• By Erik Klemetti
• March 20, 2012 |
• 8:47 am

The plume from a lateral blast at Pinatubo in the Philippines seen on June 15, 1991. The eruption may have helped stifle hurricane activity in the Atlantic for three years afterwards.

We have had many discussions over the years here on Eruptions about the relationship betweenvolcanic eruptions and weather/climate (remember, they are different things). Most of the time, the concern is how weather will become worse (i.e., much colder or much hotter) due to volcanic aerosols or ash that are kicked high into the atmosphere during large eruptions. Remember, ash plumes from manyplinian eruptions can tower over 35-50 km up, so material can be injected into the upper atmosphere and spread around the world in a matter of weeks. It would be very surprising if these sorts of eruptions – which are relatively rare, only occurring maybe once or twice a decade – didn’t effect weather and climate for years until the aerosols can all settle out.

So, I was quite interested when I saw a new paper in the Journal of Geophysical Research titled “Atlantic hurricane activity following two major volcanic eruptions” by Amato Evan. My instant thought was I actually wasn’t sure what to expect – I mean, how would a large eruption effect the activity of such major, hemisphere-spanning events like hurricanes? Would it make hurricanes worse? As it turns out, this study suggests that major eruptions in the tropics (or close) might actually subdue Atlantic hurricane activity for years after the eruption.

Figure 2B from Evan (2012) showing the drops in sea surface temperature (SST) in the Atlantic after the eruptions of El Chichón (1982) and Pinatubo (1991)

Evan (2012) looks at two eruptions in particular – the1982 eruption of El Chichónin Mexico and the 1991 eruption of Pinatubo* in the Philippines. Both were large eruptions, ranking as VEI 5-6. Both eruptions injected large volumes of aerosols and ash into the upper atmosphere in the tropics, reducing theoptical depth of the atmosphere to 0.1-0.2 (normally it should be closer to 0.01). To give you an idea, that is almost as bad as other large eruptions such as Krakatau in 1883, famous for the vibrant skies it produced worldwide. All these aerosols in the atmosphere increase the albedo of the planet – that is, the planet will reflect more sunlight back into space. This means less sunlight hitting the surface of the Earth, and in particular, less on the oceans in the tropics. This produces colder surface and near-surface waters in what is called the Atlantic Main Development Region (MDR) for hurricanes – between 8-20°N/20-65°W (see right). This decrease in sea surface temperature, in turn, lead to an increase in vertical wind shear in the MDR.

What Evan (2012) found was that the total number of hurricanes in the three years before each eruption and three years after the eruption were markedly different – ~12 per season prior to the eruption and 6-8 per season after the eruption. Not only that, but the storms in the three years after the eruption were weaker and didn’t last as long as prior to the eruption. Even beyond this, the location that hurricanes formed changed as well, where before the eruptions most hurricanes were found in the MDR, after the eruptions there were dominantly found along the eastern United States. So, the long and short becomes large volcanic eruption leads to lower sea surface temperatures and higher vertical wind shear in the locations where hurricanes form, thus fewer hurricanes occur and those that do are weaker.

Now, bear in mind, this study only looked at two major eruptions in the last 35 years – and unfortunately both coincided with an El Niño, so one can’t conclusively link the eruptions and the change in hurricane activity. Evan (2012) mentions that there are at least three other major eruptions that could effect hurricane activity – Agung in 1963**, Santa Maria in 1902 and Krakatau in 1883. However, no pattern emerges from these eruptions as hurricane activity did decrease after Krakatau, it wasn’t effected by Santa Maria and appeared to increase after Agung. Evan (2012) suggests that the Agung eruption might have cooled the South Atlantic preferentially, causing the increase in North Atlantic hurricane activity.

Hurricane Irene off of Cuba and Florida, seen on August 24, 2011. Can volcanic eruptions help or hinder hurricane activity? It is still unclear.

Clearly, there is still a lot of noise in these correlations of hurricane activity and volcanic eruptions. The eruptions that Evan (2012) examined are the big ones – so, what if any effect would smaller eruptions in the tropics have (such as Merapi in 2010 or Nabro in 2011). Taking a look at the hurricane counts for the past century, you can see a number of periods of lower hurricane activity – can these all be correlated with eruptions like Katmai in 1912 (well out of the tropics) and what is causing the low hurricane counts in 2005-08? There are many unanswered questions here – but clearly, a closer examination looks to be in order – or, as the author of the paper suggests, maybe we need a large eruption in the tropics to test this theory out.

* Lockwood and Hazlett (2010) note that a typhoon/hurricane might have helped cause the cataclysmic eruption of Pinatubo in 1991. The lowest atmosphere pressure from the Typhoon Yunya passed over Pinatubo just 3 hours before the largest eruption. It likely didn’t cause the eruption (that was an injection of magma into the system over the prior few weeks), but it could have played a role in pushing the volcano pass the “tipping point” for an eruption.

** This eruption is listed in the paper as 1964, but the activity lasted from February 1963 to January 1964.

{Hat tip to Alex Witze for pointing out this article to me.}

Image 1: Pinatubo erupting in 1991. Image by Richard Hoblitt/USGS
Image 2: Figure 2B from Evan (2012), Journal of Geophysical Research
Image 3: Hurricane Irene in 2011. Image from the NASA Earth Observatory.

from:    http://www.wired.com/wiredscience/2012/03/volcanoes-and-hurricanes-mortal-enemies-best-friends/#more-101606