Very Little Evidence for a Direct Effect of Volcano Aerosols on Global Tropical Cyclone Activity

Evan (2012) examined the impacts of the eruptions of volcanoes El Chichón (1982) and Mount Pinatubo (1991) in the North Atlantic tropical cyclone activity, and suggested a reduction of frequency, duration and intensity in the years following these eruptions. However, as these two eruptions coincided with El Niño events, a clear attribution of this reduction in TC activity to the volcanic eruptions was not possible. More recently, Guevara-Murua et al. (2015) suggested that volcano eruptions reduced Atlantic tropical cyclone frequency in the last 3 centuries during the 3 years following major eruptions compared with the preceding 3 years using documentary and proxy reconstructions. However this proxy evidence is not clear from El Niño effects either, given the higher likelihood of occurrence of El Niño events following volcanic eruptions in proxy data (Adams et al. 2003) and models (Pausata et al. 2015).

Here we examine in depth the TC global response to volcanic eruptions using a combination of observations, reanalysis and model simulations. We analyze if the changes in TC activity in the first season in each hemisphere following large volcanic eruptions is statistically significant different from climatology. First, we consider various measures of observed TC activity, namely the observed frequency of the storms (named TCs storms category 1-5 and 3-5 in the Saffir-Simpson scale) and accumulated cyclone energy (ACE) in the first TC season following a large volcanic eruption in each hemisphere, as well as the individual North hemisphere basins. Then we examine the changes in the environmental conditions associated with TC activity, in particular potential intensity (PI) and two different formulations of genesis indices in four reanalysis products in the years following large volcanic eruptions. Next we consider the NCAR Large Ensemble simulations (42 ensemble members) in the period 1921-2005 and analyze the anomalies in PI and the genesis indices after volcanic eruptions. A similar analysis is performed in the large-scale environmental fields of 47 CMIP5 models’ historical simulations in the period 1850-2005 (which include two additional volcanic eruptions in the 19^th Century). Finally, we compare the frequency and ACE of explicit TC-like storms (Camargo 2013) and downscaled synthetic TCs (Emanuel 2013) generated by a subset of the CMIP5 models in the years following volcanic eruptions with the corresponding climatology

Observations show no significant reduction of TC activity in the first season after three large volcanic eruptions in the 20^th Century (Agung, El Chichón, Pinatubo), with the exception of the North Atlantic. The response to these three volcanic eruptions cannot be separated from the coinciding El Niño events. Similarly, the anomalies in PI and genesis indices in reanalysis cannot be distinguished from the response to ENSO in these environmental fields. Both the NCAR Large Ensemble and CMIP5 models show a strong reduction in the PI following large volcanic eruptions. However, there is no significant reduction in the genesis indices anomalies. But, it is well known that the models response to volcanic aerosols is too strong. When a bias correction is considered, the PI signal after the volcanic eruptions becomes much smaller. Furthermore, there is no statistically significant reduction in TC activity for either the explicit or synthetic downscaled CMIP5 storms following the volcanic eruptions, even though the downscaling technique uses as one of its inputs the PI.

In summary, there is little evidence of a global reduction of TC activity from direct volcanic aerosols in observations and models. It should be noted that our analysis does not take into account the indirect effect of volcanic aerosols on TC activity through the El Niño events that develop 2-3 years after the volcanic eruptions. This will be the topic of a follow up study.

References:

Adams, J.B., M.E. Mann, C.M. Ammann, 2003: Proxy evidence for an El Niño-like response to volcanic forcing. Nature 426, 274-278, doi: 10.1038/nature02101.

Camargo, S.J., 2013. Global and regional aspects of tropical cyclone activity in the CMIP5 models. J. Climate, 26, 9880-9902.

Emanuel, K.A., 2013: Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21^st century. Proc. Nat. Acad. Sci., 110, 12219-12224.

Evan, A. T., 2012: Atlantic hurricane activity following two major volcanic eruptions. J. Geophys. Res., 117, D06 101.

Guevara-Murua, A., E.J. Hendy, A.C. Rust, and K.V. Cashman, 2015: Consistent decrease in North Atlantic tropical cyclone frequency following major volcanic eruptions in the last three centuries. Geophys. Res. Lett., 42, 9425-9432.

Pausata, F.S.R., L. Chafik, R. Caballero, and D.S. Battisti, 2015: Impacts of high-latitude volcanic eruptions on ENSO and AMOC. Proc. Nat. Acad. Sci., 112, 13784-13788, doi: 10.1073/pnas.1509153112.