Seasonal cycles of Stratospheric Aerosol and Gas Experiment II near-background aerosol in the lower stratosphere

Extinction data from the Stratospheric Aerosol and Gas Experiment (SAGE) II in the lower stratosphere were analyzed for seasonal cycles in the near-background levels of stratospheric aerosol. The data analyzed were the extinction coefficient at 0.525 mu m (beta(0.525)) and the extinction ratio at 0.525 mu m (E-0.525) on the basis of climatological zonal monthly mean for the years 1998-2004. Distinct seasonal cycles were found for beta(0.525) at 35-15 degrees S above 28 km (region A) and at 20 degrees S-30 degrees N from 16 to 20.5 km (region B). In the A region, the seasonal cycle of E-0.525 was characterized by a maximum in local fall and can be explained by the ascent of mean meridional circulation in local summer and descent in local winter. In the B region, the seasonal cycles of E-0.525 were characterized by a maximum in October-January, which can be interpreted by meridional transport and mixing. The amplitude of the seasonal cycles for E-0.525 exhibited asymmetry between the Northern Hemisphere (NH) and the Southern Hemisphere (SH); the amplitudes at latitudes of 20-30 degrees were larger in the SH than in the NH above 29 km, whereas they were larger in the NH than in the SH below 18 km. Comparison of the distribution of E-0.525 with that of SAGE II water vapor suggested that the E-0.525 distribution is controlled by the stratospheric circulation and troposphere-originated gases. One difference between E-0.525 and water vapor was found in the E-0.525 maximum that appears over the winter subtropics. The E-0.525 maximum can be attributed to the dominance of temperature and microphysical effects compared to transport effects, whereas the water vapor distribution can be attributed to transport effects. Another difference is that an upward propagation of the seasonal cycle of E-0.525 at 5 degrees S-30 degrees N disappeared near 23 km. This difference is explained by the fact that the chemical and microphysical processes of aerosol formation become significant above 23 km.