Scott Power, Matthieu Lengaigne, Antonietta Capotondi, Myriam Khodri, Jérôme Vialard, Beyrem Jebri, Eric Guilyardi, Shayne McGregor, Jong-Seong Kug, Matthew Newman, Michael J. McPhaden, Gerald Meehl, Doug Smith, Julia Cole, Julien Emile-Geay, Daniel Vimont, Andrew T. Wittenberg, Mat Collins, Geon-Il Kim, Wenju Cai, Yuko Okumura, Christine Chung, Kim M. Cobb, François Delage, Yann Y. Planton, Aaron Levine, Feng Zhu, Janet Sprintall, Emanuele Di Lorenzo, Xuebin Zhang, Jing-Jia Luo, Xiaopei Lin, Magdalena Balmaseda, Guojian Wang, Benjamin J. Henley

Published in Science, Oct 2021

BACKGROUND

Tropical Pacific decadal climate variability and change (TPDV) affects the global climate system, extreme weather events, agricultural production, streamflow, marine and terrestrial ecosystems, and biodiversity. Although major international efforts are underway to provide decadal climate predictions, there is still a great deal of uncertainty about the characteristics and causes of TPDV and the accuracy to which it can be simulated and predicted. Here, we critically synthesize what, as of now, is known and not known and provide recommendations to improve our understanding of TPDV and our ability to predict it.

ADVANCES

TPDV is evident in instrumental records, paleoclimate records over past millennia, and climate models. TPDV can occur spontaneously as “internal” variability, as is largely the case in the central equatorial Pacific, or in response to “external” forcing. Although internal TPDV arises to a large extent as a residual of independent El Niño–Southern Oscillation (ENSO) events, it can also result from oceanic processes occurring at decadal time scales involving the upper-ocean overturning circulation known as subtropical-tropical cells and in response to internal atmospheric variability in the extra-tropical Pacific and changes in sea surface temperature in other ocean basins. Externally forced TPDV, in the form of mean-state changes that unfold on decadal time scales or forced decadal variability, can be driven by anthropogenic [e.g., greenhouse gas (GHG) increases, sulfate aerosols changes] and natural processes (e.g., volcanic eruptions). External forcing can also affect the behavior and characteristics of internal TPDV.

In the western tropical Pacific, GHG-forced warming has reached levels that are unprecedented in the historical record. Further greenhouse warming in the equatorial Pacific will ensure that record-setting high temperatures will be experienced for decades to come. Increases in equatorial precipitation and in precipitation variability in parts of the tropical Pacific, and a southward expansion of the southern hemisphere Hadley cell, are projected by climate models with some confidence. Yet projected changes in eastern equatorial Pacific surface temperature, and changes in the strength of the Walker circulation and trade winds, remain very uncertain.

Skill in decadal predictions of temperature in the western Pacific is apparent, though it appears to be largely underpinned by GHG warming. There are also indications of multiyear skill in predicting some biogeochemical quantities important for fisheries and the global carbon budget.

The limited length of the instrumental records, the scarcity of paleoclimate data, and TPDV representation biases in climate models have so far prevented a complete characterization and understanding of TPDV. These issues have also limited our ability to predict TPDV.

OUTLOOK

Although several mechanisms have been proposed to explain TPDV, their relative importance as sources of decadal prediction remains unclear. Issues in need of greater understanding include the role played by the upper ocean overturning circulation in controlling tropical Pacific sea surface temperatures at decadal time scales, the impact of external forcing on the Walker circulation and characteristics of internally generated TPDV, and the extent to which sea surface temperature variability in other basins drives TPDV. A better understanding of the origin and spatial pattern of current predictive skill is also needed. Improving predictions and projections requires improvements in the quality, quantity, and length of instrumental and paleoclimate records and in the performance of climate models and data assimilation methods used to make predictions.

Power, S., Lengaigne, M., Capotondi, A., Khodri, M., Vialard, J., Jebri, B., et al. (2021). Decadal climate variability in the tropical Pacific: Characteristics, causes, predictability, and prospects. Science, 374(6563), eaay9165. https://doi.org/10.1126/science.aay9165