<p>Stratospheric hydrogen chloride (HCl) is the main stratospheric reservoir of chlorine, deriving from the decomposition of chlorine-containing source gases. Its trend has been used as a metrics of ozone depletion or recovery. Using the latest satellite observations, the authors find that a significant increase of Northern Hemisphere stratospheric HCl during 2010&#8211;2011 can mislead trends of HCl in recent decades. Agree with previous studies, HCl increased from 2005 to 2011; while when removing the large increase of stratospheric HCl during 2010&#8211;2011, the increasing linear trend from 2005 to 2011 becomes weak and insignificant, in addition, the linear trend of Northern Hemisphere stratospheric HCl from 2005 to 2016 also shows weak and insignificant. The significant increase of HCl during 2010&#8211;2011 is attributed to a super strong north polar vortex and a reduced residual circulation during 2010&#8211;2011, which slowed down the transport of HCl from the low&#8211;mid latitudes to the high latitudes, leading to accumulation of HCl in the middle latitudes of the stratosphere during 2010&#8211;2011. Further analysis suggests that the strong polar vortex and the reduced residual circulation were caused by the joint effect of a La Ni&#241;a event and the west phase of the quasi-biennial oscillation.</p>
Cassini ISS observation of Saturn’s north polar vortex and comparison to the south polar vortex
