#6 La Niña 2017
"Global warming?! But it's snowing?!" - I don't
know how many times people have said something along these lines
to me, denying that we are making our planet warmer based on some extreme
events that suggest the opposite. This is a theory (...a very bad
one) that I wish to dispel with this post.
Bit of background, there exists major, short-period climatic signals which can have significant effects on sea-level and atmospheric pressure, such as the North Atlantic Oscillation and, of particular importance to this post, an irregular inter-annual cycle called the El Niño Southern Oscillation (ENSO). ENSO is actually the planet’s most significant cause of interannual climate variability, alternating between a “positive” El Nino and a “negative” La Nina. These 2 events have extremely widespread impacts on weather patterns. The cause of these alternations are still not fully understood, but 2 hypotheses suggest it is either a self-sustained and naturally oscillatory mode of the ocean-atmosphere system, or triggered by stochastic forcing. Either way, it is down to a positive feedback in the ocean-atmosphere.
'El Niño' is the warming of sea surface temperature typically concentrated in the central-east equatorial Pacific - and 'La Niña' - what we will experience in the coming months - is the opposite side of the fluctuation, showing cooler-than-normal sea surface temperature in the equatorial/central Pacific.
I mentioned in the EEA blog that there are certain natural variabilities in the climate system that can influence extreme weather, so when this article popped up yesterday I couldn't resist sharing it with you for a recent example.
I don't want to go into this in too much detail as when I
discussed the blog's key question with Dr Brierley, he suggested it would be
more important to focus specifically on the science behind extreme event
attribution and climate change's role in weather patterns, but that these
natural variations are something he "keeps an eye on". So, this is
what this post aims to do! The article mentions the most powerful La Nina on
record (2010) and its impacts ranging from drought in East Africa to record
snowfalls in the US – pretty global right?
Bit of background, there exists major, short-period climatic signals which can have significant effects on sea-level and atmospheric pressure, such as the North Atlantic Oscillation and, of particular importance to this post, an irregular inter-annual cycle called the El Niño Southern Oscillation (ENSO). ENSO is actually the planet’s most significant cause of interannual climate variability, alternating between a “positive” El Nino and a “negative” La Nina. These 2 events have extremely widespread impacts on weather patterns. The cause of these alternations are still not fully understood, but 2 hypotheses suggest it is either a self-sustained and naturally oscillatory mode of the ocean-atmosphere system, or triggered by stochastic forcing. Either way, it is down to a positive feedback in the ocean-atmosphere.
'El Niño' is the warming of sea surface temperature typically concentrated in the central-east equatorial Pacific - and 'La Niña' - what we will experience in the coming months - is the opposite side of the fluctuation, showing cooler-than-normal sea surface temperature in the equatorial/central Pacific.
Let’s take a slightly closer look
Take the Ethiopian drought of 2015 (influenced by a strong
El Nino) and Somalian drought of 2016 (influenced by La Nina). In both cases,
the natural variations, although technically ‘opposites’ caused the same drying
in neighbouring regions. I think this exemplifies quite neatly how complex they
can be.
In both cases the variations created a drier environment, in
Ethiopia delaying the important Kiremt rains of the region and turning the
event’s return period to 1-in-260 years from a 1-in-80 year return period, and
in Somalia proven to reduce precipitation by 33%. However, it should also be
noted that there is not only limited existing data, but also very high natural variability and large-scale teleconnection patterns in these regions, and thus if
anthropogenic climate change had been responsible it would be difficult to identify its signal.
Hi Louise, great blog.
ReplyDeleteI have a question regarding this post.
When you mention the creation of a drier environment delaying the important Kiremt rains in Ethiopia you mention the event’s return period becoming a 1 in 260 year event from a 1 in 80 year event.
I am slightly confused. Your blog is concerned with the negative influence mankind has on the climate yet at first sight the above numbers surely indicate a positive change, as the delayed rains are becoming a LESS frequent occurrence?
Hope you can help me understand this point.
Hello Bryan! Thank you, so happy to see you are reading them!
DeleteSo, the El Nino is a natural phenomenon that would still occur on a Planet Earth that had no anthropogenic climate change. Scientists found through various models that the Ethiopia drought was rare, and that the El Nino had made this rare drought, even rarer, as it made the drought even drier. As a result, the event was made even more extreme as a result of El Nino. However, the high natural variability that these regions experiences produce a lot of 'noise'. Therefore, if anthropogenic factors were involved in either reducing the rains, delaying them, or increasing drought, due to its relatively low role vs natural variability it would have been impossible to spot in the models and as a result they cannot say climate change had a role.
Keep the questions coming!
Louise