Valérie Masson

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

The origin of Antarctic precipitation: a modelling approach

The contribution of different moisture sources to Antarctic precipitation for present-day and glacial conditions is estimated with the NASA/GISS Atmospheric General Circulation Model. Despite its low horizontal resolution (8°×10°), this model simulates reasonably well the broad features of the observed present-day hydrological cycle. Simulated present-day Antarctic precipitation is dominated throughout the year by moisture from a subtropical/midlatitude band (30°S-60°S). The moisture supplied to a given coastal area of Antarctica originates mostly in the adjacent oceanic basin; closer to the pole, other oceanic basins can also contribute significantly. Replacing the present-day sea surface temperatures (SSTs) and sea ice cover in the GCM with those from the CLIMAP oceanic reconstruction for the last glacial maximum (LGM), greatly increases the simulated latitudinal temperature gradient, with the consequence of slightly enhancing the contribution of low latitude moisture to Antarctic precipitation. It also changes the seasonality of the different contributions and thus their budget, particularly in coastal regions. Because the nature of LGM tropical SSTs is still under debate, we performed an additional LGM simulation in which the tropical SSTs are reduced relative to those of CLIMAP. The resulting decrease in the latitudinal gradient brings the relative contributions to Antarctic precipitation more in line with those of the present-day simulation.

A 420,000 year deuterium excess record from East Antarctica: Information on past changes in the origin of precipitation at Vostok

Here we present and analyze the complete Vostok ice cores deuterium excess record which spans the last four climatic cycles, back to ∼420,000 years B.P. To extract paleoclimate information from this record, we use isotopic modeling showing that changes in deuterium excess (d = δD-8δ18O) of Vostok precipitation reflect changes in the average temperature of oceanic moisture sources. Over the last 250,000 years the deuterium excess is strongly anticorrelated with changes in Earths obliquity. This reflects changes in the relative contribution of low and high latitudes to the Vostok precipitation, resulting from changes in the latitudinal annual mean insolation gradient governed by the obliquity. However, this modulation by obliquity is not observed prior to 250,000 years B.P. We attribute this difference to the ice flow, the deeper ice under Vostok station having accumulated in a location upstream Vostok, receiving precipitation from different oceanic origins. Despite this difference between the earlier and the later portions of the record, the deuterium excess changes during all the glacial inceptions are similar in amplitude and timing relative to the deuterium decrease. The glacial inceptions are characterized by high deuterium excess, indicating the significant role of the tropics in supplying moisture to the already cold poles.

Sensitivity of simulated Asian and African summer monsoons to orbitally induced variations in insolation 126, 115 and 6 kBP

We have conducted four numerical experiments with an atmospheric general circulation model (AGCM) to investigate the sensitivity of Asian and African monsoons to small changes (−5 to +12%), with respect to present-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the Northern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) however, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. This present study shows that summer warming/cooling of Eurasia and North Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summer months 6 and 126 kBP), the monsoon winds converged further inland bringing more moisture into northern India, western China and the southern Sahara. The southern tips of India, Indochina and southeastern China, as well as equatorial North Africa became drier. When land was cooler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dry (wet) 6 and 126 kBP. The location and intensity of the simulated precipitation maxima were therefore very sensitive to changes in insolation. However the total amount of monsoon rain in Asia as well as in Africa remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by changes in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (> 5 mm day −1), and fewer light showers (≤1 mm day−). Rainy days with rates between 1 and 5 mm day−1 were almost unchanged.

Relationships between δ18O in precipitation and surface air temperature in the Urumqi River Basin, East Tianshan Mountains, China

In order to evaluate the dependency of present-day δ18O in precipitation with temperature, individual precipitation events have been continuously sampled since 1996 at three stations along the Urumqi River Basin, eastern Tianshan mountains, north High China. Both the correlation and the positive slope of δ18O versus temperature increase with elevation for monthly averages, indicating that on long time scales high altitude precipitation δ18O should be a reliable indicator of regional temperature fluctuations, and supporting future ice core drilling.

Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records

Abstract Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation. The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere model.

A seasonal deuterium excess signal at Law Dome, coastal eastern Antarctica: A southern ocean signature

The snow isotopic composition (δ18O and δD) of two shallow cores from the high accumulation summit region of Law Dome, east Antarctica, was measured at monthly resolution over the 1980–1992 period. While the δ18O or δD signals clearly reflect the local temperature cycle, the deuterium excess (d = δD - 8δ18O) is shifted with respect to δ18O cycle by a 4 months lag. Interpretation of this phase lag is investigated using both an Atmospheric General Circulation Model (AGCM), which includes the water isotopic cycles, and a simple isotopic model, which better describes the microphysical processes within the cloud. Using this dual approach, we show that the seasonality of δ18O and d at Law Dome summit results from a combination of the southern ocean temperature cycle (shifted by 2–3 months with respect to the local insolation) and seasonal moisture origin changes due to a strong contribution of the local ocean when ice free. Both approaches are consistent with a dominant temperate to subtropical moisture origin. We thus demonstrate from our present-day subseasonal study that the record of d in the Dome Summit South (DSS) deep ice core represents a potential tool for identifying changes in Southern Ocean temperatures and/or sea ice cover at the scale of the past thousand years.

Simulation of intense monsoons under glacial conditions

Paleoenvironmental and paleoclimatological proxy data indicate that strong continental monsoons took place under glacial conditions during marine isotopic stage 6.5 (175 ka BP). So far, no climate model has explored the possible coexistence of glacial conditions at mid and high latitudes and of interglacial monsoons in the tropics. Here we use an atmospheric general circulation model and clearly demonstrate that high insolation can generate increased monsoon activity even with surface glacial conditions. Our experiments show that Indian and African monsoons at 175 ka were stronger than nowadays and induced an increase in outflow of the Nile river to the Mediterranean sea. This freshwater supply combined with the local low glacial evaporation may explain the stagnation of the Eastern Mediterranean sea leading to the deposit of sapropel S6. Our simulations show also increased surface winds in the northern Atlantic and Pacific oceans which may have affected the bioproductivity in these areas.

Validity of the isotopic thermometer in central Antarctica: Limited impact of glacial precipitation seasonality and moisture origin

The classical interpretation of water stable isotopes (deuterium or oxygen 18) retrieved from ice cores into past local temperature relies on the use of the spatial isotope/temperature slope as a surrogate of the temporal slope. Whereas this assumption has been challenged by independent methods in central Greenland, it is still considered as valid in central Antarctica. We use an atmospheric General Circulation Model (GCM) to study two parameters highlighted by previous studies as being among the most important with respect to this assumption. We show that in the GCM, between present-day and Last Glacial Maximum, the change in precipitation seasonality and the cooling of the moisture sources have limited and opposite effects on the isotopic record of the Antarctic local temperature. This conclusion strengthens the validity of the classical interpretation of isotope records in central Antarctica.

Energetics of the 6000-yr BP Atmospheric Circulation in Boreal Summer, from Large-Scale to Monsoon Areas: A Study with Two Versions of the LMD AGCM

Abstract The mechanisms of northern summer monsoon changes at 6 kyr BP and their dependency to model parameterizations is investigated using two versions of the atmospheric general circulation model developed by the Laboratoire de Meteorologie Dynamique, CNRS, Paris (LMD), within the Paleoclimate Modeling Intercomparison Project (PMIP). These two versions differ in their horizontal resolution and in their treatment of some surface processes: one model has a low horizontal resolution and rather simple surface treatments; the other model has a better resolution and uses more complex surface parameterizations. The authors focus on the energy budgets, which are useful tools to understand the impact of the model parameterizations. As a response to mid-Holocene insolation change, the June–September monsoon is enhanced over northern Africa, northern India, and the western Pacific, both in terms of precipitation and low-level convergence changes;convection decreases over Central America. These changes, which are ...