Monsoons Climate Change Assessment

The frequency of extreme rainfall events in Southern Brazil is impacted by El Niño—Southern Oscillation (ENSO) episodes, especially in austral spring. There are two areas in which this impact is more significant: one is on the coast, where extreme events are more frequent during El Niño (EN) and the other one extends inland, where extreme events increase during EN and decrease during La Niña (LN). Atmospheric circulation patterns associated with severe rainfall in those areas are similar (opposite) to anomalous patterns characteristic of EN (LN) episodes, indicating why increase (decrease) of extreme events in EN (LN) episodes is favoured. The most recurrent precipitation patterns during extreme rainfall events in each of these areas are disclosed by Principal Component Analysis (PCA) and evidence the separation between extreme events in these areas: a severe precipitation event generally does not occur simultaneously in the coast and inland, although they may occur inland and in the coastal region in sequence. Although EN predominantly enhances extreme rainfall, there are EN years in which fewer severe events occur than the average of neutral years, and also the enhancement of extreme rainfall is not uniform for different EN episodes, because the interdecadal non‐ENSO variability also modulates significantly the frequency of extreme events in Southern Brazil. The inland region, which is more affected, shows increase (decrease) of extreme rainfall in association with the negative (positive) phase of the Atlantic Multidecadal Variability, with the negative (positive) phase of the Pacific Multidecadal Variability and with the positive (negative) phase of the Pacific Interdecadal Variability. Copyright © 2008 Royal Meteorological Society

The spatial and temporal variability of extreme rainfall events over India and their long‐term trend have been analysed during the southwest monsoon season from June to September (JJAS) by taking daily rainfall data for 55 years (1951–2005). The contribution of extreme rainfall events (Category iii with rainfall > 124.4 mm day−1) to the total seasonal rainfall is also analysed for the same period. The analysis shows that the occurrence of extreme rainfall events over India during JJAS shows spatial variations with preferred regions of occurrence over the west coast, central parts of India and northeast India. The average frequency of extreme rainfall events along with the contribution of extreme rainfall events to the seasonal rainfall shows a significant increasing trend (above the 98% confidence level) over the Indian region during JJAS, and also during June and July. It is also found that the increasing trend of contribution from extreme rainfall events during JJAS is balanced by a decreasing trend in Category i (rainfall ≤ 64.4 mm day−1) rainfall events. Using the filtered data it is found that the increasing trend of extreme rainfall over central India is mainly contributed by synoptic scale systems (periodicity between 3 and 7 days). The seasonal mean moist convective instability (CI) during JJAS averaged over central India indicates a significant increasing trend (99.9% confidence level) and is basically due to the increasing trend of the number of days with greater degree of moist CI during JJAS, which may be one possible cause for the increasing trend in the frequency of extreme rainfall events over the region. Copyright © 2009 Royal Meteorological Society

Soil fertility and precipitation are major factors regulating transitions from grasslands to forests. Biotic regulation may influence the effects of these abiotic drivers. In this study, we examined the effects of extreme rainfall events, anthropogenic nutrient loading and insect herbivory on the ability of Chinese tallow tree (Sapium sebiferum) to invade coastal prairie to determine how these factors may influence woody invasion of a grassland. We manipulated soil fertility (NPK addition) and simulated variation in frequency of extreme rainfall events in a three growing season, full factorial field experiment. Adding water to or pumping water out of plots simulated increased and decreased rainfall frequencies. We added Sapium seeds and seedlings to each plot and manipulated insect herbivory on transplanted Sapium seedlings with insecticide. We measured soil moisture, Sapium performance, vegetation mass, and carbon and nitrogen in vegetation and soils (0–10 cm deep, 10–20 cm deep). Fertilization increased Sapium invasion intensity by increasing seedling survival, height growth and biomass. Insect damage was low and insect suppression had little effect in all conditions. Recruitment of Sapium from seed was very low and independent of treatments. Vegetation mass was increased by fertilization in both rainfall treatments but not in the ambient moisture treatment. The amount of carbon and nitrogen in plants was increased by fertilization, especially in modified moisture plots. Soil carbon and nitrogen were independent of all treatments. These results suggest that coastal tallgrass prairies are more likely to be impacted by nutrient loading, in terms of invasion severity and nutrient cycling, than by changes in the frequency of extreme rainfall events.

Global warming and anthropogenic activities have imposed noticeable impacts on rainfall pattern changes at both spatial and temporal scales in recent decades. Systematic diagnosis of rainfall pattern changes is urgently needed at spatiotemporal scales for a deeper understanding of how climate change produces variations in rainfall patterns. The objective of this study was to identify rainfall pattern changes systematically under climate change at a subcontinental scale along a rainfall gradient ranging from 1800 to 200 mm yr−1 by analyzing centennial rainfall data covering 230 sites from 1910 to 2017 in the Northern Territory of Australia. Rainfall pattern changes were characterized by considering aspects of trends and periodicity of annual rainfall, abrupt changes, rainfall distribution, and extreme rainfall events. Our results illustrated that rainfall patterns in northern Australia have changed significantly compared with the early period of the twentieth century. Specifically, 1) a significant increasing trend in annual precipitation associated with greater variation in recent decades was observed over the entire study area, 2) temporal variations represented a mean rainfall periodicity of 27 years over wet to dry regions, 3) an abrupt change of annual rainfall amount occurred consistently in both humid and arid regions during the 1966–75 period, and 4) partitioned long-term time series of rainfall demonstrated a wetter rainfall distribution trend across coastal to inland areas that was associated with more frequent extreme rainfall events in recent decades. The findings of this study could facilitate further studies on the mechanisms of climate change that influence rainfall pattern changes. Significance Statement Characterizing long-term rainfall pattern changes under different rainfall conditions is important to understand the impacts of climate change. We conducted diagnosis of centennial rainfall pattern changes across wet to dry regions in northern Australia and found that rainfall patterns have noticeably changed in recent decades. The entire region has a consistent increasing trend of annual rainfall with higher variation. Meanwhile, the main shifting period of rainfall pattern was during 1966–75. Although annual rainfall seems to become wetter with an increasing trend, more frequent extreme rainfall events should also be noticed for assessing the impacts of climate changes. The findings support further study to understand long-term rainfall pattern changes under climate change.

Unimodal response of N2O flux to changing rainfall amount and frequency in a wet meadow in the Tibetan Plateau

In recent years, extreme weather phenomena have occurred worldwide, resulting in many catastrophic disasters. Under the impact of climate change, the frequency of extreme rainfall events in Taiwan will increase, according to a report on climate change in Taiwan. This study analyzed riverbed migrations, such as degradation and aggradation, caused by extreme rainfall events under climate change for the Choshui River, Taiwan. We used the CCHE1D model to simulate changes in flow discharge and riverbed caused by typhoon events for the base period (1979–2003) and the end of the 21st century (2075–2099) according to the climate change scenario of representative concentration pathways 8.5 (RCP8.5) and dynamical downscaling of rainfall data in Taiwan. According to the results on flow discharge, at the end of the 21st century, the average peak flow during extreme rainfall events will increase by 20% relative to the base period, but the time required to reach the peak will be 8 h shorter than that in the base period. In terms of the results of degradation and aggradation of the riverbed, at the end of the 21st century, the amount of aggradation will increase by 33% over that of the base period. In the future, upstream sediment will be blocked by the Chichi weir, increasing the severity of scouring downstream. In addition, due to the increased peak flow discharge in the future, the scouring of the pier may be more serious than it is currently. More detailed 2D or 3D hydrological models are necessary in future works, which could adequately address the erosive phenomena created by bridge piers. Our results indicate that not only will flood disasters occur within a shorter time duration, but the catchment will also face more severe degradation and aggradation in the future.

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

In recent years, climate change has increased the frequency of extreme rainfall events, significantly impacting surface water quality (SWQ). This study focuses on Hangzhou, utilizing rainfall data from June 2021 to May 2024 to calculate a series of rainfall extreme indices (REIs). It explores the spatiotemporal variations in these REIs alongside SWQ parameters, including water temperature (WT), dissolved oxygen (DO), pH, total phosphorus (TP), total nitrogen (TN), and turbidity. This research also analyzes the correlations between SWQ parameters and REIs for the first time. The results show that extreme rainfall events primarily occur in July, with increases in both intensity and frequency during the study period. Influenced by human activities, natural conditions, and environmental policies, SWQ parameters in Hangzhou exhibit notable spatiotemporal variability. Correlation analyses reveal significant positive relationships between TP, TN, and turbidity in most areas with REIs. However, the correlations between pH, WT, and turbidity with REIs differ between the eastern and western regions, resulting from variations in land use. These findings will provide a theoretical basis for developing models to predict changes in SWQ based on REIs, contributing to the safeguarding of surface water quality.

Recently, there has been a growing interest in extreme and intense rainfall events due to climate change, which leads to increased urban damage from flooding, landslides, and more. Rainfall exceeding design frequencies is a primary cause of natural disasters that significantly affect human life, such as floods and landslides. As the frequency of extreme rainfall events increases due to climate change, this study collected rainfall data from meteorological stations to analyze flood risks in Kangwon province, and identified periods rich and poor in floods using the GEV distribution and time series analysis, which are common methods for analyzing extreme events. In addition, using the actual frequency and design frequency, the flood load that can quantify the risk of flooding was calculated in this paper. As a result of the analysis, cases in which the flood load exceeded 1 occurred in all target areas, and the area with the highest flood load was analyzed as a factor of 3.83. In order to prevent floods caused by extremely frequent heavy rains due to climate change, it is necessary to increase the design frequency of repair facilities.

Synthetic design storms are often used to plan new drainage systems or assess flood impacts on infrastructure. To simulate extreme rainfall events under climate change, design storms can be modified to match a different return frequency of extreme rainfall events as well as a modified temporal distribution of rainfall intensities. However, the same magnitude of change to the rainfall intensities is often applied in space. Several hydrological applications are limited by this. Climate change impacts on urban pluvial floods, for example, require the use of 2D design storms (rainfall fields) at sub-kilometer and sub-hourly scales. Recent kilometer scale climate models, also known as convection-permitting climate models (CPM), provide rainfall outputs at a high spatial resolution, although rainfall simulations are still restricted to a limited number of climate scenarios and time periods. We nevertheless explored the potential use of rainfall data obtained from these models for hydrological flood impact studies by introducing a method of spatial quantile mapping (SQM). To demonstrate the new methodology, we extracted high-resolution rainfall simulations from a CPM for four domains representing different urban areas in Switzerland. Extreme storms that are plausible under the present climate conditions were simulated with a 2D stochastic rainfall model. Based on the CPM-informed stochastically generated rainfall fields, we modified the design storms to fit the future climate scenario using three different methods: the SQM, a uniform quantile mapping, and a uniform adjustment based on a rainfall–temperature relationship. Throughout all storms, the temporal distribution of rainfall was the same. Using a flood model, we assessed the impact of different rainfall adjustment methods on urban flooding. Significant differences were found in the flood water depths and areas between the three methods. In general, the SQM method results in a higher flood impact than the storms that were modified otherwise. The results suggest that spatial storm profiles may need to be re-adjusted when assessing flood impacts.

The El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are widely recognized as major drivers of inter-annual variability of the Indian monsoon (IM) and extreme rainfall events (EREs). We assess the time-varying strength and non-linearity of these linkages using dynamic linear regression and Generalized Additive Models. Our results suggest that IOD has evolved independently of ENSO, with its influence on IM and EREs strengthening in recent decades when compared to ENSO, whose relationship with IM seems to be weakening and more uncertain. A unit change in IOD currently has a proportionately greater impact on IM. ENSO positively influences EREs only below a threshold of 100 mm day−1. Furthermore, there is a non-linear and positive relationship between IOD and IM totals and the frequency of EREs (>100 mm day−1). Improvements in modeling this complex system can enhance the forecasting accuracy of the IM and EREs.

Land use and land cover change (LULCC) directly affect the temporal and spatial change of soil erosion. As a typical governance watershed in the hilly and gully area of the Loess Plateau, the Jiuyuangou watershed has experienced significant LULCC in the past 10 years due to conversion of farmland to forests, economic construction, and cropland abandonment. However, the evolution process of soil erosion change and LULCC in the watershed is unclear, as is the relationship between the two. This study used satellite images to extract information on LULCC in the watershed and the Chinese soil loss equation (CSLE) model to evaluate the temporal and spatial evolution of soil erosion in the watershed from 2010 to 2020. The main results showed that (1) the continuous vegetation restoration project in the watershed reduced soil erosion from 2010 to 2015; however, the increased frequency of extreme rainfall events after 2015 reduced its impact. The annual average soil erosion modulus decreased from 10.85 t ha −1 year −1 in 2010 to 8.03 t ha −1 year −1 in 2015 but then increased to 10.57 t ha −1 year −1 in 2020; (2) the main land use and land cover (LULC) type in the Jiuyuangou watershed is grassland, accounting for 62% of the total area, followed by forestland, cropland, buildings, and water. Cropland has the largest multi‐year average soil erosion modulus, followed by grassland and buildings, with forestland having the smallest; (3) significant spatial correlations occurred between soil erosion change and LULCC for common ‘no change’ and common ‘gain’ in the settlements, roads, and areas near the human influences with good soil and water conservation, but not other regions due to the influence of climatic factors (heavy rain events). Thus, we should repair terraces, control dams in the watershed, and actively conserve water and soil. This study provides a scientific reference for planning and managing water and soil conservation and ecological environment construction in the watershed.

<strong class="journal-contentHeaderColor">Abstract.</strong> Orographic precipitation is a key driver of flooding in mountainous areas. This article investigates the microphysical response of orographic rainfall to perturbations of temperature and cloud condensation nuclei (CCN) concentration. The study is motivated by the increased water vapour capacity of the atmosphere in a warming climate and the increasing frequency of extreme rainfall events. A case study for the Cumbria flood in December 2015 is performed with sensitivities using a realization of the `piggybacking' method implemented into a limited-area setup of the ICON model. A 6 % K<sup>-1</sup> enhancement of rainfall results for the highest altitudes, caused by a `mixed-phase seeder-feeder mechanism', i.e. the interplay of melting and accretion. Total 24 h rainfall is found to increase by only 2 % K^-1, significantly less than the 7 % K^-1&nbsp;increase in atmospheric water vapour. A rain budget analysis reveals that the negative temperature sensitivity of the condensation ratio and the increase of rain evaporation dampen the rainfall enhancement. Decreasing the CCN concentration speeds up the microphysical processing, which leads to an increase in total rainfall. At low CCN concentration the rainfall sensitivity to temperature is systematically smaller. It is shown that the CCN and temperature sensitivities are to a large extent independent (with a &plusmn;3 % relative error) and additive.

Control of risks associated with floods in recent years has a central role in urban planning scientific research. This is related to the increased incidence of these phenomena whose the root cause was found in the greater frequency of extreme rainfall events as a result of climate change. In Italy the phenomenon became dramatic because of the numerous human casualties that occur each year. Without prejudice to the need for interventions that effectively act on climate change, there is a need to provide interventions to mitigate the consequences in the short-term. The Department of Urban Planning at Naples University has conducted a thorough research on the case, objectively verifying the impact that human activity has on the dramatic consequences of rainfall events, also highlighting the lack of efficiency of the traditional mitigation measures based on the construction of huge detention basins dug in the ground. This is due to three main reasons: such interventions require financial investments that local communities often do not have; the creation of detention basins cause the removal of large quantities of soil for agricultural use; the continuous increase in impervious surfaces due to urbanization implies the need to cope with the masses of water ever more impressive and therefore the dry ponds already created quickly become insufficient. The research proposes some interventions and best practices that, if adopted, can radically reduce the impact of extreme rainfall on public safety, enabling an efficient participation of private capital in investments through the use of leverage financial and fiscal incentives.

Due to the increased frequency of extreme rainfall events caused by climate change, flooding in urban areas is becoming increasingly recurrent. Nevertheless, mitigation and response actions to flood events are still defined according to the best judgments of civil protection authorities, based on their experience and on simple flood modelling tools. In this paper we present the methodological structure of an innovative prototype tool for dynamic pluvial-flood emergency planning. The tool is aimed at helping civil protection authorities (and the population) in the preparation, mitigation and response to flood events. The 2009 flood in the Agualva village (Terceira Island, Azores, Portugal), is used to exemplify the model's calibration and to illustrate the prototype capabilities. The results highlight the importance of considering a dynamic approach in the design of pluvial-flood emergency planning.