double and single itczs with and without clouds abstracta major bias in tropical precipitation over the pacific in climate simulations stems from the models tendency to produce two strong distinct intertropical convergence zones itczs too often several mechanisms have been proposed that may contribute to the emergence of two itczs but current theories cannot fully explain the bias this problem is tackled by investigating how the interaction between atmospheric cloud radiative effects acres and the large scale circulation influences the itcz position in an atmospheric general circulation model simulations are performed in an idealized aqua planet setup and the longwave and the shortwave acres are turned off individually or jointly the low level moist static energy mse is shown to be a good predictor of the itcz position therefore a mechanism is proposed that explains the changes in mse and thus itcz position due to acres consistently across simulations the mechanism implies that the itcz moves equatorward if the hadley circulation strengt

responses of tropical deep convection to the qbo cloud resolving simulations abstractobservational studies suggest that the stratospheric quasi biennial oscillation qbo can modulate tropical deep convection the authors use a cloud resolving model with a limited domain representing a convective column in the tropics to study the mechanisms of this modulation the large scale circulation is parameterized using the weak temperature gradient wtg approximation under which the parameterized large scale vertical motion acts to relax the horizontal mean temperature toward a specified reference profile temperature variations typically seen in easterly and westerly phases are imposed in the upper troposphere and lower stratosphere of this reference profile the responses of convection are studied over different sea surface temperatures holding the reference temperature profile fixed this can be thought of as studying the response of convection to the qbo over different relative ssts and also corresponds to different equilibrium precipitation rates in the control simulation the

geographic variability in the export of moist static energy and vertical motion profiles in the tropical pacific 1 column integrated moist static energy mse budgets were calculated using daily data from three reanalyses 1998 2001 and isccp radiative cooling the mse export by the circulation was separated into vertical and horizontal mse advection components in order to examine how observationally derived vertical motion profiles affect gross moist stability calculations in a 2000 km wide region in the central eastern pacific itcz vertical motions import mse implying a negative gross moist stability horizontal mse advection is negative and of similar magnitude to vertical mse advection in rainy regions geographic differences in export of mse by the vertical circulation are primarily due to differences in the shape of the vertical motion profile in the west pacific warm pool rainy regions mean horizontal convergence extends up to 300 mb while in parts of the pacific itcz where meridional sst gradients are strong the vertical motion profile is bottom heavy with convergence below 800 mb and divergence above on daily timescales the mode of vertical motion variability has little dependence on precipitation rate and is consistent with observed regional differences in the vertical structure of trmm pr reflectivities

idealized simulations of the intertropical convergence zone and its multilevel flows abstract a mesoscale numerical model with an idealized tropical channel environment is used to study the dynamics of intertropical convergence zones itczs and the recently identified shallow return flow srf and midlevel inflow mli four idealized sea surface temperature sst distributions are used a meridionally symmetric sst profile with a sharply peaked sst maximum at the equator a similar profile with the maximum sst shifted off the equator a cosine shaped sst profile with zero gradient at the equator and an idealized sst profile modeled after the observed sst of the eastern pacific the simulations show that both the srf and the mli are robust features of the itcz the prior result that the srf is a sea breeze like response to surface temperature gradients is further supported whereas the mli is caused by the upper level maxima in diabatic heating and vertical motion simulations with the sst maximum at the equator generate long lasting convectively coupled kelvin waves when the sst maxi

seasonal and interannual variations of the energy flux equator and itcz part ii zonally varying shifts of the itcz the itcz lies at the ascending branch of the tropical meridional overturning circulation where near surface meridional mass fluxes vanish near the itcz column integrated energy fluxes vanish forming an atmospheric energy flux equator efe this paper extends existing approximations relating the itcz position and efe to the atmospheric energy budget by allowing for zonal variations the resulting relations are tested using reanalysis data for 1979 2014 the zonally varying efe is found as the latitude where the meridional component of the divergent atmospheric energy transport aet vanishes a taylor expansion of the aet around the equator relates the itcz position to derivatives of the aet to a first order the itcz position is proportional to the divergent aet across the equator it is inversely proportional to the local atmospheric net energy input nei that consists of the net energy fluxes at the surface at the top of the atmosphere and zonally across longitudes the first order approximation captures the seasonal migrations of the itcz in the african asian and atlantic sectors in the eastern pacific a third order approximation captures the bifurcation from single to double itcz states that occurs during boreal spring in contrast to linear efe theory during boreal winter in the eastern pacific northward cross equatorial aet goes along with an itcz north of the equator efe and itcz variations driven by enso are characterized by an equatorward poleward shift in the pacific during el nino la nina episodes which are associated with variations in equatorial ocean energy uptake

impacts of vertical structure of large scale vertical motion in tropical climate moist static energy framework abstractinteractions between cumulus convection and its large scale environment have been recognized as crucial to the understanding of tropical climate and its variability in this study the moist static energy mse budget is employed to investigate the potential impact of the vertical structure of large scale vertical motion in tropical climate based on results from both reanalysis data and model simulation two domains are selected over the western and eastern pacific with vertical motion profiles that are dominated by top heavy and bottom heavy structures respectively the bottom heavy structure is climatologically associated with more shallow convection while the top heavy structure is related to more deep convection the column integrated vertical mse advection of top heavy vertical motion is positive while that of bottom heavy vertical motion tends to be negative controlling factors responsible for the above vertical mse advection contrast are discussed based on a simple decomposition of the

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

structure of the annual mean equatorial planetary waves in the era 40 reanalyses abstract the three dimensional structure of the annual mean equatorial planetary waves in the 40 yr ecmwf re analysis era 40 is documented the features in the free atmosphere are predominantly equatorially symmetric driven by east west heating gradients the geopotential height and wind perturbations are strongest at or just below the 150 hpa level below the level of maximum amplitude the circulations in the waves are thermally direct with latent heat release in deep convective clouds and radiative cooling in the intervening cloud free regions within the overlying capping layer the wave related circulations are thermally indirect with rising of the coldest air and sinking of air that is less cold at the cold point just above the 100 hpa 17 km level the era 40 annual mean vertical velocity in the equatorial belt ranges up to 3 mm s 1 over the equatorial western pacific during the boreal winter implying diabatic heating rates of up to 3 c day 1 an order of magnitude larger than typical clear

energetic constraints on the intertropical convergence zone position in the observed seasonal cycle from modern era retrospective analysis for research and applications version 2 merra 2 idealized simulations show that the approximate colocation between the itcz and the energy flux equator efe which holds on the annual and zonal average breaks down on subseasonal timescales as the hadley cell develops a shallow return ow and negative gross moist stability gms here we explore if similar mechanisms are seen in re analysis data in the zonal mean a temporal offset exists between the itcz and the efe as the itcz is retreating from the northern to the southern hemisphere and the hadley cell transports energy northward across the equator despite a northward shifted itcz at these times the southern cell has a bottom heavy structure with a distorted cell boundary and northward energy transport in the eastern pacific while bottom heavy structures exist throughout the year the bottom heaviness is stronger at times with negative gms and weak ssts and large negative sst laplacian below the itcz

response of convection to relative sea surface temperature cloud resolving simulations in two and three dimensions 1 the properties of equilibrated tropical convection are studied using a cloud resolving model with large scale dynamics parameterized by the weak temperature gradient wtg approximation model integrations are performed in both 2 d and 3 d geometries the target profile toward which horizontal mean free tropospheric temperature is relaxed is held fixed while sea surface temperature sst is varied consistent with previous studies large scale ascent and precipitation increase under wtg as the sst is increased but more rapidly in two dimensions than in three dimensions this is related to greater extremes of near surface buoyancy in two dimensions as well as a lower gross moist stability and perhaps also to weaker entrainment in both two and three dimensions the vertical profiles of large scale vertical velocity are top heavy and remarkably self similar in shape as sst is increased when all integrations are analyzed together precipitation increases with column integrated relative humidity once the latter reaches a threshold as in observations and other models however within each integration the two quantities are correlated negatively albeit over a very narrow range

weaker links between zonal convective clustering and itcz width in climate models than in observations strong links are seen in observations between convective clustering and several properties of the intertropical convergence zone itcz these links suggest that biases in how climate models simulate the itcz may be related to model biases in convective clustering or that there may be biases in how models represent the relationship between clustering and the itcz we investigate these issues by analyzing convective clustering and the link between clustering and itcz properties in 18 climate models we find that the links between variability in convective clustering and variability of itcz properties are generally weaker and less robust in models than in observations by contrast model biases in the climatological convective clustering explain a substantial fraction of the climatological double itcz bias though they do not explain biases in the climatological itcz width

a new mechanism of the slow eastward propagation of unstable disturbances with convection in the tropics implications for the mjo abstractto understand the slow eastward propagation of the madden julian oscillation mjo the authors propose a new mechanism of slow eastward propagation of an unstable disturbance slow eastward propagating disturbance sed in the tropics assuming a simplified instability source two conditions are necessary over the convective region associated with the sed for this mechanism i moderately negative gross moist stability gms and ii strong stretching in the lower layer the instability due to i needs to be moderate for causing a propagating convective disturbance under the structure of twin cyclonic vortices to the west and convection to the east stretching of planetary vorticity associated with the convection produces cyclonic vorticity which results in eastward propagation of the vortices equatorial plane dynamics explain the positional relation between the twin vortices and convection coherently strong equatorial low level westerly winds accompanying the vortices induce convergence

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

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

kinematic analyses of the hsuehshan range taiwan a large scale pop up structure the hsuehshan range exposed in the northern and central taiwan slate belt is a fault bounded structural high cored by biotite grade slates and metasandstones syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern hsuehshan range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland near the eastern boundary of the hsuehshan range however pressure shadows record noncoaxial strain histories consistent with a top to the east sense of shear along a steep nw dipping shear zone this noncoaxiality is attributed to se directed backthrusting on the lishan fault which separates the higher grade eo oligocene rocks of the hsuehshan range from the lower grade miocene rocks of the backbone range because it is bounded to the east by the se vergent lishan fault and to the west by a series of nw vergent thrusts e g the chuchih fault the hsuehshan range is envisaged as a pop up structure strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces e g the lishan fault the hsuehshan range tapers in width to the south thus the pop up may be buried or die out to the south where the collision is younger

on large scale circulations in convecting atmospheres the dominant thinking about the interaction between large scale atmospheric circulations and moist convection holds that convection acts as a heat source for the large scale circulations while the latter supply water vapour to the convection we show that this idea has led to fundamental misconceptions about this interaction and offer an alternative paradigm based on the idea that convection is nearly in statistical equilibrium with its environment according to the alternative paradigm the vertical temperature profile itself rather than the heating is controlled by the convection which ties the temperature directly to the subcloud layer entropy the understanding of large scale circulations in convecting atmospheres can therefore be regarded as a problem of understanding the distribution in space and time of the subcloud layer entropy we show that the subcloud layer entropy is controlled by the sea surface temperature the surface wind speed and the large scale vertical velocity in the convecting layer and demonstrate how the recognition of this control leads to a simple physically consistent view of large scale flows ranging from the hadley and walker circulations to the 30 50 day oscillation in particular we argue that the direct effect of convection on large scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations and to damp all of them

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

atmospheric dynamic and thermodynamic processes driving the western north pacific anomalous anticyclone during el nino part ii formation processes abstractin part i the authors showed that northerly anomalies associated with the rossby wave response to el nino heating anomalies in the equatorial central pacific lead to the southward advection of low moist enthalpy air forming the western north pacific anomalous anticyclone wnpac why does such a remote forcing not cause the formation of the anomalous anticyclone in el nino developing summer the physical mechanism responsible for the timing of the wnpac formation is investigated in part ii through both an observational analysis and idealized numerical model experiments the authors find that the onset timing of the wnpac relies on the following three factors the first is a sign change from positive to negative of the meridional gradient of background low level specific humidity over the key tropical western north pacific wnp region in november the second is a sign change from positive to negative of the meridional gradient of background relative vorticity which efficiently reduces the w

comparison of atmospheric hydrology over convective continental regions using water vapor isotope measurements from space 1 global measurements of the 500 825 hpa layer mean hdo h2o ratio from the tropospheric emission spectrometer tes are used to expose differences in the dominant hydrologic processes in the amazon north australian and asian monsoon regions the data show high regional isotopic variability and numerous values unexpected from classical rayleigh theory correlation analysis shows that mixing with boundary layer air enhanced isotopic fractionation during precipitation and subsiding air parcels contribute to intraseasonal isotopic variability these local controls explain only 8 30 of total regional variance which suggests that the isotopes are primarily indicators of moist processes that occur upstream seasonal trajectory analysis demonstrates that rayleigh distillation in a lagrangrian framework underestimates the observed isotopic depletion during the monsoons and suggests substantial recycling of water within or below clouds the trajectory results for the dry seasons reveal that subsiding air parcels periodically introduce isotopically depleted air into the north australia and asian monsoon regions whereas vigorous low level convection over the amazon basin acts to quickly enrich and moisten dry subsiding air the analysis indicates variations in the strength of convective detrainment into the lower to middle troposphere over all regions which during the dry seasons of the north australian and asian monsoon regions correlate with increases in relative humidity this study shows that isotopic measurements provide unique diagnostics of mechanisms that control the seasonal sources of water and that these provide a refined understanding of the differences in the characteristics of hydrologic budgets in these monsoonal regions