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1.
The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.  相似文献   
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3.
The simulation of the mean seasonal cycle of sea surface temperature (SST) remains a challenge for coupled ocean–atmosphere general circulation models (OAGCMs). Here we investigate how the numerical representation of clouds and convection affects the simulation of the seasonal variations of tropical SST. For this purpose, we compare simulations performed with two versions of the same OAGCM differing only by their convection and cloud schemes. Most of the atmospheric temperature and precipitation differences between the two simulations reflect differences found in atmosphere-alone simulations. They affect the ocean interior down to 1,000 m. Substantial differences are found between the two coupled simulations in the seasonal march of the Intertropical Convergence Zone in the eastern part of the Pacific and Atlantic basins, where the equatorial upwelling develops. The results confirm that the distribution of atmospheric convection between ocean and land during the American and African boreal summer monsoons plays a key role in maintaining a cross equatorial flow and a strong windstress along the equator, and thereby the equatorial upwelling. Feedbacks between convection, large-scale circulation, SST and clouds are highlighted from the differences between the two simulations. In one case, these feedbacks maintain the ITCZ in a quite realistic position, whereas in the other case the ITCZ is located too far south close to the equator.  相似文献   
4.
Crétat  Julien  Braconnot  Pascale  Terray  Pascal  Marti  Olivier  Falasca  Fabrizio 《Climate Dynamics》2020,55(9-10):2761-2784

The low-frequency evolution of Indian rainfall mean-state and associated interannual-to-decadal variability is discussed for the last 6000 years from a multi-configuration ensemble of fully coupled global transient simulations. This period is marked by a shift of Indian Summer Monsoon Rainfall (ISMR) distribution towards drier conditions, including extremes, and a contraction of the rainy season. The drying is larger in simulations with higher horizontal resolution of the atmosphere and revised land surface hydrology. Vegetation–climate interactions and the way runoff is routed to ocean modulate the timing of the monsoon onset but have negligible effects on the evolution of seasonal rainfall amounts in our modeling framework in which carbon cycling is always active. This drying trend is accompanied by changes in ISMR interannual-to-decadal variability decreasing over north and south India but increasing over central India (20°–25° N). The ISMR interannual-to-decadal variability is decomposed into six physically consistent regimes using a clustering technique to further characterize its changes and associated teleconnections. From 6 to 3.8 kyr bp, the century-to-century modulations in the frequency of occurrence associated to the regimes are asynchronous between the simulations. Orbitally-driven trends can only be detected for two regimes over the whole 6–0 kyr bp period. These two regimes reflect increased influence of ENSO on both ISMR and Indian Ocean Dipole as the inter-hemispheric energy gradient weakens. Severe long-term droughts are also shown to be a combination of long-term drying and internally generated low-frequency modulations of the interannual-to-decadal variability.

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5.
 Within the framework of the PMIP (Paleoclimate Modelling Intercomparison Project), we have compared mid-Holocene climate simulations from 16 atmospheric general circulation models (AGCMs) with new pollen-based reconstructions of the European bioclimatic variables for winter and growing season temperatures as well as annual water budget changes. In winter, some models are able to simulate the reconstructed northeastern warming, due to an increased heat transport from the ocean, associated with a larger north-south pressure gradient over the northern Atlantic. Whereas most models are only able to simulate a strong summer warming, data indicate a shorter and/ or colder growing season in southern Europe and a longer and/or warmer growing season in northwestern Europe. The reconstructed change in annual water budget indicates drier conditions in northwestern Europe and wetter conditions in southern Europe. Some models simulate such moisture changes, due to more summer evaporation over Scandinavia during summer, and more autumn-winter-spring precipitation over southern Europe. To address the PMIP approximation of no change in ocean and land boundary conditions, we have performed short sensitivity experiments to surface boundary conditions (sea-surface-temperatures, vegetation) using one single model. The model-data disagreements over Europe are probably due to the local influence of the surrounding oceans which are not taken into account in the first PMIP simulations. We therefore stress the need for more mid-Holocene SST reconstructions and further analysis of pollen data in the Mediterranean region. Received: 23 February 1998 /Accepted: 19 September 1998  相似文献   
6.
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.  相似文献   
7.
The climates of the mid-Holocene (MH), 6,000 years ago, and of the Last Glacial Maximum (LGM), 21,000 years ago, have extensively been simulated, in particular in the framework of the Palaeoclimate Modelling Intercomparion Project. These periods are well documented by paleo-records, which can be used for evaluating model results for climates different from the present one. Here, we present new simulations of the MH and the LGM climates obtained with the IPSL_CM5A model and compare them to our previous results obtained with the IPSL_CM4 model. Compared to IPSL_CM4, IPSL_CM5A includes two new features: the interactive representation of the plant phenology and marine biogeochemistry. But one of the most important differences between these models is the latitudinal resolution and vertical domain of their atmospheric component, which have been improved in IPSL_CM5A and results in a better representation of the mid-latitude jet-streams. The Asian monsoon’s representation is also substantially improved. The global average mean annual temperature simulated for the pre-industrial (PI) period is colder in IPSL_CM5A than in IPSL_CM4 but their climate sensitivity to a CO2 doubling is similar. Here we show that these differences in the simulated PI climate have an impact on the simulated MH and LGM climatic anomalies. The larger cooling response to LGM boundary conditions in IPSL_CM5A appears to be mainly due to differences between the PMIP3 and PMIP2 boundary conditions, as shown by a short wave radiative forcing/feedback analysis based on a simplified perturbation method. It is found that the sensitivity computed from the LGM climate is lower than that computed from 2 × CO2 simulations, confirming previous studies based on different models. For the MH, the Asian monsoon, stronger in the IPSL_CM5A PI simulation, is also more sensitive to the insolation changes. The African monsoon is also further amplified in IPSL_CM5A due to the impact of the interactive phenology. Finally the changes in variability for both models and for MH and LGM are presented taking the example of the El-Niño Southern Oscillation (ENSO), which is very different in the PI simulations. ENSO variability is damped in both model versions at the MH, whereas inconsistent responses are found between the two versions for the LGM. Part 2 of this paper examines whether these differences between IPSL_CM4 and IPSL_CM5A can be distinguished when comparing those results to palaeo-climatic reconstructions and investigates new approaches for model-data comparisons made possible by the inclusion of new components in IPSL_CM5A.  相似文献   
8.
Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm−2, which translates to equilibrium climate sensitivity for doubling of atmospheric CO2 of 2.6–3.1°C.  相似文献   
9.
We investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid-Holocene (6,000 year BP) using results from seven coupled ocean–atmosphere general circulation models. We examine how the dipole in late summer sea-surface temperature (SST) anomalies in the tropical Atlantic increases the length of the African monsoon, how this dipole structure is created and maintained, and how the late summer SST warming in the northwest Indian Ocean affects the monsoon retreat in this sector. Similar mechanisms are found in all of the models, including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing, as well as increased Ekman transport in the Atlantic that sharpens the Atlantic dipole pattern. We also consider changes in interannual variability over West Africa and the Indian Ocean. The teleconnection between variations in SST and Sahelian precipitation favor a larger impact of the Atlantic dipole mode in this region. In the Indian Ocean, the strengthening of the Indian dipole structure in autumn has a damping effect on the Indian dipole mode at the interannual time scale.  相似文献   
10.
Results from nine coupled ocean-atmosphere simulations have been used to investigate changes in the relationship between the variability of monsoon precipitation over western Africa and tropical sea surface temperatures (SSTs) between the mid-Holocene and the present day. Although the influence of tropical SSTs on the African monsoon is generally overestimated in the control simulations, the models reproduce aspects of the observed modes of variability. Thus, most models reproduce the observed negative correlation between western Sahelian precipitation and SST anomalies in the eastern tropical Pacific, and many of them capture the positive correlation between SST anomalies in the eastern tropical Atlantic and precipitation over the Guinea coastal region. Although the response of individual model to the change in orbital forcing between 6 ka and present differs somewhat, eight of the models show that the strength of the teleconnection between SSTs in the eastern tropical Pacific and Sahelian precipitation is weaker in the mid-Holocene. Some of the models imply that this weakening was associated with a shift towards longer time periods (from 3–5 years in the control simulations toward 4–10 years in the mid-Holocene simulations). The simulated reduction in the teleconnection between eastern tropical Pacific SSTs and Sahelian precipitation appears to be primarily related to a reduction in the atmospheric circulation bridge between the Pacific and West Africa but, depending on the model, other mechanisms such as increased importance of other modes of tropical ocean variability or increased local recycling of monsoonal precipitation can also play a role.  相似文献   
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