In the Atlantic Ocean, there operates a large system of ocean currents, through which the waters are circulated between the north and the south. Termed Atlantic Meridional Overturning Current, or AMOC, this system is such that the waters of the oceans are continually mixed, and heat and energy get distributed around the Earth. A recent study states that warming of the Indian Ocean waters is causing a series of cascading effects that is providing AMOC a “jump start”, i.e. intensifying the circulation, according to a report in September 2019.
AMOC has been stable for thousands of years. The last time AMOC weakened substantially was 15,000 to 17,000 years ago, and it most probably had a global impact. There would have been harsh winters in Europe, with more storms, or a drier Sahel in Africa due to the downward shift of the tropical rain belt. Data since 2004, as well as projections, however, have given some scientists a fresh concern. Indications are that AMOC may be slowing, something that could have severe effects on global climate. But it is not known whether the signs of slowing in AMOC are a result of global warming or only a short-term anomaly. The concern is regarding whether the warming of the Indian Ocean is boosting a key system of currents in the world’s oceans and how long this enhanced Indian Ocean warming would continue. If other tropical oceans’ warming, especially the Pacific Ocean, catches up with the Indian Ocean, the advantage for AMOC would stop.
The new study has been authored by Fedorov and Shineng Hu of Scripps Institution of Oceanography, and it has been published in Nature Climate Change. Yale University collaborated with Shineng Hu of the Scripps Institution of Oceanography at the University of California-San Diego to model different AMOC scenarios.
How AMOC works
The AMOC has been likened to a conveyor belt in its working. As warm water flows northwards in the Atlantic, it cools, even as evaporation brings about an increase in its salt content. The density of the water is raised by the low temperature and a high salt content, and this makes it sink deep into the ocean. The cold, dense water deep below slowly spreads southward. Eventually, it gets pulled back to the surface and warms again, and the circulation is complete. This continual mixing of the oceans, and distribution of heat and energy around the planet, has an impact on global climate.
Details of the Study
The work involved climate mechanisms that may be shifting due to global warming. The researchers wanted to investigate the impacts of rising global temperatures on critical climate systems like the AMOC. The AMOC transports warm water to the North Atlantic and sends cold water at deeper levels farther south.
Researchers combined observational data and computer modelling to map the effects of rising temperatures in the Indian Ocean on the AMOC. They plotted out what effect such shifts might have over time. They looked at warming in the Indian Ocean, as the ocean is seen as one of the fingerprints of global warming.
It was found that as the Indian Ocean warms at a rapid pace, it generates additional precipitation. This draws more air from other parts of the world, including the Atlantic Oceam, to the Indian Ocean. So there may be very high precipitation in the Indian Ocean, and less precipitation over the Atlantic Ocean and others. Less precipitation will lead to higher salinity in the waters of the tropical portion of the Atlantic , as there won’t be as much rainwater to dilute it. This saltier water in the Atlantic, as it comes north through the AMOC, one of the largest water circulation systems in the world, will get cold much quicker than usual and sink faster as it travels along the AMOC. This is referred to as the AMOC slowing down. Researchers agree that the rippling consequences of the AMOC slowing down would be felt intensely in Europe and along the Atlantic Ocean rim.
The study thus identifies a climate variable that could have a profound impact on ocean currents and weather patterns as temperatures increase and climate change intensifies.
Another oceanic system in news is the El Nino-Southern Oscillation (ENSO) which involves temperature changes of 1°-3°C in the central and eastern tropical Pacific Ocean, over periods between 3 and 7 years. El Nino refers to warming of the ocean surface and La Nina, to cooling of the ocean surface; ‘neutral’ refers to something between these extremes. The alternating pattern has an impact on distribution of rainfall in the tropics and can influence weather in other parts of the world.