The Coming of the Dry Season: The Failed Rains, the Christ Child, and Renewed Hope in the Indian Ocean

 

The current drought has been a devastating one - one of the worst in living memory. Entire communities' livelihoods have been destroyed, and we've been warned that these long rains will offer little relief in quantity and duration. But what's going on? What's behind it?

There are some terms you need to keep in your front pocket, as we shall refer to them continuously. These are the Indian Ocean Dipole (IOD), El Niño (EN) and La Niña (LN). Also, the El Niño-Southern Oscillation (ENSO). All these are phenomena related to the temperatures in the oceans. They have huge impact on rain. 

Here is how it works. Sea surface temperatures (SST) are a significant factor in precipitation (rain and snow). How? Remember your high school physics. Warm air has molecules further apart, meaning that they have space to 'host' more water molecules. Warm air holds more water. At the same time, you all know that when you have warm and cold water in close proximity, there will be movement, because cold water tends to sink and warm water rise. At very large (i.e. global) scale, that leads to ocean currents between warm (tropical) and cold (polar) regions.

Still here? Good. So what does that have to do with the current drought? Good you asked.

It has been known for a while now that temperature differentials between different parts of the ocean have huge impact on precipitation in different parts of the world. Let us start with EN. Most of us first heard of El Niño in 1997/98. It caused massive rains and flooding in East Africa, and @gatwiri_c has been documenting some long term changes that it brought about. Rivers changed their courses. Entire food crops disappeared. But what is it?

El Niño literally means 'Little Boy' in Spanish, and is named after the Christ Child. This is because when Spanish sailors first encountered the phenomenon in the 1600s in the Pacific, near Christmas. They named it El Niño de Navidad - the Little Boy of Christmas. An EN develops when the unusual warming of ocean water leads to changes in (typically weakening of) ocean currents. Trade winds, as well, change in intensity. As we said, warmer air retains more moisture, hence dumps more of it in the form of rain. 

Crucially for us, ENs typically see increased rainfall in the western Indian Ocean (WIO), east Africa included, and drier conditions in the eastern Indian Ocean (EIO), especially western Australia. You notice this see-saw effect? The WIO and EIO having opposite levels of precipitation? Think of them as two poles. In the Indian Ocean. Hence the Indian Ocean Dipole. The IOD has positive (WIO is warmer than the EIO), and negative (the opposite (or obost, as Kisiis would say)) phases. Each results in unusual precipitation patterns, and these swing from one to the other.

Now, cast your mind back to the end of 2019. Because of the onset of Covid a couple of months later, we forget the unusual weather then. It rained continuously in east Africa for months on end, with floods in some places. The rains continued relentlessly. At the same time, Australia recorded the worst bushfires in its history, with tens of millions of acres burned down, dozens of humans dead and at least 3 billion animals killed. Smoke from these fires moved halfway across the world to South America, causing some unusual sunsets.

These concurrent phenomena were the result of a positive IOD. If you were paying attention in this class (instead of hitting on your cute deskmate), you would remember that this is warm SST in the WIO, thus greater moisture in the air, hence more rain in the region. 

Ah, I can see your mind's gears grinding. Is the drought over the last three years as a result of a negative IOD? Congratulations (or congs, as Ujandans say). You get a cookie! (You can share it with your hot deskmate). Yes, there has been a negative IOD (which also coincides with La Niña). Cooler waters in the WIO, and warmer water in the EIO. As we have been baking to death, Australians have been drowning. As our short rains failed, they had devastatingly wet October and November 2022.

So, to the most important question. Has the IOD swung back to the positive? Is the current La Niña at an end, and is an El Niño possible? The answer you want to hear is 'yes', but the more accurate one would be 'perhaps'.  Weather and climate are fiendishly difficult to predict and model. Even the most powerful supercomputers struggle to incorporate all the inputs and spit out 100% accurate predictions. As an indication, the Indian Ocean Dipole was first described only in 1999. Indications are, to a certain degree of probability, that this 3 year-long LN and negative IOD are at an end. How quickly they will turn is uncertain, but there are signs of a possible EN and a positive IOD towards the end of this year. 

But this is too late to affect the long rains (or what are called March-April-May - MAM rains). As you saw in the news, these MAM rains will start late, and will be in much lower quantities than we require. We may see rain later this month, and even a few storms, but not enough. What about later in the year? Could the short rains (October-November-December, or OND), see a recovery? There are increasing (although yet uncertain) signs that we may indeed see much more intensive rains than we have seen this decade. If a strong El Niño forms like we had in 1997-98, there could be devastation to farmlands, urban infrastructure (roads being washed away, slums being inundated and the like), which is obviously just as catastrophic as the die-off in the Amboseli and southern Kajiado.

What then? That is the subject of  the next essay.