This map shows the historical probability (given in percentile) of seasonal average monthly rainfall falling within the upper (wet), middle (normal), or bottom (dry) one-third ("tercile") of the 1981-2019 historical distribution in the country given the state of ENSO (El Niño, Neutral, La Niña) during that same season.
Here, the ENSO state for each season is defined according to the Oceanic Niño Index (ONI). It is calculated using Sea Surface Temperature (SST) anomalies, based on centered 30-year base periods updated every 5 years, in the geographical box (170˚W, 5˚S, 120˚W, 5˚N). A season is considered El Niño (La Niña) if it is part of at least 5 consecutive overlapping 3-month long seasons where the ONI is above 0.5˚C (below -0.5˚C). Use the controls on the page to select the season, rainfall tercile category of interest, and ENSO state. The analysis reproduces, using same SST dataset, the following definition from NOAA.
Clicking on the map will then display, for the selected point, yearly seasonal rainfall averages time series. The color of the bars depict what ENSO phase it was that year, and the horizontal lines show the historical terciles limits. This allows to quickly picture what years fell into what ENSO Phase and into what Rainfall Tercile category.
NB: This is not a forecast. It is based just on historical observations of rainfall and SST. However, it would be a good tool for exploring the effect of different ENSO phases on seasonal rainfall.
Reference for ENSO phases definition: V. E. Kousky and R. W. Higgins, 2007: An Alert Classification System for Monitoring and Assessing the ENSO Cycle. Wea. Forecasting, 22, 353–371. doi: http://dx.doi.org/10.1175/WAF987.1
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