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Earth's climate is a dynamic system of interconnected physical processes and events occurring around the globe. Many of these processes are coupled so that changes in one event or process in one geographical area will result in changes in many other geographical areas. It is one of the challenges in climate science to understand the various processes, how they affect each other and how they change over time and space.

One of the major influences on climate variability is the El Niño–Southern Oscillation (ENSO). It in turn affects the tropical basins in the Atlantic and Indian oceans, which for the latter then feeds back to ENSO. This is an immense, coupled climate system that affects everything from oceanic temperatures to rainfall, to hurricane development or suppression in several areas of the globe.

With the increasing scrutiny on our climate systems, researchers around the globe, spearheaded by the Climate and Global Dynamics Laboratory at the NSF National Center for Atmospheric Research (NSF NCAR) in the US, have undertaken a review of recent research on the links between ENSO and the Atlantic and Indian tropical basins, and between tropics and mid-high latitudes.

This review is in Ocean-Land-Atmosphere Research.

Aixue Hu, first author and project scientist from NSF NCAR, said their "paper reviews the interactions among tropical ocean basins as well as those between the tropics and mid-to-high latitudes, with the aim of advancing our understanding of the multi-scale coupling processes within the global climate system."

In recent decades, ENSO has been classified as the central (CP) and the eastern equatorial Pacific Ocean (EP) ENSO. It can last from one to several years and has two phases, El Niño and La Niña.

In El Niño, the central or eastern equatorial waters in the Pacific Ocean are warmer than normal and the regular upwelling of colder, nutrient-rich waters is diminished.

In La Niña, the upwelling of the colder water is much stronger, and the water is colder than normal. Variations of ENSO's effect on weather and climate occur depending on many factors, including but not limited to when it began, how long it lasts, and what is occurring at other sources of climate variability, such as the Indian Ocean Dipole and the Atlantic Niño.

Looking at the recent research on ENSO, the researchers noted several trends. In the central equatorial Pacific Ocean, the El Niño events have become more frequent and the ability to predict these events has decreased. In addition, modeling studies predict there may be a double occurrence of the strong El Niños, while separate studies predict that multi-year La Niñas will also become more frequent.

As ENSO changes, so too do the tropical Atlantic Ocean and Indian Ocean basins that it influences, which then feed back to ENSO. For example, ENSO in the El Niño phase affects atmospheric circulation in the tropical Atlantic basin, which in turn increases wind shear and suppresses hurricane development. During La Niña years the opposite occurs, and hurricane development escalates.

"With global warming, feedback from the equatorial Atlantic and Indian Ocean to ENSO has intensified, thereby strengthening the two-way coupling among tropical ocean basins," Hu said.

One other important area of climate variability is the mid-to-high latitude Atlantic multi-decadal variability (AMV). This is a measurement of multi-decadal scale variability in anomalous sea surface temperatures in the Atlantic, either above normal or below.

The time scale for this variability is 60 to 80 years, and it has a wide-ranging effect on global climate systems, although there is still much that is not well understood about its underlying physical processes.

One of those processes is the Atlantic meridional overturning circulation (AMOC). It circulates relatively warm and salty water into the subpolar North Atlantic where it then cools, sinks and flows south again. The changes in its strength can modulate AMV by altering the amount of heat it transports into the North Atlantic.

In another example of systems coupling, increasing temperatures in the Indian Ocean basin have been shown, via modeling, to stabilize the AMOC.

There are two processes that cause this coupling. As the sea surface temperature increases in the Indian Ocean, a forced northward shift of the jet stream occurs with a resulting cooling of the water circulating south in the AMOC-associated sinking region and, in a second process, an increase in atmospheric vertical stability occurs. Both processes act to stabilize the AMOC.

The scientists also highlight several key areas for future research. Just a few of the questions they raise include: What are the processes underlying the other AMOC-induced changes that are affecting the tropical basins? What changes within these systems are caused by anthropogenic effects such as greenhouse gases?

"In addition, machine learning and artificial intelligence methods offer new opportunities to explore potential bidirectional coupling mechanisms between the tropics and mid-to-high latitudes, which may help advance our understanding of the complexity of the global climate system," Hu said.

"The ultimate goal of this research is to enhance our ability to predict the complex interactions among tropical basins and between the tropics and mid-to-high latitudes on sub-seasonal to decadal timescales, thereby better serving society."