Rotated Empirical Orthogonal Function Analysis Research Articles

Modes of North Atlantic Western boundary current variability at 36° N

The surface-intensified, poleward-flowing Gulf Stream (GS) encounters the equatorward-flowing Deep Western Boundary Current (DWBC) at 36° N off Cape Hatteras. In this study, daily output from a data-assimilative, high-resolution (800 m), regional ocean reanalysis was examined to quantify variability in the velocity structure of the GS and DWBC during 2017–2018. The validity of this reanalysis was confirmed with independent observations of ocean velocity and density that demonstrate a high level of realism in the model’s representation of the regional circulation. The model’s daily velocity time series across a transect off Cape Hatteras was examined using rotated Empirical Orthogonal Function analysis, and analysis suggests three leading modes that characterize the variability of the western boundary currents throughout the water column. The first mode, related to meandering of the GS current, accounts for 55.3% of the variance, followed by a “wind-forced mode”, which accounts for 12.5% of the variance. The third mode, influenced by the DWBC and upper-ocean eddies, accounts for 7.1% of the variance.

Operational forecasting inundation extents using REOF analysis (FIER) over lower Mekong and its potential economic impact on agriculture

In the Lower Mekong River Basin floodplains, rice cultivation is highly crucial for regional and global food security. However, prolonged flooding can pose damage to rice cultivation and other socio-economic aspects. Yet, there is no rapid operational inundation forecasting system that can help decision-makers proactively mitigate flood damages. Here, we integrated the so-called Forecasting Inundation Extents using Rotated empirical orthogonal function analysis (FIER) framework with an altimetry-based operational Mekong River level forecasting system and built an operational web application, FIER-Mekong, (https://fier-mekong.streamlit.app/) that generates daily skillful forecasted inundation extents (>70% of critical success index) and depths in about 3 and 30 seconds, respectively, with up to 18-day lead times. One of its applications, predicting flood-induced rice economic losses, is also presented. Had FIER-Mekong being adopted, we estimated that the rice damages, up to 87 and 53 million US dollars during the 2020 and 2021 harvest time, respectively, could have been avoided.

Rainfall regimes in Sri Lanka

ABSTRACT Understanding spatio-temporal variation in rainfall is important to water resources planning and management activities. Empirical orthogonal function (EOF) analysis and harmonic analysis are considered state-of-the-art techniques for analysing rainfall variability. This study considered monthly rainfall data from 18 principal stations and 21 secondary stations in Sri Lanka. K-means clustering was applied for identifying rainfall regimes. Rotated EOF analysis provided four key factors related to climate seasons of which the first two factors were more prominent, explaining more than 80% of the total variance. Harmonic analysis showed that northeastern and southwestern parts of the country possess strong annual and semi-annual variation, respectively explaining 90% of the total variance. Both EOF and harmonic analysis can be considered effective tools for identifying rainfall characteristics. However, harmonic analysis results successfully identified seven well-defined rainfall regime characteristics, which could be explained using tendency in data, amplitude, phase angle, and seasonal contribution.

Understanding spatiotemporal variability of drought in recent decades and its drivers over identified homogeneous regions of India

AbstractDrought is a recurring hydroclimatic extreme whose frequency and intensity have increased over India in recent decades, with a detrimental effect on regional water resources. This study addresses the spatiotemporal variability of drought and its plausible mechanism over India from 1951 to 2018. Firstly, six drought‐homogeneous regions are adequately ascertained by applying rotated empirical orthogonal function analysis using the Standardized Precipitation Evapotranspiration Index that captures most of the reported drought and regional hydroclimatic patterns. For the study period, a drying trend is witnessed across the regions though not significant, whereas in recent decades, northeast India (NEI) and some parts of the Indo‐Gangetic plain (IGP) exhibit higher drought frequency. This anomalous drying is attributed to a weakening of the monsoon circulation and accelerated warming caused by changes in the land‐use pattern. The interplay between El Niño Southern Oscillation and the Indian Ocean Dipole largely affects drought's interannual variability, which shows a modified response in recent decades. However, the long‐term decadal drought pattern is found to be strongly teleconnected with a newly discovered Southern Atlantic Oscillation (SAO) index, which reveals a statistically quite significant relationship (>50%) with drought variability. The positive phase of the SAO index is generally associated with drought across the regions except for IGP and NEI. Despite the recent overall wetting trend, drought frequency has enhanced over most of the regions, modulating the regional hydrological cycle.

Regionalization based on spatial and seasonal variation in ground-level ozone concentrations across China

Owing to the vast territory of China and strong regional characteristic of ozone pollution, it's desirable for policy makers to have a targeted and prioritized regulation and ozone pollution control strategy in China based on scientific evidences. It's important to assess its current pollution status as well as spatial and temporal variation patterns across China. Recent advances of national monitoring networks provide an opportunity to insight the actions of ozone pollution. Here, we present rotated empirical orthogonal function (REOF) analysis that was used on studying the spatiotemporal characteristics of daily ozone concentrations. Based on results of REOF analysis in pollution seasons for 3years' observations, twelve regions with clear patterns were identified in China. The patterns of temporal variation of ozone in each region were separated well and different from each other, reflecting local meteorological, photochemical or pollution features. A rising trend in annual averaged Eight-hour Average Ozone Concentrations (O3-8hr) from 2014 to 2016 was observed for all regions, except for the Tibetan Plateau. The mean values of annual and 90 percentile concentrations for all 338 cities were 82.6±14.6 and 133.9±25.8μg/m3, respectively, in 2015. The regionalization results of ozone were found to be influenced greatly by terrain features, indicating significant terrain and landform effects on ozone spatial correlations. Among 12 regions, North China Plain, Huanghuai Plain, Central Yangtze River Plain, Pearl River Delta and Sichuan Basin were realized as priority regions for mitigation strategies, due to their higher ozone concentrations and dense population.

Investigation of medium-term barred beach behavior using 28-year beach profile data and Rotated Empirical Orthogonal Function analysis

A 28-year beach profile dataset for a stretch of the Hasaki coast in Japan was examined using Rotated Empirical Orthogonal Function (REOF) analysis to investigate the cross-shore variation in the characteristics of beach profile change. The data were obtained weekly, on a micro-tidal wave-dominated intermediate beach, along a survey line extending from the backshore to a water depth of approximately 5m. REOF analysis using the first eight empirical orthogonal functions led to the study area being divided into five unique zones based on beach profile change patterns, namely the backshore, the foreshore, the inner and outer transition zones and the bar-trough zone. Although these zones were notably distinct from one another, the profiles in foreshore and the shoreward part of the inner transition zone changed in the same way over periods of 6 and 12months.

Spatial distribution, seasonal variation and regionalization of PM2.5 concentrations in China

In order to provide scientific support to policy makers in the regulation of PM2.5 pollution in China, it is important to accurately assess the current status, spatiotemporal characteristics and regionalization data for this air pollutant. An analysis of the pollution status of PM2.5 was conducted using daily averaged mass concentration data recorded in 74 cities in 2013 and 161 cities in 2014. The rotated empirical orthogonal function (REOF) method was applied to analyze this data. Results showed that the average annual PM2.5 concentration in urban areas of China is 62.2±21.5 µg/m3, and that the distribution is spatially heterogeneous. The North China Plain, middle and lower Yangtze River Plain, Si Chuan Basin and Guanzhong Plain had relatively high annual PM2.5 concentrations compared with the southeast coastal region, the Tibetan Plateau and the Yungui Plateau. PM2.5 mass concentrations tended to be higher in winter than in summer, however, the data for many cities showed a small peak in concentrations from May to July. An analysis of the spatial correlation of PM2.5 indicated a significant influence of topographic conditions. A lower correlation was observed where terrain features varied greatly. Based on the results of the REOF analysis and topographic characteristics, ten regions were identified in mid-eastern China, which could be considered as basic pollution prevention divisions for PM2.5; these include the North China Plain region, Pearl River Delta region, Jianghuai Plain region, middle Yangtze River Plain region, Northeast Plain region, Jiangnan coastal region, Si chuan Basin region, Qiantao Plain region, Guanzhong-Central Plain region and Yungui Plateau region. Seasonal variations in the regionalization data were observed, especially for the North China Plain and Pearl River Delta regions. Among the ten regions identified in this study, the North China Plain, Guanzhong-Central Plain, middle Yangtze River Plain and Jianghuai Plain had relatively high PM2.5 mass concentrations in comparison with the others. Therefore, these regions should be considered as the key regions to target in developing PM2.5 pollution prevention strategies. This study improves the present understanding of the spatial distribution, seasonal changes and regional status of PM2.5 pollution in China and helps establish possible control strategies for the reduction of this air pollutant.

Spatial and temporal variability of drought in the arid region of China and its relationships to teleconnection indices

Summary We studied the drought patterns in the arid region of northwestern China between 1960 and 2010 using the Palmer Drought Severity Index (PDSI). The general evolution of drought was obtained by empirical orthogonal function (EOF), rotated empirical orthogonal function (REOF), the Mann–Kendall test, and the continuous wavelet transform method. Additionally, relationships between rotated principal component time series (RPCs) and seven selected climate indices were analyzed. The results showed that: (1) Four moisture-related spatial patterns (North Xinjiang, western South Xinjiang, Central Xinjiang, and the Hexi Corridor) were objectively defined by REOF analysis. These patterns are related to distinct geographical areas and are associated with distinct temporal variations. (2) The PDSI increased significantly in most regions of Xinjiang, while decreased in the eastern Hexi Corridor. The significant 4–8 year band is the major period band for the annual and seasonal PDSI derived. (3) The seasonal REOFs (RPCs) and EOFs (PCs) have consistent spatial distribution patterns with the annual REOF. The seasonal trends of PDSI are also the same as the annual PDSI trends, indicating space–time consistency between annual PDSI and seasonal PDSI. (4) The drought evolution in this region is affected by the area of northern hemisphere polar vortex, the Arctic Oscillation, and the North Atlantic Oscillation. In addition, the changes of drought in South Xinjiang and the Hexi Corridor may also be associated with the Tibetan Plateau High. Changes in drought pattern are expected to have a strong impact on the economic livelihood of the region, especially for agricultural production.

Interdecadal Variability of the South American Precipitation in the Monsoon Season

AbstractInterdecadal variability modes of monsoon precipitation over South America (SA) are provided by a continental-scale rotated empirical orthogonal function analysis, and their connections to well-known climatic indices and SST anomalies are examined. The analysis, carried out for austral spring and summer, uses a comprehensive set of station data assembled and verified for the period 1950–2000. The presented modes are robust, consistent with previous regional-scale studies and with modes obtained from longer time series over smaller domains. Opposite phases of the main modes show differences around 50% in monthly precipitation. There are significant relationships between the interdecadal variability in spring and summer, indicating local and remote influences. The first modes for both seasons are dipole-like, displaying opposite anomalies in central-east and southeast SA. They tend to reverse polarity from spring to summer. Yet the summer second mode and its related spring fourth mode, which affect the core monsoon region in central Brazil and central-northwestern Argentina, show similar factor loadings, indicating persistence of anomalies from one season to the other, contrary to the first modes. The other presented modes describe the variability in different regions with great monsoon precipitation. Significant connections with different combinations of climatic indices and SST anomalies provide physical basis for the presented modes: three show the strongest connections with SST-based indices, and two have the strongest connections with atmospheric indices. However, the main modes show connections with more than one climatic index and more than one oceanic region, stressing the importance of combined influence.

Evaluation of observed and simulated teleconnections over the Euro-Atlantic region on the basis of partial least squares regression

An interesting topic in climate research is to determine the variability of teleconnection patterns under warming conditions. The North Atlantic Oscillation (NAO), the East Atlantic, the East Atlantic-West Russian and the Scandinavian (SCAND) patterns are the most important teleconnection patterns affecting Europe. Results associated with traditional methodologies for capturing these patterns, such as conventional and rotated empirical orthogonal function analysis, are difficult to intercompare when using different datasets. Therefore, we employed the method of partial least squares (PLS) regression to find a plausible representation of the teleconnections using the standard set of teleconnection patterns defined by National Oceanic and Atmospheric Administration’s Climate Prediction Center as a reference. The variability and trend of the teleconnection indices and patterns over the twentieth and twenty-first centuries were investigated for 20C3M and SRES A1B experiments from the third phase of the Coupled Model Intercomparison Project (CMIP) and compared with the twentieth-century reanalysis data. The results of this study show a positive trend for the NAO and a negative trend for the SCAND under a future climate scenario. With this study, we were able to extract consistent teleconnection patterns across different models, demonstrating the usefulness of the PLS regression in evaluating models and establishing the basis for future work using the fifth phase of CMIP data to assess atmospheric circulation trends and causes of regional climate change.

Pattern-based statistical downscaling of East Asian Summer Monsoon precipitation

ABSTRACTThis study identifies daily Meiyu-like East Asian Summer Monsoon patterns that are linked to precipitation observations in the Poyang lake catchment. This analysis provides insight into the dynamics of strong, local precipitation events and has the potential to improve projections of precipitation from coarse-grid numerical simulations. Precipitation observations between 1960 and 1999 are taken from 13 rain gauges located in the Poyang lake catchment, which is a sub-catchment of the Yangtze River. The analysis shows that the observations are linked to daily patterns of relative vorticity at 850 hPa (Vo850) and vertical velocity at 500 hPa (W500) taken from the ERA-40 re-analysis data set. The patterns are derived by two approaches: (a) empirical orthogonal function (EOF) analysis and (b) rotated EOF analysis. Vo850 and W500 refer to geostrophic and ageostrophic processes, respectively. A logistic regression connects the large-scale dynamics to the local observations, whereby a forward regression selects the patterns best suited as predictors for the probability of exceeding thresholds of 24 h accumulated rainfall at the gauges. The regression model is verified by cross-validation.The spatial structure of the detected patterns can be interpreted in terms of well-known meso-α-scale disturbances called Southwest vortices. Overall, the proposed EOF and rotated EOF patterns are both related to physical processes and have the potential to work as predictors for exceedance rates of local precipitation in the Poyang catchment.

An Evaluation of Rotated EOF Analysis and Its Application to Tropical Pacific SST Variability

Abstract As an effective eigen method for phenomenon identification and space reduction, empirical orthogonal function (EOF) analysis is widely used in climate research. However, because of its orthorgonality constraint, EOF analysis has a tendency to produce unphysical modes. Previous studies have shown that the drawbacks of EOF analysis could be partly alleviated by rotated EOF (REOF) analysis, but such studies are always based on specific cases. This paper provides a thorough statistical evaluation of REOF analysis by comparing its ability with that of EOF analysis in reproducing a large number of randomly selected stationary modes of variability. The synthetic experiments indicate that REOF analysis is overwhelmingly a better choice in terms of accuracy and effectiveness, especially for picking up localized patterns. When applied to the tropical Pacific sea surface temperature variability, REOF and EOF analyses show obvious discrepancies, with the former making much better physical sense. This challenges the validity of the so-called sea surface temperature cooling mode and the spatial structure of “El Niño Modoki,” both of which are recently identified by EOF analysis. At any rate, one has to be cautious when claiming new discoveries of climate modes based on EOF analysis alone.

Understanding space–time patterns of groundwater system by empirical orthogonal functions: A case study in the Choshui River alluvial fan, Taiwan

Summary Natural or anthropogenic activities contribute to changes of groundwater levels in space and time. Understanding the major and significant driving forces to changes in space–time patterns of groundwater levels is essential to groundwater management. This study analyzes monthly observations of piezometric heads from 66 wells during 1997–2002 located in the Choshui River alluvial fan of Taiwan, where groundwater has been the important local water resource for myriads of agricultural or industrial demands. Following spatiotemporal estimations of piezometric heads by Bayesian Maximum Entropy method (BME), this work performs rotated empirical orthogonal function (REOF) analysis to decompose the obtained space–time heads into a set of spatially distributed empirical orthogonal functions (EOFs) and their associated uncorrelated time series. Results show that the leading EOFs represent the most significant driving forces to spatiotemporal changes of groundwater levels in the Choshui River aquifer. These include rainfall recharges from upstream Choshui and Pei-Kang River, pumping activities from aquaculture usages in the coastal areas, as well as water exchanges between surface and subsurface flow of Choshui River. In summary, this study shows the strength of the REOF analysis which can effectively provide integrative views of spatiotemporal changes of groundwater, gaining insights of interactions between the groundwater system and other natural and human activities.

The role of the South Indian and Pacific oceans in South American monsoon variability

This work has investigated the impact of three different low-frequency sea surface temperature (SST) variability modes located in the Indian and the Pacific Oceans on the interannual variability of the South American Monsoon System (SAMS) using observed and numerical data. Rotated Empirical Orthogonal Function (REOF) analysis and numerical simulations with a General Circulation Model (GCM) were used. One of the three SST variability modes is located close to southeastern Africa. According to the composites, warmer waters over this region are associated with enhanced austral summer precipitation over the sub-tropics. The GCM is able to reproduce this anomalous precipitation pattern, simulating a wave train emanating from the Indian Ocean towards South America (SA). A second SST variability mode was located in the western Pacific Ocean. REOF analysis indicates that warmer waters are associated with drought conditions over the South Atlantic Convergence Zone (SACZ) and enhanced precipitation over the sub-tropics. The GCM indicates that the warmer waters over Indonesia generate drought conditions over tropical SA through a Pacific South America-like (PSA) wave pattern emanating from the western Pacific. Finally, the third SST variability mode is located over the southwestern South Pacific. The composites indicate that warmer waters are associated with enhanced precipitation over the SACZ and drought conditions over the sub-tropics. There is a PSA-like wave train emanating from Indonesia towards SA, and another crossing the Southern Hemisphere in the extra-tropics, probably associated with transient activity. The GCM is able to reproduce the anomalous precipitation pattern, although it is weaker than observed. The PSA-like pattern is simulated, but the model fails in reproducing the extra-tropical wave activity.

Ocean Model Diagnosis of Low-Frequency Climate Variability in the South Atlantic Region

Abstract South Atlantic Ocean variability is investigated by means of an ocean general circulation model (ORCA2), forced with the NCEP–NCAR reanalyses for the 1948–99 period. A rotated EOF analysis of the mixed layer temperature suggests a breakdown of the South Atlantic into the following four subdomains, with characteristic spatial and temporal scales: (a) the tropical Atlantic, with mainly interannual fluctuations; (b) the northeastern subtropics, with variability on an interannual to decadal scale; (c) the midlatitudes, with interannual and multidecadal variability; and (d) the southwestern subtropics/midlatitudes with a mixture of interannual and decadal variability. These modes are closely connected to anomalous atmospheric circulation patterns, which induce typical forcing mechanisms for each region. Temperature changes in the western to central Tropics are found to be driven by changes in surface heat fluxes and the horizontal advection of heat, while in the central to eastern Tropics and the northern Benguela region temperature changes are connected to reduced vertical entrainment, altering the depth of the mixed layer and leading to reduced upwelling. In the western and eastern subtropics, changes in the net surface fluxes drive the upper-ocean temperature anomalies, and wind-induced vertical mixing dissipates them, inducing changes in the depth of the mixed layer. Anomalous heat and volume transports are found to be related to anomalous Ekman and geostrophic currents in the eastern subtropics. A wind-driven mechanism is suggested, whereby changes in Ekman-related heat and volume transport lead to modulations of the subtropical gyre and thus to changes in the geostrophic-related heat and volume transport. Temporal variability in the midlatitudes is mainly due to horizontal advection and wind-induced vertical mixing, whereby geostrophic advection of heat dominates in the western to central area, and Ekman-induced heat transports are confined to the eastern midlatitudes.

Reconstruction of long-term winter streamflow in the Gulf of California continental watershed

Applying multiple regression techniques to tree-ring series from Sierra de la Laguna in Baja California Sur, Mexico and Sierra Madre Occidental in Durango, and Tlaxcala, Mexico (A.D. 1384–1997), reconstruction of winter streamflow was performed for data from the central and southern regions of the Gulf of California continental watershed. Each region was defined after applying Varimax Rotated Empirical Orthogonal Function analysis. Streamflow reconstruction was possible from 1840 to 1992 for the central region and from 1712 to 1993 for the southern region. The power spectrums of the reconstructed streamflow series and the Index of the Pacific Decadal Oscillation series (IPDO) show evidence of low frequency (decadal-scale) variations around 40 and 80 years. Tendencies in reconstructed streamflow series and in the IPDO are analyzed and discussed. We conclude that streamflow reconstruction was attainable on a decadal temporal scale. In this sense, the IPDO may be considered a good indicator of future tendencies (in decades) in winter streamflow in the study zone.

Extended Reconstruction of Global Sea Surface Temperatures Based on COADS Data (1854–1997)

Abstract A monthly extended reconstruction of global SST (ERSST) is produced based on Comprehensive Ocean–Atmosphere Data Set (COADS) release 2 observations from the 1854–1997 period. Improvements come from the use of updated COADS observations with new quality control procedures and from improved reconstruction methods. In addition error estimates are computed, which include uncertainty from both sampling and analysis errors. Using this method, little global variance can be reconstructed before the 1880s because data are too sparse to resolve enough modes for that period. Error estimates indicate that except in the North Atlantic ERSST is of limited value before 1880, when the uncertainty of the near-global average is almost as large as the signal. In most regions, the uncertainty decreases through most of the period and is smallest after 1950. The large-scale variations of ERSST are broadly consistent with those associated with the Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST) reconstruction produced by the Met Office. There are differences due to both the use of different historical bias corrections as well as different data and analysis procedures, but these differences do not change the overall character of the SST variations. Procedures used here produce a smoother analysis compared to HadISST. The smoother ERSST has the advantage of filtering out more noise at the possible cost of filtering out some real variations when sampling is sparse. A rotated EOF analysis of the ERSST anomalies shows that the dominant modes of variation include ENSO and modes associated with trends. Projection of the HadISST data onto the rotated eigenvectors produces time series similar to those for ERSST, indicating that the dominant modes of variation are consistent in both.

Temporal and spatial variability of precipitation in Sweden and its link with the large-scale atmospheric circulation

The main characteristics of spatial and temporal variability of the precipitation regime inSweden were studied by using the long-term monthly precipitation amount (1890–1990) at 33stations. The data were filtered by using Empirical Orthogonal Function (EOF) analysis, whichprovides principal modes of both spatial variability and time coefficient series describing thedominant temporal variability. Canonical correlation analysis (CCA) was used to reveal associationbetween the atmospheric circulation and the characteristics of the climate variability.Statistically significant upward shifts in the mean precipitation have been found during coldmonths (March, September, November and December) and only a downward shift (less significant)for August. Simultaneous changes in the time series associated to the optimally correlatedcirculation patterns were found, indicating an important role of the circulation. The circulationpatterns are given by the North Atlantic Oscillation (NAO) in March and December and acyclonic structure centred over southern Scandinavia in September and November. Thesechanges may have induced changes in the mean precipitation seasonality reflected by a shift ofthe maximum precipitation from August to July (after 1931 for western part and after 1961 forthe southeastern coast) and after 1961 to September, October or November for other regions.Combining rotated EOF analysis with cluster analysis, 4 regions with similar climate variabilitywere objectively identified. For these regions the standardised monthly precipitation anomalieswere computed. The frequency of the extreme events (very dry/wet and dry/wet months) over5-year consecutive intervals was analysed. It has been concluded that extreme wet months weremore frequent than extreme dry months over the entire country, especially in the northern andsoutheastern part.

Wintertime Low-Frequency Weather Variability in the North Pacific–American Sector 1949–93

Abstract A study of the impact of ENSO in the Hadley Centre’s atmospheric climate model HADAM1 is presented, with emphasis on the North Pacific–American (NPA) sector. The study is based both on observational data and an ensemble of six integrations for the period 1949–93, forced with observed global sea-ice and sea surface temperature data. The model is shown to reproduce most of the known features of the worldwide atmospheric response to ENSO in boreal winter (January–March). Focusing on the NPA sector, the leading modes of low-frequency weather variability in the winter season are identified on their natural timescales for both the modeled and observed atmospheres. These modes are analyzed via rotated EOF analysis of daily 500-hPa height data, filtered to remove synoptic timescale variations. The model gives a reasonably skillful simulation of the main features of the four leading modes in the NPA region:the Pacific–North American (PNA), the west Pacific (WP), the east Pacific (EP), and the North Pacifi...

過去30年間(1964-1993)における北半球の地表面および下部対流圏気温の長期傾向と10年スケールの変動

In this study, we demonstrate remarkable warming trends in northern winter and spring not only at the surface but also in the lower troposphere during a recent 30 year period (1964-93). In winter, major warming has happened over central Siberia and the northwestern Canada/Alaska region. The vertical structure of the warming tendency is somewhat different between these two regions. In spring, tropospheric warming is most noticeable over the northern part of North America. A three-dimensional rotated EOF analysis of the temperature anomaly field in these seasons confirmed the warming trends as the most dominant mode at the surface and in the lower troposphere.The rotated EOF analysis also shows that an interdecadal-scale fluctuation with abrupt changes in the periods of 1976/77 and 1988/89 is another dominant mode in the temperature field in winter. The spatial structure of this mode shows a wavenumber-three spatial pattern with positive factorloadings over North America, Northern Europe and the eastern part of Eurasia. In spring, a fluctuation of 10-13 year period is noticed in the second largest mode, which suggests a possible relation with the solar cyle of the same period. Possible forcing factors of these tropospheric temperature trends and fluctuations are briefly discussed.