Showing papers in "International Journal of Climatology in 1995"

A new method for detecting undocumented discontinuities in climatological time series

Abstract: The development of homogeneous climatological time series is a crucial step in examining climate fluctuations and change. We review and test methods that have been proposed previously for detecting inhomogeneities, and introduce a new method we have developed. This method is based on a combination of regression analysis and non-parametric statistics. After evaluation against other techniques, using both simulated and observed data, our technique appears to have the best overall performance.

Climatologically aided interpolation (cai) of terrestrial air temperature

Abstract: A new and relatively straightforward approach to interpolating and spatially averaging air temperature from weather-station observations is introduced and evaluated using yearly station averages taken from the Jones et al. archive. All available terrestrial station records over the period from 1881 through to 1988 are examined. Called climatologically aided interpolation, or CAI, our procedure makes combined use of (i) a spatially high-resolution air-temperature climatology recently compiled by Legates and Willmott, as well as (ii) spatially interpolated yearly temperature deviations (evaluated at the stations) from the climatology. Spherically based inverse-distance-weighting and triangular-decomposition interpolation algorithms are used to interpolate yearly station temperatures and temperature deviations to the nodes of a regular, spherical lattice. Interpolation errors are estimated using a cross-validation methodology. Interpolation errors associated with CAI estimates of annual-average air temperatures over the terrestrial surface are quite low. On average, CAI errors are of the order of 0.8°C, whereas interpolations made directly (and only) from the yearly station temperatures exhibit average errors between 1.3°C and 1.9°C. Although both the high-resolution climatology and the interpolated temperature-deviation fields explain non-trivial portions of the space-time variability in terrestrial air temperature, most of CAI's accuracy can be attributed to the spatial variability captured by the high-resolution (Legates and Willmott's) climatology. Our results suggest that raw air-temperature fields as well as temperature anomaly fields can be interpolated reliably.

Sea-surface temperature — South African rainfall associations, 1910–1989

Abstract: The main features of sea-surface temperature variability in the South Atlantic and the south-west Indian Oceans are identified and their interaction with the Southern Oscillation discussed. Most of the variance is explained by coherent features of variability in the South Atlantic Ocean. Tropical and subtropical features dominate the variance, but this may be partly a reflection of data availability. Many of the principal components are associated with rainfall over southern Africa and the strongest associations occur with sea-surface temperatures in the western equatorial Indian Ocean, the Agulhas system, and the central South Atlantic Ocean.

Construction of a 1961-1990 European climatology for climate change modelling and impact applications

Abstract: A 1961–1990 mean monthly climatology for a ‘greater European’ region extending from 32°W to 66°E and from 25° to 81°N has been constructed at a resolution of 0.5°latitude by 0.5° longitude for a suite of nine surface climate variables: minimum, maximum, and mean air temperature; precipitation totals; sunshine hours; vapour pressure; wind speed; and (ground) frost day and rain day ( > 0.1 mm) frequencies. This climatology has been constructed from observed station data distributed across the region. Station frequencies range from 936 (wind speed) to 3078 (precipitation). Over 95 per cent of these data are based on observations between 1961 and 1990 and over 90 per cent were supplied by individual national meteorological agencies (NMAs) on specific request. For four variables, some standardization of the data had to be performed because different countries supplied data under different definitions. Thus cloud cover had to be converted to sunshine hours, relative humidity to vapour pressure, air frost days to ground frost days and rain days > 1 mm to rain days > 0.1 mm. The interpolation of the station data to the grid used elevation as one of the predictor variables and thus enabled three climate surfaces to be produced for each variable, reflecting the minimum, mean, and maximum elevation within each 0.5° by 0.5° cell. Subsets of stations were used for the interpolation of each variable, the selection being based on optimizing the spatial distribution, source priority and length of record. The accuracy of the various interpolations was assessed using validation sets of independent station data (i.e. those not used in the interpolation). Estimated mean absolute errors (MAE) ranged from under 4 per cent for vapour pressure to about 10 per cent for precipitation and up to 20 per cent for wind speed. The accuracy of the interpolated surfaces for minimum and maximum temperature was between 0.5°C and 0.8°C. We believe these results constitute the first climatology that has been constructed for this extensive European region at such a fine spatial resolution (0.5° by 0.5°) from relatively dense station networks, for three different elevation surfaces and for a wide range of surface climate variables, all expressed with respect to a standard 30-year period. The climatology is already being used by researchers for applications in the areas of ecosystem modelling, climate change impact assessment and climate model validation, and is available from the authors.

Fuzzy rule-based classification of atmospheric circulation patterns

Abstract: A fuzzy rule-based methodology is applied to the problem of classifying daily atmospheric circulation patterns (CP). The purpose of the approach is to produce a semi-automated classification that combines the expert knowledge of the meteorologist and the speed and objectivity of the computer. Rules are defined corresponding to the geographical location of pressure anomalies. A CP is described by the location of four different possible pressure anomalies. The rules are formulated with fuzzy sets, allowing a certain flexibility because slightly different pressure maps may correspond to a given CP. Accordingly the degree of fulfilment of a rule is defined in order to measure the extent to which a pressure map may indeed belong to a CP type. As an output of the analysis, the CP on any given day is assigned to one, and only one, CP type to a varying degree of credibility. The methodology is applied to a European case study. The subjective classification of European CPs given by Hess and Brezowsky provides a basis for constructing the rules. The classification obtained can be used, for example, to simulate local precipitation conditioned on the 700 hPa pressure field. The information content of the fuzzy classification as measured by precipitation-related indices is similar to that of existing subjective classifications. The fuzzy rule-based approach thus has potential to be applicable to the classification of GCM produced daily CPs for the purpose of predicting the effect of climate change on space-time precipitation over areas where only a rudimentary classification exists or where none at all exists.

Climate trends in the South-west Pacific

Abstract: Temperature and precipitation trends are described for newly homogenized historical climate data sets for the South-west Pacific. Regions that exhibit similar temperature and precipitation trends and variability are defined, and the temperature and precipitation time series aggregated according to these regions. Four temperature regions show distinctive trends: two regions south-west of the South Pacific Convergence Zone (SPCZ), which display steady climate warming; two regions north-east of the SPCZ, which cooled during the 1970s, and warmed in the 1980s. Annual anomalies differ in response to the El Nino-Southern Oscillation (ENSO) phenomena, depending on the region's position with respect to a pivotal line along the SPCZ. The climate warming apparent throughout much of the south-west Pacific comes from sites where there can be no question of any urban influence. Five main South-west Pacific precipitation regions show distinctive trends that are connected to the main climatological features. Four New Zealand precipitation subregions relate to the interaction of the main climatological features with local orography. Annual precipitation anomalies show marked variability and are also affected by ENSO in most regions. The pivotal line for the response of precipitation regions lies just to the north-east of the SPCZ. The ENSO relationships with precipitation appear consistent on both annual and interdecadal time-scales. From these climatic trends four climatic response regions are recognized in the South-west Pacific.

Large-scale circulation regimes and surface climatic anomalies over the Mediterranean

Abstract: Significant relationships between atmospheric circulation anomalies, in terms of monthly mean 500 hPa geopotential height (Z500) and sea-level pressure (SLP) over Atlantic-Europe, and surface climatic anomalies, in terms of monthly rainfall (Rm) and monthly mean temperature (Tm), over the extended Mediterranean area, are examined statistically by canonical correlation analysis (CCA), with time lags from zero to four months. At a confidence level of 99.99 per cent, CCA yields five pairs of patterns that describe simultaneous responses of the monthly rainfall field to anomalies of surface circulation (SLP). Between mid-tropospheric circulation (Z500) and rainfall fields, six concurrent CCA pairs were also identified to be significant at the same confidence level. From the synoptic point view, these results seem very reasonable. Six concurrent (hereafter lag 0) CCA pairs and one CCA pair for a lag of 1 month (hereafter lag 1) were identified for the Z500—Tm fields at the confidence level of 99 per cent. The SLP—Tm fields, also six concurrent CCA pairs and three pairs for lag 1, were found to be significant at the same confidence level. Quite similar temperature anomaly patterns are represented in the first CCA pairs of the Z500—Tm fields for both lag 0 and lag 1, and in the first lag-1 and second lag-0 CCA pairs of the SLP-Tm fields; the patterns of air temperature anomalies revealed in the third CCA pair of SLP—Tm fields for lag 1, as well as in the fourth concurrent CCA pairs of the Z500—Tm and SLP—Tm fields, also resemble each other. Similar temperature anomaly patterns can be found for other CCA pairs. These similarities reveal that some slow-changing processes in large-scale anomalies of atmosphere are responsible for producing the same type of temperature anomalies.

On the linearity and stability of Southern Oscillation-climate relationships for New Zealand

Abstract: Previous research has highlighted the existence of significant linear correlations of the Southern Oscillation Index (SOI) with mean sea-level pressure in the Australasian region and with patterns of rainfall and temperature anomalies in New Zealand. The issue of the Southern Oscillation influence on New Zealand climate is revisited here, with emphasis on whether climate anomalies during the El Nino and La Nina extremes are indeed equal and opposite, as assumed in any linear analysis. The consistency of SOI-climate relationships over time is also assessed by comparing analyses before and after 1950. A linear regression analysis of seasonally stratified rainfall and temperature data from a number of New Zealand sites is carried out, but in addition a test for a simple form of non-linearity (namely bilinearity) is applied. In a separate but complementary analysis, which does not impose a linear structure on the anomalies, climate data are composited for periods representing extremes of the Southern Oscillation. Both analyses show that although it is often reasonable to assume a reversal between El Nino and La Nina, non-linearities are also evident and particularly so in southern parts of New Zealand in the winter and spring seasons. At all the central and southern South Island sites analysed, winter temperatures were colder than average at both extremes of the Southern Oscillation. In the earlier record prior to about 1950, the pattern of SOI-related climate anomalies also appears substantially different to more recent teleconnection patterns.

Delineation of Uganda into climatological rainfall zones using the method of principal component analysis

Abstract: This paper attempts to delineate Uganda into climatological rainfall zones using the method of principal component analysis (PCA). Monthly rainfall records from 102 Uganda rain-gauge stations for the years 1940–1975 inclusive are used in the study. The rotated dominant PCA modes were used to delineate the temporal rainfall characteristics into homogeneous spatial regions. Statistical and other methods were then used to determine the physical reality of the spatial patterns of the delineated zones of the rainfall network in describing the climatological rainfall patterns of Uganda. The PCA varimax rotated solutions showed that for the climatological data a maximum of four PCA modes, accounting for about 65 per cent of annual rainfall variance, were significant. However, a maximum of 16 PCA modes, accounting for over 81 per cent of the rainfall variance were obtained with the seasonal/monthly records. The PCA regional patterns delineated Uganda into 14 homogeneous climatological zones, which were also indicated by the spatial patterns of the physical features of Uganda and other tests. The derived rainfall zones would be useful in the planning and management of rainfall dependent activities in the country.

June-september rainfall in north-eastern Africa and atmospheric signals over the tropics: A zonal perspective

Abstract: The connection between rainfall in northeast Africa (Djibouti, Eritrea, Ethiopia, Kenya, Somalia, Sudan, and Uganda) and various atmospheric indicators from the tropics have been investigated for the northern summer season. Variables used include zonal wind at 700 hPa and 200 hPa, sea-level pressure, and rainfall over other tropical areas, for the period 1951–1988. Strong significant correlations are shown with the Southern Oscillation and the components of the Walker cell in the Pacific Ocean. Droughts in the Ethiopia-Uganda area are associated with El Nino events and, moreover, with above normal pressure and droughts over India. Other significant relationships exist with the African monsoon. Westerly (easterly) anomalies in the lower (upper) levels are associated with abundant summer rainfall years in north-east Africa. These results are all consistent with the previously defined Walker-type circulations over the tropics, particularly as far as the Indo-Pacific and African cells are concerned. Although the Southern Oscillation has a dominant impact in this respect, the specific coherence of the African cell is further evidenced. Preliminary findings suggest that the summer rainfall areas of East Africa could qualify as a location for the rising branch of this cell.

Recent frost date trends in the north‐eastern USA

Abstract: Uncertainty in projections of future climate, along with a need for policy makers and management to respond to change in climate, highlight the importance of the detection of trends in climatological time series. Selected indicators of regional climate status and change may well be related to indicators of ecological condition. Climate factors are known to stress ecosystems. With the identification and description of trends in biologically important climate descriptors this knowledge could be used to provide scenarios of biological changes. One aspect of climate which stresses forest ecosystems is described by the annual date of the last hard spring-freeze (minimum temperature ⩽ −2·2°C). Averaged for New England, there was a significant linear trend indicating an earlier initiation of frost-free conditions (negative slope) for the period 1961–1990. Significant time trends were detected locally at stations throughout much of the New England regions as well. Temporal discontinuities in trend, spatial relationships, and urban influence were considered. The presence of trends in time does not appear to be related to geographical location. The direction of trend (positive or negative) exhibits some geographical coherency, which may be related to climatological variables such as percentage of possible sunshine and cloud cover.

Rainfall variability in subequatorial America and Africa and relationships with the main sea-surface temperature modes (1951–1990)

Abstract: The rainfall variability of subequatorial South America and Africa is poorly documented owing to the scarcity of data. We present a new land-only data set of monthly precipitation from 1951 to 1990, focusing on subequatorial South America and Africa, which improves the knowledge of rainfall variability and allows comparisons with GCM outputs. The results of multivariate analyses are compared with those performed on the best actual global rainfall data set developed by Mike Hulme. The main modes of bimonthly rainfall variability are not located in the major rain-forest basins of Za'ke and Amazonia, but rather on the tropical margins, such as Venezuela or Sudan, and near-coastal equatorial areas, such as Guyana, Nordeste, Guinea, and Gabon. A regionalization into 13 homogeneous areas selected from the multivariate analyses is proposed. The statistical links between the rainfall variability and the four main sea-surface temperature modes indicate a strong influence of the El NiHeSouthem Oscillation (ENSO) phenomenon upon South America (lesslmore rainfall during an El Niiio/La NiHa event) and a weaker impact, modulated by the Atlantic thermal state, upon Africa. The impact of ENSO events seems stronger since 1965 than before.

Towards a consistent global climatological rawinsonde data‐base

Abstract: An archive of monthly temperatures, dew points, geopotentials, and winds at standard tropospheric and stratospheric levels from about 800 rawinsonde stations has been developed from routinely transmitted monthly data, and supplemented with published or national archive data. Many stations' data commence in the 1950s and the archive is continually updated. The raw data, however, contain both random and systematic errors. Quality control of random errors includes hydrostatic, wind-shear, and climatological checks, and comparisons with neighbouring stations and operational model analyses. Systematic errors in the wind speeds have resulted from the inadequately documented use of knots by some nations and metres per second by others in monthly messages. These errors are being amended by using geopotential height gradients, by comparing wind speeds reported from opposite sides of national borders, by averaging independently coded and transmitted daily data, and by the acquisition of original data from national archives. Other systematic errors result from changes of rawinsonde instrumentation, evolving operationally applied radiation and lag corrections, and changes of ascent times. Accordingly, the archive includes station histories where available, but these are far from complete. If the data are to be used in trustworthy analyses of interannual and longer term climatic variations, adjustments will need to be applied, using fully documented station histories and a knowledge of the effects of instrumental and other changes. Techniques for estimation of systematic adjustments include comparisons between neighbouring stations, comparisons with operational model analyses, the use of extended international radiosonde comparisons, and models of the thermodynamics of radiosonde instruments.

Solar dimming and urban heating at Hong Kong

Abstract: Over the last 35 years solar radiation in Hong Kong has decreased by more than one-third. During the same period the population doubled and the total use of fossil fuels increased more than sevenfold. A surface energy balance model was used to demonstrate the opposing effects of the decline in irradiance and the increase in anthropogenic heat production on air temperature. Differences between air temperatures measured in the city and at an upwind, unpopulated island were used to verify the simulated decrease in the urban heat-island by day and increase at night, leading to an unchanged mean air temperature difference. The implications of these results for climate change in the rapidly urbanizing Pacific Rim region are discussed briefly.

Climatic response of the Indian subcontinent to doubled CO2 concentrations

Abstract: Results from the United Kingdom Meteorological Office (UKMO) coupled climate model have been analysed over the Indian subcontinent in order to validate the model's performance and to assess the changes in climate and its variability in a simulation with a 1 per cent increase per year in CO2 (compound). The model produces a reasonable simulation of present-day climate over the Indian subcontinent. At the time of CO2 doubling, the model simulates temperature increases of the order of 1 K to 4 K over the Indian subcontinent during winter and monsoon seasons. The model-simulated monsoon circulation shifts by 10° latitude towards the north and intensifies by approximately 10 per cent in the warmer atmosphere. The interannual variability of monsoon onset dates and intraseasonal variability of monsoon rainfall are not significantly different when the CO2 concentration doubles. However, the model simulates increased interannual variability of the monsoon rainfall and a greater number of heavy rainfall days during the monsoon.

Variations and trends in annual mean air temperatures in Turkey with respect to climatic variability

Abstract: The purpose of this study is to investigate the variations and trends in the long-term annual mean air temperatures by using graphical and statistical time-series methods. The study covers a 63-year period starting from 1930 and uses temperature records from 85 climate stations. First, spatial distributions of the annual mean temperatures and coefficients of variation are studied in order to show normal conditions of the long-term annual mean temperatures. Then variations and trends observed in the annual mean temperatures are investigated using temperature data from 71 climate stations and regional mean series. Various non-parametric tests are used to detect abrupt changes and trends in the long-term mean temperatures of both geographical regions within Turkey and individual stations. The analyses indicate some noticeable variations and significant trends in the long-term annual mean temperatures. Among the geographical regions, only Eastern Anatolia appears to show similar behaviour to the global warming trends, except in the last 5 years. All the coastal regions, however, are characterized by cooling trends in the last two decades. Considering the results of the statistical tests applied to the 71 individual stations data, it could be concluded that annual mean temperatures are generally dominated by a cooling tendency in Turkey. The coldest years of the temperature records of the majority of the stations were 1933 and 1992, respectively.

Interannual variability of deep convection over the tropical South American sector as deduced from ISCCP C2 data

Abstract: The interannual variability of deep convection over the South American sector and regional hydrometeorological anomalies are studied using International Satellite Cloud Climatology Project (ISCCP) data for 1984–1988. Satellite-derived convection anomalies are compared with regional rainfall and river runoff anomalies. At the height of the austral summer, the deep convective clouds (DCC) and ISCCP depict the centres of intense convection over central Amazonia and the mouth of the River Amazon, as well as the enhanced convection over north-east Brazil, which is related to the Atlantic Intertropical Convergence Zone (ITCZ). Positive/negative regional hydrometeorological anomalies are consistent with positive/negative anomalies in the deep convection over northern Amazonia, the mouth of the River Amazon and north north-east Brazil, whereas this is not always true over southern Amazonia and southern Brazil. Over north-east Brazil and the mouth of the River Amazon, abundant rainfall and large DCC are both in agreement with variations in the latitudinal position and intensity of the Atlantic ITCZ, which extends between 4°N and 5°S along the Atlantic coast, where the precipitation maximum is found. The related rainfall anomalies over Amazonia and north-east Brazil during years with anomalously warm or cool surface waters in the central equatorial Pacific (e.g. the El Nino event in 1987), are consistent with the convection anomalies as depicted by the ISCCP deep convective clouds. For the Amazon Basin and north-east Brazil, DCC based on 2.5°×2.5° horizontal resolution explained about 50 per cent of the variance of rainfall at individual surface stations. A new finding is the enhanced convective activity located at the east side of the central Andes and over central Amazonia around 5°S, with a region of reduced convection in between. These two subcentres are oriented west-east and are part of the summertime semi-permanent centre of convection over the Amazon Basin, and have not been identified previously from the analysis of outgoing longwave radiation (OLR) or highly reflective clouds. These subcentres of large DCC exhibit interannual variations in intensity and location, where the centre located near the east flank of the Andes shows a wider range of latitudinal variations than that located over central Amazonia. South of this area convection and rainfall decreases rapidly.

Surface temperature trends and variations in New Zealand and the surrounding ocean, 1871–1993

Abstract: We compare homogenized series of maximum, minimum, and mean air temperature averaged over New Zealand, measured between 1871 and 1993, with rigorously quality controlled marine temperature data measured over the surrounding ocean surface. The marine data are those of sea-surface temperature (SST) and air temperature measured at night (NMAT) on board ship, both corrected for time-varying instrumental biases. There is mostly very good agreement between the variations in the three data sets on time-scales down to a season. Some disagreements are related to short periods of sparsely observed marine data, particularly during the World Wars. Differences also exist between trends in the maximum and minimum New Zealand temperatures (NZT), particularly in winter. In addition, interannual variations in winter NZT tend not to track those of nearby marine temperatures quite as well as happens in other seasons. Although it is not the main purpose of the paper to discuss the causes of NZT change, New Zealand temperature is known to be influenced by the El Nino-Southern Oscillation (ENSO) phenomena. Tropical East Pacific SST variations are strongly related to ENSO and so are well correlated with NZT on time-scales of a few years to near a decade. We discuss these relationships, and also associations with Southern Hemisphere SST. We conclude that annual NZT and NMAT over the nearby ocean surface have both warmed by about 0.7°C since the beginning of the century, with a slightly smaller increase in SST. This confirms previous work on the magnitude of the warming of New Zealand climate this century. Warming in NMAT and NZT in each season varies in a similar way, with consistently slightly smaller increases in SST. We also conclude that the UK Meteorological Office seasonal historical marine temperature data set appears to be generally of very good quality in the New Zealand region.

Spatio-temporal patterns of drought in morocco

Abstract: Past climatic moisture adomalies can be reconstructed from climatically sensitive tree-ring chronologies of Cedrus uhntica in Morocco. A map-pattern technique was utilized to derive 10 patterns of variability for the period 1845-1 974. The individual patterns were examined, in space using geostatistical techniques, and in time using correlation coefficients. The results indicate a clear alternation of favourable-unfavourable climatic regimes lasting 2&25 years. The spatial extent of the climatic moisture anomalies can be explained by a combination of three factors of the atmospheric circulation related to the Azores High, local cyclogenesis, and north-eastern perturbations.

Greenhouse warming and threshold temperature events in Victoria, Australia

Abstract: Small changes in the mean climate can produce relatively large changes in the frequency of extreme events. Two experiments were performed: (i) a sensitivity study where observed temperatures at climate stations were increased by either 1°, 2° or 3°C, and (ii) a scenario study for the year 2030 where temperatures were increased according to warming scenarios which vary spatially and seasonally. The sensitivity study shows that at some sites, up to four more summer days over 35°C occur for a 1°C warming and the frequency is more than doubled for a 3°C warming. The frequency of subzero winter days is halved for a 1°C warming. At selected sites, the current probability of a run of at least five summer days over 35°C is doubled for a 1°C warming and increases by a factor of five for a 3°C warming. Where the current probability of a run of at least five winter days below 0°C is less than 50 per cent, a 1°C warming reduces the probability by a factor of about three. A 10 per cent change in temperature variance is unlikely to significantly alter these results. A low warming scenario for 2030 gives at least 25 per cent more days over 35°C in summer and spring, and at least 25 per cent fewer winter days below 0°C. A high warming scenario produces 50–100 per cent more extremely hot summer and spring days and 50–100 per cent fewer extremely cold winter days. Larger percentae changes occur in regions where absolute changes are smaller. The high scenario also gives a 20 per cent increase in the probability of a run of five summer days over 35°C in the northern half of the State, and the risk of a run of five winter days below 0°C is reduced by less than 20 per cent in the western and coastal areas and by 25–40 per cent in the north-east highlands. Although a reduction in frost frequency may benefit agriculture, more extremely hot temperatures may increase bushfire potential, human mortality, and heat stress to livestock and crops.

Trends in air temperature and precipitation for Canada and north‐eastern USA

Abstract: Using the univariate, non-parametric, Kendall's test for monotonic trends from the 1949 to 1989 period, monthly maximum, minimum, average temperature, and precipitation data of some carefully selected stations in Canada and north-eastern USA, it is found that western Canada for the past 40 years has experienced significant warming in January and March, and to a limited extent in April, May, and June, but virtually no warming trend was detected in eastern Canada and north-eastern USA. Instead, some cooling trends were found, particularly in October. The analysis on the annual cycle of temperature data also shows the seasonal and regional nature of warming and cooling trends. Unlike temperature, no obvious trend is found in the precipitation data, but on the whole there are more stations (up to 10 per cent) with upward trends (increase in precipitation) during springs and summers. A bivariate test was used to detect a shift in the mean of precipitation series with respect to maximum temperature series but no obvious indication of such a shift was detected.

Relationships between 700 hPa height anomalies and 1 April snowpack accumulations in the western USA

Abstract: Relationships between atmospheric circulation and the temporal and spatial distributions of snowpack accumulations in the western USA are examined. Winter mean 700 hPa height anomalies, representing the average atmospheric circulation during the snow season, are compared with snowpack measurements made on or about 1 April at 31 1 snowcourse stations in the western USA during the winters of 1947-1948 through 19861987. Correlation and anomaly pattern analysis are used to identify relations between atmospheric circulation and the temporal and spatial distributions of snowpack accumulations, and to quantify the degree to which the temporal and spatial variability in snowpack accumulations can be attributed to variations in atmospheric circulation. Results indicate that winter mean 700 hPa height anomalies account for a statistically significant portion of both the temporal and spatial variability in the snowpack accumulations. In general, above-average snowpack accumulations are associated with negative 700 hPa height anomalies over the eastern North Pacific Ocean and the western USA. These anomalies are indicative of anomalous cyclonic circulation, which produces an anomalous westerly flow of moist air from the eastern North Pacific Ocean into the western USA and increases winter precipitation and snowpack accumulations. Below-average snowpack accumulations at most of the snowcourse stations are associated with positive 700 hPa height anomalies over the western USA. These positive anomalies indicate anomalous anticyclonic circulation which prevents the intrusion of moist air from the eastern North Pacific Ocean into the western USA, increases subsidence, and decreases winter precipitation. Five winter mean 700 hPa height anomaly patterns also were identified that explain the spatial variability in snowpack accumulations.

Seasonal phase lag between rainfall and vegetation activity in tropical Africa as revealed by NOAA satellite data

Abstract: A seasonal phase lag in the response of vegetation to rainfall and its dependence on the type of vegetation in tropical Africa have been investigated, using weekly-basis data of the NOAA/AVHRR-derived NDVI (normalized difference vegetation index) and OLR (outgoing longwave radiation) for April 1985 to September 1987. The lag-correlation analysis of the NDVI and OLR data demonstrates that the seasonal phase lag, by which the NDVI responds most markedly to the OLR, tends to increase from the Equator to the pole in the Northern and Southern Hemispheres. The lag ranges from 1–3 weeks in the equatorial rain forest to about 2–6 weeks in the wet savanna and about 6–9 weeks in the dry savanna. The strongest negative lag-correlation coefficients at the above lags are characterized by a spatial pattern which has a maximum absolute value near the latitudes 7.5°–10° in the savanna areas of both hemispheres. The equatorial rain forest, especially its north-eastern part, shows the weakest correlation. The results suggest that the response of vegetation to rainfall is more pronounced and has a longer lag in the savanna than in the equatorial rain forest. A regional analysis of the NDVI-rainfall relationship for Cameroon confirms the above NDVI-OLR lag-correlation on the continental scale.

Long-term trends of precipitation and runoff in Louisiana, USA

Abstract: Changing precipitation patterns and their impact on surface water resources is an important climatic problem facing society today, especially as it relates to potential global change. Therefore, this paper analyses temporal variability and trends in the empirical precipitation record and modelled runoff in Louisiana, USA, during the past century. Long-term trends in precipitation were identified and impacts on surface water resources were found. If general circulation model predictions for increasing precipitation in the south-eastern US prove to be correct, surface water will likely increase at a disproportionately higher rate which should be of concern to water resource planners in the region.

Chaotic influences and the problem of deterministic seasonal predictions

Abstract: An analysis has been performed of three simulations made by forcing a global climatic model with observed sea-surface temperature and sea-ice distributions for the period 1979–1988. These simulations were started from different years of an extended control run, otherwise all conditions were identical. The analysis concentrates on the regional and local differences in the simulations caused by chaos attributable to the differing initial conditions interacting with the non-linearities of the model climatic system. Of particular interest is whether the presence of sea-surface temperature variations in individual years is able to overcome chaotic influences. This interest arises because of the increasing international appreciation that such temperature variations provide the basis for the development of multi-seasonal prediction schemes. The analysis shows that between about 20°N and 20°S a coherent climatic reponse is obtainable owing to the boundary value forcing associated with the sea-surface temperature field. At high latitudes the signal-to-noise ratio rapidly deteriorates. Even at low latitudes, in a sensitive variable such as rainfall, chaos creates substantial differences between the three simulations, except over the Pacific Ocean where a very coherent response is found. In general, over land areas, even those within recognized regions of ENSO (El Nino-Southern Oscillation) influence, the impact of chaos is much higher. Examination of Pacific North American teleconnection patterns for 1988 reveals substantial differences between the three simulations, and the associated impact on rainfall and surface temperatures over the North American continent. Agreement with observations ranges from rather good to poor, indicating the need for multiple runs for any given situation. Overall the results establish that sea-surface temperature variations can overwhelm chaos in appropriate circumstances and specific regions. Importantly, this indicates that given accurate predictions of sea-surface temperature anomalies a few seasons in advance, then model-based predictions will be possible over many regions of the world despite the omnipresence of chaos.

On the severe drought of 1993 in North-East Brazil

Abstract: During the period 1990 through to 1993 a dry spell occurred over north-east Brazil. The drought of 1993 was very severe. Rainfall series for north-east Brazil are updated to 1993 and the drought conditions during 1993 are discussed. The 1993 drought seems to be connected at least partially to the unusual ENSO conditions during that year.

Growing season temperatures over the former Soviet Union

Abstract: Analyses of daily mean temperatures during the growing season (days >5°C) from about 200 stations over the former Soviet Union indicate that little change has taken place in a number of growing-season-related variables during the last 110 years. We have considered the start (STR), end (END), duration (DUR), and the number of degree-days above 5°C (DD). Annual average temperatures in the region have warmed by about 1°C, but this increase has been confined to the October-to-April period. Time series of the variables at individual stations show that there is little correlation between the start and end dates of the growing season and between the duration and the number of degree-days in the season. Mean May-to-September temperatures at individual stations are shown to be highly correlated with the number of degree-days, implying that the average of an appropriate combination of monthly temperatures can be used as a proxy for the number of degree-days when daily data are not available. The low correlation between degree-day counts and growing season duration in time series at all the stations is surprising given the strong spatial dependence between the 1950–1989 averages of the variables at all the stations. The results have implications for studies concerned with the impacts on growing season temperatures of future climate change.

Temperature trends and change points in the northern Spanish Plateau during the last 100 years

Abstract: In this work we have analysed the mean, maximum, and minimum temperature records of five localities in the northern Spanish Plateau during the last century. We have used the short-cut Bartlett test to examine their homogeneity. Our main proposal is the detection of trends and abrupt changes, using the Mann-Kendall test and the estimation of the magnitude of these. We have found that the minimum and mean temperatures have increased. The maxima do not present special behaviour. So the minimum temperatures show more influence over the mean temperatures than the others. Briefly, we have studied the possible linkage between the increase of minimum temperatures and urban growth in the area studied.

Variability of summer rainfall over Madagascar: Climatic determinants at interannual scales

Abstract: Variability of convective rainfall in the austral summer season over central Madagascar is studied using an area-rainfall index, local radiosonde data, and gridded information on outgoing longwave radiation (OLR), sea-surface temperature, and tropospheric winds. Seasonal rainfall patterns are influenced by topography, monsoon and trade wind circulations, and tropical cyclone events. The Inter Tropical Convergence Zone overlies the north-west coast, where summer rainfall averages 47 cm month−1. Climatic conditions that affect convective rainfall at interannual time-scales are studied through spatial lag correlation analysis. Sea-surface temperature is weakly correlated with rainfall departures, with positive values in the central Indian Ocean reaching +0.41 at lags −4 and 0 months. Strongest positive correlations (+0.45) gradually shift to the central South Atlantic Ocean at lag 0 and +2 months. Wind correlations imply that increased upper level tropical easterly flow overlying enhanced low-level north-west monsoon flow favours above normal rainfall. Regional teleconnection patterns identified by rainfall-OLR correlations are remarkably similar to those for Quasi-biennial oscillation (QBO)-OLR correlations, suggesting that up to one-third of the interannual convective variance can be explained by the phase of the QBO.

Complex demodulation and the estimation of the changing continentality of Europe's climate

Abstract: Six long records of European mean monthly temperature have been analysed for changes in continentality of climate using the time series technique of complex demodulation. Variations in amplitude and phase of the annual temperature cycle as estimated by the complex demodulation, in combination with low-pass filtering, are found to be coherent across much of Europe, from distinctly maritime to distinctly continental locations. The most significant departure of continentality found during the last 460 years is the maritime period of the 1920s when cool summers accompanied mild winters along with a retardation of the seasons throughout Europe.