Asian Summer Monsoon Precipitation Research Articles

Thermal Contrast between the Middle-Latitude Asian Continent and Adjacent Ocean and Its Connection to the East Asian Summer Precipitation

Abstract To analyze the middle-to-lower-troposphere atmospheric thermal contrast between the middle latitude over the Asian continent and over its eastern adjacent ocean near Japan, an empirical orthogonal function (EOF) analysis of the 40-yr ECMWF Re-Analysis (ERA-40) data of the June–August (JJA) 500-hPa geopotential height over the Asia–Pacific area (10°–80°N, 60°–180°E) during 1958–2000 was done. It shows that the dominating pattern of the thermal contrast may well be represented by a “seesaw” of 500-hPa geopotential height anomalies between a land area (40°–55°N, 75°–90°E) and an oceanic area (35°–42.5°N, 140°– 150°E). An index showing the difference between the two areas is defined as the middle-latitude land–sea thermal contrast index (LSI). The LSI has significant interannual and interdecadal variability. Its interannual variation is mainly attributed to the atmospheric thermal condition over the ocean, which has a remarkably regional unique feature, while the interdecadal variability is greatly attributed to that over the land. The LSI has a close connection to the East Asian summer precipitation. The results show that large (small) LSI is related to high (low) summer precipitation in the middle to lower reaches of the Yangtze River, Korea, Japan, and its eastern adjacent ocean at the same latitude, and low (high) precipitation in the South China Sea and tropical western Pacific, as well as low (high) precipitation in north China and high-latitude northeast Asia. The pattern of correlation between LSI and precipitation resembles the spatial distribution of the principle EOF mode of year-to-year precipitation variations. Furthermore, the variation of LSI is highly correlated to the time series of the first EOF mode of summer precipitation anomalies. This suggests that the middle-latitude land–sea thermal contrast is one of important factors to influence on the summer precipitation variations over the area from the whole East Asia to the western Pacific. The possible physical mechanisms of the land–sea thermal contrast impacting the East Asian summer monsoon precipitation are also investigated.

Regional summer precipitation events in Asia and their changes in the past decades

A recently derived data set of daily precipitation is used to study the summer precipitation events over Asia and their changes in the decades of 1978–2002. Regional features of the precipitation over entire tropical‐subtropical Asia are investigated, exploiting the increased resolution and improved accuracy of the data set relative to other estimates. The changes in precipitation amount and precipitation days for total, extreme, heavy, and light‐moderate precipitations are examined. Although the Asian summer monsoon precipitation falls mostly in the form of light‐moderate rainfalls, regions of relatively frequent extreme precipitation events are found over South Asia and East‐Southeast Asia. These regions are separated by a narrow zone over the Indo‐China peninsula, along 100°E, where extreme precipitation rarely occurs. During the period examined, the amount of total precipitation and light‐moderate precipitation exhibits positive trends over southeastern and northwestern China, separated by negative trends over central China and southwestern and northeastern Asia. This sandwich‐like pattern, which also appears in the fields of precipitation days and soil moisture content, is associated with the enhanced water vapor supply related to the strengthened monsoon flow over southeastern China and the anomalous easterlies over northwestern China. It is also associated with the decreased water vapor supply linked to the weakened monsoon flows over southern‐southwestern Asia and central China and to the anomalous northerly flow over northeastern Asia. Over the entire tropical‐subtropical Asia, the largest changes in precipitation, atmospheric circulation, and water vapor transport occur over southern China. On the other hand, the changes over India are much smaller.

Interdecadal variations of East Asian summer monsoon northward propagation and influences on summer precipitation over East China

The interdecadal variation of northward propagation of the East Asian Summer Monsoon (EASM) and summer precipitation in East China have been investigated using daily surface rainfall from a dense rain gauge network in China for 1957–2001, National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, European Center for Medium-Range Weather Forecast (ECMWF) reanalysis, and Global Mean Sea Level Pressure Dataset (GMSLP2) from Climatic Research Unit (CRU). Results in general show a consistent agreement on the interdecadal variability of EASM northward propagations. However, it appears that the interdecadal variation is stronger in NCEP than in ECMWF and CRU datasets. A newly defined normalized precipitation index (NPI), a 5-day running mean rainfall normalized with its standard deviation, clearly depicts the characteristics of summer rainbelt activities in East China in terms of jumps and durations during its northward propagations. The EASM northward propagation shows a prominent interdecadal variation. EASM before late 1970s had a rapid northward advance and a northern edge beyond its normal position. As a result, more summer rainfall occurred for the North China rainy season, Huaihe-River Mei-Yu, and South China Mei-Yu. In contrast, EASM after late 1970s had a slow northward movement and a northern edge located south of its normal position. Less summer precipitation occurred in East China except in Yangtze River basin. The EASM northernmost position (ENP), northernmost intensity (ENI), and EASM have a complex and good relationship at interdecadal timescales. They have significant influences on interdecadal variation of the large-scale precipitation anomalies in East China.

Asian summer monsoon precipitation recorded by stalagmite oxygen isotopic composition in the western Loess Plateau during AD1875–2003 and its linkage with ocean-atmosphere system

Based on 5 high-precision 230Th dates and 103 stable oxygen isotope ratios (δ18O) obtained from the top 16 mm of a stalagmite collected from Wanxiang Cave, Wudu, Gansu, variation of monsoonal precipitation in the modern Asian Monsoon (AM) marginal zone over the past 100 years was reconstructed. Comparison of the speleothem δ18O record with instrumental precipitation data at Wudu in the past 50 years indicates a high parallelism between the two curves, suggesting that the speleothem δ18O is a good proxy for the AM strength and associated precipitation, controlled by “amount effect” of the precipitation. Variation of the monsoonal precipitation during the past 100 years can be divided into three stages, increasing from AD 1875 to 1900, then decreasing from AD 1901 to 1946, and increasing again thereafter. This variation is quite similar to that of the Drought/Flooding index archived from Chinese historical documents. This speleothem-derived AM record shows a close association with the Pacific Decadal Oscillation (PDO) between AD 1875 and 1977, with higher monsoonal precipitation corresponding to cold PDO phase and vice versa at decadal timescale. The monsoonal precipitation variation is out of phase with the PDO after AD 1977, probably resulting from the decadal climate jump in the north Pacific occurring at around AD 1976/77. These results demonstrate a strong linkage between the AM and associated precipitation and the Pacific Ocean via ocean/atmosphere interaction. This relationship will aid to forecast future hydrological cycle for the AM monsoon region, and to improve forecasting potential of climatic model with observation data from cave.

Monsoon precipitation variation recorded by tree-ring δ18O in arid Northwest China since AD 1878

The East Asian summer monsoon (EASM) precipitation from May to August over the Helan Mountains, located in the northwest margin of the EASM, has been investigated through tree-ring δ 18O for the last 120 years. The dry and wet intervals derived from the δ 18O series can be verified by the tree-ring width-based May–July precipitation reconstruction. Most of the time, the δ 18O series and ring width-derived precipitation reconstruction vary synchronously at low frequency. High values of δ 18O correspond to high precipitation periods, and vice versa. Since the 1970s, the East Asian summer monsoon precipitation has been increasing, which was confirmed by other records and documents. Our results show that tree-ring δ 18O can serve as an indicator of the strength of the East Asian summer monsoon variation in the past. We also found that tree-ring δ 18O varies inversely to δ 13C in the study area.

ENSO Impact on the Space–Time Evolution of the Regional Asian Summer Monsoons

Abstract This study investigates how ENSO affects the space–time evolution of the Asian summer monsoon (ASM) precipitation and synoptic variables on a 5-day resolution over the entire ASM area. Cyclostationary EOF and regression methods were used to investigate the detailed evolution features associated with ENSO during the prominent life cycle of the ASM (21 May–17 September). This ENSO mode is identified as the third largest component (next to the seasonal cycle and the intraseasonal oscillations with a 40–50-day period) of the ASM rainfall variation. The ENSO mode reveals that the individual regional monsoons over the ASM domain respond to ENSO in a complex manner. 1) Under the El Niño condition, the early monsoon stage over India, the Bay of Bengal, and the Indochina peninsula is characterized by rainfall deficit, along with a delayed monsoon onset by one or two pentads. This is the result of weakened diabatic heating over the Asian continent and meridional pressure gradient over the Indian Ocean, causing a weak low-tropospheric westerly monsoonal flow and the ensuing moisture transport decrease toward the regional monsoon areas. Onsets of the subsequent regional monsoons are delayed successively by this poorly developed ASM system in the early stage. 2) The Walker circulation anomaly persistently induces an enhanced subsidence over the Maritime Continent, resulting in a drought condition over this region for the entire ASM period. 3) The Hadley circulation anomaly linked to the Walker circulation anomaly over the Tropics drives a rising motion over the subtropical western Pacific, resulting in a wetter south China monsoon. The negative sea level pressure anomaly over the subtropical western Pacific associated with this anomalous Hadley circulation provides an unfavorable condition for the moisture transport toward East Asia, causing drier monsoons over north China, Japan, and Korea regions. 4) This negative sea level pressure anomaly intrudes into India, the Bay of Bengal, and the Indochina peninsula during late July and early August, developing a brief wet period over these regions. In contrast, the physical changes including the onset variation and the monsoon strength addressed above are reversed during La Niña events. In reality, the observed ASM rainfall anomaly does not necessarily follow the ENSO-related patterns addressed above because of other impacts contributing to the rainfall variations. Although the impact of ENSO is moderately important, a comparison with other impacts demonstrates that the rainfall variations are controlled more by regional-scale intraseasonal oscillations.

South Asian summer monsoon precipitation variability: Coupled climate model simulations and projections under IPCC AR4

South Asian summer monsoon precipitation and its variability are examined from the outputs of the coupled climate models assessed as part of the Intergovernmental Panel on Climate Change Fourth Assessment. Out of the 22 models examined, 19 are able to capture the maximum rainfall during the summer monsoon period (June through September) with varying amplitude. While two models are unable to reproduce the annual cycle well, one model is unable to simulate the summer monsoon season. The simulated inter-annual variability from the 19 models is examined with respect to the mean precipitation, coefficient of variation, long-term trends and the biennial tendency. The model simulated mean precipitation varies from 500 mm to 900 mm and coefficient of variation from 3 to 13%. While seven models exhibit long-term trends, eight are able to simulate the biennial nature of the monsoon rainfall. Six models, which generate the most realistic 20th century monsoon climate over south Asia, are selected to examine future projections under the doubling CO2 scenario.

Convection suppression criteria applied to the MIT cumulus parameterization scheme for simulating the Asian summer monsoon

It is shown in this study that applying some convection suppression criteria to the MIT or Emanuel cumulus parameterization scheme may significantly improve the performance of a regional climate model to simulate the Asian summer monsoon precipitation, particularly the precipitation over southeastern China and the Mei‐yu rainband over the East Asian region. With the original MIT cumulus scheme, the precipitation over the ocean particularly the South China Sea (SCS) region is generally over‐estimated, which may result in the under‐estimation of precipitation over China. It is found that the relative vorticity criterion (which shuts down the convection when the low‐level flow is anticyclonic and stronger than a certain threshold value) has the largest impact on suppressing the over‐estimated precipitation over the SCS.

A possible impact of the North Atlantic Oscillation on the east Asian summer monsoon precipitation

This letter reports on a possible delayed impact of the winter North Atlantic Oscillation (NAO) on the following east Asian summer monsoon precipitation. An analysis of weather station data shows significant correlations between the December NAO index and precipitation over Korea and China in the subsequent summer. It appears that the correlation may be related to a wave train pattern which originates from the North Atlantic. The east Asian branch of this wave train can affect large‐scale circulation and the precipitation over east Asia in early summer. We also found a significant interdecadal change of this relationship, which is possibly linked to a climatological change of the east Asian jet stream.

A New Perspective on the Climate Prediction of Asian Summer Monsoon Precipitation

Abstract A new paradigm for climate (one month and longer) prediction is developed and is applied to the 5-day-averaged Asian summer monsoon (ASM) precipitation. The foundation of the method is to identify climate signals (deterministic components) that constitute the ASM system and predict the temporal fluctuations of the amplitudes (stochastic components) of the individual signals. Climate signals were identified via cyclostationary empirical orthogonal function (CSEOF) analysis of the Xie–Arkin pentad precipitation in this study and include the annual cycle, El Niño/La Niña, and the intraseasonal oscillations of the 40–50-day period band (the Madden–Julian oscillation). Prediction is much facilitated by forecasting the slowly undulating amplitude time series of each climate signal rather than the raw precipitation data directly. The new prediction method results in reasonable forecasts of the pentad precipitation for the test period of 1999–2001. Specifically, the propagation of the intraseasonal oscillations is predicted successfully 60 days in advance. The performance of the new method is significantly better than persistence and that of conventional prediction methods in which raw data is predicted directly.

Space-time variability of low frequency oscillations and Asian summer monsoon precipitation

In this study, we provide an observational synthesis of several aspects of MJO (Madden Julian Oscillation) and ISO (Intra Seasonal Oscillation) waves and the manner in which they seem to affect the Asian summer monsoon variability. We examine a number of features such as the passage of MJO waves around the globe from west to east, excitation of ISO waves over the equatorial Indian Ocean, meridional propagation of ISO waves and the role of the local Hadley cell over the monsoon domain in its propagation. The geographical distribution of the amplitude and phase speed of ISO waves over the Asian summer monsoon domain shows that the ISO has prominent centers of action, with small scales of the order of a few 1000 km and that these features vary interannually in amplitude and in their geographical locations. It was also seen that some of the interesting features of the WET and DRY season rainfall distribution can be obtained by compositing the extremes of the amplitude oscillations of these ISO centers. We also noted that the amplitudes of ISO waves generally decay as they propagate north but have an occasional built up in their amplitudes when they seem to encounter and interact with heavy rain producing monsoon disturbances. The issue of these low frequency modes and the modulation of monsoonal dry and wet spells are examined in the context of this broad scenario.

Response of the East Asian summer monsoon to doubled atmospheric CO2: Coupled climate model simulations and projections under IPCC AR4

The East Asian (China, Korea and Japan) summer monsoon precipitation and its variability are examined from the outputs of the coupled climate models performing coordinated experiments leading to the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). Out of the 22 models examined, 14 reproduce the observed shape of the annual cycle well with peak during the boreal summer (June through August), but with varying magnitude. Three models simulate the maximum a month later and with lower magnitudes. Only one model considerably underestimates the magnitude of the annual cycle. The remaining 4 models show some deviations from the observed. Models are unable to simulate the minimum in July with peaks in June and August associated with northward shifts of the Meiyu-Changma-Baiu precipitation band. The realistic simulation of the annual cycle does not appear to depend on the model resolution. The inter-model variation is slightly larger during summer, implying larger diversity of the models in simulating summer monsoon precipitation.

High-resolution evidence from southern China of an early Holocene optimum and a mid-Holocene dry event during the past 18,000 years

Computer models suggest that the Holocene Optimum for East Asian summer monsoon precipitation occurred at different times in different regions of China. Previous studies indicate that this time-transgressive Holocene Optimum should have been experienced about 3000 yr ago in southern China. In this study we describe a section which allows us to test this timing directly. We have closely examined high-resolution eutrophic peat/mud sequences covering the past 18,000 cal yr at Dahu, Jiangxi, on the southern boundary of the mid subtropical zone in China. Late Pleistocene successions in the Dahu record indicate cooler and much wetter conditions relative to synchronous events in north-central China. Our results indicate that the Holocene Optimum occurred between ca. 10,000 and 6000 cal yr ago in southern China, consistent with the global pattern. Conditions were relatively dry and cold from 6000 to 4000 cal yr ago. Our data also support the conclusion that the last deglaciation to early Holocene in the south was much wetter, resulting in the formation of dense broad-leaved forests, which could have acted to moderate land temperature ∼10,000 to 6000 cal yr ago, yielding a stable early-Holocene climate. After 6000 cal yr, forest reduction led to unstable land temperatures, and possibly to a northerly shift of the subtropical high-pressure system. Whatever the mechanism, these changes resulted in decreased precipitation between 6000 and 4000 cal yr B.P. in southern China.

Role of land surface processes in monsoon development: East Asia and West Africa

Evidence is presented that exchanges of water and energy between the vegetation and the atmosphere play an important role in east Asian and West African monsoon development and are among the most important mechanisms governing the development of the monsoon. The results were obtained by conducting simulations for five months of 1987 using a general circulation model (GCM) coupled with two different land surface parameterizations, with and without explicit vegetation representations, referred to as the GCM/vegetation and the GCM/soil, respectively. The two land surface models produced similar results at the planetary scale but substantial differences at regional scales, especially in the monsoon regions and some of the large continental areas. In the simulation with GCM/soil, the east Asian summer monsoon moisture transport and precipitation were too strong in the premonsoon season, and an important east Asian monsoon feature, the abrupt monsoon northward jump, was unclear. In the GCM/vegetation simulation, the abrupt northward jump and other monsoon evolution processes were simulated, such as the large‐scale turning of the low‐level airflow during the early monsoon stage in both regions. With improved initial soil moisture and vegetation maps, the intensity and spatial distribution of the summer precipitation were also improved. The two land surface representations produced different longitudinal and latitudinal sensible heat gradients at the surface that, in turn, influenced the low‐level temperature and pressure gradients, wind flow (through geostrophic balance), and moisture transport. It is suggested that the great east‐west thermal gradient may contribute to the abrupt northward jump and the latitudinal heating gradient may contribute to the clockwise and counterclockwise turning of the low‐level wind. The results showed that under unstable atmospheric conditions, not only low‐frequency mean forcings from the land surface, such as monthly mean albedo, but also the perturbation processes of vegetation were important to the monsoon evolution, affecting its intensity, the spatial distribution of precipitation, and associated circulation at the continental scale.

Monsoonal precipitation variation in the East Asia since A.D. 1840

Three tree-ring rainfall reconstructions from China and Korea are used in this paper to investigate the East Asian summer monsoon-related precipitation variation over the past 160 years. Statistically, there is no linear correlation on a year-by-year basis between Chinese and Korean monsoon rainfall, but region-wide synchronous variation on a decadal-scale was observed. More rainfall intervals were 1860–1890, 1910–1925, and 1940–1960, and dry or even drought periods were 1890–1910, 1925–1940, and 1960–present. Reconstructions also display that the East Asian summer monsoon precipitation suddenly changed from more into less around mid-1920. These tree-ring precipitation records were also confirmed by Chinese historical dryness/wetness index and Korean historical rain gauge data.

A continuous 3000-year precipitation record of ENSO variability during LGM from a stalagmite in Nanjing

A glacial stalagmite chronology from Nanjing has been established by the TIMS-U series dating and annual band counting methods. The annually layering sequence spanning the 3000-year period from 18179 to 14900 calendar years before the present (aBP) was analyzed for evidence of East Asian summer monsoon precipitation variability during the Last Glacial Maximum (LGM). Power spectral analysis of the sequence shows a distinct interannual (2-7 years) band of enhanced variability suggestive of El Nino-Southern Oscillation (ENSO) teleconnections into East China during the LGM when climatic boundary conditions were different from those of today. The lower frequency bands (4-7 years) variability becomes weaker from 18179 to 14900 aBP, supporting the precession forcing model. The reappearance of the ENSO band in the coldest climatic boundary conditions during the Heinrich Event 1, however, suggests the stimulation of the enhanced East Asia winter monsoon to the El Nino events.

Monsoonal precipitation variation in the East Asia since A.D. 1840??Tree-ring evidences from China and Korea

Three tree-ring rainfall reconstructions from China and Korea are used in this paper to investigate the East Asian summer monsoon-related precipitation variation over the past 160 years. Statistically, there is no linear correlation on a year-by-year basis between Chinese and Korean monsoon rainfall, but region-wide synchronous variation on a decadal-scale was observed. More rainfall intervals were 1860-1890, 1910-1925, and 1940-1960, and dry or even drought peri-ods were 1890-1910, 1925-1940, and 1960-present. Reconstructions also display that the East Asian summer monsoon precipitation suddenly changed from more into less around mid-1920. These tree-ring precipitation records were also confirmed by Chinese historical dry-ness/wetness index and Korean historical rain gauge data.

Intercomparison of the climatological variations of Asian summer monsoon precipitation simulated by 10 GCMs

We assesses the overall performance of state-of-the-art atmospheric GCMs in simulating the climatological variations of summer monsoon rainfall over the Asian-Western Pacific region and the systematic errors that are common to a group of GCMs. The GCM data utilized are obtained from 10 GCM groups participated in the CLIVAR/Monsoon GCM Intercomparison Project. The model composite shows that the overall spatial pattern of summer monsoon rainfall is similar to the observed, although the western Pacific rainfall is relatively weak. For the simulated precipitation over the western Pacific, the models can be classified into two categories. The first category of models simulates the precipitation more confined to the equatorial region and weaker precipitation in the subtropical western Pacific compared to the observed. The second category of models simulates large precipitation in the subtropical western Pacific but the region is shifted to the north by 5–10°. None of the models realistically reproduce the observed Mei-yu rain band in the region from the East China Sea to the mid Pacific. Most of the models produce a rain band along the continental side of East Asia. The climatological variations of simulated summer rainfall are examined in terms of their amplitude and their principal EOF modes. All models simulate larger amplitudes of the climatological seasonal variation of Indian summer monsoon than the observed, though most models simulate smaller amplitudes in the western Pacific. The ten model composite produces four leading EOF modes over the Asian-western Pacific region, which are remarkably similar to the observed counterparts. The first and second eigenmodes, respectively, represent the smoothed seasonal march of broad-scale monsoon and the onsets of the Indian and East Asian summer monsoon. The third and fourth modes relate to the climatological intraseasonal oscillation (CISO). In contrast to the model composite, several models fail to reproduce the first principal mode, and most models do not reproduce the observed modes higher than the second. The CISO of precipitation is also examined over the Indian monsoon and the East Asia-western Pacific monsoon regions separately.

Asynchronous Holocene optimum of the East Asian monsoon

The spatial and temporal distribution of summer monsoon precipitation (or effective moisture) during the Holocene has been reconstructed on the basis of geological data, including lake levels, pollen profiles, and loess/paleosol records. In addition, the summer (July) precipitation increment, effective precipitation, and monsoon strength index have been obtained from numerical modeling experiments. Both geological data and numerical modeling indicate that the Holocene optimum, as defined by peak East Asian summer monsoon precipitation, was asynchronous in central and eastern China, reaching a maximum at different times in different regions, e.g., ca. 10,000–8000 yr ago in northeastern China, 10,000–7000 yr ago in north-central and northern east-central China, ca. 7000–5000 yr ago in the middle and lower reaches of the Yangtze River, and ca. 3000 yr ago in southern China. In southwestern China the maximum appeared ca. 11,000 yr ago, but probably was related to the maximum landward extension of the Indian summer monsoon. The regional shift in the maximum precipitation belt from northwest to southeast over the past 10,000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate. The southeastward shift of the East Asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the Holocene and the East Asian monsoon index declined, while the early maximum in southwestern China matches the maximum of the Indian monsoon index.

Evidence for Asian summer monsoon precipitation instability of the Younger Dryas phase

The loess and desert transitional zone of China responses sensitively to the Asian monsoon fluctuations. A high resolution loess-dust and aeolian sand-paleosols section records the climatic history of the Younger Dryas. Based upon AMS14C dating of pollen concentrates and proxy climate indices including magnetic susceptibilty, organic carbon content and grain size, it is demonstrated that the Asian monsoon climate manifests not only cooling as in other regions of the Northern Hemisphere, but also unusual oscillations in precipitation which were probably associated with atmospheric interactions between both the Northern and Southern Hemispheres and atmospheric pressure anomalies of topic Pacific Ocean (ENSO).