Intraseasonal oscillations of the monsoon circulation over South Asia

The space–time structure of the three-dimensional circulation over the South Asian monsoon region has been studied using the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Applying multi-channel singular spectrum analysis on combined daily values of horizontal winds and pressure vertical velocity at ten vertical levels for the period 1958–2001, two leading intraseasonal nonlinear oscillations were extracted. The first oscillation has an average period of 50?days and propagates northeastward from the Indian Ocean to the Indian subcontinent. The second oscillation has a period of 30?days and propagates northwestward from the West Pacific to the Indian region. Both the oscillations exhibit the oscillatory and propagation features at all vertical levels from 1,000 to 100?hPa. The two oscillations correspond well with similar oscillations found in outgoing longwave radiation and precipitation in earlier studies. The wind oscillations also account for the active and break phases of the Indian monsoon. The vertical structures and propagation of specific humidity and temperature are found to be consistent with those of the winds in each oscillation. The structure and movement of regional Hadley and Walker circulations have also been described. The analyses provide further strong evidence for the existence of two distinct monsoon intraseasonal oscillations.     

Observed lead-lag relationships between Indian summer monsoon and some meteorological variables

Lagged relationships between the Indian summer monsoon and several climate variables are investigated. The variables examined are gridded fields of snow cover (14 years), sea surface temperature (41 years) and 500 hPa geopotential height north of 20 °N (42 years). We also used series of global air temperature (108 years) and Southern Oscillation index (112 years). Precipitation over all India during June–September over a 112 year period are used as Indian monsoon index. Emphasis is put on early monsoon precursors. In agreement with the tendency for a low frequency oscillation in the ocean-atmosphere system, several precursor patterns are identified as early as the year preceding the monsoon. The most important key regions and seasons of largest correlations are selected and the corresponding series are used to perform a monsoon prediction. The prediction shows however a relatively moderate score mainly due to the not highly significant correlations. To improve the predictions we filtered the variables into their biennial (1.5–3.5 years) and low frequency (3.5–7.5 years) modes. Correlations between the monsoon and the filtered variables are higher than those obtained without filtering especially for the biennial mode. The two modes are out-of-phase before the monsoon and in-phase during and after. This phasing is found in all variables except for snow cover for which the two modes are in-phase before the monsoon and out-of-phase during and after. It is suggested that such phasing may be important for the formation of snow and could explain the higher correlations when variables are concomitant or are lagging the monsoon. Early predictions of the monsoon based on those two modes show improved scores with highly significant correlations with the actual monsoon. Received: 19 June 1996 / Accepted: 11 April 1997     

The upper-level circulation anomaly over Central Asia and its relationship to the Asian monsoon and mid-latitude wave train in early summer

A large intraseasonal variation in geopotential height over the Central Asia region, where the Asian subtropical jet is located, occurs between May and June, and the most dominant variation has a wave-like distribution. This variation in geopotential height influences precipitation across South and Southeast Asia. In this paper, we use composite analysis to determine the causes of this intraseasonal variation over Central Asia. The wave train propagates from the northern Atlantic Ocean to Central Asia over a period of a week, and generates an anomaly in geopotential height over the region. The tropical disturbance, which is similar to the Madden–Julian oscillation, appears a few days before the maximum of the anticyclonic anomaly over Central Asia, and is accompanied by active convection over the Indian Ocean and suppressed convection over Central America. Results of numerical experiments using a linear baroclinic model show that the active convection over the northern Indian Ocean causes the anticyclonic anomaly over Central Asia. The wave train that extends from the northern Atlantic Ocean to Central Asia is generated by negative thermal forcing over Central America, and the phase distribution of this wave train is similar to that observed in the composite analysis. Central Asia is the region where the effects of the tropics and middle latitudes overlap, and it is an important connection point between the Asian monsoon and middle latitudes.     

Assessment of CORDEX-SA experiments in representing precipitation climatology of summer monsoon over India

The present work assesses the performance of 11 regional climate simulations in representing the precipitation patterns of summer monsoon over India for the period 1970–2005. These simulations have been carried out under Coordinated Regional Climate Downscaling Experiment–South Asia (CORDEX-SA) project. The regional climate models (RCMs) have been inter-compared as well as evaluated against the observation to identify the common weaknesses and differences between them. For this, a number of statistical analysis has been carried out to compare the model precipitation field with the corresponding observation. Model uncertainty has been also evaluated through bias studies and analysis of the spread in the ensemble mean (hereafter, ensemble). The models which perform better than the rest are identified and studied to look for any improvement in the ensemble performance. These better performing experiments (best RCM experiments) are further assessed over the monsoon core region (MCR) of India. This has been done to understand how well the models perform in a spatially homogeneous zone of precipitation which is considered to be a representative region of Indian summer monsoon characteristics. Finally, an additional analysis has been done to quantify the skill of models based on two different metrics—performance and convergence including a combination of the two. The experiment with regional model RegCM4 forced with the global model GFDL-ESM2M shows the highest combined mean skill in capturing the seasonal mean precipitation. In general, a significant dry bias is found over a larger part of India in all the experiments which seems most pronounced over the central Indian region. Ensemble on an average tends to outperform many of the individual experiments with bias of smaller magnitude and an improved spatial correlation compared with the observation. Experiments which perform better over India improve the results but only slightly in terms of agreement among experiments and bias.     

Impact of East Asian summer monsoon circulation on the regional aerosol distribution in observations and models

Theoretical and Applied Climatology - The East Asian summer monsoon (EASM) can change the spatio-temporal distribution of aerosols by influencing the aerosol horizontal and vertical transports and...     

新的高原季风指数与四川盆地夏季降水的关系

用NCEP/NCAR逐月再分析资料和中国560站月降水资料,定义了一个高原季风指数IPM2。结果表明:与原有高原季风指数相比,该指数与四川盆地夏季降水的相关性更好,能够较好地反映四川盆地夏季降水的异常变化。当高原夏季风偏弱时,巴尔喀什湖至贝加尔湖低压槽、亚洲东岸高压脊、印度低压均加强,同时西太平洋副热带高压偏北,来自孟加拉湾的西南风水汽输送和源于西太平洋的偏南风水汽输送均加强,这种环流形式有利于四川盆地西(东)部夏季降水偏多(少);当高原夏季风偏强时,情况相反。IPM2弱(强)大(小)年与四川盆地西(东)部涝年环流背景相似,表明IPM2能够很好反映四川盆地夏季降水异常的环流场特征。     

Simulation of the anthropogenic aerosols over South Asia and their effects on Indian summer monsoon

A regional climate model coupled with a chemistry-aerosol model is employed to simulate the anthropogenic aerosols including sulfate, black carbon and organic carbon and their direct effect on climate over South Asia. The model is driven by the NCAR/NCEP re-analysis data. Multi-year simulations with half, normal and double emission fluxes are conducted. Results show that the model performs well in reproducing present climate over the region. Simulations of the aerosol optical depth and surface concentration of aerosols are also reasonable although to a less extent. The negative radiative forcing is found at the top of atmosphere and largely depended on emission concentration. Surface air temperature decreases by 0.1?C0.5°C both in pre-monsoon and monsoon seasons. The range and intensity of cooling areas enlarge while aerosol emission increases. Changes in precipitation are between ?25 and 25%. Different diversifications of rainfall are showed with three emission scenarios. The changes of precipitation are consistent with varieties of monsoon onset dates in pre-monsoon season. In the regions of increasing precipitation, monsoon onset is advanced and vice versa. In northeast India and Myanmar, aerosols lead the India summer monsoon onset advancing 1?C2 pentads, and delaying by 1?C2 pentads in central and southeast India. These changes are mainly caused by the anomaly of local Hadley circulations and enhancive precipitation. Tibetan Plateau played a crucial role in the circulation changes.     

Spring Arctic Oscillation-East Asian summer monsoon connection through circulation changes over the western North Pacific

In the present study the links between spring Arctic Oscillation (AO) and East Asian summer monsoon (EASM) was investigated with focus on the importance of the North Pacific atmospheric circulation and sea surface temperature (SST). To reduce the statistical uncertainty, we analyzed high-pass filtered data with the inter-annual time scales, and excluded the El Ni?o/Southern Oscillation signals in the climate fields using a linear fitting method. The significant relationship between spring AO and EASM are supported by the changes of multi-monsoon components, including monsoon indices, precipitation, and three-dimensional atmospheric circulations. Following a stronger positive spring AO, an anomalous cyclonic circulation at 850?hPa appears in southeastern Asia and the western North Pacific in summer, with the easterly anomalies spanning from the Pacific to Asian continent along 25°N?C30°N and the westerly anomalies south of 15°N. At the same time, the summer western North Pacific subtropical high becomes weaker. Consistently, the positive precipitation anomalies are developed over a broad region south of 30°N stretching from southern China to the western Pacific and the negative precipitation anomalies appear in the lower valley of the Yangtze River and southern Japan. The anomalous cyclone in the western North Pacific persisting from spring to summer plays a key role in modulating EASM and monsoon precipitation by a positive air-sea feedback mechanism. During spring the AO-associated atmospheric circulation change produces warmer SSTs between 150°E?C180° near the equator. The anomalous sensible and latent heating, in turn, intensifies the cyclone through a Gill-type response of the atmosphere. Through this positive feedback, the tropical atmosphere and SST patterns sustain their strength from spring to summer, that consequently modifies the monsoon trough and the western North Pacific subtropical high and eventually the EASM precipitation. Moreover, the SST response to AO-circulation is supported by the numerical simulations of an ocean model, and the anomalous atmospheric circulation over the western North Pacific is also reproduced by the dedicated numerical simulations using the coupled atmosphere?Cocean model. The observation evidence and numerical simulations suggest the spring AO can impact the EASM via triggering tropical air-sea feedback over the western North Pacific.     

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

Summary 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.     

1998年东亚夏季风降水的模拟

利用改进后的p-σ9层混合坐标系模型，采用不同的土壤温度与太阳短波辐射参数化方案，对1998年夏季风降水进行模拟，发现改进的土壤温度计算法和改进的Lacis和Hansen(NEW)短波辐射参数化方法相结合能较好地模拟降水，利用不同的统计方法对模拟结果进行统计检验发现，土壤温度对降水的显著性影响主要体现在降水场的空间分布，通过与实测降水的比较，发现该模式对些时段的降水模拟拟得较好，对有些时段的降水特别是对强降水过程模拟较差，降水量普遍偏小。     

Zonal circulation indices, 500 hPa April ridge position along 75° E and Indian summer monsoon rainfall: Statistical relationships

Summary This paper presents an examination of the statistical relationship between summer monsoon rainfall over all India, and two sub-regions (north west India and peninsular India) and the indices of mid-latitude (35° to 70° N) zonal circulation at 500 hPa level, over different sectors of the hemisphere, based on 19 years (1971–1989) data. The results indicate that summer monsoon rainfall (June–September) over India shows; (i) a significant and direct relationship with the strength of the zonal circulation index during concurrent July over the sector 90° E to 160° E. (ii) a significant inverse relationship with the strength of the zonal index during the previous April over the sector 160° E to 45° W and a similar relationship with the whole northern hemisphere and (iii) a significant and direct relationship with the frequency of the zonal index during the previous January over the sector 45° W to 90° E.Significant relationships are also observed between the zonal circulation indices of the above mentioned months and sectors with the 500 hPa ridge location in April at 75° E over India.With 4 Figures     

A reappraisal of relationships between northern hemispheric surface air temperatures and Indian summer monsoon rainfall

Summary Zonally averaged surface air temperatures have been analysed to form time series of surface air temperature anomalies over the tropics (TTA), extratropics (ETA), the poles (PTA) and the whole northern hemisphere (NHTA) for the period 1901–1990. The temporal statistical relationships between these temperature time series and Indian monsoon rainfall over all India (AIR), northwest India (NWR) and peninsular India (PIR) have been examined for the above period.The northern hemispheric January–February (JF) temperature correlates significantly and positively with all the three monsoon rainfall series, the regional peninsular rainfall series (PIR) displaying the best correlation. The Strongest correlation is observed during 1951–1980 for both AIR and NWR but weakened in 1961–1990. For PIR, the highest correlation is observed during 1961–1990, remaining almost stable since 1951–1980. The JF series AIR monsoon relationship showed the highest correlation over the tropics during 1901–1940, over the polar region during 1941–1980 and over the northern hemisphere during 1951–1980. AIR and NWR moreover show a significant negative relationship with simultaneous, succeeding autumn and following year TTA series, while AIR and PIR monsoon rainfall series show significant positive association with the following year PTA series.The results also suggest that cooler January–February NHTA not only lead to a poor monsoon, but a poor monsoon also leads to warmer temperatures over the tropics and cooler temperatures over the polar region in the following year.With 1 Figure     

Climatological characteristics of summer precipitation over East Asia measured by TRMM PR: A review

Precipitation is an important indicator of climate change and a critical process in the hydrological cycle, on both the global and regional scales. Methods of precipitation observation and associated analyses are of strategic importance in global climate change research. As the first space-based radar, the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has been in operation for almost 17 years and has acquired a huge amount of cloud and precipitation data that provide a distinctive view to help expose the nature of cloud and precipitation in the tropics and subtropics. In this paper we review recent advances in summer East Asian precipitation climatology studies based on long-term TRMM PR measurements in the following three aspects: (1) the three-dimensional structure of precipitation, (2) the diurnal variation of precipitation, and (3) the recent precipitation trend. Additionally, some important prospects regarding satellite remote sensing of precipitation and its application in the near future are discussed.     

Onset of the regional monsoon over Southeast Asia

Summary ?This is an observational study in which regional features of the different summer monsoon components over Asia especially the South China Sea (SCS) are examined. The authors use various data sets including satellite measurements to understand the onset, maintenance, and retreat of monsoon and explain the connection and independence among the variabilities in the monsoon components. It is shown that while outgoing longwave radiation (OLR) data can only measure tropical convection, upper-tropospheric water vapor band brightness temperature (BT) represents appropriately convective precipitation in both the tropics and the extratropics. The authors define criteria for measuring the SCS monsoon using precipitation, BT, OLR, and lower-tropospheric winds and suggest that multi-variables should be considered to depict regional monsoon features adequately. Under the criteria defined in this study, the SCS summer monsoon is considered as an expansion of deep convection from the tropics. The onset of the monsoon occurs in mid-May, with its precursory signal found over the Indochina peninsula. It is characterized by an abrupt establishment, especially over the central SCS. Although the role of convection over the southern SCS in the monsoon onset is unclear, the early precipitation over the northern SCS and South China, resulted from the effect of subtropical fronts, is separated from the tropical monsoon rainfall. The relative independence from one monsoon component to another is explained by the effects from local topography and land-sea thermal contrast. Received November 5, 1999/Revised April 13, 2000     

The summer circulation over the eastern Mediterranean and the Middle East: influence of the South Asian monsoon

The summer circulation over the eastern Mediterranean and the Middle East (EMME) is dominated by persistent northerly winds (Etesians) whose ventilating effect counteracts the adiabatic warming induced by large scale subsidence. The ERA40 dataset is used to study the vertical distribution of these circulation features, which both appear to be reconciled manifestations of the South Asian monsoon influence. As predicted by past idealized modeling studies, in late spring a westward expanding upper level warm structure and subsidence areas are associated with Rossby waves excited by the monsoon convection. Steep sloping isentropes that develop over the EMME facilitate further subsidence on the western and northern periphery of the warm structure, which is exposed to the midlatitude westerlies. The northerly flow and descent over the eastern Mediterranean have maxima in July that are strikingly synchronous to the monsoon convection over northern India, where the weaker easterly jet favors a stronger Rossby wave response and consequent impact on the EMME circulation. The pronounced EMME topography modifies the monsoon induced structure, firstly, by inducing orographically locked summer anticyclones. These enhance the mid and low level northwesterly flow at their eastern flanks, leading to distinct subsidence maxima over the eastern Mediterranean and Iran. Secondly, topography amplifies the subsidence and the northerly flow over the Aegean, Red Sea, the Iraq—Gulf region and to the east of the Caspian Sea.     

Projected change in East Asian summer monsoon precipitation under RCP scenario

Future changes in East Asian summer monsoon precipitation climatology, frequency, and intensity are analyzed using historical climate simulations and future climate simulations under the RCP4.5 scenario using the World Climate Research Programme’s (WCRP) Coupled Model Intercomparison Project 5 (CMIP5) multi-model dataset. The model reproducibility is evaluated, and well performance in the present-day climate simulation can be obtained by most of the studied models. However, underestimation is obvious over the East Asian region for precipitation climatology and precipitation intensity, and overestimation is observed for precipitation frequency. The overestimation of precipitation frequency is mainly due to the large positive bias of the light precipitation (precipitation <10 mm/day) days, and the underestimation of precipitation intensity is mainly caused by the negative bias of the intense precipitation (precipitation >10 mm/day) intensity. For the future climate simulations, simple multi-model ensemble (MME) averages using all of the models show increases in precipitation and its intensity over almost all of East Asia, while the precipitation frequency is projected to decrease over eastern China and around Japan and increase in other regions. When the weighted MME is considered, no large difference can be observed compared with the simple MME. For the MME using the six best models that have good performance in simulating the present-day climate, the future climate changes over East Asia are very similar to those predicted using all of the models. Further analysis shows that the frequency and intensity of intense precipitation events are also projected to significantly increase over East Asia. Increases in precipitation frequency and intensity are the main contributors to increases in precipitation, and the contribution of frequency increases (contributed by 40.8 % in the near future and by 58.9 % by the end of the twenty-first century) is much larger than that of intensity increases (contributed by 29.9 % in the near future and by 30.1 % by the end of the twenty-first century). This finding also implies an increased risk of intense precipitation events over the East Asian region under global warming scenario. These results regarding future climate simulations show much greater reliability than those using CMIP3 simulations.     

Summer monsoon circulation and precipitation over the tropical Indian Ocean during ENSO in the NCEP climate forecast system

This study investigates the El Niño Southern Oscillation (ENSO) teleconnections to tropical Indian Ocean (TIO) and their relationship with the Indian summer monsoon in the coupled general circulation model climate forecast system (CFS). The model shows good skill in simulating the impact of El Niño over the Indian Oceanic rim during its decay phase (the summer following peak phase of El Niño). Summer surface circulation patterns during the developing phase of El Niño are more influenced by local Sea Surface Temperature (SST) anomalies in the model unlike in observations. Eastern TIO cooling similar to that of Indian Ocean Dipole (IOD) is a dominant model feature in summer. This anomalous SST pattern therefore is attributed to the tendency of the model to simulate more frequent IOD events. On the other hand, in the model baroclinic response to the diabatic heating anomalies induced by the El Niño related warm SSTs is weak, resulting in reduced zonal extension of the Rossby wave response. This is mostly due to weak eastern Pacific summer time SST anomalies in the model during the developing phase of El Niño as compared to observations. Both eastern TIO cooling and weak SST warming in El Niño region combined together undermine the ENSO teleconnections to the TIO and south Asia regions. The model is able to capture the spatial patterns of SST, circulation and precipitation well during the decay phase of El Niño over the Indo-western Pacific including the typical spring asymmetric mode and summer basin-wide warming in TIO. The model simulated El Niño decay one or two seasons later, resulting long persistent warm SST and circulation anomalies mainly over the southwest TIO. In response to the late decay of El Niño, Ekman pumping shows two maxima over the southern TIO. In conjunction with this unrealistic Ekman pumping, westward propagating Rossby waves display two peaks, which play key role in the long-persistence of the TIO warming in the model (for more than a season after summer). This study strongly supports the need of simulating the correct onset and decay phases of El Niño/La Niña for capturing the realistic ENSO teleconnections. These results have strong implications for the forecasting of Indian summer monsoon as this model is currently being adopted as an operational model in India.