dynamical mechanisms for african monsoon changes during the mid holocene 1 during the mid holocene about 6 ka bp the african monsoon was stronger and extended farther north than in present day climate associated with differences in orbital parameters affecting insolation earlier research suggests that land surface vegetation interaction and atmosphere ocean coupling may be important to the northward extension of the holocene monsoon rainfall but different climate models produce different results in terms of rainfall distribution this study examines the dynamical mechanisms that modulate the orbital forcing effect on the african monsoon convection a series of simulations with 6 ka bp orbital parameters modified land surface albedo that mimics grassland extending over north africa and alterations of the model dynamics are used it is found that the ventilation mechanism inflow of low moist static energy air from oceanic or poleward regions is a key process in setting the northern rainfall boundary for the african monsoon in particular advection and diffusion of less moist air oppose the increase of moisture that would be required to meet the increased convective instability criteria set by the warmer tropospheric air above the continent while changes in heat fluxes into the atmospheric column favor convection in principle the ventilation mechanism limits the poleward extent of the region that can actually convect

mechanisms limiting the northward extent of the northern summer monsoons over north america asia and africa abstract mechanisms determining the poleward extent of summer monsoon convergence zones for north america asia and africa are examined in an intermediate atmospheric model coupled with a simple land model and a mixed layer ocean observations show that thermodynamical factors associated with the net heat flux into the atmospheric column provide favorable conditions for the monsoon convergence zone to extend farther poleward than actually occurs to understand the discrepancy a series of experiments are designed to test the importance of mechanisms previously examined in the south american case by the authors namely soil moisture ventilation and the interactive rodwell hoskins mechanism the latter refers to the interaction between baroclinic rossby wave dynamics and convective heating in north america experiments suggest that ventilation by both temperature and moisture advection is a leading effect the interactive rodwell hoskins mechanism tends to favor east coast rainfall and west coast dryne

orographic effects of the tibetan plateau on the east asian summer monsoon an energetic perspective this paper investigates the dynamical processes through which the tibetan plateau tp influences the east asian summer monsoon easm within the framework of the moist static energy mse budget using both observations and atmospheric general circulation model agcm simulations the focus is on the most prominent feature of the easm the so called meiyu baiu mb which is characterized by a well defined southwest northeast elongated quasi stationary rainfall band spanning from eastern china to japan and into the northwestern pacific ocean between mid june and mid july observational analyses of the mse budget of the mb front indicate that horizontal advection of moist enthalpy and primarily of dry enthalpy sustains the front in a region of otherwise negative net energy input into the atmospheric column a decomposition of the horizontal dry enthalpy advection into mean transient and stationary eddy fluxes identifies the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity as the most influential factors determining the pattern of horizontal moist enthalpy advection numerical simulations in which the tp is either retained or removed show that the tp influences the stationary enthalpy flux and hence the mb front primarily by changing the meridional stationary eddy velocity with reinforced southerly wind over the mb region and northerly wind to its north changes in the longitudinal thermal gradient are mainly confined to the near downstream of the tp with the resulting changes in zonal warm air advection having a lesser impact on the rainfall in the extended mb region

subtropical anticyclones and summer monsoons the summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans in winter the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography here the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations by prescribing in the model the heatings associated with several of the world s monsoons it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the kelvin wave response to the monsoon heating over the continent to the west a poleward flowing low level jet into a monsoon such as the great plains jet is required for sverdrup vorticity balance this jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region the low level jet into north america induced by its monsoon heating is augmented by a remote response to the asian monsoon heating the rossby wave response to the west of subtropical monsoon heating interacting with the midlatitude westerlies produces a region of adiabatic descent it is demonstrated here that a local diabatic enhancement can lead to a strengthening of the descent longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region below the descent sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through ekman transport feedbacks involving for example sea surface temperatures may further enhance the descent in these regions the conclusion is that the mediterranean type climates of regions such as california and chile may be induced remotely by the monsoon to the east hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems each involving monsoon rains a low level poleward jet a subtropical anticyclone to the east and descent and equatorward flow to the west in winter it is demonstrated how the nonlinear interaction between the strong zonal mean circulation associated with the winter hadley cell and the mountains can define many of the large scale features of the subtropical circulation the blocking effect of the longitudinal mountain chains is shown to be very important subsequent diabatic effects such as a local diabatic enhancement would appear to be essential for producing the observed amplitude of these features

hadley circulation response to orbital precession part ii subtropical continent the responseof the monsoonaland annual meanhadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with a simplified representation of land surface processes in subtropical latitudes when perihelion occurs in the summer of a hemisphere with a subtropical continent changesin the top of atmosphereenergy balance togetherwith a polewardshift ofthe monsoonalcirculation boundary lead to a strengthening of the monsoonal circulation spatial variations in surface heat capacity determine whether radiative perturbations are balanced by energy storage or by atmospheric energy fluxes althoughorbitalprecessiondoesnotaffectannual meaninsolation theannual meanhadleycirculationdoes respond to orbital precession because its sensitivity to radiative changes varies over the course of the year the monsoonal circulation in summer is near the angular momentum conserving limit and responds directly to radiative changes whereas in winter the circulation is affected by the momentum fluxes of extratropical eddies and is less sensitive to radiative changes

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

interaction of the westerlies with the tibetan plateau in determining the mei yu termination abstractthis study explores how the termination of the mei yu is dynamically linked to the westerlies impinging on the tibetan plateau it is found that the mei yu stage terminates when the maximum

variety and spatial heterogeneity of tectonic tremor worldwide 1 deep tectonic tremor occurs at various sites worldwide and the source characteristics are heterogeneous even at small scales in this study tremor sources were determined using data sets of seismic waveforms from various locations some of which are not well recognized as being a site of tremor activity the regions of interest are subduction zones at nankai and kyushu japan cascadia in western north america mexico southern chile and new zealand tremor locations are consistent with the geometry of subducting plates and tremor depth tends to be shallower where younger plate is subducting tremor duration shows a negative correlation with tidal sensitivity as reported previously in western shikoku in the nankai subduction zone the duration is also related to the width of the tremor zone in the dip direction of the subducting plate linear structures resembling striations are observed in hypocenter distributions in western shikoku and in the probability of short term migration direction these spatial heterogeneities in tremor activity may be controlled by the heterogeneous frictional properties of the downgoing slab due to long term plate subduction thereby providing information on the maturity of the plate interface

the mechanical impact of the tibetan plateau on the seasonal evolution of the south asian monsoon the impact of the tibetan plateau on the south asian monsoon is examined using a hierarchy of atmospheric general circulation models during the premonsoon season and monsoon onset april june when westerly winds over the southern tibetan plateau are still strong the tibetan plateau triggers early monsoon rainfalldownstream particularlyoverthebay ofbengalandsouth china the downstreammoist convection is accompanied by strong monsoonal low level winds in experiments where the tibetan plateau is removed monsoon onset occurs about a month later but the monsoon circulation becomes progressively stronger and reaches comparable strength during the mature phase during the mature and decaying phase of monsoon july september when westerly winds over the southern tibetan plateau almost disappear monsoon circulation strength is not much affected by the presence of the tibetan plateau a dry dynamical core with east west oriented narrow mountains in the subtropics consistently simulates downstream convergence with background zonal westerlies over the mountain in a moist atmosphere the mechanically driven downstream convergence is expected to be associated with significant moisture convergence the authors speculate that the mechanically driven downstream convergence in the presence of the

variations of indian and african monsoons induced by insolation changes at 6 and 9 5 kyr bp this study investigates the role of insolation in controlling the indian and african monsoon evolutions during the holocene using coupled ocean atmosphere simulations of 0 6 9 5 kyr bp climates for which only the variations of earth s orbital configuration are considered the two monsoon systems are enhanced at 6 and 9 5 kyr bp compared to 0 kyr bp as a result of the intensified seasonal cycle of insolation in the northern hemisphere the analysis of daily climatologies indicates that even though the length of the celestial summer season is shorter at 9 5 kyr bp the rainy season is longer than at present emphasis is put on the impact of the precession on the seasonality which partly explains why the relative amplification of the indian and african monsoon varies between 9 5 and 6 kyr bp moreover the changes in snow cover over the tibetan plateau play a critical role in reinforcing the 9 5 kyr bp monsoon in india during spring the results suggest that the teleconnection between convection over india and subsidence over the mediterranean regions through the rodwell and hoskins mechanism has an impact on the development of the african monsoon at 9 5 kyr bp

time dependent modeling of slow slip events and associated seismicity and tremor at the hikurangi subduction zone new zealand we present a time dependent slip model of 12 slow slip events sses occurring in the hikurangi margin of new zealand during 2010 and 2011 this model is obtained by inverting daily gps solutions from geonet s continuous gps network on the north island and northern south island we compare the properties of these sses to observations in japan cascadia and mexico and find that hikurangi sses have comparatively large amounts of slip up to 27 cm high slip rates up to 1 4 cm d and a large range of depths 10 40 km durations 7 270 days and sizes mw 5 9 6 9 we further investigate the relationship between the cape turnagain sse and an associated seismic swarm and find that observations are consistent with stress triggering outside the slowly slipping region however other explanations cannot be ruled out we also compare slip during the long term manawatu sse with the tremor epicenters found by ide 2012 and note that tremor locations are offset in the downdip direction relative to the slipping region similar to observations in the bungo channel of japan and guerrero mexico

upscale effects of deep convection during the north american monsoon abstractthe ability of deep monsoon convection to influence the larger scale circulation over north america is investigated for a 6 day long case study during the 2006 north american monsoon results from rossby wave ray tracing and numerical simulations using the advanced research weather research and forecasting model indicate that north american monsoon convection provides a source region for stationary rossby waves two wave trains are seen in the numerical model simulations with behaviors that agree well with expectations from theory and ray tracing the shorter and faster moving wave train moves eastward from the source region in mexico and reaches the western atlantic within 4 days the longer and slower moving wave train travels northeastward and reaches the coastal new england region within 6 days an upstream tail of anticyclonic vorticity extends westward from the source region into the central pacific ocean the monsoon convection appears to help cut off the low level anticyclonic flow by deve

the effects of orography on midlatitude northern hemisphere dry climates abstract the role of mountains in maintaining extensive midlatitude arid regions in the northern hemisphere was investigated using simulations from the gfdl global climate model with and without orography in the integration with mountains dry climates were simulated over central asia and the interior of north america in good agreement with the observed climate in contrast moist climates were simulated in the same regions in the integration without mountains during all season but summer large amplitude stationary waves occur in response to the tibetan plateau and rocky mountains the midlatitude dry regions are located upstream of the troughs of these waves where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited in summer this mechanism contributes to the dryness of interior north america as a stationary wave trough remains east of the rockies but is not effective in eurasia due to seasonal changes in the atmospheric circulation the dryness

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

characterizing the north american monsoon in the community atmosphere model sensitivity to resolution and topography abstractthis work examines the effect of horizontal resolution and topography on the north american monsoon nam in experiments with an atmospheric general circulation model observations are used

non volcanic tremor associated with the march 2010 gisborne slow slip event at the hikurangi subduction margin new zealand 1 non volcanic tremor accompanies slow slip events at most circum pacific subduction zones previously the hikurangi subduction margin new zealand and the boso peninsula of japan have been noted as exceptions where slow slip which occurs at shallow depth compared to other subduction zones does not generate seismic tremor the lack of tremor observations raises questions whether tremor and slow slip are manifestations of the same process and whether different temperature and pressure conditions influence the physical mechanism of tremor production analysis of seismic data during the march 2010 gisborne slow slip event new zealand reveals that this slow slip event at the northern hikurangi margin is accompanied by tremor with distinctive characteristics compared to local microearthquakes both the gisborne area of new zealand and the boso peninsula of japan are underlain by thick accumulations of sedimentary rocks that likely inhibit the observation of low amplitude tremor due to high attenuation discovery of tremor in new zealand suggests that tremor may always accompany slow slip and that its apparent absence at the boso peninsula the hikurangi margin prior to this study and any future regions discovered to experience slow slip without concurrent tremor may be the effect of highly attenuating sediments

dynamics of wet and dry years in west africa abstract this paper proposes a theoretical framework for describing interannual climatic variability over west africa the dynamical theory of zonally symmetrical thermally direct circulations suggests that a meridional monsoon circulation must develop over any tropical region off the equator when the absolute vorticity near the tropopause reaches a threshold value of zero however for a moist atmosphere that satisfies a quasi equilibrium balance between moist convection and the radiative forcing the absolute vorticity at upper tropospheric levels is a function of both latitude and the meridional distribution of boundary layer entropy hence the onset of a monsoon circulation depends in a nonlinear fashion on these two factors the theory predicts that a flat distribution of entropy does not drive any circulation and that a relatively large gradient of entropy should drive a strong monsoon circulation the location of the region of west africa relatively close to the equator dictates that the dynam

mechanism of heating and the boundary layer over the tibetan plateau abstract the structure of the boundary layer and the mechanism of heating over the tibetan plateau are examined using the data obtained from the first garp global atmospheric research program global experiment and the chinese qinghai xizang tibet plateau meteorological experiment from may to august 1979 the meteorological elements near the plateau surface exhibit pronounced diurnal variations there is a large ground air temperature difference during the daytime generating a thin layer of superadiabatic lapse rates near the surface the horizontal wind speed attains the minimum in the morning and the maximum in the evening a deep and well mixed layer of potential temperature is observed over the western and central plateau in the evening 1200 utc however moisture is not well mixed vertically and water vapor mixing ratio is larger in the morning 0000 utc than in the evening the mixed layer becomes shallower from the western to the central plateau and it disappears over the eastern plateau the

a prototype for convective margin shifts 1 regional precipitation anomalies under el nino or global warming scenarios manifest considerable spatial variability especially in the vicinity of tropical deep convective zones using a simplified set of equations we develop an analytic prototype of convective margins for tropical land regions subject to inflow from adjacent ocean regions the approach yields an expression for the location of the convective margin from which it is straightforward to deduce how climate perturbations impact the margin we compare the results of the prototype to full model simulations of the equatorial south american precipitation response to el nino forcing and find general agreement analysis of the observations supports the convective inflow margin prototype s applicability to appropriately simple basic state configurations

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

modeling tropical precipitation in a single column abstract a modified formulation of the traditional single column model for representing a limited area near the equator is proposed this formulation can also be considered a two column model in the limit as the area represented by one of the columns becomes very large compared to the other only a single column is explicitly modeled but its free tropospheric temperature rather than its mean vertical velocity is prescribed this allows the precipitation and vertical velocity to be true prognostic variables as in prior analytical theories of tropical precipitation two models developed by other authors are modified according to the proposed formulation the first is the intermediate atmospheric model of j d neelin and n zeng but with the horizontal connections between columns broken rendering it a set of disconnected column models the second is the column model of n o renno k a emanuel and p h stone in the first model the set of disconnected column models is run with a fixed temperature

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

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

the principal mode of interannual variability of the north american monsoon system nonseasonal behavior of the north american monsoon system nams during the summer months june september is investigated based on monthly mean satellite data and the national centers for environmental prediction national center for atmospheric research reanalyses for the period 1979 97 the leading principal component of the monthly mean precipitation over the domain 58 358n 808 1258w is used as a reference time series the associated variations in fields other than the rainfall are identified based on linear regression analysis the reference time series is highly correlated with summer rainfall averaged over this broadly defined nams domain and is therefore referred to as the spatially integrated precipitation index ipi it is also correlated with hurricane activity over the tropical and subtropical northeast pacific in its positive polarity the ipi is characterized by an intensification and northward expansion of the itcz toward the mexican coast with enhanced rainfall throughout mexico but particularly in the south and in a relative sense in the semiarid northwest it has a well defined sea level pressure and surface wind signature with negative sea level pressure anomalies to the northwest of the region of enhanced rainfall and westerly wind anomalies converging into the region from the west along 108 208n these features extend upward to the 500 hpa level and are overlaid by weak anomalies of opposing sign in the upper troposphere there are indications of an associated deep barotropic planetarywave signature over the united states but the geopotential anomalies are only weakly correlated with the ipi the ipi is only weakly correlated with summer rainfall over the southwestern united states with enso and with sst anomalies within the nams domain

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

regional energy budget control of the intertropical convergence zone and application to mid holocene rainfall shifts in the latitude of the intertropical convergence zone a region of intense tropical rainfall have often been explained through changes in the atmospheric energy budget specifically through theories that tie rainfall to meridional energy fluxes these quantitative theories can explain shifts in the zonal mean but often have limited relevance for regional climate shifts such as a period of enhanced precipitation over saharan africa during the mid holocene here we present a theory for regional tropical rainfall shifts that utilizes both zonal and meridional energy fluxes we first identify a qualitative link between zonal and meridional energy fluxes and rainfall variations associated with the seasonal cycle and the el nino southern oscillation we then develop a quantitative theory based on these fluxes that relates atmospheric energy transport to tropical rainfall when applied to the orbital configuration of the mid holocene our theory predicts continental rainfall shifts over africa and southeast asia that are consistent with complex model simulations however the predicted rainfall over the sahara is not sufficient to sustain vegetation at a level seen in the palaeo record which instead requires an additional large energy source such as that due to reductions in saharan surface albedo we thus conclude that additional feedbacks such as those involving changes in vegetation or soil type are required to explain changes in rainfall over africa during the mid holocene shifts in the latitude of the tropical rainfall band are constrained by meridional energy fluxes calculations show that combining zonal and meridional energy fluxes can explain past regional rainfall variations like the african humid period

mechanisms for spatially inhomogeneous changes in east asian summer monsoon precipitation during the mid holocene abstractthe east asian summer monsoon easm intensified during the early to mid holocene relative to the present primarily due to orbital forcing however on the regional scale changes in the mo

multistage graphite precipitation through protracted fluid flow in sheared metagranitoid digana sri lanka evidence from stable isotopes abstract the digana metagranitoid situated within the digana shear zone a critical junction between the highland and wanni complexes of sri lanka hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain here we discuss their occurrence association and stable isotopes the carbon isotopes of graphite reported in this study from the sri lankan high grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other precambrian terrains the stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range 1 6 to 8 suggesting precipitation by reduction from co2 rich fluids intracrystalline microscale traverses made on graphite crystals indicate the 13c composition of the rims becoming progressively lighter in 13c in metagranitoid hosted samples with the variation reaching up to 1 5 consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth and shows the preservation of core at higher temperatures a highly enriched carbon isotopic composition 0 6 with a rimward enrichment of 13c observed in migmatites contrary to the values in metagranitoid indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution reprecipitation of graphite the oxygen isotopic composition of a quartz calcite vein show values in the range 18 5 to 20 typical for recycled crustal source the tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits prolonged fluid flow in this region is envisaged in this study which is related to the multistage deformation of the crustal scale shear zone

dynamic effect of last glacial maximum ice sheet topography on the east asian summer monsoon the effect of ice sheet topography on the east asian summer monsoon easm during the last glacial maximum is studied using ccsm3 in a hierarchy of model configurations it is found that receding ice sheets result in a weakened easm with the reduced ice sheet thickness playing a major role the lower ice sheet topography weakens the easm through shifting the position of the midlatitude jet and through altering northern hemisphere stationary waves in the jet shifting mechanism the lowering of ice sheets shifts the westerly jet northward and decreases the westerly jet over the subtropics in summer which reduces the advection of dry enthalpy and in turn precipitation over the easm region in the stationary wave mechanism the lowering of ice sheets induces an anomalous stationary wave train along the westerly waveguide that propagates into the easm region generating an equivalent barotropic low response this leads to reduced lower tropospheric southerlies which in turn reduces the dry enthalpy advection into east asia and hence the easm precipitation less

graphite mineralization in paleoproterozoic khondalites of the north china craton a carbon isotope study abstract graphite mineralization in precambrian supracrustal rocks provide important information on the source characteristics of carbon and has been the focus of studies tracking primitive life in the early earth as well as a geochemical tracer in fluid rock interaction processes here we present results from a high precision carbon stable isotope study of the different associations of graphites from a graphite deposit in the paleoproterozoic khondalite belt of the north china craton our results show that disseminated graphite occurring as stratabound deposits along the compositional planes of the khondalites possess the lowest 13c values of ca 25 5 graphite in the massive ores also show identical composition with 13c values in the narrow range of ca 25 3 to 25 7 coarse graphite flakes in quartz pods and veins show slightly heavier values with 13c in the range of 19 1 to 20 9 such flakes with 13c composition of ca 20 6 were also deposited in some domains of the host khondalites the graphite in melt pockets within felsic leucosomes show the heaviest 13c values in the range of 15 8 to 16 8 micro sampling perpendicular to the c axis of the graphite crystals indicates a remarkable homogeneity of carbon isotopic composition within the individual graphite types the highly negative 13c values of the dominant graphite type in this deposit as well as similar values reported for those from other deposits in the region compare well with the isotopic signature of biogenic carbon occurring in other precambrian terranes and suggest that microbial life was flourishing in the paleoproterozoic oceans that closed during the final assembly of the north china craton the 13c enriched graphite in veins and melt pockets might have been deposited from mixed fluids derived through degassing of organic material during high grade metamorphism and externally derived co2 rich fluids

the equatorial energy balance itcz position and double itcz bifurcations the intertropical convergence zone itcz migrates north south on seasonal and longer time scales previous studies have shown that the zonal mean itcz displacement off the equator is negatively correlated with the energy flux across the equator when the itcz lies in the northern hemisphere energy flows southward across the equator and vice versa the hemisphere that exports energy across the equator is the hemisphere with more net energy input and it is usually the warmer hemisphere but states with a double itcz straddling the equator also occur for example seasonally over the eastern pacific and frequently in climate models here it is shown how the itcz position is connected to the energy balance near the equator in a broad range of circumstances including states with single and double itczs taylor expansion of the variation of the meridional energy flux around the equator leads to the conclusion that for large positive net energy input into the equatorial atmosphere the itcz position depends linearly on the cross equatorial energy flux for small positive equatorial net energy input the dependence of the itcz position on the cross equatorial energy flux weakens to the third root when the equatorial net energy input or its curvature become negative a bifurcation to double itcz states occurs simulations with an idealized aquaplanet general circulation model gcm confirm the quantitative adequacy of these relations the results provide a framework for assessing and understanding causes of common climate model biases and for interpreting tropical precipitation changes such as those evident in records of climates of the past

establishment of the low level wind anomalies over the western north pacific during enso development abstract the anomalous low level anticyclone over the western north pacific presents a link between el nino and the western north pacific east asian climate during a la nina event however the low level wind anomalies over the western north pacific reverse sign the low level wind anomalies move eastward from south asia and are established over the western north pacific in the fall of the el nino la nina developing year the anomalous low level anticyclone associated with el nino is a response to suppressed convection this suppressed convection is mainly induced by a cooling tendency associated with the vertical average of the anomalous horizontal advection of moist static energy that is defined as the mechanism of the horizontal advection of moist static energy the el nino related sea surface temperature sst anomalies over the eastern pacific in particular create negative meridional gradients of temperature and moisture over the western north pacific when the winter monsoon starts to dominate

maintenance of the african easterly jet the maintenance of the african easterly jet aej has been examined using a zonally symmetric general circulation model with simple parametrizations it is shown that the aej is maintained in association with two diabatically forced meridional circulations one associated with surface fluxes and dry convection in the saharan heat low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this the heat low heating which reaches the height of the aej around 700 mb is particularly important in maintaining the aej and its associated meridional gradients in potential vorticity it is concluded that the mean observed aej results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it

mechanisms limiting the southward extent of the south american summer monsoon an intermediate atmospheric model coupled with a simple land surface model and a mixed layer ocean model is used to examine effects that determine the southward extension of summer precipitation over south america the extent of the continental convergence zone is mainly determined by two mechanisms which we term ventilation and the interactive rodwell hoskins mechanism ventilation refers to the import into south america of low moist static energy air from the cooler ocean primarily the pacific in the interactive rodwell hoskins mechanism rossby wave induced subsidence to the west of the diabatic heating interacts with the convection zone because of the shape of south america the interactive rodwell hoskins mechanism is of comparable importance to ventilation soil moisture feedback also helps limit poleward movement of the continental convergence zone but its effect is relatively weak compared to the above two effects the characteristic northwest southeast tilt of the continental convergence zone appears to be due to a combination of ventilation and the interactive rodwell hoskins mechanism

an idealized two dimensional framework to study the west african monsoon part i validation and key controlling factors abstract an idealized vertical meridional zonally symmetric model is developed in order to recover a july typical monsoon regime over west africa in response to surface conditions the model includes a parameterization to account for heat and momentum fluxes associated with eddies the sensitivity of the simulated west african monsoon equilibrium regime to some major processes is explored it allows confirmation of the important role played by the sun s latitudinal position the aerosols the albedo and the sst s magnitude in the gulf of guinea and in the mediterranean sea the important role of aerosols in warming the saharan lower layers and their effect on the whole monsoon is underlined model results also stress the importance of the mediterranean sea which is needed to obtain the extreme dryness of the sahara the use of this idealized model is finally discussed for studying the scale interactions and coupling involved in the west african monsoon as explored in a companion paper

on the role of planetary scale waves in the abrupt seasonal transition of the northern hemisphere general circulation the role of planetary scale waves in the abrupt seasonal transition of the northern hemisphere nh general circulation is studied in reanalysis data the winter to summer transition involves the growth of planetary scale wave latent heat and momentum transports in the region of monsoons and anticyclones that dominate over the zonal mean transport beginning in midspring the wave dominated regime coincides with an abrupt northward expansion of the cross equatorial circulation and reversal of the trade winds in the upper troposphere the transition coincides with the growth of cross equatorial planetary scale wave momentum transport and a poleward shift of subplanetary scale wave transport and jet stream the dynamics of the seasonal transition are captured by idealized aquaplanet model simulations with a prescribed subtropical planetary scale wave sea surface temperature sst perturbation the sst perturbation generates subtropical planetary scale wave streamfunction variance and transport in the lower and uppertroposphereconsistentwithquasigeostrophictheory beyondathresholdsst atransitionofthezonalmean circulation occurs which coincides with a localized reversal of absolute vorticity in the nh tropical upper troposphere the transition is abrupt in the lower troposphere because of the quadratic dependence of the wave transport on the sst perturbation and involves seasonal time scale feedbacks between the wave and zonal meanflow in theupper troposphere including cross equatorial wave propagation thezonal mean vertical and meridional flows associated with the circulation response are in balance with the planetary scale wave momentumand latentheat meridionalflux divergences the resultshighlightthe leading order roleofmonsoon anticyclone transport in the seasonal transition including its impact on the meridional extent of the hadley and ferrel cells they can also be used to explain why the transition is less abrupt in the southern hemisphere

interhemispheric teleconnections from tropical heat sources in intermediate and simple models themechanismsthatcontroltheinterhemisphericteleconnectionsfromtropicalheatsourcesareinvestigated using an intermediate complexity model a quasi equilibrium tropical circulation model qtcm and a simple linear two level model with dry dynamics illustrating the interhemispheric teleconnection process with an atlantic warm pool principal case the heat source directly excites a baroclinic response that spreads across the equator then threeprocesses involvingbaroclinic barotropicinteractions shearadvection surfacedrag and vertical advection forcea cross equatorial barotropicrossbywaveresponse ananalysis ofthese processes in qtcm simulations indicates that 1 shear advection has a pattern that roughly coincides with the baroclinic signal in the tropics and subtropics 2 surface drag has large amplitude and spatial extent and can be very effective in forcing barotropic motions around the globe and 3 vertical advection has a significantcontribution locally and remotely where large vertical motions and vertical shear occur the simple model is modified to perform experiments in which each of these three mechanisms may be included or omitted by adding surface drag and vertical advection and comparing each to shear advection the effects of the three mechanisms on the generationandpropagationofthebarotropicrossbywavesareshowntobequalitativelysimilartotheresultsin qtcm it is also found that the moist processes included in the qtcm can feed back on the teleconnection process and alter the teleconnection pattern by enlarging the prescribed tropical heating in both intensity and geographical extent and by inducing remote precipitation anomalies by interaction with the basic state

glacial reduction of the north american monsoon via surface cooling and atmospheric ventilation nsf ags 1204011 ocn 1304910 oce 1651034 david and lucile packard foundation fellowship in science and engineering

observational evaluation of a convective quasi equilibrium view of monsoons abstract idealized dynamical theories that employ a convective quasi equilibrium qe treatment for the diabatic effects of moist convection have been used to explain the location intensity and intraseasonal evolution of monsoons this paper examines whether observations of the earth s regional monsoons are consistent with the assumption of qe it is shown here that in local summer climatologies based on reanalysis data maxima of free tropospheric temperature are indeed nearly collocated with maxima of subcloud equivalent potential temperature eb in all monsoon regions except the north and south american monsoons free tropospheric temperatures over north africa also exhibit a strong remote influence from the south asian monsoon consistent with idealized dynamical theories peak precipitation falls slightly equatorward of the maxima in eb and free tropospheric temperature in regions where qe seems to hold vertical structures of temperature and wind reveal two types of monsoon circulations one

primary igneous graphite in ultramafic xenoliths ii isotopic composition of the carbonaceous phases present in xenoliths and host lava at tissemt eggere alegerian sahara

seasonal variation abrupt transition and intraseasonal variability associated with the asian summer monsoon in the gla gcm abstract in this paper a description of the global and regional features of the asian summer monsoon in the goddard laboratory atmospheres gla general circulation model gcm is presented based on a 10 yr 1979 1988 integration of the model under the atmospheric model intercomparison project it is found that the earliest signal of onset of the asian monsoon is represented by the emergence of convection over the indo china land bridge in early may followed by the sudden jump of the equatorial itcz intertropical convergence zone to 10 n over the south china sea and indian ocean in mid may this sudden jump is linked to an abrupt northward shift of the ascending branch of the local hadley circulation possibly stemming from symmetric instability of the basic flow in may in agreement with observations the model shows a pronounced intraseasonal oscillation along the equator and enhanced synoptic scale activities identified as easterly waves in the tropical and subtropical western pacific these int

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

deep tremor in new zealand triggered by the 2010 mw8 8 chile earthquake 1 deep non volcanic tremor nvt is usually associated with episodic slow slip events new zealand is one notable exception where numerous slow slip events have been identified yet nvt has remained undetected here we present the first known case of triggered nvt at new zealand s hikurangi subduction margin following the mw8 8 chilean earthquake of february 27 2010 we identify coherent high frequency tremor signals that are in phase with and modulated by the passing rayleigh waves this is consistent with the surface wave triggering potential for strike normal incidence on a low angle thrust fault after constraining the tremor depth on the plate interface we locate the tremor source within 20 km of the source area of episodic slow slip the tremor location is also near the edge of a region with high seismic attenuation that marks the boundary between dehydrated subducted slab and inferred hydrated underplated sediment we speculate that reduced interface friction and high fluid pressures provided by fluid rich underplated sediment facilitates the tremor generation

eddy mediated regime transitions in the seasonal cycle of a hadley circulation and implications for monsoon dynamics in a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions the hadley cells undergo transitions between two regimes distinguishable according to whether large scale eddy momentum fluxes strongly or weakly influence the strength of a cell the center of the summer and equinox hadley cell lies in a latitude zone of upper level westerlies and significant eddy momentum flux divergence the influence of eddy momentum fluxes on the strength of the cell is strong the center of the cross equatorial winter hadley cell lies in a latitude zone of upper level easterlies and is shielded from the energy containing midlatitude eddies the influence of eddy momentum fluxes on the strength of the cell is weak mediated by feedbacks between eddy fluxes mean zonal winds at upper levels and the mean meridional circulation the dominant balance in the zonal momentum equation at the center of a hadley cell shifts at the transitions between the regimes from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell at the transitions a feedback involving changes in the strength of the lower level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross equatorial winter cell the transitions resemble the onset and end of monsoons for example in the shift in the dominant zonal momentum balance rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels rapid changes in strength of the upward mass flux and changes in direction and strength of the zonal wind at upper and lower levels in the monsoonal regime the maximum upward mass flux occurs in an off equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear frictional to geostrophic similar dynamic mechanisms as at the regime transitions in the simulation mechanisms that can act irrespective of land sea contrasts and other inhomogeneities in lower boundary conditions may be implicated in large scale monsoon dynamics in earth s atmosphere

space time correlation of slip and tremor during the 2009 cascadia slow slip event 1 episodic tremor and slip ets involving transient deformations accompanied by emergent low frequency tremor occurs in subduction zones around the world ets events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes despite the clear association of tremor and slip the physical relationship between them is unresolved tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault or stress increases ahead of the propagating slow slip front previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models to address this we invert gps data from the august 2009 ets event in central cascadia for the space time evolution of fault slip rate we find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip rate this supports the first hypothesis that tremor asperities are loaded directly by slow slip rather than by stress increases ahead of the slip front and provides new insights into the mechanics of ets

west african monsoon dynamics and eastern equatorial atlantic and pacific sst anomalies 1970 88 abstract the laboratoire de meteorologie dynamique atmospheric gcm is used to investigate relationships between west african monsoon dynamics and sst anomalies in the eastern equatorial atlantic and pacific for the period 1970 88 positive sst anomalies in the eastern equatorial pacific mainly associated with a larger east west divergent circulation over the tropical atlantic are found to coincide with negative rainfall anomalies over west africa this is the case for the composite enso warm episodes of 1972 1976 1982 and 1983 by contrast positive sst anomalies in the eastern equatorial atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the sahel and positive rainfall anomalies in the guinean region this was the case in 1987 the enso warm event during this year had apparently no significant impact on west african monsoon dynamics a zonal atmospheric coupling associated with differences of sst anomalies between the eastern

ocean atmosphere land feedbacks in an idealized monsoon summary an intermediate complexity atmospheric model coupled with a simple land surface model and a mixedlayer ocean model is used to investigate the processes involved in an idealized monsoon occumng on a single rectangular continent idealized divergences of ocean heat transports are specified as an annual average q flux in this simple coupled configuration the mechanisms that affect land ocean contrast and in turn the seasonal movement of the continental convergence zones are examined these include soil moisture feedbacks cooling of tropical oceans by ocean transport ventilation defined as the import into continental regions of low moist static energy air from ocean regions where heat storage opposes summer warming and the interactive rodwellhoskins mechanism in which rossby wave induced subsidence to the west of monsoon heating interacts with the convection zone the fixed ocean transports have a substantial impact on the continental convection if q flux is set to zero subtropical subsidence and ventilation tend to substantially limit the poleward movement of summer monsoon rainfall when land hydrology feedbacks are active the drying of subtropical continents disfavours continental convection even in the tropics when ocean transports are included tropical oceans are slightly disfavoured as regions for producing convection which by contrast favours continental convection the monsoon circulation then produces moisture transport from the ocean regions that allows substantial progression of convection into the subtropics over the eastern portion of the continent the western portion of the continent tends to have a dry region of characteristic shape this east west asymmetry is partly due to the interactive rodwell hoskins mechanism the ventilation is of at least equal importance in producing east west asymmetry and is the single most important process in limiting the poleward extent of the continental convection zone

generation of the african easterly jet and its role in determining west african precipitation abstract an examination of analyses and model simulations is used to show that the african easterly jet forms over west africa in summer as a result of strong meridional soil moisture gradients in a series of gcm experiments the imposition of realistic surface wetness contrasts between the sahara and equatorial africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere the associated easterly shear in the atmosphere is strong enough to establish easterly flow the african easterly jet above the monsoon westerlies at the surface positive temperature gradients associated with the summertime distributions of solar radiation ssts or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients a thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet while moisture converges throughout the lower troposphere over east africa moisture divergenc

land sea heating contrast in an idealized asian summer monsoon mechanisms determining the tropospheric temperature gradient that is related to the intensity of the asian summer monsoon are examined in an intermediate atmospheric model coupled with a mixed layer ocean and a simple land surface model with an idealized afro eurasian continent and no physical topography these include processes involving in the influence of the eurasian continent thermal effects of the tibetan plateau and effects of sea surface temperature the mechanical effect on the large scale flow induced by the plateau is not included in this study the idealized land sea geometry without topography induces a positive meridional tropospheric temperature gradient thus a weak asian summer monsoon circulation higher prescribed heating and weaker surface albedo over eurasia and the tibetan plateau which mimic effects of different land surface processes and the thermal effect of the uplift of the tibetan plateau strengthens the meridional temperature gradient and so as cold tropical sst anomalies the strengthened meridional temperature gradient enhances the asian summer monsoon circulation and favors the strong convection the corresponding monsoon rainbelt extends northward and northeastward and creates variations of the monsoon rainfall anomalies in different subregions the surface albedo over the tibetan plateau has a relatively weak inverse relation with the intensity of the asian summer monsoon the longitudinal gradient of enso like sst anomalies induces a more complicated pattern of the tropospheric temperature anomalies first the positive negative longitudinal gradient induced by the el nino la nina like sst anomalies weakens strengthens the walker circulation and the circulation between south asia and northern africa and therefore the intensity of the asian summer monsoon while the corresponding monsoon rainbelt extends northward southward the el nino la nina like sst anomalies also induces colder warmer tropospheric temperature over eurasia and warmer colder tropospheric temperature over the indian ocean the associated negative positive meridional gradient of the tropospheric temperature anomalies is consistent with the existence of the weak strong asian summer monsoon

slow slip evolves into megathrust earthquakes in 2d numerical simulations 1 slow slip events sse in many subduction zones incrementally stress the adjacent locked megathrust suggesting that they could potentially either trigger or evolve into damaging earthquakes we explore this with 2d quasi dynamic simulations with rate state friction dilatancy and coupled 1d pore fluid and heat transport steady state weakening friction allows transient slip to nucleate but is inhibited by dilatant strengthening and destabilized by thermal pressurization sse spontaneously nucleate in low effective stress velocity weakening lesvw regions if the dimension of the lesvw is relatively small the sse are trapped at its updip end imparting a strong stress concentration in the locked zone after several centuries sse penetrate into the region of higher effective stress where thermal pressurization eventually leads to dynamic rupture for larger lesvw regions sse tend to increase in length with time ultimately higher slip speeds enhance thermal weakening leading to dynamic instability within the sse zone in both cases the onset of the ultimate sse is essentially indistinguishable from preceding events