interpentadal variability of temperature and salinity at intermediate depths of the north atlantic ocean 1970 1974 versus 1955 1959 we have composited historical hydrographic observations for the world ocean for two pentads 1955 1959 and 1970 1974 and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the north atlantic ocean the subtropical gyre 500 1300 m of the north atlantic was colder and fresher during 1970 1974 compared to 1955 1959 the relatively large changes in temperature and salinity on constant depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces at intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970 1974 pentad in contrast the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970 1974 compared to 1955 1959 characteristic differences in temperature and salinity for both gyres were 0 5 c and 0 025 respectively analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26 5 surface suggesting that convection in this region may have been shallower in 1970 1974 compared to 1955 1959 below the 26 5 surface potential vorticity decreased through the 27 3 surface indicating that deeper than normal convection may have formed the water masses in these layers at 250 m depth relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest across the north atlantic ocean

investigation of the causes of historical changes in the subsurface salinity minimum of the south atlantic in this study we investigate the subsurface salinity changes on decadal timescales across the subtropical south atlantic ocean using two ocean reanalysis products the latest version of the simple ocean data assimilation and the estimating the circulation and climate of the ocean phase ii as well as using additional climate model experiments results show that there is a recent significant salinity increase at the core of the salinity minimum at intermediate levels the main underlying mechanism for this subsurface salinity increase is the lateral advective gyre changes due to the southern annular mode variability which conditions an increased contribution from the indian ocean high salinity waters into the atlantic the global warming signal has a secondary but complementary contribution latitudinal differences at intermediate depth in response to large scale forcing are in part caused by local variation of westward propagation features and by compensating contributions of salinity and temperature to density changes

global sea level rise published values for the long term global mean sea level rise determined from tide gauge records exhibit considerable scatter from about 1 mm to 3 mm yr this disparity is not attributable to instrument error long term trends computed at adjacent sites often agree to within a few tenths of a millimeter per year instead the differing estimates of global sea level rise appear to be in large part due to authors using data from gauges located at convergent tectonic plate boundaries where changes of land elevation give fictitious sea level trends in addition virtually all gauges undergo subsidence or uplift due to postglacial rebound pgr from the last deglaciation at a rate comparable to or greater than the secular rise of sea level modeling pgr by the ice 3g model of tushingham and peltier 1991 and avoiding tide gauge records in areas of converging tectonic plates produces a highly consistent set of long sea level records the value for mean sea level rise obtained from a global set of 21 such stations in nine oceanic regions with an average record length of 76 years during the period 1880 1980 is 1 8 mm yr 0 1 this result provides confidence that carefully selected long tide gauge records measure the same underlying trend of sea level and that many old tide gauge records are of very high quality

apparent changes in the climatic state of the deep north atlantic ocean determination of any long term changes in the large scale characteristics of the deep ocean circulation would be an important clue in understanding the climatic interactions of the ocean and atmosphere in the summer of 1981 the rv atlantis ii reoccupied two transatlantic sections at nominal latitudes of 24 30 n and 36 16 n with a conductivity temperature depth instrument ctd one purpose of the work was to make a comparison with previous surveys conducted during the international geophysical year igy 1 when sections were obtained in october 1957 and april may 1959 we report here that significant warming occurred in an ocean wide band from 700 m to 3 000 m with a maximum temperature difference of 0 2 c these changes are sufficient to expand the water column by several centimetres the historical temperature salinity curve was apparently unchanged interannual changes in local water mass characteristics have been proposed previously2 3 perhaps it would be most surprising if no changes were seen to occur what remains obscure is the significance of these changes and the extent to which they represent long term climate trends or merely the minor and random fluctuations to be expected in any complex fluid system

the long term hydrographic record at bermuda abstract a long by oceanographic standards time series of hydrographic observations at bermuda was begun in 1954 and continues to the present analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer 0 200 dbar where integral timescales for salinity are 50 longer than for temperature and surface salinity changes are correlated with decadal changes in the 18 c water production temperature salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of 50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of 300 kim salinity changes in the deepest layer observed at bermuda station s 1500 2500 dbar are uncorrelated with temperature masked by measurement errors in the early years of the time series inclusion of earlier

using a cfc effective age to estimate propagation and storage of climate anomalies in the deep western north atlantic ocean 1 1990s chlorofluorocarbon cfc observations are used to estimate an effective age of 20 years for north atlantic deep water nadw at the equator the effective age is found by subtracting a relic age from pcfc 11 age the effective age is several to 10 years younger at the equator than other methods from the effective age equatorward effective spreading rates of 1 2 cm s are found which are similar for the nadw components effective spreading rates take into account recirculation gyres along equator flow and mixing which slow spreading of climate anomalies from the north atlantic to the southern hemisphere the difference between direct velocities and tracer effective spreading rates suggests that the buffering effect due to storage in the western north atlantic and equatorial region is about 20 years even so warming and freshening due to greenhouse gases will be first felt in the deep waters of the western north atlantic

antarctic intermediate water circulation and variability in a coupled climate model the variability of antarctic intermediate water aaiw in a long term natural integration of a coupled climate model is examined the mean state of the climate model includes a realistic representation of aaiw which appears centered on the su 5 27 2 kg m23 density surface hereinafter s 27 2 both in observations and the model an assessment of ventilation rates on the s 27 2 surface suggests that this particular climate model forms aaiw in a mostly circumpolar fashion with a significant contribution from antarctic surface water this motivates the assessment of oceanic variability along this core aaiw isopycnal surface complex empirical orthogonal function analyses decompose the variability into three dominant modes showing circumpolar patterns of zonal wavenumber 1 2 and 3 on the s 27 2 density surface the modes contain eastward propagating signals at interannual to centennial time scales mechanisms forcing this variability are investigated using heat and salt budget analyses at the wintertime outcrop of the s 27 2 surface such an approach ignores the mechanism of aaiw variability sourced by subantarctic mode water variations which has been examined previously and is for the most part beyond the present study variability in meltwater rates and atmospheric heat and freshwater fluxes are found to dominate the intermediate water variability at the outcrop region in contrast northward ekman transport of heat and salt plays a significant but localized role in aaiw temperature salinity variability there is also an important contribution from the antarctic circumpolar current to the variability at the outcrop region via zonal transport of heat and salt while the magnitude of aaiw natural variability can be large near the outcrop of the salinity minimum layer recent observations of cooling and freshening at depth are suggested to be beyond that of the unperturbed system

the passive and active nature of ocean heat uptake in idealized climate change experiments the influence of ocean circulation changes on heat uptake is explored using a simply configured primitive equation ocean model resembling a very idealized atlantic ocean we focus on the relative importance of the redistribution of the existing heat reservoir due to changes in the circulation and the contribution from anomalous surface heat flux in experiments in which the surface boundary conditions are changed we perform and analyze numerical experiments over a wide range of parameters including experiments that simulate global warming and others that explore the robustness of our results to more general changes in surface boundary conditions we find that over a wide range of values of diapycnal diffusivity and southern ocean winds and with a variety of changes in surface boundary conditions the spatial patterns of ocean temperature anomaly are nearly always determined as much or more by the existing heat reservoir redistribution than by the nearly passive uptake of temperature due to changes in the surface boundary conditions calculating heat uptake by neglecting the existing reservoir redistribution which is similar to treating temperature as a passive tracer leads to significant quantitative errors notably at high latitudes and secondarily in parts of the main thermocline experiments with larger circulation changes tend to produce a relatively larger magnitude of existing reservoir redistribution and a faster growing effective heat capacity of the system the effective heat capacity is found to be sensitive to both vertical diffusivity and southern ocean wind

recent changes in subsurface temperature and salinity in the canary region 1 based on hydrographic sections carried out during the last decade in the canary region at 29 10 n we show that there has been a statistically significant rise in temperature and salinity on isobars between 1500 and 2300 db the maximum increase found at 1600 db is occurring at a rate of 0 29 c and 0 047 per decade isobaric change decomposition into changes on neutral surfaces and changes due to the vertical displacement of the isoneutrals was performed results reveal that the lower part of north atlantic central water nacw cooled and freshened on neutral surfaces suggesting changes in the freshwater fluxes at the outcropping region however the signal in deep waters 1500 2300 db was principally due to a downward displacement of the isoneutrals although water mass modification is observed in the range of mediterranean water mw influence

decadal changes in water mass characteristics at 24 n in the subtropical north atlantic ocean abstract three transatlantic hydrographic sections across 24 n through the center of the subtropical gyre are analyzed for evidence of decadal changes in water mass characteristics in the main thermocline there has been a steady increase in salinity from 1957 to 1981 to 1992 the salinity at a given potential temperature between 12 c and 17 c has increased at a nearly constant rate of 0 010 per decade at constant potential density salinity and temperature both increased at rates of 0 018 per decade and 0 065 c per decade in the intermediate waters between 800 and 1500 dbar salinity has increased as well on average by as much as 0 025 from 1981 to 1992 either on constant potential temperature or constant density surfaces these changes cannot be due to vertical heaving of a constant water mass structure and they cannot be easily explained in terms of horizontal shifts in the subtropical gyre water mass structure throughout the water column down to 2000 dbar and across the zonal extent of the 24 n

climatic warming of atlantic intermediate waters abstract interdecadal temperature variability of the atlantic ocean is investigated by differencing hydrographic sections taken from the 1920s through the 1990s a comprehensive reanalysis of north atlantic sections and the inclusion of south atlantic sections show that warming seen previously in the north atlantic extends to the south atlantic the largest statistically significant changes occur on pressure surfaces between 1000 and 2000 decibars db over this pressure range and for latitudes between 32 s and 36 n temperatures have warmed by 0 5 c century 1 at 48 n a cooling of 3 c century 1 occurred between the 1950s and 1980s these isobaric temperature trends are decomposed into ones along surfaces of constant neutral density and ones due to the vertical movement of neutral surfaces the two components are associated with different processes in the southern north atlantic 8 36 n the subthermocline warming between the 1950s and 1980s appears to be due primarily to downward displacements of n

investigating error structure of shuttle radar topography mission elevation data product 1 an attempt was made to experimentally assess the instrumental component of error of the c band srtm srtm this was achieved by comparing elevation data of 302 runways from airports all over the world with the shuttle radar topography mission data product srtm it was found that the rms of the instrumental error is about 1 55 m modeling of the remaining srtm error sources including terrain relief and pixel size shows that downsampling from 30 m to 90 m 1 to 3 arc sec pixels worsened srtm vertical accuracy threefold it is suspected that the proximity of large metallic objects is a source of large srtm errors the achieved error estimates allow a pixel based accuracy assessment of the srtm elevation data product to be constructed vegetation induced errors were not considered in this work

cooling and freshening of the subpolar north atlantic ocean since the 1960s little is known of the interdecadal variability in the thermohaline circulation of the world s oceans yet such knowledge is essential as a background to studies of the effects of natural and anthropogenic climate change the subpolar north atlantic is an area of extensive water mass modification by heat loss to the atmosphere lying as it does at the northern limit of the global thermohaline conveyor belt 12 changes in this region may ultimately have global consequences here we report that in august 1991 the waters between greenland and the united kingdom were on average 0 08 c and 0 15 c colder than in 1962 and 1981 respectively and slightly less saline than in 1962 the cause appears to be renewed formation of intermediate water in the labrador sea from cooler and fresher source waters and the spreading of this water mass from the west variations in the source characteristics of labrador sea water can be traced across the north atlantic with a circulation time of 18 19 years between the labrador sea and rockall trough more recently formed labrador sea water with even lower temperature and salinity should cool and freshen the north atlantic still further as it circulates around the ocean in the coming decade

intense warming and salinity modification of intermediate water masses in the southeastern corner of the bay of biscay for the period 1992 2003 1 the evolution of the intermediate water masses within the southeastern corner of the bay of biscay is studied from the shelf edge and slope stations of a hydrographic standard section northward from santander 43 30 43 54 n 3 47 w the section was sampled monthly from 1991 to autumn 2003 with some gaps more frequent in the early stages due to weather conditions data were systematically collected up to a 1000 m depth or to the maximum depth at the shallower stations showing a detailed picture of the variability in water masses below the mixing layer eastern north atlantic central water has warmed as a consequence of continuous deepening of the isopycnal levels changes in water properties at fixed isopycnal levels are less relevant and highly biased by a 1998 positive peak in temperature and salinity the warming trend found is 0 032 c per year on average on the other hand the mediterranean water has been modified along isopycnals variations 10 times greater than those due to isopycnal displacement increasing progressively both temperature and salinity for the period 1994 2001 the values found are around 0 020 c per year for temperature and 0 005 for salinity as a final picture all water masses below the mixing layer and down to a 1000 m depth in the southern bay of biscay have warmed up during the last decade at rates from two to six times greater than the those accepted for the north atlantic during the last half century

long term hydrographic changes at 52 and 66 w in the north atlantic subtropical gyre caribbean abstract in july september 1997 two hydrographic lines were done in the western n atlantic along longitudes of 52 and 66 w as part of the woce one time hydrographic survey of the oceans each of these two lines approximately repeated earlier ones done during the international geophysical year s igy and the mid 1980s because of this repeated sampling long term hydrographic changes in the water masses can be examined in this report we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35 n and compare our results to those presented by bryden et al 1996 who examined changes along a zonal line at 24 n most recently occupied in 1992 since this most recent 24 n section in 1992 substantial changes have occurred in the western part of the subtropical gyre at the depths of the labrador sea water lsw in particular we see clear evidence for colder fresher labrador sea water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24 n at shallower depths inhabited by waters that are an admixture of mediterranean mw and antarctic intermediate waters aaiw our recent survey shows an increase in salinity which can only be attributed to changes in water masses on potential temperature or neutral density surfaces furthermore waters above the mw aaiw layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40 year period spanned by our sections these latter changes have been dominantly due to a vertical sinking of density surfaces as t s changes in density surfaces are small but depths of individual t s horizons have increased with time the net change since the igy shows a mean temperature increase between 800 and 2500 m depth at a rate of 0 57 c century with a corresponding steric sea level rise of 1 mm yr and a net downward heave with small values near the top and bottom and a maximum rate of 2 7 m yr at 1800 m depth changes in the deep caribbean indicate a warming since the igy due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth but no significant change in the deep salinity

temperature and salinity increase in the eastern north atlantic along the 24 5 n in the last ten years 1 the ocean section along the 24 5 n has been one of the most frequently sampled of the world s oceans it has been sampled in 1957 1981 1992 and 1998 previous works showed a progressive warming and salt increase of the thermocline and intermediate waters from around 800 m to 3000 m the maximum warming rate was 0 009 c yr at 1100 m the eastern part of this section was sampled in 2002 the warming of the main thermocline has continued at a higher rate 0 042 yr at 400 m in the last 10 years salinity has also increased along the thermocline conserving the 1992 s relationship from 100 to 1000 db the warming is mainly due to downward displacement of the isopycnals though some cooling along isopycnals is also observed in the upper 350 db

wind forcing of the atlantic thermocline along 32 n at low frequencies abstract the bermuda tide gauge record extends back to the early 1930s that sea level fluctuations there are highly coherent with dynamic height from hydrographic data has two interesting implications first it should contain information about the low frequency circulation of the atlantic furthermore because dynamic height contains information on heat storage it might on the limited timescales accessible in the record also contain clues about climate a simple model of wind forcing of the atlantic from the african coast to bermuda uses the levitus mean density data to estimate the long rossby wave speed as a function of longitude sea level and thermocline variability estimated this way are in remarkably good agreement with observations at periods of more than a few years duration the peak to peak sea level signal is 18 cm which is nearly 25 of the slope across the gulf stream at this latitude the model results suggest that the variability is largest somewhat to the east of bermuda fluctuatio

observation of an abrupt disruption of the long term warming trend at the balearic sea western mediterranean sea in summer 2005 1 the properties of the western mediterranean deep water in a wide area located at the western boundary of the mediterranean sea including the balearic sea have revealed intense changes when observed in summer 2005 between february and june 2005 a temperature drop of 0 14 c reverted dramatically the progressive warming trend of 0 011 c yr 1 that had been observed since 1996 in the waters below 600 dbar north of the balearic channels a similar temperature drop has been observed east of the minorca island below the levantine intermediate water and down to 1500 dbar in the deepest levels a complex thermohaline structure which implies different waters masses as sources was tracked more than 400 km along the western boundary of the mediterranean sea from barcelona to the algerian basin it is suggested that the changes may be linked to the severe 2004 2005 winter occurred at the northwestern mediterranean sea

abyssal water carbon 14 distribution and the age of the world oceans the carbon 14 distribution in the abyssal waters of the world oceans indicates replacement times for pacific indian and atlantic ocean deep waters more than 1500 meters deep of approximately 510 250 and 275 years respectively the deep waters of the entire world ocean are replaced on average every 500 years

multidecadal variability of north atlantic temperature and salinity during the twentieth century abstract substantial changes occurred in the north atlantic during the twentieth century here the authors demonstrate through the analysis of a vast collection of observational data that multidecadal fluctuations on time scales of 50 80 yr are prevalent in the upper 3000 m of the north atlantic ocean spatially averaged temperature and salinity from the 0 300 and 1000 3000 m layers vary in opposition prolonged periods of cooling and freshening warming and salinification in one layer are generally associated with opposite tendencies in the other layer consistent with the notion of thermohaline overturning circulation in the 1990s widespread cooling and freshening was a dominant feature in the 1000 3000 m layer whereas warming and salinification generally dominated in the upper 300 m except for the subpolar north atlantic where complex exchanges with the arctic ocean occur the single signed basin scale pattern of multidecadal variability is evident from decadal 1000 3000 m temperature and salin

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

multidecadal warming and shoaling of antarctic intermediate water antarctic intermediate water aaiw is a dominant southern hemisphere water mass that spreads from its formation regions just north of the antarctic circumpolar current acc to at least 20 s in all oceans this study uses an isopycnal climatology constructed from argo conductivity temperature depth ctd profile data to define the current state of the aaiw salinity minimum its core and thence compute anomalies of aaiw core pressure potential temperature salinity and potential density since the mid 1970s from ship based ctd profiles the results are used to calculate maps of temporal property trends at the aaiw core where statistically significant strong circumpolar shoaling 30 50 dbar decade 1 warming 0 05 0 15 c decade 1 and density reductions up to 0 03 kg m 3 decade 1 are found these trends are strongest just north of the acc in the southeast pacific and atlantic oceans and decrease equatorward salinity trends are generally small with their sign varying regionally bottle data are used to extend the aaiw core potential temperature anomaly analysis back to 1925 in the atlantic and to 1960 elsewhere the modern warm aaiw core conditions appear largely unprecedented in the historical record biennially and zonally binned median aaiw core potential temperatures within each ocean basin are with the notable exception of the subtropical south atlantic in the 1950s 70s 0 2 1 c colder than modern values zonally averaged sea surface temperature anomalies around the aaiw formation latitudes in each ocean and sectoral southern annular mode indices are used to put the aaiw core property trends and variations into context

estimated decadal changes in the north atlantic meridional overturning circulation and heat flux 1993 2004 results from a global 1 model constrained by least squares to a multiplicity of datasets over the interval 1992 2004 are used to describe apparent changes in the north atlantic ocean meridional overturning circulation and associated heat fluxes at 26 n the least squares fit is both stable and adequately close to the data to make the analysis worthwhile changes over the 12 yr are spatially and temporally complex a weak statistically significant trend is found in net north atlantic volume flux above about 1200 m which drops slightly 0 19 0 05 sv yr 1 1s v 10 6 m 3 s 1 but with a corresponding strengthening of the outflow of north atlantic deep water and inflow of abyssal waters the slight associated trend in meridional heat flux is very small and not statistically significant the month to month variability implies that single section determinations of heat and volume flux are subject to serious aliasing errors

a twenty year reversal in water mass trends in the subtropical north atlantic temperature and salinity changes have been compared along three repeat sections at 36 n in the north atlantic the principal changes cooling of the upper waters and warming of the intermediate waters observed between 1959 and 1981 were reversed between 1981 and 2005 the controlling mechanisms for the changes over the two time periods differed through the water column neutral density surfaces over the upper 800 m were firstly uplifted and secondly depressed by typically 50 m which is broadly consistent with the changes in thermocline thickness implied by the temporal changes in ekman pumping in contrast the intermediate waters 800 2500 m firstly became warmer and saltier and secondly became cooler and fresher this change in the intermediate waters was controlled by water mass changes along neutral density surfaces suggesting a change in the source waters principally labrador sea water and mediterranean outflow water

southwest pacific ocean water mass changes between 1968 69 and 1990 91 abstract water mass changes are estimated in the southwest pacific ocean by comparing a meridional hydrographic section along 170 w between 60 s and the equator occupied in 1968 69 during the southern cross cruise and again in 1990 during a noaa climate and global change cruise another comparison is made using a hydrographic section along 35 s between the date line and 169 w occupied in 1969 during usns eltanin cruise 40 and again in 1991 during a mapkiwi cruise the most robust change consists of cooling and freshening on isopycnals with peak differences exceeding 1 0 c 0 25 pss at the base of the subtropical thermocline the cooling and freshening starts above the stratification minimum of the subantarctic mode water and persists to below the salinity minimum of the antarctic intermediate water amplitudes are largest at 48 s near where these water masses subduct and decay toward 20 s near the axis of the subtropical gyre this change is likely the result of surface warming and or freshening

timely detection of changes in the meridional overturning circulation at 26 n in the atlantic it is investigated how changes in the north atlantic meridional overturning circulation moc might be reliably detected within a few decades using the observations provided by the rapid moc 26 n array previously detectability of moc changes had been investigated with a univariate moc time series exhibiting strong internal variability which would prohibit the detection of moc changes within a few decades here a modification of k hasselmann s fingerprint technique is used simulated observations are projected onto a time independent spatial pattern of natural variability to derive a time dependent detection variable the fixed spatial pattern of natural variability is derived by regressing the zonal density gradient along 26 n against the strength of the moc at 26 n within the coupled echam5 max planck institute ocean model s mpi om control climate simulation this pattern is confirmed against the observed anomalies found between the 1957 and the 2004 hydrographic occupations of the section onto this fixed spatial pattern of natural variability both the existing hydrographic observations and simulated observations mimicking the rapid moc 26 n array in three realizations of the intergovernmental panel on climate change ipcc scenario a1b are projected for a random observation error of 0 01 kg m 3 and only using zonal density gradients between 1700 and 3100 m depth statistically significant detection occurs with 95 reliability after about 30 yr in the model and climate change scenario analyzed here compared to using a single moc time series as the detection variable continuous observations of zonal density gradients reduce the detection time by 50 for the five hydrographic occupations of the 26 n transect none of the analyzed depth ranges shows a significant trend between 1957 and 2004 implying that there was no moc trend over the past 50 yr

rheology of mg2geo4 olivine and spinel harzburgite implications for earth s mantle transition zone geophysical modeling requires an upper mantle low viscosity zone lvz recent results suggest that the lvz could be within the mantle transition zone mtz or above it depending on boundary conditions the rheology of the olivine dominated uppermost mantle is well constrained experimentally but not the mtz here we report first time measurement of the rheology of mg2geo4 spinel analogue of ringwoodite the dominant mineral of the lower mtz in the dislocation creep regime and compare it with that of mg2geo4 olivine in both cases we incorporate 15 20 pyroxene the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies under similar conditions spinel dominated material is about twice as strong as olivine dominated material our results suggest that unless h2o weakening of ringwoodite or phase transformation induced grain size reduction occurs in the lower mtz it will be more viscous than above the 410 km discontinuity

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

parallel decadal variability of inferred water temperatures for northern and southern hemisphere intermediate water masses we use a novel proxy growth rates of long lived deep water fish orange roughy to reconstruct inferred water temperatures of intermediate water masses in both northern and southern hemispheres since the mid 1800s the data are consistent with instrumental records showing long term warming in the northern hemisphere but also indicate decadal variability of intermediate depth temperatures that is coherent across the two hemispheres this variability correlates with the dominant subpolar annular mode in each hemisphere and implies a bihemispheric oceanic response to external forcing that influences the properties of intermediate depth water masses

rising temperature and salinity fields to the north of the almeria oran front during the past decade 1 time series of temperature and salinity profiles in the south western mediterranean sea are analyzed from june 1996 to november 2006 previous studies have addressed the detection of changes on isobaric levels in this paper we decompose thermohaline changes into those caused by vertical displacements of the isopycnals more likely linked to changes in the dynamics of the region and dominant winds and those changes on isopycnal surfaces the latter give a more accurate idea about changes in water mass properties due to ocean atmosphere interaction our analysis reveals that there has been a warming and salting on isopycnal surfaces the warming during this 10 year period is quite intense around 0 032 c yr 1 and the salinity increase is around 0 009 yr 1 the depth of isopycnals shows an upward trend rising 45 m over the study period compensating for the warming on isobaric surfaces

recirculating components to the deep boundary current of the northern north atlantic abstract the meridional overturning system of the north atlantic is conventionally thought of as transporting warm water 4 c to high latitudes and covd water to low latitudes the northern cold water sources for this system are dense overflow from the nordic seas and less dense water from the labrador sea lsw the overflow components are carried westward by a deep northern boundary current dnbc that is shown here actually to begin southeast of the overflow regions the dnbc lies along the southern side of a system of islands and submarine ridges that divides the nordic seas from the subpolar basins the overflows act to increase the transport of the dnbc and this transport is further increased through the entrainment of warm waters once inside the labrador sea the dnbc is joined by lsw before continuing south to low latitudes as a cold deep western boundary current dwbc however published transport estimates are larger than can be explained by warm water entrainment alone thus indicating entrainment also of recirculating cold components the subject of this paper their sources are shown to be the lsw and cold abyssal waters originating from the southern hemisphere the lsw is entrained downward into the denser part of the dnbc and laterally into the upper part of the dwbc whereas the cold abyssal waters are supplied by an eastern intensified northward flow in the eastern atlantic that serves as the initial source of the dnbc and by a similar northward flow in the western atlantic the meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system the cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin but their influence in the northwestern atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the newfoundland and labrador basins this influence is detected by a deep silicate maximum extending poleward from the mid latitude western atlantic through the two gyres with eastern concentration and recirculating back diluted toward the south in the dwbc within the respective basins such a maximum also extends northward through the irminger basin into the dnbc but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin or if it comes from the mid latitude eastern basin by way of westward flow through the charlie gibbs fracture zone it is perhaps consequence of both estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north

intense warming and salinification of intermediate waters of southern origin in the eastern subpolar north atlantic in the 1990s to mid 2000s 1 recent thermohaline changes in the layer of intermediate waters iw advected into the eastern subpolar north atlantic from lower latitudes are quantified using the data from the repeated transatlantic sections positive trends in temperature and salinity in the iw density class at 53 n 0 049 c a and 0 0088 a 1992 2002 and 60 n 0 044 c a and 0 0085 a 1997 2005 are derived the unexpectedly high rates of the iw warming and salinification cannot be explained solely by the long term and recent decadal changes at the intermediate levels in the midlatitude north atlantic and appear to be a consequence of the northward advance of the source water masses caused by the north atlantic oscillation induced contraction of the subpolar gyre

on the sub decadal variability of south atlantic antarctic intermediate water variability of antarctic intermediate water salinity in the south atlantic is investigated on interannual and intradecadal timescales novel observations of slow westward propagating salinity anomalies in argo data are presented the features have no corresponding signal in temperature and are anomalous in density analysis of 40 years of model output supports the existence of these westward propagating salinity anomalies and indicates that they are typical occurrences in time the features are intensified in a latitude band around 30 s associated with the propagation of agulhas rings however the features are much larger than agulhas rings and occur on decadal timescales in the model they propagate westward with speeds of 2 3 cm s in observations and 1 7 cm s in model data they are more consistent with planetary waves than with the advection of large scale salinity anomalies the observation of these features has implications for the interpretation of salinity anomalies such as the linking of hydrological cycle changes to salinity changes

radiocarbon decay and oxygen utilization in the deep atlantic ocean a parameter based on the sum of the concentrations of po4 and o2 divided by the redfield coefficient o2 po4 is used to separate the contributions of the northern and southern components to deep waters in the atlantic this separation allows the amount of radiocarbon lost by radiodecay and the amount of oxygen lost to respiration during residence in the deep atlantic to be calculated maps of these quantities reveal strong west to east gradients and weak north to south gradients consistent with ventilation along the western boundary from both ends of the ocean coupled with mixing outward from the boundary the o2 and 14c deficiences are highly correlated suggesting an o2 utilization rate of 12 m kg per century the apparent mean isolation time of water in the deep atlantic is about 200 years

deep and intermediate layer warming in the western mediterranean water mass changes and heaving 1 temperature and salinity have increased in the western mediterranean deep water during the second half of the 20th century but surprisingly the two water masses contributing to its formation atlantic water and levantine intermediate water show no clear warming trends at least for the same period of time previous studies concerned with long term ts property changes in the western mediterranean had dealt with changes on isobaric surfaces in this study we decompose these changes into their two natural contributions changes on isopycnal surfaces and changes induced by heaving vertical displacements of density surfaces this decomposition shows that liw temperature and salinity have increased when changes on density surfaces are analyzed the upward movement of deep layers has induced cooling on isobaric levels masking liw temperature trends and underestimating deep layers temperature trends

transatlantic temperature and salinity changes at 24 5 n from 1957 to 2004 in 2004 a fifth occupation of a transatlantic section at 24 5 n allows us to examine decadal temperature and salinity changes using high quality full depth hydrographic data since 1957 waters shallower than 1750 dbar have been warming and salinifying at least since 1981 and in 2004 are significantly warmer and saltier than at any time since 1957 while deeper than 3000 dbar there has been continuous cooling and freshening since 1957 temperature and salinity changes at constant pressures are partitioned into changes on isopycnal surfaces and changes due to the vertical movement of the isopycnals warming in the western atlantic thermocline since 1957 at a rate of 0 0111 c yr dominates the transatlantic average while deep water has cooled and freshened at rates of 0 0021 c yr and 0 0003 psu yr respectively we argue that the shallower and deeper changes are consistent with a recently reported increased southward thermocline circulation and reduced southward flux of deep water

coastal sea surface temperature warming trend along the continental part of the atlantic arc 1985 2005 1 coastal warming was analyzed by means of satellite derived sea surface temperature sst along the continental part of the atlantic arc extending from 37 n to 48 n for the period 1985 2005 interannual sst variability calculated from anomalies shows an inhomogeneous warming trend in coastal sst sst coastal warming trend ranges from 3 5 c 100 a 1 at latitudes close to 48 n to 1 2 c 100 a 1 at latitudes close to 37 n this warming is observed to reflect oceanic sst which increases northward in the atlantic area in addition a westward increase was also observed across the north atlantic coastal warming is not constant all year long it only occurs during spring and summer in the entire area under scope following a trend similar to the annual one the mean spring sst values are 5 1 c 100 a 1 in the french coast 4 4 c 100 a 1 in the cantabrian coast and 3 8 c 100 a 1 in the western iberian peninsula coast similar values 5 0 c 100 a 1 3 7 c 100 a 1 and 2 7 c 100 a 1 are observed in summer

interannual variability of antarctic intermediate water in the tropical north atlantic interannual variability of antarctic intermediate water aaiw in the tropical north atlantic is investigated using the gecco2 ocean state estimate and argo data aaiw salinity variability near the western boundary is highly correlated with the transport along the western boundary on interannual timescales northward propagating anomalies are associated with the western boundary transport variability that to some extent is related to the large scale wind stress curl forcing by means of the sverdrup balance aaiw anomalies also propagate westward with the speed of baroclinic rossby waves indicating that the displacement of the meridional salinity gradient by westward propagation of baroclinic rossby waves plays a role in the variability of aaiw characteristics slower eastward spreading of aaiw anomalies is identified on decadal timescales likely associated with the advection of salinity anomalies by weak eastward current bands understanding the observed interannual and decadal variability of aaiw salinity is important to properly interpret salinity changes reported in response to changes in the hydrological cycle

spiciness anomalies in the upper south indian ocean abstractthis study investigates an isopycnal temperature salinity t s or spiciness anomaly in the upper south indian ocean for the period from 2004 to 2015 using observations and reanalyses spic

late winter generation of spiciness on subducted isopycnals the origins of density compensating anomalies of temperature and salinity spice are investigated using a model forced with the most realistic surface products available over the 40 years 1958 97 in this hindcast the largest interannual spiciness anomalies are found in the pacific ocean near the isopycnal s 0 5 25 5 where deviations as great as 1 28c and 0 6 psu are generated equatorward of winter outcropping in the eastern subtropics in both hemispheres these source regions are characterized by very unstable salinity gradients and low mean density stratification in winter two related signatures of winter mixing in the southeast pacific sep are density that is well mixed deeper than either temperature or salinity and subsurface density ratios that approach 1 all ocean basins in the model are shown to have regions with these characteristics and signatures however the resultant spiciness signals are focused on different isopycnals ranging from s 0 5 25 0 in the northeast pacific to s 0 5 26 5 in the south indian ocean a detailed examination of the sep finds that large positive anomalies are generated by diapycnal mixing across subducted isopycnals e g s 0 5 25 5 whereas negative anomalies are the result of a steady isopycnal advection moderated by vertical advection and heave there is considerable interannual variability in the strength of anomalies and in the density on which they occur historical observations are consistent with the model results but are insufficient to verify all aspects of the hindcast including the processes of anomaly generation in the sep it was not possible to relate isopycnal anomaly genesis to local surface forcing of any kind a complex scenario involving basinwide circulation of both the ocean and atmosphere especially of surface water through the subtropical evaporation zones is put forward to explain the decadal time scale evident in sep salinity anomalies on s 0 5 25 5 pacific anomalies generated on s 0 5 25 5 can be traced along mean geostrophic streamlines to the western boundary where decadal salinity variations at 78s are about 2 times as large order 60 1 psu as at 128n although there may be more variance on shallower isopycnals in the north at least portions of the s 0 5 25 5 signals appear to continue along the boundary to a convergence at the equator suggesting that the most robust sources of pacific spiciness variance coincide with equatorial exchange pathways

an anomalous recent acceleration of global sea level rise abstract tide gauge data are used to estimate trends in global sea level for the period from 1955 to 2007 linear trends over 15 yr segments are computed for each tide gauge record averaged over latitude bands and combined to form an area weighted global mean trend the uncertainty of the global trend is specified as a sampling error plus a random vertical land motion component but land motion corrections do not change the results the average global sea level trend for the time segments centered on 1962 90 is 1 5 0 5 mm yr 1 standard error in agreement with previous estimates of late twentieth century sea level rise after 1990 the global trend increases to the most recent rate of 3 2 0 4 mm yr 1 matching estimates obtained from satellite altimetry the acceleration is distinct from decadal variations in global sea level that have been reported in previous studies increased rates in the tropical and southern oceans primarily account for the acceleration the timing of the global acceleration

north atlantic climate responses to perturbations in antarctic intermediate water recent observations suggest antarctic intermediate water aaiw properties are changing the impact of such variations is explored using idealised perturbation experiments with a coupled climate model hadcm3 aaiw properties are altered between 10 and 20 s in the south atlantic maintaining constant potential density the perturbed aaiw remains subsurface in the south atlantic but as it moves northwards it surfaces and interacts with the atmosphere leading to density anomalies due to heat exchanges for a cooler fresher aaiw there is a significant decrease in the mean north atlantic sea surface temperature sst of up to 1 c during years 51 100 in the north atlantic current region there are persistent cold anomalies from 2 000 m depth to the surface and in the overlying atmosphere atmospheric surface pressure increases over the mid latitude atlantic and precipitation decreases over northwest africa and southwest europe surface heat flux anomalies show that these impacts are caused by changes in the ocean rather than atmospheric forcing the sst response is associated with significant changes in the atlantic meridional overturning circulation moc after 50 years there is a decrease in the moc that persists for the remainder of the simulation resulting from changes in the column averaged density difference between 30 s and 60 n rather than showing a linear response a warmer saltier aaiw also leads to a decreased moc strength for years 51 100 and resulting cooling in the north atlantic the non linearity can be attributed to opposing density responses as the perturbed water masses interact with the atmosphere

intermediate depth warming in the tropical atlantic related to weakened thermohaline circulation combining paleoclimate data and modeling results for the last deglaciation 1 benthic foraminiferal oxygen isotope ratios from two sediment cores recovered at 426 and 1299 m water depth in the eastern and western tropical atlantic show that a slowdown of the thermohaline circulation thc during heinrich event h1 and the younger dryas was accompanied by rapid and intense warming of intermediate depth waters millennial scale covariations of low paleosalinities in the subpolar north atlantic with decreased benthic oxygen isotope ratios in the eastern tropical atlantic throughout the past 10 000 years suggest that thc weakening might be related to middepth warming during the holocene period as well climate model experiments simulating a strong reduction of the thc in the atlantic ocean under present day and glacial conditions reveal that the increase of temperature in the middepth tropical and south atlantic is a common feature for both climatic states caused by a reduced ventilation of cold intermediate and deep waters in conjunction with downward mixing of heat from the thermocline from the similarity of the paleoclimatic records with the model simulations we infer that the characteristic pattern of temperature change in the atlantic ocean related to weakened thermohaline circulation can serve as an indicator of present day and future thc slowdown

decadal variability in the outflow from the nordic seas to the deep atlantic ocean the global thermohaline circulation is the oceanic overturning mode which is manifested in the north atlantic ocean as northward flowing surface waters which sink in the nordic greenland iceland and norwegian seas and return southwards after overflowing the greenland scotland ridge as deep water this process has been termed the conveyor belt and is believed to keep europe 5 8 c warmer than it would be if the conveyor were to shut down the variability of today s conveyor belt is therefore an important component of climate regulation the nordic seas are the only northern hemisphere source of deep water and a previous study has revealed no long term variability in the outflow of deep water from the nordic seas to the atlantic ocean here i use flows derived from hydrographic data to show that this outflow has approximately doubled and then returned to previous values over the past four decades i present evidence which suggests that this variability is forced by variability in polar air temperature which in turn may be connected to the recently reported arctic warming

decadal wind forcing of the north atlantic subtropical gyre abstract in the central north atlantic ocean there are large decadal scale fluctuations of sea level and of the depth of the thermocline this variability can be explained by low frequency rossby waves forced by wind the authors have used a simple model of ocean fluctuations driven by the coads wind stress curl and find that their model results and hydrographic data to the limited extent they can be compared meaningfully agree rather well the variation in wind curl over the ocean leads to surprisingly large north south variability in the computed oceanic response over the subtropical gyre the peak to peak sea level differences are as great as 20 cm and persist for many years these large variations could induce major errors in calculations of the mean ocean flow when hydrographic sections from many years are combined it appears possible however to correct for these wind induced effects to allow the lower frequency signals to be determined the westernmost edge of our oceanic calculations is at the