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O bservations of global average surface air temperature (SAT) show an unequivocal warming over the twentieth century 1 , however the overall trend has been interrupted by periods of weak warming or even cooling (Fig. 1). For example, warming largely stalled from the 1940s to the 1970s. Between 1975 and 2000 the overall upward SAT trend resumed, but it was not uniform, with a decade of accelerated warming from about 1975-1985 (ref. 2), as well as periods of little warming 3 . Since around 2001 a marked hiatus in global surface warming has occurred, raising questions about its cause, its likely duration and the implications for global climate change.Decadal periods of minimal warming, or even cooling, interspersing decades of rapid warming, are not inconsistent with a long-term warming trend; indeed this characterizes the interplay between steadily increasing greenhouse gas forcing and internally generated climate variability. Factors other than internal variability, such as volcanoes and changes in solar radiation, can also drive cooler decades against the backdrop of ongoing warming. Indeed, hiatus decades are expected to punctuate future warming trends, even under scenarios of rapid global warming 4,5 . Mechanisms proposed to explain the most recent observed hiatus include increased ocean heat uptake 2,3,6,7 , the prolonged solar minimum 4 and changes in atmospheric water vapour 8 and aerosols 9,10 . The cool surface waters of the eastern Pacific have also been linked to the global temperature hiatus 11 and consensus is building that the subsurface ocean, with its vast capacity for heat storage, is playing a significant role through enhanced heat uptake 2,3,7,12,13 . It remains unclear, however, where the bulk of anomalous ocean heat uptake has occurred, with the Pacific 2,3 , Atlantic 13,14 and Southern [14][15][16][17] Oceans all potential candidates. One notable aspect of the two most recent extended hiatus periods (1940-1975 and 2001-present), in contrast to periods of global SAT warming (1910SAT warming ( -1940SAT warming ( and 1976SAT warming ( -2000, is that they correspond closely to periods when the Interdecadal Pacific Oscillation 18-20 (IPO) has been in a negative phase (Fig. 1a). The IPO manifests as a low-frequency El Niño-like pattern of climate variability, with a warm tropical Pacific and weakened trade winds during its positive phase, and a cool tropical Pacific and strengthened winds during its negative phase. Recent analyses of climate model simulations suggest that hiatus decades are linked to negative phases of the IPO (refs 2,3,11). Here we examine the most recent hiatus in this context, particularly in relation to altered ocean dynamics and enhanced ocean heat uptake, and assess implications for the coming decades.To examine the ongoing hiatus compared with a period of warming, we start by considering climatic trends over the past two decades, spanning the transition from a period of global surface warming in the 1990s to the post-2000 hiatus. During this time the Pacific trade winds ...

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El Niño events are characterized by surface warming of the tropical Pacific Ocean and weakening of equatorial trade winds that occur every few years. Such conditions are accompanied by changes in atmospheric and oceanic circulation, affecting global climate, marine and terrestrial ecosystems, fisheries and human activities. The alternation of warm El Niño and cold La Niña conditions, referred to as the El Niño-Southern Oscillation (ENSO), represents the strongest year-to-year fluctuation of the global climate system. Here we provide a synopsis of our current understanding of the spatio-temporal complexity of this important climate mode and its influence on the Earth system.

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Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake 1-4 . The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns 5-7 , but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

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Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents-in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow-has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.