temporal spatial distribution of thin moist layers in the midtroposphere over the tropical eastern pacific abstract the temporal spatial distribution of thin moist layers in the midtroposphere over the tropical eastern pacific is studied by data analyses of radiosonde soundings and downscaling numerical experiments with a regional model radiosonde soundings at san cristobal galapagos show frequent existence of thin moist layers between 2 and 10 km in altitude with a local minimum at 7 8 km the downscaling experiments with global objective analyses are completed for 2005 06 september and december of 1999 2004 and march of 2000 04 the vertical distribution of thin moist layers has three local maxima at 5 10 and 16 km where bimodality of the frequency distribution of water vapor is evident between 4 and 7 km an annual variation is dominant in the occurrence ratio of thin moist layers which tend to appear in nonconvective regions in boreal winter the layers appear to the north of the intertropical convergence zone itcz whereas in boreal summer the layers appear in the equator side of the itcz i

the annual cycle in equatorial convection and sea surface temperature abstract the coupled atmosphere ocean system in the equatorial eastern pacific and atlantic exhibits a distinct annual cycle that is reflected in contrasting conditions at the times of the two equinoxes the contrasts are so strong that they dominate the annual march of zonally averaged outgoing longwave radiation for the equatorial belt the march equinox corresponds to the warm season when the equatorial cold tongues in the eastern pacific and atlantic are absent with the onset of summer monsoon convection over colombia central america and west africa in may june northward surface winds strengthen over the eastern pacific and atlantic the equatorial cold tongues reappear and the marine convection shifts from the equatorial belt to the intertropical convergence zones itczs along 8 n as the northern summer program the itczs remain strong and shift northward to new 10 n while sea surface temperature sst continues to drop over the cold tongues and the southern tropics perhaps in response to the

the seasonal cycle in the lower troposphere over west africa from sounding observations in situ sounding observations from selected sites in west and central africa were used to document the seasonal cycle in the lower troposphere associated with the annual migration of precipitation the main purpose of this exercise is to provide a description of the seasonal cycle free of the errors and biases of numerical models which can be substantial in this region many features in the circulation typically associated with the west african monsoon wam exist through almost the entire seasonal cycle in the region the low level easterly jet known as the african easterly jet during the monsoon season remains prominent and closely related to the rain band in other phases of the seasonal cycle even when the rain band is located near the equator the surface southerly monsoon onshore flow penetrates through the rain band in the entire season cycle the depth of the southerly surface monsoon flow undergoes a seasonal variation and is greatest during the peak season of the monsoon a unique feature is a low level 700 hpa northerly flow penetrating through the rain band in various phases of the seasonal cycle risking possible biases of land bound sounding data these observations are synthesized into a schematic describing the salient features in the low level circulation associated with the rain band in different phases of the seasonal cycle this synthesized circulation pattern in west africa is in contrast to the traditional conceptual circulation pattern associated with the marine itcz and other monsoon systems the results from this study call for more research attention to the low level meridional circulation for a full understanding of the dynamics of the wam this study also advocates the study of the wam to be conducted in a broad context of the seasonal cycle in the region and in comparison to the marine itcz copyright 2006 royal meteorological society

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

a further study of the tropical western hemisphere warm pool abstract variability of the tropical western hemisphere warm pool whwp of water warmer than 28 5 c which extends seasonally over parts of the eastern north pacific the gulf of mexico the caribbean and the western tropical north atlantic tna was previously studied by wang and enfield using the da silva data from 1945 93 using additional datasets of the ncep ncar reanalysis field and the ncep sst from 1950 99 and the levitus climatological subsurface temperature the present paper confirms and extends the previous study of wang and enfield the whwp alternates with northern south america as the seasonal heating source for the walker and hadley circulations in the western hemisphere during the boreal winter a strong hadley cell emanates northward from the amazon heat source with subsidence over the subtropical north atlantic north of 20 n sustaining a strong north atlantic anticyclone and associated northeast ne trade winds over its southern limb in the tna this circulation including the ne

convection in the east pacific intertropical convergence zone the eastern tropical pacific exhibits a strong cross equatorial sea surface temperature sst gradient which drives a southerly flow in the atmospheric boundary layer convergence in this flow is generally considered to drive deep convection in the east pacific intertropical convergence zone however results from cloud modeling and recent field programs provide an alternative thermodynamic mechanism for controlling this convection while shallow convection responds to boundary layer convergence deep convection appears to be controlled by a combination of convective inhibition surface moist entropy fluxes tropospheric relative humidity and moist convective instability these factors explain the sharp minimum in infrared brightness temperature near 8 n while boundary layer convergence occurs over a much broader range of latitudes

large scale atmospheric forcing by southeast pacific boundary layer clouds a regional model study abstract a regional model is used to study the radiative effect of boundary layer clouds over the southeast pacific on large scale atmosphere circulation during august october 1999 with the standard settings the model simulates reasonably well the large scale circulation over the eastern pacific precipitation in the intertropical convergence zone itcz north of the equator and marine boundary layer stratocumulus clouds to the south in a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern pacific artificially removed boundary layer clouds south of the equator almost disappear and precipitation in the itcz is reduced by 15 20 indicating that the stratocumulus clouds over the southeast pacific have both local and cross equatorial effects examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer in response to this cloud induced cooling an in situ anom

regional variation of morphology of organized convection in the tropics and subtropics 1 properties of organized convection with large horizontal area 1000 km2 and with different horizontal structures in the tropics and subtropics are investigated by using 14 years of tropical rainfall measuring mission observations first the convective features cfs are defined as contiguous areas of convective precipitation detected by the tropical rainfall measuring mission precipitation radar using the minor and major axes of fitted ellipses the morphology of the cfs are described as closer to a circular or a line shape regional variations and the properties of organized convection are examined with cfs with area 1000 km2 after categorizing them by their shapes organized convection tends to have larger extent and a higher fraction of near circular shapes over land than over ocean shallow organized convection with maximum radar echo top height below 4 5 km is found mainly over ocean and some coastal regions of all tropical oceans most shallow organized convection is found over the east pacific the fraction of line shaped organized convection is higher over the ocean than over land and is higher in the subtropics than in the tropics more convective lines are found in winter than in summer over oceans but more in summer over land organized convective lines are slightly less convectively intense indicated by lower 30 dbz echo top heights and warmer 37 ghz brightness temperatures than those with near circular shapes orientations of organized convective lines are often aligned with fronts dry lines warm ocean currents coastlines and mountain slopes over the subtropics organized convective lines are tilted more east west over land and more north south over oceans the largest and the most intense convective lines are found over central africa argentina and southeast u s over land and over several warm currents in subtropical oceans

boundary layer cloud and drizzle variability in the southeast pacific stratocumulus regime abstract the southeast pacific stratocumulus regime is an important component of the earth s climate system because of its substantial impact on albedo observational studies of this cloud regime have been limited but during the past 5 yr a series of cruises with research vessels equipped with in situ and remote sensing systems have provided unprecedented observations of boundary layer cloud and drizzle structures these cruises started with the east pacific investigation of climate epic 2001 field experiment followed by cruises in a similar area in 2003 and 2004 pan american climate studies pacs stratus cruises the sampling from these three cruises provides a sufficient dataset to study the variability occurring over this region this study compares observations from the 2004 cruise with those obtained during the previous two cruises observations from the ship provide information about boundary layer structure fractional cloudiness cloud depth and drizzle characteristics this study indicat

dynamics of the shallow meridional circulation around intertropical convergence zones the generally accepted view of the meridional circulation in the tropical east pacific is that of a single deep overturning cell driven by deep convective heating in the intertropical convergence zone itcz similar to the zonal mean hadley circulation however recent observations of the atmosphere from the tropical eastern pacific have called this view into question in several independent datasets significant meridional return flows out of the itcz region were observed not only at high altitudes but also at low altitudes just above the atmospheric boundary layer this paper presents a theory and idealized simulations to understand the causes and dynamics of this shallow meridional circulation smc fundamentally the smc can be seen as a large scale sea breeze circulation driven by sea surface temperature gradients when deep convection is absent in the itcz region a simple model of this circulation is presented using observed values the sea breeze model shows that the pressure gradient above the boundary can indeed reverse leading to the pressure force that drives the shallow return flow out of the itcz the weather research and forecast model wrf is used to simulate an idealized hadley circulation driven by moist convection in a tropical channel the smc is reproduced with reasonable similarity to the circulation observed in the east pacific the simulations confirm that the smc is driven by a reversal of the pressure gradient above the boundary layer and that the return flow is strongest when deep convection is absent in the itcz and weakest when deep convection is active the model also shows that moisture transport out of the itcz region is far greater in the low level shallow return flow than in the high altitude return flow associated with the deep overturning and that a budget for water transport in and out of the itcz region is grossly incomplete without it much of the moisture carried in the shallow return flow is recycled into the boundary layer but does not appear to contribute to enhanced cloudiness in the subtropical stratocumulus poleward of the itcz

detecting the itcz in instantaneous satellite data using spatiotemporal statistical modeling itcz climatology in the east pacific abstract a markov random field mrf statistical model is introduced developed and validated for detecting the east pacific intertropical convergence zone in instantaneous satellite data from may through october the mrf statistical model uses satellite data at a given location as well as information from its neighboring points in time and space to decide whether the given point is classified as itcz or non itcz two different labels of itcz occurrence are produced ir only labels result from running the model with 3 hourly infrared data available for a 30 yr period 1980 2009 all data labels result from running the model with additional satellite data visible and total precipitable water available from 1995 to 2008 ir only labels detect less area of itcz than all data labels especially where the itcz is shallower yet qualitatively the results for the two sets of labels are similar the seasonal distribution of the itcz through the summer half year is presented showing typical location and e

on the relationship between sst gradients boundary layer winds and convergence over the tropical oceans a linear mixed layer model that skillfully reproduces observed surface winds and convergence over the tropicaloceans is used to examine the relative influenceof boundarylayer and free tropospheric processes on the distribution of climatological surface winds and convergence the semiempirical model assumes a subcloud layer momentum force balance between pressure gradients coriolis acceleration linearized friction and downward momentum mixing and it utilizes boundary conditions from the 40 yr ecmwf re analysis era 40 observed pressure gradients are linearly decomposed into boundary layer defined as the region below 850 hpa and free tropospheric components and the surface winds and convergence associated with these components are computed results show that surface zonal winds are predominantly associated with a combination of free tropospheric pressure gradients and downward momentum mixing whereas the distribution of convergence is primarily due to boundary layer temperature gradients which are closely related to sst gradients the authors conclude that the climatological distribution of boundary layer convergence is primarily a function of the pattern of sst gradients and is better regarded as a cause rather than a consequence of deep convection

the vertical structure of the eastern pacific itczs and associated circulation using the trmm precipitation radar and in situ data the atmospheric circulation associated with the eastern pacific single and double itczs particularly its vertical structure is little known due to the sparce observations using precipitation profiles from the tropical rainfall measure mission trmm precipitation radar with approximations to the liquid water and energy budget equations we estimated vertical profiles of latent heating and vertical velocity in the far eastern pacific 95 w 85 w itczs in the 800 730 hpa layer we combined this with eastern pacific investigation of climate campaign epic2001 and other in situ data to produce a preliminary characterization of the meridional vertical circulation we found evidence of a double cell structure in boreal fall between the itcz and the equator with both shallow and upper level peaks in vertical velocity in spring the flow poleward of the two itczs has a single cell structure although around the equator it shows some hints of the double cells reanalysis and satellite based data are shown to be unreliable for describing the vertical structure of the circulation

the interaction of clouds and dry air in the eastern tropical pacific cloud radar observations of eastern pacific intertropical convergence zone cloud vertical structure are interpreted in light of soundings 100 km scale divergence profiles calculated from precipitation radar doppler velocities and surface rain gauge data from a ship at 10 n 95 w during the 2001 east pacific investigation of climate epic experiment the transition from convective to stratiform rain is clear in all four datasets indicating a coherence from local to 100 km scale a novel finding is dry air intrusions at altitudes of 6 8 km often undercutting upper level ice clouds two distinct dry air source regions are identified one is a relatively dry area overlying the cooler waters of the costa rica oceanic thermocline dome centered approximately 400 km east northeast of the ship site the other is the even drier nearequatorial subsidence zone south of 6 7 n the former source is somewhat peculiar to this specific ship location so that the ship sample is not entirely representative of the region the 20 25 september period is studied in detail as it depicts two influences of the dry air on cloud vertical structure one is the modulation of small scale surface based convection evident as a weakening and narrowing of cloud radar reflectivity features the other springs from intense sublimation cooling as differential advection brought snowing anvil clouds over the dry layers during one half day period of strong sublimation the cooling rate is inferred to be several tens of degrees per day over a 100 hpa layer based on a heat budget estimate at 100 km scale involving the horizontal wind divergence data this is consistent with fluxing ice water contents of 0 05 0 10 g m 3 derived from the cloud radar reflectivities the temperature profile shows the dynamically expected response to this cooling a positive negative positive temperature anomaly pattern centered on the sublimating layer a buoyancy sorting diagnostic model of convection indicates that these upper troposphere temperature anomalies can cause premature detrainment of updrafts into the lower part of the cloudy layer a feedback that may actively maintain these long lasting dense anvils middle troposphere southerly dry air inflow is also evident in large scale analysis given the proximity of the dry equatorial subsidence zone to the eastern tropical pacific the differential advection of dry and cloudy air the ensuing sublimation and its dynamical aftereffects may play a role in establishing the region s climate although the extent of their significance needs to be further established

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

energetics of east pacific easterly waves during intraseasonal events abstractthe background atmospheric state of the east pacific warm pool in which easterly waves develop varies dramatically on intraseasonal time scales east pacific intraseasonal variability is well known to modulate local convective and circulation patterns westerly easterly intraseasonal phases are associated with westerly easterly low level wind and positive negative convective anomalies this study investigates the perturbation available potential energy pape and perturbation kinetic energy pke budgets of easterly waves composited during westerly easterly and neutral intraseasonal phases respectively the mechanisms magnitudes locations and vertical structures of easterly waves are shown to strongly vary as a function of intraseasonal phase easterly waves draw energy from low level barotropic conversion regardless of phase although the location and magnitude of the conversion varies strongly during neutral and westerly intraseasonal phases the generation of pape associated with

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

diagnosing monthly mean boundary layer properties from reanalysis data using a bulk boundary layer model abstract the mixed layer approach to modeling the planetary boundary layer pbl is particularly well suited to inversion topped pbls such as the stratocumulus topped boundary layer found off the west coast of america in the subtropical pacific ocean at northern and southern latitudes however a strong temperature inversion near 850 hpa the trade wind inversion is not confined to the stratocumulus regimes but has been observed over most parts of the subtropical tropical pacific ocean in this paper the authors test the ability of a simple bulk boundary layer model bblm to diagnose entrainment velocity cumulus mass flux and surface latent heat flux from monthly mean reanalysis data the pbl depth is estimated from geoscience laser altimeter system data the model is based on the conservation equations for mass total water mixing ratio and moist static energy the bblm diagnoses entrainment velocities between 1 and 8 mm s 1 in the stratocumulus and trade wind regions with increasing rates towar

the 2007 niigata earthquake effect of arc magma and fluids to clarify the generation mechanism of large crustal earthquakes in the amur okhotsk plate boundary we determined high resolution 3d seismic velocity v p v s and poisson s ratio structures in niigata northeast japan using a large number of p and s wave arrival times from the aftershocks of the 2004 and 2007 niigata earthquakes as well as many intermediate depth earthquakes in this area our results show that the niigata mainshocks occurred around zones of low velocity and high poisson s ratio which suggest that fluids may have influenced the nucleation and rupture processes of the 2004 and 2007 niigata earthquakes a zone with pronounced low velocity and high poisson s ratio is revealed under the source areas of the 2004 and 2007 niigata earthquakes in the lower crust and uppermost mantle which may reflect the arc magma and fluids ascending from the upper mantle wedge these results indicate that the generation of the 2004 and 2007 niigata earthquakes was affected by the ascending magma and fluids released by the dehydration process of the subducting pacific slab c 2008 elsevier b v all rights reserved

trmm observed shallow versus deep convection in the eastern pacific related to large scale circulations in reanalysis datasets abstractover the eastern pacific recent studies have shown that a shallow large scale meridional circulation with its return flow just above the boundary layer coexists with a deep hadley circulation this study examines how the vertical structure of large scale circulations is related to satellite observed individual precipitation properties over the eastern pacific in boreal autumn three reanalysis datasets are used to describe differences in their behavior the results are compared among reanalyses and three distinctly different convection periods which are defined according to their radar echo depths shallow and deep circulations are shown to often coexist for each of the three periods resulting in the multicell circulation structure deep shallow circulations preferentially appear in the mostly deep shallow convection period of radar echo depths thus depth of convection basically corresponds to which circulation branch is dominant this anticipated relationship between the circulation stru

processes leading to double intertropical convergence zone bias in cesm1 cam5 abstractthe double intertropical convergence zone itcz bias in the eastern pacific in the community earth system model version 1 with community atmosphere model version 5 cesm1 cam5 is diagnosed in cam5 standalone the northern itcz is associated with inertial instability and the southern itcz is thermally forced after air sea coupling the processes on both hemispheres are switched because the spatial pattern of sea surface temperature sst is changed biases occur during boreal spring in both cam5 and the ocean model in cam5 alone weaker than observed equatorial easterly in the tropical eastern south pacific leads to weaker evaporation and an increase in local sst the shallow meridional circulation overly converges in the same region in the cam5 standalone simulation the planetary boundary layer and middle troposphere are too humid and the large scale subsidence is too weak at the middle levels these biases may result from excessive shallow convection behavior in cam5 the extra moisture wou

the global monsoon as seen through the divergent atmospheric circulation a comprehensive description is given of the global monsoon as seen through the large scale overturning in the atmosphere that changes with the seasons and it provides a basis for delimiting the monsoon regions of the world the analysis focuses on the mean annual cycle of the divergent winds and associated vertical motions as given by the monthly mean fields for 1979 93 reanalyses from the national centers for environmental prediction national center for atmospheric research ncep ncar and european centre for medium range weather forecasts ecmwf which are able to reproduce the dominant modes a complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year the first mode which depicts the global monsoon has a simple vertical structure with a maximum in vertical motion at about 400 mb divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitude above 70 mb and convergence in the lower troposphere with a maximum at 925 mb ecmwf or 850 mb ncep however this mode has a rich three dimensional spatial structure that evolves with the seasons it accounts for 60 of the annual cycle variance of the divergent mass circulation and dominates the hadley circulation as well as three overturning transverse cells these include the pacific walker circulation an americas atlantic walker circulation both of which comprise rising motion in the west and sinking in the east and a transverse cell over asia the middle east north africa and the indian ocean that has rising motion in the east and sinking toward the west these exist year round but migrate and evolve considerably with the seasons and have about a third to half of the mass flux of the peak hadley cell the annual cycle of the two hadley cells reveals peak strength in early february and early august in both reanalyses a second monsoon mode which accounts for 20 of the variance features relatively shallow but vigorous overturning with the maximum vertical velocities near 800 mb outflow from 750 to 350 mb and inflow peaking at 925 mb it is especially strong over africa where the shallow mostly meridional overturning migrates back and forth across the equator with the seasons it influences the middle east has a signature over australia and is also an important component of the overturning in the tropical eastern pacific and atlantic and thus of the convergence zones in these regions the relationship of the global monsoon to the regional monsoons is described over six zonal sectors africa australia asia north america south america and the pacific and atlantic oceans only the two ocean areas do not undergo a seasonal reversal required for monsoons although they have direct overturning cells and they nevertheless participate in the global monsoon through the changes in large scale overturning the regional meridional cross sections highlight the importance of the shallow overturning cell in lower troposphere monsoon activity the steadiness of the overturning circulation is determined by comparing the signal of the seasonal mean vertical motions at 500 mb with the standard deviation of the transient daily variations locations where this signal exceeds 60 of the daily noise correspond closely with the regional centers of the monsoon

spatial variability of atmospheric boundary layer structure over the eastern equatorial pacific abstract variations in the atmospheric boundary layer structure over the eastern equatorial pacific are analyzed using 916 soundings collected during the first global atmospheric research program global experiment unstable boundary layer structures are observed much more frequently in soundings north of the ocean front located near 2 5 n in the eastern equatorial pacific than in soundings south of the front an objective criterion is applied to identify the presence of the transition layer a weak stable layer near cloud base in the soundings the transition is observed in about 45 of the soundings in both the unstable and the inversion categories a comparison of soundings over the cold tongue with those over the itcz indicates that differences in static stability between these regions are limited to the layer from the surface to about 850 mb which is the mean height of the inversions capping the cloud layer over the cold tongue the cold tongue soundings on average are found to be drier from the sur

physical processes associated with the tropical atlantic sst meridional gradient abstract the major modes of seasonal sea surface temperature sst meridional gradient and their connection with some regional mean sst indices in the atlantic ocean are examined using reanalysis data the focus of the work is on the evolution of the dominant mode of the meridional sst gradient in boreal spring and the associated physical processes the spatial distribution of the dominant mode in boreal spring is a seesaw pattern reflecting the opposite variation of the meridional sst gradient between the subtropical and tropical north atlantic which resulted from a coherent warming or cooling with maxima along 10 15 n it is confirmed that this mode is dominated by the wind evaporation sst feedback the feedback persists a longer time in the western atlantic than in the eastern the contribution to the sst variation is mainly from latent heat flux the surface longwave and shortwave cloud radiative forcings are mainly determined by low cloud cover variations the authors also found that the thermodyn

oscillations of the intertropical convergence zone and the genesis of easterly waves part i diagnostics and theory we examine the mean and transient state of the intertropical convergence zone itcz by analyzing data and using simple theory we concentrate on the tropical eastern pacific ocean noting that there exists in this region a well developed mean itcz furthermore it is a region where there has been considerable discussion in the literature of whether easterly waves develop in situ or propagate westwards from the atlantic ocean the region is typical of tropical regions where there is a strong cross equatorial pressure gradient cepg mean convection well removed from the equator but located equatorward of the maximum sea surface temperature sst and minimum sea level pressure mslp further to the west near the dateline where the cepg is much smaller convection is weaker and collocated with sst and mslp extrema it is argued that in regions of significant cepg that the near equatorial tropical system is inertially unstable and that the rectification of the instability for a given cepg determines the location and intensity of the climatological itcz using simple theoretical arguments we develop an expression for the mean latitude of the itcz as a function of the cepg we note on a day by day basis that the itcz is highly transient state with variability occurring on 3 8 day time scales transients with amplitudes about half of the mean itcz propagate northwards from the near equatorial southern hemisphere as anomalous meridional oscillations eventually amplifying convection in the vicinity of the mean itcz it is argued that in these longitudes of strong cepg the mean itcz is continually inertially unstable with advections of anticylonic vorticity across the equator resulting in the creation of an oscillating divergence convergence doublet the low level convergence produces convection and the resultant vortex tube stretching generates cyclonic vorticity which counteracts the northward advection of anticylonic vorticity during a cycle the mid troposphere heating near 10on oscillates between 6 and 12 k day at the inertial frequency of the latitude of the mean convection as a result there exists an anomalous and shallower oscillating meridional circulation with a magnitude about 50 of the mean itcz associated with the stable state following the generation of anticylonic vorticity further it is argued that the instabilities of the itcz are directly associated with in situ development of easterly waves which occur with the inertial period of the latitude of the mean itcz the dynamical sequences and the genesis of easterly waves are absent in the regions further to the east where the cepg is much weaker or absent altogether in a companion study part ii numerical experiments are conducted to test the hypothesis raised in the present study

estimating vertical motion profile shape within tropical weather states over the oceans abstractthe vertical structure of tropical deep convection strongly influences interactions with larger scale circulations and climate this paper focuses on investigating this vertical structure and its relationship with mesoscale tropical weather states the authors test the hypothesis that latent heating plus turbulent flux convergence varies in space and time in association with weather state type the authors estimate mean state vertical motion profile shape and latent heating plus turbulent flux convergence for six weather states defined using cloud top pressure and optical depth properties from the international satellite cloud climatology project isccp dataset assuming two modes of vertical motion profile variability these modes are statistically extracted from reanalysis data using a principal component analysis using these modes and the relationship between vertical motion the dry static energy budget and mass continuity the authors estimate vertical motion profile shape in these estim

regional changes in the annual mean hadley circulation in recent decades previous studies suggest that the zonally averaged hadley circulation zahc has experienced a robust poleward expansion and its trend in intensity displays inconsistency among different data sets this study examines changes in regional hc intensity and poleward edge using six reanalyses outgoing longwave radiation and precipitation data sets hcs in six regions including africa afhc the indian ocean iohc the western pacific wphc the eastern pacific ephc south america sahc and the atlantic athc are investigated intensity trends in the northern hemisphere nh wphc and athc and the southern hemisphere sh ephc and athc are in agreement with each other in the six reanalyses furthermore regional hcs in these domains appear to be intensifying although not all of the reanalyses show statistically significant trends for the poleward edge its trend in the nh afhc iohc ephc sahc and athc is significantly larger than zero and the northern hc poleward edge exhibits uniform poleward migrations in these five regions in the sh only the trend in the sahc poleward edge is significantly different from zero furthermore the trend in the sh sahc poleward edge is significantly larger than those in the sh afhc iohc and athc the results indicate that the poleward migration of the southern zahc poleward edge during recent decades that has been identified by previous studies may be attributed mainly to the poleward migration of the southern sahc poleward edge further analyses suggest that changes in regional hc poleward edges could have a significant impact on regional precipitation anomalies

shallow meridional circulation in the tropical eastern pacific abstract based on four independent datasets this study presents observational evidence of a shallow meridional circulation cell in the eastern tropical pacific in this shallow meridional circulation cell the northerly cross equatorial return flow from the itcz into the southern hemisphere is found immediately above the atmospheric boundary layer in contrast to the classic concept of the hadley type deep meridional circulation whose northerly return flow resides in the upper troposphere the strength and depth of this shallow meridional circulation undergo a distinct annual cycle possible causes and climatic implications of this shallow meridional circulation are discussed

the vertical structure of tropical convection and its impact on the budgets of water vapor and ozone abstract convective clouds in the tropics that penetrate the boundary layer inversion preferentially detrain into a shallow outflow layer 2 5 km or a deep outflow layer 10 17 km the properties of these layers are diagnosed from a one dimensional model of the tropics constrained by observed mean temperature and water vapor profiles the mass flux divergence of the shallow cumuli 2 5 km is balanced by a mass flux convergence of evaporatively forced descent downdrafts while the mass flux divergence of deep cumulonimbus clouds 10 17 km is balanced by a mass flux convergence of clear sky radiative descent the pseudoadiabatic temperature stratification of the midtroposphere 5 10 km suppresses cloud outflow in this interval the detrainment profile in the deep outflow layer is shifted downward by about 1 5 km from the profile one would anticipate based on undilute pseudoadiabatic ascent of air from the boundary layer the main source of water vapor to most of the tropical troposphere is evaporativ

observational evidence of crystalline iron oxides on mars visible to near ir 0 4 1 0 mu m spectral reflectance observations of mars during the 1988 opposition were performed at mauna kea observatory using a circular variable filter spectrometer at a spectral resolution r lambda delta lambda approx 80 on august 13 and 14 1988 ut 41 regions 500 600 km in diameter were observed on mars the data have been reduced both to reflectance relative to solar analog mars 16 cyg b and to relative reflectance spot spot the spectra show the strong near uv reflectance dropoff characteristic of mars as well as absorptions at 0 62 0 72 mu m and 0 81 0 94 mu m both seen here clearly for the first time these absorption features are interpreted as fe sup 3 electronic transition bands that indicate the presence of crystalline ferric oxide or hydroxide minerals on the martian surface comparison of these data with laboratory spectra obtained by other workers supports the conclusion that a single iron oxide phase most likely hematite could account for all of the observed spectral behavior of the martian surface soils and airborne dust in the 0 4 1 0 mu m region this possibility must be reconciled with data from other possible spectral analogs and other wavelength regions as well as geochemical and mineral stability considerations to arrive at amore more complete understanding of the role of ferric minerals in martian surface mineralogy and weathering less

the itcz in the central and eastern pacific on synoptic time scales the itcz in the central and eastern pacific on synoptic time scales is highly dynamic the active season extends roughly from may through october during the active season the itcz continuously breaks down and re forms and produces a series of tropical disturbances the life span of the itcz varies from several days to 3 weeks sixty five cases of itcz breakdown have been visually identified over five active seasons 1999 2003 in three independent datasets itcz breakdown can be triggered by two mechanisms 1 interaction with westward propagating disturbances wpds and 2 the vortex rollup vr mechanism results show that the frequency of occurrence of itcz breakdown from these two mechanisms is the same the vr mechanism may have been neglected because the produced disturbances are rather weak and they may dissipate quickly the itcz shows a strong tendency to re form within 1 2 days in the same location the itcz may break down via the vr mechanism without any other support and thus it may continuously generate numerous tropical disturbances throughout the season there are two main differences between the two mechanisms 1 the wpds induced itcz breakdown tends to create one or two vortices that may be of tropical depression strength the vr induced itcz breakdown generates several nearly equal sized weak disturbances 2 the wpds tend to disturb the itcz in the eastern pacific only disturbances generally move along the mexican coast after shedding off from the itcz and do not further disturb the itcz in the central pacific therefore the vr mechanism is observed more clearly and is the dominating mechanism for itcz breakdown in the central pacific

itcz breakdown in three dimensional flows the intertropical convergence zone itcz is observed to undulate and at times break down into a series of tropical disturbances in several days some of these disturbances may develop into tropical cyclones and move to higher latitudes while others dissipate and the itcz may reform in the original region it has been proposed that the itcz may break down because of its heating induced potential vorticity pv anomalies here this process is examined in three dimensional simulations using a primitive equation model a simulation of the itcz in a background state of rest is compared to simulations in different background flows the effect of different vertical structures of the prescribed heating is also examined deep heating induces a positive pv anomaly in the lower troposphere leading to a reversal of the pv gradient on the poleward side of the heating while the induced pv anomaly at upper levels is negative leading to a reversal of the pv gradient on the equatorward side of the heating the response at upper levels leads to a weaker pv gradient change but the response is greater in areal extent than the lowertropospheric response for shallow heating the lower tropospheric pv response is greater than that for deep heating and there is no upper tropospheric pv response the itcz lasts longer before breaking in this case than in the deep heating case effects of the background flow are mainly felt in the deep heating cases when the background flow enforces the pv induced wind field itcz breakdown occurs more rapidly whereas when the background flow is opposite to the pv induced flow itcz breakdown takes longer and the itcz may dissipate before breakdown

low level inversions over the tropical pacific thermodynamic structure of the boundary layer and the above inversion moisture structure abstract the structure of the boundary layer over a broad region of the equatorial pacific is studied using dropwinsonde measurements made in january february may and june of 1979 low level inversions of sufficient strength to inhibit deep convection are found to be present in more than 50 of the soundings these inversions appear to play a critical role in regulating convective activity over the central and eastern pacific the tops of the inversions have an average pressure level of approximately 300 mb and show little latitudinal or longitudinal variation the majority of the inversion soundings approximately 70 have a reversal in the mixing ratio profile qreversal above the inversion that appears as a dry layer at the top of the inversion layer capped by a relatively moist layer this moist layer is on the average 2 g kg 1 more moist than the corresponding soundings that have no qreversal no systematic regional or temporal variations in the frequency of occurrence of the qreversal or the str

the drying tendency of shallow meridional circulations in monsoons shallow meridional overturning circulations are superimposed on the deep circulations that produce precipitation in nearly all monsoon regions and these shallow circulations transport subtropical mid tropospheric dry air into the tropical monsoon precipitation maxima here horizontal moisture advection produced by shallow meridional circulations is characterized in the monsoon regions of west africa south asia australia and southern africa during local summer horizontal flow in the upper and lower branches of the shallow meridional circulations consistently dries and moistens air respectively in the continental precipitation maxima of each region the peak drying by horizontal advection occurs at a lower altitude than peak winds in the upper branch of the shallow circulations consistent with the small scale height of water vapor advection of time mean moisture by time mean wind dominates horizontal moisture advection in south asia and west africa while most horizontal moisture advection in australia and southern africa is produced by transient eddies much of the transient eddy advection can be accurately represented as a first order horizontal diffusion with a constant globally uniform diffusivity these results suggest that horizontal moisture advection in theoretical and conceptual models of seasonal mean monsoons can be adequately represented in terms of time mean winds plus a simple horizontal moisture diffusion finally interannual variations in the summer mean regional averages of monsoon precipitation and horizontal advective drying in the lower free troposphere are shown to be negatively correlated in most regions consistent with the hypothesis that advective drying by shallow meridional circulations inhibits monsoon precipitation

enso s impact on the annual cycle the view from galapagos uhf profiler observations of lower tropospheric winds at san cristobal in ecuador s galapagos islands commenced in october 1994 the available data give a detailed picture of the lower troposphere s annual cycle over this portion of the east pacific cold tongue the profiler tracks the low level southerly wind maximum from its weakest state in austral summer when it may briefly rise above 1 km to its strongest state mid year when it is often found below 400 m the profiler operated during much of the 1997 98 el nino and gives us an unprecedented look at dramatic changes in the flow over the cold tongue during a warm event the seasonal deepening of the low level flow was enhanced as the itcz moved into the southern hemisphere for a few months and the lower tropospheric winds became northerly

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

diurnal cycle of the itcz in dynamo abstractduring the 2011 special observing period of the dynamics of the madden julian oscillation dynamo field experiment two sounding arrays were established over the central indian ocean one

variability in the southerly flow into the eastern pacific itcz abstract during boreal summer and fall there is a strong southerly boundary layer flow across the equator into the east pacific intertropical convergence zone itcz the modulation of this flow on synoptic to seasonal time scales is studied using an index of meridional pressure difference between the equator and the itcz along 95 w two complementary datasets from the east pacific investigation of climate epic are used to study eastern pacific variability daily measurements of sea level pressure slp from tropical atmosphere ocean toa array buoys from may to november 2001 provide temporal coverage and eight flights by a c 130 aircraft during september to october 2001 document the associated modulation of lower tropospheric vertical structure the principal mode of variability of the perturbation slp along 95 w from 1 s to 12 n derived by principal component analysis from either the eight flights pc1c 130 or from daily tao buoy observations pc1 explains 77 of the meridional pressure gradien

entrainment rayleigh friction and boundary layer winds over the tropical pacific abstract winds over the tropical pacific are interpreted using mixed layer theory the theory which posits that the surface winds can be derived in terms of a force balance among surface drag pressure gradients coriolis forces and the vertical mixing of momentum into the boundary layer entrainment is very successful in predicting the seasonal climatology of the surface winds the model is also used as a basis for interpreting previous results in particular the model illustrates why studies that model the momentum flux divergence as a rayleigh damping find optimal damping coefficients that are anisotropic a linear variant of the model which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress is also found to predict the surface winds skillfully in addition to improving the representation of the winds it leads to realistic representations of the divergence of the vector wind if the key parameters of the model the entrainment rate and t

the role of inertial instability in determining the location and strength of near equatorial convection there are two major organized cloud configurations in the vicinity of the equator where there is a small cross equatorial surface pressure gradient convection is close to the equator and is generally tied to the location of the lowest sea level pressure slp and warmest sea surface temperature sst in agreement with arguments based upon simple thermodynamical considerations however when there is a substantial cross equatorial pressure gradient such as occurs in the monsoon regions organized convection appears off the equator in the summer hemisphere equatorward of the slp minimum and not necessarily collocated with the warmest ssts thus in this instance simple thermodynamical considerations alone cannot explain 2he location of the convection in this situation the zero absolute vorticity contour 0 also lies in the summer hemisphere therefore between the equator and the 0 contour is a region of locally anticyclonic absolute vorticity and an inertially unstable regime it is argued that the convection results from the low level divergence convergence doublet centred about the 0 contour which is the mitigating response to the inertial instability the associated latitude height secondary circulation should provide subsidence suppressed convection over the equator and rising motion enhanced convection to the north of the zero absolute vorticity contour signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis wherever a strong cross equatorial pressure gradient exists the 0 contour bisects a maximum in the divergent wind field divergence is found equatorward of the zero contour and convergence on the poleward side latitude height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of 0 as the regions where the rising motions occur are conditionally unstable there is deep convection and the vertical circulations extend throughout the troposphere it is noted that the intensity of the off equator convection is deeper and probably stronger than convection located at the equator this is probably because the convection associated with the inertial instability is more efficient necessary conditions for the location of near equatorial convection are listed arguments are presented whereby inertial instability is established as the cause rather than an effect of off equatorial convection these include an outline of the sequence of processes leading up to the convection the factors that limit the encroachment of the 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low level westerly monsoon wind maximum is suggested the possible role played by the instability mechanism or the lack of it in coupled model simulations that produce seasonally migrating and or double itczs in the eastern pacific ocean is discussed finally it is proposed that the instability mechanism is important in the initiation of westward moving disturbances found in the eastern pacific and in determining active and break periods in the summer indian monsoon

stable water isotopes and large scale vertical motions in the tropics a complete understanding of the interaction between convection and the large scale circulation in the tropics remains an outstanding problem although there is evidence that the vertical structure of convective heating has great influence in the large scale response and that this structure also presents considerable geographical variations more need to be done one of the questions that are still unanswered is how the vertical structure of the convective heating or similarly of vertical velocity varies across the tropical pacific here it is suggested that some light can be shed on this debate by considering stable water isotopes because these tend to be progressively less abundant with increasing height precipitation associated with top heavy profiles should be expected to be more depleted than that associated with bottom heavy profiles this claim is verified with a variety of data first using observations from iaea wmo global network of isotopes in precipitation stations then using a simple model based on the budget of water isotopes in precipitation finally using a more complex isotope enabled general circulation model evidence provided by these sources confirms that different structures of vertical velocities are associated with different isotopic abundances with top heavy profiles giving rise to more depleted rainfall finally the data from over the pacific although scarce seem to suggest that precipitation in the eastern part is more enriched than in the western thus hinting at velocity profiles over the east being more bottom heavy than over the west pacific

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

on the tropical atmospheric signature of el nino abstractthe linear atmospheric signature of enso obtained by regressing fields of geopotential height z wind vertical velocity and rainfall upon the nino 3 4 sea surface temperature sst index is partitioned into zonally symmetric and eddy components the zonally symmetric component is thermally forced by the narrowing and intensification of the zonally averaged equatorial rain belt during el nino and mechanically forced by the weakening of the upper tropospheric equatorial stationary waves and their associated flux of wave activity the eddy component of the enso signature is decomposed into barotropic bt and baroclinic bc contributions the latter into first and second modal structures bc1 and bc2 separable functions of space x y and pressure p using eigenvector analysis bc1 exhibits a nearly equatorially symmetric planetary wave structure comprising three dumbbell shaped features suggestive of equatorial rossby waves with out of phase wind and geopotential height perturbations in the

links between annual variations of peruvian stratocumulus clouds and of sst in the eastern equatorial pacific abstract the hypothesis that peruvian stratocumulus play an important role on both the annual mean and annual variations of sea surface temperature sst in the eastern equatorial pacific is examined the problem is addressed by performing sensitivity experiments using the university of california los angeles coupled atmosphere ocean gcm with different idealized temporal variations of stratocumulus in a region along the coast of peru the results obtained are consistent with the notion that peruvian stratocumulus are a key component of the interhemispherically asymmetric features that characterize the annual mean climate of the eastern equatorial pacific including the cold ssts off peru and the absence of a southern itcz the principal new finding of this study is that the annual variations i e deviations from the annual mean of peruvian stratocumulus are linked to the differences between the amplitude duration and westward propagation of the warm and cold phases of the equatorial cold tongue in

dynamics of the intertropical convergence zone of the east pacific abstract the dynamical factors controlling the mean state and variability of the east pacific intertropical convergence zone itcz and the associated cross equatorial boundary layer flow are investigated using observations from the east pacific investigation of climate epic2001 project the tropical east pacific exhibits a southerly boundary layer flow that terminates in the itcz this flow is induced by the strong meridional sea surface temperature sst gradient in the region away from the equator and from deep convection it is reasonably well described on a day to day basis by an extended ekman balance model variability in the strength and northward extent of this flow is caused by variations in free tropospheric pressure gradients that either reinforce or oppose the pressure gradient associated with the sst gradient these free tropospheric gradients are caused by easterly waves tropical cyclones and the madden julian oscillation convergence in the boundary layer flow is often assumed to be r

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

a simple model of climatological rainfall and vertical motion patterns over the tropical oceans abstract a simple model is developed that predicts climatological rainfall vertical motion and diabatic heating profiles over the tropical oceans given the sea surface temperature sst using statistical relationships deduced from the 40 yr ecmwf re analysis era 40 the model allows for two modes of variability in the vertical motion profiles a shallow mode responsible for all boundary layer convergence between 850 hpa and the surface and a deep mode with no boundary layer convergence the model is based on the argument expressed in the authors companion paper that boundary layer convergence can be usefully viewed as a forcing on deep convection not just a result thereof the shallow mode is either specified from satellite observations or modeled using a simple mixed layer model that has sst as well as 850 hpa geopotential height winds and temperature as boundary conditions the deep mode amplitude is empirically shown to be proportional to a simple measure of conditional instability in conve

the variable nature of convection in the tropics and subtropics a legacy of 16 years of the tropical rainfall measuring mission satellite for over 16 years the precipitation radar of the tropical rainfall measuring mission trmm satellite detected the three dimensional structure of significantly precipitating clouds in the tropics and subtropics this paper reviews and synthesizes studies using the trmm radar data to present a global picture of the variation of convection throughout low latitudes the multiyear data set shows convection varying not only in amount but also in its very nature across the oceans continents islands and mountain ranges of the tropics and subtropics shallow isolated raining clouds are overwhelmingly an oceanic phenomenon extremely deep and intense convective elements occur almost exclusively over land upscale growth of convection into mesoscale systems takes a variety of forms oceanic cloud systems generally have less intense embedded convection but can form very wide stratiform regions continental mesoscale systems often have more intense embedded convection some of the most intense convective cells and mesoscale systems occur near the great mountain ranges of low latitudes the maritime continent and amazonia exhibit convective clouds with maritime characteristics although they are partially or wholly land convective systems containing broad stratiform areas manifest most strongly over oceans the stratiform precipitation occurs in various forms often it occurs as quasi uniform precipitation with strong melting layers connected with intense convection in monsoons and the intertropical convergence zone it takes the form of closely packed weak convective elements where fronts extend into the subtropics broad stratiform regions are larger and have lower and sloping melting layers related to the baroclinic origin of the precipitation

intertropical convergence zones during the active season in daily data synoptic scale variability of vorticity structures in the lower troposphere of the tropics is analyzed in 23 yr of daily averaged high resolution reanalysis data the vorticity structures can be divided into zonally elongated vorticity strips classified as intertropical convergence zones itczs and more localized maxima termed westward propagating disturbances a composite of such variability is presented for the east to central pacific and for the east atlantic africa region both in summer the composite in the east pacific is zonally elongated and itcz like propagating westward over a number of days before dissipating the spatial structure of the vorticity strip shows the characteristic cyclonic tilt into the latitudinal direction with time that is also seen in modeling experiments the composite over the atlantic africa region shows two active regions that are correlated on synoptic time scales the disturbances in the southern region are better developed and longer lasting even though the time and space scales are smaller than over the east pacific overall variability over the atlantic is consistent with variability due to african easterly waves the double itcz in spring in the east pacific is different from the few earlier studies available it is stronger south of the equator and located at 10 s which is farther poleward than earlier studies have indicated the northern branch that is weak in comparison is located at 5 n the two branches of the double itcz tend to appear in tandem on the 2 week time scale

evolution of marine atmospheric boundary layer structure across the cold tongue itcz complex the structure of the marine atmospheric boundary layer mabl over the tropical eastern pacific ocean is influenced by spatial variations of sea surface temperature sst in the region as the mabl air is advected across a strong sst gradient associated with the cold tongue itcz complex ctic substantial changes occur in the thermodynamic structure surface fluxes and cloud properties this study attempts to define and explain the variability in the mabl structure and clouds over the ctic using data collected on research cruises from the fall seasons of 1999 2001 composite soundings were created for both the cold and warm sides of the sst front to describe the mean atmospheric boundary layer abl structure and its evolution across this front the average difference in sst across this front was 6 c much of this difference was concentrated in a band only 50 km wide during the fall seasons on the cold side of the gradient a well defined inversion exists in all years below this inversion both fair weather cumulus and stratiform clouds are observed as the mabl air moves over the sst front to warmer waters the inversion weakens and increases in height the mabl also moistens and eventually supports deeper convection over the itcz both the latent and sensible heat fluxes increase dramatically across the sst front because of both an increase in sst and surface wind speed cloudiness is variable on the cold side of the sst front ranging from 0 2 to 0 9 coverage on the warm side cloud fraction was quite constant in time with values generally greater than 0 8 the highest cloud top heights 3 km are found well north of the sst front indicating areas of deeper convection an analysis using energy and moisture budgets identifies the roles of various physical processes in the mabl evolution

hadley cell dynamics in a virtually dry snowball earth atmosphere the hadley cell of a virtually dry snowball earth atmosphere under equinox insolation is studied in a comprehensive atmospheric general circulation model in contrast to the hadley cell of modern earth momentum transport by dry convection which is modeled as vertical diffusion of momentum is important in the upper branch of the snowball earth hadley cell in the zonal momentum balance mean meridional advection of mean absolute vorticity is not only balanced by eddies but also by vertical diffusion of zonal momentum vertical diffusion also contributes to the meridional momentum balance by decelerating the hadley cell through downgradient mixing of meridional momentum between its upper and lower branches when vertical diffusion of momentum is suppressed in the upper branch the hadley cell strengthens by a factor of about 2 this is in line with the effect of vertical diffusion in the meridional momentum balance but incontrastwithitseffectinthezonalmomentumbalance neitheraxisymmetrichadleycelltheoriesbasedon angular momentum conservation nor eddy permitting hadley cell theories that neglect vertical diffusion of momentum are applicable to the snowball earth hadley cell because the snowball earth hadley cell is a particular realization of a dry hadley cell these results show that an appropriate description of dry hadley cells should take into account vertical transport of momentum by dry convection