Energetic constraints on tropical precipitation changes under stratospheric aerosol geoengineering: a topical review

Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.

All of the representative concentration pathways (RCPs) assume that future emissions of aerosols and aerosol precursors will decline sharply. There is considerable evidence that historically increasing aerosols have substantially affected tropical precipitation, but the effects of projected aerosol declines have received little attention. We compare projections forced by the medium-low RCP4.5 pathway in two subsets of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5): one group (HiForc) includes treatments of indirect aerosol effects on cloud albedo and cloud lifetime as well as direct aerosol effects, while the other group (LoForc) only treats direct aerosol effects. In this scenario we find that models in the HiForc group consistently project larger increases in both the mean and inter-hemispheric (north minus south) asymmetry of tropical sea-surface temperature (SST) and precipitation than do models in the LoForc group. Earlier projections from CMIP3, in which future aerosol declines were assumed to be smaller, behave more like the CMIP5 LoForc group. These results show that projected tropical SST and precipitation changes are sensitive to assumptions about aerosol emissions and indirect aerosol effects. If the real world resembles the HiForc group, then future aerosol changes are likely to be an important (even dominant) driver of tropical precipitation changes under low to moderate forcing scenarios.

Stratospheric aerosol geoengineering (SAG) has been proposed as one of the potential options to offset the impacts of anthropogenically induced climate change. Previous modelling studies have shown that the efficacy of the cooling via SAG increases with altitude of the aerosol layer. It has been also shown that the stratospheric heating associated with SAG could stabilize the tropical atmosphere and weaken the tropical hydrological cycle. Using a global climate model, we perform a systematic study by prescribing volcanic sulphate aerosols at three different altitudes (22 km, 18 km and 16 km) and assess the sensitivity of the global and tropical mean precipitation to the altitude. We find that even though the efficacy of cooling increases with altitude of the aerosol layer, the global and tropical mean precipitation changes are less sensitive to the height of the aerosol layer. This is because the magnitude of both the global and tropical mean precipitation reduction increases with aerosol altitude in response to increasing efficacy of aerosols, but this sensitivity related to the slow response is nearly offset by the sensitivity of fast precipitation adjustments to aerosol altitude. A perspective and analysis based on atmospheric energy budget is presented to explain the lack of sensitivity of the hydrological cycle to the altitude of the stratospheric sulphate aerosol layer.

Stratospheric Aerosol Geoengineering (SAG) is one of the solar geoengineering approaches that have been proposed to offset some of the impacts of anthropogenic climate change. Past studies have shown that SAG may have adverse impacts on the global hydrological cycle. Using a climate model, we quantify the sensitivity of the tropical monsoon precipitation to the meridional distribution of volcanic sulfate aerosols prescribed in the stratosphere in terms of the changes in aerosol optical depth (AOD). In our experiments, large changes in summer monsoon precipitation in the tropical monsoon regions are simulated, especially over the Indian region, in association with meridional shifts in the location of the intertropical convergence zone (ITCZ) caused by changes in interhemispheric AOD differences. Based on our simulations, we estimate a sensitivity of − 1.8° ± 0.0° meridional shift in global mean ITCZ and a 6.9 ± 0.4% reduction in northern hemisphere (NH) monsoon index (NHMI; summer monsoon precipitation over NH monsoon regions) per 0.1 interhemispheric AOD difference (NH minus southern hemisphere). We also quantify this sensitivity in terms of interhemispheric differences in effective radiative forcing and interhemispheric temperature differences: 3.5 ± 0.3% change in NHMI per unit (Wm−2) interhemispheric radiative forcing difference and 5.9 ± 0.4% change per unit (°C) interhemispheric temperature difference. Similar sensitivity estimates are also made for the Indian monsoon precipitation. The establishment of the relationship between interhemispheric AOD (or radiative forcing) differences and ITCZ shift as discussed in this paper will further facilitate and simplify our understanding of the effects of SAG on tropical monsoon rainfall.

Summertime insolation intensified in the Northern Hemisphere during the mid-Holocene, resulting in enhanced monsoonal precipitation. In this study, the authors examine the changes in the annual-mean tropical precipitation as well as changes in atmospheric circulation and upper-ocean circulation in the mid-Holocene compared to the preindustrial climate, as simulated by 12 coupled climate models from PMIP3. In addition to the predominant zonally asymmetric changes in tropical precipitation, there is a small northward shift in the location of intense zonal-mean precipitation (mean ITCZ) in the mid-Holocene in the majority (9 out of 12) of the coupled climate models. In contrast, the shift is southward in simulations using an atmospheric model coupled to a slab ocean. The northward mean ITCZ shift in the coupled simulations is due to enhanced northward ocean heat transport across the equator [OHT(EQ)], which demands a compensating southward atmospheric energy transport across the equator, accomplished by shifting the Hadley cell and hence the mean ITCZ northward. The increased northward OHT(EQ) is primarily accomplished by changes in the upper-ocean gyre circulation in the tropical Pacific acting on the zonally asymmetric climatological temperature distribution. The gyre intensification results from the intensification of the monsoonal winds in the Northern Hemisphere and the weakening of the winds in the Southern Hemisphere, both of which are forced directly by the insolation changes.

The zonal and annual mean tropical precipitation response to paleoclimate and anthropogenic forcing scenarios ranging from the Last Glacial Maximum (LGM), CO\(_{2}\) quadrupling (4XCO\(_{2}\)), mid-Holocene, North Atlantic freshwater hosing and volcanic forcing is analyzed in an ensemble of global climate models. Zonally averaged tropical precipitation changes are characterized in terms of three geometric manipulations of the climatological precipitation (hereafter, modes): meridional shifts, intensifications, and meridional contractions. We employ an optimization procedure that quantifies the magnitude and robustness (across different models) of changes in each mode in response to each forcing type. Additionally, the fraction of precipitation changes that are explained by the modes—in isolation and combined—is quantified. Shifts are generally less than 1\(^{\circ }\) latitude in magnitude and explain a small fraction (< 10\(\%\)) of tropical precipitation changes. Contractions and intensifications are strongly correlated across all simulations with a robust intensification and contraction of precipitation under global warming and a robust reduction and expansion under global cooling during the Last Glacial Maximum. The near constant scaling between contractions and intensifications across all simulations is used to define a joint contraction/intensification (CI) mode of tropical precipitation. The CI mode explains nearly 50\(\%\) of the precipitation change under 4XCO\(_{2}\) and LGM forcing by optimizing a single parameter. These results suggest the shifting mode that has been extensively used to interpret paleo-rainfall reconstructions is of limited use for characterizing forced zonal mean precipitation changes and advocates for a reinterpretation of past precipitation changes to account for the CI mode.

Projected tropical precipitation changes by the end of the century include increased net precipitation over the Pacific Ocean and drying over the Indian Ocean, prompting ongoing debate about the underlying mechanisms. Previous studies argued for the importance of the zonal circulation in the longitudinally dependent tropical precipitation response, as the meridional circulation is often defined and analyzed as the zonal mean. Here we show that the projected changes in the meridional circulation are highly longitudinally dependent, and explain the zonally dependent changes in net precipitation. Our analysis exposes a zonal shift in the ascending branch of the meridional circulation, associated with a strengthened net precipitation over the central Pacific and weakened precipitation in the Indo Pacific. The zonal circulation has minor influence on these projected tropical precipitation changes. These results point to the importance of monitoring the longitudinal changes in the meridional circulation for improving our preparedness for climate change impacts.

Current changes in tropical precipitation from satellite data and climate models areassessed. Wet and dry regions of the tropics are defined as the highest 30% and lowest 70%of monthly precipitation values. Observed tropical ocean trends in the wet regime (1.8%/decade) and the dryregions (−2.6%/decade) according to the Global Precipitation Climatology Project (GPCP) over theperiod including Special Sensor Microwave Imager (SSM/I) data (1988–2008),where GPCP is believed to be more reliable, are of smaller magnitude than whenincluding the entire time series (1979–2008) and closer to model simulations thanprevious comparisons. Analysing changes in extreme precipitation using dailydata within the wet regions, an increase in the frequency of the heaviest 6% ofevents with warming for the SSM/I observations and model ensemble mean isidentified. The SSM/I data indicate an increased frequency of the heaviest events withwarming, several times larger than the expected Clausius–Clapeyron scaling and atthe upper limit of the substantial range in responses in the model simulations.

Tropical precipitation is projected to substantially change by the end of this century, including increased net precipitation over the Pacific Ocean and drying over the Indian Ocean. The underlying mechanisms responsible for these changes are still debated. Previous studies argued for the importance of the zonal circulation in the longitudinally-dependent tropical precipitation response, as the meridional circulation is often analyzed as the zonal mean. Here we show that the projected changes in the meridional circulation are highly longitudinally dependent, and explain the zonally dependent changes in net precipitation. Our analysis exposes a zonal shift in the meridional circulation's ascending branch, associated with a strengthened net precipitation over the central Pacific and weakened precipitation in the Indo-Pacific. Zonal circulation has minor influence on these projected tropical precipitation changes. These results point to the importance of monitoring the longitudinal changes in the meridional circulation for improving our preparedness for climate change impacts.

&amp;lt;p&amp;gt;Models from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the mid-Pliocene Warm Period (mPWP) was a warmer and wetter world than today. However, there is not strong model agreement as to how tropical precipitation was different in the mPWP. Although PlioMIP2 models agree that there was more precipitation associated with the African Monsoon and the Asian Monsoon, away from these regions models do not show a consistent and robust change in precipitation between the mPWP and the preindustrial.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Here we use the HadGEM2 model to explore changes in tropical precipitation between the mPWP and the preindustrial, particularly those associated with the position and strength of the Intertropical Convergence Zone (ITCZ). Reasons for these changes within HadGEM2 will be discussed. We will also expand our discussion of the ITCZ to the PlioMIP2 ensemble in order to show the differing factors that could influence ITCZ characteristics in a warmer world.&amp;lt;/p&amp;gt;

Abstract. The mid-Pliocene Warm Period (mPWP, 3.3–3.0 Ma) was characterised by an atmospheric CO2 concentration exceeding 400 ppmv with minor changes in continental and orbital configurations. Simulations of this past climate state have improved with newer models but still show some substantial differences from proxy reconstructions. There is little information about atmospheric aerosol concentrations during the Pliocene, but previous work suggests that it could have been quite different from the modern period. Here we apply idealised aerosol scenario experiments to examine the importance of aerosol forcing on mPWP tropical precipitation and the possibility of aerosol uncertainty explaining the mismatch between reconstructions and simulations. The absence of industrial pollutants leads to further warming, especially in the Northern Hemisphere. The Intertropical Convergence Zone (ITCZ) becomes narrower and stronger and shifts northward after removal of anthropogenic aerosols. Though not affecting the location of monsoon domain boundary, removal of anthropogenic aerosol alters the amount of rainfall within the domain, increasing summer rain rate over eastern and southern Asia and western Africa. This work demonstrates that uncertainty in aerosol forcing could be the dominant driver in tropical precipitation changes during the mid-Pliocene: causing larger impacts than the changes in topography and greenhouse gases.

It has been shown that asymmetric warming between the Northern and Southern Hemisphere extratropics induces a meridional displacement of tropical precipitation. This shift is believed to be due to the extra energy transported from the differentially heated hemisphere through changes in the Hadley circulation. Generally, the column-integrated energy flux in the mean meridional overturning circulation follows the direction of the upper, relatively dry branch, and tropical precipitation tends to be intensified in the hemisphere with greater warming. This framework was originally applied to simulations that did not include ocean dynamical feedback, but was recently extended to take the ocean heat transport change into account. In the current study, an atmosphere–ocean general circulation model applied with a regional nudging technique is used to investigate the impact of extratropical warming on tropical precipitation change under realistic future climate projections. It is shown that warming at latitudes poleward of 40° causes the northward displacement of tropical precipitation from October to January. Warming at latitudes poleward of 60° alone has a much smaller effect. This change in the tropical precipitation is largely explained by the atmospheric moisture transport caused by changes in the atmospheric circulation. The larger change in ocean heat transport near the equator, relative to the atmosphere, is consistent with the extended energy framework. The current study provides a complementary dynamical framework that highlights the importance of midlatitude atmospheric eddies and equatorial ocean upwelling, where the atmospheric eddy feedback modifies the Hadley circulation resulting in the northward migration of precipitation and the ocean dynamical feedback damps the northward migration from the equator.

Abstract. This study used the Community Atmospheric Model 3.5 (CAM3.5) to investigate the effects of carbonaceous aerosols on climate. The simulations include control runs with 3 times the mass of carbonaceous aerosols as compared to the model's default carbonaceous aerosol mass, as well as no-carbon runs in which carbonaceous aerosols were removed. The slab ocean model (SOM) and the fixed sea surface temperature (SST) were used to examine effects of ocean boundary conditions. Throughout this study, climate response induced by aerosol forcing was mainly analyzed in the following three terms: (1) aerosol radiative effects under fixed SST, (2) effects of aerosol-induced SST feedbacks, and (3) total effects including effects of aerosol forcing and SST feedbacks. The change of SST induced by aerosols has large impacts on distribution of climate response; the magnitudes in response patterns such as temperature, precipitation, zonal winds, mean meridional circulation, radiative fluxes, and cloud coverage are different between the SOM and fixed SST runs. Moreover, different spatial responses between the SOM and fixed SST runs can also be seen in some local areas. This implies the importance of SST feedbacks on simulated climate response. The aerosol dimming effects cause a cooling predicted at low layers near the surface in most carbonaceous aerosol source regions. The temperature response shows a warming (cooling) predicted in the north (south) high latitudes, suggesting that aerosol forcing can cause climate change in regions far away from its origins. Our simulation results show that direct and semidirect radiative forcing due to carbonaceous aerosols decreases rainfall in the tropics. This implies that carbonaceous aerosols have possibly strong influence on weakening of the tropical circulation. Most changes in precipitation are negatively correlated with changes of radiative fluxes at the top of model. The changes in radiative fluxes at top of model are physically consistent with the response patterns in cloud fields. On global average, low-level cloud coverage increases, and mid- and high-level cloud coverage decreases in response to changes in radiative energy induced by aerosol forcing. An approximated moisture budget equation was analyzed in order to understand physical mechanism of precipitation changes induced by carbonaceous aerosols. Our results show that changes in tropical precipitation are mainly dominated are mainly dominated by the dynamic effect (i.e., vertical moisture transport carried by the perturbed flow).

The regional response of climate models to small perturbations is shown to be highly dependent on the unperturbed simulation. an experiment in which CO2 concentrations are doubled and sea surface temperatures are enhanced by 2 K has been carried out with two general circulation models which differ considerably in their control climates. the resulting changes in tropical precipitation in each model simulation are related to the increase in atmospheric water vapour which leads to enhanced precipitation in the main regions of low‐level atmospheric convergence. Since these regions of convergence occur in slightly different locations in the unperturbed simulations, the distribution of changes is also different.Differences in control simulations must be taken into account when comparing results from different models (for example, on doubling atmospheric CO2); otherwise unduly pessimistic conclusions may be reached concerning the consistency of model results. One may be able to make subjective allowance for the effect of known deficiencies in the unperturbed simulation on the model's response before using the simulated changes in, for example, impact studies.A detailed examination of one of the experiments reveals that the change in precipitation is limited by the heat balance of the atmosphere, and indicates the importance of treating accurately the radiative perturbation due to changes in water vapour. the magnitude of the model's response is shown to be consistent with that found in three‐dimensional climate models which include a simple representation of the ocean.

Future precipitation changes include contributions from both thermodynamic and dynamic processes. Given that precipitation in the tropics is commonly associated with convergence lines, we construct a simple linear regression model relating the convergence line frequency and strength to precipitation at subdaily time scales, and use it to show that changes in the convergence lines are related to the dynamic change in the precipitation. Given GCM‐predicted convergence line changes, we predict precipitation changes using the regression model. The so‐predicted precipitation change is equivalent to the dynamic component of the precipitation change identified in earlier studies that used very different methods. The difference between the precipitation change in GCMs and that predicted from changes in convergence lines accounts for thermodynamic and other potentially important dynamic contributions. More accurate predictions of future precipitation therefore require the accurate simulations of the relatively short‐lived weather features responsible for convergence lines in the tropics in GCMs.