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Rainfall Trends (rainfall + trend)
Selected AbstractsTrends in the southern oscillation phenomenon and Australian rainfall and changes in their relationshipINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2004Ramasamy Suppiah Abstract An attempt has been made to investigate decadal-scale trends in Australian rainfall and in the southern oscillation index (SOI) and their influence on the relationship between them. Monthly rainfall data from high-quality stations in Australia (from 1900 to 1995), India, Sri Lanka and Tahiti are used. The relationship between the SOI and Australian rainfall is positive, but shows decadal-scale variations during the past century. Although there were extended and severe El Niño events in the early 1990s and in 1997, Australia did not experience the expected severe rainfall deficiencies characteristic of previous events. However, severe drought conditions over eastern Australia were associated with a moderate El Niño event during 2002,03. Long-term fluctuations of March,May (MAM) rainfall show high-frequency variations, but trends during June,August (JJA), September,November (SON) and December,February (DJF) show low-frequency or decadal-scale variations. Trends and multi-decadal fluctuations in all-Australian spring (SON) and summer (DJF) rainfall are strongly dominated by rainfall trend fluctuations in northern and eastern Australia. Austral summer rainfall shows an increasing trend during the 1980s and 1990s, particularly in Queensland and New South Wales, despite the occurrence of extended and severe El Niños. However, some parts of New South Wales and Queensland experienced severe rainfall deficiencies during 2002,03 in conjunction with an El Niño event. The relationship between the SOI and rainfall on the interannual time scale is strong when the SOI and rainfall follow the same direction, but it is weak when they follow opposite directions on a decadal-time scale. The poor correlation during the 1920s and 1930s was due to a slightly increasing trend in the SOI and a stronger decreasing trend in rainfall. A weakening in the relationship between the SOI and rainfall in recent years, after the mid-1970s, is due to a small increase in rainfall in the 1980s and 1990s and a strong decrease in the SOI. Rainfall trends were enhanced (stronger decreases or increases) when the influence of the SOI (or El Niño-southern oscillation (ENSO)) was removed. Enhanced increases and decreases are particularly strong during SON and DJF, when the ENSO phenomenon is at the mature stage and also the influence on Australian rainfall is strong. The increasing trend in rainfall during the 1980s and 1990s in some parts of eastern Australia and the decreasing trend in the SOI result in more rainfall for a given SOI compared with the same SOI during the previous period, i.e. before the mid-1970s. A similar analysis was carried out for two periods, before and after 1972, for Tahiti, India and Sri Lanka. The upward or downward shift in regression lines is very clear during the season, that shows a strong relationship between rainfall and the SOI. Moreover, strengthening or weakening of the relationship between rainfall and the SOI is largely dependent on their multi-decadal variations and trends during the past century. Increases in rainfall during the 1980s and the 1990s and decreases in the SOI have weakened their relationship, both in Australia and India. Such a relationship gives more rainfall for a given SOI after 1973. The pattern was reversed for Sri Lanka, where rainfall during the second intermonsoon season has decreased. Analyses of trends in temperature at Darwin and Tahiti and of rainfall over Australia, India, Tahiti and Sri Lanka suggest a regional-scale change in climate, whereas the SOI reflects a change in the large-scale circulation pattern over the Indo-Pacific region after the mid-1970s. Copyright © 2004 Royal Meteorological Society [source] Analysis of historical landslide time series in the Emilia-Romagna region, northern ItalyEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2010Mauro Rossi Abstract A catalogue of historical landslides, 1951,2002, for three provinces in the Emilia-Romagna region of northern Italy is presented and its statistical properties studied. The catalogue consists of 2255 reported landslides and is based on historical archives and chronicles. We use two measures for the intensity of landsliding over time: (i) the number of reported landslides in a day (DL) and (ii) the number of reported landslides in an event (Sevent), where an event is one or more consecutive days with landsliding. From 1951,2002 in our study area there were 1057 days with 1 , DL ,?45 landslides per day, and 596 events with 1 , Sevent , 129 landslides per event. In the first set of analyses, we find that the probability density of landslide intensities in the time series are power-law distributed over at least two-orders of magnitude, with exponent of about ,2·0. Although our data is a proxy for landsliding built from newspaper reports, it is the first tentative evidence that the frequency-size of triggered landslide events over time (not just the landslides in a given triggered event), like earthquakes, scale as a power-law or other heavy-tailed distributions. If confirmed, this could have important implications for risk assessment and erosion modelling in a given area. In our second set of analyses, we find that for short antecedent rainfall periods, the minimum amount of rainfall necessary to trigger landslides varies considerably with the intensity of the landsliding (DL and Sevent); whereas for long antecedent periods the magnitude is largely independent of the cumulative amount of rainfall, and the largest values of landslide intensity are always preceded by abundant rainfall. Further, the analysis of the rainfall trend suggests that the trigger of landslides in the study area is related to seasonal rainfall. Copyright © 2010 John Wiley & Sons, Ltd. [source] Spatial and temporal characteristics of droughts in the western part of BangladeshHYDROLOGICAL PROCESSES, Issue 13 2008Shamsuddin Shahid Abstract Spatial and temporal characteristics of droughts in the western part of Bangladesh have been analysed. Standardized precipitation index method is used to compute the severity of droughts from the rainfall data recorded in 12 rainfall gauge stations for the period of 1961,1999. An artificial neural network is used to estimate missing rainfall data. Geographic Information System (GIS) is used to map the spatial extent of droughts of different severities in multiple time scales. Critical analysis of rainfall is also carried to find the minimum monsoon and dry months rainfall require in different parts of the study area to avoid rainfall deficit. The study shows that the north and north-western parts of Bangladesh are most vulnerable to droughts. A significant negative relationship between multiple ENSO index and rainfall is observed in some stations. Analysis of seasonal rainfall distribution, rainfall reliability and long-term rainfall trend is also conducted to aid prediction of future droughts in the area. Copyright © 2007 John Wiley & Sons, Ltd. [source] An analysis of late twentieth century trends in Australian rainfallINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2009Andréa S. Taschetto Abstract Trends in Australian precipitation from 1970 to 2006 are examined using a daily rainfall dataset. Results suggest a linkage between changes in the monsoon trough and rainfall trends over northwestern Australia. The late twentieth century drought observed along the Queensland coast is a response to changes in the atmospheric circulation that generates anomalous subsidence at high and middle levels of the atmosphere, thus inhibiting convection over the region. In addition, an anomalous anticyclonic circulation at low levels over Queensland tends to weaken the easterlies in the tropical western Pacific, thus diminishing the transport of moist air onto the coast. Trends in the frequency and magnitude of different rainfall events are also examined. This reveals that changes in total rainfall are dominated by trends in very heavy rainfall events across Australia. For example, some parts of western Australia reveal an increase in heavy rainfall events that are not accompanied by a rise in modest rainfall events, resulting in changes in the shape of the distribution towards a more skewed precipitation distribution. On the other hand, the frequency of extreme rainfall events along the Queensland coast has declined during summer and autumn consistently with the total rainfall decrease, indicating changes in the position of the precipitation distribution rather than its shape. Copyright © 2008 Royal Meteorological Society [source] The recent Sahel drought is realINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2004Aiguo Dai Abstract Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10,20°N and 18°W,20°E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi-year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology24: 547,554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded. Copyright © 2004 Royal Meteorological Society [source] Circulation dynamics of Mediterranean precipitation variability 1948,98INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 15 2003A. Dünkeloh Abstract Canonical correlation analysis is used to identify main coupled circulation,rainfall patterns and to relate recent variability and trends of Mediterranean precipitation to large-scale circulation dynamics. Analyses are based on geopotential heights (500 and 1000 hPa levels) for the North Atlantic,European area (National Centers for Environmental Prediction,National Center for Atmospheric Research reanalysis) and on highly resolved (0.5° × 0.5° ) monthly rainfall grids (Climatic Research Unit, Norwich) selected for the Mediterranean area during the 1948,98 period. Combining monthly analyses with similar characteristics to seasonal samples yields winter (October,March), spring (April,May) and summer (June,September) types of coupled variability; a particular autumn type for the whole Mediterranean does not occur on the monthly time scale. Coupled patterns specifically linked to one or two seasons include an east Atlantic jet (EA-Jet) related pattern for summer and a Mediterranean meridional circulation (MMC) pattern for winter and spring. The most important pattern recurring with dynamical adjustments throughout the whole year reflects the seasonal cycle of the Mediterranean oscillation (MO), which is linked (with seasonal dependence) to the Northern Hemisphere teleconnection modes of the Arctic oscillation (AO) and North Atlantic oscillation (NAO). Winter rainfall trends of the recent decades marked by widespread decreases in the Mediterranean area and by opposite conditions in the southeastern part are linked to particular changes over time in several of the associated circulation patterns. Thus, different regional rainfall changes are integrated into an overall interrelation between Mediterranean rainfall patterns and large-scale atmospheric circulation dynamics. Copyright © 2003 Royal Meteorological Society [source] Trends in extreme daily rainfall and temperature in Southeast Asia and the South Pacific: 1961,1998INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2001M.J. Manton Abstract Trends in extreme daily temperature and rainfall have been analysed from 1961 to 1998 for Southeast Asia and the South Pacific. This 38-year period was chosen to optimize data availability across the region. Using high-quality data from 91 stations in 15 countries, significant increases were detected in the annual number of hot days and warm nights, with significant decreases in the annual number of cool days and cold nights. These trends in extreme temperatures showed considerable consistency across the region. Extreme rainfall trends were generally less spatially coherent than were those for extreme temperature. The number of rain days (with at least 2 mm of rain) has decreased significantly throughout Southeast Asia and the western and central South Pacific, but increased in the north of French Polynesia, in Fiji, and at some stations in Australia. The proportion of annual rainfall from extreme events has increased at a majority of stations. The frequency of extreme rainfall events has declined at most stations (but not significantly), although significant increases were detected in French Polynesia. Trends in the average intensity of the wettest rainfall events each year were generally weak and not significant. Copyright © 2001 Royal Meteorological Society [source] | ||||||||||