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Showing papers in "International Journal of Climatology in 1993"


Journal ArticleDOI
TL;DR: In this paper, the Canadian Land Surface Scheme (CLSS) is described and the accompanying vegetation model is described, including physically based treatment of energy and moisture fluxes from the canopy as well as radiation and precipitation cascades through it, and incorporating explicit thermal separation of the vegetation from the underlying ground.
Abstract: In the companion to the present paper, the soil model associated with CLASS (Canadian Land Surface Scheme) was outlined. In this paper, the accompanying vegetation model is described. This model includes physically based treatment of energy and moisture fluxes from the canopy as well as radiation and precipitation cascades through it, and incorporates explicit thermal separation of the vegetation from the underlying ground. Seasonal variations of canopy parameters are accounted for. The morphological characteristics of the ‘composite canopy’ associated with each grid square are calculated as weighted averages over the vegetation types present. Each grid square is divided into a maximum of four separate subareas: bare soil, snow-covered, vegetation-covered, and snow-and-vegetation covered. Test runs were done in coupled mode with the Canadian Climate Centre GCM, to evaluate the performance of CLASS compared with that of the simpler land surface scheme previously used. Two versions of CLASS were run: one with ponded surface water saved between time steps, and one with it discarded. For the seasons of June—July—August and December—January—February, diagnostic calculations showed that the old scheme underestimated the globally averaged land surface screen temperature by as much as 3.0°C, and overestimated the globally averaged precipitation rate over land by up to 1.0 mm day−1. CLASS, on the other hand, produced screen temperature anomalies, varying in sign, of 0.2–0.3°C, and positive precipitation anomalies of 0.6–0.7 mm day−1. The relatively poor performance of the old model was attributed to its neglect of vegetation stomatal resistance, its assumption that the contents of the soil moisture ‘bucket’ had to be completely frozen before the surface temperature could fall below 0°C, and its use of the force-restore method for soil temperatures, which systematically neglects long-term thermal forcing from the soil substrate. The assumption made in most GCMs that excess surface water immediately becomes overland runoff is shown to result in substantial overestimates of surface screen temperatures in continental interiors.

552 citations


Journal ArticleDOI
TL;DR: In this paper, an objective scheme, initially developed by Jenkinson and Collison, is used to classify daily circulation types over the British Isles, along the lines of the subjective method devised by Lamb.
Abstract: An objective scheme, initially developed by Jenkinson and Collison, is used to classify daily circulation types over the British Isles, along the lines of the subjective method devised by Lamb. The scheme uses daily grid-point mean sea-level pressure data for the region. The results of the analysis over the period 1881-1989 are compared with ‘true’ Lamb weather types. The frequencies of objectively developed types are highly correlated with traditional Lamb types, especially so for synoptic (cyclonic and anticyclonic) types, although still good for wind directional types. Comparison of the two classification schemes reveals negligible differences between the correlations of the counts of circulation types and regional temperature and rainfall. The major difference between the two classification schemes is that the decline of the westerlies since 1940 is less evident with the objective scheme.

443 citations


Journal ArticleDOI
TL;DR: In this paper, the characteristics of rainfall variability in West Africa are analyzed using time series of June, July, August, and September and annual rainfall anomalies for 19 regions, most of which are in the Sahel-Soudan zones of West Africa.
Abstract: This article deals with the characteristics of rainfall variability in West Africa. Time series of June—July, August—September and annual rainfall anomalies are derived for 19 regions, most of which are in the Sahel—Soudan zones of West Africa. The temporal characteristics of the series, such as trend, persistence, and low frequency variance are evaluated using regression, lag-1 autocorrelation, and spectral analysis. High-frequency-filtered series are also derived. The spatial teleconnections among the regions are examined using linear correlations and principal component analysis. The study shows that there are primarily three spatial modes of rainfall variability in the region and these differ with respect to the part of the rainy season in which they are most important and to what extent they describe high and low-frequency components of the variability. The study also defines three broad homogeneous sectors and demonstrates that the characteristics of rainfall variability in these sectors differ markedly. A strong contrast between the bulk of the Sahel—Soudan and its West Coast sector is apparent.

209 citations


Journal ArticleDOI
TL;DR: In this paper, five statistical tests are applied to the historical annual streamflow series of 30 unregulated Australian rivers, located in protected natural catchments, in order to detect trends or changes in the means.
Abstract: Changes in streamflow volumes resulting from climatic change can affect the management of water resources. However, the detection of a statistically significant trend can be difficult because of the large interannual variability and the relatively short period of instrumental record. In this paper, five statistical tests are applied to the historical annual streamflow series of 30 unregulated Australian rivers, located in protected natural catchments, in order to detect trends or changes in the means. The Student t-statistic is also used to describe the relationship between interannual variability and the magnitude of change and length of data required to identify a statistically significant trend. The analysis indicates that, with the current data set, there is no clear evidence to suggest that the greenhouse effect is impacting on Australian streamflow volumes. The change required to identify a statistically significant change in the mean is directly proportional to the interannual variability, and less dependent on the length of data available. The interannual variability of Australian streams is twice that of Northern Hemisphere streams, therefore, a greater change is required in Australian flow volumes before a statistically significant change can be identified. The analysis of available historical records, however, indicates that statistically significant trends can be detected in the future flow volumes if the scenarios predicted by the G'CMs are realized.

166 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of long-term trends in mean wave height in the North Atlantic with values of annual mean atmospheric pressure gradient was made for the period of overlap between the data set of Bacon and Carter and of Sahsamanoglou (early 1950s to 1980), showing a correlation between these two measures.
Abstract: Comparison of long-term trends in mean wave height in the North Atlantic with values of annual mean atmospheric pressure gradient in the North Atlantic measured between the Iceland Low and the Azores High suggests, for the period of overlap between the data set of Bacon and Carter and of Sahsamanoglou (early 1950s to 1980), a correlation between these two measures. This paper, using monthly mean values of significant wave height (HS) derived from measurements at two sites, one oceanic (Ocean Weather Station Lima) and one coastal (Seven Stones Light Vessel), covering the period 1962–1988, investigates correlation between (firstly) annual mean values and (secondly) monthly mean values of HS and of pressure gradient. The annual mean values of pressure gradient are shown to be linked to the long-term changes in wave climate described by Bacon and Carter. The large annual cycles in the monthly mean values of both variates are found to be in phase; furthermore, correlation between the two variates independent of the annual cycle is found. Finally, the correlation between the annual means is used to generate a hindcast mean wave climate for Seven Stones Light Vessel (and also, tentatively, for the whole north-east Atlantic) from 1873 to the present, which suggests that present conditions are as high as have been observed since 1873.

135 citations


Journal ArticleDOI
TL;DR: In this paper, the first comparative study of climate over Amazonian tropical forest and an embedded deforested clearing is presented, which includes a continuous 60-day run of data from mid-October to mid-December 1990, covering the end of the dry season and the beginning of the wet season.
Abstract: Measurements from the first comparative study of climate over Amazonian tropical forest and an embedded deforested clearing are presented. Observations comprise a continuous 60-day run of data from mid-October to mid-December 1990, covering the end of the dry season and the beginning of the wet season. Mean hourly observations are calculated for the whole period; and for two 10-day periods, one in the dry season and one at the start of the wet season. Much greater variation in weather variables was observed at the clearing compared with over the forest. While the mean values of temperature and specific humidity deficit differed by less than 1°C and 1 g kg-' respectively, their daily ranges at the clearing were twice those at the forest. Mean daily albedo of the forest was 13.1 per cent, agreeing well with other tropical forest measurements, and of the clearing was 16.3 per cent, somewhat lower than the values currently being used in GCMs. The surface energy balance was investigated and mean available energy calculated for each site. The significant difference in the daily pattern of net radiation between the sites was found to be at least as much due to differences in the longwave radiation balance as to differences in albedo. The diurnal pattern of net radiation therefore changed between dry and wet periods as the higher plant water stress experienced by clearing vegetation altered the daily temperature cycle.

129 citations


Journal ArticleDOI
TL;DR: In this paper, the relationship between rainfall variations in the Eastern Horn of Africa, the Southern Oscillation, and the Indian Ocean temperature and pressure surface fields are studied for the period 1932-83.
Abstract: Relationships between rainfall variations in the Eastern Horn of Africa, the Southern Oscillation, and the Indian Ocean temperature and pressure surface fields are studied for the period 1932–83. Rainfall data consist of stations and regional indices calculated for three selected areas experiencing quite different rainfall patterns. The results indicate significant negative correlations between northern autumn rains in Somalia and the Southern Oscillation during the same season. These rains are also negatively correlated with pressure in the Western Indian Ocean, and positively correlated in the Eastern Indian Ocean. The reverse pattern is shown with sea-surface temperature. This agrees very well with the observations made in Kenya as far as the short rains of October-November are concerned. Central Ethiopia summer rains, in contrast, indicate significant positive correlations with the Southern Oscillation, at zero and 3 months lag. El Nino years often correspond to drought years in this region. Evidence of an inverse relationship between the amount of rainfall in Ethiopia during summer (especially during September) and both the pressure and sea temperature over the Indian Ocean is also given. Correlations with the Arabian Sea are particularly strong. However, teleconnections between summer rains in Northern Ethiopia (Eritrea) and the ENSO or the Indian Ocean fields are much weaker. These results suggest that, although summer rains over Ethiopia are said to be related to the monsoon air flow from the Congo Basin and the Atlantic Ocean, there also exist quite strong connections with the surface conditions prevailing in the Indian and Pacific Oceans.

122 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed analysis of the variability of Australian district rainfall on seasonal time-scales over the period 1950-1987 is described, where the major analysis tool is rotated principal component analysis (RPCA), used in both the S and T modes.
Abstract: A detailed analysis of the variability of Australian district rainfall on seasonal time-scales over the period 1950–1987 is described. This paper, Part I, describes the dominant spatial modes or patterns of variability. The major analysis tool is rotated principal component analysis (RPCA), used in both the S and T modes. Various criteria are examined to determine the number of components to rotate, including conducting trial rotations and comparison of the resulting patterns with the corresponding one-point correlation maps. The stability of the chosen solutions is examined by repeating the analysis on various subsets of the data. The S-mode analysis, which groups districts with similar temporal variation, provides a regionalization of the continent into eight coherent and approximately equally sized regions. The results of this analysis closely resemble those obtained from cluster analysis. The T-mode analysis clusters seasons with similar large-scale spatial variations (anomaly patterns). The similarity measure used in the T-mode analysis is the congruence coefficient, rather than the correlation or covariance. The patterns produced by this analysis consist of continental-scale anomalies, similar in some respects to the unrotated S-mode patterns, but more amenable to meteorological interpretation. In particular the first pattern, which accounts for approximately 25 per cent of the total variance consists of anomalies of the same sign over the entire continent centred on south-east Australia. The relationship between the two modes of representation is also explored. Regression equations are developed to express the spatially complex T-mode patterns in terms of the localized S-mode patterns, and alternatively, to partition the variance of each of the S-mode patterns between the T-mode components. In Part II the temporal variability and the relationship of these patterns to the Southern Oscillation and other large-scale circulation anomalies are examined.

102 citations


Journal ArticleDOI
TL;DR: In this paper, the authors compared years of below average rainfall over West Africa, sea-surface temperatures (SST) over the Atlantic Ocean and the world ocean, and latitudinal positions of the Inter Tropical Convergence Zone (ITCZ), and showed that the relationships depend mainly on the rainfall anomaly patterns.
Abstract: Comparisons between years of below average rainfall over West Africa, sea-surface temperatures (SST) over the Atlantic Ocean and the world ocean, and latitudinal positions of the Inter Tropical Convergence Zone (ITCZ) over the Atlantic Ocean show that the relationships depend mainly on the rainfall anomaly patterns. The well-known SST dipole (cold northern ocean and warm equatorial and southern ocean) is only apparent during those August months with below average Sudano-Sahelian rainfall and above average Guinean rainfall (rainfall type ‘−/+’) and an abnormal southward position of the ITCZ. In contrast, those August months that experience rainfall deficits over the whole of West Africa (rainfall type ‘−/−’) are associated with warm SST anomalies over the eastern Pacific Ocean, cold persistant SST anomalies over the equatorial Atlantic Ocean and a more northward position of the ITCZ. Those patterns first appear in northern spring before the Sahelian rainy season. The composite SST differences for the ‘−/+’ and ‘−/+’ rainfall types computed with August or July-September amounts have a good resemblance with each other. Comparison of results related to the reverse July-September rainfall patterns (the ‘+/−’ and ‘+/+’ patterns) during the 1950s shows that the SST anomalies were globally colder when, on average, the Sahelian rainy seasons experienced significant excesses while the Guinean little dry seasons were more marked.

101 citations


Journal ArticleDOI
TL;DR: In this article, the effect of known site changes on temperature and rainfall measurements is estimated by comparing equal periods before and after the site change, and a variety of graphical and analytical techniques are used for deciding how to adjust for a site change.
Abstract: Methods are presented for estimating the effect of known site changes on temperature and rainfall measurements. Parallel cumulative sums of seasonally adjusted series from neighbouring stations are a useful exploratory tool for recognizing site-change effects at a station that has a number of near neighbours. For temperature data, a site-change effect can be estimated by a difference between the target station and weighted mean of neighbouring stations, comparing equal periods before and after the site change. For rainfall the method is similar, except for a logarithmic transformation. Examples are given. In the case of isolated stations, the estimation is necessarily more subjective, but a variety of graphical and analytical techniques are useful aids for deciding how to adjust for a site change.

98 citations


Journal ArticleDOI
TL;DR: In this paper, it is shown that important changes in vapour fluxes occur between October (early summer) and January (late summer), with zonal fluxes being more important in October and meridional fluxes in January.
Abstract: Southern Africa is semi-arid to arid, and the moisture that contributes to rainfall over the summer rainfall region is largely imported from other areas. Interseasonal and interannual variations in rainfall must result from changes in the circulation and in vapour fluxes over the subcontinent. It is shown that important changes in vapour fluxes occur between October (early summer) and January (late summer), with zonal fluxes being more important in October and meridional fluxes in January. Wet and dry months of October and January are characterized by enhanced zonal (meridional) flow in wet Octobers (Januaries), and reduced importance of these flows in the dry months. Adjustments in the areas of vapour flux convergence and divergence are as important as changes in the fluxes. The convergence and divergence changes between wet and dry months are confirmed by decreases in outgoing longwave radiation over central southern Africa in wet months and increases in dry months. Tropical zonal circulations, and tropical—subtropical meridional circulation cells, conform to the patterns of enhanced (reduced) convection over the central subcontinent in wet (dry) months.

Journal ArticleDOI
TL;DR: In this paper, a classification of extratropical storms in the North Atlantic Ocean is developed based upon the storm's origin, track, and intensification, and all storms producing at least 1.6m deep-water waves at Cape Hatteras, North Carolina from 1943 to 1984 are included.
Abstract: A classification of extratropical storms in the north-western Atlantic Ocean is developed based upon the storm's origin, track, and intensification. All storms producing at least 1.6-m deep-water waves at Cape Hatteras, North Carolina from 1943 to 1984 are included. Eight distinct storm types are identified, and their seasonality, interannual variation, and relationship to a previously devised storm-intensity scale are discussed. The most dangerous storms are cyclones which originate either over Florida or north of Cuba from October through to April, travel northward, and are blocked by a stagnating anticyclone over New England or the North Atlantic. Coastal storm frequencies declined from the mid-1960s through to the mid-1970s and increased through to 1984, but the frequency of potentially damaging storms has increased since 1965.

Journal ArticleDOI
TL;DR: The south-west of Western Australia is characterized by a Mediterranean climate with dominantly winter rainfall, and the only Australian region where the annual rainfall has significantly decreased during the period 1911-1990 as mentioned in this paper.
Abstract: The south‐west of Western Australia is characterized by a ‘Mediterranean’ climate with dominantly winter rainfall. It is the only Australian region where the annual rainfall has significantly decreased during the period 1911–1990. Based on daily rainfall records from 70 stations in the area, high intensity rainfall (which is closely related to flooding, soil erosion, and gully formation) does not show a concurrent decrease. Reasons for this unconformable trend include a greater temporal variability of high intensity rainfall and an increase in the frequency of occurrence of rainfall events at high intensities during the summer. This tends to balance out a decrease during the winter months. While it is unwise to assume that there will be similar trends for average rainfall and high intensity rainfall for a given region, evidence from the instrumental records supports, to some extent, the notion that in a CO‐warmed world, high intensity rainfall could occur more frequently irrespective of local change to average rainfall. Copyright

Journal ArticleDOI
TL;DR: In this article, a detailed examination of the variability of Australian district rainfall of seasonal time-scales over the period 1950-1987 is described, and the temporal variability of these spatial patterns is examined, and their relationship to some large-scale circulation indices, such as the Southern Oscillation (SO) and Pittock's L index are consistent with previous studies.
Abstract: A detailed examination of the variability of Australian district rainfall of seasonal time-scales over the period 1950–1987 is described. In Part I, S- and T-mode rotated principal component analysis (RPCA) was used to obtain a compressed representation of Australian district rainfall anomalies on seasonal time-scales. In this paper, the temporal variability of these spatial patterns is examined, and their relationship to some large-scale circulation indices, such as the Southern Oscillation (SO) and Pittock's L index. The S-mode components show large intercomponent correlations as a result of the rotation of the principal components, although much of this is due to a common relationship to the SO. The T-mode components are less strongly interrelated and only the first two show large correlations with the SO. The relationship with the SO and the L index are consistent with previous studies. Spectral analysis shows that most components do not have any dominant periodicity. Two distinct biennial tendencies are identified; in the north-east associated with the SO, and in the south-east associated with the L index. The large-scale circulation anomalies associated with both the S- and T-mode patterns are examined using the Comprehensive Ocean Atmosphere Data Set (COADS) and a network of rawindsonde stations over Australia and the South-west Pacific. Three dominant teleconnection patterns are identified; the well-known Southern Oscillation anomalies associated with the dominant T1 pattern of continental-scale rainfall anomalies, a predominantly winter season Indonesian-Indian Ocean pattern previously documented by Nicholls, and a spring/summer south Tasman Sea pattern. The evolution of these patterns is examined through composites, and the physical mechanisms for the initiation and maintenance of the anomaly patterns are discussed. In the latter two cases both the rainfall and SST anomalies appear to be driven by anomalies in the large-scale mid-latitude circulation.

Journal ArticleDOI
TL;DR: In this paper, the long-term precipitation series (1926-1990) from a 10 rain gauge station network in the Marathon area, Greece, have been statistically examined for trends.
Abstract: The long-term precipitation series (1926–1990) from a 10 rain gauge station network in the Marathon area, Greece, have been statistically examined for trends. Application of the Mann—Kendall rank statistic test to the annual time series revealed a significant decreasing trend in the precipitation over the area. Similar decreasing trends are observed in neighbouring regions to the Marathon area (Athens basin and Beotia). The differences in precipitation trends during the transitional seasons among the three areas seem to be caused by the urbanization process. The seasonal variations and the periodicities of precipitation in the study areas also are examined. Finally, the spatial distribution of precipitation and the relation of precipitation amounts to the station height in the Marathon area are discussed.

Journal ArticleDOI
TL;DR: In this article, daily data of 19 stations in Bangladesh for the months May through to October, for the period 1958 through to 1987, have been analyzed in order to determine various spatio-temporal patterns of the arrival and withdrawal dates of the summer monsoon in the country.
Abstract: Daily data of (a) rainfall amount, (b) prevailing wind direction, (c) wind speed, (d) mean temperature, and (e) mean relative humidity of 19 stations in Bangladesh for the months May through to October, for the period 1958 through to 1987, have been analysed in order to determine various spatio-temporal patterns of the arrival and withdrawal dates of the summer monsoon in the country. Statistical characteristics of these dates have been presented. The summer monsoon arrives first in the extreme south-eastern part of the country, and initially moves towards the north. After being diverted by the Meghalaya Plateau, the flow turns towards the west. The mean arrival date of the summer monsoon in the extreme south-eastern coastal part and in the extreme north-western part are 2 June and 15 June, respectively. Withdrawal of the summer monsoon proceeds in the opposite direction to the arrival. Mean withdrawal dates of the summer monsoon from the extreme north-western part and extreme south-eastern part of the country are 30 September and 17 October, respectively. Standard deviations of both arrival and withdrawal dates in different parts of the country vary from 7 to 10 days. Probabilistic early dates of arrival of the summer monsoon in Cox's Bazar area in 1 year out of 4 years, 10 years, and 25 years are 28 May, 23 May, and 19 May, respectively. Probabilistic late dates of arrival in 1 year out of 4 years, 10 years, and 25 years are 8 June, 13 June, and 16 June, respectively. Probabilistic early dates of complete withdrawal of the summer monsoon from Bangladesh in 1 year out of 4 years, 10 years, and 25 years are 10 October, 5 October, and 1 October, respectively. Probabilistic late dates of complete withdrawal in 1 year out of 4 years, 10 years, and 25 years are 23 October, 28 October, and November 1, respectively.

Journal ArticleDOI
TL;DR: In this article, the extent and nature of non-random changes, such as fluctuations, trend, and persistence, are investigated for a 72-year period (1916-1987).
Abstract: Variations of monthly and annual rainfall over the southern, middle belt and northern regions of Nigeria as well as the country as a whole are examined for a 72–year period (1916–1987). In this way the zonal distribution of the country's rainfall is taken into account. The extent and nature of non-random changes, such as fluctuations, trend, and persistence, are investigated. The fluctuations were filtered by a Gaussian low-pass filter and the resulting curves are discussed. Trend analysis showed a tendency towards decreasing annual rainfall totals in all the regions, with rates of decrease being greatest after 1961 in the southern region. All regions experienced the reverse of the trend during the 1931–1960 period. Overall, no significant persistence was evident in all the series. Power spectrum analysis revealed the existence of significant oscillations in the rainfall totals. These have time periods of 2·00–2·82 years in the middle belt and northern regions. 3·00–4·80 years in the middle belt, and 8·009·60 years in all the regions. The implications of these fluctuations, and continued application of the 1931–1960 normal for planning purposes, are pointed out, as well as suggestions for further lines of inquiry.

Journal ArticleDOI
TL;DR: In this paper, the authors have delineated rainfall zones for the Indian region that are coherent with respect to the variations of the summer monsoon rainfall, using an objective method specifically developed for the purpose.
Abstract: We have delineated rainfall zones for the Indian region that are coherent with respect to the variations of the summer monsoon rainfall. Within each zone, the time series of the summer monsoon rainfall at every pair of stations are significantly positively correlated, and the mean interseries correlation for each zone is high. The interseries correlation data set is analysed in order to delineate the rainfall zones, using an objective method specifically developed for the purpose. Each of the zonal averages are shown to be representative of the zone as a whole. We suggest that this regionalization is appropriate for study of the variation of the summer monsoon rainfall over the Indian region on interannual and larger scales.

Journal ArticleDOI
TL;DR: In this article, spectrum analysis of 1197 USA air temperature records yields evidence for two peaks with periods 18.8 ± 1.7 and 10.4 ± 0.5 years, identified as the luni-solar 18.6-year Mn and solar cycle Sc 10-11-year signals in climate.
Abstract: Spectrum analysis of 1197 USA air temperature records yields evidence for two peaks with periods 18.8 ± 1.7 and 10.4 ± 0.5 years. Tests by the t-statistic show that both are significant at confidence levels of 99.9 per cent, and both account for 23 per cent of total variance in the raw data. They are identified as the luni-solar 18.6-year Mn and solar cycle Sc 10–11-year signals in climate, induced by the twelfth largest constituent tide acting on the Earth and a variation of 10 to 11 years in the Sun's luminosity of the order of 0.1 per cent. Amplitude and phase of Mn wavetrains are highly non-stationary with respect to both time and geography; in particular, abrupt 180° phase changes in wave polarity are often observed. Amplitude and phase of the Sc waves are also highly non-stationary, with those east of the Rocky Mountains out of phase with waves to the west. These results consolidate and greatly extend and clarify earlier studies of Currie based on far less American data.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the relationship between anomalous North Pacific sea surface temperatures and the occurrence of growing season extended dry spells on the Canadian Prairies and found a significant correlation between extended dry spell and a positive SST anomaly gradient in the east-central North Pacific.
Abstract: This investigation documents the relationships between anomalous North Pacific sea-surface temperatures (SSTs) and the occurrence of growing season extended dry spells on the Canadian Prairies. Results show a significant correlation between extended dry spells and a positive SST anomaly gradient in the east-central North Pacific. The gradient exists between an area of anomalously cold water in the east-central North Pacific in the area 30°N–40°N latitude and 165°W-135°W longitude, and an area of anomalously warm water along the central west coast of North America in the area 45°N–55°N latitude and 130°W-125°W longitude. A probability model shows that the longer this positive gradient persists, the greater the probability of a major extended dry spell on the Canadian Prairies.


Journal ArticleDOI
TL;DR: In this paper, the mean monthly values of the zonal index of surface pressure for the zone 35°-65°N during the period 1899-1990 are studied and the correlation and correspondence between zonal indexes and air temperature changes of the Northern Hemisphere and over Europe has been also examined.
Abstract: Mean monthly values of the zonal index of surface pressure for the zone 35°–65°N during the period 1899–1990 are studied. The extreme values – 11·5 hPa in February 1947 and 16·5 hPa in January 1978 define the range of zonal index variability. The highest annual value occurred in 1990. On average, the maximum of zonal index occurs in October (mean value 7·5 hPa), the minimum in May (2·5 hPa). Spectral variance analysis shows 2-month, 3-month, 6-month, annual, quasi-biennial, and 13·2-year cyclicity in time series of monthly values. The quasi-biennial and 13-year cyclicity appear also in seasonal values of the zonal index. Instability of the zonal index spectra and autocorrelation for monthly values has been noted. The period under study is divided into three subperiods characterized by high (1899–1938 and 1972–1990) and low (1939–1971) zonal indexes. The correlation and correspondence between zonal index and air temperature changes of the Northern Hemisphere and over Europe has been also examined.

Journal ArticleDOI
TL;DR: In this paper, the Lamb's Weather Type register was used to determine daily precipitation event probabilities and magnitudes for eight dominant synoptic classes and the stochastic rainfall series thus generated were then used to drive a robust hydrochemical model.
Abstract: There is growing awareness as to the potential sensitivity of hydrological systems to climate change. In particular long-term changes in precipitation variability will have consequences for runoff regimes and thence solute transport. By utilizing the Lamb's Weather Type register in conjunction with observed 24 h rainfall totals it was possible to determine daily precipitation event probabilities and magnitudes for eight dominant synoptic classes. The stochastic rainfall series thus generated were then used to drive a robust hydrochemical model of an experimental catchment in the East Midlands, UK. The model results indicate that the frequencies of flood and drought are dependent upon the synoptic scenario, and especially the historical prevalence of both the anticyclonic and cyclonic patterns. Given that there have been marked fluctuations in the annual frequencies of these classes since the mid-nineteenth century, long-term variations in the severity and incidence of extreme flows may be anticipated. Clearly this will have major implications for the episodic export and dilution of solutes, sediments and microorganisms originating from both pristine and polluted hydrosystems.

Journal ArticleDOI
TL;DR: This study may provide a basis for nucleated clustering, which will enable the computer-assisted classification of weather to deal with longer data series and to study the long-term trends of the properties of weather types.
Abstract: The computer-assisted classification of weather at Prague-Clementinum used the average linkage clustering technique. Since the results of the clustering exhibit the snowballing effect, the usual methods of determining the threshold aggregation level (i.e. the level at which the clustering procedure is to be terminated) appeared to be inapplicable. A new method based on Monte Carlo simulations of the means was developed. Its key idea is the termination of the clustering procedure at different aggregation levels in different parts of the data set. This ensures that the number of resultant clusters is reasonable, while minimizing the numbers of very small clusters and unclustered days. The weather categorization resulted in 44 clusters for 14 winters in the period 1965–1978. Thirty-one of the clusters had sizes of 5 or more days. The Monte Carlo scores, comparing the means and variances of the clusters with those of a large number of subsets chosen randomly, indicate that all the resulting clusters represent meaningful weather types. This study may provide a basis for nucleated clustering, which will enable us to deal with longer data series and to study the long-term trends of the properties of weather types.

Journal ArticleDOI
Abstract: Summer and winter indices of rainfall total, intensity, and contributions of rain-days of different intensities were developed from 70 stations in Queensland over the period 1921–1987. In both summer and winter, only a few days of relatively high intensity rainfall contribute most to the seasonal rainfall total. In wetter summers, although the numbers of rain-days of different magnitude rainfall all increase, it is days with rainfall in excess of 50 mm day−1 that contribute most to the increased seasonal total. Year-to-year and longer time-scale variations in seasonal rainfall totals are matched by similar variations in secondary rainfall characteristics, i.e. wetter years or periods are likely to have more rain-days and rainfall of greater intensity than drier years or periods. Variations of the secondary rainfall characteristics show similar relationships with the Southern Oscillation as total rainfall. The correlations with the SOI are, with one exception, insignificant for the period, 1921–1950, of ‘weak’ SOI teleconnections. Only the date by which 15 per cent of the summer seasonal rainfall is accumulated (representing pre-monsoon rainfall) has maintained a significant and stable relationship with the SOI over time. Rainfall in Queensland is more closely linked with the number of tropical cyclones actually making landfall than the total number of cyclones in the region. Even in years when the large-scale circulation is not conducive to tropical cyclone activity, a single tropical cyclone crossing the coast can lead to substantial rainfall amounts.

Journal ArticleDOI
TL;DR: Spectrum analysis of 59 South African rain-gauge records yields evidence for two peaks with periods 18.2·1.6 and 10.3·0.5 years (in 37 series covering almost all except easternmost South Africa).
Abstract: Spectrum analysis of 59 South African rain-gauge records yields evidence for two peaks with periods 18.2·1.4 years (57 out of 59 series covering virtually the entire country) and 10.3·0.5 years (in 37 series covering almost all except easternmost South Africa). They are identified as the luni-solar 18.6-year Mn and 10–11-year solar cycle Sc signals in climate; their phases and amplitudes are non-stationary with respect to both time and geography and, in particular, 180° phase changes in wave polarity are observed. These phase changes occurred in the early part of the century and by mid-epochs 1964.0 and 1982.6 (mid-epochs are dates of minimum in Mn tidal forcing) a unipolar cell of drought encompassed virtually the entire nation. Constructive interference by the Sc wave, which reached a minimum near 1981, exacerbated the droughts of mid-epoch 1982.6 which were more severe than even those of the 1960s. The results confirm and extend in many respects those found by P. D. Tyson and his co-workers in the 1970s.

Journal ArticleDOI
TL;DR: In this article, the authors used data from 10 pyranometer stations in 1987 and 1988 to build a satellite insolation model for the western tropical Pacific and found that aerosol depletion is site-specific, with typical long-term values ranging from 19·8 per cent of the extraterrestrial value for Kuala Lampur, to 4·8 percent for Willis Island.
Abstract: Concurrent pyranometer and GMS (Japanese Geostationary Meteorological Satellite) data were collected for the western tropical Pacific during a 21-month period in 1987 and 1988. Data from 10 pyranometer stations in 1987 were used to build a satellite insolation model. Such processes as surface albedo, narrow band to broad band conversion, and depletion by water vapour and aerosols were taken into account. The model was then tested with an independent pyranometer data set comprising 9 months in 1988. Results show that aerosol depletion is site-specific, with typical long-term values ranging from 19·8 per cent of the extraterrestrial value for Kuala Lampur, to 4·8 per cent for Willis Island. Monthly insolation may be mapped over the oceanic regions of the study area assuming that the Willis Island aerosol depletion values apply. The model gave root-mean-square (RMS) errors of 14·08 W m−2 when tested against all monthly average pyranometer data for 1988. Corresponding RMS errors are 11·67 W m−2 when only Willis Island data are used. Sources of errors are discussed, as are suggestions for future research.

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TL;DR: In this paper, daily rainfall affinity areas are derived for the island of Mallorca using four years of data, and a 96 × 96 intersite correlation matrix is used to map correlation links between sites.
Abstract: Daily rainfall affinity areas are derived for the island of Mallorca using four years of data, and a 96 × 96 intersite correlation matrix. An initial visual impression of the nature of rainfall organization is afforded by the mapping of correlation links between sites. The Serra de Tramontana form a climatological as well as topographic barrier, producing a marked alignment in spatial rainfall occurrence in that part of the island. Elsewhere ‘core’ sites are identified, about which rainfall correlation is high for distances of up to 15 km. These centre on higher elevation locations in the Serra de Llevant and in the central area, and also the Palma and Campos coastal embayments. Use of rotated principal components and cluster analyses applied to the correlation matrices permits the division of the island into seven major rainfall areas. The uplands of the north-west and east of the island provide clear units in terms of both exposure and altitude, and their general exposure to prevailing rain-bearing winds appears important, whilst an urban effect due to Palma and clear sea-breeze convergence effects near the centre of the island are also implied.

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TL;DR: In this paper, the temporal variability of USA snowfall is investigated for the period 1945-1946 through to 1984-1985 using linear trend and principal components analyses, showing that two regions of the USA evidence significant changes in monthly snowfall over the period.
Abstract: The temporal variability of USA snowfall is investigated for the period 1945–1946 through to 1984–1985 using linear trend and principal components analyses. The results of the linear trend analysis indicate that two regions of the USA evidence significant changes in monthly snowfall over the period. These areas include the Great Lakes/upper mid-west and high plains regions of the USA. In the Great Lakes/upper mid-west sector, positive linear trends are found in monthly snowfall totals for the mid-winter months (December, January, February). For the high plains region, positive linear trends are found for the month of December. Principal components analysis (PCA) is used with seasonal snowfall data in order to better understand the spatial and temporal nature of seasonal snowfall variations across the USA. The PCA isolates six spatially coherent regions in which seasonal snowfall varied similarly over the 40-year period. Only one of these regions, centred on the Great Lakes and upper mid-west, displays any long-term change in seasonal snowfall, a positive trend during the period 1945–1946 through to 1984–1985. These results are discussed in the context of man-induced and natural environmental changes.

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TL;DR: In this paper, the authors used a combination of principal components analysis and cluster analysis to identify 13 distinct synoptic types (clusters) that correspond to seasonal variations in the atmosphere's general circulation.
Abstract: Daily particulate concentrations in the Desert South-west from 1984 to 1988 are analysed using a synoptic climatology based upon tropospheric thermal, moisture, and flow conditions. Using a combination of principal components analysis and cluster analysis, 13 distinct synoptic types (clusters) are identified that correspond to seasonal variations in the atmosphere's general circulation. These include summer clusters, which reflect monsoonal and non-monsoonal conditions, winter clusters defined by the positions of the polar and subtropical jets and shortwave ridges and troughs, and clusters that predominate in the transition seasons. Particulate concentrations vary significantly by synoptic type. Three clusters have poor air quality. ‘Summer monsoon’ days have high humidity and cloud cover, conditions that encourage aerosol growth. ‘Continental high’ days have a slow-moving anticyclone centred over the Great Basin that traps particulates within the boundary layer. ‘Rockies ridge’ days are associated with strong south-westerly flow and the advection of regional haze from southern California into the Desert South-west. In all of these clusters, high humidity or cloud cover enhance particulate growth. Good air quality in the summer occurs on monsoonal ‘break’ days when relative humidity and cloud cover decrease. In the winter, good air quality occurs when Arctic or continental polar air masses are present, or when either the polar or subtropical jets are strong and positioned over the South-west. In some of these latter situations, widespread precipitation removes particulates through wet deposition. These results suggest that the Navajo Generating Station (NGS), a large coal-fired power plant on the Arizona-Utah border, does not contribute significantly to air quality degradation in the Grand Canyon National Park, Arizona. Most of the days with high particulate levels are associated with transport from southern California or unstable summer days with high humidity and contributions from many point and regional sources. There may be some contribution from the NGS on ‘continental high’ days, but when this cluster occurs on consecutive days, air quality improves rather than declines. This research provides a useful framework for future mesoscale modelling studies of air quality in south-western USA.