sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

a microwave technique for mapping thin sea ice a technique is presented for mapping the distribution of new young and first year sea ice in seasonal sea ice zones that utilizes microwave spectral and polarization information from the defense meteorological satellite program special sensor microwave imager the motivation for this work stems from the need for accurate estimates of open water and thin ice within the arctic ice pack the technique utilizes the microwave polarization and spectral characteristics of these three ice types through two microwave radiance ratios the 19 4 ghz polarization and the spectral gradient ratio which is a measure of the spectral difference between the 19 4 ghz and the 37 0 ghz vertically polarized radiance components the combined use of the spectral gradient ratio and polarization reduces the low ice concentration bias generally associated with the presence of thin ice types the microwave polarization which is sensitive to changes in ice thickness and ice surface characteristics is used to classify new young and first year ice types the polarization varies from about 0 3 for calm ice free water to about 0 15 for new ice and to 0 03 for thick first year ice on the basis of this parameterization improved sea ice concentration maps and maps of new young and first year ice type distributions are derived examples from the bering sea are presented and comparisons are made with analyzed noaa avhrr imagery

a simple model for the skewness of global sea surface winds abstract a strong linear relationship between the mean and skewness of global sea surface winds both zonal and meridional is shown to exist such that where the wind component is on average positive it is negatively skewed and vice versa this relationship is observed in reanalysis satellite and buoy data this relationship between the mean and skewness fields of sea surface winds follows from the nonlinear surface drag predicted for a turbulent boundary layer by monin obukhov similarity theory since forcing perturbations speeding the wind up are subject to a stronger drag force than perturbations slowing it down furthermore it is demonstrated that the results of an empirical fit of observed surface winds to a stochastic differential equation presented in a recent study by sura are consistent with the white noise limit of the momentum equations for a turbulent boundary layer subject to fluctuating forcing albeit with a somewhat different physical interpretation

calibrating a viscoelastic sea ice model for wave propagation in the arctic fall marginal ice zone this paper presents a wave in ice model calibration study data used were collected in the thin ice of the advancing autumn marginal ice zone of the western arctic ocean in 2015 where pancake ice was found to be prevalent multiple buoys were deployed in seven wave experiments data from four of these experiments are used in the present study wave attenuation coefficients are calculated utilizing wave energy decay between two buoys measuring simultaneously within the ice covered region wavenumbers are measured in one of these experiments forcing parameters are obtained from simultaneous in situ and remote sensing observations as well as forecast hindcast models cases from three wave experiments are used to calibrate a viscoelastic model for wave attenuation dispersion in ice cover the calibration is done by minimizing the difference between modeled and measured complex wavenumber using a multi objective genetic algorithm the calibrated results are validated using two methods one is to directly apply the calibrated viscoelastic parameters to one of the wave experiments not used in the calibration and then compare the attenuation from the model with measured data the other is to use the calibrated viscoelastic model in wavewatch iii over the entire western beaufort sea and then compare the wave spectra at two remote sites not used in the calibration both validations show reasonable agreement between the model and the measured data the completed viscoelastic model is believed to be applicable to the fall marginal ice zone dominated by pancake ice

the normality of el nino the amplitude of the el nino southern oscillation enso would be normally distributed if the coupled pacific ocean atmosphere were a linear system forced by gaussian weather noise moment estimates of skewness and kurtosis demonstrate that this is not the case for monthly mean anomalies in pacific sea surface temperatures during 1950 97 the noted predominance of el nino events compared to la nina events is related to the high skewness in the eastern pacific skewness and kurtosis both exhibit an intriguing geographical variation from positive in the eastern to negative in the western pacific we have also examined nino 3 indices generated by three climate models having widely different complexity these exhibit a wide range of skewness and kurtosis values rather different from those found for the observations skewness and kurtosis can be used to diagnose non linear processes and provide powerful tools for validating models and for testing observed sea surface temperatures for the presence of possible climate change

a 10 15 yr modulation cycle of enso intensity abstract this study examines the slow modulation of el nino southern oscillation enso intensity and its underlying mechanism a 10 15 yr enso intensity modulation cycle is identified from historical and paleoclimate data by calculating the envelope function of boreal winter nino 3 4 and nino 3 sea surface temperature sst indices composite analyses reveal interesting spatial asymmetries between el nino and la nina events within the modulation cycle in the enhanced intensity periods of the cycle el nino is located in the eastern tropical pacific and la nina in the central tropical pacific the asymmetry is reversed in the weakened intensity periods el nino centers in the central pacific and la nina in the eastern pacific el nino and la nina centered in the eastern pacific are accompanied with basin scale surface wind and thermocline anomalies whereas those centered in the central pacific are accompanied with local wind and thermocline anomalies the el nino la nina asymmetries provide a possible m

estimation of sea ice thickness in ross and weddell seas from ssm i brightness temperatures in polar regions ocean atmosphere interactions are strongly influenced by sea ice and its thickness since satellite passive microwave observations became available in the 1970s significant progress has been made in the study of snow depth and sea ice concentration and extent in these regions estimating sea ice thickness sit instead turned out to be considerably more difficult we present a new empirical algorithm to estimate sit in the ross and weddell seas from special sensor microwave imager brightness temperatures this algorithm combines brightness temperature polarization difference and ratio values to obtain sit for seasonal ice up to a thickness of about 90 cm during freezing conditions a series of filters accounts for open water new ice and snow on sea ice our sit estimates are consistent with colocated visual ship based sit observations made according to the antarctic sea ice processes and climate project showing linear correlation values between 0 73 and 0 96 and root mean square error values between 14 and 24 cm the seasonal development of the region average sit derived with our approach agrees with the corresponding values derived from u s national ice center ice charts comparison with colocated polynya distribution maps suggests that the algorithm could be optimized for its performance with regard to sit values around 50 cm and that a closer investigation of the snow impact on the sit retrieval is required

feasibility of sea ice typing with synthetic aperture radar sar merging of landsat thematic mapper and ers 1 sar satellite imagery ers 1 synthetic aperture radar sar and landsat thematic mapper tm images were acquired for the same area in the beaufort sea april 16 and 18 1992 the two image pairs were colocated to the same grid 25 m resolution and a supervised ice type classification was performed on the tm images in order to classify ice free nilas gray ice gray white ice thin first year ice medium and thick first year ice and old ice comparison of the collocated sar pixels showed that ice free areas can only be classified under calm wind conditions 10 m s 1 based on the backscattering coefficient alone this is true for pack ice regions during the cold months of the year where ice free areas are spatially limited and where the capillary waves that cause sar backscatter are dampened by entrained ice crystals for nilas two distinct backscatter classes were found at 17 db and at 10 db the higher backscattering coefficient is attributed to the presence of frost flowers on light nilas gray and gray white ice have a backscatter signature similar to first year ice and therefore can not be distinguished by sar alone first year and old ice can be clearly separated based on their backscattering coefficient the performance of the geophysical processor system ice classifier was tested against the landsat derived ice products it was found that smooth first year ice and rough first year ice were not significantly different in the backscatter domain ice concentration estimates based on ers 1 c band sar showed an error range of 5 to 8 for high ice concentration regions mainly due to misclassified ice free and smooth first year ice areas this error is expected to increase for areas of lower ice concentration the combination of c band sar and tm channels 2 4 and 6 resulted in ice typing performance with an estimated accuracy of 90 for all seven ice classes

mapping of sea ice production in the arctic coastal polynyas 1 high ice production in coastal polynyas over the continental shelves in the arctic ocean is responsible for the formation of cold saline water which contributes to the maintenance of the arctic ocean halocline the accurate detection of coastal polynyas including an estimate of thin ice thickness is essential for the estimation of sea ice production this paper presents an algorithm that estimates thin ice thickness using special sensor microwave imager ssm i data in the arctic ocean detection and estimation of sea ice thicknesses of 0 15 m are based on the ssm i 85 and 37 ghz polarization ratios pr85 and pr37 through a comparison with sea ice thicknesses estimated from the advanced very high resolution radiometer avhrr data in the three different arctic coastal polynyas thus for the entire arctic ocean the algorithm can be used for the detection of coastal polynyas and for the estimation of sea ice production through combination with heat flux calculation this study provides the first circumpolar mapping of sea ice production in coastal polynyas over the entire arctic ocean high ice production is confined to the major arctic coastal polynyas with the highest ice production rate being in the north water polynya this study also presents the interannual variability of sea ice production in the 10 major coastal polynyas from 1992 to 2007 in general interannual variability in sea ice production has good correlation with polynya extent rather than surface air temperature the mapping also provides surface heat and salt flux conditions in the ice covered region which have not been well understood to date

nonlinear principal component analysis tropical indo pacific sea surface temperature and sea level pressure abstract nonlinear principal component analysis nlpca is a generalization of traditional principal component analysis pca that allows for the detection and characterization of low dimensional nonlinear structure in multivariate datasets the authors consider the application of nlpca to two datasets tropical pacific sea surface temperature sst and tropical indo pacific sea level pressure slp it is found that for the sst data the low dimensional nlpca approximations characterize the data better than do pca approximations of the same dimensionality in particular the one dimensional nlpca approximation characterizes the asymmetry between spatial patterns characteristic of average el nino and la nina events which the 1d pca approximation cannot the differences between nlpca and pca results are more modest for the slp data indicating that the lower dimensional structures of this dataset are nearly linear

helicopter borne observations with portable microwave radiometer in the southern ocean and the sea of okhotsk accurately measuring and monitoring the thickness distribution of thin ice is crucial for accurate estimation of ocean atmosphere heat fluxes and rates of ice production and salt flux in ice affected oceans here we present results from helicopter borne brightness temperature tb measurements in the southern ocean in october 2012 and in the sea of okhotsk in february 2009 carried out with a portable passive microwave pmw radiometer operating at a frequency of 36 ghz the goal of these measurements is to aid evaluation of a satellite thin ice thickness algorithm which uses data from the spaceborne advanced microwave scanning radiometer earth observing system amsr e or the advanced microwave scanning radiometer ii amsr ii amsr e and amsr ii tb agree with the spatially collocated mean tb from the helicopter borne measurements within the radiometers precision in the sea of okhotsk in february 2009 the amsr e 36ghz tb values are closer to the mean than the modal tb values measured by the helicopter borne radiometer in an antarctic coastal polynya in october 2012 the polarization ratio of 36ghz vertical and horizontal tb is estimated to be 0 137 on average our measurements of the tb at 36ghz over an iceberg tongue suggest a way to discriminate it from sea ice by its unique pmw signature

reconstruction of historical sea surface temperatures using empirical orthogonal functions abstract studies of climate variability often rely on high quality sea surface temperature sst anomalies although the high resolution national centers for environmental prediction formerly the national meteorological center optimum interpolation oi sst analysis is satisfactory for these studies the oi resolution cannot be maintained before november 1931 due to the lack of satellite data longer periods of ssts have come from traditional analyses of in situ ship and buoy sst observations alone a new interpolation method is developed using spatial patterns from empirical orthogonal functions e0fs that is a principal component analysis to improve analyses of sst anomalies from 1950 to 1981 the method uses the more accurate oi analyses from 1982 to 1993 to produce the spatial eofs the dominant eof modes which correspond to the largest variance are used as basis functions and are fit in a least squares sense to the in situ data to determine the time dependence of each mode a complete field

circumpolar mapping of antarctic coastal polynyas and landfast sea ice relationship and variability abstractsinking of dense water from antarctic coastal polynyas produces antarctic bottom water aabw which is the densest water in the global overturning circulation and is a key player in climate change as a significant sink for heat and carbon dioxide very recent studies have suggested that landfast sea ice fast ice plays an important role in the formation and variability of the polynyas and possibly aabw however they have been limited to regional and case investigations only this study provides the first coincident circumpolar mapping of antarctic coastal polynyas and fast ice the map reveals that most of the polynyas are formed on the western side of fast ice indicating an important role of fast ice in the polynya formation winds diverging from a boundary comprising both coastline and fast ice are the primary determinant of polynya formation the blocking effect of fast ice on westward sea ice advection by the coastal current would be another key factor these effects on the variability in

numerical study of winter water formation in the chukchi sea roles and impacts of coastal polynyas winter water formation is examined in the chukchi sea for the winters of 1992 2006 using a primitive equation ocean model forced by ncep wind and surface salinity flux derived from ssm i thin ice thickness estimates the model is also forced by an external inflow of 0 8 sv through the bering strait the model successfully reproduces the oceanic circulation on the chukchi shelf thus providing numerous insights into behaviors of salt enriched water produced on the shelf the experiments show that under northeasterly winds northward throughflow across barrow canyon is reduced this results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the arctic basin the flow and salinity enhancement of the flow through herald canyon is less extreme but more steady than through barrow canyon together with moored salinity in the bering strait the model results estimate the actual salinity to be 32 9 0 8 psu and 32 7 0 3 psu respectively for waters moving through the barrow and herald canyons both estimates are less than 33 1 psu that is typically observed for the cold halostad layer in the canada basin suggesting the importance of diapycnal mixing with saltier atlantic origin water

new palaeomagnetic results from the kerguelen islands summary new palaeomagnetic analyses have been carried out in the kerguelen islands on 32 lava flows of well established age 20 22 ma combined with previous studies they yield a reliable pole for the lower miocene for the antarctic plate n 59 flows 349 9 e 83 5 s a 6 1 a reversal sequence reversed normal has been identified in the port jeanne d arc section

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

nonlinear modes of decadal and interannual variability of the subsurface thermal structure in the pacific ocean 1 the nonlinear principal component analysis a neural network technique is applied to the observed upper ocean heat content anomalies hca in the pacific basin from 1961 to 2000 by applying the analysis to high passed and low passed data nonlinear interannual and decadal modes are extracted separately the first nonlinear interannual mode is mainly characterized by the el nino southern oscillation enso structure in the tropical pacific with considerable asymmetry between warm el nino and cool la nina episodes for example during strong el nino the negative hca in the western tropical pacific is much stronger than the corresponding positive hca during strong la nina the first nonlinear decadal mode goes through several notable phases two of the phases are related to decadal changes in the la nina and el nino characteristics revealing that the decadal changes for la nina episodes are much weaker than the changes for el nino episodes other phases of the decadal mode show a possible anomaly link from the middle latitudes to the western tropical pacific via the subtropical gyre the decadal changes in the hca around 1980 and around 1990 were compared and contrasted

non antipodal directions in magnetostratigraphy an overprint bias s u m m a r y paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal as one would expect for an axial geocentric dipole model specifically in magnetostratigraphic studies of tertiary sediments which include large data sets of paleomagnetic directions of both polarities it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions we investigate the conventional explanation for such asymmetry which involves the presence of a persistent partially unremoved present day field magnetization in the studied rocks both plate motion history and inclination shallowing which we evaluate in detail can play an important role in the paleomagnetic record our analysis shows that alternatively it is plausible that the inclination anomaly is due to a contribution of an axial octupole in accordance with observations of the recent 0 5 ma geomagnetic field recorded in lava flows

the retrospective prediction of el nino southern oscillation from 1881 to 2000 by a hybrid coupled model i sea surface temperature assimilation with ensemble kalman filter in this study we assimilated sea surface temperature sst data of the past 120 years into an oceanic general circulation model ogcm for el nino southern oscillation enso retrospective predictions using ensemble kalman filter enkf it was found that the ensemble covariance matrix in enkf can act as a time variant transfer operator to project the sst corrections onto the subsurface temperatures effectively when initial perturbations of ensemble were constructed using vertically coherent random fields as such the increments of subsurface temperatures can be obtained via the transfer operator during assimilation cycles the results show that the sst assimilation improves the model simulation skills significantly not only for the sst anomalies over the whole assimilated domain but also for the subsurface temperature anomalies of the upper 100 m over the tropical pacific off the equator along the equator the improvement of the assimilation is confined within the mixing layer because strong upwelling motions there prevent the downward transfer of sst information the retrospective prediction skills of enso over the past 120 years from 1881 to 2000 were significantly improved by the sst assimilation at all leads of 1 12 months especially for the 3 6 months leads compared with those initialized by the control run without assimilation the skilful predictions by the assimilation allow us to further study enso predictability using this coupled model

reversed spatial asymmetries between el nino and la nina and their linkage to decadal enso modulation in cmip3 models abstractthis study examines preindustrial simulations from coupled model intercomparison project phase 3 cmip3 models to show that a tendency exists for el nino sea surface temperature anomalies to be located farther eastward than la nina anomalies during strong el nino southern oscillation enso events but farther westward than la nina anomalies during weak enso events such reversed spatial asymmetries are shown to force a slow change in the tropical pacific ocean mean state that in return modulates enso amplitude cmip3 models that produce strong reversed asymmetries experience cyclic modulations of enso intensity in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries it is concluded that the reversed spatial asymmetries enable an enso tropical pacific mean state interaction mechanism that gives rise to a decadal modulation of enso intensity and that at least three cmip3 models realistically simu

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

the classification of arctic sea ice types and the determination of surface temperature using advanced very high resolution radiometer data the accurate quantification of new ice and open water areas and surface temperatures within the sea ice packs is a key to the realistic parameterization of heat moisture and turbulence fluxes between ocean and atmosphere in the polar regions multispectral noaa advanced very high resolution radiometer 2 avhrr 2 satellite images are analyzed to evaluate how effectively the data can be used to characterize sea ice in the bering and greenland seas both in terms of surface type and physical temperature the basis of the classification algorithm which is developed using a late wintertime bering sea ice cover data is that frequency distributions of 10 8 m radiances provide four distinct peaks representing open water new ice young ice and thick ice with a snow cover the results are found to be spatially and temporally consistent possible sources of ambiguity especially associated with wider temporal and spatial application of the technique are discussed an ice surface temperature algorithm is developed for the same study area by regressing thermal infrared data from 10 8 and 12 0 m channels against station air temperatures which are assumed to approximate the skin temperatures of adjacent snow and ice the standard deviations of the results when compared with in situ data are about 0 5 k over leads and polynyas to about 0 5 1 5 k over thick ice this study is based upon a set of in situ data limited in scope and coverage cloud masks are applied using a thresholding technique that utilizes 3 74 and 10 8 m channel data the temperature maps produced show coherence with surface features like new ice and leads and consistency with corresponding surface type maps further studies are needed to better understand the effects of both the spatial and temporal variability in emissivity aerosol and precipitable atmospheric ice particle distribution and atmospheric temperature inversions

contrasting eastern pacific and central pacific types of enso abstract surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of el nino southern oscillation enso in the tropical pacific an eastern pacific ep type and a central pacific cp type an analysis method combining empirical orthogonal function eof analysis and linear regression is used to separate these two types correlation and composite analyses based on the principal components of the eof were performed to examine the structure evolution and teleconnection of these two enso types the ep type of enso is found to have its sst anomaly center located in the eastern equatorial pacific attached to the coast of south america this type of enso is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical indian ocean in contrast the cp type of enso has most of its surface wind sst and subsurface anomalies confined in the central pacific and tends to onset develop and decay in situ

rare palaeomagnetic evidence of long term mantle control of the geodynamo and possible role of the nad field in the reversal process the degree to which the lower most mantle influences behavior of the geodynamo has been debated over the past quarter century our analysis of a comprehensive set of 17 cenozoic paleomagnetic transitional field records obtained from lavas in the southern hemisphere provides robust evidence of stable mantle control since the pliocene the records come from a region where given a significantly weakened axial dipole the magnetic field today would be largely controlled by the non axial dipole nad flux patch currently emanating from earth s outer core beneath western australia the paleomagnetic recording sites from west to east include the south indian ocean eastern downloaded from https academic oup com gji advance article abstract doi 10 1093 gji ggz480 5602599 by guest on 17 november 2019 australia new zealand and french polynesia the analyzed records contain from 2 to 25 sequential transitional virtual geomagnetic poles vgps ten of the 17 records supply at least one vgp within a narrow longitudinal band 10 wide between 60 s and the equator centered along longitude 102 4 e that is transitional data from 59 of the cenozoic recordings are found to reside in a region that encompasses a mere 2 8 of the vgp transitional area on earth s surface a robust monte carlo approach applied to this dataset one that takes into account the number of transitional vgps contained in each record finds this result highly improbable p value 0 0006 the present day pattern of vertical flux at the core mantle boundary shows an anomalously strong thin southern hemisphere longitudinal band off the west coast of australia that strikingly coincides with this unusual paleomagnetic finding we conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 myr seemingly independent of the behavior of the axial dipole our findings indicate it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity

ross sea polynyas response of ice concentration retrievals to large areas of thin ice 1 for a 3 month period between may and july of 2005 we examine the response of the advanced microwave scanning radiometer amsr e enhanced nasa team 2 nt2 and amsr e bootstrap aba ice concentration algorithms to large areas of thin ice of the ross sea polynyas coincident envisat synthetic aperture radar sar coverage of the region during this period offers a detailed look at the development of the polynyas within several hundred kilometers of the ice front the high resolution imagery and derived ice motion fields show bands of polynya ice covering up to 105 km2 of the ross sea that are associated with wind forced advection in this study ice thickness from amsr e 36 ghz polarization information serves as the basis for examination of the response the quality of the thickness of newly formed sea ice 10 cm from amsr e is first assessed with thickness estimates derived from ice surface temperatures from the moderate resolution imaging spectroradiometer modis instrument the effect of large areas of thin ice in lowering the ice concentration estimates from both nt2 aba approaches is clearly demonstrated results show relatively robust relationships between retrieved ice concentrations and thin ice thickness estimates that differ between the two algorithms these relationships define the approximate spatial coincidence of ice concentration and thickness isopleths using the 83 aba and 91 nt2 isopleths as polynya boundaries we show that the computed coverage compares well with that using the estimated 10 cm thickness contour the thin ice response characterized here suggests that in regions with polynyas the retrieval results could be used to provide useful geophysical information namely thickness and coverage

estimation of thin sea ice thickness from noaa avhrr data in a polynya off the wilkes land coast east antarctica antarctic coastal polynyas are major areas of intense ocean atmosphere heat and moisture flux and associated high sea ice production and dense water formation their accurate detection including an estimate of thin ice thickness is therefore very important in this paper we apply a technique originally developed in the arctic to an estimation of sea ice thickness using us national oceanic and atmospheric administration noaa advanced very high resolution radiometer avhrr data and meteorological data in the vincennes bay polynya off wilkes land east antarctica the method is based upon the heat flux calculation using sea ice surface temperature estimates from the satellite thermal infrared data combined with global objective analysis european centre for medium range weather forecasts ecmwf data the validity of this method is assessed by comparing results with independent ice surface temperature and ice thickness data obtained during an australian led research cruise to the region in 2003 in thin ice polynya regions ice thicknesses estimated by the heat flux calculation using avhrr and ecmwf data show reasonable agreement with those estimated by a applying the heat flux calculation to in situ radiation thermometer and meteorological data and b in situ observations the standard deviation of the difference between the avhrr derived and in situ data is 0 02 m comparison of the avhrr ice thickness retrievals with coincident satellite passive microwave polarization ratio data confirms the potential of the latter as a means of deriving maps of thin sea ice thickness on the wider scale uninterrupted by darkness and cloud cover

paleomagnetism and dating of a thick lava pile in the permian bakaly formation of eastern kazakhstan regularities and singularities of the paleomagnetic record in thick lava series abstract paleomagnetic results on thick lava series are among the most important sources of information on the characteristics of ancient geomagnetic fields most paleo secular variation data from lavas psvl are of late cenozoic age there are far fewer results from lavas older than 5 ma the central asia orogenic belt that occupies several million square kilometers in asia is probably the world s largest area of paleozoic volcanism and is thus an attractive target for psvl studies we studied a ca 1700 m thick lava pile in eastern kazakhstan of early permian age magmatic zircons successfully separated from an acid flow in this predominantly basaltic sequence yielded an early permian age of 286 3 3 5 ma oriented samples were collected from 125 flows resulting in 88 acceptable quality flow means n 4 samples radius of confidence circle 95 15 of the high temperature magnetization component the uniformly reversed component is pre tilting and arguably of a primary origin the overall mean direction has a declination 242 0 and an inclination 56 2 k 71 5 95 1 8 n 88 sites pole at 44 1 n 160 6 e a95 2 2 our pole agrees well with the early permian reference data for baltica in accord with the radiometric age of the lava pile and geological views on evolution of the western part of the central asia orogenic belt the new early permian result indicates a comparatively low level of secular variation especially when compared to psvl data from intervals with frequent reversals still the overall scatter of dispersion estimates that are used as proxies for sv magnitudes elongation values and elongation orientations for psvl data is high and cannot be fitted into any particular field model with fixed parameters both observed values and numerical simulations indicate that the main cause for the scatter of form parameters elongation values and elongation orientations is the too small size of collections dispersion estimates concentration parameter and standard angular deviation are more robust and their scatter stems from other sources which may include non stochastic features of datasets like clusters loops etc or non stationary behavior of secular variation magnitude over time intervals of many million years

development of seasonal pack ice in the beaufort sea during the winter of 1991 1992 a view from below subsea sonars moored in the beaufort sea acquired a spatial section of draft across 941 km of sea ice during the winter of 1991 1992 these observations document the development of seasonal sea ice from an open sea surface in october to an average accumulation exceeding 2 5 m draft by early april initially level ice occupied 85 of the profile but continued ridging reduced this fraction to 50 in late winter three modes of level ice were tracked that of seasonal ice whose growth was initiated at freeze up and two others created during openings of the ice field in january and in march the growth of these modes can be closely matched by calculations based on a slab model of ice formation forced using surface meteorological data initially ridge keels were small and widely spaced by midwinter an exponential dependence of keel frequency on draft was obvious e folding scale 2 16 m and the frequency and mean draft of keels had stabilized the maximum keel draft was a modest 17 4 m the low incidence of deep keels at all times is a statistically significant departure from the exponential dependence valid at lesser drafts the truncation point of the exponential relation is related to the draft of the thinnest level ice present in late winter the formation of deeper keels from thick first year ice was apparently precluded by the presence of younger thinner ice which limited the force available for ridge building through calculation of the seaward transport of ice over the sonars the total production of ice in the coastal flaw lead during 1991 1992 was determined to be about 60 of earlier indirect estimates in general the observations revealed winter ice conditions significantly less severe than those found on the periphery of the polar pack only a few tens of kilometers to the north but more so than conditions in a marginal first year ice zone at the same latitude in davis strait

analysis of the non linearity in the pattern and time evolution of el nino southern oscillation in this study the observed non linearity in the spatial pattern and time evolution of el nino southern oscillation enso events is analyzed it is shown that enso skewness is not only a characteristic of the amplitude of events el ninos being stronger than la ninas but also of the spatial pattern and time evolution it is demonstrated that these non linearities can be related to the non linear response of the zonal winds to sea surface temperature sst anomalies it is shown in observations as well as in coupled model simulations that significant differences in the spatial pattern between positive el nino versus negative la nina and strong versus weak events exist which is mostly describing the difference between central and east pacific events central pacific events tend to be weak el nino or strong la nina events in turn east pacific events tend to be strong el nino or weak la nina events a rotation of the two leading empirical orthogonal function modes illustrates that for both el nino and la nina extreme events are more likely than expected from a normal distribution the bjerknes feedbacks and time evolution of strong enso events in observations as well as in coupled model simulations also show strong asymmetries with strong el ninos being forced more strongly by zonal wind than by thermocline depth anomalies and are followed by la nina events in turn strong la nina events are preceded by el nino events and are more strongly forced by thermocline depth anomalies than by wind anomalies further the zonal wind response to sea surface temperature anomalies during strong el nino events is stronger and shifted to the east relative to strong la nina events supporting the eastward shifted el nino pattern and the asymmetric time evolution based on the simplified hybrid coupled rechoz model of enso it can be shown that the non linear zonal wind response to sst anomalies causes the asymmetric forcings of enso events this also implies that strong el ninos are mostly wind driven and less predictable and strong la ninas are mostly thermocline depth driven and better predictable which is demonstrated by a set of 100 perfect model forecast ensembles

east antarctic sea ice albedo thickness distribution and snow cover characteristics of springtime sea ice off east antarctica were investigated during a cruise of the australian national antarctic research expedition in october through december 1988 the fractional coverage of the ocean surface the ice thickness and the snow cover thickness for each of several ice types were estimated hourly for the region near the ship these observations were carried out continuously during the 4 weeks the ship was in the ice thin and young ice types were prevalent throughout the region and the observations show a systematic increase in the total area weighted pack ice thickness including open water area from only 0 2 m within 50 km of the ice edge to 0 45 m close to the coast ice thickness averaged over the ice covered region only is also relatively thin ranging from 0 35 m near the ice edge to 0 65 m in the interior these values are probably typical of average winter thickness for the area the average snow cover thickness on the ice increased from 0 05 m near the ice edge to 0 15 m in the interior average ice concentration increased from less than 6 10 near the ice edge to 8 10 in the interior the ship observed concentrations were in good agreement with concentrations derived from passive microwave satellite imagery except in some regions of high concentration in these regions the satellite derived concentrations were consistently lower than those estimated from the ship possibly because of the inability of the satellite sensors to discriminate the appreciable percentage of very thin ice observed within the total area spectral albedo was measured for nilas young grey ice grey white ice snow covered ice and open water at wavelengths from 420 to 1000 nm allwave albedo was computed by using the spectral measurements together with estimates of near infrared albedo and modeled spectral solar flux area averaged albedos for the east antarctic sea ice zone in spring were derived from representative allwave albedos together with the hourly observations of ice types these area averaged surface albedos increased from about 0 35 at the ice edge to about 0 5 at 350 km from the edge remaining at 0 5 to the coast of antarctica the low average albedo is in part due to the large fraction of open water within the pack but extensive fractions of almost snow free thin ice also play an important role

some aspects of magnetic viscosity in subaerial and submarine volcanic rocks summary the magnetic viscosity of 334 upper tertiary and pre bruhnes quaternary volcanic rocks from the massif central france and the steens mountain oregon and of 40 basaltic cores from dsdp leg 37 has been investigated thellier s viscosity index follows a log normal distribution with mean values equal to 6 and 3 5 per cent for subaerial and submarine rocks respectively for subaerial rocks the average intensity of the viscous remanent magnetization vrm acquired in the earth s field since the beginning of the bruhnes polarity epoch t 0 7 myr is estimated to be equal to one quarter of the average intensity of the primary remanence alternating field demagnetization of vrms acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t neel s diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard vrm is carried by single domain grains and soft vrm carried by multidomain particles single domain particles carrying hard vrm in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold resulting from high temperature oxidation of titanomagnetite soft vrm is carried by low curie point homogeneous titanomagnetite unlike his single domain theory neel s multidomain theory of magnetic viscosity does not account quantitatively for the resistance of vrm to alternating fields

40ar 39ar geochronology of flood basalts from the kerguelen archipelago southern indian ocean implications for cenozoic eruption rates of the kerguelen plume the 6500 km2 kerguelen archipelago formed on the northern kerguelen plateau nkp 4 105 km2 which is a shallow submarine plateau belonging to the kerguelen large igneous province in the southern indian ocean flood basalts make up 85 of the archipelago and are interpreted as the most recent volcanism 40 ma from the kerguelen hotspot which has erupted basalt for the last 115 million years based on 40ar 39ar incremental heating of acid leached groundmass separates we report isochron ages ranging from 29 26 0 87 ma to 24 53 0 29 ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago the oldest dated basalt from the archipelago 29 ma is much younger than the 40 ma age of conjunction between the hotspot and the southeast indian ridge basalt eruption seems to have ceased shortly after 24 ma although small volume highly evolved lavas and plutons continued to form in the archipelago the basalt age data suggest an average lava accumulation rate of 1 6 0 9 km my during the oligocene the archipelago s volumetric eruption rate 0 009 km3 yr is lower than estimates made for the cretaceous kerguelen plateau 1 7 km3 yr and the ninetyeast ridge hotspot track 0 18 km3 yr suggesting that the late cenozoic extrusive activity of the kerguelen plume is waning cenozoic volcanism attributed to the kerguelen plume occurs over a diffuse area with quaternary eruptions at heard and mcdonald islands and within the kerguelen archipelago the decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the cretaceous kerguelen plateau overriding and insulating the plume however if the undated nkp which underlies the archipelago formed during the cenozoic then the crustal production rate of the plume from 40 ma to the present 0 25 km3 yr would be similar to the crustal production rate 0 23 km3 my previously estimated for the formation of the ninetyeast ridge 82 38 ma

palaeomagnetic study of oligocene 24 30 ma lava flows from the kerguelen archipelago southern indian ocean directional analysis and magnetostratigraphy we report palaeomagnetic determinations in order to estimate the palaeosecular variation of the geomagnetic field for the upper oligocene period from a southern site we measured the anisotropy of magnetic susceptibility ams of basalts from five sections to check the reliability of applying tectonic correction and concluded that no stratigraphic corrections were needed a positive reversal test implies that secondary components were sufficiently removed from the characteristic remanent magnetization also the sampling was sufficiently random to accurately reflect upper oligocene palaeosecular variation calculations of the between flow angular standard deviation sb from the geographic pole show values between 21 2 and 21 8 this palaeosecular variation estimate agrees with model g for palaeosecular variation j geophys res 96 b3 1991 3923 which is axisymmetric and assumes an equatorial symmetry of the geomagnetic field the virtual geomagnetic poles vgps from the mont des ruches section seem to exhibit two polarity changes reverse normal r n first and in more detail a second normal reverse n r one finally assuming that no polarity chrons are missed by any of the sections we compare the magnetostratigraphy we defined to the global geomagnetic polarity scale of geophys j int 129 1 1997 176 using previously obtained radiometric dates to correlate the sections j petrol 39 4 1998 711 earth planet sci lett 174 2000 313 j petrol 43 7 2002 1341 the precise age constraints furnished by these correlations allow an estimation of eruption rates from 0 30 to at least 1 31 km my with wide variations from section to section and an increase of eruption rate for younger sections however due to the non uniqueness of the correlations the age limits of the sections and consequently the extrusion rate estimations can vary moreover assuming the mont des ruches section that has probably recorded all the magnetic polarity intervals the mont de la rabouillere and mont de la tourmente intermediate vgp directions may indicate unrecorded polarity chrons the eruption rates calculated can thus be overestimated 2002 elsevier science b v all rights reserved

temporal geochemical trends in kerguelen archipelago basalts evidence for decreasing magma supply from the kerguelen plume abstract the kerguelen archipelago a 39 ma to recent volcanic plutonic complex is interpreted to be a manifestation of the kerguelen plume most 85 of the surface area is covered by flood basalts ranging in age from 29 to 25 ma the youngest 25 ma studied flood basalts are in the southeast se province of the archipelago a composite 460 m section of this southeast flood basalt dominantly consists of evolved 3 to 6 mgo alkalic basalt and trachybasalt with a few interbedded highly evolved lavas trachyandesites a 40 70 m conglomerate which contains lignite beds and a trachytic breccia tuff unit all of the lavas in this composite section have sr and nd isotopic ratios that are typical of the kerguelen plume e g 80 of the 115 analyzed archipelago lavas with 2 3 mgo have 87 sr 86 sr i 0 70515 12 and 143 nd 144 nd i 0 51259 5 these ranges include the southeast flood basalts pb isotopes however are more variable these 25 ma lavas have high 206 pb 204 pb at 18 4 to 18 6 relative to other archipelago lavas the temporal trend of the archipelago flood basalt from older 29 ma transitional basalts to younger 25 ma alkalic basalt with an increasing proportion of highly evolved lavas and intra bedded sediments in the relatively young southeast section indicates a a temporal decrease in extent of melting and b a decreasing supply of magma from the plume to the crust these temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge centered plume at 43 ma to its intraplate setting supporting evidence for this interpretation is a the absence of a morb geochemical signature in these 25 ma lavas and b the relatively low abundances of heavy rare earth elements in these southeast lavas which reflect partial melting within the garnet stability field

estimation of thin ice thickness from the advanced microwave scanning radiometer eos for coastal polynyas in the chukchi and beaufort seas in this paper a thin ice thickness algorithm for coastal polynyas in the chukchi and beaufort seas has been attempted which uses the advanced microwave scanning radiometer earth observing system amsr e 89 ghz brightness temperature data at a 6 25 km resolution thermal ice thickness estimated from the moderate resolution imaging spectroradiometer modis ice surface temperature data for a set of control points is used in conjunction with the amsr e brightness temperature data to develop a regression relationship between the thermal ice thickness and the passive microwave polarization ratio the modis thermal ice thickness has been determined from the calculation of conductive heat flux through the ice air interface the relationship is then applied across the polynyas to derive thin ice thickness information at different points in the development of the algorithm the modis derived thermal thickness is assumed to be the standard reference validation of the algorithm has been carried out using an independent modis derived thermal ice thickness data set it is observed that the model error is 1 3 cm within the thickness range of 1 10 cm this method is expected to be useful in high latitudes where in situ observations are sparse

thin ice impacts on surface salt flux and ice strength inferences from advanced very high resolution radiometer temperatures and albedos derived from satellite imagery are combined with a thermodynamic ice model to estimate thin ice thickness distributions over the beaufort and the northern greenland seas the study shows that thin ice thinner than 1 m occupied over half the area in the seasonal ice zones in november and december of 1990 but dropped significantly in april 1991 the beaufort shelf showed the largest seasonal change the aggregate properties of surface salt flux and compressive ice strength both strongly dependent on the thin end of the ice thickness distribution were estimated with data from advanced very high resolution radiometer to reveal the spatial and temporal variations over a large scale the salt flux from growing thin ice was 1 2 orders of magnitude larger on the beaufort and greenland shelves than in the deep basins on the shelves flux from thin ice accounted for over 90 of the total surface salt budget these satellite derived estimates provided detailed spatial information on salt flux which can be of great use in studies of surface patterns of salinity forcing and shelf basin interaction also revealed by the satellite data was the wide range in values of compressive strength which affects how freely the ice cover can deform strengths were low in early winter and in seasonal ice zones and nearly doubled in spring both thin ice fraction and compressive ice strength estimated from satellite imagery were in good agreement with those simulated by a coupled ice ocean model monitoring the seasonal and interannual behavior of sea ice properties with noaa satellites is both an opportunity and a challenge the opportunity arises from twice daily overpasses which provide excellent coverage of the polar ocean the challenge lies with the surface being sampled sea ice appears radiometrically similar in the imagery of advanced very high resolution radiometer avhrr making it difficult to monitor for example the mass of the sea ice cover compromises in which useful ice properties match what satellites can distinguish must be made one useful property is the surface area and thickness of thin ice defined here as ice with thickness 1 m in winter thin ice has a warmer surface and appears darker than the surrounding thicker ice these thermal contrasts form the basis for the approach followed in this paper an example of a clear sky avhrr image over the southern canada basin shows the leads as linear dark red and orange features that are 10o 20oc warmer than the thick floes plate 1 since thin ice in leads responds rapidly to changes in wind and temperature and is generally covered with no or little snow the thickness of thin ice not just its presence can be inferred directly from the radiative surface temperature detected by the satellite fortuitously several aggregate properties are sensitive to thin ice and can be largely or completely determined by thin strength of the ice cover figure 1 shows an annual cycle of salt flux from both thin and thick ice while the thick ice continues melting negative salt flux in september thin ice grows vigorously as the air cools the growing thin ice dominates the total salt budget until january when thicker ice reaches the peak of its growth cycle in the annual mean brine rejection from thin ice is about 8 times larger than that from thicker ice profoundly affecting the density structure of underlying water thin ice also controls the compressive strength p and thus the mechanical behavior of sea ice high percentages of thin ice reduce the stresses transmitted among ice floes and make pressure ridges more likely to form the compressive ice strength can be related explicitly to the thin end of the ice thickness distribution e g rothrock 1975

numerical models of the geodynamo and observational constraints 1 the past few years have seen the emergence of a large number of numerical simulations of the geodynamo in parallel both new and old geomagnetic archeomagnetic and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models naturally model predictions should be tested against actual characteristics of the geomagnetic field despite huge differences sometimes in excess of a billion between the values of parameters used in the simulations and those estimated for the earth it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures in agreement with the first order characteristics of the geomagnetic field yet when considering finer characteristics there are significant differences and failures to actually produce a number of fundamental characteristic features in this presentation we first review numerical results obtained to date then we attempt to summarize which field characteristics derived from observational data sets can be considered robust on the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum we sort presumably characteristic geomagnetic features into three categories robust controversial and unlikely we conclude that numerical models should be illustrated with a number of key predictions averaged over at least 10 dipole diffusion times these predictions should be tested against the subset of robust observations only controversial observations should await additional confirmation

the under ice thickness distribution of the arctic basin as recorded in 1958 and 1970 the uss nautilus ssn 571 was the first vessel to cross the arctic basin via the north pole in early august 1958 during this expedition almost continuous acoustic under ice thickness profiles were recorded in august 1970 uss queenfish ssn 651 retraced nautilus route yielding the only duplicate transect of under ice topography across the arctic basin comparisons of the statistical analysis of the under ice draft measurements obtained through use of wide beam and narrow beam and narrow beam only acoustic profilers by nautilus and queenfish respectively along the coincidental route are presented geographic areas found to have distinct under ice characteristics and ice composition are identified the under ice statistics of both cruises are considered in relation to representative field observations and modeling results of other researchers principal findings are 1 nautilus recorded generally more severe ice conditions within the canada basin than did queenfish 12 years later overall mean drafts were 3 08 and 2 39 m respectively 2 the under ice topography becomes progressively more severe when proceeding from the canadian to the eurasian side of the arctic basin 3 the canada basin may contain the most moderate under ice topography and the greatest number of open water and refrozen polynyas and leads within the central arctic basin 4 the makarov and amundsen basins and the arctic mid ocean ridge may contain some of the most severe under ice topography within the arctic basin and 5 the present study indicates an overall arctic basin mean of 3 4 open water new ice less than 30 cm in summer

magnetic fabric and flow direction in basaltic pahoehoe lava of xitle volcano mexico abstract we sampled five basaltic lava flow units from xitle volcano mexico city to study the variation of anisotropy of magnetic susceptibility within their cooling boundaries we find that the mean maximum susceptibility parallels the geologically inferred flow direction in the units that were emplaced on a steeper slope whereas for those on a negligible slope the mean intermediate susceptibility points in the flow direction we propose however that the maximum susceptibility always points in the direction of local movement and that a change in slope produces a deviation of the local motion from that of the unit as a whole the axis of susceptibility closest to the geologically inferred flow direction usually plunges upflow in the basal part of the flow unit comprising an imbrication which clearly marks the flow azimuth of the lava thus the scenario of emplacement may influence the results in a predictable way we suggest that the degree of anisotropy could bear a direct relationship to either the viscosity of the lava the morphology of the flows or both based on a comparison with lavas from azufre argentina and ko olau o ahu volcanoes also we suggest that the shape of the susceptibility ellipsoid may be related to the degree of internal deformation of the lava flows we also compare the two methods currently available to calculate regions of confidence around the mean principal susceptibilities

determination of flow directions by combining ams and thin section analyses implications for oligocene volcanism in the kerguelen archipelago southern indian ocean we report an anisotropy of magnetic susceptibility ams study carried out on seven oligocene volcanic sections widely distributed on the surface of the kerguelen archipelago the statistical results at flow scale and at section scale were checked with ams density diagrams and thin section analysis because of the axis permutations frequently observed for lava flows two possible flow directions were nicely defined for the majority of the sections we then used two alternative approaches imbrication of the minimum axis and symmetry of the ams distribution to infer a flow direction for each section these two methods and thin section analysis not being decisive for choosing between the two solutions geological and geomorphological observations were finally used to obtain confident interpretations the results of this study enable the supposed eruptive centres of the kerguelen archipelago to be located more precisely

snow thickness retrieval from l band brightness temperatures a model comparison the soil moisture and ocean salinity smos satellite s l band 1 4 ghz measurements have been used to retrieve snow thickness over thick sea ice in a previous study here we consider brightness temperature simulations for 2 5 4 5 m thick arctic multi year ice and compare the results of the relatively simple emission model m2013 used previously for the retrieval with simulations from a more complex model t2011 that combines a sea ice version of the microwave emission model for layered snowpacks memls with a thermodynamic model we find that l band brightness temperature is mainly determined by ice temperature in the m2013 model ice temperature in turn is mainly determined by surface temperature and snow thickness and this dependence has been used previously to explain the potential for a snow thickness retrieval our comparisons suggest that the m2013 retrieval model may benefit from a more sophisticated thermodynamic calculation of the ice temperature or from using independent temperature data e g from 6 ghz channels in both models horizontally polarized brightness temperatures increase with snow thickness while holding surface temperature ice thickness and snow density near constant the increase in the t2011 model is steeper than in m2013 suggesting a higher sensitivity to snow thickness than found earlier

the internal structure of lava flows insights from ams measurements ii hawaiian pahoehoe toothpaste lava and a ā abstract we studied the anisotropy of magnetic susceptibility ams of 22 basaltic flow units including s type pahoehoe p type pahoehoe toothpaste lava and a emplaced over different slopes in two hawaiian islands systematic differences occur in several aspects of ams mean susceptibility degree of anisotropy magnetic fabric and orientation of the principal susceptibilities among the morphological types that can be related to different modes of lava emplacement ams also detects systematic changes in the rate of shear with position in a unit allowing us to infer local flow direction and some other aspects of the velocity field of each unit a flows are subject to stronger deformation than pahoehoe and also their internal parts behave more like a unit according to ams the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes probably resulting from endogenous growth

thickness and production of sea ice in the okhotsk sea coastal polynyas from amsr e 1 from comparisons with thickness of sea ice from advanced very high resolution radiometer avhrr and ice profiling sonar data we have developed an advanced microwave scanning radiometer eos amsr e thin ice thickness algorithm for the sea of okhotsk this algorithm can estimate ice thickness of 0 2 m without snow using the polarization ratio of amsr e brightness temperature at a 36 5 ghz channel from a linear relationship with avhrr ice thickness when a snow cover exists on the thin ice surface as occurred a few times in each winter it is shown that the algorithm cannot detect the thin ice sea ice and dense shelf water dsw production in coastal polynya are estimated on the basis of heat flux calculation with the daily amsr e ice thickness for three winters december march of 2002 2003 to 2004 2005 the ice production is largest in the northwest shelf nws polynya which accounts for 45 of the sum of ice production in major coastal polynyas the ice production in major coastal polynyas would cover the maximum ice area of the okhotsk sea if the average ice thickness is assumed to be 1 m variability of the ice production is mainly modulated by air temperature in the nws polynya which is the main dsw production area the annual dsw formation rate is estimated to be 0 36 sv

estimation of sea ice production in the bering sea from amsr e and amsr2 data with special emphasis on the anadyr polynya

on the construction of geomagnetic timescales from non prejudicial treatment of magnetic anomaly data from multiple ridges summary when marine magnetic anomaly data are used to construct geomagnetic polarity timescales the usual assumption of a smooth spreading rate function at one seafloor spreading ridge forces much more erratic rate functions at other ridges to eliminate this problem we propose a formalism for the timescale problem that penalizes non smooth spreading behaviour equally for all ridges specifically we establish a non linear lagrange multiplier optimization problem for finding the timescale that 1 agrees with known chron ages and with anomaly interval distance data from multiple ridges and 2 allows the rate functions for each ridge to be as nearly constant as possible according to a cumulative penalty function the method is applied to a synthetic data set reconstructed from the timescale and rate functions for seven ridges derived by cande kent 1992 under the assumption of smooth spreading in the south atlantic we find that only modest changes in the timescale less than 5 per cent for each reversal are needed if no one ridge is singled out for the preferential assumption of smoothness future implementation of this non prejudicial treatment of spreading rate data from multiple ridges to large anomaly distance data sets should lead to the next incremental improvement to the pre quaternary geomagnetic polarity timescale as well as allow a more accurate assessment of global and local changes in seafloor spreading rates over time

response of sea ice drift to wind forcing on the northeastern newfoundland shelf observations of sea ice drift obtained with satellite tracked ice beacons in march april of 1988 and 1989 have been used to examine the response of sea ice drift to wind forcing over the northern newfoundland continental shelf the short term 5 20 days response of sea ice drift ranges from 2 6 to 5 4 of the local wind over much of the inner continental shelf which is comparable to that in the bering sea and the antarctic but larger than that in arctic sea ice drifts to the right of the local wind at angles ranging from 10 to 63 the response to wind forcing is largest near the ice edge both over the middle portions of the shelf and along the southern margins of the seasonal ice zone and during strong and steady wind of several days duration the large wind driven response of ice drift observed in this study in comparison with the arctic may result from 1 reduced levels of internal ice stress associated with the generally thin ice cover and lower areal concentration of sea ice 2 large atmospheric drag coefficients associated with the small ice floes in areas of comparatively higher ice concentration and 3 smooth ice bottom caused by melting in nearshore areas the ice to wind coupling is reduced owing to larger internal ice stresses experienced locally due to ice pileup

sea ice thickness distribution in the greenland sea and eurasian basin may 1987 upward sonar data from a 3400 km track in the greenland sea and the eurasian basin of the arctic ocean obtained by a british submarine in may 1987 have been statistically analyzed mean ice thicknesses in the greenland sea show significant melting with downstream distance at an average rate of 0 38 m per degree of latitude equivalent to about 2140 km3 yr 1 of freshwater input between 80 n and 75 n in the region north of fram strait between 82 n and 80 n there is remarkable consistency between mean ice drafts in 1987 and in three previous cruises 1976 1979 1985 while between greenland and the north pole there was a significant thinning in 1987 relative to 1976 in fram strait the mean ice draft shows evidence of thinning within the region of influence of a semipermanent eddy ice draft distributions show that the probability density function of deformed ice follows a negative exponential distribution and that the change of shape of the distribution with downstream distance in the greenland sea is due to preferential melting of thicker ice pressure ridge draft distributions also fit a negative exponential while their spacings fit a lognormal distribution lead width distribution follows a power law with an exponent of 1 45 for narrow leads and 2 50 for wider leads if leads are defined as having a 1 m maximum draft 1 67 and 2 76 for a 0 5 m maximum draft lead spacings follow a negative exponential distribution for moderate spacings 400 1500 m but with an excess of lead pairs at small and very large spacings

thin ice thickness from satellite thermal imagery sea ice thickness is an important parameter in the surface energy exchange and mass balance of the polar oceans remote sensing offers the opportunity to monitor thin ice routinely we combine measurements of surface temperature from an advanced very high resolution radiometer avhrr with a thermodynamic ice growth model to estimate thin ice and its thickness distribution during the cold season in a beaufort sea area of perennial ice we find that thin ice up to a meter thick had a concentration of 18 in december 1990 and 3 in april 1991 thin ice concentrations near the ice edge in the greenland sea range from 20 to 50 these avhrr derived estimates generally compare well with thickness observations from upward looking sonar given the uncertainties in all the model inputs estimated from avhrr such as radiative and turbulent heat flux components and surface albedo we find that ice thickness is estimated with an uncertainty of roughly 50 the uncertainty in the cumulative thickness distribution is about 3 for ice less than 20 cm thick and about 9 for ice around 1 m thick these magnitudes are significantly smaller than the variations in thin ice observed by satellite in different months and regions we conclude that avhrr is capable of resolving the regional and seasonal variations in thin ice to a level of accuracy useful both for establishing a climatology and for testing ice models

petrogenesis of the flood basalts forming the northern kerguelen archipelago implications for the kerguelen plume the thick 20 km crust of the kerguelen archipelago formed as the tectonic setting of the kerguelen plume changed from an oceanic ridge centered location at 43 ma to its present location beneath the antarctic plate the uppermost crust is dominantly flood basalt with a thickness of up to 10 km inverse isochron 40ar 39ar ages for upper and lower lavas in a 630 m section of basalt flows from mont bureau are 30 4 and 29 0 ma re os isotopic systematics are consistent with this age most of the lavas in two stratigraphic sections mont bureau and mont rabouillere from the northern part of the archipelago have sr nd and pb isotopic characteristics similar to the youngest upper miocene to pleistocene lavas erupted in the southeast part of the archipelago i e initial 87sr 86sr 0 7050 143nd 144nd 10 mgo alkalic lavas in the southeast province of the archipelago in these northern sections it is confined to transitional lavas with

a nonlinear analysis of the enso cycle and its interdecadal changes abstract the nonlinear principal component analysis nlpca via a neural network approach was applied to thermocline anomalies in the tropical pacific while the tropical sea surface temperature sst anomalies had been nonlinearly mapped by the nlpca mode 1 onto an open curve in the data space the thermocline anomalies were mapped to a closed curve suggesting that enso is a cyclic phenomenon the nlpca mode 1 of the thermocline anomalies reveals the nonlinear evolution of the enso cycle with much asymmetry for the different phases the weak heat accumulation in the whole equatorial pacific is followed by the strong el nino and the subsequent strong drain of equatorial heat content toward the off equatorial region precedes a weak la nina this asymmetric enso evolution implies that the nonlinear instability enhances the growth of el nino but dwarfs the growth of la nina the nonlinear enso cycle was found to have changed since the late 1970s for the pre 1980s the enso cycle associated with the ther

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

absolute palaeointensity of oligocene 28 30 ma lava flows from the kerguelen archipelago southern indian ocean we report palaeointensity estimates obtained from three oligocene volcanic sections from the kerguelen archipelago mont des ruches mont des tempetes and mont rabouillere of 402 available samples 102 were suitable for a palaeofield strength determination after a preliminary selection among which 49 provide a reliable estimate application of strict a posteriori criteria make us confident about the quality of the 12 new mean flow determinations which are the first reliable data available for the kerguelen archipelago the virtual dipole moments vdm calculated for these flows vary from 2 78 to 9 47 10 2 2 am 2 with an arithmetic mean value of 6 15 2 1 10 2 2 am 2 compilation of these results with a selection of the 2002 updated iaga palaeointensity database lead to a higher 5 4 2 3 10 2 2 am 2 oligocene mean vdm than previously reported goguitchaichvili et al 2001 riisager 1999 identical to the 5 5 2 4 10 2 2 am 2 mean vdm obtained for the 0 3 5 ma time window however these kerguelen palaeointensity estimates represent half of the reliable oligocene determinations and thus a bias toward higher values nonetheless the new estimates reported here strengthen the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0 3 ma

arctic sea ice leads from advanced very high resolution radiometer images a large number of advanced very high resolution radiometer avhrr images from throughout 1989 are analyzed to determine lead characteristics the units of analysis are square 200 km cells and there are 270 such cells in the data set clouds are masked manually leads are determined from images of the potential open water a scaled version of the surface temperature or albedo that weights thin ice by its thermal or brightness impact the lead fraction is determined as the mean the monthly mean lead fraction ranges from 0 02 in winter to 0 06 in summer in the central arctic and is near 0 08 in the winter in the peripheral seas a method of accounting for lead width sampling errors due to the finite sample areas is introduced in the central arctic the observed mean lead width for a threshold of 0 1 ranges from 2 or 3 km near the resolution of the instrument in the winter to 6 km in the summer in the peripheral seas it is about 5 km in the winter width distributions are often more heavily weighted in the tail than exponential distributions and are well approximated by a power law the along track number density power law n aw b has a mean exponent of b 1 60 standard deviation 0 18 and shows some seasonal variability mean floe widths in the central arctic are 40 to 50 km in the winter dropping to about 10 km in the summer for floes the power law has a mean exponent of 0 93 and exhibits a clearer annual cycle lead orientation is determined with a method based on the direction of maximum extent

statistical assessment of preferred transitional vgp longitudes based on palaeomagnetic lava data summary we examine the statistical significance of a recent report indicating that palaeomagnetic lava data recording reversals and excursions over the past 20 myr yield transitional virtual geomagnetic poles vgps tending to fall along american and asian longitudes using a bootstrap analysis of spherical densitygrams we find that this result is neither a sensitive function of nor heavily dependent upon particular transitional records previous analyses used 2 and kuiper tests of longitudinal uniformity of transitional vgps however the bootstrap analysis presented here takes such statistical tests one step further showing specifically that american and asian longitudes are preferred with significance at about the 95 per cent confidence level furthermore provided the appropriate polarity convention is used we confirm that icelandic transitional vgps tend to fall along asian longitudes these results are indicative of non dipolar transitional fields whose structure is governed by some form of core mantle coupling

dynamics of the terra nova bay polynya the potential of multi sensor satellite observations research on processes leading to formation maintenance and disappearance of polynyas in the polar regions benefits significantly from the use of different types of remote sensing data the sentinels of the european space agency esa together with other satellite missions provide a variety of data from different parts of the electromagnetic spectrum at different spatial scales and with different temporal resolutions in a case study we demonstrate the advantage of merging data from different spaceborne instruments for analysing ice conditions and ice dynamics in and around the frequently occurring terra nova bay polynya tnbp in the ross sea in the antarctic starting with a list of polynya parameters that are typically retrieved from satellite images we assess the usefulness of different sensor types on regional scales several 100 km passive microwave radiometers provide a view on the mutual influence of the three ross sea polynyas on sea ice drift and deformation patterns optical sensors with meter scale resolution on the other hand allow very localized analyses of different polynya zones the combination of different ranges of the electromagnetic spectrum is essential for recognition and classification of ice types and structures radar images together with data from thermal infrared sensors operated at tens to hundreds of meters resolution improve the separation of the outlet zone of the polynya from the adjacent pack ice the direct comparison of radar and passive microwave images reveals the visibility of deformed ice zone in the latter a sequence of radar images was employed to retrieve ice drift around the tnb which allows analysing the temporal changes of the polynya area and the extension and structure of the outlet zone as well as ice movements and deformation that are influenced by the katabatic winds

nonlinear modes of north american winter climate variability derived from a general circulation model simulation abstract nonlinear principal component analysis nlpca via a neural network nn approach was applied to an ensemble of six 47 yr simulations conducted by the canadian centre for climate modelling and analysis cccma second generation atmospheric general circulation model agcm2 each simulation was forced with the observed sea surface temperature from the global sea ice and sea surface temperature dataset gisst from january 1948 to november 1994 the nlpca modes reveal nonlinear structures in both the winter 500 mb geopotential height z500 anomalies and surface air temperature sat anomalies over north america with asymmetric spatial anomaly patterns during the opposite phases of an nlpca mode only during its negative phase is the first nlpca mode related to the el nino southern oscillation enso the positive phase is related to a weakened jet stream spatial patterns of the nlpca mode for the z500 anomalies generally agree with those for the sat anomalies nonlinear canonical correlation

magnetic interactions at the origin of abnormal magnetic fabrics in lava flows a case study from kerguelen flood basalts summary anisotropy of low field magnetic susceptibility ams of basaltic lava flows can give some clues about post emplacement tilting occurring in volcanic sequences such a study has been carried out on a sequence of 19 successive lava flows from kerguelen archipelago surprisingly two different patterns were observed the first one herein called normal fabric is consistent with the lava flow direction inferred from geological observations whereas the second one observed for about 70 per cent of the analysed samples herein called abnormal fabric appears to be unrelated to the shear history of the lava flows during their emplacements these abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin the unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high resolution electron backscattered diffraction measurements the aim of this study is to propose a physical interpretation of these fabrics a first step in our understanding was to clearly identify the nature and the magnetic properties of thefe tioxidesassemblage thus wepresentacomprehensive rockmagneticanalysisrelying on low and high temperature thermomagnetic curves k t crossover diagrams first order

el nino la nina and the nonlinearity of their teleconnections the paradigm of an atmospheric system varying linearly with respect to extreme phases of the el nino southern oscillation is questioned it is argued that the global response to tropical pacific sea surface temperature forcing will be inherently nonlinear a physical basis for this intrinsic nonlinearity is the thermodynamic control on deep convection climate statistics for warm and cold events of the tropical pacific are analyzed separately for the northern winter periods during 1950 96 composite analysis of 500 mb heights reveal planetary scale teleconnection patterns as noted in earlier studies a new result is the evidence for an appreciable 358 longitude phase shift between the warm and cold event circulation composites and the two wave trains appear to have different tropical origins a large nonlinear component in north american surface climate anomalies is also found which is consistent with such a phase shift in teleconnections in the tropics rainfall anomalies also show evidence of nonlinear behavior the maximum rain anomalies along the equator are located east of the date line during warm events but west of the date line during cold events the interpretation of this behavior is complicated however by the fact that composite warm event sst anomalies are not the exact inverse of their cold event counterparts idealized atmospheric general circulation model agcm experiments are performed in order to test the question of whether the observed nonlinearity is an intrinsic property of the atmospheric system the model is forced with a composite sst anomaly that undergoes a realistic seasonally varying enso life cycle as described by e rasmusson and t carpenter both positive and negative phases of the sst anomaly are used and a 40 member ensemble of warm and cold event model simulations is conducted a nonlinear climate response in the agcm is found that closely resembles the observed composites including a shift in the equatorial positions of the maxmium rain responses and a phase shift of teleconnection patterns in the upper troposphere barotropic model experiments indicate that the inherent nonlinearity in the tropical rain response may itself be responsible for the phase shift in the extratropical teleconnection patterns

more stable yet bimodal geodynamo during the cretaceous superchron we report palaeomagnetic and ar 40 ar 39 dating results from two sequences of basaltic lava flows deposited at the same locality in western china yet separated in time by similar to 50 myr one set lies within the cretaceous normal superchron at 112 115 ma and a second at 59 70 ma spanning the cretaceous palaeogene boundary we find that magnetic field directions during the superchron exhibit bimodal populations one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex multipolar field however the time dependent variability in field directions was 50 lower during the superchron than after which implies greater field stability during the superchron our results suggest that episodes of less dipolar field behavior occurred within the cretaceous superchron and raise the question whether a second more multipolar field state is more persistent than previously thought

assimilating satellite concentration data into an arctic sea ice mass balance model 1979 1985 using satellite ice concentration data observed buoy velocities and a thermodynamic ice growth model we compute the time histories of the thickness distributions of four ice types level and ridged first year ice and level and ridged multiyear ice these histories are computed for seven regions covering the arctic ocean with a 10 day time step for 1979 through 1985 a blend of observed buoy motions and wind driven free drift velocities drives the advective and ridging parts of the model while climatological growth and melt rates determine the ice thickness the model adjusts the thickness distributions so that the modeled concentrations of open water first year ice and multiyear ice match the concentrations derived by kalman filtering of satellite passive microwave observations the major adjustment is to reduce modeled ice concentrations in summer and particularly in the peripheral seas to account for unmodeled melt the model includes an ice salinity parameterization which along with changes in ice volume gives the salt or freshwater flux from the ice our results show that the arctic ice cover consists primarily of multiyear ice 60 by area and 82 by volume and that most of this ice is ridged the average ice thickness is 2 7 m with a seasonal variation of 30 and an interannual variation of 10 the study also shows that for each cell and the arctic as a whole the changes in ice volume caused by growth minus melt approximately balance the changes caused by import minus export the annual ice export through fram strait varies from 1100 to 3000 km3 primarily because of variations in ice velocity all regions except the chukchi sea have a net ice growth a net export and a net salt input to the ocean surface the annual mean salt flux to the ocean surface from net ice production is 7 3 kg m 2 downward fluxes along the transpolar drift stream are larger than elsewhere the largest downward flux of 33 kg m 2 yr 1 in the nansen basin can be attributed to the strong divergence near fram strait a companion paper steele et al this issue uses these fluxes and ice velocities to derive the arctic ocean freshwater balance

growth rate and salinity profile of first year sea ice in the high arctic this paper describes the growth of sea ice and the salinity profiles observed in eclipse sound near pond inlet baffin island canada during the winter of 1977 78 a numerical method of calculation has been developed to incorporate the variations in snow conditions and physical properties of ice and snow during the growth season it is shown that the growth rate can be predicted reasonably well it is also shown that the vertical salinity profile in the ice towards the end of the season provides a record of previous climatological conditions a dependence has been shown between the predicted growth rate and the measured salinity

ice production and brine formation in storfjorden svalbard brines which appear as salty and cold bottom water layers on arctic shelves form by salt rejection during sea ice formation high ice production occurs in latent heat polynyas such as the one that appears in storfjorden under northerly winds using ers 2 synthetic aperture radar imagery we observed the ice cover in storfjorden over the winter 1997 1998 revealing an area of over 6000 km 2 of the open water and thin ice that are characteristic of a polynya changes in the polynya extent correlated well with a simple wind driven polynya size algorithm furthermore using a frazil ice formation algorithm and the hypothesis of ice accumulation at the lee side of the polynya we distinguished between open water thin ice and fast pack ice the average polynya width during the 6 winter months was about 30 km with a maximum of 130 km in march thus storfjorden was composed of 5 6 fast and pack ice and 1 6 polynya open water occupied about half of the polynya the other half was composed of brash ice and thin ice involved in ridging and rafting a total ice volume of 40 km 3 was produced in storfjorden between november 15 1997 and may 15 1998 about 57 of it was formed in the open water area 9 in the thin ice area and 34 in the fast pack ice area this ice production released more than 1 gt gigaton of salt able to increase the salinity of the storfjorden waters by 1 4 practical salinity units psu assuming that brine enriched waters are 0 3 psu more saline than the parent waters the above salt release is able to ventilate storfjorden 4 5 times during the course of the productive season

pacific sea surface temperatures in the twentieth century an evolution centric analysis of variability and trend abstract a consistent analysis of natural variability and secular trend in pacific ssts in the twentieth century is presented by focusing on spatial and temporal recurrence but without imposition of periodicity constraints this single analysis discriminates between biennial enso and decadal variabilities leading to refined evolutionary descriptions and between these natural variability modes and secular trend all without advance filtering and potential aliasing of the sst record sst anomalies of all four seasons are analyzed together using the extended eof technique canonical enso variability is encapsulated in two modes that depict the growth east to west along the equator and decay near simultaneous amplitude loss across the basin phases another interannual mode energetic in recent decades is shown linked to the west to east sst development seen in post climate shift ensos the noncanonical enso mode the mode is closely related to chiang and vimont s meridional mode and leads to som

post emplacement tilting of lava flows inferred from magnetic fabric study the example of oligocene lavas in the jeanne d arc peninsula kerguelen islands abstract statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results a difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement for 2 successive series of flows on the jeanne d arc peninsula presently nnw dipping this method shows lava emplacement along a ssw nne direction this indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago additionally the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series

observations of ice thickness and frazil ice in the st lawrence island polynya from satellite imagery upward looking sonar and salinity temperature moorings 1 for the 1999 winter this paper examines the behavior of the bering sea st lawrence island polynya using a combination of advanced very high resolution radiometer avhrr radarsat synthetic aperture radar sar meteorological data over winter moored upward looking sonars uls and seabird salinity temperature sensors we define a thermal ice thickness from the avhrr retrieval of ice surface temperature combined with meteorological observations and a heat flux model south of the island we compare the uls and thermal thicknesses for congelation and frazil ice when the satellites observe congelation ice over the ulss the uls and thermal ice thicknesses generally agree when sar observes langmuir plumes over the ulss which indicate frazil ice formation the ulss show scatterers at 5 20 m depths in the water column and the seawater temperatures are either within 0 01 c of freezing or are slightly supercooled this suggests that during frazil events crystals either nucleate at depth or are transported to depth by the langmuir circulation the combination of the sar imagery and uls observations also allow measurement of the pack ice advection velocity the polynya width and the downwind frazil accumulation thickness giving widths of 10 to 30 km and thicknesses of 0 1 0 2 m substitution of these observed values with the heat flux into the pease polynya model yields polynya widths that approximately agree with the observed