Sampriti Roy, University of Calcutta
In the images above, on the left, we see predicted change of ocean temperatures in 2050-2099 that’s relative to 1950-1999 by using an ensemble of climate models. On the right, we have predicted rainfall change in amplitude of rainfall fluctuations (year-to-year standard deviation) in 2050-2099 which’s relative to 1950-1999.
When speaking of ocean temperatures and rainfall variability, we will often come across the following terms: El Niño and La Niña. So, what are these phenomena?
ENSO: La Niña is the cool phase and El Niño is the warm phases of a climate pattern across the tropical Pacific—the El Niño-Southern Oscillation (“ENSO” for short).
La Niña: During La Niña, the surface winds across the entire tropical Pacific are stronger than usual. For the most part, the tropical Pacific Ocean is cooler than average during this phenomenon. Rainfall decreases over the central tropical Pacific (which is cool) and increases over Indonesia (where waters remain warm).
El Niño: During El Niño, we find that the surface winds across the entire tropical Pacific are weaker than they usually are. Ocean temperatures in the eastern and central tropical Pacific Ocean are warmer than average and rainfall is below average over Indonesia and above average over the central or eastern Pacific.
These swings between the wet and warm El Niño and the dry and cold La Niña hold significance because they impact weather conditions worldwide and consequently, impact agriculture, ecosystems, and economies. This is what makes it important for us to know what has been predicted by climate models about the aforementioned variations. It has been said that over the next 80 years, though there may only be very few temperature differences between El Niño and La Niña, there is said to be an increase in tropical rainfall in response to global warming.
In a study published in Communications Earth & Environment, we find reasons proposed regarding this prediction. Lead author Dr. Kyung-Sook Yun (from the IBS Centre for Climate Physics) has stated that all the climate models they studied have shown a pronounced intensification of yearly fluctuations in tropical rainfall in response to global warming. The study that comprises researchers from Environment and Climate Change Canada, the Korea Polar Research Institute, IBS Centre for Climate Physics at Pusan National University and the University of Hawaii at Mānoa used the latest crop of climate models to base their findings on.
Findings:
As found out by the research team, the key to understanding mechanisms that link future ocean warming to extreme rainfall in the tropical Pacific lies in the relationship between ocean surface temperature and tropical rainfall. They have provided us with 2 important aspects to consider:
- Rainfall sensitivity: The rainfall response to ocean surface temperature change.
- The temperature threshold for ocean surface for rainfall occurrence: Prof. Malte Stuecker (University of Hawaii) says: “In the tropics, heavy rainfall is typically associated with thunderstorms and deep clouds shaped like anvils. These only form once the ocean surface is warmer than approximately 27.5 degrees Celsius (81 degrees Fahrenheit) in our current climate.” The temperature threshold in case of intense tropical rainfall has been found shifting towards a higher value in warmer worlds.
In the warmer world, the atmosphere has a special characteristic. As stated by Prof. June-Yi Lee (a co-author from IBS Centre for Climate Physics), “a warmer atmosphere can hold more moisture which means that when it rains, rainfall will be more intense. Moreover, enhanced warming of the equatorial oceans leads to the upward atmospheric motion on the equator. Rising air sucks in moist air from the off-equatorial regions, which can further increase precipitation, in case other meteorological conditions for a rain event are met.”
The increase in sensitivity of rainfall, a key explanation as to why there more extreme ENSO- related swings in rainfall in a warmer world is an important area of research for climate scientists. With ocean warming being predicted to result in a twofold increase in the amplitude of rainfall fluctuations over the tropical Pacific, studying predictions made by climate models and research are the need of the hour.
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Source: “Increasing ENSO–rainfall variability due to changes in future tropical temperature–rainfall relationship” by Kyung-Sook Yun, June-Yi Lee, Axel Timmermann, Karl Stein, Malte F. Stuecker, John C. Fyfe and Eui-Seok Chung, 22 February 2021, Communications Earth & Environment. https://doi.org/10.1038/s43247-021-00108-8
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