Checking the El Niño effect using R

Today in San Cristóbal it rained heavily during the afternoon. This is very unusual in early February. There is statistical evidence that unusually heavy or unseasonal rainfall in Southern Mexico is associated with cold sea temperatures in the South Pacific (“La Niña”). R can be used to automatically download and conduct a simple graphical analysis of the el niño south pacific sea surface temperature data sets from noaa using stl to adjust for seasonal effects.

conect<-url("http://www.cpc.ncep.noaa.gov/data/indices/sstoi.indices")
d<-read.table(conect,header=T)
nino<-ts(d$NINO3.4,start=c(min(d$YR),1),frequency=12)
plot(stl(nino,s.window=12))

fiig12.png

(Click on the thumbnail to see a larger graph)

The top panel shows the raw data. The second shows the common seasonal pattern. The third panel is the most interesting as it shows the smoothed trend after seasonal effects have been accounted for. We can conclude that the seasonally adjusted sea surface temperature in January 2008 is the lowest for years. A closer look at the last decade can be made in R using a time window.

nino2<-window(nino,start=c(1990,1))
plot(stl(nino2,s.window=12))

fiig13.png

This result could indicate that the rains will begin early in 2008 and that early season storms could be intense. Although dry years tend to be associated with deforestation through uncontrolled fires, this may not be all good news for the subsistence farmers in Chiapas as models suggest that early season rain is responsible for severe erosion of top soil from fields planted with maize.

One thought on “Checking the El Niño effect using R

  1. Pingback: La niña and early rainfall « Duncan Golicher’s weblog

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