Tropical Belt Width Proportionately More Sensitive to Aerosols Than
The tropical belt has widened during the last several decades, and both
internal variability and anthropogenic forcings have contributed.
Although greenhouse gases and stratospheric ozone depletion have been
implicated as primary anthropogenic drivers of tropical expansion, the
possible role of other drivers remains uncertain. Here, we analyze the
tropical belt width response to idealized perturbations in multiple
models. Our results show that absorbing black carbon (BC) aerosol drives
tropical expansion, and scattering sulfate aerosol drives contraction.
BC, especially from Asia, is more ecient per unit radiative forcing than
greenhouse gases in driving tropical expansion, particularly in the
Northern Hemisphere. Tropical belt expansion (contraction) is associated
with an increase (decrease) in extratropical static stability induced by
absorbing (scattering) aerosol. Although a formal attribution is dicult,
scaling the normalized expansion rates to the historical time period
suggests that BC is the largest driver of the Northern Hemisphere
tropical widening but with relatively large uncertainty.