modulation of boreal summer intraseasonal oscillations over the western north pacific by enso abstractthe authors investigate the effects of el nino and la nina on the intraseasonal oscillation iso in the boreal summer may october over the western north pacific wnp it is found that during el nino summers the iso is dominated by a higher frequency oscillation with a period of around 20 40 days whereas during la nina summers the iso is dominated by a lower frequency period of around 40 70 days the former is characterized by northwestward propagating convection anomalies in the wnp and the latter is characterized by northward and eastward propagating convective signals over the tropical indian ocean maritime continent the possible mechanisms through which el nino southern oscillation enso induced background mean state changes influence the iso behavior are examined through idealized numerical experiments it is found that enhanced weakened mean moisture and easterly westerly vertical wind shear in the wnp during el nino la nina are the main causes of the strengthened weakened 2

the role of bay of bengal convection in the onset of the 1998 south china sea summer monsoon abstract assimilated analysis fields from the south china sea monsoon experiment and the outgoing longwave radiation data from the national center for atmospheric research ncar have been employed to describe the large scale and synoptic features of the subtropical circulation during the bay of bengal bob 6 20 n 80 100 e and south china sea scs 7 20 n 110 120 e monsoon onsets in 1998 the results show that the asian monsoon onset during may 1998 exhibited a typical eastward development from the bob region to the scs domain the weakening and retreat of the subtropical anticyclone from the scs were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the scs nscs 15 20 n 110 120 e as the vertical shear of zonal wind changes in sign the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the scs based on these observational results version 4 of the ncar climate model ccm3

long time evolution of models of aeolian sand dune fields influence of dune formation and collision abstract theoretical models which approximate individual sand dunes as particles that move and interact via simple rules are currently the only viable method for examining whether large dune fields will evolve into a patterned structure we find that these types of simulations are sensitive to the influx condition and interaction function and are not necessarily robust under common assumptions in this paper we review continuum dune models and how they connect to models of dune fields that approximate dunes as interacting particles with collision and coalescence dynamics this type of simple dune field model is examined under different boundary and initial conditions we identify different long term behaviors depending on model parameters as well as the way in which dunes are initialized and collide a rule for predicting the end state of a modelled dune field is derived based on the statistics of a uniform influx dune size distribution and the interaction function possible future adjustments to the multiscale model such as the use of a gaussian influx dune size distribution and their effect on the prediction rule are also discussed

effects of intraseasonal oscillation on south china sea summer monsoon onset understanding of south china sea scs summer monsoon scssm onset response to tropical intraseasonal oscillation iso is critical for extended range prediction of scssm onset in this study we investigate the effect of iso on scssm onset for the period of 1980 2013 the 34 onset cases are classified into three groups early onsets around may 6th normal onsets around may 21st and late onsets around june 8th and the late onsets are even later than the indian monsoon onsets before each onset the scs experiences a dry iso phase to precondition the convective energy due to the easterly wind anomalies of the wet iso phase over the tropical indian ocean for the group of early onsets over the southern bay of bengal monsoon region for the group of normal onsets and over the southern indian monsoon region for the group of late onsets after each onset the scssm is supported by the westerly wind anomalies of the dry iso phase over these associated regions each early scssm onset is triggered by the northwestward propagating rossby wave of the wet iso in the western pacific which comes from the indian ocean for each normal late onset the scssm is triggered by synoptic scale low level westerlies in conjunction with seasonal low level westerlies when the wet iso moves to the northern bay of bengal region indian monsoon region since this convection to the north of 10 n cannot excite the easterly wind anomalies associated with kelvin wave responses over the scs to suppress the convection the mechanisms explaining the mean state controlled iso scssm onset relationship are also discussed

tropical circulations associated with southwest monsoon onset and westerly surges over the south china sea abstract the earliest onset of the asian summer monsoon occurs in early to middle may over the south china sea this onset is signified by the development of low level westerlies and leads to heavy convective rainfall over southern china pre mei yu in june low level westerly surges over the northern south china sea are associated with the mei yu rainfall system in the yangtze region and southern japan in this work the ecmwf data for 1981 86 are used to study the tropical circulations associated with the development of low level westerlies during both events composites of horizontal wind geopotential height moisture and vertical velocity during six may onsets and nine june surges respectively indicate that both events occur with the approach of a midlatitude trough front system the possible triggering of the south china sea summer monsoon onset by the midlatitude system may explain why the south china sea onset occurs prior to other regions of the asian monsoon during boreal spring this is t

mechanisms of northward propagating intraseasonal oscillation over the south china sea during the pre monsoon period abstractin the present study the spatiotemporal structures of the northward propagating intraseasonal oscillation iso over the south china sea scs in the premonsoon period are analyzed by usin

real time multivariate indices for the boreal summer intraseasonal oscillation over the asian summer monsoon region the boreal summer intraseasonal oscillation bsiso of the asian summer monsoon asm is one of the most prominent sources of short term climate variability in the global monsoon system compared with the related madden julian oscillation mjo it is more complex in nature with prominent northward propagation and variability extending much further from the equator in order to facilitate detection monitoring and prediction of the bsiso we suggest two real time indices bsiso1 and bsiso2 based on multivariate empirical orthogonal function mv eof analysis of daily anomalies of outgoing longwave radiation olr and zonal wind at 850 hpa u850 in the region 10 s 40 n 40 160 e for the extended boreal summer may october season over the 30 year period 1981 2010 bsiso1 is defined by the first two principal components pcs of the mv eof analysis which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward mjo with quasi oscillating periods of 30 60 days bsiso2 is defined by the third and fourth pcs which together mainly capture the northward northwestward propagating variability with periods of 10 30 days during primarily the pre monsoon and monsoon onset season the bsiso1 circulation cells are more rossby wave like with a northwest to southeast slope whereas the circulation associated with bsiso2 is more elongated and front like with a southwest to northeast slope bsiso2 is shown to modulate the timing of the onset of indian and south china sea monsoons together the two bsiso indices are capable of describing a large fraction of the total intraseasonal variability in the asm region and better represent the northward and northwestward propagation than the real time multivariate mjo rmm index of wheeler and hendon

mechanisms of global teleconnections associated with the asian summer monsoon an intermediate model analysis the indian summer monsoon ism and western north pacific summer monsoon wnpsm are two subsystems of the asian summer monsoon and they exhibit different global teleconnection patterns the enhanced ism strengthens the south asian high and mascarene high and the wnpsm excites a meridional tripolar wave train in the northern hemisphere and affects the australian high in the southern hemisphere to understand the dynamics behind these global teleconnections especially the processes responsible for the cross equatorial teleconnection an intermediate model describing a two level troposphere and a steady planetary boundary layer pbl is linearized from the background horizontal wind field the model results indicate that the ism heating located under the strong easterly vertical shear vs and close to the westerly jet in the northern hemisphere can excite a barotropic rossby wave that emanates northwestward and then propagates downstream along the westerly jet since the wnpsm heating is far away from the westerly jet over the north pacific it only excites a weak rossby wave train which cannot explain the meridional tripolar teleconnection associated with the wnpsm it is found that both the ism and wnpsm heating excite strong teleconnections in the southernhemisphere via an advectionmechanism that is the background upper level northerly winds can transport energy across the equator from the northern hemisphere summer monsoon to the southern hemisphere westerly jet in addition the pbl enhances monsoon teleconnections by trapping more energy in the upper troposphere the elevated maximum monsoon heating also reinforces upper level perturbations and enhances the teleconnection pattern

asian summer monsoon onset barrier and its formation mechanism the onset process of asian summer monsoon asm is investigated based on diagnostic analysis of observations of precipitation and synoptic circulation results show that after the asm commences over the eastern bay of bengal bob around early may the onset can propagate eastwards towards the south china sea and western pacific but is blocked on its westward propagation along the eastern coast of india this blocking termed the monsoon onset barrier mob presents a gill type circulation response to the latent heating released by bob monsoon convection this convective condensation heating generates summertime wintertime vertical easterly westerly shear to its east west and facilitates air ascent descent the convection then propagates eastward but gets trapped on its westward path to the east of the central bob the surface air temperature sat cools faster than the underlying sea surface temperature sst due to monsoon onset thus more sensible heat flux supports the onset convection to propagate eastward to the west of the central bob however the land surface sensible heating over the indian peninsula is strengthened by the enhanced anticyclone circulation and air descent induced by the bob monsoon heating the strengthened upstream warm horizontal advection then produces a warm sat center over the mob region which together with the in situ cooled sst reduces the surface sensible heating and atmospheric available potential energy to prevent the occurrence of free convection therefore it is the change in both large scale circulation and air sea interaction due to bob summer monsoon onset that contributes to the mob formation

an observational study of the south china sea monsoon during the 1979 summer onset and life cycle abstract the onset and life cycle of the 1979 south china sea summer monsoon were examined in the context of the 30 60 and 12 24 day monsoon modes the former intraseasonal mode formed the northward migrating monsoon trough ridge while the latter intraseasonal mode propagated westward in the south china sea the monsoon in this region exhibited three cycles over the summer may august with the onset taking place about one cycle ahead of the onset of the indian and japanese monsoons climatologically a summer trough line radiated out from the indian monsoon trough region across indochina to the northern south china sea the monsoon onset occurred when the 30 60 day monsoon trough and the 12 24 day low center arrived simultaneously at the northern south china sea close to the climatological summer trough line in the middle of may the breaks occurred when the 30 60 day monsoon ridge lines and the 12 24 day high center met near 15 20 n in the northern south china sea the south china sea monsoon wa

two types of interannual variability of south china sea summer monsoon onset related to the sst anomalies before and after 1993 94 abstractan advance in the timing of the onset of the south china sea scs summer monsoon scssm during the period 1980 2014 can be detected after 1993 94 in the present study the interannual variability of the scssm onset is classified into two types for the periods before and after 1993 94 based on their different characteristics of vertical coupling between the upper and lower tropospheric circulation and the differences in their related sea surface temperature anomalies sstas on the interannual time scale type i scssm onset is characterized by anomalous low level circulation over the northern scs during 1980 93 whereas type ii scssm onset is associated with anomalies of upper level circulation in the tropics during 1994 2014 the upper tropospheric thermodynamic field and circulation structures over the scs are distinct between the two types of scssm onset and this investigation shows the importance of the role played by the spring sstas in the southern indian ocean sio and that of enso e

relationship of boreal summer 10 20 day and 30 60 day intraseasonal oscillation intensity over the tropical western north pacific to tropical indo pacific sst the present study contrasts interannual variations in the intensity of boreal summer 10 20 day and 30 60 day intraseasonal oscillations isos over the tropical western north pacific and their factors a pronounced difference is found in the relationship of the two isos to el nino southern oscillation the 10 20 day iso intensity is enhanced during el nino developing summer whereas the 30 60 day iso intensity is enhanced during la nina decaying summer the above different relationship is interpreted as follows the equatorial central and eastern pacific sst anomalies modify vertical wind shear lower level moisture and vertical motion in a southeast northwest oriented band from the equatorial western pacific to the tropical western north pacific where the 10 20 day isos originate and propagate these background field changes modulate the amplitude of 10 20 day isos preceding equatorial central and eastern pacific sst anomalies induce sst anomalies in the north indian ocean in summer which in turn modify vertical wind shear and vertical motion over the tropical western north pacific the modified background fields influence the amplitude of the 30 60 day isos when they reach the tropical western north pacific from the equatorial region a feedback of iso intensity on local sst change is identified in the tropical western north pacific likely due to a net effect of isos on surface heat flux anomalies this feedback is more prominent from the 10 20 day than the 30 60 day iso intensity change

interannual variability of summer monsoon onset over the western north pacific and the underlying processes climatological summer monsoon onset over the south china sea scs and the western north pacific wnp defined as the region of 108 208n 1208 1608e displays three distinct stages around mid may monsoon rain commences in the scs and the philippines in early to mid june the monsoon rain extends to the southwestern philippine sea after mid july the rainy season starts in the northeastern part of the wnp the onset anomaly however displays an in phase interannual variation across the entire wnp domain the standard deviation of the onset date increases eastward from 3 pentads in the scs to 5 pentads in the northeastern part of the domain the large onset variability in the wnp is mainly attributed to large year to year changes of the seasonal cycle

the northward propagating intraseasonal oscillations in the northern indian ocean during spring early summer abstractthe intraseasonal oscillations isos activate in the tropical indian ocean io exhibiting distinct seasonal contrasts in active regions and propagating features the seasonal northward m

multi stage onset of the summer monsoon over the western north pacific the climatological summer monsoon onset displays a distinct step wise northeastward movement over the south china sea and the western north pacific wnp 110 160 e 10 20 n monsoon rain commences over the south china sea philippines region in mid may extends abruptly to the southwestern philippine sea in early to mid june and finally penetrates to the northeastern part of the domain around mid july in association three abrupt changes are identified in the atmospheric circulation specifically the wnp subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastward or northeastward at the onset of the three stages the step wise movement of the onset results from the slow northeastward seasonal evolution of large scale circulation and the phase locked intraseasonal oscillation iso the seasonal evolution establishes a large scale background for the development of convection and the iso triggers deep convection the iso over the wnp has a dominant period of about 20 30 days this determines up the time interval between the consecutive stages of the monsoon onset from the atmospheric perspective the seasonal sea surface temperature sst change in the wnp plays a critical role in the northeastward advance of the onset the seasonal northeastward march of the warmest sst tongue sst exceeding 29 5 c favors the northeastward movement of the monsoon trough and the high convective instability region the seasonal sst change in turn is affected by the monsoon through cloud radiation and wind evaporation feedbacks

roles of barotropic convective momentum transport in the intraseasonal oscillation both observational data analysis and model simulations suggest that convective momentum transport cmt by cumulus convection may play a significant role in the intraseasonal oscillations iso by redistributing atmospheric momentum vertically through fast convective mixing process the authors present a simple theoretical model for the iso by parameterizing the cumulus momentum transport process in which the cmt tends to produce barotropic wind anomalies that will affect the frictional planetary boundary layer pbl in the model with equatorial easterly vertical wind shear vws it is found that the barotropic cmt tends to select most unstable planetary scale waves because cmt suppresses the equatorial ekman pumping of short waves which reduces the shortwave instability from the pbl moisture convergence and accelerates the shortwave propagation the model with subtropical easterly vws has behavior that can be qualitatively different from the model with equatorial easterly vws and has robust northward propagation the basic mechanism of this northward propagation is that the cmt accelerates the barotropic cyclonic wind to the north of iso which will enhance the precipitation by pbl ekman pumping and favor the northward propagation the simulated northward propagation is sensitive to the strength and location of the seasonal mean easterly vws these results suggest that accurate simulation of the climatological mean state is critical for reproducing the realistic iso in general circulation models

mesozoic carbonate platforms and banks of the eastern north american margin abstract the jurassic lower cretaceous carbonate platforms and banks form a discontinuous belt extending over distance of 6000 km from the grand banks up to the bahamas six types of carbonate buildups are recognized and document the variability of depositional paleo oceanographic and tectonic processes on the eastern north american margin the composition of the carbonates closely resembles the recent deposits of the western great bahama bank since oolitic shoals were present near to the shelf edge and skeletal peloid wackestones and biomicrites were deposited in the inner part of the platform coralstromatoporoid and sponge bioherms were only rare constituents of the carbonate banks the thickness of carbonate buildups progressively increases southward along the margin attaining a thickness of more than 5 km on the bahamas the carbonate platforms also become younger southwards which is thought to reflect the northward movement of the north american plate of less than 1 5 cm yr the carbonate platforms were seeded over the continental basement following the taphrogenic period of plate tectonics building of carbonate ramps which characterized the early jurassic has began during the transitional period between continental rifting and early drift the second stage in construction of the carbonate platforms and offshore banks proceeded mainly after separation of the continental plates

intraseasonal variability of the south china sea summer monsoon the objective of this study is to explore based on the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis data the intraseasonal variability of the south china sea scs summer monsoon sm in terms of its structure and propagation as well as interannual variations a possible mechanism that is responsible for the origin of the 10 20 day oscillation of the scs sm is also proposed the 30 60 day hereafter the 3 6 mode and 10 20 day hereafter the 1 2 mode oscillations are found to be the two intraseasonal modes that control the behavior of the scssm activities for most of the years both the 3 6 and 1 2 modes are distinct but may not always exist simultaneously in a particular year and their contributions to the overall variations differ among different years thus the interannual variability in the intraseasonal oscillation activity of the scs sm may be categorized as follows the 3 6 category in which the 3 6 mode is more significant in terms of the percentage of variance explained than the 1 2 mode the 1 2 category in which the 1 2 mode is dominant and the dual category in which both the 3 6 and 1 2 modes are pronounced composite analyses of the 3 6 category cases indicate that the 30 60 day oscillation of the scs sm exhibits a trough ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the scs with anomalous cyclones anticyclones along with enhanced suppressed convection migrating northward from the equator to the midlatitudes the northward migrating 3 6 mode monsoon trough ridge in the lower troposphere is coupled with the eastward propagating 3 6 mode divergence convergence in the upper troposphere it is also found that for the years in the dual category the scs sm activities are basically controlled by the 3 6 mode but modified by the 1 2 mode composite results of the 1 2 mode category cases show that the 10 20 day oscillation is manifest as an anticyclone cyclone system over the western tropical pacific propagating northwestward into the scs a close coupling also exists between the upper level convergence divergence and the low level anticyclone cyclone it is found that the 1 2 mode of the scs sm mainly originates from the equatorial central pacific although a disturbance from the northeast of the scs also contributes to this mode the flow patterns from an inactive to an active period resemble those associated with a mixed rossby gravity wave observed in previous studies

structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation abstract the spatial and temporal structures of the northward propagating boreal summer intraseasonal oscillation bsiso are revealed based on the analysis of both the echam4 model simulation and the ncep ncar reanalysis the bsiso structure and evolution characteristics simulated by the model bear many similarities to those derived from the ncep ncar reanalysis the most notable features are the remarkable meridional asymmetries relative to the bsiso convection in the vorticity and specific humidity fields a positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center the maximum specific humidity also shows a clear northward shift in the lower troposphere two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the bsiso the first is the vertical shear mechanism the key process associated with this mechanism is the generation of barotropic vorticity due to the couplin

influences of typhoon chanchu on the 2006 south china sea summer monsoon onset 1 this study reveals some unique features associated with the impact of typhoon chanchu on the south china sea scs summer monsoon scssm onset in 2006 with typhoon chanchu entering the scs in mid may southwesterlies were induced over the scs and its upstream region due to a thermally forced rossby wave response leading to a reversal of low level winds and a positive meridional temperature gradient mtg in the upper troposphere over the scs typhoon chanchu thus acted as an immediate trigger for the scssm onset and the onset date was suggested to occur in the third pentad of may after the typhoon landed in the coast of southern china it still contributed to the establishment of the scssm due to warming the air column over the northern scs resulting in a persistent positive mtg with the ridge surface tilting northward

strong modulations on the bay of bengal monsoon onset vortex by the first northward propagating intra seasonal oscillation monsoon onset vortex ov in the form of tropical cyclone is often observed in the pre monsoon period and contributes to the subsequent abrupt establishment of summer monsoon over the bay of bengal bob it is identified here that all historical ovs occurred during the convection enhanced phase of the first northward propagating intra seasonal oscillation fniso the individual contributions from the four large scale environmental fields associated with the intra seasonal variations to the cyclone genesis are diagnosed with the aid of the genesis potential index the significant moistening of mid level atmosphere which is embedded in the fniso convection enhanced phase is shown to be the primary factor leading to the cyclone genesis the water vapor budget analysis is further done to understand the governing process for the mid level humidity increase it is clearly seen that the vertical advection process dominated by the anomalous vertical advection of the mean vertical water vapor gradient plays the critical role hence the ovs are shown to be strongly modulated by fnisos both of which are important elements of the complex story of the bob monsoon onset

structures and mechanisms of the first branch northward propagating intraseasonal oscillation over the tropical indian ocean the first branch northward propagating intra seasonal oscillation fniso over the tropical indian ocean io often triggers the onset of the asian summer monsoon in this study we investigate the structures and mechanisms associated with fniso through the diagnosis of era interim reanalysis data for the period of 1990 2009 a composite analysis is conducted to reveal the structure and evolution characteristics of the fniso and associated background circulation changes it is found that the fniso convection originates from the southwest ern io and propagates eastward after reaching the eastern io the major convective branch moves northward toward the northern bay of bengal bob two possible mecha nisms may contribute to the northward propagation of the fniso one is the meridional asymmetry of the back ground convective instability a greater background con vective instability over the northern bob may destabilize rossby waves and cause convection to shift northward the other is the meridional phase leading of perturbation humidity in the planetary boundary layer pbl maximum pbl moisture appears to the north of the convection center which promotes a convectively unstable stratification ahead of the convection and leads to the northward prop agation of the fniso a pbl moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry

anticorrelated intensity change of the quasi biweekly and 30 50 day oscillations over the south china sea 1 over the south china sea the year to year variations in the intensity of the quasi biweekly qbw and 30 50 day oscillations during june july are anti correlated an explanation is offered for the out of phase relationship we found during strong qbw years the june july mean convection and easterly vertical shear are enhanced over the equatorial western central pacific with corresponding low level cyclonic meridional shear to the northwest these conditions concurring with the equatorial eastern pacific warming are favorable for emanation of moist rossby waves from the equatorial western pacific on the other hand the 30 50 day becomes active when the mean convection is enhanced over the eastern indian ocean and maritime continent where this mode stems from and develops which often corresponds to cold western io one of these two large scale settings often occurs in the absence of the other so that when one mode is strong during june july the other mode tends to be weak

the role of mjo and mid latitude fronts in the south china sea summer monsoon onset previous studies have suggested that the south china sea scs summer monsoon onset is concurrent with the arrival of a 30 60 day northward propagating trough on the other hand from a synoptic viewpoint some studies pointed out that the arrival of a mid latitude front may be the triggering mechanism of the scssm onset this study attempts to link these two viewpoints and to investigate their relative role in inducing the scssm onset composites of low level zonal winds geopotential heights and temperatures during the 1991 1999 scssm onsets based on the european centre for medium range weather forecast era 40 data indicate that both the madden and julian oscillation mjo kelvin waves and mid latitude trough are apparently involved in the onset the mjo kelvin waves play a major role in inducing the large scale easterly westerly shift over the central scs while the effect of the acceleration of westerlies ahead of the mid latitude trough is limited to the northern scs only numerical experiments using a regional climate model further demonstrate that the mjo kelvin waves control the timing of the onset by changing the background meridional geopotential height gradient over the scs when the mjo is at its peak phase over the maritime continent it imposes a positive meridional geopotential height gradient over the scs such that easterly winds are induced which significantly reduces the strength of a mid latitude trough after the equatorial convection has dissipated a rossby wave response is induced leading to the formation of a northward moving trough when this trough moves northward the meridional geopotential height gradient is reversed and westerly winds are induced at the same time if a mid latitude trough arrives in south china the westerlies associated with the mid latitude trough will strengthen because of the background meridional geopotential height gradient which gives the impression that both the northward moving trough and mid latitude trough are in phase and work together to induce the onset

northward propagation mechanisms of the boreal summer intraseasonal oscillation in the era interim and sp ccsm abstractmechanisms for the northward propagation np of the boreal summer intraseasonal oscillation bsiso and associated asian summer monsoon asm are investigated using data from the interim ecmwf re analysis era interim herein called erai and the superparameterized community climate system model sp ccsm analyzed mechanisms are 1 destabilization of the lower troposphere by sea surface temperature anomalies 2 boundary layer moisture advection and boundary layer convergence associated with 3 sst gradients and 4 barotropic vorticity anomalies mechanism indices are regressed onto filtered olr anomaly time series to study their relationships to the intraseasonal oscillation iso and to equatorial rossby er waves northward propagation in erai and sp ccsm is promoted by several mechanisms but is dominated by boundary layer moisture advection and the barotropic vorticity effect sst linked mechanisms are of secondary importance but are nonnegligible the magnitudes of np mechanisms vary from

extremely early summer monsoon onset in the south china sea in 2019 following an el nino event abstractthe el nino southern oscillation enso is regarded as one of the most important factors for onset of the south china sea summer monsoon scssm previous studies generally indicated that a

mechanisms for the advanced asian summer monsoon onset since the mid to late 1990s abstractunderstanding the variability and change of monsoon onset is of utmost importance for agriculture planning and water management in the last three decades the asian summer monsoon onset asmo has remarkably advanced but the physical mechanisms underlying the change remain elusive since the overall asmo occurs in may this paper focuses on the change of mean fields in may and considers enhanced mean precipitation and monsoon westerly winds as signs of advanced asmo the results reveal that the advanced asmo mainly represents a robust decadal shift in the mid to late 1990s which is attributed to the mean state change in the pacific basin characterized by a grand la nina like pattern the la nina like mean state change controls the asmo through the westward propagation of rossby waves and its interaction with the asymmetric background mean states in the indian ocean and western pacific which intensifies the northern hemispheric perturbations and westerly winds intriguingly the abrupt decadal

left hand rule for synoptic eddy feedback on low frequency flow 1 in this study scale interaction between synoptic eddies and low frequency flow is investigated the synoptic eddy feedback is a key process in sustaining the low frequency flow we show clear evidence using ncep reanalysis data that there is a general underlying rule the left hand rule governing the synoptic eddy feedback onto low frequency flow this rule states that eddy vorticity fluxes are directed preferentially about 90 degrees toward their left hand side so that they converge into cyclonic flow and diverge from anticyclonic flow therefore the eddy vorticity flux plays a positive role in reinforcing low frequency flow citation kug j s and f f jin 2009 left hand rule for synoptic eddy feedback on low frequency flow geophys res lett 36 l05709 doi 10 1029 2008gl036435

decadal change in intraseasonal variability over the south china sea 1 evidence is presented to reveal a decadal change around mid 1990s in the behavior of intraseasonal variability isv over the south china sea scs during 1979 1993 the isv has a spectral peak around 64 days which is longer than in the recent epoch of 1994 2007 around 42 days the isv event in 1979 1993 involves a merging process of the northward and westward propagating convection anomalies over the western north pacific the isv in 1994 2007 has no such a merging process but exhibits a tilted band structure extending from the northern indian ocean to the scs which is strongly connected to the equatorial eastward propagating madden julian oscillation the merging process during 1979 1993 modified the isv over the scs resulting in the prolonged period enhanced convective activity and a weakened relationship with the eastward propagating mjo the possible cause of this change is discussed

northern hemisphere summer monsoon singularities and climatological intraseasonal oscillation abstract using climatological pentad mean outgoing longwave radiation olr and european centre for medium range weather forecasts analysis winds the authors show that the northern hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations cisos the extreme phases of ciso characterize monsoon singularities monsoon events that occur on a fixed pentad with usual regularity whereas the transitional phases of ciso represent the largest year to year monsoon variations the ciso results from a phase locking of transient intraseasonal oscillation to annual cycle it exhibits a dynamically coherent structure between enhanced convection and low level convergent upper level divergent cyclonic anticyclonic circulation its phase propagates primarily northward from the equator to the northern philippines during early summer may july and westward along 15 n from 170 e to the bay of bengal during august and september the propagation of ciso links monsoon singular

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

paleoenvironmental history of the north atlantic region from mineralogical and geochemical data abstract principles and applications of inorganic mineralogy and geochemistry in reconstructing past environments in marine basins are summarized by means of two examples from north atlantic deep sea drilling sites the first approach is chronological and permits by comparing the hatteras and cape verde basins since the oxfordian to identify the effects of continental and marine influences land climate and morphological evolution alternation of tectonically active and stable periods ocean deepening and widening stages and latitudinal migration of lithospheric plates the second approach more recently developed concerns the geographic distribution of paleoenvironments during albian time particularly the role of the bermuda area as a submarine barrier separating the western and eastern detrital input and the contrast between the stable american margin that favored pedologic processes on land and the unstable african margin characterized by strong erosion resedimentation and volcanic processes