texture analysis of sar sea ice imagery using gray level co occurrence matrices this paper presents a preliminary study for mapping sea ice patterns texture with 100 m ers 1 synthetic aperture radar sar imagery the authors used gray level co occurrence matrices glcm to quantitatively evaluate textural parameters and representations and to determine which parameter values and representations are best for mapping sea ice texture they conducted experiments on the quantization levels of the image and the displacement and orientation values of the glcm by examining the effects textural descriptors such as entropy have in the representation of different sea ice textures they showed that a complete gray level representation of the image is not necessary for texture mapping an eight level quantization representation is undesirable for textural representation and the displacement factor in texture measurements is more important than orientation in addition they developed three glcm implementations and evaluated them by a supervised bayesian classifier on sea ice textural contexts this experiment concludes that the best glcm implementation in representing sea ice texture is one that utilizes a range of displacement values such that both microtextures and macrotextures of sea ice can be adequately captured these findings define the quantization displacement and orientation values that are the best for sar sea ice texture analysis using glcm

comparison of ice water classification in fram strait from c and l band sar imagery in this paper an algorithm for ice water classification of c a nd l band dual polarization synthetic aperture radar data is presented a comparison of the two different frequencies is made in order to investigate the potential to improve classification results with multi frequency data the algorithm is based on backscatter intensities in co a nd cross polarization and autocorrelation as a texture feature the mapping between image features and ice water classification is made with a neural network accurate ice water maps for both frequencies are produced by the algorithm and the results of two frequencies generally agree very well differences are found in the marginal ice zone where the time difference between acquisitions causes motion of the ice pack c band reliably reproduces the outline of the ice edge while l band has its strengths for thin ice calm water areas within the icepack the classification shows good agreement with ice water maps derived from met no ice charts and radiometer data from amsr 2 variations are found in the marginal ice zone where the generalization of the ice charts and lower accuracy of ice concentration from radiometer data introduce deviations usage of high resolution dual frequency data could be beneficial for improving ice cover information for navigation and modelling

baltic sea ice sar segmentation and classification using modified pulse coupled neural networks a method for segmentation and classification of baltic sea ice synthetic aperture radar sar images based on pulse coupled neural networks pcnns is presented also automated training which is based on decomposing the total pixel value distribution into a mixture of class distributions is presented and discussed the algorithms have been trained and tested using logarithmic scale radarsat 1 scansar wide mode images over the baltic sea ice before the decomposition into mixture of class distributions an incidence angle correction specifically designed for these baltic sea ice sar images is applied because the data distributions in the uniform areas of these images are very close to gaussian distributions the data are decomposed into a mixture of gaussian distributions using the expectation maximazation algorithm only uniform image areas are used in the decomposition phase the mixture of distributions is compared to the distributions of the baltic sea ice classes based on earlier scatterometer measurements and visual video interpretations of the sea ice classes the parameter values for the pcnn segmentation are defined based on this mixture of distributions the pcnn segmentation results are also compared to the operational sea ice information of digitized ice charts and to visual interpretation of the sea ice class

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

barents sea seasonal ice zone features and processes from ers 1 synthetic aperture radar seasonal ice zone experiment 1992 sea ice features and processes in the barents sea were studied during the seasonal ice zone experiment 1992 sizex92 a dedicated ers 1 satellite synthetic aperture radar sar field campaign carried out in march 1992 sizex92 was based around the research vessels polarsyssel and hakon mosby in situ oceanographic meteorological and ice measurements were made coordinated with low altitude aerial observations and ers 1 sar data acquired in near real time fifty eight low and full resolution sar scenes were obtained during sizex92 which provided a validation data set for ers 1 sar backscatter under winter conditions in the barents sea analysis of sizex92 data has provided geophysical insights including a better understanding of barents sea ice edge freezing processes and ice edge development in response to atmospheric forcings in particular areas of new ice formation were found to be related to bathymetric features through their influence on the circulation of arctic and atlantic water masses ers 1 sar image sequences revealed rapid mesoscale variations in new ice areas and the ice edge in response to wind conditions in addition to geophysical insight sizex92 demonstrated some of the technical capabilities and limitations of ers 1 sar to identify new ice areas the ice edge ice floes and ice types mapping new ice areas using satellite sar data may be considered the most promising new application the limitations of ers 1 sar derived ice classification and ice motion estimates in the marginal ice zone are identified

mapping of different sea ice regimes using images from sentinel 1 and alos synthetic aperture radar airborne c and l band synthetic aperture radar sar images were acquired for three test sites over different sea ice regimes around svalbard in march 2007 complemented by optical imagery environmental data and ice observations one objective was to use the high resolution low noise radar data for investigations on the technical performance of european space agency s sentinel 1 mission for sea ice mapping the other to assess the potential gain of additional use of l band sar data currently available from advanced land observing satellite alos the airborne sar images were employed to simulate data products resembling the interferometric wide swath mode iwsm of sentinel 1 and alos palsar fine resolution mode frm in order to quantify the information loss due to the higher noise level and coarser spatial resolution at the iwsm noise level zones of deformation level ice and new ice can be well separated at like polarization at cross polarization the noise level is too high for a robust automated classification including thin ice but deformed and level ice can be discriminated palsar frm is on average better suited to distinguish deformed and level ice over all ice regimes it was worse for separating new from thicker level ice at two test sites for the third test site with very few patches of new ice the palsar frm showed larger intensity contrasts between new and level ice than sentinel 1 iwsm the spatial resolution provided by the iwsm and frm is sufficient to identify most ice features and types without ambiguities typical characteristics of the imaged ice regimes are described

major storm periods and climate forcing in the western mediterranean during the late holocene abstract big storm events represent a major risk for populations and infrastructures settled on coastal lowlands in the western mediterranean where human societies colonized and occupied the coastal areas since the ancient times the variability of storm activity for the past three millennia was investigated with a multi proxy sedimentological and geochemical study from a lagoonal sequence mappings of the geochemistry and magnetic susceptibility of detrital sources in the watershed of the lagoon and from the coastal barriers were undertaken in order to track the terrestrial or coastal marine origin of sediments deposited into the lagoon the multi proxy analysis shows that coarser material low magnetic susceptibility and high strontium content characterize the sedimentological signature of the paleostorm levels identified in the lagoonal sequence a comparison with north atlantic and western mediterranean paleoclimate proxies shows that the phases of high storm activity occurred during cold periods suggesting a climatically controlled mechanism for the occurrence of these storm periods besides an in phase storm activity pattern is found between the western mediterranean and northern europe spectral analyses performed on the sr content revealed a new 270 year solar driven pattern of storm cyclicity for the last 3000 years this 270 year cycle defines a succession of ten major storm periods sp with a mean duration of 96 54 yr periods of higher storm activity are recorded from 680 to 560 cal yr bc sp10 end of the iron age cold period from 140 to 820 cal yr ad sp7 to sp5 with a climax of storminess between 400 and 800 cal yr ad dark ages cold period and from 1230 to 1800 cal yr ad sp3 to sp1 little ice age periods of low storm activity occurred from 560 cal yr bc to 140 cal yr ad sp9 and sp8 roman warm period and from 820 to 1230 cal yr ad sp4 medieval warm period

classification of sea ice types in envisat synthetic aperture radar images in this paper sea ice in the central arctic has been classified in synthetic aperture radar sar images from envisat using a neural network nn based algorithm and a bayesian algorithm since different sea ice types can have similar backscattering coefficients at c band hh polarization it is necessary to use textural features in addition to the backscattering coefficients the analysis revealed that the most informative texture features for the classification of multiyear ice myi deformed first year ice fyi dfyi and level fyi lfyi and open water nilas are correlation inertia cluster prominence energy homogeneity and entropy as well as third and fourth central statistical moments of image brightness the optimal topology of the nn trained for envisat wide swath sar sea ice classification consists of nine neurons in input layer six neurons in hidden layer and three neurons in output layer the classification results for a series of 20 sar images acquired in the central part of the arctic ocean during winter months were compared to expert analysis of the images and ice charts the results of the nn classification show that the average correspondences with the expert analysis amount to 85 83 and 80 for lfyi dfyi and myi respectively the bayesian pixel based method can provide a higher resolution in the classified image and therefore better capability to identify leads compared to the nn method both methods may be effectively used in the central arctic where myi is predominant

glacial and tidal strain of landfast sea ice in terra nova bay east antarctica observed by interferometric sar techniques abstract the dynamics of landfast sea ice also called fast ice for short has a large influence on the variability of polynyas and marine ecosystems and the logistics for research stations near the antarctic coast therefore it is important to accurately measure the strain of fast ice and its seasonal variations and to identify the cause of stresses on the ice in this paper we separate the strains from glacial stress and tidal stress of fast ice near the campbell glacier tongue cgt in terra nova bay east antarctica this was done using observations from a series of one day tandem cosmo skymed interferometric synthetic aperture radar insar images obtained from december 2010 to january 2012 firstly we discriminated fast ice from pack ice and open water by analyzing the interferometric coherence values we then identified the characteristics of the strains by investigating the equi displacement lines of fringes in weekly insar and double differential insar ddinsar images the weekly insar images predominantly showed glacial shear strain of the fast ice with fringes parallel to the sides of the cgt this was due to the cumulative flow of the cgt for a week while oscillating tidal signals were relatively small the ddinsar images which cancelled glacial strain rates in two one day insar images showed a deformation of the fast ice by tidal sea surface tilt with the fringes parallel to the coastline based on the unique characteristics of these strains we separated them from the one day insar images by decomposing the fringe patterns into glacial and tidal strain glacial shear strain rates of fast ice attached to the east of the cgt decreased from may to august owing to ice thickening and then stabilized until december those to the west of the cgt increased from may to july this was possibly due to bottom melting of the ice by the increased ocean circulation during the expansion period of the nearby polynya the glacial strain then decreased until december because of reduced polynya activity the fast ice near the jang bogo station jbs only showed tidal strain as it was isolated from the cgt by cracks and leads tidal strain rates of the fast ice were strongly correlated with the magnitude of tidal variations in all these regions which represents shows that the tidal strain represents tidal sea surface tilt the tidal response of fast ice to the west of the cgt and near the jbs was stronger than that to the east of the cgt probably owing to thinner ice thickness there

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

method for detection of leads from sentinel 1 sar images the university of bremen the german excellence initiative deutsche forschungsgemeinschaft dfg

investigating enso and wpwp modulated typhoon variability in the south china sea during the mid late holocene using sedimentological evidence from southeastern hainan island china abstract the link between tropical cyclone tc activity in the south china sea scs and global climate change is commonly debated and there is a clear need for long term geological records of tc activity if we are to clarify this connection multi millennial high resolution paleo storm records from the scs are rare in the region and this causes difficulties for those exploring potential climate drivers of tc variability over centennial to millennial timescales and reduces our ability to fully assess the risks associated with future tc activity this paper presents an age constrained mud dominated sedimentary sequence from a coastal lagoon on hainan island china multiproxy analyses incorporating chronological lithological sedimentological and geochemical evidence were used to infer storm deposits preserved within the sequence and to reconstruct a time series of storm activity in the scs during the mid to late holocene this long sedimentary record shows that tcs were highly active over the periods 5500 to 3500 and 1700 to 0 cal yrs bp and these periods contrast with a relatively quiet period 3500 1700 cal yrs bp an apparent inverse correlation between tc reconstructions from the scs and those from the korean peninsula and japan implies an oscillating pattern across the scs and western north pacific wnp over centennial to millennial timescales a comparison between the sedimentary and paleoclimatic records implies that the el nino southern oscillation was not the only mechanism responsible for typhoon variability over the past 7500 cal yrs bp suggesting that other factors such as the thermal state of the western pacific warm pool likely also had a strong influence on both scs and wnp tc variability

paleoenvironmental and paleobiogeographic implications of a new ediacaran body fossil from the neoproterozoic carolina terrane stanly county north carolina new partial swartpuntiid from neoproterozoic age rocks of stanly county north carolina herein assigned to cf swartpuntia sp along with previously described pteridinium carolinaensis aspidella other metazoan trace fossils and previously reported radiometeric ages suggest nama assemblage type affinities for the carolina terrane the new specimen cf swartpuntia sp and associated radiometric ages add further information pertaining to proposed geographic linkages between the carolina terrane and the avalon terrane this new swartpuntiid may be evolutionarily convergent in terms of habitat preference to the infaunal or partially infaunal habitat preferred by other members of the nama type ediacaran biota such as pteridinium and rangea

depositional evidence for the kamikaze typhoons and links to changes in typhoon climatology in the late 13 th century kublai khan ruler of the mongol empire launched one of the world s largest armadas of its time in an attempt to conquer japan early narratives described the decimation and dispersal of these fleets by the kamikaze of 1274 ce and 1281 ce a pair of intense typhoons divinely sent to protect japan from invasion these historical accounts are prone to exaggeration and significant questions remain regarding the occurrence and true intensity of these legendary typhoons to provide independent insight we present a new 2000 yr sedimentary reconstruction of extreme coastal flooding from a coastal lake near the location of the mongol invasions two marine sourced flood deposits date to the kamikaze typhoons and are the events of record in the reconstruction the complete reconstruction indicates periods of greater flood activity relative to modern beginning ca 250 ce and extending past the timing of the kamikaze events to 1600 ce comparisons with additional reconstructions are consistent with greater regional typhoon activity during the mongol invasions due to the preferential steering of storms toward japan and driven by greater el nino activity relative to modern results are consistent with the paired kamikaze typhoons being of significant intensity and support accounts of them playing an important role in preventing the conquering of japan by mongol fleets the kamikaze typhoons may therefore serve as a prominent example for how past increases in severe weather associated with changing climate have had significant geopolitical impacts

exploring typhoon variability over the mid to late holocene evidence of extreme coastal flooding from kamikoshiki japan sediment cores from two coastal lakes located on the island of kamikoshiki in southwestern japan lake namakoike and lake kaiike provide evidence for the response of a backbarrier beach system to episodic coastal inundation over the last 6400 years sub bottom seismic surveys exhibit acoustically laminated parallel to subparallel seismic reflectors intermittently truncated by erosional unconformities sediment cores collected from targeted depocenters in both lakes contain finely laminated organic mud interbedded with coarse grained units with depths of coarse deposits concurrent with prominent seismic reflectors the timing of the youngest deposit at kamikoshiki correlates to the most recently documented breach in the barrier during a typhoon in 1951 ad assuming that this modern deposit provides an analog for identifying past events paleo typhoons may be reconstructed from layers exhibiting an increase in grain size a break in fine scale stratigraphy and elevated sr concentrations

can we extend local sea ice measurements to satellite scale an example from the n ice2015 expedition knowledge of arctic sea ice conditions is of great interest for arctic residents as well as for commercial usage and to study the effects of climate change information gained from analysis of satellite data contributes to this understanding in the course of using in situ data in combination with remotely sensed data the question of how representative local scale measurements are of a wider region may arise we compare in situ total sea ice thickness measurements from the norwegian young sea ice expedition in the area north of svalbard with airborne derived total sea ice thickness from electromagnetic soundings a segmented and classified synthetic aperture radar sar quad pol alos 2 palsar 2 satellite scene was grouped into three simplified ice classes the area fractions of the three classes are 11 2 thin 74 4 level and 14 4 deformed the area fractions of the simplified classes from ground and helicopter based measurements are comparable with those achieved from the sar data thus this study shows that there is potential for a stepwise upscaling from in situ to airborne to satellite data which allow us to assess whether in situ data collected are representative of a wider region as observed by satellites

shell and gravel layers caused by storm induced rip currents during the medieval warm period and little ice age in south korea abstract five shell gravel layers 2 2 2 9 m in elevation 70 cm in thickness formed by storm induced rip currents along the southern coast of the korean peninsula over the past 1300 years are described the three radiocarbon dates s1 s3 and the two optically stimulated luminescence s4 and s5 dates correspond well with the succession of the five shell gravel layers the age of s1 ce 720 60 corresponds to the early medieval warm period mwp the dates of the three overlying layers s2 s4 ce 880 110 950 70 and 995 120 respectively coincide with the middle mwp the fifth date of the uppermost shell gravel layer s5 ce 1535 40 coincides with the little ice age lia of western europe the characteristics of shell habitats based on 16 shell species from each layer suggest that the first four layers formed in a slightly erosional to dominantly depositional environment during the mwp whereas the youngest layer formed in a dominantly erosional environment during the lia analyses of grain size distribution magnetic susceptibility and geochemistry of the sediments and gravel morphology indicate that storm activity at this site during the mwp and lia occurred within active storm periods in the western north atlantic wna comparison of the five storm events and related climatic factors suggests that although the typhoon track type at the site is similar to recurving tracks typical for el nino conditions in north china and japan tracks during the mwp and lia were likely shifted not only by changes in el nino conditions but by other climatic factors such as sea surface temperature the north pacific high and the westerly jet stream however storm energy under el nino years during the middle mwp was likely stronger than that produced in normal years during the early mwp

holocene wetland environmental change based on major element concentrations and organic contents from the cheollipo coast korea abstract organic and inorganic geochemical and mineralogical analyses from core cl4 drilled in the cheollipo wetland were carried out to investigate the holocene climatic optimum the formation of pyrite and the humidity of the korean western coast during the holocene time lithostratigraphy mineralogy major element concentrations and organic contents of the sediments of the cl4 borehole core indicate that the holocene climatic optimum occurred from about 7400 to 4500 bp and that pyrite formed in brackish water tidal flat or salt marsh at least during 7400 6400 bp in addition sediments coarser in mean grain size and larger in standard deviation corresponded with periods of 8500 5400 5000 and 1700 bp during the periods the coarse sediments seem to have been delivered by high energy storm events the periods partially overlap with those of high ratios of si al na al and na ti these phenomena indicate that the weakly weathered terrigenous sediments were delivered to the cheollipo borehole core site by storm events

asynchronous holocene climatic change across china a review of holocene climatic variations in different parts of china shows that they were asynchronous proxy data from ice cores pollen loess lacustrine sediments and changes of sea and lake levels demonstrate that many warm and cold oscillations have occurred in china during the holocene including a most important climatic event known as the holocene optimum a milder and wetter period and that the duration and amplitude of the optimum period as well as its start and end times differed in different parts of china uplift of the tibetan plateau over the past millions of years led to the development of the monsoon climate and to complex atmospheric circulation over continental china during the holocene as a result the holocene optimum began and terminated earlier in high altitude regions of western china than at lower elevations in eastern china and the amplitude of the variations was lower in the east this suggests that the western higher altitude areas were more sensitive to climatic change than were the eastern lower altitude areas holocene climatic records in the dunde and guliya ice cores do not correspond inverse 18o variations between the two cores indicate that the effects of climate and atmospheric processes on the stable isotopes at the two sites differed the correlation between the isotopic composition of carbonates in lake deposits in western china and climatic variations is similar to that in the ice cores the climatic resolution in ice cores and lake sediments is higher than that in other media the lack of precise correspondence of climatic records constructed on the basis of proxy data from different parts of china is a result of the different locations and elevations of the sampling sites the different resolutions of the source material and the varied climatic conditions within china further work is needed to confirm both the conclusions and the inferences presented here

a case for two late proterozoic earliest cambrian faunal province loci comparison of the geographic ranges of certain benthic members of the ediacaran fauna and the earliest shelly metazoan assemblages suggests that the benthic ediacaran fauna and the first shelly faunas inhabited different parts of the late proterozoic world although ediacaran cnidarians seem to have been cosmopolitan the benthic ediacaran fauna appears to have been restricted to proto gondwana baltica and gondwana and remained isolated from proto laurasia siberia china and north america where shelly metazoans first appeared late proterozoic rifting events and continental motions perhaps corresponding to fragmentation of a proterozoic supercontinent provide a mechanism for isolation of proto gondwana and proto laurasia and resulting benthic metazoan provincialization

sea ice type maps from alaska synthetic aperture radar facility imagery an assessment synthetic aperture radar sar imagery received at the alaska sar facility is routinely and automatically classified on the geophysical processor system gps to create ice type maps we evaluated the wintertime performance of the gps classification algorithm by comparing ice type percentages from supervised classification with percentages from the algorithm the rms difference for multiyear ice is about 6 while the inconsistency in supervised classification is about 3 the algorithm separates first year from multiyear ice well although it sometimes fails to correctly classify new ice and open water owing to the wide distribution of backscatter for these classes our results imply a high degree of accuracy and consistency in the growing archive of multiyear and first year ice distribution maps these results have implications for heat and mass balance studies which are furthered by the ability to accurately characterize ice type distributions over a large part of the arctic

x c and l band sar signatures of newly formed sea ice in arctic leads during winter and spring abstract we examine an extensive synthetic aperture radar sar data set from the arctic ocean spanning a time period from january to june 2015 with the aim of identifying multi polarization parameters that can be used to accurately separate newly formed sea ice from the surroundings newly formed sea ice areas both provide favourable routing for ship traffic and are key to arctic climate science because they enable heat exchange between the ocean and the atmosphere our data set encompasses three different frequencies x c and l band at a range of incidence angles and were acquired under different environmental conditions our results suggest that by combining the scattering entropy and the co polarization ratio we can successfully separate the newly formed sea ice from open water and thicker sea ice within all three frequencies throughout the winter and spring season we observe a high correlation between scattering entropy values calculated using quad polarization c and l band data and scattering entropy values calculated using the same scenes reduced to the co polarization channels hh and vv we therefore conclude that dual polarization hh and vv x band scenes can be directly used to complement quad polarimetric c and l band scenes for studies of newly formed sea ice to confine the quad polarimetric data sets to their co polarization channels one can ensure a higher signal to noise ratio incidence angles below 35 are needed to keep the signal to noise ratios sufficiently high for the scattering entropy and co polarization ratio due to its lack of incidence angle dependency the polarization difference can provide additional support in newly formed sea ice studies the regular coverage of the arctic ocean with c band sar means that such scenes should to be included in any automatic monitoring however x and l band sar can based on their difference in penetration depth provide additional information about newly formed sea ice types and surface structure

magnitude and frequency of extra tropical north atlantic cyclones a chronology from cliff top storm deposits cliff top storm deposits ctsds are known to occur on cliffs at elevations of up to 50 m above sea level at exposed sites on the deep water coasts of the british isles eye witness accounts and time series field mapping of ctsds demonstrate their formation over the historical period and their continued modification during major storms with ctsds burying a range of modern human derived debris older deposits show surface lichen growth and weathering effects that relate to the relative age of ctsds over longer time spans buried sand and peat deposits provide dates on phases of ctsd emplacement and mobilisation and so provide an indication of the frequency and timing of the most severe storms two adjacent sites in shetland provide a combination of 14c dates on peat buried beneath storm deposited boulders 14c dates on marine shells buried inside ctsd boulder ridges and optically stimulated luminescence osl ages on sands intercalated with storm derived boulders although the precision is low this evidence identifies phases of exceptional storm activity spanning approximately 400 550 700 1050 1300 1900 and since 1950 ad by the very nature of cliff top erosion and deposition these records are fragmentary yet the timeline of major storms recorded in the shetland cliffs matches fairly well with the more general record of storminess in the north atlantic derived from the sea salt record in the gisp2 ice core as such ctsds represent an overlooked archive of storm sedimentation with the potential to elucidate local storm chronologies as well as informing debate surrounding the palaeo tempestology and impact of major cyclones and hurricanes

laboratory measurements of sea ice connections to microwave remote sensing the connections between laboratory measurements and remote sensing observations of sea ice are explored the focus of this paper is on thin ice which is more easily simulated in a laboratory environment the authors summarize results of c band scatterometer measurements and discuss how they may help in the interpretation of remote sensing data they compare the measurements with observations of thin ice from ers and airborne radar data sets they suggest that laboratory backscatter signatures should serve as bounds on the interpretation of remote sensing data they examine these bounds from the perspective of thin ice signatures the effect of temperature and surface processes such as frost flowers and slush on these signatures controlled experiments also suggest new directions in remote sensing measurements the potential of polarimetric radar measurements in the retrieval of thickness of thin ice is discussed in addition to the radar results the authors discuss the importance of low frequency passive measurements with respect to the thickness of thin ice

a tsunami about 1000 years ago in puget sound washington water surged from puget sound sometime between 1000 and 1100 years ago overrunning tidal marshes and mantling them with centimeters of sand one overrun site is 10 kilometers northwest of downtown seattle another is on whidbey island some 30 kilometers farther north neither site has been widely mantled with sand at any other time in the past 2000 years deposition of the sand coincided to the year or less with abrupt probably tectonic subsidence at the seattle site and with landsliding into nearby lake washington these findings show that a tsunami was generated in puget sound and they tend to confirm that a large shallow earthquake occurred in the seattle area about 1000 years ago

7000 years of paleostorm activity in the nw mediterranean sea in response to holocene climate events abstract a high resolution record of paleostorm events along the french mediterranean coast over the past 7000 years was established from a lagoonal sediment core in the gulf of lions integrating grain size faunal analysis clay mineralogy and geochemistry data with a chronology derived from radiocarbon dating we recorded seven periods of increased storm activity at 6300 6100 5650 5400 4400 4050 3650 3200 2800 2600 1950 1400 and 400 50 cal yr bp in the little ice age in contrast our results show that the medieval climate anomaly 1150 650 cal yr bp was characterised by low storm activity the evidence for high storm activity in the nw mediterranean sea is in agreement with the changes in coastal hydrodynamics observed over the eastern north atlantic and seems to correspond to holocene cooling in the north atlantic periods of low ssts there may have led to a stronger meridional temperature gradient and a southward migration of the westerlies we hypothesise that the increase in storm activity during holocene cold events over the north atlantic and mediterranean regions was probably due to an increase in the thermal gradient that led to an enhanced lower tropospheric baroclinicity over a large central atlantic european domain

backscatter characteristics of the winter ice cover in the beaufort sea the microwave backscatter statistics of the sea ice cover in the winter beaufort sea are characterized using c band synthetic aperture radar sar data collected by the european earth resources satellite 1 sea ice backscatter signatures were sampled from sar image data collected during the winter of 1991 1992 and winter of 1992 1993 the spatial and temporal variabilities of the backscatter signatures of different ice types are discussed the results show considerable seasonal stability of the backscatter signature of multiyear ice as well as of first year ice small amplitude regional variations of the multiyear ice backscatter can be observed consistent contrast between multiyear ice and first year ice is maintained throughout the season the highest radiometric variability is observed in sea ice in leads on the basis of these observations the backscatter from the principal ice types multiyear and first year are consistent with scatterometer observations and can be easily identified under winter conditions correlations of the regional variations of multiyear ice signatures to physical processes are suggested

incidence angle dependence of hh polarized c and l band wintertime backscatter over arctic sea ice synthetic aperture radar sar incidence angle has a significant effect on the microwave backscatter from sea ice this paper investigates the incidence angle dependence of c and l band hh polarized microwave backscatter coefficient over arctic first year sea ice fyi and multiyear sea ice myi in winter advanced land observation satellite phased array type l band sar l band and radarsat 2 c band images are used to derive ice type specific incidence angle dependencies calculated using linear regression models for l band mean ice type specific incidence angle dependencies for fyi and myi are 0 21 and 0 30 db 1 respectively and for c band they are 0 22 and 0 16 db 1 respectively to validate our results we calculated root mean square deviation rmsd by comparing the ice type specific dependence from 2010 with individual dependencies from 2009 based on ice types and frequencies the rmsd is found to be smaller than the standard deviation of ice type specific dependencies for both frequencies the rmsd values for the l band incidence angle dependencies are 0 03 and 0 04 db 1 for fyi and myi respectively for c band the rmsd values for the fyi and myi dependencies are 0 03 and 0 01 db 1 respectively subsequently we demonstrate that after applying incidence angle normalization the variability of c and l band sar backscatter reduces and separability of ice types increase substantially

object based landfast sea ice detection over west antarctica using time series alos palsar data abstract landfast sea ice fast ice is an important feature prevalent around the antarctic coast which is affected by climate change and energy exchanges with the atmosphere and ocean this study proposed a method for detection of the west antarctic fast ice using the advanced land observing satellite phased array l band sar alos palsar images the algorithm has combined image segmentation image correlation analysis and machine learning techniques i e random forest rf extremely randomized trees ert and logistic regression lr we used sar images with a baseline of 5 days that are not in the same orbit but overlap each other as overlaps between swaths in adjacent orbits are often available in the polar regions the underlying assumption for the proposed fast ice detection algorithm is that fast ice regions in sar images with a time interval of 5 days are highly correlated the object based approach proposed in this study was well suited to high resolution sar images in deriving spatially homogeneous fast ice regions the image segmentation results using the optimized parameters showed a distinct difference in the backscatter temporal evolution between fast ice and pack ice regions correlation and std of backscattering coefficients were found to be the most significant variables for the object based fast ice detection from two temporally separated images in overall the quantitative and qualitative evaluation demonstrated that the algorithm was an effective approach to detect fast ice with high accuracies the models well detected various fast ice regions in the west antarctica but misclassified some objects the misclassifications occurred toward the edge of fast ice regions with relatively rapid changes in backscattering between both data acquisitions on the other hand few fast ice objects were misclassified as uniform backscattering over time occurred by chance on very small objects far from the coast very old multi year fast ice regions with high backscattered signals were also a source for some misclassifications this may be due to the sensitivity of l band to snow structure to some extent and a thinner ice over the region with either ice growth no deformation or closing slight deformation between both images heavy snow load on the ice could be another error source for some misclassification as well the approach allowed for the reliable detection of fast ice regions by using l band sar images with a small local incidence angle difference

geochemical evolution of closed basin lakes general model and application to lakes qinghai and turkana in contrast to most previous models for the evolution of closed basin lakes we present an integrated model which considers various water budget patterns clay regradation so4 reduction and subbottom leakage in addition to the classical equilibrium approach of mineral precipitation the model was applied to lakes qinghai and turkana which significantly differ in the lithologies of their drainage areas but are representative of the carbonate rich sedimentary rock province of the tibet qinghai plateau and the silicate rock province of eastern africa both lakes are now topographically closed but to some degree hydrologically open subbottom leakage major results of the mode calculations show that the lithology controls the ultimate brine which is of na k cl type for qinghai lake and of na hco3 cl type for lake turkana so4 reduction delays the onset of sulfatic mineral precipitation and favours the formation of na carbonates such as trona at the expense of calcite clay mineral regradation plays an important role before the saturation of sulfatic or chlorine minerals is reached in particular magnesite formation may be in competition with mg bearing clay minerals finally simulations with various hydrological scenarios have shown that the modern hydrochemistry of both lakes cannot be reproduced by simply evaporating inflow water but reflects long term accumulation and evolution of solutes by continuous inflow over several thousand years the diversity of lake water composition within a uniform lithological province can thus be largely ascribed to varying hydrological conditions

an arctic sea ice multi step classification based on gnss r data from the tds 1 mission abstract this study examines the potential of using bistatic radar reflections from the global navigation satellite system gnss to classify sea ice types in the arctic ocean during the sea ice formation period for this study we used data obtained from the united kingdom uk techdemosat 1 tds 1 satellite mission the main objective of the tds 1 mission is to provide l band radar reflections of the ocean surface in order to infer wind speed through an estimation of the ocean surface roughness given the orbit inclination of tds 1 polar coverage is obtained providing datasets over the arctic sea ice cover recent studies demonstrated the use of tds 1 data for accurately deriving ocean ice detection using the shape of the bistatic radar waveforms derived from delay doppler maps we aim to advance this previous research by demonstrating the sensitivity of gnss bistatic radar signals to sea ice types the originality of the study presented in this manuscript is the classification of sea ice types never implemented before with any gnss r mission for this study we examined the fall period of october 2015 in the beaufort and chukchi seas region when considerable expanses of young ice first year ice fyi and multi year ice myi were present we developed a sea ice multi step classification approach based on bistatic radar reflections to generate not only an ocean ice classification but to classify between the three dominant ice types present the classification was done in multiple steps based on gnss bistatic radar observations only and includes checks for spatio temporal consistency with each dominant class validation results are based on the comparison of the gnss classification against sar derived sea ice type maps produced at the us national ice center first we derived classifications of sea ice open water samples with a success rate of 97 comparable to previous studies next the sea ice type classification results identified fyi myi and young ice with success rates of 70 82 and 81 respectively indicating a strong sensitivity of the l band gnss bistatic radar signals to the different surface scattering properties of these primary ice types with this manuscript we demonstrate the potential of the gnss bistatic radar signals to classify ice types during the sea ice formation period gnss bistatic radar signals provide unique forward scattered measurements at l band potentially on a daily temporal revisit that can be used to produce enhanced information on sea ice characteristics such as those currently generated by other sensors in order to determine the use of gnss r measurements on an operational basis further studies are needed over a year long period of sea ice growth and melt to determine the full benefit of gnss for improving and providing complementary information to that generated by both microwave and optical sensors

evaluation of second order texture parameters for sea ice classification from radar images with the advent of airborne and spaceborne synthetic aperture radar sar systems sea ice classification from sar images has become an important research subject since gray tone alone has proven to be of limited capability in differentiating ice types texture has naturally become an attractive avenue to explore accordingly performance of texture quantification parameters as related to their ability to discriminate ice types has to be examined sar image appearance depends on radar parameters involved in the image construction procedures from the doppler history record therefore the feasibility of using universal texture ice type relationships that hold for all combinations of radar parameters also has to be investigated to that end imagery data from three different sar systems were used in this study five conventional texture parameters derived from the gray level co occurrence matrix glcm were examined two of them were modified to ensure their invariant character under linear gray tone transformations results indicated that all parameters were highly correlated the parameters did not in general vary with the computational variables used in generating co occurrence matrices ice types can be identified uniquely by the mean value of any texture parameter the relatively high variability of texture parameters however confuses ice discrimination particularly of smoother ice types ice classification was conducted using a per pixel maximum likelihood supervised scheme when texture was combined with gray tone the overall average classification accuracy was improved texture was successful in improving the classification accuracy of multiyear ice but was less promising in discriminating first season ice types the best two glcm texture parameters according to the computed overall average classification accuracies were the inverse difference moment and the entropy a brief description of glcm texture parameters as related to ice s physical characteristics is presented

persistent non solar forcing of holocene storm dynamics in coastal sedimentary archives considerable climatic variability on decadal to millennial timescales has been documented for the past 11 500 years of interglacial climate1 2 3 this variability has been particularly pronounced at a frequency of about 1 500 years with repeated cold intervals in the north atlantic1 3 however there is growing evidence that these oscillations originate from a cluster of different spectral signatures4 ranging from a 2 500 year cycle throughout the period to a 1 000 year cycle during the earliest millennia here we present a reappraisal of high energy estuarine and coastal sedimentary records from the southern coast of the english channel and report evidence for five distinct periods during the holocene when storminess was enhanced during the past 6 500 years we find that high storm activity occurred periodically with a frequency of about 1 500 years closely related to cold and windy periods diagnosed earlier1 2 3 we show that millennial scale storm extremes in northern europe are phase locked with the period of internal ocean variability in the north atlantic of about 1 500 years4 however no consistent correlation emerges between spectral maxima in records of storminess and solar irradiation we conclude that solar activity changes are unlikely to be a primary forcing mechanism of millennial scale variability in storminess

cluster analysis of typhoon tracks part i general properties a new probabilistic clustering technique based on a regression mixture model is used to describe tropical cyclone trajectories in the western north pacific each component of the mixture model consists of a quadratic regression curve of cyclone position against time the best track 1950 2002 dataset is described by seven distinct clusters these clusters are then analyzed in terms of genesis location trajectory landfall intensity and seasonality both genesis location and trajectory play important roles in defining the clusters several distinct types of straight moving as well as recurving trajectories are identified thus enriching this main distinction found in previous studies intensity and seasonality of cyclones though not used by the clustering algorithm are both highly stratified from cluster to cluster three straight moving trajectory types have very small withincluster spread while the recurving types are more diffuse tropical cyclone landfalls over east and southeast asia are found to be strongly cluster dependent both in terms of frequency and region of impact the relationships of each cluster type with the large scale circulation sea surface temperatures and the phase of the el nino southern oscillation are studied in a companion paper

middle to late holocene storminess in brittany nw france part ii the chronology of events and climate forcing this study focuses on the recurring climate conditions required for the largest storms occurring in nw france brittany it is based on the analysed records of storm events along western brittany coast see part i in this manuscript part ii storm recurrence is explored along with forcing mechanisms periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms north atlantic oscillation nao and atlantic multidecadal oscillation amo modes triggering the most destructive storms then palaeostorm events are discussed at the holocene scale from 6000 yr bp to present to verify the forcing mechanisms most recorded events appear to be linked with cooling episodes mostly in winter a transition to or from a negative winter nao mode a positive amo mode extreme storms occur immediately prior to the medieval warm period mwp maximum effects are reached prior to the onset of the mwp and during the maunder and dalton solar minima low storm activity occurred during the sporer minimum linked to an acceleration of the atlantic meridional overturning circulation amoc main storm triggers seem to correspond to a positive amo mode with an unstable jetstream configuration driving a negative nao in this study four specific weather configurations were defined to explain each type of recorded storminess the strongest storms correspond to low amo and decennial negative nao modes e g little ice age or high amo in association with dominant low nao modes as during the early middle age and present day period fresh or warm oceans in association with a positive nao mode are stormy but with very low sting storms frequency although in agreement with the orbital forcing and the holocene glacial history increasing storm frequency and intensity is most probably partly biased by continuous sea level rise and resulting erosion

sea ice conditions in the adelie depression antarctica during besetment of the icebreaker rv xuelong during the 30th chinese antarctic expedition in 2013 14 the chinese icebreaker rv xuelong answered a rescue call from the russian rv akademik shokalskiy while assisting the repatriation of personnel from the russian vessel to the australian rv aurora australis rv xuelong itself became entrapped within the compacted ice in the adelie depression region analysis of modis and sar imagery provides a detailed description of the regional sea ice conditions which led to the 6day long besetment of rv xuelong the remotely sensed imagery revealed four stages of sea ice characteristics during the entrapment the gathering compaction dispersion and calving stages four factors characterizing the local sea ice conditions during late december 2013 and early january 2014 were identified surface component of the coastal current near surface wind ocean tides and surface air temperature this study demonstrates that shipping activity in ice invested waters should be underpinned by general knowledge of the ice situation in addition during such activity high spatio temporal resolution remotely sensed data should be acquired regularly to monitor local and regional sea ice changes with a view to avoiding the besetment of vessels

sea ice deformation state from synthetic aperture radar imagery part i comparison of c and l band and different polarization in this paper we present a quantitative comparison of l and c band airborne synthetic aperture radar imagery acquired at like and cross polarizations over deformed sea ice under winter conditions the parameters characterizing the deformation state of the ice are determined at both radar bands and at different polarizations the separation of deformed and level ice is based on a target detection technique the threshold is set such that image pixels with intensities equal to or larger than the highest 2 of the level ice intensity distribution are classified as deformed ice independent of the radar configuration and ice conditions optical imagery of sufficient quality for comparison is available only in a very few cases to characterize the deformation state the areal fraction of deformation features and the average distance between these features are evaluated the values obtained for both parameters are very sensitive to the radar frequency aeral fractions are larger and average distances are smaller at l band than at c band because of the much higher intensity contrast between the deformed and level ice at l band the differences between polarizations at one radar band are smaller but not always negligible in comparison to optical images it was observed that deformed ice areas can be distinguished from level ice over their whole length and extension at l band whereas at c band often only prominent parts are visible

tropical cyclone activity in the northwest pacific in relation to the el nino southern oscillation phenomenon abstract the interannual variations in tropical cyclone activity in the northwest pacific nwpac and their relationships with the el nino southern oscillation enso phenomenon were studied using the method of spectral analyses time series of a southern oscillation index soi defined as the sea level pressure difference between easter island and darwin and tropical cyclone activity in the entire nwpac ocean basin as well as in different regions of the nwpac were analyzed two spectral peaks are apparent in all these time series one corresponds to the generally accepted southern oscillation with a period of 3 to 3 5 years and another at the quasi biennial oscillation qbo frequency cross spectral analyses between the soi and tropical cyclone activity show significant coherence in these two spectral peaks the dominant peak is at the southern oscillation frequency with the soi leading typhoon activity by almost a year at the qbo frequency the two series are almost in phase cyclone activity in t

holocene environmental changes of the songji lagoon south korea and its linkage to sea level and enso changes abstract as an important geomorphologic element of the coastal areas lagoons receive relatively less attention than estuaries and deltas in the nw pacific marginal sea in this study based on proxies of 13c pollen and diatoms from a sediment core we attempt to understand the holocene sedimentary environment of the songji lagoon south korea and the possible mechanism for its response to global or regional sea level and climate changes the sedimentary environment was phased in different periods an estuary an estuarine lagoon an intermediate environment and an intermediate to brackish lagoon in early holocene to 6 7 cal kyr bp 6 7 6 3 cal kyr bp 6 3 4 5 cal kyr bp and 4 5 1 3 cal kyr bp respectively afterward abundant freshwater entered the lagoon from the stream the environmental evolutions were triggered by sea level and climate changes the fluctuation of the sedimentary environment implies that the sea level rose from 4 5 to 2 2 cal kyr bp and declined from 2 2 to 0 8 cal kyr bp the sedimentary environmental change is also probably connected with el nino activity especially in the periods 6 7 6 3 cal kyr bp 4 5 3 6 cal kyr bp and 3 0 2 0 cal kyr bp

c band polarimetric backscattering signatures of newly formed sea ice during fall freeze up a study of the polarimetric backscattering response of newly formed sea ice types under a large assortment of surface coverage was conducted using a ship based c band polarimetric radar system polarimetric backscattering results and physical data for 40 stations during the fall freeze up of 2003 2006 and 2007 are presented analysis of the copolarized correlation coefficient showed its sensitivity to both sea ice thickness and surface coverage and resulted in a statistically significant separation of ice thickness into two regimes ice less than 6 cm thick and ice greater than 8 cm thick a case study quantified the backscatter of a layer of snow infiltrated frost flowers on new sea ice showing that the presence of the old frost flowers can enhance the backscatter by more than 6 db finally a statistical analysis of a series of temporal spatial measurements over a visually homogeneous frost flower covered ice floe identified temperature as a significant but not exclusive factor in the backscattering measurements

the storegga slides evidence from eastern scotland for a possible tsunami abstract the second storegga slide on the continental slope off western norway has been dated at between 8000 and 5000 yrs b p a prominent sand layer in flandrian holocene deposits along the coast of eastern scotland and dated at approximately 7000 yrs b p may have been deposited by a tsunami generated by the slide the altitude and stratigraphy of the layer allow estimates to be made of the magnitude of the earthquake which initiated the slide

dual polarization c band radar observations of sea ice in the amundsen gulf polarimetric observations of sea ice from synthetic aperture radar can in principle assist in sea ice classification and ice water discrimination in this paper we use dual polarization ground based scatterometer observations of sea ice to assess the potential value of spaceborne dual polarization observations of sea ice for operational ice analysis focusing on c band and in particular the contribution of the hv backscatter coefficient and hh vv polarization ratios results show that signature variability resulting from frost flowers ice deformation and snow cover can overwhelm systematic differences between younger ice types up to first year thin as a result of this and noise floor limitations of spaceborne sensors the hv backscatter coefficient makes visual ice type and open water discrimination easier only below about 30 incidence angle the hh vv ratio is less impacted by the noise floor of spaceborne sensors but retains similar ambiguities for sea ice classification

mapping sea ice types from sentinel 1 considering the surface type dependent effect of incidence angle automated classification of sea ice types in synthetic aperture radar sar imagery is complicated by the class dependent decrease of backscatter intensity with incidence angle ia in the log domain this decrease is approximately linear over the typical range of space borne sar instruments a global correction does not consider that different surface types show different rates of decrease in backscatter intensity here we introduce a supervised classification algorithm that directly incorporates the surface type dependent effect of ia we replace the constant mean vector of a gaussian probability density function in a bayesian classifier with a linearly variable mean during training the classifier first retrieves the slope and intercept of the linear function describing the mean value and then calculates the covariance matrix as the mean squared deviation relative to this function the ia dependence is no longer treated as an image property but as a class property based on training and validation data selected from overlapping sar and optical images we evaluate the proposed method in several case studies and compare to other classification algorithms for which a global ia correction is applied during pre processing our results show that the inclusion of the per class ia sensitivity can significantly improve the performance of the classifier

detection of first year and multi year sea ice from dual polarization sar images under cold conditions this paper presents a new technique for automated detection of multi year my and first year fy sea ice from radarsat 2 dual polarization hh hv scansar wide images under cold environmental conditions the approach is applied to 2 05 km times2 05 km 41 times 41 pixels spatial window in the situation where the area is labeled as ice by our recently introduced ice and open water detection approach the probability of the presence of my ice is modeled as a function of the two selected predictor parameters computed over each spatial window the hv hh polarization ratio and the standard deviation of the hv signal the proposed my ice probability model was built based on thousands of synthetic aperture radar sar images and corresponding canadian ice service cis image analysis products covering the 2010 2016 time period not including 2013 our verification results for the independent testing subset for the year 2013 against the cis image analysis products suggest that approximately 50 of pure my and fy ice samples were classified with an accuracy of 98 2 the incidence angle correction of hh and hv backscatter does not improve my and fy ice detection results in the space of the selected predictor parameters the proposed technique will be used as part of the environment and climate change canada regional ice ocean prediction system in support of assimilation of ice thickness retrievals from cryosat 2 and soil moisture and ocean salinity mission data

the dual nature of the alpine himalayan system progress problems and prospects abstract two distinct groups of sutures belonging to two different orogenic complexes have been recognized within the alpine himalayan system associated with the development of the tethyan chains they are interpreted as places where two independent tethyan ocean complexes viz palaeo and neo tethys vanished during the permo carboniferous to early cretaceous and the late cretaceous to the present intervals respectively the older orogen is called the cimmerides and the younger the alpides cimmerides and alpides together constitute the super orogenic complex tethysides tethyside orogenic events also generated vast cratonic fields of deformation in eurasia until recently the recognition of these two orogenic complexes hidden within the alpine himalayan system had been hindered because of the artificial tripartite grouping of orogenic events of the phanerozoic into caledonian hercynian and alpine orogenic eras the most important current problems in the study of the tethysides are the relative importance of the various suture strands as sites of consumption of main tethyan oceanic branches the precise timing of ocean opening and closing along these sutures and the original provenance of the continental blocks constituting the tethyside orogenic collage a number of avenues of future research are suggested to tackle these problems

interdecadal variability of western north pacific tropical cyclone tracks abstract this study examines the interdecadal variability of the tropical cyclone tc tracks over the western north pacific wnp during the 1960 2005 period an empirical orthogonal function analysis of the 10 yr gaussian filtered annual frequency of tc occurrence shows three leading modes of tc occurrence patterns the first mode is related to the variation of tc activity in the areas near japan and its east the second mode is characterized by a northeast southwest dipole of tc occurrence anomalies along the southeast coast of china and an east west dipole near japan and its east the third mode is similar to the second mode except for the absence of the east west dipole these patterns are shown to be related to the decadal changes in the prevailing tc tracks two characteristic flow patterns related to the first and third modes of tc occurrence pattern are identified the first pattern is characterized by a north south dipole of 500 hpa geopotential anomalies over the wnp such a pattern may affect

a high resolution stalagmite record of the holocene east asian monsoon from mt shennongjia central china high resolution oxygen isotope 18o profiles of six stalagmites from sanbao cave in hubei province central china established with 1413 oxygen isotope data and 65 230th ages provide a continuous history of east asian summer monsoon easm intensity for the period from 13 0 2 thousand years before present ky bp relative to ad 1950 the 18o record includes four distinct stages in the evolution of the easm 1 an abrupt transition 11 5 ky bp into the holocene 2 a period of gradual increase in monsoon intensity 11 5 9 5 ky bp 3 the maximum humid period 9 5 6 5 ky bp and 4 a period of gradual decline in monsoon intensity 6 5 0 2 ky bp comparison of sanbao with regional records of comparable resolution reveals that the timing of the beginning and end of the holocene optimum as defined by the minimum in 18 o was similar in the indian and east asian monsoon systems this supports the idea that shifts in the monsoon tied to shifts in the mean position of the inter tropical convergen