Summer Precipitation Anomalies Research Articles

Role of Soil Moisture Feedback in the Development of Extreme Summer Drought and Flood in the United States

Abstract This study tackles the contribution of soil moisture feedback to the development of extreme summer precipitation anomalies over the conterminous United States using a regional climate model. The model performs well in reproducing both the mean climate and extremes associated with drought and flood. A large set of experiments using the model are conducted that involve swapped initial soil moisture between flood and drought years using the 1988 and 2012 droughts and 1993 flood as examples. The starting time of these experiments includes 1 May (late spring) and 1 June (early summer). For all three years, the impact of 1 May soil moisture swapping is much weaker than the 1 June soil moisture swapping. In 1988 and 2012, replacing the 1 June soil moisture with that from 1993 reduces both the spatial extent and the severity of the simulated summer drought and heat. The impact is especially strong in 2012. In 1993, however, replacing the 1 June soil moisture with that from 1988 has little impact on precipitation. The contribution of soil moisture feedback to summer extremes is larger in 2012 than in 1988 and 1993. This may be because of the presence of strong anomalies in large-scale forcing in 1988 and 1993 that prohibit or favor precipitation, and the lack of such in 2012. This study demonstrates how the contribution of land–atmosphere feedback to the development of seasonal climate anomalies may vary from year to year and highlights its importance in the 2012 drought.

Recent speed‐up of an alpine rock glacier: an updated chronology of the kinematics of outer hochebenkar rock glacier based on geodetic measurements

Surface velocities have been regularly monitored at the rock glacier in Outer Hochebenkar, Ötztal Alps, Austria since the early 1950s. This study provides an update to previously published surface velocity time series, showing mean profile velocities of four cross profiles since the beginning of the measurements (1951,1954, 1997; depending on the profile), as well as single block displacements from 1998 to 2015. Profiles P1, P2 and P3 have moved between 42 and 90 m, at mean velocities between 0.70 and 1.48 m yr–1, since they were first established in the early 1950s (1951/54). Profile P0, established in 1997, has since moved 13 m or 0.75 m yr–1. An acceleration can be observed at all profiles since the late 1990s, with a particularly sharp velocity increase since 2010. All profiles reached a new maximum velocity in 2015, with 1.98 m yr–1 at the slowest profile (P0) and 6.37 m yr–1 at the fastest profile (P1). Year‐to‐year variations in profile velocities cannot be clearly attributed to inter‐annual variations of climatic parameters like mean annual air temperature, summer temperature, positive degree days, or precipitation. However, higher correlation is found between velocities and cumulative anomalies of air temperature (mean annual air temperature and positive degree days) and summer precipitation, suggesting that these parameters play a key role for the movement of the rock glacier. The lower profiles (P0, P1) show more pronounced year‐to‐year variations than the upper profiles (P2, P3). It is considered likely that processes other than climatic forcing (e.g. sliding, topography) contribute to the different velocity patterns at the four profiles.

On the relationship between the winter Eurasian teleconnection pattern and the following summer precipitation over China

The Eurasian teleconnection pattern (EU) is an important low-frequency pattern with well-known impacts on climate anomalies in Eurasia. The difference of low-level v-winds in several regions in the Eurasian mid–high latitudes is defined as the EU index (EUIV). In this study, the relationship between the winter EUIV and precipitation in the following summer over China is investigated. Results show that there is a significant positive (negative) correlation between the winter EUIV and the following summer precipitation over North China (the Yangtze River–Huaihe River basins). Meanwhile, an interdecadal variability exists in the interannual relationship, and the correlation has become significantly enhanced since the early 1980s. Thus, the proposed EUIV may have implications for the prediction of summer precipitation anomalies over China. In positive winter EUIV years, three cyclonic circulation anomalies are observed—over the Ural Mountains, the Okhotsk Sea, and the subtropical western North Pacific. That is, the Ural blocking and Okhotsk blocking are inactive, zonal circulation prevails in the mid–high latitudes, and the western Pacific subtropical high tends to be weaker and locates to the north of its normal position in the following summer. This leads to above-normal moisture penetrating into the northern part of East China, and significant positive (negative) precipitation anomalies over North China (the Yangtze River–Huaihe River basins), and vice versa. Further examination shows that the SST anomalies over the Northwest Pacific and subtropical central North Pacific may both contribute to the formation of EUIV-related circulation anomalies over the western North Pacific.

Summer precipitation anomalies in Asia and North America induced by Eurasian non-monsoon land heating versus ENSO.

When floods ravage Asian monsoon regions in summer, megadroughts often attack extratropical North America, which feature an intercontinental contrasting precipitation anomaly between Asia and North America. However, the characteristics of the contrasting Asian-North American (CANA) precipitation anomalies and associated mechanisms have not been investigated specifically. In this article, we firmly establish this summer CANA pattern, providing evidence for a significant effect of the land surface thermal forcing over Eurasian non-monsoon regions on the CANA precipitation anomalies by observations and numerical experiments. We show that the origin of the CANA precipitation anomalies and associated anomalous anticyclones over the subtropical North Pacific and Atlantic has a deeper root in Eurasian non-monsoon land surface heating than in North American land surface heating. The ocean forcing from the ENSO is secondary and tends to be confined in the tropics. Our results have strong implications to interpretation of the feedback of global warming on hydrological cycle over Asia and North America. Under the projected global warming due to the anthropogenic forcing, the prominent surface warming over Eurasian non-monsoon regions is a robust feature which, through the mechanism discussed here, would favor a precipitation increase over Asian monsoon regions and a precipitation decrease over extratropical North America.

On the Relationship between the Northern Limit of Southerly Wind and Summer Precipitation over East China

AbstractThe relationship between the summer northernmost position of southerly wind and precipitation over East China is investigated. The northern limit of summer southerly wind index (INLSSW) over East China is defined as the latitude where the zonal-averaged (105–120°E) low-level meridional wind is equal to zero. Results show that there is a significant negative (positive) correlation between INLSSW and summer precipitation over the Yangtze River (North China) region. Thus, the proposed INLSSW may have implications for the prediction of summer precipitation anomalies in these regions. In positive INLSSW years, a cyclonic circulation anomaly is observed over the tropical western North Pacific and an anticyclonic circulation anomaly is seen over the subtopics of East China, accompanied by southerly anomalies over East China. This leads to above-normal moisture penetrating into the northern part of East China. In addition, significant upward (downward) motion anomalies can be found over the North China (Y...

Potential Connection between the Australian Summer Monsoon Circulation and Summer Precipitation over Central China

AbstractThis study investigated the connection between the Australian summer monsoon (ASM) and summer precipitation over central China. It was found that, following a weaker-than-normal ASM, the East Asian summer monsoon and western North Pacific subtropical high tend to be stronger, yielding anomalous northward moisture to be transported from the western Pacific to central China. Besides, anomalous upwelling motion emerges over 30–37.5°N, along 110°E. Consequently, significant positive summer precipitation anomalies are located over central China. Further analysis indicated that the boreal winter sea surface temperature (SST) in the Indian Ocean and South China Sea shows positive anomalies in association with a weaker-than-normal ASM. The Indian Ocean warming in boreal winter could persist into the following summer because of its own long memory, emanating a baroclinic Kelvin wave into the Pacific that triggers suppressed convection and an anomalous anticyclone. Besides, the abnormal SST signal in the So...

Climate change assessment for a small island: a Tahiti downscaling experiment

This study addresses the projected 21st century changes in austral summer precipitation and future precipitation anomalies related to El Nino Southern Oscillation (ENSO) for a South Pacific island: Tahiti, French Polynesia. An approach involving 2 successive downscaling steps is proposed here as a first attempt to simulate precipitation over a small orographic island. First, the 50 km mesh global atmospheric model ARPEGE is forced by bias-corrected sea surface temperatures (SSTs) from a CMIP5 scenario. This model drives the limited area model ALADIN. The final 12 km mesh regional simulation coarsely captures the island of Tahiti. Historical and scenario runs (RCP4.5 an d RCP8.5) are therefore available at the island scale. Linking station data and historical model outputs using quantile-quantile plots allows a correction of the biases of the regional simulation and an assessment of precipitation changes over the 21st century. For both scenarios, mean austral summer precipitation tend to increase over the century on the south-east side of Tahiti, following the present El Nino-like precipitation pattern. This trend could be a local manifestation of the large-scale drift of the tropical Pacific Ocean towards an El Nino-like spatial structure. This drift is indeed verified in the SST fields. Then, superimposed upon this slowly evolving mean state, future El Nino events would cause positive precipitation anomalies, whereas future La Nina events would cause negative precipitation anomalies. However, in the last period of RCP8.5, future El Nino events do not seem to affect precipitation anymore, suggesting a damping of the warm events, probably due to a warmer mean state.

Can the tropical storms originated from the Bay of Bengal impact the precipitation and soil moisture over the Tibetan Plateau?

This study investigates the impacts of tropical storms originated from the Bay of Bengal (BOBTSs) on the precipitation and soil moisture over the Tibetan Plateau (TP) in April-June (AMJ) and September-December (SOND) during 1981–2011 based on the best track dataset provided by Joint Typhoon Warning Centre (JTWC). Results indicate that there are about 1.35 BOBTSs influence the TP in each year and most of them occurred in May and October, and the BOBTSs in AMJ influence the TP with larger extension and higher latitudes than those in SOND. The maximum regional precipitation induced by the BOBTSs accounts for more than 50% for the total precipitation in the corresponding month and about 20% for the season. Further analysis reveals that the surface soil moisture anomalies induced by the BOBTSs can persist only 20–25 days in AMJ, and the case is also true for the snow depth in SOND. Numerical simulations by using the regional climate model of Weather Research and Forecasting (WRF) suggest that the soil moisture anomalies in the sub-surface can last 2 months whereas for the surface it can persist only about 20 days, which agrees well with the observation analysis. Overall, the effect of the preceding BOBTSs on the snow depth and soil moisture anomalies over the TP cannot maintain to summer, and there is no robust connection between the BOBTSs and summer precipitation anomalies in East China. Moreover, since the mid-1990s, the spring rainfall induced by the BOBTSs over the TP seems to be enhanced to a certain degree because of the intensified BOBTSs.

Asymmetric Relationship between Indian Ocean SST and the Western North Pacific Summer Monsoon

Abstract The summer precipitation anomalies over the tropical western North Pacific (WNP), which greatly affect East Asian climate, are closely related to Indian Ocean (IO) SST anomalies, and this WNP–IO relationship is widely assumed to be linear. This study indicates that the IO SST–WNP precipitation relationship is generally linear only when the IO SST anomalies are positive and not when the IO SST anomalies are negative, that is, a strongly cooler IO, in comparison with a moderately cooler IO, does not correspond to stronger precipitation enhancement over the WNP. Further analysis suggests that the phases of ENSO play a crucial role in modifying the impacts of IO SSTs on WNP anomalies. The reverse IO SST–WNP precipitation relationship, which exists without apparent ENSO development/decay, is intensified by El Niño decay through the enhancement of IO SST anomalies, but weakened by El Niño development and La Niña decay through the concurrence of SST anomalies in the tropical central and eastern Pacific. After removing El Niño developing and La Niña decaying cases, the IO SST and WNP precipitation anomalies show a clear linear relationship. Because of the effects of the phases of ENSO, the years of negative precipitation or anticyclonic anomalies over the WNP are highly concentrated over strongly warmer IO and El Niño decaying years, which is consistent with previous studies. However, the years of positive precipitation anomalies are scattered over cooler IO and moderately warmer IO years, implying a complexity of tropical SST forcing on positive WNP precipitation anomalies.

Potential Connection between the Australian Summer Monsoon Circulation and Summer Precipitation over Central China

This study investigated the connection between the Australian summer monsoon(ASM) and summer precipitation over central China. It was found that,following a weaker-than-normal ASM, the East Asian summer monsoon and western North Pacific subtropical high tend to be stronger, yielding anomalous northward moisture to be transported from the western Pacific to central China. Besides, anomalous upwelling motion emerges over 30–37.5°N, along 110°E. Consequently,significant positive summer precipitation anomalies are located over central China. Further analysis indicated that the boreal winter sea surface temperature(SST) in the Indian Ocean and South China Sea shows positive anomalies in association with a weaker-than-normal ASM. The Indian Ocean warming in boreal winter could persist into the following summer because of its own long memory, emanating a baroclinic Kelvin wave into the Pacific that triggers suppressed convection and an anomalous anticyclone. Besides, the abnormal SST signal in the South China Sea develops eastward with time because of local air-sea interaction, causing summer SST warming in the western Pacific. The SST warming can further affect East Asian atmospheric circulation and precipitation through its impact on convection.

The possible influence of tropical Pacific subsurface temperature anomalies on summer precipitation in eastern China

Using NCEP (National Centers for Environmental Prediction) GODAS (Global Ocean Data Assimilation System) monthly subsurface temperature (SsT) from 1980 to 2009, a 3-D four-step model was established to categorize the SsT interannual variations in the tropical Pacific. In combination with NCEP/National Center for Atmospheric Research (NCAR) reanalysis 850-hPa wind field and 160 stations' precipitation data in China, an intimate connection between the tropical Pacific SsT and eastern China summer precipitation anomalies is discovered by examining the time-lagged and simultaneous correlations among the interannual anomalies of SsT in the tropical Pacific, the 850-hPa wind pattern and summer rainfall in China. In previous winter, if the SsT in the tropical Pacific indicates an El Niño-type anomaly, an anticyclonic circulation anomaly over the East Asian coast, South China Sea and east of Taiwan along with a cyclonic circulation anomaly over the Japanese archipelago emerges in following summer. At the same time, a cyclonic circulation and an anticyclonic circulation prevail over the Lake Baikal and Kuril Islands in the high latitude. These combined circulation patterns facilitate the southward transport of cold air from high latitude and northward transport of warm and moist air through the western edge of the subtropical high. The meet between two air masses results in significantly abundant anomalous precipitation in the Sichuan Basin, Dongting and Poyang Lake Basin, and possibly deficit precipitation occurs over the Hetao region and lower Yellow River Basin, northern Zhejiang, as well as the coastal region of southern China and southern Yunnan. These findings offer a very useful predictive power for eastern China summer precipitation based on tropical Pacific SsT status in previous winter and subsequent developments of SsT and East Asian large-scale circulations.

Relationship between summer rainfall anomalies and sub-seasonal oscillations in South China

Sub-seasonal variability of summer (May–October) rainfall over South China exhibits two dominant timescales, one with a quasi-biweekly (QBW) period (10–20 days) and the other with an intraseasonal oscillation (ISO) period (20–60 days). A significant positive correlation (at a 99 % confidence level) was found between the summer precipitation anomalies and the intensity of the QBW and ISO modes. By examining the composite structure and evolution characteristics, we note that the QBW and ISO modes are characterized by a northwest-southeast oriented wave train pattern with a pronounced baroclinic vertical structure, moving northwestward. A marked feature is the phase leading of low-level moisture relative to convection. For the QBW mode, such a phase leading feature appears in both the strong and weak composites. However, for the ISO mode, this feature is only clearly seen in the strong composite. The high positive correlation between the summer precipitation and the sub-seasonal variability suggests that the summer mean state may exert a large-scale control on the sub-seasonal modes. It is found that when South China is anomalously wet, large-scale atmospheric conditions in the key QBW/ISO activity region are characterized by deeper moist layer, more convectively unstable stratification, and greater ascending motion. Such environmental conditions favor the growth of the QBW and ISO perturbations.

Modelling the effect of soil moisture variability on summer precipitation variability over East Asia

ABSTRACTThe potential role of surface soil moisture (SSM) in improving the simulation of interannual East Asian summer precipitation is investigated using a coupled land‐atmosphere climate model, the Community Atmosphere Model version 3 (CAM3). Forced by specified observational sea surface temperature (SST), two ensemble simulations for 22 boreal‐summer seasons (1979–2000) are conducted, one with interannually varying SSM (SSMinter) forcing and another with climatological SSM (SSMclim) forcing. Results show that, relative to the SSMclim run, interannual variability of East Asian summer precipitation is better simulated in the SSMinter run over the mid‐ and high‐latitudes of East Asia, especially in northwest China, where the correlation coefficient between precipitation simulated and observed from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) is increased from 0.12 to 0.56 during 1979–2000. Meanwhile, positive relationships between anomalies of local soil evaporation and summer precipitation in northwest China can be better reproduced in the SSMinter run than in the SSMclim run. Possible mechanisms for the improved simulations of the interannual summer precipitation variability in northwest China are analysed; the improvement is resulted from the reasonable reproduction of anomalous atmospheric circulation in Siberia and Iranian plateau.

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.

NCEP 계절예측시스템과 정준상관분석을 이용한 북동아시아 여름철 강수의 예측

현재 최고 수준의 대순환 모형에서 북동아시아 여름몬순 강도의 계절예측 능력은 낮으나 북서태평양 아열대 고기압 강도의 예측률은 상대적으로 높다. 북서태평양 아열대 고기압은 북서태평양 지역 및 동아시아 지역에서 가장 주된 기후 변동성이다. 본 연구에서 NCEP 계절예측시스템에서 예측된 북서태평양 아열대 고기압의 예측성에 대해 논의될 것이다. 한편, 북동아시아 여름몬순의 경년변동성은 북서태평양 아열대 고기압과 높은 상관성을 가지고 있다. 본 연구에서는 이 관계에 근거하여, NCEP 계절예측시스템과 정준상관분석을 이용한 계절예측 모형을 제안하고 그 예측률을 평가하였다. 이 방법은 북동아시아 지역 여름철 강수량 편차에 대한 계절예측에 있어 통계적으로 유의한 예측성능을 제공한다. The seasonal predictability of the intensity of the Northeast Asian summer monsoon is low while that of the western North subtropical high variability is, when state-of-the-art general circulation models are used, relatively high. The western North Pacific subtropical high dominates the climate anomalies in the western North Pacific-East Asian region. This study discusses the predictability of the western North Pacific subtropical High variability in the National Centers for Environmental Prediction Climate Forecast System (NCEP CFS). The interannual variability of the Northeast Asian summer monsoon is highly correlated with one of the western North Pacific subtropical Highs. Based on this relationship, we suggest a seasonal prediction model using NCEP CFS and canonical correlation analysis for Northeast Asian summer precipitation anomalies and assess the predictability of the prediction model. This methodology provides significant skill in the seasonal prediction of the Northeast Asian summer rainfall anomalies.

Oxygen and carbon stable isotopes in coast redwood tree rings respond to spring and summer climate signals

AbstractInterannual variability in the oxygen and carbon isotope composition of tree ring cellulose was investigated in coast redwood (Sequoia sempervirens) from three sites in coastal Northern California. Middle and late wood samples from annual tree rings were compared to regional climate indices and gridded ocean‐atmosphere fields for the years 1952–2003. The strongest climate‐isotope relationship (r = 0.72) was found with summer (June–September) daily maximum temperature and middle wood δ13, which also responds positively to coastal sea surface temperature and negatively to summer low cloud frequency. Late wood δ18O reflects a balance between 18O‐enriched summer fog drip and depleted summer rainwater, while a combined analysis of late wood δ18O and δ13C revealed sensitivity to the sign of summer precipitation anomalies. Empirical orthogonal function analysis of regional summer climate indices and coast redwood stable isotopes identified multivariate isotopic responses to summer fog and drought that correspond to atmospheric circulation anomalies over the NE Pacific and NW U.S. The presence of regional climate signals in coast redwood stable isotope composition, consistent with known mechanistic processes and prior studies, offers the potential for high‐resolution paleoclimate reconstructions of the California current system from this long‐lived tree species.

Different relationships between spring SST in the Indian and pacific oceans and summer precipitation in China

Observational and reanalysis data are used to investigate the different relationships between boreal spring sea surface temperature (SST) in the Indian and Pacific oceans and summer precipitation in China. Partial correlation analysis reveals that the effects of spring Indian Ocean SST (IO SST) and Pacific SST (PSST) anomalies on summer precipitation in China are qualitatively opposite. When IO SST anomalies are considered independently of PSST anomalies, precipitation decreases south of the Yangtze River, in most areas of Inner Mongolia, and in some parts of Liaoning Province, and increases in the Yangtze River valley, parts of southwestern and northern China, northeastern Inner Mongolia, and Heilongjiang Province. This results in a negative-positive-negative-positive pattern of precipitation anomalies in China from south to north. When PSST anomalies (particularly those in the Nino3.4 region) are considered independently of IO SST anomalies, the pattern of precipitation anomalies in China is positive-negative-positive-negative from south to north. The genesis of summer precipitation anomalies in China is also examined when El Nino-Southern Oscillation (ENSO) signals are removed from the ocean and atmosphere. An anticyclonic low-level wind anomaly forms in the South China Sea-Northwest Pacific area when the IO SST anomaly (SSTA) is warm and the Northwest Pacific SSTA is cold. This anticyclonic anomaly substantially influences summer precipitation in China. Anomalous warming of tropical IO SST induces positive geopotential height anomalies in the subtropics and an east-west dipole pattern in midlatitudes over Asia. These anomalies also affect summer precipitation in China.

Influences of the Atlantic Ocean on the summer precipitation of the southeastern Tibetan Plateau

The southeastern Tibetan Plateau is one of the predominant summer rainfall regions in the world and is also the crucial water vapor channel of the Asian summer monsoon. The rainfall variability in the region influences not only the local communities but also downstream communities in East Asia. However, previous studies have exhibited large rainfall biases in this region in state‐of‐the‐art climate models. Understanding the observed rainfall variability provides an opportunity to identify the origin of model biases and to lay a foundation for improving model performance. In this study, the interannual variability of the summer precipitation (May–September) over the southeastern Tibetan Plateau was investigated based on National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis monthly mean data from 1979 to 2010. The associated atmospheric circulation anomalies of the southeastern Tibetan Plateau summer precipitation (SET_PR) display a North Atlantic Ocean‐Europe‐Asia teleconnection pattern, indicating a possible role of the Atlantic climate in the SET_PR. Further studies have revealed that the Atlantic sea surface temperature (SST) anomalies have the greatest influence on the SET_PR via the Rossby wave response, whereas the SST anomalies in the Indo‐Pacific have less of an influence on the SET_PR because their main impacts are confined to the western North Pacific subtropical high and the monsoonal circulation there. This paper also documents the detailed spatial pattern of the atmospheric circulation anomalies associated with the SET_PR and Atlantic SST year‐to‐year variability.

Spring and summer precipitation changes from 1880 to 2011 and the future projections from CMIP5 models in the Yangtze River Basin, China

Precipitation changes in spring (MAM) and summer (JJA) over the Yangtze River Basin during 1880–2011 were analyzed based on observation datasets from 21 gauge stations. The projected changes in precipitation were evaluated using nine Coupled Model Intercomparison Project Phase 5 (CMIP5) datasets. The results suggest that interannual and interdecadal changes in spring and summer precipitation occurred during 1880–2011. The Dry–Dry events of seasonal evolution generally occurred over the past 132 years. Empirical mode decomposition (EMD) was applied to detect the oscillation periods, in which quasi – 3a, 5a, 15a, and 40a periods of spring and quasi – 3a, 5a, 11a, 22a, and 40a periods of summer were identified. S-EOF analysis was employed to identify the major modes of seasonal precipitation evolution from spring to summer. The abnormal location of the western Pacific subtropical high (WPSH) and insufficient water vapor was found to be associated with the 2011 spring drought, and their sudden change is also responsible for the flood in June. A Taylor diagram showed an improved capability to simulate monthly precipitation using the nine CMIP5 datasets. The projected precipitation changes show that the significant positive linear trends of spring precipitation will occur at representative concentration pathways (RCP)2.6 and RCP8.5, whereas summer precipitation will mainly undergo interannual change. For the projected spatial pattern of precipitation anomaly, spring precipitation will increase in the northern basin but decrease in the south under RCP2.6 and RCP8.5 in the 2020s and 2040s. Under RCP4.5, positive anomalies can be found in the middle and lower basins. In contrast to those in the spring pattern, the negative anomalies of summer precipitation are concentrated in the middle and lower basins under each RCP.

Enhanced Central European summer precipitation in the late 19th century: a link to the Tropics

AbstractIn Central Europe, an unusually large number of floods were recorded in the summers and autumns of the late 19th century, causing considerable damage. Different factors contributing to the recurrence of these floods have been discussed in the literature, including the unusually high precipitation anomalies recorded at the time. Based on the frequency and spatial pattern of these floods, previous studies suggest that changes in the large‐scale circulation may have played a relevant role. Here, we use an atmospheric global climate model forced with observed sea surface temperatures (SSTs) and atmospheric forcings (greenhouse gas concentrations and aerosol emissions) to test this hypothesis and identify the causes for the associated atmospheric circulation pattern.We find that our model is able to reproduce the high summer precipitation anomalies observed in Central Europe in the late 19th century, and we show that between 1875 and 1890, transient SSTs, as opposed to climatological ones, are the primary driver for this precipitation increase. Using a series of numerical experiments, we find that the SST variability in the central and eastern tropical Pacific Ocean (associated with the El Niño–Southern Oscillation region) was the primary driver for these anomalies. In addition, SST variability in the Indian Ocean as well as increasing anthropogenic aerosol emissions substantially enhanced European precipitation.The atmospheric circulation anomalies associated with this precipitation increase show a Pacific North American (PNA)‐like pattern over North America and a substantially weakened mid‐troposphere westerly flow over Europe and the North Atlantic, as well as increased pressure over Greenland and reduced pressure over Europe, resembling the negative phase of the summer North Atlantic Oscillation (NAO).