Southeast Asian Summer Monsoon Research Articles

Difference in the influence of Indo-Pacific Ocean heat content on South Asian Summer Monsoon intensity before and after 1976/1977

Monthly ocean temperature from ORAS4 datasets and atmospheric data from NCEP/NCAR Reanalysis I/II were used to analyze the relationship between the intensity of the South Asian summer monsoon (SASM) and upper ocean heat content (HC) in the tropical Indo-Pacific Ocean. The monsoon was differentiated into a Southwest Asian Summer Monsoon (SWASM) (2.5°–20°N, 35°–70°E) and Southeast Asian Summer Monsoon (SEASM) (2.5°–20°N, 70°–110°E). Results show that before the 1976/77 climate shift, the SWASM was strongly related to HC in the southern Indian Ocean and tropical Pacific Ocean. The southern Indian Ocean affected SWASM by altering the pressure gradient between southern Africa and the northern Indian Ocean and by enhancing the Somali cross-equatorial flow. The tropical Pacific impacted the SWASM through the remote forcing of ENSO. After the 1976/77 shift, there was a close relationship between equatorial central Pacific HC and the SEASM. However, before that shift, their relationship was weak.

Performance of the seasonal forecasting of the Asian summer monsoon by BCC_CSM1.1(m)

This paper provides a comprehensive assessment of Asian summer monsoon prediction skill as a function of lead time and its relationship to sea surface temperature prediction using the seasonal hindcasts of the Beijing Climate Center Climate System Model, BCC CSM1.1(m). For the South and Southeast Asian summer monsoon, reasonable skill is found in the model’s forecasting of certain aspects of monsoon climatology and spatiotemporal variability. Nevertheless, deficiencies such as significant forecast errors over the tropical western North Pacific and the eastern equatorial Indian Ocean are also found. In particular, overestimation of the connections of some dynamical monsoon indices with large-scale circulation and precipitation patterns exists in most ensemble mean forecasts, even for short lead-time forecasts. Variations of SST, measured by the first mode over the tropical Pacific and Indian oceans, as well as the spatiotemporal features over the Ni˜no3.4 region, are overall well predicted. However, this does not necessarily translate into successful forecasts of the Asian summer monsoon by the model. Diagnostics of the relationships between monsoon and SST show that difficulties in predicting the South Asian monsoon can be mainly attributed to the limited regional response of monsoon in observations but the extensive and exaggerated response in predictions due partially to the application of ensemble average forecasting methods. In contrast, in spite of a similar deficiency, the Southeast Asian monsoon can still be forecasted reasonably, probably because of its closer relationship with large-scale circulation patterns and El Ni˜no–Southern Oscillation.

Extreme value analysis of streamflow time series in Poyang Lake Basin, China

Extreme meteorological and hydrological events may cause major disasters and heavy social and economic losses. Therefore, more and more studies have focused on extreme hydro-meteorological events in various climates and geographic regions. Based on nearly 50 years of observed records of the Poyang Lake Basin, the occurrence and changing trends of extreme streamflow indices, including the annual maximum flow, annual peak-over-threshold flows, and low flows, were analyzed for ten hydrological stations. The results indicate that most annual maximum flows occurred from April to July, highly attributed to the Southeast Asian summer monsoons, whereas the annual minimum flows were concentrated between January and February. As for the low flow indices (the annual minimum flow, annual minimum 7-d flow, and annual minimum 30-d flow), a significant increasing trend was detected in most parts of the Poyang Lake Basin. The trends illustrate the potential effects of climate change and human activities on the hydrological cycle over the Poyang Lake Basin.

Precessional Cycles and Their Influence on the North Pacific and North Atlantic Summer Anticyclones

Abstract The response of the Northern Hemisphere summer anticyclones to a change in the timing of perihelion is investigated using the GFDL Climate Model version 2.1 (CM2.1). The orbital forcing consists of changes in the seasonal cycle of the top-of-atmosphere insolation as the perihelion shifts from the Northern Hemisphere winter to the Northern Hemisphere summer solstice. The North Pacific summer anticyclone experiences a large strengthening as well as a northward and westward expansion. The North Atlantic subtropical high experiences a smaller change that consists of a slight westward expansion but little change in strength. Experiments with a primitive equation atmospheric model show that these changes represent the circulation response to changes in the diabatic heating, both local and remotely. The remote diabatic forcing is associated with changes in the Southeast Asian and African summer monsoons, and the local forcing is dominated by a combined effect of a change in low clouds and local precipitation.

The seasonal predictability of the Asian summer monsoon in a two-tiered forecast system

An extensive set of boreal summer seasonal hindcasts from a two tier system is compared with corresponding seasonal hindcasts from two other coupled ocean–atmosphere models for their seasonal prediction skill (for precipitation and surface temperature) of the Asian summer monsoon. The unique aspect of the two-tier system is that it is at relatively high resolution and the SST forcing is uniquely bias corrected from the multi-model averaged forecasted SST from the two coupled ocean–atmosphere models. Our analysis reveals: (a) The two-tier forecast system has seasonal prediction skill for precipitation that is comparable (over the Southeast Asian monsoon) or even higher (over the South Asian monsoon) than the coupled ocean–atmosphere. For seasonal anomalies of the surface temperature the results are more comparable across models, with all of them showing higher skill than that for precipitation. (b) Despite the improvement from the uncoupled AGCM all models in this study display a deterministic skill for seasonal precipitation anomalies over the Asian summer monsoon region to be weak. But there is useful probabilistic skill for tercile anomalies of precipitation and surface temperature that could be harvested from both the coupled and the uncoupled climate models. (c) Seasonal predictability of the South Asian summer monsoon (rainfall and temperature) does seem to stem from the remote ENSO forcing especially over the Indian monsoon region and the relatively weaker seasonal predictability in the Southeast Asian summer monsoon could be related to the comparatively weaker teleconnection with ENSO. The uncoupled AGCM with the bias corrected SST is able to leverage this teleconnection for improved seasonal prediction skill of the South Asian monsoon relative to the coupled models which display large systematic errors of the tropical SST’s.

The variability of the Southeast Asian summer monsoon

ABSTRACTIn this article, we focus on the analysis of the climate variability of the Southeastern Asian Summer Monsoon (SEAM) region encompassing Myanmar, Thailand, Cambodia, Vietnam, Laos and parts of southern China. This region is climatologically found to have one of the longest wet seasons in the Asian monsoon region (of nearly 160 d) and also exhibits one of the strongest interannual variations in the length of the monsoon (wet) season. The interannual variations of the length of the SEAM are characterized by corresponding variations in the onset and demise pentad dates of the wet season, with the former dominating slightly over the latter except over Myanmar. Our study reveals that the pentad of late onset of SEAM is characterized by anomalous increase in remote moisture source from Bay of Bengal and Arabian Sea while a substantial decrease of moisture source from the near Andaman Sea and Gulfs of Martaban and Thailand. Furthermore, anomalously strong June–August Somali Jet is found to be associated with earlier than normal onset of the SEAM. Similarly, the pentad of late demise of the SEAM features excess moisture source from the South China Sea associated with a slow eastward withdrawal of the north Pacific subtropical high.We suggest on the basis of the findings of this study that careful monitoring of the onset the SEAM season will provide important information on the evolution of an ongoing SEAM. Likewise observing low level winds over the northern equatorial Indian Ocean, Bay of Bengal, Gulfs of Martaban and Thailand and South China Sea could be very useful in understanding the seasonal variability of the SEAM. Finally, monitoring of the demise would be equally helpful in characterizing the variation of the concluded SEAM as the length of the wet season seems to be a very robust climate feature of the region. © 2013 Royal Meteorological Society

Teleconnection between Winter Arctic Oscillation and Southeast Asian Summer Monsoon in the Pre-Industry Simulation of a Coupled Climate Model

有 Bergen 气候模型(BCM ) 版本 2 的 600 年的工业化前的模拟被用来在内部十的 timescale 上调查在冬季北极摆动(AO ) 和东南亚洲夏天季风(SEASM ) 之间的连接。结果与近似 16-32 和 60-80 年的时期显示在 AO 和 SEASM 之间的一种同相的关系。在冬季 AO 的积极阶段期间，异常表面逆旋风的空气循环在冬季出现在诺思太平洋上。在海洋循环和表面的相应异例加热流动，特别地潜伏、理智的热流动，类似于象海表面温度(SST ) 一样的否定和平的十的摆动(PDO ) 模式。联系 AO 的象 PDO 一样冬季 SST 能坚持直到夏天并且能因此导致内部 -- 在东方和东南亚洲的夏天季风降雨的十的可变性。

Does the Asian monsoon modulate tropical cyclone activity over the South China Sea?

To investigate whether the Asian monsoon influences tropical cyclone (TC) activity over the South China Sea (SCS), TCs (including tropical storms and typhoons) over the SCS are analyzed using the Joint Typhoon Warning Center dataset from 1945 to 2009. Results show an increasing trend in the frequencies of TC-all (all TCs over the SCS) and TY-all (all typhoons over the SCS), due mainly to an increase in the number of TCs moving into the SCS after development elsewhere. Little change is seen in the number of TCs that form in the SCS. The results of wavelet analysis indicate that the frequency of typhoons (TY) shows a similar oscillation as that of TCs, i.e., a dominant periodicity of 8-16 years around the 1970s for all TC activity, except for TC-mov (TCs that moved into the SCS from the western North Pacific). To examine the relationship between typhoon activity and the summer monsoon, a correlation analysis was performed that considered typhoons, TCs, and five monsoon indexes. The analysis reveals statistically significant negative correlation between the strength of the Southwest Asian summer monsoon and typhoon activity over the SCS, which likely reflects the effect of the monsoon on TC formation in the western North Pacific (WNP) and subsequent movement into the SCS. There is a statistically significant negative correlation between TY-loc (typhoons that developed from TCs formed over the SCS) and the South China Sea summer monsoon and Southeast Asian summer monsoon.

Horizontal transport affecting trace gas seasonality in the Tropical Tropopause Layer (TTL)

We analyze horizontal transport from midlatitudes into the tropics (in‐mixing) and its impact on seasonal variations of ozone, carbon monoxide and water vapor in the Tropical Tropopause Layer (TTL). For this purpose, we use three‐dimensional backward trajectories, driven by ECMWF ERA‐Interim winds, and a conceptual one‐dimensional model of the chemical composition of the TTL. We find that the fraction of in‐mixed midlatitude air shows an annual cycle with maximum during NH summer, resulting from the superposition of two inversely phased annual cycles for in‐mixing from the NH and SH, respectively. In‐mixing is driven by the monsoonal upper‐level anticyclonic circulations. This circulation pattern is dominated by the Southeast Asian summer monsoon and, correspondingly, in‐mixing shows an annual cycle. The impact of in‐mixing on TTL mixing ratios depends on the in‐mixed fraction of midlatitude air and on the meridional gradient of the particular species. For CO the meridional gradient and consequently the effect of in‐mixing is weak. For water vapor, in‐mixing effects are negligible. For ozone, the meridional gradient is large and the contribution of in‐mixing to the ozone maximum during NH summer is about 50%. This in‐mixing contribution is not sensitive to the tropical ascent velocity, which is about 40% too fast in ERA‐Interim. As photochemically produced ozone in the TTL shows no distinct summer maximum, the ozone annual anomaly in the upper TTL turns out to be mainly forced by in‐mixing of ozone‐rich extratropical air during NH summer.

Modeling the effects of Southeast Asian monsoon outflow on subtropical anticyclones and midlatitude ozone over the Southern Indian Ocean

This observational and modeling study explores how the Southeast Asian summer monsoon outflow into the Southern Indian Ocean influences the life cycle of local anticyclones and leads to changes in the distribution of column ozone in the Southern Hemisphere (SH). A case study of the evolution of synoptic fields during August 1998 was performed to characterize the circulation leading to the average column ozone distribution known as the “ozone croissant,” with a characteristic ozone maximum south of Australia and a minimum near South America. During this month, five phases of SH circulation were found to lead to distinctive ozone patterns, explaining the monthly location and extent of the SH column ozone maximum. An isentropic trajectory model was used to show the cross‐equatorial flow from the Tibetan High into the SH at the near‐tropopause level of 360 K. Outflow pulses are shown to be responsible for the amplification of observed anticyclones over the SH Indian Ocean and intensification of troughs south of Australia. This couplet establishes an ozone transport pathway from tropical lower stratospheric regions around the edges of anticyclones into the ozone maximum in the amplified troughs. An idealized modeling experiment using the University of Wisconsin‐Madison Nonhydrostatic Modeling System was performed to model strong outflow pulse from tropical convection. Together with model trajectory computations, the modeling study showed a strong anticyclonic response over the Indian Ocean and increased ozone transport into the amplified troughs in a perturbed wave pattern.

Role of convective heating in the seasonal evolution of the Asian summer monsoon

The seasonal evolution of the Asian summer monsoon (ASM) is closely related to the reversal of the meridional temperature gradient between the Tibetan Plateau (TP) and the adjacent ocean in the upper troposphere. Sensible heating has been suggested to play a crucial role in tropospheric warming over the TP. More recently, adiabatic subsidence associated with the Matsuno‐Gill type atmospheric response was shown to warm the upper troposphere downward from the tropopause around the TP at the onset phase of the ASM. Here, we suggest the crucial role of convective heating in the seasonal evolution of the ASM, focusing on the seasonal march of the monsoon trough and seasonal upper‐level warming around the TP. Correlation analyses and numerical experiments showed that convective heating around the Maritime Continent, which induces upper tropospheric warming over the southern slope of the TP together with southward extension of the monsoon trough around the Bay of Bengal forming the twin‐cyclone structure, is indispensable to the Southeast Asian summer monsoon onset. Meanwhile, convective heating around the Bay of Bengal, which induces upper tropospheric warming to the southwest of the TP together with westward extension of the monsoon trough from the Bay of Bengal to the Arabian Sea, is necessary for the Indian summer monsoon onset. Thus, although the TP has been emphasized previously, convective heating is a necessary factor in explaining the seasonal upper‐level warming around the TP and the seasonal march of the ASM.

Impact of the Southeast Asian summer monsoon strength on the outflow of aerosols from South Asia

Abstract. We chose a relatively weak Southeast Asian summer monsoon (SEASM) year (1998) and a relatively strong year (2002) to examine the impact of the monsoon strength on the transport of organic carbon (OC) aerosol emitted from the South Asia (75°–105° E, 10°–25° N) by using the global 3-D chemical transport model GEOS-Chem driven by the assimilated meteorological fields. Simulated surface layer concentrations and column burdens of OC indicate that OC levels are much higher in the weak SEASM year 1998 than in the strong SEASM year 2002. The sensitivity experiments with global OC emissions turned off except those over the South Asia show that OC aerosol emitted from South Asia contributes to 50–70% of OC mass over southern China and 20–50% of OC over the western North Pacific between 850 hPa and 400 hPa in 1998. The outflow of OC from the South Asia is larger in 1998 than in 2002. Three factors contribute to the larger buildup of summer time OC in the weak SEASM year of 1998. The first is the weakened summer monsoon rainfall over the Southeast Asia that leads to less wet deposition and higher OC concentrations. The second is the enhancement of deep convection in the western Indian continent and the weakened upward lifting over the western North Pacific. The last and the most important factor is the abnormal circulation in the lower and middle troposphere that contributes to the long-range transport of OC from South Asia to Southern China and the western North Pacific.

Modeling the time-dependent response of the Asian summer monsoon to obliquity forcing in a coupled GCM: a PHASEMAP sensitivity experiment

To study the time-dependent response of the Asian summer monsoon to obliquity forcing, we analyze a 284,000-year long transient simulation produced by a fully coupled global climate model (GCM) using a new phase mapping (PHASEMAP) approach. Here we focus on understanding the phase response of monsoonal circulation to insolation forcing at the Earth-orbital obliquity band (41 Kyr). Our results show that the East Asian summer monsoon (EASM) can be divided into two geographic regions: the North East Asian summer monsoon (NEASM) and the South East Asian summer monsoon (SEASM). The Indian summer monsoon (ISM) and the SEASM are in phase at the obliquity band, strengthened with an increase in obliquity from Obliquity minima (Omin) to Obliquity maxima (Omax). The NEASM is out of phase with the ISM and SEASM, weakened with an increase in obliquity from Omin to Omax. We hypothesize that the inverse phase between the NEASM and the ISM at the obliquity band results from an ISM–NEASM teleconnection linked to the formation mechanism of the Bonin High.

Environmental magnetism and climate change

A major and pressing problem is to understand how, and how fast, the Earth's climate has changed in the past, with and without human influences on the global carbon cycle. Magnetic, remanence-acquiring, minerals, mostly iron oxides and sulphides, occur ubiquitously in sediments. They can act as sensitive recorders of past climates, because as climate has varied (from glacial to interglacial, for example), the mineralogy, magnetic domain state, composition and source of these minerals has varied. Here, the magnetic properties of windblown dust and interbedded soil layers of the Chinese Loess Plateau are used to calculate rainfall for the last million years, identifying the waxing and waning of the Southeast Asian summer monsoon. Comparison of our magnetic rainfall record on land with environmental records from the deep-sea shows that summer monsoon intensity is linked with growth and decay of continental-sized ice sheets, in turn reflecting changes in the Earth's orbit around the Sun.

Impact of the Atlantic Multidecadal Oscillation on the Asian summer monsoon

The impact of the Atlantic Multidecadal Oscillation (AMO) on the Asian summer monsoon is investigated using a coupled atmosphere‐ocean global general circulation model by imposing the AMO‐associated sea surface temperature anomalies in the Atlantic as boundary forcing, and allowing atmosphere‐ocean interactions outside the Atlantic. The positive AMO phase, characterized by anomalous warm North Atlantic and cold South Atlantic, leads to strong Southeast and east Asian summer monsoons, and late withdrawal of the Indian summer monsoon. These changes of monsoons are mainly through coupled atmosphere‐ocean feedbacks in the western Pacific and Indian Oceans and tropospheric temperature changes over Eurasia in response to the imposed forcing in the Atlantic. The results are in agreement with the observed climate changes in China corresponded to the AMO phases. They suggest a non‐local mechanism for the Asian summer monsoon variability and provide an alternative view to understanding its interdecadal variation during the twentieth century.

A high-resolution record of Holocene rainfall variations from the western Chinese Loess Plateau: antiphase behaviour of the African/Indian and East Asian summer monsoons

Tropical monsoons are key mechanisms for transfer of heat and moisture to higher latitudes. Here we present a high-resolution, terrestrial proxy summer monsoon record for the southeast Asian monsoon, from a rapidly accumulating Holocene loess/soil sequence in the western Chinese Loess Plateau. We use magnetic and clastic grain size proxies to make quantitative estimates of Holocene rainfall and identify variations in winter monsoon intensity. Our record reveals cyclical millennial and multimillennial rainfall changes. As with the northwest African/southwest Asian monsoon records, a short arid interval at - 12.5 to 11.5 ka BP (the Younger Dryas) and subsequent summer monsoon intensification are recorded. However, at 6 ka BP, the southeast Asian summer monsoon weakened, when the northwest African/ southwest Asian monsoons strengthened, and then, from - 5 ka BP, intensified, when northwest Africa/ southwest Asia became dry. These antiphase monsoonal relationships may reflect competition between sea surface temperature changes and solar forcing. The intensity of the southeast Asian winter monsoon intensified from - 9 ka BP onwards, varying in its phase relationships with summer monsoon intensity. After - 2.25 ka BP, extreme climatic instability is indicated by both climate proxies.

Sediment sources and East Asian monsoon intensity over the last 450 ky. Mineralogical and geochemical investigations on South China Sea sediments

A coupled approach based on clay mineral assemblages and isotopic data (Sr and Nd) of sediment from Ocean Drilling Program (ODP) Site 1145 has been used to trace the sources of sediment feeding the northern part of the South China Sea, and to investigate the evolution of East Asian monsoon intensity over the last 450 ky. Clay mineral assemblages are dominated by illite and smectite, with lesser abundance of chlorite and kaolinite. 87Sr / 86Sr and ɛ Nd(0) isotopic data, combined with the smectite / (illite + chlorite) ratio, indicate that the Pearl River is the main contributor for detrital material to the northern margin of the SCS, with variable continental input of volcanic material derived from the erosion of the Luzon Arc. These inputs follow the low-latitude solar insolation with a 23 ky periodicity, as recorded by a periodic change of the clay mineralogy. For the first time, this study presents a SCS deep-sea sediment high-resolution record of climatic changes in the last 450 ky, showing that smectite / (illite + chlorite) variations are mainly related to changes in the East Asian summer monsoon intensity. Because the mineralogical record is independent of other external forcing phenomena such as global glacial/interglacial oscillations, the smectite / (illite + chlorite) ratio has been used here as a proxy to reconstruct past changes of the South-East Asian summer monsoon intensity.

Connections of Siberian snow onset dates to the following summer's monsoon conditions over Southeast Asia

This is an exploratory study of possible links between the conditions of early season Eurasian snowfall and the following year's Southeast Asian summer monsoon. Forty years (1950–1995) of historical records are used to examine the statistical connections between early season snow cover onset dates over northern Eurasia and the following year's summer monsoon over Southeast Asia. We found that the time of snow onset is significantly associated with warm season rainfall over Southeast Asia. The most persistent connection is between northeastern Siberian snow onsets and summer monsoon strength over India and northeastern China. This connection seems to be more clearly shown during the mature stage and monsoon withdrawal and is reflected in all three aspects of monsoon characteristics. In other words, the earlier snow cover onset (more snow cover during the early season) over northeastern Siberia, the more precipitation and moisture convergence, the higher prevalence of a southwesterly monsoon wind, and the later monsoon withdrawal over Southeast Asia. The revealed connection is likely through atmospheric circulation associated with early season land surface snow cover processes independent of El Nino conditions. The authors suggest that more studies are needed to fully understand the circuitous connections between Eurasian snowfall and the Southeastern Asian monsoon. Copyright © 2005 Royal Meteorological Society.

Differences in rainfall variability in the South and Southeast Asian summer monsoons

AbstractLong‐term rainfall variability in the South and Southeast Asian summer monsoons is investigated using observational data for a 50 year period. Linear trends indicate a general level of summer rainfall decrease over northeast India that has intensified during recent decades. In contrast to the South Asian Monsoon (SAM), there is no significant rainfall trend in the Southeast Asian Monsoon (SEAM).The result of the empirical orthogonal functions (EOFs) analysis has shown that the leading EOF modes explain about 25% of total rainfall variability in both regions. Low correlation between respective principal components implies that rainfall variations in the two monsoon subsystems are not related, suggesting that different mechanisms are responsible for the interannual rainfall variability in the SAM and the SEAM.The interannual variability of the SAM rainfall is linked to sea‐surface temperature (SST) fluctuations in the equatorial eastern Pacific during the monsoon season and the following autumn. As revealed by lead–lag correlations, the warming of the Arabian Sea and a region located northwest of Australia during the boreal winter may lead to a strong SAM. The interannual behaviour of the SAM rainfall shows a feature of the quasi‐biennial oscillation with the reversal of correlations between months preceding the monsoon and the monsoon–post‐monsoon months. In comparison with the SAM, the interannual variability of the SEAM rainfall is associated with distinctly different SST patterns. Enhanced SEAM rainfall is preceded by a high SST in the northern Indian Ocean and the South China Sea in the previous January and February. Significant correlations between the SEAM rainfall and SST, which form a tripole pattern in the North Pacific Ocean, are detected during the preceding winter and early spring, suggesting that wintertime tropics–extratropics interactions may play a role in the SEAM rainfall variability. Copyright © 2004 Royal Meteorological Society

Evolution and Structure of the Mesoscale Convection and Its Environment: A Case Study during the Early Onset of the Southeast Asian Summer Monsoon

Abstract The evolution and structure of mesoscale convection in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast Asian monsoon (SEAM). For the case of 15 May 1998, interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection. In the prefrontal region, the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast-to-southwest-oriented convection. In the postfrontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly postfrontal flow gener...