Showing papers in "International Journal of Climatology in 2001"

North atlantic climate variability: Phenomena, impacts and mechanisms

Abstract: Variability of the North Atlantic Oscillation and the Tropical Atlantic dominate the climate of the North Atlantic sector, the underlying ocean and surrounding continents on interannual to decadal time scales. Here we review these phenomena, their climatic impacts and our present state of understanding of their underlying cause. Copyright © 2001 Royal Meteorological Society.

Trends in extreme daily rainfall and temperature in Southeast Asia and the South Pacific: 1961–1998

Abstract: Trends in extreme daily temperature and rainfall have been analysed from 1961 to 1998 for Southeast Asia and the South Pacific. This 38-year period was chosen to optimize data availability across the region. Using high-quality data from 91 stations in 15 countries, significant increases were detected in the annual number of hot days and warm nights, with significant decreases in the annual number of cool days and cold nights. These trends in extreme temperatures showed considerable consistency across the region. Extreme rainfall trends were generally less spatially coherent than were those for extreme temperature. The number of rain days (with at least 2 mm of rain) has decreased significantly throughout Southeast Asia and the western and central South Pacific, but increased in the north of French Polynesia, in Fiji, and at some stations in Australia. The proportion of annual rainfall from extreme events has increased at a majority of stations. The frequency of extreme rainfall events has declined at most stations (but not significantly), although significant increases were detected in French Polynesia. Trends in the average intensity of the wettest rainfall events each year were generally weak and not significant. Copyright © 2001 Royal Meteorological Society.

Current Approaches to Seasonal to Interannual Climate Predictions

Abstract: This review paper presents an assessment of the current state of knowledge and capability in seasonal climate prediction at the end of the 20th century. The discussion covers the full range of issues involved in climate forecasting, including (1) the theory and empirical evidence for predictability; (2) predictions of surface boundary conditions, such as sea surface temperatures (SSTs) that drive the predictable part of the climate; (3) predictions of the climate; and (4) a brief consideration of the application of climate forecasts. Within this context, the research of the coming decades that seeks to address shortcomings in each area is described. Copyright © 2001 Royal Meteorological Society

Precipitation measurements and trends in the twentieth century

Abstract: Concern about anthropogenic climate change has heightened the need for accurate information about spatial and temporal variations in precipitation at the Earth’s surface. Large-scale precipitation estimates can be derived from either surface gauge measurements or by satellite remote sensing, both of which have shortcomings. Gauge measurements provide information about trends and variability of monthly precipitation throughout the entire twentieth century, but because of the lack of data from most ocean regions, this information is representative of only about 25–30% of the Earth’s surface. In contrast, satellite (especially multi-platform) measurements provide spatially complete coverage at monthly to subdaily resolution, but do not extend back beyond 1974. Merged gauge–satellite datasets maximize (and minimize) the relative benefits (and shortcomings) of each source type. While these merged products only extend back to 1979, their importance will grow as we move into the new century. Precipitation gauge data indicate that global land precipitation (excluding Antarctica) has increased by about 9 mm over the twentieth century (a trend of 0.89 mm/decade), which is relatively small compared with interannual and multi-decadal variability. Within this century-long trend, global precipitation exhibits considerable variability on decadal time-scales, with departures of up to 40 mm from the century mean of about 950 mm. Regionally, precipitation has increased over most land areas, with the exception of tropical North Africa, and parts of southern Africa, Amazonia and western South America. The dominant mode of interannual variability in global and

ENSO variability, teleconnections and climate change

Abstract: An overview is presented of the principal features of the El Nino-Southern Oscillation (ENSO) teleconnections in terms of regional patterns of surface temperature, precipitation and mid-tropospheric atmospheric circulation. The discussion is cast in the context of variations in the associations over time, with decadal scale changes emphasized. In the five decades or so for which we have adequate records to reliably analyse the global aspects of ENSO effects on regional climates around the world, we have witnessed one major decadal scale change in the overall pattern of sea-surface temperatures (SST) in the global ocean, and concomitant changes in the atmospheric response to those changes. The analysis underscores the connection between low frequency changes in tropical SST, ENSO and decadal scale changes in the general atmospheric circulation, pointing to the complex interplay between the canonical ENSO system, slow changes in SST in the Indo-Pacific over the last century, and long-term changes in the atmospheric circulation itself. Published in 2001 by John Wiley & Sons, Ltd.

Regional temperature variability in the european alps: 1760 -1998 from homogenized instrumental time series

Abstract: This paper investigates temperature variability in the Alps and their surroundings based on 97 instrumental series of monthly mean temperatures. A discussion of the initial homogenizing procedure illustrates its advantages and risks. A comparison of the homogenized series with the original series clearly shows the necessity to homogenize. Each of the original series had breaks (an average of five per series) and the mean of all series was systematically biased by non-climatic noise. This noise has subdued the long-term amplitude of the temperature evolution in the region by 0.5 K. The relatively high spatial resolution of the data enabled a regionalization within the study area of 680000 km 2 into six sub-regions based on principal component analysis of the monthly series. Long-term temperature evolution proved to be highly similar across the region—thus making a mean series (averaged over all 97 single series) representative of the study area. Trend analysis (based on progressive forward and backward Mann–Kendall statistics and on progressive analysis of linear regression coefficients) was performed on seasonal and annual series. The results diverge from those of global datasets. This is mainly due to the extension of the 240-year Alpine dataset by 100 years prior to the mid-19th century, and also due to the advantages of a dense and homogenized regional dataset. The long-term features include an initial decrease of the annual and seasonal series to a minimum followed by a positive trend until 1998. The minima are 1890 for the entire year and winter, 1840 for spring and 1920 for summer and autumn, respectively. The initial decreasing trend is more evident in spring and summer, less in autumn and smallest in winter. The mean annual temperature increase since 1890 in the Alps is 1.1 K, which is twice as much as the 0.55 K in the respective grid boxes of the most frequently used global dataset of the Climatic Research Unit (CRU), University of East Anglia. To enable an easier and more systematic handling of the dataset, these data have been interpolated to a 1° ×1° longitude–latitude grid. The 105 low-elevation and 16 high-elevation grid point series are widely available without restrictions for scientific research and can be obtained from the authors. Copyright © 2001 Royal Meteorological Society.

Seasonality and atmospheric dynamics of the teleconnection between African rainfall and tropical sea‐surface temperature: Atlantic vs. ENSO

Abstract: A 47-year record (1951-1997) of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea-surface temperatures (SST) in key tropical areas, as depicted by the NIN O3, South Atlantic and North Atlantic indices. El Nino -Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to different parts of the rainfall variance. The correlation with South Atlantic SST appears as a south-north dipole (positive/negative correlation) which shifts northward following the Inter-tropical Convergence Zone (ITCZ) translation between the northern low-sun and high-sun periods. A typing of the seasonal correlation patterns and a mapping of the multiple correlation coefficients are carried out in order to synthesize the space-time impacts of the three SST indices. Decadal-scale changes affect the strength of the teleconnections with both Atlantic and East Pacific SST, as reflected for instance by a small rise of the correlation with the NIN O3 index since 1970-1975 in the Sahel and southern Africa, and additional shifts for the Atlantic Ocean, but the main patterns remain generally apparent over the whole period. The circulation anomalies associated with the teleconnections were assessed using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. A study of the dataset accuracy in depicting long-term climatic variations revealed that a major shift, mainly artificial, is found in 1967-1968 in the time-series of most of the variables. The rest of the work thus concentrated on the 1968-1997 period. A number of changes in east-west circulation patterns have been found to be associated to ENSO variations. Over West Africa, El Nino events tend to result in enhanced northeasterlies/reduced monsoon flow, coupled to weakened upper easterlies, and hence dry conditions over West Africa close to the surface position of the ITCZ, in July-September, as well as January-March. Over the southwestern Indian Ocean, the positive equatorial temperature/geopotential height anomalies, which at 200 hPa accompany El Nino events, are conducive to an eastward shift of the mid-latitude upper troughs, thus being detrimental to summer rainfall over South Africa. Abnormally wet 'short rains' in East Africa can be accounted for by an ENSO-forced weakening of the equatorial Walker-type (east-west) cell which is found over the Indian Ocean during that season. By contrast, the impact of South Atlantic warmings is mostly shown in low-level dynamics, as exemplified by the weakened trades and monsoon flow which directly result in a southward shift of the ITCZ. The combination of ENSO and Atlantic SST anomalies are found to give rise to complex wind flow changes in the near-equatorial Atlantic. In addition to large-scale SST-forced atmospheric dynamics, a few regional atmospheric signals are found to explain residual parts of rainfall variance. For instance, a strengthening of the African Easterly Jet, or northerly wind anomalies across the Sahara, are shown to be related to drought conditions in the Sahel (July-September) and the Gulf of Guinea area (January-March), once the remote effect of SST anomalies is removed. Copyright © 2001 Royal Meteorological Society.

Have streamflow droughts in Europe become more severe or frequent

Abstract: Changes in the magnitude and frequency of droughts will have extensive impacts on water management, agriculture and aquatic ecosystems. With the projected global temperature increase, scientists generally agree that the global hydrological cycle will intensify and suggest that extremes will become or have already become more common. In this study, a pan-European dataset of more than 600 daily streamflow records from the European Water Archive (EWA) was analysed to detect spatial and temporal changes in streamflow droughts. Four different time periods were analysed: 1962–1990, 1962–1995, 1930–1995 and 1911–1995. The focus was on hydrological droughts derived by applying the threshold level approach, which defines droughts as periods during which the streamflow is below a certain threshold. The Annual Maximum Series (AMS) of drought severity and the frequency of droughts in Partial Duration Series (PDS) were studied. Despite several reports on recent droughts in Europe, the non-parametric Mann–Kendall test and a resampling test for trend detection showed that it is not possible to conclude that drought conditions in general have become more severe or frequent. The period analysed and the selection of stations strongly influenced the regional pattern. For most stations, no significant changes were detected. However, distinct regional differences were found. Within the period 1962–1990 examples of increasing drought deficit volumes were found in Spain, the eastern part of Eastern Europe and in large parts of the UK, whereas decreasing drought deficit volumes occurred in large parts of Central Europe and in the western part of Eastern Europe. Trends in drought deficit volumes or durations could, to a large extent, be explained through changes in precipitation or artificial influences in the catchment. Changes in the number of drought events per year were determined by the combined effect of climate and catchment characteristics such as storage capacity. The importance of the time period chosen for trend analysis is illustrated using two very long time series. Copyright © 2001 Royal Meteorological Society

An evaluation of the Standardized Precipitation Index, the China-Z Index and the statistical Z-Score

Abstract: The Standardized Precipitation Index (SPI) was developed to detect drought and wet periods at different time scales, an important characteristic that is not accomplished with typical drought indices. More and more users employ the SPI to monitor droughts. Although calculation of the SPI is easier than other drought indices, such as the Palmer Drought Index, it is still relatively complex. In China, an index called the China-Z Index (CZI) has been used since 1995 by the National Climate Centre of China to monitor moisture conditions across the country. The calculation of this index is easier than the SPI. A third index, the statistical Z-Score, can also be used to monitor droughts. This paper evaluates the SPI, CZI and Z-Score on 1-, 3-, 6-, 9- and 12-month time scales using monthly precipitation totals for four locations in China from January 1951 to December 1998 representing humid and arid climates, and cases of drought and flood. Advantages and disadvantages for the application of each index are compared. Study results indicate that the CZI and Z-Score can provide results similar to the SPI for all time scales, and that the calculations of the CZI and Z-Score are relatively easy compared with the SPI, possibly offering better tools to monitor moisture conditions. Copyright © 2001 Royal Meteorological Society

Interdecadal pacific oscillation and south pacific climate

Abstract: The Interdecadal Pacific Oscillation (IPO) has been shown to be associated with decadal climate variability over parts of the Pacific Basin, and to modulate interannual El Nino-Southern Oscillation (ENSO)-related climate variability over Australia. Three phases of the IPO have been identified during the 20th century: a positive phase (1922-1944), a negative phase (1946-1977) and another positive phase (1978-1998). Climate data are analysed for the two most recent periods to describe the influence of the IPO on decadal climate trends and interannual modulation of ENSO teleconnections throughout the South West Pacific region (from the equator to 55°S, and 150°E to 140°W). Data coverage was insufficient to include the earliest period in the analysis. Mean sea level pressure (SLP) in the region west of 170°W increased for the most recent positive IPO period, compared with the previous negative phase. SLP decreased to the east of 170°W, with generally more southerly quarter geostrophic flow over the region. Annual surface temperature increased significantly southwest of the South Pacific Convergence Zone (SPCZ) at a rate similar to the average Southern Hemisphere warming. Northwest of the SPCZ temperature increases were less, and northeast of the SPCZ more than the hemispheric warming in surface temperature. Increases of annual precipitation of 30% or more occurred northeast of the SPCZ, with smaller decreases to the southwest, associated with a movement in the mean location of the SPCZ northeastwards. The IPO modulates teleconnections with ENSO in a complex way, strengthening relationships in some areas and weakening them in others. For New Zealand, there is a consistent bias towards stronger teleconnections for the positive IPO period. These results demonstrate that the IPO is a significant source of climate variation on decadal time scales throughout the South West Pacific region, on a background which includes global mean surface temperature increases. The IPO also modulates interannual ENSO climate variability over the region. Copyright © 2001 Royal Meteorological Society.

Precipitation and atmospheric circulation patterns at mid-latitudes of Asia

Abstract: Analyses of the coupling between large-scale atmospheric patterns and modifications of regional precipitation regimes at seasonal and annual time scales in different terrain of mid-latitudes in Asia, including western Siberia, Tien Shan and Pamir mountains, and plains of middle Asia and Japanese Islands, were examined based on data from 57 and 88 hydro-climatic stations with 100 and 60 year records, respectively. For the past 100 years, a positive trend in precipitation was revealed in western Siberia, northern regions of Tien Shan and Japanese Islands. North Atlantic Oscillation (NAO) and West Pacific Oscillation (WPO) indices have inverse associations, with average amount of precipitation in western Siberia and in mountains and plains of middle Asia, and positive correlation in central and western regions of Japanese Islands. The Pacific North American (PNA) index is positively correlated with annual precipitation over most of the Japanese Islands. Northern Asian (NA) positive anomalies lead to decrease in winter precipitation in the western and eastern regions of Japanese Islands. We did not find significant impact of PNA or NA on precipitation in middle Asia. We suggest that during the last century, impacts of the western jet stream increased in the northern regions of Tien Shan and Japanese Islands, and weakened in the eastern Japanese Islands. There is a suggestion that conditions are more favourable for precipitation development over continental regions of Asia when the Siberian High is positioned further to the east than further to the west. During dominant development of a zonal atmospheric pattern, the annual and seasonal precipitation decreased over most regions in continental Asia and central Japan. Copyright © 2001 Royal Meteorological Society

Developments and prospects in synoptic climatology

Abstract: Developments in synoptic climatology in the 1990s included advances in traditional synoptic climatology, empirical downscaling, and dynamical downscaling (i.e. regional climate modelling). The research emphasis in traditional, empirical–statistical approaches to synoptic climatology shifted from methodological development to applications of widely accepted classification techniques, including manual, correlation-based, eigenvector-based, compositing and indexing schemes. In contrast, most efforts in empirical downscaling, which became a well-established field of synoptic climatology during the 1990s, were directed to model development; applications were of secondary concern. Similarly, regional climate models (RCMs) burst onto the scene during the decade and focused on model development, although important progress was made in linking or coupling RCMs to regional or local surface climate systems. This paper discusses prospects for the future of traditional synoptic climatology, empirical downscaling and regional climate modelling. It concludes by looking at the present role of geographic information system (GIS) concepts in synoptic climatology and the potential future role of GIS to the field. Copyright © 2001 Royal Meteorological Society

The Pacific–South American modes and their downstream effects

Abstract: There are two pervasive modes of atmospheric variability in the Southern Hemisphere (SH) that influence circulation and rainfall anomalies over South America. They appear as leading empirical orthogonal functions (EOFs) of 500-hPa height or 200-hPa streamfunction anomalies and are found from intraseasonal to decadal time scales. Both patterns exhibit wave 3 hemispheric patterns in mid to high latitudes, and a well-defined wave train with large amplitude in the Pacific–South American (PSA) sector. Therefore, they are referred to as the PSA modes (PSA1 and PSA2). PSA1 is related to sea surface temperature anomalies (SSTAs) over the central and eastern Pacific at decadal scales, and it is the response to El Nino–Southern Oscillation (ENSO) in the interannual band. The associated rainfall summer pattern shows rainfall deficits over northeastern Brazil and enhanced rainfall over southeastern South America similar to rainfall anomalies during ENSO. PSA2 is associated with the quasi-biennial component of ENSO, with a period of 22–28 months and the strongest connections occur during the austral spring. The associated rainfall pattern shows a dipole pattern with anomalies out of phase between the South Atlantic Convergence Zone (SACZ) extending from central South America into the Atlantic and the subtropical plains centred at 35°S. These two modes are also apparent in tropical intraseasonal oscillations for both summer and winter. Eastward propagation of enhanced convection from the Indian Ocean through the western Pacific to the central Pacific is accompanied by a wave train that appears to originate in the convective regions. The positive PSA1 pattern is associated with enhanced convection over the Pacific from 150°E to the date line. The convection pattern associated with PSA2 is in quadrature with that of PSA1. Both PSA modes are influenced by the Madden Julian Oscillation and influence rainfall over South America. Copyright © 2001 Royal Meteorological Society

Downscaling temperature and precipitation: a comparison of regression‐based methods and artificial neural networks

Abstract: A comparison of two statistical downscaling methods for daily maximum and minimum surface air temperature, total daily precipitation and total monthly precipitation at Indianapolis, IN, USA, is presented. The analysis is conducted for two seasons, the growing season and the non-growing season, defined based on variability of surface air temperature. The predictors used in the downscaling are indices of the synoptic scale circulation derived from rotated principal components analysis (PCA) and cluster analysis of variables extracted from an 18-year record from seven rawinsonde stations in the Midwest region of the United States. PCA yielded seven significant components for the growing season and five significant components for the non-growing season. These PCs explained 86% and 83% of the original rawinsonde data for the growing and non-growing seasons, respectively. Cluster analysis of the PC scores using the average linkage method resulted in eight growing season synoptic types and twelve non-growing synoptic types. The downscaling of temperature and precipitation is conducted using PC scores and cluster frequencies in regression models and artificial neural networks (ANNs). Regression models and ANNs yielded similar results, but the data for each regression model violated at least one of the assumptions of regression analysis. As expected, the accuracy of the downscaling models for temperature was superior to that for precipitation. The accuracy of all temperature models was improved by adding an autoregressive term, which also changed the relative importance of the dominant anomaly patterns as manifest in the PC scores. Application of the transfer functions to model daily maximum and minimum temperature data from an independent time series resulted in correlation coefficients of 0.34–0.89. In accord with previous studies, the precipitation models exhibited lesser predictive capabilities. The correlation coefficient for predicted versus observed daily precipitation totals was less than 0.5 for both seasons, while that for monthly total precipitation was below 0.65. The downscaling techniques are discussed in terms of model performance, comparison of techniques and possible model improvements. Copyright © 2001 Royal Meteorological Society.

Changes in total precipitation, rainy days and extreme events in northeastern Italy

Abstract: A new daily precipitation dataset, for the period 1920–1998, comprising seven stations located in the northeastern Italy is presented. Seasonal and annual precipitation and number of rainy days have been analysed and a study aimed at investigating precipitation intensity and extreme events has been performed. Precipitation intensity has been analysed through studying both the mean precipitation amount per wet day and dividing precipitation into heavy and non-heavy classes. In addition, the return period of the extreme events has been calculated for 30-year running windows and its variations have been examined. The results show a negative trend in the number of wet days associated with an increase in the contribution of heavy rainfall events to total precipitation. This is in agreement with a reduction in return period for extreme events since 1920. Copyright © 2001 Royal Meteorological Society

Spatial variations in the average rainfall–altitude relationship in Great Britain: an approach using geographically weighted regression

Abstract: The relationship between annual rainfall totals and gauge elevation over Great Britain is re-examined using the recently developed technique of geographically weighted regression (GWR). This enables the spatial drift of regression parameters to be identified, estimated and mapped. It is shown that the rate of increase of precipitation with height, or height coefficient, varies from around 4.5 mm:m in the northwest to almost zero in the southeast. There is a particularly rapid change in this value across the English Midlands. The predicted sea level precipitation varies from 1250 mm to less than 600 mm in much the same way. Copyright © 2001 Royal Meteorological Society.

Principal modes of interannual and decadal variability of summer rainfall over South America

Abstract: Using the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) product together with the Goddard Earth Observing System (GEOS) reanalysis and the National Center for Environmental Prediction (NCEP) sea-surface temperature (SST) data, we have conducted a diagnostic study of the interannual and decadal scale variability of principal modes of summer rainfall over South America for the period 1979–1995. By filtering the annual and short (<12 months) time-scale variations, results of empirical orthogonal function analysis show three leading modes of rainfall variation identified with interannual, decadal and long-term variability. Together, these modes explain more than half the total variance of the filtered data. The first mode is highly correlated with El Nino–Southern Oscillation (ENSO), showing a regional rainfall anomaly pattern largely consistent with previous results. This mode captures the summer season interannual variability, not only the Northeast Brazil drought but also its connection with excessive rainfall over Southern Brazil and the Ecuador coast in El Nino years. Another distinctive feature is the strengthening of the low-level flow along the eastern foothills of the eastern Andes, signifying an enhancement of the South American summer monsoon in response to an El Nino anomaly. The decadal variation displays a meridional shift of the Inter-Tropical Convergence Zone (ITCZ), which is tied to the anomalous cross-equatorial SST gradient over the Atlantic and the eastern Pacific. Associated with this mode is a large-scale mass swing between polar regions and the mid-latitudes. Over the South Atlantic and the South Pacific, the anomalous subtropical high and the associated anomalous surface wind are dynamically consistent with the distribution of local SST anomalies, suggesting the importance of atmospheric forcing at the decadal time scale. The long-term variation shows that since 1980 there has been a decrease of rainfall from the northwest coast to the southeast subtropical region and a southwards shift of the Atlantic ITCZ, leading to increased rainfall over northern and eastern Brazil. Possible links of this mode to the observed SST warming trend over the subtropical South Atlantic and to the interdecadal SST variation over the extratropical North Atlantic are discussed. Copyright © 2001 Royal Meteorological Society

Trends in the daily intensity of precipitation in Italy from 1951 to 1996

Abstract: An analysis of 67 sites of daily precipitation records over the 1951–1996 period for Italy is presented Seasonal and yearly total precipitation (TP), number of wet days (WDs) and precipitation intensity (PI) are investigated, and the trends both for the single station records, and for some different area average series are studied PI is analysed by attributing precipitation to ten class-intervals, removing the influence of variations in the number of WDs to yield changes in the underlying shape of the WD amount distribution The results show that the trend for the number of WDs in the year is significantly negative throughout Italy, stronger in the north than in the south: this trend is mainly a result of the winter Moreover, they show that there is a tendency toward an increase in PI This increase is globally less strong and significant than the decrease in the number of WDs It is not concentrated in one specific season, but changes from area to area, and is generally weak in winter, due to a significant decrease of winter TP In northern Italy, the increase in PI is mainly owing to a strong increase in the heaviest events, while in central–southern Italy, it depends on a larger part of the distribution of WD amounts The analysis of the evolution of the class-interval contributions shows that the positive trend of the heaviest events starts in the 1970s, as does the negative trend of lightest events Copyright © 2001 Royal Meteorological Society

Monsoon rainfall variations and teleconnections over South and East Asia

Abstract: Seasonal summer monsoon (June–September) data for 120 stations over East Asia (China, Japan, Mongolia, Korea) varying from 1881 to 1998 are utilized to understand their interannual and climate characteristics, and to investigate their teleconnections with South Asian (in particular India's) monsoon rainfall. Contemporaneous relations on an interannual time-scale reveal that the rainfall variations over north China (southern Japan) are in-phase (out-of-phase) with South Asian rainfall. Based on the instrumental data available, regional rainfall anomaly time series for the 118-year period for the two coherent regions, over north China and southern Japan are prepared. All the three series (India, China, Japan) have been subjected to statistical tests. Results reveal that while there are year-to-year fluctuations, the Mann–Kendall rank statistic suggests no significant long-term trends. However, the application of Cramer's statistic to study the short-term climate variability depicts decadal variability with certain epochs of above and below normal rainfall over each region. The epochs tend to last for about three decades over India and China, and about five decades over Japan. The turning points for China follow those of India about a decade later. The relationships of South and East Asian monsoon rainfall exhibit secular variations. The inter-connections between the monsoon-related events (rainfall over South Asia, rainfall over East Asia, Northern Hemisphere circulation, tropical Pacific circulation) appear to strengthen (or weaken) around the same time, implying that the monsoon related events over geographically separated regions seem to get linked (or delinked) around the same time. Copyright © 2001 Royal Meteorological Society

A 40 year objective climatology of surface cyclones in the Mediterranean region: spatial and temporal distribution

Abstract: A climatological analysis of the synoptic scale cyclones that occur in the Mediterranean region is carried out for an extended period of 40 years (1958–1997). The cyclones are identified with the aid of an objective method based on grid point values, available every 6 h. The study of the seasonal and diurnal variations of the frequency of cyclonic occurrences, with respect to the value of the central pressure, revealed that the major cyclonic centres appear to be different in frequency and location, reflecting the different triggering mechanisms. In the western Mediterranean region, the frequency of occurrence is higher in winter, more so for the intense cyclones, with no substantial diurnal difference in the Gulf of Genoa, and with a preference during the night in southern Italy. The Cyprus centre reveals diurnal variability in location, especially in the cold period, in accordance with the triggering mechanism, being associated principally with weak and moderate cyclones. In northwestern Africa, there appears to be two marked regions of high cyclonic frequency, not only in spring, but also in summer and autumn. The extremely high frequency of cyclonic occurrences in summer in the Mediterranean basin is attributed mainly to the generation of weak thermal lows. The number of rapidly deepening cyclones within 12 h and 24 h increases from the south to the north part of Mediterranean, being mostly favoured near the lee of Alps and the Gulf of Venice, occurring mainly after 06:00 co-ordinated universal time (UTC). Copyright © 2001 Royal Meteorological Society

Rapid methods to estimate sky‐view factors applied to urban areas

Abstract: A common parameter used to characterize the geometry of urban canyons is the sky-view factor (ψs). Here, two simple alternatives are presented that can be used both objectively and rapidly to estimate ψs. The first method uses a Nikon CoolPix 950 digital camera fitted with a Nikon FC-E8 fisheye lens. The second method involves a LI-COR LAI-2000 Plant Canopy Analyzer to measure automatically diffuse non-interceptance (DIFN) light using a fisheye optical sensor. Through a series of intercomparisons for urban canyons of known geometry, the digital camera is shown to provide accurate estimates of ψs. The LAI-2000 also performs well, although it tends to over-estimate ψs (for the conditions considered here, the mean absolute error is 0.04), and has a more restricted set of sky conditions under which it performs well (ideally, uniform overcast skies). For both methods, data collection and post-processing is rapid, and storage of data is straightforward. Thus, mobile data collection is possible which allows detailed information on the spatial variability of ψs in urban areas to be determined. An example of such an application for a small US city, Bloomington, IN, is presented. Copyright © 2001 Royal Meteorological Society

The relationship between the wintertime north Atlantic oscillation and blocking episodes in the north Atlantic

Abstract: A systematic examination of the dynamical relationship between the North Atlantic Oscillation (NAO) and atmospheric blocking episodes in the North Atlantic during winter is undertaken. Employing the blocking criteria, as defined by Tibaldi and Molteni (1990), we first establish a statistical relationship, through compositing and linear regression analysis, between the two phenomena. The results show that the frequency of blocking formations in the North Atlantic region is sensitive to the phase of the NAO. Sixty-seven percent more winter blocking days are observed during the negative than the positive phase of the NAO. The lifetime of blocking episodes is also sensitive to the phase of the NAO. When the NAO is in the negative phase, the distribution of the length of blocking varies considerably. The average length of blocking during the negative phase is about 11 days, which is nearly twice as long as the 6-day length observed during the positive phase of the NAO. The NAO accounts for about 30% of the variation in the wintertime North Atlantic blocking episodes. We propose a conceptual model that strengthens the statistical association and offers an explanation for a dynamical connection between the occurrences of blocking and the NAO in the North Atlantic. Application of a low-order theoretical model by Charney and DeVore (1979) and an analysis of Northern Hemisphere observed surface temperature suggest that the NAO-related difference in blocking frequency and persistence are associated with changes in the zonally asymmetric thermal forcing which, to a large extent, is determined by the phase of the NAO. For the negative phase of the NAO, the distribution of the surface air temperature anomaly is the distinctive ‘warm ocean/cold land’ pattern related to the resonance forcing of topography and creates a dynamical environment favourable for the formation and persistence of blocks. For the positive phase of the NAO, on the other hand, the distribution of the surface air temperature anomalies is the distinctive ‘cold ocean/warm land’ pattern, which reduces or destroys the resonance forcing of topography and is unfavourable for the development and persistence of blocks. Copyright © 2001 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.

Patterns of monthly rainfall shortage and excess in terms of the standardized precipitation index for Catalonia (NE Spain)

Abstract: Spatial and time behaviours of rainfall shortage and excess are analysed for Catalonia (NE Spain) using a database obtained from 99 rain gauges with monthly totals collected from 1961 to 1990. The distribution of monthly amounts for each rain gauge is modelled by means of the gamma or Poisson-gamma distributions. Then, using an equiprobable transformation, monthly amounts described with these distributions are substituted by values given by the Standardized Precipitation Index (SPI), which follows a standardized normal distribution and provides a unique pluviometric scale. After that, principal component analysis (PCA) is applied to the set of monthly SPIs. A double regionalization of the 99 rain gauges, distinguishing between episodes of rainfall shortage and excess, is achieved by taking into account the rotated factor loadings (RFL) correlating rain gauges and principal components (PC). A time classification of rainfall shortage and excess episodes is also established, considering in this case the factor scores (FS) obtained after a PCA of variables based on monthly SPIs. The spatial regionalization achieved becomes a rough picture of the different topographic domains (Pyrenees, Pre-Pyrenees, Central Basin, Littoral and Pre-Littoral chains and Mediterranean coast), the climatic diversity of Catalonia being enhanced by these results. The time clustering suggests a quite complex behaviour of the rainfall shortage and excess episodes. Moreover, the spatial distribution of these time clusters is very disperse, in such a way that monthly shortage and excess sometimes affect the whole of Catalonia and sometimes just a small area. Besides results obtained from PCA and clustering algorithms, it is worth noticing that the severity of the episodes increases remarkably only for rainfall shortage. In addition, an analysis of the number of rain gauges affected by monthly shortage and excess shows an interesting fact: whereas the number of rain gauges associated with a shortage has an increasing tendency, a significant decreasing tendency for excess is detected in the period 1961–1990. Copyright © 2001 Royal Meteorological Society

An objective classification of rainfall events on the basis of their convective features: application to rainfall intensity in the northeast of spain

Abstract: A system of rainfall events classification on the basis of their hydrometeorological characteristics is proposed. The objective is the characterization of the different event classes and their application in modelling intensity–duration–frequency curves and design hyetographs. A parameter related to the greater or lesser convective character of the precipitation, designated as β*, is defined, while its distribution throughout the entire series of the sample is studied. In addition, the main features of the different classes obtained and their relationship to floods and rainfall damage events have been analysed. The intensity series of the Jardi pluviograph (Barcelona, Spain) between 1927 and 1981 is used as a sample series. Copyright © 2001 Royal Meteorological Society

Atmospheric circulation and surface temperature in Europe from the 18th century to 1995

Abstract: The relationship between surface atmospheric circulation and temperature in Europe from the 1770s to 1995 is examined using correlation analysis. The atmospheric circulation is represented by six indices: the three leading principal components (PCs) of an empirical orthogonal function (EOF) analysis of 20 European pressure series from 1822 to 1995, which represent the central tendency of European pressure (EOF 1), a zonal circulation pattern (EOF 2) and a meridional pattern (EOF 3), a North Atlantic zonal index constructed from Gibraltar and Reykjavik pressure series for 1821–1995; a Western European zonal index constructed from Madrid, Barcelona, Lund and Trondheim for 1786–1995; and an index constructed from Paris and London, 1774–1995. Eight long temperature series from northwestern and central Europe were correlated with these circulation indices. European temperatures in general had the highest correlations with the zonal circulation indices in winter, with almost 70% of the variability in the temperature records explained by variations in the zonal index. The correlation coefficients between PC 3 (representing meridional circulation) and temperatures were highest in spring and autumn, particularly for Scandinavia. Running correlation series calculated over 25-year windows reveal significant non-stationarities in the relationship between surface temperature and atmospheric circulation on decadal time scales, suggesting caution must be used in extrapolating current relationships between circulation and temperature for future climate predictions based on downscaling or past palaeoclimatic reconstructions. Copyright © 2001 Royal Meteorological Society

20th century droughts in southern Africa: spatial and temporal variability, teleconnections with oceanic and atmospheric conditions

Abstract: Southern African rainfall does not show any trend to desiccation during the 20th century. However, the subcontinent experienced particularly severe droughts in the 1980s and at the beginning of the 1990s and the magnitude of the interannual summer rainfall variability shows significant changes. Modifications of the intensity and spatial extension of droughts is associated with changes in ocean–atmosphere teleconnection patterns. This paper focuses mostly on the well-documented 1950–1988 period and on late summer season (January–March). A principal component analysis on southern African rainfall highlights modifications of the rainfall variability magnitude. The 1970–1988 period had more variable rainfall, and more widespread and intense droughts than the 1950–1969 period. To investigate the potential modifications of the associated ocean–atmosphere teleconnection patterns, a composite analysis is performed on sea-surface temperature (SST) and National Center for Environmental Protection (NCEP) atmospheric parameters, according to the 5 driest years of both sub-periods. Significant changes are shown in ocean–atmosphere anomaly patterns coincident with droughts for both sub-periods. The 1950–1969 droughts were associated with regional ocean–atmosphere anomalies, mainly over the southwest Indian Ocean region. In contrast, during the 1970–1988 droughts near-global anomalies were observed in the tropical zone, corresponding to El Nino–Southern Oscillation (ENSO) phenomenon. Within the whole century, significant correlations between Southern Oscillation Index (SOI) and southern African Rainfall Index (SARI) were found in the periods (1900–1933 and 1970–1998) when SOI and SARI experienced high variability, and when southern Africa was affected by intense and extended droughts. During periods of low SOI (1934–1969), correlations became less significant and droughts were less intense and widespread. Copyright © 2001 Royal Meteorological Society

Satellite rainfall climatology: a review

Abstract: The monitoring of the world's climate using conventional means is well established, incorporating numerous historical data sets to ensure a long-term record. Parameters such as temperature have been quite accurately recorded over the last 200 years or so, but other parameters such as rainfall are less well recorded. This paper introduces satellite rainfall climatology and why it is important, not only from the longer term study of rainfall, but also for the future monitoring and estimation of rainfall. A summary of the range of satellites and their instruments is reviewed to provide a background to the techniques. These are discussed and their relative merits explored. Finally, examples of the techniques are shown, illustrating the applications to which the satellite-derived data sets may be put. Copyright © 2001 Royal Meteorological Society

Trends in New Zealand daily temperature and rainfall extremes

Abstract: Trends in daily temperature and rainfall indices are described for New Zealand. Two periods were examined: 1951–1998, to describe significant trends in temperature and rainfall parameters; and 1930–1998, to ascertain the effects of two main circulation changes that have occurred in the New Zealand region around 1950 and 1976. Indices examined included frequencies of daily maximum and minimum temperatures, above and below specified percentile levels and at those levels, as well as frequencies of these above and below fixed temperature thresholds. Extreme daily rainfall intensity and frequency above the 95th percentile and the length of consecutive dry day sequences were the rainfall indices selected. There were no significant trends in maximum temperature extremes (‘hot days’) but a significant increase in minimum temperatures was associated with decreases in the frequency of extreme ‘cold nights’ over the 48-year period. There was a non-significant tendency for an increase in the frequency of maximum temperature extremes in the north and northeast of New Zealand. A decline occurred in frequency of the minimum temperature 5th percentile (‘cold nights’) of 10–20 days a year in many locations. Trends in rainfall indices show a zonal pattern of response, with the frequency of 1-day 95th percentile extremes decreasing in the north and east, and increasing in the west over the 1951–1996 period. Changes in the frequency of threshold temperatures above 24.9°C (25°C days) and below 0°C (frost days) are strongly linked to atmospheric circulation changes, coupled with regional warming. From 1930–1950 more south to southwest anomalous flow occurred relative to later years. In this period, 25°C days were less frequent in all areas except the northeast, and there was markedly more frost days in all but inland areas of the South Island compared with the 1951–1975 period. There was more airflow from the east and northeast from 1951 to 1975, the frequency of 25°C days increased and frost days decreased in many areas of New Zealand. In the final period examined (1976–1998), more prevalent airflow from the west and southwest was accompanied by more anticyclonic conditions. Days with a temperature of 25°C increased in the northeast only. Frost day frequencies decreased between 5 and 15 days a year in many localities, with little change in the west of the South Island and at higher elevation locations. Copyright © 2001 Royal Meteorological Society

A conceptual model for understanding rainfall variability in the West African Sahel on interannual and interdecadal timescales

Abstract: This article describes and validates a new conceptual model for understanding Sahel rainfall variability. This conceptual model provides a framework that can readily incorporate and synthesize the roles played by the oceans, the African landmass and local meteorological factors. The most important local factors are the location of the African Easterly Jet (AEJ) and the associated shears. The position of the AEJ helps to distinguish between a wet mode and a dry mode in the Sahel, while other factors determine which of two spatial patterns prevail during years of the dry regime. We test the paradigm by contrasting selected circulation parameters for the years 1958-1967 (representing the wet mode) and 1968-1997 (representing the dry mode). In doing so, we have identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions. The AEJ is further southward and more intense, the Inter-tropical Convergence Zone (ITCZ) is further south, the Tropical Easterly Jet (TEJ) is weaker, the equatorial westerlies are shallower and weaker, the southwesterly monsoon flow is weaker, and the relative humidity is lower (but not consistently so). The results of this study suggest that the key factor controlling the occurrence of the wet Sahel mode versus the dry mode is the presence of deep, well-developed equatorial westerlies. These displace the AEJ northward into Sahelian latitudes and increase the shear instabilities. The westerlies appear to be at least partially responsible for the well-known association between a weaker AEJ and wetter conditions in the Sahel, because the thermal wind induced by the Sahara/Atlantic temperature gradient is imposed upon a westerly basic state. Since one of the strongest contrasts between the wet Sahel and dry Sahel modes is the strength of the TEJ, the TEJ probably also plays a pivotal role in rainfall variability. In the dry mode, the equatorial westerlies are poorly developed and the core of the AEJ lies well to the south of the Sahel. The dry mode consists of two basic spatial patterns, depending on whether the Guinea Coast Region is anomalously wet or dry (the well-known dipole and no-dipole patterns, respectively). Which occurs is determined by other factors acting to reduce the intensity of the rainbelt. One of the relevant factors appears to be sea-surface temperatures (SSTs) in the Gulf of Guinea.