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The El Niño Southern Oscillation (ENSO) is one of the most prominent modes of coupled variability with sizable impacts on global climate and weather patterns, which makes the ability to predict the occurrence and development of ENSO events of fundamental importance. In order to achieve accurate and timely predictions, a well-established strategy is to understand and monitor known ENSO precursors. In this paper, we focus on North Pacific Oscillation (NPO)-related precursors, namely the trade wind charging and the Northern Pacific meridional mode (TWC/NPMM). In particular, we assess whether the TWC/NPMM mode and its relationship with ENSO is reconstructed across the CMIP6 protocol-driven ensemble High Resolution Model Intercomparison Project, which was developed to systematically test the impact of increased horizontal resolution. Here, we see that the TWC/NPMM is a consistent precursor of ENSO across the ensemble, notwithstanding some spatial variations in the reconstruction. Furthermore, previous analyses on observationally-based data show that the TWC/NPMM-ENSO relationship is robust, albeit not stationary, and its variations can influence the characteristic variability of ENSO itself. In particular, during those years when the TWC/NPMM-ENSO coupling is weak, ENSO oscillates regularly with constant periodicity; whereas, when the coupling is strong, ENSO shows a more stochastic behavior. A selected subset of better-performing HighResMIP models are able to reproduce the non-stationarity of the TWC/NPMM-ENSO coupling and to recreate how these variations are reflected in the characteristics of ENSO variability, similar to what was recorded in the observational analysis. In parallel with these analyses, we also assess their sensitivity to horizontal resolution and find that there is no consistent impact of resolution on the results described above. [ABSTRACT FROM AUTHOR]

The Kuroshio Extension (KE) is the inertial meandering jet formed by the convergence of the Kuroshio and Oyashio currents in the Northern Pacific. It is widely mentioned in the literature that the KE variability is bimodal on interannual to decadal time scale. The nature of this low frequency variability (LFV) is still under debate; intrinsic oceanic mechanisms are known to play a fundamental role in the phenomenon but there is also evidence from observations that the KE LFV is connected with changes in broader patterns associated with the Pacific Decadal Oscillation, which is in its turn generated by the dominant decadal mode of the sea level pressure variability in the North Pacific. We investigate the respective contributions of oceanic and atmospheric drivers of the KE variability by taking advantage of the OCCIPUT 1/4° global model dataset: it consists in an ensemble of 50 ocean–sea ice hindcasts performed over the period 1960–2015 (hereafter OCCITENS), and in a one-member 330-year climatological simulation (hereafter OCCICLIM). In this context, OCCITENS simulates both the intrinsic and forced variability, while OCCICLIM simulates the "pure" intrinsic variability. We explore several features of the KE, finding analogies between the OCCICLIM and OCCITENS datasets with autonomous and non-autonomous dynamical systems respectively. This approach aims to apply concepts from the dynamical systems theory on complex and realistic ocean simulations. In this framework, the results suggest that both oceanic and atmospheric drivers control the KE LFV, and that the effect of the low-frequency atmospheric forcing reduces the phase space region explored by the system through synchronization mechanisms. The system's intrinsic variability can be paced, and therefore clustered over the system's pullback attractor under the effect of the time dependent forcing. [ABSTRACT FROM AUTHOR]

Atlantic multidecadal variability (AMV) has been linked to the observed slowdown of global warming over 1998–2012 through its impact on the tropical Pacific. Given the global importance of tropical Pacific variability, better understanding this Atlantic–Pacific teleconnection is key for improving climate predictions, but the robustness and strength of this link are uncertain. Analyzing a multi-model set of sensitivity experiments, we find that models differ by a factor of 10 in simulating the amplitude of the Equatorial Pacific cooling response to observed AMV warming. The inter-model spread is mainly driven by different amounts of moist static energy injection from the tropical Atlantic surface into the upper troposphere. We reduce this inter-model uncertainty by analytically correcting models for their mean precipitation biases and we quantify that, following an observed 0.26 °C AMV warming, the equatorial Pacific cools by 0.11 °C with an inter-model standard deviation of 0.03 °C. [ABSTRACT FROM AUTHOR]

We study the impact of horizontal resolution in setting the North Atlantic gyre circulation and representing the ocean–atmosphere interactions that modulate the low-frequency variability in the region. Simulations from five state-of-the-art climate models performed at standard and high-resolution as part of the High-Resolution Model Inter-comparison Project (HighResMIP) were analysed. In some models, the resolution is enhanced in the atmospheric and oceanic components whereas, in some other models, the resolution is increased only in the atmosphere. Enhancing the horizontal resolution from non-eddy to eddy-permitting ocean produces stronger barotropic mass transports inside the subpolar and subtropical gyres. The first mode of inter-annual variability is associated with the North Atlantic Oscillation (NAO) in all the cases. The rapid ocean response to it consists of a shift in the position of the inter-gyre zone and it is better captured by the non-eddy models. The delayed ocean response consists of an intensification of the subpolar gyre (SPG) after around 3 years of a positive phase of NAO and it is better represented by the eddy-permitting oceans. A lagged relationship between the intensity of the SPG and the Atlantic Meridional Overturning Circulation (AMOC) is stronger in the cases of the non-eddy ocean. Then, the SPG is more tightly coupled to the AMOC in low-resolution models. [ABSTRACT FROM AUTHOR]

A dominant paradigm for mid-latitude air-sea interaction identifies the synoptic-scale atmospheric "noise" as the main driver for the observed ocean surface variability. While this conceptual model successfully holds over most of the mid-latitude ocean surface, its soundness over frontal zones (including western boundary currents; WBC) characterized by intense mesoscale activity, has been questioned in a number of studies suggesting a driving role for the small scale ocean dynamics (mesoscale oceanic eddies) in the modulation of air-sea interaction. In this context, climate models provide a powerful experimental device to inspect the emerging scale-dependent nature of mid-latitude air-sea interaction. This study assesses the impact of model resolution on the representation of air-sea interaction over the Gulf Stream region, in a multi-model ensemble of present-climate simulations performed using a common experimental design. Lead-lag correlation and covariance patterns between sea surface temperature (SST) and turbulent heat flux (THF) are diagnosed to identify the leading regimes of air-sea interaction in a region encompassing both the Gulf Stream system and the North Atlantic subtropical basin. Based on these statistical metrics it is found that coupled models based on "laminar" (eddy-parameterised) and eddy-permitting oceans are able to discriminate between an ocean-driven regime, dominating the region controlled by the Gulf Stream dynamics, and an atmosphere-driven regime, typical of the open ocean regions. However, the increase of model resolution leads to a better representation of SST and THF cross-covariance patterns and functional forms, and the major improvements can be largely ascribed to a refinement of the oceanic model component. [ABSTRACT FROM AUTHOR]

The Kuroshio Extension (KE) shifts between elongated and convoluted states on interannual to decadal time scales. The nature of this low frequency variability (LFV) is still under debate since it is known to be driven by intrinsic oceanic mechanisms, but it is also synchronized with the Pacific Decadal Oscillation (PDO). In this analysis we present the results from two present-climate coupled simulations performed with the CMCC-CM2 model under the CMIP6 HighResMIP protocol and differing only by their atmospheric component resolution. The impact of increased atmospheric resolution on the KE LFV is assessed inspecting several aspects: the KE bimodality, the large-scale variability and the air–sea interactions. The KE LFV and the teleconnection mechanism that connects the KE and the PDO are well captured by both configurations. However, higher atmospheric resolution favors the occurrence of the elongated state and leads to a more realistic PDO representation. Moreover, both simulations qualitatively capture the signatures of atmosphere-driven and ocean-driven regimes over the North Pacific Ocean, even if the higher resolution induces an excessively strong ocean–atmosphere coupling that leads to an overestimation of the air–sea feedbacks. This work highlights that the small scale atmospheric variability (resolution lower than 1°) does not substantially contribute to improve the realism of the KE LFV, but causes significant differences in the air–sea interaction over the KE region likely related to the strengthening of the coupling. The eddy-permitting ocean resolution shared by both configurations is likely responsible for the degree of realism exhibited by the simulated KE LFV in the two analyzed simulations. [ABSTRACT FROM AUTHOR]

This study investigates how teleconnections linking tropical rainfall anomalies and wintertime circulation in the northern extra-tropics are represented in historical simulations for the period 1950–2010 run by partners of the EU-funded PRIMAVERA project, following the HighResMIP protocol of CMIP6. The analysis focusses on teleconnections from the western/central Indian Ocean in mid-winter and from the NINO4 region in both the early and the late part of winter; this choice is justified by a substantial change in the relationship between ENSO and the North Atlantic Oscillation (NAO) in the two parts of the season. Model results for both coupled integrations and runs with prescribed sea-surface temperature (SST) are validated against data from the latest ECMWF 20th-century re-analysis, CERA20C. Simulations from six modelling groups are considered, comparing the impact of increasing atmospheric resolution in runs with prescribed SST, and of moving from uncoupled to coupled simulations in the high-resolution version of each model. Single runs were available for each model configurations at the time of writing, with one centre (ECMWF) also providing a 6-member ensemble. Results from this ensemble are compared with those of a 6-member multi-model ensemble (MME) formed by including one simulation from each model. Using only a single historical simulation from each model configuration, it is difficult to detect a consistent change in the fidelity of model-generated teleconnections when either atmospheric resolution is increased or ocean coupling is introduced. However, when simulations from six different models are pooled together in the MME, some improvements in teleconnection patterns can be seen when moving from uncoupled to coupled simulations. For the ECMWF ensemble, improvements in the coupled simulations are only apparent for the late-winter NINO4 teleconnection. While the Indian Ocean teleconnection and the late-winter NINO4 teleconnection appear equally robust in the re-analysis record, the latter is well simulated in the majority of both uncoupled and coupled runs, while the former is reproduced with (generally) much larger errors, and a high degree of variability between individual models and ensemble members. Most of the simulations with prescribed SST fail to produce a realistic estimate of multi-decadal changes between the first and the second part of the 60-year record. This is (at least partially) due to their inability to simulate an Indian Ocean rainfall change which, in observations, has a zonal gradient out of phase with SST changes. In coupled runs, at least one model run with both realistic teleconnections and a good simulation of the inter-decadal pattern of Indian Ocean rainfall also shows a realistic NAO signal in extratropical multi-decadal variability. [ABSTRACT FROM AUTHOR]

Can multi-annual variations in the frequency of North Atlantic atmospheric blocking and mid-latitude circulation regimes be skilfully predicted? Recent advances in seasonal forecasting have shown that mid-latitude climate variability does exhibit significant predictability. However, atmospheric predictability has generally been found to be quite limited on multi-annual timescales. New decadal prediction experiments from NCAR are found to exhibit remarkable skill in reproducing the observed multi-annual variations of wintertime blocking frequency over the North Atlantic and of the North Atlantic Oscillation (NAO) itself. This is partly due to the large ensemble size that allows the predictable component of the atmospheric variability to emerge from the background chaotic component. The predictable atmospheric anomalies represent a forced response to oceanic low-frequency variability that strongly resembles the Atlantic Multi-decadal Variability (AMV), correctly reproduced in the decadal hindcasts thanks to realistic ocean initialization and ocean dynamics. The occurrence of blocking in certain areas of the Euro-Atlantic domain determines the concurrent circulation regime and the phase of known teleconnections, such as the NAO, consequently affecting the stormtrack and the frequency and intensity of extreme weather events. Therefore, skilfully predicting the decadal fluctuations of blocking frequency and the NAO may be used in statistical predictions of near-term climate anomalies, and it provides a strong indication that impactful climate anomalies may also be predictable with improved dynamical models. [ABSTRACT FROM AUTHOR]

Middle East surface air temperature (ME-SAT), during boreal summer (June to August: JJA), shows robust multidecadal variations for the period 1948−2016. Here, using observational and reanalysis datasets, as well as coupled atmosphere–ocean model simulations, we linked the observed summer ME-SAT variability to the multidecadal variability of sea surface temperature (SST) in the North Atlantic Ocean (AMV). This Atlantic−ME connection during summer involves ocean–atmosphere interactions through multiple ocean basins, with an influence from the Indian Ocean and the Arabian Sea. The downstream response to Atlantic SST is a weakening of the subtropical westerly jet stream that impacts summer ME-SAT variability through a wave-like pattern in the upper tropospheric levels. The Atlantic SST response is further characterized by positive geopotential height anomalies in the upper levels over the Eurasian region and a dipole-like pressure distribution over the ME in lower levels. For positive Atlantic SST anomalies this pressure gradient initiates anomalous low-level southerly flow, which transports moisture from the neighboring water bodies toward the extremely hot and dry ME landmass. The increase in atmospheric moisture reduces the longwave radiation damping of the SAT anomaly, increasing further ME-SAT. A suite of Atlantic Pacemaker experiments skillfully reproduces the North Atlantic–ME teleconnection. Our findings reveal that in observations and models the Atlantic Ocean acts as a critical pacemaker for summer ME-SAT multidecadal variability and that a positive AMV can lead to enhanced summer warming over the Middle East. [ABSTRACT FROM AUTHOR]

Arctic sea-ice area and volume have substantially decreased since the beginning of the satellite era. Concurrently, the poleward heat transport from the North Atlantic Ocean into the Arctic has increased, partly contributing to the loss of sea ice. Increasing the horizontal resolution of general circulation models (GCMs) improves their ability to represent the complex interplay of processes at high latitudes. Here, we investigate the impact of model resolution on Arctic sea ice and Atlantic Ocean heat transport (OHT) by using five different state-of-the-art coupled GCMs (12 model configurations in total) that include dynamic representations of the ocean, atmosphere and sea ice. The models participate in the High Resolution Model Intercomparison Project (HighResMIP) of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Model results over the period 1950–2014 are compared to different observational datasets. In the models studied, a finer ocean resolution drives lower Arctic sea-ice area and volume and generally enhances Atlantic OHT. The representation of ocean surface characteristics, such as sea-surface temperature (SST) and velocity, is greatly improved by using a finer ocean resolution. This study highlights a clear anticorrelation at interannual time scales between Arctic sea ice (area and volume) and Atlantic OHT north of 60 ∘ N in the models studied. However, the strength of this relationship is not systematically impacted by model resolution. The higher the latitude to compute OHT, the stronger the relationship between sea-ice area/volume and OHT. Sea ice in the Barents/Kara and Greenland–Iceland–Norwegian (GIN) Seas is more strongly connected to Atlantic OHT than other Arctic seas. [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal variability (AMV) is a coherent pattern of variability of the North Atlantic sea surface temperature field affecting several components of the climate system in the Atlantic region and the surrounding areas. The relatively short observational record severely limits our understanding of the physical mechanisms leading to the AMV. The present study shows that the spatial and temporal characteristics of the AMV, as assessed from the historical records, should also be considered as highly uncertain. Using 11 multi-century preindustrial climate simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database, we show that the AMV characteristics are not constant along the simulation when assessed from different 200-year-long periods to match the observed period length. An objective method is proposed to test whether the variations of the AMV characteristics are consistent with stochastic internal variability. For 7 out of the 11 models analysed, the results indicate a non-stationary behaviour for the AMV time series. However, the possibility that the non-stationarity arises from sampling errors can be excluded with high confidence only for one of the 7 models. Therefore, longer time series are needed to robustly assess the AMV characteristics. In addition to any changes imposed to the AMV by external forcings, the detected dependence on the time interval identified in most models suggests that the character of the observed AMV may undergo significant changes in the future. [ABSTRACT FROM AUTHOR]

Possible changes in the intensity of heavy precipitation events at the end of the twenty-first century over the Euro-Mediterranean region are investigated, using a subset of numerical climate simulations taking part to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). As a measure of the intensity associated with heavy precipitation events, we use the difference between the 99th and the 90th percentiles. Despite a slight tendency to underestimate the observed heavy precipitation intensity during summer and to overestimate it during winter, the considered CMIP5 models well represent the observed patterns of the defined 99th-90th percentile metric during both seasons for the 1997-2005 period over the Euro-Mediterranean region. Over the investigated domain, an increase of the width of the right tail of the precipitation distribution is projected in a warmer climate, even over regions where nearly the entire precipitation distribution becomes dryer. This is the case of the European domain within the 45N-55N belt. [ABSTRACT FROM AUTHOR]

A multi-model analysis of Atlantic multidecadal variability is performed with the following aims: to investigate the similarities to observations; to assess the strength and relative importance of the different elements of the mechanism proposed by Delworth et al. (J Clim 6:1993-2011, 1993) (hereafter D93) among coupled general circulation models (CGCMs); and to relate model differences to mean systematic error. The analysis is performed with long control simulations from ten CGCMs, with lengths ranging between 500 and 3600 years. In most models the variations of sea surface temperature (SST) averaged over North Atlantic show considerable power on multidecadal time scales, but with different periodicity. The SST variations are largest in the mid-latitude region, consistent with the short instrumental record. Despite large differences in model configurations, we find quite some consistency among the models in terms of processes. In eight of the ten models the mid-latitude SST variations are significantly correlated with fluctuations in the Atlantic meridional overturning circulation (AMOC), suggesting a link to northward heat transport changes. Consistent with this link, the three models with the weakest AMOC have the largest cold SST bias in the North Atlantic. There is no linear relationship on decadal timescales between AMOC and North Atlantic Oscillation in the models. Analysis of the key elements of the D93 mechanisms revealed the following: Most models present strong evidence that high-latitude winter mixing precede AMOC changes. However, the regions of wintertime convection differ among models. In most models salinity-induced density anomalies in the convective region tend to lead AMOC, while temperature-induced density anomalies lead AMOC only in one model. However, analysis shows that salinity may play an overly important role in most models, because of cold temperature biases in their relevant convective regions. In most models subpolar gyre variations tend to lead AMOC changes, and this relation is strong in more than half of the models. [ABSTRACT FROM AUTHOR]

Impact studies of the Atlantic Multidecadal Variability (AMV) on the climate system are severely limited by the lack of sufficiently long observational records. Relying on a model-based approach is therefore mandatory to overcome this limitation. Here, a novel experimental setup, designed in the framework of the CMIP6-endorsed Decadal Climate Prediction Project, is applied to the CMCC climate model to analyse the remote climate impact of the AMV on the Northern Eurasian continent. Model results show that, during Boreal summer, an enhanced warming associated to a positive phase of the AMV, induces a hemispheric-scale wave-train response in the atmospheric circulation, affecting vast portions of Northern Eurasia. The overall AMV-induced response consists in an upper-tropospheric anomalous flows leading to a rainfall increase over Scandinavia and Siberia and to an intensified river runoff by the major Siberian rivers. A strengthening of Eurasian shelves' stratification, broadly consistent with the anomalous river discharge, is found in the proximity of the river mouths during positive-AMV years. Considering that Siberian rivers (Ob', Yenisei and Lena) account for almost half of the Arctic freshwater input provided by terrestrial sources, the implications of these findings for decadal variability and predictability of the Arctic environment are also discussed. [ABSTRACT FROM AUTHOR]