Showing papers on "Precipitation published in 2007"

Future extreme events in European climate: an exploration of regional climate model projections

Abstract: This paper presents an overview of changes in the extreme events that are most likely to affect Europe in forthcoming decades. A variety of diagnostic methods are used to determine how heat waves, heavy precipitation, drought, wind storms, and storm surges change between present (1961–90) and future (2071–2100) climate on the basis of regional climate model simulations produced by the PRUDENCE project. A summary of the main results follows. Heat waves – Regional surface warming causes the frequency, intensity and duration of heat waves to increase over Europe. By the end of the twenty first century, countries in central Europe will experience the same number of hot days as are currently experienced in southern Europe. The intensity of extreme temperatures increases more rapidly than the intensity of more moderate temperatures over the continental interior due to increases in temperature variability. Precipitation – Heavy winter precipitation increases in central and northern Europe and decreases in the south; heavy summer precipitation increases in north-eastern Europe and decreases in the south. Mediterranean droughts start earlier in the year and last longer. Winter storms – Extreme wind speeds increase between 45°N and 55°N, except over and south of the Alps, and become more north-westerly than cuurently. These changes are associated with reductions in mean sea-level pressure, leading to more North Sea storms and a corresponding increase in storm surges along coastal regions of Holland, Germany and Denmark, in particular. These results are found to depend to different degrees on model formulation. While the responses of heat waves are robust to model formulation, the magnitudes of changes in precipitation and wind speed are sensitive to the choice of regional model, and the detailed patterns of these changes are sensitive to the choice of the driving global model. In the case of precipitation, variation between models can exceed both internal variability and variability between different emissions scenarios.

Detection of human influence on twentieth-century precipitation trends

Abstract: Human influence on climate has been detected in surface air temperature, sea level pressure, free atmospheric temperature, tropopause height and ocean heat content. Human-induced changes have not, however, previously been detected in precipitation at the global scale, partly because changes in precipitation in different regions cancel each other out and thereby reduce the strength of the global average signal. Models suggest that anthropogenic forcing should have caused a small increase in global mean precipitation and a latitudinal redistribution of precipitation, increasing precipitation at high latitudes, decreasing precipitation at sub-tropical latitudes, and possibly changing the distribution of precipitation within the tropics by shifting the position of the Intertropical Convergence Zone. Here we compare observed changes in land precipitation during the twentieth century averaged over latitudinal bands with changes simulated by fourteen climate models. We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation, such as the Sahel.

How Much More Rain Will Global Warming Bring

Abstract: Climate models and satellite observations both indicate that the total amount of water in the atmosphere will increase at a rate of 7% per kelvin of surface warming. However, the climate models predict that global precipitation will increase at a much slower rate of 1 to 3% per kelvin. A recent analysis of satellite observations does not support this prediction of a muted response of precipitation to global warming. Rather, the observations suggest that precipitation and total atmospheric water have increased at about the same rate over the past two decades.

Future long-term changes in global water resources driven by socio-economic and climatic changes

Abstract: A global water model is used to analyse the impacts of climate change and socio-economic driving forces (derived from the A2 and B2 scenarios of IPCC) on future global water stress. This work extends previous global water research by analysing not only the impact of climate change and population, but also the effects of income, electricity production, water-use efficiency and other driving forces, on water stress. Depending on the scenario and climate model, water stress increases (between current conditions and the 2050s) over 62.0–75.8% of total river basin area and decreases over 19.7–29.0% of this area. The remaining areas have small changes. The principal cause of decreasing water stress (where it occurs) is the greater availability of water due to increased annual precipitation related to climate change. The principal cause of increasing water stress is growing water withdrawals, and the most important factor for this increase is the growth of domestic water use stimulated by income growth....

Recent and Future Climate Change in Northwest China

Abstract: As a consequence of global warming and an enhanced water cycle, the climate changed in northwest China, most notably in the Xinjiang area in the year 1987. Precipitation, glacial melt water and river runoff and air temperature increased continuously during the last decades, as did also the water level of inland lakes and the frequency of flood disasters. As a result, the vegetation cover is improved, number of days with sand-dust storms reduced. From the end of the 19th century to the 1970s, the climate was warm and dry, and then changed to warm and wet. The effects on northwest China can be classified into three classes by using the relation between precipitation and evaporation increase. If precipitation increases more than evaporation, runoff increases and lake water levels rise. We identify regions with: (1) notable change, (2) slight change and (3) no change. The future climate for doubled CO2 concentration is simulated in a nested approach with the regional climate model-RegCM2. The annual temperature will increase by 2.7 ◦ C and annual precipitation by 25%. The cooling effect of aerosols and natural factors will reduce this increase to 2.0 ◦ C and 19% of precipitation. As a consequence, annual runoff may increase by more than 10%.

Responses of permafrost to climate change and their environmental significance, Qinghai-Tibet Plateau

Abstract: [1] In this paper we summarize recent research in geocryological studies carried out on the Qinghai-Tibet Plateau that show responses of permafrost to climate change and their environmental implications. Long-term temperature measurements indicate that the lower altitudinal limit of permafrost has moved up by 25 m in the north during the last 30 years and between 50 and 80 m in the south over the last 20 years. Furthermore, the thickness of the active layer has increased by 0.15 to 0.50 m and ground temperature at a depth of 6 m has risen by about 0.1° to 0.3°C between 1996 and 2001. Recent studies show that freeze-thaw cycles in the ground intensify the heat exchange between the atmosphere and the ground surface. The greater the moisture content in the soil, the greater is the influence of freeze-thaw cycling on heat exchange. The water and heat exchange between the atmosphere and the ground surface due to soil freezing and thawing has a significant influence on the climate in eastern Asia. A negative correlation exists between soil moisture and heat balance on the plateau and the amount of summer precipitation in most regions of China. A simple frozen soil parameterization scheme was developed to simulate the interaction between permafrost and climate change. This model, combined with the NCAR Community Climate Model 3.6, is suitable for the simulation of permafrost changes on the plateau. In addition, permafrost degradation is one of the main causes responsible for a dropping groundwater table at the source areas of the Yangtze River and Yellow River, which in turn results in lowering lake water levels, drying swamps and shrinking grasslands.

Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data

Abstract: A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979–2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental landmasses, zonal means over land, hemispheric land means, and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high-frequency information taken from atmosphe...

An inter-comparison of regional climate models for Europe: model performance in present-day climate

Abstract: The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.

Evaluation of the AR4 Climate Models’ Simulated Daily Maximum Temperature, Minimum Temperature, and Precipitation over Australia Using Probability Density Functions

Abstract: The coupled climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change are evaluated The evaluation is focused on 12 regions of Australia for the daily simulation of precipitation, minimum temperature, and maximum temperature The evaluation is based on probability density functions and a simple quantitative measure of how well each climate model can capture the observed probability density functions for each variable and each region is introduced Across all three variables, the coupled climate models perform better than expected Precipitation is simulated reasonably by most and very well by a small number of models, although the problem with excessive drizzle is apparent in most models Averaged over Australia, 3 of the 14 climate models capture more than 80% of the observed probability density functions for precipitation Minimum temperature is simulated well, with 10 of the 13 climate models capturing more than 80% of the observed probability densit

Contribution of land‐atmosphere coupling to recent European summer heat waves

Abstract: [1] Most of the recent European summer heat waves have been preceded by a pronounced spring precipitation deficit. The lack of precipitation and the associated depletion of soil moisture result in reduced latent cooling and thereby amplify the summer temperature extremes. In order to quantify the contribution of land-atmosphere interactions, we conduct regional climate simulations with and without land-atmosphere coupling for four selected major summer heat waves in 1976, 1994, 2003, and 2005. The coupled simulation uses a fully coupled land-surface model, while in the uncoupled simulation the mean seasonal cycle of soil moisture is prescribed. The experiments reveal that land-atmosphere coupling plays an important role for the evolution of the investigated heat waves both through local and remote effects. During all simulated events soil moisture-temperature interactions increase the heat wave duration and account for typically 50–80% of the number of hot summer days. The largest impact is found for daily maximum temperatures during heat wave episodes.

Testing the Clausius–Clapeyron constraint on changes in extreme precipitation under CO2 warming

Abstract: Increases in extreme precipitation greater than in the mean under increased greenhouse gases have been reported in many climate models both on global and regional scales. It has been proposed in a previous study that whereas global-mean precipitation change is primarily constrained by the global energy budget, the heaviest events can be expected when effectively all the moisture in a volume of air is precipitated out, suggesting the intensity of these events increases with availability of moisture, and significantly faster than the global mean. Thus under conditions of constant relative humidity one might expect the Clausius–Clapeyron relation to give a constraint on changes in the uppermost quantiles of precipitation distributions. This study examines if the phenomenon manifests on regional and seasonal scales also. Zonal analysis of daily precipitation in the HadCM3 model under a transient CO2 forcing scenario shows increased extreme precipitation in the tropics accompanied by increased drying at lower percentiles. At mid- to high-latitudes there is increased precipitation over all percentiles. The greatest agreement with Clausius–Clapeyron predicted change occurs at mid-latitudes. This pattern is consistent with other climate model projections, and suggests that regions in which the nature of the ambient flows change little give the greatest agreement with Clausius–Clapeyron prediction. This is borne out by repeating the analyses at gridbox level and over season. Furthermore, it is found that Clausius–Clapeyron predicted change in extreme precipitation is a better predictor than directly using the change in mean precipitation, particularly between 60°N and 60°S. This could explain why extreme precipitation changes may be more detectable then mean changes.

Importance of rain evaporation and continental convection in the tropical water cycle

Abstract: Atmospheric moisture cycling is an important aspect of the Earth's climate system, yet the processes determining atmospheric humidity are poorly understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.

Precipitation pulses and soil CO2flux in a Sonoran Desert ecosystem

Abstract: Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water-limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole-system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO2 efflux increased up to 30-fold, but generally returned to background levels within 48 h. CO2 production integrated over 48 h ranged from 2.5 to 19.3 g C m−2 and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO2 were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short-term losses of CO2. In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO2 efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long-term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.

Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum – Part 2: feedbacks with emphasis on the location of the ITCZ and mid- and high latitudes heat budget

Abstract: A set of coupled ocean-atmosphere(-vegetation) simulations using state of the art climate models is now available for the Last Glacial Maximum (LGM) and the Mid-Holocene (MH) through the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP2). Here we quantify the latitudinal shift of the location of the Intertropical Convergence Zone (ITCZ) in the tropical regions during boreal summer and the change in precipitation in the northern part of the ITCZ. For both periods the shift is more pronounced over the continents and East Asia. The maritime continent is the region where the largest spread is found between models. We also clearly establish that the larger the increase in the meridional temperature gradient in the tropical Atlantic during summer at the MH, the larger the change in precipitation over West Africa. The vegetation feedback is however not as large as found in previous studies, probably due to model differences in the control simulation. Finally, we show that the feedback from snow and sea-ice at mid and high latitudes contributes for half of the cooling in the Northern Hemisphere for the LGM, with the remaining being achieved by the reduced CO2 and water vapour in the atmosphere. For the MH the snow and albedo feedbacks strengthen the spring cooling and enhance the boreal summer warming, whereas water vapour reinforces the late summer warming. These feedbacks are modest in the Southern Hemisphere. For the LGM most of the surface cooling is due to CO2 and water vapour.

How Often Will It Rain

Abstract: Daily precipitation data from climate change simulations using the latest generation of coupled climate system models are analyzed for potential future changes in precipitation characteristics. For the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) B1 (a low projection), A1B (a medium projection), and A2 (a high projection) during the twenty-first century, all the models consistently show a shift toward more intense and extreme precipitation for the globe as a whole and over various regions. For both SRES B1 and A2, most models show decreased daily precipitation frequency and all the models show increased daily precipitation intensity. The multimodel averaged percentage increase in the precipitation intensity (2.0% K 1 ) is larger than the magnitude of the precipitation frequency decrease (0.7% K 1 ). However, the shift in precipitation frequency distribution toward extremes results in large increases in very heavy precipitation events (50 mm day 1 ), so that for very heavy precipitation, the percentage increase in frequency is much larger than the increase in intensity (31.2% versus 2.4%). The climate model projected increases in daily precipitation intensity are, however, smaller than that based on simple thermodynamics (7% K 1 ). Multimodel ensemble means show that precipitation amount increases during the twenty-first century over high latitudes, as well as over currently wet regions in low- and midlatitudes more than other regions. This increase mostly results from a combination of increased frequency and intensity. Over the dry regions in the subtropics, the precipitation amount generally declines because of decreases in both frequency and intensity. This indicates that wet regions may get wetter and dry regions may become drier mostly because of a simultaneous increase (decrease) of precipitation frequency and intensity.

South Asian summer monsoon precipitation variability: Coupled climate model simulations and projections under IPCC AR4

Abstract: South Asian summer monsoon precipitation and its variability are examined from the outputs of the coupled climate models assessed as part of the Intergovernmental Panel on Climate Change Fourth Assessment. Out of the 22 models examined, 19 are able to capture the maximum rainfall during the summer monsoon period (June through September) with varying amplitude. While two models are unable to reproduce the annual cycle well, one model is unable to simulate the summer monsoon season. The simulated inter-annual variability from the 19 models is examined with respect to the mean precipitation, coefficient of variation, long-term trends and the biennial tendency. The model simulated mean precipitation varies from 500 mm to 900 mm and coefficient of variation from 3 to 13%. While seven models exhibit long-term trends, eight are able to simulate the biennial nature of the monsoon rainfall. Six models, which generate the most realistic 20th century monsoon climate over south Asia, are selected to examine future projections under the doubling CO2 scenario.

Assessing risks of climate variability and climate change for Indonesian rice agriculture.

Abstract: El Nino events typically lead to delayed rainfall and decreased rice planting in Indonesia's main rice-growing regions, thus prolonging the hungry season and increasing the risk of annual rice deficits. Here we use a risk assessment framework to examine the potential impact of El Nino events and natural variability on rice agriculture in 2050 under conditions of climate change, with a focus on two main rice-producing areas: Java and Bali. We select a 30-day delay in monsoon onset as a threshold beyond which significant impact on the country's rice economy is likely to occur. To project the future probability of monsoon delay and changes in the annual cycle of rainfall, we use output from the Intergovernmental Panel on Climate Change AR4 suite of climate models, forced by increasing greenhouse gases, and scale it to the regional level by using empirical downscaling models. Our results reveal a marked increase in the probability of a 30-day delay in monsoon onset in 2050, as a result of changes in the mean climate, from 9–18% today (depending on the region) to 30–40% at the upper tail of the distribution. Predictions of the annual cycle of precipitation suggest an increase in precipitation later in the crop year (April–June) of ≈10% but a substantial decrease (up to 75% at the tail) in precipitation later in the dry season (July–September). These results indicate a need for adaptation strategies in Indonesian rice agriculture, including increased investments in water storage, drought-tolerant crops, crop diversification, and early warning systems.

Effects of 20th century warming and climate variability on flood risk in the western U.S.

Abstract: [1] Using precipitation and temperature data for the 20th century in combination with a macroscale hydrologic model, we evaluate changes in flood risk in the western U.S. associated both with century-scale warming and interannual climate variations. In addition, we examine the implications of apparent increases in precipitation variability over the region since the mid-1970s. We use detrended temperature data representing early and late 20th century climate to force the variable infiltration capacity hydrologic model and show that spatially homogeneous temperature changes over the western U.S. in the 20th century on the order of +1°C per century have resulted in substantial changes in flood risks over much of the region. Although changes specific to particular geographic areas are apparent in some cases, the overall changes due to observed warming trends are well categorized by midwinter temperature regimes in each watershed. Cold river basins where snow processes dominate the annual hydrologic cycle ( 5°C average in midwinter) show little systematic change. Intermediate or transient basins show a wide range of effects depending on competing factors such as the relative role of antecedent snow and contributing basin area during storms that cause flooding. Warmer transient basins along the coast in Washington, Oregon, and California, in particular, tend to show increased flood risk. While the absolute value of simulated changes in flood risk is affected by basin scale, the nature of the relationship of flood risk to basin temperatures in midwinter is largely scale-independent. Climate variations associated with Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO) also have strong effects on flood risks. In contrast to the effects associated with 20th century warming, the climate variability signal is characterized by regional scale patterns related to the geographic distribution of cool season precipitation also identified in many previous studies. In general, the largest changes in simulated flood risks are associated with years when PDO and ENSO are "in phase," particularly in the southwest. Changes in the variability of cool season precipitation after about 1973, the causes of which are uncertain, are shown to result in increased flood risk over much of the western U.S. in the simulations.

Observed Characteristics of the MJO Relative to Maximum Rainfall

Abstract: This study examines various dynamical and thermodynamical processes that characterize the Madden–Julian oscillation (MJO). Episodes of deep convection related to the MJO based on rainfall data from the Tropical Rainfall Measuring Mission (TRMM) satellite and the Global Precipitation Climatology Project (GPCP) are identified. Although broad convective envelopes are located utilizing spectrally filtered precipitation, analyses of the features within the envelopes are carried out using unfiltered rainfall and 40-yr ECMWF Re-Analysis (ERA-40) fields. The events are composited and categorized based on geographic location and relative intensity. The composited fields illustrate that, prior to the onset of deep convection, shallow cumulus and cumulus congestus clouds are actively involved in vertical convective transport of heat and moisture. Drying, first accomplished immediately following deep convection in the lower troposphere, is associated with an enhanced horizontal (westerly) advective component...

Summer Precipitation Frequency, Intensity, and Diurnal Cycle over China: A Comparison of Satellite Data with Rain Gauge Observations

Abstract: Hourly or 3-hourly precipitation data from Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) and Tropical Rainfall Measuring Mission (TRMM) 3B42 satellite products and rain gauge records are used to characterize East Asian summer monsoon rainfall, including spatial patterns in June–August (JJA) mean precipitation amount, frequency, and intensity, as well as the diurnal and semidiurnal cycles. The results show that the satellite products are comparable to rain gauge data in revealing the spatial patterns of JJA precipitation amount, frequency, and intensity, with pattern correlation coefficients for five subregions ranging from 0.66 to 0.94. The pattern correlation of rainfall amount is higher than that of frequency and intensity. Relative to PERSIANN, the TRMM product has a better resemblance with rain gauge observations in terms of both the pattern correlation and rootmean-square error. The satellite products overestimate rainfall frequency but underestimate its intensity. The diurnal (24 h) harmonic dominates subdaily variations of precipitation over most of eastern China. A late-afternoon maximum over southeastern and northeastern China and a near-midnight maximum over the eastern periphery of the Tibetan Plateau are seen in the rain gauge data. The diurnal phases of precipitation frequency and intensity are similar to those of rainfall amount in most regions, except for the middle Yangtze River valley. Both frequency and intensity contribute to the diurnal variation of rainfall amount over most of eastern China. The contribution of frequency to the diurnal cycle of rainfall amount is generally overestimated in both satellite products. Both satellite products capture well the nocturnal peak over the eastern periphery of the Tibetan Plateau and the late-afternoon peak in southern and northeastern China. Rain gauge data over the region between the Yangtze and Yellow Rivers show two peaks, with one in the early morning and the other later in the afternoon. The satellite products only capture the major late-afternoon peak.

Climate Response to Rapid Urban Growth: Evidence of a Human-Induced Precipitation Deficit

Abstract: The authors establish the effect of urbanization on precipitation in the Pearl River Delta of China with data from an annual land use map (1988–96) derived from Landsat images and monthly climate data from 16 local meteorological stations. A statistical analysis of the relationship between climate and urban land use in concentric buffers around the stations indicates that there is a causal relationship from temporal and spatial patterns of urbanization to temporal and spatial patterns of precipitation during the dry season. Results suggest an urban precipitation deficit in which urbanization reduces local precipitation. This reduction may be caused by changes in surface hydrology that extend beyond the urban heat island effect and energy-related aerosol emissions.

Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance of Dust Compared to Sea Surface Temperature Variations, Vegetation Changes, and Greenhouse Gas Warming

Abstract: The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.

Synoptic aspects of the central chile rainfall variability associated with the southern oscillation

Abstract: Central Chile winter (June, July, August (JJA)) rainfall shows positive anomalies during the developing stage of warm events of the Southern Oscillation. Conversely, cold events correspond quite closely to dry conditions. A synoptic characterization of major storms during the most recent warm events is presented. Dry months during coldevent years are described in terms of average 500-hPa contour anomaly fields. Significant departures from this general behaviour are also discussed. It is found that major winter storms associated with warm events are related to blocking highs frequently located around the Bellingshausen Sea (9OOW) within hemispheric circulation anomaly patterns where zonal wavenumbers 4 and 3 dominate. This phenomenon seems consistent with observed teleconnection wavetrains stemming from the anomalous atmospheric heat source above the equatorial Pacific during ENS0 events. Cold years, often immediately preceding or following a warm event, bring dry conditions in the study area owing to a well-developed south-east subtropical anticyclone with enhanced zonal westerly flow at middle latitudes. Frequency distributions of 500-hPa daily blocking indices (BI) at 90°W, derived from 1980 to 1987 European Centre for Medium Range Weather Forecasts hemispheric analyses, show a significant departure towards positive BI values for the available warm-event winters; the opposite being also true. However, the JJA rainfall variability at Santiago (33.53) also seems to be related to the regional strength of the south-east Pacific anticyclone, as represented by seasonal 500-hPa geopotential anomalies at Puerto Montt, Chile (413"s). The apparent relationship between the phases of the Southern Oscillation (SO) and rainfall anomalies in central Chile (30-35"s) has been reported by several authors. Rubin (1955), while taking into consideration pressure anomalies in the Southern Hemisphere, found that the precipitation in central Chile stays below normal during the positive phase of the SO, namely when the south-east Pacific subtropical anticyclone is stronger than average. The strength and position of the aforementioned anticyclone has also been related by Pittock (1980) to the interannual rainfall variability in the central part of Chile. Quinn and Neal (1983), in an attempt to relate long series of annual precipitation in Santiago (33*5"S, 70.7"W) and in Valparaiso (33.0°S, 71.6"W) with an El Niiio index, obtained a good correspondence of the interannual rainfall variability with area averaged sea-surface temperatures in the south-eastern tropical Pacific. More recently, Aceituno (1987) has correlated the pressure, temperature, wind, and precipitation fields with a Southern Oscillation Index (SOI), concluding that the tendency to positive rainfall anomalies in central Chile during the negative phase of the SO is associated with a weak and northerly displaced southeast Pacific subtropical anticyclone; together with an overall increase in baroclinicity at the subtropical latitudes produced by tropospheric cooling in the southern part of South America and a corresponding warming in tropical latitudes. At present, the Chilean rainfall anomaly seems to have gained a place in the world-wide sequence of major climatic anomalies related 0899-84 1 8/9 1/0 10063-1 4$07.OO 0 1991 by the Royal Meteorological Society

Unexpected impacts of climate change on alpine vegetation

Abstract: The vegetation in a high alpine site of the European Alps experienced changes in area between 1953 and 2003 as a result of climate change. Shrubs showed rapid expansion rates of 5.6% per decade at altitudes between 2400 m and 2500 m. Above 2500 m, vegetation coverage exhibited unexpected patterns of regression associated with increased precipitation and permafrost degradation. As these changes follow a sharp increase in both summer and annual temperatures after 1980, we suggest that vegetation of the alpine (2400–2800 m) and nival (above 2800 m) belts respond in a fast and flexible way, contradicting previous hypotheses that alpine and nival species appear to have a natural inertia and are able to tolerate an increase of 1–2°C in mean air temperature.

Recent decline in precipitation and tree growth in the eastern Mediterranean

Abstract: We present evidence of a recent drying in the eastern Mediterranean, based on weather and tree-ring data for Samos, an island of the eastern Aegean Sea. Rainfall declined rapidly after the late 1970s following trends for the entire Mediterranean and was associated with reduced tree-ring width in Pinus brutia. The most recent decline led to the lowest annual radial stem increment after the last 100 years (as far as records reach). As moisture availability decreased best correlations of tree growth with rainfall were obtained for progressively longer integration periods (1–2 years in moister periods, 5–6 years during the severe dryness of 20th century's last decades), suggesting increasing dependency in deep soil water. Such long-term integration periods of tree-growth responses to precipitation have not been reported before. They may reflect a tree-rooting pattern adapted to cope with even several successive dry years. In late summer 2000, moisture reserves became exhausted, however, and a substantial fraction of low altitude pines died, including some 80-year-old trees, which underlines the exceptional extent this trend had reached. Our findings provide empirical support for Intergovernmental Panel on Climate Change projections derived from global circulation models that the Mediterranean, its eastern basin in particular, should become drier as temperature rises, as was the case in the recent past.