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Climate impact of the Atlantic Multidecadal Variability (AMV) on the Northern Eurasia is poorly documented in literature due to the lack of observations in space, limited by the local fighting weather conditions, and in time, i.e. the analysed timescale is too long compared to the observational record (~120 years). To overcome this limitation, an idealized modelling approach designed in CMIP6 Decadal Climate Prediction Project is applied, based on the CMCC-CM2-SR5 coupled model: the North Atlantic sea surface temperature are continuously nudged towards the positive (negative) AMV anomalies to simulate the warm (cold) AMV phase. An ensemble of 32 members is performed for each experiment, in which all the forcings are set at the pre-industrial values for a 10-year integration time.Model results shows that, during Boreal summer, a positive phase of the AMV induces a circumglobal east-west wave train across the entire Northern Hemisphere. Enhanced westerly winds alters the climatological upper-troposphere circulation, advecting moisture from the North Atlantic Ocean along the northern part of the Eurasian continent. In particular, over Siberia, an anomalous geopotential dipole promotes the poleward movement of specific humidity from the Pacific Ocean, converging at high latitudes with the aforementioned westerly flux. Moisture anomalies regionally contribute to May-to-August rainfall, intensifying multidecadal precipitation over Scandinavia and Eastern Russia. Interestingly, rainfall rises by 10% of its climatology over Siberia, where the AMV-induced moisture fluxes converges.Furthermore, this study also points out that the AMV has a role in modulating the freshwater balance of the Arctic Ocean, due to the anomalous precipitation over the Siberian rivers' watersheds (Ob', Yenisei and Lena). Siberia is a key region for the hydrological cycle of the Arctic Ocean and the catchment areas of these three large rivers accounts for about 46% of the Arctic freshwater input provided by terrestrial sources. We find a decrease of sea surface salinity (about 12% of model climatology, broadly consistent with the anomalous river discharge) in the proximity of the river mouths during positive-AMV years.These results disclose the potential predictability of precipitation over northern Eurasian via tropical-extratropical pathway, associated with multidecadal fluctuations of the North Atlantic sea surface temperatures, in agreement with recent observational studies.This work is supported by EUCP (European Climate Prediction system) under Horizon 2020, grant agreement 776613. [ABSTRACT FROM AUTHOR]

In this study the relationship between the poleward transport of moisture and heat by atmospheric eddies and North Atlantic weather regimes is assessed in reanalysis data. Weather regimes are patterns of large scale atmospheric circulation that are often regarded as recurrent, quasi-stationary states of the atmospheric flow. Transport of heat by transient atmospheric eddies is a key component of the heat budget of the Arctic and high latitude regions. Recent studies suggest that heat transport into the pole happens in "bursts" of warm air. Time scales involved are between the midlatitude baroclinic life cycle and the sub-seasonal range. In our work, the four optimal North Atlantic weather regimes, namely the two phases of the NAO, Atlantic ridge and Scandinavian blocking, are identified, then their relationship with storminess, baroclinicity and transport of moist static energy (MSE) by transient synoptic and intraseasonal eddies is analysed. The focus of the analysis is on the extended winter season and on the lower troposphere. We demonstrate that the spatial distribution of aforementioned quantities is substantially modulated by the occurrence of weather regimes and that the convergence of the MSE flux into the pole is significantly affected. Particular emphasis is posed on link between Scandinavian blocking and heat flux convergence in the Nordic Seas. The link between WRs and surface temperature, sea ice cover, snow cover and soil moisture is also documented and the implications of our findings for sub-seasonal predictability and systematic errors in coupled models are discussed. Applications of the proposed approach to seasonal forecast systems (Blue-Action) and multi-model frameworks (PRIMAVERA) is ongoing. This study is supported by the Blue-Action project (European Union's Horizon 2020 research and innovation programme, grant: 727852) [ABSTRACT FROM AUTHOR]

The Kuroshio Extension (KE) is the eastward-flowing, free, inertial meandering jet formed by the confluence of the Kuroshio and Oyashio western boundary currents. The KE jet is known for its low-frequency variability (LFV) which connects a zonally elongated, fairly stable and energetic meandering jet (elongated state) and a much weaker, highly variable and convoluted jet with a reduced zonal penetration (contracted state). The LFV of the KE is therefore often referred to as being bimodal (Qiu, 2002). The nature of this variability is still under debate; some authors suggest that non-linear internal oceanic mechanisms can play a fundamental role in the phenomenon (Pierini, 2006) but there is also evidence from the observations that the KE LFV is connected with changes in atmospheric patterns of variability as the Aleutian Low and the North Pacific Oscillation (Nathan J. and Mantua Steven R. Hare, 2002; Qiu B., 2003; L. Ceballos et al, 2009; S. Pierini, 2014; M. Newman et al, 2016). In the first part of our analysis, we assess the impact of model resolution on the low-frequency variability of the KE jet. Results from two present-climate coupled simulations differing by their atmospheric component resolution are compared. For this analysis, results from the CMCC-CM2 model are used, under the CMIP6 HighResMIP protocol. Several aspects of the KE LFV are inspected, including the KE bimodality and the role of ocean weather (mesoscale oceanic eddies) in modulating the air-sea interactions. In the second part of the work, the role of the intrinsic variability and atmospherically-forced variability of the KE LFV is investigated taking advantage of the OCCIPUT ensemble of 1/4° global oceanic hindcasts provided by the Institut des Géosciences de l'Environnement (IGE). This dataset is built in order to simulate simultaneously both contributions (intrinsic and forced) and to allow their separation via ensemble statistics. [ABSTRACT FROM AUTHOR]

A large inter-annual variability exists in the strength of the Northern stratospheric polar-vortex in the extra-tropics. Variations of the polar vortex are influenced by external factors and by internal atmospheric dynamics. Part of variability may be linked to the sudden stratospheric warming (SSWs) in the extra-tropics and to the quasi-biennial oscillation (QBO) in the equatorial stratosphere which appears to affect the zonal mean circulation. SSW correspond to extreme 'polar vortex events' influencing the Arctic Oscillation and mid-latitude extreme weather. The QBO is the major wind variability source in the tropical lower stratosphere.We examine the decadal variability and predictability of the climate system, associated with these processes occurring in the stratosphere. We base our analysis on a set of near-term predictions performed with state-of-the-art stratosphere-resolving climate models, initialized with an estimated state of the ocean and atmosphere. We conclude that the atmosphere initialization has a key role on the variability and predictability of stratospheric processes providing evidence that a proper initialization of the QBO might be beneficial not only for the predictability of the tropics but also in the extra-tropics. [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]

The Maritime Continent plays a role in the global circulation pattern, due to the energy released by convective condensation over the region which influences the global atmospheric circulation. We demonstrate that tropical cyclones contribute to drying the Maritime Continent atmosphere, influencing the definition of the onset of the dry season. The process was investigated using observational data and reanalysis. Our findings were confirmed by numerical experiments using low- and high-resolution versions of the CMCC-CM2 General Circulation Model contributing to the HighResMIP CMIP6 effort. [ABSTRACT FROM AUTHOR]

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]

Meridional transport of heat by transient atmospheric eddies is a key component of the energy budget of the middle- and high-latitude regions. The heat flux at relevant frequencies is also part of a dynamical interaction between eddies and mean flow. In this study we investigate how the poleward heat flux by high-frequency atmospheric transient eddies is modulated by North Atlantic weather regimes in reanalysis data. Circulation regimes are estimated via a clustering method, a jet-latitude index, and a blocking index. Heat transport is defined as advection of moist static energy. The focus of the analysis is on synoptic frequencies but results for slightly longer time scales are reported. Results show that the synoptic eddy heat flux is substantially modulated by midlatitude weather regimes on a regional scale in midlatitude and polar regions. In a zonal-mean sense, the phases of the North Atlantic Oscillation do not significantly change the high-latitude synoptic heat flux, whereas Scandinavian blocking and the Atlantic ridge are associated with an intensification. A close relationship between high-latitude (midlatitude) heat flux and Atlantic jet speed (latitude) is found. The relationship between extreme events of synoptic heat flux and circulation regimes is also assessed and reveals contrasting behaviors in the polar regions. The perspective that emerges complements the traditional view of the interaction between synoptic eddies and the extratropical flow and reveals relationships with the high-latitude climate. [ABSTRACT FROM AUTHOR]

A multimodel, multiresolution set of simulations over the period 1950–2014 using a common forcing protocol from CMIP6 HighResMIP have been completed by six modeling groups. Analysis of tropical cyclone performance using two different tracking algorithms suggests that enhanced resolution toward 25 km typically leads to more frequent and stronger tropical cyclones, together with improvements in spatial distribution and storm structure. Both of these factors reduce typical GCM biases seen at lower resolution. Using single ensemble members of each model, there is little evidence of systematic improvement in interannual variability in either storm frequency or accumulated cyclone energy as compared with observations when resolution is increased. Changes in the relationships between large-scale drivers of climate variability and tropical cyclone variability in the Atlantic Ocean are also not robust to model resolution. However, using a larger ensemble of simulations (of up to 14 members) with one model at different resolutions does show evidence of increased skill at higher resolution. The ensemble mean correlation of Atlantic interannual tropical cyclone variability increases from ~0.5 to ~0.65 when resolution increases from 250 to 100 km. In the northwestern Pacific Ocean the skill keeps increasing with 50-km resolution to 0.7. These calculations also suggest that more than six members are required to adequately distinguish the impact of resolution within the forced signal from the weather noise. [ABSTRACT FROM AUTHOR]

It is well established that climate model projections indicate a tendency towards more extreme daily rainfall events. It is however uncertain how this changing intensity translates at the sub-daily time scales. The main goal of the present study is to examine possible differences in projected changes in intense precipitation events over Europe at the daily and sub-daily (3-hourly) time scales using a state-of-the-science climate model. There are large differences in intense precipitation projections when comparing the results at the daily and sub-daily time scales. Over north-eastern Europe, projected precipitation intensification at the 3-hour scale is lower than at the daily scale. On the other hand, Spain and the western seaboard exhibit an opposite behaviour, with stronger intensification at the daily rather than 3-hour scale. While the mean properties of the precipitation distributions are independent of the analysed frequency, projected precipitation intensification exhibit regional differences. [ABSTRACT FROM AUTHOR]