Showing papers in "International Journal of Climatology in 1996"

Resistant, Robust and Non-Parametric Techniques for the Analysis of Climate Data: Theory and Examples, Including Applications to Historical Radiosonde Station Data

Abstract: Basic traditional parametric statistical techniques are used widely in climatic studies for characterizing the level (central tendency) and variability of variables, assessing linear relationships (including trends), detection of climate change, quality control and assessment, identification of extreme events, etc. These techniques may involve estimation of parameters such as the mean (a measure of location), variance (a measure of scale) and correlatiodregression coefficients (measures of linear association); in addition, it is often desirable to estimate the statistical significance of the difference between estimates of the mean from two different samples as well as the significance of estimated measures of association. The validity of these estimates is based on underlying assumptions that sometimes are not met by real climate data. Two of these assumptions are addressed here: normality and homogeneity (and as a special case statistical stationarity); in particular, contamination from a relatively few ‘outlying values’ may greatly distort the estimates. Sometimes these common techniques are used in order to identify outliers; ironically they may fail because of the presence of the outliers! Alternative techniques drawn from the fields of resistant, robust and non-parametric statistics are usually much less affected by the presence of ‘outliers’ and other forms of non-normality. Some of the theoretical basis for the alternative techniques is presented as motivation for their use and to provide quantitative measures for their performance as compared with the traditional techniques that they may replace. Although this work is by no means exhaustive, typically a couple of suitable alternatives are presented for each of the common statistical quantitiedtests mentioned above. All of the technical details needed to apply these techniques are presented in an extensive appendix. With regard to the issue of homogeneity of the climate record, a powerfd non-parametric technique is introduced for the objective identification of ‘change-points’ (discontinuities) in the mean. These may arise either naturally (abrupt climate change) or as the result of errors or changes in instruments, recording practices, data transmission, processing, etc. The change-point test is able to identify multiple discontinuities and requires no ‘metadata’ or comparison with neighbouring stations; these are important considerations because instrumental changes are not always documented and, particularly with regard to radiosonde observations, suitable neighbouring stations for ‘buddy checks’ may not exist. However, when such auxiliary information is available it may be used as independent confirmation of the artificial nature of the discontinuities. The application and practical advantages of these alternative techniques are demonstrated using primarily actual radiosonde station data and in a few cases using some simulated (artificial) data as well. The ease with which suitable examples were obtained from the radiosonde archive begs for serious consideration of these techniques in the analysis of climate data.

Calculating regional climatic time series for temperature and precipitation: Methods and illustrations

Abstract: Various methods for combining station temperature and precipitation time series into regional series are examined. Interpolation of the station series on to regular grid-boxes of some kind reduces the effects of both spatial and temporal changes in station coverage. Regional time series are best produced by using anomaly or standardized anomaly values rather than the raw values. For temperature, and for spatially coherent regions in terms of precipitation variance, the exact method does not seriously affect the resulting time series, provided anomalies are used, although the magnitudes of trends may differ. For regions with large spatial variations in precipitation variance, the additional step of standardizing the anomaly values is recommended. Both anomaly and standardized anomaly series can be easily transformed back to the original units, although the exact method for doing so can alter the resulting time series in non-trivial ways.

Spatial and temporal analysis of annual rainfall variations in turkey

Abstract: This paper presents the spatial and temporal characteristics of Turkey's annual rainfall data in the context of climatic variability. Basic data consists of the monthly rainfall totals from 91 stations with a record length ranging from 54 to 64 years, during the period 1930–1993. Basic elements of the rainfall climatology have been examined and then normalized rainfall anomaly series have been analysed for long-term trend and fluctuation and changes in runs of dry and wet years, for all of Turkey, rainfall regime regions, and individual stations. All regional mean rainfall anomalies have tended to vary in a statistically coherent manner over the rainfall regions. Area-averaged annual rainfall series have decreased slightly over all of Turkey and apparently over the Black Sea and Mediterrane an rainfall regions. Results of the Mann–Kendall test, however, have indicated that none of the decreasing trends in the area-averaged rainfall series were significant. Annual series of 17 stations showed a significant trend in the mean and a majority of these trends are downward. Many of the stations have experienced marked low-frequency fluctuations in the annual rainfall. The concurrences of the dry conditions between the rainfall regions and the rest of Turkey have appeared generally during the early 1930s, the late 1950s, the early 1970s, around the 1980s and the early 1990s and of wet conditions generally during the period 1935–1945, around the 1960s and the late 1970s. The change points for the beginning of drier than normal conditions occurred in the early 1970s and early 1980s over most of the country. Annual rainfall variations over the rainfall regions except the Black Sea region were related closely to those of the rest of Turkey.

A new spatial synoptic classification: application to air-mass analysis

Abstract: Despite recent advances in the classification of synoptic-scale events, there remains the need for development of a simple, automated, continental- scale air mass-based procedure. We present a new method of analysis to identify resident air masses at first-order weather stations to facilitate intersite comparison of daily air masses across a very large region. The ‘spatial synoptic classification’ (SSC) requires initial identification of the major air masses and their typical meteorological characteristics at each site. ‘Seed days’, which exhibit these typical characteristics for each air mass, are used as input for a linear discriminant function analysis, which produces a daily categorization of air masses with spatially continuous results. A second discriminant function analysis is used to determine whether a day is to be considered transitional between air masses. Using the SSC, air-mass frequencies were developed for winter across the USA east of the Rockies, and rates of modification were determined as each air mass traversed the region. The impact of snow cover on modification rates was determined by comparing two winter months with very different snow cover characteristics across the area. Keeping other factors constant, it appears that extended snow cover can reduce the temperature of a dry polar air mass by up to 11°C.

An intercomparison of computer‐assisted circulation classification methods

Abstract: Five different methods that have been used for classification of circulation patterns (correlation method, sums-of-squares method, average linkage, K-means, and rotated principal component analysis) are examined as to their ability to detect dominant circulation types. The performance of the methods is evaluated according to the degree of meeting the following demands made on the groups formed: The groups should (i) be consistent when pre-set parameters are changed, (ii) be well separated both from each other and from the entire data set, (iii) be stable in space and time, and (iv) reproduce the predefined types. All the methods proved to be capable of yielding meaningful classifications. None of them can be thought of as the best in all aspects. Which method to use will depend mainly on the aim of the classification. Nevertheless, the principal component analysis is most successful in reproducing the predefined types and is therefore considered as the most promising method among those examined.

Global distribution of plant‐extractable water capacity of soil

Abstract: Plant-extractable water capacity of soil is the amount of water that can be extracted from the soil to fulfill evapotranspiration demands. It is often assumed to be spatially invariant in large-scale computations of the soil-water balance. Empirical evidence, however, suggests that this assumption is incorrect. In this paper, we estimate the global distribution of the plant-extractable water capacity of soil. A representative soil profile, characterized by horizon (layer) particle size data and thickness, was created for each soil unit mapped by FAO (Food and Agriculture Organization of the United Nations)/Unesco. Soil organic matter was estimated empirically from climate data. Plant rooting depths and ground coverages were obtained from a vegetation characteristic data set. At each 0.5°×0.5° grid cell where vegetation is present, unit available water capacity (cm water per cm soil) was estimated from the sand, clay, and organic content of each profile horizon, and integrated over horizon thickness. Summation of the integrated values over the lesser of profile depth and root depth produced an estimate of the plant-extractable water capacity of soil. The global average of the estimated plant-extractable water capacities of soil is 8ċ6cm (Greenland, Antarctica and bare soil areas excluded). Estimates are less than 5, 10 and 15 cm—over approximately 30, 60, and 89 per cent of the area, respectively. Estimates reflect the combined effects of soil texture, soil organic content, and plant root depth or profile depth. The most influential and uncertain parameter is the depth over which the plant- extractable water capacity of soil is computed, which is usually limited by root depth. Soil texture exerts a lesser, but still substantial, influence. Organic content, except where concentrations are very high, has relatively little effect.

An air transport climatology for subtropical southern africa

Abstract: An air transport climatology is derived for subtropical southern Africa (Africa south of 15°S) by classifying daily synoptic situations into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentage zonal transport in easterly and westerly directions, levels of transport, and times of transit are derived from forward trajectory analyses using European Centre for Medium- range Weather Forecasts (ECMWF) data for a 7-year period. It is shown that semi-permanent subtropical continental anticyclones, transient mid-latitude ridging anticyclones and mid-latitude westerly disturbances produce major transport into the south-western Indian Ocean in the Natal plume. Only quasi-stationary tropical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a function of circulation type and frequency, as well as plume dimensions. Transport in continental highs follows an annual cycle reaching peak values in excess of 70 per cent in winter. That in easterly waves also exhibits an annual cycle, but one peaking in summer, when up to 55 per cent transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout the year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Monthly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circulation types and from measurements of aerosol concentrations. An annual mass flux of aerosols some 134 Mtons is generated over the subcontinent. About 60 Mtons year−1 are deposited, and approximately 29 Mtons year−1 are exported westward over the Atlantic Ocean and 45 Mtons year−1 eastward over the Indian Ocean. Twenty-six million tons of the 74 Mtons of aerosols exported annually to the adjacent oceans on each coast are a product of recirculation. Deposition within 10° latitude of the coast is nearly 10 times greater on the east than on the west coast.

Circulation and extreme rainfall conditions in the eastern mediterranean during the last century

Abstract: Dry and wet years, seasons, and sequences at four eastern Mediterranean stations, Thessaloniki, Athens, Nicosia, and Jerusalem, were identified and compared for circulation types. The definition of dry/wet year or season was done by means of z scores. A year or a season was defined as dry when z≤− 0ċ5 and as extremely dry when z≤−1ċ5, similarly as wet when z≥ 0ċ5 and as extremely wet when z\≥1ċ5. Dry/wet sequences at each station were identified and their timings were compared. Circulation types were defined by means of zonal and meridional indices calculated independently in two regions. Region 1: ϕ=35° N and 55°N; λ=10°E and 30°E for Thessaloniki and Athens. Region 2: =30°N and 50°N; λ=20 deg;E and 40°E for Nicosia and Jerusalem. Time series of both indices in both regions were calculated and are discussed. Correlations between the z scores of the precipitation with both indices were calculated. Prevailing flows for dry and wet conditions for each of the stations were defined and are discussed.

The impact of mount pinatubo on world-wide temperatures

Abstract: We monitor and model the effects on world-wide temperatures of the June 1991 volcanic eruption of Mount Pinatubo in the Philippines. Global mean air temperatures were reduced, by up to 0·5°C at the surface and 0·6°C in the troposphere, for some months in mid-1992, in approximate accord with model predictions. Differences from these predictions occurred in the Northern Hemisphere winters of 1991–1992 and 1992–1993, as a result of atmospheric circulation changes that yielded continental surface warmings not fully reproduced by the model. The effects of the eruption were less evident by 1994. A superposed-epoch composite for five major tropical eruptions shows significant global post-eruption cooling at the surface when the effects of the El Ni7o–Southern Oscillation are removed from the data. Stratospheric warmth following Pinatubo lasted until early 1993 according to Microwave Sounding Unit data.

On the relationship between eurasian snow cover and the asian summer monsoon

Abstract: In this paper, we document the possible linkage between Eurasian snow cover and the Asian summer monsoon by analysing the National Oceanic and Atmospheric Administmtion snow data and the atmospheric circulation fields from the National Meteorological Center analyses. Using updated data, we confirm the inverse relationship between the Eurasian snow cover and Indian monsoon rainfall reported in previous studies. We find that the relationship becomes stronger in the partial correlation calculations excluding the El Niiio years. It is found by comparing years of more snow cover with years of less snow cover that the atmosphere is colder over land and warmer over the oceans during the winter seasons with more snow. During the summer following the winter with more snow cover, the lower atmosphere is colder over the Asian continent, especially north of India. Associated with these temperature changes, a weaker Asian monsoon circulation appears. KEY WORDS: Eurasian snow cover; Asian monsoon; atmospheric circulation; interannual variability

Enso–snow–monsoon associations and seasonal–interannual predictions

Abstract: Relationships between the El Nino–Southern Oscillation (ENSO), Eurasian winter snow cover (EWSC), and Indian summer monsoon rainfall (ISMR) are studied. The information provided may be useful for improving seasonal–interannual predictions of the coupled ocean–atmosphere–land system. Results show that more extensive EWSC occurs during El Nino winters rather than La Nina winters. A precursory signal of snow variability can be found in the Southern Oscillation Index. An inverse relationship between EWSC and ISMR, reported in previous studies, is generally confirmed by this study. However, an important finding is that the general snow–monsoon relationship is disrupted by El Nino events. Considering both ENSO's direct impact on ISMR and its role in the break-down of the EWSC–ISMR relationship, it may be reasonable to say that ENSO plays a more important role than EWSC in influencing the variability of the Indian monsoon.

Trends of Maximum and Minimum Daily Temperatures in Central and Southeastern Europe

Abstract: Changes in maximum and minimum daily temperatures (TMAX and TMIN, respectively) in nine selected regions of central Europe and in Bulgaria during 1951–1990 are investigated. Average series for central Europe are compiled and analyzed by linear trend analysis and the kernel smoothing. The increase in the annual TMAX in central Europe was, during 1951–1990, slightly lower than that of TMIN (0ċ52°C and 0ċ60°C, respectively). This results in a small decrease in the daily temperature range (DTR) by −0ċ08°C. With the exception of the spring TMIN other linear trends are insignificant. The observed insignificant trends in DTR in the central European region are related to small cloudiness changes. Long-term fluctuations of annual TMAX, TMIN, and DTR for eight selected series during the twentieth century are also investigated.

The spatial response of the climate system to explosive volcanic eruptions

Abstract: Determining the spatial response of the climate system to volcanic forcing is of importance in the development of short-term climate prediction and in the assesment of anthropogenic factors such as global warming. The June 1991 eruption of the Phillippine volcano, Mount Pinatubo, provides an important opportunity to test existing understanding and extend previous emperical analyses of volcanic effect. We identify the spatial climate response to historic eruptions in the surface air temperature and mean-sea- level pressure record and use this information to assess the impact of the Pinatubo eruption. The Pinatubo eruption clearly generated significant global cooling during the years after the event. The magnitude and timing of the cooling is similar to that associated with previous equatorial eruptions. There is good agreement between the spatial patterns of tempurature and circulation anomalies associated with the historic eruptions and those following the Mount Pinatubo event. Evidence of limited higher latitude warming and a major change in the atmospheric circulation is found over the Northern Hemisphere during the first winter after the equatorial eruptions analysed, followed by widespread cooling, but limited change in the atmosphere circulation, during the subsquent 2 years.

The antarctic temperature inversion

Abstract: In the interior of the Antarctic ice sheet the surface temperature inversion averages over 25°C in the winter months. The negative buoyancy of the near-surface air drives the katabatic windflow, which has important consequences for the climate of Antarctica. Radiosonde measurements of the inversion are combined with recent GCM results in an attempt to assess the accuracy of proposed connections between the surface temperature and the inversion strength by comparing the limited observational verification data with the much wider coverage that a climate model allows. This indicates that, using multi-annual data, the continent-wide RMS error of deducing the inversion strength from a regression technique is approximately 2ċ9°C, whereas using a method based upon differences between summer and winter temperaures has a RMS error of approximately 2ċ5°C.

Rainfall variability over bangladesh and nepal: comparison and connections with features over india

Abstract: In this study monthly rainfall data for 14 stations over Bangladesh for the period 1901-1977 are used to investigate and understand the interannual variability of the summer monsoon rainfall. Monthly, seasonal, and annual spatial rainfall patterns, and the spatial patterns of variability, are presented. Dominant structures of seasonal rainfall are determined through the empirical orthogonal functions. A homogeneous series for All Bangladesh Monsoon Rainfall is prepared and its temporal characteristics are studied. It is observed that the standardized rainfall for this series shows random fluctuations up to 1963, thereafter the standardized values are much above the normal values. Further the rainfall variations over Bangladesh are not related to large-scale variables such as the Northern Hemisphere surface temperature, Darwin pressure tendency, and the subtropical ridge over the Indian region. However, the rainfall variations over Bangladesh are related well with rainfall variations over north-east India. Similar analysis is done for the Nepal region by examining the monthly rainfall data over Kathmandu for a 105 year period (1851-1900, 1921-1975). Results reveal that Nepal rainfall is well related with rainfall variations over northern and central parts of India.

North–south shifts of the gulf stream: ocean‐atmosphere interactions in the north atlantic

Abstract: Year-to-year changes in the latitude of the north wall of the Gulf Stream are very similar to those seen in the abundances of zooplankton observed by the Continuous Plankton Recorder Survey around the British Isles and also to those in the abundance of zooplankton in Lake Windermere. These connections must reflect changing weather patterns across the North Atlantic. The index of Gulf Stream position was constructed from the north wall data by principal components analysis. The first principal component, the index used, has eigenvector coefficients that all have the same sign, and is a measure of the latitude of the whole of the north wall. However, the component may represent the occurrences of meanders that are extensive in space and time rather than displacements of the Gulf Stream as a whole. This principal component has been used to calculate weighted averages of monthly mean sea-level pressure and of monthly mean numbers of cyclone tracks in order to show the changes in weather patterns associated with displacements of the north wall. Northward displacements of the north wall were accompanied by significantly reduced cyclone numbers in the northernmost regions of the Atlantic (annually and in the autumn) and, in spring, summer, and autumn, a region of reduced atmospheric pressure in the central Atlantic area 40°–60°N, 30°–50°W (locally significant). There was some tendency (not significant) for storm tracks to be deflected around the south side of this region. The pattern in winter is less clear and shows no statistical significance. Changes in the vicinity of the British Isles were generally too small to be statistically significant but were generally consistent with a lower frequency of storms in spring and autumn. As the biological changes appear to be caused by variations in the onset of thermal stratification during the spring they may be the result of relatively small changes in the atmospheric forcing. The atmospheric changes show no indications of the sources of the Gulf Stream displacements, the anomaly winds opposing the displacements. This may be because meanders of the Gulf Stream are not simply related to any single atmospheric variable. The clearest and most statistically significant meteorological signals were all well downstream from the north wall. Although the displacements of the north wall are caused by changing weather patterns over the North Atlantic, the Gulf Stream is also a region of strong heat transfers from the ocean to the atmosphere. Sawyer's criterion indicates that distortions of this heat source could cause noticeable disturbances to the atmospheric circulation over the North Atlantic. A numerical model based on the analytical model of Smagorinsky is used to investigate the perturbations of the zonal circulation that might be caused by displacements of this local heat source. The predictions are in agreement with the changes seen in the central Atlantic during summer, spring, and autumn (but not those during winter). In the region where the model predicts atmospheric pressure reductions should occur, there are no positive correlation coefficients between the position of the north wall and the surface atmospheric pressure but a significant excess of negative correlation coefficients compared with chance, and northward shifts of the Gulf Stream were accompanied by significant reductions in atmospheric pressure. It is therefore possible that displacements of the north wall could influence weather patterns further east. The model predicts that any changes over the European continental shelf will be weak. An accurate description of the dynamics of the Gulf Stream may be an important requirement of coupled ocean–atmosphere models.

Ring-width variations in cedrus deodara and its climatic response over the western himalaya

Abstract: Tree-ring anlaysis of Cedrus deodara from three different sites of western Himalaya has been carried out. The chronologies include 47 cores (26 trees) from Manali, 33 cores (18 trees) from Kufri (Shimla) and 25 cores (13 trees) from Kanasar forest sites. Moderately high values of common variance exhibited by all three chronologies indicate the great potential of the species for dendroclimatic studies. Response function and correlation analyses using the above tree-ring-width data and Shimla climate show a significant negative relationship with summer temperature and positive relationship with summer precipitation. Based on these results, calibration equations have been developed for different periods, and appropriately verified using independent data, to reconstruct the summer (March–April–May) temperature at Shimla. The reconstruction has extended the temperature record of the region back to the eighteenth century.

Regional teleconnection patterns associated with summer rainfall over south africa, namibia and zimbabwe

Abstract: Climatic determinants of southern African summer rainfall are analysed using statistical techniques. Summer rainfall time series are formulated for South Africa, Namibia and Zimbabwe areas and correlated with global indices and with field variables: sea-surface temperature (SST), outgoing longwave radiation (OLR), and tropospheric winds at 200 and 700 hPa levels. Linear regression correlations are performed using monthly standardized departures at various lags before and during the summer season. The SSTs in the central equatorial Indian Ocean (CEI) are identified as significant predictors/modulators of southern African rainfall. The SSTs at the CEI are best correlated with South Africa rainfall at r≶ − 0ċ6 at lags −2 and 0 months and are associated with the El Nino. The SSTs of the CEI modulate the overlying monsoon trough, as indicated by the OLR correlation maps. A centre of convective action alternates between southern Africa and the south-west Indian Ocean from year-to-year. A useful circulation index that emerges in the statistical analysis is spring-time zonal upper wind anomalies over the equatorial central Atlantic. This index is correlated with South Africa rainfall at r≶ −0ċ 8 at lags −4 and −2 months. Westerly (easterly) 200 hPa anomalies in spring are followed by a summer of below (above) normal rainfall. Other patterns that have a bearing on summer rainfall include a circulation gyre identified in 700 hPa wind correlations off the coast of south-east Africa. This circulation feature controls the flux of moisture between southern Africa and the northern Mozambique Channel. The correlation patterns offer statistical guidance in long-range forecasts and insights to climatic processes that govern the interannual variability of summer rainfall over southern Africa.

Temporal and spatial variability of annual precipitation in costa rica and the southern oscillation

Abstract: Annual precipitation totals from over 100 stations in Costa Rica are analysed to provide estimates of the nature of their response to El Niiio-Southern Oscillation (ENSO) events Responses are found to be varied in terms of their signs (droughts or excessive precipitation), magnitudes, and durations The results of simple interstation correlations and lag cross-correlations with the Southern Oscillation Index suggest a marked difference in response in those areas draining towards the Pacific and those towards the Caribbean, as well as latitudinal variations, particularly along the Pacific coast The marked regional differences in statistical properties over so small an area are related to complex physical processes of precipitation generation, the differing provenances of the humidity laden winds, and the fluctuations of local atmosphere-ocean interactions in response to ENSO

Observed changes in maximum and minimum temperatures in turkey

Abstract: The aim of this study is to examine the nature and magnitude of the changes in mean seasonal daily maximum and minimum temperatures and diurnal temperature ranges for all of Turkey, its regions and the 59 stations during the period 1930–1993. Maximum temperatures show significant cooling trends in many stations in summer and autumn. Most of the regional mean maximum temperatures have decreased in all seasons, except spring. None of the observed trends in regions was significant statistically. Mean seasonal minimum temperatures have indicated significant warming trends in many stations in all seasons except winter. Majorities of the regional mean minimum temperatures have also increased markedly in all seasons. Warming trends of spring minimum temperatures were significant over the Black Sea, Aegean and South-eastern Anatolia regions. Spring minimum temperat ures for all of Turkey have shown a significant linear trend, with an increase of +0·145°C per decade. Diurnal temperature ranges have decreased significantly over most of the regions in all seasons, except partly in autumn and in so many stations in all seasons. Most of the decreasing trends in summer were significant at the 1 per cent level. It seems that changes of the diurnal temperature range in spring, summer and autumn were related closely to changes of maximum temperatures. Thus the decrease of the regional mean diurnal range was associated mainly with decreases of maximum temperatures in summer and autumn, and with slightly increased maximum temperatures in spring. Many stations that experienced a significant trend in diurnal ranges had a significant positive serial correlation. Our first results for Ankara have shown no significant trend in the mean monthly cloudiness and monthly cloudiness at 1400 hours local time, except a decrease in May cloudiness at 1400 hours.

An abrupt change in the intensity of the east asian summer monsoon index and its relationship with temperature and precipitation over east china

Abstract: In this study, an East Asian summer monsoon intensity index is defined and the summer monthly indices of the East Asian monsoon are calculated for the period 1873–1989. Studies of the climate change of the index indicate that around 1918 the East Asian monsoon experienced an abrupt change, from weak to strong. This abrupt change occurred synchronously with that of the Northern Hemisphere air temperature. Studies of the relation of this East Asian summer monsoon index to the rainfall and temperature over China by using the data for the recent 40 years shows that when a strong monsoon occurs, China will experience an extensive high temperature in summer. The relationship between the East Asian summer monsoon and China's summer rainfall manifests itself on the north–south position of the rain belt. The position of the summer belt is northward when the monsoon is strong and southward when the monsoon is weak. Wet years over the middle and lower reaches of Chingjiang River are related to a weak monsoon whereas with a strong summer monsoon, dry years are often observed.

GRID BOX OR GRID POINT: WHAT TYPE OF DATA DO GCMs DELIVER TO CLIMATE IMPACTS RESEARCHERS?

Abstract: Simulated data from General Circulation Models (GCM) are an important source of climate data and appropriate interpolation and subsequent interpretation of these data are required when used at the subgrid scales demanded in the study of climate change impacts. Although the techniques used to interpolate observed climate data can also be used to interpolate GCM output, there has been some hesitancy in interpreting subgrid-scale GCM data. We believe that a lack of understanding of this data source contributes to this hesitancy, in particular the spatial nature of GCM data is unclear. Are GCM data point or areal quantities; how does this affect spatial interpol ation; and, how should climate impacts researchers use and interpret subgrid scale GCM data? Above all, it bears remembering that interpolation does not create new information, it only increases the spatial precision of the existing data and it does this at the cost of increasing uncertainty. We believe that one can adopt either an areal or point approach, at least until there is some definitive evidence to support one approach over the other, but users of this data need to interpret simulated climate surfaces in light of whether they have adopted a grid-box or a grid-point approach to GCM data. In particular, the larger the area represented by a GCM value the smoother the resulting climate surface is likely to be and the more different from a similarly constructed observed climate surface. Although the grid-box approach seems more intuitive, at least to us, researchers will not find many commercially available interpolation packages for this type of data.

Synoptic mechanisms associated with snowfall increases to the lee of lakes erie and ontario

Abstract: Snowfall is a cyrospheric variable that impacts nearly every sector of society. Because of its societal importance, snowfall is a logical variable to be used as an indicator of potential global environmental change. This study investigates the mechanisms responsible for large observed snowfall increases across the eastern Great Lakes region of the USA. Results indicate that mean snowfall amounts across sections of western New York and north-western Pennsylvania have increased by up to 100 cm over the 60-year period encompassing the snowfall seasons 1930–1931 through to 1989–1990. A synoptic climatological approach is utilized to identify consistent synoptic-scale atmospheric patterns responsible for snowfall across the region. Nine synoptic types are identified as producing significan t snowfall in the study area; five with synoptic characteristics indicative of lake-effect snowfall and four evidencing characteristics of snowfall associated with cyclonic influence. An examination of the seasonal frequency of the nine synoptic types indicates a substantial increase in the frequency of the five lake-effect synoptic types and a long-term decrease in the numbers of cyclone synoptic types over the period 1950–1951 through to 1981–1982. Information concerning trends in the frequency and the intensity of each of the nine snowfall-producing synoptic types was combined to produce a modelled snowfall change due to frequency and intensity variations over the period. Trends in the frequency and intensity of the synoptic patterns associated with lake- effect snowfall explain the majority of the observed snowfall increase across the region. Variations in the synoptic types associated with cyclonically induced snowfall are shown to be unimportant to snowfall changes across the eastern Great Lakes area. Possible reasons for increases in the frequency and the intensity of the lake-effect synoptic types are discussed.

Climatic change over the mediterranean and links to the more general atmospheric circulation

Abstract: The marine climate over the Mediterranean Sea has been examined for the winter and summer seasons over the period 1946–1987. Little change was found in the winter over this period, apart from some evidence of a decline in the strength of the surface circulation. However, the summer months have seen a general rise in atmospheric pressure over the basin, with strengthening north-westerly winds in the eastern Mediterranean. Changes in the eastern and western basin are frequently out-of-phase because different processes tend to affect the climates of the two basins. These changes have been validated by analysis of the climate at land-based stations around the periphery of the Mediterranean. Combining the marine and terrestrial analyses shows possible links between climate in the eastern Mediterranean, changes in the intensity of the South Asian Monsoon (via reorientation of the summer-time trough over the Middle East), and climatic change over sub-Saharan Africa.

Continental polar outbreaks into the levant and eastern mediterranean

Abstract: Continental polar outbreaks into the Levant and eastern Mediterranean regions originate in vast continental regions over Asia and Europe, causing respective unique weather conditions--cold, dry, and stable. These north-easterly winds have important environmental implications for agriculture, cold stress, and air pollution, which were the motivation for this study. They are exemplified here by statistical and synoptic study of such events over Israel. Seventy-two days of such outbreaks were observed during 6 years (1 983-1 988), all of them between October-May associated with different synoptic patterns. The typical duration of these events was of two categories: short events of 1-2 days, and long events of 4-5 days. The latter are the result of ‘quasistationary’ systems whereas the short ones are due to the passage of disturbances over the eastern Mediterranean. The highest frequency is in December, with 28 days (between 1983 and 1988), compared with 13 days in January and 8 days in February. The decrease in frequency towards February‘can be attributed to changes in the cyclone’s trajectories, and to pressure gradient magnitude between the anticyclone over Asia and Turkey and the lower barometric pressure over the Mediterranean. These events have a typical course of pressure increase preceding the outbreak and during its first 2 days, and then a pressure decrease that is mostly accompanied with temperature increase.

Spatial and temporal variations in the relationships between the southern oscillation phenomenon and the rainfall of sri lanka

Abstract: Changes in temporal and spatial relationships between rainfall of Sri Lanka and the Southern Oscillation Index (SOI) during the 110 year period (1881–1990) are presented. The relationships during the first intermonsoon (FIM) and south-west monsoon (SWM) seasons indicate periods of weak positive and negative correlations. On the basis of major turning points in the summer rainfall over India and Sri Lanka, four distinct epochs were identified which cover the years 1881–1900, 1901–1930, 1931–1960 and 1961–1990. Indian and Sri Lankan summer rainfall shows a weak in-phase relationship from 1881 to 1900, a strong in-phase relationship from 1961 to 1990, and an out-of-phase relationship from 1901 to 1960. The correlation between the SOI and SWM season's rainfall is primarily negative from 1931 to 1960 and positive during other epochs. Strong negative correlations are generally present for the SIM season and strengthening and weakening of negative correlations during this season coincide with changes in the SWM season rainfall and its relationships with the SOI. The north-east monsoon (NEM) season correlations are negative and insignificant. Major changes in spatial patterns of correlations between seasonal rainfall and the SOI have occurred in Sri Lanka during SWM and SIM seasons. The periods of strong positive (negative) correlations during the SWM season coincide with weak (strong) negative correlations during the SIM season. This contrasting pattern is clear when the Indian and Sri Lankan summer monsoon rainfalls were out of phase between 1900 and 1960, but not before 1900, or after 1960. The sudden change in correlations around 1960 suggests a change in the coupled ocean–atmosphere system that dominates the climate of these regions. Changes in temporal and spatial patterns of correlations between the SOI and rainfall have been linked to changes in the location of the active intertropical convergence zone.

Decadal variations of the southern hemisphere circulation

Abstract: A data set of monthly mean surface and sea-level pressure observations from a number of stations in the Southern Hemisphere (SH) for the period since 1901 has been used to investigate interannual and interdecadal variations of the SH circulation. A fairly uniform network of 62 stations was selected with data for the period 1955–1985, with a reduced network of 35 stations having data for the longer period 1901–1985. The sparse network has stations mainly on the SH tropical and mid-latitude land masses, with few islands or high-latitude stations. A rotated principal component analysis of the annual mean station pressure anomalies has been used to describe the dominant modes of interannual and interdecadal variations of the SH circulation that can be resolved with the available station network. We show that the sparse station network is able to capture the dominant modes of variability found in the period 1955–1985 using the denser network. The leading mode is associated with the El Nino–Southern Oscillation and is well resolved by the sparse network. This is the leading mode for interannual and interdecadal variations throughout the last century, but there are periods when it has reduced variance and a somewhat different structure, notably 1916–1935. Other modes of interannual variations show marked changes in importance on decadal or longer time-scales.

Empiriacal study on nimbus‐7 snow mass and indian summer monsoon rainfall

Abstract: In this study the relationship between Nimbus-7 snow mass and Indian summer monsoon rainfall (ISMR) for the period 1979-1987 is examined. Results reveal that the January snow mass over two regions of the former USSR - one located north-east of Moscow and the other lying between Mongolia and Siberia - shows a high inverse relationship with subsequent ISMR. However, the snow cover over Eurasia for the month of April shows a significant relation with ISMR. Implications of these connections in long-range forecasting of ISMR are also examined.

Albedo and depth of melt ponds on sea-ice

Abstract: For a 1-month period during the spring–summer season, over 500 in situ measurements of the albedo, depth and selected properties of melt ponds were made at four sea-ice sites in the Canadian Arctic Archipelago. There are three aims of the investigation: to examine the variation in spectral reflectance of melt ponds in relation primarily to clouds, pond depth, and ice type; to compute and assess with reliability, and for the first time, the statistical properties of pond depth; and to derive pond albedo parameterizations for implementation in sea-ice model simulations. It was found that pond reflectance curves vary significantly with cloud cover, pond depth, and ice type. A simple univariate statistical analysis showed that surface ice morphology controls pond depth. Application of the Kruskal–Wallis test evidenced that differences between first-year ice, multi-year ice, and landfast ice-pond-depth distributions are statistically significant. Lastly, calibrated non-linear regression functions show that visible and near-infrared pond albedos decrease exponentially with depth until some critical depth value is exceeded. The relationship is weakest in the visible spectrum, particularly under cloudy sky conditions.