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The emerging interest in decadal climate prediction highlights the importance of understanding the mechanisms of decadal to interdecadal climate variability. The purpose of this paper is to provide a review of our understanding of interdecadal climate variability in the Pacific and Atlantic Oceans. In particular, the dynamics of interdecadal variability in both oceans will be discussed in a unified framework and in light of historical development. General mechanisms responsible for interdecadal variability, including the role of ocean dynamics, are reviewed first. A hierarchy of increasingly complex paradigms is used to explain variability. This hierarchy ranges from a simple red noise model to a complex stochastically driven coupled ocean–atmosphere mode. The review suggests that stochastic forcing is the major driving mechanism for almost all interdecadal variability, while ocean–atmosphere feedback plays a relatively minor role. Interdecadal variability can be generated independently in the tropics or extratropics, and in the Pacific or Atlantic. In the Pacific, decadal–interdecadal variability is associated with changes in the wind-driven upper-ocean circulation. In the North Atlantic, some of the multidecadal variability is associated with changes in the Atlantic meridional overturning circulation (AMOC). In both the Pacific and Atlantic, the time scale of interdecadal variability seems to be determined mainly by Rossby wave propagation in the extratropics; in the Atlantic, the time scale could also be determined by the advection of the returning branch of AMOC in the Atlantic. One significant advancement of the last two decades is the recognition of the stochastic forcing as the dominant generation mechanism for almost all interdecadal variability. Finally, outstanding issues regarding the cause of interdecadal climate variability are discussed. The mechanism that determines the time scale of each interdecadal mode remains one of the key issues not understood. It is suggested that much further understanding can be gained in the future by performing specifically designed sensitivity experiments in coupled ocean–atmosphere general circulation models, by further analysis of observations and cross-model comparisons, and by combining mechanistic studies with decadal prediction studies.

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic forcing has been difficult to detect from the short direct measurements available due to strong interdecadal variability. Here, we present observational and modeling evidence for a likely accelerated weakening of the AMOC since the 1980s under the combined forcing of anthropogenic greenhouse gases and aerosols. This likely accelerated AMOC weakening signal can be detected in the AMOC fingerprint of salinity pileup remotely in the South Atlantic, but not in the classic warming hole fingerprint locally in the North Atlantic, because the latter is contaminated by the “noise” of interdecadal variability. Our optimal salinity fingerprint retains much of the signal of the long-term AMOC trend response to anthropogenic forcing, while dynamically filtering out shorter climate variability. Given the ongoing anthropogenic forcing, our study indicates a potential further acceleration of AMOC weakening with associated climate impacts in the coming decades. An optimal salinity fingerprint is proposed to detect the long-term Atlantic meridional overturning circulation (AMOC) response to anthropogenic forcing. A real-word application suggests a likely accelerated weakening of the AMOC in recent decades.

The Atlantic Meridional Overturning Circulation (AMOC) is an active component of the Earth’s climate system1 and its response to global warming is of critical importance to society. Climate models have shown an AMOC slowdown under anthropogenic warming since the industrial revolution2–4, but this slowdown has been difficult to detect in the short observational record5–10 because of substantial interdecadal climate variability. This has led to the indirect detection of the slowdown from longer-term fingerprints11–14 such as the subpolar North Atlantic ‘warming hole’11. However, these fingerprints, which exhibit some uncertainties15, are all local indicators of AMOC slowdown around the subpolar North Atlantic. Here we show observational and modelling evidence of a remote indicator of AMOC slowdown outside the North Atlantic. Under global warming, the weakening AMOC reduces the salinity divergence and then leads to a ‘salinity pile-up’ remotely in the South Atlantic. This evidence is consistent with the AMOC slowdown under anthropogenic warming and, furthermore, suggests that this weakening has likely occurred all the way into the South Atlantic.The slowdown in the Atlantic Meridional Overturning Circulation (AMOC) is remotely detected in an increasing South Atlantic salinity trend. This salinity pile-up is caused by reduced divergence of surface salinity transport under a weakened AMOC.

Proxy reconstructions from marine sediment cores indicate peak temperatures in the first half of the last and current interglacial periods (the thermal maxima of the Holocene epoch, 10,000 to 6,000 years ago, and the last interglacial period, 128,000 to 123,000 years ago) that arguably exceed modern warmth 1 – 3 . By contrast, climate models simulate monotonic warming throughout both periods 4 – 7 . This substantial model–data discrepancy undermines confidence in both proxy reconstructions and climate models, and inhibits a mechanistic understanding of recent climate change. Here we show that previous global reconstructions of temperature in the Holocene 1 – 3 and the last interglacial period 8 reflect the evolution of seasonal, rather than annual, temperatures and we develop a method of transforming them to mean annual temperatures. We further demonstrate that global mean annual sea surface temperatures have been steadily increasing since the start of the Holocene (about 12,000 years ago), first in response to retreating ice sheets (12 to 6.5 thousand years ago), and then as a result of rising greenhouse gas concentrations (0.25 ± 0.21 degrees Celsius over the past 6,500 years or so). However, mean annual temperatures during the last interglacial period were stable and warmer than estimates of temperatures during the Holocene, and we attribute this to the near-constant greenhouse gas levels and the reduced extent of ice sheets. We therefore argue that the climate of the Holocene differed from that of the last interglacial period in two ways: first, larger remnant glacial ice sheets acted to cool the early Holocene, and second, rising greenhouse gas levels in the late Holocene warmed the planet. Furthermore, our reconstructions demonstrate that the modern global temperature has exceeded annual levels over the past 12,000 years and probably approaches the warmth of the last interglacial period (128,000 to 115,000 years ago). Reanalysis of Holocene sea surface temperature records affirms the role of retreating ice and rising greenhouse gases in driving a steady increase in global temperatures over the past 12,000 years.

The Atlantic Meridional Overturning Circulation (AMOC) has changed dramatically during the glacial–interglacial cycle. One leading hypothesis for these abrupt changes is thermohaline instability. Here, I review recent progress towards understanding thermohaline instability in both observations and modelling. Proxy records available seem to favor thermohaline instability as the cause of the abrupt climate changes during the glacial–deglacial period because the deep North Atlantic water mass and AMOC seemed to have changed before the North Atlantic climate. However, most fully Coupled General Circulation Models (CGCMs) so far seem to exhibit monostable AMOC, because (1) these models have failed to simulate abrupt AMOC changes unless they are forced by an abrupt change of external forcing and, (2) these models have shown opposite freshwater convergence from the current observations. This potential model bias in the AMOC stability leaves the model projection of the future AMOC change uncertain.

The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) platform is used to simulate Lagrangian trajectories of air parcels in East China during the summer monsoon. The investigation includes four distinct stages of the East Asian summer monsoon (EASM) during its seasonal migration from south to north. Correspondingly, the main water vapor channel migrates from the west Pacific Ocean (PO) for the premonsoon in South China (SC) to the Indian Ocean (IO) for the monsoon in SC and in the Yangtze–Huaihe River basin, and finally back to the PO for the terminal stage of monsoon in North China. Further calculations permit us to determine water vapor source regions and water vapor contribution to precipitation in East China. To a large extent, moisture leading to precipitation does not come from the strongest water vapor pathways. For example, the proportions of trajectories from the IO are larger than 25% all of the time, but moisture contributions to actual precipitation are smaller than 10%. This can be explained by the large amount of water vapor lost in the pathways across moisture-losing areas such as the Indian and Indochina Peninsulas. Local water vapor recycling inside East China (EC) contributes significantly to regional precipitation, with contributions mostly over 30%, although the trajectory proportions from subregions in EC are all under 10%. This contribution rate can even exceed 55% for the terminal stage of the monsoon in North China. Such a result provides important guidance to understand the role of land surface conditions in modulating rainfall in North China.

The interaction between sea surface temperatures (SST) and surface heat flux (SHF) is vital for atmospheric and oceanic variabilities. This study investigates SST‐SHF relationship in the framework of a coupled oscillatory model, extending beyond previous research that predominantly used AR‐1 type simple stochastic climate models. In contrast to the AR‐1 type model, we reveal distinct features of SST‐SHF relationships in the oscillatory model: sign reversals occur in the imaginary part of SST‐SHF coherence and the low‐pass SST tendency‐SHF correlation. However, these sign reversals are absent in the real part of SST‐SHF coherence and in the low‐pass SST‐SHF correlation. We find these features are robust across both the twentieth Century Reanalysis and GFDL SPEAR model for El Niño‐Southern Oscillation (ENSO) variability. Furthermore, we develop a new scheme to assess ENSO's climate forcing magnitude and natural frequency. Our findings thus provide novel insights into understanding ENSO dynamics from the perspective of heat flux. Plain Language Summary Understanding the interaction between sea surface temperature and the heat exchange between the ocean and atmosphere is crucial for predicting weather and climate patterns. Traditionally, this interaction is understood in the AR‐1 framework of stochastic model. Here, we extend the theory on sea surface temperature and surface heat flux (SHF) relation to a more advanced oscillatory model. Our new theory suggests distinct features in the sea surface temperature and SHF relationship, in contrast to those in the previous studies. These novel features are crucial for inferring climate forcing magnitude and natural frequency of the El Niño‐Southern Oscillation (ENSO). This theory sheds new light on the dynamics of ENSO variability particularly from the perspective of heat flux. Key Points Advanced idealized model uncovers distinct sea surface temperatures (SST)‐surface heat flux (SHF) relationship patterns Sign reversals observed in SST tendency‐SHF correlation and SST‐SHF coherence's imaginary part Novel schemes for assessing climate forcing magnitude and natural frequency of oscillatory variability

The Harris hawks optimization (HHO) algorithm is a new swarm-based natural heuristic algorithm that has previously shown excellent performance. However, HHO still has some shortcomings, which are premature convergence and falling into local optima due to an imbalance of the exploration and exploitation capabilities. To overcome these shortcomings, a new HHO variant algorithm based on a chaotic sequence and an opposite elite learning mechanism (HHO-CS-OELM) is proposed in this paper. The chaotic sequence can improve the global search ability of the HHO algorithm due to enhancing the diversity of the population, and the opposite elite learning can enhance the local search ability of the HHO algorithm by maintaining the optimal individual. Meanwhile, it also overcomes the shortcoming that the exploration cannot be carried out at the late iteration in the HHO algorithm and balances the exploration and exploitation capabilities of the HHO algorithm. The performance of the HHO-CS-OELM algorithm is verified by comparison with 14 optimization algorithms on 23 benchmark functions and an engineering problem. Experimental results show that the HHO-CS-OELM algorithm performs better than the state-of-the-art swarm intelligence optimization algorithms.

Understanding the past significant changes of the East Asia Summer Monsoon (EASM) and Winter Monsoon (EAWM) is critical for improving the projections of future climate over East Asia. One key issue that has remained outstanding from the paleo-climatic records is whether the evolution of the EASM and EAWM are correlated. Here, using a set of long-term transient simulations of the climate evolution of the last 21,000 years, we show that the EASM and EAWM are positively correlated on the orbital timescale in response to the precessional forcing, but are anti-correlated on millennial timescales in response to North Atlantic melt water forcing. The relation between EASM and EAWM can differ dramatically for different timescales because of the different response mechanisms, highlighting the complex dynamics of the East Asian monsoon system and the challenges for future projection. Future projection of changes in the East Asia Summer and Winter Monsoon are hindered by a lack of understanding of past variability. Here, using longterm transient simulations, the authors show that the monsoons respond in phase to precessional forcing, yet out of phase millennial-scale North Atlantic forcing.

In the developing stage of ENSO, the East Asia summer precipitation (EASP) shows a large variability that is significantly different from that in the decaying summer. In this study, we will focus on understanding the direct El Niño impact on the precipitation over East Asia in the developing summer in the observation. It is found that in its developing summer, the El Niño sea surface temperature anomaly affects the EASP directly from the eastern-central tropical Pacific, with little interference from the rest of the global ocean. The corresponding precipitation anomaly exhibits a tri-pole pattern, with two positive nodes in northeast and southeast China, sandwiched by a negative node in northern/central China. The tri-pole precipitation response is mainly attributed to the El Niño-induced cyclonic anomaly in Northeast Asia and the anticyclonic anomaly in the western North Pacific, which are part of the circulation anomalies of a circumglobal wave teleconnection in the subtropical jet in the Northern Hemisphere and a low level meridional wave train along East Asia coast. These circulation anomalies are generated by the summer El Niño in three pathways: (1) the vertical motion-induced perturbation over the central-eastern tropical Pacific entering into the subtropical jet excites a circumglobal wave train propagation eastward along the jet; (2) the El Niño-induced dipole heating across the equatorial Maritime Continent is mainly responsible for the meridional wave propagation along East Asia coast; (3) the El Niño-induced indirect heating over Northwest India triggers another perturbation in the jet waveguide, all contributing to the precipitation variation in East Asia. Further demonstration indicates the atmospheric response to the El Niño direct heating and perturbation over the tropical Pacific has the major contribution to the El Niño-induced circulation anomaly. As to the El Niño indirect heating over Northwset India, a zonal wave train response in the upper midlatitude which is mainly confined in the Eurasia sector makes a competing contribution to the circulation anomaly over East Asia.