a parametrization based on sea ice morphology of the neutral atmospheric drag coefficients for weather prediction and climate models 1 a hierarchy of parametrizations of the neutral 10 m drag coefficients over polar sea ice with different morphology regimes is derived on the basis of a partitioning concept that splits the total surface drag into contributions of skin drag and form drag the new derivation which provides drag coefficients as a function of sea ice concentration and characteristic length scales of roughness elements needs fewer assumptions than previous similar approaches it is shown that form drag variability can explain the variability of surface drag in the marginal sea ice zone miz and in the summertime inner arctic regions in the miz form drag is generated by floe edges in the inner arctic it is generated by edges at melt ponds and leads due to the elevation of the ice surface relative to the open water surface it is shown that an earlier fit of observed neutral drag coefficients is obtained as a special case within the new concept when specific simplifications are made which concern the floe and melt pond geometry due to the different surface morphologies in the miz and summertime arctic different functional dependencies of the drag coefficients on the sea ice concentration result these differences cause only minor differences between the miz and summertime drag coefficients in average conditions but they might be locally important for atmospheric momentum transport to sea ice the new parametrization formulae can be used for present conditions but also for future climate scenarios with changing sea ice conditions

variability of arctic sea ice topography and its impact on the atmospheric surface drag over the polar oceans near surface atmospheric transport of momentum is strongly influenced by sea ice surface topography the latter is analyzed on the basis of laser altimeter data obtained during airborne campaigns between 1995 and 2011 over more than 10 000 km of flight distance in different regions of the arctic ocean spectra of height and spacing between topographic features averaged over 10 km flight sections show that typical values are 0 45 m for the mean height and about 20 m for the mean spacing nevertheless the variability is high and the spatial variability is stronger than the temporal one the total topography spectrum is divided into a range with small obstacles between 0 2 m and 0 8 m height and large obstacles 0 8 m results show that large pressure ridges represent the dominant topographic feature only along the coast of greenland in the central arctic the concentration of large ridges decreased over the years accompanied by an increase of small obstacles concentration and this might be related to decreasing multiyear ice the application of a topography dependent parameterization of neutral atmospheric drag coefficients reflects the large variability in the sea ice topography and reveals characteristic differences between the regions based on the analysis of the two spectral ranges we find that the consideration of only large pressure ridges is not enough to characterize the roughness degree of an ice field and the values of drag coefficients are in most regions strongly influenced by small obstacles

a parameterization of the ice ocean drag coefficient 1 a parameterization of the ice ocean drag coefficient cw was developed through partitioning the oceanic drag force into three components 1 form drag on the floe edge 2 form drag on the ridge keel and 3 skin friction on the ice bottom through these quantities cw was expressed as a function of observable sea ice geometric parameters sensitivity studies were carried out to investigate the influence of varying sea ice conditions on cw the results revealed that cw increases first and then decreases with increasing ice concentration a similar to the observations of the air ice drag coefficient and which is mainly attributed to the nonmonotonic variation of the form drag on the floe edge with ice concentration moreover the form drag on the floe edge is always the dominant component having a proportion of more than 60 in sea ice with a large aspect ratio draft length 1 100 indicating the necessity of including this term in sea ice dynamic models particularly for the marginal ice zone miz the form drag on the ridge keel becomes dominant only when the ridging intensity is extremely high depth spacing 1 20 additionally a large value of cw cannot be caused only by the inclusion of form drag terms but also by large skin friction over rough ice bottoms finally for typical situations in the miz with moderate ridging intensity the parameterization will underestimate cw by approximately 30 for a rough ice bottom and by over 80 for a smooth ice bottom if no form drags are considered

isotopic evidence of holocene climatic change in the san juan mountains colorado abstract the d of cellulose from 14c dated wood collected in the san juan mountains of southwestern colorado decreased by about 45 from 9600 to 3100 yr b p and an additional 25 to the present the wood samples are from trees that grew above present day tree line and reflect a time of warmer average summer temperatures these changes in d are interpreted to indicate a major change during the holocene in the sources of moisture in the seasonality of precipitation or in both

lower timberline in central colorado during the past 15 000 yr a 15 000 yr paleoenvironmental record at the elevation of lower timberline in central colorado suggests two major paleoclimatic changes during that interval 1 at 10 000 yr b p from cool moist conditions to warm moist conditions and 2 at 4 000 yr b p to warm dry conditions this sequence can be interpreted as a shift from precipitation predominantly originating from pacific winter storms before 10 000 yr b p to a predominantly monsoonal type regime lasting until 4 000 yr b p to the present pattern contemporaneity of higher upper timberline and lower lower timberline during the 10 000 to 4 000 yr b p interval is not contradictory but a result of changes in those circulation patterns

influence of a sea ice ridge on low level airflow the influence of a single pressure ridge of 4 5 m height on the structure of the atmospheric surface layer is studied the field of the mean wind velocity is governed by typical features of a bernoulli effect with a speedup over the crest and a shadowing effect downwind of the ridge it is found that the turbulence generated by the ridge compensates for the deformation of the flow field by mixing momentum downward both mean and turbulent fields are restored to their upwind values at a distance of 300 m downwind of the ridge which is equivalent to an aspect ratio of 0 015 the level of maximum turbulence generated by the ridge is characterized by a linear relationship a formulation for the determination of the form drag of a single ridge is proposed and generalized toward an ensemble of ridges we estimate that the form drag contributes 50 to the total drag exerted by a typically ice covered sea surface on the atmospheric flow

pattern and forcing of northern hemisphere glacier variations during the last millennium time series depicting mountain glacier fluctuations in the alps display generally similar patterns over the last two centuries as do chronologies of glacier variations for the same interval from elsewhere in the northern hemisphere episodes of glacier advance consistently are associated with intervals of high average volcanic aerosol production as inferred from acidity variations in a greenland ice core advances occur whenever acidity levels rise sharply from background values to reach concentrations 1 2 equiv h kg above background a phase lag of about 10 15 yr equivalent to reported response lags of alpine glacier termini separates the beginning of acidity increases from the beginning of subsequent ice advances a similar relationship but based on limited and less reliable historical data and on lichenometric ages is found for the preceding 2 centuries calibrated radiocarbon dates related to advances of non calving and non surging glaciers during the earlier part of the little ice age display a comparable consistent pattern an interval of reduced acidity values between about 1090 and 1230 a d correlates with a time of inferred glacier contraction during the medieval optimum the observed close relation between noothern hemisphere glacier fluctuations and variations in greenland ice core acidity suggests that sulfur rich aerosols generated by volcanic eruptions are a primary forcing mechanism of glacier fluctuations and therefore of climate on a decadal scale the amount of surface cooling attributable to individual large eruptions or to episodes of eruptions is simlar to the probable average temperature reduction during culminations of little ice age alacier advances ca 0 5 1 2 c as inferred from depression of equilibrium line altitudes

weddell gyre temperature maximum stratum at depths below 200 m relatively warm salty water spreads poleward from the antarctic circumpolar current acc this deep water mass is cooled through interaction with the atmosphere feeding the formation of dense bottom water which in turn influences much of the world ocean significant poleward spreading is accomplished within the weddell gyre of the atlantic sector of the southern ocean an extensive data set obtained in the 1980s within the weddell gyre provides a detailed view of the subsurface temperature maximum t max stratum defining the top of the deep water mass the seasonal sea ice growth and retreat pattern reflects the t max distribution there is evidence for subdivision of the t max into two cold centers which may reflect secondary circulation features within the gyre the primary inflow of warm deep water is derived from the southern edge of the acc near 20 30 e the inflow spreads to the west along 65 s and is divided into smaller pools of relatively warm water west of maud rise with a cold feature directly over the rise in a manner shown by models of topographically generated eddies variability of the t max stratum in the vicinity of maud rise arising from changes in the vigor of the large scale circulation may influence the regional vertical heat fluxes and be related to interannual variability of the sea ice cover and to the recurring polynya feature typical of the region

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

st john s island red sea a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins abstract new gravity and magnetic data from the northern red sea reveal the extent of the large gravity anomaly 164 mgal and the presence of significant magnetic anomalies over st john s island spectral transformation and three dimensional potential field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required 1 2 g cm 3 suggests that it is composed of unserpentinised peridotite density 3 4 g cm 3 to a depth of at least 8 km st john s island is uniquely located not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation this precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones thermal contraction serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent to continent fracture zone a post mesozoic age of emplacement associated with the onset of continental rifting and the rejuvenation of a pre existing continental fracture seems most probable

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

upper mantle heterogeneities in africa deduced from rayleigh wave dispersion abstract rayleigh wave group velocities have been measured along 11 paths on the african continent north of the equator the results lead to a division of this part of the continent into three regions cratonic areas non cratonic areas mobile zones and a region located to the east of the meridian 31 e red sea neighbourhood ethiopia djibouti the highest velocities are found beneath the cratonic areas at 200 km depth 5 at greater depths the mobile zones exhibit higher velocities than the cratonic regions but the difference is not as marked 2 and the resolution is poor velocities in the eastern region are similar to those found under young oceanic regions

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

late pleistocene glacial equilibrium line altitudes in the colorado front range a comparison of methods abstract six methods for approximating late pleistocene pinedale equilibrium line altitudes elas are compared for rapidity of data collection and error rmse from first order trend surfaces using the colorado front range trend surfaces computed from rapidly applied techniques such as glaciation threshold median altitude of small reconstructed glaciers and altitude of lowest cirque floors have relatively high rmses 97 186 m because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability surfaces computed for accumulation area ratios aars and toe to headwall altitude ratios thars of large reconstructed glaciers show that an aar of 0 65 and a thar of 0 40 have the lowest rmses about 80 m and provide the same mean ela estimate about 3160 m as that of the more subjectively derived maximum altitudes of pinedale lateral moraines rmse 149 m second order trend surfaces demonstrate low elas in the latitudinal center of the front range perhaps due to higher winter accumulation there the mountains do not presently reach the ela for large glaciers and small front range cirque glaciers are not comparable to small glaciers existing during pinedale time therefore pleistocene ela depression and consequent temperature depression cannot reliably be ascertained from the calculated ela surfaces

shear velocity structure of the laterally heterogeneous crust and uppermost mantle beneath the indian region abstract the shear velocity structure of the indian lithosphere is mapped by inverting regionalized rayleigh wave group velocities in time periods of 15 60 s the regionalized maps are used to subdivide the indian plate into several geologic units and determine the variation of velocity with depth in each unit the hedgehog monte carlo technique is used to obtain the shear wave velocity structure for each geologic unit revealing distinct velocity variations in the lower crust and uppermost mantle the indian shield has a high velocity 4 4 4 6 km s upper mantle which however is slower than other shields in the world the central indian platform comprised of proterozoic basins and cratons is marked by a distinct low velocity 4 0 4 2 km s upper mantle lower crustal velocities in the indian lithosphere generally range between 3 8 and 4 0 km s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities respectively a remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the indian plate the south bruma region along the eastern subduction front of the indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle high velocities 4 8 km s are also observed in the upper mantle beneath the ninetyeast ridge in the northeastern indian ocean

on the origin of the seismic anisotropy of the lithosphere summary it is known that flow in the mantle can produce preferred orientation in olivine crystals with seismic anisotropy as a consequence flow in the subcrustal lithosphere is unlikely because of the high viscosity lenses of high temperature and low viscosity anomalous mantle are located under the crust in many tectonically active regions and viscous flow can easily arise in such material resulting in seismic anisotropy after cooling such anomalous mantle acquires high viscosity and becomes incorporated into the lithospheric layer preserving the anisotropy produced by the flows which existed previously the interaction of the stresses with cracks in the upper crust can be one of the causes of anisotropy in this layer

late quaternary snowline reconstructions for the southern and central sierra nevada california and a reassessment of the recess peak glaciation abstract in the sierra nevada the recess peak glaciation has been previously defined on the basis of deposits exhibiting relative age characteristics intermediate between those of the little ice age deposits and those of early holocene or older moraines in the absence of reliable chronological control the recess peak deposits were assigned an early neoglacial age although numerous moraines in the central and southern sierra have been attributed to this interval regional snowline gradients reconstructed from these deposits lack internal consistency and appear to represent several distinctly different episodes of glacier advance as a basis for comparison with the recess peak data modern and late pleistocene regional snowlines were reconstructed using accumulation area ratios and cirque floor altitudes these reconstructions display regionally consistent gradients rising gradually southward and more steeply eastward based on these data the full glacial late pleistocene snowline depression is estimated to have been 800 m estimates of recess peak snowline depression vary widely ranging from 140 to 500 m and a reconstructed regional gradient rises northward in opposition to the late pleistocene and modern snowlines limited radiocarbon dating and the irregular pattern derived from the recess peak snowline data suggest that even in the type area these deposits resulted from both pre and post hypsithermal glacier advances

evidence for anisotropy in north east africa from geographical and azimuthal distribution of rayleigh wave velocities and average upper mantle structure long period surface waves from wwssn gdsn sro and geoscope stations are analysed to obtain the dispersion of surface waves in the north east of africa lateral variations of rayleigh wave velocities are retrieved simultaneously with the average azimuthal anisotropy as a first result the low velocity anomaly under the red sea is apparent at all periods a second result is a nearly north south direction of maximum group and phase velocities this azimuthal anisotropy direction is in agreement with the direction of absolute plate motion in this region anisotropy is also evidenced by simultaneous inversion of love and rayleigh wave dispersion along the path tam agd in the resulting models isotropic and anisotropic very low mantle sv wave velocities extend up to the moho for sh waves we find a strong discrepancy with sv waves sh wave velocities being higher by 5 down to 200 km under the crust this is characteristic of lithospheric polarization anisotropy the s wave anisotropy polarization and azimuthal found in this region including the red sea is compatible with the absolute motion of the african plate it can be interpreted considering the low lithospheric s velocities as a consequence of plastic flow in the weak lithosphere of an anomalous mantle this anisotropy evidenced here on s waves may be due as pn and lithospheric s wave anisotropy found in other continental areas to dynamical processes related to plate motion

first two station results for long period surface waves velocity from the geoscope stations in africa rayleigh and love phase and group velocity dispersion measurements are performed along a purely african path between the geoscope stations tam and agr for the leewards isl earthquake of march 16 1985 this regional study is important as it brings deep seated structure information in a seismologically blank region of africa it enables to improve the spatial resolution of larger scale models for periods longer than 160 s and up to 250 s the fundamental rayleigh wave dispersion is in agreement with prediction by global models for shorter periods the results fall below this prediction which is an indication of lithospheric s wave velocities lower than those of the models the results are also compared with previous regionalizations the rayleigh second higher mode is extracted at agr and the phase velocity dispersion obtained in the period range 50 140 s is in agreement with the theoretical dispersion for prem

investigation of upper mantle structure under young regions of the southeast pacific using long period rayleigh waves abstract long period rayleigh wave group velocity dispersion curves are presented for paths across the east pacific ocean the records are from the ipg site at pamatai tahiti and the ida station at nana for the first time direct path observations of group velocities up to 300 s have been obtained this study shows that for young oceanic regions group velocities are low even at long periods the observations are interpreted in terms of an s wave velocity model by a generalized inversion scheme in the models for young ages the low velocity zone under the asthenosphere lid is well developed with a strong velocity gradient at the bottom of this zone followed at larger depths by a plateau representing a lower velocity zone and a marked gradient at 400 km

on the structure of the crust and upper mantle beneath the afro arabian region from surface wave dispersion abstract the deep structure beneath the afro arabian region was investigated using rayleigh and love waves in the period range 20 250 s models of crustal and upper mantle structure were inferred by simultaneous inversion of both phase and group velocities our choice of model parametrization allowed for a transverse isotropic medium and a variable depth of the moho the crustal thicknesses were found to vary from 43 to 45 km on the african plate the moho is deeper in the arabian plate 45 47 km but in the vicinity of the red sea the crust becomes much thinner 30 km the upper mantle under arabia is more similar to that of an african mobile zone than to that of a shield a low velocity zone lvz with an s wave velocity of 4 14 4 2 km s was found in the depth range 120 140 km however this lvz is less pronounced than that found in the vicinity of the red sea s wave velocity 4 08 km s in agreement with previous studies the anisotropy v sh v sv v sv has been verified for the afro arabian region we interpret the anisotropies uisng the hypothesis of lattice preferred orientation of the olivine crystals in the upper mantle the inference is that a vertical flow exists up to near the surface in the vicinity of the red sea rift vsv is 8 9 faster than vsh under the african and arabian sides the flow is essentially vertical below depths of 200 40 km and becomes essentially horizontal above with a vsh from 6 to 14 faster than vsv

anisotropy of the african continent inferred from surface waves abstract a no block regionalization is performed over the african continent to retrieve simultaneously local surface wave velocities and the 2 azimuthal variation of rayleigh waves the geographical distribution of phase and group velocities shows general agreement with global models but with enhanced lateral resolution a correlation with surface tectonics is found even at long periods low velocities occur in the tectonically active regions and high velocities in the shield areas the azimuthal anisotropy analysis results in a general north south fastest direction for rayleigh waves which could be correlated with the african plate s absolute motion the amplitude is of the order of 2 an isotropic inversion with data corrected for effects of surficial layers is performed very slight differences appear between the corrected and uncorrected results lateral heterogeneities of rayleigh and love waves are then inverted simultaneously with the 2 azimuthal terms of rayleigh waves by vectorial tomography to retrieve the 3d structures of s wave velocity and anisotropy throughout the african continent good correlation between low velocities and small anisotropies is found and their location at depth suggests an origin for african hotspots