Summer Precipitation Patterns Research Articles

Assessing Global Warming Induced Changes in Summer Rainfall Variability over Eastern China Using the Latest Hadley Centre Climate Model HadGEM3-GC2

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.

Pacific sea surface temperature related influences on North American monsoon precipitation within North American Regional Climate Change Assessment Program models

Climate inter‐annual variability over the North American monsoon (NAM) region is associated with El Niño‐Southern Oscillation (ENSO) and Pacific decadal variability (PDV), which drive a warm season atmospheric teleconnection response. Using the North American Regional Climate Change Assessment Program (NARCCAP) simulations, previous studies have found that regional models forced with an atmospheric reanalysis (NARCCAP Phase I) represent the NAM reasonably well as a climatological feature. However, when these same regional models are forced with global climate model projections (NARCCAP Phase II), their ability to represent the NAM as a salient feature substantially degrades. The present study evaluates NAM inter‐annual climate variability through the continental‐scale patterns of summer precipitation within the NARCCAP simulations (Phases I and II), in relation to ENSO–PDV, and the presence of the driving atmospheric teleconnection response. Multivariate statistical analyses are applied to sea surface temperature and precipitation data sets to determine dominant variability at continental scale, with focus on the southwest. The analysis reveals that NARCCAP Phase I simulations are able to portray the spatial pattern of precipitation associated with ENSO–PDV in a similar way to observations. However, all NARCCAP Phase II simulations, with the exception of the HRM(Hadcm3) regional–global model pair, fail to reproduce this climate variability. Although including all possible NARCCAP model simulations to generate a multi‐model ensemble mean would increase the statistical degree of confidence in climate projections, this type of result would not increase confidence in the physical climatology of model representations of warm season climate variability. More physically based, process‐oriented metrics are needed to evaluate model quality in assessing the uncertainty of future climate change in multi‐model ensemble products used for climate change impacts assessments.

Effects of Permafrost Degradation on the Hydrological Regime in the Source Regions of the Yangtze and Yellow Rivers, China

Climate warming has intensified permafrost degradation, which could have a variety of implications on the hydrological regime in permafrost regions. In this study, we analyzed the effects of permafrost degradation on the hydrological regime via four hydrological variables for 10 unregulated catchments in the source regions of the Yangtze and Yellow rivers. The results demonstrate that catchments with high permafrost coverage are expected to have an increased winter discharge ratio (proportion of winter discharge contribution to total annual flow), a decreased recession coefficient and a decreased ratio of Qmax/Qmin due to permafrost degradation. However, the great storage effects of lakes and wetlands, which could contribute to more groundwater instead of direct surface discharge, may affect the hydrological effects of permafrost degradation and result in the abnormal performance at catchment scale. The correlation analysis between summer precipitation (July–September) and the following winter discharge (December–February) indicates that permafrost degradation may affect the redistribution of summer precipitation towards the following winter discharge via increasing the soil storage capacity and delaying the release of water into streams in permafrost regions. However, unlike the Arctic and sub-Arctic regions, no significant changes for the hydrological regime (four hydrological variables) are detected over the individual periods of records for each catchment. Decreased precipitation in summer seems to reduce the water infiltration to supply the groundwater, which weakens the effects of permafrost degradation on the hydrological regime. This study implies that the storage effects of lakes and wetlands and the changes of summer precipitation patterns should be considered in future permafrost hydrological simulations, which have suggested that a large increase in groundwater discharge to streams will likely occur in response to permafrost degradation due to the warming climate in the ideal scenario.

Hydroclimatic variability in loess δDwax records from the central Chinese Loess Plateau over the past 250 ka

This study reports hydrogen isotopic records from the central Chinese Loess Plateau (CLP) over the past 250 ka. After eliminating the influence of ice and local temperatures, the δDwax records extracted from two loess sites at Xifeng and Luochuan can be taken to represent arid/humid alternations in the hydrological environment in this marginal Asian Summer Monsoon (ASM) region; they also contain integrated information on summer precipitation patterns and the corresponding responses to these changes by predominant vegetation cover types. These arid/humid alternations show 100 ka, 40 ka and 20 ka cycles. An increase in precipitation in association with an enhanced summer monsoon has historically been taken to be the major factor driving a humid environment in the central CLP. However, hydroclimatic changes in δDwax records differ for the central CLP, central China and southern China. Over a 20 ka cycle, the influence of solar insolation on hydroclimatic changes can be shown to be consistent throughout the central CLP. However, changes in the relative location of the land and sea may have caused different hydroclimatic responses between southern China and the central CLP on a glacial-interglacial scale. The hydroclimatic variability in the central CLP would suggest that an enhanced summer monsoon due to climatic warming is the key to understanding decreased drought degree in this marginal monsoonal region.

Recent interdecadal changes in the interannual variability of precipitation and atmospheric circulation over northern Eurasia

This study investigated the interannual variability and trends in precipitation and atmospheric circulation patterns over northern Eurasia using long-term Precipitation REConstruction over Land and atmospheric Japanese 55-year Reanalysis data (JRA-55) from 1958 to 2012. Special emphasis was placed on the recent increase in summer (June, July and August) precipitation around the Lena river basin in eastern Siberia. We found interdecadal modulation in the relationships between interannual variability in summer precipitation and atmospheric circulation patterns among the three major Siberian river basins (Lena, Yenisei, and Ob). The interannual variations in summer precipitation over the Ob and Lena river basins were negatively correlated from the mid-1970s to the mid-1990s. However, after the mid-1990s, this negative correlation became insignificant. In contrast, a significant positive correlation was apparent between the Yenisei and Lena river basins. We also found that there has been a significant increasing (positive) trend in geopotential height in the low-level troposphere since the mid-1980s over Mongolia and European Russia, resulting in an increasing trend of westerly moisture flux into the Yenisei and Lena river basins. Summer precipitation in both basins was continuously high from 2005 to 2008 under a trough that broadly extended from the Yenisei and Lena river basins, which has been a typical pattern of interannual variation since the mid-1990s. This trough increased the meridional pressure gradient between Mongolia and eastern Siberia in combination with the trend pattern. This further enhanced the eastward moisture flux towards the Lena river basin and its convergence over the basin, resulting in high summer precipitation from 2005 to 2008.

2014, The “year without a summer” in Italy: news media coverage and implications for the climate change debate

In 2014, there was virtually no summer in northern and central-southern Italy. Storm after storm battered the peninsula, triggering floods and landslides from Veneto to Puglia. We studied the coverage of “the year without a summer” in Italy by analyzing the content of 171 news articles from two influential online newspapers. Our software-based analysis enabled us to observe that the two newspapers hardly ever mentioned climate change in their coverage of the weather anomaly that affected Italy in the summer of 2014. This type of coverage is in line with climate science, according to which there is no evidence of a climate change-related influence on summer precipitation patterns in Southern Europe—whereas such influence has been documented for northern Europe. We compared our results with a recent paper, which documented that the same online dailies chose to represent the particularly hot summer of 2012 in Italy as a direct consequence of climate change. We corroborated this comparison also on the basis of a preliminary analysis we performed on the media coverage of the exceptionally hot and arid summer of 2015 in Italy.

Modulation of monthly precipitation patterns over East China by the Pacific Decadal Oscillation

Previous studies suggest that the Pacific Decadal Oscillation (PDO) modulates annual and summer precipitation patterns over East China. In this study, the effect of the PDO on monthly precipitation anomalies over this region is investigated. The new results show that the effect is month-dependent. The well-known North–South dipole patterns of annual precipitation are dominated by the July–August precipitation. In other months, the corresponding patterns vary in strength, position, and even shape. For example, the May and June precipitation patterns show opposite signs to the July–August or annual mean patterns, whereas the September–December monthly precipitation anomalies show a triple pattern. Monthly precipitation patterns over East China are largely determined by large-scale moisture transport controlled by atmospheric circulation. The PDO affects East China precipitation patterns by modulating the large-scale circulation pattern.

Observed variability of summer precipitation pattern and extreme events in East China associated with variations of the East Asian summer monsoon

ABSTRACTThis article presents a comprehensive analysis of interannual and interdecadal variations of summer precipitation and precipitation‐related extreme events in East China associated with variations of the East Asian summer monsoon (EASM) from 1979 to 2012. A high‐quality daily precipitation data set covering 2076 observational stations in China is analysed. Based on the precipitation pattern analysis using empirical orthogonal functions and the regime shift detection method, three sub‐periods of 1979–1992 (period I), 1993–1999 (period II) and 2000–2012 (period III) are identified to be representative of the precipitation variability. Similar significant variability of the extreme precipitation indices is found across four sub‐regions in eastern China. The spatial patterns of summer mean precipitation, the number of days with daily rainfall exceeding 95th percentile precipitation (R95p) and the maximum number of consecutive wet days (CWD) anomalies are consistent, but opposite to that of maximum consecutive dry days (CDD) anomalies to some extent during the three sub‐periods. However, the spatial patterns of hydroclimatic intensity (HY‐INT) are notably different from that of the other three extreme indices, but highly correlated to the dry events. The changes of precipitation anomaly patterns are accompanied by the change of the EASM regime and the abrupt shift of the position of the west Pacific subtropical high around 1992/1993 and 1999/2000, respectively, which influence the moisture transport and contributes to the precipitation anomalies. In addition, the EASM intensity is linked to sea surface temperature anomaly over the tropical Indian and Pacific Ocean through its effects on convective activity over the west Pacific that induces the cyclonic or anticyclonic anomaly over the South China and northwest Pacific.

Modulation of the Pacific Decadal Oscillation on the summer precipitation over East China: a comparison of observations to 600-years control run of Bergen Climate Model

Observations show that the summer precipitation over East China often goes through decadal variations of opposite sign over North China and the Yangtze River valley (YRV), such as the “southern flood and northern drought” pattern that occurred during the late 1970s–1990s. In this study it is shown that a modulation of the Pacific Decadal Oscillation (PDO) on the summer precipitation pattern over East China during the last century is partly responsible for this characteristic precipitation pattern. During positive PDO phases, the warm winter sea surface temperatures (SSTs) in the eastern subtropical Pacific along the western coast of North American propagate to the tropics in the following summer due to weakened oceanic meridional circulation and the existence of a coupled wind–evaporation–SST feedback mechanism, resulting in a warming in the eastern tropical Pacific Ocean (5°N–20°N, 160°W–120°W) in summer. This in turn causes a zonal anomalous circulation over the subtropical–tropical Pacific Ocean that induces a strengthened western Pacific subtropical high (WPSH) and thus more moisture over the YRV region. The end result of these events is that the summer precipitation is increased over the YRV region while it is decreased over North China. The suggested mechanism is found both in the observations and in a 600-years fully coupled pre-industrial multi-century control simulations with Bergen Climate Model. The intensification of the WPSH due to the warming in the eastern tropical Pacific Ocean was also examined in idealized SSTA-forced AGCM experiments.

Sensitivity of summer precipitation over the Korean Peninsula to temperature gradients

ABSTRACTThe warm‐season hydroclimatology stemming primarily from the monsoon and typhoons in East Asia is an important determinant of sustainable water supplies for coupled human and natural systems. Therefore, in this study, we develop an empirical approach to the separation of summer rainfall into typhoon and monsoon types. In addition, exploratory analysis is conducted for regional patterns of summer precipitation across the Korean Peninsula (KP) to determine changes in the various types of temperature gradient. The results show that atmospheric thermal transitions lead significant and consistent changes in rainfall during the boreal summer season (June–September) in South Korea. For positive land–ocean contrast years, statistically significant positive anomalies of summer precipitation were exhibited in the interior and eastern river basins. In contrast, for negative years, suppression of seasonal precipitation patterns was detected. Moreover, during years of positive meridional temperature gradient, summers tended to be wetter relative to the long‐term average. Although the time window of typhoon impacts may vary for different episodic typhoon events, the obtained positive anomalies of seasonal precipitation were more significant after removing the effects of tropical cyclones over the KP.

Analysis of watershed topography effects on summer precipitation variability in the southwestern United States

With climate change, precipitation variability is projected to increase. The present study investigates the potential interactions between watershed characteristics and precipitation variability. The watershed is considered as a functional unit that may impact seasonal precipitation. The study uses historical precipitation data from 370 meteorological stations over the last five decades, and digital elevation data from regional watersheds in the southwestern United States. This domain is part of the North American Monsoon region, and the summer period (June–July–August, JJA) was considered. Based on an initial analysis for 1895–2011, the JJA precipitation accounts, on average, for 22–43% of the total annual precipitation, with higher percentages in the arid part of the region. The unique contribution of this research is that entropy theory is used to address precipitation variability in time and space. An entropy-based disorder index was computed for each station’s precipitation record. The JJA total precipitation and number of precipitation events were considered in the analysis. The precipitation variability potentially induced by watershed topography was investigated using spatial regionalization combining principal component and cluster analysis. It was found that the disorder in precipitation total and number of events tended to be higher in arid regions. The spatial pattern showed that the entropy-based variability in precipitation amount and number of events gradually increased from east to west in the southwestern United States. Regarding the watershed topography influence on summer precipitation patterns, hilly relief has a stabilizing effect on seasonal precipitation variability in time and space. The results show the necessity to include watershed topography in global and regional climate model parameterizations.

Spatial Modeling of Summer Precipitation over the Czech Republic Using Physiographic Variables

AbstractThis study aims to develop the spatial model of summer precipitation over the Czech Republic using auxiliary geographical variables in order to produce gridded data needed for various applications. Accordingly, the most significant geographical variables are used to model spatial patterns of summer precipitation during the period 1971–2003. They are related to latitude; longitude; slope aspects of the neighbourhood grids westward and northward from the central grid (location); the slopes of the neighbourhood grids north‐eastward, eastward, and northward from the central grid; maximum value of elevation (percentile 95%) of the neighbourhood grid north‐westward from the central grid; and minimum value of elevation (percentile 5%) of the central grid and vegetation. Different approaches used for selecting significant predictors (i.e., the stepwise and principal component analysis) are compared and considered for spatial prediction. Precipitation data from 203 stations for the period 1971–2003 are used. The kriging of residuals (KR) is used to interpolate summer precipitation during this period onto the 1 × 1 km grid over the Czech Republic. Spatial prediction is calibrated across 153 stations and validated to 50 stations. The final KR prediction model shows a high coefficient of determination (68–70%) and moderate error (5–6 mm mean absolute error or 8 mm root mean square error). The standard error of predicted maps represents about 23% of a global variance. The KR model of summer precipitation is accurately applied to model the second episode of extreme precipitation events in summer 1997 over the Czech Republic.

Influence of the circumglobal wave-train on European summer precipitation

We investigate European summer (July–August) precipitation variability and its global teleconnections using the NCEP/NCAR reanalysis data (1950–2010) and a historical Coupled Model Intercomparison Project climate simulation (1901–2005) carried out using the ECHAM6/MPIOM climate model. A wavelike pattern is found in the upper tropospheric levels (200 hPa) similar to the summer circumglobal wave train (CGT) extending from the North Pacific to the Eurasian region. The positive phase of the CGT is associated with upper level anomalous low (high) pressure over western (eastern) Europe. It is further associated with a dipole-like precipitation pattern over Europe entailing significantly enhanced (reduced) precipitation over the western (eastern) region. The anomalous circulation features and associated summer precipitation pattern over Europe inverts for the negative CGT phase. Accordingly, the global teleconnection pattern of a precipitation index summarizing summer precipitation over Western Europe entails an upper level signature which consists of a CGT-like wave pattern extending from the North Pacific to Eurasia. The imprint of the CGT on European summer precipitation is distinct from that of the summer North Atlantic Oscillation, despite the two modes of variability bear strong similarities in their upper level atmospheric pattern over Western Europe. The analysis of simulated CGT features and of its climatic implications for the European region substantiates the existence of the CGT-European summer precipitation connection. The summer CGT in the mid-latitude therefore adds to the list of the modes of large-scale atmospheric variability significantly influencing European summer precipitation variability.

Antagonism between elevated CO2, nighttime warming, and summer drought reduces the robustness of PSII performance to freezing events

Plant responses to warming, elevated CO2, and changes in summer precipitation patterns involve complex interactions. In this study we aim to reveal the single factor responses and their interactive effects on photosystem II (PSII) performance during an autumn-to-winter period. The study was carried out in the CLIMAITE multifactor experiment, which includes the combined impact of elevated CO2 (free air carbon enrichment; CO2), warming (passive nighttime warming; T) and summer drought (rain-excluding curtains; D) in a temperate heath ecosystem. PSII performance was probed by the effective quantum yield in light, Fv′/Fm′, using the pulse amplitude methodology, and the total performance index, PItotal, which integrate changes of the chlorophyll-a fluorescence transient including the maximal quantum yield in darkness, Fv/Fm.Decreasing temperature during autumn linearly reduced PItotal, both in the wavy hair-grass, Deschampsia flexuosa, and in the evergreen dwarf shrub common heather, Calluna vulgaris, and following freezing events the PItotal and Fv′/Fm′ were reduced even more. Contrary to expected, indirect effects of the previous summer drought reduced PSII performance before freezing events, particularly in Calluna. In combinations with elevated CO2 interactive effects with drought, D×CO2 and warming, T×D×CO2, were negatively skewed and caused the reduction of PSII performance in both species after occurrence of freezing events. Neither passive nighttime warming nor elevated CO2 as single factors reduced PSII performance via incomplete cold hardening as hypothesized. Instead, the passive nighttime warming strongly increased PSII performance, especially after freezing events, and when combined with elevated CO2 a strongly skewed positive T×CO2 interactive effect was seen. This indicates that these plants take advantage of the longer growing season induced by the warming in elevated CO2 until a winter frost period becomes permanent. However, if previously exposed to summer drought this positive effect reverses via interactive D×CO2 and T×D×CO2 effects immediately after freezing events, causing the full combination of TDCO2 not to differ from the control.In a future warmer climate with high CO2 and summer drought, the occurrence of freezing events thus seem highly decisive for reducing PSII performance in the autumn-to-winter period. Such a reduced robustness of PSII performance may be highly decisive for the magnitude of the late season photosynthetic carbon uptake and reduce the growing season length in these temperate heath plants.

U.S. Summer Precipitation and Temperature Patterns Following the Peak Phase of El Niño

Abstract Evidence for spatially coherent, but different, U.S. summer precipitation and surface air temperature anomalies during the evolving phase and during the summers following the peak phase of the winter El Niño is presented. The spatial patterns during the decaying phase of El Niño are distinctive from patterns in the preceding summer when El Niño is in its evolving phase, that is, the traditional “simultaneous” composite patterns associated with El Niño. The analysis of a multimodel ensemble of global atmospheric models forced by observed sea surface temperature further confirms that the differences in the U.S. summer precipitation and surface temperature anomalies between the developing and decaying phases of El Niño are a result of the atmospheric response to tropical warm SST anomalies that are shifted eastward and are confined east of 120°W during the decaying phase of El Niño. Given the distinctive pattern, and relatively large amplitude of these anomalies during the decaying phase of El Niño, the results may have implications for the seasonal prediction of U.S. summer precipitation and temperature following winter El Niños.

Increased temperature and altered summer precipitation have differential effects on biological soil crusts in a dryland ecosystem

AbstractBiological soil crusts (biocrusts) are common and ecologically important members of dryland ecosystems worldwide, where they stabilize soil surfaces and contribute newly fixed C and N to soils. To test the impacts of predicted climate change scenarios on biocrusts in a dryland ecosystem, the effects of a 2–3 °C increase in soil temperature and an increased frequency of smaller summer precipitation events were examined in a large, replicated field study conducted in the cold desert of the Colorado Plateau, USA. Surface soil biomass (DNA concentration), photosynthetically active cyanobacterial biomass (chlorophyll a concentration), cyanobacterial abundance (quantitative PCR assay), and bacterial community composition (16S rRNA gene sequencing) were monitored seasonally over 2 years. Soil microbial biomass and bacterial community composition were highly stratified between the 0–2 cm depth biocrusts and 5–10 cm depth soil beneath the biocrusts. The increase in temperature did not have a detectable effect on any of the measured parameters over 2 years. However, after the second summer of altered summer precipitation pattern, significant declines occurred in the surface soil biomass (avg. DNA concentration declined 38%), photosynthetic cyanobacterial biomass (avg. chlorophyll a concentration declined 78%), cyanobacterial abundance (avg. gene copies g−1 soil declined 95%), and proportion of Cyanobacteria in the biocrust bacterial community (avg. representation in sequence libraries declined 85%). Biocrusts are important contributors to soil stability, soil C and N stores, and plant performance, and the loss or reduction of biocrusts under an altered precipitation pattern associated with climate change could contribute significantly to lower soil fertility and increased erosion and dust production in dryland ecosystems at a regional scale.

The recent interdecadal and interannual variation of water vapor transport over eastern China

The climatological characteristics and interdecadal variability of the water vapor transport and budget over the Yellow River-Huaihe River valleys (YH1) and the Yangtze River-Huaihe River valleys (YH2) of East China were investigated in this study, using the NCEP/NCAR monthly mean reanalysis datasets from 1979 to 2009. Changes in the water vapor transport pattern occurred during the late 1990s over YH1 (YH2) that corresponded with the recent interdecadal changes in the eastern China summer precipitation pattern. The net moisture influx in the YH1 increased and the net moisture influx in the YH2 decreased during 2000–2009 in comparison to 1979–1999. Detailed features in the moisture flux and transport changes across the four boundaries were explored. The altered water vapor transport over the two domains can be principally attributed to the additive effects of the changes in the confluent southwesterly moisture flow by the Indian summer monsoon and East Asian summer monsoon (related with the eastward recession of the western Pacific subtropical high). The altered water vapor transport over YH1 was also partly caused by the weakened midlatitude westerlies.

Onset of summer flowering in a ‘Sky Island’ is driven by monsoon moisture

Temperatures for the southwestern USA are predicted to increase in coming decades, especially during the summer season; however, little is known about how summer precipitation patterns may change. We aimed to better understand how nonsucculent plants of a water-limited gradient encompassing xeric desert to mesic mountain-top may respond to changes in summer conditions. We used a species-rich 26-yr flowering record to determine species' relationships with precipitation and temperature in months coincident with and previous to flowering. The onset of summer flowering was strongly influenced by the amount and timing of July precipitation, regardless of elevation or life form, suggesting the critical importance of soil moisture in triggering summer flowering in this region. Future changes in the timing or consistency of the early monsoon will probably impact directly on the onset of flowering for many species in this region. In addition, a key implication of predicted increasing temperatures is a decrease in available soil moisture. At all elevations, many species may be expected to flower later in the summer under the decreased soil moisture conditions associated with warmer temperatures. However, impacts on summer flowering may be greater at higher elevations, because of the greater sensitivity of mesic plants to water stress.

Recent changes in the summer precipitation pattern in East China and the background circulation

This study documents the decadal changes of the summer precipitation in East China, with increased rainfall in the Huang-Huai River region (HR) and decreased in the Yangtze River region (YR) during 2000–2008 in comparison to 1979–1999. The main features of the atmospheric circulation related to the increased precipitation in the HR are the strengthened ascending motion and slightly increased air humidity, which is partly due to the weakened moisture transport out of the HR to the western tropical Pacific (associated with the weakened westerly over East Asia and the warming center over the Lake Baikal). The rainfall decrease in the YR is related to the weakened ascending motion and reduced water vapor content, which is mainly related to the weakened southwesterly moisture flux into the YR (associated with the eastward recession of the Western Pacific Subtropical High). The global sea surface temperature (SST) also shows significant changes during 2000–2008 relative to 1979–1999. The shift of the Pacific decadal oscillation (PDO) to a negative phase probably induces the warming over the Lake Baikal and the weakened westerly jet through the air-sea interaction in the Pacific, and thus changes the summer precipitation pattern in East China. Numerical experiments using an atmospheric general circulation model, with prescribed all-Pacific SST anomalies of 2000–2008 relative to 1979–1999, also lend support to the PDO’s contribution to the warming over the Lake Baikal and the weakened westerlies over East China.

Interdecadal unstationary relationship between NAO and east China's summer precipitation patterns

Based on a 120‐year Chinese station rainfall dataset and historical NAO indices, the interdecadal unstationary relationship between the North Atlantic Oscillation (NAO) and east China's summer precipitation patterns is revealed for the period 1880–1999. It is shown that on the interannual timescale, the March NAO is closely related to the first leading precipitation mode which exhibits an out‐of‐phase variation of the precipitation between the Yangtze River Valley and Southeast China, and the January NAO is closely related to the third leading mode which depicts the anomalous rainfall in North China and its out‐of‐phase variation in the Yangtze River Delta. Both of the two relationships are characterized by clear and consistent interdecadal variations, with almost the same transition points occurring in about 1905, 1925 and 1950, respectively. Further analyses imply that these interdecadal changes of the relationship are possibly associated with the SSTA in North Pacific and North Atlantic.