Positive NAO Phase Research Articles

Annual and seasonal mean daily discharge in natural and regulated rivers in Northern Finland: variability, trends, and links to climate teleconnections

ABSTRACT This study aimed to investigate long-term (1969–2023) variability and trends in annual mean daily discharge (AMDD) and seasonal mean daily discharge (SMDD) in 10 rivers throughout Northern Finland in connection to climate teleconnections. The lowest AMDDs were mostly recorded during the first 12 years (1969–1980) of our study period, while the highest ones were during the last 12 years (2012–2023). Significant trends in AMDD were found only in three natural rivers of Tornionjoki, Simojoki, and Kuivajoki during 1969–2023. Such variations and trends in AMDD were most significantly associated with the Scandinavia (SCA) pattern, which is also an influential climate teleconnection for annual precipitation anomalies across Northern Finland. The highest (lowest) SMDDs were experienced in spring (winter). Only increasing trends in SMDD were statistically significant (p < 0.05). Such substantial increases in SMDDs were detected in winter, spring, and summer (autumn) in association with the Arctic Oscillation or AO and the North Atlantic Oscillation or NAO (East Atlantic/West Russia or EA/WR). Generally speaking, recent milder and wetter climatic conditions in association with strong positive AO and NAO (negative EA/WR) phases could increase SMDD for winter, spring, and summer (autumn) seasons. Hence, both AMDD and SMDD in rivers throughout Northern Finland were primarily influenced by precipitation.

Western disturbances alter the trend of winter precipitation and its extremes over Northwest Himalayas: Kashmir Himalaya.

The perception of spatio-temporal variability of winter precipitation covering dominant water resources and environment over Kashmir Himalaya is a pre-eminent perspective under current social interests. The main objectives of the study are, firstly, to unriddle the whole (1980-2020) and post-2000 trend in the winter season precipitation (WP), frequency of low-pressure systems (Lp-S), and western disturbances (WDs) to understand the footprints of climate change over the region; secondly, to evaluate the spatio-temporal variability of winter precipitation under the dominance of global teleconnection indices, using both station and reanalysis datasets. Modified Mann-Kendell test and a linear regression model were utilized to extract the trend and significance in WP, Lp-S, and WDs. The results reveal that WP shows an insignificant decreasing trend (1980-2020), with an increasing trend post-2000, while a decreasing trend is seen during post-2000 in the frequency of Lp-S and WDs. The contribution of WP to annual precipitation shows decadal variability with a decreasing trend post-2000. The anatomy of winter extreme precipitation days has shown a decreasing trend (1980-2020), with a robust coupled increasing trend with WP post-2000. The dynamics of ten severe WD-based extreme precipitation events show that a cyclonic circulation and trough connects the Arabian Sea and the Himalayas, bringing additional moisture to the study region and resulting in extreme precipitation. The cogency of global teleconnections on the variability of WP shows that a pattern with positive phase of NAO, ENSO, and a negative phase in Siberian high intensifies the WD-based precipitation. The study discovers its applicability for the regional forecast system in predicting the winters.

Holocene palaeoenvironment of the central Baltic Sea based on sediment records from the Gotland Basin

Sediment archives are the key source of information about long-term environmental dynamics under changing climate conditions, which is crucial for reliable forecasts. The presented work aims at reconstructing the Baltic Sea palaeoenvironment during the Littorina stage with a particular interest in variations in the saline water inflows. Complex analysis (including geochemical and micropalaeontological studies) of sediment sequences, obtained in the Eastern Gotland Basin, was performed. According to the radiocarbon dating results, the combined sediment record covers the last c. 7 cal ka BP. During this time, increased inflow activity was recorded in the Holocene Thermal Maximum, the Roman Warm Period, and Dark Ages. The predominantly positive phase of NAO during these periods resulted in the prevalence of westerly winds over the Baltic region, which in turn led to more frequent inflows of North Sea waters. The decrease in the frequency of inflows during the Little Ice Age and the Modern Warm Period coincides with the negative phase of NAO and weaker westerlies. The periods of the warmer climate, characterised by an increase in surface water productivity, correspond to the Holocene Thermal Maximum, the Roman Warm Period, the Medieval Climate Anomaly, and the Modern Warm Period. Due to the relatively shallow position, the coring site was not affected by hypoxia during the latter warm periods.

Long-Term Monitoring of In-Water Abundance of Juvenile Pelagic Loggerhead Sea Turtles (Caretta caretta): Population Trends in Relation to North Atlantic Oscillation and Nesting

Sea turtles have various life-stages, typically being oceanic foragers as juveniles while shifting to more coastal habitats as they mature. The present study focuses on the least studied and well known of these, the juvenile oceanic life stage for the loggerhead sea turtle, Caretta caretta. Loggerhead sea turtles remain threatened by fisheries and their distribution and habitat change in the North Atlantic remains poorly understood. After hatching and swimming out to sea, turtles spend 7 or more years in the pelagic life stage. Madeira Island has an advantage of being situated in the middle of the North Atlantic developmental habitat for loggerheads originating both from the US, as well as, from Cape Verde and other mixed source rookeries. Understanding the demographics of this oceanic life stage has been described as a research priority. We here present a population trendline and the abundance variation of oceanic stage loggerheads, measured at a single geographic spot in Madeiran waters, over the period of 15 years. We find that the observed loggerhead distribution results from combined effects of physical and biological processes within the North Atlantic. We explore physical phenomena that influence abundance variability, and find that oscillations in climate affect the turtle migrations, as does the population recruitment from the nesting rookeries. For this, we use novel cost-effective census methods that take advantage of platforms of opportunity from the blue ecotourism industry. To study the time series and their correlations we use spectral analysis, a method not commonly used in traditional population assessments, including Wavelet and Fourier Transformations (WT and FFT), and Digital Signal Processing (DSP) techniques. A strong anti-correlation between sea turtle sightings and North Atlantic Oscillation seasonal components was found, which implies that loggerhead sea turtles are less abundant during positive NAO phases. We also detected long period trends in the sighting data which we relate to La Niña and El Niño oscillations. Source rookeries also influenced the sighting data with a time-lag of ~ 7 years, which coincides with the average time that turtles spend as oceanic juveniles.

The role of temperature and the NAO index in the changing snow-related variables in European regions in the period 1900-2010

Snow-related variables are analysed in the present paper in the period 1901‒2010 on the basis of the ERA-20C dataset. Relationships between different snow characteristics, temperature and the NAO index are investigated on monthly, yearly and decadal scales for eight regions within Europe representing different climatic types (i.e. oceanic, continental, polar) to analyse the differences and similarities between them depending on the climatic conditions. According to our results, the ratio of snow (i.e. snowfall compared to total precipitation) can reach 1 in winter in the colder, northern regions, whereas it is about 0.6 in the continental areas of Central Europe, even in the coldest months. During a strong positive phase of NAO more snow falls in the northern regions of Europe due to the large-scale circulation characteristics. When a negative NAO phase occurs, the temperature and snowfall anomalies are the opposite in northern Europe. The highest temperature values generally occurred after 2000, and the snowfall amount was smaller in the first decades of the 21st century compared to the previous decades. The relationship between temperature and snowfall is the strongest in autumn in the colder regions; in spring in the continental areas and in winter in the oceanic climate.

Wpływ znaku faz NAO w okresie zimowym na bilans wodny i możliwości wystąpienia suszy w ciepłej porze roku na obszarze Polski

The study shows that the change in the NAO phase has an impact on the formation of the area averages of the most important climatic elements during winter, with the exception of winter precipitation totals. An increase in air temperature, wind speed and sunshine duration, and a decrease in cloudiness and relative humidity during the occurrence of positive NAO phases lead to an increase in field evaporation. Evaporation in winter, calculated by Ivanov’s method, shows strong and highly significant positive correlation with the value of the winter NAO index. Thus, the water balance changes, negatively affecting the size of the soil retention. Changes in the NAO index in the winter period have such a strong influence on the water balance in Poland that their influence is detected in the course of the value of the total annual outflow of Polish rivers. Additionally, after the occurrence of a positive NAO phase in winter, in May and August the sums of precipitation (in August – highly significantly), cloudiness and relative humidity decrease, and sunshine duration increases. This is an inertial effect of the winter NAO of unidentified genesis. Both the increase in evaporation in winter and the delayed effect of winter NAO contribute to an increase in the probability of a drought in the summer following winter, during which the NAO sign was positive.

Ocean-atmosphere circulation controls on integrated meteorological and agricultural drought over Iran

Drought may affect more people than any other climate extremes worldwide, especially in (semi)arid climates such as Iran. In this paper, the empirical Multivariate Standardized Drought Index (MSDI) that incorporates the meteorological and agricultural drought conditions is first computed. The wavelet analysis is then used to identify teleconnections between ocean–atmosphere oscillations from Atlantic (AMO, AO, and NAO) and Pacific (ENSO, PDO, and PNA) oceans and Iran’s drought at a highly dense network of regularly gridded points (2485) during 1979–2016. It is found that among the droughts of 1980, 1985–86, 1990–91, 1998–99, 2000, 2007–08, and 2010–11, both the 2007–08 and 2000 are the broadest and strongest across the country. Overall, those droughts that occurred in the 2000–2016 period (average MSDI = -1.14) were, on the whole, stronger than those happened over the latest 20 years of the 20th century (average MSDI = -0.77), reflecting the strengthening of the influence that ENSO has on the Middle East climate in the most recent decades. According to the wavelet coherence results, unlike the Atlantic-based oscillations that almost have an anti-correlation with droughts, there is a positive correlation between Pacific-based oscillations and Iran’s drought. It is also identified that the severe historical droughts across the country were usually associated with a strong La Niña coupled with cold PDO and positive NAO phases in the latest 30 years. Overall, droughts in Iran tend to be substantially controlled by multiple interacting of climate indices since three-coupled indices have stronger coherence with drought than either two-coupled indices or single ones. A combination of the three indices of ENSO, PDO, and NAO has the strongest coherence with Iran’s drought compared to all the other three- and two-coupled, and all single indices, indicating that integrated impact of climate indices can better capture hydroclimate teleconnections. These multiple interacting impacts from the different large-scale climate circulation patterns mark a significant step towards an improved prediction of severe droughts across Iran and can also be used for other parts of the world.

The unusual recent streamflow declines in the Bailong River, north-central China, from a multi-century perspective

The streamflow of the Bailong River, which is a secondary tributary of the Yangtze River located on the eastern Tibetan Plateau, has declined in recent decades. Although instrumental records for the Bailong River are short, historical streamflow reconstructions enable us to evaluate the recent streamflow reductions from a multi-century perspective. Here, we present the first water-year streamflow reconstruction for the Bailong River using six tree-ring chronologies covering the period 1681–2017 CE. The reconstruction shows that the recent declines in streamflow are unprecedented over the past 337 years, with the most prolonged and severe low flows occurring in the 1990s and 2000s. Together, higher temperatures and declining precipitation have resulted in a decrease in the streamflow of the Bailong River. The mean contribution of temperature to streamflow departures is about −10.5%, while the mean contribution of precipitation to streamflow departures is about −4% for the period 1995–2011 CE. We find that temperature has a stronger influence on streamflow than precipitation, and that the influence of temperature on streamflow has increased since 1995. At the same time, the runoff efficiency has decreased dramatically. With regard to large-scale climatic forcing, ENSO and NAO exert a significant influence on the regional moisture and streamflow variations by regulating the water vapor flux and the Asian Monsoon. Extreme high-streamflow events correspond with the negative ENSO mode and the positive NAO phase. Continued warming will likely cause a continued decline in the streamflow of the Bailong River, with repercussions on the local water supply, societal well-being, and economic development.

Relationship between Water Temperature of Polish Rivers and Large-Scale Atmospheric Circulation

The objective of the paper consisted in determining the effect of macroscale types of NAO, AO, EA, EAWR, SCAND, and AMO atmospheric circulation on changes in water temperature in Polish rivers. The study has made use of a broad body of hydrometeorological materials covering daily water temperature values for 96 water gauge stations located on 53 rivers and air temperature values for 43 meteorological stations. Percentage shares of positive and negative coefficients of correlation of annual, seasonal, and monthly circulation type indices with air and river water temperature were determined, demonstrating the character of teleconnection. Determinations were made of water temperature deviations in positive and negative phases of the analyzed indices from average values from the years 1971–2015, and their statistical significance ascertained. Research has shown that relations between the temperature of river waters in Poland and macroscale circulation types are not strong, however they are noticeable, sometimes even statistically significant, and both temporally and spatially diverse. NAO, AO, EA, and AMO indices are characterized by a generally positive correlation with temperature, whereas SCAND and EWAR indices are characterized by a negative correlation. Research showed a varying impact of types of atmospheric circulation, with their effectiveness increasing in the winter season. The strongest impact on temperature was observed for the positive and negative NAO and AO phases, when deviations of water temperature from average values are correspondingly higher (up to 1.0 °C) and lower (by a maximum of 1.5 °C), and also for the positive and negative SCAND phases, when water temperature are correspondingly lower (by a maximum of 0.8 °C) and higher (by 1.2 °C) than average values. The strongest impact on water temperature in summer, mainly in July, was observed for AMO. The results point to the complexity of processes determining the thermal regime of rivers and to the possibility of additional factors—both regional and local—exerting an influence on their temporal and spatial variability.

Influence of the North Atlantic Oscillation on δD and δ18O in meteoric water in the Armenian Highland

The stable hydrogen (δD) and oxygen (δ18O) isotope composition of modern meteoric water are influenced by changes in global temperature, Rayleigh distillation processes, and moisture transport history. However, on a global scale the relative influence of these different meteorological variables is spatially heterogeneous. We collected daily precipitation samples from meteorological stations in the Armenian Highlands for a period of one year in order to determine the constraints on δD and δ18O and relationship to broader climatic conditions. These data are coupled with HYSPLIT back-trajectory analysis to determine how these compositions relate to moisture source and transport path. Multivariate analysis shows the strongest relationship between δD and δ18O and climate variables is temperature at the sampling site. Along with temperature, the intensity of the North Atlantic Oscillation, a pattern of atmospheric circulation which determines the strength and location of Northern Hemisphere midlatitude westerlies, is significant (p < 0.05) for both δ18O and δD values of precipitation. This correlation between NAO and isotopes in precipitation is also observed in four nearby GNIP stations from 2008 to 2015. Negative NAO phases are associated with weakened westerlies, which allows precipitation enriched in heavy isotopes from the Mediterranean and southerly sources to penetrate into the Armenian Highlands, whereas positive NAO phases are associated with stronger westerlies bringing more negative precipitation from the Black Sea. This relationship is most significant (p < 0.001) during the winter and spring months (December to May). These data demonstrate that paleo-archives of δD and δ18O in the Armenian Highlands could serve as a tool to measure changes in the NAO in the past, particularly when coupled with independent measurements of temperature.

Vb Cyclones Synchronized With the Arctic-/North Atlantic Oscillation.

Vb cyclones typically emerge in the Western Mediterranean and propagate to the Northeast into Central Europe. This paper explores the temporal characteristics of Vb cyclone occurrence based on cyclone tracks identified at the atmospheric levels of Z700 and sea level pressure, using JRA‐55 reanalysis data for the period 1959–2015. The risk of Vb occurrence was significantly high in the 1960s and has remained at a lower level since then. Vb cyclones do not occur fully randomly according to a Poisson point process. Eleven well‐separated and distinct clusters as well as 11 hiatus periods are identified, with average occurrence rates of 21.5 and 5.2 yrea−1, respectively. During the event of Vb, the large‐scale atmospheric circulation is changed into a state favoring the development of successive Vb cyclones. Clustering is very prominent in the case of Genoan Vb cyclones in summer as well as those Vb cyclones developing over the Iberian Peninsula or the North African Coast in winter. Superposition of the polar and the subtropical jet stream over the Western Mediterranean is identified as a main feature at the onset of Vb cyclones. Vb cyclone occurrence appears to be synchronized with the Northern Atlantic Oscillation (NAO; at Z500) and Arctic Oscillation (AO; at Z1000). Clusters have occurred when both NAO and AO were negative. This relation applies to Western Mediterranean cyclones not following a Vb track as well, however to a much weaker extent. In contrast, Vb cyclone frequency was particularly low from 1988 to 1997 during a sustained positive phase of both NAO and AO.

Pollen, δ15N and δ13C guano-derived record of late Holocene vegetation and climate in the southern Carpathians, Romania

Two cores of bat guano were recovered from Topolnița and Gura Ponicovei Caves in the south-west region of Romania and analyzed for δ15N, δ13C and pollen. Deposition of guano began in CE 1537 at Gura Ponicovei and CE 1694 at Topolnița, allowing for the development and interpretation of climate and vegetation records since the Little Ice Age. δ15N values were found to record phases of the North Atlantic Oscillation through the effect of winter precipitation on the local nitrogen cycle. Alternatively, δ13C values are interpreted to correspond with variation in summer precipitation. Both records suggest progressively drier conditions during the termination of the LIA in the region at CE 1870. The transition to landscape openness, with patchy forests of Tilia, Carpinus betulus, and Quercus during this time could be related to the lower water availability at both sites. Wetter conditions inferred from a 3‰ increase in δ15N values between CE 1879 and CE 1970 occurred contemporaneously with a similar trend in European winter precipitation and increased frequency of positive NAO phases during this interval. The δ13C values between CE 1885 and CE 1968 suggest a trend towards a wetter summer climate as well. Although there is at times evidence of forest dynamics being controlled by climate, vegetation appears to be more closely associated with human impact, particularly around Topolnița Cave. The contribution of ruderal pollen indicators from CE 1985 – present is likely related to agricultural practices.

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

Abstract. A comprehensive multi-proxy study on two sediment cores from the western and central Skagerrak was performed in order to detect the variability and causes of marine primary productivity changes in the investigated region over the last 1100 years. The cores were dated by Hg pollution records and AMS 14C dating and analysed for palaeoproductivity proxies such as total organic carbon, δ13C, total planktonic foraminifera, benthic foraminifera (total assemblages as well as abundance of Brizalina skagerrakensis and other palaeoproductivity taxa) and palaeothermometers such as Mg∕Ca and δ18O. Our results reveal two periods with changes in productivity in the Skagerrak region: (i) a moderate productivity at ∼ CE 900–1700 and (ii) a high productivity at ∼ CE 1700–present. During ∼ CE 900–1700, moderate productivity was likely driven by the nutrients transported with the warm Atlantic water inflow associated with a tendency for a persistent positive NAO phase during the warm climate of the Medieval Climate Anomaly, which continues into the LIA until ∼ CE 1450. The following lower and more variable temperature period at ∼ CE 1450–1700 was likely caused by a reduced contribution of warm Atlantic water, but stronger deep-water renewal, due to a generally more negative NAO phase and a shift to the more variable and generally cooler climate conditions of the Little Ice Age. The productivity and fluxes of organic matter to the seafloor did not correspond to the temperature and salinity changes recorded in the benthic Melonis barleeanus shells. For the period from ∼ CE 1700 to the present day, our data point to an increased nutrient content in the Skagerrak waters. This increased nutrient content was likely caused by enhanced inflow of warm Atlantic water, increased Baltic outflow, intensified river runoff, and enhanced human impact through agricultural expansion and industrial development. Intensified human impact likely increased nutrient transport to the Skagerrak and caused changes in the oceanic carbon isotope budget, known as the Suess effect, which is clearly visible in our records as a negative shift in δ13C values from ∼ CE 1800. In addition, a high appearance of S. fusiformis during the last 70 years at both studied locations suggests increased decaying organic matter at the sea floor after episodes of enhanced primary production.

Greenland monthly precipitation analysis from the Arctic System Reanalysis (ASR): 2000–2012

Arctic System Reanalysis version 1 (ASRv1) forecasts of monthly precipitation over Greenland are compared with gauge-based precipitation measured by the Danish Meteorological Institute (DMI) and precipitation retrieved from the Precipitation Occurrence Sensor System (POSS) at Summit. The ASRv1 precipitation generally agrees with the corrected DMI gauge-based precipitation measured at coastal or near-coastal stations in Greenland, but the corresponding data at Ikerasassuaq and Nuuk are not the case. ASRv1 precipitation at Summit, i.e., in a higher continental environment, is overestimated compared with the POSS observations. The North Atlantic Oscillation (NAO index and ASRv1 precipitation are moderately correlated over northern Greenland, the North Atlantic, and the Greenland Sea regions (0.32–0.49). It is presumed that local wind events have a larger influence on precipitation where smaller correlations occur. Suggested future work to understand discrepancies between ASRv1 and DMI precipitation fields in Greenland coastal regions is to include case studies of local wind events and corresponding precipitation variations utilizing in-situ measurements during both strong positive and negative NAO phases. At high-altitude and inland areas, further observations are needed to confirm the ASRv1 overestimation.

Synoptic moisture pathways associated with mean and extreme precipitation over Canada for summer and fall

Dominant synoptic moisture pathway patterns of vertically integrated water vapor transport (IVT) in winter and spring over Canada West and East were identified using the self-organizing map method. Large-scale meteorological patterns (LSMPs), together with synoptic moisture pathway patterns, were related to the variability in seasonal precipitation totals and occurrences of extreme precipitation events. Changes in both occurrences of LSMPs and seasonal precipitation occurred under those LSMPs were analyzed to explain observed changes in seasonal precipitation totals and occurrences of extreme precipitation events. The effects of large-scale climate anomalies on occurrences of LSMPs were also examined by composite analyses. Results show that synoptic moisture pathways and LSMPs exhibit the propagation of jet streams resulting from the Rossby wave resonance, as the location and direction of ridges and troughs, and the strength and center of pressure lows and highs varied considerably. Even though LSMPs resulting in positive precipitation anomalies are associated with more frequent occurrences of extreme precipitation events than those resulting in negative precipitation anomalies, the patterns featured with anomalously low IVT are sometimes associated with more frequent occurrences of extreme precipitation events. Significant decreases in occurrences of synoptic moisture pathway patterns that are favorable with positive precipitation anomalies and more precipitation extremes in winter over Canada West, and significant decreases in seasonal precipitation and occurrences of precipitation extremes under most synoptic moisture patterns resulted in decreases in seasonal precipitation and the occurrence of extreme precipitation events. LSMPs resulting in a hot and dry (cold and wet) climate and less (more) frequent extreme precipitation events over the Canadian Prairies in winter and northwestern Canada in spring are more likely to occur in years with a negative (positive) phase of PNA. Occurrences of LSMPs for a wet climate and frequent occurrences of extreme precipitation events over southeastern Canada are associated with a positive phase of NAO. In El Nino years or negative PDO years, occurrences of LSMPs tend to associate with a dry climate and less frequent precipitation extremes over western Canada.

Temporal and spatial variation in recreational catches of anadromous brown trout, Salmo trutta, in Norwegian rivers

Overall, it has been shown that production of diadromous fishes is declining within the North Atlantic basin reaching the lowest levels in recent years compared to their historic baselines. However, these decreases in abundance, ascribed to numerous factors, are species specific and might differ across each species’ distributional range. Here we used recreational angling catch data for anadromous brown trout Salmo trutta in multiple Norwegian rivers having relatively high annual reported catches to elucidate its spatio-temporal patterns since the late 1960s. Rivers lacking or having very low annual catch reports for various reasons were not included. The catches have increased at a rate of ~15% per decade though trends varied geographically with larger increases at northern latitudes and decreases at more southern latitudes. Larger catches were obtained at elevated levels of runoff during summer and early autumn, and at positive NAO phase during the year preceding the catch. Additionally, higher average catch and more pronounced increasing temporal trends in catches were obtained in larger rivers with shallower slopes, respectively. Furthermore, stronger relationships with runoff occurred in steeper rivers. Other factors such as Gyrodactylosis or fish farming seemed not to have detectable signals in the overall pattern of brown trout catches.

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.

Long-term variability of the southern Adriatic circulation in relation to North Atlantic Oscillation

Abstract. The interannual variability of the South Adriatic Gyre and its relation to the wind vorticity and the large-scale climatic pattern (North Atlantic Oscillation – NAO) was studied using the time series of satellite altimetric data and ocean surface wind products. The cyclonic circulation observed in the southern Adriatic area was partly sustained by the local wind forcing, as suggested by the positive correlation between the rate of change of the current vorticity and the wind-stress vorticity. Nevertheless, the influence of vorticity advection from the adjacent area (northern Ionian Sea) cannot be ignored and it is more significant during the anticyclonic phase of Adriatic–Ionian Bimodal Oscillation System. The geostrophic current vorticities of the southern Adriatic and northern Ionian seas are correlated with a time lag of 14 months, which approximately corresponds to an advection speed of ∼ 1 cm s−1. The different wind patterns observed during two NAO phases in the winter revealed a stronger positive vorticity during the negative NAO phase. Conversely, during the wintertime positive NAO phase the wind vorticity is characterized by lower positive or slightly negative values. Despite a statistically significant negative correlation between the NAO index and the wind vorticity, no unequivocal relationship between large climatic system and the interannual variability of the South Adriatic Gyre intensity was found due to additional effects of the vorticity advection from the Ionian. This can be explained by the fact that the Ionian circulation mode does not depend on the NAO variations. Therefore, the main result of this study is that the interannual variability of the southern Adriatic cyclonic circulation is a result of the combined influence of the vorticity advection from the Ionian and the local wind-curl effect.

Effect of the North Atlantic Oscillation on the Pattern of Lake Ice Phenology in Poland

This paper presents an analysis of the influence of the North Atlantic Oscillation on the pattern of lake ice phenology in Poland. The research embraced 22 lakes in Poland over the period 1961–2010. Strong relations were found to hold between NAO and individual characteristics of ice phenology. In a negative NAO phase, one can observe a later appearance of ice phenomena and ice cover compared with the average values, ice cover persisting even 30 days longer and being thicker even by more than 10 cm. In turn, in a positive NAO phase the duration of ice phenomena and ice cover is shorter, the cover being less thick and solid. The observed spatial differences in the effect of NAO on the pattern of ice phenomena in Poland show this matter to be fairly complex. The most significant factor changes in climatic conditions, which manifest themselves in the continentality of the climate growing eastwards.

CHANGES IN THE SNOW COVER THICKNESS AND OF DAILY SNOWFALL INTENSITY AFFECTING THE HIGHWAYS CLEANING EXPENSES IN RUSSIAN CITIES

Исследованы осадки холодных сезонов 1950-2006 гг. на территории России и их влияние на расходы по уборке снега в некоторых российских городах. Рассмотрены общее количество осадков за холодный период, а также число дней с осадками 0-2, 2-5 и более 5 мм/сут. За последнее время на территории России при общей тенденции к увеличению количества снега везде уменьшилось число слабых снегопадов; число снегопадов средней и большой интенсивности возросло на востоке Восточно-Европейской равнины и в Западной Сибири, но уменьшилось на северо-востоке Сибири. Вероятная причина наблюдаемых измене- ний - смена режима атмосферной циркуляции в середине 1970-х годов. Если в 1950-1974 гг. в формиро- вании осадков января доминировал циркуляционный механизм SCAND, то 1975-2006 гг. были периодом усиления положительной фазы NAO. В Москве и Ханты-Мансийске расходы на уборку магистралей от снега возрастали, что в первую очередь связано с развитием этих городов (а в Ханты-Мансийске и с бoльшим числом снегопадов). В Санкт-Петербурге и Петропавловске-Камчатском расходы на уборку снега в период до 2006 г. сокращались. Здесь существенную роль в изменении анализируемого экономического показа- теля сыграл метеорологический фактор.