A Neotropical Savanna

During the 1997-98 El Niño, the Panama Canal watershed experienced the worst recorded drought in the Canal history. There was a 25% reduction in runoff toward the tributary lakes of the Canal and a 58% decrease of water flow toward Gatun Lake. Draft restrictions were imposed on the ships passing through the Canal, and the number of ships passing through decreased by 4% during the second trimester of 1998 compared to the previous year.

My question: Why does Panama experience a drought during an El Niño whereas some other parts of the world experience excessive rain and floods? In this graph from the National Oceanic and Atmospheric Administration (NOAA) of the United States,

we see that the weather in Panama and some of Central America, northern parts of South America, and most of the Caribbean experience dry and warm conditions during June, July, and August, which for Panama are the early months of the rainy season. Wet regions are in the Pacific Northwest of the US and the west cost of southern South America. During December, January, and February, the beginning of the dry season in Panama, conditions in Panama may not be significantly different from normal.

To understand what causes the particular conditions in Panama, we must first understand in general what El Niño is and then see how El Niño affects the normal weather patterns.

I will not try to explain El Niño in detail since there are so many good descriptions already available on the internet, including at Wikipedia, NOAA, University of Illinois, and other sites.

For my purposes, I need only remember that Trade Winds weaken during an El Niño and the normally cold water off the west coast of South America is replaced by warm water.

To compare what happens during an El Niño versus what happens during a normal year in Panama, the key is going to lie in what happens to the Doldrums, or the Intertropical Convergence Zone (ITCZ). Remember from the last post that when the Doldrums are close to Panama, it rains. When they move south, it’s dry.

Thinking along these lines, I predicted that the El Niño warm water off South America would influence the position of the Doldrums, drawing them south, as it were. If this happened in the rainy season, then once the Doldrums were moved further away from Panama, rains would diminish. If it happened in the dry season, there would be little effect on Panama. These predictions would explain the pattern shown in the above illustration, but I had to search the web for quite awhile before I asked google the right question and found the data I wanted.

It turns out that when NOAA wrote up an analysis of that 1997-98 El Niño mentioned at the beginning of this post, one of their findings was that there was a strengthening and equatorward shift of the intertropical convergence zone (ITCZ) in the Northern Hemisphere during June, July, and August. Prediction supported!

Once again, the Doldrums explains much – our normal weather and our El Niño weather. It all depends on location.

As far as why some other parts of the world experience floods and rains when we’re experiencing drought – well, I’m going to wimp out on that. My general rule of thumb is that during an El Niño, weather is opposite what you normally expect. For instance, the Pacific Northwest of the US normally has dry weather during the summer (June-July-August) but will have rain then during an El Niño year. The “why” will depend on regional conditions and I don’t know enough about those conditions to say why it happens in that region.

This year’s (2006-2007) El Niño is weakening and will not have anything like the impact of the El Niño of 1997-98, which was particularly strong. Since El Niño events may happen roughly every 3 to 8 years, we’ll be seeing another in the not too distant future. It will be interesting to see whether we can predict its impact on Panama just by watching where the Doldrums are.