image: The upper panel illustrates how ENSO influences the equatorial Atlantic and Indian Oceans, and how these two ocean basins in turn feed back to ENSO. On the right side of the panel, the schematic shows the meridional circulation pattern during an active phase of the Atlantic Meridional Overturning Circulation (AMOC), the zonal mean sea surface temperature (SST), and the climatological position of the Intertropical Convergence Zone (ITCZ). The lower panel shows how Indian Ocean warming and ENSO influence the AMOC. It includes a plan-view schematic of AMOC and the meridional circulation pattern after AMOC weakens significantly, along with zonal mean SST anomalies and a southward shift of the ITCZ. ENSO can modulate the Atlantic Niño, North Atlantic Meridional Mode, and Indian Ocean Dipole Mode by directly altering the zonal Walker circulation (black arrows in the figure). In turn, these three modes can feed back on ENSO (indicated by red, green, and blue arrows). Warming in the Indian Ocean can alter the Walker circulation, leading to cooling in the equatorial Atlantic. It can also affect the North Atlantic Oscillation (NAO) via atmospheric teleconnections, thereby impacting AMOC. Additionally, warming-induced increases in Indian Ocean salinity can be transported to the North Atlantic by ocean currents, further influencing the AMOC. ENSO may also affect the NAO through atmospheric teleconnections and consequently modulate AMOC.
Credit: Ocean-Land-Atmosphere Research
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, undertook a review of recent research on the links between ENSO and the Atlantic and Indian tropical basins, and between tropics and mid-high latitudes.
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.”
This review was published on July 23 in Ocean-Land-Atmosphere Research.
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
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 our ability to predict these events has decreased. As well, modelling studies predict there may be a doubled 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 have 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 via
In another example of systems coupling, increasing temperatures in the Indian Ocean basin have been shown, via modelling, 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. 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,” Hu said.
Other Contributors include Ingo Richter, Application Laboratory, Institute for Value-Added-Information Generation, Japan Agency for Marine-Earth Science and Technology; Yuko Okumura, Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin; Natalie Burls, George Mason University; Noel Keenlyside, Geophysical Institute, University of Bergen and Bjerknes Centre for Climate Research and Nansen Environmental and Remote Sensing Center; Rhys Parfitt, Florida State University; Katinka Bellomo, Department of Environment, Land, and Infrastructure Engineering, Polytechnic University of Turin; Alessio Bellucci, Italian National Research Council, Institute of Atmospheric Sciences and Climate (CNR - ISAC); Riccardo Farneti, Earth System Physics Section, The Abdus Salam ICTP; Alexey V. Fedorov, Yale University, and LOCEAN, Sorbonne University; Brady S. Ferster, Yale University, and CECI, Université de Toulouse, CNRS, IRD, CERFACS; Chengfei He, Department of Marine and Environmental Sciences, Northeastern University; Qian Li, MIT, Cambridge and Florida State University, and Daniela Matei Max Planck Institute for Meteorology.
This work was stimulated by the summer school and workshop on “Theory, Mechanisms and Hierarchical Modeling of Climate Dynamics: Atlantic Variability and Tropical Basin Interactions at Interannual to Multi-Decadal Timescales”, which was hosted by ICTP and jointly funded by US CLIVAR, NSF, NOAA, NASA, CLIVAR, and ICTP. A.H. was supported by the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the U.S. Department of Energy's Office of Biological & Environmental Research (BER), via the National Science Foundation (NSF) IA 1947282 (DE-SC0022070). The National Center for Atmospheric Research is sponsored by the NSF of the United States of America under Cooperative Agreement No. 1852977. A.V.F. and B.S.F. are supported by the “Make our planet great again” program (ANR-18-MPGA-0001, France). A.V.F. was also supported by grants from the NSF (AGS-205309), NOAA (NA20OAR4310377), NASA (80NSSC21K0558), and U.S. Department of Energy (DE- SC0023134, DE-SC0024186). A.B. is supported by the Italian Ministry of Education, University and Research (MIUR) through the JPI Oceans and JPI Climate “Next Generation Climate Science in Europe for Oceans”, ROADMAP Project (D.M. 593/2016). N.K. acknowledges the support from the Trond Mohn Foundation (grant BFS2018TMT01) and the Research Council of Norway (grant 328935). B.S.F. acknowledges support from the European Union as part of the EPOC project (Explaining and Predicting the Ocean Conveyor; grant number: 101059547). However, the views and opinions expressed are only those of the authors and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them. D.M. and N.K. were partially funded by the JPI Climate & JPI Oceans ROADMAP Project (593/2016) and the European Union’s Horizon Europe Impetus4Change project (101081555). I.R. was supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (KAKENHI) (grant no. JP23K25946) and the Kyushu University Program for Collaborative Research (grant no. 2024CR-AO-2).
Method of Research
Literature review
Article Publication Date
23-Jul-2025
COI Statement
There are no conflicts of interest to declare.