Raindrops keep falling on my head

Tropical cloud-watching never gets old.  Near the equator, the warm ocean generates a ton of fuel for cloud growth.  Over the course of a few hours, you can watch a small cloud grow into a tower tens of thousands of feet tall.  If you look at satellite images of the Pacific, you’ll often see a band of these clouds just north of the Equator.  That band is called the Intertropical Convergence Zone (ITCZ), which I’ve discussed before, and is the pillar of global atmospheric circulation.

Rainbow in the Marshall Islands
A developing storm brings a rainbow to the Marshall Islands. (photo by Diane Thompson)

The seasonal movement of the ITCZ over the Marshalls provides the country’s drinking water.  Needless to say, it’s pretty important; when it’s too far from the Marshalls, wells and catchments run dry.  However, it’s hard to simulate the ITCZ accurately in climate models, which makes its future behavior difficult to predict.  As a first step, though, we can look at how its location and strength has changed in the past.

Unfortunately, I haven’t found many good visualizations of the ITCZ through time, so I made my own.  This video shows monthly precipitation from 1979 through 2016, from NOAA’s Global Precipitation Climatology Project (GPCP).

Brief rant: Mapping the Pacific is hell.  Most files and programs pretend that the world ends at the International Date Line, which runs smack-dab down the middle of the Pacific.  So, when I tried to map the Pacific Islands, everything beyond the date line simply didn’t exist, and it took nearly a week to fix it.  The result, though, is worth it, because I can finally generate plots that show the outlines of the oft-ignored Pacific Islands and reefs!

What climate patterns can we identify here?

In this plot, you can see the rainy band of the ITCZ just north of the equator, shifting a bit north in the summer and south in the winter.  South of the equator is the South Pacific Convergence Zone, converging with the ITCZ near Indonesia, where the warm waters fuel the Australasian monsoon.

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Major rainfall (blue) and wind patterns (yellow arrows) in the western Pacific (credit: PCCSP).

In the winters of 1983-84 and 1997-98, you can also see the ITCZ move south of the Marshalls entirely, leading to a significant drought in the islands.  These droughts tend to happen during El Niños, including the 2015-2016 event that led to a national state of emergency.

In short, it pays to keep an eye on the sky!

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