Influence of volcanic eruptions on midlatitude upper tropospheric aerosol and consequences for cirrus clouds

The influence of downwelling stratospheric sulfurous aerosol on the UT (upper troposphere) aerosol concentrations and on cirrus clouds is investigated using CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container observations) (between 1999-2002 and 2005-2013) and the cirrus reflectance product from Moderate Resolution Imaging Spectroradiometer (MODIS). The initial period, 1999-2002, was volcanically quiescent after which the sulfurous aerosol in the LMS (lowermost stratosphere) (S-LMS) became enhanced by several volcanic eruptions starting 2005. From 2005 to 2008 and in 2013, volcanic aerosol from several tropical eruptions increased S-LMS. Due to consequent subsidence, the sulfur loading of the upper troposphere (S-UT) was increased by a factor of 2.5 compared to background levels. Comparison of S-LMS and S-UT during the seasons March-July and August-November shows a close coupling of the UT and LMS. Finally, the relationship between S-LMS and the cirrus cloud reflectance (CR) retrieved from MODIS spectrometer (on board the satellites Terra and Aqua) is studied. S-LMS and CR show a strong anticorrelation, with a factor of 3.5 increase in S-LMS and decrease of CR by 8 +/- 2% over the period 2001-2011. We propose that the increase of S-LMS due to volcanism has caused the coinciding cirrus CR decrease, which would be associated with a negative radiative forcing in the Northern Hemisphere midlatitudes.