Air pressure fluctuations in the North Atlantic and their relationship with El Niño-southern oscillations

Seasonal and interannual variability of the pressure field and indices of the North Atlantic atmosphere zonal circulation are analysed using historical (1894–1988) observations. It is shown that fluctuations of the index of North Atlantic oscillations (NAO) and that of the eastward transport give evidence of the interannual fluctuations with the typical time scale being 2–7 years. It is shown that the magnitude of interannual NAO index variability exceeds the typical magnitude of seasonal variations, particularly in winter. The time scale of NAO index variations and eastward transport coincides with the typicalEl Niño-southern oscillations (ENSO) temporal scale. The amplitudes of the annual, semi-annual harmonics, and high-frequency fluctuations of the NAO index increase during a typical ENSO event at least by a factor of 2.Translated by V. Puchkin.     

Temporal variations of the winter mixed layer south of the Kuroshio Extension

A monthly mean time series of the temperature profile in the recirculation gyre south of the Kuroshio Extension has been produced for the period 1971–2007 to examine temporal variations of the winter mixed layer. The winter mixed layer depth (MLD) shows both interannual and decadal variations and is significantly correlated with variation of the mean net surface heat flux in late autumn to early winter. There is also a close relation with the strength of pre-existing subsurface stratification, measured as vertical temperature gradients in the preceding summer. Linear multiple regression analysis shows that a significant fraction of the variations in the winter MLD is explained by the surface heat flux and the strength of the stratification. The contribution of the two factors is comparable.     

Structure of temperature variability in the high latitudes of the Northern Hemisphere

The spatial structure of surface air temperature (SAT) anomalies in the extratropical latitudes of the Northern Hemisphere (NH) during the 20th century is studied from the data obtained over the period 1892–1999. The expansion of the mean (over the winter and summer periods) SAT anomalies into empirical orthogonal functions (EOFs) is used for analysis. It is shown that variations in the mean air temperature in the Arctic region (within the latitudes 60°–90°N) during both the winter and summer periods can be described with a high accuracy by two spatial orthogonal modes of variability. For the winter period, these are the EOF related to the leading mode of variability of large-scale atmospheric circulation in the NH, the North Atlantic Oscillation, and the spatially localized (in the Arctic) EOF, which describes the Arctic warming of the mid-20th century. The expansion coefficient of this EOF does not correlate with the indices of atmospheric circulation and is hypothetically related to variations in the area of the Arctic ice cover that are due to long-period variations in the influx of oceanic heat from the Atlantic. On the whole, a significantly weaker relation to the atmospheric circulation is characteristic of the summer period. The first leading variability mode describes a positive temperature trend of the past decades, which is hypothetically related to global warming, while the second leading EOF describes a long-period oscillation. On the whole, the results of analysis suggest a significant effect of natural climatic variability on air-temperature anomalies in the NH high latitudes and possible difficulties in isolating an anthropogenic component of climate changes.     

Climatic and hydrophysical conditions of the development of hypoxia in waters of the northwest shelf of the Black Sea

The data of meteorological and oceanographic observations on the northwest shelf of the Black Sea for 1973–2000 are used to compute the characteristics of the entire area in the presence of hypoxia of waters under the pycnocline in the summer–autumn period and the area of surface waters with a level of salinity lower than 17.5‰ in May. The time of onset of the spring warming of air (stable transition through a temperature of 5°) is determined. A statistically significant positive trend of the air temperature (0.8° per 100 yr) is revealed in Odessa. The process of warming was observed mainly for the winter (1.5° per 100 yr) and spring (0.8° per 100 yr) periods and became especially intense since the beginning of the 1990s. On the basis of the data of correlation analyses, we establish a statistically significant relationship between the large-scale atmospheric processes [the index of North Atlantic Oscillation (NAO) and the wind conditions], the area of surface waters whose salinity is lower than 17.5‰, and the total area with hypoxia in the summer–autumn periods. For positive mean values of the NAO index (in January–March), we most often observe early spring with elevated repetition of the south and west winds with subsequent development of hypoxia in large areas of the northwest shelf. We propose an empirical regression model for the prediction of the total area of summer–autumn hypoxia of waters with predictors: the onset of the spring warming of air and the area of propagation of waters whose salinity is lower than 17.5‰ in May. The maximum error of prediction of the area with hypoxia does not exceed 5.5 ⋅ 10³ km², i.e., less than 2% of the total area of the northwest shelf in the Black Sea (to the north of 45°N).     

An argo-based model for investigation of the Global Ocean (AMIGO)

We analyze the newly developed Argo-Based Model for Investigation of the Global Ocean (AMIGO), which consists of a block for variational interpolation of the profiles of drifting Argo floats to a regular grid and a block for model hydrodynamic adjustment of variationally interpolated fields. Such a method makes it possible to obtain a full set of oceanographic characteristics—temperature, salinity, density, and current velocity—using irregularly located Argo measurements. The resulting simulations are represented as monthly mean, seasonal, and annual means and climatological fields. The AMIGO oceanographic database developed at the Shirshov Institute of Oceanology from model simulations covers the 10-year period from 2005 to 2014. Analysis of transport variations in the propagation of North Atlantic Current jets to the Arctic based on the AMIGO data showed that during this period, anomalous winter transports were observed, which correlate with anomalous winter temperatures in regions of northwestern Europe, northern European Russia, and Iceland, which are subjected to the influence of these currents. Comparative analysis of variations in mass and heat transport by the currents and the North Atlantic Oscillation (NAO) index in the period of 2005–2014 shows a well pronounced correlation between them. The low winter values of the NAO index correspond to the low values of winter transports by the Faroe–Shetland branch of the North Atlantic current, and usually, to the high values of winter transports by the North Icelandic branch of the Irminger Current. High winter value of the NAO index results in a substantial increase in the winter transport by the Faroe–Shetland branch of the North Atlantic Current without notable influence on the transport of the North Icelandic branch of the Irminger Current.     

Tropospheric temperature interannual variations associated with decadal changes in the North Atlantic Oscillation

This paper studies the causes and mechanisms of the formation of extreme anomalies in the tropospheric temperature associated with the North Atlantic Oscillation (NAO). Our approach is based on understanding that, in the annual cycle, continental-scale tropospheric temperature anomalies (planetary waves with longitudinal wave numbers of 1–3) can both intensify under the direct action of heat inflow as an energy source for these anomalies (radiation cooling/heating) and weaken as a result of the destructive action of heat inflow under temperature advections with the opposite (to the heat inflow) sign [4, 5]. According to the monthly mean data of the NCEP/NCAR reanalysis over the 40-year period, seasonal air temperature anomalies have been studied at the level 850 hPa (T ₈₅₀) in different regions of Eurasia. It has been confirmed that the negative NAO phase in winter is favorable for preserving negative T ₈₅₀ anomalies in the east of the continent at this time of year, whereas the positive NAO phase is favorable for negative T ₈₅₀ anomalies in the west. However, it has been revealed that this dependence was critically violated during the winter seasons approximately two years before an extreme event. This was explained by the fact that, in those years, the NAO influence on winter T ₈₅₀ anomalies was limited. This paper formally considers a certain mechanism of anomalous heat inflow as an energy source for these anomalies with functions of the formation (intensification) of negative T ₈₅₀ anomalies in winter and positive T ₈₅₀ anomalies in summer, as well as with a function of the limitation of the influence of the predominant dynamic mode on some regions of the continent. It is shown that, in the 1960s, T ₈₅₀ anomalies with negative NAO indices in the east of the continent were governed by a hypothetic mechanism of heat inflow as an energy source for anomalies; in 1980s, at prolonged positive NAO indices, T ₈₅₀ anomalies in the west of the continent could also be governed by this mechanism. This paper, within the accepted degree of detail, demonstrates the process of limitation of the NAO influence in some years (1966, 1967, 1987, and 1988), which leads to an unbalance of the anomalies and a possible extreme phenomenon. It is demonstrated that, in some seasons, the anomalies were not governed by the hypothetic mechanism of the heat inflow under the action of large NAO changes and a complete upset of the annual cycle of anomalies. Determining the first indicators of the unbalance, which can lead to extreme anomalies, is shown to be difficult if it is based only on an analysis of winter seasons (as is the case with most of the works) without invoking the annual trends of the tropospheric temperature and the NAO index.     

Influence of the ocean surface temperature and sea ice concentration on regional climate changes in Eurasia in recent decades

Numerical experiments with the ECHAM5 atmospheric general circulation model have been performed in order to simulate the influence of changes in the ocean surface temperature (OST) and sea ice concentration (SIC) on climate characteristics in regions of Eurasia. The sensitivity of winter and summer climates to OST and SIC variations in 1998–2006 has been investigated and compared to those in 1968–1976. These two intervals correspond to the maximum and minimum of the Atlantic Long-Period Oscillation (ALO) index. Apart from the experiments on changes in the OST and SIC global fields, the experiments on OST anomalies only in the North Atlantic and SIC anomalies in the Arctic for the specified periods have been analyzed. It is established that temperature variations in Western Europe are explained by OST and SIC variations fairly well, whereas the warmings in Eastern Europe and Western Siberia, according to model experiments, are substantially (by a factor of 2–3) smaller than according to observational data. Winter changes in the temperature regime in continental regions are controlled mainly by atmospheric circulation anomalies. The model, on the whole, reproduces the empirical structure of changes in the winter field of surface pressure, in particular, the pressure decrease in the Caspian region; however, it substantially (approximately by three times) underestimates the range of changes. Summer temperature variations in the model are characterized by a higher statistical significance than winter ones. The analysis of the sensitivity of the climate in Western Europe to SIC variations alone in the Arctic is an important result of the experiments performed. It is established that the SIC decrease and a strong warming over the Barents Sea in the winter period leads to a cooling over vast regions of the northern part of Eurasia and increases the probability of anomalously cold January months by two times and more (for regions in Western Siberia). This effect is caused by the formation of the increased-pressure region with a center over the southern boundary of the Barents Sea during the SIC decrease and an anomalous advection of cold air masses from the northeast. This result indicates that, to estimate the ALO actions (as well as other long-scale climatic variability modes) on the climate of Eurasia, it is basically important to take into account (or correctly reproduce) Arctic sea ice changes in experiments with climatic models.     

Temporal variability of winter mixed layer in the mid-to high-latitude North Pacific

Temperature and salinity data from 2001 through 2005 from Argo profiling floats have been analyzed to examine the time evolution of the mixed layer depth (MLD) and density in the late fall to early spring in mid to high latitudes of the North Pacific. To examine MLD variations on various time scales from several days to seasonal, relatively small criteria (0.03 kg m⁻³ in density and 0.2°C in temperature) are used to determine MLD. Our analysis emphasizes that maximum MLD in some regions occurs much earlier than expected. We also observe systematic differences in timing between maximum mixed layer depth and density. Specifically, in the formation regions of the Subtropical and Central Mode Waters and in the Bering Sea, where the winter mixed layer is deep, MLD reaches its maximum in late winter (February and March), as expected. In the eastern subarctic North Pacific, however, the shallow, strong, permanent halocline prevents the mixed layer from deepening after early January, resulting in a range of timings of maximum MLD between January and April. In the southern subtropics from 20° to 30°N, where the winter mixed layer is relatively shallow, MLD reaches a maximum even earlier in December–January. In each region, MLD fluctuates on short time scales as it increases from late fall through early winter. Corresponding to this short-term variation, maximum MLD almost always occurs 0 to 100 days earlier than maximum mixed layer density in all regions.     

On the problem of influence of the North Atlantic and Southern Oscillations on the variability of surface air temperature in the European-Mediterranean region in summer and autumn

On the basis of reanalysis of the data of the European Center of Medium-Range Weather Forecasts for 1979–1993, the interannual and monthly variability of the surface air temperature (SAT) in the European-Mediterranean region for summer and autumn with regard for the influence of the North Atlantic (NAO) and Southern (SO) Oscillations have been studied. The NAO manifests itself on a significant level from August to October and is responsible for up to 55% of the variance of monthly average SAT in the north part of the European region. This phenomenon is caused by the intensification of zonal circulation and enhanced transport of warm Atlantic air masses to Northern Europe. A general trend toward a decrease in the variance of monthly average SAT in the mature phase of the NAO is observed over the most part of Europe. In this case, the NAO is responsible for up to 35% of monthly fluctuations of SAT in summer, up to 45% in September, and up to 65% in October. As the SO index sharply decreases and an event of El Niño develops in the Pacific, the maximum variance of SAT in the European-Mediterranean region in summer significantly increases within the two-week range of variability. Up to 25% of the variance of fluctuations of SAT in summer within the indicated range of variability are, in this case, induced by the SO.Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 64–70, September–October, 2004.This revised version was published online in May 2005 with corrections to cover date.     

Hydrometeorological characteristics of the Black-Sea region in the years with extreme values of the Sargasso–Black-Sea index

We analyze the annual average values and the anomalies of some hydrometeorological characteristics averaged over the period since January till March near the South Coast of Crimea and in the open part of the Black Sea. It is shown that the sea-surface temperature, the surface temperature of air, and the atmospheric precipitation in the Black-Sea region are lower than the climatic mean values for the extremely high values of the index of North-Atlantic Oscillation (NAO index) (NAO ≥ 1) and the surface temperature gradient between the Sargasso and Black Seas (SBS index) (SBS ≥ 1). In the years with NAO ≤ − 1 and SBS ≤ − 1, the indicated characteristics exceed the climatic mean values. The entire process of changes in the sign of anomalies of the hydrometeorological characteristics runs with a period close to 20 yr.     

北极气旋的季节、年际变化及其与北极海冰、大气遥相关的关系

The seasonal and inter-annual variations of Arctic cyclone are investigated. An automatic cyclone tracking algorithm developed by University of Reading was applied on the basis of European Center for Medium-range Weather Forecasts(ECMWF) ERA-interim mean sea level pressure field with 6 h interval for 34 a period. The maximum number of the Arctic cyclones is counted in winter, and the minimum is in spring not in summer.About 50% of Arctic cyclones in summer generated from south of 70°N, moving into the Arctic. The number of Arctic cyclones has large inter-annual and seasonal variabilities, but no significant linear trend is detected for the period 1979–2012. The spatial distribution and linear trends of the Arctic cyclones track density show that the cyclone activity extent is the widest in summer with significant increasing trend in CRU(central Russia)subregion, and the largest track density is in winter with decreasing trend in the same subregion. The linear regressions between the cyclone track density and large-scale indices for the same period and pre-period sea ice area indices show that Arctic cyclone activities are closely linked to large-scale atmospheric circulations, such as Arctic Oscillation(AO), North Atlantic Oscillation(NAO) and Pacific-North American Pattern(PNA). Moreover,the pre-period sea ice area is significantly associated with the cyclone activities in some regions.     

Cyclicity of natural processes in the last 300 years imprinted in the Tree-Ring proxies in Subarctic European Russia revealed by wavelet analysis

We studied the cyclic structure of the earlywood ring width and maximum density of larch and spruce (International Tree-Ring Data Bank) at the northern treeline in European Russia. On the basis of correlation analyses, local chronologies are combined into seven regional ones. The chronologies of maximum density correlate positively with summer (May–August) temperature. The response functions for the earlywood ring width are chaotic. However, a wavelet analysis of instrumental cold-period (October–April) precipitation series and regional earlywood chronologies shows a similarity in their cyclic pattern. On the basis of a wavelet analysis, it was found that the chronologies of the western and northern regions of European Russia have a similar cyclic structure over the last 300 years, while the Ural chronologies differ from them in some respects. The long-term (40–100 years) variations dominate in the earlywood chronologies, whereas the chronologies of maximum density also contain short-term (5–20 years) variations. The 100- and 11-year cycles, which are probably modulated by solar activity, are detectable in all regional chronologies of earlywood width and maximum density. Cycles with periods of 40, 20–22, and 5 years in the tree-ring chronologies are most likely triggered by the North Atlantic Oscillation, which has a similar cyclic structure.     

Temporal and spatial variabilities of Japan Sea surface temperature and atmospheric forcings

In this study, we used the National Centers for Environmental Prediction monthly sea surface temperature (SST) and surface air temperature (SAT) data during 1982–1994 and the National Center for Atmospheric Research surface wind stress curl data during 1982–1989 to investigate the Japan Sea SST temporal and spatial variabilities and their relations to atmospheric forcing. First, we found an asymmetry in the correlation coefficients between SST and wind stress curl, which implies that the SST variability at the scales of the order of one month is largely due to atmospheric forcing. Second, we performed three analyses on the data fields: annual mean, composite analysis to obtain the monthly anomaly relative to the annual mean, and empirical orthogonal function (EOF) analysis on the residue data relative to the summation of the annual mean and the monthly anomaly. The first EOF mode of SST accounts for 59.9% of the variance and represents the Subpolar Front. The temporal variation of the first EOF mode implies that the deep Japan Sea could be cooler in cold seasons (November–April) of 1984–1987. Third, we computed cross-correlation coefficients among various principal components and found that the atmospheric warming/cooling is the key factor causing intra-seasonal and interannual SST variabilities.     

Influence of long-term regional and large-scale atmospheric circulation on the Baltic sea level

The connection between variations in the North Atlantic Oscillation (NAO) index and the Baltic sea level has been investigated for the period 1825–1997. The association between the NAO and the strength of the zonal geostrophic wind stress over the Northwest Atlantic suggests an NAO impact on Baltic sea level variations, because the monthly mean sea level mainly is determined by externally driven variations caused by wind conditions over the North Sea. Several period bands were found to have high correlation between oscillations in the winter (JFM) NAO index and the Baltic Sea winter mean sea level. The correlation was, however, higher in the 20th century than in the 19th. During the last two decades, the correlation between the NAO index and the sea level has been exceptionally high. The winter mean of a regional atmospheric circulation index had a correlation with the Kattegat winter mean sea level of 0.93. With the Baltic sea level the correlation was 0.91, compared with the NAO index correlation for the same period of 0.74. The regional index also showed a high correlation with the mean summer and mean autumn sea levels, when the corresponding seasonal NAO indices showed a weak connection. The temporal variation of the connection with the NAO index implies a regional atmospheric circulation occasionally differing from the large-scale circulation associated with the NAO. Seasonal means of the sea level in Stockholm do, however, reflect the regional wind climate to a large extent, and the Baltic sea level is a useful proxy for identifications of climatic dependencies in the region.     

Linkage between winter air temperature over the subtropical Western Pacific and the ice extent anomaly in the Sea of Okhotsk

This study deals with the correlation between ice extent in the Sea of Okhotsk and the interannual variability of winter (December–February) air temperature over the subtropical Western Pacific from 1979 to 2008. The analysis indicates that the increase in sea ice extent coincides not only with cooling over the Sea of Okhotsk and the adjacent area, but also with significant warming over the subtropical Western Pacific that extends from the surface to the middle troposphere. This meridional dipole pattern of tropospheric temperature anomalies (cooling in the high latitudes and warming in the low latitudes) primarily results from dynamical processes driven by the large-scale atmospheric circulation change. A heat budget diagnosis reveals that when ice extent in the Sea of Okhotsk increases by one standard deviation, the tropospheric air temperature over the subtropical Western Pacific rises by about 0.25°C. It also suggests that the adiabatic heating and stationary eddy heat flux convergence may be the most important factors, which account for 30 and 15% of the warming, respectively. In addition, these two factors also coordinate to result in significant cooling over the Sea of Okhotsk and the adjacent regions.     

Impact of the winter cooling on the variability of the thermohaline characteristics of the active layer in the Black Sea

The influence of the winter atmospheric forcing on the interannual variability of the Black Sea’s active layer’s thermohaline structure during 1982–2008 is investigated. The results are based on the combined analysis of the hydrological measurements from a ship, satellite measurements of the sea’s surface temperature (SST), and the NCEP/NCAR reanalysis data for the surface air temperature (SAT). A high correlation between the variability of the winter mean SST/SAT and the thermohaline characteristics of the active layer during the following warm season was found. It is shown that the winter atmospheric forcing significantly affects the variability of the temperature, salinity, and density down to the 150–200 m depth, and this has to be considered in the analysis of the interannual and long-term variability of the Black Sea’s active layer.     

Seasonal variations in temperature,salinity and density in the southern coastal waters of the Caspian Sea

Variability in water temperature, salinity and density was investigated based on field measurements near Anzali Port, in the Southern Caspian Sea in 2008. Seasonal changes of seawater properties were mainly observed through the upper 100 m layer, while below this layer seasonal variations of the parameters were minor. Vertical structure of the temperature in the southern coastal waters of the Caspian Sea is characterized by a significant seasonal thermocline between 20–50 m depths with vertical variation in temperature about 16°C in midsummer (August). Decrease of the thermocline occurs with the general cooling of the air and sea surface water, and deepening of the mixed layer during late of autumn and winter. Seasonal averages of the salinity were estimated in a range of 12.27–12.37 PSU. The structure of thermocline and pycnocline indicated agreement between changes of temperature and density of seawater. Seasonal pycnocline was observed in position of the thermocline layer.     

Hydrographic conditions in the Tsushima Strait revisited

Long-term averaged temperature and salinity distributions in the Tsushima Strait are investigated on the basis of a concurrent dataset of the eastern and western channels during 1971–2000. Both temperature and salinity show a clear seasonal variation with weak and strong stratifications in December–April and June–October, respectively. The largest standard deviations occur in summer around the thermocline for temperature and in the surface layer for salinity. This indicates large interannual variability in the development of a thermocline and low salinity water advection from the East China Sea. The water masses in both channels are distinctly different from each other; the water in the western channel is generally colder and fresher than that in the eastern channel throughout the year. Baroclinic transport based on the density distributions shows a seasonal variation with a single peak in August for the eastern channel and double peaks in April and August for the western channel. However, this cannot explain the seasonal variation in the total volume transport estimated from the sea level differences across the channels. The spatial distribution of baroclinic transport shows a year-round negative transport towards the East China Sea behind the Iki Island in the eastern part of the eastern channel. This negative transport reflects the baroclinic structure between the offshore Tsushima Current Water and cold coastal water. The corresponding southwestward currents are found in both Acoustic Doppler Current Profiler (ADCP) and high frequency (HF) radars observations.     

Modelling and forecasting long-term dynamics of Western Baltic macrobenthic fauna in relation to climate signals and environmental change

Long-term macrobenthos data from Kiel Bight in the Western Baltic collected between 1968 and 2000 have been correlated with the winter NAO index (North Atlantic Oscillation Index) and other environmental data such as temperature, salinity and oxygen content in the bottom water in order to detect systematic patterns related to so far unexplained abiotic signals in the dynamics of zoobenthic species assemblages. The benthos data come from a cluster of five stations (Süderfahrt/ Millionenviertel) in Kiel Bay. Our investigations concentrated on the macrobenthic dynamics with a focus on the number of species m^− 2 (species richness). Using logarithms and the time series analysis approach of Box/Jenkins (ARIMA modelling, transfer function modelling) it was shown that species richness was strongly influenced by the winter NAO (adjusted for a linear time trend within the 1968-2000 period) and salinity (with a shift/lag of four years). Bootstrapping experiments (i.e. sampling from the error process) and analysis of prediction power (by means of the one- or more-years leaving-out method) showed that the parameter estimates behaved in a stable way, leading to a relatively robust model.     

Interannual and seasonal variability of hydrometeorological fields in some regions of the Atlantic-European sector as a manifestation of the abnormal behaviour of the North-Atlantic Oscillation

We study the interannual and seasonal variability of hydrometeorological fields in some regions of the Atlantic-European sector. These low-frequency processes are analyzed depending on the index ΔP (an analog of the index of the North-Atlantic Oscillation). It is shown that the average value of the index ΔP over the winter natural synoptic season can be used for the determination of stable locations of the centre of the Azorean maximum in 1971–1980 and 1981–1990, the typical state of cloudiness in the Atlantic-European sector, and the fields of precipitations and atmospheric temperature in some regions of the Crimea. Typical anomalies of the fields of precipitations and atmospheric temperature over the west and south coasts of the Crimea are described. The signs of the anomalies of precipitations in the winter natural synoptic season are in good agreement with the signs of the corresponding anomalies of the field of cloudiness in the Black-Sea region. Translated by Peter V. Malyshev and Dmitry V. Malyshev