A Seasonal Emerald in the Sahel

Posted: November 28, 2021 by oldbrew in climate, Natural Variation, satellites

Credit: geopoliticalfutures.com

A striking example of seasonal climate variation, with detailed images.
– – –
The Inland Delta of the Niger River is one of the world’s most productive wetlands says NASA, even though it is mostly dry for nearly half of each year.

Depending on the abundance and timing of rainfall upstream, the inland delta of the Niger River in Mali typically floods with water from roughly August to December.

The result is a seasonal burst of green vegetation at the intersection of the Sahara Desert and the Sahel.

Inland deltas generally occur where rivers split and branch out across inland depressions, valleys, or former lake beds, often in arid areas.

According to geomorphologist and NASA Earth scientist Justin Wilkinson, there are at least 86 inland deltas—sometimes referred to as megafans—spread across Africa. The Inland Niger Delta is the largest in western Africa.

The waters that bathe this delta originate in the Guinea Highlands, where wet season rains usually start to fall in July and then wind their way northeast into Mali on the Niger River.

Upon reaching the southern reaches of the Inland Niger Delta, the waters spread out across floodplains and swamps full of reeds and wetland grasses (particularly bourgou).

The northern portion of the delta is full of branching sand ridges and drying stream channels that emerge from the wetlands as the season progresses.

The progression of this greenup is shown in the natural-color images above [see link] from May, August, and November 2021.

Each image is a monthly composite view built from data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite. (The composite approach allows a cloud-free view in a region that can often be cloudy.)

The image below, acquired on November 16, 2021, by the Operational Land Imager (OLI) on Landsat 8, shows a closeup of the sand ridges in the northern half of the delta.

Continued here.

Comments
  1. Curious George says:

    The Lake Chad Basin is featured prominently on the map – why?

  2. oldbrew says:

    The Ups and Downs of Lake Chad
    https://www.earthobservatory.nasa.gov/images/91291/the-ups-and-downs-of-lake-chad

    Has a link to a more detailed report about its water levels…
    https://earthobservatory.nasa.gov/features/LakeChad

    That one suggests Lake Chad is in some way mirroring the climate cycles: AMO, ENSO e.g.:

    Okonkwo and colleagues showed that in the early 1970s, the AMO was in its so-called “cold phase,” meaning that oceanic and atmospheric circulation patterns shifted in such a way that the Sahel received less rain.
    . . .
    By 1999, some water filled northern Lake Chad year-round again. Okonkwo and colleagues think a transition of the AMO from a cold (dry) phase to a warmer (wet) phase has played a role. But the recovery has been slow, and lake levels are still below the 1900–2010 mean and far below levels of the 1960s.

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