Search

1a we showed the mid- to late Holocene evolution in annual mean insolation at the top of the atmosphere in the Northern and Southern Hemisphere (hereafter NH and SH). Panel a: annual mean incoming solar radiation averaged in the Northern Hemisphere (NH) [0-90 N] and Southern Hemisphere (SH) [0-90 S] in blue and orange (dashed) lines. Panel a: annual mean incoming solar radiation averaged in the Northern Hemisphere (NH) [0-90 N] and Southern Hemisphere (SH) [0-90 S]. [Extracted from the article]

This study describes time evolution of the Southern Annular Mode (SAM) in Mid- to Late Holocene simulated with a state-of-the-art transient simulation of the last 6000 years carried out with the IPSL-CM5A2 model. Simulated SAM index exhibits significant long-term linear trends of different sign depending on the season that are closely related to multi-millennial changes in insolation which was the main driver of long-term climate change in the study period. Interactions between changes in insolation and the SAM are linked to temperature and pressure changes developed through the entire Southern Hemisphere. In fact, model results suggest that insolation changes produced significant changes in extratropical temperature gradients that, in turn, induced changes in pressure gradients synthesized by significant long-term linear trends in the SAM index from Mid- to Late-Holocene. Considering that changes in the SAM index synthetize changes in hemispheric patterns of temperature, pressure and winds, results exposed in this study should be considered as reference for reconstructions of SAM evolution in the last 6000 years from climate proxy archives. [ABSTRACT FROM AUTHOR]

State-of-the-art Earth system models, like the ones used in the Coupled Model Intercomparison Project Phase 6 (CMIP6), suffer from temporal inconsistencies at the ocean–atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on the Schwarz iterative method. Despite its large computational cost compared to standard coupling methods, which makes the algorithm implementation impractical for production runs, the Schwarz method is useful to evaluate some of the errors made in state-of-the-art ocean–atmosphere coupled models (e.g., in the representation of the processes related to diurnal cycle), as illustrated by the present study. IPSL-CM6-SW-VLR is a low-resolution version of the IPSL-CM6 coupled model with a simplified land surface model, implementing a Schwarz iterative coupling scheme. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme (referred to as the parallel algorithm) show large differences after sunrise and before sunset, when the external forcing (insolation at the top of the atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100 % of the solution, even with a small coupling period, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method, which leads to a doubling of the computing cost. Besides the parallel algorithm used in IPSL-CM6, we also test a so-called sequential atmosphere-first algorithm, which is used in some coupled ocean–atmosphere models. We show that the sequential algorithm improves the numerical results compared to the parallel one at the expanse of a loss of parallelism. The present study focuses on the ocean–atmosphere interface with no sea ice. The problem with three components (ocean–sea ice–atmosphere) remains to be investigated. [ABSTRACT FROM AUTHOR]

State-of-the-art Earth System models, like the ones used in the CMIP6 intercomparison project, suffer from temporal inconsistencies at the ocean-atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere, and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on Schwarz methods. Despite the fact that the computational cost is large compared to standard coupling methods, which makes the method impractical as is for production runs, Schwarz algorithms are useful to evaluate some of the errors made in state-of-the-art ocean-atmosphere coupled models (e.g. in the representation of the processes related to diurnal cycle), as illustrated by the present study. A new coupling scheme based on such iterative method has been implemented in the IPSL coupled model. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme show large differences at sunrise and sunset, when the external forcing (insolation at top of atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100% of the solution, even with a small coupling time step, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method which leads to a doubling of the computing cost. A thorough design of the first guess to initialize the iterative process may yield a solution that reduces the error with only one iteration. The present study focuses on the ocean-atmosphere interface, with no sea-ice. The problem with three components (ocean/sea-ice/atmosphere) remains to be investigated. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) is the strongest mode of interannual climate variability in the current climate, influencing ecosystems, agriculture, and weather systems across the globe, but future projections of ENSO frequency and amplitude remain highly uncertain. A comparison of changes in ENSO in a range of past and future climate simulations can provide insights into the sensitivity of ENSO to changes in the mean state, including changes in the seasonality of incoming solar radiation, global average temperatures, and spatial patterns of sea surface temperatures. As a comprehensive set of coupled model simulations is now available for both palaeoclimate time slices (the Last Glacial Maximum, mid-Holocene, and last interglacial) and idealised future warming scenarios (1 % per year CO2 increase, abrupt four-time CO2 increase), this allows a detailed evaluation of ENSO changes in this wide range of climates. Such a comparison can assist in constraining uncertainty in future projections, providing insights into model agreement and the sensitivity of ENSO to a range of factors. The majority of models simulate a consistent weakening of ENSO activity in the last interglacial and mid-Holocene experiments, and there is an ensemble mean reduction of variability in the western equatorial Pacific in the Last Glacial Maximum experiments. Changes in global temperature produce a weaker precipitation response to ENSO in the cold Last Glacial Maximum experiments and an enhanced precipitation response to ENSO in the warm increased CO2 experiments. No consistent relationship between changes in ENSO amplitude and annual cycle was identified across experiments. [ABSTRACT FROM AUTHOR]

It has been shown that El Niño events contribute to discharge the warm pool excess of energy out of the tropical Pacific. In a different climate, the energetic budget in the tropical Pacific is altered, which might have an effect on the El Niño amplitude and/or occurrence and thereby on the role of El Niño on energy redistribution. The mid-Holocene period (6 ka BP) offers a good example of changes in the distribution of incoming solar energy. In particular, the equator-pole gradient was weaker compared to the modern period. We analyze long stable simulations of the mid-Holocene and the pre-industrial era and discuss the mean- and El Niño-related energy transports in the two climates. We show that the role of global energy pump played by the tropical Pacific is reduced in the mid-Holocene in our simulation, both in long-term mean and during El Niño years. We demonstrate that this is not only a direct response to insolation forcing but this is further amplified by changes in internal processes. We analyze the relative role of El Niño events in the Pacific discharge in the two climates and show that it is reduced in the mid-Holocene, i.e. the fraction of the Pacific discharge that is due to El Niño is reduced. This is mainly due to reduction in the occurrence of El Niño events. This work gives a new approach to address El Niño changes, from the perspective of the role of El Niño in global energy redistribution. [ABSTRACT FROM AUTHOR]

In order to better understand the evolution of the Afro-Asian monsoon in the early Holocene, we investigate the impact on boreal summer monsoon characteristics of (1) a freshwater flux in the North Atlantic from the surrounding melting ice sheets and (2) a remnant ice sheet over North America and Europe. Sensitivity experiments run with the IPSL_CM4 model show that both the meltwater flux and the remnant ice sheets induce a cooling of similar amplitude of the North Atlantic leading to a southward shift of the Inter-Tropical Convergence Zone over the tropical Atlantic and to a reduction of the African monsoon. The two perturbations have different impacts in the Asian sector. The meltwater flux results in a weakening of the Indian monsoon and no change in the East Asian monsoon, whereas the remnant ice sheets induce a strengthening of the Indian monsoon and a strong weakening of the East Asian monsoon. Despite the similar coolings in the Atlantic Ocean, the ocean heat transport is reduced only in the meltwater flux experiment, which induces slight differences between the two experiments in the role of the surface latent heat flux in the tropical energetics. In the meltwater experiment, the southward shift of the subtropical jet acts to cool the upper atmosphere over the Tibetan Plateau and hence to weaken the Indian monsoon. In the ice sheet experiment this effect is overwhelmed by the changes in extratropical stationary waves induced by the ice sheets, which are associated with a larger cooling over the Eurasian continent than in the meltwater experiment. However these sensitivity experiments suggest that insolation is the dominant factor explaining the relative changes of the African, Indian and East Asian monsoons from the early to the mid-Holocene. [ABSTRACT FROM AUTHOR]

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 CO 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 × CO 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. [ABSTRACT FROM AUTHOR]

We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes. [ABSTRACT FROM AUTHOR]

This study investigates the role of insolation in controlling the Indian and African monsoon evolutions during the Holocene using coupled ocean-atmosphere simulations of 0, 6, 9.5 kyr BP climates, for which only the variations of Earth’s orbital configuration are considered. The two monsoon systems are enhanced at 6 and 9.5 kyr BP, compared to 0 kyr BP, as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. The analysis of daily climatologies indicates that even though the length of the “celestial” summer season is shorter at 9.5 kyr BP, the rainy season is longer than at present. Emphasis is put on the impact of the precession on the seasonality, which partly explains why the relative amplification of the Indian and African monsoon varies between 9.5 and 6 kyr BP. Moreover, the changes in snow cover over the Tibetan Plateau play a critical role in reinforcing the 9.5 kyr BP monsoon in India during spring. The results suggest that the teleconnection between convection over India and subsidence over the Mediterranean regions, through the Rodwell and Hoskins mechanism, has an impact on the development of the African monsoon at 9.5 kyr BP. [ABSTRACT FROM AUTHOR]

Driven by enhanced summer insolation in the Northern Hemisphere and land-atmosphere feedbacks during the mid-Holocene (MH), most regions of China were characterized by warmer and wetter summer than present. The MH has been recognized as a benchmark period for climate simulations, but proxy-based reconstructions and model results show some key discrepancies in climate reconstructions. A lack of accurate gridded climate data is a major obstacle to land surface modelling, which is contributing to uncertainties in climate simulations. Here we developed a monthly gridded dataset of temperature and precipitation for MH China through applying eight interpolation methods to 159 pollen-based climate reconstructions for January and July from previous studies. The gridded dataset is consistent with these input records and other independent climate reconstructions at 62 sites. Then, the temperature differences between the MH and pre-industrial period are correlated with elevations in China. Results show a significant elevation-dependent temperature amplification of 0.59 °C/km for July during the MH relative to pre-industrial period. In addition, our gridded reconstructions of the MH climate were compared with climate simulations for the MH and for the future medium-emission scenario. Most simulations significantly underestimated the magnitude of climate anomalies during the MH in China. Similar to the MH, a pronounced warming in winter and a strong wetting in summer are projected in China at the end of this century (i.e., 2091–2100), suggesting analogous seasonal changes during the MH and future. This first high-resolution proxy-based gridded climate dataset for MH China can be used in land surface modelling to improve our understanding of terrestrial changes during the MH, which in turn is expected to facilitate the climate simulations. • Gridded temperature and precipitation data are developed for China in the MH. • The dataset is consistent with input records and independent reconstructions. • A significant elevation-dependent MH warming of 0.59 °C/km is revealed for July. • MH seasonal changes of winter warming and summer wetting are similar to the future. [ABSTRACT FROM AUTHOR]

The article discusses the response of the El Niño-Southern Oscillation (ENSO) to global warming, focusing on a reconstruction of El Nino in the eastern tropical Pacific ocean near Peru over the past 10,000 years. Topics include the use of oxygen isotopes in fossil mollusk shells in Peru; El Nino conditions during the early Holocene epoch; and changes in climate boundary conditions, such as insolation.

Abstract: Three sensitivity experiments using an Ocean Atmosphere General Circulation Model (OAGCM) are conducted to simulate the climate impact of precession. The relative contributions of components of the hydrological cycle including the albedo of Arctic sea ice, advection of atmospheric water vapor and sea surface temperature to the summer Arctic melt process are evaluated. Timing of the perihelion is varied in each experiment with meteorological spring (SP), winter (WP) and autumn (AP) perihelion corresponding to conditions at 110, 115 and 120 ky BP, respectively. Obliquity is unchanged at the 115 ky level which is lower than today. The experiments are assessed relative to the present day control, which has been shown to simulate current conditions based on observations. In the SP experiment, top of the atmosphere (TOA) insolation is weaker than today between the summer solstice and autumnal equinox. In the AP case representing the interglacial, it is less intense between vernal equinox and summer solstice but stronger during the remainder of the year. Although the incident solar radiation is reduced in summer in the SP experiment, increased melting of snow is found primarily as a result of feedbacks from the delayed seasonal cycle of hydrologic components. This is in contrast to both the WP and AP cases in which the perennial snow cover is simulated. At the time of the last glacial inception, 115 ky BP, the WP experiment shows lower insolation to the high northern latitudes in late spring and summer mainly as a result of lower obliquity than today. Dynamical ocean–atmosphere interactions in response to precession maintain the reduced sea ice melting in late spring, strengthen the annual equator-to-pole sea surface temperature (SST) gradient and increase atmospheric moisture convergence in glaciation-sensitive regions. In both the WP and AP experiments seasonal sea ice melting is weakened resulting in pronounced outgoing radiative flux at the locations of expanded sea ice. This leads to further cooling and increased snowfall due to the reduced atmospheric water holding capacity and increased atmospheric moisture convergence from the subtropical Atlantic. In agreement with Milankovitch theory, our results show favorable conditions for glacial inception at 115 ky BP but with obliquity unchanged, they also show perennial snow cover at 120 ky BP resulting from the reduced strength of spring insolation. [Copyright &y& Elsevier]