Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone

Changes in the width of the tropical belt due to simple radiative forcing changes in the GeoMIP simulations

“…The enhanced expansion in boreal autumn is consistent with realistic (Hu et al, 2013;Kang and Lu, 2012) and more idealized (Kang and Lu, 2012) CMIP5 forcing simulations and with historical reanalyses (Hu and Fu, 2007). While Allen et al (2012) proposed that the observed tropical expansion in Northern Hemisphere summer and autumn was driven by the combined effects of black carbon and tropospheric ozone, it appears that increased carbon dioxide concentrations alone could also drive some of this enhanced expansion. As a caveat, however, the seasonality of Northern Hemisphere tropical expansion is not particularly robust as the tropical belt contracts in some models and seasons in response to quadrupled carbon dioxide concentrations.…”

“…Stratospheric ozone depletion and its resulting polar stratospheric cooling have been argued to be a potentially dominant driver of Southern Hemisphere tropical expansion (Polvani et al, 2011b;Min and Son, 2013), and ozone recovery over the coming decades may oppose any future greenhouse-gas-driven expansion (Son et al, 2009;Polvani et al, 2011a). Black carbon, tropospheric ozone (Allen et al, 2012), and aerosols (Allen and Sherwood, 2011;Allen et al, 2014) may have also played a role in historical tropical expansion, especially in the Northern Hemisphere. While examining the response of climate models to realistic sets of past and future forcings is appealing, it is not ideal for identifying how the tropical belt responds to particular forcings.…”

Global warming-accelerated drying in the tropics

“…In the extratropics, the increased thermodynamic stability and weaker latitudinal temperature gradient were expected to reduce extratropical baroclinic instability and eddies, leading to a poleward movement of the HC boundary (9)(10)(11). This "extratropical-driven" theory for the HC change has been further supported by the strong impact of the Antarctic ozone hole on the poleward expansion of the southern edge of the HC (12) and by a connection between northern hemispheric aerosols, especially black carbon, and the northward expansion of the HC northern edge (13).…”

Comparison of elemental and black carbon measurements during normal and heavy haze periods: implications for research

“…The past three decades have seen increasing interest in black carbon (BC) and elemental carbon (EC), both of which originate from the incomplete combustion of carbonaceous combustibles, due to their strong light-absorbing properties and their possible health effects (Johnson and Huntzicker 1979;Wolff 1981;Turco et al 1983;Molnar et al 1999;Jacobson 2000Jacobson , 2001Ramanathan and Carmichael 2008;Allen et al 2012). EC represents thermally refractory carbon with a graphitic structure, whereas BC is commonly used to define the extent to which an aerosol sample exhibits light-absorbing properties (Salako et al 2012;Gray and Cass 1998;Huntzicker et al 1982;Watson et al 2005).…”