Nathalie de Noblet

Possible role of atmosphere‐biosphere interactions in triggering the Last Glaciation

We coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system, at the time of glacial inception 115,000 years ago. Orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present (atmospheric composition, oceans, biosphere and cryosphere). Summers were however cold enough to induce major vegetation shifts in high northern latitudes. Southward migration of the boreal forest/tundra limit helped to create favourable conditions for continental ice-sheet growth, with increasing snow depth and duration in Labrador, Arctic Canada and northern/western Fennoscandia. These results support a role for biogeophysical feedback in initiating glaciations.

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.

Sensitivity of the African and Asian Monsoons to Mid-Holocene Insolation and Data-Inferred Surface Changes

Abstract Orbital forcing alone is not sufficient to explain the massive northward penetration of monsoon rains in Africa shown by data during the mid-Holocene (6000 yr ago). Feedbacks associated with changes in SSTs and land surface cover may be necessary to produce a sufficient increase in the monsoon. A step toward a better understanding of the respective role of oceans and land surfaces is to design sensitivity studies with prescribed forcings, inferred from observations. In the first study, SSTs are lowered in the upwelling regions offshore of West Africa and Somalia, and increased in the Bay of Bengal and South China Sea. In the second simulation, the modern Sahara desert is replaced by a combination of xerophytic woods/scrub and grassland. In both cases the amount of water vapor advected from oceanic sources is increased north of 10°N in Africa in response to the increased land–sea temperature contrast, thereby enhancing rainfall. But the magnitude of the simulated changes is much larger when land s...

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.