Wet and Green from Hot and Dry

The biggest desert on Earth feeds the Amazon and spawns hurricanes

Mar 06, 2023

Why are ironies so appealing? There is just something compelling about apparently logical contradictions that turn out not to be contradictory at all! The climate system is not immune to such conundrums. Here is one: it is warmer air invading the upper atmosphere in the Arctic that breaks the polar vortex and leads to extremely cold weather in the temperate zone, as experienced across the U.S. this winter. Colder local weather through a warmer Arctic.

Here's another: How can the largest expanse of dry desert in the world contribute to the growth of the lush tropical forests of the Amazon basin, and spawn some of the most violent Atlantic hurricanes? The Sahara Desert in Northern Africa plays both of these roles, and holds another surprise as well - freshwater lakes that persist in the midst of this super-arid region. Can there be a science story here that encompasses all of these apparent contradictions?

There can be and is, and it starts here: The Sahara has not always been so dry nor so barren.

A Greener Sahara

The Sahara Desert is just where it should be. Textbook descriptions of global weather circulation systems describe what are called Hadley and Ferrell (or mid-latitude) Cells, generalized patterns of circulation that predict regions of warm, dry, high pressure centered at about 30 degrees north and south of the equator, or one-third of the way from the equator to the poles.

Dry surface winds heading back towards the Equator, completing the Hadley Cell cycle, are deflected to the west by the rotation of the Earth, creating the trade winds shown in this diagram, and known to sailors for centuries. Because of these dry surface winds, many of the major deserts of the northern and southern hemispheres are to be found between 20 and 30 degrees north and south of the equator (examples include the Mojave and Sonoran deserts in the U.S. and Mexico, the Namib and Kalahari in southern Africa, and the deserts of the Australian Outback).

That textbook description and figure annoy many weather professionals because the nice clean boundary between the two cells in the diagram is wildly variable on a day-to-day basis, resulting from or causing the variation in daily weather.

And even the climatological averages that now define the Sahara have varied in the past, and, geologically speaking, the recent past.

The African Humid Period, from about 15,000 to about 5,500 years ago, was a time when rainfall was more abundant, vegetation greener, and human occupation greater in the Sahara than it is now. How do we know? Sediments collected from lakes in the region contain pollen from species of trees and shrubs that could only exist in a more benign environment. The discovery of rock paintings within the heart of what is now largely uninhabitable desert show that the region once supported a greater human presence as well.

And just the fact that there are lakes at all in this arid region also points to a very different past.

The Ounianga Lakes in northeast Chad exist in this extremely dry climate because they are underlain by a huge aquifer which reaches the surface at this location, providing a continuous water source that can outpace evaporation. The pattern you can see in the shape of these lakes results from the movement of sand dunes across a once-continuous lake basin, and also shows that the dominant wind direction is from the northeast.

A huge aquifer implies a previous period of water infiltration, which in turn means the presence of a lake holding surface water that can percolate down into that aquifer.

And during the Humid Period, there was just such a lake.

At the end of the Humid Period, Megalake Chad occupied ~390,000 square miles in what is now Chad, Niger, and Nigeria, including the modern location of the Ounianga Lakes. That area is greater than today's Caspian Sea or the combined area of all the Great lakes. Flat topography defined a lake that was never more than 150 feet deep. As such, small changes in the total water content of the lake translated into large changes in surface area or extent. By 1983, the lake had been reduced to less than 10,000 square miles, and by 2000 to less than 600 square miles. The actual extent can vary widely year-to-year depending on rainfall.

Fertilizing the Amazon

It is this drying and contracting of Megalake Chad that leads to the fertilizing power of the Sahara.

The region once occupied by this huge body of water is now known as the Bodélé Depression and the sediments left behind by the evaporating waters are rich in the remains of diatoms that grew in the waters of that lake during the Humid era.

Diatoms are single-celled algae that incorporate significant amounts of silicon into their cell walls. This causes them to sink when they die, carrying silicon and other elements, notably the nutrient element phosphorous, with them. As a result, the sediments exposed by the drying of Megalake Chad are relatively rich in phosphorous. They are also loosely packed and easily launched into the air by the surface winds that tend to blow from the northeast across the Bodélé Depression, accelerated by the alignment of mountains upwind that funnel those winds across the old lakebed.

The result? Some of the biggest dust storms on the surface of the Earth that can happen as often as 100 times in a year. And while the dust lifted by those winds retains the high silica content of those diatoms, it is the phosphorous content that creates the link to fertilizing the forests of the Amazon.

NASA satellites have captured dramatic images and videos of these dust storms as they reach the west coast of Africa and spill out over the Atlantic, heading for South America. Those 100 or so storms per year transport about 50 million tons of material across the Atlantic, about 25 million tons of which settles in the Amazon. Phosphorus makes up about 0.1% of the dust, so about 25,000 tons of the element are added to the Amazon ecosystem each year.

And why is phosphorous so important? A recent essay here presented the Hawaiian Islands as a natural laboratory for studying the development of soils over millennia under tropical climates. The take home message relevant to this essay was that the oldest tropical soils tend to be very bad at providing the phosphorous needed for plant growth (phosphorous is the P in your NPK lawn and garden fertilizer). Phosphorous in native ecosystems comes mainly from the chemical breakdown (weathering) of rocks, and old tropical soils are, literally, weathered out. The dust from the Sahara provides fresh material that can weather over time - it's like a slow-release phosphorous fertilizer.

So how about hurricanes?

The NOAA site listed under Sources below speaks directly to our quest for irony by saying: "You might think this sounds a little crazy because hurricanes are very wet and deserts are very dry, but if it weren't for this huge, hot, dry region in North Africa, we would see far fewer hurricanes in the United States."

The same wind patterns that at the surface convey dust to the Atlantic can also, at a higher altitude, deliver heat, moisture and turbulence that can breed hurricanes.

Hot dry air rising from the Sahara contrasts sharply with the cooler and more humid air over the Gulf of Guinea to the south and the humid forests along the Gulf’s northern coast. Turbulence at the boundary of these air masses creates the African Easterly Jet, a high elevation, high speed air stream, again moving toward the Atlantic, carrying with it the potential for thunderstorms. If conditions are right, these thunderstorms can seed a major hurricane. Those "right" conditions would include unusually warm ocean temperatures and the absence of "wind shear" or strong upper level winds from the west that can blow a budding hurricane apart.

The importance of the African Easterly Jet in the formation of hurricanes is captured in this graphic showing the paths of tracked historical hurricanes. Note how often hurricanes are born just off the coast at the boundary between the Sahara and the moist forests to the south. Many of the strongest storms are first formed here and, traveling east to west, gain strength by passing over very warm ocean waters. Appraoching the Americas, storms are often turned to the north by interactions with the Gulf Stream and associated wind patterns.

Depending on your source, between 72% (NASA) and 83% (BBC) of major hurricanes striking the east coast of North America originate off the west coast of Africa, and can trace their origins, at least in part, to the hot, dry Sahara.

So, ironies and conundrums. Extreme heat and dryness, increased at the waning of the African Humid Period, lead to dust storms that fertilize the Amazon as well as turbulence and thunderstorms that can seed hurricanes, the most powerful energy machines in the global weather system.

And what of climate change? Will the Sahara be green again, and would that reduce both dust loss and energy for hurricanes?

That topic is too large to append to this essay, but suffice it to say that hypotheses vary. Aware that a subtle shift in the average location of major wind patterns led to a greener Sahara in the fairly recent past, some suggest that this could happen again. On the other hand, desertification of the Sahel region just to the south of the Sahara through human actions could cause the desert to expand. And whether a greener Sahara would affect hurricane formation seems unclear. In recent years, warmer waters in the Atlantic have been shown to increase the rate at which existing hurricanes intensify, and the total energy released each year by Atlantic hurricanes has been increasing. Could a greening of the Sahara offset these trends?

That entirely insufficient last paragraph is just to reinforce the idea that any one place or process in the Earth system is tightly coupled to the global weather/climate machine. The global climate system is changing. The Sahara might change again as a result, and any change will be sure to ripple across oceans and continents.

Sources

The graphic of general global air circulation is from:

https://en.wikipedia.org/wiki/Hadley_cell

https://commons.wikimedia.org/wiki/File:Earth_Global_Circulation_-_en.svg

Information on the African Humid Period is from:

https://en.wikipedia.org/wiki/African_humid_period

https://www.pnas.org/doi/10.1073/pnas.2024898118

https://www.nature.com/scitable/knowledge/library/green-sahara-african-humid-periods-paced-by-82884405/

The image of the Ounianga Lakes is from:

https://earthobservatory.nasa.gov/images/41425/ounianga-lakes-sahara-desert-chad

and of Megalake Chad is here:

https://earthobservatory.nasa.gov/images/146304/remnants-of-an-ancient-lake