Showing papers by "Paul J. Valdes published in 1998"

Intercomparison of Simulated Global Vegetation Distributions in Response to 6 kyr BP Orbital Forcing

Abstract: The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated sh...

Simulations of the Last Glacial Maximum climates using a general circulation model : prescribed versus computed sea surface temperatures

Abstract: The climate during the Last Glacial Maximum (LGM) has been simulated using the UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (GCM) with both prescribed sea surface temperatures (SSTs) based on the CLIMAP reconstruction and computed SSTs with a simple thermodynamic slab ocean. Consistent with the Paleoclimate Modelling Intercomparison Project (PMIP), the other boundary conditions include the large changes in ice-sheet topography and geography, a lower sea level, a lower concentration of CO2 in the atmosphere, and a slightly different insolation pattern at the top of the atmosphere. The results are analysed in terms of changes in atmospheric circulation. Emphasis is given to the changes in surface temperatures, planetary waves, storm tracks and the associated changes in distribution of precipitation. The model responds in a similar manner to the changes in boundary conditions to previous studies in global mean statistics, but differs in its treatment of regional climates. Results also suggest that both the land ice sheets and sea ice introduce significant changes in planetary waves and transient eddy activity, which in turn affect regional climates. The computed SST simulations predict less sea ice and cooler tropical temperatures than those based on CLIMAP SSTs. It is unclear as to whether this is a model and/or a data problem, but the resulting changes in land temperatures and precipitation can be large. Snow mass budget analysis suggests that there is net ice loss along the southern edges of the Laurentide and Fennoscandian ice sheets and net ice gain over other parts of the two ice sheets. The net accumulation is mainly due to the decrease in ablation in the cold climate rather than to the changes in snowfall. The characteristics of the Greenland ice-sheet mass balance in the LGM simulations is also quite different from those in the present-day (PD) simulations. The ablation in the LGM simulations is negligible while it is a very important process in the ice mass budget in the PD simulations.

The influence of Carboniferous palaeoatmospheres on plant function: an experimental and modelling assessment

Abstract: Geochemical models of atmospheric evolution predict that during the late Carboniferous, ca . 300 Ma, atmospheric oxygen and carbon dioxide concentrations were 35% and 0.03%, respectively. Both gases compete with each other for ribulose–1,5–bisphosphate carboxylase/oxygenase–the primary C–fixing enzyme in C3 land plants: and the absolute concentrations and the ratio of the two in the atmosphere have the potential to strongly influence land–plant function. The Carboniferous therefore represents an era of potentially strong feedback between atmospheric composition and plant function. We assessed some implications of this ratio of atmospheric gases on plant function using experimental and modelling approaches. After six weeks growth at 35% O2 and 0.03% carbon dioxide, no photosynthetic acclimation was observed in the woody species Betula pubescens and Hedera helix relative to those plants grown at 21% O2. Leaf photosynthetic rates were 29% lower in the high O2 environment compared to the controls. A global–scale analysis of the impact of the late Carboniferous climate and atmospheric composition on vegetation function was determined by driving a process–based vegetation–biogeochemistry model with a Carboniferous global palaeoclimate simulated by the Universities Global Atmospheric Modelling Programme General Circulation Model. Global patterns of net primary productivity, leaf area index and soil carbon concentration for the equilibrium model solutions showed generally low values everywhere, compared with the present day, except for a central band in the northern land mass extension of Gondwana, where high values were predicted. The areas of high soil carbon accumulation closely match the known distribution of late Carboniferous coals. Sensitivity analysis with the model indicated that the increase in O2 concentration from 21% to 35% reduced global net primary productivity by 18.7% or by 6.3 GtC yr–1. Further work is required to collate and map at the global scale the distribution of vegetation types, and evidence for wildfires, for the late Carboniferous to test our predictions.

Modelling the Asian summer monsoon rainfall and Eurasian winter/spring snow mass

Abstract: The interannual variation of the south Asian summer monsoon is analysed based on sets of present day climate simulations using the UK Universities' Global Atmospheric Modelling Programme (UGAMP) generalcirculation model with different land surface parametrization schemes, different horizontal resolutions, and sea surface temperature variations. Generally, a negative relationship is found between south Eurasian winter/spring snow mass and the amount of summer monsoon rainfall over India in all simulations. However, the significance of this relationship is dependent on the land surface parametrization scheme. The simulations using the no-flux boundary condition at the bottom of a three-layer soil model give a strong negative correlation. This inverse relationship is the strongest over north India and the foothills of the Himalayas and is statistically significant. The model results are in good agreement with observational studies. Composite analyses suggest that less winter/spring snowfall over south Eurasia is associated with a strong Indian summer monsoon, characterized by strong south-westerlies over the Arabian Sea in the lower troposphere in the June-August season and heavy precipitation in early summer over north India and the foothills of the Himalayas. In contrast, heavy winter/spring snowfall delays the onset of the Indian summer monsoon through the feedback of snowmelt, soil moisture and evaporation processes, and is associated with weak summer precipitation over the two regions. Sensitivity studies confirm that the snow mass-Indian monsoon relationship identified in the simulation at T42 is also robust in the simulation at T31. However, a negative relationship does not exist in the simulation at T21, indicating the importance of horizontal resolution in maintaining the snow-monsoon relationship in the UGAMP model.