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This study investigates the influence of the Atlantic multidecadal oscillation (AMO) on the multidecadal variability of winter surface air temperature in arid central Asia (ACASAT). Apart from a long-term warming trend, the observational analysis shows that the winter ACASAT exhibits a significant multidecadal variability, which is characterized by antiphase fluctuations with the AMO. The mechanism for this negative correlation between the AMO and the winter ACASAT is explored from the aspect of wave teleconnection. The AMO provides energy for the Scandinavian teleconnection pattern at middle and low altitudes by regulating the high-altitude wave train over the middle and high latitudes of Eurasia, and thus has an impact on the remote climate in arid central Asia. Results from the linear baroclinic model (LBM) provide evidence for the linkage between the AMO and the Scandinavian teleconnection pattern. When the AMO is in its warm periods, the Scandinavian teleconnection pattern is in a positive phase, which further makes the cold air from the northeast strengthen, leading to the anomalously colder surface air temperature in arid central Asia. Based on the relationship that the North Atlantic Oscillation (NAO) leads the AMO by 15–20 years, it is further found that there is a leading relationship between the NAO and the winter ACASAT via the AMO. On this basis, an empirical model using the NAO as a predictor was established to predict the ACASAT, and the empirical model shows good hindcast performance. Results from the model show that the winter ACASAT will continue to rise in the next 10 years and decline after 2030. [ABSTRACT FROM AUTHOR]

Tropical cyclone (TC) translation speed (TCS) over the western North Pacific (WNP) has experienced a long-term decreasing trend. To date, however, little is known about the multidecadal variability of TCS and its possible influence on this trend. This study investigated the multidecadal variability of the WNP TCS and the underlying physical mechanisms. Results show that the WNP TCS presents robust multidecadal variability during the past seven decades, which is dominated by the TCS over the extratropics. Further analysis shows that the Atlantic multidecadal oscillation (AMO) is responsible for the TCS multidecadal variability. AMO positive (negative) phases lead to favorable (unfavorable) large-scale environmental conditions for maintaining TCs over the extratropics, which results in longer (shorter) residence time for TCs having been accelerated by the midlatitude westerlies, thus, leading to higher (lower) TCS. The TCS phase shift strongly offsets its slowdown trend, leading to the inconsistent trends during past decades. This inconsistency may also relate to the influence of extratropical transitioned cyclones without being totally excluded. These cyclones may be inhomogeneously recorded due to the absence of satellite observation before the 1980s. Our results indicate that internal variation such as AMO may dominate TCS low-frequency variations over the past several decades. Previous studies have attributed the inconsistent trends of TCS during different subperiods to data inhomogeneity. This study shows that AMO can modulate the TCS trends in different subperiods with phase shift, thus providing new evidence for the recent controversial TCS slowdown. [ABSTRACT FROM AUTHOR]

The wintertime multidecadal land precipitation (WMLP) variability is obtained using the ensemble empirical mode decomposition, and then the leading spatial pattern and temporal evolution of the WMLP in the northern hemisphere during the period of 1891–2014 are investigated. The time series shows a pronounced enhancement of the variability since the mid‐1970s, coincident with the 1976/1977 climate transition, with the variance increasing from 0.33 in the former period (P1) to 2.57 in the latter period (P2). The spatial structure in P2 resembles that in the entire period. The precipitation anomaly is positive in the North America between 30° and 50°N, South America and southern Europe; while it is negative in the southern North America, majority of Russia, Arabian Peninsula, Iran and South China. However, the spatial pattern in P1 differs from the entire period. An Interdecadal Pacific Oscillation (IPO)‐like sea surface temperature (SST) variability in cold phase and an Atlantic Multidecadal Oscillation (AMO)‐like SST variability in warm phase are associated with the leading mode of WMLP in P2 and the entire period. Yet, the relationship of IPO and AMO on the WMLP is barely found in P1. The in‐phase change of IPO and AMO in P1 may contribute to the smaller amplitude of WMLP variability in P1. In the end, the coupled models from CMIP6 historical runs are evaluated. The good‐models average is superior to any single good model in capturing the observed spatial features of WMLP in both P1 and P2. The poor‐models average performs better than any single poor model. Both good‐ and poor‐models averages are able to reproduce the IPO‐ and AMO‐like SST variabilities in P2. [ABSTRACT FROM AUTHOR]

The North Pacific Subtropical Fronts (STFs), accompanied by the eastward-flowing subtropical counter-current, stretch from the western Pacific Ocean to the north of Hawaii. Previous work has detected different trends of the frontal position and strength between the western STF (WSTF; west of 180°) and the eastern STF (ESTF; east of 180°) in the past 40 years. However, whether the basin-scale STFs have zonally asymmetric variability on multidecadal time scales and what drives that change remain to be quantified. Our recent work has shown that the multidecadal variability of the WSTF is controlled by the Atlantic multidecadal oscillation via the subtropical mode water variability. The present study proposes that the variability of ESTF is modulated by the Pacific decadal oscillation (PDO) via the central mode water (CMW) variability, quasi synchronously on multidecadal time scales. During a PDO positive phase, the enhanced midlatitude westerly winds in the central North Pacific increase the local surface buoyancy loss and deepen the winter mixed layer, which enlarges the CMW formation and thus increases its volume. Meanwhile, accompanied by the southward-migrated outcropping zone, the main body of CMW shifts equatorward. In response to such CMW changes, the ESTF strengthens and shifts equatorward correspondingly. Conversely, during a PDO negative phase, the weakened midlatitude westerly winds in the central North Pacific decrease the local surface buoyancy loss and shallow the winter mixed layer, which reduces the CMW formation and thus decreases its volume. Meanwhile, accompanied by northward-migrated outcropping zone, the main body of CMW shifts poleward. In response to such CMW changes, the ESTF weakens and shifts poleward correspondingly. Our results reveal that the dominant factor controlling the low-frequency variability of the WSTF and ESTF is different, which renews the conventional picture that all of the STFs behave symmetrically, with important implications for the North Pacific climate variability. [ABSTRACT FROM AUTHOR]

The Northern hemispheric circum global teleconnection (CGT) pattern is thought to be maintained by two main forcings—viz-diabatic heating associated with the Indian summer monsoon (ISM) and barotropic instability generation over the jet exit region over the North Atlantic. The CGT and ISM impacts one another through the circulation responses over West central Asia (WCA). In this study we revisit the CGT-ISM interactions focusing on the WCA region and try to understand whether the downstream impact of CGT on ISM dominates over the ISM feedback on CGT. Analysis indicates that the Atlantic forced CGT responses play a lead role in modulating the ISM in the interannual timescale, by modulating the upper-level anticyclones over WCA and in turn affecting the ISM easterly vertical wind shear. Atlantic multi-decadal oscillation (AMO) is a major driver of ISM variability in the multi-decadal time scale and the AMO is associated with an arching wave-train of teleconnection across Eurasia. Our analysis indicates significant modulation of WCA anomalies by the AMO in the multi-decadal time scale, implying that the Atlantic-CGT-WCA-ISM pathway of teleconnection has a low frequency counterpart. We further demonstrate that the observed out of phase relationship between AMO and ISM in the recent decades, may be attributed to the relatively stronger high latitude warming over the north Atlantic during the recent AMO warm phase. The equivalent barotropic responses to the extra-tropical north Atlantic sea surface temperature (SST) anomalies alter the entire downstream teleconnection pattern producing cyclonic anomalies over WCA and in turn weakening the ISM.

This study explores the possible drivers of the recent Hadley circulation strengthening in the modern reanalyses. Predominantly, two recent generations of reanalyses provided by the European Centre for Medium-Range Weather Forecasts are used: the fifth-generation atmospheric reanalysis (ERA5) and the interim reanalysis (ERA-Interim). Some results are also evaluated against other long-term reanalyses. To assess the origins of the Hadley cell (HC) strength variability, we employ the Kuo–Eliassen (KE) equation. ERA5 shows that both HCs were strengthening prior to the 2000s, but they have been weakening or remained steady afterward. Most of the long-term variability in the strength of the HCs is explained by the meridional gradient of diabatic (latent) heating, which is related to precipitation gradients. However, the strengthening of both HCs in ERA5 is larger than the strengthening expected from the observed zonal-mean precipitation gradient [estimated from the Global Precipitation Climatology Project (GPCP)]. This suggests that the HC strength trends in the recent decades in ERA5 can be explained partly as an artifact of the misrepresentation of latent heating and partly through (physical) long-term variability. To show that the latter is true, we analyze ERA5 preliminary data for the 1950–78 period, other long-term (e.g., twentieth century) reanalyses, and sea surface temperature observational data. This reveals that the changes in the HC strength can be a consequence of the Atlantic multidecadal oscillation (AMO) and related diabatic and frictional processes, which in turn drive the global HC variability. This work has implications for further understanding of the long-term variability of the Hadley circulation. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that the Atlantic multidecadal oscillation (AMO) could affect El Niño–Southern Oscillation (ENSO) through thermocline adjustment, with a stronger ENSO sea surface temperature (SST) amplitude during a negative AMO phase than during a positive phase. In this study, we find that the ENSO atmospheric anomaly amplitudes in the tropical Pacific during different AMO phases are not necessarily consistent with these ENSO SST changes. For El Niño episodes, the low-level wind and precipitation anomalies over the tropical Pacific in the boreal winter are more pronounced during the negative AMO phase than during the positive phase, corresponding well to the stronger SST anomalies. However, La Niña events during the negative AMO phase are accompanied by weaker atmospheric anomalies in the tropical Pacific, although their SST anomalies are stronger than those during the positive phase. We suggest that this mismatch between La Niña SST and atmospheric anomalies can be largely attributed to AMO decadal modulation. A positive AMO favors intensified trade winds and weakened precipitation in the central tropical Pacific by modifying Walker circulation. Therefore, when La Niña coincides with a positive AMO, the low-level easterly and negative precipitation anomalies are superimposed, which gives rise to stronger atmospheric perturbations. In contrast, under a negative AMO background, the atmospheric anomalies induced by La Niña anomalous SST are partly counteracted by the AMO remote decadal modulation, thereby resulting in weaker anomaly amplitudes. Here, we highlight that AMO decadal forcing needs to be considered when investigating ENSO atmospheric variabilities and related regional climate impacts. [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal variability (AMV), a dominant mode of multidecadal variations in North Atlantic sea surface temperatures (NASST), has major impacts on global climate. Given that both internal variability and external forcing have contributed to the historical AMV, how future anthropogenic forcing may regulate the AMV is of concern but remains unclear. By analyzing observations and a large ensemble of model simulations [i.e., the Max Planck Institute Grand Ensemble (MPI-GE)], the internally generated (AMVIV) and externally forced (AMVEX) components of the AMV and their climatic impacts during the twenty-first century are examined. Consistent with previous findings, the AMVIV would weaken with future warming by 11%–17% in its amplitude by the end of the twenty-first century, along with reduced warming anomaly over the midlatitude North Atlantic under future warming during the positive AMVIV phases. In contrast, the AMVEX is projected to strengthen with reduced frequency under future warming. Furthermore, future AMVIV-related temperature variations would weaken over Eurasia and North Africa but strengthen over the United States, whereas AMVIV-related precipitation over parts of North America and Eurasia would weaken in a warmer climate. The AMVEX's impact on global precipitation would also weaken. The results provide new evidence that future anthropogenic forcing (i.e., nonlinear changes in GHGs and aerosols) under different scenarios can generate distinct multidecadal variations and influence the internally generated AMV, and that multidecadal changes in anthropogenic forcing are important for future AMV. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the influence of the winter NAO on the multidecadal variability of winter East Asian surface air temperature (EASAT) and EASAT decadal prediction. The observational analysis shows that the winter EASAT and East Asian minimum SAT (EAmSAT) display strong in-phase fluctuations and a significant 60–80-year multidecadal variability, apart from a long-term warming trend. The winter EASAT experienced a decreasing trend in the last two decades, which is consistent with the occurrence of extremely cold events in East Asia winters in recent years. The winter NAO leads the detrended winter EASAT by 12–18 years with the greatest significant positive correlation at the lead time of 15 years. Further analysis shows that ENSO may affect winter EASAT interannual variability, but does not affect the robust lead relationship between the winter NAO and EASAT. We present the coupled oceanic-atmospheric bridge (COAB) mechanism of the NAO influences on winter EASAT multidecadal variability through its accumulated delayed effect of ∼15 years on the Atlantic Multidecadal Oscillation (AMO) and Africa–Asia multidecadal teleconnection (AAMT) pattern. An NAO-based linear model for predicting winter decadal EASAT is constructed on the principle of the COAB mechanism, with good hindcast performance. The winter EASAT for 2020–34 is predicted to keep on fluctuating downward until ∼2025, implying a high probability of occurrence of extremely cold events in coming winters in East Asia, followed by a sudden turn towards sharp warming. The predicted 2020/21 winter EASAT is almost the same as the 2019/20 winter. [ABSTRACT FROM AUTHOR]

During the winter, the cold wave activity over the mid-high latitudes has profound impacts on agriculture, economic and human wellbeing. Such extreme weather events have been connected with the East Asia winter monsoon system and significantly influence the climate over the Eurasian continent. However, the multidecadal variabilities and regional interconnections of the cold wave activity across the Northern Hemisphere are lesser-known. In this study, we investigate the multidecadal variations in the cold wave frequency (CWF) and find an inverse relationship between Greenland and central Eurasia. Observational and modeling evidence suggests that the Atlantic Multidecadal Oscillation (AMO) is likely to be the driving force of the multidecadal seesaw in CWF, while the effects of the Arctic sea ice are very limited. The increased sea surface temperature (SST) in association with the AMO warms the subpolar troposphere and weakens the predominant westerlies over mid-high latitudes, resulting in positive geopotential height anomalies over the subpolar region. This further weakens the Icelandic Low and strengthens the Siberian High, which directly induces the warming (cooling) over Greenland (central Eurasia). There is a strong coherence between the mean state of surface air temperature and temperature extremes. The AMO-induced warming/cooling in Greenland/central Eurasia corresponds well with less/more frequent cold wave activities. Our results provide new insight into the multidecadal variability of cold wave activities and suggest that the CWF in the Northern Hemisphere may be interlinked. [ABSTRACT FROM AUTHOR]

This study investigates the characteristics and physical mechanisms of the multidecadal variations in the East Asian winter (December–February) monsoon (EAWM) since 1850 based on multiple observational and reanalysis datasets. The results indicate that the EAWM undergoes multidecadal weakening during the periods of 1869–1919 and 1986–2004 but strengthening during the period of 1920–85. Similar evolutions can be observed in the time series of the area-averaged winter surface air temperature over East Asia. Associated with the EAWM multidecadal variations, a quasi-barotropic Rossby wave train originating from the subtropical North Atlantic propagating across the Eurasian continent to Northeast Asia also experiences phase shifting at the same time. In its positive phase, the low-level anticyclonic anomaly over the northern Eurasian continent causes a stronger Siberian high; the mid- and high-level cyclonic anomalies over Northeast Asia deepen the East Asian trough and strengthen the East Asian jet stream, respectively. Thus, the positive phase of the wave train is conducive to stronger EAWMs and vice versa. The diagnostic analysis of the Rossby wave source indicates that the upper-tropospheric divergence anomalies over the North Atlantic can favor the excitation of this wave train, and the feedback forcing of high-frequency eddies plays important roles in its maintenance. In addition, the phase shifting of the Atlantic multidecadal oscillation (AMO) can induce a similar Rossby wave train across the Eurasian continent, through which it further modulates the multidecadal variations in the EAWM. Warm phases of the AMO are favorable for a stronger EAWM and colder midlatitude Eurasian continent and vice versa. [ABSTRACT FROM AUTHOR]

The recent summer surface air temperature (SAT) changes over densely populated Eurasia exhibit a non‐uniform pattern with amplified warming over Europe and East Asia (EA) but weak warming over Central Asia (CA), forming a wave train‐like structure. However, the key factors that determine this non‐uniform warming pattern remain unclear. By analyzing observations and model simulations, here, we show that more than half of the SAT multidecadal variations from 1950 to 2014 over Europe‐west Asia and EA may have resulted from external forcing, rather than from internal variability in the Atlantic as previously thought. In contrast, the recent SAT over CA is influenced mainly by internal variations in the Atlantic and Pacific oceans. Large ensemble model simulations suggest that the forced SAT multidecadal variations over Eurasia are mainly caused by changes in greenhouse gases and aerosols. Our findings provide strong evidence for major impacts of external forcing on multidecadal climate variations over Eurasia. Plain Language Summary: Previous studies have documented non‐uniform decadal warming rates in summer since the mid‐1990s over densely populated Eurasia, with amplified warming over Europe and East Asia (EA) but weak warming over Central Asia. This unique tripole warming pattern and its cause have attracted considerable attention. It may arise from internal variability (IV; e.g., Atlantic Multidecadal Oscillation [AMO]) and/or external forcing. Furthermore, it was found that the recent AMO resulted from both IV and decadal changes in aerosols. Thus, the relative contributions of IV and external forcing to the recent Eurasian warming pattern remain unclear. Here we show that the surface air temperature multidecadal variations since 1950 over Eurasia are caused by both IV and external forcing, with more than half of the variations over Europe‐west Asia and EA resulted from external forcing (i.e., greenhouse gases [GHGs] and aerosols), in contrast to the previous notion that the non‐uniform warming is mainly originated from the Atlantic Ocean. Looking ahead into the upcoming decades, aerosols will continue to decrease, whereas the AMO will likely be in its positive phase. These changes, together with continued increases in GHGs, will accelerate warming trends over Eurasia and further exacerbate summer heat waves. Key Points: More than half of the multidecadal surface air temperature (SAT) variations from 1950 to 2014 over Europe‐west Asia and East Asia resulted from external forcingThe forced SAT multidecadal variations over Eurasia are mainly caused by changes in greenhouse gases and aerosolsOur study helps reconcile human and natural drivers of the tripole pattern of multidecadal summer temperature variations over Eurasia [ABSTRACT FROM AUTHOR]

Decadal mean variables are frequently used to characterize decadal climate variabilities. Decadal means are often calculated using yearly data, which can represent variability at time scales from annual to centennial. Residuals from interannual fluctuations may contribute to the variability in decadal time series. Such variability is more difficult to be predicted at the long range. Removing it from the decadal variability means that the remaining variability is more likely to arise from slowly varying multidecadal or longer time scale external forcing and internal climate dynamics, which are more likely to be predicted. Here, a new approach is proposed to understand the uncertainty, potential predictability, and drivers of decadal mean variables. The covariance matrix of multivariate decadal running means is decomposed into unpredictable fast decadal variability and the potentially predictable slow decadal variability. EOF analysis is then applied to the decomposed matrices to find the dominant modes, which may be related to the drivers of the two types of variabilities in the multivariate decadal means. The methodology has been applied to 140-yr datasets of North Pacific sea surface temperature and the Northern Hemisphere 1000-hPa geopotential height. For sea surface temperature, the Pacific decadal oscillation is the major driver of the fast decadal variability, while the radiative forcing and the Atlantic multidecadal oscillation are major drivers of the slow decadal variability. For the 1000-hPa geopotential height, fast decadal variability is associated with the northern annular mode, the east Atlantic mode, and the Pacific decadal oscillation. Slow decadal variability is associated with the northern annular mode and the Atlantic multidecadal oscillation. [ABSTRACT FROM AUTHOR]

Chennai experienced acute water shortage during 2019 summer, and four years prior, an earlywinter deluge. Analysis of 116 years (1901-2016) of rainfall in Chennai Sub-basin shows a weak climate change signal: Winter monsoon rainfall, has slightly increased, especially in December. The much larger Cauvery basin to the south also exhibits a nondescript climate change signal in winter rainfall. Late summer (September) rainfall in the Cauvery Basin has, however, precipitously declined in recent years (1987-2016). We show that this decline, as well as the mid-20th century increase, are attributable to natural multidecadal climate variability (Atlantic Multidecadal Oscillation) - cautioning against cavalier attributions of recent-period trends and the Chennai Water Crisis to climate change. Analysis of runoff - the rainwater leftover after its hydrologic and atmospheric processing - shows that harnessing even half of the winter monsoon runoff in the Chennai Sub-basin can satiate the city's water demand for about seven months; and without needing new reservoir facilities. The present analysis suggests that Chennai's water woes arise not from insufficient rainwater, but from the suboptimal harnessing of related runoff. [ABSTRACT FROM AUTHOR]

Recent studies have found evidence of multidecadal variability in Northern Hemisphere wintertime seasonal forecast skill. Here we assess the robustness of this finding by extending the analysis to a diverse set of ensemble atmospheric model simulations. These simulations differ in either the numerical model or type of initialisation and include atmospheric model experiments initialised with reanalysis data and free‐running atmospheric model ensembles. All ensembles are forced with observed sea‐surface temperatures (SSTs) and sea‐ice boundary conditions. Analysis of large‐scale Northern Hemisphere circulation indices over the Northern Hemisphere (namely the North Atlantic Oscillation, the Pacific/North American pattern and the Arctic Oscillation) reveals that in all ensembles there is larger correlation skill in late‐century periods than in mid‐century periods. Similar multidecadal variability in skill is found in a measure of total skill integrated over the whole extratropical region. Most of the differences in large‐scale circulation skill between the skilful late period (as well as the early period) and the less skilful mid‐century period seem to be due to a reduction in skill over the North Pacific and a disappearance of skill over North America and the North Atlantic. The results are robust across different models and different types of initialisation, indicating that the multidecadal variability in Northern Hemisphere winter skill is a robust feature of twentieth‐century climate variability. Multidecadal variability in skill therefore arises from the evolution of the observed SSTs, likely related to a weakened influence of the El Niño–Southern Oscillation on the predictable extratropical circulation signal during the middle of the twentieth century, and is evident in the signal‐to‐noise ratio of the different ensembles, particularly the larger ensembles. [ABSTRACT FROM AUTHOR]

The Pacific Decadal Oscillation (PDO), the leading mode of Pacific decadal sea surface temperature variability, arises mainly from combinations of regional air-sea interaction within the North Pacific Ocean and remote forcing, such as from the tropical Pacific and the Atlantic. Because of such a combination of mechanisms, a question remains as to how much PDO variability originates from these regions. To better understand PDO variability, the equatorial Pacific and the Atlantic impacts on the PDO are examined using several 3-dimensional partial ocean data assimilation experiments conducted with two global climate models: the CESM1.0 and MIROC3.2m. In these partial assimilation experiments, the climate models are constrained by observed temperature and salinity anomalies, one solely in the Atlantic basin and the other solely in the equatorial Pacific basin, but are allowed to evolve freely in other regions. These experiments demonstrate that, in addition to the tropical Pacific's role in driving PDO variability, the Atlantic can affect PDO variability by modulating the tropical Pacific climate through two proposed processes. One is the equatorial pathway, in which tropical Atlantic sea surface temperature (SST) variability causes an El Niño-like SST response in the equatorial Pacific through the reorganization of the global Walker circulation. The other is the north tropical pathway, where low-frequency SST variability associated with the Atlantic Multidecadal Oscillation induces a Matsuno-Gill type atmospheric response in the tropical Atlantic-Pacific sectors north of the equator. These results provide a quantitative assessment suggesting that 12–29% of PDO variance originates from the Atlantic Ocean and 40–44% from the tropical Pacific. The remaining 27–48% of the variance is inferred to arise from other processes such as regional ocean-atmosphere interactions in the North Pacific and possibly teleconnections from the Indian Ocean. [ABSTRACT FROM AUTHOR]

The Atlantic and Pacific basin are found linked in the context of multidecadal SST variability from analyses of 118 years of observational data. Recurrent spatiotemporal variability, including multidecadal modes, was identified using the extended-EOF technique in a longitudinally global domain, allowing unfettered expression of interbasin interactions. The physicality of the obtained decadal modes was assessed using fishery records and analog counts. A three-mode structure with bi-directional interbasin links frames the new perspective on the cycling of multidecadal SST variability. The three modes are the Atlantic multidecadal oscillation (AMO), low-frequency North Atlantic Oscillation (LF-NAO), and North Pacific decadal variability [PDV-NP; resembling negative (–ve) PDO]. The two previously documented links AMO→PDV-NP (with ~12.5-yr lead) and LF-NAO→AMO (with 16-yr lead) are corroborated, while a third one, PDV-NP→(−LF-NAO) with ~6.5-yr lead, is uncovered. The interaction triad closes the loop on the cycling of multidecadal SST variability, generating AMO's phase reversal in ~35 years, consistent with its widely noted ~70-yr time scale. The two previously noted links—one intrabasin and one interbasin—were unsuccessful in this regard. Other findings include the deeper subsurface extensions of Atlantic multidecadal SST variability, and the hitherto unrecognized similarity of Pan-Pacific decadal variability and North Pacific Gyre Oscillation. Instrumental records, albeit short in the context of multidecadal variability, must continue to be mined for insights into the functioning of the climate system as its model representations while improving, remain inadequate. [ABSTRACT FROM AUTHOR]

Recent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia. [ABSTRACT FROM AUTHOR]

This paper addresses the dynamics of internal hemispheric-scale multidecadal climate variability by postulating an energy-balance (EBM) model comprising two deep-ocean oscillators in the Atlantic and Pacific basins, coupled through their surface mixed layers via atmospheric teleconnections. This system is linear and driven by the atmospheric noise. Two sets of the EBM model parameters are developed by fitting the EBM-based mixed-layer temperature covariance structure to best mimic basin-average North Atlantic/Pacific sea surface temperature (SST) covariability in either observations or control simulations of comprehensive climate models within the CMIP5 project. The differences between the dynamics underlying the observed and CMIP5-simulated multidecadal climate variability and predictability are encapsulated in the algebraic structure of the two EBM model versions so obtained: EBMCMIP5 and EBMOBS. The multidecadal variability in EBMCMIP5 is overall weaker and amounts to a smaller fraction of the total SST variability than in EBMOBS, pointing to a lower potential decadal predictability of virtual CMIP5 climates relative to that of the actual climate. The EBMCMIP5 decadal hemispheric teleconnections (and, by inference, those in CMIP5 models) are largely controlled by the variability of the Pacific, in which the ocean, due to its large thermal and dynamical memory, acts as a passive integrator of atmospheric noise. By contrast, EBMOBS features a stronger two-way coupling between the Atlantic and Pacific multidecadal oscillators, thereby suggesting the existence of a hemispheric-scale and, perhaps, global multidecadal mode associated with internal ocean dynamics. The inferred differences between the observed and CMIP5 simulated climate variability stem from a stronger communication between the deep ocean and surface processes implicit in the observational data. [ABSTRACT FROM AUTHOR]

Despite its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is far from settled. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a recent study showed that the atmospheric stochastic forcing is sufficient. In this study, we resolve this conundrum by partitioning the multidecadal SST tendency into a part caused by surface heat fluxes and another by ocean dynamics, using a preindustrial control simulation of a state-of-the-art coupled climate model. In the model, horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies; however, when the vertical component is also considered, the ocean dynamics overall acts to cool the region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, although the associated surface heat flux anomalies are upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling, thus leading to relative warming by increasing the ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions. [ABSTRACT FROM AUTHOR]

Instrumental observations indicate that Amazon precipitation and streamflow extremes have increased during the last 40 years, possibly due to anthropogenic changes and natural variability. How unprecedented these changes might be is difficult to determine because some paleoclimatic, instrumental, and climate model simulations suggest that Amazonian precipitation and streamflow may be subject to multidecadal variability with return intervals longer than most direct observations. A new 258‐year long tree‐ring chronology of Cedrela odorata has been developed in the eastern Amazon and has been used to reconstruct wet season precipitation totals from 1759–2016. Reconstructed drought extremes are associated with significant sea surface temperature anomalies over the tropical Pacific and Atlantic Oceans. Strong multidecadal variance is identified in the reconstruction that may reflect a component of natural rainfall variability relevant to forest ecosystem dynamics and suggesting that recent hydroclimate changes over the eastern Amazon may not be unprecedented over the past 258 years. Plain Language Summary: Drought and flood extremes in the Amazon River basin have increased in frequency during the last 40 years, but how unprecedented these recent changes might be is difficult to determine because most instrumental records do not extend before 1950. A new tree‐ring chronology of Cedrela odorata has been developed in the eastern equatorial Amazon, confirming the exact dating and climate signal of the single existing tree‐ring chronology from the region. A tree‐ring reconstruction of wet season precipitation based on this new chronology extends from 1759–2016 and documents strong multidecadal variability that impacted the frequency of reconstruction precipitation extremes over the past 258 years. Key Points: Drought and flood extremes in the Amazon River basin have increased in frequency during the last 40 yearsA new tree‐ring chronology of Cedrela odorata confirms the dating and climate signal of a previous chronology for the eastern AmazonMultidecadal variability of tree‐ring reconstructed rainfall suggests that recent eastern Amazon rainfall extremes may not be unprecedented [ABSTRACT FROM AUTHOR]

A mechanistic understanding of the Atlantic multidecadal variability (AMV) is highly desirable since it will considerably aid regional and global climate predictions. Although ocean dynamics have long been invoked to explain the AMV, recent studies have cast doubt on its influence. Here we evaluate the necessity of ocean dynamics for the AMV using an observationally based idealized model that isolates the contribution of atmospheric forcing to the AMV. By demonstrating that this model underestimates the magnitude of the observed sea surface temperature variability in the extratropical North Atlantic, we infer that ocean dynamics contribute significantly to the AMV in this region. This inference holds when we add anthropogenic aerosol forcing and the effects of mixed layer depth variability to the idealized model. Thus, our study suggests that ocean heat transport convergence is needed to explain sea surface temperature variability in the extratropical North Atlantic. Sustained ocean observing systems in the this region will help untangle the physical mechanisms involved. [ABSTRACT FROM AUTHOR]

The response of the European climate to the Atlantic multidecadal variability (AMV) remains difficult to isolate in observations because of the presence of strong internal variability and anthropogenically forced signals. Using model sensitivity experiments proposed within the CMIP6/Decadal Climate Prediction Project Component C (DCPP-C) framework, the wintertime AMV–Europe teleconnection is here investigated in large ensembles of pacemaker-type simulations conducted with the CNRM-CM5 global circulation model. To evaluate the sensitivity of the model response to the AMV amplitude, twin experiments with the AMV forcing pattern multiplied by 2 and 3 (2xAMV and 3xAMV, respectively) are performed in complement to the reference ensemble (1xAMV). Based on a flow analog method, we show that the AMV-forced atmospheric circulation tends to cool down the European continent, whereas the residual signal, mostly including thermodynamical processes, contributes to warming. In 1xAMV, both terms cancel each other, explaining the overall weak AMV-forced atmospheric signal. In 2xAMV and 3xAMV, the thermodynamical contribution overcomes the dynamical cooling and is responsible for milder and wetter conditions found at large scale over Europe. The thermodynamical term includes the advection of warmer and more humid oceanic air penetrating inland and the modification of surface radiative fluxes linked to both altered cloudiness and snow-cover reduction acting as a positive feedback with the AMV amplitude. The dynamical anomalous circulation combines 1) a remote response to enhanced diabatic heating acting as a Rossby wave source in the western tropical Atlantic and 2) a local response associated with warmer SST over the subpolar gyre favoring an anomalous high. The extratropical influence is reinforced by polar amplification due to sea ice melting in all the subarctic seas. The weight between the tropical–extratropical processes and associated feedbacks is speculated to partly explain the nonlinear sensibility of the response to the AMV forcing amplitude, challenging thus the use of the so-called pattern-scaling technique to evaluate teleconnectivity and related impacts associated with decadal variability. [ABSTRACT FROM AUTHOR]

The variability of the surface zonal and meridional wind components over northeastern North America during June–October is analyzed through a statistical downscaling (SD) approach that relates the main wind and large-scale circulation modes. An observational surface wind dataset of 525 sites over 1953–2010 provides the local information. Twelve global reanalyses provide the large-scale information. The large-to-local variability of the wind field can be explained, to a large extent, in terms of four coupled modes of circulation explaining a similar amount of variance. The SD method is mostly sensitive to the number of retained modes and subregionally to the large-scale information variable, but not to the reanalysis source. The SD methodological uncertainty based on the use of multiple configurations is directly related to the variability of the wind, similar in relative terms for both components. With an adequate choice of parameters the SD estimates provide more realistic variances than the reanalysis wind, although their correlations with respect to observations are lower than the latter. Additionally, while these different SD estimations are very similar on the reanalysis used, the various reanalysis wind fields show noticeable differences, especially in their variances. The wind variability is reconstructed back to 1850, making use of century-long reanalyses and two additional SLP gridded datasets, which allows estimating the variability at decadal to multidecadal time scales. Recent negative (significant) trends in the zonal component do not stand out in the multidecadal context, but they are consistent with a global stilling process, and are partially attributable to changes in the large-scale dynamics. [ABSTRACT FROM AUTHOR]

We observe significant changes in the ENSO–Indian summer monsoon rainfall (ISMR) relationship in past three multidecadal epochs (early epoch: 1931–1960, middle epoch: 1961–1990, and recent epoch: 1991–2015) based on consistent correlation. The rainfall during early epoch was above normal, however, in other two epochs it was relatively dry. The ENSO–ISMR relationship and its changes are mostly pronounced in the monsoon core zone. During the early epoch, the effect of La Niña was more dominant by accounting higher number of La Niña events with higher values of Niño 3.4 index, which can be attributed to the surplus rainfall over most regions of India. The rainfall during the consecutive two multidecadal epochs is below normal and this may be attributed to the strong El Niño events. The dryness in the northeast region cannot be linked with ENSO events as it has insignificant relationship with the rainfall over this region. The weakening of Low Level Jet during the recent epoch can be a reason to reduce monsoon rainfall during recent periods. It is evident that the reduced moisture transport also accounts for the reduced precipitation. The changes in equatorial Walker and regional Hadley circulations during the three epochs show the same mode of variability and in phase with the changes in rainfall and other circulation related parameters. This study emphasizes the basin mode warming of world oceans and the increased frequency of El Nino events during the recent epochs linked to the climate change and its impact of ISMR. [ABSTRACT FROM AUTHOR]

Observational evidence suggests that the sub-arctic North Pacific (SANP; 45°–60° N, 155° E–165° W) sea surface temperature (SST) shows pronounced multidecadal variability, which cannot be explained by the Pacific Decadal Oscillation (PDO). Here, we find that the SANP SST multidecadal variability is closely linked to the remote Atlantic Multidecadal Oscillation (AMO), indicating a multidecadal inter-basin teleconnection. The teleconnection can be well reproduced in a set of Atlantic Pacemaker experiments. An atmospheric bridge mechanism for the teleconnection is proposed by analyzing both observations and simulation data. The AMO warm phase generates anomalous ascent and upper-level divergence over the North Atlantic. The upper-level outflows converge towards the subarctic North Pacific, leading to compensating subsidence along with anomalous high pressure there. The enhanced adiabatic descent causes anomalous warming and moistening of the lower troposphere above the SANP basin and increases the downwelling longwave radiation. The warming of the SANP SST is further induced and amplified due to water vapor-longwave radiation-SST positive feedback. The anomalous high also weakens the climatological cyclonic flow of Aleutian low and suppresses the turbulent heat release from ocean to atmosphere, contributing to the SANP SST warming. Our findings suggest that the AMO plays a crucial role in the subarctic North Pacific SST multidecadal variability. [ABSTRACT FROM AUTHOR]

This study analyzes the multidecadal changes in the severity of extreme meteorological droughts at the regional scale in China during 1951–2017. Dominance analysis is applied to multiple linear regression models to quantify the relative influence of global warming and internal variability on the meteorological drought severity in nine climate regions of China to understand which drivers are the most significant for each region and how they are likely to influence the severity of droughts in the near future (10–20 years). The influence of internal variability is represented by the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). The PDO and global warming are found to have a stronger influence on the multidecadal variability of drought severity in China than the AMO. Global warming is found to be the more dominant driver of multidecadal variability of drought severity in the western parts of China, whereas the PDO is found to have a more dominant influence in the eastern parts of China. In the near future, global warming and the PDO are both likely to contribute to reduction of drought severity in Xinjiang, Northwest, and Tibet regions. The positive phase of the PDO is also expected to reduce the severity of droughts in Inner Mongolia and South China. On the other hand, both the PDO and global warming are expected to contribute to increase in drought severity in North and Southwest China in the near future. Key Points: Effect of global warming and PDO has been much stronger than that of AMO on the multidecadal variability of drought severity in ChinaGlobal warming is found to be dominant driver in Xinjiang, Tibet and Northwest regionsPDO is found to be the dominant driver in Inner Mongolia, North, East, and South China [ABSTRACT FROM AUTHOR]

Atlantic multidecadal variability (AMV) of sea surface temperature exhibits an important influence on the climate of surrounding continents. It remains unclear, however, the extent to which AMV is due to internal climate variability (e.g., ocean circulation variability) or changes in external forcing (e.g., volcanic/anthropogenic aerosols or greenhouse gases). Here, the sources of AMV are examined over a 340-yr period using proxy indices, instrumental data, and output from the Last Millennium Ensemble (LME) simulation. The proxy AMV closely follows the accumulated atmospheric forcing from the instrumental North Atlantic Oscillation (NAO) reconstruction (r = 0.65)—an "internal" source of AMV. This result provides strong observational evidence that much of the AMV is generated through the oceanic response to atmospheric circulation forcing, as previously demonstrated in targeted modeling studies. In the LME there is a substantial externally forced AMV component, which exhibits a modest but significant correlation with the proxy AMV (i.e., r = 0.37), implying that at least 13% of the AMV is externally forced. In the LME simulations, however, the AMV response to accumulated NAO forcing is weaker than in the proxy/observational datasets. This weak response is possibly related to the decadal NAO variability, which is substantially weaker in the LME than in observations. The externally forced component in the proxy AMV is also related to the accumulated NAO forcing, unlike in the LME. This indicates that the external forcing is likely influencing the AMV through different mechanistic pathways: via changes in radiative forcing in the LME and via changes in atmospheric circulation in the observational/proxy record. [ABSTRACT FROM AUTHOR]

Features of decadal–multidecadal variations of the Asian summer rainfall are revealed by analysis of the reconstructed Asian summer precipitation (RAP) dataset from 1470 to 2013. Significant low-frequency periodicities of the all-Asian rainfall (AAR) index (AARI) are found on decadal (8–10 yr), quasi-bidecadal (22 yr), and multidecadal (50–54 yr) time scales, as well as centennial time scales. The decadal and multidecadal peaks are mainly from the "monsoon Asia" area and the Maritime Continent, while the 22-yr peak is from the "arid Asia" area. A remarkable change of leading frequency from multidecadal to decadal after AD 1700 is detected across the entire Asian landmass. The leading empirical orthogonal function (EOF) modes on the decadal and multidecadal time scales exhibit a uniform structure similar to that on the interannual time scale, suggesting a cross-time-scale, in-phase variation of the rainfall across continental Asia and the Maritime Continent. Enhanced AAR on a decadal time scale is found associated with the mega-La Niña sea surface temperature (SST) pattern over the Pacific. The AARI–mega-ENSO (El Niño–Southern Oscillation) relationship is persistently significant except from 1820 to around 1900. Enhanced decadal AAR is also found to be associated with extratropical North Atlantic warming. The AARI–AMO (Atlantic multidecadal oscillation) relationship, however, is nonstationary. On the multidecadal time scale, the AAR is significantly related to the AMO. Mechanisms associated with the decadal–multidecadal variability of AAR are also discussed. [ABSTRACT FROM AUTHOR]

The relations of the low-level extratropical cyclones in the southeastern Pacific and South Atlantic with the sea surface temperature (SST) anomalies associated with the Atlantic multidecadal oscillation (AMO) during the summer and winter of the 1979–93 cold AMO (CAMO) and 2003–17 warm AMO (WAMO) are analyzed. During both seasons and in both AMO phases, the cyclone trajectories defined by cyclone local counts exceeding 10 events per grid box occur approximately in the areas with the AMO-related positive SST anomalies. The cyclone densities in most latitudes during both seasons are higher in the CAMO than in the WAMO. Thus, the cyclone density in the study domain presents a reduction trend during the 1979–2017 period. The large-scale northward SST anomalous gradients between the bands north and south of 40°S increase the long-wave baroclinicity in the midlatitudes in the WAMO, and the southward SST anomalous gradients decrease it in the CAMO. Consequently, the short-wave baroclinicity is higher in the WAMO than in the CAMO in the southeastern Pacific midlatitudes. Thus, the cyclones are more energetic in the WAMO than in the CAMO. In the South Atlantic region off the Argentinean coast, both the barotropic and baroclinic conversion terms are positive, indicating an increase of the kinetic energy of the short waves. The low-level cyclones in the southeastern Pacific and South Atlantic are modulated by the AMO. As far as we know, the relation of the SH low-level extratropical cyclones to the AMO documented here was not studied before. [ABSTRACT FROM AUTHOR]

Rapid warming has led to an aggregated environmental degradation over the Tibetan Plateau (TP) in the last few decades, including accelerated glacier retreat, early snowmelt, permafrost degradation, and forest fire occurrence. Attribution of this warming in recent decades has mainly been focused on anthropogenic forcing. Yet, linkages to the Atlantic multidecadal variability (AMV), an essential part of the climate system causing decadal to centennial fluctuations of temperature, remains poorly understood for the TP, especially at long time scales. Using well-replicated tree-ring width records, we reconstructed 358 years of summer minimum temperature (MinT) of the whole TP. This reconstruction matches the recent warming signal recorded since the 1980s, and captures 63% of the variance in 1950–2005 instrumental records. A teleconnection from the North Atlantic to the TP is further identified based in observations and simulations with an atmospheric general circulation model (AGCM). We propose that half of the multidecadal variability of TP summer MinT can be explained by the AMV over the past three and a half centuries. Both observations and AGCM simulations indicate that the AMV warm phase induces a zonal dipole response in sea level pressure across the Atlantic–Eurasia region, with anomalously high surface pressure and corresponding downward atmospheric motion over the TP. We propose that the descending motion during warm AMV phases causes negative rainfall and positive temperature anomalies over the TP. Our findings highlight that the AMV plays a role in the multidecadal temperature variability over the TP. [ABSTRACT FROM AUTHOR]

In this study, we analyze the nonstationarity in meteorological droughts at the multidecadal scale in different parts of the contiguous United States during 1901–2017. We develop metrics to compare the drought risk calculated under the assumptions of stationarity and nonstationarity and identify their spatial and temporal patterns. By analyzing the variability of drought risk in the past and exploring its ongoing patterns, we evaluate in which regions of the contiguous United States the assumption of stationarity can be safely used for drought risk planning and management. We find statistically significant interdecadal changes in the probability distribution functions of drought severity in parts of the Northwest, upper Midwest, the Northeast, eastern parts of Great Plains and in parts of Arizona, New Mexico, Utah, and Nevada in the Southwest. In these regions, the nonstationary risk has been significantly higher than the stationary estimate of risk in the past, which shows that the assumption of stationarity can lead to the underestimation of drought risk in these regions. The multidecadal drought risk shows low variability in California, parts of northern and western Great Plains, Ohio Valley, and in the Southeast, since the statistical properties of droughts have not changed significantly in these regions during 1901–2017. However, the meteorological drought risk has increased in California and the Southeast in the recent decades due to the influence of global warming and hence the assumption of stationarity for risk estimation may lead to underestimation of drought risk in future in these regions if this effect of global warming persists. Plain Language Summary: Traditionally, statistical approaches adopted by water resource managers for planning and design of water resource systems and infrastructure are based on the assumption of stationarity; that is, it is assumed that the probabilistic characteristics of the hydrological and meteorological processes do not change with time, and hence, the planning and designs for future can be based on the past observations. In this paper, we have evaluated the validity of the stationarity assumption for meteorological drought risk estimation at the multidecadal scale by comparing drought risk calculated under the assumptions of stationarity and nonstationarity, respectively, in different parts of the continental United States. We find statistically significant nonstationarity in meteorological droughts in the Northwest, upper Midwest, the Northeast, eastern Great Plains and in parts of Nevada, Utah, Arizona, and New Mexico in the Southwest United States, which results in high interdecadal variability of drought risk in these regions. This result demonstrates that the assumption of stationarity can lead to underestimation of drought risk in these regions, thereby exposing water resource systems to failure under severe droughts. Key Points: The assumption of stationarity for meteorological drought risk estimation is evaluated in different parts of the CONUS at the multidecadal scaleNonstationarity is identified with statistical significance in the Northwest, upper Midwest, the Northeast, eastern Great Plains and in parts of Nevada, Utah, Arizona, and New Mexico in the Southwest United StatesInterdecadal changes in drought severity have not been statistically significant in California, Ohio valley, northern and western Great Plains, and the Southeast United States [ABSTRACT FROM AUTHOR]

Meteorological droughts in the continental United States (CONUS) are known to oscillate at the multidecadal time scale in response to the sea surface temperatures (SST) variability over the Pacific Ocean and the North Atlantic Ocean. While previous studies have focused on understanding the influence of SST oscillations on drought frequency over the CONUS, this information has not been integrated with global warming for future drought risk assessment at the decadal scale. In this study, we use the support vector machines (SVMs) to handle correlation between input variables for quantifying the influence of internal variability [Atlantic multidecadal oscillation (AMO) and Pacific decadal oscillation (PDO)] and global warming on the decadal changes in the severity of seasonal droughts over the CONUS during 1901–2015. The regional drivers of drought severity identified using SVMs are used for the assessment of decadal drought risk in the near future. We find internal variability as the dominant driver of decadal changes in drought severity in the southern and central Great Plains and global warming as the dominant driver for the southeastern and southwestern United States. In the southern Plains, the existing pattern of increasing drought severity is likely to persist in the near future if AMO and PDO remain in their positive and negative phases, respectively, while global warming is likely to contribute to increasing drought severity in the Southeast and Southwest. This study suggests an emerging role of global warming in drought risk over the southern states, where near-term climate change adaptation is necessary. [ABSTRACT FROM AUTHOR]

Extracted nonstationary oscillations and nonlinear trends of precipitation and sea surface temperature (SST) data reveal that rainfall variability in Mexico City is mainly composed by a long‐term positive trend, a multidecadal oscillation highly correlated with the Atlantic Multidecadal Oscillation (AMO), and year‐to‐year variability. The precipitation trend, lasting for more than a century, cannot be attributed to global warming or urbanization alone; rather, it can be thought of as part of a natural oscillation composed of alternating wet‐dry anomalies with a period of a couple of centuries, as past evidence indicates. To further test the dependence of the AMO‐related component, yearly SST time series were derived from a simplified model of the atmosphere‐ocean system forced by white noise. The simulated SST time series exhibits AMO‐like variability entirely consistent with the observed one, implying that North Atlantic SST multidecadal variability can be seen as the integrated response of surface ocean layers to external stochastic atmospheric forcing. Plain Language Summary: In this work, nonlinear trends and nonstationary oscillations are extracted from both annual accumulated precipitation and sea surface temperature (SST) data. Our results show that the precipitation variability in Mexico City is mainly composed by a very long‐term positive trend of more than a century, a multidecadal oscillation highly correlated with the Atlantic Multidecadal Oscillation (AMO), and year‐to‐year variability. The long‐term positive trend cannot be caused by global warming or urbanization alone; rather, it can be seen as the positive part of a natural oscillation composed of alternating wet‐dry anomalies with a period of a couple of centuries, as there are past evidence supporting this interpretation. In order to get a depth understanding of the origin of the AMO‐related component, yearly SST time series were obtained by using a simple box model of the ocean‐atmosphere system forced by white noise (short timescale atmospheric forcing). This simple model is able to simulate SST anomalies that exhibit AMO‐like variability that is entirely consistent with the observed one, implying that North Atlantic SST multidecadal variability can be seen simply as the response of near‐surface layers of the ocean to stochastic atmospheric forcing. Key Points: Long‐term secular positive precipitation trends in Mexico City cannot be accounted for by man‐induced climate change aloneThe Atlantic Multidecadal Oscillation can be explained as an integrated response of the upper ocean to surface stochastic forcingLong‐term positive trends in precipitation in Mexico City can be thought of as the increasing portion of a longer period oscillation [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal oscillation (AMO) has been shown to play a major role in the multidecadal variability of the Northern Hemisphere, impacting temperature and precipitation, including intertropical convergence zone (ITCZ)-driven precipitation across Africa and South America. Studies into the location of the intertropical convergence zone have suggested that it resides in the warmer hemisphere, with the poleward branch of the Hadley cell acting to transport energy from the warmer hemisphere to the cooler one. Given the impact of the Atlantic multidecadal oscillation on Northern Hemisphere temperatures, we expect the Atlantic multidecadal oscillation to have an impact on the location of the intertropical convergence zone. We find that the positive phase of the Atlantic multidecadal oscillation warms the Northern Hemisphere, resulting in a northward shift of the intertropical convergence zone, which is evident in the Pacific climate proxy record. Using a coupled climate model, we further find that the shift in the intertropical convergence zone is consistent with the surface energy imbalance generated by the Atlantic multidecadal oscillation. In this model, the Pacific changes are driven in large part by the warming of the tropical Atlantic and not the extratropical Atlantic. [ABSTRACT FROM AUTHOR]

Two independent atmospheric general circulation models reveal that the positive (negative) phase of Atlantic multidecadal variability (AMV) can reduce (amplify) the variance of the shorter time-scale (e.g., ENSO related) precipitation fluctuations in the United States, especially in the Southwest, as well as decrease (increase) the long-term seasonal mean precipitation for the cold season. The variance is modulated because of changes in 1) dry day frequency and 2) maximum daily rainfall intensity. With positive AMV forcing, the upper-level warming originating from the increased precipitation over the tropical Atlantic Ocean changes the mean vertical thermal structure over the United States continent to a profile less favorable for rain-inducing upward motions. In addition, a northerly low-level dry advection associated with the local overturning leaves less available column moisture for condensation and precipitation. The opposite conditions occur during cold AMV periods. [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal oscillation (AMO) in sea surface temperature (SST) has been shown to influence the climate of the surrounding continents. However, it is unclear to what extent the observed impact of the AMO is related to the thermodynamical influence of the SST variability or the changes in large-scale atmospheric circulation. Here, an analog method is used to decompose the observed impact of the AMO into dynamical and residual components of surface air temperature (SAT) and precipitation over the adjacent continents. Over Europe the influence of the AMO is clearest during the summer, when the warm SAT anomalies are interpreted to be primarily thermodynamically driven by warm upstream SST anomalies but also amplified by the anomalous atmospheric circulation. The overall precipitation response to the AMO in summer is generally less significant than the SAT but is mostly dynamically driven. The decomposition is also applied to the North American summer and the Sahel rainy season. Both dynamical and residual influences on the anomalous precipitation over the Sahel are substantial, with the former dominating over the western Sahel region and the latter being largest over the eastern Sahel region. The results have potential implications for understanding the spread in AMO variability in coupled climate models and decadal prediction systems. [ABSTRACT FROM AUTHOR]

A novel Lagrangian framework is developed to attribute monthly precipitation variability to physical processes. Precipitation variability is partitioned into a combination of five factors: airmass origin location, origin surface temperature variation, ascent intensity, mass fraction of ascending air, and the number of 'wet' analysis times per month [>1 mm (6 h)−1]. Precipitation in a target region is linked to 'origin' locations of air masses where the water vapor mixing ratio was last set by boundary layer moistening and is a maximum along back trajectories. Applying the technique to the England and Wales region, the factors together account for 83%-89% of the observed summer precipitation variability. The dominant contributor is the number of wet analyses, which is shown to be associated with cyclone statistics. The wettest summer months are mainly associated with anomalous cyclone duration rather than the number of cyclones. In addition, surface temperature and saturation humidity at the origin locations are found to be below their climatological averages (1979-2013). Therefore, the direct thermodynamic effect of anomalous surface temperature on marine boundary layer humidity acts to reduce monthly precipitation anomalies. The decadal precipitation change between phases of the Atlantic multidecadal oscillation is approximately 20% of the interannual variability between summer months. Changes in cyclone statistics have an effect 6 times larger than the direct thermodynamic factor in both monthly and decadal precipitation variability. [ABSTRACT FROM AUTHOR]

Results from a study inspecting the origins of multidecadal variability in the North Atlantic sea surface temperature (NASST) are presented. The authors target in particular the 1940-75 'warm-to-cold' transition, an event that is generally framed in the context of the longer-term Atlantic multidecadal variability (AMV) cycle, in turn associated with the Atlantic meridional overturning circulation (AMOC) internal variability. Here the authors examine the ability of uninitialized, historical integrations from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) archive to retrospectively reproduce this specific episode of twentieth-century climatic history, under a hierarchy of forcing conditions. For this purpose, both standard and so-called historical Misc CMIP5 simulations of the historical climate (combining selected natural and anthropogenic forcings) are exploited. Based on this multimodel analysis, evidence is found for a significant influence of anthropogenic agents on multidecadal sea surface temperature (SST) fluctuations across the Atlantic sector, suggesting that anthropogenic aerosols and greenhouse gases might have played a key role in the 1940-75 North Atlantic cooling. However, the diagnosed forced response in CMIP5 models appears to be affected by a large uncertainty, with only a limited subset of models displaying significant skill in reproducing the mid-twentieth-century NASST cooling. Such uncertainty originates from the existence of well-defined behavioral clusters within the analyzed CMIP5 ensembles, with the bulk of the models splitting into two main clusters. Such a strong polarization calls for some caution when using a multimodel ensemble mean in climate model analyses, as averaging across fairly distinct model populations may result, through mutual cancellation, in a rather artificial description of the actual multimodel ensemble behavior. A potentially important role for both anthropogenic aerosols and greenhouse gases with regard to the observed North Atlantic multidecadal variability has clear implications for decadal predictability and predictions. The uncertainty associated with alternative aerosol and greenhouse gas emission scenarios should be duly accounted for in designing a common protocol for coordinated decadal forecast experiments. [ABSTRACT FROM AUTHOR]

Empirical orthogonal function (EOF) analysis of global sea surface temperature yields modes in which interannual variability associated with ENSO and the lower-frequency variability associated with the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are confounded with one another and with the signature of global warming. The confounded EOFs exhibit overlapping centers of action with polarities of the perturbations juxtaposed such that the respective modes are mutually orthogonal in the global domain. When physical modes with different time scales appear in the same pair of EOFs, the principal component (PC) time series tend to be positively correlated in one frequency band and negatively correlated in another. Mode mixing may be a reflection of sampling variability or it may reflect the lack of spatial orthogonality of the physical modes themselves. Using sequences of pairwise orthogonal rotations of selected PCs, it is possible, without recourse to filtering, to recover a global warming mode with a bland spatial pattern and a nearly linear upward trend, along with dynamical modes, each with its own characteristic time scale, that resemble ENSO, the PDO, and the AMO. Novel elements of this analysis include a rationale for choosing the optimal angle for pairwise rotation and a simple algorithm for eliminating mode mixing between the dynamical modes and the global warming mode by transferring the linear trends from the former to the latter. [ABSTRACT FROM AUTHOR]

This article investigates the dynamics and temporal evolution of the Atlantic multidecadal variability (AMV) in a coupled climate model. The model contains a correction to the North Atlantic flow field to improve the path of the North Atlantic Current, thereby alleviating the surface cold bias, a common problem with climate models, and offering a unique opportunity to study the AMV in a model. Changes in greenhouse gas forcing or aerosol loading are not considered. A striking feature of the results is the contrast between the western and eastern sides of the subpolar gyre in the model. On the western side, anomalous heat supply by the ocean plays a major role, with most of this heat being given up to the atmosphere in the warm phase, largely symmetrically about the time of the AMV maximum. By contrast, on the eastern side, the ocean anomalously gains heat from the atmosphere, with relatively little role for ocean heat supply in the years before the AMV maximum. Thereafter, the balance changes with heat now being anomalously removed from the eastern side by the ocean, leading to a reduced ocean heat content, behavior associated with the establishment of an intergyre gyre at the time of the AMV maximum. In the warm phase, melting sea ice leads to a freshening of surface waters northeast of Greenland that travel southward into the Irminger and Labrador Seas, shutting down convection and terminating the AMV warm phase. [ABSTRACT FROM AUTHOR]

Precipitation and surface temperature are interdependent variables, both as a response to atmospheric dynamics and due to intrinsic thermodynamic relationships and feedbacks between them. This study analyzes the covariability of seasonal temperature (T) and precipitation (P) across the Iberian Peninsula (IP) using regional climate paleosimulations for the period 1001–1990, driven by reconstructions of external forcings. Future climate (1990–2099) was simulated according to SRES scenarios A2 and B2. These simulations enable exploring, at high spatial resolution, robust and physically consistent relationships. In winter, positive P-T correlations dominate west-central IP (Pearson correlation coefficient ρ = + 0.43, for 1001–1990), due to prevalent cold-dry and warm-wet conditions, while this relationship weakens and become negative towards mountainous, northern and eastern regions. In autumn, negative correlations appear in similar regions as in winter, whereas for summer they extend also to the N/NW of the IP. In spring, the whole IP depicts significant negative correlations, strongest for eastern regions (ρ = − 0.51). This is due to prevalent frequency of warm-dry and cold-wet modes in these regions and seasons. At the temporal scale, regional correlation series between seasonal anomalies of temperature and precipitation (assessed in 31 years running windows in 1001–1990) show very large multidecadal variability. For winter and spring, periodicities of about 50–60 years arise. The frequency of warm-dry and cold-wet modes appears correlated with the North Atlantic Oscillation (NAO), explaining mainly co-variability changes in spring. For winter and some regions in autumn, maximum and minimum P-T correlations appear in periods with enhanced meridional or easterly circulation (low or high pressure anomalies in the Mediterranean and Europe). In spring and summer, the Atlantic Multidecadal Oscillation shows some fingerprint on the frequency of warm/cold modes. For future scenarios, an intensification of the negative P-T relationship is generally found, as a result of an increased frequency of the warm-dry mode. [ABSTRACT FROM AUTHOR]

A transient multicentury simulation mimicking natural variability has been performed for the Baltic Sea. The simulation is used for investigations of long-term trends of salinity in the Baltic Sea with special focus on periods of salinity reduction. Periods with decreasing salinity over 10 yr are found to appear approximately once per century. Considering extended periods of salinity reduction, as observed from 1976 to 1992, such events are found to be quite exceptional. Based on the climate simulation, a return period of 200 yr is estimated. River discharge, net precipitation (precipitation minus evaporation), and zonal wind are identified as the most important drivers for salinity variations in the Baltic Sea. For multidecadal periods, almost two-thirds of the salinity variability can be explained by annual means of river discharge, precipitation, both wind components, temperature, and the North Atlantic Oscillation, when multilinear regression techniques are used. However, the evaluation of wavelet coherences among the time series highlights that this relationship is not constant in time. At least three periods exist, each spanning roughly 50 yr, where the coherence between salinity and runoff as the common main driver is rather weak. This indicates that the importance of river discharge might be limited for certain periods, and drivers such as zonal wind may become more important. Finally, the variability of the Baltic Sea salinity shows increased power on time scales of 100 yr and longer. Such periodicity has never been shown for Baltic Sea salinity, and the driving mechanism remains unclear. [ABSTRACT FROM AUTHOR]

This paper investigates the predictive relation between the Atlantic multidecadal oscillation (AMO) and Atlantic meridional overturning circulation across different climate models. Three overturning patterns that are significantly coupled to the AMO on interannual time scales across all climate models are identified using a statistical optimization technique. Including these structures in an autoregressive model extends AMO predictability by 2-9 years, relative to an autoregressive model without these structures. [ABSTRACT FROM AUTHOR]

ABSTRACT Eastern China suffered a record-breaking heat wave throughout almost all of summer, during 2013. Recent studies found a discernible impact of anthropogenic forcing on this extreme heat wave event. In this study, we investigate the role of multidecadal variability ( MDV) in regulating the likelihood of the 2013 heat wave event. Using the ensemble empirical mode decomposition ( EEMD) method, we decomposed the heat wave index series into three components: (1) a non-linear secular trend ( ST), representing the long-term anthropogenic warming; (2) MDV; and (3) the residual high-frequency variability. For the period 1873-2013, the linear trend of heat wave strength ( HW-S, defined as the July-August mean temperatures) was 0.06 °C per decade, mainly because of the contribution of ST. MDV plays an important role in regulating changes over decades. ST and MDV together form a changing climate background ( CB) for extreme events. The 2013 heat wave event would be almost impossible under the pre-industrial CB but becomes likely under the long-term warming background, with a return period of longer than 103 years. When the positive phase of MDV is added to the secular warming background (the current CB), the return period of a super-heat wave such as the 2013 event becomes about 42 (12-103) years. The Atlantic Multidecadal Oscillation ( AMO) shows significant correlation with the MDV component of the HW-S in Shanghai. The correlation pattern between the AMO and MDV of July-August temperatures over the Northern Hemisphere is analysed to explain the AMO-Shanghai relationship. It is suggested that the likelihood of such an extreme event will increase with further long-term climate warming, modified by low-frequency oceanic variations such as the AMO. [ABSTRACT FROM AUTHOR]

A weakly damped mode of variability, corresponding to the oceanic signature of the Atlantic multidecadal oscillation (AMO) was found through the linear stability analysis of a realistic ocean general circulation model. A simple two-level model was proposed to rationalize both its period and damping rate. This model is extended here to three levels to investigate how the mode can draw energy from the mean flow, as found in various ocean and coupled models. A linear stability analysis in this three-level model shows that the positive growth rate of the oscillatory mode depends on the zonally averaged isopycnal slope. This mode corresponds to a westward propagation of density anomalies in the pycnocline, typical of large-scale baroclinic Rossby waves. The most unstable mode corresponds to the largest scale one (at least for low isopycnal slope). The mode can be described in four phases composing a full oscillation cycle: 1) basin-scale warming of the North Atlantic (AMO positive phase), 2) decrease in upper-ocean poleward transport [hence a reduction of the Atlantic meridional overturning circulation (AMOC)], 3) basin-scale cooling (negative AMO), and 4) AMOC intensification. A criterion is developed to test, in oceanic datasets or numerical models, whether this multidecadal oscillation is an unstable oceanic internal mode of variability or if it is stable and externally forced. Consistent with the classical theory of baroclinic instability, this criterion depends on the vertical structure of the mode. If the upper pycnocline signature is in advance of the deeper pycnocline signature with respect to the westward propagation, the mode is unstable and could be described as an oceanic internal mode of variability. [ABSTRACT FROM AUTHOR]

Statistical, dynamical, and statistical-dynamical hybrid models have been developed in past decades for the seasonal prediction of North Atlantic hurricane numbers. These models' prediction skills show considerable decadal variability, with particularly poor performance in the past few years. Here, environmental factors that affect hurricane activities are reevaluated to develop a new statistical model for seasonal prediction by 1 June of each year. The predictors include the April-May multivariate ENSO index (MEI) conditioned upon the Atlantic multidecadal oscillation (AMO) index, the power of the average zonal pseudo-wind stress across the North Atlantic in May, and the average March-May tropical Atlantic sea surface temperature. When compared to the actual number of hurricanes each year from 1950 to 2013, this model has a root-mean-square error (RMSE) of 1.91 with a correlation coefficient of 0.71. It shows a 39% improvement in RMSE over a no-skill metric (based on the 5-yr running mean of seasonal hurricane counts) for the period 2001-13. It also outperforms three statistical-dynamical hybrid models [CPC, Colorado State University (CSU), and Tropical Storm Risk (TSR)] by more than 25% for the same period. Furthermore, two approaches are developed to provide the uncertainty ranges around the predicted (deterministic) hurricane number per season that better encompass the range of uncertainty than does the standard method of adding/subtracting a standard deviation of the errors. [ABSTRACT FROM AUTHOR]

The ocean-atmosphere coupling in the northeastern subtropical Pacific is dominated by a Pacific meridional mode (PMM), which spans between the extratropical and tropical Pacific and plays an important role in connecting extratropical climate variability to the occurrence of El Niño. Analyses of observational data and numerical model experiments were conducted to demonstrate that the PMM (and the subtropical Pacific coupling) experienced a rapid strengthening in the early 1990s and that this strengthening is related to an intensification of the subtropical Pacific high caused by a phase change of the Atlantic multidecadal oscillation (AMO). This PMM strengthening favored the development of more central Pacific (CP)-type El Niño events. The recent shift from more conventional eastern Pacific (EP) to more CP-type El Niño events can thus be at least partly understood as a Pacific Ocean response to a phase change in the AMO. [ABSTRACT FROM AUTHOR]

The characteristics of multidecadal variability (MDV) in global land surface air temperature (SAT) are analyzed based on observations. The role of sea surface temperature (SST) variations in generating MDV in land SAT is assessed using atmospheric general circulation model simulations forced by observed SST. MDV in land SAT exhibits regional differences, with amplitude larger than 0.3 °C mainly over North America, East Asia, Northern Eurasia, Northern Africa and Greenland for the study period of 1902-2004. MDV can account for more than 30 % of long-term temperature variation during the last century in most regions, especially more than 50 % in parts of the above-mentioned regions. The SST-forced simulations reproduce the observed feature of zonal mean MDV in land SAT, though with weaker amplitude especially at the northern high-latitudes. Two types of MDV in land SAT, one of 60-year-timescale, mainly observed in the northern mid-high-latitude lands, and another of 20-30-year-timescale, mainly observed in the low-latitude lands, are also well reproduced. The SST-forced MDV accounts for more than 40 % amplitude of observed MDV in most regions. Except for some sporadically distributed regions in central Eurasia, South America and Western Australia, the SST-forced multidecadal variations are well in-phase with observations. The Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation signals are found dominant in MDV of both the observed and SST-forced land SAT, suggesting important roles of these oceanic oscillations in generating MDV in global land SAT. [ABSTRACT FROM AUTHOR]