Intense Drought in the Amazon

Communities across the western Amazon are experiencing the lowest water levels in over a century as drought intensifies across the region. Small towns that use the Amazon River and its tributaries for transportation and trade are effectively cut-off from the world. Residents grapple with dwindling water supplies, and children are stuck at home, unable to travel to school. With less water flowing in the Amazon, shipping routes up and down the river are impacted. The Amazon basin is known for its many rivers and the world’s largest rainforest, but it is no stranger to occasional drought. So what makes this dry period so extreme? The answer has to do with the current El Ni?o conditions.

El Ni?o Effects

El Ni?o, anomalously warm water in the eastern equatorial Pacific, is not just an ocean phenomenon. It is an ocean-atmosphere interconnection that affects weather across the entire globe. In tropical regions, the dominant weather pattern is the Walker circulation, which is the general movement of air in the troposphere over the tropics. The air above warm ocean water receives energy in the form of heat from the ocean and rises in updrafts, generating clouds and rain. As the air rises, it eventually reaches the tropopause - an area in the atmosphere where temperature increases with height. The tropopause acts as a lid, stopping rising motion and causing the air to spread out. The air eventually cools and sinks (subsidence), which reduces cloud cover and rain. The main areas of updrafts and subsidence move with changing sea surface temperatures, leading to significant adjustments in the weather depending on El Ni?o-Southern Oscillation (ENSO) conditions (as in, El Ni?o, neutral, or La Ni?a), as well as other large-scale changes in oceanic and atmospheric circulations.

In ENSO neutral conditions (above), the easterly trade winds over the central to western Pacific push warm surface water to the west. This causes upwelling off Peru, bringing deep, cool, nutrient-rich water to the surface. The cool water lowers the air temperature above it, which leads to subsidence and little rain. Farther east, the sun warms the South American continent, especially near the equator, causing a persistent area of rain and thunderstorms across the region. This steady source of rain feeds the Amazon River and its tributaries, as well as supports lush rain forests.

However, during El Ni?o (above), the easterlies over the western and central equatorial Pacific weaken or reverse entirely. This reduces upwelling and allows warm water to shift eastward toward Peru. The Walker circulation adjusts based on the new location of warmest water. The primary area of rain and storms moves to the central ocean, and subsidence dominates over the Amazon basin. The shifting weather patterns are felt strongly in South America, with drought being common over the northern part of the continent during El Ni?o years.

The Amazon Drought

The Amazon basin covers a large portion of South America - nearly 7 million square kilometers (2.7 million square miles), or about 35.5% of the continent. The Amazon River transports the largest volume of water of any river system, starting in the Andes Mountains to the west, and collecting water from approximately 1,100 tributaries on its journey to the Atlantic Ocean in northeast Brazil. Much of the basin is covered by dense tropical rainforest. The wet season varies from place to place across the region, but is generally December to May, when over 200 mm (8 inches) of rain can fall per month. The driest month is August, with an average of 50 mm (2 inches) of rainfall.

The river level at Manaus, the largest city in the administrative division of Amazonia, sits at the confluence of the Rio Negro and Amazon River. The city recently measured the lowest water level in the Rio Negro since records began in 1902, with a reading approximately 6 m (20 feet) below normal for October. The 90-day period from July to October is in the dry season, but due to the current El Ni?o, the 2023 dry season has been exceptionally rain-free. The rainfall map above demonstrates just how parched the region is. Almost the entire Amazon basin has had below-average rainfall since July, with some regions being 500mm (20 inches) below average. This means every tributary flowing into the Amazon River is low, with smaller ones even drying up. This has a compounding effect, impacting every part of the basin all the way to the sea. On top of lack of rainfall, the region has experienced multiple heat waves during the 90-day period, exacerbating water loss through evaporation.

The latest forecast for ENSO shows it is likely El Ni?o will continue through at least December, suggesting the Amazon drought will persist into the wet season. As river levels fall ever lower, communities in the Amazon basin will continue to struggle as vital supplies cannot reach them and drinking water quality degrades. Agriculture exports could be stranded upriver, as larger cargo vessels lose the ability to navigate the shallow waters, further impacting the local economy. Without relief in the near-future, residents of the Amazon basin will need to find creative ways to adapt to the dry conditions.