I’ve noticed it’s been awfully wet for the dry season lately here in my part of Florida. Really wet. I found this kind of interesting: there are some people saying they expect an El Niño this year or next. Now, there is some association of El Niño here in this part of Florida with wet conditions, but even so, if El Niño is the “cause” of the increase in precipitation, why is precipitation increasing as if in anticipation of El Niño? This looks like a job for phase matching!
This time, to save time, I just used the raw SOI, took the average of 11 and 13 month centered averages, and identified the start of all periods where that index was negative (El Niño) or positive (La Niña) for at least 12 continuous months. The first such incident since 1895-when the US climate division data, and NCDC’s US data in general, began-was 33 months after January 1895, which gives us a long run up to the events in our composites. I average the events aligned by start month. I do the same with the monthly precipitation values for Florida Climate Division 6, but…that basically gave me weirdly aliased seasonal cycles. So I did it with percentage departure from the long term (1895-2013) mean for each month, and I also did it with the average of 11 month and 13 month centered totals. Now, SOI is inversely correlated with ENSO as defined by temperatures, and ENSO warm events are supposed to be associated with more precipitation and cold events with less (at least here, anyway), so SOI will be expected to be inversely related to precipitation. Additionally, the data are on very different scales. So, just for purposes of easier visual comparison and a better sense of lead/lag, I use linear regression of the time evolution of average ENSO events to predict the percent departure from long term mean, and the average of 11 and 13 month running totals. Again, this is just to put the ENSO events on the right scales for comparison-as a linear transformation of the time evolution of the event, it does not alter the basic shape. For La Niña events, things ended up looking like this:
The green is the actual precipitation, the blue is the evolution predicted by the SOI event at zero lag. On the left is the average of 11 and 13 month centered totals, on the right percent departure from the mean. The SOI does appear to very slightly precede the precipitation, more so as the event starts to end than when it starts to begin. La Niña does indeed appear to either cause, or be at least correlated with some cause, of reduced precipitation in Florida Climate Division 6. But when I did the same thing with El Niño, the result was a little different in an interesting way:
The green is the actual precipitation, the red is the evolution predicted by the SOI event at zero lag. On the left is the average of 11 and 13 month centered totals, on the right percent departure from the mean. Now this is different! Precipitation does, indeed, start to increase before the direction of SOI changes toward an El Niño! But it doesn’t start to decrease again until after the El Niño peaks. This means that the direction of causation here is actually ambiguous, if one even exists, but it also means that increasing precipitation in my part of Florida can be an indicator ahead of time that an El Niño is coming! And for that reason, I am predicting we will see an El Niño, and with it there will be more rain (here, anyway).