Robert J. Allen

The vertical distribution of black carbon in CMIP5 models: Comparison to observations and the importance of convective transport

Large uncertainty in the direct radiative forcing of black carbon (BC) exists, with published estimates ranging from 0.25 to 0.9 W m−2. A significant source of this uncertainty relates to the vertical distribution of BC, particularly relative to cloud layers. We first compare the vertical distribution of BC in Coupled Model Intercomparison Project Phase 5 (CMIP5) models to aircraft measurements and find that models tend to overestimate upper tropospheric/lower stratospheric (UT/LS) BC, particularly over the central Pacific from Hiaper Pole-to-Pole Observations Flight 1 (HIPPO1). However, CMIP5 generally underestimates Arctic BC from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites campaign, implying a geographically dependent bias. Factors controlling the vertical distribution of BC in CMIP5 models, such as wet and dry deposition, precipitation, and convective mass flux (MC), are subsequently investigated. We also perform a series of sensitivity experiments with the Community Atmosphere Model version 5, including prescribed meteorology, enhanced vertical resolution, and altered convective wet scavenging efficiency and deep convection. We find that convective mass flux has opposing effects on the amount of black carbon in the atmosphere. More MC is associated with more convective precipitation, enhanced wet removal, and less BC below 500 hPa. However, more MC, particularly above 500u2009hPa, yield more BC aloft due to enhanced convective lofting. These relationships—particularly MC versus BC below 500 hPa—are generally stronger in the tropics. Compared to the Modern-Era Retrospective Analysis for Research and Applications, most CMIP5 models overestimate MC, with all models overestimating MC above 500 hPa. Our results suggest that excessive convective transport is one of the reasons for CMIP5 overestimation of UT/LS BC.

Future aerosol reductions and widening of the northern tropical belt

Observations show that the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Although part of this signal is related to natural climate variability, studies have identified an externally forced contribution primarily associated with greenhouse gases (GHGs) and stratospheric ozone loss. Here we show that the increase in aerosols over the twentieth century has led to contraction of the northern tropical belt, thereby offsetting part of the widening associated with the increase in GHGs. Over the 21st century, however, when aerosol emissions are projected to decrease, the effects of aerosols and GHGs reinforce one another, both contributing to widening of the northern tropical belt. Models that have larger aerosol forcing, by including aerosol indirect effects on cloud albedo and lifetime, yield significantly larger Northern Hemisphere (NH) tropical widening than models with direct aerosol effects only. More targeted simulations show that future reductions in aerosols can drive NH tropical widening as large as greenhouse gases, and idealized simulations show the importance of NH midlatitude aerosol forcing. Mechanistically, the 21st century reduction in aerosols peaks near 40°N, which results in a corresponding maximum increase in surface solar radiation, NH midlatitude tropospheric warming amplification, and a poleward shift in the latitude of maximum baroclinicity, implying a corresponding shift in atmospheric circulation. If models with aerosol indirect effects better represent the real world, then future aerosol changes are likely to be an important—if not dominant—driver of NH tropical belt widening.

A 21st century northward tropical precipitation shift caused by future anthropogenic aerosol reductions

The tropical rain belt is a narrow band of clouds near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves across the equator following the Sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer timescales. Through the latter half of the twentieth century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, a decrease in northward cross-equatorial moist static energy transport, and a northward shift of the Hadley circulation, including the tropical rain belt. The shift is relatively weak in the Atlantic sector, consistent with both a smaller decrease in aerosol emissions and a larger reduction in northward cross-equatorial ocean heat flux. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.

Natural variations of tropical width and recent trends

The temporal evolution of the tropical width since 1979 is investigated in observations and models by using metrics based on outgoing radiation, 6.7u2009µm brightness temperature, and atmospheric reanalysis. The maximum 20u2009year widening as seen in radiation by satellites occurred during 1993–2012 and was associated with a global sea surface temperature (SST) change that resembles the El Nino–Southern Oscillation/Pacific Decadal Oscillation in the Pacific region. Idealized experiments with Community Atmospheric Model version 5 reveal that Tropical Pacific SST pattern largely accounts for the widening. A number of Coupled Model Intercomparison Project Phase 5 coupled models can simulate, without any type of forcing, this satellite-inferred tropical widening and its associated Pacific SST pattern. While model-simulated widenings are consistent for all metrics, the two reanalysis-based widenings are inconsistent internally and with the satellite-based and model widenings. Our results reinforce suggestions that observed widenings can be explained by internal variability as captured by climate models, though this depends on whether reanalysis trends are regarded as reliable.

Understanding influences of convective transport and removal processes on aerosol vertical distribution

The vertical distribution of aerosols is an important component of aerosol radiative forcing. Here we investigate the effects of convection transport and precipitation on the vertical aerosol distribution using observations and reanalysis data. As expected, convective mass flux is positively correlated with precipitation everywhere. Both positive and negative correlations between convective mass flux, precipitation, and aerosol vertical dispersivity exist, depending on the region. In the tropics, more (less) convective mass flux is associated with more (less) precipitation and more (less) aerosol vertical dispersivity—including more (less) aerosol above 500u2009hPa. In contrast, a negative relationship exists between aerosol vertical dispersivity and both convective mass flux and precipitation over the Northern Hemisphere midlatitude ocean. Aerosol vertical dispersivity in this region is related to convective mass flux and precipitation over the emission sources. We conclude that convective transport is an important mechanism controlling the global vertical dispersivity of aerosol.

21st century California drought risk linked to model fidelity of the El Niño teleconnection

Greenhouse gas induced climate change is expected to lead to negative hydrological impacts for southwestern North America, including California (CA). This includes a decrease in the amount and frequency of precipitation, reductions in Sierra snow pack, and an increase in evapotranspiration, all of which imply a decline in surface water availability, and an increase in drought and stress on water resources. However, a recent study showed the importance of tropical Pacific sea surface temperature (SST) warming and an El Niño Southern Oscillation (ENSO)-like teleconnection in driving an increase in CA precipitation through the 21st century, particularly during winter (DJF). Here, we extend this prior work and show wetter (drier) CA conditions, based on several drought metrics, are associated with an El Niño (La Niña)-like SST pattern. Models that better simulate the observed ENSO-CA precipitation teleconnection also better simulate the ENSO-CA drought relationships, and yield negligible change in the risk of 21st century CA drought, primarily due to wetting during winter. Seasonally, however, CA drought risk is projected to increase during the non-winter months, particularly in the models that poorly simulate the observed teleconnection. Thus, future projections of CA drought are dependent on model fidelity of the El Niño teleconnection. As opposed to focusing on adapting to less water, models that better simulate the teleconnection imply adaptation measures focused on smoothing seasonal differences for affected agricultural, terrestrial, and aquatic systems, as well as effectively capturing enhanced winter runoff.HYDROCLIMATE: future winter rain reduces California drought riskCalifornia, drought-ridden between 2012 and 2016, may be less drought-prone in the future than previously thought. Robert Allen from the University of California Riverside, and co-author Ray Anderson, use a suite of climate models to show that those able to better capture relationships with El Niño—changes in the temperature of the tropical Pacific Ocean—provide more reliable estimates of future drought conditions in the US southwest. During winter, the ‘good’ models predict more rainfall, higher river levels and increasing soil moisture. This wintertime rain offsets drying trends predicted for the rest of the year, leading to small changes in overall drought risk. However, such increasing seasonality—reduced drought risk in winter, but enhanced drought risk during the dry seasons—poses new challenges to ensure year-round water security in California.