Just about every Australian knows the part from Dorothea Mackellar’s My Country about ‘droughts and flooding rains’. In the past, climate prediction has tended to focus on the drought side of the equation. But the flooding rains are just as much part of the story, and a product of the same system.
The Christ children, El Niño (named for the infant Jesus, because fishermen noticed it generally occurred around Christmas) and La Niña, are fraternal twins, different in every respect, but co-dependent. Both can alter wind and water currents worldwide. While El Niño brings drought to Australia and its neighbours, La Niña brings floods to the Asia Pacific region, increases the severity and frequency of west Pacific cyclones, causes droughts in the south-west of the United States, and hurricanes in the Atlantic.
In a 2014 paper, published, like this latest one, in Nature Climate Change, Cai and his team looked at the effect of a warming climate on the frequency and severity of El Niño events, and found that extreme El Niños were likely to be twice as frequent – rising from an average once every 20 years to once every ten. So too, it now seems, for La Niñas.
To assess potential La Niña activity, the team used 21 global climate models that can simulate extreme La Niña events. The study covered the years from 1900 to 2005 and from 2006 to 2009. It used historical data for the first period, and the Intergovernmental Panel on Climate Change forecasts on greenhouse emissions growth for the period from 2006 to 2009. The finding is that severe La Niña events will occur on average every 13 years, as against the previous average frequency of every 23 years. Not predictably on a regular 13-year schedule, but the long-term average will rise to around every 13 years.
This result was not what Cai and his co-authors expected. Their initial assumption was if extreme El Niños become more frequent, extreme La Niñas might become less so. They found, instead, that about 75 percent of extreme La Niñas will happen directly after an extreme El Niño.
The implications of this are alarming. It raises the prospect that affected regions could seesaw between drought and flood in rapid succession. Most of these are heavily populated: countries in La Niña’s sphere of influence include China and Bangladesh.
La Niñas occur when the water in the central-to-eastern equatorial Pacific is colder than normal. This means that fewer rain-producing clouds form there. At the same time, atmospheric convection and rainfall in the western equatorial Pacific increases. An extreme La Niña is one in which the cooling in the central Pacific is more than 1.5°C below normal average temperatures. This happens when land masses warm disproportionately more than the ocean, as would be the case in more frequent extreme El Niño events.
It’s a complex system, intimately entwined. Extreme La Niña events will follow extreme El Niños, because the El Niño paradoxically speeds the cooling of the central Pacific. The Maritime Continent (a region of Southeast Asia between the Indian and Pacific Oceans, which includes Indonesia, Philippines and Papua New Guinea) will warm faster than the central Pacific Ocean in a warmer world. This difference in temperature can produce unusually strong easterly winds. These drive warm water to the west and towards the poles, in turn bringing colder water from the deep ocean closer to the surface. During an El Niño, the heat in the upper ocean releases to the upper atmosphere. The cooler water below rises to the surface, causing temperatures to fall in the central Pacific.
A commentary accompanying the article says the study is a warning that, “the possibility of more frequent devastating La Niña events must be seriously considered as we prepare to face the consequences of global warming”.
Cai is more forthright, saying, “Our results call for measures to reduce greenhouse gas emissions so as to reduce such risks”.
Dr Wenju Cai leads research using climate change and variability predictions to maximise agricultural, urban and ecological water use opportunities with the CSIRO’s Oceans and Atmosphere Flagship.
(1) Source: https://www.flickr.com/photos/garry61
(2) Source: https://www.flickr.com/photos/yewenyi