application of extended empirical orthogonal function analysis to interrelationships and sequential evolution of monsoon fields abstract extended empirical orthogonal function eeof analysis has been employed to study linear relationships among the mean sea level pressure 700 mb height and rainfall over india and their low frequency sequential evolution during the peak summer monsoon months the interrelationships between these fields are strongest over central india and while the rainfall activity is colocated with the corresponding changes in the 700 mb heights it is displaced southward with respect to the pressure changes the first two eeof s of all the three fields averaged over 5 or 7 days show that the dominant low frequency sequential evolution is associated with north and northeastward movement of the anomaly centers with a recurrence period of about 40 days in addition the presence of a westward moving wave in sea level pressure anomalies located roughly near 15 n latitude is revealed by the third eeof

global tidal residual mean circulation does it affect a climate ogcm a global description of the ocean tidal residual mean circulation trmc is provided and its effect on a global ocean climate model is evaluated the trmc is computed from a global barotropic tidal model based on pure hydrodynamical equations and forced by the major tidal constituents it thus includes both stokes drift and non linear tidal dynamics the trmc map exhibits remarkable basin scale structures in the indonesian archipelago australian region and on the malvinas shelf regions of largest transport are characterized by sites of high tidal amplitudes abrupt topography and western boundaries yet transport magnitudes remain at a few tenths of a sverdrup this trmc is introduced as an external forcing in an ocean climate model at equilibrium the thermohaline structure and the poleward heat transport of the ocean are almost insensitive to the trmc it is concluded that the trmc is negligible for climate purposes in an ocean climate model

processes of 30 90 days sea surface temperature variability in the northern indian ocean during boreal summer during summer the northern indian ocean exhibits significant atmospheric intraseasonal variability associated with active and break phases of the monsoon in the 30 90 days band in this paper we investigate mechanisms of the sea surface temperature sst signature of this atmospheric variability using a combination of observational datasets and ocean general circulation model sensitivity experiments in addition to the previously reported intraseasonal sst signature in the bay of bengal observations show clear sst signals in the arabian sea related to the active break cycle of the monsoon as the atmospheric intraseasonal oscillation moves northward sst variations appear first at the southern tip of india day 0 then in the somali upwelling region day 10 northern bay of bengal day 19 and finally in the oman upwelling region day 23 the bay of bengal and oman signals are most clearly associated with the monsoon active break index whereas the relationship with signals near somali upwelling and the southern tip of india is weaker in agreement with previous studies we find that heat flux variations drive most of the intraseasonal sst variability in the bay of bengal both in our model regression coefficient 0 9 against 0 25 for wind stress and in observations 0 8 regression coefficient 60 of the heat flux variation is due do shortwave radiation and 40 due to latent heat flux on the other hand both observations and model results indicate a prominent role of dynamical oceanic processes in the arabian sea wind stress variations force about 70 100 of sst intraseasonal variations in the arabian sea through modulation of oceanic processes entrainment mixing ekman pumping lateral advection our 100 km resolution model suggests that internal oceanic variability i e eddies contributes substantially to intraseasonal variability at small scale in the somali upwelling region but does not contribute to large scale intraseasonal sst variability due to its small spatial scale and random phase relation to the active break monsoon cycle the effect of oceanic eddies however remains to be explored at a higher spatial resolution

sun ocean nuclear bombs and fossil fuels radiocarbon variations and implications for high resolution dating radiocarbon or 14c is a radiometric dating method ideally suited for providing a chronological framework in archaeology and geosciences for timescales spanning the last 50 000 years 14c is easily detectable in most common natural organic materials and has a half life 5 730 40 years relevant to these timescales 14c produced from large scale detonations of nuclear bombs between the 1950s and the early 1960s can be used for dating modern organic materials formed after the 1950s often these studies demand high resolution chronology to resolve ages within a few decades to less than a few years despite developments in modern high precision 14c analytical methods the applicability of 14c in high resolution chronology is limited by short term variations in atmospheric 14c in the past this article reviews the roles of the principal natural drivers e g solar magnetic activity and ocean circulation and the anthropogenic perturbations e g fossil fuel co2 and 14c from nuclear and thermonuclear bombs

the indian monsoon and its variability for over 300 years the monsoon has been viewed as a gigantic land sea breeze it is shown in this paper that satellite and conventional observations support an alternative hypothesis which considers the monsoon as a manifestation of seasonal migration of the intertropical convergence zone itcz with the focus on the indian monsoon the mean seasonal pattern is described and why it is difficult to simulate it is discussed some facets of the intraseasonal variation such as active weak cycles break monsoon and a special feature of intraseasonal variation over the region namely poleward propagations of the itcz at intervals of 2 6 weeks are considered vertical moist stability is shown to be a key parameter in the variation of monthly convection over ocean and land as well as poleward propagations special features of the bay of bengal and the monsoon brought out by observations during a national observational experiment in 1999 are briefly described

monsoonal influence on southern hemisphere 14co2 1 annual rings of a cross dated teak tree core from muna island sulawesi indonesia in the southern equatorial tropics were analysed for radiocarbon from 1951 1979 14c levels at muna started rising in 1956 and reached a maximum value of 667 in early 1965 the muna 14c levels are significantly higher than those derived from the other southern hemisphere sh 14c records including tree rings and atmospheric co2sampling for 1959 and 1963 1965 during the growing season of teak tree rings at this location nov apr the inter tropical convergence zone itcz moves southward of muna our results indicate that the island is strongly influenced by northern hemisphere nh air masses carried by the winter asian monsoon while the other more southerly sh sites remain covered by sh air masses this monsoonal effect on atmospheric14c at muna is evident during the periods of rapidly rising atmospheric 14c 1959 and 1963 1965 when there is an enhanced 14c contrast between northern and southern air masses

holocene sea level change and coastal landscape evolution in the southern gulf of carpentaria australia a revised holocene sea level history for the southern gulf of carpentaria is presented based on new data from the south wellesley archipelago and age recalibration of previous research results confirm that rising sea levels during the most recent post glacial marine transgression breached the arafura sill ca 11 700 cal yr bp sea levels continued to rise to ca 30 m by 10 000 cal yr bp leading to full marine conditions by 7700 cal yr bp sea level reached present mean sea level pmsl and continued to rise to an elevation of between 1 5 m and 2 m above pmsl sea level remained ca 1 5 between 7000 and 4000 cal yr bp followed by rapid regression to within 0 5 m of pmsl by ca 3500 cal yr bp when placed into a wider regional context results from this study show that coastal landscape evolution in the tropical north of australia was not only dependent on sea level change but also show a direct correlation with holocene climate variability specifically the formation and preservation of beach rock deposits intertidal successions beach and chenier ridge systems hold valuable sea level and holocene climate proxies that can contribute to the growing research into lower latitude holocene sea level and climate histories

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

observations of a convectively coupled kelvin wave in the eastern pacific itcz abstract a case study of a convectively coupled kelvin wave in the eastern pacific intertropical convergence zone itcz is presented as observed during the 1997 pan american climate studies pacs tropical eastern pacific process study tepps the large scale convective envelope associated with this disturbance with a zonal scale of approximately 1000 2000 km propagates eastward at 15 m s 1 along the mean convective axis of the itcz this envelope consists of many smaller scale westward moving convective elements with zonal scales on the order of 100 500 km as the convectively coupled kelvin wave disturbance propagates eastward it exerts a strong control on local convection radar and vertical profiler data collected aboard the noaa r v ronald h brown during the wave passage show that convection deepens rapidly as the kelvin wave approaches from the west progressing from isolated shallow cumuli to organized deep convective features within just 12 h initially rainfall in the vicinity of the s

monsoons as eddy mediated regime transitions of the tropical overturning circulation monsoons are generally viewed as planetary scale sea breeze circulations caused by contrasts in the thermal properties between oceans and land surfaces that lead to thermal contrasts upon radiative heating1 2 but the radiative heating evolves gradually with the seasons whereas the onset of monsoon precipitation and the associated circulation changes such as reversal of surface winds occur rapidly3 4 here we use reanalysis data to show that the onset of the asian monsoon marks a transition between two circulation regimes that are distinct in the degree to which eddy momentum fluxes control the strength of the tropical overturning circulation rapid transitions of the circulation between the two regimes can occur as a result of feedbacks between large scale extratropical eddies and the tropical circulation5 using simulations with an aquaplanet general circulation model we demonstrate that rapid eddy mediated monsoon transitions occur even in the absence of surface inhomogeneities provided the planet surface has sufficiently low thermal inertia on the basis of these results we propose a view of monsoons in which feedbacks between large scale extratropical eddies and the tropical circulation are essential for the development of monsoons whereas surface inhomogeneities such as land sea contrasts are not

remote influences on the indian monsoon low level jet intraseasonal variations a strong low level jet llj also known as the findlater jet develops over the arabian sea during the indian summer monsoon this jet is an essential source of moisture for monsoonal rainfall over the densely populated indian subcontinent and is a key contributor to the indian ocean oceanic productivity by sustaining the western arabian sea upwelling systems the llj intensity fluctuates intraseasonally within the 20 to 90 day band in relation with the northward propagating active and break phases of the indian summer monsoon our observational analyses reveal that these large scale regional convective perturbations only explain about half of the intraseasonal llj variance the other half being unrelated to large scale convective perturbations over the indian ocean we show that convective fluctuations in two regions outside the indian ocean can remotely force a llj intensification four days later enhanced atmospheric deep convection over the northwestern tropical pacific yields westerly wind anomalies that propagate westward to the arabian sea as baroclinic atmospheric rossby waves suppressed convection over the eastern pacific north american monsoon region yields westerly wind anomalies that propagate eastward to the indian ocean as dry baroclinic equatorial kelvin waves those largely independent remote influences jointly explain 40 of the intraseasonal llj variance that is not related to convective perturbations over the indian ocean i e 20 of the total with the northwestern pacific contributing twice as much as the eastern pacific taking into account these two remote influences should thus enhance the ability to predict the llj

the role of internal tides in mixing the deep ocean abstract internal wave theory is used to examine the generation radiation and energy dissipation of internal tides in the deep ocean estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth varying stratification and two dimensional topography specific estimates of energy flux are made for midocean ridge topography weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km the richardson number of the radiated internal tide typically exceeds unity for these motions and so direct shear instability of the generated waves is not the dominant energy transfer mechanism it also seems that wave wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux instead it appears that most of

an ocean atmosphere interaction mechanism for the active break cycle of the asian summer monsoon a typical active break cycle of the asian summer monsoon is taken as beginning with maximum sst pentad 0 over the north bay of bengal when the oceans to its west and east from longitude 40 160 e and between latitudes 10 and 25 n area a also has maximum sst during this pentad the recently found cold pool of the bay of bengal between latitudes 3 n and 10 n has its minimum sst an area of convection takes genesis over the bay of bengal immediately after pentad 0 in the zone of large sst gradient north of the cold pool and it pulls the monsoon low level jetstream llj through peninsular india convection and the llj westerlies then spread to the western pacific ocean during pentads 1 4 taken as the active phase of the monsoon during which convection and llj have grown in a positive feed back process the cyclonic vorticity to the north of the llj axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection by the end of pentad 4 the sst over area a has cooled and the convection weakens there when the llj turns clockwise over the arabian sea and flows close to the equator in the indian ocean a band of convection develops at pentad 5 between the equator and latitude 10 s over the indian ocean and it is nourished by the cyclonic vorticity of the llj now near the equator and the moisture supply through it this is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active break cycle of 40 day duration with reduced wind and convection over the area a during the break phase solar radiation and light winds make the sst there warm rapidly and a new active break cycle begins sst convection llj and the net heat flux at the ocean surface have important roles in this new way of looking at the active break cycle as a coupled ocean atmosphere phenomenon

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

multiscale phenomena in the tropical atmosphere over the western pacific abstract multiscale variabilities in the atmosphere over the tropical western pacific during the 1979 northern hemisphere winter are studied with an aim at identifying possible interactions between phenomena of different spatial and temporal scales based on the convection index information derived from satellite measurements two intra seasonal oscillations isos are identified within the equatorial belt between 0 and 10 s in the analyzed period the two isos accompanied by both rotational and divergent circulations propagate eastward from the indian ocean to the western pacific over the warm pool in the western pacific the isos develop into quasi stationary systems with an enhanced rotational circulation characterized by a strong westerly jet in the lower troposphere the isos appear to interact with a number of regional and synoptic scale phenomena in the maritime continent and western pacific region for example the onset of the monsoon coincides with the arrival of the first iso at northern a

tidal spectroscopy of the english channel with a numerical model abstract the possibility of reproducing the complexity of tides in shallow water areas with a classical finite difference numerical model is examined this hydrodynamic model is two dimensional but incorporates topography nonlinear advection and quadratic bottom friction particular care is taken to prescribe sea surface elevations at the open boundaries a one month simulation of real tides is run with a simplified spectrum restricted to only 24 constituents corresponding to the nine main astronomical tides and their nonlinear significant interactions the results are analyzed by spectral decomposition elevations and vertically integrated currents and compared with observational data from the tide gages and current meters and with other solutions produced in the literature it is found that the dominant m2 constituent greatly influences the damping of the other constituents so that it is necessary to run them together for any correct simulation however the quadratic friction law introduced in

active break cycles diagnosis of the intraseasonal variability of the asian summer monsoon in this study various diagnostics have been applied to daily observed outgoing longwave radiation olr and ecmwf reanalysis era products to provide a comprehensive description of the active break cycles associated with the asian summer monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability 10 20 days and 30 60 days composite analysis of olr based on filtered daily all india rainfall air for the 40 day 30 60 days intraseasonal mode indicates that during active phases convection is significantly enhanced over the indian continent extending over the bay of bengal maritime continent and equatorial west pacific while convection is suppressed over the equatorial indian ocean and northwest tropical pacific resulting in a quadrapole structure over the asian monsoon domain in response to this heating pattern the large scale hadley lateral and the two east west transverse tropical circulations are enhanced there is also a significant impact on the extra tropical circulation through excitation and propagation of rossby waves in contrast the 15 day mode is more regional to the monsoon domain and has a prominent east west orientation in convection only the local hadley circulation over the monsoon region is modulated by this mode the evolution of these two modes as revealed by pop principal oscillation pattern analysis shows that the 40 day mode originates over the equatorial indian ocean once formed it has poleward propagation on either side of the equator and eastward propagation into the equatorial west pacific from the equatorial west pacific northward propagation over the west pacific and westward propagation into the indian longitudes are prominent the propagative features are complex and interactive and are responsible for the quadrapole structure in convection seen from the composites the interannual variability assessed from the pop coefficient time series indicates that the 40 day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic tcz tropical convergence zone the pop analysis of the 15 day mode indicates that this event originates over the equatorial west pacific associated with westward propagating rossby waves amplifies over the northwest tropical pacific and modulates both the continental and oceanic tczs over indian longitudes simultaneously this mode is pronounced during the established phase of the monsoon due to the complexity in the propagational features of both the intraseasonal modes it is concluded that understanding the subseasonal variability of one regional component of the asian summer monsoon asm requires understanding the entire asm system

modulation of eastern north pacific hurricanes by the madden julian oscillation abstract hurricane and tropical storm statistics verify the modulation of eastern pacific tropical systems by the madden julian oscillation mjo as hypothesized by maloney and hartmann over twice as many named tropical systems hurricanes and tropical storms accompany equatorial 850 mb westerly anomalies than accompany equatorial easterly anomalies and the systems that do exist are stronger hurricanes are over four times more numerous during westerly phases of the mjo than during easterly phases the current study constructs a composite life cycle of the mjo during may november 1979 95 using an index based on the 850 mb equatorial zonal wind equatorial kelvin waves propagating eastward from convective regions of the western pacific ocean alter dynamical conditions over the eastern pacific ocean westerly easterly equatorial 850 mb wind anomalies are accompanied by enhanced suppressed convection over the eastern pacific hurricane region convection locally amplifies the wind anomalies over the ea

intraseasonal variability of the low level jet stream of the asian summer monsoon abstract the strong cross equatorial low level jet stream llj with its core around 850 hpa of the asian summer monsoon june september is found to have large intraseasonal variability during the monsoon onset over kerala india and during break monsoon periods when the convective heating of the atmosphere is over the low latitudes of the indian ocean the axis of the llj is oriented southeastward over the eastern arabian sea and it flows east between sri lanka and the equator and there is no llj through peninsular india this affects the transport of moisture produced over the indian ocean to peninsular india and the bay of bengal in contrast during active monsoon periods when there is an east west band of strong convective heating in the latitudes 10 20 n from about longitude 70 to about 120 e the llj axis passes from the central arabian sea eastward through peninsular india and it provides moisture for the increased convection in the bay of bengal and for the monsoon depressions forming there

delayed onset of the 2002 indian monsoon 1 we show that there is a set of dynamical predictors which facilitate forecasting of a delayed monsoon onset the main dynamical contributor is the early may propagation of the bogus onset intraseasonal oscillation which triggers a set of events precluding the climatological monsoon onset we analyze in detail the 2002 monsoon onset and show that it followed a pattern described in our previous study we notice that the 2003 monsoon onset followed very similar pattern and was delayed

observation of luzon strait transport using recently collected current and hydrographic data we provide a high resolution picture of the subinertial flow and estimate the volume transport through the luzon strait the distribution of the subinertial flow shows a strong westward flow around 100 m in the northern part of the luzon strait while the eastward flow is confined to the deeper layers mostly at depths around 1000 m the total volume transport is estimated to be 6 3 sv during the period of observations from october 4 to 16 2005 the observations also confirm that the luzon strait transport has a sandwiched vertical structure the net westward volume transport in the deep 1500 m layer of the luzon strait reaches 2 sv

flow asymmetry associated with astronomical tides implications for the residual transport of sediment 1 tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water generating compound tides and overtides the present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction but is also generated by astronomical tides in absence of shallow water effects in particular the k1 o1 and m2 constituents give an asymmetrical periodic flow pattern with the largest peak velocities persistently in the same direction a theoretical explanation for this phenomenon is presented and an inventory is made of other subordinate constituents that may contribute to repetitive asymmetric flow patterns tidal current asymmetry may induce residual transport of sediment analytical expressions are derived which quantify the residual transport of sediment due to the k1 o1 and m2 tides as a function of the phases and amplitudes of these constituents and the time lag between variation in suspended sediment transport and flow velocity the residual transport induced by the k1 o1 and m2 tides is compared to the residual transport due to the asymmetry associated with the m2 and m4 tides the former mechanism is particularly relevant in diurnal or mixed mainly diurnal tidal regimes where the k1 and o1 tides are dominant

observations of eastward propagation of atmospheric intraseasonal oscillations from the pacific to the atlantic received 19 april 2010 revised 20 september 2010 accepted 29 september 2010 published 19 january 2011 1 significant 40 60 day intraseasonal variability in surface winds sea level and thermocline depth were observed in the tropical atlantic during the atlantic nino year of 2002 satellite derived quikscat winds and noaa outgoing longwave radiation olr measurements for the period of 2000 2006 together with reanalysis of winds from a longer record are analyzed to understand the sources of this 40 60 day wind variability and the global propagation of the madden julian oscillation mjo surface signatures the results demonstrate that the mjo propagated eastward from the indo pacific ocean to the atlantic during winter and spring of 2002 causing 40 60 day wind variations in the equatorial atlantic the isthmus of panama appears to be a dominant pathway for these surface wind anomalies to propagate into the atlantic where they can produce important climate impacts this result is consistent with previous work that has demonstrated a strong mjo influence on the north american monsoon nam region and complements other work demonstrating the propagation of intraseasonal wind anomalies and accompanying sea level pressure signals associated with dry equatorial kelvin waves that reach the caribbean through the panama gap the mjo is also shown to have a large influence on the subtropical atlantic ocean seasonality of the mjo impact on atlantic surface winds is investigated using data from an extended record and the most direct influence is found to be during boreal winter and spring with a delayed influence during summer and fall

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

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

the boreal summer intraseasonal oscillation relationship between northward and eastward movement of convection the summertime intraseasonal oscillation iso is an important component of the south asian monsoon lagged regressions of intraseasonally filtered 25 80 days outgoing longwave radiation olr reveal that centers of convection move both northward and eastward from the central equatorial indian ocean subsequent to the initiation of an iso eastward movement of convection is also seen at indian subcontinent latitudes 10 8 208n based on the regression results the summertime iso convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 208n viewed along any meridian convection appears to propagate northward while equatorial convection propagates to the east to examine the robustness of the connection between eastward and northward movement individual isos are categorized and composited relative to the strength of the large scale eastward component of convection in the central equatorial indian ocean it is found that the majority of isos that exhibit northward movement onto the indian subcontinent 42 out of 54 isos or 78 also exhibit eastward movement into the western pacific ocean it is also found that when convection in the central indian ocean is not followed within 10 20 days by convection in the western pacific ocean 12 out of 54 isos or 22 the independent northward movement of convection in the indian ocean region is somewhat stunted the link between the eastward and northward movement of convection is consistent with an interpretation of the summertime iso in terms of propagating equatorial modes the northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the rossby cell that is excited by equatorial iso convection a similar convergence pattern is seen for the northern winter iso but it does not generate poleward movement due to relatively cool ssts underlying the surface convergence

transport and variability of the antarctic circumpolar current in drake passage the baroclinic transport of the antarctic circumpolar current acc above 3000 m through drake passage is 107 3 10 4 sv and has been steady between 1975 and 2000 for six hydrographic sections 1993 2000 along the world ocean circulation experiment woce line sr1b the baroclinic transport relative to the deepest common level is 136 7 7 8 sv the acc transport is carried in two jets the subantarctic front 53 10 sv and the polar front pf 57 5 5 7 sv southward of the acc the southern antarctic circumpolar current transports 9 3 2 4 sv we observe the pf at two latitudes separated by 90 km this bimodal distribution is related to changes in the circulation and properties of antarctic bottom water three realizations of the instantaneous velocity field were obtained with lowered adcps from these observations we obtain near bottom reference velocities for transport calculations net transport due to these reference velocities ranges from 28 to 43 sv consistent with previous estimates of variability the transport in density layers shows systematic variations due to seasonal heating in near surface layers volume transport weighted mean temperatures vary by 0 40 c from spring to summer a seasonal variation in heat flux of about 0 22 pw finally we review a series of papers from the international southern ocean studies program the average yearlong absolute transport is 134 sv and the standard deviation of the average is 11 2 sv the error of the average transport is 15 to 27 sv we emphasize that baroclinic variability is an important contribution to net variability in the acc

kelvin waves and tropical cyclogenesis a global survey abstractconvectively coupled atmospheric kelvin waves are among the most prominent sources of synoptic scale rainfall variability in the tropics but large uncertainties surround their role in tropical cyclogenesis this study identifies the modulation of tropical cyclones relative to the passage of a kelvin wave s peak rainfall i e its crest in each basin tropical cyclogenesis is generally inhibited for 3 days before the crest and enhanced for 3 days afterward composites of storms forming in the most favorable lags illustrate the dynamical impacts of the waves in most basins the tropical cyclone actually forms during the convectively suppressed phase of the wave the 850 hpa equatorial westerly anomalies provide the cyclonic vorticity for the nascent storm and 200 hpa easterly anomalies enhance the outflow the wind anomalies persist at both levels longer than the kelvin wave s period and are often related to the madden julian oscillation mjo the onset of these wind anomalies occurs with the

a model for the boreal summer intraseasonal oscillation abstract the tropical intraseasonal oscillation iso exhibits pronounced seasonality the boreal summer iso is more complex than its winter counterpart due to the coexistence of equatorial eastward off equatorial westward and northward propagating low frequency modes and their interactions based on observational evidence and results obtained from numerical experiments a mechanism is proposed for the boreal summer iso in which the northern hemisphere summer monsoon nhsm circulation and moist static energy distribution play essential roles with a climatological july mean basic state the life cycle of model low frequency waves consists of four processes an equatorial eastward propagation of a coupled kelvin rossby wave packet an emanation of moist rossby waves in the western pacific a westward propagation and amplification of the rossby waves in south asian monsoon regions and a reinitiation of the equatorial disturbances over the central indian ocean the life cycle spans about one month and p

influence of the madden julian oscillation and intraseasonal waves on surface wind and convection of the tropical atlantic ocean abstractatmospheric intraseasonal variability in the tropical atlantic is analyzed using satellite winds outgoing longwave radiation olr and reanalysis products during 2000 08 the analyses focus on assessing the effects of dominant intraseasonal atmospheric convective processes the madden julian oscillation mjo and rossby waves on surface wind and convection of the tropical atlantic ocean and african monsoon area the results show that contribution from each process varies in different regions in general the mjo events dominate the westward propagating rossby waves in affecting strong convection in the african monsoon region the rossby waves however have larger contributions to convection in the western atlantic ocean both the westward and eastward propagating signals contribute approximately equally in the central atlantic basin the effects of intraseasonal signals have evident seasonality both convection amplitude and the number of strong convective events associated with the mjo are l

primary and successive events in the madden julian oscillation conventional analyses of the mjo tend to produce a repeating cycle such that any particular feature cannot be unambiguously attributed to the current or previous event we take advantage of the sporadic nature of the mjo and classify each observed madden julian mj event as either primary with no immediately preceding mj event or successive which does immediately follow a preceding event 40 of mj events are primary events precursor features of the primary events can be unambiguously attributed to that event a suppressed convective anomaly grows and decays in situ over the indian ocean prior to the start of most primary mj events an associated mid tropospheric temperature anomaly destabilises the atmosphere leading to the generation of the active mj event hence primary mj events appear to be thermodynamically triggered by a previous dry period although stochastic forcing may also be important other theories predict that boundary layer convergence humidity propagation of dynamical structures around the equator sea surface temperatures and lateral forcing by extratropical transients may all be important in triggering an event although precursor signals from these mechanisms are diagnosed from reanalysis and satellite observational data in the successive mj events they are all absent in the primary mj events hence it appears that these apparent precursor signals are part of the mjo once it is established but do not play a role in the spontaneous generation of the mjo the most frequent starting location of the primary events is the indian ocean but over half of them start elsewhere from the maritime continent to the western pacific copyright 2008 royal meteorological society

structure genesis and scale selection of the tropical quasi biweekly mode the quasi biweekly mode qbm and the 30 60 day mode are two major intraseasonal oscillations isos in the tropics the qbm is known to have a major influence in determining the active and break conditions of the indian monsoon during the northern summer a westward propagating equatorial rossby wave with quasi biweekly period influences the australian monsoon during the northern winter universality between the summer and winter qbm is established through analysis of daily circulation and convection data for 10 years it is shown that the mean spatial structure of the qbm in circulation and convection resembles that of a gravest meridional mode equatorial rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4 5 m s 1 however the maximum zonal wind occurs at around 5 n 5 s during the northern summer winter the wave structure appears to be translated northward southward by about 5 during the northern summer winter the relationship between outgoing long wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons an acceptable mechanism for genesis and scale selection of the qbm has been lacking in the present study a mechanism for genesis and scale selection of the observed qbm is proposed a simple 2 layer model that includes a steady ekman boundary layer bl formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves without influence of the background mean flow moist feedback in the presence of frictional bl convergence drives the gravest meridional mode equatorial rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator potential temperature perturbation associated with the rossby wave is in phase with relative vorticity perturbation at low level the bl drives moisture convergence in phase with the relative vorticity at the top of the bl release of latent heat associated with the bl convergence enhances the potential temperature leading to a positive feedback the mean flow over the indian ocean and western pacific at low levels is such that the zero ambient absolute vorticity or the dynamic equator shifts to around 5 n 5 s during summer winter and results in a shift of the unstable rossby waves towards the north south by about 5 the resulting structure of the unstable rossby mode resembles the observed structure of the biweekly mode it is shown that neither evaporation wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode however these processes marginally modify the growth rate and make the structure of the unstable wave more realistic