onset of the summer monsoon over the indochina peninsula climatology and interannual variations abstract the temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the indochina peninsula were studied using the observed daily rainfall at 30 stations and the ncep ncar reanalysis from 1951 to 1996 the climatological monsoon onset over indochina is on 9 may with a standard deviation of 12 days the monsoon onset is characterized by the pronounced northeastward progression of the low level southwesterlies over the indian ocean and the intensification and northward extension of the tropical convection from sumatra it coincides with the weakening of the midlatitude westerly over south asia and the westward propagation of the intraseasonal oscillation iso originated in the south china sea scs and the western pacific with a dominant timescale of 12 25 days a close relationship between the interannual variations of the monsoon onset and el nino la nina was identified years with warm cold sea surfac

possible role of warm sst bias in the simulation of boreal summer monsoon in sintex f2 coupled model reasonably realistic climatology of atmospheric and oceanic parameters over the asian monsoon region is a pre requisite for models used for monsoon studies the biases in representing these features lead to problems in representing the strength and variability of indian summer monsoon ism this study attempts to unravel the ability of a state of the art coupled model sintex f2 in simulating these characteristics of ism the coupled model reproduces the precipitation and circulation climatology reasonably well however the mean ism is weaker than observed as evident from various monsoon indices a wavenumber frequency spectrum analysis reveals that the model intraseasonal oscillations are also weaker than observed one possible reason for the weaker than observed ism arises from the warm bias over the tropical oceans especially over the equatorial western indian ocean inherent in the model this warm bias is not only confined to the surface layers but also extends through most of the troposphere as a result of this warm bias the coupled model has too weak meridional tropospheric temperature gradient to drive a realistic monsoon circulation this in turn leads to a weakening of the moisture gradient as well as the vertical shear of easterlies required for sustained northward propagation of rain band resulting in weak monsoon circulation it is also noted that the recently documented interaction between the interannual and intraseasonal variabilities of ism through very long breaks vlbs is poor in the model this seems to be related to the inability of the model in simulating the eastward propagating madden julian oscillation during vlbs

on the structure and dynamics of indian monsoon depressions abstractera interim reanalysis data from the past 35 years have been used with a newly developed feature tracking algorithm to identify indian monsoon depressions originating in or near the bay of bengal these were then rotated centralized and combined to give a fully three dimensional 106 depression composite structure a considerably larger sample than any previous detailed study on monsoon depressions and their structure many known features of depression structure are confirmed particularly the existence of a maximum to the southwest of the center in rainfall and other fields and a westward axial tilt in others additionally the depressions are found to have significant asymmetry owing to the presence of the himalayas a bimodal midtropospheric potential vorticity core a separation into thermally cold 1 5 k and neutral 0 k cores near the surface with distinct properties and the center has very large cape and very small cin variability as a function of background state has also been explo

the dynamic and thermodynamic structure of the monsoon over southern india new observations from the incompass iop some of the highest summer monsoon rainfall in south asia falls on the windward slopes of the western ghats mountains on india s west coast and offshore over the eastern arabian sea understanding of the processes determining the spatial distribution and temporal variability of this region remains incomplete in this paper new interaction of convective organization and monsoon precipitation atmosphere surface and sea incompass aircraft and ground based measurements of the summer monsoon over the western ghats and upstream of them are presented and placed within the context of remote sensing observations and reanalysis the transition from widespread rainfall over the eastern arabian sea to rainfall over the western ghats is documented in high spatial and temporal resolution heavy rainfall offshore during the campaign was associated primarily with mid tropospheric humidity secondarily with sea surface temperature and only weakly with orographic blocking a mid tropospheric dry intrusion suppressed deep convection offshore in the latter half of the campaign allowing the build up of low level humidity in the onshore flow and enhancing rainfall over the mountains rainfall on the lee side of the western ghats occurred during the latter half of the campaign in association with enhanced mesoscale easterly upslope flow diurnal cycles in rainfall offshore maximum in the morning and on the mountains maximum in the afternoon were observed considerable zonal and temporal variability was seen in the offshore boundary layer suggesting the presence of convective downdrafts and cold pools persistent drying of the subcloud mixed layer several hundred kilometres off the coast was observed suggesting strong mixing between the boundary layer and the free troposphere these observations provide quantitative targets to test models and suggest hypotheses on the physical mechanisms determining the distribution and variability in rainfall in the western ghats region

the asian summer monsoon an intercomparison of cmip5 vs cmip3 simulations of the late 20th century the boreal summer asian monsoon has been evaluated in 25 coupled model intercomparison project 5 cmip5 and 22 cmip3 gcm simulations of the late twentieth century diagnostics and skill metrics have been calculated to assess the time mean climatological annual cycle interannual variability and intraseasonal variability progress has been made in modeling these aspects of the monsoon though there is no single model that best represents all of these aspects of the monsoon the cmip5 multi model mean mmm is more skillful than the cmip3 mmm for all diagnostics in terms of the skill of simulating pattern correlations with respect to observations additionally for rainfall convection the mmm outperforms the individual models for the time mean the interannual variability of the east asian monsoon and intraseasonal variability the pattern correlation of the time pentad of monsoon peak and withdrawal is better simulated than that of monsoon onset the onset of the monsoon over india is typically too late in the models the extension of the monsoon over eastern china korea and japan is underestimated while it is overestimated over the subtropical western central pacific ocean the anti correlation between anomalies of all india rainfall and nino3 4 sea surface temperature is overly strong in cmip3 and typically too weak in cmip5 for both the enso monsoon teleconnection and the east asian zonal wind rainfall teleconnection the mmm interannual rainfall anomalies are weak compared to observations though simulation of intraseasonal variability remains problematic several models show improved skill at representing the northward propagation of convection and the development of the tilted band of convection that extends from india to the equatorial west pacific the mmm also well represents the space time evolution of intraseasonal outgoing longwave radiation anomalies caution is necessary when using gpcp and cmap rainfall to validate 1 the time mean rainfall as there are systematic differences over ocean and land between these two data sets and 2 the timing of monsoon withdrawal over india where the smooth southward progression seen in india meteorological department data is better realized in cmap data compared to gpcp data

ensemble simulations of asian australian monsoon variability by 11 agcms ensemble simulations of asian australian monsoon a am anomalies were evaluated in 11 atmospheric general circulation models for the unprecedented el nino period of september 1996 august 1998 the models simulations of anomalous asian summer rainfall patterns in the a am region 308s 308n 408 1608e are considerably poorer than in the el nino region this is mainly due to a lack of skill over southeast asia and the western north pacific 58 308n 808 1508e which is a striking characteristic of all the models the models deficiencies result from failing to simulate correctly the relationship between the local summer rainfall and the sst anomalies over the philippine sea the south china sea and the bay of bengal the observed rainfall anomalies are negatively correlated with sst anomalies whereas in nearly all models the rainfall anomalies are positively correlated with sst anomalies while the models physical parameterizations have large uncer tainties this problem is primarily attributed to the experimental design in which the atmosphere is forced to respond passively to the specified ssts while in nature the ssts result in part from the atmospheric forcing regional monsoon dynamic indices are calculated for the indian the western north pacific and the australian monsoons respectively most models can realistically reproduce the western north pacific and australian mon soon yet fail with the indian monsoon to see whether this is generally the case a suite of five seoul national university model runs with the same observed lower boundary forcing and differing only in their initial conditions was examined for the period 1950 98 the skill in the 49 yr ensemble simulations of the indian monsoon is significantly higher than the skill for the period 1996 98 in other words for the unprecedented 1997 98 el nino period the models under study experience unusual difficulties in simulating the indian monsoon circulation anomalies moreover the observed webster yang index shows a decreasing trend over the last 50 yr a trend missed by the models ensemble simulations

the aljibes half graben active extension at the boundary between the trans mexican volcanic belt and the basin and range province mexico the aljibes half graben is located 140 km west northwest of mexico city and is characterized by several parallel 10 km long south dipping and east striking normal faults late miocene basalt flows with 40 ar 39 ar ages of 7 7 0 1 ma and 7 1 0 5 ma are displaced along these faults with reference to this basalt the composite throw and heave of the half graben are 480 m and 270 m respectively and the relative extension is 5 whereas the vertical slip rate along the faults of the aljibes half graben after the deposition of the basalt is 0 07 mm yr the aljibes half graben is located at the northern margin of the trans mexican volcanic belt and perpendicularly intersects northstriking normal faults of the basin and range province the relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area and the two systems may have been active simultaneously we explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time which requires intermittent permutations between the intermediate and least principal stresses 2 and 3 this is supported by the low stress ratio 2 3 1 3 obtained from modeling the stress tensor associated with the slip along the faults of the aljibes half graben the orientations of the fault planes and slip vectors of the aljibes half graben are practically identical to those of the nearby acambay graben and the venta de bravo fault which caused the acambay earthquake of november 19 1912 m s 6 9 and the maravatio earthquake of february 22 1979 m b 5 3 furthermore the aljibes half graben forms the extrapolated western continuation of the northern master fault of the mezquital graben which ruptured partly in the cardonal earthquake of march 26 1976 m b 5 3 this suggests the aljibes half graben may also be seismically active although no historical earthquake is known to have occurred along this structure further evidence for the likely seismic activity of the aljibes half graben are two east to eastnortheast trending epicenter alignments between the aljibes halfgraben and the mezquital graben and structural indication for stickslip behavior based on the structure of the aljibes half graben we believe the maximum credible earthquake that may occur along the faults of the aljibes half graben is on the order of 2 10 27 dyne cm m 0 6 10 25 dyne cm and 7 5 m w 6 5

onsets of the asian summer monsoon 1979 82 abstract analyses of moisture and mean tropospheric enthalpy distributions based on fgge data are described and interpreted for the months of may and june 1979 their detailed spatial characteristics are inferred using 15 day means for the region 0 150 e 40 s 40 n and the detailed temporal evolutions from time series at daily intervals for two regions the arabian sea and the larger area 0 150 e 22 5 5 41 25 n vertical motion fields are calculated using o brien s method and are used to determine the distributions of moisture convergence and heating the onset is found to consist of two main phases 1 a moisture buildup over the arabian sea during which synoptic and mesoscale transient disturbances develop the relationship of this buildup with planetary wave activity is discussed this is followed by 2 a rapid intensification of the arabian sea winds and a substantial increase in latent heat release essentially a large scale feedback process the fully established monsoonal flow is interpreted in

simulation of the east asian summer monsoon using a variable resolution atmospheric gcm the east asia summer monsoon easm is simulated with a variable resolution global atmospheric general circulation model gcm developed at the laboratoire de meteorologie dynamique france the version used has a local zoom centered on china this study validates the model s capability in reproducing the fundamental features of the easm the monsoon behaviors over east asia revealed by the ecmwf reanalysis data are also addressed systematically providing as observational evidence the mean state of the easm is generally portrayed well in the model including the large scale monsoon airflows the monsoonal meridional circulation the cross equatorial low level jets the monsoon trough in the south china sea the surface cold high in australia and the upper level northeasterly return flow while the performance of simulating large scale monsoonal climate is encouraging the model s main deficiency lies in the rainfall the marked rainbelt observed along the yangtze river valley is missed in the simulation this is due to the weakly reproduced monsoonal components in essence and is directly related to the weak western pacific subtropical high which leads to a fragile subtropical southwest monsoon on its western flank and results in a weaker convergence of the southwest monsoon flow with the midlatitude westerlies the excessively westward extension of the high together with the distorted indian low also makes the contribution of the tropical southwest monsoon to the moisture convergence over the yangtze river valley too weak in the model the insufficient plateau heating and the resulting weak land sea thermal contrast are responsible for the weakly reproduced monsoon it is the deficiency of the model in handling the low level cloud cover over the plateau rather than the horizontal resolution and the associated depiction of plateau topography that results in the insufficient plateau heating comparison with the simulation employing regular coarser mesh model reveals that the local zoom technique improves in a general manner the easm simulation

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

south asian summer monsoon and the eastern mediterranean climate the monsoon desert mechanism in cmip5 simulations abstractdry summers over the eastern mediterranean are characterized by strong descent anchored by long rossby waves which are forced by diabatic heating associated with summer monsoon rainfall over south asia the large scale teleconnection between rising and subsiding air masses is referred to as the monsoon desert mechanism this study evaluates the ability of the phase 5 of the coupled model intercomparison project cmip5 models in representing the physical processes involved in this mechanism an evaluation of statistics between summer climatologies of monsoon diabatic heating and that of vertical velocity over the eastern mediterranean suggests a linear relationship despite large spatial diversity in monsoon heating descent over the mediterranean is coherently located and realistic in intensity to measure the sensitivity of descent to the diversity in the horizontal and vertical distribution of monsoon heating a series of linear atmosphere model experiments are performed it is shown that col

the time space structure of the asian pacific summer monsoon a fast annual cycle view abstract despite the seemingly intricate and multifold time space structure of the mean asian pacific summer monsoon apsm its complexity can be greatly reduced once the significance of fast annual cycles has been recognized and put into perspective the apsm climatology is characterized by a slowly evolving seasonal transition slow annual cycle superposed by pronounced singularities in the intraseasonal timescale termed the fast annual cycle in this study the fast annual cycles show nonrepetitive features from one episode to another which are often divided by abrupt change events the apsm fast annual cycles are composed mainly of two monsoon outbreaks each marking a distinctive dry wet cycle the first cycle spans from the middle of may to early july and the second cycle from late july to early september when the first cycle reaches its peak in mid june a slingshot like convection zone described as the grand onset pattern rules an area from the arabian sea to the indochina peninsula then b

coupled model simulations of boreal summer intraseasonal 30 50 day variability part 1 systematic errors and caution on use of metrics boreal summer intraseasonal 30 50 day variability bsisv over the asian monsoon region is more complex than its boreal winter counterpart the madden julian oscillation mjo since it also exhibits northward and northwestward propagating convective components near india and over the west pacific here we analyze the bsisv in the cmip3 and two cmip2 coupled ocean atmosphere models though most models exhibit eastward propagation of convective anomalies over the indian ocean difficulty remains in simulating the life cycle of the bsisv as few represent its eastward extension into the western central pacific as such few models produce statistically significant anomalies that comprise the northwest to southeast tilted convection which results from the forced rossby waves that are excited by the near equatorial convective anomalies our results indicate that it is a necessary but not sufficient condition that the locations the time mean monsoon heat sources and the easterly wind shear be simulated correctly in order for the life cycle of the bsisv to be represented realistically extreme caution is needed when using metrics such as the pattern correlation for assessing the fidelity of model performance as models with the most physically realistic bsisv do not necessarily exhibit the highest pattern correlations with observations furthermore diagnostic latitude time plots to evaluate the northward propagation of convection from the equator to india and the bay of bengal also need to be used with caution here incorrectly representing extratropical tropical interactions can give rise to apparent northward propagation when none exists in association with the eastward propagating equatorial convection despite these cautions the use of multiple cross checking diagnostics enables the fidelity of the simulation of the bsisv to be meaningfully assessed

interannual variability of eastern china summer rainfall the origins of the meridional triple and dipole modes the eastern china summer rainfall ecsr has a strong interannual variability and the leading mode of interannual ecsr variability is characterized by either meridional triple or dipole structures as claimed by previous studies in this study decadal differences of the leading ecsr modes are investigated using observational data and long term integrations of climate models observational analyses show that the leading mode of ecsr is characterized by a meridional triple structure during 1979 1993 whereas a meridional dipole structure during 1994 2014 in the 200 year air sea coupled simulation of community climate system model version 4 ccsm4 the leading mode of ecsr is characterized by triple structure in some decades whereas dipole structure in other decades and decadal shifts between the triple and dipole structures are seen in the 200 year simulation of the stand alone atmospheric component of ccsm4 i e cam4 forced by fixed sst annual cycle the triple mode and dipole mode are also identified suggesting both of these two modes and their decadal shift can be generated by atmospheric internal dynamics without air sea interaction as agreed by observation ccsm4 and cam4 simulations the positive negative phase of the triple leading mode is associated with enhanced weakened western north pacific subtropical high southward northward shifted east asian jet and meridional wave train along the east asian coast the positive negative phase of the dipole leading mode is associated with enhanced weakened western north pacific subtropical high enhanced weakened east asian jet and zonal wave train over mid latitude eurasian continent

controls on silica in groundwater environments in the united kingdom abstract the nature and behaviour of silica has been investigated in the shallow groundwaters of the united kingdom silicon exists in solution as the monomer si oh 4 it is a stable species and chemically unreactive showing no affinity for other major dissolved constituents under normal groundwater conditions silica concentrations in british groundwaters range up to 30 mg l 1 sio2 with an input level in recharge waters of 5 mg l 1 sio2 the input concentration is derived from hydrolysis of silicates and the solution of siliceous minerals within the soil zone detailed investigations of silica in british aquifers has shown that the development of observed silica levels reflects the degree to which reactions between groundwater and silicate minerals in the aquifer matrix are allowed to take place the final concentration is attained when the groundwater reaches equilibrium with the least stable silicate this is illustrated with details from the chalk aquifer of lincolnshire silica concentrations are conservative and may be diagnostic of groundwater bodies and variations of a few mg l 1 may be regarded as significant interpretation of silica levels requires the identification of hydrochemical processes that may influence the water rock reactions and the final equilibrium between the groundwater and silicate minerals

how to measure the strength of the east asian summer monsoon defining the intensity of the east asian summer monsoon easm has been extremely controversial this paper elaborates on the meanings of 25 existing easm indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979 2006 period the existing indices can be classified into five categories the east west thermal contrast north south thermal contrast shear vorticity of zonal winds southwesterly monsoon and south china sea monsoon the last four types of indices reflect various aspects of the leading mode of interannual variability of the easm rainfall and circulations which correspond to the decaying el nino while the first category reflects the second mode that corresponds to the developing el nino the authors recommend that the easm strength can be represented by the principal component of the leading mode of the interannual variability which provides a unified index for the majority of the existing indices this new index is extremely robust captures a large portion 50 of the total variance of the precipitation and three dimensional circulation and has unique advantages over all the existing indices the authors also recommend a simple index the reversed wang and fan index which is nearly identical to the leading principal component of the easm and greatly facilitates real time monitoring the proposed index highlights the significance of the mei yu baiu changma rainfall in gauging the strength of the easm the mei yu which is produced in the primary rain bearing system the east asian ea subtropical front better represents the variability of the easm circulation system this new index reverses the traditional chinese meaning of a strong easm which corresponds to a deficient mei yu that is associated with an abnormal northward extension of southerly over northern china the new definition is consistent with the meaning used in other monsoon regions worldwide where abundant rainfall within the major local rain bearing monsoon system is considered to be a strong monsoon

a model for the boreal summer intraseasonal oscillation abstract the tropical intraseasonal oscillation iso exhibits pronounced seasonality the boreal summer iso is more complex than its winter counterpart due to the coexistence of equatorial eastward off equatorial westward and northward propagating low frequency modes and their interactions based on observational evidence and results obtained from numerical experiments a mechanism is proposed for the boreal summer iso in which the northern hemisphere summer monsoon nhsm circulation and moist static energy distribution play essential roles with a climatological july mean basic state the life cycle of model low frequency waves consists of four processes an equatorial eastward propagation of a coupled kelvin rossby wave packet an emanation of moist rossby waves in the western pacific a westward propagation and amplification of the rossby waves in south asian monsoon regions and a reinitiation of the equatorial disturbances over the central indian ocean the life cycle spans about one month and p

air sea interaction and formation of the asian summer monsoon onset vortex over the bay of bengal in spring over the southern bay of bengal bob a vortex commonly develops followed by the asian summer monsoon onset an analysis of relevant data and a case study reveals that the bob monsoon onset vortex is formed as a consequence of air sea interaction over bob which is modulated by tibetan plateau forcing and the land sea thermal contrast over the south asian area during the spring season tibetan plateau forcing in spring generates a prevailing cold northwesterly over india in the lower troposphere strong surface sensible heating is then released forming a prominent surface cyclone with a strong southwesterly along the coastal ocean in northwestern bob this southwesterly induces a local offshore current and upwelling resulting in cold sea surface temperatures ssts the southwesterly together with the near equatorial westerly also results in a surface anticyclone with descending air over most of bob and a cyclone with ascending air over the southern part of bob in the eastern part of central bob where sky is clear surface wind is weak and ocean mixed layer is shallow intense solar radiation and low energy loss due to weak surface latent and sensible heat fluxes act onto a thin ocean layer resulting in the development of a unique bob warm pool in spring near the surface water vapor is transferred from northern bob and other regions to southeastern bob where surface sensible heating is relatively high the atmospheric available potential energy is generated and converted to kinetic energy thereby resulting in vortex formation the vortex then intensifies and moves northward where sst is higher and surface sensible heating is stronger meanwhile the zonal mean kinetic energy is converted to eddy kinetic energy in the area east of the vortex and the vortex turns eastward eventually southwesterly sweeps over eastern bob and merges with the subtropical westerly leading to the onset of the asian summer monsoon

monsoonal precipitation in the paleo tethys warm pool during the latest permian abstract simulations of the late permian 251 ma are analyzed with respect to the northern hemispheric pangean monsoon we find that the presence and spatial distribution of the warm pool and not land sea temperature differences are the primary forcing agents for the monsoon the land sea temperature gradient as a monsoonal mechanism is tested by eliminating the cathyasian peninsula and is found to have little impact on the spatial character of the monsoon furthermore the response of the monsoon to the warm pool was tested by removing all paleo tethys equatorial islands which allows the warm pool to expand and migrate westward thus shifting the pattern of monsoonal precipitation given that paleogeography changes through time these results have important implications for the migration and strength of the tropical monsoon over geologic time additionally different co2 regimes are presented in which a 10 fold change in forcing produces diverging climates and therefore different warm pool and monsoon locales tropical and equatorial ocean currents impact the seasonal progression and location of the warm pool and atmospheric mass flux for the monsoonal regime is characterized and shown to change with warm pool movement experiments were conducted using the low resolution version of community climate system model version 3 ccsm3 in both coupled and fixed sea surface temperature sst context for validation purposes a modern control is briefly presented with observations and is shown to represent both the spatial and seasonal progression of the indian monsoon

origin of low frequency intraseasonal oscillations in the tropical atmosphere iii monsoon dynamics abstract this is the third in a series of papers to study the origin of intraseasonal oscillations in this paper we address the issue of the interaction of the monsoon large scale circulation and intraseasonal oscillations we show that as a result of the interaction of the large scale monsoon flow with the near equatorial intraseasonal oscillation unstable baroclinic disturbances are generated over the monsoon region these disturbances have spatial scales of approximately 3000 4000 km and periods of 5 6 days with the vertical wave axis tilting eastward with height the rapid development of these cyclonic disturbances along 15 20 n and the concomitant weakening of the equatorial disturbances are accompanied by the rapid northward shift of the rising branch of the local hadley circulation they may also be identified with the observed sudden jump of the mei yu rainband over east asia and the inverse relationship between the monsoon itcz and the equatorial itcz over india and east asia from a linear s