Nicolas Freychet

Asian Summer Monsoon in CMIP5 Projections: A Link between the Change in Extreme Precipitation and Monsoon Dynamics

AbstractChange in extreme events in climate projections is a major concern. If the frequency of dry events is expected to increase in a warmer climate (thus, the overall number of wet days will decrease), heavy and extreme precipitation are also expected to increase because of a shift of the precipitation spectrum. However, the forecasts exhibit numerous uncertainties.This study focuses on the Asian region, separated into the following three subregions: the East Asian region, the Indian region, and western North Pacific region, where the summer monsoon can bring heavy rainfall. Particularly emphasized herein is the reliability of the projection, using data from a large ensemble of 30 models from phase 5 of the Coupled Model Intercomparison Project. The scattering of the ensemble enables obtaining an optimal estimate of the uncertainties, and it is used to compute the correlation between projected changes of extreme events and circulation changes.The results show clear spatial and temporal variations in th...

Human influence on the record-breaking cold event in January of 2016 in Eastern China

Anthropogenic influences are estimated to have reduced the likelihood of an extreme cold event in midwinter with the intensity equal to or stronger than the record of 2016 in eastern China by about two‐thirds.

On the Linearity of Local and Regional Temperature Changes from 1.5 degrees C to 2 degrees C of Global Warming

AbstractGiven the Paris Agreement it is imperative there is greater understanding of the consequences of limiting global warming to the target 1.5° and 2°C levels above preindustrial conditions. It...

Central-Eastern China Persistent Heat Waves: Evaluation of the AMIP Models

AbstractLarge-scale and persistent heat waves affecting central-eastern China are investigated in 40 different simulations of sea surface temperature driven global atmospheric models. The different models are compared with results from reanalysis and ground station datasets. It is found that the dynamics of heat-wave events is well reproduced by the models. However, they tend to produce too-persistent heat-wave events (lasting more than 20 days), and several hypotheses were tested to explain this bias. The daily variability of the temperatures or the seasonal signal did not explain the persistence. However, interannual variability of the temperatures in the models, and especially the sharp transition in the mid-1990s, has a large impact on the duration of heat waves. A filtering method was applied to select the models closest to the observations in terms of events persistence. The selected models do not show a significant difference from the other models for the long-term trends. Thus, the bias on the dur...

Impacts of Anthropogenic Forcings and El Niño on Chinese Extreme Temperatures

This study investigates the potential influences of anthropogenic forcings and natural variability on the risk of summer extreme temperatures over China. We use three multi-thousand-member ensemble simulations with different forcings (with or without anthropogenic greenhouse gases and aerosol emissions) to evaluate the human impact, and with sea surface temperature patterns from three different years around the El Niño–Southern Oscillation (ENSO) 2015/16 event (years 2014, 2015 and 2016) to evaluate the impact of natural variability. A generalized extreme value (GEV) distribution is used to fit the ensemble results. Based on these model results, we find that, during the peak of ENSO (2015), daytime extreme temperatures are smaller over the central China region compared to a normal year (2014). During 2016, the risk of nighttime extreme temperatures is largely increased over the eastern coastal region. Both anomalies are of the same magnitude as the anthropogenic influence. Thus, ENSO can amplify or counterbalance (at a regional and annual scale) anthropogenic effects on extreme summer temperatures over China. Changes are mainly due to changes in the GEV location parameter. Thus, anomalies are due to a shift in the distributions and not to a change in temperature variability.摘要本研究探讨了人为强迫和自然变率对中国夏季极端高温灾害的潜在影响. 我们使用了不同强迫条件下(包括或者不包括温室气体和气溶胶排放)的三千多个成员集合模拟结果, 来评估人为强迫的影响;同时, 利用最近一次ENSO事件发展演变过程中的三个不同位相年份(2014中性年、2015厄尔尼诺年、2016拉尼娜年)对应的海表温度型态来评估自然变率的影响. 我们利用广义极值分布来分析集合结果. 基于模式结果, 我们发现在ENSO峰值期间(2015年), 日间极端气温在中国中部地区偏小. 在2016年, 夜间极端高温灾害在中国东部沿海地区大幅增加. 上述二者(自然变率的影响)都与人为影响的量级相当. 因此, 我们认为ENSO事件(在区域和年际尺度上)能够放大或者抵消人为强迫对中国夏季极端高温的影响. 此外, 本研究揭示了中国夏季极端高温的变化主要取决于广义极值分布参数的变化, 这意味着中国夏季极端高温的变化是由温度极值分布的偏移造成的, 而非温度变率本身强度的变化.

Variability of hydrological extreme events in East Asia and their dynamical control: A comparison between observations and two high-resolution global climate models

This work investigates the variability of extreme weather events (drought spells, DS15, and daily heavy rainfall, PR99) over East Asia. It particularly focuses on the large scale atmospheric circulation associated with high levels of the occurrence of these extreme events. Two observational datasets (APHRODITE and PERSIANN) are compared with two high-resolution global climate models (HiRAM and HadGEM3-GC2) and an ensemble of other lower resolution climate models from CMIP5. We first evaluate the performance of the high resolution models. They both exhibit good skill in reproducing extreme events, especially when compared with CMIP5 results. Significant differences exist between the two observational datasets, highlighting the difficulty of having a clear estimate of extreme events. The link between the variability of the extremes and the large scale circulation is investigated, on monthly and interannual timescales, using composite and correlation analyses. Both extreme indices DS15 and PR99 are significantly linked to the low level wind intensity over East Asia, i.e. the monsoon circulation. It is also found that DS15 events are strongly linked to the surface temperature over the Siberian region and to the land-sea pressure contrast, while PR99 events are linked to the sea surface temperature anomalies over the West North Pacific. These results illustrate the importance of the monsoon circulation on extremes over East Asia. The dependencies on of the surface temperature over the continent and the sea surface temperature raise the question as to what extent they could affect the occurrence of extremes over tropical regions in future projections.

Extreme Precipitation Events over East Asia: Evaluating the CMIP5 Model

Extreme hydrological events are a direct threat to society and the environment, and their study within the framework of global climate change remains crucial. However, forecasts present numerous uncertainties. This study investigates the modification of precipitation characteristics over East Asia, a region densely populated and vulnerable to extreme rainfall. The performance of the models and the confidence in their projections are analyzed using data derived from an ensemble of models from Phase 5 of the Coupled Model Intercomparison Project (CMIP5). Different factors that can affect the confidence of the projection are considered, such as the resolution, the response to radiative forcing, and the modification of large-scale atmospheric circulation. The resolution and response in radiative forcing do not exhibit a clear correlation with the change in precipitation. The moisture flux convergence (MFC), by contrast, has a clear impact on extreme events. Specifically, the change in the dynamical term of the vertical MFC exhibits a major disagreement among the models and could strongly affect the confidence of the ensemble projection. Extreme precipitation is likely to increase over East Asia and India.