Has the stratospheric HCl in the Northern Hemisphere been increasing since 2005

<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 2010–2011 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 2010–2011, 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 2010–2011 is attributed to a super strong north polar vortex and a reduced residual circulation during 2010–2011, which slowed down the transport of HCl from the low–mid latitudes to the high latitudes, leading to accumulation of HCl in the middle latitudes of the stratosphere during 2010–2011. Further analysis suggests that the strong polar vortex and the reduced residual circulation were caused by the joint effect of a La Niña event and the west phase of the quasi-biennial oscillation.</p>

On the dynamical nature of Saturn’s North Polar hexagon

An explanation of long-lived Saturn’s North Polar hexagonal circumpolar jetin terms of instability of the coupled system polar vortex - circumpolar jet isproposed in the framework of the rotating shallow water model, wherescarcely known vertical structure of the Saturn’s atmosphere is averaged out.The absence of a hexagonal structure at Saturn’s South Pole is explainedsimilarly. By using the latest state-of-the-art observed winds in Saturn’spolar regions a detailed linear stability analysis of the circumpolar jet isperformed (i) excluding (“jet-only” configuration), and (2) including(“jet+vortex” configuration) the north polar vortex in the system. A domainof parameters: latitude of the circumpolar jet and curvature of its azimuthalvelocity profile, where the most unstable mode of the system has azimuthalwavenumber 6, is identified. Fully nonlinear simulations are then performed,initialized either with the most unstable mode of small amplitude, or withthe random combination of unstable modes. It is shown that developingbarotropic instability of the “jet+vortex” system produces a long-livingstructure akin to the observed hexagon, which is not the case of the“jet-only” system, which was studied in this context in a number of papersin literature. The north polar vortex, thus, plays a decisive dynamical role.The influence of moist convection, which was recently suggested to be at theorigin of Saturn’s north polar vortex system in the literature, is investigatedin the framework of the model and does not alter the conclusions.