Decadal climate variability in the tropical Pacific: Characteristics, causes, predictability, and prospects

Description A decades-long affair Decadal climate variability and change affects nearly every aspect of our world, including weather, agriculture, ecosystems, and the economy. Predicting its expression is thus of critical importance on multiple fronts. Power et al. review what is known about tropical Pacific decadal climate variability and change, the degree to which it can be simulated and predicted, and how we might improve our understanding of it. More accurate projections will require longer and more detailed instrumental and paleoclimate records, improved climate models, and better data assimilation methods. —HJS Decadal climate variability in the tropical Pacific has global impact. 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. Schematic overview of key issues raised. The inner circle provides a few examples of TPDV, together with some of the processes responsible for TPDV originating in the tropical Pacific. These include the Interdecadal Pacific Oscillation (IPO), decadal responses to external forcing, decadal variability in ENSO and weather, and the impact of this decadal variability on the tropical Pacific. The small, two-way arrows in the inner circle indicate the possibility of two-way interactions between these phenomena. The inner ring describes internal processes that can drive or influence TPDV, indicating that this can originate in the Pacific beyond the tropical Pacific and in the (tropical) Indian and Atlantic Oceans. The outer ring represents external forcing that can drive or influence TPDV. This includes both natural (e.g., volcanoes) and anthropogenic (e.g., anthropogenic GHG emissions) factors. The large arrows extending to the bottom of the figure represent the risks associated with TPDV (e.g., drought). The arrow on the left indicates that TPDV affects such risks. The arrow is dark to indicate that many of the risks are well established. The large arrow on the right indicates that skillful predictions, which depend on the existence of predictability in the climate system, can help to reduce the risks. This arrow is gray to indicate that decadal predictions for the tropical Pacific are still at a formative stage. Climate variability in the tropical Pacific affects global climate on a wide range of time scales. On interannual time scales, the tropical Pacific is home to the El Niño–Southern Oscillation (ENSO). Decadal variations and changes in the tropical Pacific, referred to here collectively as tropical Pacific decadal variability (TPDV), also profoundly affect the climate system. Here, we use TPDV to refer to any form of decadal climate variability or change that occurs in the atmosphere, the ocean, and over land within the tropical Pacific. “Decadal,” which we use in a broad sense to encompass multiyear through multidecadal time scales, includes variability about the mean state on decadal time scales, externally forced mean-state changes that unfold on decadal time scales, and decadal variations in the behavior of higher-frequency modes like ENSO.