Showing papers on "Monsoon published in 2006"

Increasing Trend of Extreme Rain Events Over India in a Warming Environment

Abstract: Against a backdrop of rising global surface temperature, the stability of the Indian monsoon rainfall over the past century has been a puzzle By using a daily rainfall data set, we show (i) significant rising trends in the frequency and the magnitude of extreme rain events and (ii) a significant decreasing trend in the frequency of moderate events over central India during the monsoon seasons from 1951 to 2000 The seasonal mean rainfall does not show a significant trend, because the contribution from increasing heavy events is offset by decreasing moderate events A substantial increase in hazards related to heavy rain is expected over central India in the future

Asian summer monsoon anomalies induced by aerosol direct forcing: the role of the Tibetan Plateau

Abstract: In this paper we present results of a numerical study using the NASA finite-volume GCM to elucidate a plausible mechanism for aerosol impact on the Asian summer monsoon involving interaction with physical processes over the Tibetan Plateau (TP). During the pre-monsoon season of March–April, dusts from the deserts of western China, Afghanistan/Pakistan, and the Middle East are transported into and stacked up against the northern and southern slopes of the TP. The absorption of solar radiation by dust heats up the elevated surface air over the slopes. On the southern slopes, the atmospheric heating is reinforced by black carbon from local emission. The heated air rises via dry convection, creating a positive temperature anomaly in the mid-to-upper troposphere over the TP relative to the region to the south. In May through early June in a manner akin to an “elevated heat pump”, the rising hot air forced by the increasing heating in the upper troposphere, draws in warm and moist air over the Indian subcontinent, setting the stage for the onset of the South Asia summer monsoon. Our results suggest that increased dust loading coupled with black carbon emission from local sources in northern India during late spring may lead to an advance of the rainy periods and subsequently an intensification of the Indian summer monsoon. The enhanced rainfall over India is associated with the development of an aerosol-induced large-scale sea level pressure anomaly pattern, which causes the East Asia (Mei-yu) rain belt to shift northwestward, suppressing rainfall over East Asia and the adjacent oceanic regions.

High resolution daily gridded rainfall data for the Indian region: Analysis of break and active monsoon spells

Abstract: Here, we report the development of a high resolution (1° x 1° lat./long.) gridded daily rainfall dataset for the Indian region. There are only 1803 stations with mini¬mum 90% data availability during the analysis period (1951-2003). For the analysis, we have followed the in¬terpolation method proposed by Shepard. Standard quality-controls were performed before carrying out the interpolation analysis. Comparison with similar global gridded rainfall datasets revealed that the pre¬sent rainfall analysis is better in accurate representation of spatial rainfall variation. Using this gridded rainfall dataset, an analysis was made to identify the break and active periods during the southwest monsoon season (June–September). Break (active) periods during the monsoon season were iden¬tified as those in which the standardized daily rainfall anomaly averaged over Central India (21-27°N, 72¬85°E) is less than –1.0 (more than 1.0). The break peri¬ods thus identified for the period 1951-2003 were comparable with those identified by earlier studies. Contrary to a recent study, no evidence was found for any statistically significant trends in the number of break or active days during the period 1951-2003. This gridded rainfall dataset is available for non¬commercial applications.

The Asian monsoon

Abstract: 1. Elementary monsoon physics.- 2. Monsoon as coupled atmosphere-ocean-land system.- 3. Global aspects and Asian monsoon circulations systems.- 4. Mean annual cycle of the Asian monsoon.- 5. Severe weather events in Asian monsoon.- 6. Intraseasonal Oscillation.- 7. Interannual variations and ENSO-AM interaction.- 8. Interdecadal variability.- 9. Paleo-monsoon variations.- 10. Numerical modeling and prediction.- 11. Anthropogenic changes.- 12. Monsoon environments.- Appendices: Statistical analysis approaches.

Toward a unified view of the American monsoon systems

Abstract: An important goal of the Climate Variability and Predictability (CLIVAR) research on the American monsoon systems is to determine the sources and limits of predictability of warm season precipitation, with emphasis on weekly to interannual time scales. This paper reviews recent progress in the understanding of the American monsoon systems and identifies some of the future challenges that remain to improve warm season climate prediction. Much of the recent progress is derived from complementary international programs in North and South America, namely, the North American Monsoon Experiment (NAME) and the Monsoon Experiment South America (MESA), with the following common objectives: 1) to understand the key components of the American monsoon systems and their variability, 2) to determine the role of these systems in the global water cycle, 3) to improve observational datasets, and 4) to improve simulation and monthly-to-seasonal prediction of the monsoons and regional water resources. Among the recent observational advances highlighted in this paper are new insights into moisture transport processes, description of the structure and variability of the South American low-level jet, and resolution of the diurnal cycle of precipitation in the core monsoon regions. NAME and MESA are also driving major efforts in model development and hydrologic applications. Incorporated into the postfield phases of these projects are assessments of atmosphere–land surface interactions and model-based climate predictability experiments. As CLIVAR research on American monsoon systems evolves, a unified view of the climatic processes modulating continental warm season precipitation is beginning to emerge.

Unraveling the Mystery of Indian Monsoon Failure During El Niño

Abstract: The 132-year historical rainfall record reveals that severe droughts in India have always been accompanied by El Nino events. Yet El Nino events have not always produced severe droughts. We show that El Nino events with the warmest sea surface temperature (SST) anomalies in the central equatorial Pacific are more effective in focusing drought-producing subsidence over India than events with the warmest SSTs in the eastern equatorial Pacific. The physical basis for such different impacts is established using atmospheric general circulation model experiments forced with idealized tropical Pacific warmings. These findings have important implications for Indian monsoon forecasting.

Trends in Total and Extreme South American Rainfall in 1960–2000 and Links with Sea Surface Temperature

Abstract: A weeklong workshop in Brazil in August 2004 provided the opportunity for 28 scientists from southern South America to examine daily rainfall observations to determine changes in both total and extreme rainfall. Twelve annual indices of daily rainfall were calculated over the period 1960 to 2000, examining changes to both the entire distribution as well as the extremes. Maps of trends in the 12 rainfall indices showed large regions of coherent change, with many stations showing statistically significant changes in some of the indices. The pattern of trends for the extremes was generally the same as that for total annual rainfall, with a change to wetter conditions in Ecuador and northern Peru and the region of southern Brazil, Paraguay, Uruguay, and northern and central Argentina. A decrease was observed in southern Peru and southern Chile, with the latter showing significant decreases in many indices. A canonical correlation analysis between each of the indices and sea surface temperatures (SSTs) revealed two large-scale patterns that have contributed to the observed trends in the rainfall indices. A coupled pattern with ENSO-like SST loadings and rainfall loadings showing similarities with the pattern of the observed trend reveals that the change to a generally more negative Southern Oscillation index (SOI) has had an important effect on regional rainfall trends. A significant decrease in many of the rainfall indices at several stations in southern Chile and Argentina can be explained by a canonical pattern reflecting a weakening of the continental trough leading to a southward shift in storm tracks. This latter signal is a change that has been seen at similar latitudes in other parts of the Southern Hemisphere. A similar analysis was carried out for eastern Brazil using gridded indices calculated from 354 stations from the Global Historical Climatology Network (GHCN) database. The observed trend toward wetter conditions in the southwest and drier conditions in the northeast could again be explained by changes in ENSO.

Observational relationships between aerosol and Asian monsoon rainfall, and circulation

Abstract: [1] Preliminary observational evidences are presented showing that the Indian subcontinent and surrounding regions are subject to heavy loading of absorbing aerosols, i.e., dust and black carbon, which possess spatial and temporal variability that are closely linked to those of the Asian monsoon water cycle. Consistent with the Elevated Heat Pump hypothesis, we find that increased loading of absorbing aerosols over the Indo-Gangetic Plain in the pre-monsoon season is associated with a) increased heating of the upper troposphere, with the formation of a warm-core upper level anticyclone over the Tibetan Plateau in April–May, b) an advance of the monsoon rainy season in northern India in May, and c) subsequent increased rainfall over the Indian subcontinent, and decreased rainfall over East Asia in June–July.

Steady decline of east Asian monsoon winds, 1969–2000: Evidence from direct ground measurements of wind speed

Abstract: [1] It is commonly believed that greenhouse-gas-induced global warming can weaken the east Asian winter monsoon but strengthen the summer monsoon, because of stronger warming over high-latitude land as compared to low-latitude oceans. In this study, we show that the surface wind speed associated with the east Asian monsoon has significantly weakened in both winter and summer in the recent three decades. From 1969 to 2000, the annual mean wind speed over China has decreased steadily by 28%, and the prevalence of windy days (daily mean wind speed > 5 m/s) has decreased by 58%. The temperature trends during this period have not been uniform. Significant winter warming in northern China may explain the decline of the winter monsoon, while the summer cooling in central south China and warming over the South China Sea and the western North Pacific Ocean may be responsible for weakening the summer monsoon. In addition, we found that the monsoon wind speed is also highly correlated with incoming solar radiation at the surface. The present results, when interpreted together with those of recent climate model simulations, suggest two mechanisms that govern the decline of the east Asian winter and summer monsoons, both of which may be related to human activity. The winter decline is associated with global-scale warming that may be attributed to increased greenhouse gas emission, while the summer decline is associated with local cooling over south-central China that may result from air pollution.

A physical mechanism for North Atlantic SST influence on the Indian summer monsoon

Abstract: A link between the Atlantic Multidecadal Oscillation (AMO) and multidecadal variability of the Indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking Atlantic climate and monsoon has been identified. The AMO produces persistent weakening (strengthening) of the meridional gradient of tropospheric temperature (TT) by setting up negative (positive) TT anomaly over Eurasia during northern late summer/autumn resulting in early (late) withdrawal of the south west monsoon and persistent decrease (increase) of seasonal monsoon rainfall. On inter-annual time scales, strong North Atlantic Oscillation (NAO) or North Annular mode (NAM) influences the monsoon by producing similar TT anomaly over Eurasia. The AMO achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong NAO/NAM events. This mechanism also provides a basis for explaining the observed teleconnection between North Atlantic temperature and the Asian monsoon in paleoclimatic proxies. Citation: Goswami, B. N., M. S. Madhusoodanan, C. P. Neema, and D. Sengupta (2006), A physical mechanism for North Atlantic SST influence on the Indian summer monsoon

Deep convective influence on the Asian summer monsoon anticyclone and associated tracer variability observed with Atmospheric Infrared Sounder (AIRS)

Abstract: [1] The Asian summer monsoon anticyclone is linked to climatological deep convection over Southeast Asia, and the coupling of circulation and convection strongly influences constituent behavior in the upper troposphere – lower stratosphere (UTLS). This work explores the variability of the Asian monsoon circulation and trace constituents linked to transient deep convection, on the basis of dynamical fields and outgoing longwave radiation (OLR) data, plus water vapor and ozone retrievals from the Atmospheric Infrared Sounder (AIRS) instrument. Within the monsoon region, transient deep convection varies with a timescale of ∼10–20 days, linked to active/break cycles in the monsoon circulation. We show that these convective events trigger variations of the anticyclone itself, with strong correlations between OLR and the area of low potential vorticity (PV) defining the anticyclone. Relatively high PV (stratospheric air) is also advected to low latitudes to the east of the anticyclone following enhanced convection. AIRS data show that the transient convective events are associated with the vertical transport of low ozone and high water vapor into the UTLS region, with significant effects over potential temperature levels 340–360 K (∼7–13 km). Idealized transport calculations are used to demonstrate that constituent anomalies are confined within the upper tropospheric anticyclone, and this confinement contributes to the climatological constituent patterns observed during summer.

Impact of the Atlantic Multidecadal Oscillation on the Asian summer monsoon

Abstract: [1] 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. Citation: Lu, R., B. Dong, and H. Ding (2006), Impact of the Atlantic Multidecadal Oscillation on the Asian summer monsoon, Geophys. Res. Lett., 33, L24701, doi:10.1029/ 2006GL027655.

Coupled Model Simulations of the West African Monsoon System: Twentieth- and Twenty-First-Century Simulations

Abstract: The ability of coupled GCMs to correctly simulate the climatology and a prominent mode of variability of the West African monsoon is evaluated, and the results are used to make informed decisions about which models may be producing more reliable projections of future climate in this region. The integrations were made available by the Program for Climate Model Diagnosis and Intercomparison for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The evaluation emphasizes the circulation characteristics that support the precipitation climatology, and the physical processes of a “rainfall dipole” variability mode that is often associated with dry conditions in the Sahel when SSTs in the Gulf of Guinea are anomalously warm. Based on the quality of their twentieth-century simulations over West Africa in summer, three GCMs are chosen for analysis of the twenty-first century integrations under various assumptions about future greenhouse gas increases. Each of these models behav...

Weakening of North Indian SST Gradients and the Monsoon Rainfall in India and the Sahel

Abstract: Sea surface temperatures (SSTs) in the equatorial Indian Ocean have warmed by about 0.6–0.8 K since the 1950s, accompanied by very little warming or even a slight cooling trend over the northern Indian Ocean (NIO). It is reported that this differential trend has resulted in a substantial weakening of the meridional SST gradient from the equatorial region to the South Asian coast during summer, to the extent that the gradient has nearly vanished recently. Based on simulations with the Community Climate Model Version 3 (CCM3), it is shown that the summertime weakening in the SST gradient weakens the monsoon circulation, resulting in less monsoon rainfall over India and excess rainfall in sub-Saharan Africa. The observed trend in SST is decomposed into a hypothetical uniform warming and a reduction in the meridional gradient. The uniform warming of the tropical Indian Ocean in the authors’ simulations increases the Indian summer monsoon rainfall by 1–2 mm day−1, which is opposed by a larger drying t...

Short circuit of water vapor and polluted air to the global stratosphere by convective transport over the Tibetan Plateau

Abstract: During boreal summer, much of the water vapor and CO entering the global tropical stratosphere is transported over the Asian monsoon/Tibetan Plateau (TP) region. Studies have suggested that most of this transport is carried out either by tropical convection over the South Asian monsoon region or by extratropical convection over southern China. By using measurements from the newly available National Aeronautics and Space Administration Aura Microwave Limb Sounder, along with observations from the Aqua and Tropical Rainfall-Measuring Mission satellites, we establish that the TP provides the main pathway for cross-tropopause transport in this region. Tropospheric moist convection driven by elevated surface heating over the TP is deeper and detrains more water vapor, CO, and ice at the tropopause than over the monsoon area. Warmer tropopause temperatures and slower-falling, smaller cirrus cloud particles in less saturated ambient air at the tropopause also allow more water vapor to travel into the lower stratosphere over the TP, effectively short-circuiting the slower ascent of water vapor across the cold tropical tropopause over the monsoon area. Air that is high in water vapor and CO over the Asian monsoon/TP region enters the lower stratosphere primarily over the TP, and it is then transported toward the Asian monsoon area and disperses into the large-scale upward motion of the global stratospheric circulation. Thus, hydration of the global stratosphere could be especially sensitive to changes of convection over the TP.

Changes in global monsoon precipitation over the past 56 years

Abstract: [1] Changes in the global monsoon rainfall over land were examined using four sets of rain-gauge precipitation data sets compiled for the period of 1948–2003 by climate diagnostic groups around the world. Here, we define a global monsoon rain domain according to annual precipitation range, using simple objective criteria; then, we propose metrics for quantifying the intensity of the global monsoon precipitation. The results suggest an overall weakening of the global land monsoon precipitation in the last 56 years, primarily due to weakening of the summer monsoon rainfall in the Northern Hemisphere. However, since 1980, the global land monsoon rainfall has seen no significant trend, which contrasts with the rapid intensification of global warming during the same period. Meanwhile the oceanic monsoon precipitation shows an increasing trend after 1980. The results provide a rigorous test for climate models that will be used in future climate-change assessment.

Role of Narrow Mountains in Large-Scale Organization of Asian Monsoon Convection*

Abstract: The Asian summer monsoon is organized into distinct convection centers, but the mechanism for this organization is not well understood. Analysis of new satellite observations reveals that narrow mountain ranges are an important organizing agent anchoring monsoon convection centers on the windward side. The Bay of Bengal convection, in particular, features the heaviest precipitation on its eastern coast because of orographic lifting as the southwest monsoon impinges on the coastal mountains of Myanmar (also known as Burma). This is in contrast to the widely held view that this convection is centered over the open ocean as implied by coarse-resolution datasets, a view that would require an entirely different explanation for its formation. Narrow in width and modest in height (≤1 km), these mountains are hardly mentioned in conceptual depictions of the large-scale monsoon and poorly represented in global climate models. The numerical simulations of this study show that orographic rainbands are not a...

Recent trends in observed temperature and precipitation extremes in the Yangtze River basin, China

Abstract: The present study is an analysis of the observed extreme temperature and precipitation trends over Yangtze from 1960 to 2002 on the basis of the daily data from 108 meteorological stations. The intention is to identify whether or not the frequency or intensity of extreme events has increased with climate warming over Yangtze River basin in the last 40 years. Both the Mann-Kendall (MK) trend test and simple linear regression were utilized to detect monotonic trends in annual and seasonal extremes.

A penultimate glacial monsoon record from Hulu Cave and two-phase glacial terminations

Abstract: Oxygen isotope records of three stalagmites from Hulu Cave, China, extend the previous high-resolution absolute-dated Hulu Asian Monsoon record from the last to the penultimate glacial and deglacial periods. The penultimate glacial monsoon broadly follows orbitally induced insolation variations and is punctuated by at least 16 millennial-scale events. We confirm a Weak Monsoon Interval between 135.5 ± 1.0 and 129.0 ± 1.0 ka, prior to the abrupt increase in monsoon intensity at Asian Monsoon Termination II. Based on correlations with both marine ice-rafted debris and atmospheric CH 4 records, we demonstrate that most of marine Termination II, the full rise in Antarctic temperature and atmospheric CO 2 , and much of the rise in CH 4 occurred within the Weak Monsoon Interval, when the high northern latitudes were probably cold. From these relationships and similar relationships observed for Termination I, we identify a two-phase glacial termination process that was probably driven by orbital forcing in both hemispheres, affecting the atmospheric hydrological cycle, and combined with ice sheet dynamics.

The sedimentary and tectonic evolution of the Yinggehai-Song Hong basin and the southern Hainan margin, South China Sea : Implications for Tibetan uplift and monsoon intensification

Abstract: [ 1] The Yinggehai - Song Hong basin is one of the world's largest pull-apart basins, lying along the trace of the Red River fault zone in the South China Sea. South of Hainan Island this basin crosscuts the rifted margin of the northern South China Sea. In this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins. The basins started to open after similar to 45 Ma, especially after similar to 34 Ma. The Yinggehai basin was folded and inverted in the middle Miocene, after 21 Ma in the north and 14 Ma in the south, before rapidly subsiding again after similar to 5 Ma because of continued tectonism. This subsidence has caused shale diapirism, especially driven by associated sedimentation in the late Pliocene (2.6 - 2.0 Ma). Extension along the adjacent south Hainan margin shows preferential lower crustal extension, suggestive of lower crustal flow increasing toward the continent-ocean transition during breakup. Sediment supply is reconstructed to peak in the middle Miocene, then falls between 14 and 10.3 Ma to reach a low in the late Miocene. However, rates rose again in the Pliocene-Pleistocene. The Red River sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 Ma or with large-scale river capture away from the Red River after similar to 10 Ma. Both lines of evidence point to major uplift in the Red River drainage being middle Miocene or older. The recent, preindustrial Red River carried much more sediment than the average Pleistocene accumulation rate, indicating modest sediment buffering onshore, at least in recent geologic time.

Impact of anthropogenic forcing on the Asian summer monsoon as simulated by eight GCMs

Abstract: [1] The response of the Asian summer monsoon (ASM) to a transient increase in future anthropogenic radiative forcing is investigated by multi-model global warming experiments. Most models show that, in Asia, the summer monsoon rainfall increases significantly with global warming. On the other hand, the future change in the large-scale flow indicates a weakening of the ASM circulation. Enhanced moisture transport over the Asian summer monsoon region, associated with the increased moisture source from the warmer Indian Ocean, leads to a larger moisture flux convergence, which is responsible for the intensification of the mean rainfall. Pronounced warming over the tropics in the middle-to-upper troposphere causes a reduction in the meridional thermal gradient in the Asian region, which is consistent with the weakened monsoon circulation and eastward shift of the Walker circulation.