by the decoupled vegetation model. Results are summarised and discussed in Section 5. 2. Model and experiment We use the KCM (Kiel Climate Model), a state-of-the-art coupled atmosphere–ocean–sea ice general circulation model. KCM consists of the ECHAM5 (Roeckner et al., 2006) atmosphere general circulation
model run at T31 (3.75° × 3.75°) horizontal resolution and the NEMO (Madec, 2008) ocean–sea ice general circulation model with horizontal resolution based on a 2° Mercator mesh. Meridional resolution increases towards the low latitudes with a maximum resolution of 0.5° at the equator (see detailed description by (Park et al., 2009)). The meridional refinement improves the representation of tropical climate and also of the area of interest in the present study. Neither flux correction nor anomaly coupling was used. (Schneider et al., 2010) use the model for a model data comparison of Holocene temperature trends. We consider the two experiments MODERN and PLIO, which only differ in the geometry of the Indonesian Passages. Both experiments are initialised with an ocean state that has been spun-up for 1000 years under preindustrial boundary conditions (i.e. preindustrial atmospheric composition with 286 ppm CO2 concentration, modern topography and modern orbital parameters). MODERN is a 500 year control experiment using a representation of today's bathymetry, and PLIO is a 1000 year experiment, where the geometry of the Indonesian Passages is designed to resemble the Early Pliocene setting as it was assumed by (Cane and Molnar, 2001): the region of Halmahera is 1000 m deeper than today, the northern part of New Guinea has been removed so that its coast is located 2° farther south, the exit passage between Timor and Australia has been deepened and widened to the North by removing parts of the island of Timor. It has to be noted, that the design is highly idealised and that in particular the movement of the Australian tectonic plate is not represented. A consideration of limitations of the experiment design with respect to tectonic reconstructions can be found in the Discussion section. Fig. 6 shows modern coastlines, geographic expressions and differences in land sea distribution between the two experiments. For practical reasons we refer to the deepened and widened passages between Sulawesi and New Guinea as “Halmahera Strait”, which in reality is a complicated system of several passages, which cannot be resolved by the model. In order to estimate the effect of the tectonic constriction on terrestrial vegetation we use the output of KCM to force an uncoupled (offline) version of the vegetation model BIOME4 (Kaplan et al., 2003). We use monthly mean precipitation and temperature fields of the last 100 years of both experiments interpolated onto a 0.5° × 0.5° grid. For the remaining boundary conditions we apply the default boundary conditions as documented in Kaplan et al. (2003), i.e. modern values of incoming solar radiation, soil water holding capacity, soil water percolation index and annual absolute minimum temperature. The use of modern values for annual absolute minimum temperature may somewhat limit the value of our results in high latitudes while sensitivity of vegetation to changes in the minimum temperature is marginal in low latitudes (Nemani et al., 2003). 3. Climate response In the following we focus on those oceanic and atmospheric responses to ITF changes which are capable of triggering environmental change. We use the years 501 to 1000 of experiment PLIO and year 1 to 500 of experiment MODERN for analysis. The first 500 years of experiment PLIO are not considered as they are not sufficiently equilibrated to the setting with widened and deepened Indonesian Passages. Note that MODERN already has been spun-up for 1000 years before reaching the analysis period. The vertical structure of temperature anomalies and cross section velocities in the main ITF passages are shown in Fig. 1. Specifically, we consider sections InW located in Makassar Strait, InE in “Halmahera Strait” and section Out at the exit of the Australasian Mediterranean basin (Fig. 6). The associated cross section transports, transport weighted temperature and temperature in the centre of the ITF are given in Table 1. In agreement with observations the ITF in the MODERN setting transports Pacific waters, primarily through Makassar Strait, into the subsurface Indian Ocean. The Makassar Strait
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Fig. 1. The Indonesian Passages and their representation in the model: black contours depict modern coast lines, dark grey shaded areas represent land in both experiments, light grey areas represent land only in experiment MODERN. Black lines indicate the position of the three vertical sections used in Fig. 2.
transport amounts to 9.3 Sv with a transport weighted temperature of 14.6 °C. Export into the Indian Ocean via the Timor Sea is 16.0 Sv. This is in good agreement with observations both in terms of absolute ITF transport and relative contribution of Makassar Strait: Wijffels et al. (2008) estimate 20 years of averaged modern ITF transports of 8.9 ± 1.7 Sv using geostrophic measurements and Ekman transports from satellite and reanalysis derived wind fields. For the years 2004 to 2006 Gordon et al. (2010) provide an Indonesian throughflow transport estimate of 15 Sv dominated by 11 Sv through Makassar Strait with a transport weighted temperature of 15.6 °C (Gordon et al., 2008). In contrast to the flow through Makassar Strait flow through Lifamatola passage (a deep passage in the region of our “Halmahera Strait”) is not surface intensified in the observations, which is also true for the simulated transport through our Halmahera Strait (Fig. 1). Compared to experiment PLIO transport into the Australasian Mediterranean decreases in experiment MODERN in (constricted) Halmahera Strait at section InE by 1.7 Sv and by 3.5 Sv at section it InW in Makassar Strait, even though the geometry of Makassar Strait was not changed in experiment PLIO. Outflow through the Timor Sea decreases by 4.6 Sv at section Out. The transport weighted temperature of water entering through “Halmahera Strait” increases due to the reduced depth of the inflow (which is not surface intensified, Fig. 2, upper right panel) while Makassar Strait throughflow cools on average due to reduced feeding of South Pacific Waters. Both passages contribute to the Indonesian throughflow water that enters the Indian Ocean through the Timor Sea. Even though temperature at the location of maximal velocity across Out is reduced, transport weighted
Table 1 Transport (integrated over the whole water column) and temperature of inflowing waters from the Pacific into the Australasian Mediterranean (AAM) (first two row) and outflowing waters from AAM into the Indian Ocean through the Timor Sea (third row). Southward transport is integrated across a zonal section through Makassar Strait (InW) and across a zonal section through “Halmahera Strait” (InE). Westward transport is integrated across a meridional section off the Timor Sea (Out) (Fig. 1 for position of sections). Temperature T@ max is the temperature at the maximum of southward/ westward velocities respectively, and Tweighted is the mean temperature of southward/ westward waters weighted with their respective transports. Section
InW InE Out
Transport (Sv)
Tweighted (°C)
T@max (°C)
Modern
Plio
Modern
Plio
Modern
Plio
9.3 4.9 16.0
12.8 6.6 20.6
25.7 27.8 27.4
26.1 26.6 27.7
14.6 20.4 18.0
15.9 16.4 17.0
temperature is increased in the experiment MODERN which can be related to a shallower and wider ITF core (Fig. 2, lower panel). Despite warmer Indonesian throughflow the Indian Ocean exhibits a pronounced negative subsurface temperature anomaly in experiment MODERN compared to PLIO (Fig. 3). This can be explained by the reduced ITF transport in both inflow passages as idealised modelling studies consistently show a cooling in the Indian Ocean warm water sphere when ITF transport is artificially suppressed (Schneider, 1998; Wajsowicz and Schneider, 2001; Song et al., 2007). The cold water anomaly follows the topography along the western coast of Indonesia towards the Bay of Bengal. Temperature anomalies in the centre of the basin are relatively weak in the annual mean but exhibit a strong interannual variability (not shown). At 88.5° E and 11.3° S we only find negative anomalies of less than 1 °C where (Karas et al., 2009) reconstruct a 4 °C cooling. The negative temperature anomaly surfaces in the eastern tropical Indian Ocean and manifests itself as a −0.5 °C sea surface temperature (SST) anomaly. On the Pacific side of the passages we observe a surface warming spanning across the entire equatorial Pacific. The SST dipole across the Indonesian archipelago can be associated with the reduced heat transported from Pacific into the upper Indian Ocean. In the warm pool the anomaly takes values of up to 1 °C. The increased warm pool SST may provide changes of ENSO characteristics, given the importance of the mean state in the tropical Pacific. This will be addressed in a forthcoming paper. In addition to the reduction of ITF heat transport the Indo-Pacific SST dipole anomaly is intensified by enhanced upwelling/downwelling due to anomalous divergence/convergence of Ekman transports. The anomalies of annual mean wind at 10 m height in Fig. 3 can be associated with divergent Ekman transports in the ITF outflow region and the equatorial Indian Ocean and anomalous convergence of Ekman transports in the Pacific warm pool. In the region of the Indonesian Passages the anomalous wind direction is opposed to that of the ITF. The reduction in Makassar Strait transport might thus be due to a reduction of its wind driven component. The anomalous upwelling and subsurface cooling in the eastern Indian Ocean is associated with a shoaling of the thermocline while we observe a deepening of the thermocline in the eastern equatorial Pacific (not shown). In the tropics the great contribution of convective rain results in a high sensitivity of precipitation to changes in SST. The sea surface temperature dipole across the Indonesian Passages thus results in strong local precipitation dipole with reduced rainfall in the eastern Indian Ocean and increased precipitation over the warm pool (Fig. 4). Over Australia changes in precipitation are smaller than the marine precipitation anomalies but with a reduction of more than 0.2 mm/day
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Fig. 2. Temperature anomalies for experiment MODERN with respect to PLIO at vertical sections InW InE and Out located in Makassar Strait, Halmahera Strait and in the ITF outflow and velocity across these sections; black contours for experiment MODERN and green for experiment PLIO. Contour interval is 0.1 m/s, negative velocities are indicated by dashed, positive by solid contours. Negative velocities are directed to the South in the Makassar Strait and Halmahera Strait and to the West in the Timor Sea.
in most parts of the continent represent a strong relative change (N 30%, not shown) towards a drier climate. Likewise, we observe a considerably drier climate over the Indian subcontinent with respect to precipitation in experiment PLIO. In contrast, changes over Africa are comparably small. Environmental change in Australia is additionally favoured as reduced rainfall in experiment MODERN is accompanied by warmer surface temperatures (Fig. 6). 4. Vegetation response Next we investigate whether the tectonic closing of the Indonesian Passages may have contributed to the observed Pliocene aridification and desertification over Africa. To estimate the vegetation response to a constriction of the Indonesian Passages, we force the uncoupled vegetation model BIOME4 with simulated monthly temperatures and
precipitation. Climate change due to the constricted Indonesian Passages reduces net primary production (NPP) over extensive areas around the Indian Ocean: We observe the strongest vegetation response in Australia, where we obtain a reduction in terrestrial so NPP that almost everywhere exceeds 10% for experiment MODERN relative to experiment PLIO. The NPP reduction exceeds 30% in extensive areas of central Australia and along the northern and eastern coast which corresponds to absolute changes of 50–150 g C/(m 2 yr) in the desert and more than 150 g C/(m 2 yr) in the coastal regions (Fig. 5). Fundamental environmental changes manifest in changes in so called biomes which are classifying the type of vegetation dominating a certain habitat. Biome 4 distinguishes between 28 biomes. The climate change from experiment PLIO to MODERN leads to a expansion of the central Australian desert at the expense of xerophytic shrubland in the East and North West. In the South East of the continent temperate
Fig. 3. Upper panel: anomalies of surface temperature and wind at 10 m above surface for experiments MODERN–PLIO. A 1 m/s wind vector is given in the top left corner. Lower panel: anomalous temperature (MODERN–PLIO) at 129 m depth.
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Fig. 4. Annual net precipitation (precipitation minus evaporation) difference between experiment MODERN and PLIO. Contours are marine net precipitation anomalies with a contour interval of 1 mm/day. Over land, net precipitation is given in higher resolution and in filled contours. Blue colours and contours indicate drier climate conditions for experiment MODERN than for experiment PLIO.
sclerophyll woodland recedes towards the coast (Fig. 6). Another considerable response in vegetation is found in the western Indian subcontinent where NPP reduces by more than 20%. A reduction in NPP of more than 10% is also found in southern Africa, in the Arabian Desert and south of the Sahara. However, the corresponding absolute changes exceed 50 g C/m 2 only in southern Africa, since the affected regions are generally low in NPP and it is questionable whether this response is robust, as it depends only on small changes in precipitation. 5. Discussion We have studied the response of Indo-Pacific climate to the Early Pliocene geometry change of the Indonesian passages by using a stateof-the-art climate model in order to test the Cane and Molnar (2001) hypothesis that these tectonic changes caused a detectable environmental change in East Africa. In qualitative agreement with reconstructions at Deep Sea Drilling Project site 214 (Karas et al., 2009) we find decreased subsurface temperatures in the eastern Indian Ocean in response to a constriction of the passages between New Guinea and Sulawesi. At the exact site 214, however, simulated anomalies are weaker than reconstructed, which may be related to the rather poor
representation of Indonesian exit passages as a consequence of the coarse resolution of the model. We find that the simulated subsurface cooling in the Indian Ocean in the present day configuration is primarily related to the reduced volume transport through the passages and increased Ekman upwelling and associated thermocline shoaling. Changes in the water mass properties are counteracting the subsurface cooling as transport weighted temperature of the throughflow increases by about 1 °C. If volume transport change was indeed the dominant driver of subsurface temperatures, Indonesian throughflow response might be dependent only on depth and width of the passages rather than on a shift from South to North equatorial Pacific waters feeding the ITF as hypothesised by Cane and Molnar (2001). Indeed, the simulated climate response of the Indian Ocean to a complete closure of the passages (Schneider, 1998; Wajsowicz and Schneider, 2001; Song et al., 2007) qualitatively resembles this study's response to a change from Pliocene (deeper and wider) to modern (constricted) passage geometry. In these studies no-ITF scenarios are compared to their respective control experiments which yield mean ITF transport rates of 11.1 Sv to 13.8 Sv. Suppressing the throughflow results in a reduction in SST by 0.6 °C to 1.8 °C and in precipitation of 2.0–3.5 mm in the eastern Indian Ocean. Sensitivities
Fig. 5. Difference in annual NPP for experiment MODERN–PLIO: absolute change in g C/(m2 yr) (upper panel) and relative change with respect to experiment PLIO in % (lower panel).
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Fig. 6. Australian vegetation for climate forcing taken from experiment PLIO (left panel) and experiment MODERN (right panel). Biomes indicated by numbers are 1: tropical deciduous forest/woodland, 2: tropical savanna 3: warm mixed forest, 4: temperate grassland, 5: temperate conifer forest.
are within the same range in our experiments (here an ITF reduction by 4.6 Sv translates into up to 0.6 °C SST reduction and into a reduction of up to 3 mm/day in precipitation, the latter two also being centred in the tropical eastern Indian Ocean). In the equatorial Pacific the ITF blockage experiments also exhibit an upper ocean warming and positive SST anomalies but the spatial patterns and amplitudes are less coherent between the individual experiments than in the Indian Ocean. By using a vegetation model we demonstrate that the simulated climate change is capable of causing a substantial (and detectable) vegetation response in Australia and on the Indian subcontinent which qualitatively agrees with environmental reconstructions (Quade and Cerling, 1995; Barry et al., 2002; Dodson and Macphail, 2004; Martin, 2006; McLaren and Wallace, 2010). Our experiments particularly suggest that the constriction of Indonesian passages contributed to the observed Australian aridification. This climate response to tectonic changes in the Pliocene Indonesian passage might reconcile model-data inconsistencies that are pointed out in Salzmann et al. (2009): A coupled climate simulation using Pliocene boundary conditions but neglecting tectonic changes is found to particularly underestimate the relatively humid Australian climate as it is indicated in vegetation reconstructions of the Pliocene. The Australian vegetation response might be even stronger in a vegetation model which is interactively coupled to the atmosphere due to positive biogeophysical feedbacks associated with desertification. Our experiments are designed to investigate the climatic impact of a reducing south equatorial Pacific contribution to the Indonesian throughflow. The greater south equatorial influence of the Pliocene scenario is implemented by deepening the passage between New Guinea and Sulawesi and by removing the Northern parts of New Guinea. Due to its highly simplified nature, our experiment does not reflect a specific time period in the Late Neogene evolution of the Indonesian Passages. According to tectonic reconstructions the northward moving Australian plate did not result in a simple progressive restriction of the gateway between Australia and Indonesia during Late Neogene. It is likely that the Indonesian Passages were considerably shallower and/or more restricted in the Middle Miocene (around 12 million years ago) than the Early Pliocene (around 5 million years ago), since deep basins opened between Sulawesi and New Guinea (Hall, 2009; Spakman and Hall, 2010) during this period. Thus, it seems likely that the Pliocene Indonesian throughflow not only had a higher fraction of south equatorial waters than at present, but also a greater volume transport than during the Late Miocene. Even after the Pliocene, the south equatorial influence might have been subject to strong temporal changes due to small scale changes in the geometry of the passages as the northward movement of individual islands such as Halmahera (R. Hall, personal communication, 2011). It is therefore impossible to link specific changes in the Australian vegetation of the Pliocene and Pleistocene to tectonic processes. In order to explain Australian climate variations, however,
local tectonic changes should be taken into account as well as global climate shifts related to variations in ice-volume and greenhouse gas concentration. 6. Conclusion Our simulations should be understood as an idealised process study and as a test of the hypothesised link between tectonic changes of the Indonesian Passages and East African climate change. A major limitation of our results arises from the limited representation of the complex Indonesian archipelago. In particular the narrow Lombok and Ombai exit passages of the ITF which are supposed to contribute more than 50% to the ITF export (Gordon et al., 2008) are not represented due to the relative coarse zonal resolution of 2°. However, unlike most other paleo-climate models, our simulation captures important characteristics of the Indonesian throughflow: the Makassar Strait is resolved and Makassar Strait transport, its transport weighted temperature and the total Indonesian throughflow are in good agreement with observations (Gordon et al., 2008). Future studies, possibly also from earlier periods in Earth's history, might concentrate on sensitivity to the experiment design with respect to the depth and width of the passage. Modification of the whole Australian continental plate might significantly impact on ocean circulation. The effect of atmospheric feedbacks and teleconnections might be identified with partially decoupled experiments as in Krebs and Timmermann (2007). The presented experiments simulate the isolated effect of a constriction of the Indonesian Passages. In experiments with full Pliocene boundary conditions synergy effects might arise from fundamental changes in the Indo-Pacific region due to an open Central American Seaway and changed meridional and zonal temperature gradients. This study demonstrates the need for coupled modelling studies in order to link tectonic changes to changes in the complex Indo-Pacific climate. From the present coupled modelling study (note that Cane and Molnar (2001) rely on data from an ocean only model, while the only other coupled modelling study modifying the Indonesian passages by Jochum et al. (2009) finds only a very weak climate response in general), neither a strong impact on African climate nor a contribution to a global thermocline shoaling as envisioned in Karas et al. (2009) are supported in response to the Pliocene closure of the Indonesian Passages. In contrast, local tectonic changes should be considered to be a potential origin of Australian climate variations. Acknowledgements We would like in particular to thank R. Hall for helping us to interpret the tectonic reconstructions. We also thank M. Prange and K. Sniderman who provided very constructive comments that helped to
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