The 2015 El Niño continues to grow in strength and California is once more suffering flash floods and mudslides. It’s widely recognized that California gets wetter when a strong El Niño is running in the Equatorial Pacific, and the plot of the San Diego rainfall record against the Niño3.4 Index shown in Figure 1 confirms that this is indeed the case:
Figure 1: San Diego GHCNv2 rainfall record vs. the Niño3.4 Index. The data are 12-month running averages of 1870-2013 monthly means downloaded from KNMI Climate Explorer. I define a 12-month period when the Niño3.4 Index averages 1.0˚C or more or minus 1.0˚C or less as a “strong” El Niño or La Niña. The usual threshold for defining an El Niño/La Niña event is five consecutive months with the Niño3.4 Index over 0.5˚C/under minus 0.5˚C.
But what of the rest of the world? Does El Niño affect rainfall everywhere or just in a few places? And what does its widely ignored twin sister La Niña do? I looked into this question a few years ago, and here is a brief summary of the results.
First some more rainfall records. According to GHCN v2 data annual rainfall at Guayaquil, Ecuador, increases more than anywhere else on Earth during El Niños, and with a high proportion of unusually wet years:
Figure 2: Guayaquil, Ecuador GHCNv2 rainfall record vs. the Niño3.4 Index.
But there are stations where rainfall decreases during El Niños and increases during La Niñas, such as Fortaleza, Brazil:
Figure 3: Fortaleza, Brazil GHCNv2 rainfall record vs. the Niño3.4 Index.
And some where rainfall decreases during both El Niños and La Niñas, such as Tahiti. (Other stations that show this convex-up shape, including Honolulu, Christchurch and Ouagadougou, but there are none that show a well-defined concave-up shape):
Figure 4: Tahiti GHCNv2 rainfall record vs. the Niño3.4 Index.
And others that show no significant changes, such as Manchester, Seoul and New York (shown below):
Figure 5: New York GHCNv2 rainfall record vs. the Niño3.4 Index.
To summarize the impacts of El Niños on global rainfall I plotted the percent change in annual rainfall during strong El Niño events relative to average annual rainfall at 27 stations roughly evenly spaced across the Earth’s surface. The results are shown in Figure 6. Notable features are:
*Rainfall during strong El Niños increases by more than ten percent at Cape Town and Choilbasan in Mongolia, but only Guayaquil and San Diego record large increases.
*Rainfall decreases across most of the rest of the Pacific and Pacific Rim area, from Nome, Alaska to Tahiti and from Arequipa, Peru to Townsville, Australia. Significant decreases also occur at Fortaleza, Brazil and Jakarta.
*Rainfall impacts over the rest of the world are minimal. (Changes of less than 10% are judged to be within the limits of estimation error).
Figure 6: Percent change in annual rainfall relative to average annual rainfall when a strong El Niño is running , 27 stations
Figure 7 shows the same plot for strong La Niñas. The impacts are comparable to the impacts of strong El Niños. Townsville, Fortaleza and in particular Arequipa all experience rainfall increases which with the exception of Guayaquil are as large as any experienced during El Niño episodes.
Figure 7: Percent change in annual rainfall relative to average annual rainfall when a strong La Niña is running , 27 stations
A feature of El Niños is that they frequently transition directly into La Niñas, meaning that large variations in rainfall can occur in short periods of time. Figure 8, which plots the percentage differences between strong El Niño and strong La Niña average rainfall, shows the range of total variation at each station. This range provides a measure of the combined impact of El Niños and La Niñas, or if you like the overall strength of the Niño/Niña rainfall effect:
Figure 8: Percent change in annual rainfall between strong El Niño and strong La Niña conditions, 27 stations
Combined Niño/Niña effects are strongest in Northern South America and Southern California and appreciable in Northern Australia, Indonesia and Mongolia, but weak to negligible elsewhere.
These results are not totally definitive because a high-rainfall year does not necessarily guarantee floods and a low-rainfall year does not necessarily guarantee no floods. Nevertheless we can conclude that El Niños and La Niñas have a regional rather than a global impact on rainfall. But the world sees the hand of El Niño in extreme precipitation events everywhere. El Niño has already taken the blame for floods (and droughts) in areas where it has minimal impact on rainfall, as claimed in this 2002 BBC article, and it’s now being blamed for disasters that have yet to happen, such as the coming devastating floods in Africa and the coming snowy winter in UK . Curiously, however, its twin sister La Niña rarely takes the blame for anything.