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Reduction in Amazon’s discharge into the ocean may cause floods, severe drought

  • Photo : Spoorthy Raman / Research Matters
    Photo : Spoorthy Raman / Research Matters

What would happen if the water from the Amazon -- the river that discharges the largest volume of water into the ocean - were to reduce?

In their study published in Nature’s Scientific Reports, Mr. Sharif Jahfer , Dr. P. N. Vijaychandran and  Dr. Ravi S. Nanjundiah from Centre for Atmospheric and Oceanic Sciences (CAOS),  Indian Institute of Science have explored how a decrease in the discharge would affect the climate in the region both local and remote effecting scary changes with some parts of the world expected to face extreme floods and others, severe droughts.

The effect on Indian climate is apparent too. “Amazon runoff has an impact on Indian monsoon rainfall, which we are pursuing separately”, says Mr. Jahfer. The team is expanding this work to study other major river systems as well. “Preliminary results suggest that the major river discharges into the ocean do have a significant climatic impact. But the magnitude and spatial distribution of the impact depends on many factors including the magnitude of runoff into the ocean, latitude of the river mouth, total ocean surface area dominated by river plume and its spatial distribution, and local air-sea interaction”, he adds.

“Amazon is the single largest river in terms of runoff into  the ocean. Discharging about 6.6 × 103 km3 (volume equal to that carried by 2.6 billion Olympic size swimming pools!) of freshwater into the ocean annually, it accounts for about 50% of the total river runoff into the Atlantic Ocean” says Mr. Sharif Jahfer of the Centre for Atmospheric and Oceanic Sciences (CAOS) at the Indian Institute of Science, Bangalore.

Currently there are approximately 100 dams on the various tributaries of the Amazon river. In the coming decades, 100 more dams are projected to be built. This, along with the climate projections, indicates that, in the years to come, the freshwater runoff from the river into the
oceans will considerably decrease.

Why should the Amazon river runoff affect the climate at all? “In terms of freshwater budget, tropical Atlantic loses huge amount of freshwater, as more water is evaporated from the river than the amount that rains. A part of this freshwater deficit in the tropical Atlantic is compensated by the freshwater input from Amazon river. So, the Amazon runoff has a significant role in the freshwater budget of Atlantic Ocean”, says Mr. Jahfer.

Previous studies using climate models have neglected the effect of runoff into the ocean, and this study establishes that the global river discharge into the ocean has a significant impact on large-scale climate, including El Nino Southern Oscillation (ENSO) and Indian summer monsoon rainfall.

Using model simulations, the researchers found out that when the Amazon runoff was blocked, the ocean’s surface turned saltier near the Amazon’s mouth. The ocean responded by strengthening Atlantic Meridional Overturning Circulation (AMOC) -- a major current in the Atlantic Ocean which consists of a northward flow of warm salty water in the upper layers of the Atlantic, which is incredibly important for the planet’s climate patterns. Reinforcing of the AMOC would lead to warmer water going northward and cooler waters coming southward, resulting in a cooler-than-normal southern equatorial Atlantic.

The colder ocean, then would lead to weakening of the atmospheric meridional circulation, or general airflow along the Earth's longitudinal lines. This would. in turn, diminish the strength of the winds over the extratropical Atlantic and the land masses adjacent to it. A weakening of these air flows indicates an overall weakening of the Northern Atlantic Oscillation (NAO), which is a prominent mode of climate variability in the extratropics, responsible for phenomena like storms over the Mediterranean region and cold winters in North America or Europe.

In addition, the researchers observed that the mean climate over Europe and North America would change, with northern Europe and the eastern United States experiencing cooler winters, and Canada getting warmer. Northern Europe, including Iceland and Scandinavia, would have less-than-average rainfall, while southern Europe would get wetter.