zonal and vertical structure of the madden julian oscillation abstract a statistical study of the three dimensional structure of the madden julian oscillation mjo is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered outgoing longwave radiation olr data mjo convection is generally preceded by low level convergence and upward motion in the lower troposphere while subsidence cooling and drying prevail aloft this leads to moistening of the boundary layer and the development of shallow convection followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection after the passage of the heaviest rainfall a westerly wind burst region is accompanied by stratiform precipitation where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion the evolution of the heating field leads to a temperature structure that favors the growth of the mjo the analysis also reveals distinct differences in the vertical structure

modelled atmospheric response to changes in northern hemisphere snow cover the surface boundary conditions are altered in a numerical simulation of january climate by prescribing a higher and b lower than average snow extent over northern hemisphere land masses the anomalies in snow cover are shown to have quite a strong impact on the mean climatic state associated with an increase in the areal extent of the snow there is a significant reduction in temperature throughout the lower troposphere there are also large increases in sea level pressure over most land areas significant responses in the mass field are also seen at 500 hpa where reductions in atmospheric thickness lead to significant negative anomalies in the height field responses are also seen non locally over both the north pacific and north atlantic basins the impact of increased snow on cyclone tracks is also examined a reduction in cyclones is noted over both continents and over the western sectors of both ocean basins over the north atlantic basin this reduction extends across over europe significantly weakening the storm track in the north pacific cyclone density is reduced in the west while in the east there is actually a strengthening of the storm tracks there are corresponding changes in the genesis of cyclones in both of these regions the change in cyclogenesis intensity and density is demonstrated to be associated with changes in baroclinicity between the two experiments the anomalous snow boundary conditions lead to significant changes in the meridional temperature gradients over both ocean basins which impact on the baroclinic zones

a northern oscillation relating northern hemispheric pressure anomalies and the indian summer monsoon analysis of the causes of the indian monsoon have concentrated on climatological phenomena in the southern hemisphere such as el nino and the southern oscillation by contrast we have examined meteorological records for the northern hemisphere 0 180 spanning much of this century we find that zonally integrated mean sea level pressure anomalies across eurasia during january to april exhibit a statistically significant negative correlation between values for higher and sub tropical latitudes we further find that there is an association between above normal activity of the indian monsoon and a steep poleward directed pressure anomaly gradient which tends to persist through the summer and may indicate a strong zonal flow in the circumpolar westerlies on the other hand when this pressure anomaly gradient reverses zonality is disturbed and the indian monsoon tends to be drier than usual under these conditions the circumpolar flow becomes persistently meridional as a result of a predisposition to blocking and splitting of the jet

equatorial dry air intrusion and related synoptic variability in mjo initiation during dynamo abstractdynamics of the madden julian oscillation dynamo was conducted over the equatorial indian ocean io from october 2011 to march 2012 during mid to late november a strong madden julian oscillation mjo event denoted mjo 2 initiated in the western io and passed through the dynamo observation array dry air intrusions associated with synoptic variability in the equatorial region played a key role in the evolution of mjo 2 first a sharp dry air intrusion surging from the subtropics into the equatorial region suppresses convection in the itcz south of the equator this diminishes subsidence on the equator associated with the itcz convection which leads to an equatorward shift of convection it is viewed as a contributing factor for the onset of equatorial convection in mjo 2 once the mjo convection is established a second type of dry air intrusion is related to synoptic gyres within the mjo convective envelope the westward propagating gyres draw drier air from the subtropics into the equa

propagating and nonpropagating mjo events over maritime continent abstractthe observed outgoing longwave radiation olr and era interim data during 1979 2008 from november to april were analyzed to reveal fundamental differences between eastward propagating ep and nonpropagating np mjo events across the maritime continent mc it was found that when the maximum mjo convection arrives near 120 e a positive moisture tendency lies in a longitudinal zone 10 s 10 n 130 170 e for the ep cases whereas a negative tendency appears in the same region for the np cases in the latter cases there are clearly detectable westward propagating rossby wave type dry signals over the equatorial central western pacific the dry rossby wave signal may hinder the development of new convection to the east of the mjo convective center preventing the eastward propagation of the mjo a moisture budget analysis shows that the positive tendency of specific humidity in the ep composite is mainly attributed to the advection of the mean moisture by an intraseasonal ascending motion ano

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

annual change of sea surface slope along the equator of the atlantic ocean in 1983 and 1984 for two years beginning in october 1982 oceanographic observations were made in the equatorial atlantic to investigate the relationship between sea level wind stress sea surface slope zonal pressure gradient thermocline depth and equatorial currents here we describe the sea surface slope derived from three complementary methods hydrographic stations pressure gauges and inverted echo sounders the latter two have the advantage of yielding continuous time series but depend on the hydrographic stations for an absolute reference together the three provide a detailed description of the temporal variation of the sea surface slope which is then compared to a wind stress time series the dominant signal in both sea slope and wind stress is the annual cycle although amplitude and phase vary interannually the annual increase in sea surface slope along the equator in the western and central basins lags the onset of the south east trade wind during the boreal winter of 1983 84 a strong rise in sea level occurred against the african coast accompanied by a levelling of the sea surface to the west at the same time an almost complete relaxation of eastward wind stress on the equator was observed near the centre of the basin

summer monsoon rainfall variability over north east regions of india and its association with eurasian snow atlantic sea surface temperature and arctic oscillation this observational study during the 29 year period from 1979 to 2007 evaluates the potential role of eurasian snow in modulating the north east indian summer monsoon rainfall with a lead time of almost 6 months this link is manifested by the changes in high latitude atmospheric winter snow variability over eurasia associated with arctic oscillation ao excessive wintertime eurasian snow leads to an anomalous cooling of the overlying atmosphere and is associated with the negative mode of ao inducing a meridional wave train descending over the tropical north atlantic and is associated with cooling of this region once the cold anomalies are established over the tropical atlantic it persists up to the following summer leading to an anomalous zonal wave train further inducing a descending branch over ne india resulting in weak summer monsoon rainfall

an idealized semi empirical framework for modeling the madden julian oscillation abstractthe authors present a simple semi empirical model to explore the hypothesis that the madden julian oscillation can be represented as a moisture mode destabilized by surface flux and cloud radiative feedbacks the model is one dimensional in longitude the vertical and meridional structure is entirely implicit the only prognostic variable is column water vapor w the zonal wind field is an instantaneous diagnostic function of the precipitation field the linearized version of the model has only westward propagating relative to the mean flow unstable modes because wind induced surface latent heat flux anomalies occur to the west of precipitation anomalies the maximum growth rate occurs at the wavelength at which the correlation between precipitation and surface latent heat flux is maximized this wavelength lies in the synoptic to planetary scale range and is proportional to the horizontal scale associated with the assumed diagnostic wind response to precipitation anomalies the nonlinear version

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

climatology and variability of historical soviet snow depth data some new perspectives in snow indian monsoon teleconnections this study presents the monthly climatology and variability of the historical soviet snow depth data this data set was developed under the bilateral data exchange agreement between united states of america and the former union of soviet socialist republics the original data is for 284 stations for periods varying from 1881 upto 1985 the seasonal cycle of the mean snow depth has been presented both as spatial maps and as averages over key locations the deepest snow 80 cms day areas are found over siberia in particular over 80 100 e 55 70 n during march over the course of the annual cycle average snow depth over this region changes dramatically from about 10 cms in october to about 80 cms in march the variability is presented in the form of spatial maps of standard deviation to investigate the interaction of snow depth with indian monsoon rainfall imr lag and lead correlation coefficients are computed results reveal that the winter time snow depth over western eurasia surrounding moscow eastern eurasia in central siberia shows significant negative positive relationship with subsequent imr following the monsoon the signs of relationship reverse over both the regions this correlation structure is indicative of a midlatitude longwave pattern with an anomalous ridge trough over asia during the winter prior to a strong weak monsoon as the time progresses from winter to spring the coherent areas of significant relationship show southeastward propagation empirical orthogonal function analysis of the snow depth reveal that the first mode describes a dipole type structure with one centre around moscow and the other over central siberia depicting similar pattern as the spatial correlation structure the decadal scale imr variations seem to be more associated with the northern hemisphere midlatitude snow depth variations rather than with the tropical enso el nino southern oscillation variability

a diagnostic study of the evolution of the mjo from indian ocean to maritime continent wave dynamics versus advective moistening processes abstractthe evolution processes for propagating madden julian oscillations with strong magnitude over the indian ocean io and maritime continent mc are investigated through a diagnosis of ecmwf reanalysis data for november april 1982 2011 a scale separated lower tropospheric 1000 700 hpa moisture budget is analyzed for four stages of composite life cycle suppressed cloud developing convective and decaying overall the budgets in the io and mc are dominated by wave induced boundary layer convergence in the anomalous easterlies wc and advection starting from the suppressed stage in the central io moistening by wc and advection by easterly anomalies contributes to an initiation of the mjo convection in the western io while surface evaporation and or shallow convection moistens the central io in the following cloud developing and convective stage in the central io moistening by wc and advection by the downstream kelvin rossby wave east of central io lead to eastward propagation of deep conve

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

relation between eurasian snow cover snow depth and the indian summer monsoon an observational study abstract satellite derived snow cover data for 22 yr and snow depth data for 9 yr over eurasia have been analyzed to reexamine the possible relation of snow with the indian summer monsoon in contrast to the previous studies that use snow cover averaged over all of eurasia as a single number the frequency of occurrence of snow at each grid point over eurasia is correlated with the indian summer monsoon rainfall thus specific geographical regions over eurasia that are responsible for the well known inverse relationship between eurasian snow cover and indian monsoon rainfall are delineated it is found somewhat surprisingly that western eurasia is the only geographical region for which a significant inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall however composites for high and low snow cover over eurasia show spatially homogeneous large scale patterns of snow cover and surface temperature anomalies winters of high and low snow cover for eurasia are found t

continuously stratified models of the steady state equatorial ocean abstract two linearized vertically diffusive steady state models are formulated on an equatorial plane the purpose is a to investigate the vertical boundary layer structure in a continuously stratified ocean spanning the equator and b to test the sensitivity of the results to different turbulence parameterizations both models are analytically tractable in a horizontally unbounded basin one is characterized by newtonian cooling the other has biharmonic friction for either model the equations are analogous to the well known equations governing equatorial wave motion this analogy is exploited in both obtaining and interpreting the solutions in both models zonal wind forcing leads to features such as the equatorial undercurrent south equatorial current and equatorial intermediate current structures resembling the recently discovered subsurface countercurrents are also generated the depth velocity and other scales are model dependent but the basic dynamics are not specifically near the eq

mjo initiation in the real time multivariate mjo index abstractmadden julian oscillation mjo initiation in the real time multivariate mjo rmm index is explored through an analysis of observed case studies and composite events specific examples illustrate that both the dates of mjo initiation and the existence of the mjo itself can vary substantially among several well known mjo indices depending on whether the focus is on convection or circulation composites of primary mjo initiation events in which the rmm index rapidly increases in amplitude from a non mjo state to an mjo state are presented and are supplemented by two case studies from the 1985 86 winter season results illustrate that for primary mjo initiation events in the indian ocean rmm phase 1 slowly eastward propagating 850 hpa 200 hpa easterly westerly anomalies over the indian ocean precede the amplification of the rmm index by at least 10 days while suppressed convection over the western pacific ocean precedes the amplification by 5 days these local eastern hemispheric prede

propagation of the madden julian oscillation through the maritime continent and scale interaction with the diurnal cycle of precipitation the convectively active part of the madden julian oscillation mjo propagates eastward through the warm pool from the indian ocean through the maritime continent the indonesian archipelago to the western pacific the maritime continent s complex topography means the exact nature of the mjo propagation through this region is unclear model simulations of the mjo are often poor over the region leading to local errors in latent heat release and global errors in medium range weather prediction and climate simulation using 14 northern winters of trmm satellite data it is shown that where the mean diurnal cycle of precipitation is strong 80 of the mjo precipitation signal in the maritime continent is accounted for by changes in the amplitude of the diurnal cycle additionally the relationship between outgoing long wave radiation olr and precipitation is weakened here such that olr is no longer a reliable proxy for precipitation the canonical view of the mjo as the smooth eastward propagation of a large scale precipitation envelope also breaks down over the islands of the maritime continent instead a vanguard of precipitation anomalies of 2 5 mm day 1 over 10 6 km 2 jumps ahead of the main body by approximately 6 days or 2000 km hence there can be enhanced precipitation over sumatra borneo or new guinea when the large scale mjo envelope over the surrounding ocean is one of suppressed precipitation this behaviour can be accommodated into existing mjo theories frictional and topographic moisture convergence and relatively clear skies ahead of the main convective envelope combine with the low thermal inertia of the islands to allow a rapid response in the diurnal cycle which rectifies onto the lower frequency mjo hence accurate representations of the diurnal cycle and its scale interaction appear to be necessary for models to simulate the mjo successfully

propagating versus nonpropagating madden julian oscillation events abstractbasinwide convective anomalies over the indian ocean io associated with the madden julian oscillation mjo sometimes propagate eastward and reach the west pacific wp but sometimes do not long term observations and reanalysis products are used to investigate the difference between the propagating and nonpropagating mjo events io convection onset events associated with the mjo are grouped into three categories based on the strengths of the simultaneous dry anomalies over the eastern maritime continent and wp the io convection anomaly preferentially makes eastward propagation and reaches the wp when the dry anomaly is stronger analysis of the column integrated moist static energy mse budget shows that horizontal advection moistens the atmosphere to the east of the positive mse anomaly associated with the active convection over the io and is of sufficient magnitude to explain the eastward propagation of the positive mse anomaly interpretation is complicated however by lack of closure in

a linear stratified ocean model of the equatorial undercurrent a linear stratified ocean model is used to study the wind driven response of the equatorial ocean the model is an extension of the lighthill 1969 model that allows the diffusion of heat and momentum into the deeper ocean and so can develop non trivial steady solutions to retain the ability to expand solutions into sums of vertical normal modes mixing coefficients must be inversely proportional to the square of the back ground vaisala frequency the model is also similar to the earlier homogeneous ocean model of stommel i960 he extended ekman dynamics to the equator by allowing his model to generate a barotropic pressure field the present model differs in that the presence of stratification allows the generation of a baroclinic pressure field as well the most important result of this paper is that linear theory can produce a realistic equatorial current structure the model undercurrent has a reasonable width and depth scale there is westward flow both above and below the undercurrent the meridional circulation conforms to the classical picture suggested by cromwell 1953 unlike the stommel solution the response here is less sensitive to variations of parameters ocean boundaries are not necessary for the existence of the undercurrent but are necessary for the existence of the deeper equatorial intermediate current the radiation of equatorially trapped rossby and kelvin waves is essential to the development of a realistic undercurrent because the system supports the existence of these waves low order vertical modes can very nearly adjust to sverdrup balance defined below which in a bounded ocean and for winds without curl is a state of rest as a result higher order vertical modes are much more visible in the total solution this property accounts for the surface trapping and narrow width scale of the equatorial currents the high order modes tend to be in yoshida balance defined below and generate the characteristic meridional circulation pattern associated with equatorial ekman pumping

the representation of the south tropical atlantic teleconnection to the indian ocean in the ar4 coupled models a series of recent papers showed that sea surface temperature sst anomalies in the south equatorial tropical atlantic modulate the interannual variability of the african and indian monsoon rainfall physically this teleconnection can be explained by a simple gill matsuno mechanism in this work the output from five different models chosen within the cmip3 coupled model intercomparison project version 3 ensemble of coupled general circulation models cgcms are analyzed to investigate how state of the art cgcms represent the impact of the south tropical atlantic sta ssts on the indian and african region using a correlation regression technique it is found that four out of the five models display a teleconnection between sta and indian region which is generally weaker than in the observations but in agreement in the rainfall field pattern this teleconnection is also noticeable in the ensemble mean of the five models over africa however the significant changes in rainfall displayed in the observation are properly caught by only one of the cgcms additionally none of the models reproduces the symmetric upper level wind response around the equator seen over the indian ocean in the observations and all have significant biases also in the surface pressure field response to the tropical atlantic ssts nonetheless the sta response particularly over the southern hemisphere is indicative of the gill matsuno type mechanism identified in previous studies using idealized experiments with atmospheric gcms and observational data with a suite of atmospheric only gcm integrations it is shown that the differences in amplitude and pattern are not only due to the strong biases and reduced variabilities of the cgcms over the tropical atlantic but they are also caused by the different physical parameterizations used in models

circulation response to eurasian versus north american anomalous snow scenarios in the northern hemisphere with an agcm coupled to a slab ocean model abstractthe difference between snow covered and snow free conditions is the most climatically significant natural seasonal change the land surface can experience most gcm studies investigating snow atmosphere interactions have focused on impacts of eurasian snow anomalies caused by the magnitude of snow mass while north american snow has been shown to have a weaker relationship with downstream climate experiment design of recent snow atmosphere interactions studies has been limited to atmosphere only models with sea surface temperature sst and sea ice extent represented as boundary conditions the authors explore the circulation response to anomalous snow scenarios for both north america and eurasia using a slab ocean model surface response include significant sst cooling directly downstream of each individual forcing region in addition to upstream centers of remote cooling under maximum snow conditions atmospheric response to anomalous snow conditions is consistent through multiple levels in th

on the relationship between east equatorial atlantic sst and ism through eurasian wave the dominant mode of july august ja seasonal variability of indian summer monsoon rainfall ismr are obtained by performing empirical orthogonal function eof analysis the first dominant mode of ismr and its relationships with the sea surface temperature sst pressure level wind and geopotential height gph fields are examined using gridded datasets for the period 1979 2014 the principal component of the first leading mode pc1 obtained in the eof analysis of ja rainfall over indian landmass is highly correlated with north west and central india rainfall and anti correlated with east equatorial atlantic sst eeasst the positive eeasst anomaly intensifies the inter tropical convergence zone over atlantic and west equatorial africa which generates stationary wave meridionally as meridional transfer of energy is strong as the influence of background jet streams are minimal over north africa and europe the anomalous positive and negative gph are generated over sub tropics and extra tropics respectively due to the stationary wave this increases the climatological background steep pressure gradient between sub tropics and extra tropics consisting of anomalous negative gph field over north west nw europe and vice versa for negative eeasst anomaly the anomalous positive gph over nw europe acts as center of action for the propagation of a rossby wave train to nw india via europe consisting of anomalous high over nw of india this intensifies the tibetan high westward which reinforces the outbreak of monsoon activities over central and nw india

why precipitation is mostly concentrated over islands in the maritime continent abstract high resolution observations and regional climate model simulations reveal that precipitation over the maritime continent is mostly concentrated over islands analysis of the diurnal cycles of precipitation and winds indicates that this is predominantly caused by sea breeze convergence over islands reinforced by mountain valley winds and further amplified by the cumulus merger processes comparison of a regional climate model control simulation to a flat island run and an all ocean run demonstrates that the underrepresentation of islands and terrain in the maritime continent weakens the atmospheric disturbance associated with the diurnal cycle and hence underestimates precipitation the implication of these regional modeling results is that systematic errors in coarse resolution global circulation models probably result from insufficient representation of land sea breezes associated with the complex topography in the maritime continent it is found that precipitation in the maritime continent

modeling the oceanic response to westerly wind bursts in the western equatorial pacific a 3d tropical upper ocean circulation model is employed to study the generation mechanism for subsurface reversing currents forced by strong westerly wind bursts in the western equatorial pacific the westerly wind bursts last from a few days to a few weeks and reverse the surface current from westward to eastward setting up a zonal pressure gradient that generates a westward subsurface current although less affected by the local wind the eastward equatorial undercurrent euc decelerates the authors verified the hypothesis that the subsurface reversal is a local response to westerly wind bursts however the model response is very sensitive to the wind fetch the observed reversal of the subsurface current can only be reproduced by wind bursts with a zonal extent of less than 700 km this is because the zonal extent determines the timescale on which the pressure gradient is set up by equatorial waves this timescale must be shorter than the timescales of vertical mixing and downwelling in order for the pressure driven subsurface westward acceleration to overtake the eastward acceleration due to downward transport of momentum the results emphasize the importance of resolving spatial variations in simulating the upper ocean response to atmospheric forcing the influence of off equator winds on equatorial currents is also investigated it is found that the remote effect of an off equator wind can be larger on subsurface currents than on surface currents an off equator westerly easterly wind decelerates accelerates the euc but has little effect on the surface equatorial current when a cyclone with a westerly wind at the equator and an easterly off equator wind is present the local response near the surface is dominated by the westerly wind but the remote effect of the off equator wind significantly modifies the local wind effect at the depth of the euc

the persistence of winter sea surface temperature in the north atlantic abstract the impact of the seasonal variations of the mixed layer depth on the persistence of sea surface temperature sst anomalies is studied in the north atlantic using observations a significant recurrence of winter sst anomalies during the following winter occurs in most of the basin but not in the subtropical area of strong subduction when taking reemergence into account the e folding timescale of winter sst anomalies generally exceeds 1 yr and is about 16 months for the dominant sst anomaly tripole the influence of advection by the mean oceanic currents is investigated by allowing for a displacement of the maximum recurrent correlation and alternatively by considering the sst anomaly evolution along realistic mean displacement paths taking into account the nonlocality of the reemergence generally increases the wintertime persistence most notably in the northern part of the domain the passive response of the mixed layer to the atmospheric forcing thus has a red spectrum down to near dec

the hawaii to tahiti shuttle experiment the shuttle experiment conducted between hawaii and tahiti from january 1979 to june 1980 was designed to observe the changing equatorial ocean structure and circulation to study the variations and interactions of the four major equatorial ocean currents and to develop a scientific basis for their monitoring by simple observations of thermal structure and sea level preliminary analyses of the results show that the equatorial thermal structure remains intact during a normal year and that only the positions and intensities of the currents are subject to change the water transport of the equatorial undercurrent varied from 25 x 106 cubic meters per second in january to 51 x 106 cubic meters per second in july but also exhibited strong short term pulsations the equatorial surface flow responded strongly to the winds at periods of 1 month and longer an array of drifter buoys in the equatoral countercurrent was subject to very little dispersion while traveling over 4500 kilometers in 4 months low frequency fluctuations in the north equatorial countercurrent can be monitored by means of the difference in sea level between fanning and majuro

trade wind fetch related variations in equatorial undercurrent depth speed and transport a dynamical model is developed to examine the dependence of equatorial undercurrent depth speed and volume transport on trade wind fetch the model ocean is linear continuously stratified vertically diffusive and formulated on the equatorial beta plane analytical solutions are derived in closed form for realistic choices of model parameters results indicate that the undercurrent would be significantly shallower and weaker in the atlantic than in the pacific because of the shorter atlantic trade wind fetch

dynamics of the surface layer diurnal cycle in the equatorial atlantic ocean 0 23 w a 15 year time series 1999 2014 from the 0 23 w prediction and research moored array in the tropical atlantic pirata mooring which includes an 8 month record october 2008 to june 2009 of high resolution near surface velocity data is used to analyze the diurnal variability of sea surface temperature shear and stratification in the central equatorial atlantic the ocean diurnal cycle exhibits pronounced seasonality that is linked to seasonal variations in the surface wind field in boreal summer and fall steady trade winds and clear skies dominate with limited diurnal variability in sea surface temperature diurnal shear layers with reduced richardson numbers are regularly observed descending into the marginally unstable equatorial undercurrent below the mixed layer conditions favorable for the generation of deep cycle turbulence in contrast in boreal winter and spring winds are lighter and more variable mixed layers are shallow and diurnal variability of sea surface temperature is large during these conditions diurnal shear layers are less prominent and the stability of the undercurrent increases suggesting seasonal covariance between diurnal near surface shear and deep cycle turbulence modulation of the ocean diurnal cycle by tropical instability waves is also identified this work provides the first observational assessment of the diurnal cycle of near surface shear stratification and marginal instability in the equatorial atlantic confirming previous modeling results and offering a complementary perspective on similar work in the equatorial pacific

on the difference in strength between atlantic and pacific undercurrents abstract from an earlier study of the longitudinal evolution of the equatorial zonal momentum balance in a nonlinear stratified simulation of the atlantic equatorial undercurrent it was shown that the annual mean simulated undercurrent reaches its maximum zonal velocity as it emerges from an accelerated deep inertial regime extending over about 15 degrees of longitude off the western boundary further east frictional forces overcome the accelerating effect of the zonal pressure gradient form and the undercurrent keeps losing eastward momentum as it flows eastward if the accelerating deep western regime extended over a wider range of longitudes the maximum emerging velocity would be larger this is shown to occur in a nonlinear stratified simulation of the pacific equatorial undercurrent because the western pacific thermocline is thicker due to the basin size and to the longitudinal structure of the zonal winds this is offered as an explanation for the larger observed zonal velocities of the pacific und

eurasian snow cover variability and northern hemisphere climate predictability we present observational evidence demonstrating dynamic interactions and feedbacks between multi seasonal snow cover and winter time circulation anomalies over mid high latitudes the cooling effect of snow cover is associated with a strengthened and more expansive siberian high with more frequent topographically constrained intrusions west and north early season snow cover variability leads to altered general circulation patterns consistent with the dominant mode of winter variability observed in the northern hemisphere troposphere the implications of the surface atmosphere coupling for seasonal to interannual predictability are also discussed

on the persistence of decadal sst anomalies in the north atlantic abstract historical wintertime sea surface temperature sst data show that a sandwich pattern dominates on the decadal timescales in the north atlantic at least after the 1970s the authors investigated how such decadal sst anomalies can survive against local thermal feedback which acts to dampen them rapidly at the surface winter sst anomalies have a negligible projection with the subsequent summer anomalies while they show a significant projection with the sst anomalies in the next winter on the other hand observed summer temperature anomalies below the mixed layer tend to have the same sign as the previous winter sst anomalies although the magnitude of the former is roughly one third of the latter this suggests that a reemergence mechanism of sst anomalies associated with the seasonal cycle of the mixed layer depth mld which has been verified by alexander and deser helps maintain the decadal sst anomalies in order to examine this scenario a mixed layer model driven by daily atmospheric d

topographic influence on the mjo in the maritime continent abstract this study demonstrates that during the passage of the mjo through the maritime continent in the boreal winter the corresponding deep convection and near surface wind anomalies tend to skirt around mountainous islands flow bifurcation around elongated mountainous islands such as new guinea is clearly seen topographic blocking generates distinctive vorticity and convergence distributions in this specific domain mountain wave like structures are also observed throughout the maritime continent with a clear spatial relationship with the high terrains in sumatra sulawesi and new guinea the existence of topography seems to create extra lifting and sinking within the large scale circulation and thus the convective system exhibits quasi stationary features near the major topography during the mjo passage through the maritime continent it is suggested that resolving the detailed topographic effects may play a key role in simulating realistic characteristics of the mjo in the maritime continent

a physical mechanism for north atlantic sst influence on the indian summer monsoon a link between the atlantic multidecadal oscillation amo and multidecadal variability of the indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking atlantic climate and monsoon has been identified the amo produces persistent weakening strengthening of the meridional gradient of tropospheric temperature tt by setting up negative positive tt anomaly over eurasia during northern late summer autumn resulting in early late withdrawal of the south west monsoon and persistent decrease increase of seasonal monsoon rainfall on inter annual time scales strong north atlantic oscillation nao or north annular mode nam influences the monsoon by producing similar tt anomaly over eurasia the amo achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong nao nam events this mechanism also provides a basis for explaining the observed teleconnection between north atlantic temperature and the asian monsoon in paleoclimatic proxies citation goswami b n m s madhusoodanan c p neema and d sengupta 2006 a physical mechanism for north atlantic sst influence on the indian summer monsoon

rossby wave propagation and teleconnection patterns in the austral winter abstract observational evidence of and theoretical support for the northern and southern hemisphere teleconnection patterns in the austral southern hemisphere winter are examined through an upper troposphere streamfunction teleconnectivity map and time lag cross correlation analysis using ecmwf initialized analysis 2oo hpa winds for the 11 june august periods from 1979 to 1989 as was previously found for the northern hemisphere winter the regions of strong teleconnectivity particularly in the winter hemisphere tend to he oriented in the zonal direction and coincide with the location of the major jet streams although equatorward propagation from the northern and southern hemispheres is observed little evidence of cross equatorial propagation has been found for comparison the response of a barotropic model linearized about a climatological 300 hpa june august time mean flow to localized forcing is determined it is found that the activity tends to be trapped inside each of the southern hemisphere

winter surface pressure anomalies over eurasia and indian summer monsoon in this study the spatial relationships between the monthly mean sea level pressure mslp anomalies and indian summer monsoon rainfall ismr have been examined using the gridded mslp data gmslp2 for 46 years 1949 1994 winter mslp anomalies over nw europe south europe are negatively positively correlated with ismr during the strong deficient monsoon years there is enhanced weakened poleward pressure gradient over eurasia the europe pressure gradient index which is an indicator of the winter poleward pressure gradient is significantly and positively correlated with ismr this relationship also suggests a link between the north atlantic oscillation and ismr on inter annual time scale the winter pressure gradient over eurasia has physical linkage with indian summer monsoon rainfall through winter snow cover anomalies and springtime mid latitude activities like upper tropospheric blocking highs and stationary wave anomalies

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

the slope of sea level along the equator during the 1982 1983 el nino observed deviations of monthly mean sea level at seven island stations along the equator in the pacific have been superimposed on the mean dynamic topography to determine the variations of the east west slope of sea surface topography during the 1982 1983 el nino the normal east west slope is eliminated in january 1983 when the bulk of warm water flowing eastward has reached the coast of south america the sea surface remains essentially flat from january to june 1983 although zonal winds are very weak until april only the slope requires several months to be reestablished and in october and november sea level along the entire equator is 10 cm or more below normal indicating a net loss of warm water from the equatorial pacific

intraseasonal teleconnection between the summer eurasian wave train and the indian monsoon this study investigated the most recurrent coupled pattern of intraseasonal variability between midlatitude circulation and the indian summer monsoon ism the leading singular vector decomposition svd pattern reveals a significant coupled intraseasonal variation between a rossby wave train across the eurasian continent and the summer monsoon convection in northwestern india and pakistan hereafter referred to as nism the wave train associated with an active phase of nism rainfall displays two high pressure anomalies one located over central asia and the other over northeastern asia they are accompanied by increased rainfall over the western siberia plain and northern china and decreased rainfall over the eastern mediterranean sea and southern japan the circulation of the wave train shows a barotropic structure everywhere except the anomalous central asian high located to the northwest of india where a heat induced baroclinic circulation structure dominates the time lagged svd analysis shows that the midlatitude wave train originates from the northeastern atlantic and traverses europe to central asia the wave train enhances the upper level high pressure and reinforces the convection over the nism region meanwhile it propagates farther toward east asia along the waveguide provided by the westerly jet after an outbreak of nism convection the anomalous central asian high retreats westward composite analysis suggests a coupling between the central asian high and the convective fluctuation in the nism the significance of the midlatitude ism interaction is also revealed by the close resemblance between the individual empirical orthogonal functions and the coupled svd modes of the midlatitude circulation and the ism it is hypothesized that the eastward and southward propagation of the wave train originating from the northeastern atlantic contributes to the intraseasonal variability in the nism by changing the intensity of the monsoonal easterly vertical shear and its associated moist dynamic instability on the other hand the rainfall variations over the nism reinforce the variations of the central asian high through the monsoon desert mechanism thus reenergizing the downstream propagation of the wave train the coupling between the eurasian wave train and nism may be instrumental for understanding their interaction and can provide a way to predict the intraseasonal variations of the indian summer monsoon and east asian summer monsoon

three dimensional structure and evolution of the moisture field in the mjo abstractthe large scale circulation features that determine the structure and evolution of mjo related moisture and precipitation fields are examined using a linear analysis protocol based on daily 850 minus 150 hpa global velocity potential data the analysis is augmented by a compositing procedure that emphasizes the structural features over the indo pacific warm pool sector 60 e 180 that give rise to the eastward propagation of the enhanced moisture and precipitation it is found that boundary layer bl convergence in the low level easterlies to the east of the region of maximum ascent produces a deep but narrow plume of equatorial ascent that moistens the midtroposphere while weakly diffluent flow above the bl spreads moisture away from the equator vertical advection of moisture from this plume of ascent accounts for the eastward propagation of the positive moisture anomalies across the maritime continent into the western pacific when the convection is first developing over the indian ocean ho