A Neotropical Savanna

It’s going to take a while to piece everything together, so I’m starting with what we feel the current El Niño is doing to our weather pattern here in Panama.

First, a word about El Niño and La Niña. Without going into a description of what’s happening, here is how I remember the likely impact of one or the other on local conditions. During an El Niño the “little boy” opposes – the weather is opposite the average conditions. If it’s usually hot and dry in the summer, it will be cool and wet during an El Niño summer. During a La Niña, the “little girl” exaggerates – the weather is an exaggerated version of average. If it’s usually hot and dry, it will be hotter and drier during a La Niña summer.

Therefore, it now being the rainy season in Panama, and it being the beginning of an El Niño year, we would expect the weather to be drier than average.

Here’s a graph of data from weather underground for David, Panama. David is the third largest city in Panama and the closest weather station to our Neotropical Savanna. David is on the Pacific Coast of Panama. The data are for 2005 and 2006 (to date), the only historical data available at that site:

You can see that in 2006 (dashed line) we experienced more rain in May, June, and July than we did in 2005, and that beginning in August we experienced less rain than in 2005.

These numbers alone would suggest that we started feeling the effects of the El Niño in August of this year. I quote from the National Weather Service page that announced this El Niño:

Since early July weaker-than-average low-level equatorial easterly winds have been observed across most of the equatorial Pacific. In September the Southern Oscillation Index (SOI) was negative for the fifth consecutive month. Collectively, these oceanic and atmospheric anomalies are consistent with the early stages of El Niño in the tropical Pacific.

So the equatorial easterly winds weakened in July and by August we were not getting as much rain as we expected.

Of course, rainfall and its reasons are not that simple. The Environmental Science Program at the Smithsonian Tropical Research Institute has rainfall records for Barro Colorado Island (a preserve within the Panama Canal) going back to 1929. Here is rainfall from 2002 through August 2006, with the current year in blue:

The year-to-year differences are great, and it’s too early to tell whether rainfall for the remainder of 2006 will be significantly different than for those months in the previous four years. It is clear, however, that at BCI the rainfall in June was much less than average whereas in David (first figure) the rainfall that month was greater than the year before.

The real story will be revealed when we see how far this El Niño year deviated from the overall average. I found these precipitation numbers in A Guide to the Birds of Panama by Ridgely and Gwynne.

You’ll notice that the precipitation was highest for Portobelo, which is on the Caribbean side of the Canal, and lowest for Balboa, which is on the Pacific side of the Canal. I’ll have more to say about the significance of this in Part 2.

For this period of time, over all the sites sampled, the average rainfall in the dry season (Jan – Apr) was 4.18 inches per month and for the rainy seasion (May – Dec) it was 12.58 inches per month.

In researching how El Niño will affect Panama, I’ve studied general atmospheric and oceanic circulation patterns in the region and come up with some surprises (to me). For one thing, I learned that we get our precipitation from the Trade Winds that blow across the Caribbean, even those of us on the Pacific side of Panama. El Niño famously interrupts trade winds, but I anticipate myself.

I’ll be resuming my posting on plants this week and will work up the El Niño research over the next couple of months. I expect to post Atmospheric Circulation (Part 2) by the end of October and Oceanic Circulation, specifically Panama Bight (Part 3), by the end of November.