Positive North Atlantic Oscillation Index Research Articles

Palaeoecological Conditions in the South-Eastern and Western Baltic Sea during the Last Millennium

We present the reconstruction of palaeoenvironmental conditions in the Gdansk, Bornholm, and Arkona Basins of the Baltic Sea over the last millennium. A multiproxy study (including geochemical, XRF, grain size, AMS, and micropalaeontological analyses) of five short sediment cores was performed. The relative age of the sediments was determined based on the Pb distribution along the sediment sequences, as radiocarbon dating has resulted in an excessively old age. The retrieved cores cover two comparable warm periods, the Medieval Climate Anomaly and the Modern Warm Period, for which the increase in surface water productivity was reconstructed. Notably, the production of diatoms was higher during the colder periods (the Dark Ages and Little Ice Age), but this was also the case within the Modern Warm Period. In the Gdansk Basin, the initial salinity increase during the Littorina transgression started after 7.7 cal. a BP. The increased inflow activity was reconstructed during the Medieval Climate Anomaly, even in the Gdansk Basin, despite, in general, very low foraminiferal amounts and diversity. The strongly positive North Atlantic Oscillation Index during this period led to the prevalence of westerly winds over the Baltic region and stronger saltwater intrusions. In the recent sediments, the reconstructed inflow frequency demonstrates a variability against the reduction trend, and a general decline compared to the Medieval Climate Anomaly is seen.

A 7000-year record of extreme flood events reconstructed from a threshold lake in southern Norway

Recent decades have witnessed a shift in the seasonality and frequency of river floods in Norway, primarily attributed to contemporary global warming. Such changes necessitate a more comprehensive understanding of climate-flood dynamics across river systems. A significant challenge in flood risk assessment is that instrumental data records cover only the last few decades and do not capture low-frequency, high-magnitude extremes. Another challenge is the non-stationarity of the leading atmospheric pattern over Scandinavia, the North Atlantic Oscillation (NAO), which requires a spatial understanding of extreme events through time. To address this issue, we use lacustrine sediments from southernmost Norway, aiming to extend flood records beyond the scope of instrumental and historical data by exploring a threshold lake that accumulates flood deposits only when river discharge exceeds a critical level. A multi-proxy approach, which includes X-ray fluorescence (XRF), computed tomography (CT), and magnetic susceptibility (MS), was applied to fingerprint the sediments. We detected thin layers of minerogenic sediments, which we interpreted as slackwater deposits accumulated in a lake environment during extreme flood events. These fine-grained minerogenic bands were quantified using XRF and CT data to reconstruct the frequency of major floods over the past ∼7000 years. The minimum water discharge necessary for transporting sediments into the lake was assessed with the HEC-RAS hydraulic simulation software. Our record shows a high frequency of extreme floods from 7000 to 5000 cal yr BP before a significant decrease from 5000 to 2300 cal yr BP. The frequency has been high since 2300 cal yr BP, with a peak frequency after 1600 CE, coinciding with the Little Ice Age. There is a strong co-variability between a positive NAO index, winter precipitation, and major flood events over the past 120 years. A comparison between our flood record and other palaeorecords suggests that this relationship also holds on multi-millennial time scales.

Construction of a deltaic strandplain during the Roman period in the Tavoliere di Puglia plain and palaeoclimatic implications

In response to the accidental exhumation of three ancient trees by farmers, we conducted a multidisciplinary study based on the stratigraphic analysis of boreholes, carbon-14 dating, aerial photo interpretation, and analysis of palaeobotanical and archaeological evidences. We reconstructed the formation and evolution during Roman times of a first “continuous” and then “discontinuous” deltaic strandplain at the mouth of the Carapelle Stream in the Tavoliere di Puglia Plain—the second-largest plain in Italy. Ttwo main phases can be recognised in the evolution of the Carapelle deltaic strandplain: 1) a first phase, lasted until ca. the birth of Christ, was characterised by a regular and continuous construction of sand ridges one leaning against the other; 2) a second phase, lasted more or less from the birth of Christ to the termination of the construction of the deltaic strandplain, was characterised by the discontinuous construction of sand ridges/coastal barriers with the isolation of lagoons/ponds, and by evidences of alluvial events. The most probable climatic–environmental scenario to have formed the Roman-period deltaic strandplain implied that: 1) the first phase was triggered by a higher total amount of precipitations, but with less extreme alluvial events; 2) the second phase was triggered by a total amount of precipitation lower than the previous period, but with higher occurrence of extreme alluvial events and/or by extreme alluvial events separated by longer period of low precipitations. This second phase was enhanced by the opening of vegetation. The passage between the first and second phase of the Carapelle deltaic strandplain coincides with the passage from overall negative NAO index to an overall positive NAO index.

The possible links between the Barents–Kara sea‐ice area, Ural blocking, and the North Atlantic Oscillation

AbstractWe investigated the possible links between the Barents–Kara sea‐ice area (SIA), Ural blocking, and the North Atlantic Oscillation (NAO) in December–January (DJ) and February–March (FM) using the ERA5 data for the period December 1979–March 2022. The Barents–Kara SIA loss in December is correlated with an increase in geopotential height at 500 hPa (), mean sea‐level pressure (MSLP), and the frequency and intensity of blocking over the Ural in DJ. The Barents–Kara SIA loss in December is also associated with the weakening of the stratospheric polar vortex in FM (particularly in mid‐February) and the negative NAO index. However, our results show that persistent Ural blocking occurs during the transition from a neutral or positive NAO index to its negative phase. Indeed, a significant decrease in NAO index leads to the development of an area of instantaneous blocking (IB) and positive anomalies over the Ural. Persistent Ural blocking contributes significantly to Barents–Kara SIA loss, with a peak decline about seven days after the onset of Ural blocking. The onset of persistent Ural blocking also precedes the weakening of the stratospheric polar vortex by about one month. This implies that the negative correlation between the Barents–Kara SIA loss in December and the NAO index in FM might be caused by the weakening of the stratospheric polar vortex, which itself is induced by persistent Ural blocking. We conclude that the Barents–Kara SIA loss in December can be viewed as a sign rather than the cause of changes in atmospheric circulation over the high‐latitude North Atlantic in succeeding months, because the Barents–Kara SIA also largely responds to Ural blocking and the NAO.

The Summertime Circulation Types over Eurasia and Their Connections with the North Atlantic Oscillation Modulated by North Atlantic SST

ERA5 monthly averaged reanalysis data during 1979–2020 are used to analyze the anomalous characteristics of summertime circulation types over Eurasia and their connections with the North Atlantic Oscillation (NAO) modulated by North Atlantic sea surface temperature (SST). A circulation index (CI) is defined to describe the anomalous characteristics of summertime circulation types over the Eurasian mid-high latitude and classify the anomalous circulation into a double-ridge type (DR-type) and double-trough type (DT-type). The results show that these anomalous circulation types are closely related to the variation of the western Pacific subtropical high (WPSH), East Asian subtropical jet (EASJ), South Asia high (SAH) and summer precipitation anomalies in China. There is a significant negative correlation between summer NAO and circulation types over Eurasia. The positive CI is favorable for the southward movement of the EASJ and two positive height anomalies over the Ural Mountains and the Sea of Okhotsk, respectively. Accompanied by moisture convergence and a strong ascending motion over the middle and lower reaches of the Yangtze River Valley (MLYRV), the summer rainfall will be above normal. These patterns are reversed in positive NAO-index years. The connection between the NAO and circulation types over Eurasia is modulated by a tri-pole SST anomaly pattern over the North Atlantic, which may induce the NAO-like atmospheric circulation and strengthen the impacts of the NAO on Eurasian circulation types. A wave train from the North Atlantic to East Asia, which is aroused by the tri-pole SST anomaly pattern, is the potential mechanism for linking summer NAO and circulation types over Eurasia.

The March 2012 Heat Wave in Northeast America as a Possible Effect of Strong Solar Activity and Unusual Space Plasma Interactions

In the past two decades, the world has experienced an unprecedented number of extreme weather events, some causing major human suffering and economic damage. The March 2012 heat wave is one of the most known and broadly discussed events in the Northeast United States (NE-USA). The present study examines in depth the possible influence of solar activity on the historic March 2012 heat wave based on a comparison of solar/space and meteorological data. Our research suggests that the historic March 2012 heat wave (M2012HW) and the March 1910 heat wave (M1910HW), which occurred a century earlier in NE-USA, were related to Sun-generated special space plasma structures triggering large magnetic storms. Furthermore, the largest (Dst = −222 nT) magnetic storm during solar cycle 24 in March 2015 (only three years later than the March 2012 events) occurred in relation to another heat wave (M2015HW) in NE-USA. Both these heat waves, M2012HW and M2015HW, resemble each other in many ways: they were characterized by extremely huge temperature increases ΔΤΜ = 30° and 32° (with maximum temperatures ΤΜ = 28° and 23°, respectively) during a positive North Atlantic Oscillation index, the high temperatures coincided with large-scale warm air streaming from southern latitudes, they were accompanied by superstorms caused by unexpected geoeffective interplanetary coronal mass ejections (ICMEs), and the ICME-related solar energetic particle (SEP) events were characterized by a proton spectrum extending to very high (>0.5 GeV) energies. We infer that (i) all three heat waves examined (M2012HW, M2015HW, M1910HW) were related with strong magnetic storms triggered by effective solar wind plasma structures, and (b) the heat wave in March 2012 and the related solar activity was not an accidental coincidence; that is, the M2012HW was most probably affected by solar activity. Future case and statistical studies are needed to further check the hypothesis put forward here, which might improve atmospheric models in helping people’s safety, health and life.

Significant Extremal Dependence of a Daily North Atlantic Oscillation Index (NAOI) and Weighted Regionalised Rainfall in a Small Island Using the Extremogram

Extremal dependence or independence may occur among the components of univariate or bivariate random vectors. Assessing which asymptotic regime occurs and also its extent are crucial tasks when such vectors are used as statistical models for risk assessment in the field of Climatology under climate change conditions. Motivated by the poor resolution of current global climate models in North Atlantic Small Islands, the extremal dependence between a North Atlantic Oscillation index (NAOI) and rainfall was considered at multi-year dominance of negative and positive NAOI, i.e., −NAOI and +NAOI dominance subperiods, respectively. The datasets used (from 1948–2017) were daily NAOI, and three daily weighted regionalised rainfall series computed based on factor analysis and the Voronoi polygons method from 40 rain gauges in the small island of Madeira (∼740 km2), Portugal. The extremogram technique was applied for measuring the extremal dependence within the NAOI univariate series. The cross-extremogram determined the dependence between the upper tail of the weighted regionalised rainfalls, and the upper and lower tails of daily NAOI. Throughout the 70-year period, the results suggest systematic evidence of statistical dependence over Madeira between exceptionally −NAOI records and extreme rainfalls, which is stronger in the −NAOI dominance subperiods. The extremal dependence for +NAOI records is only significant in recent years, however, with a still unclear +NAOI dominance.

Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean

The Medieval Climate Anomaly (MCA; 900–1300 AD) was the most recent period of pre-industrial climatic warming in the northern hemisphere, and thus estimations of MCA signals can illuminate possible impacts of anthropogenic climate change. Current high-resolution MCA climate signals are restricted to mid- and high-latitude regions, which confounds inferences of how the MCA impacted some global/hemispheric climate mechanisms (e.g. North Atlantic Oscillation; NAO). To address this knowledge gap, we estimate seasonally-resolved sea surface temperatures (SSTs) from the oxygen isotope composition (δ18O) of serially sampled Phorcus atratus shells from archaeological sites spanning the MCA in the Canary Islands. Twelve archaeological and six modern P. atratus shells were analyzed, and archaeological shells were dated using carbonate-target radiocarbon dating. SSTs were estimated using the published aragonite-water equilibrium fractionation equation. Modern shells showed a mean SST of 20.0 ± 1.5 °C, with a seasonal amplitude of 5.3 ± 0.9 °C. Archaeological shells exhibited a mean SST of 18.2 ± 0.7 °C, with a mean seasonal amplitude of 5.5 ± 1.0 °C. Thus, shells that span the MCA in the Canary Islands recorded SSTs that were significantly cooler than the modern (P < .05), contrasting with warming estimates and model predictions elsewhere in the Northern Hemisphere. We propose that the observed cooling resulted from increased upwelling in NW Africa due to a strengthening of the prevailing westerlies and coastal winds along the African shoreline. The intensified upwelling scenario during the MCA is partially supported by in-situ carbon isotope data (δ13C) retrieved from the archaeological shells, which was compared to the δ13C values of modern shells and dissolved inorganic carbon in the ambient seawater. These results are consistent with other low-latitude temperature/precipitation anomalies associated with a positive NAO mode, suggesting a transition to a positive NAO index during the middle and late MCA that possibly extended later into the 13th century AD.

Mechanisms of Winter Precipitation Variability in the European–Mediterranean Region Associated with the North Atlantic Oscillation

AbstractThe physical mechanisms whereby the mean and transient circulation anomalies associated with the North Atlantic Oscillation (NAO) drive winter mean precipitation anomalies across the North Atlantic Ocean, Europe, and the Mediterranean Sea region are investigated using the European Centre for Medium-Range Weather Forecasts interim reanalysis. A moisture budget decomposition is used to identify the contribution of the anomalies in evaporation, the mean flow, storm tracks and the role of moisture convergence and advection. Over the eastern North Atlantic, Europe, and the Mediterranean, precipitation anomalies are primarily driven by the mean flow anomalies with, for a positive NAO, anomalous moist advection causing enhanced precipitation in the northern British Isles and Scandinavia and anomalous mean flow moisture divergence causing drying over continental Europe and the Mediterranean region. Transient eddy moisture fluxes work primarily to oppose the anomalies in precipitation minus evaporation generated by the mean flow, but shifts in storm-track location and intensity help to explain regional details of the precipitation anomaly pattern. The extreme seasonal precipitation anomalies that occurred during the two winters with the most positive (1988/89) and negative (2009/10) NAO indices are also explained by NAO-associated mean flow moisture convergence anomalies.

On the Mechanism of Arctic Climate Oscillation with a Period of About 15 Years According to Data of the INM RAS Climate Model

According to data from a preindustrial experiment, a mechanism of Arctic climate oscillation with a period of about 15 years in the INM-CM5 climate model is considered. For this purpose, a technique for computing the contribution of the different components to the generation of oscillation energy and phase variation is used. It is shown that, in the positive phase of an oscillation (the warm Arctic), a negative anomaly of the temperature and salinity occurs at middle latitudes of the North Atlantic. These anomalies are accompanied by a positive North Atlantic Oscillation index. A quarter of the period after Arctic warming, warming and salinization occur at middle latitudes of the North Atlantic. In the Arctic Ocean, the temperature anomalies are generated due to the intensifying advection of the Atlantic water. The evolution of the oscillation phase is accounted for by the heat transport from the Atlantic and the heat flux across the surface. Anomalies of the currents transporting the heat from the Atlantic to the Arctic Ocean are generated mainly by surface wind stress.

The Monsoon over the Barents Sea and Kara Sea

In the Arctic (mainly in its European sector) there is statistically detectable seasonal reversal wind pattern. The combination of seasonally warm (cold) land surfaces in arctic areas together with cool (cool) sea surface of Arctic seas not covered by ice is conducive to the formation of a monsoon like system. On the other hand, the predominance of the cyclonic regime during all seasons makes it difficult to answer the question of whether the Arctic region belongs to the monsoon type pattern. In this study, the monsoon features of atmospheric circulation over the Barents and Kara Seas were analysed. To extract specific monsoon signs, atmospheric circulation systems (separately for areas of each sea) were divided into ten weather types. Their appearance and statistics were compared with indicators of regional circulation. A significant part of intra-annual monsoon variability is associated with the configuration of such modes as the North Atlantic Oscillation and the Scandinavia teleconnection patterns. For example, during the winter season, the monsoon currents (from land to sea) occur only with a positive North Atlantic Oscillation index. With the prevalence of other modes of variability, the direction of the winds can be different, and the regular monsoon circulation pattern is changed by chaotic regime. In summer, northern streams (from sea to land) are realized on the western periphery of cyclones, regenerating and stabilizing over the Kara Sea. As for anomalies, the nature of the monsoons is manifested in the statistics of extreme winds even without selecting data on the regimes of variability. So, in winter, maximum speeds fall on the southern streams, and in the summer—on the northern ones. Large precipitation anomalies during all seasons, as one would expect, are encountered most often with the cyclonic type of circulation.

Parasite prevalence increases with temperature in an avian metapopulation in northern Norway.

Climate and weather conditions may have substantial effects on the ecology of both parasites and hosts in natural populations. The strength and shape of the effects of weather on parasites and hosts are likely to change as global warming affects local climate. These changes may in turn alter fundamental elements of parasite-host dynamics. We explored the influence of temperature and precipitation on parasite prevalence in a metapopulation of avian hosts in northern Norway. We also investigated if annual change in parasite prevalence was related to winter climate, as described by the North Atlantic Oscillation (NAO). We found that parasite prevalence increased with temperature within-years and decreased slightly with increasing precipitation. We also found that a mild winter (positive winter NAO index) was associated with higher mean parasite prevalence the following year. Our results indicate that both local and large scale weather conditions may affect the proportion of hosts that become infected by parasites in natural populations. Understanding the effect of climate and weather on parasite-host relationships in natural populations is vital in order to predict the full consequence of global warming.

Mean annual and seasonal circulation patterns and long-term variability of currents in the Baltic Sea

Abstract A hindcast modelling study conducted with the PM3D hydrodynamic model revealed some new effects. The study involved determination of seasonal characteristics of surface and subsurface currents. In addition, long-term relationships between the Baltic Sea currents and the North Atlantic Oscillation were explored. Analysis of the Baltic Sea current data spanning five decades has shown new, previously unknown, current characteristics, including higher stability (0.4–0.7) of subsurface current eddies relative to the stability of surface currents (0.2–0.5). Circulation anomaly patterns relative to the annual mean in spring and autumn present opposite flow directions, whereas winter and summer circulation patterns were similar in this respect. The spring circulation anomalies were dominated by the low sea-level outflow from the Baltic Sea, whereas the autumn circulation showed a strong inflow and infilling of the sea. In the analysed half-century (1958–2007), velocities of surface and bottom currents showed an increasing trend (0.9 and 0.1 cm s−1 per 50 years, respectively) and were related to the dominance of the positive NAO index phase. Current velocity fluctuations correlated positively with the NAO and BSI indices (correlation coefficients of 0.66 and 0.80, respectively).

Wavelet-based analysis of global index effects in air temperature and precipitation data of the Black Sea coast

AbstractThis study has been carried out to analyse the precipitation and air temperature data in the Black Sea region of Turkey to aid the understanding of the effects of global indices. Connections between the temperature or precipitation data and global atmospheric indices such as the North Sea Caspian Pattern (NCP), the Southern Oscillation Index (SOI) or the North Atlantic Oscillation (NAO) were studied. Results of the cross-correlations between air temperature and the NCP/NAO showed a strong relationship, especially in the winter period. The seasonal and annual differences for the temperature and precipitation data during the negative and positive phases of the global indices were computed. According to this, the annual total precipitations are higher during the positive NCP index than the negative NCP index, while the annual total precipitations are higher during the negative NAO index than the positive NAO index. On the other hand, wavelet analysis showed that some short-term periodicities in precipitation and temperature data are connected with the NAO and the extreme phases of the SOI. The influence of the NCP should also be considered for the short-term periodicities of the temperatures.

Potential effects of climate change on future snow avalanche activity in western Norway deduced from meteorological data

ABSTRACTThe formation of snow avalanches is controlled by the complex interaction between terrain characteristics, snowpack and meteorological conditions. Accordingly, snow avalanches exhibit a high sensitivity to climatic variations. This study focuses on the possible effects of climatic variations on snow avalanche activity within one of Norway’s most snow avalanche-prone areas. We have statistically analyzed long-term homogenized meteorological data from seven official meteorological stations, three of them with a long-term record of 120 years (1896–2015). In addition, gained results and insights from a four-year (2009–2012) high-resolution snow avalanche monitoring investigation conducted in the same study area are incorporated. Potential effects and overall implications of a changing snow avalanche activity are discussed. Statistical analyses show an increasing trend for both air temperature and precipitation with an accelerated increase for the last 30 years for the core winter period. A tendency for a relationship between a positive North Atlantic Oscillation index and higher precipitation sums during winter could be detected. Magnitude-frequency analyses of monthly precipitation sums for the winter period exhibit an increase of precipitation especially during the last three decades with the tendency that more precipitation can be expected in March and February. An increase of the monthly precipitation sums during the winter period may lead to a generally higher snow avalanche frequency. Due to more frequent periods with air temperatures close or above the freezing point during the winter period, the probability of wet snow avalanches and slush flows will increase.

North Atlantic Oscillation drives the annual occurrence of an isolated, peripheral population of the brown seaweed Fucus guiryi in the Western Mediterranean Sea.

The canopy-forming, intertidal brown (Phaeophyceae) seaweed Fucus guiryi is distributed along the cold-temperate and warm-temperate coasts of Europe and North Africa. Curiously, an isolated population develops at Punta Calaburras (Alboran Sea, Western Mediterranean) but thalli are not present in midsummer every year, unlike the closest (ca. 80 km), perennial populations at the Strait of Gibraltar. The persistence of the alga at Punta Calaburras could be due to the growth of resilient, microscopic stages as well as the arrival of few–celled stages originating from neighbouring localities, and transported by the permanent Atlantic Jet flowing from the Atlantic Ocean into the Mediterranean. A twenty-six year time series (from 1990 to 2015) of midsummer occurrence of F. guiryi thalli at Punta Calaburras has been analysed by correlating with oceanographic (sea surface temperature, an estimator of the Atlantic Jet power) and climatic factors (air temperature, rainfall, and North Atlantic Oscillation –NAO-, and Arctic Oscillation –AO- indexes). The midsummer occurrence of thalli clustered from 1990–1994 and 1999–2004, with sporadic occurrences in 2006 and 2011. Binary logistic regression showed that the occurrence of thalli at Punta Calaburras in midsummer is favoured under positive NAO index from April to June. It has been hypothesized that isolated population of F. guiryi should show greater stress than their congeners of permanent populations, and to this end, two approaches were used to evaluate stress: one based on the integrated response during ontogeny (developmental instability, based on measurements of the fractal branching pattern of algal thalli) and another based on the photosynthetic response. Although significant differences were detected in photosynthetic quantum yield and water loss under emersion conditions, with thalli from Punta Calaburras being more affected by emersion than those from Tarifa, the developmental instability showed that the population from Tarifa suffers higher stress during ontogeny than that from Punta Calaburras. In conclusion, this study demonstrates the teleconnection between atmospheric oscillations and survival and proliferation of marine macroalgae.

Winter precipitation changes during the Medieval Climate Anomaly and the Little Ice Age in arid Central Asia

The strength of the North Atlantic Oscillation (NAO) is considered to be the main driver of climate changes over the European and western Asian continents throughout the last millennium. For example, the predominantly warm Medieval Climate Anomaly (MCA) and the following cold period of the Little Ice Age (LIA) over Europe have been associated with long-lasting phases with a positive and negative NAO index. Its climatic imprint is especially pronounced in European winter seasons. However, little is known about the influence of NAO with respect to its eastern extent over the Eurasian continent.Here we present speleothem records (δ13C, δ18O and Sr/Ca) from the southern rim of Fergana Basin (Central Asia) revealing annually resolved past climate variations during the last millennium. The age control of the stalagmite relies on radiocarbon dating as large amounts of detrital material inhibit accurate 230Th dating. Present-day calcification of the stalagmite is most effective during spring when the cave atmosphere and elevated water supply by snow melting and high amount of spring precipitation provide optimal conditions. Seasonal precipitation variations cause changes of the stable isotope and Sr/Ca compositions. The simultaneous changes in these geochemical proxies, however, give also evidence for fractionation processes in the cave. By disentangling both processes, we demonstrate that the amount of winter precipitation during the MCA was generally higher than during the LIA, which is in line with climatic changes linked to the NAO index but opposite to the higher mountain records of Central Asia. Several events of strongly reduced winter precipitation are observed during the LIA in Central Asia. These dry winter events can be related to phases of a strong negative NAO index and all results reveal that winter precipitation over the central Eurasian continent is tightly linked to atmospheric NAO modes by the westerly wind systems.

Wind-Driven Atlantic Water Flow as a Direct Mode for Reduced Barents Sea Ice Cover

AbstractVariability in the Barents Sea ice cover on interannual and longer time scales has previously been shown to be governed by oceanic heat transport. Based on analysis of observations and results from an ocean circulation model during an event of reduced sea ice cover in the northeastern Barents Sea in winter 1993, it is shown that the ocean also plays a direct role within seasons. Positive wind stress curl and associated Ekman divergence causes a coherent increase in the Atlantic water transport along the negative thermal gradient through the Barents Sea. The immediate response connected to the associated local winds in the northeastern Barents Sea is a decrease in the sea ice cover due to advection. Despite a subsequent anomalous ocean-to-air heat loss on the order of 100 W m−2 due to the open water, the increase in the ocean heat content caused by the circulation anomaly reduced refreezing on a time scale of order one month. Furthermore, it is found that coherent ocean heat transport anomalies occurred more frequently in the latter part of the last five decades during periods of positive North Atlantic Oscillation index, coinciding with the Barents Sea winter sea ice cover decline from the 1990s and onward.

Exceptionally extreme drought in Madeira Archipelago in 2012: Vegetation impacts and driving conditions

This work aims at characterizing the exceptional drought that affected Madeira Archipelago (Portugal) during the 2011–2012 hydrological year while including some major impacts but also the main atmospheric circulation mechanism behind the event. Precipitation records from six meteorological stations are used to assess the extreme drought episode by means of a decile classification. The assessment of the drought duration and severity is further corroborated by the Standardized Precipitation Index (SPI) computed for the 3, 6, 12 and 24 months’ time scales which confirmed that the 2012 drought event was one of the events with higher drought intensity at all scales, being classified as extreme at the 6-month time sale (SPI6<−1.65) from December 2011 until May 2012 on the majority of the meteorological stations analysed. Long-term precipitation data available since 1865 for the station of Funchal confirm the exceptional dryness of this episode, particularly during the winter season (December to March) corresponding to the driest winter in 150 years.Vegetation activity is assessed through anomalies of NDVI (Normalized Difference Vegetation Index), confirming several large sectors of Madeira under vegetative stress. The southern sector of Madeira Island suffered up to seven months (out of nine) of extremely negative anomalies. From an operational point of view, obtained results reveal the ability of the developed methodology to monitor vegetation stress and droughts in Madeira. The extreme dryness of the Island favoured an unusually intense summer fire season of 2012 (between June and September) in Madeira being the year with highest number of fires in the last decade (with robust data). Furthermore, the main fire hotspots of the 2012 fire season are mostly coincident with the areas affected by drought.The large-scale atmospheric circulation responsible for the setting and intensification of the drought is evaluated using reanalysis data. An extensive high pressure anomaly (maximum above 12hPa in December) persisted over the North Atlantic during the extended winter months (October to April), centred between the Azores Islands and the UK. This feature is in agreement with a positive NAO index (2.25 in December), a negative EA index (−1.76 in January and −1.73 in February) and compatible with enhanced north-easterly trade winds over the region. As a consequence of this uncharacteristic dynamics there was a deficit of moisture availability over the region as evaluated by the vertically integrated horizontal water vapour transport with a negative anomaly up to −120kgm−1s−1.

A review of oceanographic and meteorological controls on the North Sea circulation and hydrodynamics with a view to the fate of North Sea methane from well site 22/4b and other seabed sources

The North Sea hydrodynamics are key to the redistribution of methane released at the 22/4b Site, located at (57°55′N, 1°38′E) in the UK Central North Sea, 200 km east of the Scottish mainland. This review summarizes the current state of knowledge on the North Sea circulation, stratification, and variability therein and briefly discusses the potential consequences for the distribution and fate of methane released from site 22/4b or other seabed sources.Astronomical tidal waves follow a counter-clockwise path and tide-topography interaction generates a residual circulation in the same direction. Wind-stress forcing can enhance, reduce, or even reverse this circulation. Variations in the strength of the Fair Isle Current (FIC) are important. The FIC enters the North Sea between The Orkneys and Shetland, follows approximately the 100-m isobath, passes along site 22/4b, and ends up in the Norwegian Trench. The North Atlantic Oscillation (NAO) also causes variability. A positive (negative) NAO index is associated with stronger (weaker) than normal westerly winds. NAO + situations strengthen the circulation in the North Sea, whereas it weakens during NAO- conditions and is directed northeastward. High positive correlations exist between the SST at site 22/4b and the NAO index. Climate change can have a long-term effect on the hydrodynamics of the North Sea.Seasonal stratification has potentially the most important imprint on methane derived from the well 22/4b Site. Summertime heating stratifies the northern part of the North Sea. In autumn, loss of heat to the atmosphere causes the stratification to breakdown until tides and storms mix the entire water column. During the period of stratification, the bulk of (dissolved) methane released from site 22/4b gets trapped below the thermocline. The loss of methane to the atmosphere thus becomes a function of the relative time scales of transport and horizontal and vertical mixing processes versus the time scale of microbial degradation (oxidation) in the water column.