Long-term Monthly Mean Research Articles

Thermal interaction between near-water atmospheric layer and surface water layer on the northeastern shelf of the Black Sea

Thermal interaction between near-water atmospheric layer and surface water layer is considered within long-term, seasonal, synoptic, and mesoscale variability ranges from long-term air and water temperature measurement data. It is established that long-term mean monthly values of water temperature within annual cycle are greater than corresponding values of air temperature. Equations of relationships are obtained for long-term mean annual, mean seasonal, and mean monthly air and water temperature values.

Classification and flow prediction in a data-scarce watershed of the equatorial Nile region

Abstract. Continuous developments and investigations in flow predictions are of interest in watershed hydrology especially where watercourses are poorly gauged and data are scarce like in most parts of Africa. Thus, this paper reports on two approaches to generate local monthly runoff of the data-scarce Semliki watershed. The Semliki River is part of the upper drainage of the Albert Nile. With an average annual local runoff of 4.622 km3/annum, the Semliki watershed contributes up to 20% of the flows of the White Nile. The watershed was sub-divided in 21 sub-catchments (S3 to S23). Using eight physiographic and meteorological variables, generated from remotely sensed acquired datasets and limited catchment data, monthly runoffs were estimated. One ordination technique, the Principal Component Analysis (PCA), and the tree cluster analysis of the landform attributes were performed to study the data structure and spot physiographic similarities between sub-catchments. The PCA revealed the existence of two major groups of sub-catchments – flat (Group I) and hilly (Group II). Linear and nonlinear regression models were used to predict the long-term monthly mean discharges for the two groups of sub-catchments, and their performance evaluated by the Nash-Sutcliffe Efficiency (NSE), Percent bias (PBIAS) and root mean square error to the standard deviation ratio (RSR). The dimensionless indices used for model evaluation indicate that the non-linear model provides better prediction of the flows than the linear one.

Seasonal dynamics of the near‐surface alongshore flow off central Chile

The seasonal cycle of the near‐surface circulation off central Chile was analyzed using satellite altimetry and an oceanic model. To evaluate the role of the wind stress curl on the circulation we performed two identical simulations except for the wind‐forcing: the “control run” used long‐term monthly mean wind stress and the “no‐curl run” used a similar wind stress field, but without curl. The observed and modeled (control run) surface currents showed a strong seasonal cycle and a well‐defined equatorward flow with a jet like‐structure. This jet develops during spring and summer, consistent with the presence of a low‐level wind jet. South of Punta Lavapie cape (∼37°S), the equatorward surface current remains close to the coast. After the flow‐passes this cape, however, it separates to become an offshore jet. In contrast, in the no‐curl simulation the separation at Punta Lavapie is not observed and the offshore jet farther north is not present, demonstrating the importance of the wind stress curl on the dynamics of this flow. Although the offshore integrated Sverdrup transport was similar to the model transport, the offshore jet was not located where the wind stress curl was maximum. Instead, the position of the jet followed approximately the zero wind stress curl, which corresponds to the climatological location of the low‐level wind jet axis. These results illustrate the importance of the offshore upwelling/downwelling associated with curl‐driven Ekman pumping, which tilts isopycnals upward (downward) toward the east (west) of the wind jet, forcing a northward flow through thermal wind balance.

The development of seasonal structural fronts in the Baltic Sea after winters of varying severity

The manifestation in the Baltic Sea of a seasonal, structural, thermally-induced front (a direct analogue of the lacustrine thermal bar) and its specific features after winters of different severity are examined. Analysis of long-term, spring-period, monthly mean SST data reveals that surface water temperatures first exceed the (salinity-dependent) temperature of maximum density (Tmd) in the southern part of the Baltic Sea. This phenomenon advances northwards at a speed of about 11 to 16 km d–1, traversing the breadth of the sea within 8 to 10 wk. Surface temperature measurements across the Tmd-isotherm demonstrate that after the severe winter of 2002-2003 the frontal zone was highly pronounced, with a horizontal temperature gradient 10 to 100 times as large as the long-term monthly mean equivalent. Although the horizontal gradient was smaller after the mild winter of 20062007, it was still 10 times as large as the long-term monthly mean gradient. A clear correlation of thermal front advancement with biological parameters is revealed: an area of elevated chlorophyll a (about 200 km in width) associated with the Tmd was observed after severe winters, with the characteristic life-time of this zone 1 mo longer than that occurring after mild winters.

Assessment of erythemal UV level in Nepal based on solar UV estimates from total ozone mapping spectrometer

Nepal lies on the southern slope of Himalaya in Asia. In a width ranging between 15150 and 250 km, the altitude varies greatly from about 100 m at its southern border to a maximum of 8848 min the northern part. Like the variation in altitude, climatic condition varies quite a lot. Long-term monthly mean erythemal UV daily dose values for Nepal are evaluated using Total Ozone Mapping Spectrometer (TOMS) estimation from the time of its overpass between 1996 and 2003 The results are presented as summer and winter maps of mean UV levels in each satellite grid. The mean winter erythemal UV daily dose ranges between 2.1 and 3.6 kJ mt−2 whereas summer values are found to lie between 4.6 and 9.7 kJ mt−2. The altitude variation increases the UV levels by about 0.2 kJ km−1 in winter months, and 0.9 kJ km−1 in summer. A multiyear monthly average erythemal daily dose in most of the areas shows that the summer value is about three times higher than that in winter. Although year-to-year variation is not pronounced in high- and mid-elevation regions, UV levels seemed to decrease from 1997 to 2002 in the southern part of the country in the low elevation region by about 5.35%. Due to the combined effects of the altitude, low ozone concentration in the troposphere, and thin air, surface UV radiation at higher altitudes is found to be higher than in the surrounding regions.

Homogeneous Climate Divisions for Peninsular Malaysia

Classification of Peninsular Malaysia was delineated by integrating in-situ temperature elements data and Geographical Information System (GIS) raster data. The principal component (PC) analysis was applied to long-term mean monthly temperature elements data for monsoon seasons. The first three principal components were chosen to be statistically significant, accounted for 96.5% of the variability in the 27 variables. These three components are related to the mean monthly variation in minimum temperature during monsoon season (first PC), the mean monthly variation in maximum and the mean temperature in southwest monsoon (second PC), and the mean monthly variation in maximum temperature during northeast monsoon (third PC). Cluster analyses were applied to create clusters of meteorological stations, of which six classes were formed. To determine cluster boundaries, interpolation analysis was applied to generate GIS raster data of factor scores. The supervised classification analysis was then performed to the generated GIS factor data. The result of a maximum likelihood classification produced three clusters when summarized by districts. Final classification results of climate divisions show rational climate regionalization that reveals control on temperature. The use of factor score GIS raster data effectively assists the generation of meteorological station clusters, grouped using only in-situ data. © 2011 Lavoisier SAS. All rights reserved

Regionalization of the Gulf of Mexico from space-time chlorophyll-a concentration variability

Regions in the Gulf of Mexico are determined based on the statistical behavior of the long-term monthly means of chlorophyll-a concentration from SeaWiFS satellite estimations. An analysis based on the four largest modes of an empirical orthogonal decomposition, which account for 84.9% of the variance, results in nine spatial patterns with different statistical behavior representing 14 connected regions. The time evolution (or principal component) of the first two modes resemble the annual cycle, but each one with a different phase; the third mode represents a semiannual period and the fourth mode shows three maxima and minima. A map of the resulting regions is obtained and the oceanographic processes taking place in each region are discussed. The largest region covers most of the deep Gulf and the continental slope. Other regions in the deep Gulf are located southeast of the Mississippi River mouth and off-shelf of southern Texas and Tabasco, all associated with seasonal offshore cross-shelf transports. The shelves are associated with specific regions, but in wide shelves the inner and outer continental platforms are separated. Among the causes that determine different regions are topographic characteristics and the seasonal variability of physical processes, mainly entrainment caused by heat and momentum fluxes, upwelling, river plumes, and cross-shelf transports associated with the convergence of the along-coast currents.

Comparação de produtos de radiação solar incidente à superfície para a América do Sul

Para algumas aplicações meteorológicas, o conhecimento da radiação solar incidente à superfície (Sin) é muito importante. Métodos convencionais de medida deste fluxo são pontuais e, portanto, representativos de pequenas áreas. Por outro lado, os satélites meteorológicos possibilitam uma cobertura regular de todo o globo terrestre, proporcionando estimativas de fluxos radiativos sobre grandes áreas com resolução espacial do sensor (apenas alguns quilômetros). Além disso, produtos de reanálise fornecem estimativas alternativas de Sin, que precisam ser adequadamente validadas. Este trabalho compara quatro bancos de dados de radiação solar incidente para a América do Sul: três produtos provenientes de reanálise (NCEP/NCAR, ERA-40 e Eta/CPTEC) e o produto gerado pelo algoritmo GL1.2, que produz estimativas da Sin a partir do conjunto de imagens do satélite GOES. Os resultados mostram que os campos de radiação solar incidente dos produtos de reanálise do NCEP/NCAR, ERA-40 e as estimativas do satélite GOES apresentam valores médios mensais bem próximos para todos os biomas e bacias estudados. Já a reanálise do Eta/CPTEC apresentou valores bem mais elevados para à radiação sobre a América do Sul, tanto para a média anual como nas médias mensais analisadas.

Reconstruction of regular time series from bi-monthly temperature data in the Yellow Sea and the northwestern East China Sea

Reconstruction of evenly-spaced, regular time series from routine survey serial data was investigated for precise analysis of spatio-temporal variations in a temperate sea at mid-latitudes where the seasonality dominates the interannual variability. Considering that the annual range of sea surface temperature in the Yellow Sea and the northwestern East China Sea can be as large as 15–20 °C, temperature data collected bi-monthly in these temperate seas were used for the assessment of reconstruction methodology. The cubic spline interpolation with a sampling interval of 0.5 months reconstructed the regular time series closest to the in-situ measurements among various interpolation schemes. Also, two computation methods for the interannual anomaly were compared; the residual method that the long-term monthly means are subtracted from the monthly serial data and the filtering method that high-frequency variations are removed using a low-pass filter. In that the high-frequency variations in frequencies greater than 1 cycle per year are comparable in magnitude to the interannual variation, the residual method proves inadequate in the temperate sea. High-frequency noises can be effectively removed through the use of a proper low-pass filter with bell-shaped weights.

Comparing Maps of Mean Monthly Surface Temperature and Precipitation for Alaska and Adjacent Areas of Canada Produced by Two Different Methods

Maps of mean monthly surface temperature and precipitation for Alaska and adjacent areas of Canada, produced by Oregon State University’s Spatial Climate Analysis Service (SCAS) and the Alaska Geospatial Data Clearinghouse (AGDC), were analyzed. Because both sets of maps are generally available and in use by the community, there is a need to document differences between the processes and input data sets used by the two groups to produce their respective set of maps and to identify similarities and differences between the two sets of maps and possible reasons for the differences. These differences do not affect the observed large-scale patterns of seasonal and annual variability. Alaska is divided into interior and coastal zones, with consistent but different variability, separated by a transition region. The transition region has high interannual variability but low long-term mean variability. Both data sets support the four major ecosystems and ecosystem transition zone identified in our earlier work. Differences between the two sets of maps do occur, however, on the regional scale; they reflect differences in physiographic domains and in the treatment of these domains by the two groups (AGDC, SCAS). These differences also provide guidance for an improved observational network for Alaska. On the basis of validation with independent in situ data, we conclude that the data set produced by SCAS provides the best spatial coverage of Alaskan long-term mean monthly surface temperature and precipitation currently available.

Modeling Near-Surface Air Temperature From Solar Radiation and Lapse Rate: New Development on Short-Term Monthly and Daily Approach

Short-term monthly mean temperature (Tm ) and short-term daily mean temperature (Td ) rather than long-term monthly and daily mean temperature (Tm and Td ) are preferred for some ecosystem studies such as carbon source and sink, pine beetle mortality, and snow melting. The recent progress of modeling Tm and Td (based on the previous work on Tm ) supported by climatologically aided interpolation (CAI) is reported over the mountainous Yellowstone National Park. With the spatial scale of a 30 m digital elevation model (DEM), the slope, aspect, and shadows cast by surrounding topography, which could not be well captured by very coarse DEM, could be taken into account. Data from 12 months (Jan-Dec 2008) and 12 dates (25 Jan-Dec 2008) were used to demonstrate the approach. Inverse distance weighting (IDW) interpolations of limited temperature anomalies were adopted to represent the deviations from normality. Tm , as a preexisting climatology surface, was added to deviations in order to model Tm . Linear temporal interpolation of adjacent Tm was used to create a climatology surface, which was then added to deviations in order to model Td . Results show the mean absolute errors (MAEs) for Tm ranged from 0.75° C to 1.78° C, while the MAEs for Td ranged from 1.14° C to 2.02° C. The four factors of elevation, seasonal change of lapse rate, temperature difference caused by variation in solar radiation, and preexisting climatology surface for the CAI approach were comprehensively considered in this approach.

Analysis of urban effects on soil temperature in Ankara

In this study, data from two different meteorology stations were analyzed in order to reveal the effects of the urbanization on the soil temperature. These stations are the Ankara Meteorology Station (AMS), showing the urban effects, and the Esenboğa Meteorology Station (EMS), showing the rural effects. The soil temperatures measured at depths of 5, 10, 20, and 50 cm at 0700, 1400, and 2100 hours between 1960 and 2005 were used in the analysis. Long-term mean monthly temperatures at each depth and at each time considered were calculated and analyzed using Sen's slope and Mann-Kendall tests. The results showed that the mean monthly urban soil temperatures were generally higher than the rural soil temperatures. The differences between temperatures measured at 5, 10, 20, and 50 cm in urban and rural stations (DeltaT(s(AMS-EMS))) ranged between 1.8 degrees C and 2.1 degrees C. As in the urban heat islands, the differences between the urban and rural soil temperatures are high at 2100 hours and low at 1400 hours. It was also observed that, due to the increasing number of buildings around the Esenboğa Station in recent years, the difference between the urban and rural soil temperatures seems to have become smaller. These show that the factors affecting the urban heat islands and those affecting the soil temperatures are similar. Also, the temperature differences were observed to be higher during the warm season than in the cold season. The frequency distributions of the temperature differences (DeltaT(s(AMS-EMS))) reveal both positive and negative values. However, the positive temperature differences are obviously prevalent.

The relationship between the isotopic content of precipitation and the precipitation amount in tropical regions

General circulation models (GCMs) fitted with stable isotope schemes are widely used to interpret the isotope–climate relationship. However, previous studies have found that the spatiotemporal isotope/precipitation correlation simulated by GCMs is stronger and more widespread than the observed value. To understand the reason for this failure, we investigated the factors influencing the empirically well-known isotope/precipitation relationship, or precipitation amount effect, in the tropics using newly obtained daily precipitation isotope monitoring data over Asia. As in previous studies, we found an apparent correlation between the long-term monthly mean isotopic content and the corresponding precipitation amount (local precipitation) observed at sub-tropical island stations. Furthermore, on a monthly timescale, the isotopic variability of precipitation for these stations was more clearly related to the regional precipitation amount than to local precipitation. This correlation of isotopic content with the regional precipitation amount was observed at the equatorial (Maritime Continent) stations. For these stations, isotope/local precipitation relationships only appeared over longer timescales, with different regression line slopes at each station. However, at the coastal stations, there was a strong linear relationship between the monthly mean isotopic content and corresponding regional precipitation, and regression line slopes were spatially uniform. For the two sub-tropical terrestrial (Indochina Peninsula) stations, the isotopic minimum appeared without any relationship to rainfall amount but usually occurred at the leeward station during the rainy season. These results suggest that the isotopic variations of precipitation did not depend on the ’local’ rain-out history but on the rain-out process in the surrounding region. However, local rainfall events were associated not only with large-scale disturbances but also with regional circulation. Thus, the scale difference of controlling factors between local rainfall amount and isotopic value results in the weakening of the rainfall amount effect at the observation site and in the discrepancy between GCM simulations and observations. This finding suggests that regional precipitation–isotope relationships should be compared with GCM results. Additionally, because the isotope signal reflects the rain-out history at a regional scale, evaluation of the isotopic field using isotopic GCMs will be useful not only to reconstruct paleoclimate conditions but also to examine how GCMs can reproduce real atmospheric circulation over the tropics.

Trends in streamflow in the Yukon River Basin from 1944 to 2005 and the influence of the Pacific Decadal Oscillation

Summary Streamflow characteristics in the Yukon River Basin of Alaska and Canada have changed from 1944 to 2005, and some of the change can be attributed to the two most recent modes of the Pacific Decadal Oscillation (PDO). Seasonal, monthly, and annual stream discharge data from 21 stations in the Yukon River Basin were analyzed for trends over the entire period of record, generally spanning 4–6 decades, and examined for differences between the two most recent modes of the PDO: cold-PDO (1944–1975) and warm-PDO (1976–2005) subsets. Between 1944 and 2005, average winter and April flow increased at 15 sites. Observed winter flow increases during the cold-PDO phase were generally limited to sites in the Upper Yukon River Basin. Positive trends in winter flow during the warm-PDO phase broadened to include stations in the Middle and Lower Yukon River drainage basins. Increases in winter streamflow most likely result from groundwater input enhanced by permafrost thawing that promotes infiltration and deeper subsurface flow paths. Increased April flow may be attributed to a combination of greater baseflow (from groundwater increases), earlier spring snowmelt and runoff, and increased winter precipitation, depending on location. Calculated deviations from long-term mean monthly discharges indicate below-average flow in the winter months during the cold PDO and above-average flow in the winter months during the warm PDO. Although not as strong a signal, results also support the reverse response during the summer months: above-average flow during the cold PDO and below-average flow during the warm PDO. Changes in the summer flows are likely an indirect consequence of the PDO, resulting from earlier spring snowmelt runoff and also perhaps increased summer infiltration and storage in a deeper active layer. Annual discharge has remained relatively unchanged in the Yukon River Basin, but a few glacier-fed rivers demonstrate positive trends, which can be attributed to enhanced glacier melting. A positive trend in annual flow during the warm PDO near the mouth of the Yukon River suggests that small increases in flow throughout the Yukon River Basin have resulted in an additive effect manifested in the downstream-most streamflow station. Many of the identified changes in streamflow patterns in the Yukon River Basin show a correlation to the PDO regime shift. This work highlights the importance of considering proximate climate forcings as well as global climate change when assessing hydrologic changes in the Arctic.

Biomass development and production created by microorganisms of lower trophic levels in the caspian sea ecosystem: Analysis of mathematical modeling results

The biohydrochemical features of the Caspian Sea ecosystem were estimated for its ten water areas with the use of a mathematical model describing the transformation of compounds of organogenic elements (C, Si, N, and P) and taking into account the morphometric characteristics of the water areas and long-term mean monthly values of the major characteristics of the aquatic environment. Variations in the estimated concentrations of mineral and aggregated biogenic substance fractions, microorganism biomasses (heterotrophic bacteria, three phytoplankton groups and two zooplankton groups) in different water areas of the sea were analyzed. The formation conditions of microorganism biomasses in different water areas are characterized by the analysis of their bioproductivity estimates based on evaluated internal biogenic substance fluxes.

Mathematical modeling of biogenic substance transformations and recycling in a subtropical coastal marine ecosystem

A mathematical model describing the biotransformations of organic and mineral compounds of biogenic elements (P, N, and Si) and dissolved organic carbon was applied for a theoretical analysis of the recycling of nutrients in the coastal zone of the Canary water upwelling. The aim of this study was to quantitatively estimate the effect of nutrient recycling on the dynamics of the biomasses of microorganisms as a matter basis for the biological productivity of the marine environment. Model calculations were carried out for four near-shore areas off the Moroccan Sahara using the morphometric data (mean depths, water volumes, and areas) and longterm monthly mean values of the parameters of the marine environment (temperature, light intensity, water transparency, and depth of the thermocline). In the calculations, no account was taken for the flow rates across the boundaries between the areas and only indirect account for the vertical exchange was taken. At the absence of nutrient transfer, the model reproduces particular features of the dynamics of nutrient and detritus concentrations and the biomasses of microorganisms resulting from the internal recycling of organic and mineral substances only. In order to characterize the processes of the biotransformation, the internal fluxes and turnover times of organic and mineral components were estimated together with the biomasses and biological productivities of microorganisms.

Temperature responses of growth and wood anatomy in European beech saplings grown in different carbon dioxide concentrations

Effects of temperature on growth and wood anatomy were studied in young European beech (Fagus sylvatica L.) grown in 7-l pots for 2.5 years in field-phytotron chambers supplied with an ambient (approximately 400 micromol mol-1) or elevated (approximately 700 micromol mol-1) carbon dioxide concentration ([CO2]). Temperatures in the chambers ranged in increments of 2 degrees C from -4 degrees C to +4 degrees C relative to the long-term mean monthly (day and night) air temperature in Berlin-Dahlem. Soil was not fertilized and soil water and air humidity were kept constant. Data were evaluated by regression analysis. At final harvest, stem diameter was significantly greater at increased temperature (Delta1 degrees C: 2.4%), stems were taller (Delta1 degrees C: 8.5%) and stem mass tree-1 (Delta1 degrees C: 10.9%) and leaf area tree-1(Delta1 degrees C: 6.5%) were greater. Allocation pattern was slightly influenced by temperature: leaf mass ratio and leaf area ratio decreased with increasing temperature (Delta1 degrees C: 2.3% and 2.2% respectively), whereas stem mass/total mass increased (Delta1 degrees C: 2.1%). Elevated [CO2] enhanced height growth by 8.8% and decreased coarse root mass/total mass by 10.3% and root/shoot ratio by 11.7%. Additional carbon was mainly invested in aboveground growth. At final harvest a synergistic interaction between elevated [CO2] and temperature yielded trees that were 3.2% taller at -4 degrees C and 12.7% taller at +4 degrees C than trees in ambient [CO2]. After 2.5 seasons, cross-sectional area of the oldest stem part was approximately 32% greater in the +4 degrees C treatment than in the -4 degrees C treatment, and in the last year approximately 67% more leaf area/unit tree ring area was produced in the highest temperature regime compared with the lowest. Elevated [CO2] decreased mean vessel area of the 120 largest vessels per mm2 by 5.8%, causing a decrease in water conducting capacity. There was a positive interaction between temperature and elevated [CO2] for relative vessel area, which was approximately 38% higher at +4 degrees C than at -4 degrees C in elevated [CO2] compared with ambient [CO2]. Overall, temperature had a greater effect on growth than [CO2], but elevated [CO2] caused quantitative changes in wood anatomy.

Wind-driven barotropic circulation off Newfoundland and Labrador

Three-dimensional (3-D) finite element models are used to investigate barotropic wind-driven circulation over the Newfoundland and Labrador Shelf, including its 3-D structure and seasonal and interannual variability, roles of regional vs. remote wind forcing, and importance of the wind-driven circulation. Long-term monthly mean and individual seasonal-mean wind-driven circulation is computed with wind stresses from the NCEP–NCAR reanalysis data (1990–1999) prescribed at the sea surface and corresponding large-scale remote forcing specified at the open boundary determined from a North Atlantic model. The shelf model solutions are discussed for annual-mean circulation features, for seasonal and interannual variations and for relative importance of regional to remote forcing. The model solutions show a shelf-edge current and an inshore current, both of which directed equatorward. The cross-shelf flows and anticyclonic circulation over shallow banks are evident as a consequence of topographic steering. There are prominent seasonal variations in the two branches of the Labrador Current, strongest in December and weakest in July. The interannual changes are also significant in the 1990s, with larger winter transport in the early 1990s and smallest winter transport in 1996. The model shelf-edge Labrador Current is predominated by the remote forcing, while the regional wind forcing plays a significant role in the inshore Current. The model transport for the primary shelf-scale flows is examined against previous model results and in situ observations. Importance of wind-driven currents in the overall circulation is evaluated, which indicates that the wind forcing contributes significantly to the inshore branch of the Labrador Current and less to the shelf-edge branch.

Beginning of the ozone recovery over Europe? − Analysis of the total ozone data from the ground-based observations, 1964−2004

Abstract. The total ozone variations over Europe (~50° N) in the period 1964–2004 are analyzed for detection of signals of ozone recovery. The ozone deviations from the long-term monthly means (1964–1980) for selected European stations, where the ozone observations (by the Dobson spectrophotometers) have been carried out continuously for at least 3–4 decades, are averaged and examined by a regression model. A new method is proposed to disclose both the ozone trend variations and date of the trend turnaround. The regression model contains a piecewise linear trend component and the terms describing the ozone response to forcing by "natural" changes in the atmosphere. Standard proxies for the dynamically driven ozone variations are used. The Multivariate Adaptive Regression Splines (MARS) methodology and principal component analysis are used to find an optimal set of the explanatory variables and the trend pattern. The turnaround of the ozone trend in 1994 is suggested from the pattern of the piecewise linear trend component. Thus, the changes in the ozone mean level are calculated over the periods 1970–1994 and 1994–2003, for both the original time series and the time series having "natural" variations removed. Statistical significance of the changes are derived by bootstrapping. A first stage of recovery (according to the definition of the International Ozone Commission), i.e. lessening of a negative trend, is found over Europe. It seems possible that the increase in the ozone mean level since 1994 of about 1–2% is due to superposition of the "natural" processes. Comparison of the total ozone ground-based network (the Dobson and Brewer spectrophotometers) and the satellite (TOMS, version 8) data over Europe shows the small bias in the mean values for the period 1996–2004, but the differences between the daily ozone values from these instruments are not trendless, and this may hamper an identification of the next stage of the ozone recovery over Europe. Keywords. Atmospheric composition and structure (Middle atmosphere-composition and chemistry) – Meteorology and atmospheric dynamics (Climatology, Middle atmosphere dynamics)

Stratospheric temperatures over the Arctic: Comparison of three data sets

Three sets of stratospheric temperatures (at the 50- and 30-hPa level) are compared for middle and particularly high northern latitudes. Two of the data sets are re-analyses (NCEP/ NCAR and ERA40) while the FU-Berlin data are historical hand-analyses. The time period covered is September 1957 till June 2001, because the FU-Berlin data span this period. As the variability and any trends over the Arctic are of profound interest in connection with Global Change and the ozone problem, it is very important to see if and how these data sets agree and to what extend the early data, before satellites became available, are reliable. Therefore, 30-hPa temperatures over the North Pole and 50-hPa temperatures at 80°N are compared, i.e. long-term monthly means, standard deviations and trends. In such a comparison one has to remember that the natural variability is very high during the arctic winter and that radiosonde stations are not available directly over the North Pole nor over the Arctic Sea. Further, the North Pole point is just one point and one shouldn't expect absolute agreement, particularly as we are dealing with three completely different analysis schemes. Considering these caveats, the agreement of the long-term mean temperatures and of the trends is in general surprisingly good. Of special interest is the agreement in the change of the sign of the trends between the two sub-periods during winter: During the first sub-period cooling is observed in December and warming in March and April. But during the second sub-period warming is observed in December and strong cooling from February till April. Therefore, the overall trend is practically zero. Time series of the original monthly mean 30-hPa temperatures are compared directly for December and March and it is of great interest to see the generally excellent agreement of most of the extremes in the analyses of FUB and NCEP throughout the whole period. This is valid also for ERA40 in the first sub-period (before satellite data became available). This gives confidence in the analyses of the early period.