Tropical Precipitation and ENSO

Let’s take a look at how tropical precipitation (as reported by ERA) behaved in two larger ENSO events. One event happened during 1996-2000 and the other 2007-2011. Both periods included a large El Nino event and a significant La Nina in short order. Here’s an ENSO time series, with the two periods circled:


I’ll break the Indo-Pacific into two regions and call them “A” and “B”. “A” covers part of the Indian and Western Pacific Oceans. This could be considered the core of the Indo-Pacific Warm Pool.

“B” is in the central and eastern Pacific and experiences sea surface temperature changes associated with ENSO changes. B is not far from the downwelling leg of the Pacific Walker circulation where large amounts of air radiatively cool and sink. Its behavior during an ENSO event might be more complicated than area A’s.



First, let’s look at how the two areas behaved in 2007-2011. Here’s a plot of area B in 2007-2011. I have added the Oceanic Nino Index (ONI) which is a measure of the strength of ENSO events.

B’s precipitation tended to rise as the Pacific moved into El Nino conditions (indicated as positive ONI values) and declined as the area switched back to La Nina. The calculated correlation (r2) between rainfall and ONI over the five years was about 0.5, which is respectable. The area behaved in a reasonable way, as warmer sea surface temperatures means more evaporation and water vapor (the “fuel” for thunderstorms) and perhaps a shift to a less-stable atmosphere. Nothing surprising to me.


How did area A act? Here’s a plot of area A rainfall and ONI:

0127151Not much of a relationship. At least in this period, precipitation in A didn’t respond to  ENSO activity.

(A comparison of rainfall and area A SST (not shown) gave an R2 of .02, which is poor. That surprises me a bit as I expect higher SST in a rain-prone region to lead to higher rainfall and vice-versa.)


This period is interesting. The super-El Nino in 1997-8 and subsequent strong La Nina are in a period when there appears to have been a step-up in global temperatures. That step-up has been followed by an extended time when global temperatures, on average, have not risen.

First, region B, the area where ENSO behavior is most pronounced. As the plot below indicates, there is a strong correlation between rainfall and ONI. This is consistent with the behavior of the 2007-2011 period and with the general description of ENSO events. So, the reanalysis data in this overwhelmingly-unmeasured region fits my preconceived notions on how the global atmosphere works.


Now, a look at region A (the western Pacific). Unlike 2007-2011, there is actually a bit of a correlation with ENSO, with a noticeable depression during the super – El Nino of 1997-1998 and some elevation in the adjacent La Ninas:


That’s interesting and raises a question for further exploration – did the relationship between ENSO and region A rainfall change at the time of the hypothesized “climate shift” at the end of the 20’th century? Inquiring minds want to know.

Overall, my confidence in the reanalysis rainfall data in remote regions is OK, at least for the moment. Next I’ll look at broader trends in the tropics and see if they fit my existing layman’s understanding.

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