The effect of ENSO-induced rainfall and circulation changes on the direct and indirect radiative forcing from Indonesian biomass-burning aerosols

Emissions of biomass-burning aerosols from the Indonesian region are known to vary in response to rainfall anomalies associated with the El Niño Southern Oscillation (ENSO). For the severe El Niño-related drought in 1997, there have been several attempts to estimate the direct radiative forcing from increased aerosol emissions over Indonesia, as well as the associated feedbacks on climate. However, these estimates have not considered indirect aerosol effects. Another question that has not been addressed is whether the effect of ENSO-related circulation and rainfall anomalies on radiative forcing is significant relative to the effect of changes in emissions. In this study, we analyse the direct and first indirect radiative forcing from El Niño-related increased emissions of Indonesian biomass-burning aerosols, with and without the influence of ENSO-related rainfall and circulation anomalies. <br><br> We compare two experiments that are performed with the CSIRO-Mk3.6 atmospheric global climate model (GCM). The first experiment (AMIP) consists of a pair of runs that respectively represent El Niño and La Niña conditions. In these runs, the distribution of aerosols is simulated under the influence of realistic Indonesian biomass-burning aerosol emissions and sea surface temperatures (SSTs) for 1997 (El Niño) and 2000 (La Niña). The second experiment (CLIM) is identical to AMIP, but is forced by climatological SSTs, so that in CLIM meteorological differences between 1997 and 2000 are suppressed. <br><br> The comparison of AMIP and CLIM shows that the aerosol radiative forcing anomalies associated with ENSO (El Niño minus La Niña) are substantially stronger when ENSO-related SST anomalies are taken into account. For the first indirect effect, the influence of SST-induced changes in rainfall and circulation exceeds that of changes in emissions. For the direct aerosol forcing, the influence of changes in SSTs and emissions are of comparable magnitude. Averaged over the Indonesian region (5.6° N–11.2° S, 96.6° E–150.9° E), the first indirect forcing is −0.7 Wm<sup>−2</sup> in CLIM and −2.2 Wm<sup>−2</sup> in AMIP during the months July to November. The direct aerosol forcing at the top of the atmosphere (surface) is −1.0 (−5.3) Wm<sup>−2</sup> in CLIM and −1.8 (−9.1) Wm<sup>−2</sup> in AMIP during the same period. <br><br> Our results suggest that (a) the indirect aerosol effect from biomass-burning aerosols is strong enough to play an important role for impact assessments, and (b) that impacts of biomass-burning aerosols would be considerably underestimated if feedbacks of ENSO-related SST variations on radiative forcing are not taken into account

Simulation of the spatial distribution of mineral dust and its direct radiative forcing over Australia

Direct radiative forcing by mineral dust is important as it significantly affects the climate system by scattering and absorbing short-wave and long-wave radiation. The multi-angle imaging spectro radiometer (MISR) and cloud–aerosol lidar with orthogonal polarisation (CALIOP) aerosol data are used to observe mineral dust distribution over Australia. In addition, the weather research and forecasting with chemistry (WRF/Chem) model is used to estimate direct radiative forcing by dust. At the surface, the model domain clear-sky short-wave and long-wave direct radiative forcing by dust averaged for a 6-month period (austral spring and summer) was estimated to be −0.67 W m−2 and 0.13 W m−2, respectively. The long-wave warming effect of dust therefore offsets 19.4% of its short-wave cooling effect. However, over Lake Eyre Basin where coarse particles are more abundant, the long-wave warming effect of dust offsets 60.9% of the short-wave cooling effect. At the top of the atmosphere (TOA), clear-sky short-wave and long-wave direct radiative forcing was estimated to be −0.26 W m−2 and −0.01 W m−2, respectively. This leads to a net negative direct radiative forcing of dust at the TOA, indicating cooling of the atmosphere by an increase in outgoing radiation. Short-wave and long-wave direct radiative forcing by dust is shown to have a diurnal variation due to changes in solar zenith angle and in the intensity of infrared radiation. Atmospheric heating due to absorption of short-wave radiation was simulated, while the interaction of dust with long-wave radiation was associated with atmospheric cooling. The net effect was cooling of the atmosphere near the surface (below 0.2 km), with warming of the atmosphere at higher altitudes

The CSIRO Mk3L climate system model version 1.0 – Part 1: Description and evaluation

The CSIRO Mk3L climate system model is a coupled general circulation model, designed primarily for millennial-scale climate simulations and palaeoclimate research. Mk3L includes components which describe the atmosphere, ocean, sea ice and land surface, and combines computational efficiency with a stable and realistic control climatology. This paper describes the model physics and software, analyses the control climatology, and evaluates the ability of the model to simulate the modern climate. <br><br> Mk3L incorporates a spectral atmospheric general circulation model, a <i>z</i>-coordinate ocean general circulation model, a dynamic-thermodynamic sea ice model and a land surface scheme with static vegetation. The source code is highly portable, and has no dependence upon proprietary software. The model distribution is freely available to the research community. A 1000-yr climate simulation can be completed in around one-and-a-half months on a typical desktop computer, with greater throughput being possible on high-performance computing facilities. <br><br> Mk3L produces realistic simulations of the larger-scale features of the modern climate, although with some biases on the regional scale. The model also produces reasonable representations of the leading modes of internal climate variability in both the tropics and extratropics. The control state of the model exhibits a high degree of stability, with only a weak cooling trend on millennial timescales. Ongoing development work aims to improve the model climatology and transform Mk3L into a comprehensive earth system model

Total aerosol effect: forcing or radiative flux perturbation

Uncertainties in aerosol radiative forcings, especially those associated with clouds, contribute to a large extent to uncertainties in the total anthropogenic forcing. The interaction of aerosols with clouds and radiation introduces feedbacks which can affect the rate of precipitation formation. In former assessments of aerosol radiative forcings, these effects have not been quantified. Also, with global aerosol-climate models simulating interactively aerosols and cloud microphysical properties, a quantification of the aerosol forcings in the traditional way is difficult to define properly. Here we argue that fast feedbacks should be included because they act quickly compared with the time scale of global warming. We show that for different forcing agents (aerosols and greenhouse gases) the radiative forcings as traditionally defined agree rather well with estimates from a method, here referred to as radiative flux perturbations (RFP), that takes these fast feedbacks and interactions into account. Based on our results, we recommend RFP as a valid option to compare different forcing agents, and to compare the effects of particular forcing agents in different models

Have Australian rainfall and cloudiness increased due to the remote effects of Asian anthropogenic aerosols?

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94749/1/jgrd13340.pd

Projected effects of declining aerosols in RCP4.5: unmasking global warming?

All the representative concentration pathways (RCPs) include declining aerosol emissions during the 21st century, but the effects of these declines on climate projections have had little attention. Here we assess the global and hemispheric-scale effects of declining anthropogenic aerosols in RCP4.5 in CSIRO-Mk3.6, a model from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results from this model are then compared with those from other CMIP5 models. We calculate the aerosol effective radiative forcing (ERF, including indirect effects) in CSIRO-Mk3.6 relative to 1850, using a series of atmospheric simulations with prescribed sea-surface temperatures (SST). Global-mean aerosol ERF at the top of the atmosphere is most negative in 2005 (−1.47 W m−2). Between 2005 and 2100 it increases by 1.46 W m−2, i.e., it approximately returns to 1850 levels. Although increasing greenhouse gases (GHGs) and declining aerosols both exert a positive ERF at the top of the atmosphere during the 21st century, they have opposing effects on radiative heating of the atmosphere: increasing GHGs warm the atmosphere, whereas declining aerosols cool the atmosphere due to reduced absorption of shortwave radiation by black carbon (BC). We then compare two projections for 2006–2100, using the coupled atmosphere-ocean version of the model. One (RCP45) follows the usual RCP4.5; the other (RCP45A2005) has identical forcing, except that emissions of anthropogenic aerosols and precursors are fixed at 2005 levels. The global-mean surface warming in RCP45 is 2.3 °C per 95 yr, of which almost half (1.1 °C) is caused by declining aerosols. The warming due to declining aerosols is almost twice as strong in the Northern Hemisphere as in the Southern Hemisphere, whereas that due to increasing GHGs is similar in the two hemispheres. For precipitation changes, the effects of declining aerosols are larger than those of increasing GHGs due to decreasing atmospheric absorption by black carbon: 63% of the projected global-mean precipitation increase of 0.16 mm per day is caused by declining aerosols. In the Northern Hemisphere, precipitation increases by 0.29 mm per day, of which 72% is caused by declining aerosols. Comparing 13 CMIP5 models, we find a correlation of –0.54 (significant at 5%) between aerosol ERF in the present climate and projected global-mean surface warming in RCP4.5; thus, models that have more negative aerosol ERF in the present climate tend to project stronger warming during 2006–2100. A similar correlation (–0.56) is found between aerosol ERF and projected changes in global-mean precipitation. These results suggest that aerosol forcing substantially modulates projected climate response in RCP4.5. In some respects, the effects of declining aerosols are quite distinct from those of increasing GHGs. Systematic efforts are needed to better quantify the role of declining aerosols in climate projections

Intercomparison of the northern hemisphere winter mid-latitude atmospheric variability of the IPCC models

We compare, for the overlapping time frame 1962-2000, the estimate of the northern hemisphere (NH) mid-latitude winter atmospheric variability within the XX century simulations of 17 global climate models (GCMs) included in the IPCC-4AR with the NCEP and ECMWF reanalyses. We compute the Hayashi spectra of the 500hPa geopotential height fields and introduce an integral measure of the variability observed in the NH on different spectral sub-domains. Only two high-resolution GCMs have a good agreement with reanalyses. Large biases, in most cases larger than 20%, are found between the wave climatologies of most GCMs and the reanalyses, with a relative span of around 50%. The travelling baroclinic waves are usually overestimated, while the planetary waves are usually underestimated, in agreement with previous studies performed on global weather forecasting models. When comparing the results of various versions of similar GCMs, it is clear that in some cases the vertical resolution of the atmosphere and, somewhat unexpectedly, of the adopted ocean model seem to be critical in determining the agreement with the reanalyses. The GCMs ensemble is biased with respect to the reanalyses but is comparable to the best 5 GCMs. This study suggests serious caveats with respect to the ability of most of the presently available GCMs in representing the statistics of the global scale atmospheric dynamics of the present climate and, a fortiori, in the perspective of modelling climate change.Comment: 39 pages, 8 figures, 2 table

Forced decadal changes in the East Asian summer monsoon: the roles of greenhouse gases and anthropogenic aerosols

Since the mid-1990s precipitation trends over eastern China display a dipole pattern, characterized by positive anomalies in the south and negative anomalies in the north, named as the Southern-Flood-Northern-Drought (SFND) pattern. This work investigates the drivers of decadal changes of the East Asian summer monsoon (EASM), and the dynamical mechanisms involved, by using a coupled climate model (specifically an atmospheric general circulation model coupled to an ocean mixed layer model) forced by changes in (1) anthropogenic greenhouse gases (GHG), (2) anthropogenic aerosol (AA) and (3) the combined effects of both GHG and AA (All Forcing) between two periods across the mid-1990s. The model experiment forced by changes in All Forcing shows a dipole pattern of response in precipitation over China that is similar to the observed SFND pattern across the mid-1990s, which suggests that anthropogenic forcing changes played an important role in the observed decadal changes. Furthermore, the experiments with separate forcings indicate that GHG and AA forcing dominate different parts of the SFND pattern. In particular, changes in GHG increase precipitation over southern China, whilst changes in AA dominate in the drought conditions over northern China. Increases in GHG cause increased moisture transport convergence over eastern China, which leads to increased precipitation. The AA forcing changes weaken the EASM, which lead to divergent wind anomalies over northern China and reduced precipitation