Tropical Precipitation

The website KMNI Climate Explorer is an excellent source of climate data. The site offers access to a wide variety of data in user-friendly ways.

Among Climate Explorer’s many offerings are daily precipitation estimates.  The word “estimate” is important as estimates are all we have for remote, sparsely-measured regions such as tropical oceans. One estimate method is known as “reanalysis”. Reanalysis has limitations in accuracy so its data must be viewed with the proverbial “grain of salt”. Nevertheless, it can be helpful.

The daily reanalysis data I use in this post is known as ERA-interim, offered by the highly-respected ECMWF . My focus is on precipitation in the tropics. The area I chose (15N to 15S) covers about 25% of Earth’s surface. Here’s the rainfall plot covering the last 25 years (1990 through 2014):


The plot shows is a regional average of about 5.1 mm/day (about 80 inches of rain a year). That accounts for roughly 50% of global precipitation, which is a significant amount.

Importantly, tropical precipitation is a process that transports energy (heat) from the surface into the mid and upper troposphere, energy that is important to maintaining the global circulation and to warming latitudes away from the tropics.

Interestingly, precipitation processes are thought to remove about 80 W/m3 of energy from Earth. Since tropical precipitation (15N-15S) accounts for about half of global precipitation we can reasonably guess that tropical precipitation is involved in roughly 40W/m3 of global heat removal. That’s an order of magnitude greater than the expected warming impact of doubling CO2 (4W/m3, assuming no feedback). It is conceivable that relatively small changes in tropical precipitation could affect global heat transport and/or the warming rate, at least in the short term.

Here, using monthly rather than daily data, is a plot of the tropical precipitation anomalies since 1979.  The change over time is interesting and worth later exploration. The decline in tropical rainfall seemed to end about the time of the 2000 “climate shift” and switch to a rise. The period of decline roughly corresponds to the period of rising global temperature and the period of rainfall increase to the “hiatus”. Relationship? I have no idea.


In future posts I’ll explore some of the wiggles and shifts in tropical precipitation. Experience outside of climate science has taught me that much can be learned by understanding the finer details and characteristics of data – what might be thought of as noise can actually be meaningful. My major goals are to improve my limited understanding of how the global atmosphere works and to see if there are patterns which could have predictive value.

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