a comparison of inner and outer spiral rainbands in a numerically simulated tropical cyclone abstractthe simulated inner and outer spiral rainbands in a tropical cyclone are compared in this study the inner rainbands are generally active immediately outside the eyewall in the rapid filamentation zone while the outer rainbands are active in regions outside about 3 times the radius of maximum wind the inner rainbands are characterized by the convectively coupled vortex rossby waves the movement of the outer rainbands follows the low level vector winds associated with the azimuthally averaged low level flow and the radially outward cross band flow caused by the downdraft induced cold pool in the boundary layer convective cells in outer rainbands are typical of convective systems and move cyclonically and radially outward inward at large small radii net upward vertical mass transports vmts appear throughout the depth of the troposphere in the whole inner rainband region while net downward vmts are found below 4 km height in the outer rainband region in the whole inner rainband region on

weak monsoon event at 4 2 ka recorded in sediment from lake rara himalayas abstract the strength of the indian summer monsoon varied significantly on centennial to millennial timescales during the holocene among these variations a weak summer monsoon event occurred approximately at 4 ka the so called 4 2 ka event this event is considered to have been a relatively sustained and widespread phenomenon at regional scales across southern asia to eastern africa however little is known about the impact of such variations in the himalayas a key location for understanding the monsoon system here we present a record of changes in summer monsoon strength during the holocene from lake rara western nepal using redox sensitive elements mn ti and mn fe measured by x ray fluorescence xrf core scanner in addition organic matter properties toc 13c and c n and changes in the 14c age offset between bulk organic matter and leaf matter namely the old carbon offset are used to augment the environmental reconstruction collectively the data indicate a period of prolonged weak monsoon interval at around 4 ka the timing of onset and recovery from the event observed at lake rara exhibits particular similarity with changes in the indus river discharge this is the first record of the 4 2 ka event from the himalayas based on a relatively well constructed age model using 14c dates that are not influenced by reservoir effects these findings provide an important step towards better understanding the spatial and temporal variability in the holocene indian monsoon

characteristics of atmospheric divergence and convergence in the indian ocean inferred from scatterometer winds abstract large scale surface atmospheric convergence and divergence patterns in the indian ocean are mapped using high spatial resolution merged scatterometer wind vectors during 1991 2000 the convergence zone evolves to north of 15 s as a result of convection promoted by warm 28 c equatorial sea surface temperature sst and it exhibits strong intensity during boreal summer and winter a divergence zone evolves to the south of 15 s as a result of subdued convection caused by colder sst

rapid filamentation zones in intense tropical cyclones intense tropical cyclones often possess relatively little convection around their cores in radar composites this surrounding region is usually echo free or contains light stratiform precipitation while subsidence is typically quite pronounced in this region it is not the only mechanism suppressing convection another possible mechanism leading to weak echo moats is presented in this paper the basic idea is that the strain dominated flow surrounding an intense vortex core creates an unfavorable environment for sustained deep moist convection strain dominated regions of a tropical cyclone can be distinguished from rotationdominated regions by the sign of s 2 s 2 2 where s1 ux y and s2 x uy are the rates of strain and x uy is the relative vorticity within the radius of maximum tangential wind the flow tends to be rotation dominated 2 s 2 s 2 so that coherent structures such as mesovortices can survive for long periods of time outside the radius of maximum tangential wind the flow tends to be strain dominated s 2 s 2 2 resulting in filaments of anomalous vorticity in the regions of strain dominated flow the filamentation time is defined as fil 2 s 2 s 2 2 1 2 in a tropical cyclone an approximately 30 kmwide annular region can exist just outside the radius of maximum tangential wind where fil is less than 30 min and even as small as 5 min this region is defined as the rapid filamentation zone since the time scale for deep moist convective overturning is approximately 30 min deep convection can be significantly distorted and even suppressed in the rapid filamentation zone a nondivergent barotropic model illustrates the effects of rapid filamentation zones in category 1 5 hurricanes and demonstrates the evolution of such zones during binary vortex interaction and mesovortex formation from a thin annular ring of enhanced vorticity

indian monsoon variability in the last 2000 years as inferred from benthic foraminifera abstract the effect of ambient conditions on living rose bengal stained benthic foraminifera has been studied from the shelf and slope of the central western bay of bengal to assess the applicability of temporal changes in benthic foraminiferal morpho groups angular asymmetrical and rounded symmetrical to reconstruct past monsoon changes we report that the riverine influx and associated processes control the relative abundance of angular asymmetrical aabf and rounded symmetrical rsbf benthic foraminiferal morpho groups in the central western bay of bengal subsequently temporal variations in relative abundance of aabf in a gravity core collected from outfall region of the pennar river are used to reconstruct decadal scale past monsoon variability during the last 1887 years an increased monsoon discharge is observed during the medieval warm period mwp and lower during the little ice age lia the periodic changes with a frequency of 123 and 238 years are also identified indicating a close relationship between monsoon and solar activity

objective convective updraft identification and tracking part 1 structure and thermodynamics of convection in the rainband regions of two hurricane simulations to better understand the life cycles of convective elements in numerical model simulations a convective updraft tracking algorithm dubbed the statistical and programmable objective updraft tracker spout is developed spout identifies updraft cores in horizontal slices through a given model data set links these cores in three dimensions to form vertically coherent updrafts and then tracks updrafts in time via estimates of the mean local advection spout is evaluated in idealized hurricane simulations from both the regional atmospheric modeling system rams and weather research and forecasting wrf in both simulations spout indicates most updrafts live less than 60 min but some can persist for over 2 h updrafts are typically manifested as shallow cumulus cumulus congestus and deep cumulonimbus overall there is reasonable agreement between the vigor of convection and the amount of localized conditional convective instability also the majority of updrafts attain a tilted structure that is in accordance with the localized vertical wind shear nonetheless there are striking differences between models with the higher resolution wrf simulation producing around twice as many updrafts as rams moreover the wrf updrafts are significantly weaker this paper highlights the utility of spout for studying convective life cycles in high resolution numerical simulations and provides some basic features of convection in two hurricane simulations

dynamical linkage of tropical and subtropical weather systems to the intraseasonal oscillations of the indian summer monsoon rainfall part i observations the indian summer monsoon rainfall ismr over the western ghats wg and the bay of bengal bob is marked by the intraseasonal oscillations isos with preferred 10 20 day and 30 50 day bands on the basis of pentad climate prediction center merged analysis precipitation and daily sea level pressure and winds at 850 hpa derived from european center for medium range weather forecast reanalysis we present the structure and evolution of the isos linked to the ismr variations over the wg and the bob and the associated anomalies of the atmospheric circulation using the approaches of wavelet analysis bandpass filtering and composite analysis this study reveals that the activities of both the intertropical convergence zone itcz and the western pacific subtropical high wpsh contribute strongly to the structure and propagation of the isos on intraseasonal time scales northward development and propagation of the itcz plays a critical role in the northward propagating isos but not in the westward propagating bob 10 20 day isos the latter isos may be linked instead to the activity of synoptic scale weather systems to the east over the western tropical pacific the enhanced itcz in the tropical indian ocean plays a strong role in the sudden strengthening of the wpsh during the transition from the break to active phase of the 30 50 day isos we find that the strong wpsh in the asian summer monsoon season with generally northward advance and eastward withdrawal promotes the formation of a northwest to southeast tilted anomalous rainfall belt over the east asian tropical summer monsoon region and the western tropical pacific in the 30 50 day low frequency band positive negative elongated rainfall anomalies with an unbroken northwest southeast tilt strong easterly westerly anomalies in the tropical pacific and northward advance and eastward movement of strong weak wpsh are favorable for maintaining the eastward propagation of the 30 50 day isos in the pacific daily high resolution sea surface temperature obtained from the national oceanic and atmospheric administration is used to explain the propagation features of the 10 20 day isos in the indian ocean

large scale control of the arabian sea monsoon inversion in august the summer monsoon inversion in the arabian sea is characterized by a large amount of low clouds and august as the peak season atmospheric stratification associated with the monsoon inversion has been considered a local system influenced by the advancement of the india pakistan monsoon empirical and numerical evidence from this study suggests that the arabian sea monsoon inversion is linked to a broader scale monsoon evolution across the african sahel south asia and east asia western north pacific wnp rather than being a mere byproduct of the india pakistan monsoon progression in august the upper tropospheric anticyclone in south asia extends sideways corresponding with the enhanced precipitation in the subtropical wnp equatorial indian ocean and african sahel while the middle part of this anticyclone weakens over the arabian sea the increased heating in the adjacent monsoon systems creates a suppression effect on the arabian sea suggesting an apparent competition among the africa asia wnp monsoon subsystems the peak sahel rainfall in august together with enhanced heating in the equatorial indian ocean produces a critical effect on strengthening the arabian sea thermal inversion by contrast the wnp monsoon onset which signifies the eastward expansion of the subtropical asian monsoon heating might play a secondary or opposite role in the arabian sea monsoon inversion

evidence for decreasing south asian summer monsoon in the past 160 years from varved sediment in lake xinluhai tibetan plateau 1 we report glacial varves in the sediment of lake xinluhai tibetan plateau independent data of 137cs and 210pb indicate that these are annually deposited varves varves appear as rhythmic units of light colored silt layer capped by a dark clay layer under microscope varve thickness in lake xinluhai is sensitive to precipitation because sediment accumulation is strongly affected by monsoon rainfall in the area a general decreasing trend can be observed in the varve thickness over the past 160 years spectral analyses of the varve record are dominated by cycles which are similar to enso periodicities it implies that the decreasing trend of the south asia monsoon may be linking with enso spatially the decreasing trend can be observed across different proxy records in the south of the tibetan plateau although arguments still remain for the dynamic mechanisms and spatial rainfall difference the south asian summer monsoon could be weakened due to rising temperatures

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

indian ocean monsoon coupled interactions and impending monsoon droughts 1 monsoon droughts over the indian subcontinent emanate from failures in the seasonal june september monsoon rains while prolonged dry spells monsoon breaks pervade on sub seasonal intra seasonal time scales the underlying causes for these long lasting anomalies remain elusive based on analyses of a suite of observed data sets we report an ocean atmosphere dynamical coupling on intra seasonal time scales in the tropical indian ocean which is pivotal in forcing extended monsoon breaks and causing droughts over the subcontinent this coupling involves a feedback between the monsoonal flow and thermocline depth in the equatorial eastern indian ocean eeio in which an anomaly of the summer monsoon circulation induces downwelling and maintains a higher than normal heat content the near equatorial anomalies induce strong and sustained suppression of monsoon rainfall over the subcontinent it is concluded that the intra seasonal evolution of the ocean monsoon coupled system is a vital key to unlocking the dynamics of monsoon droughts

a convective cell in a hurricane rainband abstract on 10 october 1983 the two noaa wp 3d aircraft completed a mission designed to provide airborne doppler radar data for a convective cell embedded in a weak rainband on the trailing side of hurricane raymond comparisons of the wind field produced from the pseudo dual doppler radar technique with in situ wind measurements suggest that the larger convective scale feature may be resolved if the sampling time is kept to a minimum the convective cell was found to move downband faster than any environmental winds but slightly slower than the winds found in the reflectivity core that delineates the cell in the core of the cell the tangential wind is increased and the radial inflow turns to outflow with respect to the circulation center the flow field demonstrates that the downband stratiform portion of a rainband is not from cells currently active since the updraft detrains upwind relative to the cell but rather it is due to the fallout from ice particles placed into the upper troposphere by clouds t

oscillations in the southern extent of the indo pacific warm pool during the mid holocene the indo pacific warm pool ipwp is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour however little is known about past changes in the size and position of the ipwp in this study we use a total of 48 modern and fossil coral records from the mentawai islands sumatra indonesia and muschu koil islands papua new guinea to reconstruct oscillations in the extent of the ipwp since the mid holocene we show that reliable estimates of mean sea surface temperature sst can be obtained from fossil corals by using low resolution sr ca analysis of a suite of corals to overcome the large uncertainties associated with mean sr ca sst estimates from individual coral colonies the coral records indicate that the southeastern and southwestern margins of the ipwp were cooler than at present between similar to 5500 and 4300 years bp similar to 1 2 degrees c 0 3 degrees c and were similarly cool before similar to 6800 years bp this mid holocene cooling was punctuated by an abrupt short lived shift to mean ssts that were warmer than at present between similar to 6600 and 6300 years bp similar to 1 3 degrees c 0 3 degrees c while similarly warm conditions may have also existed after similar to 4300 years bp we suggest that mid holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the ipwp associated with the more northerly position of the inter tropical convergence zone itcz that accompanied mid holocene strengthening of the asian summer monsoon conversely intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the itcz that caused the ipwp to expand beyond our coral sites intervals of a strengthened asian monsoon and cooling in the southwestern ipwp during the mid holocene appear to correspond with a more positive indian ocean dipole iod like mean configuration across the tropical indian ocean suggesting that the asian monsoon iod interaction that exists at interannual time scales also persists over centennial to millennial scales associated mean changes in the pacific enso modes may have also occurred during the mid holocene the dynamic and inter connected behaviour of the ipwp with tropical climate systems during the mid holocene highlights the fundamental importance of the warm pool region for understanding climate change throughout the tropics and beyond c 2009 elsevier ltd all rights reserved

rapid filamentation zone in a numerically simulated tropical cyclone abstract in a recent study rozoff et al proposed a possible mechanism to explain the formation and maintenance of the weak echo annulus or moat outside of the primary eyewall of a tropical cyclone observed in radar images by this mechanism the moat is determined to be a region of the strain dominated flow outside of the radius of maximum wind in which essentially all fields are filamented and deep convection is hypothesized to be highly distorted and even suppressed this strain dominated region is defined as the rapid filamentation zone wherein the filamentation time is shorter than the overturning time of deep convection an attempt has been made in this study to test the hypothesis in a full physics tropical cyclone model under idealized conditions and to extend the concept to the study of the inner core dynamics of tropical cyclones the foci of this paper are the evolution of the rapid filamentation zone during the storm intensification the potential roles of rapid filamentation in the organiz

low wavenumber structure and evolution of the hurricane inner core observed by airborne dual doppler radar abstract the asymmetric dynamics of the hurricane inner core region is examined through a novel analysis of high temporal resolution three dimensional wind fields derived from airborne dual doppler radar seven consecutive composites of hurricane olivia s 1994 wind field with 30 min time resolution depict a weakening storm undergoing substantial structural changes the symmetric and asymmetric mechanisms involved in this transformation are considered separately to zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of eliassen and lystad for a neutrally stratified atmosphere vertical shear however increased dramatically during the observation period leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry the role of asymmetric vorticity d

dynamical mechanisms for asymmetric ssta patterns associated with some indian ocean dipoles indian ocean dipole iod can be divided into two types according to their sst anomaly ssta patterns the symmetric iods and the asymmetric iods dynamical mechanisms for the two types of iods are investigated using an ocean general circulation model and the numerical experiments indicate that the setup time of the equatorial easterly wind anomalies eewa associated with iods is crucially important for the symmetrical characteristics of the ssta patterns early setup of the eewa in spring can induce intense cooling in the east pole but weak warming in the west pole making the two poles asymmetric the intense cooling in the east pole is linked to the seasonal variations of thermocline depth horizontal sst gradient and unusually long duration of the eewa and the weak warming in the west pole is due to unusually cold zonal advection resulted from zonal sst gradient change the numerical experiments demonstrate that the strength of the eewa also plays an important role in the symmetrical characteristics of iods the ssta in the two poles both intensify as the eewa strength increases only when the eewa speed is below a critical value above the critical value the warming in the west pole starts to decrease while the cooling in the east pole still keeps increasing leading to an asymmetric ssta pattern the mechanism behind this phenomenon is similar to the one in the situation when eewa sets up earlier

mechanisms for the generation of mesoscale vorticity features in tropical cyclone rainbands a high resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic anticyclonic mesovortices can be produced in outer bands the primary source of this vorticity is the upward tilting of system generated horizontal vorticity by diabatic heating gradients the vertical heating gradient in the stratiform cloud also creates a potential vorticity pv dipole that accelerates the tangential flow and develops a midlevel jet the strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband the fourier decomposition of the absolute vorticity field shows two counterpropagating vortex rossby waves associated with the rainband the wave located on the inner side of the band transports energy toward the vortex center the outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband the results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic pv across the freezing level which develops a midlevel jet this mechanism creates a vorticity gradient that enables the propagation of vortex rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the

the boundary layer dynamics of tropical cyclone rainbands abstractspiral bands are ubiquitous features in tropical cyclones and significantly affect boundary layer thermodynamics yet knowledge of their boundary layer dynamics is lacking prompted by rece

the indian summer monsoon drought of 2002 and its linkage with tropical convective activity over northwest pacific the indian subcontinent witnessed a severe monsoon drought in 2002 which largely resulted from a major rainfall deficiency in the month of july while moderate el nino conditions prevailed during this period the atmospheric convective activity was anomalously enhanced over northwest and north central pacific in the 10 20 n latitude belt and heavy rainfall occurred over this region in association with a series of northward moving tropical cyclones similar out of phase rainfall variations over the indian region and the northwest nw pacific have been observed during other instances of el nino southern oscillation enso the dynamical linkage corresponding to this out of phase rainfall variability is explored in this study by conducting a set of numerical experiments using an atmospheric general circulation model the results from the model simulations lend credence to the role of the tropical pacific sea surface temperature anomalies in forcing the out of phase precipitation variability over the nw pacific and the indian monsoon region it is seen that the enso induced circulation response reveals an anomalous pattern comprising of alternating highs and lows which extend meridionally from the equatorial region into the sub tropic and mid latitude regions of west central pacific this meridional pattern is associated with an anomalous cyclonic circulation over nw pacific which is found to favor enhanced tropical cyclonic activity and intensified convection over the region in turn the intensified convection over nw pacific induces subsidence and rainfall deficiency over the indian landmass through anomalous east west circulation in the 10 20 n latitude belt based on the present findings it is suggested that the convective activity over nw pacific is an important component in mediating the enso monsoon teleconnection dynamics

seychelles coral record of changes in sea surface temperature bimodality in the western indian ocean from the mid holocene to the present we report fossil coral records from the seychelles comprising individual time slices of 14 20 sclerochronological years between 2 and 6 2 kyr bp to reconstruct changes in the seasonal cycle of western indian ocean sea surface temperature sst compared to the present 1990 2003 these reconstructions allowed us to link changes in the sst bimodality to orbital changes which were causing a reorganization of the seasonal insolation pattern our results reveal the lowest seasonal sst range in the mid holocene 6 2 5 2 kyr bp and around 2 kyr bp while the highest range is observed around 4 6 kyr bp and between 1990 and 2003 the season of maximum temperature shifts from austral spring september to november to austral autumn march to may following changes in seasonal insolation over the past 6 kyr however the changes in sst bimodality do not linearly follow the insolation seasonality for example the 5 2 and 6 2 kyr bp corals show only subtle sst differences in austral spring and autumn we use paleoclimate simulations of a fully coupled atmosphere ocean general circulation model to compare with proxy data for the mid holocene around 6 kyr bp the model results show that in the mid holocene the austral winter and spring seasons in the western indian ocean were warmer while austral summer was cooler this is qualitatively consistent with the coral data from 6 2 to 5 2 kyr bp which shows a similar reduction in the seasonal amplitude compared to the present day however the pattern of the seasonal sst cycle in the model appears to follow the changes in insolation more directly than indicated by the corals our results highlight the importance of ocean atmosphere interactions for indian ocean sst seasonality throughout the holocene in order to understand holocene climate variability in the countries surrounding the indian ocean we need a much more comprehensive analysis of seasonally resolved archives from the tropical indian ocean insolation data alone only provides an incomplete picture

vortex rossby waves in a numerically simulated tropical cyclone part i overall structure potential vorticity and kinetic energy budgets abstract the asymmetric structure in the inner core of a numerically simulated tropical cyclone is analyzed in this study the simulated tropical cyclone is found to be highly asymmetric in the inner core in the mid lower troposphere the asymmetry in the core is dominated by azimuthal wavenumber 1 and wavenumber 2 vortex rossby waves these waves propagate azimuthally upwind against the azimuthal mean cyclonic tangential flow around the eyewall and thus have a much longer cyclonic rotation period by a factor of 2 than the period of a parcel moving with the cyclonic mean tangential flow around the circumference they also propagate outward against the boundary layer inflow of the azimuthal mean cyclone the waves are only visible within a radius of about 60 km from the cyclone center beyond this distance the radial gradient of potential vorticity pv of the azimuthal mean cyclone is too weak to support the vortex rossby waves although the divergent motion remains strong the geopotential height an

influence of indian ocean dipole on poleward propagation of boreal summer intraseasonal oscillations abstract the influence of the indian ocean dipole iod on the poleward propagation of boreal summer intraseasonal oscillations bsisos is examined using observed datasets this study finds that coherent incoherent poleward propagation of precipitation anomalies from 5 s to 25 n are observed during negative positive iod years disorganized poleward propagation of bsiso in the south equatorial indian ocean is observed during positive iod years the rationale behind such an anomaly in the poleward propagation of bsiso in contrasting iod years is identified based on the theory of northward propagating bsiso which suggests the influential role of air sea interaction on the genesis and propagation of bsiso it is found that the mean structure of moisture convergence and meridional specific humidity distribution undergoes radical changes in contrasting iod years which in turn influences the meridional propagation of bsiso this study assumes significance considering the critical role of bsiso in modulat

the structure and evolution of hurricane elena 1985 part ii convective asymmetries and evidence for vortex rossby waves a portable data recorder attached to the weather surveillance radar 1957 wsr 57 in apalachicola florida collected 313 radar scans of the reflectivity structure within 150 km of the center of hurricane elena in 1985 between 1310 and 2130 utc 1 september this high temporal and spatial 750 m resolution dataset was used to examine the evolution of the symmetric and asymmetric precipitation structure in elena as the storm rapidly strengthened and attained maximum intensity fourier decomposition of the reflectivity data into azimuthal wavenumbers revealed that the power in the symmetric wavenumber 0 component dominated the reflectivity pattern at all times and all radii by at least a factor of 2 the wavenumber 1 asymmetry accounted for less than 20 of the power in the reflectivity field on average and was found to be forced by the environmental vertical wind shear the small amplitude wavenumber 2 asymmetry in the core was associated with the appearance and rotation of an elliptical eyewall this structure was visible for nearly 2 h and was noted to rotate cyclonically at a speed equal to half of the local tangential wind outside of the eyewall individual peaks in the power in wavenumber 2 were associated with repeated instances of cyclonically rotating outward propagating inner spiral rainbands four separate convective bands were identified with an average azimuthal velocity of 25 m s 1 o r68 of the local tangential wind speed and an outward radial velocity of 5 2ms 1 the azimuthal propagation speeds of the elliptical eyewall and inner spiral rainbands were consistent with vortex rossby wave theory the elliptical eyewall and inner spiral rainbands were seen only in the 6 h period prior to peak intensity when rapid spinup of the vortex had produced an annular vorticity profile similar to those that have been shown to support barotropic instability the appearance of an elliptical eyewall was consistent with the breakdown of eyewall vorticity into mesovortices asymmetric mixing between the eye and eyewall and a slowing of the intensification rate the inner spiral rainbands might have arisen from high eyewall vorticity ejected from the core during the mixing process alternatively because the bands were noted to emanate from the vertical shear forced deep convection in the northern eyewall they could have formed through the axisymmetrization of the asymmetric diabatically generated eyewall vorticity

effect of the east asian westerly jet s intensity on summer rainfall in the yangtze river valley and its mechanism abstractmany studies have shown that the northward southward displacement of the east asian westerly jet eawj drastically reduces increases summer rainfall in the yangtze river valley yrv however the effect of the jet s intensity on interannual variation in summer rainfall has not been systematically studied the present study investigates the effect of the eawj s intensity on this interannual variation and analyzes the mechanism by which this process occurs in early summer the eawj consists of two branches one located over northern continental east asia western branch eawjwb and one extending from southern china to the northern pacific eastern branch eawjeb the former merges into the latter over the yellow sea a stronger eawjeb leads to increased rainfall in the yrv while the eawjwb does not significantly affect rainfall in the yrv the faster eawjeb directly strengthens midtropospheric warm advection over the yrv because the corresponding changes in the meridional wind and horizo

boundary layer structure and dynamics in outer hurricane rainbands part i mesoscale rainfall and kinematic structure abstract results of hurricane boundary layer experiments conducted in outer rainbands of hurricanes josephine 1984 and earl 1986 are presented comparisons of precipitation and kinematic structures in these storms and in hurricane floyd 1981 indicate that principal rainbands have common characteristic mesoscale and convective scale features in the boundary layer the two dimensional mesoscale structure suggests that these rainbands are made up of a linear aggregate of cellular reflectivity elements on the inner upshear side of the band and stratiform rain on the outer downshear side the bands are oriented perpendicular to the shear above the boundary layer and cells move downband at about 85 of the density weighted mean wind speed of the 0 2 6 km layer the boundary layer wind field is strongly influenced by the rainband with alongband and crossband wind maxima located on the outer side of the band axis and minima 4 8 km to the inner side maximum crossband convergence and cyclonic shear vor

a wide rainband in a developing tropical cyclone abstract on 18 january 1987 the three aircraft from the equatorial mesoscale experiment emex completed a mission designed to identify the mesoscale reflectivity kinematic and thermodynamic structure of a convectively active rainband on the leading edge of developing tropical cyclone irma the reflectivity structure determined from land based and aircraft radars reveals that the band consisted of an unusually wide 80 km region of convective clouds that did not maintain a linear organization high e air entered from both sides in the subcloud and lower cloud layer thermodynamically the rainband was benign in the sense that it did not produce cold downdrafts or large regions of low e air that would limit the energy of the low level inflow to the developing eyewall region kinematically the band was a preferred location for the enhancement of the wind component parallel to the band the low level flow was along rather than across the major axis of the band above 4 5 km flow was from the inner to

generation of moving spiral bands in tropical cyclones abstract a theory based on the shallow water equation is developed to explain the generation of moving spiral banded structures in tropical cyclones the theory proposes that fluctuation of the vorticity distribution in the compact core region can act as a source generating gravity waves that produce the banded structures this mechanism is usually called the lighthill radiation mechanism a numerical experiment using the fifth pennsylvania state university national center for atmospheric research mesoscale model mm5 code is used to test this theory the moving bands in the simulated tropical cyclone show good agreement with the theoretically calculated gravity waves that emerge from a core region with an elliptical distribution of vorticity results of the simulated cyclone also illustrate that spiral cloud bands are triggered by the propagating gravity waves

mesoscale and convective structure of a hurricane rainband abstract the mesoscale thermodynamic kinematic and radar structure of a hurricane floyd rainband observed on 7 september 1981 is presented data are from 26 aircraft passes through the rainband from 150 to 6400 m a composite technique which presents rainband structure as a function of distance from the storm circulation center reveals inflow from the outer edge of the band and a partial barrier to this flow below 3 km in the direction parallel to rainband orientation radar reveals cellular reflectivity structure on the upwind and central portions of the rainband the frequency of cellular precipitation decreases in favor of stratiform precipitation further downwind as the band spirals gradually towards the eyewall in the radial direction a decrease of 12 k in e is observed across the rainband in the subcloud layer convective scale up and downdrafts that are associated with cellular reflectivity structure are hypothesized to be responsible for the thermodynamic modification of the cloud and subc

indian ocean response to anomalous conditions in 2006 1 the equatorial indian ocean eio exhibited anomalous conditions characteristic of an indian ocean dipole iod during 2006 the eastern eio had cold sea surface temperature anomalies ssta lower sea level shallow thermocline and higher chlorophyll than normal the anomalies in the east restricted to the south of the equator were highest off sumatra the western pole of the iod was marked by warm ssta and deeper thermocline with maxima on either side of the equator an ocean general circulation model of the indian ocean forced by quikscat winds reproduces the iod of 2006 remarkably well the switch over to cooling in the east and warming in the west happened during may and july respectively in the east airsea heat flux initiated cold ssta in the model which were sustained later by oceanic processes in the west surface heat fluxes and horizontal advection caused warm ssta and contribution by the latter decreased after august citation vinayachandran p n j kurian and c p neema 2007 indian ocean response to anomalous conditions in 2006 geophys res lett 34 l15602 doi 10 1029 2007gl030194

extratropical anticyclonic rossby wave breaking and indian summer monsoon failure interactions between midlatitude disturbances and the monsoonal circulation are significant for the indian summer monsoon ism rainfall this paper presents examples of monsoon midlatitude linkage through anticyclonic rossby wave breaking rwb over west asia during june july and august of the years 1998 2010 rwb events over west asia are identified by the inversion of the potential vorticity air mass at three different isentropic levels 340 350 and 360 k using daily ncep ncar reanalysis it is observed that rwb took place over west asia before during breaks in the ism further these events occur on the anticyclonic shear side of the subtropical jet where the gradient of the zonal wind is found to be high rwb is responsible for the southward movement of high potential vorticity air from the westerly jet leading to the formation of a blocking high over the arabian region in turn this blocking high advects and causes the descent of upper tropospheric cold and dry air towards central india such an air mass with low moist static energy inhibits deep monsoonal convection and thereby leads to a dry spell in fact we find that rwb induced blocking over west asia to be one of the major causes of dry spell break episodes in ism additionally the presence of cold air over central india reduces the north south thermal contrast over the monsoon region thereby modifying the local hadley circulation over the region

kinematic structure of convective scale elements in the rainbands of hurricanes katrina and rita 2005 1 airborne doppler radar data collected during the hurricane rainband and intensity change experiment rainex show the convective scale air motions embedded in the principal rainbands of hurricanes katrina and rita these embedded convective cells have overturning updrafts and low level downdrafts originating at 2 4 km that enter the rainband on its radially outward side and cross over each other within the rainband as well as a strong downdraft emanating from upper levels 6 km on the radially inward side these vertical motion structures repeat from one convective cell to another along each rainband the resulting net vertical mass transport is upward in the upwind portion of the band and greatest in the middle sector of the principal rainband where the updraft motions contribute generally to an increase of potential vorticity below the 3 4 km level because the convective cells in the middle sector are systematically located radially just inside the secondary horizontal wind maximum shwm the local increase in vorticity implied by the convective mass transport is manifest locally as an increase in the strength of the shwm at midlevels 4 km the overturning updrafts of the convective cells tilt stretch and vertically transport vorticity such that the convergence of the vertical flux of vorticity strengthens the vorticity anomaly associated with the shwm this process could strengthen the shwm by several meters per second per hour and may explain how high wave number convective scale features can influence a low wave number feature such as the principal rainband and subsequently influence the primary vortex

on the weakening relationship between the indian monsoon and enso analysis of the 140 year historical record suggests that the inverse relationship between the el nino southern oscillation enso and the indian summer monsoon weak monsoon arising from warm enso event has broken down in recent decades two possible reasons emerge from the analyses a southeastward shift in the walker circulation anomalies associated with enso events may lead to a reduced subsidence over the indian region thus favoring normal monsoon conditions additionally increased surface temperatures over eurasia in winter and spring which are a part of the midlatitude continental warming trend may favor the enhanced land ocean thermal gradient conducive to a strong monsoon these observations raise the possibility that the eurasian warming in recent decades helps to sustain the monsoon rainfall at a normal level despite strong enso events

extremes of the indian summer monsoon rainfall enso and equatorial indian ocean oscillation it is well known that anomalies of the indian summer monsoon rainfall ismr are linked to el nino and southern oscillation enso we show that large anomalies of the ismr are also linked to the equatorial indian ocean oscillation equinoo between states with enhancement suppression of atmospheric convection over the western part of the equatorial indian ocean with suppression enhancement over the eastern part and associated changes in the anomaly of the zonal wind along the equator equinoo is the atmospheric component of the coupled indian ocean dipole mode there is a strong relation between the large anomalies of ismr and a composite index which is a linear combination of the indices for enso and equinoo with all seasons with large deficits excess characterized by small large values of the index however the variation of ismr within one standard deviation is more complex and does not appear to be related to the composite index

impact of indian ocean dipole and el nino southern oscillation wind forcing on the wyrtki jets 1 interannual variability of the wyrtki jets is studied in the context of indian ocean dipole iod and el nino and southern oscillation enso wind forcing using a three dimensional numerical ocean model and observations the boreal fall october november wyrtki jet is more significantly affected than the boreal spring april may wyrtki jet since both the iod and enso tend to peak toward the end of the calendar year various statistical methods are used in an attempt to separate the impacts of the iod and enso on these jets with emphasis on the fall jet the first two modes of an empirical orthogonal function eof decomposition account for about 90 and 85 of variability in zonal currents and wind stress respectively along the equator in the indian ocean but eof analysis does not cleanly separate out iod and enso forcing and response partial correlation analysis reveals that iod wind forcing and zonal equatorial current response are stronger on average than for enso and extend further west across the basin composite analysis of iod only enso only and combined iod and enso years provides a complementary definition of the relative contributions of these two phenomena on wyrtki jet variability and in general is consistent with the results of the partial correlation analysis

the role of intraseasonal variability in the nature of asian monsoon precipitation abstract the structure of the mean precipitation of the south asian monsoon is spatially complex embedded in a broad precipitation maximum extending eastward from 70 e to the northwest tropical pacific ocean are strong local maxima to the west of the western ghats mountain range of india in cambodia extending into the eastern china sea and over the eastern tropical indian ocean and the bay of bengal bob where the strongest large scale global maximum in precipitation is located in general the maximum precipitation occurs over the oceans and not over the land regions distinct temporal variability also exists with time scales ranging from days to decades neither the spatial nor temporal variability of the monsoon can be explained simply as the response to the cross equatorial pressure gradient force between the continental regions of asia and the oceans of the southern hemisphere as suggested in classical descriptions of the monsoon monthly 1979 2005 and daily 1997 present rainfall estimates

seasonal persistence and propagation of intraseasonal patterns over the indian monsoon region the space time evolution of convection over the monsoon region containing the indian subcontinent the indian ocean and the west pacific has been studied a multi channel singular spectrum analysis of the daily outgoing longwave radiation has yielded two intraseasonal oscillatory patterns and two large scale standing patterns as the most dominant modes of intraseasonal variability the oscillatory modes vary on time scales of about 45 and 28 days and their average cycles of variability are shown to correspond to the life cycles of active and break periods of monsoon rainfall over india during an active break cycle a convection dry anomaly zone first appears in the equatorial indian ocean subsequently expands to cover the indian subcontinent and finally contracts to disappear in the northern part of india some eastward and northward movements are found to be associated with both oscillatory modes while westward movement may also be associated with the 28 day mode the oscillatory modes are shown to have a large spatial scale extending to the west pacific one of the standing modes has anomalies of uniform sign covering the entire region and is related to el nino and southern oscillation enso pattern the other standing mode has a dipole structure in the equatorial indian ocean associated with large scale anomalies over india with the same sign as those over the western part of the dipole these two standing modes persist throughout the monsoon season each maintaining its respective pattern the seasonal mean monsoon is mainly determined by the two standing patterns without much contribution from the oscillatory modes the relative role of the standing patterns enso mode and dipole mode seems to be important in determining the seasonal mean during certain years

spiral rainbands in a numerical simulation of hurricane bill 2009 part i structures and comparisons to observations abstractthis study examines spiral rainbands in a numerical simulation of hurricane bill 2009 this paper the first part of the study evaluates the structures of spiral rainbands and compares them to previous observations four types of spiral rainbands are identified principal secondary distant and inner rainbands principal rainbands tend to be stationary relative to the storm center while secondary rainbands are more transient and move around the storm center both principal and secondary rainbands are tilted radially outward with height and have many of the commonly observed kinematic features such as overturning secondary circulation and enhanced tangential velocity on their radially outward sides principal rainbands are bounded by very dry air on their radially outward sides distant rainbands are radially inward tilting convective features that have dense cold pools near the surface inner rainbands are made of shallow convection that appears to have originated from near the eyewall dif

impact of the atlantic multidecadal oscillation on the asian summer monsoon 1 the impact of the atlantic multidecadal oscillation amo on the asian summer monsoon is investigated using a coupled atmosphere ocean global general circulation model by imposing the amo associated sea surface temperature anomalies in the atlantic as boundary forcing and allowing atmosphere ocean interactions outside the atlantic the positive amo phase characterized by anomalous warm north atlantic and cold south atlantic leads to strong southeast and east asian summer monsoons and late withdrawal of the indian summer monsoon these changes of monsoons are mainly through coupled atmosphere ocean feedbacks in the western pacific and indian oceans and tropospheric temperature changes over eurasia in response to the imposed forcing in the atlantic the results are in agreement with the observed climate changes in china corresponded to the amo phases they suggest a non local mechanism for the asian summer monsoon variability and provide an alternative view to understanding its interdecadal variation during the twentieth century citation lu r b dong and h ding 2006 impact of the atlantic multidecadal oscillation on the asian summer monsoon geophys res lett 33 l24701 doi 10 1029 2006gl027655

the organization of convection in the rainbands of tropical cyclone laurence abstract a slow moving weak tropical cyclone passed near darwin australia in december 1990 rainbands were observed by a doppler weather radar and a 50 mhz wind profiler for over 24 h the principal bands were seen to be organized on two distinct scales bands of stratiform precipitation formed at a radius of about 100 km from the center of the storm and moved outward at about 6 m s 1 these decayed after they moved past darwin over land a distinct midlevel jet extended along the bands within the bands convective lines formed at regular intervals propagated against and outward with respect to the mean flow and acted as a partial barrier to the radial inflow deep active convection was confined to these lines the vertical motion in the convection showed a distinct acceleration above the freezing level with measured updrafts of up to 10 m s 1 the convection elevated the tropopause height over the rainband it is hypothesized that an inertia gravity wave propagating from near the storm eye was resp

a glimpse of the quaternary monsoon history from india and adjoining seas abstract the evolution of the monsoon is briefly summarized taking into account previous studies conducted on both terrestrial and marine records recovered from the indian subcontinent and the adjoining seas while the initiation of the monsoon is debated it seems clear that a major intensification of the summer monsoon occurred at 8 2 ma a seasonal monsoon circulation with distinct summer and winter monsoon phases was established at 2 8 ma in the late quaternary the summer monsoon weakens during glacial periods as compared to interglacial periods the centennial to sub centennial scale changes in the monsoon during both glacial and interglacial periods suggest strong links between high latitude processes and the tropical monsoon and its role in modulating northern hemispheric climate the glacial terminations are marked by weak monsoon activity the summer monsoon weakened during the middle holocene after the early holocene optimum a change in the monsoon intensity though alters the salinity of the seas surrounding india the quantitative paleosalinity reconstructions still have large associated uncertainties and an effort should be made to lower them

tropical cyclogenesis via convectively forced vortex rossby waves in a shallow water primitive equation model abstract this work examines further the problem of tropical cyclogenesis by convective generation of vertical vorticity within a preexisting cyclonic circulation whose initial maximum tangential wind is approximately 5 m s 1 this paper validates and extends recent work examining the suggested upscale cascade mechanism in a three dimensional quasigeostrophic framework using a simple shallow water primitive equation swpe numerical model and helps clarify certain aspects of the rossby adjustment problem on a nonresting basic state for finite amplitude nonaxisymmetric disturbances the swpe approach serves as a meaningful intermediate step between the quasigeostrophic and full physics frameworks and allows a simple investigation of the effects of unbalanced dynamics contributions of gravity waves and rossby numbers of order unity the authors compare quantitative results of the two models on the storm spinup time and magnitude for asymmetric initial conditions whose mass and wind field are out of balanc

extreme monsoon rainfall signatures preserved in the invasive terrestrial gastropod lissachatina fulica indian summer monsoon ism rainfall lasts for a period of 4 months with large variations recorded in terms of rainfall intensity during its period between june and september proxy reconstructions of past ism rainfall variability are required due to the paucity of long instrumental records however reconstructing subseasonal rainfall is extremely difficult using conventional hydroclimate proxies due to inadequate sample resolution here we demonstrate the utility of the stable oxygen isotope composition of gastropod shells in reconstructing past rainfall on subseasonal timescales we present a comparative isotopic study on present day rainwater and stable isotope ratios of precipitate found in the incremental growth bands of giant african land snail lissachatina fulica bowdich from modern day 2009 and in the historical past 1918 isotopic signatures present in the growth bands allowed for the identification of ism rainfall variability in terms of its active and dry spells in the modern as well as past gastropod record our results demonstrate the utility of gastropod growth band stable isotope ratios in semiquantitative reconstructions of seasonal rainfall patterns high resolution climate records extracted from gastropod growth band stable isotopes museum and archived specimens can expand the scope for understanding past subseasonal to seasonal climate variability plain language summary we provide a new way to reconstruct sub annual rainfall variability using isotope measurements on shells of the invasive giant african land snails in india we show that these records can preserve signatures of monsoon rainfall extremes we utilize museum samples of the snails to reconstruct rainfall during 1918

forcing of recent decadal variability in the equatorial and north indian ocean recent decadal sea surface height ssh variability across the equatorial and north indian ocean enio north of 5 s is spatially coherent and related to a reversal in basin scale upper ocean temperature trends analysis of ocean and forcing fields from a data assimilating ocean synthesis eccov4 suggests that two equally important mechanisms of wind driven heat redistribution within the indian ocean account for a majority of the decadal variability the first is the cross equatorial cell cec forced by zonal wind stress curl at the equator the wind stress curl variability relates to the strength and position of the mascarene high which is influenced by the phase of the indian ocean subtropical dipole the second mechanism is deep 700 m upwelling related to zonal wind stress at the equator that causes deep cross equatorial overturning due to the unique geometry of the basin the cec acts to cool the upper ocean throughout most of the first decade of satellite altimetry while the deep upwelling delays and then amplifies the effect of the cec on ssh during the subsequent decade reversals in the forcing anomalies drive warming of the upper ocean and increasing ssh with the effect of the deep upwelling leading the cec this article is protected by copyright all rights reserved

secondary wind maxima in hurricanes airflow and relationship to rainbands abstract aircraft flight level data from 787 radial legs in 20 hurricanes are analyzed to identify the composite kinematic structure in the hurricane eyewall and especially with secondary horizontal wind maxima shwm that occur outside the eyewall similar to previous studies analysis of the flight level wind data in the eyewall reveals radial convergence near the radius of maximum wind rmw and the highest frequency of updraft and the largest upward mass transport radially inward of the rmw more than 20 of the flight legs contain substantial secondary horizontal wind maxima of specified strength and length the kinematic structure associated with shwm is similar to that of the hurricane eyewall with radial convergence near the radius of maximum wind and a preferred location for maximum upward motions and upward mass transport just inside the rmw statistical analysis confirms the similarity in characteristics between radial and vertical velocities of the eyewall and near the shwm in addition for

precessional forced evolution of the indian ocean dipole in a transient accelerated simulation of a coupled climate model we identified a zonal dipole like pattern of sea surface temperature sst anomalies in the tropical indian ocean which is forced by precessional insolation changes since 300 ka and named as the paleo iod indian ocean dipole a positive paleo iod mean state at 23 kyr precessional band exhibits warmer and wetter conditions over the western indian ocean and cooler and drier conditions over the eastern tropical indian ocean from august to october this zonal thermal seesaw at the sea surface can extend downward to the subsurface ocean between 60 and 80 m and accompanies stronger oceanic upwelling in the eastern tropical indian ocean the associated boreal summer autumn tropospheric circulation anomalies are characterized by anomalous ascent over the western indian ocean and anomalous descent over the southeastern tropical indian ocean with anomalous easterlies at the surface along the equatorial indian ocean the positive paleo iod largely originates from local air sea interactions that are induced by the increased summer insolation and is also contributed by the reduced boreal winter insolation through an oceanic heat memory effect our simulated dipole mode index dmi of sst is qualitatively consistent with the paleoceanographic reconstructed dmi based on the uk37 proxy of sst at precessional band and provides a possible explanation for the in phase precessional variation between boreal winter insolation and the uk37 proxy of sst in the eastern tropical indian ocean

a multiscale numerical study of hurricane andrew 1992 part i explicit simulation and verification abstract in this study the inner core structures of hurricane andrew 1992 are explicitly simulated using an improved version of the penn state ncar nonhydrostatic two way interactive movable triply nested grid mesoscale model mm5 a modified betts miller cumulus parameterization scheme and an explicit microphysics scheme were used simultaneously to simulate the evolution of the larger scale flows over the coarser mesh domains the intense storm itself is explicitly resolved over the finest mesh domain using a grid size of 6 km and an explicit microphysics package containing prognostic equations for cloud water ice rainwater snow and graupel the model is initialized with the national centers for environmental prediction analysis enhanced by a modified moisture field a model generated tropical storm like vortex was also incorporated a 72 h integration was made which covers the stages from the storm s initial deepening to a near category 5 hurricane intensity and the landfall over florida as

high resolution climatic monsoonal variability reconstructed from a continuous 2700 year sediment record from northwest himalaya ladakh a chronologically well constrained sedimentary archive from upshi ladakh was studied using a multi proxy approach namely palynology environmental magnetism total organic carbon toc total nit

inter annual relationship between atlantic sea surface temperature anomalies and indian summer monsoon in this study the simultaneous inter annual relationships between sst anomalies over the northwest atlantic ocean and southwest monsoon rainfall over the monsoon core region have been examined using monthly sst and atmospheric data for the period 1951 2005 statistical analyses reveal significant inter annual simultaneous relationship between the sst anomalies over north atlantic and rainfall over the monsoon core region but with significant epochal variations the relationship has become stronger after mid 1970s when the el nino indian monsoon relationship has weakened positive sst anomalies over the north atlantic ocean shift the north atlantic jet northwards and the associated circulation changes in the upper troposphere influence indian monsoon through the circumglobal teleconnection across central asia the present study thus highlights the important role of north atlantic ocean as an important source of inter annual variability of the indian summer monsoon

earthquakes near rocky mountain house alberta and their relationship to gas production facilities a 1980 field survey of earthquake activity near rocky mountain house alberta using one digital and six conventional seismographs recorded 146 microearthquakes m 3 4 in 23 days and located 67 of them the located events and probably the unlocated events as well all occurred in a small zone approximately 4 km north south by 4 km east west by 1 km thick centred near 52 12 5 n 115 15 w at a depth of 4 0 km with an uncertainty of 2 km on each of the location coordinates the activity occurred below and or in the strachan d 3a sour gas reservoir which is a devonian aged limestone reef complex in a section of nearly flat lying unfaulted sediments the earthquakes had a composite thrust faulting mechanism with generally north trending intermediate dipping planes in the presence of a regional deviatoric stress field that was horizontally compressive and oriented approximately east west the earthquake activity may be related to the extraction of the natural gas and related fluids from the reservo

equatorial indian ocean subsurface current variability in an ocean general circulation model the variability of subsurface currents in the equatorial indian ocean is studied using high resolution ocean general circulation model ogcm simulations during 1958 2009 february march eastward equatorial subsurface current esc shows weak variability whereas strong variability is observed in northern summer and fall esc an eastward subsurface current with maximum amplitude in the pycnocline is prominent right from summer to winter during strong indian ocean dipole iod years when air sea coupling is significant on the other hand during weak iod years both the air sea coupling and the esc are weak this strongly suggests the role of esc on the strength of iod the extension of the esc to the summer months during the strong iod years strengthens the oceanic response and supports intensification and maintenance of iods through modulation of air sea coupling although the esc is triggered by equatorial winds the coupled air sea interaction associated with iods strengthens the esc to persist for several seasons thereby establishing a positive feedback cycle with the surface this suggests that the esc plays a significant role in the coupled processes associated with the evolution and intensification of iod events by cooling the eastern basin and strengthening thermocline sst sea surface temperature interaction as the impact of iod events on indian summer monsoon is significant only during strong iod years understanding and monitoring the evolution of esc during these years is important for summer monsoon forecasting purposes there is a westward phase propagation of anomalous subsurface currents which persists for a year during strong iod years whereas such persistence or phase propagation is not seen during weak iod years supporting the close association between esc and strength of air sea coupling during strong iod years in this study we report the processes which strengthen the iod events and the air sea coupling associated with iod it also unravels the connection between equatorial indian ocean circulation and evolution and strengthening of iod

relation between indian monsoon variability and sst abstract the relation between the intraseasonal modes of the south asian monsoon and the sea surface temperature sst in the tropical oceans on a daily time scale has been investigated long lead lag relations of the daily sst anomalies with the dominant monsoon modes obtained from a multichannel singular spectrum analysis mssa of the daily outgoing longwave radiation olr over the south asian monsoon region are presented the dominant mssa monsoon modes consisting of two oscillatory modes at 45 and 28 day time scales and two seasonally persistent modes are found to have varying degrees of lead lag relation with the ssts in the indian and pacific oceans while the 45 day oscillatory mode has weak correlations with the ssts in the pacific and indian ocean it also reveals a possible 45 day oscillation in the sst in the northwestern pacific and northern indian ocean the 28 day oscillatory mode has negligible correlation with the tropical sst one of the persistent monsoon modes has a very strong r

vortex rossby waves in a numerically simulated tropical cyclone part ii the role in tropical cyclone structure and intensity changes abstract in part i the author analyzed the asymmetric structure in the inner core of a numerically simulated tropical cyclone and found that the asymmetry near the eyewall in the mid lower troposphere is dominated by wavenumber 1 and 2 vortex rossby waves these waves are found to be well coupled with asymmetries in eyewall convection and thus may play an important role in the life cycle of a tropical cyclone in this paper analyses are extended to include the role of these vortex rossby waves in tropical cyclone structure and intensity changes the waves are found to transport angular momentum from the eyewall to the eye accelerating tangential winds in the eye at the expense of decelerating the tangential wind in the eyewall and thus they play an important role in the inner core dynamics of the tropical cyclone convection in the eyewall is enhanced between the downstream trough and upstream ridge in the vortex rossby waves but suppressed between the downstream ridge and upstream trough this close

intraseasonal convection and air sea fluxes over the indian monsoon region revealed from the bimodal iso index the present study aims to elucidate the intraseasonal oscillations iso of atmospheric convection and air sea fluxes over the indian region during summer monsoon season to accomplish this the study employs the extended empirical orthogonal function based bimodal iso index developed by kikuchi et al clim dyn 38 9 10 1989 2000 2012 the propagation of deep convective anomalies and air sea fluxes are explored during the different phases p1 p8 of the bimodal index this is achieved by examining the tropical rainfall measuring mission satellite rainfall outgoing long wave radiation olr sea surface temperatures sst downward shortwave radiation dsr and reanalysis products of 850 hpa winds potential vorticity and latent heat fluxes lhfs composite analysis of the anomalies depicts the strong weak northward eastward propagation of convective anomalies over the indian region equatorial indian ocean over the indian region active suppressed or weak convection is evident during the phases of p4 and p5 p1 p2 and p7 p8 enhanced deep convection is lead by a phase of 850 hpa westerly winds and negative sst anomalies signatures of iso during different phases are examined from research moored array for african asian australian monson analysis and prediction rama buoy observations over the bay of bengal rainfall sst and lhf anomalies from rama buoy measurements are in concurrence with the spatial composites of bimodal iso phases plausible drivers for the variability of intraseasonal convection and air sea fluxes were reviewed using published observational and modelling studies findings from the present study advocate the applicability of kikuchi bimodal index clim dyn 38 9 10 1989 2000 2012 over the indian region and have practical application for the validation of ocean atmospheric coupled models

evolution of indian ocean dipole and its forcing mechanisms in the absence of enso the evolution of indian ocean dipole iod and its forcing mechanisms are examined based on the analysis of coupled model simulations that allow or suppress the el nino southern oscillation enso mode of variability the model can reproduce the most salient observed features of iod even without enso including the relationships between the eastern and western poles at both the surface and subsurface as well as their seasonality this suggests that enso is not fundamental for the existence of iod it is demonstrated that cold warm sea surface temperature sst anomalies in the eastern indian ocean associated with iod can be initiated by springtime indonesian rainfall deficit surplus through local surface wind response the growth of the sst anomalies depends on the initial local subsurface condition both the air sea interaction and surface subsurface interaction contribute to the development of iod the evolution of iod can be represented by two leading extended empirical orthogonal function eeof modes of tropical surface subsurface indian ocean temperatures one stationary and the other non stationary the onset growth and termination of iod as well as the transition to an opposite phase can be interpreted as alternations between the non propagating mode eeof1 and the eastward propagating kelvin wave eeof2 the evolution of iod is also accompanied by a westward propagating rossby wave which is captured in the eeof1 of the subtropical surface subsurface indian ocean temperatures therefore both bjerknes feedback and a delayed oscillator operate during the evolution of iod in the absence of enso also

the axisymmetric and asymmetric aspects of the secondary eyewall formation in a numerically simulated tropical cyclone under idealized conditions on an f plane abstractthe axisymmetric and asymmetric aspects of the secondary eyewall formation sef in a numerically simulated tropical cyclone tc under idealized conditions were analyzed consistent with p

evaluation of interannual simulations and indian ocean dipole events during 2000 2014 from a basin scale general circulation model interannual simulations performed using an eddy permitting ocean general circulation model ogcm for years 2000 2014 were validated against buoy measurements over the indian ocean io model simulated fields were evaluated extensively using multiple statistical metrics to quantify the quality of model in reproducing variability in oceanic surface and subsurface features the model simulated sea surface temperature sst at different moored buoy locations exhibits high close to 0 9 correlation coefficient r with the ranges of standard deviation sd of simulated sst consistent with the corresponding buoy observations the root mean square difference rmsd estimated between the buoy and simulated sst was found to be less than 0 6 c at most of the buoy locations the model simulated subsurface temperature profile including the thermocline resembled good agreement with the buoy profiles intraseasonal and interannual variability of 20 c isotherm d20 was simulated reasonably well as observed at the respective buoy locations mean error in surface currents was low however the meridional component from model simulations showed a better agreement rmsd 0 25 m s with the observations as compared to zonal components 0 4 m s for the periods of buoy data availability the dipole mode index dmi derived from simulated sst reproduces the positive negative indian ocean dipole iod events that occurred during the simulation period interannual variability in temperature currents and oceanic mixed layer depth was analyzed in response to iod events the anomalies in equatorial currents were found to affect the strength of coastal currents along the indian coastlines model simulations showed the enhanced suppressed coastal upwelling process along the sumatra coast that leads to anomalous cooling warming off the sumatra coast during the positive negative iod events

an air sea interaction theory for tropical cyclones part ii evolutionary study using a nonhydrostatic axisymmetric numerical model abstract in part i of this study an analytical model for a steady state tropical cyclone is constructed on the assumption that boundary layer air parcels are conditionally neutral to displacements along the angular momentum surfaces of the hurricane vortex the reversible thermodynamics implied by this assumption allows the mature storm to be thought of as a simple carnot engine acquiring heat at the high temperature ocean surface and losing heat near the low temperature tropopause although the oceanic heat source is universally recognized as the sine qua non for the mature hurricane there is also wide acceptance of conditional instability of the second kind cisk which makes no specific reference to surface heat fluxes as the formative mechanism this ambivalence is seen in that all numerical simulation studies find it essential to have transfer from the ocean surface yet all start from a conditionally unstable atmosphere the hypothesis put forward in part i based on the steady state theory is t

hurricane spiral bands abstract the spiral bands that occur in tropical cyclones can be conveniently divided into two classes outer bands and inner bands evidence is presented here that the outer bands form as the result of nonlinear effects during the breakdown of the intertropical convergence zone itcz through barotropic instability in this process a zonal strip of high potential vorticity the itcz shear zone or monsoon trough begins to distort in a varicose fashion with the potential vorticity pv becoming pooled in local regions that are connected by filaments of high pv as the pooled regions become more axisymmetric the filaments become thinner and begin to wrap around the pv centers it is argued that inner bands form in a different manner as a tropical cyclone intensifies due to latent heat release the pv field becomes nearly circular with the highest values of pv in the cyclone center the radial gradient of pv provides a state on which pv waves the generalization of rossby waves can propagate the nonline