Winter North Atlantic Oscillation Index Research Articles

Analysis of solar cycle-like signal in the North Atlantic Oscillation index

Relation of decadal variations of the North Atlantic Oscillation index (NAOI) to the 11-year solar cycle in the sunspot number is analyzed for almost 200-year observational period with the use of cross-spectral and cross-wavelet techniques. Based on wavelet transforms of a pair of time series, local cross characteristics such as the local coherency and local correlation are used to study time evolution of the solar–NAOI relationship. The analysis of the NAOI is supplemented and confirmed by similar analysis of more than 300-year long data of Central England temperature (CET). Both the solar cycle–NAOI and solar cycle–CET relationships exhibit a quasi-periodic oscillation with the mean period of about 50 years, which includes a subinterval with positive coherency and a subinterval with negative coherency between the solar cycle and the 11-year modes of the NAOI and CET. The same multidecadal modulation of the solar–NAOI relationship is manifested in the temporal structure of the local correlation between the 11-year modes of the NAOI and the sunspot number at specific time lags of the NAOI relative to the sunspot number. It is distinct in the local correlations for the extended winter period (November–March) NAOI lagging by 7 years after the sunspot number. At this lag, the NAOI anticorrelates in general with the sunspot number and the local anticorrelation reinforces periodically with the aforementioned ∼50 year period. At the 1-year lag, which differs from the 7-year lag by a half solar cycle, the 11-year mode of the winter NAOI correlates in general with the solar cycle but the correlation is mainly associated with the last 70-year time interval. The change in the sign of the solar–NAOI relationship is also the case for the extended summer periods of the year (May–September). For this period, the approximately mirror image of the local correlations is observed at the 7-year and 1-year lags. The multidecadal modulation is likely associated with periods of enhanced solar impact on the NAO.

A Euro-Mediterranean tree-ring reconstruction of the winter NAO index since 910 C.E.

We develop a new reconstruction of the winter North Atlantic Oscillation (NAO) index using a network of 97 Euro-Mediterranean tree-ring series. The reconstruction covers the period 910–2018 C.E., making it the longest annually resolved estimate of winter NAO variability available. We use nested correlation-weighted principal components regression and the Maximum Entropy Bootstrap method to generate a 2400-member ensemble of reconstructions for estimating the final reconstruction and its quantile uncertainties. Extensive validation testing of the new reconstruction against data withheld from the calibration exercise demonstrates its skill. The skill level of the new reconstruction is also an improvement over two NAO reconstructions published earlier. Spectral analyses indicate that the new reconstruction behaves like a ‘white noise’ process with intermittent band-limited power, suggesting that the winter NAO is stochastically forced. The ‘white noise’ properties of our reconstruction are also shown to be consistent with the spectral properties of long instrumental NAO indices extending back to 1781 and NAO indices extracted from a large number of forced climate model runs covering the last millennium. In contrast, an annually resolved multi-proxy NAO reconstruction of comparable length, based in part on speleothem data, behaves more like externally forced ‘red noise’ process, which is inconsistent with our reconstruction, long observations, and forced model runs.

Spatial and temporal analysis of changes in temperature extremes in the non-monsoon region of China from 1961 to 2016

Changes in temperature extremes have far-reaching consequences for fragile ecological processes, hydrologic cycles, and human society in semiarid and arid regions. Therefore, detecting the spatial and temporal variations in temperature extremes and their driving mechanisms is crucial. Based on daily temperature records from 1961 to 2016 at 154 meteorological stations in the non-monsoon region of China, the spatial and temporal variations in 12 extreme temperature indices and the diurnal temperature range (DTR) were analyzed, and the association of these variations with atmospheric circulation patterns was also investigated. The major conclusions are as follows. (1) DTR and cold temperature extremes decreased significantly except for the coldest days (TXn) and coldest nights (TNn), which increased significantly, while all the warm temperature extremes significantly increased in the past 56 years; that is, all the temperature-based indices show patterns consistent with a general warming trend. (2) Cold extremes slowed during the global warming hiatus (1998–2016) compared with 1961–2016; however, the variation trends in the warm extremes did not change dramatically between the two periods. (3) Almost all temperature indices display the largest trend magnitudes in the cold-half year (autumn and winter), the variation trends in most cold extremes are significantly higher than those in warm extremes, and the warming rates of the nighttime indices are also significantly faster than those of the daytime indices. (4) Spatially, for most significant temperature indices, the stations located on the Tibetan Plateau show larger variation trends. (5) Both annual and seasonal Atlantic multidecadal oscillation (AMO) index are significantly correlated with almost all of the temperature indices, except for TXn; further, the summer and winter western Pacific subtropical high intensity (WPI) index, the summer and winter North Atlantic oscillation (NAO) index, and the winter Arctic oscillation (AO) index display significant relationships with most of the temperature extremes, which indicates that the large atmospheric circulation patterns play significant roles in the changing temperature extremes in this region. Overall, the results provide scientific references for understanding and predicting the spatial and temporal trends of extreme climate in the future.

Signal and noise in regime systems: A hypothesis on the predictability of the North Atlantic Oscillation

Studies conducted by the UK Met Office reported significant skill in predicting the winter North Atlantic Oscillation (NAO) index with their seasonal prediction system. At the same time, a very low signal‐to‐noise ratio was observed, as measured using the “ratio of predictable components” (RPC) metric. We analyse both the skill and signal‐to‐noise ratio using a new statistical toy model, which assumes NAO predictability is driven by regime dynamics. It is shown that if the system is approximately bimodal in nature, with the model consistently underestimating the level of regime persistence each season, then both the high skill and high RPC value of the Met Office hindcasts can easily be reproduced. Underestimation of regime persistence could be attributable to any number of sources of model error, including imperfect regime structure or errors in the propagation of teleconnections. In particular, a high RPC value for a seasonal mean prediction may be expected even if the model's internal level of noise is realistic.

Structure and Variability of the Meridional Overturning Circulation in the North Atlantic Subpolar Gyre, 2007–2017

The vertical structure and interannual and long-term variability of the meridional overturning circulation in the North Atlantic Subpolar Gyre is analyzed. A close relationship between deep winter convection in the Irminger Sea and the MOC intensity is shown. The revealed significant correlation between the winter North Atlantic Oscillation Index and the summer MOC shifted by 3.5 years argues for an atmospheric circulation effect on the variability of the oceanic meridional heat flow.

Habitat‐ and species‐mediated short‐ and long‐term distributional changes in waterbird abundance linked to variation in European winter weather

AbstractAimMany species are showing distribution shifts in response to environmental change. We explored (a) the effects of inter‐annual variation in winter weather conditions on non‐breeding distributional abundance of waterbirds exploiting different habitats (deep‐water, shallow water, farmland) and (b) the long‐term shift in the population centroid of these species and investigate its link to changes in weather conditions.LocationEurope.MethodsWe fitted generalized additive mixed Models to a large‐scale, 24‐year dataset (1990–2013) describing the winter distributional abundance of 25 waterbird species. We calculated the annual and long‐term (3‐year periods) population centroid of each species and used the winter North Atlantic Oscillation (NAO) index to explain the inter‐annual and long‐term shifts in their location.Results(a) Year‐to‐year southwestwards shifts in the population centroids of deep‐ and shallow‐water species were linked to negative NAO values. Shallow‐water species shifted northeastwards associated with positive NAO values and the distance shifted increased with increasing NAO. Deep‐water species shifted northeastwards up to zero NAO indices, but showed no further increase at higher NAO values. (b) Deep‐water species showed long‐term northeastwards shifts in distributional abundance throughout the 1990s and the 2000s. Shallow‐water species, on the other hand, shifted northeastwards during the 1990s and early 2000s, but southwestwards thereafter. There were no significant links between the NAO and year‐to‐year movements or long‐term shifts in farmland species’ population centroid.Main ConclusionsWe provide evidence for a link between both year‐to‐year and long‐term changes in waterbird winter distributional abundances at large geographical scales to short‐ and long‐term changes in winter weather conditions. We also show that species using shallow water, deep‐water and farmland habitats responded differently, especially at high NAO values. As well as important ecological implications, these findings contribute to the development of future conservation measures for these species under current and future climate change.

Influence of the North Atlantic Oscillation at local and regional scales: a case study of the Belgian coast

Together, large‐scale atmospheric circulation and local winds determine the occurrence of storm surges that may result in disastrous erosion. This study investigates the interrelationships between large‐scale atmospheric flow and local wind regimes as driving factors of erosion at the Belgian coast. The large‐scale atmospheric circulation is examined in terms of the winter North Atlantic Oscillation (NAO) index, and it is analysed and compared with regional‐scale circulation patterns (i.e. circulation weather types) and local wind regimes, on both annual and seasonal timescales. The results indicate that a strengthening of the positive winter NAO phase can result in an increase in the frequencies of west (W), southwest (SW) and northwest (NW) directional weather types, leading to local wind speeds in excess of 8ms−1, propagating towards the coast. These conditions are likely to lead to erosive events at the coast. By contrast, a weakening of the winter NAO phase can favour the development of northeast (NE), north (N), east (E) and southeast (SE) weather types. The NE and E types generate particularly strong winds, which potentially worsen erosion at the coast. The E and SE types do not, however, affect wind at a local scale. Thus, the extent to which the wind regime at the Belgian coast is sensitive to the winter NAO is a function of the equilibrium between the NAO effects and the regional atmospheric flow direction. In this study, a downscaling conceptual model is developed which relates climate to large‐scale and local‐scale coastal erosion forcing factors. A better understanding of the variability of, and the interrelationships between, climate, meteorology and erosion can help us to adapt to the changes in coastal evolution driven by climate change, improving the prospects of long‐term coastal resilience.

Increased Winter‐Mean Wave Height, Variability, and Periodicity in the Northeast Atlantic Over 1949–2017

AbstractA 69‐year (1948–2017) numerical weather and wave hindcast is used to investigate the interannual variability and trend of winter wave height along the west coast of Europe. Results show that the winter‐mean wave height, variability, and periodicity all increased significantly in the northeast Atlantic over the last seven decades which primarily correlate with changes in the climate indices North Atlantic Oscillation (NAO) and West Europe Pressure Anomaly (WEPA) affecting atmospheric circulation in the North Atlantic. NAO and WEPA primarily explain the increase in winter‐mean wave height and periodicity, respectively, while both WEPA and NAO explain the increase in interannual variability. This increase in trend, variability, and periodicity resulted in more frequent high‐energy winters with high NAO and/or WEPA over the last decades. The ability of climate models to predict the winter NAO and WEPA indices a few months ahead will be crucial to anticipate coastal hazards in this region.

An analysis of changes in temperature extremes in the Three River Headwaters region of the Tibetan Plateau during 1961–2016

Regional spatiotemporal assessments of temperature extremes are essential for accurate forecasting of future climate changes, agricultural production and the regulation of water resources. This study evaluates the characteristics of the spatiotemporal variations in temperature extremes from 1961 to 2016 in the Three River Headwaters (TRH) region and its surroundings in the northeastern portion of the Tibetan Plateau in China. The association of these variations with atmospheric circulation patterns is also investigated. This study selected twenty-six meteorological stations where <2% of the data are missing and where the data pass quality control and homogeneity tests. Following the Expert Team on Climate Change Detection and Indices (ETCCDI), thirteen extreme temperature indices and two mean temperature indices are analyzed using the Mann-Kendall test and Sen's test; the indices examined here include six warm indices, six cold indices, the diurnal temperature range (DTR) and the annual minimum and maximum temperatures. Pearson correlation analysis is also used to analyze the correlations between the temperature extremes and various atmospheric circulation patterns. The results indicate that the annual minimum and maximum temperatures display significant increasing trends, whereas the DTR displays a significant decreasing trend with a rate of −0.11 °C/decade. All of the regional warm indices display significant increasing trends. On the other hand, most of the cold indices show significant decreasing trends, except for the lowest daily minimum temperature (TNn) and the lowest daily maximum temperature (TXn), which show increasing trends. These results are consistent with the trends observed by most global-scale studies. In terms of spatial patterns, most of the stations that display significant trends are located in the central and northwestern portions of the TRH region. On the other hand, all of the extreme indices show weak correlations with altitude, with the exception of the DTR, which displays a significant negative correlation with altitude (P<0.01); moreover, most of the indices are weakly correlated with latitude and longitude. Overall, the winter Arctic Oscillation index (AO), the summer and winter North Atlantic Oscillation index (NAO), and the summer and winter Western Pacific Subtropical High Intensity Index (WPI) display significant relationships with most of the extreme temperature indices, and these circulation patterns contribute to changes in temperature extremes in this region. However, the Southern Oscillation Index (SOI) is not significantly correlated with most of the extreme temperature indices.

Dietary trends of yellow-legged gulls in Galicia, north-west Spain, over the last 30 years

Dietary trends of yellow-legged gulls in Galicia, north-west Spain, over the last 30 years

Flowering of allergenically important plant species in relation to the North Atlantic Oscillation system and thermal time in the Czech Republic

This paper analyses long-term (1960-2015) onset of flowering in 16 native terrestrial plants (11 of them produce important allergens) recorded in different parts of the Czech Republic (southern, central and northern part) in relation to the North Atlantic Oscillation (NAO) index of the preceding winter and thermal data-growing degree-days (GDD) and soil temperature. Flowering occurred significantly earlier following positive winter NAO phases (causing spring to be warmer than normal in Central Europe) in nearly all early-flowering (February, March, April) species; high Pearson correlation values were recorded in, e.g. wood anemone, common snowdrop, goat willow, common hazel and common alder. There was found a difference between the southern and northern part of the country, e.g. in silver birch and pedunculate oak. Out of the later-flowering (May-July) plant species, black elder and meadow foxtail also significantly correlated with the winter NAO index, lime tree correlated less markedly. The best results of a threshold for calculation of GDD to onset of beginning of flowering were found in lime tree-it was 5 degrees C at all three stations. Results of other taxa were more variable (e.g. 4-7 degrees C in goat willow; 6-10 degrees C in silver birch). Pearson correlation coefficients between NAO index and GDD were negative in lime tree at all thresholds (5, 6, 7, 8, 9, 10 degrees C), while goat willow and silver birch were not so uniform (both positive and negative values). Correlation coefficients between phenophase onset and soil temperature (10 cm depth) had the highest values in silver birch, European larch and wood anemone. Stations situated at higher elevation showed negative correlation coefficient with soil temperature in common snowdrop, pedunculate oak, meadow foxtail and lime tree; other values were positive.

Linking annual variations of roe deer bag records to large-scale winter conditions: spatio-temporal development in Europe between 1961 and 2013

Synchrony in bag records over large spatio-temporal scales point to factors acting simultaneously on population size. In this manuscript, we analysed time series of annual roe deer (Capreolus capreolus) bag records for 17 European countries for the time span 1961–2013. Even though a synchronous increase in bag records was identified in 14 of 17 countries between 1986 and 1992, bag records were not synchronised at the European level. We detected a significant positive correlation between the North Atlantic Oscillation winter index (December–March) and annual changes in roe deer bag records for Czech Republic, Germany, Latvia, Lithuania, Norway and Sweden. Further time-series analyses revealed a significant lag length (L) in 13 of 17 time series on roe deer bag records which highlights that the number of bagged deer in year t was significantly correlated to the number of bagged deer in year t + L (negative association). We conclude that large-scale environmental variations or human-induced changes will not only induce periodic fluctuations in wildlife populations but also in wildlife harvest. By incorporating the knowledge about population dynamics of large herbivores into management concepts, wildlife management will not only decrease the chances of serious pitfalls but also optimise measures for population control and conservation.

Shipwrecks and man-made coastal structures as indicators of historical shoreline position. An interdisciplinary study in the Sancti Petri sand spit (Bay of Cádiz, SW Spain)

Shoreline evolution studies are mostly performed by comparison of vertical images (e.g. aerial photographs and satellite imagery), being limited to the last decades. However, phenomena such as longer term climatic fluctuations may cause important secular coastal changes which are mostly unknown. The use of archaeological remains as shoreline proxies could overcome this limitation by allowing to expand back in time the historical shoreline record. The present study endorses the effectiveness of the use of shipwrecks and man-made structures as proxy data for analyzing historical shoreline evolution. This was confirmed by the evaluation of nearshore evolution (historical shorelines from 1787 to 1823 and recent changes from 1956 to 2013) and the reconstruction of the palaeobathymetry along Sancti Petri sand spit (SW Spain) by means of the analysis of historical documents, historical maps, modern aerial photographs and a recent bathymetry (2011), along with the archaeological prospection supported by the numerical modelling of the dispersion of archaeological remains. The analysis of historical maps has revealed a significant shoreline retreat over the medium term scale in Sancti Petri sand spit, averaging −1.2m·year−1. Results from historical maps and archaeological remains show slight differences with respect to the most recent retreat rates (−0.9m·year−1) obtained from aerial photographs for the period 1956–2013. Spatial variations in coastal retreat along the sandspit are closely linked to local patterns of wave height and wave-induced currents. Significant temporal variation in rates of shoreline change was observed, probably related to storminess variability in the study area, as indicated by the relation between the rates of coastal change for each analyzed period and the corresponding rate of change of winter NAO index.

Decline of a montane Mediterranean pied flycatcher Ficedula hypoleuca population in relation to climate

Certain populations of long‐distance migratory birds are suffering declines, which may be attributed to effects of climate change. In this article, we have analysed a long‐term (1991–2015) data set on a pied flycatcher Ficedula hypoleuca population breeding in nest‐boxes in a Mediterranean montane oak forest, exploring the trends in population size due to changes in nestling recruitment, female survival and female immigration. We have related these changes in population parameters to local climate, winter NAO index and to breeding density. During the last 25 yr the population has declined by half, mainly in association with a decrease in nestling mass and structural size which had repercussions on the probability of nestling recruitment to the population. Lower local nestling recruitment in certain years was linked to lower female immigration rate in the same years. On the other hand, the local survival of females remained stable throughout the study period. Laying date and breeding success were negatively affected by local temperatures while breeding, recruitment rate likewise by minimum temperature prior to breeding in April. As minimum April temperatures have increased across the study period, this may have affected recruitment and immigration rates negatively. On the other hand, tarsus length and body mass of nestlings were positively associated with winter NAO index, pointing to more global climatic links. Moreover, there was also a negative temporal trend in body mass of adults, implying increasingly difficult conditions for breeding. Declining recruit production in the study area could be attributed to a mismatch between the timing of arrival and breeding in the population, and the peak of food availability in this area.

Assessment of the impacts of storm events for developing an erosion index

During a storm event, strong winds combined with low atmospheric pressure and high water level conditions generate surge which often cause significant beach erosion. Over the years, a number of storm erosion indexes have been developed combining marine factors, but none of them incorporate wind and large-scale atmospheric factors of the storm events. The occurrence of wind storms was examined along the Belgian coast within a time span of 14 years (1994–2007) with respect to the local marine and large-scale atmospheric factors (circulation weather types, CWT and the winter North Atlantic Oscillation, NAO) under which the events took place. It was shown that 181 wind storms were recorded (average of 14 events per year). This was characterized by a calm period with a range of annual events from 6 to 14 between 1994 and 2001, following by an energetic period of frequent events reaching up to 18 events per year for the most recent years. However, only 8% of the wind storms were related to the measured erosion events. Based on the assessment of the wind storm characteristics, the events were filtered with respect to the high water level, and the occurrence of primary and secondary CWTs in order to identify the potential erosive wind storm events. Results indicated that more than 63% of the wind storms were related to the measured sand volume loss of the beaches. Also, it found that the occurrence of severe wind storms is regulated by the winter NAO index. Based on these considerations, an index for the intensity of the wind storm activity is built to model the induced erosion volume. A reliable and robust erosion model can help to better prepare coastal managers and coastal communities for preventing the morphological impacts of severe storms in the future.

Potential links between the North Atlantic Oscillation and decreasing precipitation and runoff on a Mediterranean area

In the Mediterranean region, the reduction in precipitation and warmer temperatures is generating a desertification process, with dramatic consequences for both agriculture and the sustainability of water resources. On the island of Sardinia (Italy), the decrease in runoff impacts the management of water resources, resulting in water supply restrictions even for domestic consumption. In the 10 Sardinian basins with a longer database (at least 40 complete years of data, including data from the past 10years), runoff decreased drastically over the 1975–2010 period, with mean yearly runoff reduced by more than 40% compared to the previous 1922–1974 period. Trends in yearly runoff are negative, with Mann-Kendall τ values ranging from −0.39 to −0.2. Decreasing winter precipitation over the 1975–2010 period everywhere on Sardinia island has led to these decreases in runoff, as most yearly runoff in the Sardinian basins (70% on average) is produced by winter precipitation due to the seasonality typical of the Mediterranean climate regime. The trend in winter precipitation is not homogenous; the negative trend is higher (around −0.25) on the west Sardinian coast, becoming lower across the island toward the east coast (around −0.14). Winter precipitation is highly correlated with the North Atlantic Oscillation (NAO), a weather phenomenon in the North Atlantic Ocean that controls the direction and strength of westerly winds and storm tracks into Europe. High negative correlations (up to −0.45) between winter NAO index and winter precipitation are estimated along the west coast. Meanwhile, these correlations decrease east across the island toward the high mountain in the center of Sardinia, reaching the lowest values along the east coast (about −0.25). The generally decreasing correlation between winter NAO index and winter precipitation in the longitudinal direction (from the North Atlantic dipole to the east) here accelerates due to local-scale orographic effects that overlap the large-scale NAO impact on the winter precipitation regime, thus softening the precipitation reduction due to the NAO. Such local topographic effects that may attenuate large-scale climate change effects must be considered in water resource planning and management alongside such climate change effects related to large-scale circulations, such as NAO.

Factors driving patterns and trends in strandings of small cetaceans

The incidence of cetacean strandings is expected to depend on a combination of factors, including the distribution and abundance of the cetaceans, their prey, and causes of mortality (e.g. natural, fishery bycatch), as well as currents and winds which affect whether carcasses reach the shore. We investigated spatiotemporal patterns and trends in the numbers of strandings of three species of small cetacean in Galicia (NW Spain) and their relationships with meteorological, oceanographic, prey abundance and fishing-related variables, aiming to disentangle the relationship that may exist between these factors, cetacean abundance and mortality off the coast. Strandings of 1166 common dolphins (Delphinus delphis), 118 bottlenose dolphins (Tursiops truncatus) and 90 harbour porpoises (Phocoena phocoena) during 2000–2013 were analysed. Generalised additive and generalised additive-mixed model results showed that the variables which best explained the pattern of strandings of the three cetacean species were those related with local ocean meteorology (strength and direction of the North–South component of the winds and the number of days with South-West winds) and the winter North Atlantic Oscillation Index. There were no significant relationships with indices of fishing effort or landings. Only bottlenose dolphin showed possible fluctuations in local abundance over the study period. There was no evidence of long-term trends in number of strandings in any of the species and their abundances were, therefore, considered to have been relatively stable during the study period.

A 481-year chronology of oak earlywood vessels as an age-independent climatic proxy in NW Iberia

The earlywood vessels of ring-porous trees can be analyzed dendrochronologically and used as a proxy for environmental information. However, most works deal with the analysis of contemporary climate-growth relationships and do not evaluate their long-term variation.We obtained a 481-year chronology of earlywood vessel size of oak (Quercus robur L.) in the northwestern Iberian Peninsula, investigated its behavior through time, and compared it to a chronology of younger trees developed at the same site. We expressed earlywood vessel size as the hydraulically-weighted diameter (DH) and discriminated between vessels in the first row (r1) and the rest of the vessels (nr1); radial increment was assessed from latewood width (LW).Climate-growth relationships were strong and nearly identical for both age classes. Spring temperature positively affected vessel size, but only for the first row, probably mediating the onset of cambial activity. The chronology of old trees showed an almost flat age trend, except for the first decades, and series were not affected by stand dynamics. In contrast, LW had a weak response to climate, probably because of the high impact of abrupt growth changes. There was a high negative correlation between DH and the winter North Atlantic Oscillation Index (NAO), which was unstable during the 20th century.To our knowledge, this is the longest chronology of earlywood vessel size obtained to date, and offers promising results, as this proxy is shown to be independent of age and forest disturbances, and was strongly correlated to climate across long time spans.

Effect of environmental conditions on the seasonal and inter‐annual variability of small pelagic fish abundance off North‐West Africa: The case of both Senegalese sardinella

AbstractThe objective of this study was to assess the effect of environmental variations on the abundance of Sardinella aurita and Sardinella maderensis in Senegalese waters in the upwelling system. Monthly data indicating the abundance of sardinella were first estimated from commercial statistics, using Generalized Linear Model from 1966 to 2011. Abundance indices (AIs) were then compared with environmental indices, at the local scale, a Coastal Upwelling Index (CUI) and a coastal Sea Surface Temperature (SST) index, and on a large scale, the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the Multivariate El Niño Southern Oscillation Index (MEI), using correlations and times series analyses. The results showed that the abundance of sardinella is determined by a strong seasonal pattern and inter‐annual fluctuations. The abundance of S. aurita peaked in spring and in autumn, whereas that of S. maderensis peaked in the warm season (July–September). The trend of the sardinella abundance was significantly correlated with the CUI, especially in autumn and spring. Interannual fluctuations of S. maderensis and S. aurita abundance are, respectively, driven by the precocity and the duration of the upwelling season that is attributed to distinct migration patterns. Both sardinella species also respond with a delay of around 4 years to the winter NAO index and the autumn CUI, and the AMO index, respectively, both related to migration patterns. The wide variations in sardinella biomass are caused by variations in environmental conditions, which should be considered in the implementation of an ecosystem‐based approach in sardinella stocks management.

An investigation of water level fluctuations in Polish lakes in various phases of the winter North Atlantic Oscillation

The North Atlantic Oscillation is a macroscale type of circulation determining climate and hydrological conditions in Europe. The paper presents water level fluctuations in 19 Polish lakes in various phases of NAO of the winter season in the years 1976-2010. Correlations of the winter NAO index with monthly, seasonal, maximum, and minimum water stages, and annual water level amplitudes in lakes were calculated. Approximately 20% of the performed tests showed statistical significance. Lakes were recorded where the correlations were significant in a major part of the year, as well as lakes showing no significant correlations in any month. Deviations of mean water stages in the positive and negative phase of NAO from mean water stages were calculated for each of the lakes. This permitted classifying the lakes into two groups by Ward’s method based on deviations of water stages in the positive and negative phase of NAODJFMfrom average values. This resulted in the designation of 4 and 5 typological classes of lakes, respectively. Deviations of water stages in lakes in various phases of NAODJFMfrom mean values were determined to even exceed 20 cm. The study results can find practical applications in reference to water retention. In view of the observed environmental changes, it is possible that the hydrotechnical infrastructure on lakes will have to be developed in the future. Such a situation will permit the precise control of water level fluctuations for the purpose of the optimal adaptation of lakes for economic purposes.