The Rain In Spain Stays Mainly The Same – Watts Up With That?

The article, basically quoting the study’s abstract without attribution, says:

Researchers say their findings have revealed a “wet-get-wetter, dry-get-drier” pattern driven by different mechanisms. Dynamic processes largely control wet extremes under land use changes, while changes in evapotranspiration control dry extremes. However, in a warming world, dynamic processes amplify wet extremes, while a reduction in moisture and weakened atmospheric circulation influence dry extremes.

So, are they right that the wet is getting wetter and the dry is getting dryer, either in the Maritime Continent or globally? We actually have the data to determine that. A 1° latitude by 1° longitude satellite-based rainfall record since 1979 is available from Copernicus here.

Upon reflection I realized that this is actually two different questions.

• Are wet areas getting wetter and dry areas getting drier?

• Are wet times of year getting wetter and dry times of year getting drier?

Since we are considering the question of trends, let me take a slight digression. My hypothesis is that a main one of the emergent phenomena that thermoregulate the planet are the tropical thunderstorms. Thunderstorms cool the surface in a variety of ways. They keep the temperature in the Pacific Warm Pool from ever exceeding around 30° – 31°C. So per my hypothesis, the recent global warming should have been accompanied by an increase in cooling rainfall in the tropics and particularly in the area of very frequent thunderstorms, the intertropical convergence zone (ITCZ) just above the equator. Here is a look at where it’s been getting wetter and where it’s getting drier since 1979.

Figure 3. Rainfall trend. The two panels are the Pacific and Atlantic views of the same data. Red lines enclose areas which are drying at the rate of -3 mm/decade or faster. White lines enclose areas getting wetter at the rate of 3 mm/decade or more.

Note that this bears out my hypothesis, in that there is increasing thunderstorm-driven cooling in the Pacific Warm Pool and the ITCZ. But I digress into theory. Let me return to observations.

There are some fascinating and surprising things about Figure 3. The overall trend is zero. Land is drying slightly, while the ocean is growing slightly wetter. Tropical land is drying the fastest, tropical ocean is getting wetter the fastest.

Rainfall in New Zealand is decreasing. Around the equator, the largest area of decreasing rain is in between two of the largest areas of increasing rain. North America is mostly unchanged, except for some drying in the Northwest Coast. The Southern Ocean has two areas getting drier and two areas getting wetter. Most of the world’s landmass is pretty neutral, neither getting much wetter nor much dryer, except for the southern Amazon which is getting drier.

Not seeing much pattern in all of that. Well, except for the fact that the actual observations agree with my hypothesis that global warming is opposed by increasing numbers, earlier daily emergence, and greater duration (lifespans) of cooling tropical thunderstorms.

Moving on, regarding the first question about wet and dry areas, here’s a scatterplot of the decadal trend in rainfall (vertical axis) versus the average annual rainfall (horizontal axis) for each 1° latitude by 1° longitude gridcell (n = 64,800).

Figure 4. Scatterplot, rainfall trend versus average rainfall in individual 1°latitude by 1°longitude surface gridcells. Global average rainfall is ~ one meter, so “wet” and “dry” areas are based on that threshold.

As you can see in Fig. 4 above, in areas where the rainfall is less than about two meters per year, there’s no “wet gets wetter, dry gets dryer” at all. It’s only in areas of very heavy rain, more than about two meters per year, that the wet is getting wetter. Here are the areas we’re talking about.

Figure 5. The only areas of the world that are getting wetter overall are the areas where the average rainfall is over 2 meters per year..

Note that, while most of this “wet gets wetter” area is over the ocean, by a peculiar coincidence, the Maritime Continent area in the study above is also in that area.

This would tend to indicate that no matter what they discovered by aiming their models at the 0.3% of the planet that is the Maritime Continent, their conclusions are not widely applicable to the rest of the planet.

Next question is, on the Maritime Continent are the wet times of year getting wetter and the dry times of year getting drier? To investigate that, we can look at the standard deviation of the rainfall dataset. If wet gets wetter and dry gets drier, the standard deviation will increase.

So let’s start with the actual monthly rainfall on the Maritime Continent.

Figure 6. Monthly average rainfall on the Maritime Continent.

When people ask what the weather In the Solomon Islands is Iike, I say “There’s the hot wet season, followed by the hotter wetter season”. The Copernicus dataset says that is true in the Maritime Continent as well, no surprise. Note that even the driest months in the Maritime Continent record are wetter than the global average monthly rainfall of 82 mm or so.

Using my Mark I eyeball, I’m not seeing any “wet gets wetter, dry gets drier” going on. Looks like the biggest swings are in the middle of the record. But let’s look to the measurements. Here is the 10-year trailing standard deviation of the rainfall on the Maritime Continent. Each monthly data point in Figure 7 below shows the standard deviation of the previous ten years (120 months) of rainfall.

Figure 7. Ten-year trailing standard deviation of the Maritime Continent rainfall data shown in Figure 6. Each point on the yellow line shows the standard deviation of the 120 months of rainfall data prior to that time.

Again, I’m not seeing any indication of “wet gets wetter, dry gets drier”. There are changes, but no overall trend.

Finally, what’s happening with the rainfall overall? Well … nothing. Here’s the global rainfall record.

Figure 8. Monthly global average rainfall, 1979-2024. The trend of the data is 0.05 mm of increasing rainfall per decade, basically zero.

A final oddity highlighting the curious stability of the climate system is that rainfall in the northern and southern hemispheres are in opposition — even after removing the seasonal variations, when one hemisphere is wetter, the other tends to be drier, and vice versa.

Figure 9. Monthly global average rainfall (black), along with northern hemisphere rainfall (blue) and southern hemisphere rainfall (red).

And that’s what I learned about the rainfall this week. Plus now I know what the Maritime Continent is. And here in the generally dry Northern California coast, it’s raining outside my window as I write this.

What an astounding planet!

w.

It Bears Repeating: When you comment, please quote the exact words you are discussing. I choose my words with care, and I’m happy to explain and defend them. But I can’t explain or defend your restatement of what you think I meant.