Seasonal Precipitation Variability Research Articles

Pliocene flora and paleoenvironment of Zanda Basin, Tibet, China

This paper describes a plant megafossil assemblage from the Pliocene strata of Xiangzi, Zanda Basin in the western Qinghai-Tibet Plateau. Twenty-one species belonging to 12 genera and 10 families were identified. Studies show that the Pliocene vegetation in Zanda Basin was mostly deciduous shrub composed of Cotoneaster , Spiraea , Caragana , Hippophae , Rhododendron , Potentilla fruticosa , etc. Leaf sizes of these taxa were generally small. Paleoclimate reconstruction using Coexistence Analysis and CLAMP showed that this area had higher temperature and precipitation in the Pliocene than today, and distinct seasonal precipitation variability was established. The reconstructed paleoelevation of Zanda Basin in the Pliocene was similar to modern times. In the context of central Asian aridification, the gradual drought in the area beginning in the late Cenozoic caused vegetation to transition from shrub to desert, and the flora composition also changed.

Effects of a stochastic multicloud parameterization on the simulated Asian‐Australian monsoon rainfall in an AGCM

AbstractThe Asian‐Australian monsoon (AAM) plays a vital role in modulating the global and local climates. Current numerical models, including climate system models (CSMs) and atmospheric general circulation models (AGCMs), have difficulties in capturing the fundamental and intrinsic features of the AAM. By applying the stochastic multicloud model (SMCM) to the state‐of‐the‐art ECHAM6.3 atm model, we investigated the impacts of SMCM on the simulation of AAM precipitation. Though the modified model with the SMCM improves simulations of the AAM precipitation in June–July–August–September season, the solstice mode, the AAM domain, and the AAM precipitation intensity, deficiencies also exist in other aspects, that is, simulations of rainfall in December–January–February–March and April–May seasons, and the equinox mode. Analyses also unravelled the modified model improves in simulating the spatial patterns of seasonal variation of precipitation in the Indian monsoon region, western North Pacific monsoon region, and Australian monsoon region in terms of pattern correlation coefficient and normalized root‐mean square error scores. The enhanced simulation of solar radiation in the modified model favours improving the performance of the simulated solstice mode. In addition, moisture budget analysis was applied to investigate physical processes that modulate the variations and distributions of AAM precipitation. Analyses revealed that variations and distributions of the vertical integral of moisture flux convergence is closely associated with changes in precipitation, while the stratification of vertical moist transport is associated with changes in precipitation over land in the AAM region. In addition, the modified model influences the AAM precipitation mainly through the variations of moisture flux convergence. The present work potentially provides a method to improve the simulated capability of AGCMs.

Variations of Drought Tendency, Frequency, and Characteristics and Their Responses to Climate Change under CMIP5 RCP Scenarios in Huai River Basin, China

Huai River Basin (HRB) is an important food and industrial production area and a frequently drought-affected basin in eastern China. It is necessary to consider the future drought development for reducing the impact of drought disasters. Three global circulation models (GCMs) from Coupled Model Intercomparison Project phase 5 (CMIP5), such as CNRM-CM5 (CNR), HadGEM2-ES (Had) and MIROC5 (MIR), were used to assessment the future drought conditions under two Representative Concentration Pathways (RCPs) scenarios, namely, RCP4.5 and RCP8.5. The standardized precipitation evapotranspiration index (SPEI), statistical method, Mann-Kendall test, and run theory were carried out to study the variations of drought tendency, frequency, and characteristics and their responses to climate change. The research showed that the three CMIP5 models differ in describing the future seasonal and annual variations of precipitation and temperature in the basin and thus lead to the differences in describing drought trends, frequency, and drought characteristics, such as drought severity, drought duration, and drought intensity. However, the drought trend, frequency, and characteristics in the future are more serious than the history. The drought frequency and characteristics tend to be strengthened under the scenario of high concentration of RCP8.5, and the drought trend is larger than that of low concentration of RCP4.5. The lower precipitation and the higher temperature are the main factors affecting the occurrence of drought. All three CMIP5 models show that precipitation would increase in the future, but it could not offset the evapotranspiration loss caused by significant temperature rise. The serious risk of drought in the future is still higher. Considering the uncertainty of climate models for simulation and prediction, attention should be paid to distinguish the effects of different models in the future drought assessment.

The Influence of Synoptic-Scale Air Mass Conditions on Seasonal Precipitation Patterns over North Carolina

This paper characterizes the influence of synoptic-scale air mass conditions on the spatial and temporal patterns of precipitation in North Carolina over a 16-year period (2003–2018). National Center for Environmental Prediction Stage IV multi-sensor precipitation estimates were used to describe seasonal variations in precipitation in the context of prevailing air mass conditions classified using the spatial synoptic classification system. Spatial analyses identified significant clustering of high daily precipitation amounts distributed along the east side of the Appalachian Mountains and along the Coastal Plains. Significant and heterogeneous clustering was prevalent in summer months and tended to coincide with land cover boundaries and complex terrain. The summer months were dominated by maritime tropical air mass conditions, whereas dry moderate air mass conditions prevailed in the winter, spring, and fall. Between the three geographic regions of North Carolina, the highest precipitation amounts were received in western North Carolina during the winter and spring, and in eastern North Carolina in the summer and fall. Central North Carolina received the least amount of precipitation; however, there was substantial variability between regions due to prevailing air mass conditions. There was an observed shift toward warmer and more humid air mass conditions in the winter, spring, and fall months throughout the study period (2003–2018), indicating a shift toward air mass conditions conducive to higher daily average rain rates in North Carolina.

Temporal and Spatial Characteristics of Precipitation and Temperature in Punjab, Pakistan

Identifying the changes in precipitation and temperature at a regional scale is of great importance for the quantification of climate change. This research investigates the changes in precipitation and surface air temperature indices in the seven irrigation zones of Punjab Province during the last 50 years; this province is a very important region in Pakistan in terms of agriculture and irrigated farming. The reliability of the data was examined using double mass curve and autocorrelation analysis. The magnitude and significance of the precipitation and temperature were visualized by various statistical methods. The stations’ trends were spatially distributed to better understand climatic variability across the elevation gradient of the study region. The results showed a significant warming trend in annual Tmin (minimum temperature) and Tmean (mean temperature) in different irrigation zones. However, Tmax (maximum temperature) had insignificant variations except in the high elevation Thal zone. Moreover, the rate of Tmin increased faster than that of Tmax, resulting in a reduction in the diurnal temperature range (DTR). On a seasonal scale, warming was more pronounced during spring, followed by that in winter and autumn. However, the summer season exhibited insignificant negative trends in most of the zones and gauges, except in the higher-altitude Thal zone. Overall, Bahawalpur and Faisalabad are the zones most vulnerable to warming annually and in the spring, respectively. Furthermore, the elevation-dependent trend (EDT) indicated larger increments in Tmax for higher-elevation (above 500 m a.s.l.) stations, compared to the lower-elevation ones, on both annual and seasonal scales. In contrast, the Tmin showed opposite trends at higher- and lower-elevation stations, while a moderate increase was witnessed in Tmean trends from lower to higher altitude over the study region. An increasing trend in DTR was observed at higher elevation, while a decreasing trend was noticed at the lower-elevation stations. The analysis of precipitation data indicated wide variability over the entire region during the study period. Most previous studies reported no change or a decreasing trend in precipitation in this region. Conversely, our findings indicated the cumulative increase in annual and autumn precipitation amounts at zonal and regional level. However, EDT analysis identified the decrease in precipitation amounts at higher elevation (above 1000 m a.s.l.) and increase at the lower-elevation stations. Overall, our findings revealed unprecedented evidence of regional climate change from the perspectives of seasonal warming and variations in precipitation and temperature extremes (Tmax and Tmin) particularly at higher-elevation sites, resulting in a variability of the DTR, which could have a significant influence on water resources and on the phenology of vegetation and crops at zonal and station level in Punjab.

Seasonal and synoptic climatic drivers of tree growth in the Bighorn Mountains, WY, USA (1654–1983 CE)

In the United States’ (US) Northern Rockies, synoptic pressure systems and atmospheric circulation drive interannual variation in seasonal temperature and precipitation. The radial growth of high-elevation trees in this semi-arid region captures this temperature and precipitation variability and provides long time series to contextualize instrumental-era variability in synoptic-scale climate patterns. Such variability in climate patterns can trigger extreme climate events, such as droughts, floods, and forest fires, which have a damaging impact on human and natural systems. We developed 11 tree-ring width (TRW) chronologies from multiple species and sites to investigate the seasonal climatic drivers of tree growth in the Bighorn Mountains, WY. A principal component analysis of the chronologies identified 54% of shared common variance (1894–2014). Tree growth (expressed by PC1) was driven by multiple seasonal climate variables: previous October and current July temperatures, as well as previous December and current April precipitation, had a positive influence on growth, whereas growth was limited by July precipitation. These seasonal growth-climate relationships corresponded to circulation patterns at higher atmospheric levels over the Bighorn Mountains. Tree growth was enhanced when the winter jet stream was in a northward position, which led to warmer winters, and when the spring jet stream was further south, which led to wetter springs. The second principal component, explaining 19% of the variance, clustered sites by elevation and was strongly related to summer temperature. We leverage this summer temperature signal in our TRW chronologies by combining it with an existing maximum latewood density (MXD) chronology in a nested approach. This allowed us to reconstruct Bighorn Mountains summer (June, July, and August) temperature (BMST) back to 1654, thus extending the instrumental temperature record by 250 years. Our BMST reconstruction explains 39–53% of the variance in regional summer temperature variability. The 1830s were the relatively coolest decade and the 1930s were the warmest decade over the reconstructed period (1654–1983 CE) – which excludes the most recent 3 decades. Our results contextualize recent drivers and trends of climate variability in the US Northern Rockies, which contributes to the information that managers of human and natural systems need in order to prepare for potential future variability.

Climate change and its potential impact on the conservation of the Hoary Fox, Lycalopex vetulus (Mammalia: Canidae)

We aimed to assess the potential impact of climate change on the geographic distribution of areas holding suitable climatic conditions for the presence of Lycalopex vetulus, and to discuss the implications of such distribution for the conservation of the species. We employed correlative modeling analyses to infer the geographic distribution of climatically suitable conditions for the species on climatic scenarios for the present and for years 2050 (average for 2041–2060) and 2070 (average for 2061–2080). The data consisted of species occurrences and 5 bioclimatic variables containing interpolated and averaged information on seasonal variation of temperature and precipitation. Models were projected onto climatic scenarios employing three different global circulation models and two representative concentration pathways. For each of these scenarios, we quantified the expected changes in area (km2) holding suitable climatic conditions and how much of that area is expected to be within the current Brazilian system of protected areas. Our results reveal that climate change represents a major threat for the survival of L. vetulus by drastically reducing its habitat availability in a period of time no longer than five decades from now. Experimental physiological and behavioral studies are necessary to assess whether L. vetulus is able to adequately tolerate under climatic conditions different to those under which the species is currently present.

Emerging Asian aerosol patterns

Anthropogenic aerosol emissions over Asia are changing rapidly, both in composition and spatial distribution1. The Shared Socioeconomic Pathways (SSPs), potential narratives of development used by the Intergovernmental Panel for Climate Change in future projections, span a range of influences of aerosols on climate over the next decades. Several of these narratives project the continuation of a trend manifested in observations since 2010, with a clear dipole between South and East Asia. The patterns of radiative forcing that result from these distributions of aerosols will differ from those of the late 20th century. They may instigate large-scale atmospheric responses that could have wide ranging impacts on climate and society well beyond the aerosol source regions. South and East Asia are particularly vulnerable to climate change because of strong seasonal variations in precipitation, high average temperature, and very high population density. Therefore, any aerosol impacts on the strength or seasonal variations in monsoon rainfall, freshwater availability, or climate extremes, will incur large societal costs. We urge the scientific community to make definite progress towards understanding and quantifying the impacts of Asian aerosols and to tackle the potentially large regional and hemispheric implications of these emerging trends.

Performance evaluation of AR5-CMIP5 models for the representation of seasonal and multi-annual variability of precipitation in Brazilian hydropower sector basins under RCP8.5 scenario

ABSTRACTRepresentations of precipitation from CMIP5 models over the 1950–1999 period in hydrographic basins that are relevant to the Brazilian electricity sector are evaluated in this study. The majority of ensemble members adequately represented seasonal variability, although they differed about the patterns of high-frequency interannual variation. The models did not adequately represent seasonal-scale precipitation in the southern region of Brazil. Relative to other models, the CNRM_CM5 and HadGEM2-ES models demonstrated good seasonal and interannual representation over most basins, while the global CanESM2, GFDL-ESM2M and IPSL_CM5A-LR models demonstrated relatively poor performance. The models concur on the impact of the RCP8.5 scenario in the Southeast/Midwest and South sectors over the period 2015–2044, suggesting that precipitation will decrease up to 15% in the basin supplying the Furnas hydropower plant and by 12% in the basin supplying the Itaipú plant, which represents 25% of the hydroelectric production in Brazil.

Spatial pattern and seasonal variation of alkaline precipitation observed in the Gansu Province, NW China

In contrast to the term acid rain, alkaline precipitation or basic precipitation with pH value > 7 has been measured in many regions, and is considered to be an indicator of a regional atmospheric environment. Based on an observation network including 14 stations in northwestern China’s Gansu Province, the pH values in precipitation were measured during 2011–2015, and the spatial pattern and seasonal variation of precipitation acidity were investigated. In the Gansu Province, the precipitation is alkaline for most events, and precipitation with pH < 5.6 is very rare. The atmospheric environment of the study region is greatly influenced by the Asian dust sources in which the surface soil is alkaline, and the measured alkaline precipitation is related to the high dust loading. However, in some industrial cities like Jinchang and Lanzhou, the acidity in precipitation is usually stronger than the others. The seasonal variation of precipitation acidity is complex, which is jointly influenced by emission, transport and deposition. For extremely arid Northwest Gansu where deserts are located, the pH value is much high; for semi-arid and semi-humid East Gansu, the seasonal variation is not spatially coherent; for cold Southwest Gansu at high elevations, the precipitation acidity is relatively stable; for South Gansu in a humid climate, the large fluctuation is related to the complex moisture regimes of the monsoons.

Multidecadal variability in Atlas cedar growth in Northwest Africa during the last 850 years: Implications for dieback and conservation of an endangered species

Widespread forest dieback is a phenomenon of global concern that requires an improved understanding of the relationship between tree growth and climate to support conservation efforts. One priority for conservation is the Atlas cedar (Cedrus atlantica), an endangered species exhibiting dieback throughout its North African range. In this study, we evaluate the long-term context for recent dieback and develop a projection of future C. atlantica growth by exploring the periodic variability of its growth through time. First, we present a new C. atlantica tree-ring chronology (1150–2013 CE) from the Middle Atlas mountains, Morocco. We then compare the new chronology to existing C. atlantica chronologies in Morocco and use principal components analysis (PCA) to isolate the common periodic signal from the seven longest available records (PCA7, 1271–1984 CE) in the Middle and High Atlas portions of the C. atlantica range. PCA7 captures 55.7% of the variance and contains significant multidecadal (˜95 yr, ˜57 yr, ˜21 yr) periodic components, revealed through spectral and wavelet analyses. Parallel analyses of historical climate data (1901–2016 CE) suggests that the multidecadal growth signal originates primarily in growing season (spring and summer) precipitation variability, compounded by slow-changing components of summer and winter temperatures. Finally, we model the long-term growth patterns between 1271–1984 CE using a small number (three to four) of harmonic components, illustrating that suppressed growth since the 1970s – a factor implicated in the dieback of this species – is consistent with recurrent climatically-driven growth declines. Forward projection of this model suggests two climatically-favourable periods for growth in the 21st century that may enhance current conservation actions for the long-term survival of the C. atlantica in the Middle and High Atlas mountains.

Links between Tasmanian precipitation variability and the Indian Ocean subtropical high

The relationship between the Indian Ocean High (IOH) and austral autumn (March–May) precipitation variability over Tasmania during 1951–2016 is described. We present evidence that changes in autumn rainfall are influenced primarily by a strength of the Indian Ocean high pressure (IOH_P) and its longitudinal position (IOH_LN). The variation of the IOH shows a clear influence on the seasonal variability of precipitation over Tasmania. The most important factor of autumn precipitation variability is found to be IOH_P system, particularly in the regions neighboring the west coast. Positive anomalies of IOH_P are associated with less precipitation over the island. In addition, the overall eastward shift of the center of the IOH, i.e., IOH_LN, also plays a significant role in rainfall reduction. These two effects are found to be effectively statistically independent with each other. A multi-linear statistical model of autumn precipitation in Tasmania using the IOH_P and IOH_P accounts for 43% of the precipitation variability during 1951–2016. A series of well-defined atmospheric circulation composite analyses are identified during dry (wet) Tasmanian autumn mean sea level pressure, 850 hPa vector wind, and 850 hPa vertical wind velocity over the Indian Ocean. These circulations generate abnormally strong (weak) westerly winds along much of the western Tasmania which is responsible to provide more (less) moisture over Tasmania.

Change characteristics of precipitation and temperature in the Qilian Mountains and Hexi Oasis, Northwestern China

Precipitation and temperature in the Qilian Mountains and the Hexi oasis are changing with climate change. The characteristics and control factors of these changes are not well understood, affecting the ability to allocate water resources in this area judiciously. We analyzed the annual and seasonal variability in precipitation and temperature during 1960–2016 using monthly data from 25 national meteorological stations in the Qilian Mountains. Results showed that precipitation increased by 6.2, 4.1, and 8.9 mm/decade for the whole regions (including oasis and mountains), oasis, and mountains, respectively. Variability in summer precipitation made the greatest contribution to the annual change in precipitation. Moreover, the increment in precipitation was amplified with elevation. Enhanced westerly circulation and increased water vapor were the factors responsible for the increased precipitation in the study area. Temperature in the whole regions, oasis, and mountains increased at the rate of 0.32, 0.32, and 0.33 °C/decade, respectively. Changes in temperature were greatest in winter, with 0.47, 0.36 and 0.50 °C/decade in the whole regions, oasis, and mountains, respectively; changes in winter temperature made the greatest contribution to the annual temperature changes. Seasonal differences in carbon dioxide concentration may have caused the seasonal differences in the rate of temperature increase. The annual increase in temperature was mainly due to Siberian High, and Arctic Oscillation, carbon emissions and decreased cloud cover in the study area. The study results will help evaluate climatic hazards and contribute to development of water resources management strategies such as supplementary irrigation.

Influence of the flood pulse on mercury accumulation in detritivorous, herbivorous and omnivorous fish in Brazilian Amazonia.

Hg accumulation in fish is influenced by several factors including seasonality. In the Amazon, ecosystems are marked by strong seasonal variation in precipitation, which leads to drastic changes in the water level of lakes and rivers. The aim of this study was to evaluate Hg levels in muscle of detritivorous, herbivorous and omnivorous fish from an Amazon lake (Madeira River Basin, Amazonas, Brazil) over four seasons (rising water, high water, falling water and low water). We hypothesized that total Hg concentration varies during the seasons. The results indicate that total Hg levels in detritivorous fish were higher in rising and low water seasons while in herbivorous and omnivorous fish the total Hg concentration was higher during the rising water season. The hypothesis was supported by the results. Additionally, the study provides evidence that Hg levels in fish with different feeding habits are influenced by the flood pulse of the Amazon region.

Phenology and floral synchrony of Rhizophora mangle along a natural salinity gradient

AbstractRhizophora mangle is the most common species of mangrove within its range and a major component of coastal ecosystems in the tropics and subtropics. Its phenology is affected by seasonal variations in temperature, salinity, and precipitation, all of which may be altered by impending climate change. In this work, the monthly leaf, flower, and fruit phenology of R. mangle was assessed at three sites, along a natural salinity gradient for 12 months. We assessed phenological states using litter traps installed in mangrove stands, and by direct observation of leaf production at two sites, and flower, and fruit production at a single site. Phenological events were compared with seasonal climatic variations (in wind, rain, and temperature) to explore potential abiotic controls on the phenology of this species. Leaf shedding was lowest at the low salinity site. Leaf and stipule production showed a seasonal trend, decreasing during the cool and dry winter months. Flowering was highly seasonal and synchronous and peaked during the rainy season. Flowering was correlated with average temperature (r = 0.82; p &lt; 0.05) at all three study sites, but was not correlated with precipitation. At the high‐salinity site, flowering was restricted to the rainy season. Fruit abortion was associated with wind speed (r = 0.79; p &lt; 0.05). Flowering of R. mangle is induced by increasing temperatures and the onset of rains. Rising temperatures, drier summers, and more frequent and intense storms, due to climate change, may influence mangrove productivity, population dynamics, and floral synchrony.

PRECIPITACIÓN RECONSTRUIDA PARA LA PARTE ALTA DE LA CUENCA DEL RÍO NAZAS, DURANGO

&#x0D; &#x0D; &#x0D; La variabilidad referente a la precipitación de los últimos 410 años (1599 - 2008) del ciclo invierno-primavera (noviembre-junio) en la parte alta de la cuenca del río Nazas, Durango, México se reconstruyó a partir de la cronología o serie de tiempo desarrollada con base en anillos de crecimiento de Pseudotsuga menziesii, que la explicó en 64%. La reconstrucción para estos cuatro siglos, validada con registros de precipitación y datos de archivos históricos, permitió determinar la presencia de sequías severas entre los periodos 1665 - 1688, 1695 - 1718, 1774 - 1791, 1798 - 1813, 1890 - 1896 y 1945 - 1963. Así, en la correspondiente a este último, de 1948 a 1963, se presentó la más importante del siglo XX; así como de, esos 410 años, por su impacto en la sociedad y la economía; sin embargo, aquellas de 1665 - 1688 y 1695 - 1718, pudieron haber causado efectos similares por su intensidad. En la parte alta de la cuenca, la precipitación es modulada de manera significativa (p&lt;0.05) por El Niño Oscilación del Sur (ENSO, por sus siglas en inglés) tanto en su fase fría (La Niña), al producir intensa escasez de agua con repercusiones económicas, políticas y sociales para los pobladores de la Comarca Lagunera, como en su fase cálida (El Niño), con importantes incrementos en la pluviometría de la región.&#x0D; &#x0D; &#x0D;

Regionality of long-term trends and interannual variation of seasonal precipitation over India

This study investigated the long-term trends and interannual variations (IAVs) of seasonal precipitation during two different periods in India and their relation to atmospheric circulation. To focus on the relation between the regional characteristics of precipitation and the large-scale circulation system, we defined homogeneous regions based on pentad rainfall seasonality using hierarchical cluster analysis. Seven obtained clusters (regions) were used to analyze the long-term trends, IAVs, and their relation to atmospheric circulation. Seasonal precipitation showed no remarkable trend in most regions and seasons, which implies the effect of IAV in seasonal precipitation is greater than the long-term trend. Based on regression analysis, the IAV of seasonal precipitation and its relation to atmospheric circulation over the ocean surrounding India were detected for each season. Enhancement of the southwesterly monsoon flow over the Indian Ocean and the strength of the southeasterly wind related to the activity of the monsoon trough around the Gangetic Plain both affected the increase of summer monsoon precipitation in regions CL3 (central and eastern India) and CL6 (western coast of the Arabian Sea in southern India). In contrast, neither region CL5 (eastern coastal region of southern India) nor CL7 (northeastern India and part of the western coastal region of southern India) showed a relationship between IAV of summer monsoon precipitation and the southwesterly flow or the southeasterly flow over the Indian Ocean or the Gangetic Plain, respectively. Region CL5 showed strong correlation with the northeasterly monsoon flow (strength of the southeasterly flow over the Bay of Bengal) during the post-monsoon season. Other regions showed no relation with the northeasterly flow during the post-monsoon season. Furthermore, IAV of precipitation in region CL7 showed no correlation with either the southwesterly monsoon during the summer monsoon season or the northeasterly monsoon during the post-monsoon season.

Middle Eocene East Asian monsoon prevalence over southern China: Evidence from palynological records

Abstract The initiation of Asian monsoon systems is a complex and controversial issue, and the existence of monsoon systems in the Eocene has not yet been confirmed by commonly applicable proxy data. Eocene palynological records from three basins in southern China provide an ideal opportunity to investigate the early stage of the Asian monsoon due to their unique geographical location. The palynological assemblages suggest that the middle Eocene vegetation in southern China was composed of mixed evergreen and deciduous broad-leaved forests, accompanied by abundant subtropical to tropical evergreen taxa, thus bearing strong resemblance to the modern vegetation in southern China, and indicating a warm-humid subtropical climate. By analysing the palynological data using a coexistence approach (CA), we quantitatively reconstructed seven climatic parameters, which indicate a subtropical monsoon climate with a mean annual temperature (MAT) in the range 16.5 to 23.5 °C and a mean annual precipitation (MAP) in the range 1035 to 1724 mm, similar to those of the modern subtropical monsoon climate in southern China although the MAT was surprisingly lower than present. Significant seasonal variations in precipitation and temperature and as well as the spatial distribution of MAP during the middle Eocene were most similar to those of the modern East Asia Monsoon (EAM) climate of southern China, clearly indicating that the Eocene climate of southern China was mainly influenced by the EAM, rather than the Indian monsoon (IM) or Inter-tropical Convergence Zone (ITCZ) monsoons. By combining the above results with previously published geological evidence, we infer that the EAM developed in southern China as early as the middle Eocene, albeit with an intensity that may have been weaker than present. The establishment of the middle Eocene EAM in southern China was most likely driven by the northward drift of the Indian Subcontinent and its collision with the Eurasian continent, associated with the uplift of the Central-Southern Tibetan Plateau, which enlarged the land-sea thermal contrast and deepened the low pressure over continental Asia in summer, thus greatly intensifying the East Asian subtropical summer monsoon over southern China.

Tree rings as a proxy for seasonal precipitation variability and Early Neolithic settlement dynamics in Bavaria, Germany.

Studying the dynamic of Neolithic settlement on a local scale and its connection to climate variability is often difficult due to missing on-site climate reconstructions from natural archives. Here we bring together archaeological settlement data and a regional climate reconstruction from precipitation-sensitive trees. Both archives hold information about regional settlement dynamics and hydroclimate variability spanning the time of the first farming communities, the so called Linearbandkeramik (LBK) in Bavaria, Germany. Precipitation-sensitive tree-ring series from subfossil oak are used to develop a spring-summer precipitation reconstruction (5700–4800 B.C.E.) representative for southern Germany. Early Neolithic settlement data from Bavaria, mainly for the duration of the LBK settlement activities, are critically evaluated and compared to this unique regional hydroclimate reconstruction as well as to reconstructions of Greenland temperature, summer sea surface temperature, delta 18O and global solar irradiance to investigate the potential impact of climate on Neolithic settlers and their settlement dynamic during the LBK. Our hydroclimate reconstruction demonstrates an extraordinarily high frequency of severe dry and wet spring-summer seasons during the entire LBK, with particularly high year-to-year variability from 5400 to 5101 B.C.E. and with lower fluctuations until 4801 B.C.E. A significant influence of regional climate on the dynamic of the LBK is possible (e.g. around 4960 B.C.E.), but should be interpreted very carefully due to asynchronous trends in settlement dynamics. Thus, we conclude that even when a climate proxy such as tree rings that has excellent spatio-temporal resolution is available, it remains difficult to establish potential connections between the settlement dynamic of the LBK and climate variability.

Climate model projections for future seasonal rainfall cycle statistics in Northwest Costa Rica

Increasing water scarcity due to rising demand and changes in climate and land use are expected to exert significant stress on water resources in many parts of the world. In many areas, distinctive patterns of seasonal precipitation play an important role in regional ecosystems, economies, and food and energy supplies. Given the inconsistency among climate model precipitation projections, this study develops a statistical approach to better characterize seasonal patterns of precipitation and to assess the potential impact of climate change on these patterns. The method is applied to evaluate the bimodal seasonal pattern of precipitation in northwest Costa Rica as a case study. A Gaussian mixture model is used to describe the bimodal pattern and quantify changes in the seasonal precipitation cycle projected by 19 Coupled General Circulation Models. The model simulations for the current period (1979–2005) are compared with observed monthly precipitation data based on four goodness‐of‐fit metrics to select the best performing models. The monthly bias‐corrected and spatially disaggregated (BCSD) climate projections from the selected models are used to investigate the projected change in the bimodal seasonal pattern, seasonal mean precipitation and interannual variability at the late twenty‐first century. Under RCP8.5, the degree of consistency associated with these BCSD climate projections is found to be higher for the projections of the midsummer drought than that for the early portion of the wet season (early season). For the late portion of the wet season (late season) projections, the degree of consistency is found to be the lowest. The proposed method provides (a) an overall characterization of inconsistencies in climate model projections for the region, (b) more information about the possible changes in the seasonal cycle of precipitation, and (c) the possibility of performing detailed comparison tests among climate models over a set of simulations and projections.