Summer Precipitation Patterns Research Articles

Impact of May–June Antarctic Oscillation on July–August Heat-Drought Weather in Yangtze River Basin

Investigating the physical mechanism behind the formation of summer heat-drought weather (HDW) in the Yangtze River Basin (YRB) holds significant importance for predicting summer precipitation and temperature patterns in the region as well as disaster mitigation and prevention. This study focuses on spatiotemporal patterns of July–August (JA) HDW in the YRB from 1979 to 2022, which is linked partially to the preceding May–June (MJ) Antarctic Oscillation (AAO). Key findings are summarized as follows: (1) The MJ AAO displays a marked positive correlation with the JA HDW index (HDWI) in the southern part of upper YRB (UYRB), while showing a negative correlation in the area extending from the Han River to the western lower reaches of the YRB (LYRB); (2) The signal of MJ AAO persists into late JA through a specific pattern of Sea Surface Temperature anomalies in the Southern Ocean (SOSST). This, in turn, modulates the atmospheric circulation over East Asia; (3) The SST anomalies in the South Atlantic initiate Rossby waves that cross the equator, splitting into two branches. One branch propagates from the Somali-Tropical Indian Ocean, maintaining a negative-phased East Asia–Pacific (EAP) teleconnection pattern. This enhances the moisture flow from the Pacific towards the middle and lower reaches of the Yangtze River Basin (MYRB-LYRB). The other branch propagates northward, crossing the Somali region, and induces a positive geopotential height anomaly over Urals-West Asia. This reduces the southwesterlies towards the UYRB, thereby contributing to HDW variabilities in the region. (4) Partial Least Squares Regression (PLSR) demonstrated predictive capability for JA HDW in the YRB for 2022, based on Southern Ocean SST.

Variations in the Thermal Low-Pressure Location Index over the Qinghai–Tibet Plateau and Its Relationship with Summer Precipitation in China

The thermal and dynamic effects of the special topography of the Qinghai–Tibet Plateau have a significant impact on rainfall in China. Utilizing NCEP/NCAR monthly reanalysis data alongside precipitation observations from 1936 monitoring stations across China spanning from 1966 to 2022, this study establishes a location index for the thermal low-pressure center situated over the Qinghai–Tibet Plateau. Temporal variations in the location index and summer (July) precipitation patterns in China were studied. Over the past six decades, thermal low-pressure centers have been predominantly positioned near 90° E and 32.5° N within a geopotential height of 4360 gpm, with their distribution extending from east to west rather than from south to north. The longitudinal and latitudinal position indices showed the same linear trend, with a negative trend before the 21st century, and then began to turn positive. Mutation analysis highlights pronounced weakening mutations occurring in 1981 and 1973, with the longitudinal index transitioning from an interannual cycle of approximately 6–8 years, while the latitudinal index displays quasi-cyclic oscillations of 5 and 8 and 12–14 years. Strong negative correlations are evident between the location indices and precipitation along the southeastern edge of the Qinghai–Tibet Plateau and in southern China, contrasting with the positive correlations observed in the central-eastern plateau, northwest, north, and the Huang-Huai region of China. The center of the thermal low is located to the east and north, corresponding to the deeper surface thermal low in most areas east of China, and the stronger transport of warm and wet air from the southwest wind, leading to greater convergence of southwest wind and northwest wind in China’s northern region. The south of the Yangtze River is controlled by the strengthening West Pacific subtropical high and South Asia high, resulting in a significant decrease in precipitation, and the warm and humid air from the southwest on the west side of the West Pacific subtropical high is also transported to the north, increasing the precipitation in most parts of the north.

The role of Arctic sea ice loss in the interdecadal trends of the East Asian summer monsoon in a warming climate

The East Asian summer monsoon precipitation has exhibited a well-known “southern China flood and northern China drought” pattern in recent decades. The increase in aerosols and warming oceans are recognized as two important forcings that control of the precipitation trends over East Asian land. However, in this study, by using large ensemble simulations from the CMIP6 Polar Amplification Model Intercomparison Project (PAMIP), the influence of Arctic amplification, serving as the prominent feature of global warming, is very important in modulating the East Asian summer precipitation pattern, which is comparable to the influence of sea surface temperature (SST). Additionally, the observed “southern China flood and northern China drought” pattern only exists in July and August, whereas a triple pattern with the precipitation positive anomaly center over Middle China occurs in June. These patterns are closely connected with the regional differences in Arctic sea ice loss from June to July, affected through both the Rossby waves propagating in a weaker westerly jet and the decrease in the large-scale meridional thermal contrast in a warming climate.

A Persistent Coast Mode of Precipitation in Southeast China Over the Last Millennium

AbstractInstrumental data set have revealed several summer precipitation patterns in eastern China, being summarized as “tripole,” “dipole” and “coast” modes. The former two have been found to persist at different time scales, leaving the latter unclear in geological records. Here we present 1300‐year hydroclimate records in a tropical maar lake in the southern coast of China using archeal lipid GDGTs, which can reflect lower water redox conditions largely regulated by lake water depth. The down‐core GDGTs reveal a relatively drier condition during the medieval climate anomaly compared to the Little Ice Age, in‐phase with other records in southeast coast of China but opposite to the inland region, hence demonstrating a persistent “coast” mode in eastern China. The thermal state of equatorial Pacific is suggested to play an important role in shaping the “coast” mode by modulating the location and strength of the western Pacific subtropical high and tropical typhoons.

Precipitation and Soil Moisture Variation over the Tibetan Plateau to the Anomaly of Indian Summer Monsoon from 1979 to 2019

The Indian Summer Monsoon (ISM) can profoundly influence the summer precipitation patterns of the Tibetan Plateau (TP) and indirectly affect the TP’s soil humidity. This study investigates the responses of TP’s precipitation and soil moisture to the ISM in the monsoon season (June to September, JJAS) from 1979 to 2019. Precipitation in the TP and the ISM intensity generally exhibit a positive correlation in the west and a negative correlation in the east. The response of TP soil moisture to the ISM generally aligns with precipitation patterns, albeit with noted inconsistencies in certain TP regions. A region exhibiting these inconsistencies (30°–32°N, 80°–90°E) is selected as the study area, hereafter referred to as IRR. In periods of strong ISM, precipitation in IRR increases, yet soil moisture decreases. Conversely, in years with a weak ISM, the pattern is reversed. During strong ISM years, the rainfall increase in IRR is modest, and the soil remains drier compared to other TP regions. Under the combined effects of a marginal increase in precipitation and relatively rapid evaporation, soil moisture in the IRR decreased during years of strong ISM. During weak ISM years, the surface temperature in the IRR is higher compared to strong ISM years, potentially accelerating the melting of surface permafrost and snow in this region. Additionally, glacier meltwater, resulting from warmer temperatures in the northwest edge of the TP, may also result in the humidification of the soil in the IRR.

Hydroclimate Changes Based on Testate Amoebae in the Greater Khingan Mountains’ Peatland (NE China) during the Last Millennium

The driving force of climate change in the monsoon margin is complex, making it a key area for regional and global climate change research. Palaeohydrological studies in the monsoon margin have increased the resolution of research in the long term, transitioning from qualitative to quantitative studies to comprehend climate change processes, patterns, and mechanisms. Testate amoebae (TA) in peat sediments are used as a proxy indicator organism for quantitative reconstruction of palaeohydrology. Thus, their community changes are directly related to precipitation, and widely used to reconstruct the patterns of summer precipitation globally. We investigated TA species and reconstructed palaeohydrological changes in the Greater Khingan Mountains’ Hongtu (HT) peatland, located in the East Asian Summer Monsoon (EASM) margin. The result showed that the most abundant TA species were Assulina muscorum (12.4 ± 5.0%) and Nebela tincta (8.9 ± 4.9%) in the HT peat core. The increase in dry indicator species (e.g., A. muscorum and Alabasta militaris) indicated a drying pattern in the HT peatland since 150 cal yr BP. Principal component analysis (PCA) explained 47.6% of the variation in the selected TA assemblages. During 400 to 250 cal yr BP, PCA axis 1 scores ranged from 0.2 to −1.3 (reflecting a drier climate), associating with the Little Ice Age. The paleohydrology of the northern part of the Greater Khingan Mountains was mainly controlled by the EASM, which was associated with changes in North Atlantic Sea surface temperature and solar radiative forcing. The apparent drying pattern may be the result of the gradual intensification of anthropogenic activities and the increase in EASM intensity.

To what extent can the ozone valley over the Tibetan Plateau influence the East Asian summer precipitation?

The ozone valley over the Tibetan Plateau (OVTP) has experienced significant interannual variations during the past decades. Previous studies have primarily focused on the origins of OVTP rather than its climate impact. This study reveals that OVTP during its peak season (May–July) explains up to 15% of the summer precipitation variability in East Asia. The results suggest that the surface temperature (Ts) anomaly over the Tibetan Plateau (TP) acts as a link between OVTP and East Asian precipitation. Through the positive land-atmosphere feedback, the Ts anomaly over the TP is amplified. The anomalous Ts pattern persists into summer (June–August) due to the land memory effect and impacts the East Asian precipitation by modulating the local circulation. The Specified-Chemistry version of the Whole Atmosphere Community Climate Model is employed to validate that MJJ OVTP results in a substantial increase of Ts over TP and induces an anomalous anti-cyclone centered over the Yangtze-Huaihe River Basin during summer. Consequently, negative precipitation anomalies are observed in the Yangtze River Basin, while positive precipitation anomalies occur in Southern China. The linear baroclinic model further demonstrates that the diabatic heating over the TP serves as the link between MJJ OVTP and East Asian summer precipitation patterns. Our analysis of Coupled Model Intercomparison Project Phase 6 models reveals that a more accurate prediction of East Asian precipitation requires an improved understanding of the relationship between OVTP and TP Ts.

Changes in Summer Precipitation Modes over Eastern China in Simulated Warm Intervals of the Last Interglacial, Mid-Holocene, and Twenty-First Century

Abstract The East Asian summer monsoon (EASM) and associated summer precipitation patterns over eastern China have exhibited interannual and interdecadal variations in recent decades, but whether the precipitation modes vary with climate conditions remains unclear. In this study, we investigate the changes in summer precipitation modes over eastern China and associated mechanisms in simulated past and future warm periods, namely, the last interglacial (LIG), mid-Holocene, and the twenty-first century under the Shared Socioeconomic Pathway (SSP) based on representative concentration pathway 8.5 (SSP5–8.5) scenario, using Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The model evaluation indicates that only one out of nine CMIP6 models, specifically the Community Earth System Model, version 2 (CESM2), can well reproduce the observed summer precipitation modes at the present. The empirical orthogonal function (EOF) analyses of the CESM2 simulations reveal that during the warm periods induced by either orbital parameters or greenhouse gas concentrations, the first leading mode of summer precipitation switches from its present meridional tripolar structure to a dipole pattern, and the second leading mode is generally a meridional tripolar mode that differs from the present dipole pattern. This summer precipitation regime interchange in the warm intervals is associated with the formation of the dipole and tripolar circulation patterns. The large-scale circulation changes responsible for the altered precipitation modes are attributed to the enhanced EASM and relevant southerly winds due to intensified land–sea thermal contrast, although the external forcings and the magnitude of EASM enhancement are different in the interglacial and future periods.

Decade-to-decade variations in summer precipitation patterns in eastern China since the 1960s: roles of water vapor transport and sea surface temperature

Based on the observational precipitation data and multiple reanalysis datasets, this study analyzes the decade-to-decade spatiotemporal evolution of anomalous summer precipitation in eastern China (SPEC) for 1961-2019 and explores the roles of water vapor transport and global sea surface temperatures (SSTs). Results show that SPEC patterns in the last 60 yr were mainly classified into 3 categories: dipole (the 1960s and 1970s), tripole (the 1990s) and quadrupole (the 1980s and 2000s), with marked decade-to-decade variations. In the 2010s, the precipitation pattern exhibited a more complex nature. Obvious differences can be detected in the water vapor channels (WVCs) associated with the 3 SPEC patterns. Specifically, the dipole pattern of the SPEC was mainly related to the Pacific Ocean WVC (POW) and the westerly WVC (WW). The tripole pattern was mainly associated with the Tibetan Plateau Southern Side WVC (TPW), the South China Sea WVC, the POW and the WW. The quadrupole pattern was quite clearly correlated with the Indian Ocean WVC, the Bay of Bengal WVC, the TPW, the POW and the WW. Furthermore, the dipole SPEC pattern was correlated with the Pacific decadal oscillation (PDO), the Indian Ocean basin mode (IOBM) and the Atlantic multi-decadal oscillation. The tripole and quadrupole patterns were related to the IOBM and PDO, respectively. The PDO has a marked influence on the interdecadal shifting of the SPEC pattern in the late 1970s, favoring the formation of the quadrupole SPEC pattern. However, the decadal shifting of the SPEC pattern in the early 1990s is mainly influenced by the IOBM, which favored the formation of the tripole precipitation pattern.

Evaluation of the precipitation of the East Asia regional reanalysis system mainly over mainland China

AbstractIn this article, precipitation in a new regional reanalysis dataset, namely, the East Asia regional reanalysis system (EARS‐CMA) with a 12‐km resolution based on the Weather Research and Forecast (WRF) model, ERA‐Interim, ERA5, TRMM 3B42V7, Climate Prediction Center Morphing Technique (CMORPH) and CN05.1, is compared in 2008–2017. The results show that EARS‐CMA can capture the spatial features and temporal variation in precipitation over East Asia well. Focusing on mainland China, CMORPH performs worse in regard to winter precipitation and dryer areas than TRMM against CN05.1. Although ERA5 behaves better with integrated properties compared with ERA‐Interim and EARS‐CMA over East Asia, EARS‐CMA produces more reliable annual, summer and winter mean precipitation pattern than ERA5 over the subregions dominated by large scales in mainland China, including the Tibetan Plateau and northwestern China, similar to its driving fields, ERA‐Interim. In addition, the improvement from ERA‐Interim to EARS‐CMA can be obtained in reasonably reproducing precipitation seasonality in South China, Indo‐China and southern Japan. Thus, the compatibility of EARS is outstanding in maintaining the advantages of its corresponding driving fields, ERA‐Interim, as well as developing finer small scales. Although slightly behind ERA5 in the subregions affected by tropical systems and monsoons, EARS‐CMA is competitive in acting as reference data for daily, seasonal, annual and interannual precipitation estimations over mainland China.

Future precipitation changes in Egypt under the 1.5 and 2.0 °C global warming goals using CMIP6 multimodel ensemble

Rainfall projections for 1.5 and 2.0 °C warming can explain regional precipitation response to emission reductions under the Paris Agreements' goals. Assessment of such changes is vital for Egypt, a global climate change hotspot. The performance of 29 CMIP6 GCMs' hindcasts was evaluated according to their capability to replicate the spatial patterns of annual, winter, and summer precipitation for 1971–2014 to select a suitable GCM subset to form a robust multimodel ensemble (MME). The MME median was used to project precipitation and precipitation extremes of Egypt at the end of the century (2081–2100) for two shared socioeconomic pathways (SSP) scenarios, SSP1–1.9 and SSP1–2.6, representing 1.5 and 2.0 °C warming at the end of the present century, respectively. The results showed an increase in precipitation in the northern high precipitation region by 37% and 54% for SSP1–1.9 and SSP1–2.6, respectively, and a decrease in the southwestern low precipitation region by −35% for both scenarios. The projected increase would be mostly in winter and almost no change in summer. The projection of precipitation extremes revealed an increase in extreme precipitation amount in the northern coast between 0% and 14% and the longest dry spell over most of the country by 160%. The results indicate more heterogeneity in the spatial distribution in Egypt's precipitation, increasing extreme precipitation amount in some regions and dry spell length over the whole country. The results indicate a large increase in hydrological hazard susceptibility in Egypt, even if the global warming can be limited to 2 °C at the end of the century following the Paris Agreement.

Change of East-Asian Summer Precipitation Associated With Strong El Niño Under the Future Emission Scenarios

Strong Eastern-Pacific type El Niño (EP-El Niño) events have significant impacts on the decaying-summer precipitation over East Asia (EA). It has been demonstrated that frequency of strong EP-El Niños will increase and associated precipitation will become more severe and complex under future high emission scenarios. In this study, using simulations of CMIP5 and CMIP6, changes of the summer precipitation pattern related to strong EP-El Niño during its decay phase and the possible mechanism as responding to high emission scenarios are examined. Precipitation anomaly patterns over EA of strong EP events show a large inter-model spread in historical simulations between the CMIP models where CMIP6 is not superior to CMIP5. Under high emission scenarios, changes of summer precipitation anomalies related to strong EP events tend to increase over the southern EA and decrease around the northern EA from CMIP5, while there is an overall increase in the whole EA from CMIP6. The common change is featured by the increase of precipitation over southeastern China under high emission scenarios. This could be mainly attributed to the anticyclonic circulation from the South China Sea to the western North Pacific as a delayed response to more frequent strong EP-El Niños, which favors an increase in water vapor fluxes converging into the southeastern China.

Permafrost Biases Climate Signals in δ18Otree-ring Series from a Sub-Alpine Tree Stand in Val Bever/Switzerland

During recent decades, stable oxygen isotopes derived from tree-ring cellulose (δ18OTRC) have been frequently utilised as the baseline for palaeoclimatic reconstructions. In this context, numerous studies take advantage of the high sensitivity of trees close to their ecological distribution limit (high elevation or high latitudes). However, this increases the chance that indirect climatic forces such as cold ground induced by permafrost can distort the climate-proxy relationship. In this study, a tree stand of sub-alpine larch trees (Larix decidua Mill.) located in an inner alpine dry valley (Val Bever), Switzerland, was analysed for its δ18OTRC variations during the last 180 years. A total of eight L. decidua trees were analysed on an individual base, half of which are located on verified sporadic permafrost lenses approximately 500 m below the expected lower limit of discontinuous permafrost. The derived isotope time series are strongly dependent on variations in summer temperature, precipitation and large-scale circulation patterns (geopotential height fields). The results demonstrate that trees growing outside of the permafrost distribution provide a significantly stronger and more consistent climate-proxy relationship over time than permafrost-affected tree stands. The climate sensitivity of permafrost-affected trees is analogical to the permafrost-free tree stands (positive and negative correlations with temperature and precipitation, respectively) but attenuated partly leading to a complete loss of significance. In particular, decadal summer temperature variations are well reflected in δ18OTRC from permafrost-free sites (r = 0.62, p < 0.01), while permafrost-affected sites demonstrate a full lack of this dependency (r = 0.30, p > 0.05). Since both tree stands are located just a few meters away from one another and are subject to the same climatic influences, discrepancies in the isotope time series can only be attributed to variations in the trees’ source water that constraints the climatic fingerprints on δ18OTRC. If the two individual time series are merged to one local mean chronology, the climatic sensitivity reflects an intermediate between the permafrost-free and –affected δ18OTRC time series. It can be deduced, that a significant loss of information on past climate variations arises by simply averaging both tree stands without prior knowledge of differing subsurface conditions.

Multiple movement characteristics of the Meiyu rain belt in East Asia: reconstructing historical data on the southern margin from 1861 to 2017

The location of the Meiyu belt has an important influence on the summer precipitation pattern in East Asia. In this study, historical diary-recorded data and modern instrument-measured data were used to reconstruct the Meiyu sequence at its southern margin from 1861 to 2017. Combining with the variation of Meiyu intensity in the middle and lower Yangtze River Valley (YRV) and the Huai River Valley (HRV), this study discussed the change characteristics of the Meiyu rain belt. The overall shift of the Meiyu belt is basically the same as the change to the southern margin of the rain belt which has interdecadal and multidecadal north-south shifts. The northward shift of the rain belt after 2000 occurred only as an interdecadal fluctuation. The summer monsoon is the main factor for the shift of the Meiyu belt on a multidecadal-centennial scale, AMO (Atlantic Multidecadal Oscillation) has a certain role in modulation, and PDO (Pacific Decadal Oscillation) is the decisive factor for the interdecadal scale of the Meiyu belt position. When PDO is in a positive (negative) phase, the whole Meiyu belt tends to be strong (weak) and shifts to the South (North). AMO and IOBM (Indian Ocean Basin Mode) play a particular role in modulation. The shift of the Meiyu belt is usually consistent with the north-south variation of the main rain belt of eastern China. Its location may play a role in predicting summer precipitation in North China.

Impacts of land use/cover change on spatial patterns of summer precipitation at decadal scale over eastern China

AbstractThe impacts and corresponding mechanisms of land use/cover change (LUCC), mainly crops and pasture, on the spatial patterns of summer precipitation over eastern China at the decadal timescale during the last millennium are investigated, using a set of land use/cover sensitivity experiments from the Community Earth System Model (CESM) Last Millennium Ensemble (LME) archive. Two leading modes of summer precipitation over eastern China at decadal scale before (with slight LUCC) and after (with drastic LUCC) 1750 AD are compared. The results show that the LUCC plays an important role in modulating the spatial patterns of summer precipitation over eastern China at decadal scale. Before 1750 AD, the first leading mode exhibits a meridional dipole pattern, while the second leading mode exhibits a meridional tripole pattern. After 1750 AD, the order of the spatial patterns switches, that is, the tripole pattern as the first leading mode and the dipole pattern as the second leading mode. During both periods, the “dry north‐wet south” dipole pattern is associated with an anomalous “A‐C” (anticyclone‐cyclone) low‐level circulation pattern and the related water vapour divergence‐convergence pattern from north to south over the eastern part of East Asia, whereas the tripole pattern corresponds to an anomalous “A‐C‐A" (anticyclone‐cyclone‐anticyclone) circulation pattern along with the corresponding tripole pattern of water vapour divergence/convergence. The regressions of sea surface temperature (SST) during boreal winter resemble obvious Pacific decadal oscillation (PDO) or Atlantic multi‐decadal oscillation (AMO) modes. In addition, the AMO index displays a decreasing trend during the last millennium under the LUCC forcing, while the PDO index does not show an obvious changing trend. The possibility of coupled PDO‐AMO anti‐phase (in‐phase) increases (decreases) after 1750 AD, leading to the increased (decreased) possibility of the tripole (dipole) pattern in eastern China. Our findings confirm the necessity of including LUCC as an important forcing factor in predictions of future climate changes, especially at the regional scale and also at longer timescales.

How to ‘downsize’ a complex society: an agent-based modelling approach to assess the resilience of Indus Civilisation settlements to past climate change

The development, floruit and decline of the urban phase of the Indus Civilisation (c.2600/2500-1900 BC) provide an ideal opportunity to investigate social resilience and transformation in relation to a variable climate. The Indus Civilisation extended over most of the Indus River Basin, which includes a mix of diverse environments conditioned, among other factors, by partially overlapping patterns of winter and summer precipitation. These patterns likely changed towards the end of the urban phase (4.2 ka BP event), increasing aridity. The impact of this change appears to have varied at different cities and between urban and rural contexts. We present a simulation approach using agent-based modelling to address the potential diversity of agricultural strategies adopted by Indus settlements in different socio-ecological scenarios in Haryana, NW India. This is an ongoing initiative that consists of creating a modular model, Indus Village, that assesses the implications of trends in cropping strategies for the sustainability of settlements and the resilience of such strategies under different regimes of precipitation. The model aims to simulate rural settlements structured into farming households, with sub-models representing weather and land systems, food economy, demography, and land use. This model building is being carried out as part of the multi-disciplinary TwoRains project. It brings together research on material culture, settlement distribution, food production and consumption, vegetation and paleoenvironmental conditions.

A distributed analysis of lateral inflows in an Alaskan Arctic watershed underlain by continuous permafrost

AbstractLateral inflows control the spatial distribution of river discharge, and understanding their patterns is fundamental for accurately modelling instream flows and travel time distributions necessary for evaluating impacts of climate change on aquatic habitat suitability, river energy budgets, and fate of dissolved organic carbon. Yet, little is known about the spatial distribution of lateral inflows in Arctic rivers given the lack of gauging stations. With a network of stream gauging and meteorological stations within the Kuparuk River watershed in northern Alaska, we estimated precipitation and lateral inflows for nine subcatchments from 1 July to 4 August,2013, 2014, and 2015. Total precipitation, lateral inflows, runoff ratios (area‐normalized lateral inflow divided by precipitation), percent contribution to total basin discharge, and lateral inflow per river kilometre were estimated for each watershed for relatively dry, moderate, or wet summers. The results show substantial variability between years and subcatchments. Total basin lateral inflow depths ranged 24‐fold in response to a threefold change in rainfall between dry and wet years, whereas within‐basin lateral inflows varied fivefold from the coastal plain to the foothills. General spatial trends in lateral inflows were consistent with previous studies and mean summer precipitation patterns. However, the spatially distributed nature of these estimates revealed that reaches in the vicinity of a spring‐fed surficial ice feature do not follow general spatial trends and that the coastal plain, which is typically considered to produce minimal runoff, showed potential to contribute to total river discharge. These findings are used to provide a spatially distributed understanding of lateral inflows and identify watershed characteristics that influence hydrologic responses.

Seasonal Predictability and Change of Large-Scale Summer Precipitation Patterns over the Northeast United States

AbstractThis study examines space–time patterns of summer daily rainfall variability across the Northeast United States, with a focus on historical trends and the potential for long-lead predictability. A hidden Markov model based on daily data is used to define six weather states that represent distinct patterns of rainfall across the region, and composites are used to examine atmospheric circulation during each state. The states represent the occurrence of region-wide dry and wet conditions associated with a large-scale ridge and trough over the Northeast, respectively, as well as inland and coastal storm tracks. There is a positive trend in the frequency of the weather state associated with heavy, regionwide rainfall, which is mirrored by a decreasing trend in the frequency of stationary ridges and regionwide dry conditions. The frequency of state occurrences is also examined for historical Northeast droughts. Two primary drought types emerge that are characterized by region-wide dry conditions linked to a persistent ridge and an eastward-shifted storm track associated with light precipitation along the coastline. Finally, composites of May sea surface temperature anomalies (SSTAs) prior to summers with high and low frequencies of each weather state are used to assess long-lead predictability. These composites are compared against similar composites based on regional anomalies in low streamflow conditions [June–August 7-day low flows (SDLFs)]. Results indicate that springtime SSTs, particularly those in the Caribbean Sea and tropical North Atlantic Ocean, provide some predictability for summers with above-average precipitation and SDLFs, but SSTs provide little information on the occurrence of drought conditions across the Northeast.

A Study of Objective Prediction for Summer Precipitation Patterns Over Eastern China Based on a Multinomial Logistic Regression Model

The prediction of summer precipitation patterns (PPs) over eastern China is an important and topical issue in China. Predictors that are selected based on historical information may not be suitable for the future due to non-stationary relationships between summer precipitations and corresponding predictors, and might induce the instability of prediction models, especially in cases with few predictors. This study aims to investigate how to learn as much information as possible from various and numerous predictors reflecting different climate conditions. An objective prediction method based on the multinomial logistic regression (MLR) model is proposed to facilitate the study. The predictors are objectively selected from a machine learning perspective. The effectiveness of the objective prediction model is assessed by considering the influence of collinearity and number of predictors. The prediction accuracy is found to be comparable to traditionally estimated predictability, ranging between 0.6 and 0.7. The objective prediction model is capable of learning the intrinsic structure of the predictors, and is significantly superior to the prediction model with randomly-selected predictors and the single best predictor. A robust prediction can be generally obtained by learning information from plenty of predictors, although the most effective model may be constructed with fewer predictors through proper methods of predictor selection. In addition, the effectiveness of objective prediction is found to generally improve as observation increases, highlighting its potential for improvement during application as time passes.

Does El Niño play an early signal role for the south-flood north-drought pattern over eastern China?

Reconstructed hydroclimatic spatial patterns from historical documents, the chronology of El Nino events since 1525, Meiyu (a unique rainy season dominated by the East Asian summer monsoon in eastern China) series since 1736, and a 1000-year control simulation using the Community Earth System Model were used to analyze the occurrence of anomalous summer precipitation patterns over eastern China with El Nino events. The results show that the frequency of the south-flood north-drought during 1525–2005 was significantly higher during El Nino episodes. Specifically, the occurrences of the triple pattern with floods in the Yangtze River Valley but droughts in its north and south (FYRDNS) as well as the dipole pattern with floods in the south but droughts in the north (FSDN) divided by the Yangtze River both increased in El Nino developing years, while the occurrence of FSDN increased in El Nino decaying years. Among 44 severe and extreme Meiyu events over the middle and lower reaches of the Yangtze River in 1736–2016, 75% of events occurred during El Nino episodes. Especially, 80% of extreme Meiyu events occurred in El Nino decaying years. The 1000-year control simulation captured well the reconstructed triple pattern of FYRDNS in developing years and the dipole pattern of FSDN in decaying years, with more noticeable precipitation anomalies for developing years of very strong and extremely strong El Nino events. These results implicated that the El Nino is likely an early signal for the occurrence of the south-flood north-drought over eastern China.