Perspectives on study in PNAS (“Proceedings of the National Academy of Science”) by F.S. Pausata and S.J. Camargo, published in April 2019: “Tropical cyclone activity affected by volcanically induced ITCZ shifts” (PC-19)
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Background
Volcanic eruptions have long been known to affect the climate on various time and space scales. For over 20 years, climate models have been used to evaluate the impact of eruptions on atmospheric concentrations of greenhouse gases for the succeeding years with the goal of separating this episodic event from man-made sources of ongoing climate change. Tropical eruptions are known to have a specific signature, inducing local changes in the tropical latitudes and a shift in equator-to-pole temperature contrast, which in turn has implications for both tropical cyclones (hurricanes) and extra-tropical cyclones (winter storms). Thus, the timing and location of an eruption can affect the severity of the upcoming summer and winter seasons’ extreme weather.
The particulates ejected in a volcanic eruption are a complex mix of aerosols that serve as a medium for micro-chemical reactions on various time scales. In the short-term, the aerosols modulate the formation of cloud and rain droplets in developing weather systems. In the medium-term they may compound the effects of an El Nino episode, and, in the long-term, they can alter inter-decadal temperature cycles. For example, eruptions are theorized to have played a major role in the Little Ice Age. The interplay of atmospheric physics and atmospheric chemistry combined with the geological discipline of physical volcanology make the application of climate models with embedded “cloud microphysics” a vital tool in understanding the complex relationship of volcanic eruptions to hurricane risk.
Objective
The objective of PC-19 was to study the medium-term (few years following) relationship of a volcanic eruption to the Intertropical Convergence Zone (“ITCZ”) and, in turn, global hurricane activity. The study used an ensemble of numerical (climate model) simulations to understand the effect of tropical eruptions on tropical cyclone ingredients.
Key Findings
- In the year following an eruption, the study finds a clear southward (northward) shift in the ITCZ and hence preferred tropical cyclone (“TC”) genesis regions for eruptions occurring in the northern (southern) hemisphere.
- Tropical eruptions lead to a consistent decrease in TC frequency and intensity over SE Asia and NW and NE Australia, regardless of whether the eruption occurred in the northern or southern hemisphere.
- While eruptions can modulate the intensity of an ENSO event, the study does not find a robust response in TC activity.
- PC-19 found that in the years following the first post-eruption season, the impact closely follows the modified precipitation patterns through years two to four, then dissipates thereafter.
- The study shows that the response of TC activity following a major tropical eruption is distinct and independent of the ENSO cycle, and that the effects on TC genesis and development are more closely tied to the shift in the ITCZ caused by the eruption and the resulting reaction of that shift to TC formation, even in the midst of an El Nino event.
- The authors point to agreement in their results with observations and other studies noting a reduction in TC genesis following northern hemisphere eruptions, but disagree with studies finding a more general reduction in global TC activity following major eruptions.
Implications for Aeolus
PC-19 notes a clear connection between volcanic events and subsequent tropical cyclone activity in both hemispheres. Specifically, the findings indicate that eruptions shift the ITCZ in the direction opposite the hemisphere in which the eruption occurred. The effects are most apparent in the season following an eruption episode and dissipates after four years. The result is a redistribution of TC activity without a notable reduction in activity overall. Furthermore, PC-19 finds that while eruptions may modulate the global effect of an El Nino event, there is no robust signal indicating they amplify ENSO effects on tropical activity. The results of PC-19 can inform Aeolus decision-making for episodic (though unpredictable) effects of major eruptions on seasonal forecasts (or activity in-season) for the Atlantic and South Pacific tropical basins.
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