Negative Trend In Precipitation Research Articles

A seasonal study of the atmospheric dynamics over the Iberian Peninsula based on circulation types

A seasonal analysis of the atmospheric circulation over the Iberian Peninsula (IP) based on circulation types (CTs) obtained from sea level pressure and 500-hPa geopotential height is presented. The study covers the period of 1958–2008, when a high variability and important changes in winter and spring precipitation and temperature have been reported. Frequency, persistence, and the most probable transitions of the circulation types are analyzed. Among the clustering methods available in the literature, two of the most reliable classification methods have been tested, K-means and simulated annealing and diversified randomization. A comparison of both methods over the IP is presented for winter (DJF). The quality of the circulation types obtained through both methods as well as the better stability achieved by K-means suggest this method as more appropriated for our target area. Twelve CTs were obtained for each season and were analyzed. The patterns obtained were regrouped in five general situations: anticyclonic, cyclonic, zonal, summertime, and hybrid-mixed. The analysis of frequencies of these situations offers a similar characterization of the atmospheric circulation that others previously obtained by subjective methods. The analysis of the trends in frequency and persistence for each CT shows few significant trends, mainly in winter and spring with a general decrease of the cyclonic patterns and an increase of the anticyclonic situations. This can be related to the negative precipitation trends reported by other authors. Regarding the persistence, an interesting result is that there is a high interannual variability of the persistence in autumn and spring, when patterns can persist longer than in other seasons. An analysis of the most probable transitions between the CTs has been performed, revealing the existence of cyclic sequences in all seasons. These sequences are related to the high frequency of certain patterns such as the anticyclonic situations in winter. Finally, a clear seasonal dependence of the transitions between cyclonic situations associated with extratropical disturbances was found. This dependence suggests that the transitions of low-pressure systems towards the south of the IP are more likely in spring and autumn than in winter.

Climatic controls of glacier distribution and glacier changes in Austria

AbstractIn this study we aim to connect glacier extent in 1998 with general climatic conditions, and glacier changes between 1969 and 1998 with climate change in the Austrian Alps. The investigations are based on two complete glacier inventories, a homogenized gridded precipitation dataset and European Centre for Medium-Range Weather Forecasts re-analysis (ERA-40) data of air temperature at different levels. A relationship between median glacier elevation, minimal elevation, the general elevation of the surrounding mountains and mean climatic values was found. In the Austrian Alps, the existence of glaciers at low elevations can only be maintained with above-average accumulation or strong dynamic ice supply. For debris-free glaciers, we found a limit of ~2080ma.s.l., where mean summer temperatures (June–August) exceed 4°C. Glacier changes from 1969 to 1998 are strongly negative both in relative area and in mean thickness. There is a weak and regionally varying negative trend in precipitation over this period. A spatially consistent sequence of positive temperature anomalies in the early 1980s and after 1990 offers an explanation for the retreat. The study shows that the observed spatial variability of glacier changes is connected more strongly to the topographic differences than to a regionally different climate change signal.

Precipitation dynamics in southern Spain: trends and cycles

AbstractThe temporal variability of precipitation in southern Spain was analysed to detect trends and cycles. The objectives of this study were (1) to determine whether there has been a decrease in precipitation in southern Spain and (2) to evaluate if any decrease is related to temporal cycles or is a well‐defined trend. The study was based on analysis of data series from nine monitoring stations in the Andalusia area of the Spanish Mediterranean. The Pettit test and temporal variability analyses were carried out using the temporal databases (1960–2006) from the selected stations. The results indicate that (1) there were significant differences in precipitation throughout the study period based on the location of the stations; (2) a negative trend in precipitation was observed for the east coast and inland stations, and an increasing trend in precipitation was observed for stations on the west coast; and (3) a general decreasing trend in seasonal precipitation, especially in winter, autumn, and spring, was observed for all stations except those on the west coast. Copyright © 2010 Royal Meteorological Society

Attribution of the river flow growth in the Plata Basin

AbstractA regression approach was used to quantitatively estimate the attribution of the notable growth in the river flows of the Plata Basin during 1960–1999. The study was conducted in seven large basins that account for most of the Plata River discharge. Annual rainfall integrated over each basin and annual river flows at their closing points were used for the analysis. The contribution of rainfall changes during each of the three phases of El Niño‐Southern Oscillation to total rainfall change in these basins was also calculated.The two main drivers for the generalized growth of the river flows were the increased precipitation and the decreased evaporation attributable to land use change, including deforestation of natural forest and crop switch from sugarcane and coffee trees to soybean. Other evaporation changes played a minor role. There was a north‐south gradient in the respective importance of each driver, with land use change having greater weight in the northern basins and the precipitation increase in the southern ones. Thus, in the northern part of the Upper Paraná Basin, despite the negative trend in precipitation there was a strong augment of the river flow caused by land use change. The contribution to the positive trend of the stream flows in the middle of the Plata Basin came from both land use change and increased precipitation. Finally, in the south, the Uruguay River flow change was basically due to the precipitation trend that was not only observed during the El Niño phase, but also during the Neutral phase.Only in the Middle Paraguay Basin was the shift to more frequent and intense El Niño events that took place in the 1970s an important factor in the contribution of precipitation to streamflow trends. Copyright © 2010 Royal Meteorological Society

Aridity changes in the Temperate-Mediterranean transition of the Andes since ad 1346 reconstructed from tree-rings

The Andes Cordillera acts as regional “Water Towers” for several countries and encompasses a wide range of ecosystems and climates. Several hydroclimatic changes have been described for portions of the Andes during recent years, including glacier retreat, negative precipitation trends, an elevation rise in the 0° isotherm, and changes in regional streamflow regimes. The Temperate-Mediterranean transition (TMT) zone of the Andes (35.5°–39.5°S) is particularly at risk to climate change because it is a biodiversity hotspot with heavy human population pressure on water resources. In this paper we utilize a new tree-ring network of Austrocedrus chilensis to reconstruct past variations in regional moisture in the TMT of the Andes by means of the Palmer Drought Severity Index (PDSI). The reconstruction covers the past 657 years and captures interannual to decadal scales of variability in late spring–early summer PDSI. These changes are related to the north–south oscillations in moisture conditions between the Mediterranean and Temperate climates of the Andes as a consequence of the latitudinal position of the storm tracks forced by large-scale circulation modes. Kernel estimation of occurrence rates reveals an unprecedented increment of severe and extreme drought events during the last century in the context of the previous six centuries. Moisture conditions in our study region are linked to tropical and high-latitude ocean-atmospheric forcing, with PDSI positively related to Nino-3.4 SST during spring and strongly negatively correlated with the Antarctic Oscillation (AAO) during summer. Geopotential anomaly maps at 500-hPa show that extreme dry years are tightly associated with negative height anomalies in the Ross–Amundsen Seas, in concordance with the strong negative relationship between PDSI and AAO. The twentieth century increase in extreme drought events in the TMT may not be related to ENSO but to the positive AAO trend during late-spring and summer resulting from a gradual poleward shift of the mid-latitude storm tracks. This first PDSI reconstruction for South America demonstrates the highly significant hindcast skill of A. chilensis as an aridity proxy.

Validation of the abrupt change in GPCP precipitation in the Congo River Basin

AbstractThe Global Precipitation Climatology Project (GPCP) monthly precipitation exhibits a significant negative trend during 1979–2004 over southern tropical Africa from the Congo River Basin to the east coast. This trend appears as a more than 20% drop beginning in 1992 in the 6‐year and 9‐year averages of the areal mean GPCP satellite‐gauge precipitation, whose magnitude is largely determined by the gauge analyses. This paper's analysis of satellite precipitation estimates, gauge precipitation analyses, and gauge coverage information suggests that the negative precipitation trend is only true in part of southern tropical Africa but the magnitude is much smaller than that calculated from the GPCP. In the eastern portion of the region, the precipitation drop in the GPCP is confirmed by the satellite‐only estimates but the decrease of more than 16% is amplified by a change in gauge coverage. In the western portion of the region, basically the southern Congo River Basin, all gauge dependent products show a negative precipitation trend, which is much larger in the GPCP merged satellite‐gauged data set, but not supported by the satellite‐only precipitation estimates. In this study we conclude that for the Congo River Basin, where both the mean precipitation and its spatial gradient are high, the spurious negative trend detected in the GPCP precipitation is caused by a significant change in local gauge coverage and the methodology used by the GPCP to merge satellite and gauge data during the analysis period. Copyright © 2009 Royal Meteorological Society

Dry and wet rainy seasons in the Mantaro river basin (Central Peruvian Andes)

Abstract. Monthly precipitation data from the period of 1970 to 2004 from 38 meteorological stations in the Mantaro river basin were used to classify the rainy seasons (September–April) of each year into anomalously dry or wet, and to determine the basin-wide extent of the anomalies based on the Standardized Precipitation Index (SPI). The wet periods mostly occurred in the early 1970's and during the first half of the 1980's, except for the event that occurred in the 1993/94 period which was the strongest and most generalized in the analyzed period. The dry periods occurred mostly during the second half of the 1980's and the 1990's. Consistent with this, a negative trend in precipitation of 2% per decade was found for the rainy season, due mainly to a stronger trend (−4%/decade) during the peak phase (January–March). Despite previously reported significant negative correlations between El Niño-Southern Oscillation (ENSO) and rainfall during the peak of the rainfall season, the similar amplitude variability of precipitation during the onset phase of the rainfall season (September–December), which is uncorrelated with ENSO, participate to the reduction of the absolute correlation for the full rainfall season. Correlations between rainfall in the Mantaro basin and sea surface temperature (SST) in the tropical Atlantic are significant only near the end of the rainy season, with more rain associated with a weaker north-south difference in SST in the tropical Atlantic.

High‐latitude forcing of regional aridification along the subtropical west coast of South America

Precipitation in north‐central (subtropical) Chile has been declining over the last 130 years. This is of concern in a region where precipitation is already low and which hosts considerable economic activity and a large population. Interannual variability of precipitation is primarily controlled by ENSO, but the reasons for the increasing aridity have remained elusive. Here we show that the negative trend in precipitation is not related to tropical Pacific forcing and that recent El Niño activity instead helped to alleviate the worst drought conditions. Based on data from 1979–2004 we further show that sea surface temperatures and sea‐ice concentration in the Amundsen Sea and associated blocking activity in the Bellingshausen Sea are related to winter precipitation in subtropical Chile, independently of ENSO. High latitude forcing from the Amundsen Sea region may provide an alternative explanation for the observed secular drying trend.

Climatic changes in Central Italy and their potential effects on corn water consumption

This study evaluates the trends of meteorological and agrometeorological indices computed at different sites in the Region of Umbria (Central Italy). The indices were selected on the basis of whether they enable the evaluation of the potential effects of climatic changes on agricultural water requirements and management. The meteorological stations were preliminarily selected considering the length and entirety of the data series and a homogeneous spatial coverage of the region under study. The daily rainfall and minimum and maximum temperature data collected at the stations were elaborated to calculate meteorological and agrometeorological indices (for corn, widespread irrigated crop in Central Italy). The results of the analysis show that rainfall has decreased generally everywhere and the temperature has either remained unchanged or have increased. In particular, in two areas an asymmetric warming results in a decreasing evapotranspiration level. The trends, when present, are stronger in southeastern Umbria; this observation is valid for all the meteorological and agrometeorological indices analysed. At some sites the risk of crop damage due to frost has decreased and sowing time in spring can be anticipated; increasing trends in crop water requirements are expected only when high positive trends in reference evapotranspiration and negative trends in precipitation occur at the same time. Therefore, the regulating function of the soil water zone and the specific plant ability to modify its phenology seem to mitigate the water shortage peaks and the irrigation water requirement potential trends.

Integrated modelling of climate, water, soil, agricultural and socio-economic processes: A general introduction of the methodology and some exemplary results from the semi-arid north-east of Brazil

Many semi-arid regions are characterised by water scarcity and vulnerability of natural resources, pronounced climatic variability and social stress. Integrated studies including climatology, hydrology, and socio-economic studies are required both for analysing the dynamic natural conditions and to assess possible strategies to make semi-arid regions less vulnerable to the present and changing climate. The model introduced here dynamically describes the relationships between climate forcing, water availability, agriculture and selected societal processes. The model has been tailored to simulate the rather complex situation in the semi-arid north-eastern Brazil in a quantitative manner including the sensitivity to external forcing, such as climate change. The selected results presented show the general functioning of the integrated model, with a primary focus on climate change impacts. It becomes evident that due to large differences in regional climate scenarios, it is still impossible to give quantitative values for the most probable development, e.g., to assign probabilities to the simulated results. However, it becomes clear that water is a very crucial factor, and that an efficient and ecologically sound water management is a key question for the further development of that semi-arid region. The simulation results show that, independent of the differences in climate change scenarios, rain-fed farming is more vulnerable to drought impacts compared to irrigated farming. However, the capacity of irrigation and other water infrastructure systems to enhance resilience in respect to climatic fluctuations is significantly constrained given a significant negative precipitation trend.

Dynamics of the boreal summer African monsoon in the NSIPP1 atmospheric model

A nine-member ensemble of simulations with a state-of-the-art atmospheric model forced only by the observed record of sea surface temperature (SST) over 1930–2000 is shown to capture the dominant patterns of variability of boreal summer African rainfall. One pattern represents variability along the Gulf of Guinea, between the equator and 10°N. It connects rainfall over Africa to the Atlantic marine Intertropical Convergence Zone, is controlled by local, i.e., eastern equatorial Atlantic, SSTs, and is interannual in time scale. The other represents variability in the semi-arid Sahel, between 10°N and 20°N. It is a continental pattern, capturing the essence of the African summer monsoon, while at the same time displaying high sensitivity to SSTs in the global tropics. A land–atmosphere feedback associated with this pattern translates precipitation anomalies into coherent surface temperature and evaporation anomalies, as highlighted by a simulation where soil moisture is held fixed to climatology. As a consequence of such feedback, it is shown that the recent positive trend in surface temperature is consistent with the ocean-forced negative trend in precipitation, without the need to invoke the direct effect of the observed increase in anthropogenic greenhouse gases. We advance plausible mechanisms by which the balance between land–ocean temperature contrast and moisture availability that defines the monsoon could have been altered in recent decades, resulting in persistent drought. This discussion also serves to illustrate ways in which the monsoon may be perturbed, or may already have been perturbed, by anthropogenic climate change.

Secular sea level change in the Russian sector of the Arctic Ocean

Sea level is a natural integral indicator of climate variability. It reflects changes in practically all dynamic and thermodynamic processes of terrestrial, oceanic, atmospheric, and cryospheric origin. The use of estimates of sea level rise as an indicator of climate change therefore incurs the difficulty that the inferred sea level change is the net result of many individual effects of environmental forcing. Since some of these effects may offset others, the cause of the sea level response to climate change remains somewhat uncertain. This paper is focused on an attempt to provide first‐order answers to two questions, namely, what is the rate of sea level change in the Arctic Ocean, and furthermore, what is the role of each of the individual contributing factors to observed Arctic Ocean sea level change? In seeking answers to these questions we have discovered that during the period 1954–1989 the observed sea level over the Russian sector of the Arctic Ocean is rising at a rate of approximately 0.123 cm yr−1 and that after correction for the process of glacial isostatic adjustment this rate is approximately 0.185 cm yr−1. There are two major causes of this rise. The first is associated with the steric effect of ocean expansion. This effect is responsible for a contribution of approximately 0.064 cm yr−1 to the total rate of rise (35%). The second most important factor is related to the ongoing decrease of sea level atmospheric pressure over the Arctic Ocean, which contributes 0.056 cm yr−1, or approximately 30% of the net positive sea level trend. A third contribution to the sea level increase involves wind action and the increase of cyclonic winds over the Arctic Ocean, which leads to sea level rise at a rate of 0.018 cm yr−1 or approximately 10% of the total. The combined effect of the sea level rise due to an increase of river runoff and the sea level fall due to a negative trend in precipitation minus evaporation over the ocean is close to 0. For the Russian sector of the Arctic Ocean it therefore appears that approximately 25% of the trend of 0.185 cm yr−1, a contribution of 0.048 cm yr−1, may be due to the effect of increasing Arctic Ocean mass.

A comparison of the climate response to increased carbon dioxide simulated by general circulation models with mixed‐layer and dynamic ocean representations in the region of South America

The patterns of climate change in the South American continent and surrounding oceans simulated in two sets of general circulation model (GCM) climate experiments under an enhanced greenhouse effect, are compared. One set is composed of doubled CO2 concentration equilibrium climate experiments, performed by five GCMs with mixed-layer (slab) ocean representation (SGCM). The other set includes transient climate experiments simulating increasing CO2 concentration, performed by five coupled GCMs with dynamic ocean representation (CGCM). The variables considered are near-surface temperature, mean sea-level pressure, and precipitation. A statistical measure of performance, which combines the complementary information of the root-mean-square error and pattern correlation, is used to assess model capability in control runs, and it is later applied in calculating weighted mean variation fields (2×CO2−1×CO2 experiments). Climate change patterns with consensus among model results are analysed with regard to their physical consistency. The CGCM experiments confirm previous results of SGCM experiments, i.e. the precipitation increase in the eastern equatorial Pacific over the Inter Tropical Convergence Zone, extending toward the north-western part of the continent. They also confirm the negative precipitation trend between the Orinoco and the Amazon rivers in the northern part of the continent. The strengthening of the Pacific anticyclone on its south-eastern side next to the continent is seen in both sets of experiments, but the rainfall decrease and continental warming in subtropical latitudes is more extensive in CGCM than in SGCM experiments. The rainfall increase along the South Atlantic Convergence Zone in the eastern part of the continent during the Southern Hemisphere summer is another persistent feature. The CGCMs simulate lower warming rates in high latitudes than SGCMs, a feature which extends northward along the western coast of the continent in the former experiments. The intensification of the latitudinal temperature gradient between 35° and 50°S in oceanic regions surrounding the continent, and the strengthening of the mid-latitude meridional pressure gradient, is concurrent with an enhanced rainfall rate over the storm track extending along 50°S during June, July and August. © 1997 Royal Meteorological Society.

A comparison of the climate response to increased carbon dioxide simulated by general circulation models with mixed-layer and dynamic ocean representations in the region of South America

The patterns of climate change in the South American continent and surrounding oceans simulated in two sets of general circulation model (GCM) climate experiments under an enhanced greenhouse effect, are compared. One set is composed of doubled CO2 concentration equilibrium climate experiments, performed by five GCMs with mixed-layer (slab) ocean representation (SGCM). The other set includes transient climate experiments simulating increasing CO2 concentration, performed by five coupled GCMs with dynamic ocean representation (CGCM). The variables considered are near-surface temperature, mean sea-level pressure, and precipitation. A statistical measure of performance, which combines the complementary information of the root-mean-square error and pattern correlation, is used to assess model capability in control runs, and it is later applied in calculating weighted mean variation fields (2×CO2−1×CO2 experiments). Climate change patterns with consensus among model results are analysed with regard to their physical consistency. The CGCM experiments confirm previous results of SGCM experiments, i.e. the precipitation increase in the eastern equatorial Pacific over the Inter Tropical Convergence Zone, extending toward the north-western part of the continent. They also confirm the negative precipitation trend between the Orinoco and the Amazon rivers in the northern part of the continent. The strengthening of the Pacific anticyclone on its south-eastern side next to the continent is seen in both sets of experiments, but the rainfall decrease and continental warming in subtropical latitudes is more extensive in CGCM than in SGCM experiments. The rainfall increase along the South Atlantic Convergence Zone in the eastern part of the continent during the Southern Hemisphere summer is another persistent feature. The CGCMs simulate lower warming rates in high latitudes than SGCMs, a feature which extends northward along the western coast of the continent in the former experiments. The intensification of the latitudinal temperature gradient between 35° and 50°S in oceanic regions surrounding the continent, and the strengthening of the mid-latitude meridional pressure gradient, is concurrent with an enhanced rainfall rate over the storm track extending along 50°S during June, July and August. © 1997 Royal Meteorological Society.