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

 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. Received: 10 January 1997 / Accepted: 11 December 1997     

Impact of vegetation changes on the dynamics of the atmosphere at the Last Glacial Maximum

Much work is under way to identify and quantify the feedbacks between vegetation and climate. Palaeoclimate modelling may provide a mean to address this problem by comparing simulations with proxy data. We have performed a series of four simulations of the Last Glacial Maximum (LGM, 21,000 years ago) using the climate model HadSM3, to test the sensitivity of climate to various changes in vegetation: a global change (according to a previously discussed simulation of the LGM with HadSM3 coupled to the dynamical vegetation model TRIFFID); a change only north of 35°N; a change only south of 35°N; and a variation in stomatal opening induced by the reduction in atmospheric CO₂ concentration. We focus mainly on the response of temperature, precipitation, and atmosphere dynamics. The response of continental temperature and precipitation mainly results from regional interactions with vegetation. In Eurasia, particularly Siberia and Tibet, the response of the biosphere substantially enhances the glacial cooling through a positive feedback loop between vegetation, temperature, and snow-cover. In central Africa, the decrease in tree fraction reduces the amount of precipitation. Stomatal opening is not seen to play a quantifiable role. The atmosphere dynamics, and more specifically the Asian summer monsoon system, are significantly altered by remote changes in vegetation: the cooling in Siberia and Tibet act in concert to shift the summer subtropical front southwards, weaken the easterly tropical jet and the momentum transport associated with it. By virtue of momentum conservation, these changes in the mid-troposphere circulation are associated with a slowing of the Asian summer monsoon surface flow. The pattern of moisture convergence is slightly altered, with moist convection weakening in the western tropical Pacific and strengthening north of Australia.     

Simulation of the global bio-geophysical interactions during the Last Glacial Maximum

 The bio-geophysical feedbacks during the Last Glacial Maximum (LGM, 21 000 y BP) are investigated by use of an asynchronously coupled global atmosphere-biome model. It is found that the coupled model improves on the results of an atmosphere-only model especially for the Siberian region, where the inclusion of vegetation-snow-albedo interaction leads to a better agreement with geological reconstructions. Furthermore, it is shown that two stable solutions of the coupled model are possible under LGM boundary conditions. The presence of bright sand desert at the beginning of a simulation leads to more extensive subtropical deserts, whereas an initial global vegetation cover with forest, steppe, or dark desert results in a northward spread of vegetation of up to some 1000 km, mainly in the western Sahara. These differences can be explained in the framework of Charney’s theory of a “self-induction” of deserts through albedo enhancement. Moreover, it is found that the tropical easterly jet is strengthened in the case of the “green” Sahara, which in turn leads to a modification of the Indian summer monsoon. Received: 20 June 1997/Accepted: 14 January 1998     

A Nine-layer Atmospheric General Circulation Model and Its Performance

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Hydro-climatic Characteristics of Yarlung Zangbo River Basin since the Last Glacial Maximum

Global climate changes significantly impact the water condition of big rivers in glacierized high mountains. However,there is a lack of studies on hydrological changes within river basins caused by climate changes over a geological timescale due to the impossibility of direct observations. In this study, we examine the hydro-climatic variation of the Yarlung Zangbo River Basin in the Tibet Plateau since the Last Glacial Maximum(LGM) by combining δ18 O proxy records in Indian and Omani caves with the simulated Indian summer monsoon, surface temperature, precipitation, evapotranspiration and runoff via the Community Climate System Model and the reconstructed glacier coverage via the Parallel Ice Sheet Model. The mean river runoff was kept at a low level of 145 billion cubic meters per year until an abrupt increase at a rate of 8.7 million cubic meters per year in the B?lling-Aller?d interval(BA). The annual runoff reached a maximum of 250 billion cubic meters in the early Holocene and then reduced to the current value of 180 billion cubic meters at a rate of 6.4 million cubic meters per year. The low runoff in the LGM and Heinrich Stadial 1(HS1) is likely attributed to such a small contribution of precipitation to runoff and the large glacier cover. The percentage of precipitation to runoff was only 20%during the LGM and HS1. Comparison of glacier area among different periods indicates that the fastest deglaciation occurred during the late HS1, when nearly 60% of glacier area disappeared in the middle reach, 50% in the upper reach,and 30% in the lower reach. The rapid deglaciation and increasing runoff between the late HS1 and BA may have accelerated widespread ice-dam breaches and led to extreme outburst flood events. Combining local geological proxy records and regional simulations could be a useful approach for the study of paleo-hydrologic variations in big river basins.     

Coupling an ocean wave model to an atmospheric general circulation model

A coupled model, consisting of an ocean wave model and an atmospheric general circulation model (AGCM), is integrated under permanent July conditions. The wave model is forced by the AGCM wind stress, whereas the wind waves modify the AGCM surface fluxes of momentum, sensible and latent heat. We investigate the following aspects of the coupled model: how realistic are the wave fields, how strong is the coupling, and how sensitive is the atmospheric circulation to the spatially and temporally varying wave field. The wave climatology of the coupled model compares favorably with observational data. The interaction between the two models is largest (although weak) in the storm track in the Southern Hemisphere. Young windsea, which is associated with enhanced surface fluxes is generated mostly in the equatorward frontal area of an individual cyclone. However, the enhancement of the surface fluxes is too small to significantly modify the climatological mean atmospheric circulation.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max Planck Institute for Meteorology. Guest Editor for these papers is L. Dümenil     

A coupled climate model simulation of the Last Glacial Maximum,Part 2: approach to equilibrium

The climate of the last glacial maximum (LGM) is simulated with a coupled climate model. The simulated climate undergoes a rapid adjustment during the first several decades after imposition of LGM boundary conditions, as described in Part 1, and then evolves toward equilibrium over 900 model years. The climate simulated by the coupled model at this period is compared with observationally-based LGM reconstructions and with LGM results obtained with an atmosphere-mixed layer (slab) ocean version of the model in order to investigate the role of ocean dynamics in the LGM climate. Global mean surface air temperature and sea surface temperature (SST) decrease by about 10 °C and 5.6 °C in the coupled model which includes ocean dynamics, compared to decreases of 6.3 and 3.8 °C in slab ocean case. The coupled model simulates a cooling of about 6.5 °C over the tropics, which is larger than that of the CLIMAP reconstruction (1.7 °C) and larger than that of the slab ocean simulation (3.3 °C), but which is in reasonable agreement with some recent proxy estimates. The ocean dynamics of the coupled model captures features found in the CLIMAP reconstructions such as a relative maximum of ocean cooling over the tropical Pacific associated with a mean La Niña-like response and lead to a more realistic SST pattern than in the slab model case. The reduction in global mean precipitation simulated in the coupled model is larger (15%) than that simulated with the slab ocean model (~10%) in conjunction with the enhanced cooling. Some regions, such as the USA and the Mediterranean region, experience increased precipitation in accord with proxy paleoclimate evidence. The overall much drier climate over the ocean leads to higher sea surface salinity (SSS) in most ocean basins except for the North Atlantic where SSS is considerably lower due to an increase in the supply of fresh water from the Mississippi and Amazon rivers and presumably a decrease in salt transport by the weakened North Atlantic overturning circulation. The North Atlantic overturning stream function weakens to less than half of the control run value. The overturning is limited to a shallower depth (less than 1000 m) and its outflow is confined to the Northern Hemisphere. In the Southern Ocean, convection is much stronger than in the control run leading to a stronger overturning stream function associated with enhanced Antarctic Bottom Water formation. As a result, Southern Ocean water masses fill the entire deep ocean. The Antarctic Circumpolar Current (ACC) transport through the Drake Passage increases by about 25%. The ACC transport, despite weaker zonal winds, is enhanced due to changes in bottom pressure torque. The weakening of the overturning circulation in the North Atlantic and the accompanying 30% decrease in the poleward ocean heat transport contrasts with the strengthening of the overturning circulation in the Southern Ocean and a 40% increase in heat transport. As a result, sea ice coverage and thickness are affected in opposite senses in the two hemispheres. The LGM climate simulated by the coupled model is in reasonable agreement with paleoclimate proxy evidence. The dynamical response of the ocean in the coupled model plays an important role in determining the simulated, and undoubtedly, the actual, LGM climate.     

The role of temperature on treeline migration for an eastern African mountain during the Last Glacial Maximum

Paleo-data suggest that East African mountain treelines underwent an altitudinal shift during the Last Glacial Maximum (LGM). Understanding the ecological and physiological processes underlying treeline response to such past climate change will help to improve forecasts of treeline change under future global warming. In spite of significant improvements in paleoclimatic reconstruction, the climatic conditions explaining this migration are still debated and important factors such as atmospheric CO₂ concentration, the impact of lapse rate decreasing temperature along altitudinal gradients and rainfall modifications due to elevation have often been neglected or simplified. Here, we assess the effects of these different factors and estimate the influence of the most dominant factors controlling changes in past treeline position using a multi-proxy approach based on simulations from BIOME4, a coupled biogeography and biogeochemistry model, modified to account for the effect of elevation on vegetation, compared with pollen, and isotopic data. The results indicate a shift in mountain vegetation at the LGM was controlled by low pCO₂ and low temperatures promoting species morphologically and physiologically better adapted to LGM conditions than many trees composing the forest belt limit. Our estimate that the LGM climate was cooler than today’s by ?4.5 °C (range: ?4.3 to ?4.6 °C) at the upper limit of the treeline, whereas at 831 m it was cooler by ?1.4 °C (range: ?2.6 to ?0.6 °C), suggests that a possible lapse rate modification strongly constrained the upper limit of treeline, which may limit its potential extension under future global warming.     

Future climate of the Caribbean from a super-high-resolution atmospheric general circulation model

Present-day (1979–2003) and future (2075–2099) simulations of mean and extreme rainfall and temperature are examined using data from the Meteorological Research Institute super-high-resolution atmospheric general circulation model. Analyses are performed over the 20-km model grid for (1) a main Caribbean basin, (2) sub-regional zones, and (3) specific Caribbean islands. Though the model’s topography underestimates heights over the eastern Caribbean, it captures well the present-day spatial and temporal variations of seasonal and annual climates. Temperature underestimations range from 0.1 °C to 2 °C with respect to the Japanese Reanalysis and the Climatic Research Unit datasets. The model also captures fairly well sub-regional scale variations in the rainfall climatology. End-of-century projections under the Intergovernmental Panel on Climate Change SRES A1B scenario indicate declines in rainfall amounts by 10–20 % for most of the Caribbean during the early (May–July) and late (August–October) rainy seasons relative to the 1979–2003 baselines. The early dry season (November–January) is also projected to get wetter in the far north and south Caribbean by approximately 10 %. The model also projects a warming of 2–3 °C over the Caribbean region. Analysis of future climate extremes indicate a 5–10 % decrease in the simple daily precipitation intensity but no significant change in the number of consecutive dry days for Cuba, Jamaica, southern Bahamas, and Haiti. There is also indication that the number of hot days and nights will significantly increase over the main Caribbean basin.     

GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica

 The LMDz variable grid GCM was used to simulate the Last Glacial Maximum (LGM, 21 ky Bp.) climate of Greenland and Antarctica at a spatial resolution of about 100 km.The high spatial resolution allows to investigate the spatial variability of surface climate change signals, and thus to address the question whether the sparse ice core data can be viewed as representative for the regional scale climate change. This study addresses primarily surface climate parameters because these can be checked against the, limited, ice core record. The changes are generally stronger for Greenland than for Antarctica, as the imposed changes of the forcing boundary conditions (e.g., sea surface temperatures) are more important in the vicinity of Greenland. Over Greenland, and to a limited extent also in Antarctica, the climate shows stronger changes in winter than in summer. The model suggests that the linear relationship between the surface temperature and inversion strength is modified during the LGM. The temperature dependency of the moisture holding capacity of the atmosphere alone cannot explain the strong reduction in snowfall over central Greenland; atmospheric circulation changes also play a crucial role. Changes in the high frequency variability of snowfall, atmospheric pressure and temperature are investigated and possible consequences for the interpretation of ice core records are discussed. Using an objective cyclone tracking scheme, the importance of changes of the atmospheric dynamics off the coasts of the ice sheets, especially for the high frequency variability of surface climate parameters, is illustrated. The importance of the choice of the LGM ice sheet topography is illustrated for Greenland, where two different topographies have been used, yielding results that differ quite strongly in certain nontrivial respects. This means that the paleo-topography is a significant source of uncertainty for the modelled paleoclimate. The sensitivity of the Greenland LGM climate to the prescribed sea surface conditions is examined by using two different LGM North Atlantic data sets. Received: 23 October 1997 / Accepted: 17 March 1998     

Non-linear climate feedback analysis in an atmospheric general circulation model

 A method is described for evaluating the ‘partial derivatives’ of globally averaged top-of-atmosphere (TOA) radiation changes with respect to basic climate model physical parameters. This method is used to analyse feedbacks in the Australian Bureau of Meteorology Research Centre general circulation model. The parameters considered are surface temperature, water vapour, lapse rate and cloud cover. The climate forcing which produces the changes is a globally uniform sea surface temperature (SST) perturbation. The first and second order differentials of model parameters with respect to the forcing (i.e. SST changes) are estimated from quadratic least square fitting. Except for total cloud cover, variables are found to be strong functions of global SST. Strongly non-linear variations of lapse rate and high cloud amount and height appear to relate to the non-linear response in penetrative convection. Globally averaged TOA radiation differentials with respect to model parameters are also evaluated. With the exception of total cloud contributions, a high correlation is generally found to exist, on the global mean level, between TOA radiation and the respective parameter perturbations. The largest non-linear terms contributing to radiative changes are those due to lapse rate and high cloud. The contributions of linear and non-linear terms to the overall radiative response from a 4 K SST perturbation are assessed. Significant non-linear responses are found to be associated with lapse rate, water vapour and cloud changes. Although the exact magnitude of these responses is likely to be a function of the particular model as well as the imposed SST perturbation pattern, the present experiments flag these as processes which cannot properly be understood from linear theory in the evaluation of climate change sensitivity. Received: 16 January 1997/Accepted: 9 May 1997     

Synoptic-scale perturbations in AGCM simulations of the present and Last Glacial Maximum climates

 The conditions of development of mid-latitude depressions (synoptic eddies) in the winter Northern Hemisphere mid-latitudes at the Last Glacial Maximum (LGM, 21 000 years ago) are very different from the present ones: this period is characterised by a general cooling of the extra-tropics, with massive ice sheets over the Northern Hemisphere continents and sea-ice extending very far south over the North Atlantic. The present work uses regression analysis to study the characteristics of the synoptic eddies in present-day and LGM climate simulations by the Atmospheric General Circulation Model (AGCM) of the UK Universities' Global Atmospheric Programme (UGAMP). In the LGM experiment, the structure of the Pacific eddies is similar to the present-day (PD) situation, but they are weaker. On the other hand, the Atlantic eddies show an increased zonal wavelength and a much shallower structure in the temperature and vertical wind perturbations. To understand the changes of these characteristics from present-day to LGM, we compare them to those computed for the most unstable modes of the corresponding mean flows, determined using a dry primitive equation model. A normal-mode stability analysis is carried both on zonally symmetric and asymmetric flows for each of the Northern Hemisphere storm-tracks. The changes in the most unstable normal modes found by both these analyses give a good account of changes in the structure of the perturbations as retrieved from the AGCM, suggesting that changes in the mean state (especially the temperature gradient) is the main driver of these changes. However in the case of the present-day Atlantic storm-track, the growth rate of these modes is found to be very low compared to the other cases. A complementary analysis evaluates the importance of non-modal growth, in the form of downstream development of perturbations, for each of the storm-tracks. This type of growth is found to be especially important in the case of the present-day Atlantic storm-track. Received: 29 September 1999 / Accepted: 17 November 1999