The Taklamakan and Gobi Desert (TGD) region has experienced a pronounced increase in summer precipitation, including high-impact extreme events, over recent decades. Despite identifying large-scale circulation changes as a key driver of the wetting trend, understanding the relative contributions of internal variability and external forcings remains limited. Here, we approach this problem by using a hierarchy of numerical simulations, complemented by diverse statistical analysis tools. Our results offer strong evidence that the atmospheric internal variations primarily drive this observed trend. Specifically, recent changes in the North Atlantic Oscillation have redirected the storm track, leading to increased extratropical storms entering TGD and subsequently more precipitation. A clustering analysis further demonstrates that these linkages predominantly operate at the synoptic scale, with larger contributions from large precipitation events. Our analysis highlights the crucial role of internal variability, in addition to anthropogenic forcing, when seeking a comprehensive understanding of future precipitation trends in TGD.
The Coronavirus Disease 2019 (COVID‐19) pandemic led to a widespread reduction in aerosol emissions. Using satellite observations and climate model simulations, we study the underlying mechanisms of the large decreases in solar clear‐sky reflection (3.8 W m−2 or 7%) and aerosol optical depth (0.16 W m−2 or 32%) observed over the East Asian Marginal Seas in March 2020. By separating the impacts from meteorology and emissions in the model simulations, we find that about one‐third of the clear‐sky anomalies can be attributed to pandemic‐related emission reductions, and the rest to weather variability and long‐term emission trends. The model is skillful at reproducing the observed interannual variations in solar all‐sky reflection, but no COVID‐19 signal is discerned. The current observational and modeling capabilities will be critical for monitoring, understanding, and predicting the radiative forcing and climate impacts of the ongoing crisis.
Anthropogenic aerosols have been postulated to have a cooling effect on climate, but its magnitude remains uncertain. Using atmospheric general circulation model simulations, we separate the land temperature response into a fast response to radiative forcings and a slow response to changing oceanic conditions and find that the former accounts for about one fifth of the observed warming of the Northern Hemisphere land during summer and autumn since the 1960s. While small, this fast response can be constrained by observations. Spatially varying aerosol effects can be detected on the regional scale, specifically warming over Europe and cooling over Asia. These results provide empirical evidence for the important role of aerosols in setting regional land temperature trends and point to an emergent constraint that suggests strong global aerosol forcing and high transient climate response.
Li, Y, Yi Deng, S Yang, Henian Zhang, Yi Ming, and Zhaoyi Shen, June 2019: Multi-scale temporal-spatial variability of the East Asian summer monsoon frontal system: observation versus its representation in the GFDL HiRAM. Climate Dynamics, 52(11), DOI:10.1007/s00382-018-4546-z. Abstract
This study examines the representation of the multi-scale temporospatial variability of the East Asian summer monsoon stationary front (MSF) in the High-Resolution Atmospheric Model (HiRAM) of the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory. Compared with the observed variability of the MSF in the European Center for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim), HiRAM reproduces reasonably well the seasonal mean precipitation pattern and the seasonal migration of MSF. However, wet biases are found over the northern and eastern China and northern Japan, and dry biases extend from the southern China to the western North Pacific. These rainfall biases are directly tied to a northwestward bias in the model simulated seasonal mean location of MSF and this location bias is most pronounced in the month of May. In general, the MSF in HiRAM is more intense, located more northwestward, and more stationary with weaker interannual variations compared to the observed. A pronounced positive bias in the ocean-land sea level pressure contrast over East Asia, largely manifested as the westward expansion of the western North Pacific subtropical high, is hypothesized to be the main cause of the northwestward location bias of MSF in HiRAM. This bias in sea level pressure contrast likely results from the missing of realistic air-sea interactions in the HiRAM simulations.
Despite distinct geographic distributions of top-of-the-atmosphere radiative forcing, anthropogenic greenhouse gases and aerosols have been found to produce similar patterns of climate response in atmosphere-and-ocean coupled climate model simulations. Understanding surface energy flux changes, a crucial pathway by which atmospheric forcing is communicated to the ocean, is a vital bridge to explaining the similar full atmosphere-and-ocean responses to these disparate forcings. Here we analyze the fast, atmosphere-driven change in surface energy flux caused by present-day greenhouse gases vs aerosols to elucidate its role in shaping the subsequent slow, coupled response. We find that the surface energy flux response patterns achieve roughly two-thirds of the anti-correlation seen in the fully coupled response, driven by Rossby waves excited by symmetric changes to the land–sea contrast. Our results suggest that atmosphere and land surface processes are capable of achieving substantial within-hemisphere homogenization in the climate response to disparate forcers on fast, societally-relevant timescales.
This study examines how aerosol absorption affects the extratropical circulation by analyzing the response to a globally uniform increase in black carbon (BC) simulated with an atmospheric general circulation model forced by prescribed sea surface temperatures. The model includes aerosol direct and semidirect effects, but not indirect or cloud-absorption effects. BC-induced heating in the free troposphere stabilizes the mid-latitude atmospheric column, which results in less energetic baroclinic eddies and thus reduced meridional energy transport at mid-latitudes. Upper tropospheric BC also decreases the meridional temperature gradient on the equatorward flank of the tropospheric jet and yields a weakening and poleward shift of the jet, while boundary layer BC has no significant influence on the large-scale circulation since most of the heating is diffused by turbulence in the boundary layer. The effectiveness of BC in altering circulation generally increases with height.
Dry baroclinic eddy theories can explain most of the extratropical response to free troposphere BC. Specifically, the decrease in vertical eddy heat flux related to a more stable atmosphere is the main mechanism for re-establishing atmospheric energy balance in the presence of BC-induced heating. Similar temperature responses are found in a dry idealized model, which further confirms the dominant role of baroclinic eddies in driving the extratropical circulation changes. The strong atmospheric-only response to BC suggests that absorbing aerosols are capable of altering synopic-scale weather patterns. Its height dependence highlights the importance of better constraining model-simulated aerosol vertical distributions with satellite and field measurements.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
Arctic haze has a distinct seasonal cycle with peak concentrations in winter but pristine conditions in summer. It is demonstrated that the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation model AM3 can reproduce the observed seasonality of Arctic black carbon (BC), an important component of Arctic haze. We use the model to study how large-scale circulation and removal drive the seasonal cycle of Arctic BC. It is found that despite large seasonal shifts in the general circulation pattern, the transport of BC into the Arctic varies little throughout the year. The seasonal cycle of Arctic BC is attributed mostly to variations in the controlling factors of wet removal, namely the hydrophilic fraction of BC and wet deposition efficiency of hydrophilic BC. Specifically, a confluence of low hydrophilic fraction and weak wet deposition, owing to slower aging process and less efficient mixed-phase cloud scavenging, respectively, is responsible for the wintertime peak of BC. The transition to low BC in summer is the consequence of a gradual increase in the wet deposition efficiency, while the increase of BC in late fall can be explained by a sharp decrease in the hydrophilic fraction. The results presented here suggest that future changes in the aging and wet deposition processes can potentially alter the concentrations of Arctic aerosols and their climate effects.
Long-range transport of black carbon (BC) is a growing concern as a result of the efficiency of BC in warming the climate and its adverse impact on human health. We study transpacific transport of BC during HIPPO-3 using a combination of inverse modeling and sensitivity analysis. We use the GEOS-Chem chemical transport model and its adjoint to constrain Asian BC emissions and estimate the source of BC over the North Pacific. We find that different sources of BC dominate the transport to the North Pacific during the southbound (29 March 2010) and northbound (13 April 2010) measurements in HIPPO-3. While biomass burning in Southeast Asia (SE) contributes about 60% of BC in March, more than 90% of BC comes from fossil fuel and biofuel combustion in East Asia (EA) during the April mission. GEOS-Chem simulations generally resolve the spatial and temporal variation of BC concentrations over the North Pacific, but are unable to reproduce the low and high tails of the observed BC distribution. We find that the optimized BC emissions derived from inverse modeling fail to improve model simulations significantly. This failure indicates that uncertainties in BC transport, rather than in emissions, account for the major biases in GEOS-Chem simulations of BC.
The aging process, transforming BC from hydrophobic into hydrophilic form, is one of the key factors controlling wet scavenging and remote concentrations of BC. Sensitivity tests on BC aging suggest that the aging time scale of anthropogenic BC from EA is several hours, faster than assumed in most global models, while the aging process of biomass burning BC from SE may occur much slower, with a time scale of a few days. To evaluate the effects of BC aging and wet deposition on transpacific transport of BC, we develop an idealized model of BC transport. We find that the mid-latitude air masses sampled during HIPPO-3 may have experienced a series of precipitation events, particularly near the EA and SE source region. Transpacific transport of BC is sensitive to BC aging when the aging rate is fast; this sensitivity peaks when the aging time scale is in the range of 1–1.5 d. Our findings indicate that BC aging close to the source must be simulated accurately at a process level in order to simulate better the global abundance and climate forcing of BC.
