Showing papers in "International Journal of Climatology in 2000"

Climatic warming in the Tibetan Plateau during recent decades

Abstract: Adequate knowledge of climatic change over the Tibetan Plateau (TP) with an average elevation of more than 4000 m above sea level (a.s.l.) has been insufficient for a long time owing to the lack of sufficient observational data. In the present study, monthly surface air temperature data were collected from almost every meteorological station on the TP since their establishment. There are 97 stations located above 2000 m a.s.l. on the TP; the longest records at five stations began before the 1930s, but most records date from the mid-1950s. Analyses of the temperature series show that the main portion of the TP has experienced statistically significant warming since the mid-1950s, especially in winter, but the recent warming in the central and eastern TP did not reach the level of the 1940s warm period until the late 1990s. Compared with the Northern Hemisphere and the global average, the warming of the TP occurred early. The linear rates of temperature increase over the TP during the period 1955‐1996 are about 0.16°C:decade for the annual mean and 0.32°C:decade for the winter mean, which exceed those for the Northern Hemisphere and the same latitudinal zone in the same period. Furthermore, there is also a tendency for the warming trend to increase with the elevation in the TP and its surrounding areas. This suggests that the TP is one of the most sensitive areas to respond to global climate change. Copyright © 2000 Royal Meteorological Society.

A methodological approach of climatological modelling of air temperature and precipitation through GIS techniques

Abstract: This study proposes an empirical methodology for modelling and mapping the air temperature (mean maximum, mean and mean minimum) and total precipitation, all of which are monthly and annual, using geographical information systems (GIS) techniques. The method can be seen as an alternative to classical interpolation techniques when spatial information is available. The geographical area used to develop and apply this model is Catalonia (32000 km 2 , northeast Spain). We have developed a multiple regression analysis between these meteorological variables as the dependent ones, and some geographical variables (altitude (ALT), latitude (LAT), continentality (CON), solar radiation (RAD) and a cloudiness factor (CLO)) as the independent ones. Data for the dependent variables were obtained from meteorological stations, and data for the independent variables were elaborated from a 180 m resolution digital elevation model (DEM). Multiple regression coefficients (bn) were used to build final maps, using digital layers for each independent variable, and applying basic GIS techniques. The results are very satisfactory in the case of mean air temperature and mean minimum air temperature, with coefficients of determination (R 2 ) between 0.79 and 0.97, depending on the month; in the case of mean maximum air temperature, R 2 ranges between 0.70 and 0.89, while in the case of precipitation, it ranges between 0.60 and 0.91. Copyright © 2000 Royal Meteorological Society.

Observed trends in the daily intensity of United Kingdom precipitation.

Abstract: The intensity distribution of daily precipitation amounts in the UK has changed over the period 1961–1995, becoming on average more intense in winter and less intense in summer. This result is based on an analysis of 110 UK station records. In winter, and in terms of their relative contributions to total winter precipitation, there has been a decline in light and medium events and an increase in the heaviest events. This change is fairly uniform across the whole country and is apparent even when longer records (with reduced spatial coverage/detail) are analysed back to 1931 or 1908. The reverse is found in summer: over 1961–1995 there has been a decline in the proportion of the seasonal total being provided by the heaviest events. In the longer term context, however, the summer changes appear to be a return to earlier levels after a period in the 1960s when heavy summer rainfall made a greater than normal contribution. More complex changes have occurred in the intensity distribution of spring and autumn precipitation, with opposite changes in different regions of the UK. Copyright © 2000 Royal Meteorological Society

Circulation weather types and their influence on the precipitation regime in Portugal.

Abstract: An objective classification scheme of the atmospheric circulation affecting Portugal, between 1946 and 1990, is presented, where daily circulation is characterized through the use of a set of indices associated with the direction and vorticity of the geostrophic flow. The synoptic characteristics and the frequency of ten basic circulation weather types (CWTs) are discussed, as well as the amount of precipitation associated with each type between 1957 and 1986. It is shown that the anticyclonic (A) type, although being the most frequent class in winter (37%), gives a rather small (less then 16%) contribution to the winter precipitation amount, observed on a daily basis. On the other hand, the three wettest CWTs, namely the cyclonic (C), southwesterly (SW) and westerly (W) types, together representing only 32% of all winter days, account for more than 62% of the observed daily precipitation. Results obtained highlight the existence of strong links between the interannual variability of monthly precipitation and interannual variability of CWTs. Multiple regression models, developed for 18 stations, show the ability of modelling monthly winter precipitation through the exclusive use, as predictors, of the wet CWTs (i.e. C, SW and W). The observed decreasing trend of March precipitation is also analysed and shown to be especially associated with the decrease of the three wet weather types. The anomalous low (high) frequency of wet CWTs during the hydrological year is shown to be strongly related with the occurrence of extreme dry (wet) years in Portugal, which had important impacts on Portuguese agriculture. Overall, the results suggest that the precipitation regime over Portugal, including interannual variability, trends and extremes, may be adequately explained in terms of variability of a fairly small number of circulation weather patterns. On the other hand, observed contrasts in the spatial distribution of correlations between frequency of wet CWTs and rainfall amounts suggest that precipitation regimes are of a different nature in northern and southern regions of Portugal; the former possessing an orographic origin and the latter being associated to cyclogenetic activity. Copyright © 2000 Royal Meteorological Society.

ENSO signals in East African rainfall seasons

Abstract: The evolutions of ENSO modes in the seasonal rainfall patterns over East Africa are examined in this study. The study covers the period 1961–1990. Both rotated empirical orthogonal function (EOF) and simple correlation analyses were used to delineate a network of 136 stations over East Africa into homogeneous rainfall regions in order to derive rainfall indices. Time series generated from the delineated regions were later used in the rainfall/ENSO analyses. Such analyses involved the development of composite rainfall map patterns for El Nino and post-ENSO (+1) years in order to investigate the associations between seasonal evolution of El Nino–Southern Oscillation (ENSO) signals and the space-time evolution of rainfall anomalies over the region. Analyses based on both EOF and simple correlation techniques yielded eight homogeneous rainfall regions over East Africa. The results showed unique seasonal evolution patterns in rainfall during the different phases of the ENSO cycles. East African rainfall performance characteristics were stratified to identify distinct rainfall anomaly patterns associated with ENSO and post-ENSO (+1) years. These can be applied in conjunction with skilful long lead (up to 12 months) ENSO prediction to provide guidance on likely patterns of seasonal rainfall anomalies over the region. Such information can be crucial for early warning of socio-economic disasters associated with extreme rainfall anomalies over East Africa. Copyright © 2000 Royal Meteorological Society

Changes in North American spring

Abstract: Onset of the growing season in mid-latitudes is a period of rapid transition, which includes heightened interaction between living organisms and the lower atmosphere. Phenological events (seasonal plant and animal activity driven by environmental factors), such as first leaf appearance or flower bloom in plants, can serve as convenient markers to monitor the progression of this yearly shift, and assess longer-term change resulting from climate variations. We examined spring seasons across North America over the 1900–1997 period using modelled and actual lilac phenological data. Regional differences were detected, as well as an average 5–6 day advance toward earlier springs, over a 35-year period from 1959–1993. Driven by seasonally warmer temperatures, this modification agrees with earlier bird nesting times, and corresponds to a comparable advance of spring plant phenology described in Europe. These results also align with trends towards longer growing seasons, reported by recent carbon dioxide and satellite studies. North American spring warming is strongest regionally in the northwest and northeast portions. Meanwhile, slight autumn cooling is apparent in the central USA. Copyright © 2000 Royal Meteorological Society

Precipitation fluctuations in the Nepal Himalaya and its vicinity and relationship with some large scale climatological parameters.

Abstract: Precipitation records from 78 stations distributed across Nepal were analysed and all-Nepal (1948–1994) and subregional records (1959–1994) were developed. The all-Nepal and regional precipitation series showed significant variability on annual and decadal time scales. Distinct long-term trends were not found in these precipitation records. The all-Nepal record agrees well with the precipitation records from northern India, while it does not compare well with the all-India precipitation record. The all-Nepal monsoon record is highly correlated with the Southern Oscillation Index (SOI) series. The exceptionally dry year of 1992 recorded in Nepal coincides with the elongated El Nino of 1992–1993 and the Mount Pinatubo eruption. A remarkable cooling in the region covering the Tibetan Plateau also occurred in 1992, suggesting that Pinatubo aerosol played a major role in the drought of that particular year in Nepal. In other years, the correlation between the precipitation record from Nepal and the temperature of the Tibetan Plateau is not significant, while a stronger correlation with temperature over the Indian Ocean and southern India exists. This provides further support for the strong relationship between the El Nino–Southern Oscillation (ENSO) and precipitation fluctuation in Nepal. The correlation is stronger between all-Nepal monsoon precipitation and SOI averaged over seasons following the monsoon compared with seasons preceding the monsoon. Copyright © 2000 Royal Meteorological Society

Precipitation predictors for downscaling: observed and general circulation model relationships

Abstract: Because of the coarse resolution of general circulation models (GCM), ‘downscaling’ techniques have emerged as a means of relating meso-scale atmospheric variables to grid- and sub-grid-scale surface variables. This study investigates these relationships. As a precursor, inter-variable correlations were investigated within a suite of 15 potential downscaling predictor variables on a daily time-scale for six regions in the conterminous USA, and observed correlations were compared with those based on the HadCM2 coupled ocean/atmosphere GCM. A comparison was then made of observed and model correlations between daily precipitation occurrence (a time series of zeroes and ones) and wet-day amounts and the 15 predictors. These two analyses provided new insights into model performance and provide results that are central to the choice of predictor variables in downscaling of daily precipitation. Also determined were the spatial character of relationships between observed daily precipitation and both mean sea-level pressure (mslp) and atmospheric moisture and daily precipitation for selected regions. The question of whether the same relationships are replicated by HadCM2 was also examined. This allowed the assessment of the spatial consistency of key predictor–predictand relationships in observed and HadCM2 data. Finally, the temporal stability of these relationships in the GCM was examined. Little difference between results for 1980–1999 and 2080–2099 was observed. For correlations between predictor variables, observed and model results were generally similar, providing strong evidence of the overall physical realism of the model. For correlations with precipitation, the results are less satisfactory. For example, model precipitation is more strongly dependent on surface divergence and specific humidity than observed precipitation, while the latter has a stronger link to 500 hPa divergence than is evident in the model. These results suggest possible deficiencies in the model precipitation process, and may indicate that the model overestimates future changes in precipitation. Correlation field patterns for mslp versus precipitation are remarkably similar for observed data and HadCM2 output. Differences in the correlation fields for specific humidity are more noticeable, especially in summer. In many cases, maximum correlations between precipitation and mslp occurred away from the grid box; whereas correlations with specific humidity were largest when the data were propinquitous. This suggests that the choice of predictor variable and the corresponding predictor domain, in terms of location and spatial extent, are critical factors affecting the realism and stability of downscaled precipitation scenarios. Copyright © 2000 Royal Meteorological Society

North Atlantic influence on Tigris–Euphrates streamflow

Abstract: Changes in streamflow of the Tigris and the Euphrates Rivers are shown to be associated with the North Atlantic Oscillation (NAO), a large-scale mode of natural climate variability which governs the path of Atlantic mid-latitude storm tracks and precipitation in the eastern Mediterranean. Composite indices of Turkish winter (December‐March, DJFM) temperature and precipitation are developed which capture the interannual‐decadal climate variability for the Tigris‐Euphrates headwater region, a significant source of freshwater for Turkey, Syria and Iraq. These indices are significantly correlated with the NAO, with 27% of the variance in precipitation accounted for by this natural mechanism. As evidenced by the recent widespread drought events of 1984, 1989 and 1990, the Tigris‐Euphrates streamflow also exhibits significant, 940% variability, associated with extrema. Copyright © 2000 Royal Meteorological Society.

ENSO and climatic signals across the Indian Ocean Basin in the global context: part I, interannual composite patterns

Abstract: This study focuses on the interplay between mean sea level pressure (MSLP), sea surface temperature (SST), and wind and cloudiness anomalies over the Indian Ocean in seasonal composite sequences prior to, during, and after strong, near-global El Nino and La Nina episodes. It then examines MSLP and SST anomalies in the 2–2.5-year quasi-biennial (QB) and 2.5–7-year low-frequency (LF) bands that carry the bulk of the raw ENSO signal. Finally, these fields were examined in conjunction with patterns of correlations between rainfall and joint spatiotemporal empirical orthogonal function (EOF) time series band pass filtered in the QB and LF bands. The seasonal composites indicate that the El Nino-1 (La Nina-1) pattern tends to display a more robust and coherent (weaker and less organized) structure during the evolution towards the mature stage of the event. The reverse tends to be apparent in the cessation period after the peak phase of an event, when El Nino events tend to collapse quite quickly. Climatic variables over the Indian Ocean Basin linked to El Nino and La Nina events show responses varying from simultaneous, to about one season's lag. In general, SSTs tend to evolve in response to changes in cloud cover and wind strength over both the north and south Indian Ocean. There are also strong indications that the ascending (descending) branch of the Walker circulation is found over the African continent (central Indian Ocean) during La Nina phases, and that the opposite configuration occurs in El Nino events. These alternations are linked to distinct warm–cool (cool–warm) patterns in the north–south SST dipole over the western Indian Ocean region during the El Nino (La Nina) events. An examination of MSLP and SST anomaly patterns in the QB and LF bands shows that signals are more consistent during El Nino-1 and El Nino sequences than they are during La Nina-1 and La Nina sequences. The QB band has a tendency to display the opposite anomaly patterns to that seen on the LF band during the early stages of event onset, and later stage of event cessation, during both El Nino–Southern Oscillation (ENSO) phases. El Nino events tend to be reinforced by signals on both bands up to their mature phase, but are then seen to erode rapidly, as a result of the presence of distinct La Nina anomalies on the QB band after their peak phase. During La Nina events, the opposite is observed during their cessation phase. Both QB and LF bands often display SST dipole anomalies that are not clearly evident in the raw composites alone. An eastern Indian Ocean SST dipole shows a tendency to occur during the onset phase of particular El Nino or La Nina episodes, especially during the austral autumn–winter (boreal spring–summer) and, when linked to tropical-temperate cloud bands, can influence Australian rainfall patterns. Analyses of seasonal correlations between rainfall and joint MSLP and SST EOF time series on QB and LF bands and their dynamical relationship with MSLP and SST anomalies during El Nino and La Nina events, show that the interplay between atmospheric circulation and SST anomalies dictates the observed rainfall response. Instances where either, or both, QB and LF bands are the prime influence on observed rainfall regimes are evident. This ability to discriminate the finer structure of physical relationships, correlations and patterns provides a deeper insight into Indian Ocean responses to ENSO phases. Copyright © 2000 Royal Meteorological Society

Trends in extreme rainfall indices for an updated high quality data set for australia, 1910-1998

Abstract: Daily rainfall was analysed at 91 high quality stations over eastern and southwestern Australia to determine if extreme rainfall had changed between 1910 and 1998. Three indices of extreme rainfall were examined: the number of events above an extreme threshold (extreme frequency); the average intensity of rainfall from extreme events (extreme intensity); and the proportion of total rainfall from extreme events (extreme percent). Several problems are discussed associated with designing such indices under a climate with significant trends in the number of raindays. Three different methods are used for calculating the extreme intensity and extreme percent indices to account for such trends in raindays. A separate analysis was carried out for four separate regions with significant results including a decrease in the extreme frequency and extreme intensity in southwest Western Australia and an increase in the extreme percent in eastern Australia. Trends in the extreme intensity and extreme percent are largely dependent on the method used to calculate the index. Total rainfall is strongly correlated with the extreme frequency and extreme intensity indices, suggesting that extreme events are more frequent and intense during years with high rainfall. Due to an increase in the number of raindays during such years, the proportional contribution from extreme events to the total rainfall depends on the method used to calculate this index. Copyright © 2000 Royal Meteorological Society.

Is the North Atlantic Oscillation a random walk

Abstract: The North Atlantic Oscillation (NAO) is a major mode of large-scale climate variability which contains a broad spectrum of variations. There are substantial contributions from short-term 2–5 year variations, which have clearly marked teleconnections. Decadal trends are also apparent in the historical record of the NAO and may be due to either stochastic or deterministic processes. Evidence is presented that suggests the NAO exhibits ‘long-range’ dependence having winter values residually correlated over many years. Several simple stochastic models have been used to fit the NAO SLP (sea-level pressure) wintertime index over the period 1864–1998, and their performance at predicting the following year has been assessed. Long-range fractionally integrated noise provides a better fit than does either stationary red noise or a non-stationary random walk. Copyright © 2000 Royal Meteorological Society

Spatial and temporal characteristics of potential evapotranspiration trends over China

Abstract: This paper analyses the time series (1954–1993) of Penman–Monteith evapotranspiration estimates for 65 stations in mainland China and Tibet, for the country as a whole and for individual stations. The analysis shows that for China as a whole, the potential evapotranspiration (PET) has decreased in all seasons. On a regional basis, northeast and southwest China have experienced moderate evapotranspiration increases, while in northwest and southeast China evapotranspiration has decreased to a much higher extent. South of 35°N, sunshine appears to be most strongly associated with evapotranspiration changes while wind, relative humidity and maximum temperature are the primary factors in northwest, central and northeast China, respectively. In the mountains of southwest China, a positive relation between evapotranspiration change and station altitude has been observed. If observed precipitation and PET trends remain unchanged future agricultural production, particularly in south and southwest China, will have to cope with decreasing water availability in the growing season. Copyright © 2000 Royal Meteorological Society

Spatial analysis of rainfall trends in the region of Valencia (east Spain).

Abstract: This paper examines the spatial and temporal rainfall characteristics of the region of Valencia, Western Mediterranean Basin (east Spain), during the World Meteorological Organization (WMO) normal period 1961‐1990. The study used a dense and homogeneous daily precipitation database comprising 97 rain-gauge stations. Total and monthly rainfall concentrations have been studied in the context of their mean values, interannual variability and spatial diversification. Trends have been analysed using both parametric and non-parametric tests. In order to establish the spatial distribution of rainfall patterns and to detect homogeneous areas with similar rainfall evolution, %

The effect of ENSO on the intraseasonal variance of surface temperatures in winter

Abstract: The effect of the El Nino–Southern Oscillation (ENSO) on the probability distribution of daily surface air temperature over the Pacific–North American sector is investigated using the US National Centers for Environmental Prediction (NCEP) Reanalysis data for 1959–1998. The El Nino response is characterized by reduced intraseasonal variance over most of the US, western Canada and the Gulf of Alaska. Conversely, there is an increase of variance during La Nina over the US and the west coast, northward to Alaska. The sign of this response is consistent for most individual El Nino/La Nina years in regions with a strong signal. The response is also robust with respect to differing definitions of ENSO or choice of dataset. Finally, a similar response is evident in station data for an earlier period. The change of variance is associated both with altered skewness, and a change in high and low extremes. Extremes of both signs are reduced during El Nino, and are slightly increased during La Nina. These results are consistent with other studies, suggesting an increased incidence of blocking along the west coast of North America during El Nino, leading to less storm activity and less incursions of warm and cold air over the eastern US. While an understanding of the changed variance is important in itself, it also has implications for changes in exceedence statistics (e.g. heating degree days) and the occurrence of extreme values. Copyright © 2000 Royal Meteorological Society

A study of the urban heat island of Granada

Abstract: In this study we examine the Urban Heat Island (UHI) of Granada. First, we perform a study of the evolution of the recorded temperatures at a meteorological station over the last century. In this record, the minimum temperatures increase while the maximum temperatures decrease. We also compare both rural and urban temperature records, obtaining the UHI fluctuations on a smaller time scale. The results show that the UHI phenomenon is stronger in winter, and the maximum difference occurs in early morning when temperatures are at their daily minimum. Then, we examine the geographical distribution of temperature in the urban region and obtain the dependence of UHI form on meteorological conditions, urban geometry and time scale. Urban geometry plays a particular role in the UHI form. The formation of UHI phenomena depends mainly on weather conditions and on time of night. Finally, we relate both UHI form and intensity with the observed trends in the urban time series. Copyright © 2000 Royal Meteorological Society.

The influence of la nina on african rainfall

Abstract: This article examines the influence of La Nina:cold events on rainfall over the African continent, using a harmonic analysis method. As with El Nino, there is a general association between wet conditions continentally and cold temperatures in the Atlantic and Indian Oceans, and dry conditions in association with warm sea-surface temperatures (SSTs) in these oceans. La Nina appears to have the greatest influence on rainfall in southern Africa and wet episodes tend to occur throughout the subcontinent during the first few months of the post-La Nina year. There is a somewhat weaker tendency for below-normal rainfall in eastern equatorial Africa at this time. Our results provide further confirmation of an earlier conclusion that SSTs in the Atlantic and Indian Oceans are a primary influence on African rainfall. La Nina’s influence tends to be exactly opposite that of El Nino: reduced rainfall over much of the continent in the first half of the episode, abnormally high rainfall in the second half. The documented El Nino:La Nina associations with African rainfall reflect the impact of these episodes on SSTs in the oceans. Copyright © 2000 Royal Meteorological Society.

Advection and the surface energy balance across an irrigated urban park

Abstract: The surface energy balance in an irrigated urban park in suburban Sacramento, CA is observed. Three sites extend from the edge of the park to its centre, along a transect which is aligned with the prevailing wind. Direct measurements of the fluxes of net radiation, soil heat flux and evaporation are made at each site and the convective sensible heat is found by residual. Strong advective effects on evaporation are observed, especially in the afternoon and evening. The driving forces for this are the differences in surface and air temperature, and humidity, between the cool, wet park and its warmer, drier built-up surroundings. The control of the surroundings on park evaporation is demonstrated by comparing values with those from synchronous observations in the surrounding suburbs and at an irrigated sod farm just outside the city. Greatest evaporative enhancement is observed at the upwind edge. Throughout the afternoon evaporation considerably exceeds the net radiation. This is interpreted to be due to the microscale leading-edge effect which appears to be restricted to a fetch of about 20 m. Further into the park evaporation also exceeds the net radiation in the afternoon due to the oasis effect. At all sites the sensible heat flux density in the afternoon is negative. Daily and daytime total evaporation from the park is more than 300% that from the integrated suburban area, and more than 130% that from the irrigated rural grass site. The unlimited water supply and the high temperatures of the park allow it to behave like a wet leaf in that its surface temperature seems to be ‘thermostatically’ controlled—it never rises more than a few degrees above that of the park air and for much of the day is cooler than the park air. Copyright © 2000 Royal Meteorological Society

An analysis of New Zealand synoptic types and their use in defining weather regimes

Abstract: A new set of 12 daily weather types for the New Zealand region has been derived from the 40-year NCEP/NCAR reanalysis dataset. Cluster analysis of the monthly frequencies of these patterns has led to the definition of three ‘regimes’, characterized by (i) frequent troughs crossing the country, (ii) highs to the north with strong zonal flow to the south of the New Zealand, and (iii) blocking patterns with highs more prominent in the south. Blocking regimes are more frequent in summer and autumn and are associated with above-normal temperatures, less precipitation in the southwest of the country and more precipitation in the northeast. The Zonal regime, which brings below-normal precipitation to the northeast and milder conditions in the south, is less common in summer. The Trough regime is less frequent in autumn and is linked to cooler temperatures in the west and above-normal precipitation over the entire country. The monthly frequencies of individual synoptic types are only weakly related to the Southern Oscillation Index (SOI) and other indices of the hemispheric-scale flow, with variance reductions from regression equations ranging from between 3 and 33%. Similar predictions of weighted combinations of the synoptic types corresponding to each regime gave mean variance reductions 70–80% higher. The correct regime could be specified from the hemispheric indices by the use of discriminant analysis for 56% of the 474 months in the dependent dataset. The monthly frequency of synoptic types was also tested as an alternative to monthly mean 1000 hPa patterns in the selection of analogues over the New Zealand region. The success of the selection process was measured by the variance of temperature and rainfall patterns over homogeneous regions for sets of two, four and eight analogues. The mean 1000 hPa patterns scored better in nearly all cases, but gave variance reductions of only 13–19% over randomly chosen analogues. Copyright © 2000 Royal Meteorological Society

Precipitation intensity trends in northern Italy.

Abstract: Recent studies on changes in precipitation intensity encompassing North America have found evidence for an increase in the relative amount of precipitation contributed by heavy and extreme rainfall events in the last 80 years. Within this context, the purpose of this paper is to verify whether such a signal can also be detected in northern Italy, where daily precipitation data are available from the beginning of the 19th century. The analysis is performed by applying the non-parametric Mann–Kendall test to mean anomaly series obtained through averaging the anomalies of some precipitation intensity statistics over five stations: Genoa (1833–1998), Milan (1858–1998), Mantova (1868–1997), Bologna (1879–1998) and Ferrara (1879–1996). It provides evidence that in northern Italy, the number of rainy days has a stronger and more significant negative trend than the corresponding precipitation amount, both on a yearly basis and in all of the seasons. As a consequence, precipitation intensity has a positive trend. The increase in precipitation intensity causes a significant positive trend in the proportion of total precipitation contributed by heavy precipitation events (i.e. daily precipitation >25 mm and daily precipitation >50 mm). The trend is mainly caused by the last 60–80 years, and is particularly evident in the periods of 1930–1945 and 1975–1995. The increase in precipitation intensity is connected to a modification of the distribution of daily precipitation values in a year, with trends that grow from the lower to the upper percentiles, and up to 4 mm/100 years for the 95th percentile. Copyright © 2000 Royal Meteorological Society

Predictions of climate change over Europe using statistical and dynamical downscaling techniques

Abstract: Statistical and dynamical downscaling predictions of changes in surface temperature and precipitation for 2080–2100, relative to pre-industrial conditions, are compared at 976 European observing sites, for January and July. Two dynamical downscaling methods are considered, involving the use of surface temperature or precipitation simulated at the nearest grid point in a coupled ocean–atmosphere general circulation model (GCM) of resolution ∼300 km and a 50 km regional climate model (RCM) nested inside the GCM. The statistical method (STAT) is based on observed linear regression relationships between surface temperature or precipitation and a range of atmospheric predictor variables. The three methods are equally plausible a priori, in the sense that they estimate present-day natural variations with equal skill. For temperature, differences between the RCM and GCM predictions are quite small. Larger differences occur between STAT and the dynamical predictions. For precipitation, there is a wide spread between all three methods. Differences between the RCM and GCM are increased by the meso-scale detail present in the RCM. Uncertainties in the downscaling predictions are investigated by using the STAT method to estimate the grid point changes simulated by the GCM, based on regression relationships trained using simulated rather than observed values of the predictor and the predictand variables (i.e. STAT_SIM). In most areas the temperature changes predicted by STAT_SIM and the GCM itself are similar, indicating that the statistical relationships trained from present climate anomalies remain valid in the perturbed climate. However, STAT_SIM underestimates the surface warming in areas where advective predictors are important predictors of natural variability but not of climate change. For precipitation, STAT_SIM estimates the simulated changes with lower skill, especially in January when increases in simulated precipitation related to a moister atmosphere are not captured. This occurs because moisture is rarely a strong enough predictor of natural variability to be included in the specification equation. The predictor/predictand relationships found in the GCM do not always match those found in observations. In January, the link between surface and lower tropospheric temperature is too strong. This is also true in July, when the links between precipitation and various atmospheric predictors are also too strong. These biases represent a likely source of error in both dynamical and statistical downscaling predictions. For example, simulated reductions in precipitation over southern Europe in summer may be too large. Copyright © 2000 British Crown Copyright

Southwest Western Australian winter rainfall and its association with Indian Ocean climate variability

Abstract: Southwest Western Australia (SWWA) has experienced a significant decrease in winter rainfall since the late 1960s. This decrease is unexplained and the resultant problem of reduced water storage has been compounded by the lack of any useful predictive skill at the seasonal time scale. This study uses recent gridded, historical data and simple linear correlation in order to evaluate the importance of links between rainfall and both mean sea level pressure (MSLP) and sea-surface temperature (SST) patterns over the Indian Ocean. The decrease in rainfall is linked to decreases in the density of low-pressure systems in the region and to increases in both MSLP and SST over the southern Indian Ocean. Warmer SSTs and increases in MSLP are associated with the observed long-term changes, but changes in these variables do not explain a great deal of the observed interannual variability. Greenhouse-induced climate change is not regarded as a likely explanation for the observed decrease, however, the existence of links with both MSLP and SSTs suggests the existence of coupled air–sea interactions over the southern Indian Ocean which may be relevant at decadal or multi-decadal timescales. A major difficulty with defining any such processes is the relative sparseness of data at high latitudes in the Southern Hemisphere. This should be partly alleviated as more recent high quality data becomes available over time. Copyright © 2000 Royal Meteorological Society

Predicting monthly and seasonal rainfall, onset and cessation of the rainy season in West Africa using only surface data

Abstract: New empirical long-range schemes for the prediction of dates of onset and cessation and of the monthly and annual amounts of rainfall are developed for Kano, in the West African Sahel, using only surface synoptic data. They are based on variations in equivalent potential temperature, θe, which occur as a result of the seasonal, monthly and daily variations of moisture in the summer monsoon flow over West Africa. Agricultural activities may begin about 72 days after the day the anomalies of θe (i.e. θ') first become positive for at least 15 days, essentially signifying the beginning of adequate moisture supply associated with a well established monsoon flow. The new schemes ensure that both the cessation date and the annual amount of rainfall can be predicted prior to the onset of the rains, thus providing, in conjunction with the onset date, very important and useful information for reliable and effective planning of agricultural and water resource activities. Performance tests using an 11-year independent data set indicate that the schemes possess reliable skill. Because the weather over Nigeria is very typical of the entire West African region, being affected by the same wind regime and weather phenomena, these prediction schemes will provide tremendous assistance for enhanced and sustainable agriculture, as well as for efficient water resources management, if extended to the whole area. Furthermore, the methods have the important advantage that, bearing in mind the fact that the majority of West African countries have very sparse, if indeed, any upper-air data, the surface synoptic data needed for their use are readily available in all of the countries. Copyright © 2000 Royal Meteorological Society

A monthly circulation climatology for Sweden and its application to a winter temperature case study.

Abstract: Atmospheric circulation is important in determining the surface climate and environment. To quantify its effect, circulation indices or classifications of circulation type are often used. In this study, the classification system developed by Lamb (1950. Quarterly Journal of the Royal Meteorological Society76: 393–438) is applied to obtain circulation information for Sweden on a monthly basis. For that purpose, monthly mean sea-level pressure (MSLP) data from 1873 to 1995 is used to derive six circulation indices and to provide a circulation catalogue with 27 circulation types. The frequency of circulation types over different periods is computed and described. Four major types (cyclonic, C; west, W; southwest, SW; anticyclonic, A) have been identified. The catalogue and the associated indices provide a tool for interpreting the regional climate and for developing statistical downscaling models to derive regional climate change scenarios for Sweden. An example is given to illustrate the application and usefulness of this climatology. It deals with the linkage between the mean regional temperature in southwestern Sweden and the large-scale circulation. It is shown that directional flows with a westerly component clearly favour a positive temperature anomaly, while the anticyclonic condition often induces a negative anomaly. Moreover, it is the westerly or southwesterly flows that accompany extreme positive temperature anomalies, whereas anticyclonic conditions play an important role in creating extreme negative temperature anomalies. Further, the derived indices have a relationship with temperature anomalies, established via a statistical model. Stepwise multiple regression was used, leading to a successful model with only three indices. Using the model, 70% of the total variance in the temperature anomalies between 1887 and 1994 has been reconstructed from the pressure, which leads to the conclusion that the circulation is a critical determinant of the January temperature and that the derived climatology is useful in explaining the temperature anomalies. Copyright © 2000 Royal Meteorological Society

Associations between varying magnitudes of the urban heat island and the synoptic climatology in Melbourne, Australia

Abstract: This paper examines the characteristics of the urban heat island (UHI) in a large Australian city (Melbourne) using local area weather data and the US National Centers for Environmental Prediction (NCEP) reanalyses data of mean sea level pressure (MSLP). Conventional methods to determine the UHI from climatological data often involve comparing minimum temperature data between urban and rural environments. The findings presented in this paper show that comparing simultaneous measurements of temperature at one urban and three nearby airport sites at 0600 (Australian eastern standard time (EST) provided a better estimate of the UHI magnitude. Analyses of these data between 1973-1991 were grouped according to the daily UHI magnitude. For each UHI group of the daily data, mean anomalous synoptic conditions from the 19-year mean monthly MSLP were examined for the influence of the different MSLP conditions on varying UHI magnitudes. These synoptic conditions included the entire range of weather conditions over the study period. Over the 19-year period, daily analyses of the regional climatological 0600 EST temperature data revealed a UHI between - 3.16° and 6.0°C. The reanalyses of the NCEP MSLP data, in association with the local area climatological data, suggest that statistically significant anomalous anticyclonic conditions were associated with the warmest 17% and coolest 1% of UHI events. The position of the centre of the anticyclone was critical to UHI genesis and development. Statistically significant mean low pressure anomalies were associated with UHI values between 0° and 1°C. These occurred on 40% of the days between 1973-1991. Melbourne's urban area, independent of the topography, was found to inhibit early morning advection events of warm continent air and result in an urban cool island (UCI). Copyright © 2000 Royal Meteorological Society.

On the relationships between ENSO and autumn rainfall in Iran.

Abstract: An investigation of the relationships between Iranian autumn rainfall and the El Nino–Southern Oscillation (ENSO) phenomenon was explored for the period 1951–1990. A negative correlation between the Troup Southern Oscillation Index (SOI) and rainfall data was found for almost all of Iran. The relationships were found to be stronger and more consistent over some regions comprising the southern foothills of the Alborz Mountains, northwestern districts and central areas. For the other parts of the country, correlations were found to be either less significant or non-stationary. It was determined that the associations between SOI and rainfall over central parts of Iran have persistently improved for the recent period studied. The impacts of ENSO on rainfall amounts during low and high phases of the SO index were also studied. It was found that during El Nino episodes, the amount of rainfall over various parts of the country was several times more than during La Nina periods. The associations between SOI and surface air pressure data were found to be poor and insignificant. The possibility of rainfall forecasting was also explored and the results suggest that autumn rainfall could be predicted a season ahead for some parts of the country. A mechanism for the influence of the ENSO cycle on Iranian rainfall is suggested. Copyright © 2000 Royal Meteorological Society

Climate sensitivity of snow cover duration in Austria

Abstract: The number of days with snow cover at Austrian climate stations, normalized by the maximum possible snow days within a season, is denoted n. This seasonal relative snow cover duration is considered a function of station height H and of the seasonal mean temperature T over Europe. When T increases, n decreases and vice versa. The function becomes saturated both for high stations at low European temperature (‘always snow’, n=1) and for low stations at high temperature (‘never snow’, n=0). In the saturated regions, the sensitivity s≡∂n(H, T)/∂T is practically zero, while in the transition region, s is extreme. The observed interannual fluctuations of T are considered here as simulation of a possible climate shift. s is determined for the climate stations of Austria from its snow cover record [1961–1990, 84 stations between 153 and 3105 m above sea level (a.s.1.)] by fitting the data of n for each individual station (local mode) as well as for all Austrian stations (global mode) with a hyperbolic tangent function. In the global mode, s reaches an extreme value of −0.34±0.04 K−1 in winter and −0.46±0.13 K−1 in spring. The implications of these results are discussed. Included in this discussion is the fact that a rise in the European temperature by 1 K may reduce the length of the snow cover period in the Austrian Alps by about 4 weeks in winter and 6 weeks in spring. However, these extreme values apply only to the height of maximum sensitivity (575 m in winter, 1373 m in spring); the actual sensitivity of individual stations located at higher or lower levels is less. Copyright © 2000 Royal Meteorological Society

Tropical-midlatitude exchange of air masses during summer and winter in South America: climatic aspects and examples of intense events

Abstract: Meridional transport of air masses between the tropics and midlatitudes in South America are the most intense in the entire Southern Hemisphere, mainly due to the presence of the Andes. The incursions of tropical air into midlatitudes occur on the eastern side of the Andes in two preferred regions. The first is located in the tropical latitudes, close to the mountains between 20° and 30°S, and the second is a function of the position of the South Atlantic Convergence Zone (SACZ). All year long, the two preferred regions maintain their behaviour, with only small variations of their position or relative importance in different seasons of the year. The variability of the meridional transport is larger on the eastern side of the Andes, due to the presence of the mountain barrier, which favours baroclinic activity and allows an active exchange of air masses in both senses, especially during winter. The importance of the air mass transport is evident in the precipitation and surface temperature fields. During summertime, the Chaco Low (25°S and 65°W) intensifies due to the positive net radiation, favouring the transport of tropical air masses towards the south and the presence of strong convective activity, which is fed by moisture from tropical regions. During winter, the penetration of tropical air towards higher latitudes is more sporadic. The displacement of midlatitude air towards tropical latitudes occurs on both sides of the Andes. On the western side, the air associated with the subtropical Pacific anticyclone flows northward channelled by the Andes. On the eastern side, incursions of polar air towards lower latitudes are linked to cold fronts whose trajectory and movement is also favoured by the presence of the Andes. In particular, during wintertime the cold fronts are more intense and faster, and sometimes even reach tropical and equatorial latitudes which produces freezes in subtropical regions, such as the coffee growing areas in southeastern Brazil. In contrast, the incursions of cold air are notably weaker and less frequent in summer, and during these events the active cold fronts move northwards merging with the SACZ, which becomes more intense. Copyright © 2000 Royal Meteorological Society

Temporal and spatial variation of surface air temperature over the period of instrumental observations in the Arctic

Abstract: A detailed analysis of the spatial and temporal changes in mean seasonal and annual surface air temperatures over the period of instrumental observations in the Arctic is presented. In addition, the role of atmospheric circulation in controlling the instrumental and decadal-scale changes of air temperature in the Arctic is investigated. Mean monthly temperature and temperature anomalies data from 37 Arctic, 7 sub-Arctic and 30 grid-boxes were used for analysis. The presented analysis shows that the observed variations in air temperature in the real Arctic (defined on the basis of climatic as opposed to other criteria, e.g. astronomical or botanical) are in many aspects not consistent with the projected climatic changes computed by climatic models for the enhanced greenhouse effect. The highest temperatures since the beginning of instrumental observation occurred clearly in the 1930s and can be attributed to changes in atmospheric circulation. The second phase of contemporary global warming (after 1975) is, at most, weakly marked in the Arctic. For example, the mean rate of warming for the period 1991–1995 was 2–3 times lower in the Arctic than the global average. Temperature levels observed in Greenland in the last 10–20 years are similar to those observed in the 19th century. Increases of temperature in the Arctic are more significant in the warm half-year than in the cold half-year. This seasonal pattern in temperature change confirms the view that positive feedback mechanisms (e.g. sea-ice–albedo–temperature) as yet play only a small role in enhancing temperature in the Arctic. Hypotheses are presented to explain the lack of warming in the Arctic after 1975. It is shown that in some parts of the Arctic atmospheric circulation changes, in particular in the cold half-year, can explain up to 10–50% of the temperature variance. For Arctic temperature, the most important factor is a change in the atmospheric circulation over the North Atlantic. The influence of atmospheric circulation change over the Pacific (both in the northern and in the tropical parts) is significantly lower. Copyright © 2000 Royal Meteorological Society

On the El Niño teleconnection to spring precipitation in Europe

Abstract: In a statistical analysis of more than a century of data, a strong connection was found between strong warm El Nino winter events and high spring precipitation in a band from southern England eastwards into Asia. This relationship is an extension of the connection mentioned by Kiladis and Diaz (1989. ‘Global climatic anomalies associated with extremes in the Southern Oscillation’, J. Climate, 2, 1069–1090), and is much stronger than the winter season teleconnection that has been the subject of other studies. Correlation coefficients between December–January (DJF) NINO3 indices and March–May (MAM) precipitation are higher than r=0.3 for individual stations, and as high as r=0.49 for an index of precipitation anomalies around 50°N from 5°W to 35°E. The lagged correlation suggests that southeast Asian surface temperature anomalies may act as intermediate variables. Copyright © 2000 Royal Meteorological Society