break up of the atlantic deep western boundary current into eddies at 8 s the existence in the ocean of deep western boundary currents which connect the high latitude regions where deep water is formed with upwelling regions as part of the global ocean circulation was postulated more than 40 years ago1 these ocean currents have been found adjacent to the continental slopes of all ocean basins and have core depths between 1 500 and 4 000 m in the atlantic ocean the deep western boundary current is estimated to carry 10 40 times 106 m3 s 1 of water2 3 4 5 transporting north atlantic deep water from the overflow regions between greenland and scotland and from the labrador sea into the south atlantic and the antarctic circumpolar current here we present direct velocity and water mass observations obtained in the period 2000 to 2003 as well as results from a numerical ocean circulation model showing that the atlantic deep western boundary current breaks up at 8 s southward of this latitude the transport of north atlantic deep water into the south atlantic ocean is accomplished by migrating eddies rather than by a continuous flow our model simulation indicates that the deep western boundary current breaks up into eddies at the present intensity of meridional overturning circulation for weaker overturning continuation as a stable laminar boundary flow seems possible

atlantic meridional overturning circulation slowdown cooled the subtropical ocean 1 observations show that the upper 2 km of the subtropical north atlantic ocean cooled throughout 2010 and remained cold until at least december 2011 we show that these cold anomalies are partly driven by anomalous air sea exchange during the cold winters of 2009 2010 and 2010 2011 and more surprisingly by extreme interannual variability in the ocean s northward heat transport at 26 5 n this cooling driven by the ocean s meridional heat transport affects deeper layers isolated from the atmosphere on annual timescales and water that is entrained into the winter mixed layer thus lowering winter sea surface temperatures here we connect for the first time variability in the northward heat transport carried by the atlantic meridional overturning circulation to widespread sustained cooling of the subtropical north atlantic challenging the prevailing view that the ocean plays a passive role in the coupled ocean atmosphere system on monthly to seasonal timescales

what is the thermohaline circulation the term thermohaline circulation is widely used in the context of past and future climate change in his perspective wunsch argues that there are many different and inconsistent definitions for this term he defines ocean circulation in terms of mass fluxes which are largely driven by surface forces such as wind and tides and argues that thermohaline circulation should only refer to the separate transport of heat and salinity in the ocean

predictability of the mid latitude atlantic meridional overturning circulation in a multi model system assessing the skill of the atlantic meridional overturning circulation amoc in decadal hindcasts i e retrospective predictions is hampered by a lack of observations for verification models are therefore needed to reconstruct the historical amoc variability here we show that ten recent oceanic syntheses provide a common signal of amoc variability at 45 n with an increase from the 1960s to the mid 1990s and a decrease thereafter although they disagree on the exact magnitude this signal correlates with observed key processes such as the north atlantic oscillation sub polar gyre strength atlantic sea surface temperature dipole and labrador sea convection that are thought to be related to the amoc furthermore we find potential predictability of the mid latitude amoc for the first 3 6 year means when we validate decadal hindcasts for the past 50 years against the multi model signal however this predictability is not found in models driven only by external radiative changes demonstrating the need for initialization of decadal climate predictions

vertical partition of ocean heat transport in isothermal coordinates 1 the meridional overturning circulation moc associated with deep water formation in the north atlantic is thought to play an important part in the transport of heat by the climate system according to observational estimates and simple scaling arguments the transport of heat by the moc is 1 pw however in one recent modeling study boccaletti et al 2005 this estimate is reduced to 0 4 pw using a model with higher resolution the issue of heat transport partitioning between the circulation in the upper ocean and that due to the flows in the deep ocean is revisited here it is found that out of about 3 pw of heat entering the low latitude ocean through the surface approximately 65 is transported poleward by the directly wind driven circulations in the upper ocean however 0 85 0 35 pw of the peak northward heat transport at 15 20 n can be associated with the moc in the southern hemisphere the net effect of the deep flows on the heat transport is weaker so that the corresponding maximum at 15 20 s is essentially due to the flows in the upper ocean

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

low frequency variability of the north pacific ocean the roles of boundary and wind driven baroclinic rossby waves 1 the effects of the sea level variability along the eastern boundary of the north pacific ocean including those associated with el nino southern oscillation have been considered an important factor in determining the low frequency large scale variability of the ocean interior through rossby waves generated at the eastern boundary this hypothesis is examined in this study using 8 years worth of the sea surface height observations made by the topex poseidon satellite the timescales of interest are longer than 90 days with the annual cycle removed the results indicate that the influence of the eastern boundary of the north pacific ocean has limited offshore extent varying from 3000 4000 km at 10 n to 200 300 km at 50 n the variability in the ocean interior is primarily driven by wind with only a minor influence from the boundary simulations of a linear two layer model of the ocean driven by wind stress curl are correlated with the observations the effects of wind forcing accumulate along the characteristics of long nondispersive rossby waves in the time longitude domain these wind driven rossby waves overwhelm the boundary driven waves in the ocean interior where there are only some small residual effects of the boundary driven waves however these small effects become clearer after the simulated wind driven variability is removed from the observations index terms 4556 oceanography physical sea level variations 4504 oceanography physical air sea interactions 0312 4215 oceanography general climate and interannual variability 3309 keywords rossby waves satellite altimetry el nino north pacific eastern boundary

anthropogenic impact on agulhas leakage recent work suggests that changes of the southern hemisphere sh winds led to an increase in agulhas leakage and a corresponding salinification of the atlantic climate model projections for the 21st century predict a progressive southward migration and intensification of the sh westerlies the potential effects on the ocean circulation of such an anthropogenic trend in wind stress are studied here with a high resolution ocean model forced by a step function change in sh wind stress that involves a 7 increase in westerlies strength and a 2 shift in the zero wind stress curl the model simulation suggests a rapid dynamic adjustment of agulhas leakage by 4 5 sv about a third of its original value after a few years the change in leakage is reflected in a concomitant change in the transport of the south atlantic subtropical gyre but leads only to a small increase in the atlantic meridional overturning circulation amoc of o 1 sv after three decades a main effect of the increasing inflow of indian ocean waters with potential long term ramifications for the amoc is the salinification and densification of upper thermocline waters in the south atlantic which extends into the north atlantic within the first three decades

long term effect of volcanic forcing on ocean heat content 1 explosive volcanic eruptions cause episodic negative radiative forcing of the climate system using coupled atmosphere ocean general circulation models aogcms subjected to historical forcing since the late nineteenth century previous authors have shown that each large volcanic eruption is associated with a sudden drop in ocean heat content and sea level from which the subsequent recovery is slow here we show that this effect may be an artefact of experimental design caused by the aogcms not having been spun up to a steady state with volcanic forcing before the historical integrations begin because volcanic forcing has a long term negative average a cooling tendency is thus imposed on the ocean in the historical simulation we recommend that an extra experiment be carried out in parallel to the historical simulation with constant time mean historical volcanic forcing in order to correct for this effect and avoid misinterpretation of ocean heat content changes

latitudinal dependence of atlantic meridional overturning circulation amoc variations 1 amoc variations are often thought to propagate with the kelvin wave speed resulting in a short time lead between high and low latitudes amoc variations however as shown in this paper using a coupled climate model gfdl cm2 1 with the existence of interior pathways of north atlantic deep water nadw from flemish cap to cape hatteras as that observed recently amoc variations estimated in density space propagate with the advection speed in this region resulting in a much longer time lead several years between subpolar and subtropical amoc variations and providing a more useful predictability the results suggest that amoc variations have significant meridional coherence in density space and monitoring amoc variations in density space at higher latitudes might reveal a stronger signal with a several year time lead

warm and saline events embedded in the meridional circulation of the northern north atlantic 1 ocean state estimates from 1958 to 2005 from the simple ocean assimilation system soda system are analyzed to understand circulation between subtropical and subpolar atlantic and their connection with atmospheric forcing this analysis shows three periods 1960s around 1980 and 2000s with enhanced warm saline waters reaching high latitudes alternating with freshwater events originating at high latitudes it complements surface drifter and altimetry data showing the subtropical subpolar exchange leading to a significant temperature and salinity increase in the northeast atlantic after 2001 the warm water limb of the atlantic meridional overturning cell represented by soda expanded in density salinity space during these warm events tracer simulations using soda velocities also show decadal variation of the gulf stream waters reaching the subpolar gyre and nordic seas the negative phase of the north atlantic oscillation index usually invoked in such variability fails to predict the warming and salinization in the early 2000s with salinities not seen since the 1960s wind stress curl variability provided a linkage to this subtropical subpolar gyre exchange as illustrated using an idealized two layer circulation model the ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal in this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax hence the polar front moves westward opening an enhanced northward access of the subtropical waters in the eastern boundary current

estimates of global ocean volume transports during 1960 through 2001 1 an estimate of global time varying ocean volume transports is provided for the period 1960 2001 as it results from the german ecco gecco synthesis results confirm previously discussed mean state ocean circulation systems encompassing an upper meridional cell in the atlantic with sinking in the north atlantic connected to a lower meridional cell with sinking around antarctica in the pacific and atlantic decadal to interdecadal variability as well as long term trends of global ocean transport analyzed over the course of the 42 years appear as a reorganization of time mean circulation structures including the layer transports in the antarctic circumpolar current acc and the circulation involving the indo pacific exchange

interdecadal variability of the meridional overturning circulation as an ocean internal mode the meridional overturning circulation moc in the coupled echam5 mpiom exhibits variability at periods of near 30 years and near 60 years the 30 year variability referred to as interdecadal variability idv exist in an ocean model driven by climatological atmospheric forcing suggesting that it is maintained by ocean dynamics the 60 year variability the multidecadal variability mdv is only observed in the fully coupled model and therefore is interpreted as an atmosphere ocean coupled mode the coexistence of the 30 year idv and the 60 year mdv provides a possible explanation for the widespread time scales observed in climate variables further analyses of the climatologically forced ocean model shows that the idv is related to the interplay between the horizontal temperature dominated density gradients and the ocean circulation temperature anomalies move along the cyclonic subpolar gyre leading to fluctuations in horizontal density gradients and the subsequent weakening and strengthening of the moc this result is consistent with that from less complex models indicating the robustness of the idv we further show that along the north atlantic current path the sea surface temperature anomalies are determined by the slow lsw advection at the intermediate depth

dynamics of the gulf stream deep western boundary current crossover part i entrainment and recirculation abstract a regional primitive equation model is applied to the study of the interaction between the gulf stream and the deep western boundary current dwbc where they cross at cape hatteras it is found that for a wide range of forcing parameters the upper core of the dwbc is split into two mean paths at the crossover one flowing toward the south along the western boundary and the other flowing toward the cast under the gulf stream the eastward branch is entrained into the southern recirculation gyre and after being diverted into the interior for up to 1500 km eventually returns to the western boundary current and continues to flow southward this recirculation and mixing is shown to have a significant impact on the separation point and mean path of the gulf stream the basin scale stratification and the properties of the dwbc south of the crossover for most configurations the lower dwbc remains largely on the western boundary and interacts only weakly with the interior the entrainment of the upp

observation of decadal change in the atlantic meridional overturning circulation using 10 years of continuous transport data the meridional overturning circulation moc represents the main mechanism for the oceanic northward heat transport in the atlantic and fluctuations of this circulation are believed to have major impacts on northern hemisphere climate while numerical ocean and climate models and paleo records show large variability in this circulation the use of direct observations of the moc for detecting climate timescale changes has proven difficult so far this report presents the first observational record of moc measurements that is continuous and sufficiently long to exhibit decadal scale changes here a decrease by 20 over the observational period jan 2000 june 2009 and large interannual changes in the flow and its vertical structure data are from a mooring array at 16 n meridional overturning variability experiment move the observed change agrees with the amplitude of multi decadal natural fluctuations seen in numerical ocean and climate models knowledge of the existence and phasing of such internal cycles provides multi decadal climate predictability recently some numerical model simulations have produced results that show a weakening of the moc since the 1990 s and observational confirmation of this now is a high priority

ice core data evidence for a prominent near 20 year time scale of the atlantic multidecadal oscillation 1 using five ice core data sets combined into a single time series we provide for the first time strong observational evidence for two distinct time scales of arctic temperature fluctuation that are interpreted as variability associated with the atlantic multidecadal oscillation amo the dominant and the only statistically significant multidecadal signal has a time scale of about 20 years the longer multidecadal variability of 45 85 years is not well defined and none of the time scales in this band is statistically significant we compare these observed temperature fluctuations with results of coupled climate model simulations hadcm3 and gfdl cm2 1 both the 20 25 year and a variable longer amo time scale are prominent in the models long control runs this periodicity supports our conjecture that the observed ice core fluctuations are a signature of the amo the robustness of this short time scale period in both observations and model simulations has implications for understanding the dominant amo mechanisms in climate

on the currents and transports connected with the atlantic meridional overturning circulation in the subpolar north atlantic results from an interannually forced 0 08 degrees eddy resolving simulation based on the hybrid coordinate ocean model in conjunction with a small but well determined transport database are used to investigate the currents and transports associated with the atlantic meridional overturning circulation amoc in the subpolar north atlantic spna the model results yield a consistent warming in the western spna since the early 1990s along with mean transports similar to those observed for the trans basin amoc across the world ocean circulation experiment hydrographic section ar19 16 4 sv and boundary currents at the exit of the labrador sea near 53 degrees n 39 0 sv and east of the grand banks near 43 degrees n 15 9 sv over a 34 year integration the model determined amoc across the ar19 section and the western boundary current near 53 degrees n both exhibit no systematic trend but some long term interannual and longer variabilities including a decadal transport variation of 3 4 sv from relatively high in the 1990s to low in the 2000s the decadal variability of the model boundary current transport near 53 degrees n lags the observed winter time north atlantic oscillation index by about 2 years and leads the model amoc across the ar19 section by about 1 year the model results also show that the long term variabilities are low compared to those on shorter time scales thus rapid sampling of the current over long time intervals is required to filter out high frequency variabilities in order to determine the lower frequency variabilities of interest

global ocean surface velocities from drifters mean variance el nino southern oscillation response and seasonal cycle 1 global near surface currents are calculated from satellite tracked drogued drifter velocities on a 0 5 0 5 latitude longitude grid using a new methodology data used at each grid point lie within a centered bin of set area with a shape defined by the variance ellipse of current fluctuations within that bin the time mean current its annual harmonic semiannual harmonic correlation with the southern oscillation index soi spatial gradients and residuals are estimated along with formal error bars for each component the time mean field resolves the major surface current systems of the world the magnitude of the variance reveals enhanced eddy kinetic energy in the western boundary current systems in equatorial regions and along the antarctic circumpolar current as well as three large eddy deserts two in the pacific and one in the atlantic the soi component is largest in the western and central tropical pacific but can also be seen in the indian ocean seasonal variations reveal details such as the gyre scale shifts in the convergence centers of the subtropical gyres and the seasonal evolution of tropical currents and eddies in the western tropical pacific ocean the results of this study are available as a monthly climatology

propagation pathways of classical labrador sea water from its source region to 26 n 1 more than two decades of hydrography on the abaco line east of the bahamas at 26 n reveals decadal variability in the salinity of classical labrador sea water clsw despite the long distance from its source region in the north atlantic ocean hydrographic time series from the labrador sea and from the abaco line show a pronounced step like decrease in salinity between 1985 and 1995 in the labrador sea and between 1995 and 2010 at the abaco line suggesting a time lag between the two locations of approximately 9 years the amplitude of the anomaly at the abaco line is 50 of the amplitude in the labrador sea a similar time lag and reduction of amplitude is found in the high resolution ofes model in which salinity anomalies can be observed propagating through the deep western boundary current as well as through a broad interior pathway on its way south to the abaco line the clsw becomes 8 standard deviations saltier due to isopycnal mixing with mediterranean outflow water mow climatological data in the north atlantic suggests that the mixing ratio of mow to clsw at the abaco line is 1 4 and that no variability in mow is required to explain the observed variability at the abaco line the data studied here suggest that decadal clsw anomalies stay relatively coherent while getting advected despite the important role of interior pathways

a 1400 year high resolution record of atmospheric circulation over the north atlantic and asia major ion series developed from new subannual scale sampling of an ice core from central greenland are calibrated with instrumental series of atmospheric sea level pressure recording major marine icelandic low and terrestrial siberian high atmospheric circulation systems to provide proxy records of atmospheric circulation over the past 1400 years examination of the proxy records reveals major changes in behaviour of these systems c ad 1400 multidecadal and centennial scale periodic components characterization of mean sea level pressure anomaly fields during the little ice age and the mediaeval warm period the potential role of solar forcing coupled ocean atmosphere associations and a perspective within which the characteristics of instrumental era climate can be assessed

interdecadal variability in a hybrid coupled ocean atmosphere model abstract a hybrid coupled ocean atmosphere model is used to investigate low frequency variability in the climate system the model s atmospheric component is a budyko sellers north two dimensional energy balance model the oceanic component is a simplified general circulation model the coupled model is confined to an idealized rectangular north atlantic basin in the present model version the ocean density depends exclusively on temperature an interdecadal oscillation with a period of 40 50 years is found in the hybrid coupled model when model parameters are within the climatological range even though density does not depend on salinity this interdecadal oscillation is characterized by a pair of vortices of opposite signs that grow and decay in quadrature with each other in the ocean s upper layer their centers follow each other anticlockwise through the northwestern quadrant of the model domain the interdecadal oscillation s physical mechanism resembles that of the interdecadal oscillation anal

the partition of regional sea level variability abstract the existing altimetric record offers an unprecedented view of sea level variability on a global scale for more than 2 decades optimal inference from the data involves appropriate partition of signal and noise in terms of relevant scales physical processes and forcing mechanisms such partition is achieved here through fitting a general circulation model to altimeter and other datasets to produce a best estimate of variability directly forced by the atmosphere the signal of primary interest here in this context noise comes primarily from instrument errors and meso scale eddies as expected but spatial smoothing effectively reduces this noise a separate constraint is thus formulated to measure the fit to monthly large scale altimetric variability that unlike the daily pointwise constraint shows a high signal to noise ratio the estimate is explored to gain insight into dynamics forcing and other factors controlling variability contributions from thermo steric halo steric and bottom pressure terms are all important depending on region but slopes of steric spectra red and bottom pressure spectra white are nearly invariant with latitude much variability can be represented by a seasonal cycle and linear trend plus a few eofs that can be associated with known modes of climate variability and or with topographic controls both wind and buoyancy forcing are important the response is primarily basin bound in nature but uneven patterns of propagation across basin boundaries are clearly present with the pacific being able to affect large portions of the indian and atlantic basins but the atlantic affecting mostly the arctic

northern north atlantic sea surface height and ocean heat content variability 1 the evolution of nearly 20 years of altimetric sea surface height ssh is investigated to understand its association with decadal to multidecadal variability of the north atlantic heat content altimetric ssh is dominated by an increase of about 14 cm in the labrador and irminger seas from 1993 to 2011 while the opposite has occurred over the gulf stream region over the same time period during the altimeter period the observed 0 700 m ocean heat content ohc in the subpolar gyre mirrors the increased ssh by its dominantly positive trend over a longer period 1955 2011 fluctuations in the subpolar ohc reflect atlantic multidecadal variability amv and can be attributed to advection driven by the wind stress gyre mode bringing more subtropical waters into the subpolar gyre the extended subpolar warming evident in ssh and ohc during the altimeter period represents transition of the amv from cold to warm phase in addition to the dominant trend the first empirical orthogonal function ssh time series shows an abrupt change 2009 2010 reaching a new minimum in 2010 the change coincides with the change in the meridional overturning circulation at 26 5 n as observed by the rapid rapid climate change project and with extreme behavior of the wind stress gyre mode and of atmospheric blocking while the general relationship between northern warming and atlantic meridional overturning circulation amoc volume transport remains undetermined the meridional heat and salt transport carried by amoc s arteries are rich with decade to century timescale variability

multidecadal north atlantic sea surface temperature and atlantic meridional overturning circulation variability in cmip5 historical simulations received 22 may 2013 revised 4 september 2013 accepted 8 september 2013 published 23 october 2013 1 in this paper simulated variability of the atlantic multidecadal oscillation amo and the atlantic meridional overturning circulation amoc and their relationship has been investigated for the first time climate models of the coupled model intercomparison project phase 5 cmip5 provided to the intergovernmental panel on climate change fifth assessment report ipcc ar5 in historical simulations have been used for this purpose the models show the most energetic variability on the multidecadal timescale band both with respect to the amo and amoc but with a large model spread in both amplitude and frequency the relationship between the amo and amoc in most of the models resembles the delayed advective oscillation proposed for the amoc on multidecadal timescales a speed up slow down of the amoc is in favor of generating a warm cold phase of the amo by the anomalous northward southward heat transport in the upper ocean which reversely leads to a weakening strengthening of the amoc through changes in the meridional density gradient after a delayed time of ocean adjustment this suggests that on multidecadal timescales the amo and amoc are related and interact with each other

hydrographic changes in the labrador sea 1960 2005 abstract the labrador sea has exhibited significant temperature and salinity variations over the past five decades the whole basin was extremely warm and salty between the mid 1960s and early 1970s and fresh and cold between the late 1980s and mid 1990s the full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s the freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the labrador sea sustained throughout the subsequent years it took only a decade for the whole water column to lose most of its excessive freshwater reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s if the recent tendencies persist the basin s storages of salt and heat will fairly soon likely by 2008 exceed their historic highs the main process responsible for the net cooling and freshening of the labrador sea between 1987 and 1994 was deep winter convection which during this period progressively developed to its record depths it was caused by the recurrence of severe winters during these years and in its turn produced the deepest densest and most voluminous labrador sea water lsw1987 1994 ever observed the estimated annual production of this water during the period of 1987 1994 is equivalent to the average volume flux of about 4 5 sv with some individual annual rates exceeding 7 0 sv once winter convection had lost its strength in the winter of 1994 1995 the deep lsw1987 1994 layer lost communication with the mixed layer above consequently losing its volume while gaining heat and salt from the intermediate waters outside the labrador sea while the 1000 2000 m layer was steadily becoming warmer and saltier between 1994 and 2005 the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air sea heat fluxes in the winter of 1999 2000 this change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent lsw class lsw2000 penetrating deeper than 1300 m this layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass implying that lsw2000 is likely to follow the fate of its deeper precursor lsw1987 1994 the increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid depth perimeter of the labrador sea the near bottom waters originating from the denmark strait overflow exhibit strong interannual variability featuring distinct short term basin scale events or pulses of anomalously cold and fresh water separated by warm and salty overflow modifications regardless of their sign these anomalies pass through the abyss of the labrador sea first appearing at the greenland side and then about a year later at the labrador side and in the central labrador basin the northeast atlantic deep water 2500 3200 m originating from the iceland scotland overflow water reached its historically freshest state in the 2000 2001 period and has been steadily becoming saltier since then it is argued that lsw1987 1994 significantly contributed to the freshening density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening lsw the hydrostatic adjustment to expanding lsw or both

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

comment on multiyear prediction of monthly mean atlantic meridional overturning circulation at 26 5 n matei et al reports 6 january 2012 p 76 claim to show skillful multiyear predictions of the atlantic meridional overturning circulation amoc however these claims are not justified primarily because the predictions of amoc transport do not outperform simple reference forecasts based on climatological annual cycles accordingly there is no justification for the confident prediction of a stable amoc through 2014

enhanced erosion rates on mars during amazonian glaciation abstract observations of mars from the surface and from orbit suggest that erosion rates over the last 3 gyr the amazonian have been as slow as 10 5 m myr and have been dominated by aeolian processes while ancient noachian erosion rates may have been orders of magnitude higher due to impact bombardment and fluvial activity amazonian aged glacial deposits are widespread on mars but rates of erosion responsible for contributing debris to these remnant glacial deposits have not been constrained here we calculate erosion rates during amazonian glaciations using a catalog of mid latitude glacial landforms coupled with observational and theoretical constraints on the duration of glaciation these calculations suggest that erosion rates for scarps that contributed debris to glacial landforms are 4 7 orders of magnitude higher than average amazonian rates in non glaciated low slope regions these erosion rates are similar to terrestrial cold based glacier erosion and entrainment rates consistent with cold based glacier modification of parts of mars

interannual to decadal variability of outflow from the labrador sea a decade of weak convection in the labrador sea associated with decreasing water mass transformation in combination with advective and eddy fluxes into the convection area caused significant warming of the deep waters in both the central labrador sea and boundary current system along the labrador shelf break the connection to the export of deep water was studied based on moored current meter stations between 1997 and 2009 at the exit of the labrador sea near the shelf break at 5 3n more than 100 year long current meter records spanning the full water column have been analyzed with respect to high frequency variability decaying from the surface to the bottom layer and for the annual mean flow showing intra to interannual variability but no detectable decadal trend in the strength of the deep and near bottom flow out of the labrador sea

spectral characteristics of the response of the meridional overturning circulation to deep water formation abstract the response of the upper limb of the meridional overturning circulation to the variability of deep water formation is investigated analytically with a linear reduced gravity model in basins of simple geometry the spectral characteristics of the model response are first derived by prescribing white noise fluctuations in the meridional transport at the northern boundary although low frequency basin modes are solutions to the eigenproblem they are too dissipative to be significantly excited by the boundary forcing and the thermocline depth response has a red spectrum with no prevailing time scale other than that of a high frequency equatorial mode only flattening at the millennial time scale because of vertical diffusivity the meridional transport is asymmetric about the equator because the northern part of the basin is directly influenced by the boundary forcing while the southern part is mostly set in motion by long rossby waves this results in the equator acting as a low pass filter for

nonnormal thermohaline circulation dynamics in a coupled ocean atmosphere gcm abstract using the gfdl coupled atmosphere ocean general circulation model cm2 1 the transient amplification of thermohaline circulation thc anomalies due to its nonnormal dynamics is studied a reduced space based on empirical orthogonal functions eofs of temperature and salinity anomaly fields in the north atlantic is constructed under the assumption that the dynamics of this reduced space is linear the propagator of the system is then evaluated and the transient growth of thc anomalies analyzed although the linear dynamics are stable such that any initial perturbation eventually decays nonnormal effects are found to result in a significant transient growth of temperature salinity and thc anomalies the growth time scale for these anomalies is between 5 and 10 yr providing an estimate of the predictability time of the north atlantic thc in this model there are indications that these results are merely a lower bound on the nonnormality of thc dynamics in the present coupled gcm this seems

interannual variations in the atlantic meridional overturning circulation and its relationship with the net northward heat transport in the south atlantic 1 variability of the atlantic meridional overturning circulation amoc and its effect on the net northward heat transport nht in the south atlantic are examined using a trans basin expendable bathythermograph xbt high density line at 35s ax18 the time mean amoc is 17 9 2 2 sv during 2002 2007 although the geostrophic transport dominates the time mean amoc both geostrophic and ekman transports are important in explaining the amoc variability the contributions of geostrophic and ekman transports to the amoc show annual cycles but they are out ofphase resultinginweakseasonalvariabilityoftheamoc the nht variability is significantly correlated with the amoc where a 1 sv increase in the amoc would yield a 0 05 0 01 pw increase in the nht partition of transport into the western and eastern boundaries and interior suggests that to quantify changes in the amoc and nht it is critical to monitor all three regions citation dong s s garzoli m baringer c meinen andg goni 2009 interannualvariations intheatlanticmeridionaloverturningcirculationanditsrelationship with the net northward heat transport in the south atlantic geophys res lett 36 l20606 doi 10 1029 2009gl039356