FY 2019

Pantropical climate interactions

Cai, W., L. Wu, M. Lengaigne, T. Li, S. McGregor, J.-S. Kug, J.-Y. Yu, M.F. Stuecker, A. Santoso, X. Li, Y.-G. Ham, Y. Chikamoto, B. Ng, M.J. McPhaden, Y. Du, D. Dommenget, F. Jia, J.B. Kajtar, N. Keenlyside, X. Lin, J.-J. Luo, M. Martin-Rey, Y. Ruprich-Robert, G. Wang, S.-P. Xie, Y. Yang, S.M. Kang, J.-Y. Choi, B. Gan, G.-I. Kim, C.-E. Kim, S. Kim, J.-H. Kim, and P. Chang

Science, 363(6430), eaav4236, doi: 10.1126/science.aav4236, View online (2019)

The El Niño–Southern Oscillation, which originates in the tropical Pacific, affects the rest of the world's tropics by perturbing global atmospheric circulation. Less appreciated than this influence is how the tropical Atlantic and Indian Oceans affect the Pacific. Cai et al. review what we know about these pantropical interactions, discuss possible ways of improving predictions of current climate variability, and consider how projecting future climate under different anthropogenic forcing scenarios may be improved. They argue that making progress in this field will require sustained global climate observations, climate model improvements, and theoretical advances.

BACKGROUND: Ocean-atmosphere interactions in the tropics have a profound influence on the climate system. El Niño–Southern Oscillation (ENSO), which is spawned in the tropical Pacific, is the most prominent and well-known year-to-year variation on Earth. Its reach is global, and its impacts on society and the environment are legion. Because ENSO is so strong, it can excite other modes of climate variability in the Atlantic and Indian Oceans by altering the general circulation of the atmosphere. However, ocean-atmosphere interactions internal to the Atlantic and Indian Oceans are capable of generating distinct modes of climate variability as well. Whether the Atlantic and Indian Oceans can feed back onto Pacific climate has been an ongoing matter of debate. We are now beginning to realize that the tropics, as a whole, are a tightly interconnected system, with strong feedbacks from the Indian and Atlantic Oceans onto the Pacific. These two-way interactions affect the character of ENSO and Pacific decadal variability and shed new light on the recent hiatus in global warming. Here we review advances in our understanding of pantropical interbasin climate interactions and their implications for both climate prediction and future climate projections.

ADVANCES: ENSO fluctuates between warm events (El Niño) and cold events (La Niña). These events force changes in the Atlantic and Indian Oceans than can feed back onto the Pacific. Indian Ocean variations, for example, can accelerate the demise of El Niño and facilitate its transition to La Niña. ENSO events also exhibit considerable diversity in their amplitude, spatial structure, and evolution, which matters for how they affect global climate. Sea surface temperature variations in the equatorial and north tropical Atlantic can significantly contribute to the diversity of these events. In addition, tropical interbasin linkages vary on decadal time scales. Warming during a positive phase of Atlantic Multidecadal Variability over the past two decades has strengthened the Atlantic forcing of the Indo-Pacific, leading to an unprecedented intensification of the Pacific trade winds, cooling of the tropical Pacific, and warming of the Indian Ocean. The Indo-Pacific temperature contrast further strengthened the Pacific trade winds, helping to prolong the cooling in the Pacific. These interactions forced from the tropical Atlantic were largely responsible for the recent hiatus in global surface warming. Changes in Pacific mean-state conditions during this hiatus also affected ENSO diversity considerably.

OUTLOOK: There is tremendous potential for improving seasonal to decadal climate predictions and for improving projections of future climate change in the tropics though advances in our understanding of the dynamics that govern interbasin linkages. The role of the tropical Atlantic, in particular, requires special attention because all climate models exhibit systemic errors in the mean state of the tropical Atlantic that compromise their reliability for use in studies of climate variability and change. Projections based on the current generation of climate models suggest that Pacific mean-state changes in the future will involve faster warming in the east equatorial basin than in the surrounding regions, leading to an increase in the frequency of extreme El Niños. Given the presumed strength of the Atlantic influence on the pantropics, projections of future climate change could be substantially different if systematic model errors in the Atlantic were corrected. Progress on these issues will depend critically on sustaining global climate observations; climate model improvements, especially with regard to model biases; and theoretical developments that help us to better understand the underlying dynamics of pantropical interactions and their climatic impacts.