Controls over spatial and seasonal variations on isotopic composition of the precipitation along the central and eastern portion of Brazil
Didier GastmansVinícius dos SantosJuliana Aparecida GalhardiJoão Felipe GromboniLudmila Vianna BatistaKonrad MiotlińskiChang Hung KiangJosé Sílvio Govone
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Based on Global Network Isotopes in Precipitation (GNIP) isotopic data set, a review of the spatial and temporal variability of δ18O and δ2H in precipitation was conducted throughout central and eastern Brazil, indicating that dynamic interactions between Intertropical and South Atlantic Convergence Zones, Amazon rainforest, and Atlantic Ocean determine the variations on the isotopic composition of precipitation over this area. Despite the seasonality and latitude effects observed, a fair correlation with precipitation amount was found. In addition, Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) air mass back trajectories were used to quantify the factors controlling daily variability in stable isotopes in precipitation. Through a linear multiple regression analysis, it was observed that temporal variations were consistent with the meteorological parameters derived from HYSPLIT, particularly precipitation amount along the trajectory and mix depth, but are not dependent on vapour residence time in the atmosphere. These findings also indicate the importance of convective systems to control the isotopic composition of precipitation in tropical and subtropical regions.Keywords:
HYSPLIT
Intertropical Convergence Zone
Orography
δ18O
Seasonality
Speleothem
Speleothem
Stalagmite
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<p>Madagascar, an island located ~300km off the eastern coast of Africa, is a natural laboratory to study paleoclimate and paleoenvironment. It holds a key position in the Indian Ocean and in Africa, as information from it has particularly helped fill gaps in paleoclimate reconstruction in the Southern Hemisphere, where such information is still scarce. Madagascar is seasonally visited by the Intertropical Convergence Zone (ITCZ) and experiences monsoon during austral summers. Furthermore, it hosts caves where speleothems can be found. Speleothems preserve a range of continuous geochemical records, mainly stable isotopes, that allow scientists to predict changes happening in the past.&#160; In Madagascar, speleothem studies have revealed distinct early, mid, and late Holocene climatic regimes that were linked to the latitudinal migration of the ITCZ, and the monsoonal responses associated with the migration. Other speleothem studies revealed evidence of the African Humid Period, rapid climate changes, and most importantly the shift in &#948;<sup>13</sup>C<sub>c</sub> starting ca. AD 800, that was attributed to anthropogenic activities. Although information from these speleothems is unquestionably significant, there are still gaps in isotopic proxies interpretation, mainly in linking modern environments where these speleothems grew and the signals they preserve. Such modern information is however fundamental to calibrate paleo-based climate and environmental reconstructions in Madagascar, which could be a key to refine their past interpretation. In this study, we performed an in-cave spatial test to understand kinetic isotope effect in Anjohibe Cave and to define oxygen isotopic fractionation between speleothem carbonate and its parent water and carbon isotopic fractionation between speleothem carbonate and the corresponding dissolved inorganic carbon (DIC). Results have been compared with modern calibration studies on speleothems from other locations worldwide, and we found that our data fit within the empirical relationship for cave-specific CaCO<sub>3</sub>-H<sub>2</sub>O isotope fractionation, grouping a range of monitored caves worldwide, 1000 ln &#945;&#160;= 16.1 (10<sup>3</sup>T<sup>-1</sup>(&#176;K))&#8211;24.6 of Tremaine et al. (2011). Other physico-chemical parameters in Anjohibe Cave have also been measured, and they will be used to discuss potential linkages with the spatial variability in the&#160; modern speleothem stable isotopic values and their corresponding parent water and DIC.</p><p><em>Tremaine, D.M., Froelich, P.N., Wang, Y., 2011. Speleothem calcite farmed in situ: Modern calibration of &#948;<sup>18</sup>O and &#948;<sup>13</sup>C paleoclimate proxies in a continuously-monitored natural cave system. Geochim Cosmochim Acta 75, 4929-4950.</em></p>
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East Asian Monsoon
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Abstract Global Climate Models (GCMs) exhibit substantial biases in their simulation of tropical climate. One particularly problematic bias exists in GCMs' simulation of the tropical rainband known as the Intertropical Convergence Zone (ITCZ). Much of the precipitation on Earth falls within the ITCZ, which plays a key role in setting Earth's temperature by affecting global energy transports, and partially dictates dynamics of the largest interannual mode of climate variability: The El Niño‐Southern Oscillation (ENSO). Most GCMs fail to simulate the mean state of the ITCZ correctly, often exhibiting a “double ITCZ bias,” with rainbands both north and south rather than just north of the equator. These tropical mean state biases limit confidence in climate models' simulation of projected future and paleoclimate states, and reduce the utility of these models for understanding present climate dynamics. Adjusting GCM parameterizations of cloud processes and atmospheric convection can reduce tropical biases, as can artificially correcting sea surface temperatures through modifications to air‐sea fluxes (i.e., “flux adjustment”). Here, we argue that a significant portion of these rainfall and circulation biases are rooted in orographic height being biased low due to assumptions made in fitting observed orography onto GCM grids. We demonstrate that making different, and physically defensible, assumptions that raise the orographic height significantly improves model simulation of climatological features such as the ITCZ and North American rainfall as well as the simulation of ENSO. These findings suggest a simple, physically based, and computationally inexpensive method that can improve climate models and projections of future climate.
Intertropical Convergence Zone
Orography
Orographic lift
Rainband
Convergence zone
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In palaeoclimate reconstructions, the combination of proxy records measured in different climate archives is challenging because of the uncertainties associated with each proxy, but it can also help reduce some of these uncertainties. Here, we present a novel approach to combine speleothem and tree ring proxies for a drought reconstruction of the last 640 years: a fluid inclusion δ18O record from a stalagmite from Villars Cave (southwest France) and a tree ring cellulose δ18O record of Quercus spp. from the nearby Angoulême area. The δ18O of the fluid inclusions is taken as an estimate of the δ18O of the trees' source water. Then, the cellulose and source water δ18O are used to calculate the leaf water isotopic enrichment, as well as relative humidity, which is the dominant controlling factor of this enrichment. The reconstructed long-term trends in relative humidity differ from a previously published reconstruction of moisture variability based on the tree ring record alone. Further measurements will be necessary to support either reconstruction. Nevertheless, this investigation demonstrates the great potential for combining isotope proxies from speleothems and tree rings to reconstruct both the low- and high-frequency variability of drought.
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Abstract We examine the influence of southern African orography on the Namibian stratocumulus deck, the South Atlantic ocean‐to‐atmosphere energy transport, and the Intertropical Convergence Zone (ITCZ), using an atmosphere‐only model and a coupled atmosphere‐ocean model. For both models, a control simulation with realistic orography is compared to a simulation where the orography in southern Africa was removed. As in the previous studies, the removal of orography results in thinning of the Namibian stratocumulus deck. In the coupled model, the increased sea surface temperature in the southern Atlantic due to the reduction of low clouds forces the Atlantic ITCZ to shift southward toward the warmer hemisphere. However, changes in the ocean circulation cool the South Atlantic atmosphere, lessening the ITCZ shift and changing the structure of precipitation. These results show the importance of orography on shaping Atlantic rainfall and highlight the role of dynamical ocean processes in atmospheric dynamics.
Orography
Intertropical Convergence Zone
Convergence zone
Atmospheric models
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