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Summer Monsoon (summer + monsoon)
Kinds of Summer Monsoon Terms modified by Summer Monsoon Selected AbstractsWhat caused the mid-Holocene forest decline on the eastern Tibet-Qinghai Plateau?GLOBAL ECOLOGY, Issue 2 2010Ulrike Herzschuh ABSTRACT Aim, Atmospheric CO2 concentrations depend, in part, on the amount of biomass locked up in terrestrial vegetation. Information on the causes of a broad-scale vegetation transition and associated loss of biomass is thus of critical interest for understanding global palaeoclimatic changes. Pollen records from the north-eastern Tibet-Qinghai Plateau reveal a dramatic and extensive forest decline beginning c. 6000 cal. yr bp. The aim of this study is to elucidate the causes of this regional-scale change from high-biomass forest to low-biomass steppe on the Tibet-Qinghai Plateau during the second half of the Holocene. Location, Our study focuses on the north-eastern Tibet-Qinghai Plateau. Stratigraphical data used are from Qinghai Lake (3200 m a.s.l., 36°32,,37°15, N, 99°36,,100°47, E). Methods, We apply a modern pollen-precipitation transfer function from the eastern and north-eastern Tibet-Qinghai Plateau to fossil pollen spectra from Qinghai Lake to reconstruct annual precipitation changes during the Holocene. The reconstructions are compared to a stable oxygen-isotope record from the same sediment core and to results from two transient climate model simulations. Results, The pollen-based precipitation reconstruction covering the Holocene parallels moisture changes inferred from the stable oxygen-isotope record. Furthermore, these results are in close agreement with simulated model-based past annual precipitation changes. Main conclusions, In the light of these data and the model results, we conclude that it is not necessary to attribute the broad-scale forest decline to human activity. Climate change as a result of changes in the intensity of the East Asian Summer Monsoon in the mid-Holocene is the most parsimonious explanation for the widespread forest decline on the Tibet-Qinghai Plateau. Moreover, climate feedback from a reduced forest cover accentuates increasingly drier conditions in the area, indicating complex vegetation,climate interactions during this major ecological change. [source] Observational relationships between summer and winter monsoons over East Asia.INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2005Part I: Basic framework Abstract This paper is the first of a two-part study to investigate the possible relationships between summer and winter monsoons over East Asia during the period 1958 to 1999. It documents the approach employed in the study. Assuming the existence of relationships, the central theme of the study is to answer the questions: Is there evidence for the relationships and, if so, what are the relationships? In particular, the approach used to interpret the available evidence to make inferences about the conditions of the summer monsoon is described. Six winter monsoon categories are defined in terms of the monsoon strength and the conditions of the El Niño,southern oscillation. The conditions of the summer monsoons preceding and following each winter monsoon category are assessed to identify the possible summer-to-winter monsoon and winter-to-summer monsoon relationships respectively. Summer monsoons are classified into unlikely strong (notS) or unlikely weak (notW) according to the bias in the relative occurrence of positive and negative anomalies of several summer monsoon indices. The rainfall condition over China and the characteristics of the subtropical high are also used to provide supplementary evidence for the summer monsoon strength and to describe the accompanying synoptic situations. Copyright © 2005 Royal Meteorological Society. [source] Occurrence of Appias albina albina (Boisduval, 1836) (Lepidoptera: Pieridae: Pierinae) in northern Australia: phenotypic variation, life history and biology, with remarks on its taxonomic statusENTOMOLOGICAL SCIENCE, Issue 2 2010Michael F. BRABY Abstract Variation in adult phenotype, the life history and general biology of the "White Albatross", Appias albina albina (Boisduval, 1836), are described and illustrated from the monsoon tropics of the northern Australia for the first time. Like elsewhere throughout the species' wide geographical range, the population exhibits sex-limited polymorphism, with females having three distinct color morphs (white, yellow, intermediate). Variation within and among these morphs is compared with populations from elsewhere in South-East Asia, particularly Maluku (including the type locality Ambon), and comments are made on the taxonomic status of the Australian population. The species inhabits coastal semi-deciduous monsoon vine-thicket where its larval food plant Drypetes deplanchei (Brongn. & Gris) Merr. (Euphorbiaceae) grows on lateritic edges and cliffs. The early stages and behaviour are compared with those of A. albina pancheia Fruhstorfer, 1910 from South-East Asia and A. paulina ega (Boisduval, 1836) from Australia. Adults are highly seasonal, their timing of appearance coinciding with annual leaf flush of the larval food plant and onset of the summer monsoon. During this period, the broad flight season lasts about two months, the life cycle is completed in approximately four weeks, and the species is probably univoltine or partially bivoltine. We conclude that breeding populations of A. albina albina in Australia are resident, but it remains to be established how the species survives the long dry season. [source] The geoarchaeological and paleoenvironmental context of Paleoindian sites in western Middle Park, Colorado, USAGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 2 2010James H. Mayer Geoarchaeological investigations in western Middle Park provide important information for understanding the soil-stratigraphic context of Paleoindian components, as well as the latest Quaternary environmental change and landscape evolution in a Southern Rocky Mountain intermontane basin. Paleoindian components are associated with the oldest two of four latest Quaternary stratigraphic units (1,4) recognized in co-alluvial mantles (combined slopewash and colluvium) in uplands and in alluvial valley fills. Limited data suggest accumulation of unit 1 as early as ,12,500 14C yr B.P. in alluvial valleys and by at least ,11,000 14C yr B.P. in uplands was followed by brief stability and soil formation. A relatively widespread disconformity marks earliest Holocene erosion and substantial removal of latest Pleistocene deposits in upland and alluvial settings followed by unit 2 deposition ,10,000,9000 14C yr B.P., perhaps signaling the abrupt onset of an intensified summer monsoon. In situ Paleoindian components in uplands are found in a moderately developed buried soil (the Kremmling soil) formed in units 1 and 2 in thin (,1m) hillslope co-alluvial mantles. The Kremmling soil reflects geomorphic stability in upland and alluvial settings ,9000,4500 14C yr BP, and represents a buried landscape with the potential to contain additional Paleoindian components, although elsewhere in western Middle Park Early Archaic components are documented in morphologically similar soils. Kremmling soil morphology, the relative abundance of charcoal in unit 2 relative to younger units, and charcoal morphology indicate the expansion of forest cover, including Pinus, and grass cover during the early and middle Holocene, suggesting conditions moister than present. © 2010 Wiley Periodicals, Inc. [source] Large-scale summer monsoon rainfall over India and its relation to 850 hPa wind shearHYDROLOGICAL PROCESSES, Issue 15 2007V. S. Prasad Abstract The daily variations of the horizontal wind shear at the 850 hPa level between a southern region (5,15°N, 40,80°E; Zone 1) and a northern region (20,30°N, 70,90°E; Zone 2) during the period 1979,2002 were investigated. Investigations revealed that the changes of this wind shear on a daily basis are directly related to the large-scale rainfall over the Indian region during the monsoon season. The wind shear of zonal wind together with Zone 2 is useful for determining active, weak and break periods of the Indian summer monsoon (ISM). Thus, the Horizontal Wind Shear can be used as a dynamical circulation Index (HWSI) for studying ISM variability. Copyright © 2007 John Wiley & Sons, Ltd. [source] A multi-time scale Australian monsoon indexINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2010Yoshiyuki Kajikawa Abstract A broad-scale Australian monsoon index (AUSMI) describing multi-time scale variations is defined by using 850 hPa zonal wind averaged over the area (5°S,15°S, 110°E,130°E). This circulation index reflects monsoonal rainfall variability over Northern Australia and maritime continent. The index can be used to depict the seasonal cycle (for instance the onset) and measure the intraseasonal, interannual, and interdecadal variations of the Australian monsoon. The interannual variation of the Australian monsoon onset determined by the AUSMI agrees well with that derived from the rainfall and winds at Darwin in the previous studies. We found a significant anti-correlation between the monsoon onset date and the seasonal (DJF) mean AUSMI anomalies; namely an early onset is accompanied by a strong Australian summer monsoon and vice versa. These interannual variations are also strongly associated with El Niño-Southern Oscillation (ENSO). In contrast, the retreat dates are not significantly different between the strong and weak Australian summer monsoon years. The AUSMI is useful in monitoring the weather and climate variations of the Australian monsoon and validating the performance of climate models. Copyright © 2009 Royal Meteorological Society [source] Effect of late 1970's climate shift on tropospheric biennial oscillation,role of local Indian Ocean processes on Asian summer monsoonINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2010Prasanth A Pillai Abstract The tropical climate has undergone noticeable changes on interdecadal time scales. The climate shift that occurred in the late 1970s attained enormous attention owing to its global-scale variations in ocean temperature, heat content and El Nino Southern Oscillation (ENSO) properties. Earlier studies presented the effect of this shift on ENSO and the Asian summer monsoon,ENSO relationship. The present study is an attempt to investigate the effect of late 1970's climate shift on tropospheric biennial oscillation (TBO), which is an important tropical phenomenon that includes both air,sea processes in the tropical Indian and Pacific Ocean regions. TBO is the tendency for the Asian,Australian monsoon system to alternate between relatively strong and weak years. The study comprises a detailed analysis of the TBO cycle in the time periods before (1951,1975) and after (1978,2002) the climate shift in 1976 with the help of National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) data sets of 200-hPa velocity potential; the Indian Ocean sea surface temperature (SST) and circulation are more obvious after the shift, although they were significant in the Pacific Ocean before 1976. The effect of ENSO in the biennial cycle is reduced with climate shift. The persistence of Asian-to-Australian summer monsoon has weakened in recent decades, as it is controlled by ENSO. Local oceanic processes in the Indian Ocean and local monsoon Hadley circulation have an increased role in the biennial oscillation of the Asian summer monsoon after 1976. Copyright © 2009 Royal Meteorological Society [source] Influences of the Indian Ocean dipole on the Asian summer monsoon in the following yearINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2008Yuan Yuan Abstract By exploring the spatiotemporal features of the Indian Ocean Dipole (IOD) both on the sea surface and in the subsurface ocean, the present article reveals that the subsurface dipole mode, with larger amplitude than the surface one, is likely to prolong the dipole signal for a long time. Using the wind and geopotential height data from NCEP/NCAR, this article further investigates IOD impacts on the Asian summer monsoon activities in the following year. A normal (late) South China Sea summer monsoon onset is associated with the previous positive (negative) IOD. In the summer after an IOD year, a positive (negative) IOD tends to induce a stronger (weaker) 100-hPa South Asian High, with a more (less) eastward-extending high ridge, and also an enhanced (a weakened) 500-hPa western Pacific subtropical high, with a westward-advancing (an eastward-retreating) high ridge. Influenced by the anomalous 850-hPa Asian monsoon circulations and the longitudinal position of the 500-hPa subtropical high ridge, summer rainfall in China also exhibits different patterns corresponding to different phases of the IOD in the previous year. Copyright © 2008 Royal Meteorological Society [source] Impacts of the basin-wide Indian Ocean SSTA on the South China Sea summer monsoon onsetINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2008Yuan Yuan Abstract This article explores the impacts of the Indian Ocean basin-scale sea surface temperature anomaly (SSTA) on the South China Sea (SCS) summer monsoon onset. Basin-wide warming in the tropical Indian Ocean (TIO) is found to occur in the spring following an El Niño event, and the opposite occurs for a La Niña event. Such changes of the Indian Ocean SSTA apparently prolong the El Niño-Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the Philippine Sea anti-cyclone. Warming in the TIO induces an anomalous reversed Walker circulation over the tropical Indo,Pacific Ocean, which leads to descending motion, and hence suppressed convection in the western Pacific. The intensified Philippine Sea anti-cyclone in May and June advances more westward and prevents the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late SCS monsoon onset. The case is opposite for the TIO cooling such that the Philippine Sea anti-cyclone weakens and retreats eastward, thus favouring an early onset of the SCS monsoon. Copyright © 2008 Royal Meteorological Society [source] The impact of vertical resolution on regional model simulation of the west African summer monsoonINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2008Leonard M. Druyan Abstract The RM3 regional climate model is used to simulate the west African summer monsoon for six June,September seasons using NCEP reanalysis data for lateral boundary forcing. The study compares the performance of the previously published 16-level version with a newly tested 28-level version, both running on a horizontal grid with 0.5° spacing, in order to determine what improvements in simulations are achieved by increased vertical resolution. Comparisons between the performances include diagnostics of seasonal mean precipitation rates and circulation, vertical profiles of cumulus heating rates, frequencies of shallow and deep convection and diagnostics related to transient African easterly waves (AEWs). The characteristics of a composite AEW simulated at both vertical resolutions are presented. Results show that the most significant impact of increasing the vertical resolution is stronger circulation, stronger vertical wind shear and higher amplitude AEWs. The simulations with higher vertical resolution also achieve higher peaks of cumulus latent heating rates. Spatial,temporal correlations between simulated daily 700 mb meridional winds versus corresponding NCEP reanalysis data and simulated daily precipitation versus estimates from the Tropical Rainfall Measurement Mission (TRMM) archive were equally high at both vertical resolutions. Copyright © 2007 Royal Meteorological Society [source] Inter-decadal variation of the summer precipitation in East China and its association with decreasing Asian summer monsoon.INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2008Part I: Observed evidences Abstract In recent two decades, North and Northeast China have suffered from severe and persistent droughts while the Yangtze River basin and South China have undergone much more significant heavy rainfall/floods events. This long-term change in the summer precipitation and associated large-scale monsoon circulation features have been examined by using the new dataset of 740 surface stations for recent 54 years (1951,2004) and about 123-yr (1880,2002) records of precipitation in East China. The following new findings have been highlighted: (1) One dominating mode of the inter-decadal variability of the summer precipitation in China is the near-80-yr oscillation. Other modes of 12-yr and 30,40-yr oscillations also play an important role in affecting regional inter-decadal variability. (2) In recent 54 years, the spatial pattern of the inter-decadal variability of summer precipitation in China is mainly structured with two meridional modes: the dipole pattern and the positive-negative-positive ("+ , + " pattern). In this period, a regime transition of meridional precipitation mode from "+ , + " pattern to dipole pattern has been completed. In the process of southward movement of much precipitation zone, two abrupt climate changing points that occurred in 1978 and 1992, respectively, were identified. (3) Accompanying the afore-described precipitation changes, the East Asian summer monsoon have experienced significant weakening, with northward moisture transport and convergence by the East Asian summer monsoon greatly weakened, thus leading to much deficient moisture supply for precipitation in North China. (4) The significant weakening of the component of the tropical upper-level easterly jet (TEJ) has made a dominating contribution to the weakening of the Asian summer monsoon system. The cooling in the high troposphere at mid- and high latitudes and the possible warming at low latitude in the Asian region is likely to be responsible for the inter-decadal weakening of the TEJ. Copyright © 2007 Royal Meteorological Society [source] Simulation of Indian summer monsoon: sensitivity to cumulus parameterization in a GCMINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2007S. K. Deb Abstract Hindcasts for the Indian summer monsoons (ISMs) of 2002 and 2003 have been produced from a series of numerical simulations performed with a general circulation model using different cumulus parameterization schemes. Ten sets of ensemble simulations have been produced without using any vegetation scheme but by prescribing the monthly observed SST from the ECMWF (European Centre for Medium Range Weather Forecasts) analyses. For each ensemble, ten simulations have been realised with different initial conditions that are also prepared from the ECMWF data: five each from the April and May analyses of both the years. Stream function, velocity potential with divergent winds at 200 hPa, winds at 850 hPa and rainfall patterns with their anomalies have been analysed and interpreted. The large-scale upper and lower level circulation features are simulated satisfactorily. The spatial structure of predicted July monsoon rainfall over India shows a fair agreement with the GPCP (observed) pentad rainfall distribution. The variability associated with all-India June,July simulated rainfall time series matches reasonably well with the observations in 2003, but the model fails to simulate the observed variability in July 2002. Further evaluation of the model-produced precipitation in seasonal simulations is done with the help of empirical orthogonal functions (EOFs) of the GPCP rainfall over India. Since the first four EOFs explain a significant part of the total variance of the observed rainfall, the simulated precipitation is projected on to these modes. Thus, the differences in simulated and observed rainfall fields manifest in the time series of their expansion coefficients, which are utilised for inter-comparison/evaluation of model simulations. Copyright © 2006 Royal Meteorological Society [source] Simulated changes in active/break spells during the Indian summer monsoon due to enhanced CO2 concentrations: assessment from selected coupled atmosphere,ocean global climate modelsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2007Sujata K. Mandke Abstract The simulations by ten coupled GCMs under the Intergovernmental Panel on Climate Change Assessment Report-4 are used to study the implication of possible global climate change on active/break spells of the Indian summer monsoon (ISM). The validation of the mean daily cycle of the summer monsoon precipitation over the Indian core region and the spatial pattern of the ISM precipitation climatology with observation suggest that six models simulate fairly well, whereas four models differ from observation. Thus, the identification of active/break spells is confined to six models. The sensitivity to climate change has been assessed from two experiments, namely, 1% per year CO2 increase to doubling and 1% per year CO2 increase to quadrupling. The changes in the daily mean cycle and the standard deviation of precipitation, frequency, and duration of active/break spells in future climate change are uncertain among the models and at times among two experiments. The break composite precipitation anomalies strengthen and spread moderately (significantly) in the doubled (quadrupled) CO2 experiment. Copyright © 2006 Royal Meteorological Society [source] Seasonal march and its spatial difference of rainfall in the PhilippinesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2007I. Akasaka Abstract On the basis of the pentad rainfall data averaged from 1961 to 2000, the seasonal march of rainfall in the Philippines is analyzed in this study. The relation to the atmospheric circulation at the 850 hPa level is also discussed. To investigate the temporal and spatial features of rainfall, the Empirical orthogonal function (EOF) analysis was applied to rainfall data. The result showed two dominant modes in the seasonal march of rainfall. The first mode reveals the increase of rainfall amount in the entire Philippines during summer monsoon while the second mode represents the contrast between the west and east coasts in the seasonal march of rainfall. The rainy season starts simultaneously over the entire west coast in the middle of May and withdraws gradually from northern stations around November. And on the east coast, the rainfall amount increases in autumn and winter rather than in summer. These regional differences between west and east coasts are considered to correspond to the seasonal change of Asian summer monsoon and orographic effect. The seasonal march of rainfall in the Philippines is characterized by the sudden change of atmospheric circulation around the Philippines. Particularly, the onset and peak of rainy season on the west coast are influenced by the eastward shift of the subtropical high and the evolution of the monsoon trough with southwesterly, respectively. The increase of rainfall on the east coast is related with the weakened monsoon trough around early September. Copyright © 2006 Royal Meteorological Society [source] ENSO and the South China Sea summer monsoon onsetINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2007Wen Zhou Abstract This paper investigates the relationship between the onset date of the South China Sea summer monsoon (SCSSM) and the El Niño/Southern Oscillation (ENSO). The monsoon onset date (MOD) is defined on the basis of the switch of the 850-hPa zonal winds over the South China Sea (SCS) from easterly to westerly for two consecutive pentads. The ENSO signal is represented by the ocean heat content (OHC), which is proportional to the depth of the 20 °C isotherm. It is found that, in years associated with a warm (cold) ENSO event or the year after, the monsoon tends to have a late (an early) onset and the intensity of the SCSSM also tends to be weaker (stronger). During a 2-year period prior to the onset, anomalies of OHC have an obvious eastward propagation. The 850-hPa flow east of the Philippines, specifically the strength of the subtropical high, is also found to be critical in determining the MOD. The link between these two results appears to be the propagation of cold (warm) subsurface water into the western North Pacific (WNP), which strengthens (weakens) the subtropical high, and hence a late (an early) SCSSM onset. Copyright © 2006 Royal Meteorological Society. [source] Ten-year climatology of summer monsoon over South China and its surroundings simulated from a regional climate modelINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2006Yiming Liu Abstract In a previous study by the authors, a regional climate model (hereafter the RCM) developed to study the summer monsoon over South China (SC) and the South China Sea (SCS) has been tested and found to be able to simulate to a large extent the precipitation over this region for the months of May and June. To examine the interannual variability of the summer monsoon here, it is necessary to establish a model climatology to serve as a comparison and to reduce or even remove any systematic model biases. This paper presents the analyses of such a 10-year climatology (1991,2000). The model was initialized on 1 April and integrated up to the end of June for the ten years. The initial atmospheric conditions and lateral boundary data used in this study are from the European Centre for Medium-range Weather Forecasts ,40-year' reanalyses. The RCM can reproduce well the main features of the monsoon circulation and vertical structure of the atmosphere. The RCM can simulate the intensification and northwestward displacement of the south Asian upper anticyclones from May to June, as well as the low-level moisture transport from the Bay of Bengal to SC. In the simulation, the average SCS summer monsoon onset occurs in the fourth pentad of May, which is consistent with the results from previous observational research. In addition, the RCM can reproduce the main characteristics of the onset such as the change of the low-level zonal flow from easterly to westerly as well as the rapid increase in daily precipitation. The SC and SCS precipitation anomalies have the correct sign in almost all the years. The shortcomings of the model simulation include an under-prediction of the strength of the subtropical high over the Northwest Pacific and the moisture transport from the Bay of Bengal to the Indochina Peninsula (IC) and SCS. A cold bias in surface air temperature is also observed, with the 10-year mean biases of the simulated surface air temperature over SC, SCS and IC in May and June being about ,2.1 °C, ,2.4 °C and ,1.4 °C respectively. The 10-year mean biases of the simulated daily precipitation rate over SC, SCS and IC are about 2.0, ,3.8 and 3.5 mm d,1 respectively. Copyright © 2005 Royal Meteorological Society. [source] Connections of Siberian snow onset dates to the following summer's monsoon conditions over Southeast AsiaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2005Hengchun Ye Abstract 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 Ñino 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. [source] Intraseasonal oscillations and the South China Sea summer monsoon onsetINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2005Wen Zhou Abstract This paper investigates the role of intraseasonal oscillations (ISOs) in the onset of the South China Sea summer monsoon (SCSSM). Two major components of ISO (10,20-day and 30,60-day modes) are identified. The coupling of these two intraseasonal modes during the pre-monsoon period of the SCSSM are investigated by examining the filtered outgoing longwave radiation (OLR), low-level circulation, apparent heat source and apparent moisture sink from October of a previous calendar year to September of a calendar year. The zonal and meridional propagations of the 10,20-day and 30,60-day modes are found to be different, which reflects their different roles in the establishment and development of the SCSSM. The northwestward propagation of the 10,20-day mode is associated with the weakening of the subtropical high over the western Pacific, while the northeastward propagation of the 30,60-day mode originates from convection over the equatorial Indian Ocean. A hypothesis is then proposed to explain the observed variabilities in the SCSSM onset. When the equatorial Indian Ocean exhibits a 30,60-day mode oscillation, an initially weak convection develops into a large convection band (or monsoon trough). Meanwhile, a convective disturbance of the 10,20-day mode is induced when this monsoon trough extends to the western Pacific. These two processes then collaborate to cause a weakening of the subtropical anticyclone over the South China Sea. Because the monsoon trough associated with the 30,60-day mode subsequently propagates northward into the Bay of Bengal (BOB), the induced vortex together with the 10,20-day westward-migrating convection from the equatorial western Pacific will substantially increase the effect of horizontal advection of moisture and heat, thus destabilizing the atmosphere and weakening the subtropical ridge there. Westerlies can then penetrate and prevail over the SCS region, and the SCSSM onset occurs. Copyright © 2005 Royal Meteorological Society. [source] Observational relationships between summer and winter monsoons over East Asia.INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2005Part I: Basic framework Abstract This paper is the first of a two-part study to investigate the possible relationships between summer and winter monsoons over East Asia during the period 1958 to 1999. It documents the approach employed in the study. Assuming the existence of relationships, the central theme of the study is to answer the questions: Is there evidence for the relationships and, if so, what are the relationships? In particular, the approach used to interpret the available evidence to make inferences about the conditions of the summer monsoon is described. Six winter monsoon categories are defined in terms of the monsoon strength and the conditions of the El Niño,southern oscillation. The conditions of the summer monsoons preceding and following each winter monsoon category are assessed to identify the possible summer-to-winter monsoon and winter-to-summer monsoon relationships respectively. Summer monsoons are classified into unlikely strong (notS) or unlikely weak (notW) according to the bias in the relative occurrence of positive and negative anomalies of several summer monsoon indices. The rainfall condition over China and the characteristics of the subtropical high are also used to provide supplementary evidence for the summer monsoon strength and to describe the accompanying synoptic situations. Copyright © 2005 Royal Meteorological Society. [source] Observational relationships between summer and winter monsoons over East Asia.INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2005Part II: results Abstract Using the framework presented in part I of this study, three possible summer-to-winter monsoon and four possible winter-to-summer monsoon relationships are identified. A generalized relationship between summer and winter monsoons is virtually non-existent, and some of the possible relationships are in fact tied to the influence of the El Niño,southern oscillation (ENSO). Indeed, relationships between summer and winter monsoons are specific in terms of both the winter monsoon strength and the ENSO conditions. It is found that the strength of winter monsoon is unlikely to be an important forcing regarding the possible winter-to-summer monsoon relationships, since the summer monsoon is unlikely to be weak following a non-ENSO-coupled winter monsoon, regardless of the winter monsoon strength. On the other hand, possible summer-to-winter relationships are noted only when the summer monsoon is not weak, regardless of the ENSO condition. An alternation or opposite tendency in the summer monsoon strength is noted between the onset year (tends to be unlikely weak) and the following year (tends to be unlikely strong) of an El Niño. Therefore, certain possible relationships between summer and winter monsoons are obvious when the winter monsoon tends to be weaker during the mature phase of an El Niño. For a La Niña, the signature in the summer monsoon strength is less clear, as indicated from the assessment of summer monsoon indices. Nevertheless, when the winter monsoon tends to be strong when coupled with a La Niña, the following summer monsoon also tends to be weaker. A biennial alternation of the summer and winter monsoons is noted, i.e. that a stronger summer monsoon precedes a weaker winter monsoon and a weaker winter monsoon is followed by a stronger summer monsoon. This biennial alternation is associated with a transition of ENSO warm phase to ENSO cold phase, representing the biennial signal in the interannual variability of the monsoons as well as in ENSO. Concurrent with this biennial alternation is an evident variation in the subtropical-high strength. It appears that the commonly recognized tropical biennial oscillation (TBO) is not tied to the biennial signal in the interannual variability of the East Asian monsoons, because the TBO is constituted by a strong (weak) summer monsoon followed by strong (weak) winter monsoon process. Furthermore, it is suggested that a complete biennial oscillation in the interannual variability of the monsoons is not observed because of the breakdown of a cycle (or oscillation) in the summer monsoon following a La Niña onset. Copyright © 2005 Royal Meteorological Society. [source] Onset characteristics of the southwest monsoon over IndiaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2005P. V. S. Raju Abstract Dynamic and thermodynamic characteristics of the Asian summer monsoon during the onset phase over the Indian Peninsula (Kerala coast) and its variability are examined with reanalysis data sets. For this study, daily averaged (0000 and 1200 UTC) reanalysis data sets of National Centre for Environmental Prediction,National Centre for Atmospheric Research for the period 1948,99 are used. Based on 52 years of onset dates of the Indian summer monsoon, we categorized pre-onset, onset and post-onset periods (each averaged 5 days) to investigate the mean circulation characteristics and the large-scale energetics of the Asian summer monsoon. It is found that the strength of the low-level Somali jet and upper tropospheric tropical easterly jet increase rapidly during the time of evolution of the summer monsoon over India. Over the Bay of Bengal and the Arabian Sea, predominant changes are noticed in the large-scale balances of kinetic energy, heat and moisture from the pre-onset to the post-onset periods. Prior to the onset of the summer monsoon over India, a zone of flux convergence of heat and moisture is noticed over the eastern sector of the Bay of Bengal and this intensifies in the onset and post-onset periods. During onset of the monsoon over India, the horizontal flux convergence of heat and moisture, as well as diabatic heating, are enhanced over the Arabian Sea. These subsequently increase with the evolution and advancement of the monsoon over India. Further, the dynamics of the evolution processes (15 days before and 30 days after the onset date of the monsoon over Kerala for each annual cycle) are studied over various sectors, such as the Arabian Sea, Bay of Bengal and Indian Peninsula region. The study reveals that the low-level kinetic energy, vertically integrated generation of kinetic energy and net tropospheric moisture over Arabian Sea can be used as potential predictors for the prediction of the possible onset date of the summer monsoon over the Indian Peninsula. Copyright © 2005 Royal Meteorological Society [source] Characteristics, evolution and mechanisms of the summer monsoon onset over Southeast AsiaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2004Zuqiang Zhang Abstract Based on the 1979,95 mean pentad reanalysis data from the US National Centers for Environmental Prediction, the climatological characteristics and physical mechanism of the Asian summer monsoon (ASM) onset are investigated. Special focus is given to whether the ASM onset starts earlier over the Indochina Peninsula than over the South China Sea (SCS) and why the ASM is established the earliest over Southeast Asia. An examination of the composite thermodynamic and dynamic quantities confirms that the ASM onset commences earliest over the Indochina Peninsula, as highlighted by active convection and rainfall resulting from the convergence of southwesterly flow from the Bay of Bengal (BOB) vortex and easterly winds associated with the subtropical anticyclone over the SCS. Two other important characteristics not previously noted are also identified: the earliest reversal of meridional temperature gradient throughout the entire troposphere and the corresponding establishment of an easterly vertical wind shear, which are due to upper level warming caused by eddy (convective) transport of latent heat. These changes in the large-scale circulation suggest that, in addition to rainfall, a reversal in the planetary-scale circulation should be included in determining the timing of the ASM onset. With such a consideration, the climatological ASM onset occurs first over southeastern BOB and southwestern Indochina Peninsula in early May, and then advances northeastward to reach the SCS by the fourth pentad of May (16,20 May). The monsoon then covers the entire Southeast Asia region by the end of May. Subsequently, a similar onset process begins over the eastern Arabian Sea, India and western BOB, and the complete establishment of the ASM over India is accomplished in mid June. In the process of the onset of each ASM component, the reversal of the upper level planetary-scale circulation depends strongly on that of the meridional temperature gradient. Over the Indochina Peninsula, the seasonal transition of upper level temperature results from convection-induced diabatic heating, whereas over western Asia it is attributed to subsidence warming induced by the active ascending motion over the former region. The steady increase in surface sensible heating over the Indian subcontinent and the latent heating over the tropical Indian Ocean in April to early May appear to be the major impetus for the development of the cyclonic vortex over the BOB. A similar enhancement over the Arabian Peninsula and the surrounding regions is also identified to be crucial to the development of the so-called onset vortex over the Arabian Sea, and then ultimately to the ASM onset over India. Copyright © 2004 Royal Meteorological Society [source] Responses of China's summer monsoon climate to snow anomaly over the Tibetan PlateauINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2003Prof. Y. F. Qian Abstract The climatological features of the winter snow depth over the Tibetan Plateau and the summer precipitation in China are diagnosed using datasets obtained from 78 snow observation stations and 160 rainfall stations during 1957 to 1998. The climatic effects of the snow anomaly over the Tibetan Plateau on the regional summer monsoon climate in China are diagnosed and numerically simulated by use of a regional climate model (RegCM2). The singular value decomposition technique is adopted to diagnose the relationships between the previous winter and spring plateau snow depth anomalies and the spring and summer regional precipitation in China. It is found that the snow depth anomaly, especially in winter, is one of the factors influencing precipitation in China; however, it is perhaps not the only one, and even not the most important one. Nevertheless, it is proved that the winter snow anomaly over the Tibetan Plateau is relatively more important than that in spring for the regional precipitation in China. Results of numerical simulations show that the snow anomaly over the plateau has effects that are evident on China's summer monsoon climate. The increase of both snow cover and snow depth can delay the onset and weaken the intensity of the summer monsoon obviously, resulting in a decrease in precipitation in southern China and an increase in the Yangtze and Huaihe River basins. The influence of the winter snow depth is more substantial than that of both the winter snow cover and the spring snow depth. The mechanism of how the plateau snow anomaly influences the regional monsoon climate is briefly analysed. It is found that snow anomalies over the Tibetan Plateau change the soil moisture and the surface temperature through the snowmelt process at first, and subsequently alter heat, moisture and radiation fluxes from the surface to the atmosphere. Abnormal circulation conditions induced by changes of surface fluxes may affect the underlying surface properties in turn. Such a long-term interaction between the wetland and the atmosphere is the key process resulting in later climatic changes. Copyright © 2003 Royal Meteorological Society [source] Spring northward retreat of Eurasian snow cover relevant to seasonal and interannual variations of atmospheric circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2003Hiroaki Ueda Abstract An observational study is made of the seasonal and interannual variations of spring snow-disappearance over the Eurasian continent and the circulation mechanisms causing those variations. The spring northward retreat of the snow boundary over the East European Plain (EEP) between 30 and 60° E is faster (0.4° per day) than to the east of the Ural Mountain range (0.3° per day). These migrations of the snow boundary lag behind the appearance of the surface air temperature 0 °C by about 1 to 5 pentads. The analyses of the atmospheric heat and moisture budgets showed that the seasonal intrusion of warm air associated with southwesterly winds is primarily responsible for the rapid snowmelt in March and April over the EEP. In addition, the adiabatic heating of descending air plays a secondary role in the snowmelt in mid-March. On an interannual time scale, horizontal warm advection also plays an essential role in the spring northward retreat of snow cover extent. The present study confirms the previous finding that the surface air temperature anomalies, produced during the seasonal snow-disappearance period, diminished in May, suggesting a weak dynamical linkage between the EEP snow cover and Asian summer monsoon. Copyright © 2003 Royal Meteorological Society [source] Evaluation of broad scale vertical circulation and thermal indices in relation to the onset of Indian summer monsoonINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2002S. K. Roy Bhowmik Abstract The onset of the Indian summer monsoon over Kerala for an individual year of delayed (1997), early (1999) and normal onset (2000) was examined in relation to the intensity of vertical circulation and thermal indices during the pre-monsoon months (April and May). The study showed that in the delayed monsoon onset year (1997) negative anomalies of vertical zonal index dominated over the north Indian Ocean during pre-monsoon months, particularly in April. In contrast, in the early onset year (1999) the positive anomalies of this index over the north Indian Ocean during the pre-monsoon months were considerably stronger (April and May). However, the meridional vertical index did not show any appreciable difference. The gradient of the vertical thermal index anomalies over the Tibetan Plateau in the month of April was prominently stronger during the years of early and normal onset (1999 and 2000). The anomalies of geopotential height at 200 hPa over the Tibetan Plateau in the pre-monsoon months were significantly lower in the year of delayed onset (1997). The precipitable water content was found to be another major feature, which grew rapidly over the equatorial belt of the Indian Ocean extending up to the Arabian Sea and Bay of Bengal during the two weeks prior to onset. Most of these features were observed very distinctly in the month of April, well before the monsoon onset, and promise to provide important predictive signals for the onset over Kerala. Copyright © 2002 Royal Meteorological Society. [source] Interannual variability of lower-tropospheric moisture transport during the Australian monsoonINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2002Christopher R. Godfred-Spenning Abstract The interannual variability of the horizontal lower-tropospheric moisture transport associated with the Australian summer monsoon has been analysed for the 1958,99 period. The 41-season climatology of moisture flux integrated between the surface and 450 hPa showed moderate levels of westerly transport in the month before Australian monsoon onset, associated with cross-equatorial flow in the Sulawesi Sea and west of Borneo. In the month after onset the westerly moisture transport strengthened dramatically in a zonal belt stretching from the Timor Sea to the Western Equatorial Pacific, constrained between the latitudes 5 and 15 °S, and associated with a poleward shift in the Intertropical Convergence Zone and deepening of the monsoon trough. Vertical cross-sections showed this transport extending from the surface to the 500 hPa level. In the second and third months after onset the horizontal flow pattern remained similar, although flux magnitudes progressively decreased, and the influence of trade winds became more pronounced over northern Australia. Nine El Niño and six La Niña seasons were identified from the data set, and composite plots of the affected years revealed distinct, and in some cases surprising, alterations to the large-scale moisture transport in the tropical Australian,Indonesian region. During an El Niño it was shown that the month prior to onset, in which the moisture flux was weaker than average, yielded to a dramatically stronger than average flux during the following month, with a zone of westerly flux anomalies stretching across the north Australian coast and Arafura Sea. The period of enhanced moisture flux during an El Niño is relatively short-lived, with drier easterly anomalies asserting themselves during the following 2 months, suggesting a shorter than usual monsoon period in north Australia. In the La Niña composite, the initial month after onset shows a tendency to weaker horizontal moisture transport over the Northern Territory and Western Australia. The subsequent 2 months show positive anomalies in flux magnitude over these areas; the overall effect is to prolong the monsoon. Comparison of these results with past research has led us to suggest that the tendency for stronger (weaker) circulations to arise in the initial month of El Niño (La Niña) events is a result of mesoscale changes in soil moisture anomalies on land and offshore sea surface temperature (SST) anomalies, brought about by the large-scale alterations to SST and circulation patterns during the El Niño,Southern Oscillation. The soil moisture and SST anomalies initially act to enhance (suppress) the conditions necessary for deep convection in the El Niño (La Niña) cases via changes in land,sea thermal contrast and cloud cover. Copyright © 2002 Royal Meteorological Society. [source] Spatial variation in the duration of the rainy season in monsoonal AustraliaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2001Garry D. Cook Abstract Climatological research in the tropics of northern Australia has focused particularly on the Australian summer monsoon. However, the timing of many ecologically important processes is determined not by the monsoon but by extra-monsoonal rainfall events. These events produce a temporal pattern of wet and dry spells that is at least as important ecologically as the amount of rain. We defined the rainy season as that period when the probability of 10-day dry spells was less than 0.5, and the wet season as that period within the rainy season when the probability of dry spells was less than 0.1. We applied these criteria to seven stations along a strong north,south rainfall gradient spanning more than 12° of latitude in tropical north Australia. The duration of the rainy and wet seasons in northern Australia decreases with increasing latitude. The timing and duration of these seasons were also affected by the El Niño,Southern Oscillation (ENSO). The relative contribution of temporally isolated rainfall events to both the duration of the rainy season and the amount of rainfall increases with latitude. The geographic variation in these seasons corresponds to many patterns occurring in natural ecosystems. We argue that understanding the extra-monsoon rainfall events is critical to understanding how climate variation affects natural ecosystems. Copyright © 2001 Royal Meteorological Society [source] Did the Indo-Asian summer monsoon decrease during the Holocene following insolation?,JOURNAL OF QUATERNARY SCIENCE, Issue 7 2010Manish Tiwari Abstract A few studies from the western Arabian Sea indicate that the Indian summer (or southwest) monsoon (ISM), after attaining its maximum intensity at ca. 9,ka, declined during the Holocene, as did insolation. In contrast, earlier and later observations from both the eastern and the western Arabian Sea do not support this inference. Analysis of multiple proxies of productivity in a new sediment core from the western Arabian Sea fails to confirm the earlier, single-proxy (e.g. abundance of Globigerina bulloides) based, inference of the Holocene weakening of ISM, following insolation. The reason for the observed decreasing trend in foraminiferal abundance , the basis for the earlier inference , could be the favouring of silicate rather than carbonate productivity by the increased ISM wind strength. Although ISM exhibits several multi-millennial scale fluctuations, there is no evidence from several multi-proxy data to conclude that it declined during the Holocene; this is consistent with the phase lag analysis of longer time series of monsoon proxies. Thus, on sub-Milankovitch timescales, ISM did not follow insolation, highlighting the importance of internal feedbacks. A comparison with East Asian summer monsoon (EASM) records suggests that both ISM and EASM varied in unison, implying common forcing factors on such longer timescales. Copyright © 2010 John Wiley & Sons, Ltd. [source] Response of mid-latitude North Pacific surface temperatures to orbital forcing and linkage to the East Asian summer monsoon and tropical ocean,atmosphere interactions,JOURNAL OF QUATERNARY SCIENCE, Issue 8 2009Masanobu Yamamoto Abstract We present a palaeoceanographic perspective of the North Pacific during the last two glacial cycles based on U -derived palaeotemperature records of IMAGES Core MD01-2421 off the coast of central Japan and cores from the Ocean Drilling Program (ODP) Sites 1014 and 1016 off the coast of California. The sea surface temperature (SST) differences between ODP Sites 1014 and 1016 (,SSTnortheastern Pacific (NEP),=,SSTODP1014 , SSTODP1016) indicate the intensity of the California Current. Comparison of ,SSTNEP and the SST from Core MD01-2421 revealed anti-phase variation; high ,SSTNEP (indicating weakening of the California Current) corresponded to low SST at the Japan margin (indicating the southward displacement of the north-western Pacific subarctic boundary and weakening of the Kuroshio Extension), and vice versa. This finding suggests that the intensity of the North Pacific subtropical gyre circulation has varied in response to precessional forcing and that this response has been linked with changes in tropical ocean,atmosphere interactions. In the precessional cycle, the SST variation derived from Core MD01-2421 lags ca. 2.5,4,ka behind the variations shown by Hulu and Sanbao stalagmite ,18O records and by the pollen temperature index from Core MD01-2421, suggesting out-of-phase variations of the North Pacific subtropical gyre circulation and the East Asian summer monsoon. These findings indicate that the behaviour of interactions between tropical ocean,atmosphere dynamics and the East Asian summer monsoon may have varied in response to the precessional cycle. Copyright © 2009 John Wiley & Sons, Ltd. [source] PRECIPITATION CHANGES FROM 1956 TO 1996 ON THE WALNUT GULCH EXPERIMENTAL WATERSHED,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 1 2002Mary H. Nichols ABSTRACT: The climate of Southern Arizona is dominated by summer precipitation, which accounts for over 60 percent of the annual total. Summer and non-summer precipitation data from the USDA-ARS Walnut Gulch Experimental Watershed are analyzed to identify trends in precipitation characteristics from 1956 to 1996. During this period, annual precipitation increased. The annual precipitation increase can be attributed to an increase in precipitation during non-summer months, and is paralleled by an increase in the proportion of annual precipitation contributed during non-summer months. This finding is consistent with previously reported increases in non-summer precipitation in the southwestern United States. Detailed event data were analyzed to provide insight into the characteristics of precipitation events during this time period. Precipitation event data were characterized based on the number of events, event precipitation amount, 30-minute event intensity, and event duration. The trend in non-summer precipitation appears to be a result of increased event frequency since the number of events increased during nonsummer months, although the average amount per event, average event intensity, and average event duration did not. During the summer "monsoon" season, the frequency of recorded precipitation events increased but the average precipitation amount per event decreased. Knowledge of precipitation trends and the characteristics of events that make up a precipitation time series is a critical first step in understanding and managing water resources in semiarid ecosystems. [source] | ||||||||||||||||