variability in southern hemisphere ocean circulation from the 1980s to the 2000s abstractinterannual to decadal variability of ocean circulation in the southern hemisphere was examined using data from the 1980s to the 2000s in a box inverse model to estimate transport across hydrographic sections and three ocean general circulation models ogcms the westerly wind stress over the ogcm southern ocean showed a steady increase of 5 8 decade 1 the meridional overturning circulation was quantified by the transport across 30 s the ogcms suggested a slight strengthening from 0 2 1 0 to 0 8 1 3 sv decade 1 1 sv 106 m3 s 1 of the upper meridional cell deacon cell and two ogcms showed a weakening 0 8 0 6 and 1 0 0 3 sv decade 1 of the lower meridional antarctic bottom water aabw cell partly explained by contraction of the aabw volume the box inverse estimates did not contradict these two findings for antarctic circumpolar current transport quantified by zonal transport across four key sections the box inverse model estimated a decrease of 5 21 sv decomposition

antarctic ozone depletion causes an intensification of the southern ocean super gyre circulation 1 recent climate trends over the southern hemisphere sh summer feature a strengthening of the circumpolar westerly and a weakening of the midlatitude westerly extending from the stratosphere to earth s surface much of the change is attributable to antarctic ozone depletion however the consequential ocean circulation changes are unknown here i demonstrate that the observed surface wind changes have forced a southward shift and spin up of the super gyre which links the subtropical south pacific indian and atlantic ocean circulation advecting more warm water southward the circulation change includes a strengthening of the east australian current eac flow passing through the tasman sea the southward shift may be responsible for the observed unusually large warming in the sh midlatitude ocean and may contribute to the reported range extension to the south of many marine species in the south west pacific

spatiotemporal variation in exhumation of the crystallines in the nw himalaya india constraints from fission track dating analysis abstract during himalayan orogeny coeval thrusting along the main central munsiari thrust mct mt and extension along the south tibetan detachment system stds are widely responsible for rapid exhumation of the higher himalayan crystalline hhc zone apatite and zircon fission track data along the kaliganga and darma valleys in the kumaon himalaya serve to document the shallow bedrock exhumation history of the hhc taking into account sample location within the hhc with respect to the mct mt the apatite fission track aft data sets along the darma 1 0 0 1 to 2 8 0 3 ma and kaliganga 1 4 0 2 to 2 4 0 3 ma which are sharing same structural setting and rock types and being separated by 40 km show very similar patterns of exhumation histories since plio quaternary in the kumaon himalaya data sets along darma and kaliganga are very similar to data set of adjacent traverse 50 km away along the goriganga valley studied by patel and carter 2009 whole data sets within the hhc in kumaon himalaya provide clear evidence for plio quaternary tectonic activity along the vaikrita thrust vt precipitation in this region exerts a strong influence on erosional surface processes fluvial erosional unloading along the himalaya is focused on the high mountainous region of the hhc where the orographic barrier forces out the maximum percentage of annual rainfall ft cooling ages reveal coincidence between rapid erosion and exhumation that is focused in a 25 30 km wide sector of the hhc rather than covering the entire orogen similarity of aft age pattern and exhumation rates along all three major traverses goriganga darma and kaliganga indicates that the region has been experiencing constant rate of crustal uplift and erosion since long time comparison of fission track ages from the kumaon himalaya with other segments of the nw himalaya shows spatiotemporal variation in exhumation it is described due to the development of local structures such as dome window in jammu and kashmir and himachal himalayas and quaternary active thrusting along the vt and mct mt in the garhwal kumaon himalaya

the role of extension during brittle deformation within the nw indian himalaya 1 synorogenic extension has been recognized as an integral structural constituent of mountain belts and high elevation plateaus during their evolution in the himalaya both orogen parallel and orogen normal extension has been recognized however the underlying driving forces for extension and their timing are still a matter of debate here we present new fault kinematic data based on systematic measurements of hundreds of outcrop scale brittle fault planes in the nw indian himalaya this new data set as well as field observations including crosscutting relationships mineral fibers on fault planes and correlations with deformation structures in lake sediments allows us to distinguish different deformation styles the overall strain pattern derived from our data reflects the large regional contractional deformation pattern very well but also reveals significant extensional deformation in a region which is dominated by shortening in total we were able to identify six deformation styles most of which are temporally and spatially linked representing protracted shortening our observations also furnish the basis for a detailed overview of the younger deformation history in the nw himalaya which has been characterized by extension overprinting previously generated structures related to shortening the four dominant deformation styles are 1 shortening parallel to the regional convergence direction 2 arc normal extension 3 arc parallel extension and finally 4 e w extension this is the first data set where a succession of both arc normal and e w extension has been documented in the himalaya importantly our observations help differentiate e w extension triggered by processes within the tibetan plateau from arc parallel and arc normal extension originating from the curvature of the himalayan orogen

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

interseismic strain accumulation on the himalayan crustal ramp nepal the departement of mines and geology has been monitoring the seismicity of the central himalayas of nepal since 1985 intense microseismicity and frequent medium size earthquakes ml 4 tend to cluster beneath the topographic front of the higher himalaya this 10 20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data both microseismic and geodetic data indicate strain accumulation on a mid crustal ramp that had been previously inferred from geological and geophysical evidence this ramp connects a flat decollement under the lesser and sub himalaya with a deeper decollement under the higher himalaya and probably acts as a geometric asperity where strain and stress build up during the interseismic period the large himalayan earthquakes could nucleate there and probably activate the whole flat and ramp system up to the blind thrusts of the sub himalaya

focal depths and fault plane solutions of earthquakes and active tectonics of the himalaya synthetic seismograms were compared with long period body waves for nine earthquakes with epicenters in the himalayan arc to determine depths of foci and to improve fault plane solutions focal depths are shallow 10 20 km inferred slip vectors are locally perpendicular to the mountain range they plunge very gently about 10 deg in the eastern sections of the range and more steeply about 25 deg in western sections assuming india to be a rigid plate the radially oriented slip vectors imply that southern tibet extends at about half the rate of underthrusting in the himalaya and therefore probably at about 5 10 mm yr the shallow depths and gentle dips of the fault planes at least for the events in the eastern half of the range are consistent with coherent underthrusting of the indian plate beneath at least the lesser himalaya the steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the greater himalaya

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

the dynamics of large scale wind driven variations in the antarctic circumpolar current a nonlinear model is devloped and analytical results obtained to discuss the response of the antarctic circumpolar current to wind forcing over a wide range of frequencies the main results are as follows i the nonlinear equations of motion can be conveniently separated into one baroclinic and one barotropic mode ii for forcing with period t equal to less than a few years wind driven fluctuations in the antarctic circumpolar current are barotropic and governed by the linearized laplace tidal equations theory suggests that fluctuations in the transport should lag and be most strongly correlated with the circumpolar averaged wind stress these theoretical results are consistent with recent measurements made in drake passage an interested untested theoretical prediction is that the sea level fluctuations measured at the southern side of drake passage with t between one month and a few years should be coherent at zero lag with sea level fluctuations at the same latitude around the earth iii for longer period forcing baroclinic fluctuations are important the baroclinic pressure current and associated density variations all decrease exponentially with depth exponential depth decay of these baroclinic fields is in fact observed the decay scale being about 1 km iv the theory indicates thatmore significant large scale wind driven fluctuations in the strength of the baroclinic antarctic circumpolar current can only occur at frequencies with periodicity or approx 70 years climatic changes associated with such variability must therefore consist of oscillations of similar or longer period this is consistent with limited observations which suggest that wind and sea surface temperature in the region of the antarctic circumpolar current have fluctuated through one cycle over the last 100 years less

moment tensor inversion of small earthquakes in southwestern germany for the fault plane solution summary the amplitudes of direct p and s waves as well as p and other deep crustal body wave phases from events in southwestern germany are studied for the effects of the double couple earthquake radiation pattern the direct p and s wave amplitudes for six earthquakes in the southern black forest of west germany with magnitudes between 2 2 and 0 5 were inverted for the source moment tensor and the double couple components were extracted from the solutions and refined by a further inversion the inversion focal mechanisms using both p and s wave amplitudes were quite consistent with those found from a large set of first motion readings even when very different crustal models were used for the events at or above magnitude 1 4 inversions using only direct p wave amplitudes also yielded good focal mechanisms furthermore the vertical component record at a station at p distance from the largest of the events was successfully synthesized using local mechanisms representative of those found from the inversions the phases modelled included p pmp and several free surface reflections the effect of the earthquake radiation pattern was also noted in the amplitudes of the p arrivals of a set of azimuthally distributed stations for two other earthquakes this study confirms that direct p and s amplitudes can be inverted for the source focal mechanism and suggests that deeper crustal phases like p pmp etc are also affected by the earthquake radiation pattern

the role of vertical eddy flux in southern ocean heat uptake 1 the role of changing vertical eddy heat flux on southern ocean heat uptake is investigated in an idealized eddy permitting ocean model enhanced air sea heat flux drives deep reaching southern warming due to a reduction in the isopycnal meridional temperature gradient and therefore decreased upward eddy heat flux this mechanism is qualitatively similar in models with either permitted or parameterized eddies due to its dependence on the isopycnal temperature gradient rather than the dynamical response of the eddy field in contrast increased wind stress drives mid depth southern ocean cooling through an enhancement of the eddy field and resultant eddy heat divergence the transient cooling extends over multiple decades while the mean flow adjusts to balance the faster eddy response this wind driven cooling mechanism has not been captured by coarse resolution models with fixed eddy parameterizations and is a possible candidate for the recent cooling observed in the southern ocean

the global influence of localized dynamics in the southern ocean the circulation of the southern ocean connects ocean basins links the deep and shallow layers of the ocean and has a strong influence on global ocean circulation climate biogeochemical cycles and the antarctic ice sheet processes that act on local and regional scales which are often mediated by the interaction of the flow with topography are fundamental in shaping the large scale three dimensional circulation of the southern ocean recent advances provide insight into the response of the southern ocean to future change and the implications for climate the carbon cycle and sea level rise

an equivalent barotropic mode in the fine resolution antarctic model abstract both the 6 year time mean flow and the eddy kinetic energy in the fine resolution antarctic model are found to be approximately self similar in the vertical with the velocity showing a rapid decay with depth a simple argument is given as to why this would be expected to occur in this and other numerical models and an analysis is given as to the degree of steering of the mean flow by topography

observed subsurface signature of southern ocean sea level rise satellite altimetry data show a strong increase in sea level in various parts of the southern ocean over the 1990s in this paper we examine the causes of the observed sea level rise in the region south of australia using 13 years of repeat hydrographic data from the woce sr3 sections and the survostral xbt and surface salinity data the hydrographic data show a poleward shift in the position of the subtropical and the subantarctic fronts over the period in the antarctic zone the antarctic surface water has become warmer and fresher and the winter water tongue has become warmer fresher thinner and shallower increased freshening south of the polar front is linked to increased precipitation over the 1990s temperature changes over the upper 500 m account for only part of the altimetric sea level rise the ctd sections show that the deeper layers are also warmer and slightly saltier and the observed sea level can be explained by steric expansion over the upper 2000 m enso variability impacts on the northern part of the section and a simple sverdrup transport model shows how large scale changes in the wind forcing related to the southern annular mode may contribute to the deeper warming to the south

an altimetry based gravest empirical mode south of africa 1 development and validation 1 hydrographic transects of the antarctic circumpolar current acc south of africa are projected into baroclinic stream function space parameterized by pressure and dynamic height this produces a two dimensional gravest empirical mode gem that captures more than 97 of the total density and temperature variance in the acc domain weekly maps of absolute dynamic topography data derived from satellite altimetry are combined with the gem to obtain a 16 year time series of temperature and salinity fields the time series of thermohaline fields are compared with independent in situ observations the residuals decrease sharply below the thermocline and through the entire water column the mean root mean square rms error is 0 15c 0 02 and 0 02 kg m 3 for temperature salinity and density respectively the positions of acc fronts are followed in time using satellite altimetry data these locations correspond to both the observed and gem based positions the available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar this is compared with velocity measurements from repeat hydrographic transects at the goodhope line the net accumulated transports of the acc derived from these different methods are within 1 3 sv of each other similarly gem produced cross sectional velocities at 300 dbar compare closely to the observed data with the rms difference not exceeding 0 03 m s 1 the continuous time series of thermohaline fields described here are further exploited to understand the dynamic nature of the acc fronts in the region and which is given by swart and speich 2010

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

seismotectonics and crustal stress field in the kumaon garhwal himalaya abstract we present fault plane solutions of 94 well located small to moderate sized 1 5 ml 5 4 earthquakes which occurred in the kumaon garhwal himalaya during 2005 2008 using p wave polarity and body wave amplitudes these earthquakes show a mixture of thrust normal and strike slip type mechanism with a majority of thrust type most of the thrust earthquakes occur at a depth of 8 22 km in the main central thrust mct zone and the lower himalaya the spatial distribution of these earthquakes suggest that the strain resulting from the ongoing collision of the indian plate with the eurasian plate is being consumed by thrust fault movement mainly on the north dipping munsiari thrust and south dipping tons thrust the strike slip earthquakes are mainly observed in the lower himalaya as well as around the munsiari region in the mct zone the normal earthquakes are also observed in different parts of the kumaon garhwal himalaya and the gangetic plain their occurrence is attributed to the local structure s as well as the flexure of the indian plate stress tensor inversion of the calculated fault plane solutions indicates that the maximum compressive stress in the gangetic plain is n s directed and near vertical whereas in the kumaon garhwal himalaya it is near horizontal and nne ssw directed and correlating with the prevailing stress condition due to northward movement of indian plate

circumpolar structure and distribution of the antarctic circumpolar current fronts 2 variability and relationship to sea surface height 1 in part 1 of this study we showed that the antarctic circumpolar current acc consisted of multiple fronts each of which was consistently associated with a particular contour of sea surface height ssh or approximate streamline in part 2 we have used maps of ssh to examine the variability of the acc fronts between 1992 and 2007 the ssh label associated with each frontal branch is nearly constant around the circumpolar belt the front labels are also nearly constant in time the bands of enhanced ssh gradient i e fronts occur along the same streamlines throughout the 15 year period of observations both short and long period changes of the ssh frontal labels of the acc are small based on a tight relationship between dynamic height and cumulative baroclinic transport of the acc the baroclinic transport variability of the individual branches of the acc is also expected to be small the major change in the total acc baroclinic transport occurs in the drake passage the streamline associated with the northern branch of the saf saf n does not pass through drake passage and the waters carried by this branch are not observed there instead the transport of the saf n turns north in the pacific to supply the export of water to the indian ocean north and south of australia strong eddy activity in the southeast pacific acts to dissipate the hydrographic signature of the saf n there in the atlantic the saf n reappears as the same streamline is again associated with enhanced ssh gradients to the east of the brazil malvinas confluence zone while the large changes in ssh have occurred in the southern ocean between 1992 and 2007 there are strong regional differences because the acc fronts are robustly associated with particular ssh contours the changes in ssh reflect shifts in the position of the acc fronts in the circumpolar average each of the acc fronts has shifted to the south by about 60 km the changes in ssh in the southern ocean are largely due to changes in ocean circulation rather than warming and freshening by atmospheric fluxes much larger changes in ssh are observed in some locations of the southern ocean particularly where the fronts interact with large scale topography the northern branch of the pf pf n near the kerguelen plateau is an extreme example where the pf n followed a path around the northern end of kerguelen plateau between 1992 and 1997 passed through the fawn trough after 2003 and oscillated between the two paths between 1997 and 2003

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

interdecadal water mass changes in the southern ocean between 30 e and 160 e interdecadal water mass changes in the indian western pacific sectors of the southern ocean were investigated using the japanese antarctic research expeditions and historical hydrographic observations from the 1950s to 1990s freshening and cooling occurred on the neutral density surfaces of 27 0 kgm3 equatorward of sub antarctic front results for the area south of the polar front show warm and saline anomalies and oxygen decreases on the surfaces around 27 9 kgm3 which correspond to the upper circumpolar deep water these latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters steric sea level has also increased with an average change of 1mmyr1 from the 1970s to 1990s the changes are larger north of the sub antarctic front implying a strengthening of the antarctic circumpolar current it appears that the observed changes are consistent with the results from coupled climate model results for a similar period

the effects of enhanced sea ice export from the ross sea on recent cooling and freshening of the southeast pacific abstractthe top 2000 m of the southern ocean has freshened and warmed over recent decades however the high latitude south of 50 s southeast pacific was observed to be cooler and fresher in the

warming and freshening in the abyssal southeastern indian ocean abstract warming and freshening of abyssal waters in the eastern indian ocean between 1994 95 and 2007 are quantified using data from two closely sampled high quality occupations of a hydrographic section extending from antarctica northward to the equator these changes are limited to abyssal waters in the princess elizabeth trough and the australian antarctic basin with little abyssal change evident north of the southeast indian ridge as in previous studies significant cooling and freshening is observed in the bottom potential temperature salinity relations in these two southern basins in addition analysis on pressure surfaces shows abyssal warming of about 0 05 c and freshening of about 0 01 practical salinity scale 1978 pss 78 in the princess elizabeth trough and warming of 0 1 c with freshening of about 0 005 in the abyssal australian antarctic basin these 12 yr differences are statistically significant from zero at 95 confidence intervals over the bottom few to several hundred decibars of t

rapid freshening of antarctic bottom water formed in the indian and pacific oceans 1 repeat hydrographic sections occupied in 1995 and 2005 reveal a rapid decline in the salinity and density of antarctic bottom water throughout the australian antarctic basin the basin wide shift of the deep potential temperature salinity s relationship reflects freshening of both the indian and pacific sources of antarctic bottom water the s curves diverge for waters cooler than 0 1 c corresponding to a layer up to 1000 m thick over the antarctic continental slope and rise changes over the last decade are in the same direction but more rapid than those observed between the late 1960s and the 1990s when combined with recent observations of similar freshening of north atlantic deep water these results demonstrate that dense water formed in both hemispheres is freshening in response to changes in the high latitude freshwater balance and rapidly transmitting the signature of changes in surface climate into the deep ocean

microearthquake activity in some parts of the himalaya and the tectonic model abstract microearthquake data from temporary permanent networks in different parts of the himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region the microearthquake activity in arunachal pradesh northeastern himalaya is found to be pronounced at the main boundary thrust mbt and to its south the subcrustal earthquakes depth 50 80 km occur much below the plane of detachment of the tectonic models proposed by seeber et al 1981 ni and barazangi 1984 the mbt is not the seismogenic fault the earthquakes are generated by strike slip movement on deep seated hidden faults transverse to the mbt the high seismic activity in the shillong plateau about 200 km south of the mbt in the northeast region is due to the influence of himalayan collision tectonics to the north and burmese arc subduction tectonics to the east the activity in the plateau is not directly related to the himalayan thrust belt or seismic belt these are mostly crustal earthquakes depth 10 30 km and are caused by local active faults lineaments in the eastern himalaya in the sikkim and darjeeling area the seismic activity is found to be clustered mostly to the north of the mbt the earthquakes occur at a depth range 0 50 km the majority of them occur below the detachment plane by thrust faulting in the central part in the nepal himalaya lateral variations of the seismic activity are observed which represent lateral segmentation of the mbt by transverse tectonic features in the western himalaya however the tectonic models fit well with the microearthquake data in the himachal pradesh of the western himalaya the microearthquakes are mostly recorded in the mbt zone and the hypocentres depth 0 20 km are confined above the plane of detachment or on the basement thrust the earthquakes mostly occur to the south of the main central thrust mct which suggests that the mct is not seismogenic it is rather a dormant fault no single tectonic model explains the himalayan earthquakes

aftershocks of the march 1999 chamoli earthquake and seismotectonic structure of the garhwal himalaya the aftershocks of the chamoli earthquake m b 6 3 of 28 march 1999 are analyzed to understand the seismotectonic structure of the garhwal himalaya the permanent as well as temporary microearthquake network data are used the mainshock occurred on the basement thrust by thrust faulting the aftershocks on the other hand are triggered at the seismogenic faults to the south of the main central thrust mct by thrust faulting as well as by strike slip faulting and all the events occurred above the plane of detachment basement thrust the mct is not the seismogenic fault

electrical structure of the himalaya of central nepal high conductivity around the mid crustal ramp along the mht twelve broadband magnetotelluric mt soundings were performed across the himalaya of central nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics the mt impedance tensors were obtained for frequencies between 0 001 and 500 hz the 2 d section derived from joint inversion of te and tm mode after rri and groom bailey decomposition shows high conductivity in the foreland basin 30 m that contrasts with the resistive indian basement 300 m and lesser himalaya 1000 m in addition our mt sounding reveals a major conductive feature beneath the front of the higher himalaya also characterized by intense microseismic activity and the position of a mid crustal ramp along the major active thrust fault mht this high conductivity zone probably reflects metamorphic fluids released during underthrusting of the indian basement and pervading well connected microcracks induced by interseismic stress build up or distributed brittle deformation around the ramp

the imprint of southern ocean overturning on seasonal water mass variability in drake passage seasonal changes in water mass properties are discussed in thermohaline coordinates from a seasonal climatology and repeat hydrographic sections the sr1b ctd transects along drake passage are used as a case study the amount of water within temperature and salinity classes and changes therein are used to estimate dia thermal and dia haline transformations these transformations are considered in combination with climatologies of surface buoyancy flux to determine the relative contributions of surface buoyancy fluxes and subsurface mixing to changes in the distribution of water in thermohaline coordinates the framework developed provides unique insights into the thermohaline circulation of the water masses that are present within drake passage including the erosion of antarctic winter water aaww during the summer months and the interaction between the circumpolar deep waters cdw and antarctic intermediate water aaiw the results presented are consistent with summertime wind driven inflation of the cdw layer and deflation of the aaiw layer and with new aaiw produced in the winter as a mixture of cdw remnant aaww and surface waters this analysis therefore highlights the role of surface buoyancy fluxes in the southern ocean overturning

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

the status of central seismic gap a perspective based on the spatial and temporal aspects of the large himalayan earthquakes abstract the central himalaya considered as a prominent seismic gap is generally believed to be the most vulnerable segment due for a great plate boundary earthquake m 8 two significant historical earthquakes are known to have occurred in this region in ad 1505 and 1803 interpreted by some workers as great earthquakes occurrences of these events question the validity of central himalaya being a seismic gap here we analyze the macroseismal data from the central himalaya and the gangetic plains and suggest that neither of these earthquakes can be qualified as a plate boundary type event and the size of at least the 1803 does not exceed mw 7 7 our studies further indicate that the central himalayan frontal thrusts may be undergoing a quiescence of 1000 years in terms of generation of great earthquakes assuming a steady elastic strain accumulation due to plate convergence and its simple translation as periodic great earthquakes such long quiescence may appear intriguing we suggest that the long term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the himalaya is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt our analyses also indicate that the large moderate earthquakes are mostly concentrated on higher level thrusts these structures probably accommodate most of the seismogenic slip in the himalaya and the rest of the strain could possibly be expended on passive folding consequently great earthquakes m 8 are relatively infrequent as in the case of central himalaya apparently leading to long periods of quiescence

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

iapetus geophysics rotation rate shape and equatorial ridge iapetus has preserved evidence that constrains the modeling of its geophysical history from the time of its accretion until now the evidence is a its present 79 33 day rotation or spin rate b its shape that corresponds to the equilibrium figure for a hydrostatic body rotating with a period of 16 h and c its high equatorial ridge which is unique in the solar system this paper reports the results of an investigation into the coupling between iapetus thermal and orbital evolution for a wide range of conditions including the spatial distributions with time of composition porosity short lived radioactive isotopes slri and temperature the thermal model uses conductive heat transfer with temperature dependent conductivity only models with a thick lithosphere and an interior viscosity in the range of about the water ice melting point can explain the observed shape short lived radioactive isotopes provide the heat needed to decrease porosity in iapetus early history this increases thermal conductivity and allows the development of the strong lithosphere that is required to preserve the 16 h rotational shape and the high vertical relief of the topography long lived radioactive isotopes and slri raise internal temperatures high enough that significant tidal dissipation can start and despin iapetus to synchronous rotation this occurred several hundred million years after iapetus formed the models also constrain the time when iapetus formed because the successful models are critically dependent upon having just the right amount of heat added by slri decay in this early period the amount of heat available from short lived radioactivity is not a free parameter but is fixed by the time when iapetus accreted by the canonical concentration of 26al and to a lesser extent by the concentration of 60fe the needed amount of heat is available only if iapetus accreted between 2 5 and 5 0 myr after the formation of the calcium aluminum inclusions as found in meteorites models with these features allow us to explain iapetus present synchronous rotation its fossil 16 h shape and the context within which the equatorial ridge arose

the response of the southern annular mode the east australian current and the southern mid latitude ocean circulation to global warming 1 climate models predict an upward trend of the southern annular mode sam in response to increasing atmospheric co2 concentration however the consequential impact of this change on oceanic circulation has not been explored here we analyse the outputs of a series of global warming experiments from the csiro mark 3 climate model we show that although for the zonal wind stress change the maximum is located at approximately 60 s in terms of the change in surface wind stress curl the maximum is situated at approximately 48 s this change in the wind stress curl causes a spin up of the entire southern midlatitude ocean circulation including a southward strengthening of the subtropical gyres particularly the east australia current eac the intensified eac generates a warming rate in the tasman sea that is the greatest in the southern hemisphere sh with significant implications for sea level rise the pan southern ocean scale suggests a broad impact on the marine ecosystem of the entire southern midlatitude ocean

multiple jets of the antarctic circumpolar current south of australia abstract maps of the gradient of sea surface height ssh and sea surface temperature sst reveal that the antarctic circumpolar current acc consists of multiple jets or frontal filaments the braided and patchy nature of the gradient fields seems at odds with the traditional view derived from hydrographic sections that the acc is made up of three continuous circumpolar fronts by applying a nonlinear fitting procedure to 638 weekly maps of ssh gradient ssh it is shown that the distribution of maxima in ssh i e fronts is strongly peaked at particular values of absolute ssh i e streamlines the association between the jets and particular streamlines persists despite strong topographic and eddy mean flow interactions which cause the jets to merge diverge and fluctuate in intensity along their path the ssh values corresponding to each frontal branch are nearly constant over the sector of the southern ocean between 100 e and 180 the front positions inferred from ssh agree closely with

late miocene topographic inversion in southwest tibet based on integrated paleoelevation reconstructions and structural history article investigations of the deformation history of the himalayan orogen support interpretations of rapid and striking changes in the landscape of the tibetan himalaya and high himalaya we examine this issue by integrating oxygen isotope based paleoelevation reconstructions of the zada basin in southwestern tibet with information on the structural evolution of the high and tibetan himalaya between 79 e and 84 30 e 18 opsw values were calculated from 18 occ values from pristine fluvial miocene gastropod shells analyses comparing the most negative 18 osw and reconstructed 18 opsw values to 18 osw versus elevation relationships based on both thermodynamic models and an empirical data set suggest a decrease in the mean watershed elevation of 1 to 1 5 km since the late miocene geologic mapping and structural data from crustal scale fault systems in the zada region and regions to its east indicate a phase of arc normal shortening and vertical thickening since the middle miocene followed by ongoing arc parallel extension and vertical thinning these results suggest that regions in this part of the orogen transitioned from undergoing arc normal shortening to arc parallel extension in the late miocene and that arc parallel extensional structures root deeply within the himalayan thrust wedge when combined with data on the distribution age and provenance of sedimentary basins our geologic mapping structural data and paleoelevation results suggest that this transition from shortening to extension was accompanied by a topographic inversion from mountains to basins in b4 m y these observations can be explained by a foreland propagating fault system that accommodates outward radial expansion of the himalayan orogen

paleoseismic evidence of great surface rupture earthquakes along the indian himalaya 1 toward understanding the relationship between strain accumulation and strain release in the context of the mechanics of the earthquake and mountain building process and quantifying the seismic hazard associated with the globes largest continental thrust system we describe the late quaternary expression and paleoseismic evidence for great surface rupture earthquakes at six sites along the himalayan frontal thrust hft system of india our observations span a distance of 250 km along strike of the hft uplifted and truncated fluvial terrace deposits resulting from the holocene displacements on the hft are preserved along canyons of the ghaggar markanda shajahanpur and kosi rivers dividing the elevation of the bedrock straths at each site by their ages yields estimates of the vertical uplift rate of 4 6 mm yr which when assumed to be the result of slip on an underlying thrust dipping at 20 45 are equivalent to fault slip rates of 6 18 mm yr or shortening rates of 4 16 mm yr trench exposures reveal the hft to fold and break late holocene surface sediments near the cities and villages of chandigarh kala amb rampur ganda lal dhang and ramnagar radiocarbon ages of samples obtained from the displaced sediments indicate surface rupture at each site took place after a d 1200 and before a d 1700 uncertainties attendant to the radiocarbon dating currently do not allow an unambiguous definition of the capping bound on the age of the displacement at each site and hence whether or not the displacements at all sites were contemporaneous trench exposures and vertical separations measured across scarps at rampur ganda lal dhang and ramnagar are interpreted to indicate single event displacements of 11 38 m dividing the observed single event vertical components of displacement by the estimated longer term uplift rates indicates 1330 3250 or more years should be required to accumulate the slip sufficient to produce similar sized displacements surface rupture appears to not have occurred during the historical 1905 kangra mw 7 7 1934 bihar nepal mw 8 1 and 1950 assam mw 8 4 earthquakes which also occurred along the himalayan front yet we observe clear evidence of fault scarps and displacements in young alluvium and progressive and continued offset of fluvial terrace deposits along the hft we suggest on this basis and the size and possible synchroneity of displacements recorded in the trenches that there exists the potential for earthquakes larger than recorded in the historical record and with the potential to rupture lengths of the hft greater than the 250 km we have studied

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

the role of eddies in the southern ocean temperature response to the southern annular mode the role of eddies in modulating the southern ocean response to the southern annular mode sam is examined usinganoceanmodelrunatmultipleresolutionsfromcoarsetoeddyresolving thehigh resolution versionsof the model showan increase in eddy kinetic energy that peaks 2 3 yr after a positive anomalyin the sam index previous work has shown that the instantaneous temperature response to the sam is characterized by predominant cooling south of 458s and warming to the north at all resolutions the model captures this temperatureresponse this responseis also evidentin the coarse resolution implementation of the model with no eddy mixing parameterization showing that eddiesdo not play an important role in the instantaneous response on the longer time scales an intensification of the mesoscale eddy field occurs which causes enhanced poleward heat flux and drives warming south of the oceanic polar front this warming is of greater magnitude and occurs for a longer period than the initial cooling response the results demonstrate that this warming is surface intensified and strongest in the mixed layer non eddy resolving models are unable to capturethedelayededdy driventemperatureresponsetothesam theauthorsthereforequestiontheability

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

what can we learn from global altimetry hydrography comparisons differences and complementarities between altimeter and hydrographic data are studied to describe the sea level and ocean circulation variations in terms of steric and non steric contributions altimeter sea level anomalies sla and hydrographic dynamic height anomalies dha agree remarkably well they also reveal however systematic large scale differences in several areas of the pacific and indian oceans that are related to a barotropic sverdrup like response of the ocean to wind forcing in addition the use of the two data sets provides a good estimation of the partition between barotropic and baroclinic modes needed for data synthesis or assimilation studies

comparison of synoptic and climatologically mapped sections in the south pacific ocean abstract to understand the extent to which oceanic climate shifts could be detected a south pacific climatology has been used to create pseudosections of temperature salinity and other tracers along a zonal and meridional lines at 15 s and 90 w respectively interpolations from the climatology were made using combined empirical orthogonal functions and objective mapping comparisons are made with independent measurements taken in 1987 of temperature and salinity at 15 s temperature and salinity fields between the surface and 300 db along the 15 s section are predicted with an uncertainty sufficiently small to display significant differenccs in temperature and salinity related to el nino of 1987 the 90 w pseudosection is a forecast of a synoptic section to be obtained as part of woce in 1992 explicit values for the smallest temperature shift with depth that could be detected are produced

sea level rise during past 40 years determined from satellite and in situ observations the 3 2 0 2 millimeter per year global mean sea level rise observed by the topex poseidon satellite over 1993 98 is fully explained by thermal expansion of the oceans for the period 1955 96 sea level rise derived from tide gauge data agrees well with thermal expansion computed at the same locations however we find that subsampling the thermosteric sea level at usual tide gauge positions leads to a thermosteric sea level rise twice as large as the true global mean as a possible consequence the 20th century sea level rise estimated from tide gauge records may have been overestimated

frontal movements and property fluxes contributions to heat and freshwater trends in the southern ocean we examine the synoptic variability of temperature and salinity in the southern ocean using a static gravest empirical mode gem mapping based on historical hydrography and a time evolving projection of the gem created using satellite altimetry satgem the gem and satgem projections allow the separation of observed trends into a component due to a shift of the circumpolar fronts adiabatic using the satgem and a component due to changes in water masses diabatic expressed as a temporal trend in residual between historical hydrography and the static gem the mean southward movement of the antarctic circumpolar current acc fronts drives an adiabatic warming of 1 196 0 090 w m 2 distributed over most of the acc and at all depths this is strongest where the meridional temperature gradient is largest such as in the upper 1000 dbar of the subantarctic front saf there is a weak adiabatic freshening of 6 57 0 18 mm yr 1 m 2 concentrated mainly below the antarctic intermediate water and south of the saf residuals between historical hydrography and the static gem field driven by diabatic changes in the water mass structure have temporal trends in temperature and salinity when these trends are integrated over the whole acc there is a net cooling of 0 628 0 129 w m 2 and strong freshening of 30 27 0 70 mm yr 1 m 2 by combining the diabatic and adiabatic components and integrating over the acc the net increase in heat and freshwater in the region is 0 570 0 099 w m 2 and 36 91 0 72 mm yr 1 m 2 respectively the sum of the adiabatic and diabatic changes are consistent with previous trends inferred from observations

subsurface structure of interannual temperature anomalies in the australian sector of the southern ocean 1 a 7 year time series of austral summer expendable bathythermograph xbt sections between tasmania and antarctica is used to describe the subsurface structure of southern ocean interannual temperature anomalies comparison of the discontinuous xbt record with a continuous weekly average satellite based sst data set confirms that the xbt sampling is adequate to resolve interannual variability significant correlations are found between changes in surface and subsurface temperature along much of the section in the subantarctic zone surface and subsurface changes are coherent because deep convection in winter homogenizes the upper 600 m and the summer mixed layer temperature reflects the preconditioning of the previous winter at other latitudes the regions of high correlation generally extend well beyond the depth of direct atmospheric influence i e the depth of the winter mixed layer negative positive temperature anomalies at interannual timescales are coherent with poleward equatorward displacements of fronts the simplest explanation for the correspondence between deep vertically coherent temperature anomalies and the location of acc fronts is that the observed features reflect meridional shifts of temperature gradients associated with the fronts the link between temperature anomalies and front displacements suggests southern ocean sea surface temperature sst anomalies primarily reflect a dynamical response of the ocean to wind or other forcing rather than surface trapped features produced by anomalous air sea heat flux the xbt record contains both interannual and longer unresolved period signals which cannot be isolated in the short discontinuous xbt time series spectral analysis of the 17 year sst data set reveals roughly equal variability in 2 7 year and 7 year bands

a circumpolar gravest empirical mode for the southern ocean hydrography historical hydrographic data from the southern ocean are projected into a baroclinic stream function space generating a three dimensional gravest empirical mode gem parameterized by pressure geopotential height and longitude the low dimensional gem projection is intrinsically steady vertically coherent and equivalent barotropic the gem fields capture more than 97 of the total density and temperature variance in the antarctic circumpolar current region

a two dimensional gravest empirical mode determined from hydrographic observations in the subantarctic front abstract south of australia where the baroclinicity in the subantarctic front extends almost to the seafloor the geopotential height of the sea surface and the vertical acoustic travel time exhibit a tight empirical relationship to each other and to the entire vertical structure of temperature t p salinity s p and specific volume anomaly p measurements of provide proxy estimates of the profiles tg p sg p and g p based on a two dimensional gravest empirical mode gem representation of the vertical structure fitted to 212 hydrographic stations a seasonal model was fitted in the near surface layers using additional historical data at each depth in the range 150 3000 dbar more than 96 of the variance in both the t and fields is captured by the gem representation during the subantarctic flux and dynamics experiment mar 1995 mar 1997 inverted echo sounders ies and current meters were moored in a 2d array along the woce sr3 transect south of australia at each i

synchronous variability in the southern hemisphere atmosphere sea ice and ocean resulting from the annular mode zonally symmetric fluctuations of the midlatitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons this is true not only in observations but also in an unforced 15 000 yr integration of a coarse resolution r15 coupled ocean atmosphere model here it is documented how this mode of atmospheric variability known as the southern annular mode sam generates ocean circulation and sea ice variations in the model integration on interannual to centennial timescales that are tightly in phase with the sam the positive phase of the sam is associated with an intensification of the surface westerlies over the circumpolar ocean around 608s and a weakening of the surface westerlies farther north this induces ekman drift to the north at all longitudes of the circumpolar ocean and ekman drift to the south at around 308s through mass continuity the ekman drift generates anomalous upwelling along the margins of the antarctic continent and downwelling around 458s the anomalous flow diverging from the antarctic continent also increases the vertical tilt of the isopycnals in the southern ocean so that a more intense circumpolar current is also closely associated with positive sam in addition the anomalous divergent flow advects sea ice farther north resulting in an increase in sea ice coverage finally positive sam drives increases in poleward heat transport at about 308s while decreases occur in the circumpolar region ocean and sea ice anomalies of the opposite sign occur when the sam is negative the ocean and sea ice fluctuations associated with the sam constitute a significant fraction of simulated ocean variability poleward of 308s year round the robustness of the mechanisms relating the sam to oceanic variability suggests that the sam is likely an important source of large scale variability in the real southern hemisphere ocean

deep temperature variability in drake passage observations made on 21 occupations between 1993 and 2016 of go ship line sr1b in eastern drake passage show an average temperature of 0 53 c deeper than 2000 dbar with no significant trend but substantial year to year variability standard deviation 0 08 c using a neutral density framework to decompose the temperature variability into isopycnal displacement heave and isopycnal property change components shows that approximately 95 of the year to year variance in deep temperature is due to heave changes on isopycnals make a small contribution to year to year variability but contribute a significant trend of 1 4 0 6 m c per year largest for density n 28 1 south of the polar front pf the heave component is depth coherent and results from either vertical or horizontal motions of neutral density surfaces which trend upward and northward around the pf downward for the densest levels in the southern section and downward and southward in the subantarctic front and southern antarctic circumpolar current front saccf a proxy for the locations of the antarctic circumpolar current acc fronts is constructed from the repeat hydrographic data and has a strong relationship with deep ocean heat content explaining 76 of deep temperature variance the same frontal position proxy based on satellite altimeter derived surface velocities explains 73 of deep temperature variance the position of the pf plays the strongest role in this relationship between acc fronts and deep temperature variability in drake passage although much of the temperature variability in the southern half of the section can be explained by the position of the saccf this article is protected by copyright all rights reserved

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

on the nonequivalent barotropic structure of the antarctic circumpolar current an observational perspective we examine the vertical structure of the horizontal flow and diagnose vertical velocities in the antarctic circumpolar current acc near the kerguelen plateau using em apex profiling floats eight floats measured horizontal velocity temperature and salinity profiles to 1600 dbar with a vertical spacing of 3 5 dbar four times per day over a period of approximately 3 months horizontal velocity profiles show a complex vertical structure with strong rotation of the velocity vector through the water column the distribution of rotation angles from 1247 profiles is approximately gaussian and rotations of either sign are equally likely forty percent of profiles with speed greater than 5 cm s 1 have a depth integrated rotation of less than 15 degrees over 1300 dbar while the other 60 demonstrate significantly stronger rotation consequently most profiles do not conform to the equivalent barotropic model deep flow parallel and proportional to the surface flow used in simplified dynamical models and in gravest empirical mode climatologies of the acc nevertheless since we find the mean rotation to be zero an equivalent barotropic assumption is valid to first order vertical velocities inferred using conservation of mass and a gradient wind balance in natural coordinates have magnitudes on the order of 100 m day we find robust patterns of upwelling and downwelling phase locked to meanders in the flow as found in earlier studies with the advent of high resolution observations such as those presented here and high resolution models we can advance to a more complete understanding of the rich variability in acc structure that is neglected in the equivalent barotropic model

dissolved oxygen change and freshening of antarctic bottom water along 62 s in the australian antarctic basin between 1995 1996 and 2012 2013 abstract significant warming and freshening signals observed in the world s deep oceans are generally stronger southward in the australian antarctic basin along 62 s near the southern edge of the western pacific and indian oceans we compared water properties of antarctic bottom water observed in 1995 1996 and 2012 2013 the basin collects newly ventilated dense water from the western pacific sector of the antarctic coast and the ross sea compared on pressure surfaces freshening of more than 0 003 g kg in absolute salinity 0 003 in practical salinity scale 1978 or pss 78 was found below 3000 dbar the freshening was stronger 0 02 g kg or 0 02 in pss 78 eastward and towards the bottom suggesting that it originated from the regions of new bottom water production we also detected warming 0 2 c on pressure surfaces below the surface layer freshening of similar magnitudes were observed on density surfaces neutral densities 1028 1 kg m 3 adiabatic southward migration of water masses observed by satellite altimetry can explain the warming but not the freshening a notable increase in dissolved oxygen on density surfaces was found near the bottom the increase appears due to density decrease of near bottom water which can be explained by a change in the mixing ratio of bottom water from near the adelie depression adelie land bottom water albw higher oxygen with that from the more distant ross sea ross sea bottom water rsbw lower oxygen an increased contribution of albw and a decreased contribution of rsbw this change might be associated with a decrease in the rsbw supply following an earlier ice calving event in the polynya region oxygen rich 242 mol kg and dense 1028 35 kg m 3 bottom water observed in 1995 1996 in troughs near 130 e disappeared in 2012 2013 which also might reflect changes in bottom water production along the coast possibly in mertz and dibble polynyas

precursor seismicity foreshocks and aftershocks of the uttarkashi earthquake of october 20 1991 at garhwal himalaya abstract the uttarkashi earthquake m b 6 6 of october 20 1991 occurred in a complicated geological tectonic setting at the main central thrust mct zone of the himalaya the main shock was preceded by a precursory earthquake activity which continued for about eight years and then there was a quiescent period for about three years three foreshocks m 4 0 occurred 48 h before the main shock two aftershocks m 4 0 occurred within 12 h of the main shock which was then followed by another two aftershocks m 4 0 after about seven days on october 27 1991 the largest aftershock m 5 2 occurred within 12 h the aftershocks magnitude less than 4 0 continued for about two months and it decayed at a rate t 0 87 about 1000 earthquakes magnitude less than 1 0 and above were recorded by a closely spaced five station temporary microearthquake meq network out of these 124 aftershocks which occurred inside the network are located by homogeneous station method spatial variation of the aftershocks with time was observed fault plane solutions of the main shock and aftershocks indicate dominantly thrust faulting based on the hypocentral data depth estimate of the plane of detachment is made and a fault interaction model is presented the model shows that the main shock occurred at the juncture of mct zone plane of detachment and the basement thrust the north dipping uttarkashi fault a parallel fault at the mct zone has been activated by the tectonic stress generated due to relative movement of the tibetan plate over the indian plate

heat flux carried by the antarctic circumpolar current mean flow 1 a stream function projection of historical hydrographic data is applied to study the heat flux problem in the antarctic circumpolar current acc the acc is defined as a circumpolar band consisting of mean streamlines passing through drake passage its mean path exhibits a globally meandering pattern the calculation of zonal heat transport shows that the acc warms along its equatorward segments south atlantic and indian ocean and cools along its poleward segment south pacific the primary heat sources for the acc system are two western boundary currents the brazil current and the agulhas current the mean baroclinic flow relative to 3000 dbar carries 0 14 pw poleward heat flux across 56 s and 0 08 pw across 60 s

plio plistocene in sequence thrust propagation along the main central thrust zone kumaon garhwal himalaya india new thermochronological data abstract the kumaon and garhwal himalayas nw india have similar geologic tectonic topographic and precipitation pattern however low temperature thermochronological age pattern varies significantly along the strike as well as across the strike of major faults here we interpret the new set of thermochronological age data across the strike of major faults namely vaikrita thrust vt munsiari thrust mt berinag thrust bt and the baijnath nappe along pindari valley in the kumaon garhwal himalaya in the present work 18 new apatite fission track aft ages of samples collected along a north south transect have been reported ages in the hanging wall of vt which range from 2 1 0 2 to 4 2 0 7 ma have been found to be becoming younger linearly with distance from north to the vt this trend crosses the vt with significant jump in ages in the south of the vt ages are lying between 0 3 0 1 and 3 9 0 8 ma and show linear variation with distance from the vt to bt no significant jump in ages across the mt is observed and the linear trend of younging southward is continuing till the bt the break and displacement in age pattern with youngest ages close to thrusts possibly indicate progressive late thrust movement it reflects sequential uplift and cooling towards the south from the vt to bt which is consistent with an in sequence style of thrust propagation we compare our new data to the published thermochronological data from its adjacent traverses along the goriganga and dhauliganga valleys 25 km away from the present traverse towards east and west respectively in the kumaon garhwal himalaya in order to understand potential along strike variations in the kinematics our observations indicate a dynamic coupling between tectonic and surface processes in the garhwal kumaon himalaya and support that the geometry of crustal scale faults and their associated kinematics control exhumation pathways of rocks

multidecadal warming of subsurface temperature in the indian sector of the southern ocean 1 long term variations of subsurface layer temperature were examined for 32 years 1966 1998 of continuous observations in the indian sector of the southern ocean the data analyzed consist of observations from japanese antarctic research expeditions and also from other countries significant warming trends of about 0 004 c yr 1 were observed at all depths from 200 to 900 m to the north of the southern antarctic circumpolar current acc front orsi et al 1995 the rates of warming increase with increasing depth from 0 003 c yr 1 at 200 m to 0 012 c yr 1 statistically significant at 900 m in the region close to the front these warming trends were also found after applying a technique to correct for differences in the data sampling locations the warming tendency in the frontal region is consistent with the assumption of a southward shift of the front and may have a relationship with the observed retreat of sea ice edge

vertical structure and transport on a transect across the north atlantic current near 42 n time series and mean a new method is presented for combining historical hydrography with the acoustic travel time measurements from an inverted echo sounder ies to obtain time series of full water column profiles of temperature t and specific volume anomaly hydrography is used to demonstrate that profiles constructed from the ies measurement account for more than 90 of the t and variance through the main thermocline range horizontal gradients between neighboring pairs of instruments when combined with measurements from bottom pressure sensors and deep current meters provide accurate estimates of absolute geostrophic velocity profiles a section of iess and current meters across the north atlantic current nac along world ocean circulation experiment line acm6 near 42 5 n is analyzed using these methods and the eulerian mean temperature and absolute velocity sections are calculated the mean transport for the combined nac and northward flow within the inshore portion of the mann eddy is 146 13 106 m3 s 1 the temporal standard deviation of the northward transport is 41 106 m3 s 1 arising not only from changes in the transport of the nac but also from lateral shifts of the currents and eddies captured by the fixed span of moorings assumption of a level of no motion at the bottom and at 2000 dbar underestimates the true absolute transport by 33 or 60 respectively historical measurements indicate that 50 60 106 m3 s 1 circulates within the mann eddy adjacent to the north atlantic current thus 86 96 106 m3 s 1 appears to be throughput at the acm6 line because the throughput observed on the acm6 line considerably exceeds the combined historical estimates of northward and eastward outflow from the newfoundland basin of 30 106 m3 s 1 these measurements suggest a substantial 50 106 m3 s 1 recirculation or southward flow within the newfoundland basin in addition to the historical recirculation within the mann eddy

himalayan seismic hazard the devastating earthquake in bhuj on 26 january 2001 highlighted the high earthquake risk in the indian region of gujarat in their perspective bilham et al warn that major earthquakes may be expected in another part of india the himalayan arc an analysis of historic earthquakes suggests that several major earthquakes are overdue in this region but that recent low activity has focused attention away from the himalayan arc projections from earlier earthquakes suggest that just one of the possibly overdue himalayan earthquakes may lead to 200 000 predictable fatalities

linear trends in salinity for the world ocean 1955 1998 1 quality controlled oceanographic profile salinity measurements from the world ocean database 2001 wod01 were used to calculate linear trends of zonally averaged salinity anomalies for running five year periods from 1955 1959 through 1994 1998 for the world ocean and the atlantic pacific and indian ocean basins from the surface to 3000 meters depth each basin exhibits large scale coherent trends most of the pacific is freshening with the exception of the subtropical south pacific the atlantic exhibits a deep freshening in the subpolar gyre and a shallower more intense increase in salinity in the tropics and subtropics the indian ocean is becoming more saline at all latitudes in the upper 150 meter layer with a subsurface freshening between 40 s and the equator in the 250 1000 meter layer there is freshening in both the weddell and ross seas

seismicity and the state of stress from investigations of local earthquakes in the kumaon himalaya abstract results of investigations of local earthquakes in the region of the main central thrust mct in the kumaon himalaya between and adjacent to the valleys of the bhagirathi and yamuna rivers are presented records of over 250 earthquakes were analysed and the following facts emerged 1 1 earthquakes in the himalayas occur in specific areas and belts one such belt has been identified in the region under investigation hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt crossing the yamuna river between the villages of barkot and syanachatti 2 2 all but a few epicentres in this middle segment lie to the southwest of the surface trace of the mct in a zone with a width of 10 30 km 3 3 most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface the maximum estimated depth being 32 km using observations of first motion for a composite focal mechanism solution the nodal planes were observed to be near vertical and the compression axis near horizontal and normal to the local strike of the seismic belt and of the mct we conclude that although the main central thrust itself is not seismically active in this region there is considerable activity immediately to the southwest of it furthermore the mode of faulting as inferred from the records of these earthquakes is strike slip