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Rainfall, with its intensity, duration, and seasonal distribution, is among the main factors causing soil erosion, which is a widespread environmental problem in Bulgaria. Rainfall erosivity shows the potential of precipitation to generate erosion processes and is an essential indicator of the climate vulnerability of a region. This paper aims to evaluate rainfall erosivity in a part of the Eastern Rhodopes Mountains, an area that is characterised by high-intensity erosion processes and high erosion risk. Local peculiarities of rainfall erosivity were revealed by the calculation of some precipitation indices based on the monthly precipitation for the period 2000–2021, such as the precipitation concentration index (PCI), Angot precipitation index, Fournier index (FI), and modified Fournier index (MFI). The analysis of the extremely wet and extremely dry months at the annual and seasonal (October–March and April–September) levels was performed to evaluate the susceptibility to erosion. The results from the study show that rainfall erosivity in the studied area varies from low to moderate in the northern part of the study area and from high to very high in the south. According to the MFI, high and very high erosivities have been observed mainly since 2012. The erosivity increases from north to south, to the area with a complex relief, where the combination of orography and atmospheric circulation make favourable conditions for the occurrence of extreme precipitation. The analyses of the calculated indices show that the precipitations in most of the studied area generally have from a low to a moderate erosivity, but this does not exclude the occurrence of cases with high and very high erosivities, which are characteristic of recent years and are related to the increase in annual precipitations and extreme precipitation months. The results of this study can contribute to the development and implementation of measures and preventive activities for the reduction and possible elimination of the negative impacts of extreme precipitation. [ABSTRACT FROM AUTHOR]

In the paper, the frequency and spatial distribution of atmospheric and hydrological droughts over the territory of Belarus in 1945–2020 is assessed. Aridity is evaluated using the standardized precipitation and streamflow indices (SPI and SSFI). Trends in monthly total precipitation are determined, the features of the streamflow formation during the open-channel period are revealed. The frequency of anticyclones that determines hydrometeorological conditions on the territory of Belarus is calculated. The statistically significant increase in the number of the days with anticyclonic weather over the country during the warm season is found. The increasing frequency of dry conditions in summer months against the background of a rise in the precipitation intensity is highlighted. A simultaneous increase in the frequency of very-low-flow periods on rivers and an increase in the height of warm rainfall floods in some river basins are revealed. [ABSTRACT FROM AUTHOR]

Heat lows are key features of subtropical climates and monsoon systems. In southern Africa, they are pivotal to understanding divergent climate change projections, in particular the veracity of future rainfall decline. Compared to other heat lows, including in West Africa and Australia, the southern African heat low remains poorly documented. Here, we analyze the diurnal cycle, seasonal variability, and trends of the heat low in reanalysis data. In ERA5, 462 strong heat low days are detected between September and March from 1990 to 2019, equating to 7.3% of days sampled. These events feature ascent (exceeding −0.2 Pa s−1) at low levels (strongest between 800 and 600 hPa) and subsidence aloft, generating low-level cyclonic flow with anticyclonic flow above. This flow exhibits strong diurnal variability, with peak windspeeds between 0600 and 0900 UTC and maximum ascent at ∼2300 UTC. Heat lows form preferentially over Angola in September (∼14°S) and October (15°–20°S), and in Namibia from November to March (∼20°–26°S). Strongest ascent occurs over areas of high elevation. Finally, we show a rapidly increasing frequency of strong heat low days, with a 175% increase between 1960–89 and 1990–2019. The greatest increase (459%) has occurred in the early summer months of September and October, consistent with projections of delayed rainfall onset. Strikingly, more strong heat lows are detected in the most recent 5 years of analysis (2014–19) than in the 30-yr period from 1960 to 1989. These results suggest the heat low is an important feature in determining drying trends over southern Africa and is a vital indicator of climate model accuracy. Significance Statement: This work documents the heat low that forms in southern Africa in the lowest levels of the atmosphere. The feature is present during austral summer (from September to March) and is associated with below average rainfall across much of the subcontinent. The frequency of strong heat lows has rapidly increased in line with regional amplified warming trends. The heat low is identified as an important control on circulation and precipitation patterns and changes in the frequency or intensity of the feature in the future are likely to influence the strength of declining rainfall trends across southern Africa. [ABSTRACT FROM AUTHOR]

This paper scrutinizes the zonal oscillation of the western North Pacific subtropical high (WNPSH) via diagnosing its two extreme phases, which are defined by the top 10% strongest (positive phase) and the weakest (negative phase) WNPSH index (WNPSHI) days during summers in 1979–2016. Key findings include the following: a tripole pattern consisting of intensified (weakened) precipitation over the Maritime Continent and the East Asian summer monsoon regions, and suppressed (strengthened) precipitation over the western North Pacific summer monsoon region during positive (negative) WNPSH phases; a westward movement of WNPSH-induced precipitation anomalies that subsequently affects eastern China, Japan, and the Korean Peninsula at different time lags; an OLR–vorticity pattern explained by atmospheric responses to thermal sources is suggested to drive the oscillation; and the competitive interaction of local air–sea feedbacks, especially during the positive phase. In addition, moderate-to-strong positive correlations between the WNPSHI and the Niño-3.4 index are found on 1–2-, 2–3-, and 3–6-yr time scales; both exhibit decadal shifts to a higher-frequency mode, suggesting the intensification of both the zonal WNPSH oscillation and the ENSO under the changing climate and their close interdecadal association. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) WNPSH phase sometimes occurs during 1) a decaying El Niño (La Niña) in the preceding summer/autumn, and/or 2) a developing La Niña (El Niño) in the current summer/autumn. A full ENSO transition from moderate-to-strong El Niño to La Niña is often seen during the positive phase, offering potential in predicting ENSO events and extreme WNPSH phases and thereby the summer monsoon rainfall in East Asia. [ABSTRACT FROM AUTHOR]

The paper tests if the two La Niña (LN) types can be distinguished from each other using their climate impacts on northeastern Brazil (NEB). To this end, all LN events during the 1901–2010 period followed by a wet or dry rainy season (from February to April) in NEB are classified into two categories: WET‐LN and DRY‐LN. The global and regional anomalous atmospheric circulation patterns and the rainfall anomaly patterns in South America associated with the two cases are analysed. The WET‐LN and DRY‐LN events present, respectively, the eastern Pacific LN and central Pacific LN sea surface temperature (SST) anomaly features in the tropical Pacific. On the other hand, the WET‐LN features an interhemispheric SST dipole pattern in the tropical Atlantic, and the DRY‐LN, colder‐than‐normal surface waters in the tropical South Atlantic (TSA) and equatorial Atlantic during MAM(+1). The two analysed cases show regional differential circulation patterns in all seasons. The anomalous wetness over northern and northwestern South America occurs for the WET‐LN type during JJA(0)–DJF(+1) and for the DRY‐LN type, during DJF(+1)–MAM(+1). The anomalous dryness over SESA is more evident for the WET‐LN during JJA(0) and MAM(+1), and for the DRY‐LN, during SON(0). Anomalous dryness occurs in central and eastern South America noted during JJA(0) and DJF(+1) for both cases analysed. The precipitation anomalies in northern South America during DJF(+1) are stronger and more extensive for the DRY‐LN than for the WET‐LN events due to the action of both the anomalous (double) Walker and Hadley cells for the DRY‐LN, in contrast with the exclusive action of an anomalous Walker cell for the WET‐LN case. Also, a double Walker cell drives the dry–wet dipole between northern South America and NEB during MAM(+1) for the DRY‐LN. Our results might be useful mainly for climate monitoring purposes. The two La Niña (LN) types were distinguished from each other using their climate impacts on northeastern Brazil (NEB) and classified in two categories WET‐LN (LN events followed by a wet rainy season in NEB) and DRY‐LN (LN events followed by a dry rainy season in NEB) that present, respectively, the eastern Pacific LN and central Pacific LN sea surface temperature (SST) anomaly features in the tropical Pacific. The illustration shows the LN‐related SST and precipitation anomaly patterns evolutions associated with the two LN types. [ABSTRACT FROM AUTHOR]

There is an urgent need to understand and predict how extreme precipitation events (EPEs) will change at high latitudes, both for local climate change adaptation plans and risk mitigation and as a potential proxy "anticipating" the impact of climate change elsewhere in the world. This paper illustrates that a combination of non‐stationary modelling approaches can be adopted to evaluate trends in EPEs under uncertainty. A large database of daily rainfall events from 281 sparsely distributed weather stations in Finland between 1961 and 2016 was analysed. Among the tested methods, Poisson distributions provided a powerful method to evaluate the impacts of multiple physical covariates, including temperature and atmospheric circulation patterns (ACPs), on the resulting trends. The analysis demonstrates that non‐stationarity is statistically valid for the majority of observations, independently of their location in the country and the season of the year. However, subsampling can severely hinder the statistical validity of the trends, which can be easily confused with random noise and therefore complicate the decision‐making processes regarding long‐term planning. Scaling effects have a strong impact on the estimates of non‐stationary parameters, as homogenizing the data in space and time reduces the statistical validity of the trends. Trends in EPE statistics (mean, 90 and 99% percentiles) and best‐fitted Generalized Pareto parameters in the tails of the distributions appear to be stronger when approaching the Polar region (Lapland) than away from it, consistent with the Arctic amplification of climate change. ACPs are key covariates in physically explaining these trends. In particular, the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) can explain statistically significant increases in extreme precipitation in Lapland, Bothnian and South regions of Finland, particularly during summer and fall seasons. Non‐stationarity is statistically valid in the majority of observations but masked by subsampling and spatio‐temporal heterogeneity. Trends on extremes and non‐stationary generalized Pareto parameters stronger when approaching the Polar region (Lapland). Artic Oscillation (AO) and North Atlantic Oscillation (NAO) can explain statistically significant trends. [ABSTRACT FROM AUTHOR]

Summer precipitation anomalies over eastern China are characterized spatially by meridionally banded structures fluctuating on interannual and interdecadal timescales, leading to regional droughts and floods. In addition to long-term trends, how these patterns may change under global warming has important implications for agricultural planning and water resources over this densely populated area. Using the latest Hadley Centre climate model, HadGEM3-GC2, this paper investigates the potential response of summer precipitation patterns over this region, by comparing the leading modes between a 4×CO2 simulation and the model’s pre-industrial control simulation. Empirical Orthogonal Function (EOF) analyses show that the first two leading modes account for about 20% of summer rainfall variability. EOF1 is a monopole mode associated with the developing phase of ENSO events and EOF2 is a dipole mode associated with the decaying phase of ENSO. Under 4×CO2 forcing, the dipole mode with a south-north orientation becomes dominant because of a strengthened influence from excessive warming of the Indian Ocean. On interdecadal time scales, the first EOF looks very different from the control simulation, showing a dipole mode of east-west contrast with enhanced influence from high latitudes. [ABSTRACT FROM AUTHOR]

The IPCC Fifth Assessment Report highlighted large uncertainty in European precipitation changes in the coming century. This paper investigates the sources of intermodel differences using CMIP5 model European precipitation data. The contribution of atmospheric circulation to differences in precipitation trends is investigated by applying cluster analysis to daily mean sea level pressure (MSLP) data. The resulting classification is used to reconstruct monthly precipitation time series, thereby isolating the component of precipitation variability directly related to atmospheric circulation. Reconstructed observed precipitation and reconstructions of simulated historical and projection data are well correlated with the original precipitation series, showing that circulation variability accounts for a substantial fraction of European precipitation variability. Removing the reconstructed precipitation from the original precipitation leaves a residual component related to noncirculation effects (and any small remaining circulation effects). Intermodel spread in residual future European precipitation trends is substantially reduced compared to the spread of the original precipitation trends. Uncertainty in future atmospheric circulation accounts for more than half of the intermodel variance in twenty-first-century precipitation trends for winter months for both northern and southern Europe. Furthermore, a substantial part of this variance is related to different forced dynamical responses in different models and is therefore potentially reducible. These results highlight the importance of understanding future changes in atmospheric dynamics in achieving more robust projections of regional climate change. Finally, the possible dynamical mechanisms that may drive the future differences in regional circulation and precipitation are illustrated by examining simulated teleconnections with tropical precipitation. [ABSTRACT FROM AUTHOR]

Potential and real predictive skill of the frequency of extreme rainfall in southeastern South America for the December-February season are evaluated in this paper, finding evidence indicating that mechanisms of climate variability at one time scale contribute to the predictability at another scale; that is, taking into account the interference of different potential sources of predictability at different time scales increases the predictive skill. Part I of this study suggested that a set of daily atmospheric circulation regimes, or weather types, was sensitive to these cross-time scale interferences, conducive to the occurrence of extreme rainfall events in the region, and could be used as a potential predictor. At seasonal scale, a combination of those weather types indeed tends to outperform all the other candidate predictors explored (i.e., sea surface temperature patterns, phases of the Madden-Julian oscillation, and combinations of both). Spatially averaged Kendall's τ improvements of 43% for the potential predictability and 23% for real-time predictions are attained with respect to standard models considering sea surface temperature fields alone. A new subseasonal-to-seasonal predictive methodology for extreme rainfall events is proposed based on probability forecasts of seasonal sequences of these weather types. The cross-validated real-time skill of the new probabilistic approach, as measured by the hit score and the Heidke skill score, is on the order of twice that associated with climatological values. The approach is designed to offer useful subseasonal-to-seasonal climate information to decision-makers interested not only in how many extreme events will happen in the season but also in how, when, and where those events will probably occur. [ABSTRACT FROM AUTHOR]

This paper studies operators inspired by the fluctuation-dissipation theorem that consider the seasonality (nonstationarity) of the climate system under conditions of limited sample size relevant to application of the method to observational records. The approach is used to predict the steady-state response of an atmospheric general circulation model to localized temperature perturbations. A seasonal operator nominally requires a much larger data sample than a stationary operator; the authors study some strategies to overcome this. First, two methods for approximating the seasonality of the system are examined. Second, an alternative 'transpose approach' to the standard dimension reduction is considered that is more efficient and accurate for small sample sizes and additionally enables the use of a kernel, which provides a convenient way to incorporate prior physical understanding into the operator. All operators show considerable skill in predicting seasonal responses for a variety of variables (temperature, winds, rainfall, and cloud cover) and better skill in predicting the annual-mean ones. A comparison of these predictions to ones done on the same system with temporally fixed boundary conditions shows unexpectedly that skill is, if anything, improved by the presence of a seasonal cycle. The authors suggest that the extra complexity due to a seasonal system is outweighed by the added information due to the seasonal forcing and the effect of seasonality in smoothing out prediction errors. [ABSTRACT FROM AUTHOR]

The early season rainfall (ESR) over southern China, usually occurring from April to June, is a prominent meteorological phenomenon of the East Asian monsoon system. In this paper, output from the 45-day hindcast by the NCEP Climate Forecast System, version 2 (CFSv2), and various observational datasets are analyzed to assess the predictability of the ESR and associated atmospheric circulation. Results show that CFSv2 can successfully predict the ESR and associated circulation patterns over southern China. The lower-tropospheric convergence and upper-tropospheric divergence as well as the local upward motion over southern China lead to the formation of ESR. Analysis of bias shows small differences and close relationships between the predicted and observed ESR values when the forecast lead time is less than 2 weeks. The skill in the ESR predictions by CFSv2 decreases significantly when the lead time is longer than 2 weeks. Overall, CFSv2 has a higher level of skill when predicting the southern China ESR compared to the rainfall over other Asian regions during the same period of time. [ABSTRACT FROM AUTHOR]

The response over West Africa to uniform warming of the Atlantic Ocean is analyzed using idealized simulations with a regional climate model. With warming of 1 and 1.5 K, rainfall rates increase by 30%-50% over most of West Africa. With Atlantic warming of 2 K and higher, coastal precipitation increases but Sahel rainfall decreases substantially. This nonlinear response in Sahel rainfall is the focus of this analysis. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Gulf of Guinea and in the large-scale meridional geopotential height gradient. This leads to easterly wind anomalies in the central Sahel. With Atlantic warming below 2 K, these easterly anomalies support moisture transport from the Gulf of Guinea and precipitation increases. With Atlantic warming over 2 K, the easterly anomalies reverse the westerly flow over the Sahel. The resulting dry air advection into the Sahel reduces precipitation. Increased low-level moisture provides moist static energy to initiate convection with Atlantic warming at 1.5 K and below, while decreased moisture and stable thermal profiles suppress convection with additional warming. In all simulations, the southerly monsoon flow onto the Guinean coast is maintained and precipitation in that region increases. The relevance of these results to the global warming problem is limited by the focus on Atlantic warming alone. However, confident prediction of climate change requires an understanding of the physical processes of change, and this paper contributes to that goal. [ABSTRACT FROM AUTHOR]

Coupled ocean-atmosphere general circulation models (GCMs) lack sufficient resolution to model the regional detail of changes to mean circulation and rainfall with projected climate warming. In this paper, changes in mean circulation and rainfall in GCMs are compared to those in a variable resolution regional climate model, the Conformal Cubic Atmospheric Model (CCAM), under a high greenhouse gas emissions scenario. The study site is Tasmania, Australia, which is positioned within the mid-latitude westerlies of the southern hemisphere. CCAM projects a different response in mean sea level pressure and mid-latitude westerly circulation to climate warming to the GCMs used as input, and shows greater regional detail of the boundaries between regions of increasing and decreasing rainfall. Changes in mean circulation dominate the mean rainfall response in western Tasmania, whereas changes to rainfall in the East Coast are less related to mean circulation changes. CCAM projects an amplification of the dominant westerly circulation over Tasmania and this amplifies the seasonal cycle of wet winters and dry summers in the west. There is a larger change in the strength than in the incidence of westerly circulation and rainfall events. We propose the regional climate model displays a more sensitive atmospheric response to the different rates of warming of land and sea than the GCMs as input. The regional variation in these results highlight the need for dynamical downscaling of coupled general circulation models to finely resolve the influence of mean circulation and boundaries between regions of projected increases and decreases in rainfall. [ABSTRACT FROM AUTHOR]

Marine Isotope Stage (MIS) 13, an interglacial about 500,000 years ago, is unique due to an exceptionally strong East Asia summer monsoon (EASM) occurring in a relatively cool climate with low greenhouse gas concentrations (GHG). This paper attempts to find one of the possible mechanisms for this seeming paradox. Simulations with an Earth System model LOVECLIM show that the presence of ice sheets over North America and Eurasia during MIS-13 induces a positive phase of the winter North Atlantic Oscillation (NAO) like feature. The ocean having a longer memory than the atmosphere, the oceanic anomalies associated with NAO persists until summer. The signals of summer NAO are transmitted to East Asia to reinforce the monsoon there through the stationary waves excited at the Asian Jet entrance. The geopotential height shows clearly a mid-latitude wave train with positive anomalies over the eastern Mediterranean/Caspian Sea and the Okhotsk Sea and a negative anomaly over Lake Baikal. This reinforces the effect of the high-latitude wave train induced independently by the Eurasian ice sheet topography as shown in previous study. These features reinforce the Meiyu front and enhance the precipitation over East Asia. The results obtained from LOVECLIM are further confirmed by an atmospheric general circulation model, ARPEGE. [ABSTRACT FROM AUTHOR]

Large and continuing reductions in southern Australian winter rainfall are shown to be associated with major shifts in the Southern Hemisphere circulation since the mid-1970s. In particular, these changes have been linked to a reduction in winter storm formation with the growth rate of the leading storm track modes affecting southern Australia being more than 30% lower during the last three decades compared to the period between 1949 and 1968. These effects have become more pronounced with time. In this paper, we focus on the changes in southern Australian winter rainfall and relate them to circulation changes that are directly associated with storm formation. We employ a useful diagnostic of storm development which is the vertical shear in the atmospheric winds, commonly known as baroclinic instability, and is encapsulated in the Phillips (1954) criterion. The relationship between changes in the Phillips criterion and changes in rainfall during the twentieth century is discussed. We also consider projected changes and trends in rainfall and baroclinic instability in SRES scenarios using results from CMIP3 models. We elucidate the roles of anthropogenic forcing and internal variability. Our results show that the impact of further increases in anthropogenic CO2 concentrations can lead to further large reductions in baroclinic instability, with model trends during the 21st century similar to those simulated during the second half of the 20th century. Associated reductions in modelled southern Australian rainfall can be as much as twice those seen at the end of the 20th century. [ABSTRACT FROM AUTHOR]

Postglacial expansion of deciduous oak woodlands of the Zagros-Anti-Taurus Mountains, a major biome of the Near East, was delayed until the middle Holocene at ~6300 cal. yr BP. The current hypotheses explain this delay as a consequence of a regional aridity during the early Holocene, slow migration rates of forest trees, and/or a long history of land use and agro-pastoralism in this region. In the present paper, support is given to a hypothesis that suggests different precipitation seasonalities during the early Holocene compared with the late Holocene. The oak species of the Zagros-Anti-Taurus Mts, particularly Quercus brantii Lindl., are strongly dependent on spring precipitation for regeneration and are sensitive to a long dry season. Detailed analysis of modern atmospheric circulation patterns in SW Asia during the late spring suggests that the Indian Summer Monsoon (ISM) intensification can modify the amount of late spring and/or early summer rainfall in western/northwestern Iran and eastern Anatolia, which could in turn have controlled the development of the Zagros-Anti-Taurus deciduous oak woodlands. During the early Holocene, the northwestward shift of the Inter-Tropical Convergence Zone (ITCZ) could have displaced the subtropical anticyclonic belt or associated high pressure ridges to the northwest. The latter could, in turn, have prevented the southeastward penetration of low pressure systems originating from the North Atlantic and Black Sea regions. Such atmospheric configuration could have reduced or eliminated the spring precipitation creating a typical Mediterranean continental climate characterized by winter-dominated precipitation. This scenario highlights the complexity of biome response to climate system interactions in transitional climatic and biogeographical regions. [ABSTRACT FROM AUTHOR]

The Las Vegas metropolitan area in Nevada has experienced extensive urban growth since 1950 coincident with regional and local climate change. This study explores the nonstationary flood history of the Las Vegas Wash (LVW) watershed by deconstructing it into its constituent physical drivers. Observations and reanalysis products are used to examine the hydroclimatology, hydrometeorology, and hydrology of flash flooding in the watershed. Annual peak flows have increased nonlinearly over the past seven decades, with an abrupt changepoint detected in the mid-1990s, which is attributed to the implementation of flood conveyance systems rather than changes in land use. The LVW watershed exhibits two pronounced flood seasons, associated with distinct synoptic atmospheric circulations: winter floods linked to inland-penetrating atmospheric rivers and summer floods linked to the North American monsoon. El Niño–Southern Oscillation also plays a role in modulating extreme rainfall and the resultant floods because annual maximum daily rainfall totals positively correlate with El Niño, with Spearman's correlation coefficient of 0.36 (p value < 0.05). Winter maximum daily rainfall totals have increased since 1950, whereas summer daily rainfall maxima have decreased. The trends in hydrometeorological drivers interact with urbanization to shift flood seasonality toward more frequent winter floods in the LVW watershed. A process-based understanding of the flood hydrology of the watershed also provides insights into flood frequency analysis and flood forecasting. [ABSTRACT FROM AUTHOR]

Understanding the spatial–temporal changes in glacier mass balance and associated drivers on the Tibetan Plateau (TP) is important for predicting future water supplies and glacier-related hazards. However, the comparative study of changes in glacier mass balance in different regions of the same glacierized massif on the TP remains scarce. Combining the reconstructed detailed mass balance time-series from 1970 to 2015 for Ningchan No.1 Glacier in the eastern Qilian Mountains and Qiyi Glacier in the western Qilian Mountains using the energy and mass balance model in this work with the published mass balance data from different glaciers, we find that interannual changes in glacier mass balance are broadly similar in different regions of the Qilian Mountains. These interannual changes are primarily driven by variations in ablation-season (June–September) air temperature (Ta), which impact albedo and melt by changing snowfall and incoming longwave radiation (Lin). We link such interannual mass balance variability to the combination of changes in atmospheric circulation over Europe and changes in sea surface temperature (SST) in the Northwest Pacific during the ablation season which can cause the changes in Ta across the Qilian Mountains. In addition, we find a trend of increasingly negative glacier mass balance across the Qilian Mountains from 1970–1994 compared to 1995–2015. This interdecadal trend is driven by higher ablation-season Ta through increasing Lin and through increasing precipitation falling as rain. Lastly, higher glacier mass loss in the east than in the west Qilian Mountains from 1970–1994 and 1995–2015 was mainly caused by lower glacier area-altitude distribution, as well as a reduction in ablation-season precipitation in the eastern Qilian Mountains. [ABSTRACT FROM AUTHOR]

Under the intensification of global warming, the characteristics of the Three Rivers source region (TRSR; i.e., headwaters of the Yellow River, the Yangtze River, and the Lancang River) in China were diagnosed in the summer season from 1979 to 2015 using observations and reanalysis data. The diagnoses indicate that summer precipitation decreased from 1979 to 2002 [by 9.01 mm day−1 (10 yr)−1; p < 0.05 by Student's t test] and increased significantly after 2002 [by 5.52 mm day−1 (10 yr)−1]. This abrupt change year (2002) was further confirmed by the cumulative anomaly method, the moving t-test method, and the Yamamoto method. By compositing the thermodynamics before and after the abrupt change year (2002), the results reveal that increased water vapor and more substantial lower-level convergence were present over the TRSR during 2003–15. This marked interdecadal variability in the TRSR summer precipitation responded to the interdecadal position and intensity of the large-scale forcing East Asian westerly jet (EAWJ), which is significantly modulated by the low-frequency variability associated with Southern Oscillation index. The connection between the interannual TRSR precipitation and the location and intensity of EAWJ was also explored. The position index of the EAWJ is negatively (with correlation coefficient R of −0.446; p < 0.05 by Student's t test) correlated with the precipitation over the TRSR, implying that southward and northward years of EAWJ are respectively associated with intensifying and weakening the TRSR summer precipitation, whereas the intensity of EAWJ is insignificantly correlated with the TRSR summer precipitation. [ABSTRACT FROM AUTHOR]

Large-scale climate response to the spatial and temporal changes of anthropogenic aerosols are investigated in comparison with greenhouse gas (GHG) forcing using Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations during 1930–2010. Globally, increasing anthropogenic aerosols in the Northern Hemisphere (NH) since the industrial revolution generates a North–South interhemispherically asymmetric climate response pattern. But this feature only dominates the historical climate change early in the twentieth century. In the second half of the twentieth century, anthropogenic aerosol emissions increase monotonically in Asia, but increase first and then decrease rapidly over Europe and North America. The shift of aerosol emissions from the western mid-latitude NH to the eastern low-latitude NH induces distinct climate response patterns compared with its global increase effect, anchoring an equatorial symmetric tropical sea surface temperature (SST) cooling and precipitation decrease, which resembles that in the GHG-induced response patterns with sign reversed. After the mid-1970s, with declining aerosol emissions over Europe and North America and increasing emissions in Asia, anthropogenic aerosol forcing generates a northward recovery of the tropical rainfall with little response in the tropical SST anomalies due to the cancellation effect of the distinct aerosol emissions conditions between the western and the eastern NH. While GHG forcing dominates observed climate change in the twentieth century, the distinct climate response patterns to the spatial and temporal evolutions of anthropogenic aerosol forcing highlight its important role in shaping the regional climate changes. [ABSTRACT FROM AUTHOR]

In earlier work, it was proposed that the reliability of climate change projections, particularly of regional rainfall, could be improved if such projections were calibrated using quantitative measures of reliability obtained by running the same model in seasonal forecast mode. This proposal is tested for fast atmospheric processes (such as clouds and convection) by considering output from versions of the same atmospheric general circulation model run at two different resolutions and forced with prescribed sea surface temperatures and sea ice. Here output from the high-resolution version of the model is treated as a proxy for truth. The reason for using this approach is simply that the twenty-first-century climate change signal is not yet known and, hence, no climate change projections can be verified using observations. Quantitative assessments of reliability of the low-resolution model, run in seasonal hindcast mode, are used to calibrate climate change time-slice projections made with the same low-resolution model. Results show that the calibrated climate change probabilities are closer to the proxy truth than the uncalibrated probabilities. Given that seasonal forecasts are performed operationally already at several centers around the world, in a seamless forecast system they provide a resource that can be used without cost to help calibrate climate change projections and make them more reliable for users. [ABSTRACT FROM AUTHOR]

Interdecadal changes, over the last seventy years, in winter synoptic weather systems that grow and decay in the extratropical storm tracks of the Southern Hemisphere are investigated. The statistics of the intensity of growing and decaying eddies, and their preferred structures and growth rates, are determined from 6-hourly lower-tropospheric reanalysis data for high-pass filtered (period less than 4 days) and band-pass filtered (period between 4 and 8 days) disturbances. A new data-driven methodology is used in which the perturbations are spectrally analysed in both space and time, their growth rates are calculated and statistics are accumulated for growing and decaying instabilities. Rainfall variability in Southern Australia is found to be closely related to the changing statistics of weather system intensity and to interannual and decadal variability in indices or predictors that characterize local, hemispheric or global aspects of the atmospheric circulation. In investigations into the mid-1970s drying of southwestern Western Australia, the Australian Millennium Drought and Southern Australian rainfall anomalies of the early twenty-first century it is found that decadal changes in rainfall are most related to the intensity of the fast growing weather systems. Two new indices, the Subtropical Atmospheric Jet (SAJ) index and the Southern Ocean Regional Dipole (SORD) index, are introduced and shown to have some skill in characterizing aspects of Southern Australian winter rainfall variability particularly on decadal timescales. [ABSTRACT FROM AUTHOR]

The El Niño–Southern Oscillation (ENSO) has previously been shown to influence the winter North Atlantic Oscillation (NAO). Here we investigate the ENSO‐NAO teleconnection in historical and RCP8.5 scenario CMIP5 simulations and show a future strengthening of the teleconnection under RCP8.5. The teleconnection strength is associated with increased East Pacific tropical rainfall variability. Stratospheric and tropospheric teleconnection pathways are examined, with both pathways having stronger links in the future. The stratospheric pathway involves the Aleutian Low and the stratospheric polar vortex with a downward influence on the NAO. This pathway is clearest in the high‐top models that better resolve the stratosphere. The tropospheric pathway is driven by the Pacific subtropical jet strengthening and extending further into the Atlantic in the future, generating increased baroclinicity in the Caribbean and influencing the Atlantic storm track. These results suggest increasing influence of tropical rainfall on extratropical circulation in the future. Plain Language Summary: Winter weather over Europe is driven by the North Atlantic Oscillation (NAO) an atmospheric circulation pattern linked to the fluctuating strength of westerly winds across the North Atlantic. Previous work has shown that the NAO is influenced by rainfall in the tropical East Pacific linked to the El Niño–Southern Oscillation (ENSO) an irregular warming and cooling of the tropical Pacific Ocean. We show that the ENSO‐NAO link strengthens in a future warmer climate according to climate models. Models with stronger links simulate bigger year‐to‐year changes in tropical east Pacific rainfall. We investigate two pathways in which the tropical rainfall influences the NAO. In the first pathway, the rainfall is linked to a persistent low‐pressure system over the North Pacific; this then weakens the winter stratospheric polar vortex, which in turn affects the NAO. The models that best simulate the stratosphere show this pathway is stronger in the future. In the second pathway, the jet stream over the Pacific extends further into the Atlantic in the future. This affects the Atlantic storm track, which is itself closely linked to the NAO. These results suggest that tropical rainfall has an increasing influence on the winter weather over Europe in the future, which may lead to better future predictability. Key Points: CMIP5 models show a stronger link between NAO and ENSO in the future under climate changeModels with highest interannual tropical East Pacific rainfall variability show strongest effectResults show potential for increased seasonal predictability of the NAO in the future [ABSTRACT FROM AUTHOR]

While long‐term rainfall trends and related atmospheric dynamics have been researched over the past several decades across equatorial Africa, little is known about rainfall in western Uganda, a transition zone in the middle of the continent. Using satellite‐derived rainfall and reanalysis data from 1983 to 2017, this study examines atmospheric characteristics of seasons and multidecadal trends in rainfall. Most of the region has a biannual rainfall regime (i.e., the first rains within March–May and the second rains during August–November). Ascending (descending) air and increased (decreased) specific humidity are observed over western Uganda during the rainy (dry) seasons. Southeasterly air‐parcel back trajectories are common throughout western Uganda at all times except for the first dry season (i.e., December–February). For all seasons, wet days in western Uganda are characterized by increases in ascending air and specific humidity in addition to westerly flow anomalies. Wet days in most seasons also have a disproportionately high frequency of westerly back trajectories extending over the Congo Basin. These Congo westerlies are associated with more vertical ascent and a more humid middle troposphere compared to the other trajectories. Rainy seasons, especially the first rains, have gotten longer and wetter throughout western Uganda. The duration of the first rains increased by about 1 month over the 35 years; in turn, the rainfall total increased by approximately 70%. Rainfall also has increased for climatological seasons, with the exception being December–February. Increases in middle‐troposphere specific humidity and vertical ascent over time provide support for the wetting trends derived from the satellite‐derived rainfall data. Key Points: The two rainy seasons have gotten longer and wetter, with significant increases in duration and rainfall total for the first rainsRainfall increased in all seasons, except DJF, throughout the region, and southwestern Uganda had the largest increase in annual rainfallCongo westerlies contribute substantially to rainfall, especially during JJA and SON, but were not responsible for the wetting trends [ABSTRACT FROM AUTHOR]

The impacts of global warming/climate change on extreme rainfall events during the Baiu period in Japan and their dependency on weather patterns (WPs) were examined using the self-organizing map (SOM) method. To investigate the differences in climate change impacts on daily rainfall among the WPs, a SOM was applied to surface atmospheric circulation data from the Database for Policy Decision Making for Future Climate Change (d4PDF) to determine the dominant heavy rainfall WPs. The obtained SOM shows that different WPs are associated with regional variations in extreme rainfall events. Projected future changes in the occurrence of heavy rainfall displayed a non-uniform spatial distribution. To understand the spatial heterogeneous rainfall changes, the sensitivity of heavy rainfall WPs to climate forcing was evaluated. Results of the SOM analysis suggest that this regional variation in the future changes in extreme rainfall events could be attributed to sensitivity differences between WPs to the climate changes. These sensitivity differences can be attributed to the non-uniform spatial changes in the large-scale climatological background state in East Asia via modulations in the moist air intrusion into Japan. [ABSTRACT FROM AUTHOR]

Analysis of observations indicates that there was a significant decadal change in summer (June–August) mean rainfall over South Asia and Southeast Asia across the mid‐1990s, which is characterized by less rainfall over central‐northern India and northern Indo‐China Peninsula. This study investigates impacts of anthropogenic forcing on the observed decadal change across the mid‐1990s. A set of experiments using the coupled atmosphere‐ocean‐mixed‐layer model MetUM‐GOML2 has been performed to quantify the relative roles of changes in anthropogenic greenhouse gases and anthropogenic aerosols (AA). Results indicate a dominant role of anthropogenic changes in the observed decadal changes. Separately, the changes in greenhouse gases forcing play an important role in the reduction of rainfall over central‐northern India through the changes of atmospheric circulation (i.e., the local Hadley circulation and the Walker circulation), with additional contribution from changes in AA forcing. The changes in AA forcing dominate the reduction of rainfall over northern Indo‐China Peninsula due to high‐pressure anomalies over northern South Asia and the western subtropical Pacific. These high‐pressure anomalies are induced by the surface cooling, mainly via aerosol‐radiation interaction that decreases downward clear‐sky shortwave radiation over South Asia during summer and aerosol‐radiation interaction and aerosol‐cloud interaction that decrease downward shortwave radiation over the western subtropical Pacific during presummer seasons. Key Points: Anthropogenic forcing plays a dominant role for the decadal change of the SASM across the mid‐1990sGHG forcing plays an important role in the reduction of rainfall over central‐northern India via weakening the Walker circulationAA forcing dominates the reduction of rainfall over the northern Indo‐China Peninsula due to high‐pressure anomalies [ABSTRACT FROM AUTHOR]

The boreal summer monsoon (BSM), which includes the monsoons over India (IND), the western North Pacific (WNP), East Asia (EA), North America (NAM), and North Africa (NAF), shows prominent interannual variation (IAV) in summer precipitation and affects the areas with the largest populations in the world. In the present, the EOF analysis is used to extract the BSM dominant covariation mode during 1979-2014. This mode is featured by the out-of-phase rainfall IAV over the WNP compared with the other BSM members. The BSM covariation mode is closely associated with the upper- and lower-level coupled circulations, which are characterized by two anomalous zonal circulations over the tropical oceans coupled near the date line and an abnormal meridional cell over the WNP and EA regions, respectively. Furthermore, the strength of this mode depends on the phase relationship of rainfall IAV between the WNP and NAM monsoon regions, which is modulated by the seasonal evolution of ENSO events and the resultant SST anomalies (SSTAs) in the tropical Indian Ocean (TIO). The weaker mode is accompanied by the in-phase rainfall IAV between these two regions, along with the persisting ENSO events and stronger SSTAs in the TIO from winter to summer. In the years with fast-decaying ENSO events and the related weaker TIO SSTA, the out-of-phase rainfall IAV between the WNP and NAMregion takes place to enhance this mode.A series of AGCM sensitivity experiments could reproduce the anomalies of atmospheric circulation related to the distinct seasonal evolution of ENSO events. [ABSTRACT FROM AUTHOR]

Abstract: Long‐term climate simulations are performed using the IITM Earth System Model version 2 (IITM‐ESMv2), a sequel to the earlier version (IITM‐ESMv1), to assess climate variability and change with a special focus on the South Asian monsoon. Substantial improvements are incorporated in IITM‐ESMv2 to obtain a radiatively balanced global climate modeling framework. The IITM‐ESMv2 includes time‐varying aerosol forcing and land‐use land‐cover changes. Multicentury simulations corresponding to preindustrial and present‐day conditions show major improvements in capturing key aspects of time‐mean atmosphere and ocean large‐scale circulation. Representations of the Atlantic Meridional Overturning Circulation and poleward ocean heat transport, and major global climate drivers are superior to ESMv1. Teleconnections of the South Asian monsoon with climate drivers such as the El‐Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are found to be more robust in IITM‐ESMv2, thus leading to improved simulation of South Asian monsoon and its variability. The IITM‐ESMv2 takes a novel pursuit from India to contribute in the Intergovernmental Panel for Climate Change (IPCC) sixth assessment report (AR6). [ABSTRACT FROM AUTHOR]

East African hydroclimate exhibits considerable variability across a range of timescales, with implications for its population that depends on the region’s two rainy seasons. Recent work demonstrated that current state-of-the-art climate models consistently underestimate the long rains in boreal spring over the Horn of Africa while overestimating the short rains in autumn. This inability to represent the seasonal cycle makes it problematic for climate models to project changes in East African precipitation. Here we consider whether this bias also has implications for understanding interannual and decadal variability in the East African long and short rains. Using a consistent framework with an unforced multi-century global coupled climate model simulation, the role of Indo-Pacific variability for East African rainfall is compared across timescales and related to observations. The dominant driver of East African rainfall anomalies critically depends on the timescale under consideration: Interannual variations in East African hydroclimate coincide with significant sea surface temperature (SST) anomalies across the Indo-Pacific, including those associated with the El Niño-Southern Oscillation (ENSO) in the eastern Pacific, and are linked to changes in the Walker circulation, regional winds and vertical velocities over East Africa. Prolonged drought/pluvial periods in contrast exhibit anomalous SST predominantly in the Indian Ocean and Indo-Pacific warm pool (IPWP) region, while eastern Pacific anomalies are insignificant. We assessed dominant frequencies in Indo-Pacific SST and found the eastern equatorial Pacific dominated by higher-frequency variability in the ENSO band, while the tropical Indian Ocean and IPWP exhibit lower-frequency variability beyond 10 years. This is consistent with the different contribution to regional precipitation anomalies for the eastern Pacific versus Indian Ocean and IPWP on interannual and decadal timescales, respectively. In the model, the dominant low-frequency signal seen in the observations in the Indo-Pacific is not well-represented as it instead exhibits overly strong variability on subdecadal timescales. The overly strong ENSO-teleconnection likely contributes to the overestimated role of the short rains in the seasonal cycle in the model compared to observations. [ABSTRACT FROM AUTHOR]

We investigate the daily variability of the East Asian summer monsoon (EASM) by projecting daily wind anomaly data onto the two major modes of an interannual multivariate Empirical Orthogonal Functions analysis. Mode 1, closely resembling the Pacific–Japan (PJ) pattern and referred to as PJ‐mode, transits from positive to negative phase around mid‐summer consistent with the Meiyu rains predominantly being an early summer phenomenon. Mode 2, which is influenced by the Indian summer monsoon (ISM) and referred to as ISM‐mode, peaks in late July and early August and is associated with rainfall farther north over China. We then analyze the relation between the intraseasonal variation of the EASM and the Madden–Julian Oscillation (MJO) by analyzing circulation anomalies following MJO events. In the lower troposphere, the circulation anomalies associated with the MJO most strongly project on the PJ‐mode. MJO phases 1–4 (5–8) favor the positive (negative) phase of the PJ‐mode by favoring the anticyclonic (cyclonic) anomalies over the subtropical western North Pacific. In the upper troposphere, the circulation anomalies associated with the MJO project mainly on the ISM‐mode. [ABSTRACT FROM AUTHOR]

The General Circulation Models experience a significant challenge in realistically simulating rainfall over the tropical Maritime Continent (hereafter, MC). Here, we investigate the mechanisms of an improvement in monthly rainfall simulation over the MC region in the UK Met Office Unified Model (version Global Atmosphere 6.0), which occurs when the horizontal resolution is increased from N96 (grid spacing ~135 km) to N216 (~60 km). The increased resolution enhances the area-averaged rainfall rate over the MC, thereby reducing the dry rainfall bias seen in the model at the N96 resolution. We find that the enhanced area-averaged rainfall is mostly due to an increase in the medium rainfall rates that occurs over the MC islands in the N216 experiment. The rainfall change is predominantly associated with changes in the atmospheric convective circulation and the related horizontal moisture flux convergence. The vertical profiles of convective circulation show a strong sensitivity to the increased horizontal resolution over the MC islands, but not over the surrounding oceans. It is shown that a significant underestimation of the deep convection (as opposed to the shallow convection) in the N96 experiment is primarily responsible for the stronger dry bias in this experiment. We present evidence that the dry bias, and the associated weaker deep convection, are in part caused by the strongly smoothed orography used in the N96 experiment, which provides a weaker orographic lifting of the moist surface air (in a conditionally unstable atmosphere) than that in the N216 experiment. [ABSTRACT FROM AUTHOR]

ABSTRACT The seasonal and interannual variability of the mid-tropospheric Botswana High and its relationships with various characteristics of rainfall and temperature over southern Africa are examined. This High typically forms in August and then strengthens and moves southward over southern Africa during the spring and summer. It also expands in zonal extent so that by March it is part of an anticyclonic ridge extending from the South Atlantic Convergence Zone eastwards over this ocean, across southern Africa and to the western Indian Ocean, with its greatest intensity in February. Its position is always to the south and southeast of the region of heavy precipitation across tropical southern Africa associated with the Inter Tropical Convergence Zone ( ITCZ) and its meridional arm that passes through the eastern Congo Basin. Substantial interannual variability exists in the strength of the Botswana High. While the High is typically stronger (weaker) during El Niño (La Niña) events, there are also a number of neutral El Niño Southern Oscillation (ENSO) summers with large anomalies in the Botswana High. Neutral summers with an anomalously weak Botswana High are also characterized by a cyclonic anomaly extending from Angola south to the Antarctic and an anticyclonic anomaly over the South West Indian Ocean. These circulation patterns favour the development of tropical extratropical cloud bands, the main synoptic summer rainfall producing system, and the inflow of more moisture from the Indian Ocean towards the source of the cloud bands and hence increased rainfall. Roughly, the reverse circulation anomalies occur during neutral summers with anomalously large values of the Botswana High. Relationships are also found between the Botswana High and dry spell frequency, maximum temperature, diurnal temperature range and the number of days with extreme temperatures during the summer. [ABSTRACT FROM AUTHOR]

ABSTRACT This study investigates impacts of two types of La Niña events, eastern Pacific ( EP) La Niña and central Pacific ( CP) La Niña, on Australian summer rainfall during 1951-2009. Results show that Australian summer rainfall is sensitive to the change in the location of sea surface temperature ( SST) anomalies in equatorial Pacific. During CP La Niña, maximum cold SST anomaly is located in the equatorial CP west of 150°W, and significant northeasterly wind anomalies tend to prevail over northeastern Australia during austral summer. This brings more moist and warm flow from the tropics to Australia and leads to significant positive rainfall anomalies over northern and eastern Australia. In contrast, during EP La Niña, maximum cold SST anomaly is confined to equatorial EP east of 150°W and atmospheric circulation anomalies tend to be weak. As a result, rainfall anomalies are generally weak over Australia in EP La Niña. The differences in the Australian summer rainfall anomalies between CP La Niña and EP La Niña are attributed to the differences in atmospheric circulation anomalies. Specifically, the atmospheric circulation anomalies over tropical Pacific tend to be stronger and located more westward in CP La Niña. Higher climatological SST in the equatorial CP than equatorial EP, larger magnitude and westward shift of cold SST anomaly centre in CP La Niña than EP Niña may explain stronger and westward shift of the atmospheric anomalies in CP La Niña. Atmospheric model numerical experiments confirm the contribution of stronger circulation response in CP La Niña to the positive rainfall anomalies in Australia. Results in this study suggest that it is important to classify the La Niña events into different types when predicting Australian summer rainfall. [ABSTRACT FROM AUTHOR]

A 20-yr simulation with a fine-resolution regional atmospheric model for projected late twenty-first-century conditions in Hawaii is presented. The pseudo-global-warming method is employed, and the boundary conditions are based on a multimodel mean of projections made with global coupled models run with a moderate greenhouse gas emissions scenario. Results show that surface air temperature over land increases ~2°-4°C with the greatest warming at the highest topographic heights. A modest tendency for the warming to be larger on the leeward sides of the major islands is also apparent. Climatological rainfall is projected to change up to ~25% at many locations. The currently wet windward sides of the major islands will have more clouds and receive more rainfall, while the currently dry leeward sides will generally have even less clouds and rainfall. The average trade wind inversion-base height and the mean marine boundary layer cloud heights are projected to exhibit only small changes. However, the frequency of days with clearly defined trade wind inversions is predicted to increase substantially (~83% to ~91%). The extreme rainfall events are projected to increase significantly. An analysis of the model's moisture budget in the lower troposphere shows that the increased mean rainfall on the windward sides of the islands is largely attributable to increased boundary layer moisture in the warmer climate. Rainfall changes attributable to mean low-level circulation changes are modest in comparison. [ABSTRACT FROM AUTHOR]

This study was conducted to examine the correlation between the South Asian High (SAH) shift and the summer (June-August) rainfall variability in Korea over the course of 45 years (1958-2002). It is found that the shift in the west-east direction is closely related to the summer rainfall variability in Korea. The atmospheric circulations in the east shift years showed that at 850 hPa, an intensification of the western North Pacific subtropical high allowed a sufficient amount of warm and humid air to be supplied to the lower atmosphere near Korea. On the other hand at 200 hPa, the SAH expanded toward the east, near the southern part of Korea, causing the intensification of the northwesterlies, and this allowed the supply of cold and dry air to the upper atmosphere near Korea. Meanwhile, in the latitudinal zone in which Korea is located, there were anomalous upward flows with warm and humid characteristics in the lower atmosphere and anomalous downward flows with cold and dry characteristics in the upper atmosphere. As such, the cold and warm air masses present in the upper and lower atmosphere near Korea led to thermal instability, and this was analyzed to be a major factor that could potentially increase the amount rainfall in Korea in summer in the east shift years. Moreover, a warm SST anomaly was formed in the mid-latitude zones of East Asia, and it heated the lower troposphere, thereby intensifying the thermal instability even further. Furthermore, TCs frequently had an impact on Korea in the east shift years, and were another factor that caused the increased rainfall in summer. [ABSTRACT FROM AUTHOR]

In order to find out the optimal setting for downscaling the Coupled Model Inter Comparison Project Phase 5 (CMIP5) database, this study describes the most suitable domain, convection and land-surface schemes within Regional Climate Model version 4 (RegCM4) over the Middle East and North Africa (MENA) domain. The activity is carried out in compliance with COordinated Regional climate Downscaling EXperiment (CORDEX) initiative, where domain suitability is assessed through seven simulations for the period 2001-2005. After the selection of domain, eight simulations are carried out to find better performing convection and land-surface schemes within RegCM4 for the same time period of 5 years. These experiments are conducted using ERA-Interim reanalysis datasets; however, the selection of optimal settings is also validated using other reanalysis datasets. Finally, using the same settings, data of five CMIP5 general circulation models (GCMs) are also downscaled to assess the applicability of RegCM4. Statistical measures, such as correlation coefficient, bias, root mean square difference and standard deviation, are taken into consideration to compare the simulated data against the gridded Climatic Research Unit (CRU) and Tropical Rainfall Measuring Mission (TRMM) datasets. RegCM4-simulated rainfall and temperature show better spatial distributions and magnitudes against observations, as compared to ERA-Interim data, which may be considered as an added value of the model. In analysing rainfall and temperature data for the 11 sub-domains (each an 8° x 8° box), the region encompassing the area 7°S-45°N and 27°W-76°E, named CORDEX-MENA/Arab domain, is found to be the most cost effective and suitable domain. Further analysis over this domain shows Biosphere and Atmosphere Transfer Scheme (BATS) to be the better performing land-surface scheme, whilst Grell with Fritsch--Chappell closure (GFC) comes up as the most suitable option for convection scheme in the model. Of five CMIP5 datasets downscaled using RegCM4, rainfall and temperature patterns of four are similar to the reanalysis and observations. Therefore, it is recommended that RegCM4 may be employed with the aforementioned settings in downscaling the CMIP5 multi-model database for climate change impact and adaptation studies for the region in future. [ABSTRACT FROM AUTHOR]

Numerous studies have reported significant declines in snow resources in Finland and elsewhere during the 20th century. To identify the main climate factors controlling these declines in Finland, this study evaluated long-term variations and trends in wintertime climate, snowpack hydrological processes (SHPs) and continuous snow cover duration (CSCD), and their links to atmospheric circulation patterns (ACPs). Analyses were conducted using observed daily climatological time series and simulated SHPs at three stations in southern (Kaisaniemi), central (Kajaani) and northern (Sodankylä) Finland with about 100 years of data. The Mann-Kendall nonparametric test was used to detect significant trends, the Pearson's coefficient (r) to identify relationships within snow-related variables, and Spearman's coefficient (ρ) to measure correlations of these variables with ACPs. Sensitivity of the snow-related variables with projected changes in temperature and precipitation was assessed. The results showed increases in wintertime temperature only at Kaisaniemi, but decreases in wintertime precipitation, snowfall and snow water equivalent (SWE) and shorter CSCD at all stations. In general, variations in wintertime temperature were positively associated with the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). However, wintertime precipitation showed significant relationships with the East Atlantic/West Russia (EA/WR), AO and West Pacific (WP) patterns in southern, central and northern Finland, respectively. SHPs and CSCD in southern Finland were associated with the same ACP influencing wintertime temperature (AO), and those in central and northern areas with the patterns influencing wintertime precipitation (EA, EA/WR and AO). Thus, declines in snow resources in Finland are mainly the result of reductions in snowfall owing to both wintertime warming and decreased precipitation at Kaisaniemi, while only to decreases in wintertime precipitation at Kajaani and Sodankylä stations. However, increase in precipitation (up to 30%) plays an important role in offsetting effects of temperature warming (up to 4 °C) on snow resource decline in Finland. [ABSTRACT FROM AUTHOR]

This review provides a synopsis of the major progress that has been made in monsoon studies in China and to further bridge the gap between the Chinese and international meteorological community. It consists of seven major sections. After the introduction, the second section begins with the global monsoon systems and their seasonal variation, based on some new methods proposed in recent years. Besides, some major intraseasonal features of East Asian monsoon, including the onset of South China Sea summer monsoon are discussed. In the third section, we review the interactions between ENSO and the East Asian monsoon, focusing in particular on the results of Chinese meteorologists that indicate the influence of ENSO on the East Asian summer monsoon (EASM) is obviously different from that on the tropical monsoon. Besides the tropical Pacific, other ocean basins, such as the Indian Ocean and the Atlantic Ocean, are also important to the East Asian monsoon, and this topic is discussed in the fourth section. In the fifth section, we address the role of land surface processes in East Asian monsoon. For example, we describe work that has shown more snow cover in spring on the Tibetan Plateau is followed by a weakened EASM and more summer rainfall in the Yangtze River valleys. The sixth section focuses on the influence of atmospheric circulation in the Southern Hemisphere (SH) on EASM, demonstrating how the signal from the SH is likely to provide new clues for the seasonal forecasting of summer rainfall in China. Finally, in the seventh section, we concentrate on the interdecadal variations of EASM. In particular, we look at a significant interdecadal variation that occurred at the end of the 1970s, and how our understanding of this feature could affect forecasting ability. [ABSTRACT FROM AUTHOR]

The present study investigates the relationships between the December-February (DJF) South Pacific tripole (SPT) sea surface temperature anomaly (SSTA) pattern and the following March-May (MAM) rainfall over eastern China based on multiple datasets. It is found that the relationships between the DJF SPT and the following MAM rainfall over eastern China are modulated by the El Niño-Southern Oscillation (ENSO). When the ENSO signal is removed, the positive DJF SPT is significantly associated with more rainfall over eastern China during the following boreal spring. However, such significant relationships disappear if ENSO is considered. After removing ENSO impacts, the possible mechanisms through which the DJF SPT impacts the following MAM rainfall over eastern China are investigated. The positive DJF SPT is associated with the significantly positive SSTA in the tropical western Pacific, which can persist to the following MAM. In response to the positive SSTA in the tropical western Pacific, a wave-like train in the low-level troposphere extends from the tropical western Pacific (an anomalous cyclone) to the western North Pacific (an anomalous anticyclone) during the following MAM. The anomalous anticyclone over the western North Pacific enhances the anomalous southwesterly over eastern China, which can bring more moisture and favor anomalous increased rainfall. It should be pointed out that La Niña (El Niño) could induce an anomalous cyclone (anticyclone) over the western North Pacific, which offsets the MAM anomalous anticyclone (cyclone) caused by the positive (negative) SPT in the preceding DJF and thus weakens the relationship between the SPT and the rainfall over eastern China. [ABSTRACT FROM AUTHOR]

Previous studies revealed the double-peak mode (DPM) in South China precipitation, corresponding to the two stages in the rainy season, i.e. the first rainy stage (FRS) and the second rainy stage (SRS). But observations in recent two decades show that the DPM has changed to a single-peak mode (SPM). Both the precipitation amount and the heavy rainfall event frequency enhanced significantly in the gap between the FRS and the SRS in 1991–2010, compared to those in 1961-1990. This change can be linked to the effects of the global warming. During the warmer period, the July sea surface temperature over the western Pacific has greater increases than that over the central and eastern Pacific, especially west of 140°E. It may generate more tropical cyclones (TCs) in the inshore areas and then more typhoon rainfall over South China. On the other hand, the increments of the air temperature over the East Asian continent are greater than those of the SST over the western Pacific under the global warming, which enlarges the land-ocean temperature/pressure contrast and leads to a trend of the earlier onset dates of the Asian summer monsoon (ASM) in recent two decades. Then, the earlier ASM will facilitate the western Pacific subtropical high (WPSH) to retreat earlier from the South China Sea and enhance the convective precipitation in South China between the FRS and the SRS. Also, due to the warmer ocean, the WPSH locates more westward in July, and more moisture will be transported to South China from the southwest side to the WPSTH. All these influences favor a remarkably increasing precipitation in the gap in the warmer period and changes the seasonal cycle from double-peak mode to single-peak mode. [ABSTRACT FROM AUTHOR]

Projection of future climate changes and their regional impact is critical for long-term planning at the national and regional levels aimed at adaptation and mitigation. This study assesses the future changes in precipitation in China and the associated atmospheric circulation patterns using the Couple Model Intercomparison Project 5 Phase (CMIP5) simulations under the RCP4.5 and RCP8.5 scenarios. The results consistently indicate that the annual precipitation in China is projected to significantly increase at the end of the 21st century compared to the present-day levels. The number of days and the intensity of medium rain, large rain and heavy rain are obviously increased, while the number of trace rain days is projected to decrease over the entire area of China. Further analysis indicates that the significant increase of annual precipitation in Northwest China is primarily due to the increase of light rain and the increases in North and Northeast China are primarily due to the increase of medium rain. In the region of southern China, the increases of large rain and heavy rain play an important role in the increase of annual precipitation, while light rain events play a negative role. Analysis of the changes in atmospheric circulation indicates that the East Asian summer monsoon circulation is projected to be considerably stronger, and the local atmospheric stratification is projected to be more unstable, all of which provide a background benefit for the increase of precipitation and extreme rainfall events in China under global warming scenarios. [ABSTRACT FROM AUTHOR]

Data analysis based on station observations reveals that many meteorological variables averaged over the Tibetan Plateau (TP) are closely correlated, and their trends during the past decades are well correlated with the rainfall trend of the Asian summer monsoon. However, such correlation does not necessarily imply causality. Further diagnosis confirms the existence of a weakening trend in TP thermal forcing, characterized by weakened surface sensible heat flux in spring and summer during the past decades. This weakening trend is associated with decreasing summer precipitation over northern South Asia and North China and increasing precipitation over northwestern China, South China, and Korea. An atmospheric general circulation model, the HadAM3, is employed to elucidate the causality between the weakening TP forcing and the change in the Asian summer monsoon rainfall. Results demonstrate that a weakening in surface sensible heating over the TP results in reduced summer precipitation in the plateau region and a reduction in the associated latent heat release in summer. These changes in turn result in the weakening of the near-surface cyclonic circulation surrounding the plateau and the subtropical anticyclone over the subtropical western North Pacific, similar to the results obtained from the idealized TP experiment in Part I of this study. The southerly that normally dominates East Asia, ranging from the South China Sea to North China, weakens, resulting in a weaker equilibrated Sverdrup balance between positive vorticity generation and latent heat release. Consequently, the convergence of water vapor transport is confined to South China, forming a unique anomaly pattern in monsoon rainfall, the so-called 'south wet and north dry.' Because the weakening trend in TP thermal forcing is associated with global warming, the present results provide an effective means for assessing projections of regional climate over Asia in the context of global warming. [ABSTRACT FROM AUTHOR]

Numerical experiments with different idealized land and mountain distributions are carried out to study the formation of the Asian monsoon and related coupling processes. Results demonstrate that when there is only extratropical continent located between 0 and 120°E and between 20/30°N and the North Pole, a rather weak monsoon rainband appears along the southern border of the continent, coexisting with an intense intertropical convergence zone (ITCZ). The continuous ITCZ surrounds the whole globe, prohibits the development of near-surface cross-equatorial flow, and collects water vapor from tropical oceans, resulting in very weak monsoon rainfall. When tropical lands are integrated, the ITCZ over the longitude domain where the extratropical continent exists disappears as a consequence of the development of a strong surface cross-equatorial flow from the winter hemisphere to the summer hemisphere. In addition, an intense interaction between the two hemispheres develops, tropical water vapor is transported to the subtropics by the enhanced poleward flow, and a prototype of the Asian monsoon appears. The Tibetan Plateau acts to enhance the coupling between the lower and upper tropospheric circulations and between the subtropical and tropical monsoon circulations, resulting in an intensification of the East Asian summer monsoon and a weakening of the South Asian summer monsoon. Linking the Iranian Plateau to the Tibetan Plateau substantially reduces the precipitation over Africa and increases the precipitation over the Arabian Sea and the northern Indian subcontinent, effectively contributing to the development of the South Asian summer monsoon. [ABSTRACT FROM AUTHOR]

With the twentieth century analysis data (1901-2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon. Associated with a positive APO phase are the warm troposphere over the Eurasian land and the relatively cool troposphere over the North Pacific, the North Atlantic, and the Indian Ocean. Such an amplified land-ocean thermal contrast between the Eurasian land and its adjacent oceans signifies a stronger than normal NH summer monsoon, with the strengthened southerly or southwesterly monsoon prevailing over tropical Africa, South Asia, and East Asia. A positive APO implies an enhanced summer monsoon rainfall over all major NH land monsoon regions: West Africa, South Asia, East Asia, and Mexico. Thus, APO is a sensible measure of the NH land monsoon rainfall intensity. Meanwhile, reduced precipitation appears over the arid and semiarid regions of northern Africa, the Middle East, and West Asia, manifesting the monsoon-desert coupling. On the other hand, surrounded by the cool troposphere over the North Pacific and North Atlantic, the extratropical North America has weakened low-level continental low and upper-level ridge, hence a deficient summer rainfall. Corresponding to a high APO index, the African and South Asian monsoon regions are wet and cool, the East Asian monsoon region is wet and hot, and the extratropical North America is dry and hot. Wet and dry climates correspond to wet and dry soil conditions, respectively. The APO is also associated with significant variations of SST in the entire Pacific and the extratropical North Atlantic during boreal summer, which resembles the Interdecadal Pacific Oscillation in SST. Of note is that the Pacific SST anomalies are not present throughout the year, rather, mainly occur in late spring, peak at late summer, and are nearly absent during boreal winter. The season-dependent APO-SST relationship and the origin of the APO remain elusive. [ABSTRACT FROM AUTHOR]

We use a coupled atmosphere-ocean global climate model (CSIRO-Mk3.6) to investigate the drivers of trends in summer rainfall and circulation in the vicinity of northern Australia. As part of the Coupled Model Inter comparison Project Phase 5 (CMIP5), we perform a 10-member 21st century ensemble driven by Representative Concentration Pathway 4.5 (RCP4.5). To investigate the roles of different forcing agents, we also perform multiple 10-member ensembles of historical climate change, which are analysed for the period 1951-2010. The historical runs include ensembles driven by "all forcings" (HIST), all forcings except anthropogenic aerosols (NO AA) and forcing only from long lived greenhouse gases (GHGAS). Anthropogenic aerosol- induced effects in a warming climate are calculated from the difference of HIST minus NO AA. CSIRO-Mk3.6 simulates a strong summer rainfall decrease over north-western Australia (NWA) in RCP4.5, whereas simulated trends in HIST are weakly positive (but insignificant) during 1951-2010. The weak rainfall trends in HIST are due to compensating effects of different forcing agents: there is a significant decrease in GHGAS, offset by an aerosol-induced increase. Observations show a significant increase of summer rainfall over NWA during the last few decades. The large magnitude of the observed NWA rainfall trend is not captured by 440 unforced 60-yr trends calculated from a 500-yr preindustrial control run, even though the model's decadal variability appears to be realistic. This suggests that the observed trend includes a forced component, despite the fact that the model does not simulate the magnitude of the observed rainfall increase in response to "all forcings" (HIST). We investigate the mechanism of simulated and observed NWA rainfall changes by exploring changes in circulation over the Indo-Pacific region. The key circulation feature associated with the rainfall increase in reanalyses is a lower-tropospheric cyclonic circulation trend off the coast of NWA, which enhances the monsoonal flow. The model shows an aerosol-induced cyclonic circulation trend off the coast of NWA in HIST minus NO AA, whereas GHGAS shows an anticyclonic circulation trend. This explains why the aerosol-induced effect is an increase of rainfall over NWA, and the greenhouse gas-induced effect is of opposite sign. Possible explanations for the cyclonic (anticyclonic) circulation trend in HIST minus NO AA (GHGAS) involve changes in the Walker circulation or the local Hadley circulation. In either case, a plausible atmospheric mechanism is that the circulation anomaly is a Rossby wave response to convective heating anomalies south of the Equator. We also discuss the possible role of air-sea interactions, e.g. an increase (decrease) of sea-surface temperatures off the coast of NWA in HIST minus NO AA (GHGAS). Further research is needed to better understand the mechanisms and the extent to which these are model-dependent. In summary, our results suggest that anthropogenic aerosols may have "masked" greenhouse gas-induced changes in rainfall over NWA and in circulation over the wider Indo-Pacific region. Due to the opposing effects of greenhouse gases and anthropogenic aerosols, future trends may be very different from trends observed over the last few decades. [ABSTRACT FROM AUTHOR]

Widely distributed proxy records indicate that the Medieval Climate Anomaly (MCA; ~900-1350 AD) was characterized by coherent shifts in large-scale Northern Hemisphere atmospheric circulation patterns. Although cooler sea surface temperatures in the central and eastern equatorial Pacific can explain some aspects of medieval circulation changes, they are not sufficient to account for other notable features, including widespread aridity through the Eurasian sub-tropics, stronger winter westerlies across the North Atlantic and Western Europe, and shifts in monsoon rainfall patterns across Africa and South Asia. We present results from a full-physics coupled climate model showing that a slight warming of the tropical Indian and western Pacific Oceans relative to the other tropical ocean basins can induce a broad range of the medieval circulation and climate changes indicated by proxy data, including many of those not explained by a cooler tropical Pacific alone. Important aspects of the results resemble those from previous simulations examining the climatic response to the rapid Indian Ocean warming during the late twentieth century, and to results from climate warming simulations-especially in indicating an expansion of the Northern Hemisphere Hadley circulation. Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST) change responsible for producing the proxy-model similarity in our results agrees well with MCA-LIA SST differences obtained in a recent proxy-based climate field reconstruction. Though much remains unclear, our results indicate that the MCA was characterized by an enhanced zonal Indo-Pacific SST gradient with resulting changes in Northern Hemisphere tropical and extra-tropical circulation patterns and hydroclimate regimes, linkages that may explain the coherent regional climate shifts indicated by proxy records from across the planet. The findings provide new perspectives on the nature and possible causes of the MCA-a remarkable, yet incompletely understood episode of Late Holocene climatic change. [ABSTRACT FROM AUTHOR]

Following diagnostic work two decades ago, this study examines the long-term variations of rainfall in the West African Sahel and the underlying circulation mechanisms over the past century. Raingauge measurements bear out DRY regimes during 1910-1919 and 1979-1988 and WET regimes during 1950-1959 and 1999-2008. In the WET as compared to the DRY regimes the cross-equatorial winds from the Southern Hemisphere are enhanced, consistent with lower surface pressure over the tropical North Atlantic, which in terms of hydrostatic forcing plausibly accompanies warmer surface waters, with cooling/warming leading pressure rise/drop in the tropical North Atlantic and DRY/WET in the Sahel. The controls of the observed long-term variations of sea surface temperature in the tropical North Atlantic must remain target of further diagnostic evaluation. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Three formulations of a non-parametric stochastic downscaling framework for simulation of daily rainfall at multiple point locations for catchment-scale climate change impact assessment under enhanced greenhouse conditions are evaluated. In the first case, rainfall is downscaled using a system of variables that combine atmospheric circulation indicators (geo-potential height, pressure and derived variables) with time-lagged wetness indicators reflecting a continuous rainfall state. In the second and third cases, the above system of variables is augmented using indicators of atmospheric moisture found relevant in defining the downscaling relationship. The downscaling framework is specified using 43 years of daily rainfall observations at 30 locations near Sydney, Australia, with reanalysis data being used to represent observed atmospheric variables. A single ensemble member of the CSIRO Mark 3 general circulation model (GCM) (corresponding to the SRES A2 emission scenario) is used for comparison of downscaled rainfall for current and future (year 2070) conditions. While all the downscaling formulations show an overall similarity in the downscaled rainfall for the current climate, variations for some rainfall attributes are observed for year 2070 conditions. A variable convergence score (VCS) is employed to assess the relative skill of atmospheric predictors used in the different formulations. The VCS indicates use of temperature depression (difference of dew point and atmospheric temperatures) as an indicator of atmospheric moisture to obtain more realistic estimates of rainfall for year 2070. In general, a decrease in winter and spring and an increase in summer in the frequency of wet days with a decrease in the winter rainfall amount in the year 2070 is projected. Also, inland stations are found to be more sensitive to the expected future climate changes in comparison to the coastal ones. Copyright © 2009 Royal Meteorological Society [ABSTRACT FROM AUTHOR]