Little evidence of reduced global tropical cyclone activity following recent volcanic eruptions.

The impact of volcanic aerosols on recent global tropical cyclone (TC) activity is examined in observations, reanalysis, and models (the Coupled Model Intercomparison Project phase 5 - CMIP5 multi-model, and one single model large ensemble). In observations, we find a reduction of TC activity only in the North Atlantic following the last three strong volcanic eruptions; that signal, however, cannot be clearly attributed to volcanoes, as all three eruptions were simultaneous with El Niño events. In reanalyses, we find no robust impact of volcanic eruptions on potential intensity (PI) and genesis indices. In models, we find a reduction in PI after volcanic eruptions in the historical simulations, but this effect is significantly reduced when differences between the model environment and observations are accounted for. Morever, the CMIP5 multi-model historical ensemble shows no effect of volcanic eruptions on a TC genesis index. Finally, there is no robust and consistent reduction in recent TC activity following recent volcanic eruptions in a large set of synthetic TCs downscaled from these simulations. Taken together, these results show that in recent eruptions volcanic aerosols did not reduce global TC activity. No global effect of recent volcanic eruptions on tropical cyclones: Past volcanic eruptions did not reduce the intensity or number of tropical cyclones globally. Volcanic aerosols cool sea surface temperatures and warm the lower stratosphere, thereby weakening the vertical temperature gradient that, in theory, governs the development and intensification of tropical cyclones. Suzana Camargo and Lorenzo Polvani from Columbia University systematically examine observations, atmospheric reanalyses, and climate models and show that — contrary to prior work — there is no clear evidence that eruptions reduce the occurrence or strength of tropical cyclones. The differing results arise because the researchers assessed the global, rather than the North Atlantic only response. Furthermore, the reduction of North Atlantic hurricane activity could not be separated from the response to simultenous El Niño events. Climate models do suggest that eruptions can potentially weaken tropical cyclones. But these model-based conclusions are inconsistent with observations and mainly arise from model biases in simulations of both surface and stratospheric temperatures. [ABSTRACT FROM AUTHOR]