Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized1,2, but in the Indian Ocean region a similar ocean–atmosphere interaction causing interannual climate variability has not yet been found3. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air–sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Nino/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean—and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia—brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.

A Dipole Mode in the Tropical Indian Ocean

The Tropical Ocean-Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean-atmosphere system. The World Climate Research Programme's (WCRP) Global Ocean-Atmosphere-Land System (GOALS) program seeks to explore predictability of the global climate system through investigation of the major planetary heat sources and sinks, and interactions between them. The Asian-Australian monsoon system, which undergoes aperiodic and high amplitude variations on intraseasonal, annual, biennial and interannual timescales is a major focus of GOALS. Empirical seasonal forecasts of the monsoon have been made with moderate success for over 100 years. More recent modeling efforts have not been successful. Even simulation of the mean structure of the Asian monsoon has proven elusive and the observed ENSO-monsoon relationships has been difficult to replicate. Divergence in simulation skill occurs between integrations by different models or between members of ensembles of the same model. This degree of spread is surprising given the relative success of empirical forecast techniques. Two possible explanations are presented: difficulty in modeling the monsoon regions and nonlinear error growth due to regional hydrodynamical instabilities. It is argued that the reconciliation of these explanations is imperative for prediction of the monsoon to be improved. To this end, a thorough description of observed monsoon variability and the physical processes that are thought to be important is presented. Prospects of improving prediction and some strategies that may help achieve improvement are discussed.

/pdf/monsoons-processes-predictability-and-the-prospects-for-1auwiao0qt.pdf

Monsoons: Processes, predictability, and the prospects for prediction

Artificial neural networks (the multilayer perceptron)—a review of applications in the atmospheric sciences

Titanium and iron concentration data from the anoxic Cariaco Basin, off the Venezuelan coast, can be used to infer variations in the hydrological cycle over northern South America during the past 14,000 years with subdecadal resolution. Following a dry Younger Dryas, a period of increased precipitation and riverine discharge occurred during the Holocene “thermal maximum.” Since ∼5400 years ago, a trend toward drier conditions is evident from the data, with high-amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the “Little Ice Age.” These regional changes in precipitation are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone (ITCZ), potentially driven by Pacific-based climate variability. The Cariaco Basin record exhibits strong correlations with climate records from distant regions, including the high-latitude Northern Hemisphere, providing evidence for global teleconnections among regional climates.

https://science.sciencemag.org/content/sci/293/5533/1304.full.pdf

Southward Migration of the Intertropical Convergence Zone Through the Holocene

Departure characteristics of the large-scale circulation are studied in relation to extreme drought and flood years in northeastern Brazil identified from a collective of long-term rainfall stations and series of river discharge. Ship observations during 1911–72 compiled with a one degree square resolution and extending between 30°N and 30°S from the African coast to the eastern Pacific form a major observational basis.



The rainy season of northeastern Brazil is narrowly centred around March/April and is related to the southernmost seasonal migration of a lower-tropospheric confluence axis over the adjacent eastern tropical Atlantic. the ‘Sěcas’ of Ceara province are characterized by an equatorward expansion of the South Atlantic, and a poleward retraction of the North Atlantic, high - associated with a northward displacement of the enclosed near-equatorial trough of low pressure. Concurrently, the zonally oriented bands of maximum cloudiness and precipitation frequency stay farther north, the North Atlantic trades weaken, and the South Atlantic trades become stronger than in the long-term mean. the sea surface temperature pattern during a deficient rainy season in northeast Brazil is characterized by positive departures in a broad band across the North Atlantic and in the eastern Pacific, and anomalously cold waters in most of the South and equatorial Atlantic. During abundant rainy seasons in northeast Brazil, departure patterns are approximately inverse to those typical of drought years.



A strong negative linkage exists between northeast Brazil rainfall and sea surface temperature along the Ecuador/Peru coast. This seems to be caused by inverse long-term pressure variations over the eastern South Pacific and South Atlantic oceans, which in turn may be part of large-scale mass adjustments of the Southern Oscillation type.



Development of the equatorward expansions of the North and South Atlantic highs, pre-season precipitation in northeast Brazil and the Guayanas, and location of the cloudiness and precipitation belts over the adjacent western tropical Atlantic, are among the more promising predictors for seasonal foreshadowing of extreme rainy season behaviour in northeast Brazil.

Dynamics of climatic hazards in northeast Brazil

1. Introduction.- References.- 2. Diurnal Forcings and Local Circulations.- 2.1. Insolation and Heat Budget Forcing.- 2.2. Atmospheric Tides.- 2.3. Circulations on the Local and Meso-Scale.- 2.4. Diurnal Marches of Sea-Air Exchange, Cloudiness, and Precipitation.- 2.5. Synthesis.- References.- 3. Planetary Scale Atmospheric Circulation.- 3.1. Mean Meridional Distribution of Temperature and Humidity.- 3.2. Zonal Wind Regime.- 3.3. Mean Meridional Circulation.- 3.4. Maintenance of the Global Circulation: Angular Momentum.- 3.5. Maintenance of the Global Circulation: Kinetic Energy.- 3.6. Synthesis.- References.- 4. Ocean Circulation.- 4.1. Wind Stress and Motion Field in the Upper Ocean.- 4.2. The Subtropical Gyres.- 4.3. Equatorial Current Systems.- 4.3.1. Overview of Surface Circulation.- 4.3.2. Balance of Forces.- 4.3.3. The North Equatorial Countercurrent.- 4.3.4. Wind Stress, Vertical Motion, Thermocline and Surface Topography.- 4.3.5. The Equatorial Undercurrent.- 4.3.6. Equatorial Waves and Remote Forcing.- 4.3.7. Recent Discoveries of Subsurface Currents.- 4.4. The Monsoon Ocean.- 4.5. Deep Circulation.- 4.6. Synthesis.- References.- 5. Heat and Water Budgets.- 5.1. Basic Theory.- 5.2. Net Radiation at the Top of the Atmosphere.- 5.3. Oceanic Heat Budget.- 5.4. Atmospheric Heat Budget.- 5.5. Water Budget.- 5.6. Relative Roles of Oceanic Versus Atmospheric Heat Transports.- 5.7. Synthesis.- References.- 6. Regional Circulation Systems.- 6.1. Overview of the Global Tropics.- 6.2. Jet Streams.- 6.2.1. Basic Dynamics.- 6.2.2. Subtropical Westerly Jet.- 6.2.3. Tropical Easterly Jet.- 6.2.4. West African Mid-Tropospheric Jet.- 6.2.5. East African Low Level Jet.- 6.3. Subtropical Highs.- 6.4. Trades.- 6.5. Trade Inversion.- 6.5.1. Spatial Patterns.- 6.5.2. Origin and Maintenance.- 6.5.3. Climatic Implications.- 6.6. Mid-Tropospheric Inversions.- 6.7. Equatorial Trough Zone.- 6.7.1. The Large-Scale Setting.- 6.7.2. Structure of the Intertropical Convergence Zone.- 6.7.2.1. Atlantic and Pacific Oceans.- 6.7.2.2. Indian Ocean.- 6.7.2.3. Africa.- 6.7.2.4. On Atmospheric and Oceanic Controls.- 6.7.3. Dynamics of Cross-Equatorial Flow.- 6.7.4. Equatorial Dry Zone.- 6.8. Monsoons.- 6.8.1. Definition and Global Perspective.- 6.8.2. Africa.- 6.8.3. Indian Ocean Sector.- 6.8.4. On the Heat and Moisture Budget of the Indian Monsoons.- 6.8.5. Numerical Modelling of the Indian Summer Monsoon.- 6.9. Zonal Circulations.- 6.10. Upper-Tropospheric Anticyclones.- 6.11. Wind Regimes of the Equatorial Stratosphere.- 6.12. Synthesis.- References.- 7. Climatology of Weather Systems.- 7.1. Clouds and Convection.- 7.2. Tropical Storms.- 7.3. Waves in the Easterlies.- 7.4. Squall Lines.- 7.5. Dust Storms of the Sudan.- 7.6. Monsoon Depressions.- 7.7. Subtropical Cyclones.- 7.8. Temporales of Pacific Central America.- 7.9. Cold Surges.- 7.10. Interannual Variability.- 7.11. Synthesis.- References.- 8. Interannual Variability of the Atmosphere-Ocean System.- 8.1. Surface Patterns of the Southern Oscillation.- 8.2. El Nino.- 8.3. Upper-Air Patterns of the Southern Oscillation.- 8.4. Rainfall Anomalies in Indonesia.- 8.5. Vagaries of the Indian Monsoon.- 8.6. The Secas of Northeast Brazil.- 8.7. Rainfall Variations in the Central American - Caribbean Region.- 8.8. Drought and Flood Regimes in Subsaharan Africa.- 8.9. Climate Anomalies at the Angola Coast.- 8.10. Hydrometeorological Anomalies in the Zaire (Congo) Basin.- 8.11. Time Scales of Climate Variability.- 8.12. Synthesis.- References.- 9. Climate Prediction.- 9.1. Indian Monsoon.- 9.2. Indonesian Rainfall.- 9.3. Hong Kong Climate.- 9.4. Southern Africa.- 9.5. Kenya Rainfall.- 9.6. Sahel Drought.- 9.7. The Droughts of Northeast Brazil.- 9.8. Rivers of Northern South America.- 9.9. El Nino.- 9.10. Tropical Storms.- 9.11. Numerical Modelling.- 9.12. Biological Consequences.- 9.13. Synthesis and Outlook.- References.- 10. The Human Impact.- 10.1. Deforestation.- 10.2. Land Use and Surface Albedo.- 10.3. Effects of Dust on Climate.- 10.4. Intentional Climate Modification.- 10.5. Ocean Climate and Fisheries.- 10.6. India.- 10.7. Northeast Brazil.- 10.8. Subsaharan Africa.- 10.9. The Use of Climate Prediction.- 10.10. Synthesis.- References.- 11. Tropical Glaciers and Climate.- 11.1. Spatial Patterns.- 11.2. History of Glacier Variations.- 11.3. Climatic Forcing and Terminus Response.- 11.4. Ice Cores.- 11.5. Synthesis.- References.- 12. Past Climates Of The Tropics.- 12.1. Causes of Climate Variations.- 12.2. Vegetation.- 12.3. Lakes.- 12.4. Glaciers.- 12.5. Deep-Sea Cores.- 12.6. Pacific and Australasia.- 12.7. The Indian Ocean and Surrounding Continents.- 12.8. Africa and the Adjacent Tropical Atlantic.- 12.9. Americas.- 12.10. Numerical Modelling.- 12.11. Synthesis.- References.- Author Index.

Climate dynamics of the tropics

General circulation mechanisms instrumental in both annual cycle and interannual variability of rainfall are studied with reference to key regions of the tropical Americas and Africa, including the Central American–Caribbean area, northern Northeast Brazil, Subsaharan Africa, the Angola coast and the zaire (Congo) and Amazon basins. For most of these regions, rainfall anomalies tend to be associated with departures in the large-scale atmospheric and oceanic fields that correspond to the pattern changes in the annual alternation of dry and rainy seasons. The interannual variability of climate and circulation thus appears largely as enhancement and reduction of the annual cycle.

Interannual Variability and Annual Cycle: Mechanisms of Circulation and Climate in the Tropical Atlantic Sector

Atmosphere-ocean mechanisms of rainfall anomalies at the coast of eastern Africa are studied using long-term ship observations in the Indian Ocean, surface current measurements, subsurface casts, upper air analyses by the European Centre for Medium Range Weather Forecasts, rain gauge series in eastern Africa and India, and an index of the Southern Oscillation (SO), the high-SO phase being defined by anomalously high/low pressure at Tahiti/Darwin. The causalities of precipitation anomalies at the coast of eastern Africa differ for the two rainy seasons centered on April-May and October-November, and only the latter is strongly related to the SO. In the high-SO phase, April-May pressure is low over the entire Indian Ocean domain, whereas in October-November, pressure is high in the west and low in the east. Concomitantly, surface waters are anomalously cold in the west, and strong westerlies sweep the equatorial zone of the Indian Ocean. Eastern African rainfall anomalies are related to the SO through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern Africa. (1) Equatorial westerly winds are conducive to lower tropospheric divergence over equatorial East Africa, and in the high-SO phase these are accelerated, especially in October-November, owing to the anomalous eastward pressure gradient. (2) The equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere, which entails cold-water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds. (3) In addition, cold-water anomalies in the western Indian Ocean, most pronounced in October-November during the high-SO phase, also suppress convection. (4) In the high-SO phase, the Indian summer monsoon tends to be strong, leaving behind an anomalously cold western Indian Ocean, which in turn feeds into mechanisms 1 to 3. The eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern Africa.

Atmospheric-hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean

Tropical atmosphere and ocean are receiving increased attention in relation to the functioning of the global climate system, the remarkable climatic variability in low latitudes, and the associated manifold environmental and societal consequences. Beyond the traditional emphasis of meteor ology on weather analysis and forecasting, there is a growing interest in the climate and large scale circulation of the tropics. This book may serve as a text for graduate and upper-division undergraduate students in meteorology, and is also intended as a reference work for practicing meteorologists, and researchers in the atmospheric, oceanic, and other environmental sciences. I began writing this book in 1979, but the roots reach further back. Early experiences in North Africa fuelled my curiosity about the low latitudes. In 1960 I seized the opportunity to work in the National Meteorological Service of El Salvador in Central America. My interest in the tropics continued after joining the University of Wisconsin in 1963. Field research brought me to the equatorial Pacific, and many times to the tropical Americas and Africa. This involved visits and correspondence with many weather services. My acquaintance with Australasia and South Asia is limited to short study visits, but includes continuous contacts with colleagues at key research institutions in India, namely the India Meteorological Department, the Indian Institute of Tropical Meteorology, and Andhra University. A guest semester at the University of the Witwatersrand in 1971 and related travels provided a perspective on the problems of Southern Africa."

Stefan Hastenrath

Papers

Dynamics of climatic hazards in northeast Brazil

Climate dynamics of the tropics

Interannual Variability and Annual Cycle: Mechanisms of Circulation and Climate in the Tropical Atlantic Sector

Atmospheric-hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean

Climate and circulation of the tropics