Changes in Northern Hemisphere extra-tropical cyclone frequency following volcanic eruptions

Explosive volcanic eruptions are well known to influence Earth’s temperature. Changes in Earth’s
temperature can affect temperature gradients which in turn could affect the isentropic slope and
hence Northern Hemisphere high and mid-latitude weather. Yet, the possible influence of volcanic
eruptions on these atmospheric circulation patterns and the potential spatial extent are not well
understood. To address this issue, we pursue two independent lines of evidence. Firstly, we simulate
volcanic eruptions with the MPI-ESM1.2 Earth System Model and use the TRACK algorithm to
explore how extra-tropical cyclone (ETC) frequency is affected in the model experiments.
Secondly, we query the Greenland ice core NEEM-2011-S1 for indications of increased Northern
Hemisphere ETC frequency correlating with evidence for explosive volcanism by comparing the
storm proxies sodium and calcium; with the eruption proxy sulphur. Both the model and proxy
evidence suggest that large explosive volcanic eruptions increase storminess around the location of
the ice core. Furthermore, the simulations indicate that the number of ETCs increases in the
subtropics and at high latitudes, while they decrease in the mid-latitudes. A detailed interrogation
of the simulated eruptions reveals that increases in cyclone frequency are linked to steepening of
the isentropic slope due to a larger meridional temperature gradient and to a lower tropopause.
The steepening is driven by a combination of warming of the tropical stratosphere from absorption
of longwave radiation by volcanic aerosols and surface cooling due to the scattering of sunlight by
the same aerosols, whereas the lower tropopause may be attributed to a warmer stratosphere.