Didier M. Roche

Orbital Asian summer monsoon dynamics revealed using an isotope-enabled global climate model

The Asian summer monsoon dynamics at the orbital scale are a subject of considerable debate. The validity of Asian speleothem δ(18)O records as a proxy for summer monsoon intensity is questioned together with the ultimate forcing and timing of the monsoon. Here, using the results of a 150,000-year transient simulation including water isotopes, we demonstrate that Asian speleothem δ(18)O records are not a valid proxy for summer monsoon intensity only at the orbital timescale. Rather, our results show that these records reflect annual variations in hydrologic processes and circulation regime over a large part of the Indo-Asian region. Our results support the role of internal forcing, such as sea surface temperature in the equatorial Pacific, to modulate the timing of monsoon precipitation recorded in paleo-proxies inside the Asian region.

Oceanic oxygen-18 at the present day and LGM: equilibrium simulations with a coupled climate model of intermediate complexity

Abstract An Earth system model of intermediate complexity, CLIMBER-2, is used to simulate the oxygen-18 content of the water masses (H182O) in the oceans. Firstly, we forced CLIMBER-2 with the fluxes from the atmospheric general circulation model ECHAM. Simulated oceanic 18O fields for the present day are in good agreement with data. Secondly, a water isotope module was developed to transport δ18O in the atmosphere on a large scale and compute the 18O fluxes to the ocean at the atmosphere–ocean interface using only variables already computed by CLIMBER-2. For the present day, we successfully represent oxygen-18 distribution in the Atlantic, Indian and Pacific oceans, and close agreement is also found when we compare modelled and observed δ18Ow:salinity relationships. During the Last Glacial Maximum (LGM), we find that the major differences in the 18O oceanic fields (apart from the global oceanic enrichment due to ice-sheet build-up) are due to surface condition changes (surface temperature, shift in bottom water formation zones) and that no drastic changes occurred in the δ18Ow:salinity spatial relationship. In addition, we compute a calcite δ18Oc field for the Atlantic and compare it to the available data to assess the variation between the LGM and the present day.

Could meltwater pulses have been sneaked unnoticed into the deep ocean during the last glacial

The lack of climatic imprint l eft by t he Meltwa t er Pulse-1A ( ’ 14.5 ka BP), equivalent to a sea-level rise of 14 to 20 meters, is puzzling . Recent studies sugge st the event might have occurre d as a hyperpycnal flow i n t he Gulf of Me xico, preve nting its detection in oc eanic records throughout the North Atlantic. We present a suite of simula tions with the LOVECLIM climate model, which mimic th e e ffect of h yperp ycnal f low u nder LGM conditions, in a first attempt to constrain its climatic effects. Analysing the ocean dynamics associated with the anomalous freshwater input, we show that the proposed mechanism is capable of sneaking a signif i cant propo rtion of the MWP into the ocean ( ’ 6 meters equivalent sea-level rise using our model under LGM boundary cond itions). We also de monstrate t hat, in our model, the meridional circulati on i s more sensitive t o such inputs i n t he Arctic Ocean than in the Gulf of Mexico.

Simulating Heinrich event 1 with interactive icebergs

During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are thus associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean. We simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere–ocean model. In our model, dynamic-thermodynamic icebergs influence the climate through two direct effects. First, melting of the icebergs causes freshening of the upper ocean, and second, the latent heat used in the phase-transition of ice to water results in cooling of the iceberg surroundings. This cooling effect of icebergs is generally neglected in models. We investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off. At the peak of the simulated Heinrich event, icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects. The cause of this intriguing result must be sought in the involvement of a secondary mechanism: facilitation of sea-ice formation, which can disturb deep water production at key convection sites, with consequences for the thermohaline circulation. We performed additional sensitivity experiments, designed to explore the effect of the more plausible distribution of the dynamic icebergs’ melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid-latitude North Atlantic (NA) Ocean. The early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments, which must be a distribution-effect: the dynamically distributed icebergs quickly affect western NADW formation, which synergizes with direct sea-ice facilitation, causing an earlier sea-ice expansion and climatic response. Furthermore, compared to dynamic-thermodynamic icebergs, a homogeneous hosing overestimates the fresh water flux in the Eastern Ruddiman belt, causing a fresh anomaly in the Eastern North Atlantic, leading to a delayed recovery of the circulation after the event.

Statistical modelling of a new global potential vegetation distribution

The potential natural vegetation (PNV) distribution is required for several studies in environmental sciences. Most of the available databases are quite subjective or depend on vegetation models. We have built a new high-resolution world-wide PNV map using a objective statistical methodology based on multinomial logistic models. Our method appears as a fast and robust alternative in vegetation modelling, independent of any vegetation model. In comparison with other databases, our method provides a realistic PNV distribution in agreement with respect to BIOME 6000 data. Among several advantages, the use of probabilities allows us to estimate the uncertainty, bringing some confidence in the modelled PNV, or to highlight the regions needing some data to improve the PNV modelling. Despite our PNV map being highly dependent on the distribution of data points, it is easily updatable as soon as additional data are available and provides very useful additional information for further applications.

Sensitivity of South American Tropical Climate to Last Glacial Maximum Boundary Conditions: Focus on Teleconnections with Tropics and Extratropics

We explore how the moist deep convection over the Amazonian region responds to glacial forcings compared to the pre-indrustrial climate and how this change might interact with the meridional shift of rainfall over Nordeste, Pacific and Atlantic tropical Oceans. The objective is to assess and investigate the individual contributions of greenhouse gases concentration, ice sheet topography and/or albedo on the hydrological changes over tropical South America and their links to the Hadley and Walker circulations. We employ coupled ocean-atmosphere simulations for the Last Glacial Maximum and sensitivity experiments for each Last Glacial Maximum forcing. We show that the Last Glacial Maximum reduced greenhouse gases alone can explain the observed rainfall changes over tropical South America through the induced increase in tropical and northern extra-tropical dry static stability and altered Hadley circulation. Furthermore, we show that the topography of the North American ice sheet reinforces the equatorward shift of the descending branch of the Hadley cell leading to stronger subsidence and drying over the northern tropics. However, we show that the Laurentide ice sheet has also a significant influence on the simulated enhanced rainfall over Nordeste and Southeastern Brazil via the eastward shift of the Walker circulation, with a mechanism analogous to the atmospheric thermodynamical response to El-Nino conditions.

Rapid termination of the African Humid Period triggered by northern high-latitude cooling

The rapidity and synchrony of the African Humid Period (AHP) termination at around 5.5 ka are debated, and it is unclear what caused a rapid hydroclimate response. Here we analysed the hydrogen isotopic composition of sedimentary leaf-waxes (δDwax) from the Gulf of Guinea, a proxy for regional precipitation in Cameroon and the central Sahel-Sahara. Our record indicates high precipitation during the AHP followed by a rapid decrease at 5.8–4.8 ka. The similarity with a δDwax record from northern East Africa suggests a large-scale atmospheric mechanism. We show that northern high- and mid-latitude cooling weakened the Tropical Easterly Jet and, through feedbacks, strengthened the African Easterly Jet. The associated decrease in precipitation triggered the AHP termination and combined with biogeophysical feedbacks to result in aridification. Our findings suggest that extratropical temperature changes, albeit smaller than during the glacial and deglacial, were important in triggering rapid African aridification during the Holocene.The synchrony, spatial distribution and causes of the African Humid Period termination at 5.5 ka remain debated. Here, the authors show that rapid aridification in Cameroon and the central Sahel-Sahara took place between 5.8–4.8 ka and was likely triggered by high- and mid-latitude cooling.

A coupled model for carbon and radiocarbon evolution during the last deglaciation

Changes in the ventilation of the Southern Ocean are thought to play an important role on deglacial carbon and radiocarbon evolution but have not been tested within a coupled climate-carbon model. Here we present such a simulation based on a simple scenario of transient deglacial sinking of brines—sea ice salt rejections—around Antarctica, which modulates Southern Ocean ventilation. This experiment is able to reproduce deglacial atmospheric changes in carbon and radiocarbon and also ocean radiocarbon records measured in the Atlantic, Southern, and Pacific Oceans. Simulated for the first time in a fully coupled climate-carbon model of intermediate complexity including radiocarbon, our modeling results suggest that the deglacial changes in atmospheric carbon dioxide and radiocarbon were achieved by means of a breakdown in the glacial brine-induced stratification of the Southern Ocean.

Modeling water isotopologues during the last glacial: Implications for quantitative paleosalinity reconstruction

Quantitative paleosalinity reconstructions with reasonable uncertainties remain a challenge in paleoceanography. In this study, we focus on stable isotope-based methods (δ18O and δ2H) to derive paleosalinity. We use the water isotopes-enabled fully coupled atmosphere/ocean/vegetation/land surface three-dimensional model of intermediate complexity iLOVECLIM to simulate the climate and water isotopes during the Last Glacial Maximum (LGM) and Heinrich event 1. We investigate how the isotopes in water can be used as reliable proxies to quantitatively reconstruct past changes in seawater salinity. Our results demonstrate that a quantitative salinity reconstruction during glacial conditions based on present-day regional δ18O-salinity relationships can lead to considerable errors (up to 25 g/kg in certain regions). However, we show that these eventual uncertainties on paleosalinity reconstruction can be reduced by allowing for model-derived regional δ18O-salinity relationships to vary through time. Our results indicate a rather stable dependence between δ2H and δ18O in surface seawater during the LGM. This suggest that quantitative reconstruction of salinity based on the δ2H measurement of alkenones (δ2Ha) might be possible if the slope and the intercept of the regression between the fractionation factor δ2Ha-δ2Hsw and salinity can be sufficiently tightly constrained in open ocean conditions. We confirm that pairing water isotopologues has a strong potential to reduce uncertainties on quantitative paleosalinity reconstructions, also under glacial boundary conditions.

Stereoselective synthesis of (4S,5R,6S)-4-(5,6-epoxy-6-phenyl)-γ-lactone

Abstract A short (7 steps) and efficient (45% overall yield) synthesis of (4S,5R,6S)-4-(5,6-epoxy-6-phenyl)-γ-lactone, a versatile intermediate toward possible HIV-1 protease inhibitors, is described. Two examples of trans-α-benzylation of the lactonic ring followed by a regioselective opening of the epoxide (with thiopropanamide) as well as an opening of the lactone ring with L-valine (2-methoxy-ethyl)-amide are also given.