A Neotropical Savanna

Back in October, I started looking at some precipitation data for our region and wondered whether we were beginning to see any effects of the current El Niño, which is predicted to continue through May 2007. Since I’m so new to the region, I needed to learn the normal patterns before I could understand how they might be affected by an El Niño-Southern Oscillation (ENSO) event.

My first questions were: 1) where do our rains come from, and 2) why do we have a wet season and a dry season in Panama? For questions like this, I step back as far as possible to get an overview. In this case, I started with the Sun and the Earth.

[For a professional illustration, click here.]

Because the Earth is a sphere, more or less, the Sun heats it unevenly, with the most radiation hitting near the equator and the least reaching the poles. As a consequence, the warm air at the equator tends to rise, and 3-dimensional convection cells are set up on either side of the equator. The warm air that rises moves poleward in the upper atmosphere, cools, and eventually sinks at approximately 30 degrees latitude, both in the northern and the southern hemispheres.

Meantime, that air rising from the equator has to be replaced. It is, by winds moving equatorward across the surface of the Earth. The motion of the air is affected by the rotation of the earth, so the winds, in both the northern and southern hemisphere are slanted from the east. Thus we have the NE Trades in the Northern Hemisphere and the SE Trades in the Southern Hemisphere.

Finally, the warm air that has risen from the equator holds plenty of moisture. As it rises, it cools and is unable to hold all that moisture. It begins to rain. This place between the Trade Winds, and where there is no wind and where it is raining, was called by sailors “The Doldrums.” Scientists call this region the Intertropical Convergenze Zone, or ITCZ, but for this exercise I will call it the Doldrums.

[For a professional illustration, click here.] It turns out that the Doldrums are responsible for our rain, and the movement of the Doldrums with the seasons, is responsible for our alternating wet and dry seasons.

As the Earth revolves around the sun, the latitude at which the maximum amount of heat reaches the surface, and therefore the latitude of the Doldrums, shifts.

The position of the Doldrums (the ITCZ) is also influenced by factors such as the presence of continents and prevailing pressure systems, their location in the Western Hemisphere is near the equator (far from Panama) in January

but near or above the equator (close to Panama) in July .

[For the source of these images, illustration, click here and scroll about halfway down the page.]

Given the migration of the Doldrums, then, how is Panama weather affected? First, winds over the ocean tend to blow in the direction of the highest sea surface temperature, and in general the highest temperature will be in the Doldrums belt. The NE Trade Winds become very strong when the Doldrums shift south, blowing toward that high temperature zone and holding onto their moisture, if not continuing to pick moisture up as the air moves across the surface of the ocean. At this time, the dry season for Panama and much of Central America arrives.

When the Doldrums shift back north, the NE trades are a little weaker in the area of Central America. The winds don’t have to travel so far to get to the high temperature zone. Remember that the high temperature zone, the Doldrums, is where warm, moisture-laden air rises from the earth and then, as it cools, drops its moisture in the form of rain. So as the winds approach the high temperature zone, that air, too rises and becomes part of the convection cell.

In a very general sense, the wet season is the time when Panama is itself virtually in the Doldrums. This season is also called the Central American monsoon season.

There is an interruption in the rainy season when the Trade Winds strengthen over the Caribbean during July and August.

How much rain is received depends on regional factors such as topography.

The weak winds of the rainy season blow across the isthmus if mountains are not in the way, and they drop their moisture load there. Even so, in the region of the Canal, where elevations are the lowest in the country, there is a distinct difference in the amount of rain dropped on the Caribbean and the Pacific sides of the ocean. In the precipitation graph above, Portobello is on the Caribbean side and Balboa on the Pacific side of the Canal. There can be as much as 15 inches per month more rainfall in Portobello than in Balboa.

When mountains are in the way, the differences can be even greater.

The mountains block the moisture-laden air as it tries to move across the isthmus. Rain will fall at higher elevations on both sides of the mountains. Some friends have kindly lent me 3-years’ worth of their data from a private weather station in Jaramillo at an elevation of about 3700 ft on the Pacific side of Volcan Baru, the tallest mountain in Panama. The city of David is south of Jaramillo, on the Pacific lowlands. In the graph above, the rainfall for Balboa and David, both on the Pacific lowlands, are seen to be fairly similar whereas the rainfall for Jaramillo can be as much as 25 inches per month greater than for David.

In a further complication of the monsoon picture, some rain comes in from the Pacific Doldrums as well. On the Pacific slopes of the mountains, air moves from the ocean onto land and then up the mountains, usually in the afternoon, bringing moisture and rainfall. Therefore, the isthmus can receive its moisture from both sides. Rain usually comes from the Pacific only during the rainy or monsoon season, and only in the parts of the country where the Caribbean wind and rain is not blocked by mountains.

There are some regional differences in rainfall during the dry season as well. Rain is more persistent on the Caribbean side – it is said that the Caribbean side of Panama has two seasons – wet and wetter.

If you stand at a lower elevation on the Pacific slope and look up at Volcan Baru in the dry season, especially in March or April, you can often see clouds massed behind the mountain, almost hugging it. Those clouds are on the Caribbean side, and enough of that moisture wraps around the higher elevations to bring some rain when none reaches the lowlands.

So the general picture of a wet and dry season is certainly made more complex when we begin looking at the details. Nevertheless, the global influences are worth understanding and will help shed light on the possible effects that an El Niño will bring to the region.

In summary, then,

1) Where do our rains come from? They come from the ocean – mostly from the Atlantic Ocean or Caribbean Sea – because the warm air over the ocean takes up water, rises, cools and loses its ability to hold moisture, thereby releasing that water back to the surface of the Earth. Some of it reaches Panama.

2) Why do we have a wet season and a dry season? Because as the Earth revolves around the Sun, the latitude at which the maximum solar energy reaches the Earth changes. When that latitude is near the equator, Trade Winds are strong and Panama experiences a dry season. When that latitude is north of the equator, Panama is near the Doldrums and experiences a rainy, or monsoon, season.

These events are explained by the Sun’s uneven heating of the Earth, which causes movement of air in the atmosphere, the establishment of convection cells, Trade Winds, and The Doldrums.

In the next post I’m going to look at how El Niño affects these seasonal patterns.