Showing papers in "International Journal of Climatology in 2002"

Daily dataset of 20th‐century surface air temperature and precipitation series for the European Climate Assessment

Abstract: We present a dataset of daily resolution climatic time series that has been compiled for the European Climate Assessment (ECA). As of December 2001, this ECA dataset comprises 199 series of minimum, maximum and/or daily mean temperature and 195 series of daily precipitation amount observed at meteorological stations in Europe and the Middle East. Almost all series cover the standard normal period 1961–90, and about 50% extends back to at least 1925. Part of the dataset (90%) is made available for climate research on CDROM and through the Internet (at http://www.knmi.nl/samenw/eca). A comparison of the ECA dataset with existing gridded datasets, having monthly resolution, shows that correlation coefficients between ECA stations and nearest land grid boxes between 1946 and 1999 are higher than 0.8 for 93% of the temperature series and for 51% of the precipitation series. The overall trends in the ECA dataset are of comparable magnitude to those in the gridded datasets. The potential of the ECA dataset for climate studies is demonstrated in two examples. In the first example, it is shown that the winter (October–March) warming in Europe in the 1976–99 period is accompanied by a positive trend in the number of warm-spell days at most stations, but not by a negative trend in the number of cold-spell days. Instead, the number of cold-spell days increases over Europe. In the second example, it is shown for winter precipitation between 1946 and 1999 that positive trends in the mean amount per wet day prevail in areas that are getting drier and wetter. Because of its daily resolution, the ECA dataset enables a variety of empirical climate studies, including detailed analyses of changes in the occurrence of extremes in relation to changes in mean temperature and total precipitation. Copyright  2002 Royal Meteorological Society.

A drought climatology for Europe

Abstract: We present a high spatial resolution, multi-temporal climatology for the incidence of 20th century European drought. The climatology provides, for a given location or region, the time series of drought strength, the number, the mean duration, and the maximum duration of droughts of a given intensity, and the trend in drought incidence. The drought climatology is based on monthly standardized precipitation indices (SPIs) calculated on a 0.5degrees grid over the European region 35-70degreesN and 35degreesE-10degreesW at time scales of 3, 6, 9, 12, 18, and 24 months for the period 1901-99. The standardized property facilitates the quantitative comparison of drought incidence at different locations and over different time scales. The standardization procedure (probability transformation) has been tested rigorously assuming normal, log-normal, and gamma statistics for precipitation. Near equivalence is demonstrated between the Palmer drought severity index (PDSI) and SPIs on time scales of 9 to 12 months. The mean number and duration by grid cell of extreme European drought events (SPI less than or equal to -2) on a time scale of 12 months is 6 +/- 2 months and 27 +/- 8 months respectively. The mean maximum drought duration is 48 +/- 17 months. Trends in SPI and PDSI values indicate that the proportion of Europe experiencing extreme and/or moderate drought conditions has changed insignificantly during the 20th century. We hope the climatology will provide a useful resource for assessing both the regional vulnerability to drought and the seasonal predictability of the phenomenon. Copyright (C) 2002 Royal Meteorological Society.

Observed changes in seasons: An overview

Abstract: Within the last decade the study of phenology has taken on a new legitimacy in the area of climate change research. A growing literature reveals that a change in the timing of natural events is occurring in a wide range of locations and affecting a wide range of species. Changes in spring have been those most commonly reported, with the emphasis on an advance in spring linked to an increase in temperature. Detection of change in autumn is hampered by a smaller pool of available data, events that are harder to define (such as leaf coloration), and various influencing environmental factors triggering autumnal phases. Despite this, the general pattern may be towards a delay in autumn. Plant, animal and abiotic responses, especially in spring, are quite similar. Thus, it would appear that winter is being squeezed at both ends, and this effect, of increasing the growing season, should become more pronounced in the face of predicted global warming. Copyright © 2002 Royal Meteorological Society.

The redevelopment of a weather-type classification scheme for north america

Abstract: Synoptic weather-typing, or the classification of weather conditions into categories, is a useful tool for climate impact applications. Numerous procedures have been developed to accomplish this goal. Before the advent of high-speed computers, manual methods were most common; more recently, more automated methods have come into wide use. Both types of classification have shortcomings; manual methods are time consuming and difficult to reproduce, whereas automated methods may not produce easily interpretable results. Several recent methods have incorporated the advantages of both methodologies into a hybrid scheme. This paper describes the redevelopment of one such hybrid scheme, the Spatial Synoptic Classification (SSC). The SSC, originally developed in the mid-1990s, classifies each day at a location into one of six weather types, or a transition. It has been utilized for several applications, from climate trends to human health. Despite its utility, it has several shortcomings, most notably a lower-than-desired match percentage among adjacent stations and a framework that only allows for classification during winter and summer. The new SSC (SSC2) has been altered in several important ways. The most notable changes involve the procedure for selecting seed days, days that typify a particular weather type at a particular location. With the new procedures, the SSC can now produce weather-type classifications year-round, instead of only winter and summer. The spatial cohesiveness among stations has also been improved. The SSC has been expanded to include Canada, Alaska, and Hawaii in addition to the lower 48 US states. SSC calendars are now available for 327 stations with a mean length of 44.6 years, and are updated daily on a website. This paper also presents an important application of the redesigned SSC. It has been used in several heat-stress warning systems worldwide. The synoptic approach is considered to be superior to a traditional apparent temperature approach, as it considers more parameters in its holistic assessment. At each location, one or two of the weather types is associated with mortality levels significantly above the mean. Copyright  2002 Royal Meteorological Society.

Links between circulation and changes in the characteristics of Iberian rainfall

Abstract: Investigation of the links between atmospheric circulation patterns and rainfall is important for the understanding of climatic variability and for the development of empirical circulation-based downscaling methods. Here, spatial and temporal variations in circulation-rainfall relationships over the Iberian Peninsula during the period 1958–97 are explored using an automated circulation classification scheme and daily rainfall totals for 18 stations. Links between the circulation classification scheme and the North Atlantic oscillation (NAO) are also considered, as are the direct links between rainfall and the NAO. Trends in rainfall and circulation-type frequency are explored. A general tendency towards decreasing mean seasonal rainfall over the peninsula, with the exception of the southeastern Mediterranean coast, hides larger changes in wet day amount and rainfall probability. There is a tendency towards more, less-intensive rain days across much of Iberia, with a tendency towards more, more-intensive rain days along the southeastern Mediterranean coast, both of which are reflected in changes in rainfall amount quantiles. A preliminary analysis indicates that these changes may have occurred systematically across all circulation types. Comparison of the trends in rainfall and in circulation-type frequency suggests possible links. These links are supported by linear regression analyses using circulation-type frequencies as predictor variables and rainfall totals for winter months as the predictands. The selected predictor variables reflect the main circulation features influencing winter rainfall across the peninsula, i.e. the strong influence of Atlantic westerly and southwesterly airmasses over much of the peninsula, of northerly and northwesterly surface flow over northern/northwestern Spain and northern Portugal and the stronger effect of Mediterranean rather than Atlantic influences in southeastern Spain. The observed rainfall changes cannot, however, be explained by changes in circulation alone. Copyright © 2002 Royal Meteorological Society.

Synoptic climatology of major floods in the Negev Desert, Israel

Abstract: This study examines the extent to which the floods in the Negev Desert, an area that constitutes the southern half of Israel, are not the outcome of purely local weather conditions but are, rather, the result of distinct synoptic-scale events. This was done through compiling and analysing a hydro-climatological database of all the major floods in the Negev, and then categorizing them manually into synoptic types that cause the major floods. The type analysis is based on the US National Meteorological Center data sets with 2.5° × 2.5° resolution analysed by GrADS. Data were compiled and studied for 52 floods for the period 1965–94, with peak discharge above the magnitude of 5 year recurrence intervals (RI > 5 years) in at least one drainage basin. Distinct extreme synoptic patterns are indeed associated with 42 of the 52 floods. They were grouped into four synoptic types, two of which were associated with 37 events: (a) an active Red Sea trough, defined as a surface trough extending from East Africa through the Red Sea toward the eastern Mediterranean, accompanied by a pronounced trough at the 500 hPa level over eastern Egypt: (b) a Syrian low, defined as a well-developed Mediterranean cyclone accompanied by a pronounced upper-level trough, both located over Syria. Each of the four synoptic types has its own evolution course, and a unique seasonal and spatial distribution of its associated flooded basins. These findings imply that the major floods in the Negev can be considered as signatures of exceptional synoptic-scale evolutions, and that major floods reflect extreme climatic events. Our results indicate that it is possible to use a set of dynamic and thermodynamic variables for predicting the occurrence and location of major flash floods. Copyright © 2002 Royal Meteorological Society.

Changes in European spring phenology

Abstract: The European phyto-phenological database of the EU 5th Framework project ‘POSITIVE’ facilitated an examination of the rate and spatial pattern of changes in spring phenology across Europe. This database was collected, evaluated and composed from different national databases of Eastern and Western Europe covering the time period 1951–1998. Results show that spring phases have advanced four weeks in Western and Central Europe, and have been delayed up to two weeks in Eastern Europe. Western European spring starts earlier because of the intensive flow of warmer Atlantic air masses; the Eastern part of Europe has a different phenological rhythm and trends, that can be explained by the influence of the Siberian high. The highest rate of significant ( p< 0.05) phenological change (−0. 3t o−0.4 days per year) occurs in the Western Europe and Baltic Sea regions for early spring phases of hazel and colts-foot. Spring phases of birch, apple and lilac, and summer phases, such as the flowering of linden, tend to occur earlier with an average rate of −0.1 to 0.3 days per year. Copyright  2002 Royal Meteorological Society.

Interannual rainfall variability in the Amazon Basin and sea-surface temperatures in the Equatorial pacific and the Tropical Atlantic oceans

Abstract: Rainfall variability in the Amazon basin is studied in relation to sea-surface temperatures (SSTs) in the equatorial Pacific and the northern and southern tropical Atlantic during the 1977–99 period, using the HiBAm original rainfall data set and complementary cluster and composite analyses. The northeastern part of the basin, north of 5 °S and east of 60 °W, is significantly related with tropical SSTs: a rainier wet season is observed when the equatorial Pacific and the northern (southern) tropical Atlantic are anomalously cold (warm). A shorter and drier wet season is observed during El Ni˜ no events and negative rainfall anomalies are also significantly associated with a warm northern Atlantic in the austral autumn and a cold southern Atlantic in the spring. The northeastern Amazon rainfall anomalies are closely related with El Ni˜ no–southern oscillation during the whole year, whereas the relationships with the tropical Atlantic SST anomalies are mainly observed during the autumn. A time–space continuity is observed between El Ni˜ no-related rainfall anomalies in the northeastern Amazon, those in the northern Amazon and south-eastern Amazon, and those in northern South America and in the Nordeste of Brazil. A reinforcement of certain rainfall anomalies is observed when specific oceanic events combine. For instance, when El Ni˜ no and cold SSTs in the southern Atlantic are associated, very strong negative anomalies are observed in the whole northern Amazon basin. Nonetheless, the comparison of the cluster and the composite analyses results shows that the rainfall anomalies in the northeastern Amazon are not always associated with tropical SST anomalies. In the southern and western Amazon, significant tropical SST-related rainfall anomalies are very few and spatially variable. The precipitation origins differ from those of the northeastern Amazon: land temperature variability, extratropical perturbations and moisture advection are important rainfall factors, as well as SSTs. This could partially explain why: (a) the above-mentioned signals weaken or disappear, with the exception of the relative dryness that is observed at the peak of an El Ni˜ no event and during the dry season when northern Atlantic SSTs are warmer than usual; (b) rainfall anomalies tend to resemble those of southeastern South America, noticeably at the beginning and the end of El Ni˜ no a %

Using the standardized precipitation index for flood risk monitoring

Abstract: Flood risk and the critical time of occurrence are difficult to monitor and detect. The availability and application of adapted indices may allow not only continuous monitoring of hydrological conditions in flood-prone areas, but also the potential threat of possible flood events in order to promote preventive actions to mitigate the impacts of the phenomenon. The standardized precipitation index (SPI) was developed for drought detection and monitoring. However, because of its characteristics, the SPI can also be used as a tool to monitor wetter-than-normal conditions. The aim of this paper, based on the recurrent floods affecting the southern Cordoba Province in Argentina, is to analyse the potential of the SPI as a tool for monitoring flood risk in that region. Results indicate that the SPI satisfactorily explains the development of conditions leading up to the three main flood events to occur in the region during the past 25 years. This fact supports the proposal for applying the SPI as an effective component of a regional system for climate risk monitoring. Copyright © 2002 Royal Meteorological Society.

Assessing satellite-derived start-of-season measures in the conterminous USA

Abstract: National Oceanic and Atmospheric Administration (NOAA)-series satellites, carrying advanced very high-resolution radiometer (AVHRR) sensors, have allowed moderate resolution (1 km) measurements of the normalized difference vegetation index (NDVI) to be collected from the Earth's land surfaces for over 20 years. Across the conterminous USA, a readily accessible and decade-long data set is now available to study many aspects of vegetation activity in this region. One feature, the onset of deciduous plant growth at the start of the spring season (SOS) is of special interest, as it appears to be crucial for accurate computation of several important biospheric processes, and a sensitive measure of the impacts of global change. In this study, satellite-derived SOS dates produced by the delayed moving average (DMA) and seasonal midpoint NDVI (SMN) methods, and modelled surface phenology (spring indices, SI) were compared at widespread deciduous forest and mixed woodland sites during 1990-93 and 1995-99, and these three measures were also matched to native species bud-break data collected at the Harvard Forest (Massachusetts) over the same time period. The results show that both SOS methods are doing a modestly accurate job of tracking the general pattern of surface phenology, but highlight the temporal limitations of biweekly satellite data. Specifically, at deciduous forest sites: (1) SMN SOS dates are close in time to SI first bloom dates (average bias of +0.74 days), whereas DMA SOS dates are considerably earlier (average bias of -41.24 days) and also systematically earlier in late spring than in early spring; (2) SMN SOS tracks overall yearly trends in deciduous forests somewhat better than DMA SOS, but with larger average error (MAEs 8.64 days and 7.37 days respectively); and (3) error in both SOS techniques varies considerably by year.

Survey of Greenland instrumental temperature records: 1873–2001

Abstract: Temporal and spatial variability are analysed in Greenland instrumental temperature records from 24 coastal and three ice sheet locations. Trends over the longest period available, 1873–2001, at Ilulissat/Jakobshavn indicate statistically significant warming in all seasons: 5°C in winter. Trends over the 1901–2000 century in southern Greenland indicate statistically significant spring and summer cooling. General periods of warming occurred from 1885 to 1947 and 1984 to 2001, and cooling occurred from 1955 to 1984. The standard period 1961–90 was marked by 1–2°C statistically significant cooling. In contrast to Northern Hemisphere mean temperatures, the 1990s do not contain the warmest years on record in Greenland. The warmest years in Greenland were 1932, 1947, 1960, and 1941. The coldest years were 1918, 1984, 1993, and 1972, several of which coincide with major volcanic eruptions. Over 1991–2000, statistically significant 2–4°C warming was observed in western Greenland, 1.1°C warming at the ice sheet summit (3200 m), although this is statistically insignificant. Annual temperature trends are dominated by winter variability. Much of the observed variability is shown to be linked with the North Atlantic oscillation (NAO), sea ice extent, and volcanism. The correlation of coastal temperature anomalies with the NAO is statistically significant, in autumn and winter at western and southern sites. Warming from 1873 to 1930 and subsequent cooling persists after the removal of the NAO signal. Temperature trends are often opposite between west and east Greenland. This apparent teleconnection is spurious, however, given insignificant east–west correlation values. Frequency peaks correspond with periods of 3.7, 14.3, 9.1, 5.5–6.0, 11.1, and 7.1 years in both temperature and NAO. Copyright © 2002 Royal Meteorological Society.

Re-evaluation of trends and changes in mean, maximum and minimum temperatures of Turkey for the period 1929-1999

Abstract: Mean, maximum and minimum surface air temperatures recorded at 70 climatological stations in Turkey during the period from 1929 to 1999 were analysed to reveal spatial and temporal patterns of long-term trends, change points, significant warming (cooling) periods and linear trend rates per decade. Annual, winter and spring mean temperatures have tended to increase, particularly over the southern regions of Turkey, whereas summer and particularly autumn mean temperatures have decreased over the northern and continental inner regions. Annual, winter, spring and summer maximum temperatures have indicated a positive trend at many stations, except those in the Central Anatolia and Black Sea regions and partly in the Eastern Anatolia region. Autumn maximum temperatures, however, have decreased slightly over much of Turkey, except for the Aegean region. The majority of the urbanized and rapidly urbanizing stations in Turkey have been experiencing an apparent night-time warming, especially during the warm and dry period of the year. Minimum temperatures have shown a statistically significant positive trend at 31 stations annually, 30 stations in spring and 33 stations in summer; most are significant at the 0.01 level. Winter and autumn minimum temperatures have indicated a general negative trend in some portions of the Marmara, Black Sea and Eastern Anatolia regions, whereas a general positive trend has been seen over much of Turkey along with the significant trends in the Mediterranean region of the country. Summer night-time warming rates are generally larger than in spring and autumn night-time temperatures. On the other hand, the night-time warming rates of spring and summer are generally stronger than those in spring and summer daytime temperatures. By considering the marked increasing trends in spring, summer and annual minimum temperatures of urban stations, we have recognized that this is a clear and significant indication for the existence of a strong night-time urban warming in Turkey. Copyright © 2002 Royal Meteorological Society.

Regional streamflow trend detection with consideration of both temporal and spatial correlation

Abstract: It is known that serial correlation within time series at sites and cross-correlation among sites in a specific region will influence the ability of statistical tests to assess the field significance of trends over the region. However, serial and/or cross-correlation has been ignored in field trend-analyses. This study attempts to develop a methodology that takes into account both serial and cross-correlation in the assessment of the field significance of trends. The regional average Mann–Kendall (RAMK) statistic is used to represent the regional properties of trends at a regional scale. The null distribution of the RAMK statistic is derived on the basis that the joint probability distribution of m independent normal variables is also normally distributed. The variance of the RAMK statistic is then modified by serial and cross-correlation. The applicability of the method was demonstrated by applying it to assess the field significance of trends in annual mean, annual maximum, and annual minimum daily streamflow from 1967 to 1996 in ten major homogeneous climate regions of Canada. The results indicate that the method developed provides more accurate assessment of the field significance of trends than that without consideration of serial and cross-correlation. At the significance level of 0.10, annual mean daily flow increased significantly in the region of Yukon and northern BC mountains whereas it decreased significantly in the Pacific and the Prairie regions. Annual maximum daily flow decreased significantly across southern Canada, except in the Pacific region. Annual minimum daily flow decreased significantly in the Pacific region and in southeastern Canada, with the exception of the region of Great Lakes and St Lawrence river basin, whereas it increased significantly in the region of Yukon and northern BC mountains. Copyright © 2002 Royal Meteorological Society

Investigating the link between early season Caribbean rainfall and the El Niño + 1 year

Abstract: The Caribbean rainfall season is best characterized by its bimodal nature, with an initial peak in May–June and a second more prominent one in September–October. This allows for a convenient division into an early and a late rainfall season. In this study we examine the rainfall patterns of the early rainfall season (mid April to July) for links with El Ni˜ no–Southern Oscillation (ENSO) events. Whereas traditionally ENSO events have been identified with dry conditions during the later Caribbean rainfall season, recent research suggests a second signal that manifests itself as a wet early rainfall season of the year of ENSO decline (the El Ni˜ no+1 year). Two leading empirical orthogonal function modes of early season Caribbean rainfall are examined for evidence of this. Strong correlations are shown to exist between the first mode and wintertime equatorial Pacific anomalies. The first mode explains nearly half of the early season variability. The idea that the wintertime Pacific anomalies alter the early Caribbean rainfall season via the warm spring sea surface temperature anomalies they induce in the north tropical Atlantic is also investigated. An atmospheric general circulation model is also used to show that, when warm/cold anomalies exist across the north tropical Atlantic, this results in a large-scale atmospheric circulation that is more/less favourable to rainfall production over the Caribbean. Copyright  2002 Royal Meteorological Society. no; ENSO; EOF; AGCM

The influence of temperature and climate change on the timing of pollen release in the Netherlands

Abstract: In the last decade it has become clear that the timing of many phenological processes, like the start of flowering and leaf unfolding in spring, have changed. The increase in temperature is believed to be the main cause. The earlier start of flowering will have consequences for the start of the pollen season, and thus for the start of the hay fever season. Millions of people world-wide will therefore experience the impact of climate change in their daily lives during spring and summer. In this paper we analyse the relation between climate parameters, especially temperature, and the start of the pollen season in the western part of the Netherlands based on daily pollen counts of the Leiden University Medical Centre and temperature measurements from 1969 till 2000 by the Royal Netherlands Meteorological Institute in De Bill. The results indicate that there is a strong correlation between temperature and start of the pollen season. An advance of the start of the pollen season of 3 to 22 days has been observed. The potential future changes in the start of the pollen season under climate change scenarios are also discussed. Copyright (C) 2002 Royal Meteorological Society.

Droughts and extreme events in regional daily Italian precipitation series

Abstract: This paper proposes a methodology to study daily precipitation series that include a significant proportion of missing data, without resorting to completion methods based on randomly generated numbers. It is applied to a data set consisting of 75 station records (1951–2000) covering the Italian territory. They are clustered by principal component analysis into six regions: the north-west, the northern part of the north-east, the southern part of the north-east, the centre, the south and the islands (i.e. Sicily and Sardinia). Complete annual and seasonal regional average series are obtained from the incomplete station records, and analysed for droughts and extreme precipitation events. Droughts are identified by means of two indicators: the longest dry period and the proportion of dry days. The most remarkable result is a systematic increase in winter droughts over all of Italy, especially in the north, due mainly to the very dry 1987–93 period. Extreme events are analysed considering 5 day regional totals. In this case, however, an attempt to search for a statistically significant trend is not successful because of the scarcity of events in such a short period. The reliability of the regional series is checked by computing some basic statistics concerning total precipitation, rainy days and precipitation intensity and comparing them with the same statistics computed for regional series obtained by station records completed with methods based on random number generators. Copyright © 2002 Royal Meteorological Society.

Recent trends in temperature and precipitation over South Korea

Abstract: Temperature and precipitation changes over South Korea have been studied since 1954, both in terms of means and extreme events, using observational station data. Recurrence intervals are defined to examine trends in extreme events at each station. Generally, it is possible to describe trends in temperature and precipitation over South Korea as follows. The annual mean temperature shows an upward trend at a rate of 0.23 °C decade−1 in the past four to five decades. There are some indications that climatic extremes have increased during recent decades. The diurnal temperature range increased in the later part of the time series (except for summer) as a result of a faster increase in maximum temperature than in minimum temperature. This is not consistent with the results reported elsewhere in the world. A wavelet analysis of wintertime temperatures indicates that the rapid warming in the recent decade may be associated with the warm phase of a decadal–interdecadal variation. The frequency of occurrence of extreme maximum temperature events shows an increasing trend, with higher values in the 1980s and 1990s. The frequency of occurrence of extreme minimum temperature events shows the opposite, with a statistically significant decreasing trend. Copyright © 2002 Royal Meteorological Society.

Climate change in the Colorado Rocky Mountains: free air versus surface temperature trends

Abstract: A high elevation data set of surface temperatures from the Front Range of the Rocky Mountains in Colorado, USA, is analysed for evidence of long-term change (1952–98). Sites range from the high plains of Colorado (1509 m) to the alpine tundra (3749 m). Systematic changes in surface-based lapse rates are uncovered, with absolute cooling at the highest elevations, but little temperature change on the high plains. There is lapse-rate steepening at the higher elevations (>3000 m). A synoptic analysis using gridded pressure data shows lapse rate changes to be largely independent of synoptic type. Radiosonde ascents from Denver (1956–98) and Grand Junction (1946–98) are used to derive air equivalent temperatures (AETs) at the same elevations as the surface records. AETs show a contrasting temporal trend, with absolute warming at all levels. Furthermore, free-air lapse rates are weakening at higher elevations, the warming becoming stronger with height. A comparison of the two data sets through derivation of free-air–surface temperature differences shows that the alpine tundra zone of the high Rockies is becoming a progressively stronger heat sink. Possible explanations include increased snow cover, enhanced air movement over the surface and decreased solar radiation input. The heat sink enhancement has led to rapid cooling in the alpine tundra that could not be predicted from the free-air record, casting doubt upon the strong dependence on free-air temperature changes in climate modelling when investigating the potential effects of global warming in mountainous regions. In addition, these local surface trends are of the opposite sign to global and other regional trends identified in many recent observational and modelling studies. Copyright © 2002 Royal Meteorological Society.

Sensitivity of the southern African circulation to dipole sea-surface temperature patterns in the south Indian Ocean

Abstract: Previous observational work suggests that when sea-surface temperature (SST) is warm (cool) in the southwest Indian Ocean and cool (warm) in the southeast Indian Ocean, increased (decreased) summer rains may occur over large areas of southeastern Africa. In this study, an atmospheric general circulation model is used to investigate the sensitivity of the regional circulation and rainfall over southern Africa to these dipole SST anomalies in the subtropical south Indian Ocean. When the model is forced with positive SST anomalies in the west and negative SST anomalies in the east, increased rainfall occurs over southeastern Africa as a result of the enhanced convergence of moister than average air over the region. Enhanced evaporation occurs over the warm pole in the southwest Indian Ocean and this moist air is advected towards southeastern Africa as a result of the low-pressure anomaly generated over this pole, which strengthens the onshore flow. Increased and more intense extra-tropical cyclones occur to the southeast of South Africa, favourable for tropical–temperate trough formation. When the SST poles are reversed in sign, decreased precipitation occurs over southeastern Africa as a result of increased low-level divergence of low-level flow and this flow being drier than average. Weaker and fewer extra-tropical cyclones occur southeast of South Africa in this case. The model results are sensitive to the proximity of the southwest Indian Ocean pole to southeastern Africa. There is also sensitivity in the model low-level wind changes and precipitation anomaly over low-latitude southern Africa (but not over South Africa to any significant extent) to the presence or absence of the SST pole over the southeast Indian Ocean. Although the model resolution does not capture the details of the local SST and topographic gradients as well as one would like, the changes in model circulation and precipitation in the experiments with different SST anomalies are consistent with previous observational and theoretical work, hence increasing confidence in the robustness of the results. Copyright © 2002 Royal Meteorological Society

Historical changes in lake ice-out dates as indicators of climate change in New England, 1850-2000

Abstract: Various studies have shown that changes over time in spring ice-out dates can be used as indicators of climate change. Ice-out dates from 29 lakes in New England (USA) with 64 to 163 years of record were assembled and analysed for this study. Ice-out dates have become significantly earlier in New England since the 1800s. Changes in ice-out dates between 1850 and 2000 were 9 days and 16 days in the northern/mountainous and southern regions of New England respectively. The changes in the ice-out data over time were very consistent within each of the two regions of New England, and more consistent than four air-temperature records in each region. The ice-out dates of the two regions had a different response to changes in air temperature. The inferred late winter–early spring air-temperature warming in both regions of New England since 1850, based on linear regression analysis, was about 1.5°C. Published in 2002 by John Wiley & Sons, Ltd.

Enhanced resolution modelling study on anthropogenic climate change: changes in extremes of the hydrological cycle

Abstract: Changes in variability and extremes of the hydrological cycle are studied in two 30 year simulations using a general circulation model at high horizontal resolution. The simulations represent the present-day climate and a period in which the radiative forcing corresponds to a doubling of the present-day concentrations of atmospheric greenhouse gases. In most regions and seasons the probability density function of daily precipitation experiences a stretching associated with a higher probability of heavy precipitation events in the warmer climate. Whereas extremely long wet spells show only moderate changes, the extremely long dry spells are extended at middle latitudes over most land areas. At high latitudes the changes in annual maximum river runoff are mainly controlled by changes in snow budget. Eight out of 14 selected major rivers show a statistically significant change in 10 year return values of the annual maximum discharge. In two cases a significant decrease is found and in six cases there is a significant increase. Copyright © 2002 Royal Meteorological Society

A comparison of methods to determine the onset of the growing season in northern Nigeria

Abstract: In the sudan savanna of northern Nigeria, with its semi-arid climate, the ability to determine effectively or predict the start of actual productive rains cannot be overemphasized. Several methods exist for calculating the date of onset of the rains that may be taken as the start of the growing season. Five methods currently in use, which are relatively easy to apply on a large scale, were selected for comparison. One is a traditional technique (Ramadan method), two use accumulated rainfall totals (Walter’s and Sivakumar’s methods), and two use rainfall–evapotranspiration relationships (Kowal’s and Benoit’s methods). For the period 1961–91, the traditional technique performed most poorly. Walter’s method gave quite early onsets and Sivakumar’s method gave very late onsets, thereby seriously shortening the growing season. Kowal’s and Benoit’s determinations fell most often in between the results of Walter’s and Sivakumar’s methods in their performance. However, although generally to a lesser extent than the other methods, they are still significantly affected by false starts. To avoid incorrectly predicting the growing season’s onset as far as possible, but to prevent an unacceptable shortening of the growing season, a combination of Kowal’s and Sivakumar’s criteria was used to develop an improved technique. This proved to work well for determining the onset date in the study area. However, because some false starts remain when using average onset dates, it is suggested that an operational advisory team should be constituted by the government. This team would be responsible for calculating onset dates in any year, on-line for the ongoing season, in a participatory approach with farmers, and for disseminating such dates to the farmers. This could be done for any place for which the appropriate data can be made available. In the future, improved climate prediction skill may replace the classical probabilistic approaches presently suffering from increasing rainfall variabilities. Copyright  2002 Royal Meteorological Society.

Influence of eurasian spring snow cover on asian summer rainfall

Abstract: The Eurasian snow cover anomaly in spring has been considered as one of the important factors affecting Asian summer monsoon variability. Using the long time series (1922–98) of Eurasian spring (March–April) snow cover (ESSC) reconstructed by Brown (2000. Journal of Climate13: 2339) and snow cover (1973–98) and depth (1979–87) data from satellite observation, the influences of ESSC on the all-India monsoon (June–September) rainfall (AIMR) and the summer rainfall over all parts of Asia are examined. It is found that the statistical relation between AIMR and ESSC changes over a multi-decadal time scale. The negative correlation between them has increased markedly since the mid 1970s. The region where the summer rainfall has the strongest and most stable negative correlation with the preceding ESSC is located in northern Mongolia, south of Lake Baikal. The correlation between the summer rainfall and ESSC increases after the data are treated with a low-pass filter, showing that the impact of snow cover may be seen more clearly with the removal of the effect of El Nino–southern oscillation. Comparative analyses for contrasting years with excessive and deficient snow cover show that the anomalies of ESSC occur mainly in northwestern Eurasia. In the years of excessive ESSC anomalies, cooling and a cyclonic circulation anomaly in the lower troposphere appear over the northern part of Eurasia, leading to a Rossby-wave-train-like circulation response, then a weakened East Asia summer monsoon and deficient rainfall with an anticyclonic circulation anomaly south of Lake Baikal. Anomalies with opposite signs occur in the years of deficient snow cover. Copyright © 2002 Royal Meteorological Society.

Mesoscale precipitation variability in the region of the European Alps during the 20th century

Abstract: The purpose of this study is to construct and evaluate a new gridded analysis of precipitation that covers the entire region of the European Alps (43.2–48.8 °N, 3.2–16.2 °E), resolves the most prominent mesoscale variations (grid spacing 25 km) and extends with a monthly time-resolution over most of the 20th century (1901–90). The analysis is based on a reconstruction using the reduced-space optimal interpolation technique. It combines data from a high-resolution network over a restricted time period (1971–90) with homogeneous centennial records from a sparse sample of stations. The reconstructed fields account for 78% of the total variance in a cross-validation with independent data. The explained variance for individual grid points varies between 60 and 95%, with lower skills over the southern and western parts of the domain. For averages over 100 × 100 km 2 subdomains, the explained variance increases to 90–99%. Comparison of the reconstruction with the CRU05 global analysis reveals good agreement with respect to the interannual variations of large subdomain averages (10 000–50 000 km 2 ), some differences in decadal variations, especially for recent decades, and physically more plausible spatial patterns in the present analysis. The new dataset is exploited to depict 20th century precipitation variations and their correlations with the North Atlantic oscillation (NAO). A linear trend analysis (1901–90) reveals an increase of winter precipitation by 20–30% per 100 years in the western part of the Alps, and a decrease of autumn precipitation by 20–40% to the south of the main ridge. Correlations with the NAO index (NAOI) are weak and highly intermittent to the north and weak and more robust to the south of the main Alpine crest, indicating that changes in the NAOI in recent decades are not of primary importance in explaining observed precipitation changes. Copyright  2002 Royal Meteorological Society.

A mid-latitude Asian circulation anomaly pattern in boreal summer and its connection with the Indian and East Asian summer monsoons

Abstract: Using the NCEP–NCAR reanalysis for 1948–98, this study identifies a dominant pattern for the interannual variation of upper-level winds over mid-latitude Asia in boreal summer This pattern, called the mid-latitude Asian summer (MAS) pattern, features two anomalous anticyclones: one centred at 375° N, 65° E and the other at 425° N, 130° E The MAS pattern significantly influences East Asian summer monsoon variability In the positive phase of the MAS pattern, contrasting meridional wind anomalies between eastern China and Japan lead to above- and below-normal summer rainfall in north China and south Japan respectively The year-to-year change of the MAS pattern is related to that of the Indian summer rainfall, especially in central and northern India Thus, the MAS pattern plays a role in connecting anomalous Indian heating with the East Asian summer monsoon variability The East Asian anomalous anticyclone displays a southeastward shift after the late 1970s This results in a similar change of anomalous summer rainfall regions in East Asia The West Asian anomalous anticyclone moves northeastward after the late 1970s The relation of the MAS pattern with the Indian summer rainfall experienced an obvious weakening in the late 1970s As a result, the statistical relation between the Indian and north China summer rainfall becomes weak after the late 1970s Copyright © 2002 Royal Meteorological Society

Atmospheric mechanisms governing the spatial and temporal variability of phenological phases in central Europe

Abstract: A data set of 17 phenological phases from Germany, Austria, Switzerland and Slovenia spanning the time period from 1951 to 1998 has been made available for analysis together with a gridded temperature data set (1° × 1° grid) and the North Atlantic Oscillation (NAO) index time series. The disturbances of the westerlies constitute the main atmospheric source for the temporal variability of phenological events in Europe. The trend, the standard deviation and the discontinuity of the phenological time series at the end of the 1980s can, to a great extent, be explained by the NAO. A number of factors modulate the influence of the NAO in time and space. The seasonal northward shift of the westerlies overlaps with the sequence of phenological spring phases, thereby gradually reducing its influence on the temporal variability of phenological events with progression of spring (temporal loss of influence). This temporal process is reflected by a pronounced decrease in trend and standard deviation values and common variability with the NAO with increasing year-day. The reduced influence of the NAO with increasing distance from the Atlantic coast is not only apparent in studies based on the data set of the International Phenological Gardens, but also in the data set of this study with a smaller spatial extent (large-scale loss of influence). The common variance between phenological and NAO time series displays a discontinuous drop from the European Atlantic coast towards the Alps. On a local and regional scale, mountainous terrain reduces the influence of the large-scale atmospheric flow from the Atlantic via a proposed ‘decoupling mechanism’. Valleys in mountainous terrain have the inclination to harbour temperature inversions over extended periods of time during the cold season, which isolate the valley climate from the large-scale atmospheric flow at higher altitudes. Most phenological stations reside at valley bottoms and are thus largely decoupled in their temporal variability from the influence of the westerly flow regime (local-scale loss of influence). This study corroborates an increasing number of similar investigations that find that vegetation does react in a sensitive way to variations of its atmospheric environment across various temporal and spatial scales. Copyright © 2002 Royal Meteorological Society.

Temporal fluctuations in heat waves at Prague–Klementinum, the Czech Republic, from 1901–97, and their relationships to atmospheric circulation

Abstract: Temporal fluctuations in heat wave occurrence and severity are analysed in long-term daily temperature series at Prague–Klementinum, the Czech Republic. Although the observations have been continuous since 1775, the period 1901–97 with the most credible data is mainly examined. Most of the warmest summers of the 20th century appeared within the periods 1943–52 and 1992–95; the temporal distribution of heat waves corresponds to this pattern and shows two maxima, in the 1940s to early 1950s and in the 1990s. A very low occurrence of heat waves was typical of the beginning of the 20th century and around 1980. The peak of heat wave severity in the 1940s–early 1950s, as well as their almost total absence in the first two decades of the 20th century, may be a common feature for a large area, as indicated by the comparison between two stations within central Europe, Prague–Klementinum and Basel (Switzerland), and by other studies. An extraordinary heat wave occurred in July and August 1994. It greatly exceeded other heat waves, mainly in the much higher cumulative temperature excess above 30°C and a record-breaking duration of a continuous period of tropical days. Relationships between heat wave characteristics in warm and cold decades and circulation conditions were analysed using the subjective Hess–Brezowsky catalogue of weather types (Grosswetterlagen). The link to the atmospheric circulation is evident, e.g. situations with an anticyclone or a ridge over central Europe were more (less) frequent during all the warm (cold) decades. Moreover, the occurrence of long and severe heat waves in the 1990s may reflect an enhanced persistence of the atmospheric circulation over Europe in the summer season because all groups of weather types have considerably increased residence times in 1988–97 compared with long-term means. Copyright © 2002 Royal Meteorological Society

Dependence between extreme sea surge, river flow and precipitation in eastern Britain

Abstract: Flooding in estuaries may be caused by both high river flows and by high sea levels. In order to investigate whether these tend to occur simultaneously in eastern Britain, the dependence between high sea surge (observed sea level minus predicted astronomical tide), river flow and precipitation was studied using a measure of dependence specially developed for extremal dependence. Extreme events were interpreted using meteorological maps. This new analysis found that the strongest flow–surge dependence occurs between river flow on the north shore of the Firth of Forth and sea surge at Aberdeen, Wick and Lerwick. In contrast to most other catchments in eastern Britain, the area to the north of the Firth of Forth is not sheltered from south-westerly winds by any major topographical barrier. Therefore, precipitation from this direction may be orographically enhanced as it encounters the hills on the northern side of the firth, and high river flows may ensue. Events resulting in both high river flow and surge in the northern part of the study area were found to be caused by cyclones travelling north-eastward to the north of Scotland. High surge events, only, were associated with similar storm tracks, but without much precipitation from the fronts. High river flows, only, were associated with rain-bearing east–west-directed fronts over northern Britain, with slow-moving depressions located over or to the west of the British Isles where they are unable to generate a strong surge in the North Sea. The dependence between river flow and surge was found to be stronger during winter than summer, and a lagged analysis revealed that the dependence is strongest when flow and surge occur on the same day, but was also strong for lags of plus and minus 1 day. For precipitation, the dependence with both flow and surge is strongest when precipitation precedes them by 1 day. Copyright © 2002 Royal Meteorological Society.

El Niño–southern oscillation influences on rice production in Sri Lanka

Abstract: Despite advances in the capacity to predict the evolution of the El Ni˜ no–southern oscillation (ENSO) phenomenon and advances in understanding the influence of ENSO on rainfall in tropical regions such as Sri Lanka, there has been limited use of climate predictions for agricultural decision-making. Climatic fluctuations have a profound influence on the cultivation of crops such as rice, which is the staple food in Sri Lanka. Here, the relationship between the sea-surface temperature-based ENSO index of NINO3.4, rainfall and the departure of Sri Lankan rice production from long-term trends, is analysed for the ‘Maha’ (October to March) and ‘Yala ’ (April to September) cultivation seasons between 1952 and 1997. During the El Ni˜ no phase, the Maha rice production frequently increased (10 out of 15 seasons) and the Yala production frequently decreased (10 out of 14 seasons). Conversely, during the La Ni˜ na phase, the Maha production decreased (seven out of ten seasons) and Yala production increased (six out of eight seasons). Floods, state interventions, civil disturbances, fertilizer price hikes and extreme anomalies in the previous season were noted in the majority of seasons in which these ENSO–production linkages were violated. The correlation of the Maha rice production anomaly with the average NINO3.4 from October to December was significant at the 5% level and that with the aggregate October to December rainfall was significant at the 1% level. Yala rice production showed a significant relationship with concurrent NINO3.4 and a strong correlation (r = 0.60) with the previous season’s rainfall. Yala cultivation is water constrained, and carryover storage from the previous season is often used to determine the extent of planting. The relationships between ENSO and seasonal rice production and the relationship between Yala rice production and previous Maha rainfall could be used for agricultural management and policy formulation. Copyright  2002 Royal Meteorological Society.

The intertropical convergence zone of the eastern Pacific revisited

Abstract: The Intertropical Convergence Zone (ITCZ) complex over the eastern Pacific is re-examined from the NCEP–NCAR 40 year reanalysis and other data. Consistent with earlier work, the new analysis yields for July–August: a surface position of the near-equatorial wind confluence at 12 °N; the cross-equatorial flow changing from divergent to convergent at the latitude of recurvature from southeasterly to southwesterly; strongest convergence in the surface layer to the south of the wind confluence, with this feeding vigorous ascending motion and compensating divergence in the upper troposphere; and the interface between cross-equatorial flow and northeast trades sloping southward at 1 : 1500. An easterly speed maximum in the mid troposphere over the equator is consistent with the 4 year rawin record at Galapagos. The new coarse-resolution dataset fails to capture a well-developed Intertropical Divergence Zone. The annual cycle features for the wind confluence and the ITCZ a position closest to the equator in February, a northward migration to June, southward shift in July and August, and a northernmost location in September. This behaviour is paralleled by maxima in the frequency of the Temporal weather systems and rainfall on the Pacific side of Central America in June and September. Regarding the climatic variability, with an anomalously far southerly position of the wind confluence, Central American rainfall tends to be deficient, and such a combination of departures is common in the warm phase of the southern oscillation. The long-term evolution is characterized by a warming trend of equatorial Pacific waters and weakening of cross-equatorial wind, while trends in the latitude position of the wind confluence and in Central American rainfall are not significant. Copyright  2002 Royal Meteorological Society.