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This study aims to determine the role of the shallow and deep circulations in the Tropical Pacific Ocean heat budget, under both unperturbed and transient climate conditions. The transient response of the ocean is evaluated by performing explicit flux experiments, whereby perturbations to the surface fluxes of momentum and heat are applied to the surface of the ocean. In equilibrium conditions, the overall balance of the column is achieved as the residual between advective cooling and warming by vertical diffusive mixing. The main contribution to the balance is by far achieved by the shallow ocean, with nearly 85% and 72% of the balance in the Northern and Southern Hemisphere, respectively. Under wind stress perturbations, net imbalance in both hemispheres is due to enhanced advective cooling largely compensated by vertical diffusive warming (i.e. a strengthened upwelling-diffusive balance). In both hemispheres, imbalances are mainly confined within the shallow ocean (60% and 70% in the Northern and Southern Hemispheres respectively). The last is in accordance with the observed strengthening of the Shallow Overturning Cell and a significant weakening of higher latitudes dense water production. So strengthened shallow overturn in the North Pacific reduce northward transport out of the cell, reducing the deep ocean ventilation. The Wind stress perturbation enhances the dominant upwelling-diffusive balance with the cell cooling the shallow ocean. Under heat flux perturbations, total warming imbalance is set primarily by strengthened vertical diffusive warming in both hemispheres. However, while in the Northern Hemisphere the Shallow Ocean held nearly 70% (0.66 PW) of the total net warming imbalance, the Southern Hemisphere only accounts for 10% of the imbalance (1.20 PW). Observed warming tendencies are arguably explained in terms of mass and heat transport changes. Under the wind-stress perturbation changes in the Shallow Overturning Cell in both hemispheres are in agreement with the net heat imbalance resulting from intensified advective cooling compensated by vertical diffusive warming. In the case of the heat flux perturbation, the overall Shallow Overturning Cell circulation strengthening in both hemispheres, and the subsequent reduction in mass export by the cell towards deeper levels seem to be responsible for the shallow and deep ocean warming. This study aims to determine the role of the shallow and deep circulations in the Tropical Pacific Ocean heat budget, under both unperturbed and transient climate conditions. The transient response of the ocean is evaluated by performing explicit flux experiments, whereby perturbations to the surface fluxes of momentum and heat are applied to the surface of the ocean. In equilibrium conditions, the overall balance of the column is achieved as the residual between advective cooling and warming by vertical diffusive mixing. The main contribution to the balance is by far achieved by the shallow ocean, with nearly 85% and 72% of the balance in the Northern and Southern Hemisphere, respectively. Under wind stress perturbations, net imbalance in both hemispheres is due to enhanced advective cooling largely compensated by vertical diffusive warming (i.e. a strengthened upwelling-diffusive balance). In both hemispheres, imbalances are mainly confined within the shallow ocean (60% and 70% in the Northern and Southern Hemispheres respectively). The last is in accordance with the observed strengthening of the Shallow Overturning Cell and a significant weakening of higher latitudes dense water production. So strengthened shallow overturn in the North Pacific reduce northward transport out of the cell, reducing the deep ocean ventilation. The Wind stress perturbation enhances the dominant upwelling-diffusive balance with the cell cooling the shallow ocean. Under heat flux perturbations, total warming imbalance is set primarily by strengthened vertical diffusive warming in both hemispheres. However, while in the Northern Hemisphere the Shallow Ocean held nearly 70% (0.66 PW) of the total net warming imbalance, the Southern Hemisphere only accounts for 10% of the imbalance (1.20 PW). Observed warming tendencies are arguably explained in terms of mass and heat transport changes. Under the wind-stress perturbation changes in the Shallow Overturning Cell in both hemispheres are in agreement with the net heat imbalance resulting from intensified advective cooling compensated by vertical diffusive warming. In the case of the heat flux perturbation, the overall Shallow Overturning Cell circulation strengthening in both hemispheres, and the subsequent reduction in mass export by the cell towards deeper levels seem to be responsible for the shallow and deep ocean warming.

International audience; A vivid scientific debate exists on the nature of the Atlantic Multidecadal Variability (AMV) as an intrinsic rather than predominantly forced climatic phenomenon, and on the role of ocean circulation. Here, we use a multi-millennial unperturbed control simulation and a Holocene simulation with slow-varying greenhouse gas and orbital forcing performed with the low-resolution version of the Max Planck Institute Earth System Model to illustrate thermohaline conditions associated with twelve events of strong AMV that are comparable, in the surface anomalies, to observations in their amplitudes (~ 0.3 °C) and periods (~ 80 years). The events are associated with recurrent yet spatially diverse same-sign anomalous sea-surface temperature and salinity fields that are substantially symmetric in the warm-to-cold and following cold-to-warm transitions and only partly superpose with the long-term spatial AMV pattern. Subpolar cold-fresh anomalies develop in the deep layers during the peak cold phase of strong AMV events, often in association with subtropical warm-salty anomalies yielding a meridional dipole pattern. The Atlantic meridional overturning circulation (AMOC) robustly weakens during the warm-to-cold transition of a strong AMV event and recovers thereafter, with surface salinity anomalies being potential precursors of such overturning changes. A Holocene simulation with the same model including volcanic forcing can disrupt the intrinsic AMV–AMOC connection as post-eruption periods often feature an AMOC strengthening forced by the volcanically induced surface cooling. Overall, our results support the AMV as a potential intrinsic feature of climate, whose episodic strong anomalous events can display different shades of spatial patterns and timings for the warm-to-cold and subsequent cold-to-warm transitions. Attribution of historical AMV fluctuations thus requires full consideration of the associated surface and subsurface thermohaline conditions and assessing the AMOC–AMV relation.

Large explosive volcanic eruptions can have major impacts on global climate, affecting both radiative balance and inducing interannual-to-decadal dynamical alterations of the atmospheric and ocean circulation. Despite some discrepancies across studies regarding the response of ENSO to volcanism based on paleoclimate data, the majority of ENSO reconstructions display an El Niño–like warming in the year of eruption, while none display a significant La Niña–like response, Furthermore, there has been an emerging consensus from the numerous coupled General Circulation Model studies investigating the impact of tropical volcanism on ENSO, with the overwhelming majority displaying an El Niño–like warming occurring in the year following the eruption. However, the mechanisms that trigger a change in the ENSO state following volcanic eruptions are still debated. The center of the argument is understanding how volcanism can affect the trade winds along the equatorial Pacific.Here, we shed light on the processes that govern the ENSO response to tropical volcanic eruptions through a series of sensitivity experiments with an Earth System Model where a uniform stratospheric volcanic aerosol loading is imposed over different parts of the tropics. Three tropical mechanisms are tested: the “ocean dynamical thermostat” (ODT); the cooling of the Maritime Continent; and the cooling of tropical northern Africa (NAFR). We find that the NAFR mechanism plays the largest role, while the ODT mechanism is absent in our simulations as La Niña-like rather than El-Niño-like conditions develop following a uniform radiative forcing over the equatorial Pacific.

Venice is an iconic place and a paradigm of huge historical and cultural values at risk. The frequency of the flooding of the city centre has dramatically increased in recent decades, and this threat is expected to continue to grow – and even accelerate – through this century. This special issue is a collection of three review articles addressing different and complementary aspects of the hazards causing the floods of Venice, namely (1) the relative sea level rise, (2) the occurrence of extreme water heights, and (3) the prediction of extreme water heights and floods. It emerges that the effect of compound events poses critical challenges to the forecast of floods, particularly from the perspective of effectively operating the new mobile barriers (Modulo Sperimentale Elettromeccanico – MoSE) in Venice and that the relative sea level rise is the key factor determining the future growth of the flood hazard, so that the present defence strategy is likely to become inadequate within this century under a high-emission scenario. Two strands of research are needed in the future. First, there is a need to better understand and reduce the uncertainty of the future evolution of the relative sea level and its extremes at Venice. However, this uncertainty might not be substantially reduced in the near future, reflecting the uncertain anthropogenic emissions and structural model features. Hence, complementary adaptive planning strategies appropriate for conditions of uncertainty should be explored and developed in the future.

In this work, we want to investigate the influences of water masses on the basal melting under the RIS at present and in the past. In particular, the research aimed at understanding the influences of Ross Sea water masses variability on the RIS basal melting both at present and in the past. A regional adaptation of the Massachusetts Institute of Technology general circulation model (MITgcm) was implemented on the Ross Sea to simulate ocean circulation on the continental shelf and under the RIS. A present-day transient run, forced by ocean (GLORYS12V1) and atmospheric (ERA5) reanalysis over the period 1993-2018, shows that: [1] simulated water masses present different timescales of variability in their properties: Circumpolar Deep Water and Antarctic Surface Waters show a strong seasonal cycle, modulated by strong interannual variability. High Salinity Shelf Water and Low Salinity Shelf Water, on the other hand, show a weaker seasonal cycle and a decadal oscillation in their salinity. Variability of CDW and AASW is probably related to wind variability associated with the Southern Annular Mode, the Amundsen Sea Low, and El-Niño Southern Oscillation, mediated by sea ice. Variability of HSSW and LSSW is probably related to variability of the sea ice and meltwater input, and katabatic wind strength, in turn associated with the Polar Cell. The same variability is observed for the water masses beneath the RIS. [2] Basal melting presents a distinct pattern, related to the current at draft level, and variability related to the changing water masses properties. A new method based on mixing of water masses was developed to disentangle the effect of mixing, and highlight the melting variability associated to each water mass. Results show basal melting of ∼78 Gt/yr, in line with the observations, and presenting variability at the seasonal, interannual and decadal scale indicative of changing water masses properties or volume expansion inside the cavity. Then, we run 21 snapshots at intervals of 1000 years, over the Last Deglaciation (∼21-0 kyears BP): each snapshot was 26 years long and branched on a separate 120 years-long spinup. Simulations are forced by the outputs from an existent transient global paleoclimate experiment TraCE-21ka. The purpose of the paleo experiment was: 1) to analyse the evolution of the water masses with varying deglacial climatic conditions, and 2) how circulation resumed on the continental shelf, starting from a condition restricted by a grounded ice sheet at LGM (∼21 ka), and retreating during the deglaciation. Results show that: [1] initially, circulation was limited to three sub-ice shelf cavities in the Western Ross Sea. In Pennel trough warm CDW water reached the cavity, whereas in the Drygaslki and Joides troughs, HSSW filled the bottom level. [2] During the millenium following the Meltwater Pulse 1-A (14.6-14.3 ka), deep ocean warming and sub-surface ocean freshening caused a weakening of the Antarctic Slope Front, fostered CDW flow in Pennel and the Whales Deep cavity, which experienced high rates of basal melting. HSSW production in the Drygaslki and Joides stopped during this event. [3] In the Early Holocene (∼11.8 ka) grounding line retreat uncovered growingly portions of the continental shelf, allowing stronger atmospheric cooling and resumption of HSSW production. At ∼10ka the RIS cavity began to form, and was melted on the Westward side by HSSW, and on the Eastward side by advected mCDW; therefore, the stronger melting role shifted to the HSSW at that time. In this work, we want to investigate the influences of water masses on the basal melting under the RIS at present and in the past. In particular, the research aimed at understanding the influences of Ross Sea water masses variability on the RIS basal melting both at present and in the past. A regional adaptation of the Massachusetts Institute of Technology general circulation model (MITgcm) was implemented on the Ross Sea to simulate ocean circulation on the continental shelf and under the RIS. A present-day transient run, forced by ocean (GLORYS12V1) and atmospheric (ERA5) reanalysis over the period 1993-2018, shows that: [1] simulated water masses present different timescales of variability in their properties: Circumpolar Deep Water and Antarctic Surface Waters show a strong seasonal cycle, modulated by strong interannual variability. High Salinity Shelf Water and Low Salinity Shelf Water, on the other hand, show a weaker seasonal cycle and a decadal oscillation in their salinity. Variability of CDW and AASW is probably related to wind variability associated with the Southern Annular Mode, the Amundsen Sea Low, and El-Niño Southern Oscillation, mediated by sea ice. Variability of HSSW and LSSW is probably related to variability of the sea ice and meltwater input, and katabatic wind strength, in turn associated with the Polar Cell. The same variability is observed for the water masses beneath the RIS. [2] Basal melting presents a distinct pattern, related to the current at draft level, and variability related to the changing water masses properties. A new method based on mixing of water masses was developed to disentangle the effect of mixing, and highlight the melting variability associated to each water mass. Results show basal melting of ∼78 Gt/yr, in line with the observations, and presenting variability at the seasonal, interannual and decadal scale indicative of changing water masses properties or volume expansion inside the cavity. Then, we run 21 snapshots at intervals of 1000 years, over the Last Deglaciation (∼21-0 kyears BP): each snapshot was 26 years long and branched on a separate 120 years-long spinup. Simulations are forced by the outputs from an existent transient global paleoclimate experiment TraCE-21ka. The purpose of the paleo experiment was: 1) to analyse the evolution of the water masses with varying deglacial climatic conditions, and 2) how circulation resumed on the continental shelf, starting from a condition restricted by a grounded ice sheet at LGM (∼21 ka), and retreating during the deglaciation. Results show that: [1] initially, circulation was limited to three sub-ice shelf cavities in the Western Ross Sea. In Pennel trough warm CDW water reached the cavity, whereas in the Drygaslki and Joides troughs, HSSW filled the bottom level. [2] During the millenium following the Meltwater Pulse 1-A (14.6-14.3 ka), deep ocean warming and sub-surface ocean freshening caused a weakening of the Antarctic Slope Front, fostered CDW flow in Pennel and the Whales Deep cavity, which experienced high rates of basal melting. HSSW production in the Drygaslki and Joides stopped during this event. [3] In the Early Holocene (∼11.8 ka) grounding line retreat uncovered growingly portions of the continental shelf, allowing stronger atmospheric cooling and resumption of HSSW production. At ∼10ka the RIS cavity began to form, and was melted on the Westward side by HSSW, and on the Eastward side by advected mCDW; therefore, the stronger melting role shifted to the HSSW at that time.

The marginal ice zone is the dynamic interface between the open ocean and consolidated inner pack ice. Surface gravity waves regulate marginal ice zone extent and properties, and, hence, atmosphere-ocean fluxes and ice advance/retreat. Over the past decade, seminal experimental campaigns have generated much needed measurements of wave evolution in the marginal ice zone, which, notwithstanding the prominent knowledge gaps that remain, are underpinning major advances in understanding the region's role in the climate system. Here, we report three-dimensional imaging of waves from a moving vessel and simultaneous imaging of floe sizes, with the potential to enhance the marginal ice zone database substantially. The images give the direction-frequency wave spectrum, which we combine with concurrent measurements of wind speeds and reanalysis products to reveal the complex multi-component wind-plus-swell nature of a cyclone-driven wave field, and quantify evolution of large-amplitude waves in sea ice. ispartof: NATURE COMMUNICATIONS vol:13 issue:1 ispartof: location:England status: published

In recent years, there is increasing attention on the contaminants of emerging concern (CECs), which include plasticizers, flame retardants, industrial chemicals, pharmaceuticals, and personal care products, since they have been detected even far away from pollution sources. The polar regions are not exempt from the presence of anthropogenic contaminants, and they are employed as a model for understanding the pollutant fate and impact. During the 2021 spring campaign, sixteen surface snow samples were collected close to the research station of Ny-Ålesund located on the Spitsbergen Island of the Norwegian Svalbard Archipelago. The samples were extracted by solid-phase extraction and analyzed by liquid chromatography-high-resolution tandem mass spectrometry (LC-HRMS/MS) following an untargeted approach. Compound tentative identification was obtained with the aid of the software Compound Discoverer, using both mass spectral database search and manual validation. Among the 114 compounds identified with a high confidence level in the snow samples,80 have some commercial or industrial use (drugs, plasticizers, fragrances, etc.), therefore they could be of anthropogenic origin. Nonetheless, a clear contamination trend did not appear in the snow samples collected on eight different days during one month. The comparison with aerosol samples collected in the same area did not help identifying the source, either, since only a few compounds were in common, and they were mainly of natural origin. As such, the analysis of aerosol sample did not support possible long-range transport, also considering that compounds were detected mostly in the coarse fraction.

On global and hemispheric scales, sea-surface temperature (SST) anomalies are assumed to be good surrogates for near-surface marine air temperature (MAT) anomalies. In fact, global gridded temperature datasets commonly blend SST and near-surface air temperature anomalies to overcome the lack of geographically homogeneous and reliable MAT observations. Here, we show that SST and MAT anomalies differ regarding crucial statistical properties such as multiannual trends and probabilistic distributions of daily and monthly averages. We provide evidence of the lack of interchangeability from an array of moored buoys in the tropical Pacific Ocean. We identify statistically significant discrepancies between SST and MAT anomalies for single as well as groups of such buoys. Thus, caution is required when characterizing and interpreting MAT variability through SST observations, especially at shorter than decadal timescale.

The characteristic wind field of a certain region is mostly governed by the climatology of its larger scale area. In the case of mid-latitude regions (e.g. Europe), their climatology is determined by the extra-tropical cyclones at the larger synoptic scale. Atmospheric boundary layer (ABL) winds based on synoptic-scale structures are hence considered as the foundation for codes and standards used to assess the wind loading of structures and to design structures to prevent wind-related damage accordingly. In addition to the ABL winds, the mid-latitude regions are also prone to winds of a non-synoptic origin at the mesoscale level, with thunderstorm outflows or downbursts being the representative of such non-synoptic wind action. Since they are determined by a set of features that makes them fundamentally different from the ABL winds, downbursts can produce the corresponding wind action that is often fatal to low-rise and mid-rise structures. On these grounds, a comprehensive initiative to enable a better understanding of fundamental downburst flow features relevant for the structural loading was framed under the umbrella of the ERC THUNDERR Project. The present thesis, as the numerical modeling part of the THUNDERR Project framework, aims to address the physical characteristics of thunderstorm downbursts through the application of Computational Fluid Dynamics (CFD) technique. The focus of this work is placed on the CFD reconstruction of experimental tests of the reduced-scale thunderstorm downbursts carried out in the WindEEE Dome Research Institute (University of Western Ontario, Canada). Although they recreate the downburst flow field, the experimental analysis is restricted to the limited number of probe points. In that perspective, CFD allows expanding the analysis of experimental tests to the entire flow field, which can reveal phenomenological aspects that are either challenging or impossible to retrieve from experimental tests only. Two fundamental downburst scenarios were analyzed: (i) an isolated vertical downburst, and (ii) a downburst embedded inside the approaching ABL flow. For that purpose, three CFD approaches of a ranging complexity level were adopted. The unsteady Reynolds-Averaged Navier-Stokes (URANS), hybrid Scale-Adaptive Simulations (SAS), and Large-Eddy Simulations were used, and their overall reliability was examined. Theimplications of the WindEEE Dome specific geometrical features (i.e. bell-mouth inflow nozzle) on the downburst flow reconstruction by the facility were further discussed. The bulk of the thesis discusses the dominant flow features of the downburst with the particular emphasis on the dynamics of dominant vortex structures (i.e. primary vortex, secondary vortex, trailing ring vortices) and their spatio-temporal influence on the vertical profiles of radial velocity component. The non-dimensional flow characteristics of interest were evaluated such as the trajectory of the primary vortex and the spatial dependence of the velocity of primary vortex propagation. Analyses were further extended for the case of a joint downburst and ABL wind interaction to address the dynamics between two different wind fields, and the genesis of the worst condition in terms of the maximum radial velocity due to the ABL wind entrainment was discussed. The flow field was analyzed across various azimuth angles with respect to the ABL flow to report on the flow asymmetry, and general implications of such downburst configuration on spatio-temporal evolution of wind velocity profiles which can produce severe conditions for low-rise and mid-rise structures.

The focus of the Ph.D. project was the implementation of a network for monitoring significant sea wave heights (Hs) along the Ligurian coasts (north-western Mediterranean Sea), in order to provide near-real time data to weather forecasters and sea users (vessel captains, pilots, Coast Guard, Port Authorities and yachtsmen). The project was divided into a part dedicated to the use of microseismic data, interfaced with data derived from modelling hindcasts and from the wave buoy of Capo Mele, for the determination of Hs, and a part dedicated to the Interreg Italy-France Maritime 2014-2020 SINAPSI "Assistance to navigation for safe access to ports? Project, which involves the installation of instruments for measuring weather and sea parameters along the coasts of the Ligurian Sea. The first phase of the Ph.D. project involved the study and updating of a mathematical procedure for the estimation of Hs from the microseismic data recorded by the network of seismic stations along the Ligurian arch. The procedure exploited the existing relationship between sea wave heights and microseismic signal and included the use of hindcast data to calibration steps and data from the Capo Mele wave buoy to verify the reliability of the estimated Hs data. In October 2018, an extreme sea storm hit the Ligurian coast with waves characterised by Hs greater than 6 m; these wave heights were not matched by an equivalent energy in the microseismic signal, which therefore led to an underestimation of the Hs estimated by the procedure. It was therefore necessary to investigate the extreme event of 2018, which led to the evidence of a lack in the energy of the microseismic signal and the need for a data compensation, which was possible thanks to the use of weather data (wind speed and atmospheric pressure) that were included in an additional element to the estimation procedure. The second phase of the Ph.D. project therefore involved verifying the reliability of the data estimated by the procedure, which led to the evidence of a generic underestimation of Hs. This led to the study of a first modification of the procedure and to the consequent monitoring of its validity on a large time scale. The monitoring of the reliability of the estimated data will be continued after the end of the Ph.D., as well as the updating/refinement of the procedure. The Ph.D. project involved the participation in the SINAPSI Project, which started in April 2019, in all its components, from the management to the implementation of the monitoring network, also including the communication and dissemination of the project activities to the stakeholders. The Ph.D. has achieved the improvement of the Hs estimation procedure. The research activities will continue beyond the Ph.D. natural end to implement the results with data from a radar antenna system that will be installed in the Genoa area in the framework of the SINAPSI Project.

Kongsfjorden is an Arctic fjord located in the Svalbard archipelago. Its hydrography is influenced by the warm and saline Atlantic Water (AW) in the West Spitsbergen Current and the cold and fresh Polar Water circulating on the shelf. We assess the so-called atlantification of Kongsfjorden in the 2010–2020 decade by inspecting modifications in water properties and water masses variability through moored data and summer CTD surveys. Atlantification in this fjord has emerged as an increasing temperature and salinity, resulting from enhanced advection of Atlantic waters from the West Spitsbergen Current. The water column in inner Kongsfjorden warmed by 0.13°C/yr at 35 m and 0.06°C/yr at 85 m depth from 2010 to 2020, while salinity increased by 0.3 PSU. Depth-averaged temperatures have increased by 0.26°C/yr in the warmest months of the year, whereas they appear relatively stable in the coldest months. Both temperature and salinity present a linear regression change point in January 2017, with latter years featuring decreasing values. Highly diluted AW is found at the beginning of the decade, which give way to more and more pure AW in latter years, culminating in extensive intrusions in 2016 and 2017 determining the warmest and saltiest conditions over the decade in inner Kongsfjorden. Observations in the 2010–2020 decade confirm that Kongsfjorden has transitioned to an Atlantic-type fjord, featuring depleted sea ice conditions and rather regular shallow intrusions of AW in summer and frequently also in winter. Although single intrusions of AW are associated with dynamical events on the shelf, we found that the long-term temperature evolution in the inner Kongsfjord is consistent with the meridional temperature transport of the West Spitsbergen Current. The AW current flowing northward from lower latitudes along the western Svalbard archipelago thus has profoundly driven local conditions in the inner fjord in this decade.

Sea surface temperature (SST) is of paramount importance for comprehending ocean dynamics and hence the Earth’s climate system. Accordingly, it is also the most measured oceanographic parameter. However, until the end of the XIX century, no continuous time series of SST seems to exist, with most of the available data deriving from measurements on ships. Here, we present a continuous digitalized record of surface water measurements originally written in a book published in 1853. The measurements were retrieved thrice daily in the Venice lagoon, in the northeastern part of the Italian peninsula, from June to August 1851 and 1852. To the best of our knowledge, these data constitute the oldest time series of the entire world ocean. The measurements were performed by immersing a Réaumur thermometer a few meters deep in the lagoon water at 8 a.m., 12 p.m., and 8 p.m. Despite several limitations affecting these data (e.g., lacking information regarding the exact water depth where measurements were performed and instrumental metadata), they are of utmost significance, as they put many decades backward the date of the development of a fundamental aspect of oceanographic observations. Moreover, the data were collected close to the Punta della Salute site, where actual sea water temperature measurements have been performed since 2002. Therefore, a unique comparison between surface water temperatures within the Lagoon of Venice across three centuries is possible.

Paleoclimate reconstructions have identified a period of exceptional summer and winter cooling in the North Atlantic region following the eruption of the tropical volcano Huaynaputina (Peru) in 1600 CE. A previous study based on numerical climate simulations has indicated a potential mechanism for the persistent cooling in a slowdown of the North Atlantic subpolar gyre (SPG) and consequent ocean–atmosphere feedbacks. To examine whether this mechanism could have been triggered by the Huaynaputina eruption, this study compares the simulations used in the previous study both with and without volcanic forcing and this SPG shift to reconstructions from annual proxies in natural archives and historical written records as well as contemporary historical observations of relevant climate and environmental conditions. These reconstructions and observations demonstrate patterns of cooling and sea-ice expansion consistent with, but not indicative of, an eruption trigger for the proposed SPG slowdown mechanism. The results point to possible improvements in future model–data comparison studies utilizing historical written records. Moreover, we consider historical societal impacts and adaptations associated with the reconstructed climatic and environmental anomalies. This research has been supported by the Swiss National Science Foundation (grant no. P2BEP1_175214), the Swiss National Science Foundation through the SNSF Sinergia CALDERA project (grant no. CRSII5_183571), the Spanish Sci-ence and Innovation Ministry (Ministerio de Ciencia e Innovación) through the STREAM project (grant no. PID2020-114746GBI00), Fonds de la Recherche Scientifique – FNRS and the FWO under the Excellence of Science (EOS) program through the PARAMOUR project (grant no. O0100718F, EOS ID no. 30454083), and the Georgetown Environment Initiative.

This paper provides initial results from a multi-model ensemble analysis based on the volc-pinatubo-full experiment performed within the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The volc-pinatubo-full experiment is based on an ensemble of volcanic forcing-only climate simulations with the same volcanic aerosol dataset across the participating models (the 1991–1993 Pinatubo period from the CMIP6-GloSSAC dataset). The simulations are conducted within an idealized experimental design where initial states are sampled consistently across models from the CMIP6-piControl simulation providing unperturbed preindustrial background conditions. The multi-model ensemble includes output from an initial set of six participating Earth system models (CanESM5, GISS-E2.1-G, IPSL-CM6A-LR, MIROC-E2SL, MPI-ESM1.2-LR and UKESM1). The results show overall good agreement between the different models on the global and hemispheric scales concerning the surface climate responses, thus demonstrating the overall effectiveness of VolMIP's experimental design. However, small yet significant inter-model discrepancies are found in radiative fluxes, especially in the tropics, that preliminary analyses link with minor differences in forcing implementation; model physics, notably aerosol–radiation interactions; the simulation and sampling of El Niño–Southern Oscillation (ENSO); and, possibly, the simulation of climate feedbacks operating in the tropics. We discuss the volc-pinatubo-full protocol and highlight the advantages of volcanic forcing experiments defined within a carefully designed protocol with respect to emerging modelling approaches based on large ensemble transient simulations. We identify how the VolMIP strategy could be improved in future phases of the initiative to ensure a cleaner sampling protocol with greater focus on the evolving state of ENSO in the pre-eruption period.

Northern Hemisphere (NH) climate has experienced various coherent wintertime multidecadal climate trends in stratosphere, troposphere, ocean, and cryosphere. However, the overall mechanistic framework linking these trends is not well established. Here we show, using long-term transient forced coupled climate simulation, that large parts of the coherent NH-multidecadal changes can be understood within a damped coupled stratosphere/troposphere/ocean-oscillation framework. Wave-induced downward propagating positive stratosphere/troposphere-coupled Northern Annular Mode (NAM) and associated stratospheric cooling initiate delayed thermohaline strengthening of Atlantic overturning circulation and extratropical Atlantic-gyres. These increase the poleward oceanic heat transport leading to Arctic sea-ice melting, Arctic warming amplification, and large-scale Atlantic warming, which in turn initiates wave-induced downward propagating negative NAM and stratospheric warming and therefore reverse the oscillation phase. This coupled variability improves the performance of statistical models, which project further weakening of North Atlantic Oscillation, North Atlantic cooling and hiatus in wintertime North Atlantic-Arctic sea-ice and global surface temperature just like the 1950s–1970s.

One of the most promising techniques for the analysis of Spatio-Temporal ocean wave fields is stereo vision. Indeed, the reconstruction accuracy and resolution typically outperform other approaches like radars, satellites, etc. However, it is computationally expensive so its application is typically restricted to the analysis of short pre-recorded sequences. What prevents such methodology from being truly real-time is the final 3D surface estimation from a scattered, non-equispaced point cloud. Recently, we studied a novel approach exploiting the temporal dependence of subsequent frames to iteratively update the wave spectrum over time. Albeit substantially faster, the unpredictable convergence time of the optimization involved still prevents its usage as a continuously running remote sensing infrastructure. In this work, we build upon the same idea, but investigating the feasibility of a fully data-driven Machine Learning (ML) approach. We designed a novel Convolutional Neural Network that learns how to produce an accurate surface from the scattered elevation data of three subsequent frames. The key idea is to embed the linear dispersion relation into the model itself to physically relate the sparse points observed at different times. Assuming that the scattered data are uniformly distributed in the spatial domain, this has the same effect of increasing the sample density of each single frame. Experiments demonstrate how the proposed technique, even if trained with purely synthetic data, can produce accurate and physically consistent surfaces at five frames per second on a modern PC.

The most prominent oceanographic features in the Central Mediterranean are decadal reversals of the Northern Ionian Gyre (NIG), interpreted in terms of internal processes. Altimetry data, drifter data and model surface salinity products are used in this paper to define some specific features of the circulation related to the anticyclonic or cyclonic NIG modes. Results not only highlight different shapes and intensities of the Mid-Ionian Jet and northern Ionian meander among the two circulation mode, but emphasize distinct behaviour within the same mode, imputable to the variability of the mesoscale quasi-permanent features. The wind-driven Messina Rise Vortex, on the western side of the northern Ionian, and the Pelops Gyre, on the eastern side of the northern Ionian, show different behaviour during the first (1993–1996) and the second (2006–2010) anticyclonic periods, related to the cyclonic activity along the dense water pathway. When the dense water was of Aegean origin (1993–1996; “Aegean” anticyclone), the cyclonic mesoscale activity on the eastern flank of the Ionian overwhelmed the anticyclonic wind forcing, and led to the disappearance of the Pelops Gyre. When the dense water was of Adriatic origin (2006–2010; “Adriatic” anticyclone), the strengthening of cyclonic mesoscale activity on the western flank of the Ionian caused the shape change of the Messina Rise Vortex, favouring its longitudinal extension. The zonally-elongated Messina Rise Vortex reduced the inflow of Atlantic Water in the northern Ionian, and induced a strengthening of the eastward flowing Mid-Ionian Jet in the southern Ionian and Cretan Passage. From these results, it emerges that the interannual variability of the wind-driven quasi-permanent mesoscale gyres in the Ionian is influenced by internal forcing (location of the deep water formation site – Aegean or Adriatic Sea), in turn related to the major local climatic shift occurred in the last decades (the Eastern Mediterranean Transient). We show that the interplay between decadal and interannual variability, or rather between basin-wide and mesoscale circulation, affected the intrinsic characteristics of the NIG reversals.

The “1809 eruption” is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the third largest since 1500 with a sulfur emission strength estimated to be 2 times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ∼ 1809 eruption that is similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous. We test the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSIs) and uncertainties from ice-core records. Three of the forcing reconstructions represent a tropical eruption with an approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12 and 19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure. While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble-mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability.

L'ultimo massimo glaciale (LGM in inglese) è stato un evento globale avvenuto da 26 a 21 ka, segnato dall'espansione di calotte glaciali, ghiacciai montani, permafrost e ghiaccio marino e associato ad un abbassamento del livello medio del mare di circa 120 m. Le temperature da 3 a 6°C più basse rispetto ad oggi e i diversi regimi di precipitazione hanno profondamente modificato l'ambiente. Durante l'LGM il ghiacciaio del Tagliamento si espandeva nella pianura Friulana con un lobo pedemontano. I ghiacciai montani sono particolarmente sensibili ai cambiamenti delle condizioni climatiche e sono fortemente influenzati dalle variazioni di temperatura e precipitazioni e quindi sono considerati importanti indicatori climatici. I processi che hanno portato alla formazione del ghiacciaio del Tagliamento all'LGM non sono ancora del tutto chiari, ma sono probabilmente ascrivibili alla riduzione della fusione del manto nevoso e dell'evaporazione legate a temperature più basse e a cambiamenti nella circolazione atmosferica. La bassa quota e il bacino di accumulo relativamente piccolo fanno del ghiacciaio del Tagliamento un caso studio ideale per l'applicazione di un modello climatico regionale ad alta risoluzione. Questo studio combina un modello climatico regionale ad alta risoluzione con ricostruzioni ambientali basate su proxy paleoclimatici di topografia, vegetazione e ghiacciai per l'LGM. Obiettivo primario di questo studio è analizzare la circolazione atmosferica che ha portato allo sviluppo del ghiacciaio del Tagliamento all'LGM. Questo lavoro è basato su un nesting multiplo. Il modello climatico regionale RegCM4, sviluppato dall'ICTP, è stato forzato prima dal modello climatico globale del Max Planck Institute for Meteorology (ECHAM) e poi dallo stesso RegCM4 con una risoluzione di 50 km. Le simulazioni finali hanno fornito informazioni quantitative sul clima dell'LGM e del preindustriale per le regioni alpina e appenninica con una risoluzione di 12 km. Al fine di validare il modello, è stata eseguita una simulazione, forzata con i dati di rianalisi di ERA-Interim che è stata confrontata poi con dataset grigliati di misure dirette di precipitazione e temperatura. Il sistema RegCM4 è stato adattato per simulare il clima della regione alpina. Per le simulazioni all'LGM, i parametri orbitali e le concentrazioni di gas serra sono stati fissati ai valori dell'LGM. I risultati di questo studio evidenziano che RegCM4 a 12 km di risoluzione è in grado di rappresentare la variabilità spazio-temporale della circolazione atmosferica che hanno portato allo sviluppo dei principali ghiacciai delle Alpi sudorientali, degli Appennini e dei Balcani all'LGM. I risultati sono compatibili con proxy ambientali e ricostruzioni da modelli. I risultati di RegCM4 per l'LGM, mostrano temperature 6,6°C più fredde rispetto al periodo preindustriale e condizioni generalmente più secche. La regione a sud delle Alpi era relativamente più umida rispetto alla regione a nord. Precipitazioni stratiformi e convettive hanno contribuito ad alimentare il ghiacciaio del Tagliamento, con predominanza di fenomeni convettivi dalla tarda primavera all'inizio dell'autunno. Durante il resto dell'anno prevalevano precipitazioni stratiformi provenienti dai quadranti meridionali e occidentali, causate da una circolazione ciclonica. Nelle Alpi sud orientali le precipitazioni estive spesso avvenivano sotto forma di nevicate, limitando la fusione dei ghiacciai. Le Alpi sudorientali ricevevano una maggiore quantità di precipitazioni (sia liquide che solide) rispetto al resto della catena alpina. Questo fatto ha favorito lo sviluppo di un ghiacciaio così grande in un bacino esposto a sud e a bassa quota. Questo progetto rappresenta un passo avanti nel contesto degli studi paleoclimatici, aggiungendo nuove informazioni quantitative ad alta risoluzione sulla circolazione atmosferica all'LGM e aprendo la strada a diversi sviluppi futuri. The Last Glacial Maximum (LGM) is a global event that occurred 26 to 21 ka BP. It was characterised by the expansion of ice sheets, mountain glaciers, permafrost and sea ice and was associated with a mean sea level drop of about 120 m. The environment was deeply modified by decreased temperature (3 to 6°C globally) and different precipitation regimes compared with today's conditions. In the southeastern Alps, the Tagliamento glacier expanded into the Friuli plane with a piedmont lobe. Mountain glaciers are inherently sensitive to changes in climate conditions and are strongly affected by temperature and precipitation variations and are thus considered crucial climate indicators. For the Tagliamento glacier during the LGM, the physical processes leading to its extension and mass balance are not yet fully understood. Contributions likely derive from reduced snowmelt and evaporation related to lower temperature and changes in regional circulations. Disentangling these contributions is thus important for a full understanding of the dynamics of the glacier. The low elevation and relatively small accumulation basin of the Tagliamento glacier make it an ideal case study for the application of a high-resolution regional climate model, which can provide climate information at fine spatial scales. Therefore this thesis combines a high-resolution regional climate model (RCM), the ICTP RegCM4 model system, with LGM topography, vegetation and glacier morphology reconstructed from paleoclimate proxies. The primary aim of the thesis is to improve our knowledge about the atmospheric circulation and thermodynamic conditions that sustained the Tagliamento glacier at the LGM. A multiple nesting approach is adopted, with the RegCM4 run at high spatial resolution (12 km) over a greater Alpine domain, with boundary conditions from an intermediate resolution (50 km) RegCM4 simulation, itself driven by the atmospheric component of the MPI Earth System Model (ECHAM). Two 20-year long simulations are conducted and intercompared, one for the climate state at the LGM standard (21 ka BP) and one for pre-industrial conditions. In order to validate and customise the RegCM4 simulation, an experiment forced with ERA-Interim observations reanalysis data is performed and compared with observational precipitation and temperature datasets. For the LGM simulation orbital parameters and greenhouse gas concentrations are set at the LGM values. This is one of the few applications of RCMs to paleoclimate studies today. The results show that RegCM4 at 12 km of resolution is able to capture the spatio-temporal variability of the atmospheric circulation patterns which were responsible for the development of several glaciers in the domain at the LGM, with simulated climate fields compatible with available proxies and other model reconstructions. LGM temperatures are simulated to be about 6.6°C lower than at pre-industrial times, with drier conditions over the study domain, especially north of the alpine chain, and with the southern alpine region relatively wetter due to increasing southerly moist flow. Stratiform and convective precipitation contributed to feed the Tagliamento glacier, with the predominance of convective phenomena from late spring to early autumn, while during the rest of the year southerly and westerly stratiform precipitation prevailed, as originated from a cyclonic circulation. In the southeastern Alps, summer precipitation often occurred as snowfall, limiting extensive melting phenomena, and in general precipitation was higher than in the whole alpine chain. These conditions favoured the development of a large glacier in a southerly exposed, low elevation basin. This project represents a step forward in the framework of paleoclimate studies, adding new high-resolution quantitative information about the atmospheric circulation feeding glacier formation at the LGM and paving the way for several future developments.

In recent years, there has been growing interest in remote and proximal observation of sea surface waves. This has been partially driven by new technologies allowing the characterization of wave fields in both their spatial and temporal aspects. Typical examples are radar systems and stereo-imaging that permit remote monitoring of oceanic waves (from satellites, platforms, or vessels) with remarkable accuracy and range of use. These new exciting possibilities usually come at the price of being relatively harder to master with respect to traditional "point-like" approaches providing measurements limited to a temporal perspective. This difficulty is not restricted to the technology itself (see, for example, the delicate camera-calibration process required in stereo-imaging) but also on how to properly process, analyze, and assimilate spatio-temporal data. Therefore, in this Special Issue, we decided to embrace a wide range of topics that have led a multitude of multi-disciplinary works in the recent past, including: o Wave mechanics and sea surface dynamics; o Analysis of the wave climate and its extremes; o Data fusion and signal processing; o Statistical and probabilistic methods; o Assessment of wave models. We did our best to propose recent advancements, not only on the technological aspect of spatially distributed sea waves acquisition but also on the characterization of wave statistics from measured and assimilated data. For the former aspect, we included the work of Vieira et al. [1], proposing the first cheap and simple stereo-based technique to estimate the 3D sea surface elevation from inexpensive smartphones. For the latter, the paper of Serebryany et al. [2] investigates internal waves on a narrow steep shelf of the northeastern coast of the Black Sea using the spatial antenna of line temperature sensors. We also included a discussion on space-time wave extremes in the paper of Benetazzo et al. [3] and a comparison of assimilated coastal wave data by Yukiharu Hisaki [4]. Finally, the work of Ciurana and Aguilar [5] provides an overview of how an ensemble of meteorological buoys and citizen science data can help economic activities to achieve optimal performances (in a case study, to predict optimal surfing days in the Iberian Peninsula). We hope that these works will be interesting both for researchers already working on this topic and for those who want to embrace the new possibilities offered by modern sea wave acquisition techniques.

Reliable predictions of oceanic waves during storms have always been foremost for offshore design and operation, coastal hazards, and navigation safety. Indeed, many accidents that occurred during storms were ascribed to the impact with unforeseen large waves. In this context, the purpose of this study is to improve the present state extreme wave estimate from spectral wave models. We describe an implementation for the WAM model, and we investigate the use of WAM and WAVEWATCH III fed with common routines designed to evaluate the short-term/range maximum wave statistics. An extensive assessment of models' results in the Adriatic and North Sea is performed using time and space-time wave measurements, and through an intercomparison between WAM and WAVEWATCH III applied with three different input/dissipation source term parametrizations (ST3/4/6). Further, models’ capabilities are investigated, and extreme waves characterized, in the Mediterranean Sea, aiming also at disentangling the wave spectrum bulk parameters that may point to favorable conditions for the generation of high waves. Based on the comparisons between model results and measurements, we conclude that for the model characterization of extremes, the accuracy of the significant wave height is pivotal; differences between models of other spectral parameters seem to have a minor effect.

Dominant Euro-Atlantic climate modes such as the North Atlantic Oscillation (NAO), the Eastern Atlantic pattern (EA), the Eastern Atlantic Western Russian pattern (EAWR), and the Scandinavian pattern (SCA) significantly affect interannual-to-decadal Euro-Mediterranean climate fluctuations, especially in winter.In this contribution, we will present and discuss results from a CMIP6 multi-model analysis performed to investigate the robustness of historical and projected state and variability of such modes under the historical and ssp585 future scenario of anthropogenic forcing (fossil-fueled development with 8.5W/m2 forcing level) simulations, focusing on the winter season.Toward this goal, we first search for a reliable box-based index definition for each of the abovementioned observed climate modes and, then, we perform a comparative assessment of the temporal, spectral and distributional properties of the so-defined indices during the historical (1850-2014) and ssp585 future scenario (2015-2099) time periods, with a special focus on the two interdecadal periods 1960-1999 and 2060-2099.Results show overall good skills of the historical ensemble to reproduce the observed temporal, spectral and distributional properties of all considered modes. At the end of the 21st Century the ssp585 ensemble yields non-significant distributional changes for NAO, EAWR, and SCA indices and a transition to a stronger baroclinic structure for EA, with persistent positive anomalies in the mid-troposphere enhancing globally-driven warming over the Euro-Mediterranean region. The hemispheric spatial correlation patterns with temperature and precipitation significantly change for all modes, that is, we observe a significant modulation of the teleconnections associated with each index.

Principles of the spatio-temporal statistics are used to investigate the characteristics of short-term/range extreme sea waves and related sea-state parameters under cyclone winds (northern hemisphere). We base our analysis upon consistent stereo-imaging observations of the 3D (2D space + time) sea surface elevation field, and spectral wave model results in the Northwestern Pacific during tropical storm Kong-rey (2018). The focus is on the extreme value analysis of individual maximum sea surface elevations (crest heights) and maximum crest-to-trough wave heights. Results highlight the sea areas around the storm centre where the spatio-temporal highest waves are more likely, and, via scale analysis, the principal mechanisms responsible for the occurrence of extreme conditions in bimodal (composed of wind-sea and swell) and short-crested storm seas. We find that individual waves are the highest to the north-east of the translating cyclone, where sea states are more energetic. However, in the south/south-west of the centre, where opposing wind-sea/swell sea states dominate, directional spread and bound nonlinear interactions are enhanced. In this area, more extreme waves may occur, having the maximum crest and wave heights mean values in excess of 1.3 and 2.1 times the significant wave height, respectively. This set of results provides insights into the role of the dispersive and directional focusing enhanced by nonlinearities up to the second order as an effective mechanism for the formation of extreme waves under cyclone winds. To examine what physical mechanism is behind the generation of extreme waves in different regions around the cyclone, we also explore for comparison areas where nonlinear four-wave interactions are more likely to occur.

The North Ionian Gyre (NIG) displays prominent inversions on decadal scales. We investigate the role of internal forcing induced by changes in the horizontal pressure gradient due to the varying density of Adriatic Deep Water (AdDW), which spreads into the deep layers of the northern Ionian Sea. In turn, the AdDW density fluctuates according to the circulation of the NIG through a feedback mechanism known as the bimodal oscillating system. We set up laboratory experiments with a two-layer ambient fluid in a circular rotating tank, where densities of 1000 and 1015 kg m−3 characterize the upper and lower layers, respectively. From the potential vorticity evolution during the dense-water outflow from a marginal sea, we analyze the response of the open-sea circulation to the along-slope dense-water flow. In addition, we show some features of the cyclonic and anticyclonic eddies that form in the upper layer over the slope area. We illustrate the outcome of the experiments of varying density and varying discharge rates associated with dense-water injection. When the density is high (1020 kg m−3) and the discharge is large, the kinetic energy of the mean flow is stronger than the eddy kinetic energy. Conversely, when the density is lower (1010 kg m−3) and the discharge is reduced, vortices are more energetic than the mean flow – that is, the eddy kinetic energy is larger than the kinetic energy of the mean flow. In general, over the slope, following the onset of dense-water injection, the cyclonic vorticity associated with current shear develops in the upper layer. The vorticity behaves in a two-layer fashion, thereby becoming anticyclonic in the lower layer of the slope area. Concurrently, over the deep flat-bottom portion of the basin, a large-scale anticyclonic gyre forms in the upper layer extending partly toward a sloping rim. The density record shows the rise of the pycnocline due to the dense-water sinking toward the flat-bottom portion of the tank. We show that the rate of increase in the anticyclonic potential vorticity is proportional to the rate of the rise of the interface, namely to the rate of decrease in the upper-layer thickness (i.e., the upper-layer squeezing). The comparison of laboratory experiments with the Ionian Sea is made for a situation when the sudden switch from cyclonic to anticyclonic basin-wide circulation took place following extremely dense Adriatic water overflow after the harsh winter in 2012. We show how similar the temporal evolution and the vertical structure are in both laboratory and oceanic conditions. The demonstrated similarity further supports the assertion that the wind-stress curl over the Ionian Sea is not of paramount importance in generating basin-wide circulation inversions compared with the internal forcing.

This paper reviews the state of the art in storm surge forecasting and its particular application in the northern Adriatic Sea. The city of Venice already depends on operational storm surge forecasting systems to warn the population and economy of imminent flood threats, as well as help to protect the extensive cultural heritage. This will be more important in the future, with the new mobile barriers called MOSE (MOdulo Sperimentale Elettromeccanico, Experimental Electromechanical Module) that will be completed by 2021. The barriers will depend on accurate storm surge forecasting to control their operation. In this paper, the physics behind the flooding of Venice is discussed, and the state of the art of storm surge forecasting in Europe is reviewed. The challenges for the surge forecasting systems are analyzed, especially in view of uncertainty. This includes consideration of selected historic extreme events that were particularly difficult to forecast. Four potential improvements are identified: (1) improve meteorological forecasts, (2) develop ensemble forecasting, (3) assimilation of water level measurements and (4) develop a multimodel approach.

Floods in the Venice city centre result from the superposition of several factors: astronomical tides; seiches; and atmospherically forced fluctuations, which include storm surges, meteotsunamis, and surges caused by atmospheric planetary waves. All these factors can contribute to positive water height anomalies individually and can increase the probability of extreme events when they act constructively. The largest extreme water heights are mostly caused by the storm surges produced by the sirocco winds, leading to a characteristic seasonal cycle, with the largest and most frequent events occurring from November to March. Storm surges can be produced by cyclones whose centres are located either north or south of the Alps. Historically, the most intense events have been produced by cyclogenesis in the western Mediterranean, to the west of the main cyclogenetic area of the Mediterranean region in the Gulf of Genoa. Only a small fraction of the inter-annual variability in extreme water heights is described by fluctuations in the dominant patterns of atmospheric circulation variability over the Euro-Atlantic sector. Therefore, decadal fluctuations in water height extremes remain largely unexplained. In particular, the effect of the 11-year solar cycle does not appear to be steadily present if more than 100 years of observations are considered. The historic increase in the frequency of floods since the mid-19th century is explained by relative mean sea level rise. Analogously, future regional relative mean sea level rise will be the most important driver of increasing duration and intensity of Venice floods through this century, overcompensating for the small projected decrease in marine storminess. The future increase in extreme water heights covers a wide range, largely reflecting the highly uncertain mass contributions to future mean sea level rise from the melting of Antarctica and Greenland ice sheets, especially towards the end of the century. For a high-emission scenario (RCP8.5), the magnitude of 1-in-100-year water height values at the northern Adriatic coast is projected to increase by 26–35 cm by 2050 and by 53–171 cm by 2100 with respect to the present value and is subject to continued increase thereafter. For a moderate-emission scenario (RCP4.5), these values are 12–17 cm by 2050 and 24–56 cm by 2100. Local subsidence (which is not included in these estimates) will further contribute to the future increase in extreme water heights. This analysis shows the need for adaptive long-term planning of coastal defences using flexible solutions that are appropriate across the large range of plausible future water height extremes.

Stereo imaging of the sea surface elevation provides unique field data to investigate the geometry and dynamics of oceanic waves. Typically, this technique allows retrieving the 4-D ocean topography (3-D space + time) at high frequency (up to 15–20 Hz) over a sea surface region of area ~104 m2. Stereo data fill the existing wide gap between sea surface elevation time-measurements, like the local observation provided by wave-buoys, and large-scale ocean observations by satellites. The analysis of stereo images provides a direct measurement of the wavefield without the need of any linear-wave theory assumption, so it is particularly interesting to investigate the nonlinearities of the surface, wave-current interaction, rogue waves, wave breaking, air-sea interaction, and potentially other processes not explored yet. In this context, this open dataset aims to provide, for the first time, valuable stereo measurements collected in different seas and wave conditions to invite the ocean-wave scientific community to continue exploring these data and to contribute to a better understanding of the nature of the sea surface dynamics., Measurement(s)stereo image • wave height • peak wave period • wave directionTechnology Type(s)Camera Device • computational modeling techniqueFactor Type(s)geographic locationSample Characteristic - Environmentsea surface layerSample Characteristic - LocationBlack Sea • Adriatic Sea • Yellow Sea • Iroise Sea Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12181158

Meteorological human discomfort indices or bioclimatic indices are important metrics to gauge potential risks to human health under varying environmental thermal exposures. Derived using sub-daily meteorological variables from a quality-controlled reanalysis data product (Global Land Data Assimilation System&mdash, GLDAS), a new high-resolution global dataset referred to as &ldquo, HDI_0p25_1970_2018&rdquo, is presented in this study. The dataset includes the following daily indices at 0.25&deg, &times, 0.25&deg, gridded resolution: (i) Apparent Temperature indoors (ATind), (ii) two variants of Apparent Temperature outdoors in shade (ATot), (iii) Heat Index (HI), (iv) Humidex (HDEX), (v) Wet Bulb Temperature (WBT), (vi) two variants of Wet Bulb Globe Temperature (WBGT), (vii) Thom Discomfort Index (DI), and (viii) Windchill Temperature (WCT). Spanning 49 years over the period 1970&ndash, 2018, HDI_0p25_1970_2018 fills gaps in existing climate indices datasets by being the only high-resolution historical global-gridded daily time-series of multiple human discomfort indices based on different meteorological parameters, thus offering applications in wide-ranging climate zones and thermal-comfort environments.

The El Niño Southern Oscillation (ENSO) produces global marine environment conditions that can cause changes in abundance and distribution of distant fish populations worldwide. Understanding mechanisms acting locally on fish population dynamics is crucial to develop forecast skill useful for fisheries management. The present work addresses the role played by ENSO on the round sardinella population biomass and distribution in the central-southern portion of the Canary Current Upwelling System (CCUS). A combined physical-biogeochemical framework is used to understand the climate influence on the hydrodynamical conditions in the study area. Then, an evolutionary individual-based model is used to simulate the round sardinella spatio-temporal biomass variability. According to model experiments, anomalous oceanographic conditions forced by El Niño along the African coast cause anomalies in the latitudinal migration pattern of the species. A robust anomalous increase and decrease of the simulated round sardinella biomass is identified in winter off the Cape Blanc and the Saharan coast region, respectively, in response to El Niño variations. The resultant anomalous pattern is an alteration of the normal migration between the Saharan and the Mauritanian waters. It is primarily explained by the modulating role that El Niño exerts on the currents off Cape Blanc, modifying therefore the normal migration of round sardinella in the search of acceptable temperature conditions. This climate signature can be potentially predicted up to six months in advance based on El Niño conditions in the Pacific.

Volcanic eruptions trigger ENSO response through shifts in the ITCZ and extratropical-to-tropical teleconnections., The mechanisms through which volcanic eruptions affect the El Niño–Southern Oscillation (ENSO) state are still controversial. Previous studies have invoked direct radiative forcing, an ocean dynamical thermostat (ODT) mechanism, and shifts of the Intertropical Convergence Zone (ITCZ), among others, to explain the ENSO response to tropical eruptions. Here, these mechanisms are tested using ensemble simulations with an Earth system model in which volcanic aerosols from a Tambora-like eruption are confined either in the Northern or the Southern Hemisphere. We show that the primary drivers of the ENSO response are the shifts of the ITCZ together with extratropical circulation changes, which affect the tropics; the ODT mechanism does not operate in our simulations. Our study highlights the importance of initial conditions in the ENSO response to tropical volcanic eruptions and provides explanations for the predominance of posteruption El Niño events and for the occasional posteruption La Niña in observations and reconstructions.

1. Main oceanographic characteristics of Patagonian fjords The Chilean Patagonia (41�-56�S) encompasses one of the most extensive fjord regions in the world (240000 km2) with oceanographic conditions that can sustain unique ecosystems. The region is made up mainly of fjords and channels, characterized by intertangled geomorphologies where water inputs from terrestrial and marine ecosystems overlap and mix (Gonz�lez et al., 2013). Patagonian fjords are characterized by highly complex geomorphology and hydrographic conditions, besides strong seasonal and latitudinal patterns in precipitation, freshwater discharge, glacier coverage, and light regime (Aracena et al., 2011). These systems receive Sub-Antarctic Water (SAAW) with high loads of nitrate and phosphate from the ocean, and freshwater with high loads of silicic acid from land (Silva, 2008). The surface freshwater layer that is formed by river discharges, high precipitation and glacier melting, gradually mixes with the deeper and salty SAAW layer through estuarine circulation (Chaigneau and Pizarro, 2005; Silva et al., 2009; Schneider et al., 2014). The interplay between oceanic waters and freshwater produces a vertical and horizontal gradient in salinity, nutrients and structure of microplanktonic community, making these fjords highly heterogeneous ecosystems. In addition, this interaction allows the transport and exchange of large amounts of organic matter between terrestrial and open-ocean environments (Sievers and Silva, 2009; Gonz�lez et al., 2011). Patagonian fjords play an important role in biological productivity and in coastal carbon cycling (Gonz�lez et al., 2013; Iriarte et al., 2014). These highly productive ecosystems have a great potential in terms of transfer of food to higher trophic levels, and vertical carbon export (Gonz�lez et al., 2010, 2011; Montero et al., 2011, 2017a,b). Previous studies within this region have highlighted the role of light (Jacob et al., 2014), winds, low-pressure systems, and freshwater discharge in driving cycles of biological productivity and composition of the phytoplankton community at seasonal and shorter time scales (Montero et al., 2011, 2017a,b). 2. Factors that modulate primary production in Chilean Patagonia The hydrodynamic mechanism that usually controls total phytoplankton production is the alternation, in time and/or space, between the destabilization and re-stratification of the water column (Legendre and Razzoulzadegan, 1996). In fjords, fresh water inputs tends to stabilise stratification while wind stress can oppose the stabilising effects of freshwater and is likely to be an important mechanism for mixing the upper water column (Goebel et al., 2005). In general, it is accepted that the interplay between wind-forced vertical mixing, solar radiation, and nutrient availability determines the occurrence of phytoplankton blooms (Sverdrup, 1953). Iriarte and Gonz�lez (2008) have suggested that an improvement in the light regime towards the end of winter is the main factor triggering phytoplankton production in the southern Pacific coastal area. However, this seasonal improvement in light alone may not be enough to cause phytoplankton blooms. Montero et al. (2011) suggests that the annual solar radiation cycle interact with mesoscale patterns of wind variability to modulate primary productivity in fjords. These authors reported that the onset of the productive season in Reloncav� Fjord (41�S; 72�W) coincided with seasonal changes in the direction and intensity of meridional winds. Low-pressure synoptic (LPS) events have also been shown to be important drivers of phytoplankton productivity during winter in Puyuhuapi fjord (44�S; 72�W) (Montero et al., 2017b). In that study, the area was under low surface irradiance levels and was subjected to the passage of several LPS separated by intervals of 2 to 4 days. During these events, strong northern winds (10-20 m s-1) contributed to the mixing of the water column, resulting (subsequent to water column re- stratification) in enhanced phytoplankton productivity at very low irradiance (1-10 ?E m-2 s-1) (Montero et al., 2017b). These results are novel and challenge the so far established paradigm of low levels of irradiance as a key factor limiting phytoplankton blooms in fjords ecosystems. This research has also highlighted the importance of LPS events to ecosystem productivity. During these events, periods of intensive freshwater inputs also appear to modulate pulses of primary production (including the onset of productive season) in the Puyuhuapi fjord (44�S; 72�W) (Montero et al., 2017a). Freshwater input stabilizes the water column and increase the concentration of silicic acid in the upper layers, favouring the occurrence of diatom dominated phytoplankton blooms (Montero et al., 2017a). 3. Primary production cycle in Chilean Patagonia The productivity cycle within a number of Patagonian fjords (41-51oS) has been typically described as a two-phase system consisting of a short non-productive winter phase (May to July) and a productive phase extending from late winter (August) to autumn (April) (Iriarte et al., 2007; Montero et al., 2011, 2017a). Nevertheless, more recently, the occurrence of highly productive winter blooms of phytoplankton challenges our earlier perception of winter as a low production season in these fjords (Montero et al., 2017b). The productive season has been characterized by the occurrence of diatom blooms associated with high primary production rates (1-3 g C m-2 d-1), while winter phytoplankton productivity, characterized by the dominance of small phytoplankton cells, has been reported to be a small fraction (

Processes on both synoptic scale and mesoscale contribute to variations in the atmospheric water vapor isotopic signal in the lower troposphere. The drivers of the isotopic composition of water vapor at these temporal scales is a key topic of discussion. The aim of this paper is to decouple mesoscale from synoptic processes that affect the water vapor isotopic composition over a coastal lagoon and to quantify the vertical mixing and sea breeze contributions to the daily water vapor cycle. We monitored the water vapor isotopic composition at the beginning of the 2017 spring season for 21 days, at the inland boundary of the Venetian Lagoon (Italy) with a commercially available cavity ring-down spectroscopy analyzer. Mean atmospheric pressure fields and back trajectories are used to investigate the long-range transport of water vapor to the study area. Moisture source areas and d-excess correlation maps show that the control on the d-excess parameter is dominated by the mesoscale contribution. The daily cycle of water vapor is studied in detail, showing that the intrusion of air from the free atmosphere represents the main process that occurs in the morning. Under those circumstances, the morning variability of the water vapor isotopic composition can be used as a proxy for the Planetary Boundary Layer height. The study period was characterized by a constant development of the sea breeze circulation that conserved the marine signature of water vapor until the afternoon. However, the water vapor isotopic signal tends to stabilize when atmospheric conditions such as high relative humidity and a weak gradient in the atmospheric pressure field occur, leading to the establishment of an isotopic equilibrium between the atmospheric water vapor and surface waters.

Levantine intermediate water (LIW) is formed in the Levantine Sea (Eastern Mediterranean) and spreads throughout the Mediterranean at intermediate depths, following the general circulation. The LIW, characterized by high salinity and relatively high temperatures, is one of the main contributors of the Mediterranean Overturning Circulation and influences the mechanisms of deep water formation in the Western and Eastern Mediterranean sub-basins. In this study, the LIW and Levantine deep water (LDW) formation processes are investigated using Argo float data from 2001 to 2017 in the Northwestern Levantine Sea (NWLS), the larger area around Rhodes Gyre (RG). To find pronounced events of LIW and LDW formation, more than 800 Argo profiles were analyzed visually. Events of LIW and LDW formation captured by the Argo float data are compared to buoyancy, heat and freshwater fluxes, sea surface height (SSH), and sea surface temperature (SST). All pronounced events (with a mixed layer depth (MLD) deeper than 250 m) of dense water formation were characterized by low surface temperatures and strongly negative SSH. The formation of intermediate water with typical LIW characteristics (potential temperature > 15 °C, salinity > 39 psu) occurred mainly along the Northern coastline, while LDW formation (13.7 °C < potential temperature < 14.5 °C, 38.8 psu < salinity < 38.9 psu) occurred during strong convection events within temporary and strongly depressed mesoscale eddies in the center of RG. This study reveals and confirms the important contribution of boundary currents in ventilating the interior ocean and therefore underlines the need to rethink the drivers and contributors of the thermohaline circulation of the Mediterranean Sea.

This study aims at characterizing the distinctive features of a spatio-temporal nonlinear wave surface. We analyze wind-generated 3-D wave fields observed during the passage of an atmospheric front, which led to a wide directional spreading of wave energy. Data were acquired from the ocean research station Socheongcho-ORS (Yellow Sea) with a stereo wave imaging system. They include 3-D (i.e. 2-D + time) measurements of the sea surface elevation with high spatial and temporal resolution over a swath larger than any previous similar deployment. We examine the shape and the nonlinear properties of the wavenumber/frequency 3-D wave spectrum, and the characteristic spatial, temporal and spatio-temporal length scales of the wave field. We then focus on analyzing the probability of occurrence and the spatio-temporal size of the rogue waves we identified in the data. In particular, we provide for the first time an empirical estimate of the extent of the horizontal sea surface spanned by rogue waves. We also propose and assess a novel strategy to determine from the 3-D wave spectrum the vertical current profile, to be then used to map the spectrum on intrinsic frequencies.

The heat contained in the ocean (OHC) dominates the Earth’s energy budget and hence represents a fundamental parameter for understanding climate changes. However, paucity of observational data hampers our knowledge on OHC variability, particularly in abyssal areas. Here, we analyze water characteristics, observed during the last three decades in the abyssal Ionian Sea (Eastern Mediterranean), where two competing convective sources of bottom water exist. We find a heat storage of ~1.6 W/m2 – twice that assessed globally in the same period – exceptionally well-spread throughout the local abyssal layers. Such an OHC accumulation stems from progressive warming and salinification of the Eastern Mediterranean, producing warmer near-bottom waters. We analyze a new process that involves convectively-generated waters reaching the abyss as well as the triggering of a diapycnal mixing due to rough bathymetry, which brings to a warming and thickening of the bottom layer, also influencing water-column potential vorticity. This may affect the prevailing circulation, altering the local cyclonic/anticyclonic long-term variability and hence precondition future water-masses formation and the redistribution of heat along the entire water-column.

We assess the radiative heating error affecting marine air temperature (MAT) measurements in the Tropical Atmosphere Ocean array. The error in historical observations is found to be ubiquitous across the array, spatially variable and approximately stationary in time. The error induces spurious warming during daytime hours, but does not affect night-time temperatures. The range encompassing the real, unknown daily- and monthly-mean values is determined using daytime and night-time mean temperatures as upper and lower limits. The uncertainty in MAT is less than or equal to 0.5 °C and 0.2 °C for 95% of daily and monthly estimates, respectively. Uncertainties impact surface turbulent heat flux estimates, with potentially significant influences on the quantification of coupled ocean-atmosphere processes.

We consider the observation and analysis of oceanic rogue waves collected within spatio-temporal (ST) records of 3D wave fields. This class of records, allowing a sea surface region to be retrieved, is appropriate for the observation of rogue waves, which come up as a random phenomenon that can occur at any time and location of the sea surface. To verify this aspect, we used three stereo wave imaging systems to gather ST records of the sea surface elevation, which were collected in different sea conditions. The wave with the ST maximum elevation (happening to be larger than the rogue threshold 1.25Hs) was then isolated within each record, along with its temporal profile. The rogue waves show similar profiles, in agreement with the theory of extreme wave groups. We analyze the rogue wave probability of occurrence, also in the context of ST extreme value distributions, and we conclude that rogue waves are more likely than previously reported; the key point is coming across them, in space as well as in time. The dependence of the rogue wave profile and likelihood on the sea state conditions is also investigated. Results may prove useful in predicting extreme wave occurrence probability and strength during oceanic storms.

The expanding interest in decadal climate variability, predictability, and prediction highlights the importance of understanding the sources and mechanisms of decadal and interdecadal climate fluctuations. The purpose of this paper is to provide a critical review of our current understanding of externally forced decadal climate variability. In particular, proposed mechanisms determining decadal climate responses to variations in solar activity, stratospheric volcanic aerosols, and natural as well as anthropogenic tropospheric aerosols are discussed, both separately and in a unified framework. The review suggests that the excitation of internal modes of interdecadal climate variability, particularly centered in the Pacific and North Atlantic sectors, remains a paradigm to characterize externally forced decadal climate variability and to interpret the associated dynamics. Significant recent advancements are the improved understanding of the critical dependency of volcanically forced decadal climate variability on the relative phase of ongoing internal variability and on additional external perturbations, and the recognition that associated uncertainty may represent a serious obstacle to identifying the climatic consequences even of very strong eruptions. Particularly relevant is also the recent development of hypotheses about potential mechanisms (reemergence and synchronization) underlying solar forced decadal climate variability. Finally, outstanding issues and, hence, major opportunities for progress regarding externally forced decadal climate variability are discussed. Uncertain characterization of forcing and climate histories, imperfect implementation of complex forcings in climate models, limited understanding of the internal component of interdecadal climate variability, and poor quality of its simulation are some of the enduring critical obstacles on which to progress. It is suggested that much further understanding can be gained through identification and investigation of relevant periods of forced decadal climate variability during the preindustrial past millennium. Another upcoming opportunity for progress is the analysis of focused experiments with coupled ocean–atmosphere general circulation models within the umbrella of the next phase of the coupled model intercomparison project.

Many Holocene hydroclimate records show rainfall changes that vary with local orbital insolation. However, some tropical regions display rainfall evolution that differs from gradual precessional pacing, suggesting that direct rainfall forcing effects were predominantly driven by sea-surface temperature thresholds or inter-ocean temperature gradients. Here we present a 12,000 yr continuous U/Th-dated precipitation record from a Guatemalan speleothem showing that Central American rainfall increased within a 2000 yr period from a persistently dry state to an active convective regime at 9000 yr BP and has remained strong thereafter. Our data suggest that the Holocene evolution of Central American rainfall was driven by exceeding a temperature threshold in the nearby tropical oceans. The sensitivity of this region to slow changes in radiative forcing is thus strongly mediated by internal dynamics acting on much faster time scales., What drives hydroclimate changes in tropical regions is not well known. Here, the authors present a 12,000 year long precipitation record from Guetemala which shows that exceeding a threshold in sea surface temperatures caused Central American rainfall to change from a dry to an active convective regime around 9000 years ago.

The study assesses the performance of 24 model runs from five COordinated Regional climate Downscaling Experiment (CORDEX) regional climate models (RCMs) in simulating East Africa’s spatio-temporal precipitation characteristics using a set of eight descriptors: consecutive dry days (CDD), consecutive wet days (CWD), simple precipitation intensity index (SDII), mean daily annual (pr_ANN), seasonal (pr_MAM and pr_OND) precipitation, and representatives of heavy precipitation (90p) and very intense precipitation (99p) events. Relatively better performing RCM runs are then used to assess projected precipitation changes (for the period 2071–2099 relative to 1977–2005) over the study domain under the representative concentration pathway (RCP) 8.5 scenario. The performance of RCMs is found to be descriptor and scope specific. Overall, RCA4 (r1i1p1) forced by CNRM-CERFACS-CNRM-CM5 and MPI-M-MPI-ESM-LR, REMO2009 (r1i1p1) forced by MPI-M-MPI-ESM-LR, and RCA4 (r2i1p1) forced by MPI-M-MPI-ESM-LR emerge as the top four RCM runs. We show that an ensemble mean of the top four model runs outperforms an ensemble mean of 24 model simulations and ensemble means for all runs in an RCM. Our analysis of projections shows a reduction (increase) in mean daily precipitation for MAM(OND), an increase(decrease) in CDD(CWD) events, and a general increase in SDII and the width of the right tail of the precipitation distribution (99p–90p). An increase in SDII and 99p–90p implies a possibility of occurrence of heavy and extreme precipitation incidences by the end of the twenty-first century. Our findings provide important information to support the region’s climate change adaptation and mitigation efforts.