Showing papers in "International Journal of Climatology in 2004"

North Atlantic oscillation influence on precipitation, river flow and water resources in the Iberian Peninsula

Abstract: The Iberian Peninsula precipitation and river flow regimes are characterized by large values of inter-annual variability, with large disparities between wet and dry years, especially in southern Iberia. This situation is a major problem for water resources management in general, and for the production of hydroelectricity in particular. Hydroelectric production represents, in an average year of precipitation, 20% of the total Spanish electricity production (and 35% for Portuguese production). Its absolute value, however, can vary by a factor of three between wet and dry years. We have assessed the impact of the North Atlantic oscillation (NAO) on winter precipitation and river flow regimes for the three main international Iberian river basins, namely the Douro (north), the Tejo (centre) and the Guadiana (south). Results show that the large inter-annual variability in the flows of these three rivers is largely modulated by the NAO phenomenon. Throughout most of the 20th century, the January-to-March river flow is better correlated with the December to February (DJF) NAO index than is the simultaneous (DJF) river flow. Correlation values for the period 1973-98 are highly significant (-0.76 for Douro, -0.77 for Tejo and -0.79 for Guadiana), being consistently of higher magnitude than those obtained over previous decades. This impact of the NAO on winter river flow was quantified in terms of total Spanish potential hydroelectricity production. The important control exerted by the NAO and the recent positive trend in the NAO index contribute to a significant decrease in the available flow. This reduction represents an important hazard for the two Iberian economies because of its negative impact on water-dependent resources, such as intensive agriculture and hydroelectric power production.

Recent changes in rainfall and rainy days in Ethiopia

Abstract: Changes in annual, June–September and March–May rainfall and rainy days herein (defined as a day with rainfall greater than 1 mm) have been analysed based on 11 key stations located in different climatic zones of Ethiopia over the common period 1965–2002. The progressive Mann–Kendall trend test shows that there is no trend in the annual rainfall total, the seasonal rainfall total or rainy days over central, northern and northwestern Ethiopia in the period 1965–2002. In contrast, the annual and the June–September total rainfalls for the eastern (Jijiga, 137 mm/decade), southern (Negele, 119 mm/decade) and southwestern (Gore 257 mm/decade) stations show a significant decline since about 1982. Correlation analysis shows that the decline of rainfall in eastern, south and southwestern Ethiopia is caused by the corresponding persistent warming of the South Atlantic Ocean over the period approximately from 1986 to 2002. The sea-surface temperature (SST) over the tropical eastern Pacific Ocean is not significantly correlated with the main rainfall of the semi-arid lowland areas of eastern, southern, and southwestern Ethiopia, except at marginal zones in transition to the Ethiopian Highlands. June–September rainfall over the Ethiopian Highlands is positively correlated to the equatorial east Pacific sea-level pressure and the southern oscillation index, and negatively correlated to SST over the tropical eastern Pacific Ocean as expected, confirming again that warm El Nino–southern oscillation episodes are associated with below-average June–September rainfall over the Ethiopian Highlands. Copyright © 2004 Royal Meteorological Society

The El Niño–southern oscillation and Antarctica

Abstract: This paper reviews our understanding of how the effects of the El Nino–southern oscillation (ENSO) might be transmitted from the tropical Pacific Ocean to the Antarctic, and examines the evidence for such signals in the Antarctic meteorological, sea ice, ice core and biological records. Many scientific disciples concerned with the Antarctic require an understanding of how the climatic conditions in the tropical and mid-latitude regions affect the Antarctic, and it is hoped that this review will aid their work. The most pronounced signals of ENSO are found over the southeast Pacific as a result of a climatological Rossby wave train that gives positive (negative) height anomalies over the Amundsen–Bellingshausen Sea during El Nino (La Nina) events. However, the extra-tropical signature can sometimes show a high degree of variability between events in this area. In West Antarctica, links between ENSO and precipitation have shown variability on the decadal time scale. Across the continent itself, it is even more difficult to relate meteorological conditions to ENSO, yet analyses of the long meteorological records from the stations do indicate a distinct switch in sign of the pressure anomalies from positive to negative across the minimum in the southern oscillation index. The oceanic signals of ENSO around the Antarctic are less clear, but it has been suggested that the Antarctic circumpolar wave could be forced by the phenomenon. Ice-core data offer the potential to help in understanding the long-term relationship between ENSO and the climate of the Antarctic, but there are difficulties because of the need to smooth the ice-core data to overcome the mixing of snow on the surface. Nevertheless, analysis of methylsulphonic acid in a South Pole core has shown high variability on ENSO time scales. It is clear that some evidence of ENSO can be found in the Antarctic meteorological and ice-core records; however, many of the relationships tend not to be stable with time, and we currently have a poor understanding of the transfer functions by which such signals arrive at the Antarctic from the tropical Pacific. Copyright © 2004 Royal Meteorological Society

The recent Sahel drought is real

Abstract: Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10–20 °N and 18 °W–20 °E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi-year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology 24: 547–554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded. Copyright  2004 Royal Meteorological Society.

Energy and radiation balance of a central European city

Abstract: Results from an experimental network of seven energy balance stations in and around a European city are presented. The network of micrometeorological stations was part of the Basel Urban Boundary Layer Experiment (BUBBLE) carried out in the city of Basel, Switzerland. Three urban sites provided turbulent flux densities and radiation data over dense urban surfaces. Together with a suburban site and three rural reference sites, this network allowed the simultaneous comparison of urban, suburban, and rural energy balance partitioning during one month of summertime measurements. The partitioning is analysed together with long-term data to evaluate the magnitude of the urban flux density modification, and to document characteristic values in their diurnal and yearly course. Simple empirical relations between flux densities and surface characteristics are presented. The energy balance partitioning is addressed separately for daytime and nocturnal situations. All four components of the surface radiation budget are analysed. Moreover, the vertical flux density divergences within the urban canopy layer are discussed. Copyright © 2004 Royal Meteorological Society

Climate impacts on indian agriculture

Abstract: Agriculture (arguably the backbone of India’s economy) is highly dependent on the spatial and temporal distribution of monsoon rainfall. This paper presents an analysis of crop–climate relationships for India, using historic production statistics for major crops (rice, wheat, sorghum, groundnut and sugarcane) and for aggregate food grain, cereal, pulses and oilseed production. Correlation analysis provides an indication of the influence of monsoon rainfall and some of its potential predictors (Pacific and Indian Ocean sea-surface temperatures, Darwin sea-level pressure) on crop production. All-India annual total production (except sorghum and sugarcane), and production in the monsoon (except sorghum) and post-monsoon seasons (except rice and sorghum) were significantly correlated to all-India summer monsoon rainfall. Monsoon season crops (except sorghum) were strongly associated with the three potential monsoon predictors. Results using state-level crop production statistics and subdivisional monsoon rainfall were generally consistent with the all-India results, but demonstrated some surprising spatial variations. Whereas the impact of subdivisional monsoon rainfall is strong in most of the country, the influence of concurrent predictors related to El Ni˜ no–southern oscillation and the Indian Ocean sea-surface temperatures at a long lead time seem greatest in the western to central peninsula. Copyright  2004 Royal Meteorological Society. Agriculture is the backbone of India’s economy. Its contribution to gross domestic product (GDP) has declined from 57% in 1950–51 to around 28% (1998–99) due primarily to growth in other sectors of the economy. The declining share of the agricultural sector, however, has not affected the importance of the sector in the Indian economy. Owing to both the direct value of agricultural products and agriculture’s indirect impact on employment, rural livelihoods and other sectors that use agricultural products, the growth of India’s GDP has largely been determined by the trend in agricultural production. Its impact on the welfare of the country is much greater than the macroeconomic indicators suggest: as nearly 70% of the working population depends on agricultural activities for their livelihood. The majority of India’s population depends on cereal and pulse production for sustenance. Agriculture is also a major supplier of raw materials for industry. Examples include cotton and jute for textiles, sugar and vegetable oil. Some 50% of all the income generated in the manufacturing sector in India can be attributed directly or indirectly to agricultural production. Agricultural commodities, and products that depend on agriculture, account for nearly 70% of the value of exports. Tea, sugar, oilseeds, tobacco and spices are major export commodities. Cereals dominate India’s agricultural output, accounting for more than 90% of the food grains; pulses account for the rest. Rice (44% of production) and wheat (37%) are the main cereals, with coarse cereals (e.g. maize, sorghum, millet) accounting for about 18% (Central Statistical Organization, 1998). Table I gives the areas under the principal crops considered in this study and their changes over the years (Figure 1).

Quality control of daily meteorological data in China, 1951-2000: a new dataset

Abstract: Long-term observational data are essential for understanding local and regional climate and climate change. These data are also important for hydrological designs and agricultural decision making. This study examined the daily meteorological data from 726 stations in China from 1951 to 2000, and developed an unprecedented climatic dataset that contains 10 daily variables: maximum and minimum surface air temperatures, mean surface air temperature, skin surface temperature, surface air relative humidity, wind speed, wind gust, sunshine duration hours, precipitation, and pan evaporation. The characteristics of the original stations’ data and quality-control methods designed and used in developing this dataset are detailed. The quality-control procedures identified less than 0.05% of the data records as being erroneous because of typos and incorrect units, reading, or data coding. When the spatial and temporal consistency of the variables’ time series were inspected, nearly 37.9% of the stations were found to have one or more variables with inconsistent changes. The sources causing the temporal inconsistency/discontinuity were evaluated, and a method was developed and applied to adjust those data segments containing inconsistent values. The resulting data series, as an alternative to the original quality-controlled series, showed both spatially and temporally consistent trends in the occurrence frequency of extreme climate events compared with the unadjusted data series. Finally, the quality-controlled daily data were gridded to a 1.0°×1.0° grid system covering China after the erroneous and missing data were estimated. This new dataset opens up opportunities for analysing and understanding the climate variability and climate change in China. Copyright  2004 Royal Meteorological Society.

Changes in Australian pan evaporation from 1970 to 2002

Abstract: Contrary to expectations, measurements of pan evaporation show decreases in many parts of the Northern Hemisphere over the last 50 years. When combined with rainfall measurements, these data show that much of the Northern Hemisphere’s terrestrial surface has become less arid over the last 50 years. However, whether the decrease in pan evaporation is a phenomenon limited to the Northern Hemisphere has until now been unknown because there have been no reports from the Southern Hemisphere. Here, we report a decrease in pan evaporation rate over the last 30 years across Australia of the same magnitude as the Northern Hemisphere trends (approximately − 4m m a −2 ). The results show that the terrestrial surface in Australia has, on average, become less arid over the recent past, just like much of the Northern Hemisphere. Copyright  2004 Royal Meteorological Society.

Trends in extreme daily precipitation indices in India

Abstract: We assembled daily precipitation records, initially for 3838 stations, throughout India and ultimately identified 129 stations with reasonably complete records over the period 1910 to 2000. From these daily records, we generated annual time series of seven different indices of extreme precipitation events, including total precipitation, largest 1, 5, and 30 day totals, and the number of daily events above the amount that marks the 90th, 95th, and 97.5th percentiles of all precipitation at each station. Of the 903 different time series (seven variables for 129 stations), 114 had a significant upward trend and 61 had a significant downward trend; overall, 61% of the time series showed an upward trend. The standard regression coefficients showing the strength and sign of the trend were highly correlated across the network. They generally showed increasing values in a contiguous region extending from the northwestern Himalayas in Kashmir through most of the Deccan Plateau in the south and decreasing values in the eastern part of the Gangetic Plain and parts of Uttaranchal. Our results are in general agreement with the prediction from numerical models for an increase in extreme precipitation events in India given the ongoing build-up of greenhouse gases. Copyright © 2004 Royal Meteorological Society

Temperature trends in South Africa: 1960–2003

Abstract: Time series of South African temperatures were investigated for temporal and spatial trends for the period 1960 to 2003. For this purpose a total of 26 climate stations were utilized, with each having sufficient data available and not having undergone major moves or changes in exposure that would influence the homogeneity of their data series. The vast majority, a total of 23 stations, showed positive trends in their annual mean maximum temperature series, 13 of them significant, with trends higher for central stations than those closer to the coast. Annual mean minimum temperatures showed 21 stations having positive trends, with 18 significant. Stations not showing significantly positive trends in annual mean minimum temperatures were mostly situated in the central interior. The annual average temperature data series of 24 of the stations showed positive trends, with 18 of them significant. Trends of mean seasonal temperature showed that temperature trends are not consistent throughout the year, with the average trend for autumn showing a maximum and spring a minimum. Monthly trends of average annual temperatures showed large differences in trend between stations, and for each station between months, but similar tendencies in trend between months were found to exist for stations close by and also for groups of stations on a regional basis. Trends in diurnal temperature range are almost equally divided between positive and negative, with the positive trends in the central interior mainly being caused by large positive trends in maximum temperature. It is also shown that, in general, days and nights with relatively high temperatures have increased, while days and nights with relatively low temperatures have decreased. The effects of urbanization on temperature trends are investigated, and the conclusion is that most stations regarded as urban stations are still useful for trend analysis; being situated on the outskirts of cities they are, therefore, not substantially influenced by the urban heat island. El Nino and La Nina events do not seem to play a significant role in the increasing temperatures observed. Copyright © 2004 Royal Meteorological Society

Spatial distribution of a daily precipitation concentration index in peninsular Spain

Abstract: Given its concentration in a few days and its modest total volume, knowledge about precipitation with daily resolution in Spain is highly important. In this article, a concentration index (CI) that evaluates the varying weight of daily precipitation, i.e. the contribution of the days of greatest rainfall to the total amount, is presented. The index is applied to exponential curves such as Y = aXexp(bX), which adjust the accumulated percentages of precipitation Y contributed by the accumulated percentage of days X on which it took place. The index was applied to 32 meteorological stations across peninsular Spain with quality data for the period 1951 to 1990, which enables the spatial patterns of daily pluviometric concentration in the territory to be determined. Copyright © 2004 Royal Meteorological Society

Trend and Variability of China Precipitation in Spring and Summer: Linkage to Sea Surface Temperatures

Abstract: Observational records in the past 50 years show an upward trend of boreal-summer precipitation over central eastern China and a downward trend over northern China. During boreal spring, the trend is upward over southeastern China and downward over central eastern China. This study explores the forcing mechanism of these trends in association with the global sea-surface temperature (SST) variations on the interannual and inter-decadal timescales. Results based on Singular Value Decomposition analyses (SVD) show that the interannual variability of China precipitation in boreal spring and summer can be well defined by two centers of actions for each season, which are co-varying with two interannual modes of SSTs. The first SVD modes of precipitation in spring and summer, which are centered in southeastern China and northern China, respectively, are linked to an ENSO-like mode of SSTs. The second SVD modes of precipitation in both seasons are confined to central eastern China, and are primarily linked to SST variations over the warm pool and Indian Ocean. Features of the anomalous 850-hPa winds and 700-Wa geopotential height corresponding to these modes support a physical mechanism that explains the causal links between the modal variations of precipitation and SSTs. On the decadal and longer timescale, similar causal links are found between the same modes of precipitation and SSTs, except for the case of springtime precipitation over central eastern China. For this case, while the interannual mode of precipitation is positively correlated with the interannual variations of SSTs over the warm pool and Indian Ocean; the inter-decadal mode is negatively correlated with a different SST mode, the North Pacific mode. The later is responsible for the observed downward trend of springtime precipitation over central eastern China. For all other cases, both the interannual and inter-decadal variations of precipitation can be explained by the same mode of SSTs. The upward trend of springtime precipitation over southeastern China and downward trend of summertime precipitation over northern China are attributable to the warming trend of the ENSO-like mode. The recent frequent summertime floods over central eastern China are linked to the warming trend of SSTs over the warm pool and Indian Ocean.

The factors governing the summer regime of the eastern mediterranean

Abstract: The synoptic scale features over the eastern Mediterranean (EM) for July–August are examined using National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. The region is subjected to two primary factors: mid–upper level subsidence and lower level cool advection, associated with the Etesian winds. The interdiurnal variations of these factors were found to be correlated with each other, with a maximum of r = 0.76, found between the 700 hPa subsidence and the 925 hPa wind speed. The impact of these factors on the temperature regime was examined through their contributions in the temperature tendency equation at 32.5 °N, 35 °E. A significant correlation was found between them at the 850 hPa level, indicating that they tend to balance each other. This explains the low interdiurnal temperature variations there in summer. Zonal-vertical and isentropic cross-sections indicate the existence of a closed circulation connecting the EM with the Asian monsoon, and a meridional-vertical cross-section indicates a signature of the Hadley cell across eastern North Africa. Air back-trajectories demonstrate that the EM is connected at the lower troposphere with Europe, at the midtroposphere with eastern North Africa and at the higher troposphere with the Asian monsoon. Significant correlation was found between the interdiurnal variations in the upward motion over the Asian monsoon and the subsidence over the Levant, with a 1 day lag, implying that the Asian monsoon controls the interdiurnal variations over the Levant. A detailed analysis shows that the correlation between the two dynamic factors governing the EM results from a linkage existing between each one of them and the Asian monsoon. An intensification of the Asian monsoon enhances both the subsidence over the Levant, via the circulation connecting them, and the Etesian winds, due to the enhanced pressure gradient between the two regions. Copyright  2004 Royal Meteorological Society.

Semi-objective classification for daily synoptic systems: Application to the eastern mediterranean climate change

Abstract: Semi-objective classification of daily 1200 UTC synoptic systems was carried out by a discriminant-like analysis of the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis data over the eastern Mediterranean (EM) for 1948–2000. An example for a climate change application is given by the analysis of trends in the annual frequencies of the synoptic systems. The frequencies of the mostly dry Red Sea trough (RST) systems nearly doubled since the 1960s from 50 to about 100 days per year. This explains a dominant decreasing trend of rainfall in most of the EM, along with an increase in the southern part of the EM region, when the RST is deep enough to bring tropical moisture over this area. Also, the increasing tendency towards heavier daily rainfall in spite of the general decrease in the totals may be explained by the increase in the active and stormy types of RST situations. The annual frequency of the Cyprus lows was noticed to drop slightly in 1983–98 to 26, compared with about 30 during 1967–82. The high positive correlation between the recent increase in the North Atlantic oscillation index and the pressure over most of the EM countries is linked to these tendencies in the synoptic systems. Copyright  2004 Royal Meteorological Society.

Variability of the recent climate of eastern Africa

Abstract: The primary objective of this study is to investigate the recent variability of the eastern African climate. The region of interest is also known as the Greater Horn of Africa (GHA), and comprises the countries of Burundi, Djibouti, Eritrea, Ethiopia, Kenya, Rwanda, Somalia, Sudan, Uganda, and Tanzania. The analysis was based primarily on the construction of empirical orthogonal functions (EOFs) of gauge rainfall data and on CPC Merged Analysis of Precipitation (CMAP) data, derived from a combination of rain-gauge observations and satellite estimates. The investigation is based on the period 1961–2001 for the ‘short rains’ season of eastern Africa of October through to December. The EOF analysis was supplemented by projection of National Centers for Environmental Prediction wind data onto the rainfall eigenmodes to understand the rainfall–circulation relationships. Furthermore, correlation and composite analyses have been performed with the Climatic Research Unit globally averaged surface-temperature time series to explore the potential relationship between the climate of eastern Africa and global warming. The most dominant mode of variability (EOF1) based on CMAP data over eastern Africa corresponds to El Nino–southern oscillation (ENSO) climate variability. It is associated with above-normal rainfall amounts during the short rains throughout the entire region, except for Sudan. The corresponding anomalous low-level circulation is dominated by easterly inflow from the Indian Ocean, and to a lesser extent the Congo tropical rain forest, into the positive rainfall anomaly region that extends across most of eastern Africa. The easterly inflow into eastern Africa is part of diffluent outflow from the maritime continent during the warm ENSO events. The second eastern African EOF (trend mode) is associated with decadal variability. In distinct contrast from the ENSO mode pattern, the trend mode is characterized by positive rainfall anomalies over the northern sector of eastern Africa and opposite conditions over the southern sector. This rainfall trend mode eluded detection in previous studies that did not include recent decades of data, because the signal was still relatively weak. The wind projection onto this mode indicates that the primary flow that feeds the positive anomaly region over the northern part of eastern Africa emanates primarily from the rainfall-deficient southern region of eastern Africa and Sudan. Although we do not assign attribution of the trend mode to global warming (in part because of the relatively short period of analysis), the evidence, based on our results and previous studies, strongly suggests a potential connection. Copyright © 2004 Royal Meteorological Society.

Interannual variability of European extreme winter rainfall and links with mean large‐scale circulation

Abstract: December–February (DJF) extreme rainfall was analysed at 347 European stations for the period 1958–2000. Two indices of extreme rainfall were examined: the maximum number of consecutive dry days (CDD); and the number of days above the 1961–90 90th percentile of wet-day amounts (R90N). A principal component analysis of CDD found six components that accounted for 52.4% of the total variance. Six components of DJF R90N were also retained that accounted for 39.1% of the total variance. The second component of R90N has a very significant trend and the factor loadings closely resemble the observed linear trend in this index, suggesting that the analysis has isolated the mode of variability causing the trend as a separate component. The principal components of the indices were correlated with surface and upper-air observations over the North Atlantic. The best correlations were generally found to be with sea-level pressure (SLP) observations. A separate canonical correlation analysis of each of the two indices with SLP revealed several coupled modes of variability. The North Atlantic oscillation (NAO) was isolated as the first canonical pattern for R90N. For CDD the first two canonical coefficients of CDD were significantly correlated with the NAO index. Generally, the canonical coefficients with the highest correlations with the NAO had the most significant trends, suggesting that the observed trend in the NAO has strongly contributed to the observed trends in the indices. Two other important canonical patterns were isolated: a pattern of anomalous mean SLP (MSLP) centred over the North Sea, which seems to be related to local sea-surface temperature over this region; and a dipole-like pattern of MSLP with poles over the eastern Mediterranean and the central North Atlantic. Repeating the canonical correlation analysis with two other indices of extreme rainfall, the 90th percentile of wet day amounts and the maximum 10 day rainfall total, gives very similar coupled patterns. Copyright  2004 Royal Meteorological Society.

Seasonal forecasting of the Ethiopian summer rains

Abstract: We present a new seasonal forecasting model for the June–September rains in Ethiopia. It has previously been found that the total June–September rainfall over the whole country is difficult to predict using statistical methods. A detailed study of all available data shows the rainfall seasonality varies greatly from one region to another, which would explain why the total June–September rainfall over all regions is a difficult property to forecast. In addition, the correlation between rainfall and the southern oscillation index varies spatially, with a strong teleconnection present only in some regions. This study accounts for the spatial variability in rainfall by grouping the rain gauge stations into four geographical clusters based on seasonality and cross-correlation of rainfall anomalies. Linear regression equations are then developed separately for each cluster. The variables we use for the regressions are sea-surface temperature anomalies in the preceding March, April and May of the tropical western Indian Ocean, the tropical eastern Indian Ocean, and Nino3.4. Formal skill testing of the equations shows that the new forecasting scheme is more effective in central western Ethiopia than either climatology or persistence—the methods currently used by the Ethiopian National Meteorological Services Agency. Copyright © 2004 Royal Meteorological Society

Northeast monsoon rainfall variability over south peninsular India vis‐à‐vis the Indian Ocean dipole mode

Abstract: Whereas the June through to September southwest summer monsoon season over the Indian subcontinent has been the main focus of research, the October through to December northeast monsoon season over south peninsular India has received less attention. Hence, this study focuses on the northeast monsoon rainfall (NEMR) variability using historical data for a 131 year period (1871–2001) and its relationship with the neighbouring Indian Ocean sea-surface temperature (SST). Based on the observed available rainfall data, a time series of NEMR over the coherent south peninsular India has been developed. Similarly, an index to quantify the Indian Ocean dipole mode (IODM), exhibiting warm (cool) waters over the equatorial western (southeastern) Indian Ocean, has been developed using GISST data for the same period. The dynamics of the NEMR–IODM relationship are examined with the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. Interannual and decadal variabilities in NEMR reveal alternate epochs of above- and below-normal rainfall. The epochs tend to last for about a decade or two. Rainfall epochs near the equatorial Indian Ocean tend to last for about a decade. No long-term trends in NEMR are noted. On the other hand, the decadal variability in the IODM reveals a dominance of the negative phase during the earlier decades (1880–1920) and a positive phase during recent decades (1960–2000), with suppressed activity in between. Long-term variability in the IODM clearly reveals an increasing trend, which could be related to greenhouse warming. Further, the NEMR variability is enhanced during the decades when the IODM exhibits its active phase, and is suppressed during the decades when the IODM is inactive. The NEMR and the IODM are directly related, suggesting that the positive (negative) phase enhances (suppresses) the northeast monsoon activity. During the positive phase, the anomalous flow pattern shows winds converging and suggesting moisture transport from the southeast Indian Ocean and the Bay of Bengal towards south peninsular India. In contrast, the negative phase reveals winds diverging and transporting moisture away from the south Indian region. The anomalous SST and circulation features associated with the positive IOD/excess rainfall and negative IOD/deficient rainfall are consistent and clearly bring out the coupled ocean–atmosphere dynamics over the Indian Ocean. These results show the direct influence of the IODM phenomenon on the interannual and decadal NEMR variability over south India. Copyright  2004 Royal Meteorological Society.

Spatial pattern in the precipitation regime of Nepal

Abstract: A large-scale perspective is provided upon the nature of precipitation regimes across Nepal by classifying long-term mean monthly precipitation data for 222 stations. The classification methodology is shown to be a useful tool for identifying underlying spatial structure in precipitation regime shape (seasonal variability) and magnitude (size) in an extreme physical environment where climatological patterns are complex and poorly known. Four precipitation regime shape classes are identified: (a) July–August peaks are typical of western regions; (b) marked July peaks are characteristic of the central region and eastern lowlands; (c) July peaks with gradual onset are mainly concentrated in eastern Middle Mountains; and (d) July–August peaks with winter precipitation are confined to western higher mountains, particularly the TransHimalayan region. Four precipitation regime magnitude classes are found: (a) low regimes clearly dominate western high-mountain areas but occur more widely across the High Mountains and High Himalaya; (b) intermediate regimes are distributed from east to west at low altitude, dominating the western Terai/Siwaliks; (c) moderately high regimes are widespread but most evident in the Terai (plains) to Middle Mountains of central and eastern parts and eastern high mountains; and (d) high precipitation zones are situated near Pokhara and northeast of the Kathmandu Valley around Langtang. A composite (shape and magnitude) regime classification indicates the key controls upon spatial patterns in Nepalese precipitation to be: length and timing of the summer monsoon (duration decreases east to west with later onset and earlier withdrawal in the west); successively higher altitude, east–west-trending mountain ranges causing rainfall to decline broadly south–north; topographic barriers that induce local rain shadows (lee) and precipitation hotspots (windward side); and westerly weather systems supplying winter precipitation to the northwest mountains. Although general patterns relating to zonal movement of the summer monsoon and physiographic (mountain ranges) controls upon precipitation may be identified, the role of mountainous relief in yielding localized precipitation patterns is significant. Copyright  2004 Royal Meteorological Society.

A new seasons definition based on classified daily synoptic systems: an example for the eastern Mediterranean

Abstract: A new definition for seasons based on a synoptic classification is introduced. It uses the automatically classified daily synoptic systems. For the eastern Mediterranean (EM), the temporal distribution of the synoptic systems over 53 years enables a proper definition of the timing and duration of the cold rainy, warm dry and of the transition seasons. Comparisons with the astronomical, meteorological and the temperature-based seasons definitions following Trenberth is performed. According to the synoptic definition proposed here and applied to the EM, the winter and summer seasons each last about 4 months (3 months and 23 days). The EM ‘synoptic summer’ and ‘synoptic winter’ defined here begin at about the earliest starting date, i.e. the meteorological start, and they end at about the latest ending date, i.e. the astronomical end. Copyright © 2004 Royal Meteorological Society

Vegetation as an urban climate control in the subtropical city of Gaborone, Botswana

Abstract: The influence of vegetation on the urban climate was studied in the subtropical city Gaborone, the rapidly expanding capital of Botswana with approximately 200 000 inhabitants. Temperature records from an urban and a rural station were analysed for the period 1985–96. In an attempt to explain possible seasonal change in vegetation, NOAA satellite normalized difference vegetation index imagery was analysed. The present urban influence was investigated with temperature loggers at selected urban and rural sites. In addition, mobile measurements revealed spatial patterns in temperature and humidity for different land uses. Seasonal patterns of urban–rural differences in minimum temperatures emerge during the period 1994–96, especially during the winter when the heat island effect is largest. It is shown that differences in urban and rural vegetation over the year partly explain this variation. Mobile measurements reveal a weak nocturnal heat island of 2–3 °C during clear and calm nights. There are intra-urban temperature differences that are in the same range as the urban–rural relationship due to the role of vegetation. Evapotranspiration lowers the temperature, which was detected by high humidity in areas of lush vegetation. This becomes apparent at midday, when densely vegetated areas were up to 2 °C cooler than rural sites. An oasis effect, therefore, only exists on a highly local basis. In contrast, parts of the city with sparse vegetation are warmer than the countryside. There is an apparent opposed effect of rural and urban vegetation, whereby the former is hindering the temperature from falling and the latter is cooling the environment through evapotranspiration. This can be explained by the overwhelming amounts of imported water in the city promoting evaporative cooling. Copyright © 2004 Royal Meteorological Society

Over one century of rainfall and temperature observations in Addis Ababa, Ethiopia

Abstract: A detailed historical reconstruction and analysis is presented of the longest record of climate observations for Ethiopia, from 1898 to 2002 in Addis Ababa. Prior to 1951 the record comprises rainfall and minimum and maximum temperatures recorded in different locations by different observers. The rainfall series is complete except for 1899 and 1900, but the temperature series are very incomplete. Using documentary evidence, we attempt as far as is possible to establish the origins of all the pre-1951 observations. Rainfall observations originate from at least six different sites. After establishment of an Ethiopian meteorological department in 1951 the records are complete and, to our understanding, originate from the same location, the Addis Ababa Observatory (AAO). A revised rainfall series for 1898-1950 is derived using observations from sites with the longest records. The minimum and maximum temperature records show evidence of statistically significant inhomogeneities. Homogeneity tests on the full rainfall record (the revised series plus AAO) show it is reliable, with evidence of minor but not statistically significant breaks in the record before establishment of the AAO. Some, but not all, breaks can be accounted for using the historical information. Analysis of the records shows increasing trends in annual minimum and maximum temperatures from 1951 to 2002 (0.4 °C/decade and 0.2 °C/decade, respectively). There is little trend in rainfall from 1901-50, 1951-2002 and 1901-2002, dry years do not correspond with known drought years elsewhere in Ethiopia, and interannual variability is poorly correlated with another long rainfall series in Ethiopia (Gore), Blue Nile river flows and the southern oscillation index. This suggests strongly that the record for Addis Ababa should not be used as a proxy for conditions in Ethiopia, particularly the more drought-prone areas to the north and east. We conclude that the temperature series are suspect but that the full rainfall record is useful for analysis of long-term rainfall conditions in Addis Ababa.

Modern glacier retreat on Kilimanjaro as evidence of climate change: observations and facts

Abstract: In recent years, Kilimanjaro and its vanishing glaciers have become an ‘icon’ of global warming, attracting broad interest. In this paper, a synopsis of (a) field observations made by the authors and (b) climatic data as reported in the literature (proxy and long-term instrumental data) is presented to develop a new concept for investigating the retreat of Kilimanjaro's glaciers, based on the physical understanding of glacier–climate interactions. The concept considers the peculiarities of the mountain and implies that climatological processes other than air temperature control the ice recession in a direct manner. A drastic drop in atmospheric moisture at the end of the 19th century and the ensuing drier climatic conditions are likely forcing glacier retreat on Kilimanjaro. Future investigations using the concept as a governing hypothesis will require research at different climatological scales. Copyright © 2004 Royal Meteorological Society

Trend analysis of precipitation and drought in Basilicata from 1923 to 2000 within a southern Italy context

Abstract: Series of annual, monthly and seasonal precipitation from 50 stations, distributed all over the Basilicata territory (southern Italy), were studied for the period 1923–2000. All the series were checked for homogeneity using MASH v. 1.0.1 software and the time series analysis was performed with the Mann–Kendall nonparametric test in order to detect possible trends. The results show that the annual total rainfall decreased by about 156 mm during the period investigated; the decrease becomes stronger in the last 30 years. There are considerably different trends for the different seasons. In particular, only the winter trend, which is downward, is statistically significant. In this season the total rainfall has decreased by about 133 mm. For this same period the standardized precipitation index (SPI) for multiple time scales of 12, 24 and 48 months has been computed. It appears that periods of drought have been quite frequent starting from 1975, with SPI ranging from about −1 to about −3. Copyright © 2004 Royal Meteorological Society

The upwelling regime off the West Portuguese Coast, 1941–2000

Abstract: The evolution of the upwelling regime off west Portugal between 1941 and 2000 was investigated. Monthly averages of the longshore (meridional) wind component at four coastal stations of the Institute for Meteorology were computed and subject to linear regression analysis. Several comparisons were made among stations until a final regression model was reached. The resulting residuals were checked for the presence of red noise, and pairwise correlation coefficients were estimated for residuals of different stations. To complement this study, monthly sea-surface temperature averages were computed for six regions off west Portugal and subject to a similar procedure. In both analyses, it was concluded that the Portuguese upwelling regime has weakened since the 1940s. The waning of the northerly, upwelling-favourable winds was significant throughout the traditional upwelling season (April–September). Sea-surface temperature showed a steady year-round increase from 1941 onwards, in both offshore (+0.002°C/year) and coastal (+0.010°C/year) regions.

Middle east climate simulation and dominant precipitation processes

Abstract: The ability of both the ECMWF-TOGA analyses and regional model RegCM2 to simulate the climate of the Middle East is examined. The climate of the region displays high spatial, seasonal and interannual variability, providing a strong test of a climate model’s abilities. The higher resolution of RegCM2, compared with the ECMWF analyses, allows it to capture the spatial variability of temperature and precipitation better despite model biases being present. Both RegCM2 and the analyses have a cold bias, exacerbated in RegCM2 by a bias present in the prescribed sea-surface-temperature forcing. RegCM2 does not capture the annual cycle of precipitation on the Black and Caspian Sea coasts, where very steep topography exists, nor on the eastern Mediterranean coast, where the coastal mountains are not resolved. RegCM2 does capture the seasonal cycle in the Fertile Crescent and Zagros Mountains, where it is strongly influenced by a plateau circulation above the Iranian plateau. It is shown that accurate simulation of precipitation in these regions requires the correct simulation of storm tracks, topographic interactions and atmospheric stability. RegCM2 is better able to simulate the interannual variability averaged over the entire domain compared with the ECMWF analyses; however, they both have difficulty reproducing the interannual variability in particular subregions. It is also shown that processes controlling the seasonality of precipitation differ in different subregions and are often different from the processes controlling interannual variability. This suggests that, in order to model precipitation successfully, a horizontal scale that allows differentiation of precipitation zones dominated by different precipitation processes, both seasonally and interannually, is required. Copyright  2004 Royal Meteorological Society.

Hydro-meteorological variability in the greater Ganges -Brahmaputra-Meghna basins

Abstract: The flows of the Ganges, Brahmaputra and Meghna (GBM) are highly seasonal, and heavily influenced by monsoon rainfall. As a result, these rivers swell to their banks and often overflow during the monsoon months. This is most pronounced in the downstream regions, particularly in Bangladesh, which is the lowest riparian country. The objective of this paper is to study this hydro-meteorological variability in the greater GBM regions, including the headwater regions in India and their role in streamflows in Bangladesh, and explore the large-scale oceanic factors affecting this hydro-meteorological variability. Global precipitation data, Bangladesh rainfall and streamflow records have been analysed and related to large-scale climate patterns, including upstream rainfall, regional atmospheric circulation and patterns of sea-surface temperature. The findings have quantified how the streamflows of these rivers in Bangladesh are highly correlated with the rainfall in the upper catchments with typically a lag of about 1 month. Therefore, streamflows in Bangladesh could be reasonably estimated for 1 to 3 months in advance (especially for the Ganges and Brahmaputra rivers) by employing simple correlation, if rainfall data from countries further up are available on a real-time and continuous basis. In the absence of rainfall data, streamflow forecasts are still possible from unusually warm or cold sea-surface temperatures in the tropics. The study concludes that hydro-meteorological information flow between Bangladesh and other neighbouring countries is essential for developing a knowledge base for evaluating the potential implications of seasonal streamflow forecast in the GBM basins in Bangladesh. Copyright © 2004 Royal Meteorological Society

Rainfall and the length of the growing season in Nigeria

Abstract: This study examines the length of the growing season in Nigeria using the daily rainfall data of Ikeja, Ondo, Ilorin, Kaduna and Kano The data were collected from the archives of the Nigerian Meteorological Services, Oshodi, Lagos The length of the growing season was determined using the cumulative percentage mean rainfall and daily rainfall probability methods Although rainfall in Ikeja, Ondo, Ilorin, Kaduna, and Kano appears to commence around the end of the second dekad of March, middle of the third dekad of March, mid April, end of the first dekad of May, and early June respectively, its distribution characteristics at the respective stations remain inadequate for crop germination, establishment, and development till the end of the second dekad of May, early third dekad of May, mid third dekad of May, end of May, and end of the first dekad of July respectively Also, rainfall at the various stations appears to retreat starting from the early third dekad of October, early third dekad of October, end of the first dekad of October, end of September, and early second dekad of September respectively, but its distribution characteristics only remain adequate for crop development at the respective stations till around the end of the second dekad of October, end of the second dekad of October, middle of the first dekad of October, early October, and middle of the first dekad of September respectively Thus, the active lengths of the growing season are approximately 5 months, 5 months, 4 months, 4 months, and 2 months respectively Plants that are short-dry-spell tolerant may thrive early in the rainy season, ie from the end of the second dekad of March to the end of the second dekad of May (in Ikeja), middle of the third dekad of March to the early third dekad of May (in Ondo), mid April to the middle of the third dekad of May (in Ilorin), end of the first dekad of May to end of May (in Kaduna), and early June to the end of the first dekad in July (in Kano), but other less tolerant plants should be planted starting from the end of the second dekad of May, early third dekad of May, mid third dekad of May, end of May, and end of the first dekad of July respectively The daily rainfall probability method is recommended as more efficient in the assessment of the nature and length of the growing season Copyright © 2004 Royal Meteorological Society

Changes of air temperature and precipitation in Poland in the period 1951–2000 and their relationship to atmospheric circulation

Abstract: An analysis of trends of mean monthly temperature and precipitation totals in Poland in the period 1951–2000 was carried out. Areal means of temperature and precipitation were used, averaged for 51 meteorological stations evenly distributed within Poland's borders. Sensitivity of air temperature and precipitation variations towards circulation was assessed. Circulation variations were expressed by sea-level pressure in the 52.5°N, 20°E grid point and geostrophic wind calculated from meridional (45–60°N) and latitudinal (10–30°E) pressure gradients. It was shown that the circulation factor explains up to 77% and 44% of temperature and precipitation variance respectively. Significant upward trends of temperature in March and May were detected. Also, the precipitation total in March was on the increase. The last decade of the 20th century was slightly ‘too warm’ in comparison with the circulation-induced temperature level, as well as with the temperature change scenario according to HadCM2 GS model. An attempt was made to evaluate the contribution of the frequency of snow cover occurrence to the temperature increase in winter, i.e. the temperature–albedo feedback. Copyright © 2004 Royal Meteorological Society

An analysis of Icelandic climate since the nineteenth century

Abstract: New, long monthly series of Icelandic air pressure, temperature, precipitation and sunshine data are presented and analysed to determine possible evidence of recent climatic changes in Iceland. Climatic series are compared with the North Atlantic oscillation (NAO) indices; Icelandic temperature and precipitation are moderately but significantly correlated with the NAO. An updated south-north Iceland temperature index is discussed in relation to 20th century reductions in sea-ice coverage. Net warming over Iceland occurred over all long-term records from the mid 19th century to the present, consistent with observed global warming trends, but superimposed on this was a marked cooling between the 1940s and early 1980s; Icelandic warming resumed around 1985. The mid-late 20th century cooling is in agreement with observed cooling in southern Greenland, suggesting that large-scale changes in atmospheric circulation were probably responsible. The 1930s was the warmest decade of the 20th century in Iceland, in contrast to the Northern Hemisphere land average. There was a distinct 20th century dipole in temperatures between Iceland and northwestern Europe, with 1941 serving as an extreme year, i.e. cold Europe and warm Iceland and Greenland. There are also signs of a precipitation increase since the late 19th century, although this is significant for only one out of three stations. Moreover, precipitation rates exhibit a positive correlation with temperature. There were no statistically significant overall long-term changes in pressure or sunshine duration. However, there are statistically significant negative correlations of precipitation with the sunshine data. There is evidence of possible solar forcing of Icelandic temperature and pressure. Results from the analysis aid our understanding of recent and ongoing changes in Icelandic and North Atlantic climate. The results should help us interpret these changes in the context of larger scale atmospheric/subpolar variability and future climate-change predictions. © 2004 Royal Meteorological Society.