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Major storm systems over Europe frequently have a tropical origin. This paper analyses the characteristics and dynamics of such cyclones in the observational record, using MERRA reanalysis data for the period 1979-2013. By stratifying the cyclones along three key phases of their development (tropical phase, extratropical transition and final re-intensification), we identify four radically different life cycles: the tropical cyclone and extratropical cyclone life cycles, the classic extratropical transition and the warm seclusion life cycle. More than 50% of the storms reaching Europe from low latitudes follow the warm seclusion life cycle. It also contains the strongest cyclones. They are characterized by a warm core and a frontal T-bone structure, with a northwestward warm conveyor belt and the effects of dry intrusion. Rapid deepening occurs in the latest phase, around their arrival in Europe. Both baroclinic instability and release of latent heat contribute to the strong intensification. The pressure minimum occurs often a day after entering Europe, which enhances the potential threat of warm seclusion storms for Europe. The impact of a future warmer climate on the development of these storms is discussed. [ABSTRACT FROM AUTHOR]

This paper addresses the effect of interannual variability in jet stream orientation on weather systems over the North Atlantic basin (NAB). The observational analysis relies on 65 yr of NCEP-NCAR reanalysis (1948-2012). The total daily kinetic energy of the geostrophic wind (GTKE) is taken as a measure of storm activity over the North Atlantic. The NAB is partitioned into four rectangular regions, and the winter average of GTKE is calculated for each quadrant. The spatial GTKE average over all four quadrants shows striking year-to-year variability and is strongly correlated with the North Atlantic Oscillation (NAO). The GTKE strength in the northeast quadrant is closely related to the diffluence angle of the jet stream in the northwest quadrant. To gain insight into the relationship between the diffluence angle and its downstream impact, a quasigeostrophic baroclinic model is used. The results show that an initially zonal jet persists at its initial latitude over 30 days or longer, while a tilted jet propagates meridionally according to the Rossby wave group velocity, unless kept stationary by external forcing. A Gulf Stream-like narrow sea surface temperature (SST) front provides the requisite forcing for an analytical steady-state solution to this problem. This SST front influences the atmospheric jet in the northwest quadrant: it both strengthens the jet and tilts it northward at higher levels, while its effect is opposite at lower levels. Reanalysis data confirm these effects, which are consistent with thermal wind balance. The results suggest that the interannual variability found in the GTKE may be caused by intrinsic variability of the thermal Gulf Stream front. [ABSTRACT FROM AUTHOR]

We demonstrate for the first time a direct oceanic link between climate‐driven change in the North Atlantic and Arctic Oceans and the circulation of the northwest European shelf seas. Downscaled scenarios show a shutdown of the exchange between the Atlantic and the North Sea and a substantial decrease in the circulation of the North Sea in the second half of the 21st century. The northern North Sea inflow decreases from 1.2–1.3 Sv (1 Sv = 106 m3/s) to 0.0–0.6 Sv with Atlantic water largely bypassing the North Sea. This is traced to changes in oceanic haline stratification and gyre structure and to a newly identified circulation‐salinity feedback. The scenario presented here is of a novel potential future state for the North Sea, with wide‐ranging environmental management and societal impacts. Specifically, the sea would become more estuarine and susceptible to anthropogenic influence with an enhanced risk of coastal eutrophication. Plain Language Summary: Little is known about how climate change might impact the long‐term circulation of shelf seas. In this paper, we use a high‐resolution shelf sea model to demonstrate how end‐of‐century changes in the wider ocean can lead to a substantial reduction in the flow of water from the North Atlantic into the North Sea. This, in turn, reduces the circulation of this sea, which becomes more influenced by rivers and less by oceanic waters. River water generally contains higher levels of nutrients, and our simulations show that this future scenario leads to enhanced levels of phytoplankton growth in local regions of the North Sea. This may lead to undesirable disturbances to the marine ecosystems, such as depletion of oxygen near the seabed. The reduced circulation would also disrupt the transport of larvae around the sea and lead to increased retention of pollutants. The reduction in circulation arises from several causes relating to increased density layering at the continental shelf edge, changes in the large‐scale ocean circulation and salinity, and disruption of the density‐driven circulation of the North Sea. By exploring these novel future scenarios, we emphasize the need to understand better the many ways climate change can influence the marine environment and its ecosystems. Key Points: Potential end‐of‐century scenarios of dramatically reduced North Sea inflow and circulation are demonstrated by downscaling experimentsThis reduction is traced to increased shelf‐slope salinity stratification and modified North Atlantic and Arctic circulation and salinityThe North Sea then becomes more estuarine, with some regions of substantially enhanced nutrient content and primary production [ABSTRACT FROM AUTHOR]

This paper presents a new palynological record from a 146 cm long finely laminated sediment core obtained in 2009 from the deep-water meromictic Lake Shira (54°30′38″N, 90°12″09′E; ca. 353 m a.s.l.) situated in the Khakassian steppe region of southern Siberia between the rivers Ob' and Yenisei. The area is rich in lakes and represents an exceptionally well preserved sequence of Bronze and Iron Age archeological cultures. Little is known about the changes in vegetation and climate of the region during the Holocene. The palynological analysis of the core allows us to partly fill up this gap in current knowledge. The record of pollen and non-pollen palynomorphs presented here covers the past 2450 year interval with an average resolution of 22 years. The results obtained support the interpretation that the late Holocene vegetation changes around Lake Shira are mainly associated with large-scale atmospheric circulation processes controlling the regional water balance rather than with human activities. An attempt to trace human impact in the pollen assemblages provides no clear evidence for anthropogenic activity, except for the last few decades since ca. 1955, though the region has a long history of mobile pastoralists. For explanation of decadal-scale changes in the regional vegetation cover, the Artemisia /Chenopodiaceae (A/C) pollen ratio proved to be a reliable indicator of effective moisture availability. Using available fossil and published instrumental data our study suggests a link between the North Atlantic warmer/colder temperatures and higher/lower atmospheric precipitation (or moisture availability) in southern Siberia at multi-decadal to centennial scales. [ABSTRACT FROM AUTHOR]

This paper reviews the current knowledge on the ecology of widely distributed pelagic fish stocks in the North Atlantic basin with emphasis on their role in the food web and the factors determining their relationship with the environment. We consider herring ( Clupea harengus ), mackerel ( Scomber scombrus ), capelin ( Mallotus villosus ), blue whiting ( Micromesistius poutassou ), and horse mackerel ( Trachurus trachurus ), which have distributions extending beyond the continental shelf and predominantly occur on both sides of the North Atlantic. We also include albacore ( Thunnus alalunga ), bluefin tuna ( Thunnus thynnus ), swordfish ( Xiphias gladius ), and blue marlin ( Makaira nigricans ), which, by contrast, show large-scale migrations at the basin scale. We focus on the links between life history processes and the environment, horizontal and vertical distribution, spatial structure and trophic role. Many of these species carry out extensive migrations from spawning grounds to nursery and feeding areas. Large oceanographic features such as the North Atlantic subpolar gyre play an important role in determining spatial distributions and driving variations in stock size. Given the large biomasses of especially the smaller species considered here, these stocks can exert significant top-down pressures on the food web and are important in supporting higher trophic levels. The review reveals commonalities and differences between the ecology of widely distributed pelagic fish in the NE and NW Atlantic basins, identifies knowledge gaps and modelling needs that the EURO-BASIN project attempts to address. [ABSTRACT FROM AUTHOR]

It has long been recognised that there are strong interactions and feedbacks between climate, upper ocean biogeochemistry and marine food webs, and also that food web structure and phytoplankton community distribution are important determinants of variability in carbon production and export from the euphotic zone. Numerical models provide a vital tool to explore these interactions, given their capability to investigate multiple connected components of the system and the sensitivity to multiple drivers, including potential future conditions. A major driver for ecosystem model development is the demand for quantitative tools to support ecosystem-based management initiatives. The purpose of this paper is to review approaches to the modelling of marine ecosystems with a focus on the North Atlantic Ocean and its adjacent shelf seas, and to highlight the challenges they face and suggest ways forward. We consider the state of the art in simulating oceans and shelf sea physics, planktonic and higher trophic level ecosystems, and look towards building an integrative approach with these existing tools. We note how the different approaches have evolved historically and that many of the previous obstacles to harmonisation may no longer be present. We illustrate this with examples from the on-going and planned modelling effort in the Integrative Modelling Work Package of the EURO-BASIN programme. [ABSTRACT FROM AUTHOR]

The representation of the Northern Hemisphere (NH) storm tracks and jet streams and their response to climate change have been evaluated in climate model simulations from Phases 3, 5, and 6 of the Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6, respectively). The spatial patterns of the multimodel biases in CMIP3, CMIP5, and CMIP6 are similar; however, the magnitudes of the biases in the CMIP6 models are substantially lower. For instance, the multimodel mean RMSE of the North Atlantic storm track for the CMIP6 models (as measured by time‐filtered sea‐level pressure variance) is over 50% smaller than that of the CMIP3 models in both winter and summer, and over 40% smaller for the North Pacific. The magnitude of the jet stream biases is also reduced in CMIP6, but by a lesser extent. Despite this improved representation of the current climate, the spatial patterns of the climate change response of the NH storm tracks and jet streams remain similar in the CMIP3, CMIP5, and CMIP6 models. The SSP2‐4.5 scenario responses in the CMIP6 models are substantially larger than in the RCP4.5 CMIP5 models, which is consistent with the larger climate sensitivities of the CMIP6 models compared to CMIP5. Plain Language Summary: The ability of the climate models used in the CMIP (Coupled Model Intercomparison Project) models to represent the Northern Hemisphere storm tracks and jet streams is evaluated. The newest models (CMIP6) are found to have a much better representation, especially for the North Atlantic. The response of the storm tracks and jet streams to climate change is also assessed. The spatial pattern of the response in the CMIP3, CMIP5, and CMIP6 models is found to be similar. The response of the new CMIP6 models to the SSP2‐4.5 scenario is found to be larger than that of the very similar RCP4.5 scenario in the CMIP5 models, suggesting the CMIP6 models are more sensitive to the forcing from climate change. Key Points: The spatial patterns of the historical CMIP3, CMIP5, and CMIP6 biases are similar, but their magnitudes are substantially smaller in CMIP6The spatial patterns of the climate change responses in the NH storm tracks and jet streams are similar in the CMIP3, CMIP5, and CMIP6 modelsThe SSP2‐4.5 response in CMIP6 is larger than the RCP4.5 CMIP5 response, consistent with the larger climate sensitivities in CMIP6 [ABSTRACT FROM AUTHOR]

A conventional parameterization of midlatitude warm fog occurrence, based on in situ observations, is employed to estimate marine surface visibility in the Arctic and North Atlantic from three datasets: an ensemble member of the Hadley Earth System (HadGEM2) model and a nested regional WRF simulation that follow historical and future emissions scenarios for 1979-2100, and the ERA-Interim reanalysis for 1979-2004. Over large scales (of an entire year and region), all three gridded datasets agree well in terms of variables like surface air temperature, whose systematic differences seem small by comparison with its predicted change over the course of this century. On the other hand, systematic differences are more apparent in large-scale estimates of relative humidity and visibility. Large differences are attributed to a sensitivity to representation bias that is inherent in the formulation of each individual model and analysis. Two simple linear calibrations are examined, both of which assume that an in situ based parameterization is broadly consistent with the use of marine (ICOADS) observations of air and dew point temperature as an errorfree reference. A single-step calibration is considered that takes the mean and variance of ICOADS frequency distributions as a reference. A two-step calibration is also performed in which ICOADS collocations are taken as a reference for the ERA reanalysis, which in turn is taken as a large-scale reference for the 1979-2004 HadGEM2 and WRF simulations. Both linear calibrations are applied (locally in time and space to air and dew point temperature) to the future climate scenarios of HadGEM2 and WRF. Although ICOADS observations are not error-free and parameterized visibility estimates are unlikely to capture much more than half the variance found in observations, attempts are made to present consistent regional changes in the frequency of high relative humidity, as a proxy for warm fog occurrence. The large-scale decrease in visibility over the 21st century is in the range of 8%-12% in the Arctic and 0%-5% in the North Atlantic. [ABSTRACT FROM AUTHOR]

Contributions of atmospheric factors to the variability of the calculated theoretical maximum potential intensity (MPI) of tropical cyclones (TCs) over the North Atlantic are explored using the 6‐hourly atmospheric reanalysis and TC best track data from 1980 to 2015. The results show that for a given sea surface temperature (SST), the calculated theoretical MPI between the medians of top 10% and bottom 10% samples can vary by as large as 10–15 m/s, which accounts for 20–25% of the median of the MPI. It is shown that the drier (moister) and colder (warmer) environment favors higher (lower) MPI, and the TC‐MPI is more sensitive to atmospheric temperature at lower SSTs but more sensitive to atmospheric humidity at higher SSTs. Results from sensitivity experiments show that the tropospheric temperature and humidity profiles and the outflow layer temperature are all responsible for the MPI variability, but their relative importance vary with SST. The atmospheric humidity accounts for 12–13 (7–11) m/s at SSTs over (below) 28 °C, the tropospheric temperature accounts for about 7–12 (5–6) m/s at SSTs below (above) 28 °C, and the outflow temperature accounts for 7–8 m/s almost independent of SST. These results strongly suggest that the modulation of MPI by synoptic variability needs to be considered when MPI is calculated and used as a predictor/parameter in operational TC intensity prediction schemes, especially for strong TCs. Some other implications of the results are also discussed. Plain Language Summary: Contributions of atmospheric factors to the variability of the theoretical maximum potential intensity (MPI) of tropical cyclones (TCs) over the North Atlantic are explored using the 6‐hourly atmospheric reanalysis and TC best track data from 1980 to 2015. The results show that for a given sea surface temperature (SST), the theoretical MPI between the medians of top 10% and bottom 10% samples can vary by as large as 10–15 m/s, which accounts for 20–25% of the median of the MPI. It is shown that the drier (moister) and colder (warmer) environment favors higher (lower) MPI, and the MPI is more sensitive to atmospheric temperature at lower SSTs but more sensitive to atmospheric humidity at higher SSTs. Results strongly suggest that the modulation of MPI by synoptic variability needs to be considered when MPI is calculated and used as a predictor in operational TC intensity prediction schemes, especially for strong TCs. Key Points: The theoretical TC maximum potential intensity (MPI) can vary by as large as 20–25% for a given SST due to synoptic variabilityThe drier and colder environment favors higher MPI, and the MPI is more sensitive to atmospheric temperature (humidity) at low (high) SSTsThe modulation of MPI by synoptic variability should be considered if the MPI is used as a predictor in a TC intensity prediction scheme [ABSTRACT FROM AUTHOR]

This study investigates the changing relationship between the December North Atlantic Oscillation (NAO) and the following February East Asian trough (EAT) throughout the past 60 years. We found that the relationship between the December NAO and the following February EAT is significantly enhanced after the late 1980s compared with the period before the late 1980s. The changing relationship mainly results from the enhanced relationship between the December NAO and the following February North Atlantic mid-latitudes' sea surface temperature (SST) anomalies (NAMA) during the same period. During the period after the late 1980s, the persistent positive (negative) NAO pattern from December to the following January contributes to a positive (negative) NAMA, which reaches its maximum magnitude in the following February and excites an anomalous wave train along the North Atlantic and northern Eurasia, and significantly impacts the EAT. During the period before the late 1980s, the positive (negative) NAO pattern during December cannot persist into the following January, and the related positive (negative) NAMA is insignificant during the following February, causing the response of the simultaneous EAT to be insignificant as well. Moreover, there exists a significant impact of the December NAO on the December-January NAMA after the late 1980s, while the December-January NAMA is relatively less affected by the December NAO before the late 1980s. As a result, the simultaneous response of the atmospheric circulation anomalies to the December-January NAMA are evident before the late 1980s, and the positive (negative) NAMA can excite an anomalous wave train along the North Atlantic and northern Eurasia and significantly deepen (shallow) the downstream EAT. By contrast, after involving a feature of atmosphere forcing of SST, the simultaneous feedback of the December-January NAMA on EAT is significantly decreased after the 1980s. [ABSTRACT FROM AUTHOR]

During the second half of the 20th century, in the littoral and pre-littoral areas of the Valencia region, the diurnal temperature range (DTR) registered a significant drop in summer (July and August). Meanwhile, in the same period in the lower Ebro Valley (Ebro Observatory), to the north of the Valencia region, the maximum temperature and DTR increased steeply. In order to explain the DTR drop in the coastal areas of the Valencia region, some studies have proposed an increase in the summer sea-surface temperature (SST) and others have attributed it to the urban heat island effect. Nevertheless, this drop occurred well before the current climate change was evident and therefore, before the rise of the SST. Furthermore, regarding the second proposed explanation, the drop in the DTR does not disappear when working with selected stations away from heavily urbanized areas. In this work, we propose that both the DTR rise at the Ebro Observatory and the DTR drop in the Valencia region are linked to the same process: changes in atmospheric circulation on a synoptic-scale in the North Atlantic that occurred in the late 1960s. [ABSTRACT FROM AUTHOR]

Obtaining reliable reconstructions of long-term atmospheric circulation changes in the North Atlantic region presents a persistent challenge to contemporary paleoclimate research, which has been addressed by a multitude of recent studies. In order to contribute a novel methodological aspect to this active field, we apply here evolving functional network analysis, a recently developed tool for studying temporal changes of the spatial co-variability structure of the Earth's climate system, to a set of Late Holocene paleoclimate proxy records covering the last two millennia. The emerging patterns obtained by our analysis are related to long-term changes in the dominant mode of atmospheric circulation in the region, the North Atlantic Oscillation (NAO). By comparing the time-dependent inter-regional linkage structures of the obtained functional paleoclimate network representations to a recent multi-centennial NAO reconstruction, we identify co-variability between southern Greenland, Svalbard, and Fennoscandia as being indicative of a positive NAO phase, while connections from Greenland and Fennoscandia to central Europe are more pronounced during negative NAO phases. By drawing upon this correspondence, we use some key parameters of the evolving network structure to obtain a qualitative reconstruction of the NAO long-term variability over the entire Common Era (last 2000 years) using a linear regression model trained upon the existing shorter reconstruction. [ABSTRACT FROM AUTHOR]

The present study investigates changes in the location, frequency, intensity, and dynamical processes of North Atlantic extratropical cyclones with warming consistent with the IPCC Fifth Assessment Report (AR5) representative concentration pathway 8.5 (RCP8.5) scenario. The modeling, analysis, and prediction (MAP) climatology of midlatitude storminess (MCMS) feature-tracking algorithm was utilized to analyze 10 cold-season high-resolution atmospheric simulations over the North Atlantic region in current and future climates. Enhanced extratropical cyclone activity is most evident in the northeast North Atlantic and off the U.S. East Coast. These changes in cyclone activity are offset from changes in eddy kinetic energy and eddy heat flux. Investigation of the minimum SLP reached at each grid point reveals a lack of correspondence between the strongest events in the current and future simulations, indicating the future simulations produced a different population of storms. Examination of the percent change of storms in the storm-track region shows a reduction in the number of strong storms (i.e., those reaching a minimum SLP perturbation of at least −51 hPa). Storm-relative composites of strong and moderate storms show an increase in precipitation, associated with enhanced latent heat release and strengthening of the 900-700-hPa layer-average potential vorticity (PV). Other structural changes found for cyclones in a future climate include weakened upper-level PV for strong storms and a weakened near-surface potential temperature anomaly for moderate storms, demonstrating a change in storm dynamics. Furthermore, the impacts associated with extratropical cyclones, such as strong near-surface winds and heavy precipitation, strengthen and become more frequent with warming. [ABSTRACT FROM AUTHOR]

High-resolution reconstructions based on productivity proxies and magnetic properties of core LV63-41-2 (off Kamchatka) reveal prevailing centennial productivity/climate variability in the northwestern (NW) Pacific from the Last Glacial Maximum (LGM) to the early Holocene (EH). The age model of the core is established by AMS 14C dating and by projections of AMS 14C data of the nearby core SO-201-12KL through correlation of the productivity proxies and relative paleomagnetic intensity. The resulting sequence of centennial productivity increases/climate warming events in the NW Pacific occurred synchronously with the East Asian summer monsoon (EASM) sub-interstadials during the LGM (four events), Heinrich Event 1 (HE1) (four events), Bølling-Allerød (B/A) warming (four events), and over the EH (four events). Remarkable similarity of the sequence of the NW Pacific increased-productivity events with the EASM sub-interstadials over the LGM-HE1 implies that the Siberian High is a strong and common driver. The comparison with the δ18O record from Antarctica suggests that another mechanism associated with the temperature gradient in the Southern Hemisphere may also be responsible for the EASM/NW Pacific centennial events over the LGMHE1. During the B/A warming and resumption of the Atlantic Meridional Overturning Circulation (AMOC), clear synchronicity between the NW Pacific, EASM and Greenland sub-interstadials was mainly controlled by changes in the atmospheric circulation. During the EH the linkages between solar forcing, ocean circulation, and climate changes likely control the synchronicity of abrupt climate changes in the NW Pacific and North Atlantic. The sequence of centennial events recorded in this study is a persistent regional feature during the LGM-EH, which may serve as a template in high-resolution paleoceanography and sediment stratigraphy in the NW Pacific. [ABSTRACT FROM AUTHOR]

Recent work has provided ample evidence that nonlinear methods of time series analysis potentially allow for detecting periods of anomalous dynamics in paleoclimate proxy records that are otherwise hidden to classical statistical analysis. Following upon these ideas, in this study, we systematically test a set of Late Holocene terrestrial paleoclimate records from Northern Europe for indications of intermittent periods of time- irreversibility during which the data are incompatible with a stationary linear-stochastic process. Our analysis reveals that the onsets of both the Medieval Climate Anomaly and the Little Ice Age, the end of the Roman Warm Period, and the Late Antique Little Ice Age have been characterized by such dynamical anomalies. These findings may indicate qualitative changes in the dominant regime of interannual climate variability in terms of large-scale atmospheric circulation patterns, ocean-atmosphere interactions, and external forcings affecting the climate of the North Atlantic region. [ABSTRACT FROM AUTHOR]

A novel statistical-deterministic method is applied to generate thousands of synthetic tracks of North Atlantic (NA) polar lows and Mediterranean hurricanes ("medicanes"); these synthetic storms are compatible with the climates simulated by 30 CMIP5 models in both historical and RCP8.5 simulations for a recent (1986-2005) and a future (2081-2100) period, respectively. Present-to-future multimodel mean changes in storm risk are analyzed, with special attention to robust patterns (in terms of consensus among individual models) and privileging in each case the subset of models exhibiting the highest agreement with the results yielded by two reanalyses. A reduction of about 10%-15% in the overall frequency of NA polar lows that would uniformly affect the full spectrum of storm intensities is expected. In addition a very robust regional redistribution of cases is obtained, namely a tendency to shift part of the polar low activity from the south Greenland-Icelandic sector toward the Nordic seas closer to Scandinavia. In contrast, the future change in the number of medicanes is unclear (on average the total frequency of storms does not vary) but a profound reshaping of the spectrum of lifetime maximum winds is found; the results project a higher number of moderate and violent medicanes at the expense of weak storms. Spatially, the method projects an increased occurrence of medicanes in the western Mediterranean and Black Sea that is balanced by a reduction of storm tracks in contiguous areas, particularly in the central Mediterranean; however, future extreme events (winds > 60kt; 1 kt = 0.51 ms-1) become more probable in all Mediterranean subbasins. [ABSTRACT FROM AUTHOR]

Mineral dust has a large impact on regional and global climate, depending on its particle size. Especially in the Atlantic Ocean downwind of the Sahara, the largest dust source on earth, the effects can be substantial but are poorly understood. This study focuses on seasonal and spatial variations in particle size of Saharan dust deposition across the Atlantic Ocean, using an array of submarine sediment traps moored along a transect at 12 N. We show that the particle size decreases downwind with increased distance from the Saharan source, due to higher gravitational settling velocities of coarse particles in the atmosphere. Modal grain sizes vary between 4 and 32 µm throughout the different seasons and at five locations along the transect. This is much coarser than previously suggested and incorporated into climate models. In addition, seasonal changes are prominent, with coarser dust in summer and finer dust in winter and spring. Such seasonal changes are caused by transport at higher altitudes and at greater wind velocities during summer than in winter. Also, the latitudinal migration of the dust cloud, associated with the Intertropical Convergence Zone, causes seasonal differences in deposition as the summer dust cloud is located more to the north and more directly above the sampled transect. Furthermore, increased precipitation and more frequent dust storms in summer coincide with coarser dust deposition. Our findings contribute to understanding Saharan dust transport and deposition relevant for the interpretation of sedimentary records for climate reconstructions, as well as for global and regional models for improved prediction of future climate. [ABSTRACT FROM AUTHOR]

The atmospheric seasonal cycle of the North Atlantic region is dominated by meridional movements of the circulation systems: from the tropics, where the West African Monsoon and extreme tropical weather events take place, to the extratropics, where the circulation is dominated by seasonal changes in the jetstream and extratropical cyclones. Climate variability over the North Atlantic is controlled by various mechanisms. Atmospheric internal variability plays a crucial role in the mid-latitudes. However, El Niño-Southern Oscillation (ENSO) is still the main source of predictability in this region situated far away from the Pacific. Although the ENSO influence over tropical and extra-tropical areas is related to different physical mechanisms, in both regions this teleconnection seems to be non-stationary in time and modulated by multidecadal changes of the mean flow. Nowadays, long observational records (greater than 100 years) and modeling projects (e.g., CMIP) permit detecting non-stationarities in the influence of ENSO over the Atlantic basin, and further analyzing its potential mechanisms. The present article reviews the ENSO influence over the Atlantic region, paying special attention to the stability of this teleconnection over time and the possible modulators. Evidence is given that the ENSO–Atlantic teleconnection is weak over the North Atlantic. In this regard, the multidecadal ocean variability seems to modulate the presence of teleconnections, which can lead to important impacts of ENSO and to open windows of opportunity for seasonal predictability. [ABSTRACT FROM AUTHOR]

Small glaciers and ice caps respond rapidly to climate variations, and records of their past extent provide information on the natural envelope of past climate variability. Millennial-scale trends in Holocene glacier size are well documented and correspond with changes in Northern Hemisphere summer insolation. However, there is only sparse and fragmentary evidence for higher-frequency variations in glacier size because in many Northern Hemisphere regions glacier advances of the past few hundred years were the most extensive and destroyed the geomorphic evidence of ice growth and retreat during the past several thousand years. Thus, most glacier records have been of limited use for investigating centennial-scale climate forcing and feedback mechanisms. Here we report a continuous record of glacier activity for the last 9.5 ka from southeast Greenland derived from high-resolution measurements on a proglacial lake sediment sequence. Physical and geochemical parameters show that the glaciers responded to previously documented Northern Hemisphere climatic excursions, including the "8.2 ka" cooling event, the Holocene Thermal Maximum, Neoglacial cooling, and 20th century warming. In addition, the sediments indicate centennial-scale oscillations in glacier size during the late Holocene. Beginning at 4.1 ka, a series of abrupt glacier advances occurred, each lasting 100 years and followed by a period of retreat, that were superimposed on a gradual trend toward larger glacier size. Thus, while declining summer insolation caused long-term cooling and glacier expansion during the late Holocene, climate system dynamics resulted in repeated episodes of glacier expansion and retreat on multi-decadal to centennial timescales. These episodes coincided with ice rafting events in the North Atlantic Ocean and periods of regional ice cap expansion, which confirms their regional significance and indicates that considerable glacier activity on these timescales is a normal feature of the cryosphere. The data provide a longer-term perspective on the rate of 20th century glacier retreat and indicate that recent anthropogenic-driven warming has already impacted the regional cryosphere in a manner outside the natural range of Holocene variability. [ABSTRACT FROM AUTHOR]

Sea Surface Salinity (SSS) is an essential climate variable that requires long term in situ observation. The French SSS Observation Service (SSS-OS) manages a network of Voluntary Observing Ships equipped with thermosalinographs (TSG). The network is global though more concentrated in the tropical Pacific and North Atlantic oceanic basins. The acquisition system is autonomous with real time transmission and is regularly serviced at harbor calls. There are distinct real time and delayed time processing chains. Real time processing includes automatic alerts to detect potential instrument problems, in case raw data are outside of climatic limits, and graphical monitoring tools. Delayed time processing relies on a dedicated software for attribution of data quality flags by visual inspection, and correction of TSG time series by comparison with daily water samples and collocated Argo data. A method for optimizing the automatic attribution of quality flags in real time, based on testing different thresholds for data deviation from climatology and retroactively comparing the resulting flags to delayed time flags, is presented. The SSS-OS real time data feed the Coriolis operational oceanography database, while the research-quality delayed time data can be extracted for selected time and geographical ranges through a graphical web interface. Delayed time data have been also combined with other SSS data sources to produce gridded files for the Pacific and Atlantic oceans. A short review of the research activities conducted with such data is given. It includes observation-based process-oriented and climate studies from regional to global scale as well as studies where in situ SSS is used for calibration/validation of models, coral proxies or satellite data. [ABSTRACT FROM AUTHOR]

Long- and short-term climate variations in the North Atlantic have been of sufficient magnitude to leave a discernible mark on the history of vegetation and landscape stability in Iceland during the Holocene. A reconstruction of early- and mid-Holocene vegetation around Lake Kagaðarhóll, Northwest Iceland, examines how climate fluctuations have affected the terrestrial ecosystem. A thorough reconstruction has been made using pollen and plant macrofossil analyses combined with proxies for organic and inorganic matter. The record shows the development from a period of pioneer vegetation towards a woodland ecosystem. The deposition of the Saksunarvatn tephra at c. 10,300 cal. yr BP caused a 100-year period of instability, followed by a gradual trend of stabilization over several centuries while material left behind by retreating glaciers and tephra was being contained by expanding and developing vegetation. Early-Holocene warmth is indicated by high pollen production of Juniperus communis around the lake by c. 10,100 cal. yr BP and birch woodland being established around the lake by c. 9200 cal. yr BP, much earlier than previously believed for this locale. Cooling climate between c. 8700 and 8200 cal. yr BP halted woodland development, with reduced plant reproduction likely caused by cold spring and summer temperatures. Woodlands became re-established from c. 7900 cal. yr BP before entering a decline from c. 6000 cal. yr BP, with harsher environmental conditions apparent after c. 4200 cal. yr BP. The Kagaðarhóll record compares favourably with other palaeoclimatic data from the North Atlantic, demonstrating the potential of pollen and macrofossil data for reconstructions of environmental change in Iceland and as an indicator of climate variability in the North Atlantic during the Holocene. [ABSTRACT FROM AUTHOR]

Data analysis indicates that the interannual variability of the spring atmospheric heat source over the Tibetan Plateau (TP) depends largely on the intensity of the overlying subtropical westerly jet (WJ), which is closely related to the early spring (February-March-April) sea surface temperature anomaly (SSTA) tripole pattern over the North Atlantic, i.e., the SSTA tripole pattern with a warm core to the southeast of Newfoundland and two cold cores to the south of Iceland and southeast of Bermuda, respectively. Such an SSTA pattern can be regarded as a response to the atmospheric forcing of the preceding January-February-March North Atlantic Oscillation. Numerical experiment results from both linear baroclinic model with an idealized diabatic heating profile and an atmospheric general circulation model with prescribed SSTA forcing demonstrate that the warm core alone of the tripole pattern can stimulate a steady downstream Rossby wave train, which further intensifies the spring WJ over the TP. As a result, a positive anomaly of surface sensible heating occurs over most parts of the TP, whereas the precipitation and corresponding latent heating is characterized by a seesaw pattern with a positive/negative anomaly over the northern/southern TP. Meanwhile, the air column radiation cooling effect is enhanced to a certain degree over the plateau. Further analysis suggests that the early spring SSTA over the North Atlantic may exert a seasonal-lagged impact upon the East Asian summer monsoon by modulating the thermal forcing over the TP. [ABSTRACT FROM AUTHOR]

The detection of anthropogenic climate change can be improved by recognizing the seasonality in the climate change response. This is demonstrated for the North Atlantic jet [zonal wind at 850 hPa (U850)] and European precipitation responses projected by the climate models from phase 5 of CMIP (CMIP5). The U850 future response is characterized by a marked seasonality: an eastward extension of the North Atlantic jet into Europe in November-April and a poleward shift in May-October. Under the RCP8.5 scenario, the multimodel mean response in U850 in these two extended seasonal means emerges by 2035-40 for the lower-latitude features and by 2050-70 for the higher-latitude features, relative to the 1960-90 climate. This is 5-15 years earlier than when evaluated in the traditional meteorological seasons (December-February and June-August), and it results from an increase in the signal-to-noise ratio associated with the spatial coherence of the response within the extended seasons. The annual mean response lacks important information on the seasonality of the response without improving the signal-to-noise ratio. The same two extended seasons are demonstrated to capture the seasonality of the European precipitation response to climate change and to anticipate its emergence by 10-20 years. Furthermore, some of the regional responses (such as the Mediterranean precipitation decline and the U850 response in North Africa in the extended winter) are projected to emerge by 2020-25, according to the models with a strong response. Therefore, observations might soon be useful to test aspects of the atmospheric circulation response predicted by some of the CMIP5 models. [ABSTRACT FROM AUTHOR]

The North Atlantic Oscillation (NAO) response to the northeast Pacific climate variability is examined using the ERA-40 dataset. The main objective is to validate a mechanism involving downstream wave propagation processes proposed in a recent idealized companion study: a low-frequency planetary-scale ridge (trough) anomaly located in the eastern Pacific-North American sector induces more equatorward (poleward) propagation of synoptic-scale wave packets on its downstream side, which favors the occurrence of anticyclonic (cyclonic) wave breakings in the Atlantic sector and the positive (negative) NAO phase. The mechanism first provides an interpretation of the canonical impact of the El Niño-Southern Oscillation on the NAO in late winter. The wintertime relationship between the Pacific-North American oscillation (PNA) and the NAO is also investigated. For out-of-phase fluctuations between the PNA and NAO indices (i.e., the most recurrent situation in late winter), the eastern Pacific PNA ridge (trough) anomaly modifies the direction of downstream wave propagation, triggering more anticyclonic (cyclonic) wave breakings over the North Atlantic. For in-phase fluctuations, the effect of the eastern Pacific PNA anomalies is cancelled out by the North American PNA anomalies. The latter anomalies being deeper and more centered in the latitudinal band of downstream wave propagation, they are able to reverse the direction of wave propagation just before waves enter the Atlantic domain. The contrasting relationship between the PNA and NAO is similar to what occurs for the two leading hemispheric EOFs of geopotential height: the northern annular mode (NAM) and the cold ocean-warm land (COWL) pattern. The proposed mechanism provides a physical meaning for the NAM and COWL patterns. [ABSTRACT FROM AUTHOR]

The observed North Atlantic multi-decadal variability for the period 1872-2009 is reconstructed with the Max Planck Institute ocean model, which is forced with an ensemble of the atmospheric twentieth century reanalysis. Special emphasis is put on the early part of the experiments, which includes a prominent climate variation during the 1920s. The experiments are in agreement with selected hydrographic records, indicating a transition from cold and fresh North Atlantic water properties, prior to the 1920 climate variation, towards warm and saline waters afterwards. Examining the variation reveals that sea level pressure (SLP) anomalies prior to the 1900s resemble a negative phase of North Atlantic Oscillation and associated weak winds result in a weak North Atlantic Current (NAC) and sub-polar gyre (SPG). This leads to a reduced transport of warm and saline waters into the higher latitudes. Simultaneously, Arctic freshwater release results in the accumulation of cold and fresh water properties, which cover the upper layers in the Labrador Sea and subsequently suppress convection. From the 1910s, the Arctic freshwater export is reduced, and, NAC and SPG are strengthened as a result of an increased SLP gradient over the North Atlantic. Concurrently, Labrador Sea convection and Atlantic meridional overturning circulation (AMOC) increase. The intensified NAC, SPG, and AMOC redistribute sub-tropical water into the North Atlantic and Nordic Seas, thereby increasing observed and modelled temperature and salinity during the 1920s. [ABSTRACT FROM AUTHOR]

Late Holocene coastal dune successions in north-western Europe contain evidence of episodic aeolian sand movement in the recent past. If previous periods of increased sand movement can be dated sufficiently precisely and placed in a correct cultural and geomorphological context, they may add to our understanding of storminess variation and climate change in the North Atlantic during the later part of the Holocene. In this study, coastal cliff sections of Holocene dune sand were investigated in the north-western part of the Skagen Odde spit system in northern Denmark. Four units of aeolian sand were recognized. Optically stimulated luminescence (OSL) dating indicates that aeolian sand movement took place in four phases: around AD 1460, between AD 1730 and 1780, around AD 1870, and since about AD 1935. The first phase of sand movement occurred during cooling in the first part of the Little Ice Age. A change in the atmospheric circulation, so that both the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) were negative, apparently led to an increased number of intense cyclones causing inland sand movement and dune building. The second and third phase of aeolian sand movement during the Little Ice Age also took place in periods of increased storminess, but during these events it appears that negative NAO values were coupled with positive AMO values. The final phase of sand movement is intimately linked to the modern formation of frontal dunes which takes place during moderate storminess. These findings are important as they indicate three major periods of aeolian sand movement and storminess during the Little Ice Age. [ABSTRACT FROM AUTHOR]

Analyses of diatoms, grain size, magnetic susceptibility, total organic carbon, and total nitrogen were applied to a 9.26 m long sediment core, spanning the last 12.2 kyr, from a small treeline lake (Tiancai Lake, ~3898 m a.s.l.) in southwest China. Diatom assemblages are dominated by Cyclotella distinguenda, Aulacoseira species, and small fragilarioid taxa, all of which are sensitive to changes in water pH and light conditions that are probably related to vegetation development and runoff processes triggered by variations in the Asian monsoon. High abundances of C. distinguenda and Pseudostaurosira brevistriata reflected cold and dry climates during the Late Glacial (12.2–11.4 kyr BP). In the early Holocene (11.4–9.4 kyr BP), a steep decline in C. distinguenda and a visible increase in Aulacoseira alpigena responded to a strengthening monsoon intensity. The persistent increases in A. alpigena mirrored strong monsoon intensity in the middle Holocene (9.4–4.6 kyr BP). After 4.6 kyr BP, the reduction of A. alpigena was related to weak monsoon intensity in the late Holocene. The main trends of diatom evolution show a general correspondence to variations in solar insolation. Three visible excursions, with an increase in P. brevistriata and a drop in A. alpigena, centered at around 8.4, 2.5, and 0.3 kyr BP, correlate with low sunspot numbers and known cold events in the North Atlantic. Some similarities and correlations between the Holocene diatom data, the North Atlantic record, and solar insolation indicate that variations in the Asian monsoon response to changes in solar forcing and the North Atlantic climate. [ABSTRACT FROM AUTHOR]

Discharge time series of major large-catchment European rivers are known to display significant decadal and interdecadal fluctuations. However, the hydroclimate variability causing such fluctuations remains poorly understood, particularly due to a lack of a spatio-temporal integrated assessment. Here, we demonstrate for the first time that European hydroclimate variability is dominated by a meridional delayed oscillation characterized by a lag of approximately 5 years in interdecadal discharge fluctuations of continental (northern) European rivers with respect to those of Euro-Mediterranean (southern) rivers. We demonstrate a connection of this coherent signal with the large-scale atmospheric circulation over the North Atlantic, and suggest a hitherto unexplored multiannual atmosphere-ocean mechanism in the subpolar North Atlantic at its root. [ABSTRACT FROM AUTHOR]