Processes shaping the spatial pattern and seasonality of the surface air temperature response to anthropogenic forcing

In the period 1960-2010, the land surface air temperature (SAT) warmed more rapidly over some regions relative to the global mean. Using a set of time-slice experiments, we highlight how different physical processes shape the regional pattern of SAT warming. The results indicate an essential role of anthropogenic forcing in regional SAT changes from the 1970s to 2000s, and show that both surface-atmosphere interactions and large-scale atmospheric circulation changes can shape regional responses to forcing. Single forcing experiments show that an increase in greenhouse gases (GHG) can lead to regional changes in land surface warming in winter (DJF) due to snow-albedo feedbacks, and in summer (JJA) due to soil-moisture and cloud feedbacks. Changes in anthropogenic aerosol and precursor (AA) emissions induce large spatial variations in SAT, characterized by warming over western Europe, Eurasia, and Alaska. In western Europe, SAT warming is stronger in JJA than in DJF due to substantial increases in clear sky shortwave radiation over Europe, associated with decreases in local AA emissions since the 1980s. In Alaska, the amplified SAT warming in DJF is due to increased downward longwave radiation, which is related to increased water vapor and cloud cover. In this case, although the model was able to capture the regional pattern of SAT change, and the associated local processes, it did not simulate all processes and anomalies correctly. For the Alaskan warming, the model is seen to achieve the correct regional response in the context of a wider North Pacific anomaly that is not consistent with observations. This demonstrates the importance of model evaluation that goes beyond the target variable in detection and attribution studies

Anthropogenically forced decadal change of South Asian summer monsoon across the mid‐1990s

Analysis of observations indicates that there was a significant decadal change in summer (June‐August) mean rainfall over South Asia and Southeast Asia across the mid‐1990s, which is characterized by less rainfall over central‐northern India and northern Indo‐China Peninsula. This study investigates impacts of anthropogenic forcing on the observed decadal change across the mid‐1990s. A set of experiments using the coupled atmosphere‐ocean‐mixed‐layer model MetUM‐GOML2 has been performed to quantify the relative roles of changes in anthropogenic greenhouse gases (GHG) and anthropogenic aerosols (AA). Results indicate a dominant role of anthropogenic changes in the observed decadal changes. Separately, the changes in GHG forcing play an important role in the reduction of rainfall over central‐northern India through the changes of atmospheric circulation (i.e. the local Hadley circulation and the Walker circulation), with additional contribution from changes in AA forcing. The changes in AA forcing dominate the reduction of rainfall over northern Indo‐China Peninsula due to high‐pressure anomalies over northern South Asia and the western subtropical Pacific. These high‐pressure anomalies are induced by the surface cooling mainly via aerosol‐radiation interaction that decreases downward clear sky shortwave radiation over South Asia during summer, and aerosol‐radiation interaction and aerosol‐cloud interaction that decrease downward shortwave radiation over the western subtropical Pacific during pre‐summer seasons

Air–sea fluxes in a climate model using hourly coupling between the atmospheric and the oceanic components

We analyse the changes in the air–sea fluxes of momentum, heat and fresh water flux caused by increasing the ocean–atmosphere coupling frequency from once per day to once per hour in the Max Planck Institute Earth System Model. We diagnose the relative influences of daily averaging and high-frequency feedbacks on the basic statistics of the air–sea fluxes at grid point level and quantify feedback modes responsible for large scale changes in fluxes over the Southern Ocean and the Equatorial Pacific. Coupling once per hour instead of once per day reduces the mean of the momentum-flux magnitude by up to 7 % in the tropics and increases it by up to 10 % in the Southern Ocean. These changes result solely from feedbacks between atmosphere and ocean occurring on time scales shorter than 1 day. The variance and extremes of all the fluxes are increased in most parts of the oceans. Exceptions are found for the momentum and fresh water fluxes in the tropics. The increases result mainly from the daily averaging, while the decreases in the tropics are caused by the high-frequency feedbacks. The variance increases are substantial, reaching up to 50 % for the momentum flux, 100 % for the fresh water flux, and a factor of 15 for the net heat flux. These diurnal and intra-diurnal variations account for up to 50–90 % of the total variances and exhibit distinct seasonality. The high-frequency coupling can influence the large-scale feedback modes that lead to large-scale changes in the magnitude of wind stress over the Southern Ocean and Equatorial Pacific. In the Southern Ocean, the dependence of the SST-wind-stress feedback on the mean state of SST, which is colder in the experiment with hourly coupling than in the experiment with daily coupling, leads to an increase of westerlies. In the Equatorial Pacific, Bjerknes feedback in the hourly coupled experiment reveals a diurnal cycle during the El Niño events, with the feedback being stronger in the nighttime than in the daytime and no clear diurnal cycle during the La Niña events. This asymmetry might lead to the decrease of wind stress in the Equatorial Pacific in the hourly coupled experiment. © 2016 The Author(s

Projected near term changes in the East Asian summer monsoon and its uncertainty

Changes in the East Asian summer monsoon (EASM) during the mid-21st century relative to present day are simulated in two related models GOML1 and GOML2. Both models are the atmospheric components of two state-of-the-art climate models coupled to a multi-level mixed-layer ocean model, following the RCP 4.5 scenario. Both show that the EASM is enhanced due to the amplified land-sea thermal contrast. Summer precipitation over northern China is projected to increase by 5-10% in both models mainly driven by enhancement of the monsoon circulation. Over south-eastern China the two models project different signs of precipitation change: a decrease in GOML1 with the maximum of about -1.0 mm day-1 and an increase in GOML2 with a maximum of around 1.0 mm day-1. Though the thermal effect of climate warming leads to a projected increase in precipitation over south-eastern China in both models, circulation changes are opposite and dominate the precipitation response. This indicates that uncertainty in changes in projected precipitation largely arises from uncertainly in projected circulation changes. The different circulation changes in the two models are likely related to differences in projected Sea Surface Temperature (SST) in the Western tropical Pacific and North Pacific. In GOML1, the SST warming in the tropical Pacific is associated with an anomalous local Hadley circulation, characterized by anomalous ascent in the tropics and southern subtropics, and anomalous descent with less precipitation over south-eastern China. In GOML2, the large decrease in the meridional SST gradient between the South China Sea and Western North Pacific is associated with an anomalous local Hadley circulation with anomalous ascent at 20°N-30°N and anomalous descent at 5°N-15°N, leading to an anti-cyclonic circulation anomaly over the South China Sea and increased precipitation over south-eastern China

Anthropogenic warming has substantially increased the likelihood of July 2017-like heat waves over Central-Eastern China

Heat waves in Central-Eastern China like the record-breaking July 2017 event were rare in natural worlds, but have now become approximately one-in-five-year events due to anthropogenic forcings

Physical processes of summer extreme rainfall interannual variability in Eastern China—part II: evaluation of CMIP6 models

Eastern China is regularly exposed to extreme precipitation with significant socio-economical consequences. Following an observational analysis in a first part of this study, here the ability of the Coupled Model Intercomparison Project Phase 6 models to reproduce the main modes of interannual variability of 5-day summer extreme precipitation over eastern China is evaluated, using an empirical orthogonal teleconnection (EOT) method. These models capture the main patterns and magnitudes of the different EOT patterns, although the North China mode is less well represented. Models also reproduce the dynamical features associated with each mode. There is no systematic improvement in the ability of models to simulate either the pattern or the 5-day intensity when using higher resolution models compared to coarser resolution ones. Instead, multi-member or multi-model ensembles lead to results closer to observations. Using a low mitigation projection pathway (SSP-370), it is shown that the risk of the most extreme 5-day precipitation events by about 40%, 80% and more than 150% for global-mean warming levels, relative to 1850–1900, of + 1.5, + 2 and + 3 ∘C respectively. This increase is found to be more significant for 5-days events than for seasonal scale precipitation, consistent with previous studies

Anthropogenic Influences on 2019 July Precipitation Extremes Over the Mid–Lower Reaches of the Yangtze River

Understanding the driving factors for precipitation extremes matters for adaptation and mitigation measures against the changing hydrometeorological hazards in Yangtze River basin, a habitable area that provides water resources for domestic, farming, and industrial needs. However, the region is naturally subject to major floods linked to monsoonal heavy precipitation during May–September. This study aims to quantify anthropogenic influences on the changing risk of 2-week-long precipitation extremes such as the July 2019 extreme cases, as well as events of shorter durations, over the middle and lower reaches of Yangtze River basin (MLYRB). Precipitation extremes with different durations ranging from 1-day to 14-days maximum precipitation accumulations are investigated. Gridded daily precipitations based on nearly 2,400 meteorological stations across China are used to define maximum accumulated precipitation extremes over the MLYRB in July during 1961–2019. Attribution analysis is conducted by using the Met Office HadGEM3-GA6 modeling system, which comprises two sets of 525-member ensembles for 2019. One is forced with observed sea-surface temperatures (SSTs), sea-ice and all forcings, and the other is forced with preindustrialized SSTs and natural forcings only. The risk ratio between the exceedance probabilities estimated from all-forcing and natural-forcing simulations is calculated to quantify the anthropogenic contribution to the changing risks of the July 2019–like precipitation extremes. The results reveal that anthropogenic warming has reduced the likelihood of 2019-like 14-days heavy precipitation over the mid–lower reaches of the Yangtze River by 20%, but increased that of 2-days extremes by 30%