Rain, Clouds, Waves, and Sun -- what more could we want?

After several days that were characterized by relatively calm seas and only brief showers, we are now in a more tempestuous patch. The seas are rougher (lots of rocking and rolling like on the first days of the cruise), the rain showers are stronger, and the clouds are more persistent.

Such is the nature of the tropics -- and specifically, of the Inter-Tropical Convergence Zone. The ITCZ, as it is called (when it is not called the doldrums, that is!), shows up on a satellite images as a band of clouds that encircles the globe near the equator. This band of clouds can be thick and solid, extending hundreds of miles. Or, it may be broken into smaller lines of clouds with sunny skies in between.

The Intertropical Convergenze Zone (ITCZ) -- image from http://radar.weather.gov/jetstream/tropics/itcz.htm.

Along with the clouds, showers and occasional thunderstorms are common in the ITCZ. We've been in the region of the ITCZ since arriving at our study area, so we're not surprised by the changes in the weather. The calm seas with patchy clouds and brief mild showers that we experienced in the last days of February and first of March are as characteristic of the ITCZ as the more consistent cloudiness and stronger showers (squalls) we are experiencing now. We have not yet seen any lightening or heard any thunder (although distant thunder would certainly be drowned out by the sound of the ship), but we won't be too surprised if we do.

Large-scale movement of the ITCZ occurs seasonally. It is farther north in June, July and August; farther south in January, February, and March. This seasonal movement is what causes the rainy seasons in the tropics. But, although the large-scale movements of the ITCZ occur on seasonal timescales, the ITCZ also moves from day to day, albeit over much shorter distances.

The map below shows the position of the ITCZ today (March 4th) and two days ago. As you can see from our cruise track, we were just south the line marking the center of the zone of ITCZ cloudiness (the "axis" of the ITCZ) at the start of our work, but are now north of the axis.

Early March positions of the ITCZ relative to our cruise track (ITCZ positions from the NWS
tropical weather discussions web pages: http://www.weather.gov/view/national.php?prodtype=tropical).

This rapid small-scale movement of the ITCZ is not uncommon and is a cause of much of the variability of tropic weather in this region. Yes, we've been in the ITCZ since early on, but our position relative to the densest bands of clouds and the heaviest rains, is constantly changing.

So, although yesterday morning I managed to get a solid hour and a half of reading time out in the sunshine, this morning has been socked in with heavy clouds and squalls. Such is life in the tropics!