decadal variability of subpolar gyre transport and its reverberation in the north atlantic overturning 1 analyses of sea surface height ssh records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the north atlantic subpolar gyre during the 1990s here we report hindcast simulations with high resolution ocean circulation models that demonstrate a close correspondence of the ssh changes with the volume transport of the boundary current system in the labrador sea the 1990s decline of about 15 of the long term mean appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the north atlantic oscillation nao the changes in the subpolar gyre as manifested in the deep western boundary current off labrador reverberate in the strength of the meridional overturning circulation moc in the subtropical north atlantic suggesting the potential of a subpolar transport index as an element of a moc monitoring system

basinwide integrated volume transports in an eddy filled ocean the temporal evolution of the strength of the atlantic meridional overturning circulation amoc in the subtropical north atlantic is affected by both remotely forced basin scale meridionally coherent climate relevant transport anomalies such as changes in high latitude deep water formation rates and locally forced transport anomalies such as eddies or rossby waves possibly associated with small meridional coherence scales which can be considered as noise the focus of this paper is on the extent to which local eddies and rossby waves when impinging on the western boundary of the atlantic affect the temporal variability of the amoc at 26 5 degrees n continuous estimates of the amoc at this latitude have been made since april 2004 by combining the florida current ekman and midocean transports with the latter obtained from continuous density measurements between the coasts of the bahamas and morocco representing respectively the western and eastern boundaries of the atlantic at this latitude within 100 km of the western boundary there is a threefold decrease in sea surface height variability toward the boundary observed in both dynamic heights from in situ density measurements and altimetric heights as a consequence the basinwide zonally integrated upper midocean transport shallower than 1000 m as observed continuously between april 2004 and october 2006 varies by only 3 0 sv 1 sv 10 6 m 3 s 1 rms instead upper midocean transports integrated from western boundary stations 16 40 and 500 km offshore to the eastern boundary vary by 3 6 6 0 and 10 7 sv rms respectively the reduction in eddy energy toward the western boundary is reproduced in a nonlinear reduced gravity model suggesting that boundary trapped waves may account for the observed decline in variability in the coastal zone because they provide a mechanism for the fast equatorward export of transport anomalies associated with eddies impinging on the western boundary an analytical model of linear rossby waves suggests a simple scaling for the reduction in thermocline thickness variability toward the boundary physically the reduction in amplitude is understood as along boundary pressure gradients accelerating the fluid and rapidly propagating pressure anomalies along the boundary the results suggest that the local eddy field does not dominate upper midocean transport or amoc variability at 26 5 degrees n on interannual to decadal time scales

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

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

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

variability of shallow and deep western boundary currents off the bahamas during 2004 05 results from the 26 n rapid moc array data from an array of six moorings deployed east of abaco bahamas along 26 5 n during march 2004 may 2005 are analyzed these moorings formed the western boundary array of a transbasin observing system designed to continuously monitor the meridional overturning circulation and meridional heat flux in the subtropical north atlantic under the framework of the joint u k u s rapid climate change rapid meridional overturning circulation moc program important features of the western boundary circulation include the southward flowing deep western boundary current dwbc below 1000 m and the northward flowing antilles current in the upper 1000 m transports in the western boundary layer are estimated from direct current meter observations and from dynamic height moorings that measure the spatially integrated geostrophic flow between moorings the results of these methods are combined to estimate the time varying transports in the upper and deep ocean over the width of the western boundary layer to a distance of 500 km offshore of the bahamas escarpment the net southward transport of the dwbc across this region inclusive of northward deep recirculation is 26 5 sv sv 10 6 m 3 s 1 which is divided nearly equally between upper 13 9 sv and lower 12 6 sv north atlantic deep water nadw in the top 1000 m 6 0 sv flows northward in a thermocline intensified jet near the western boundary these transports are found to agree well with historical current meter data in the region collected between 1986 and 1997 variability in both shallow and deep components of the circulation is large with transports above 1000 m varying between 15 and 25 sv and deep transports varying between 60 and 3 sv much of this transport variability associated with barotropic fluctuations occurs on relatively short time scales of several days to a few weeks upon removal of the barotropic fluctuations slower baroclinic transport variations are revealed including a temporary stoppage of the lower nadw transport in the dwbc during november 2004

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

wind forced variability of the remote meridional overturning circulation abstractthe mechanisms by which time dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored it is shown that

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

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

basinwide response of the atlantic meridional overturning circulation to interannual wind forcing an eddy resolving ocean general circulation model for the earth simulator ofes and a simple wind driven two layer model are used to investigate the role of momentum fluxes in driving the atlantic meridional overturning circulation amoc variability throughout the atlantic basin from 1950 to 2010 diagnostic analysis using the ofes results suggests that interior baroclinic rossby waves and coastal topographic waves play essential roles in modulating the amoc interannual variability the proposed mechanisms are verified in the context of a simple two layer model with realistic topography and only forced by surface wind the topographic waves communicate high latitude anomalies into lower latitudes and account for about 50 of the amoc interannual variability in the subtropics in addition the large scale rossby waves excited by wind forcing together with topographic waves set up coherent amoc interannual variability patterns across the tropics and subtropics the comparisons between the simple model and ofes results suggest that a large fraction of the amoc interannual variability in the atlantic basin can be explained by wind driven dynamics

variability of the simulated meridional heat transport in the north atlantic for the period 1951 1993 a 43 year ocean model simulation for the period 1951 1993 is analyzed with a focus on the meridional heat transport mht as a proxy for the strength of meridional overturning cell moc at 25 n the surface heat flux forcing associated with the north atlantic oscillation nao pattern is related to the variability in mht both on interannual and longer timescales the manifestation of the interannual and decadal oceanic response to nao is nonlocal as evidenced by the concentrated heat content and sea level variability at the gulf stream and north atlantic current regions and shown by the comparison of the model sea level variability to the altimeter data between low mht years in mid 1980s and high mht years in early 1990s the same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level mht and time series of the leading sea level mode are highly correlated which reflects the importance of the moc in determining the variability of the heat content of the whole basin also the model suggests that the mht moc has entered a very strong phase since the mid 1980s and this trend has continued up to the end of 1993 this behavior follows the general trend in the nao index

magnetic polarity stratigraphy of siwalik group sediments of karnali river section in western nepal the remanent magnetization of siltstones and sandstones sampled at 476 levels sites throughout a 3560 m thick molasse sequence belonging to the siwalik group 0 2015 m lower 2015 3560 m middle has been studied by stepwise thermal demagnetization this section is exposed along the karnali river in nepal the natural remanent magnetization nrm usually consists of two components a viscous or thermoviscous component of recent field origin and an ancient characteristic component chrm the former component is of normal polarity and resides either in goethite unblocking temperature 150 c resistant to afd up to 150 mt or in maghemite unblocking temperature 150 400 c goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section maghemite content is significant in the grey mud free lithologies from the upper half of the section the main component unblocked in the high temperature range commonly 610 680 c and believed to reside in haematite presumably of mostly detrital origin represents a characteristic remanence chrm the tilt corrected chrm directions at individual sites show antipodal clusters ratio of normal to reverse polarity sites 0 62 and yield mean inclinations recording significant inclination shallowing a feature well recorded in the siwaliks this chrm is interpreted to represent a largely primary detrital remanence the chrm data from 430 sites yield the karnali river magnetic polarity sequence whose correlation with the geomagnetic polarity timescale cande kent 1995 suggests a depositional age of 16 ma younger than chron c5cn 1n to 5 2 ma around the top of chron c3r for the 3560 m section sampled hence the karnali river exposes the oldest part of the siwalik group in nepal estimates of the sediment accumulation rate sar average to 32 9 cm kyr 1 for the 10 8 myr time span of deposition

subannual seasonal and interannual variability of the north atlantic meridional overturning circulation abstract an eddy permitting numerical ocean model is used to investigate the variability of the meridional overturning circulation moc both wind stress and fluctuations of the seawater density contribute to moc changes on subannual and seasonal time scales whereas the interannual variability mainly reflects changes in the density field even on subannual and seasonal time scales a significant fraction of the total moc variability is due to changes of the density field in the upper 1000 m of the ocean these changes reflect perturbations of the isopycnal structure that travel westward as rossby waves because of a temporally changing phase difference between the eastern and western boundaries the rossby waves affect the moc by modifying the basinwide east west density gradient both the numerical model used in this study and calculations based on rossby wave theory suggest that this effect can account for an moc variability of several sverdrups sv 106 m3 s 1 these results have implications for t

evidence for a recent change in the link between the north atlantic oscillation and arctic sea ice export evidence for a recent change in the link between the north atlantic oscillation nao and arctic sea ice export through fram strait during wintertime djfm is presented from the analysis of simulated arctic sea ice and observations for the period 1958 1997 whereas no correlation between the two time series is found from 1958 to 1977 r 0 1 the correlation increased significantly thereafter r 0 7 1978 1997 the increased coherency between the nao and arctic sea ice export through fram strait during the last two decades resulted from anomalous meridional wind components near fram strait which were associated with the more easterly position of the nao s centers of interannual variability compared to 1958 1977

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

wind forced interannual variability of the atlantic meridional overturning circulation at 26 5 n the observed atlantic meridional overturning circulation amoc at 26 5 n shows interannual variability consisting of an increase from early 2004 to late 2005 and a following downtrend which reaches a minimum in the winter of 2009 2010 these interannual amoc fluctuations are dominated by changes in the upper mid ocean geostrophic flow except during the winter of 2009 2010 when the anomalous wind driven ekman transport also has a significant contribution the physical mechanisms for the interannual changes of the amoc are proposed and evaluated in a two layer model while the ekman transport is linked to the north atlantic oscillation nao the anomalous geostrophic transport involves the oceanic adjustment to surface wind forcing in particular the intensification and weakening of the southward interior geostrophic flow is modulated by the internal rossby wave adjustment to the surface wind forcing the gulf stream on the other hand is controlled by both topographic waves along the us coast and westward propagating planetary waves our study suggests that a large part of the observed amoc interannual variability at 26 5 n can be explained by wind driven dynamics

a sea change in our view of overturning in the subpolar north atlantic to provide an observational basis for the intergovernmental panel on climate change projections of a slowing atlantic meridional overturning circulation moc in the 21st century the overturning in the subpolar north atlantic program osnap observing system was launched in the summer of 2014 the first 21 month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the osnap line in a departure from the prevailing view that changes in deep water formation in the labrador sea dominate moc variability these results suggest that the conversion of warm salty shallow atlantic waters into colder fresher deep waters that move southward in the irminger and iceland basins is largely responsible for overturning and its variability in the subpolar basin

spatial pattern and zonal shift of the north atlantic oscillation part i a dynamical interpretation abstract this paper presents a possible dynamical explanation for why the north atlantic oscillation nao pattern exhibits an eastward shift from the period 1958 77 p1 to the period 1978 97 p2 or 1998 2007 p3 first the empirical orthogonal function analysis of winter mean geopotential heights during p1 p2 and p3 reveals that the nao dipole anomaly exhibits a northwest southeast nw se tilting during p1 but a northeast southwest ne sw tilting during p2 and p3 the nao pattern especially its northern center undergoes a more pronounced eastward shift from p1 to p2 the composite calculation of nao events during p1 and p2 also indicates that the negative positive nao phase dipole anomaly can indeed exhibit such a nw se ne sw tilting second a linear rossby wave formula derived in a slowly varying basic flow with a meridional shear is used to qualitatively show that the zonal phase speed of the nao dipole anomaly is larger smaller in higher latitudes and smaller larger in lower latitud

anomalies of meridional overturning mechanisms in the north atlantic abstract optimal observations are used to investigate the overturning streamfunction in the north atlantic at 30 n and 900 m depth those observations are designed to impact the meridional overturning circulation moc in numerical models maximally when assimilated and therefore establish the most efficient observation network for studying changes in the moc they are also ideally suited for studying the related physical mechanisms in a general circulation model optimal observations are evaluated here in the framework of a global 1 model over a 10 yr period hydrographic observations useful to monitor the moc are primarily located along the western boundary north of 30 n and along the eastern boundary south of 30 n additional locations are in the labrador irminger and iberian seas on time scales of less than a year variations in moc are mainly wind driven and are made up through changes in ekman transport and coastal up and downwelling only a small fraction is buoyancy driven and constitutes a sl

monitoring the atlantic meridional overturning circulation the rapid climate change programme rapid has established a prototype system to continuously observe the strength and structure of the atlantic meridional overturning circulation moc at 26 5 n here we provide a detailed description of the rapid moc monitoring array and how it has evolved during the first four deployment years as well as an overview of the main findings so far the rapid moc monitoring array measures 1 gulf stream transport through florida strait by cable and repeat direct velocity measurements 2 ekman transports by satellite scatterometer measurements 3 deep western boundary currents by direct velocity measurements 4 the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the mid atlantic ridge and 5 barotropic fluctuations using bottom pressure recorders the array became operational in late march 2004 and is expected to continue until at least 2014 the first 4 years of observations april 2004 april 2008 have provided an unprecedented insight into the moc structure and variability we show that the zonally integrated meridional flow tends to conserve mass with the fluctuations of the different transport components largely compensating at periods longer than 10 days we take this as experimental confirmation of the monitoring strategy which was initially tested in numerical models the moc at 26 5 n is characterised by a large variability even on timescales as short as weeks to months the mean maximum moc transport for the first 4 years of observations is 18 7 sv with a standard deviation of 4 8 sv the mechanisms causing the moc variability are not yet fully understood part of the observed moc variability consists of a seasonal cycle which can be linked to the seasonal variability of the wind stress curl close to the african coast close to the western boundary fluctuations in the gulf stream and in the north atlantic deep water nadw coincide with bottom pressure variations at the western margin thus suggesting a barotropic compensation ongoing and future research will put these local transport variations into a wider spatial and climatic context

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

signal propagation related to the north atlantic overturning 1 changes of the meridional overturning circulation moc due to surface heat flux variability related to the north atlantic oscillation nao are analyzed in various ocean models i e eddying and non eddying cases a prime signature of the forcing is variability of the winter time convection in the labrador sea the associated changes in the strength of the moc near the subpolar front 45 n are closely related to the nao index leading moc anomalies by about 2 3 years in both the eddying and non eddying simulation further south the speed of the meridional signal propagation depends on model resolution with lower resolution non eddying case 4 3 resolution the moc signal propagates equatorward with a mean speed of about 0 6 cm s similar as spreading rates of passive tracer anomalies eddy permitting experiments 1 3 show a significantly faster propagation with speeds corresponding to boundary waves thus leading to an almost in phase variation of the moc transport over the subtropical to subpolar north atlantic

mechanisms of interannual variations of the meridional overturning circulation of the north atlantic ocean abstract the authors investigate the nature of the interannual variability of the meridional overturning circulation moc of the north atlantic ocean using an estimating the circulation and climate of the ocean ecco assimilation product for the period of 1993 2003 the time series of the first empirical orthogonal function of the moc is found to be correlated with the north atlantic oscillation nao index while the associated circulation anomalies correspond to cells extending over the full ocean depth model sensitivity experiments suggest that the wind is responsible for most of this interannual variability at least south of 40 n a dynamical decomposition of the meridional streamfunction allows a further look into the mechanisms in particular the contributions associated with 1 the ekman flow and its depth independent compensation 2 the vertical shear flow and 3 the barotropic gyre flowing over zonally varying topography are examined ekman processes are found to dominate the shorter time

locally and remotely forced subtropical amoc variability a matter of time scales mechanisms driving the north atlantic meridional overturning circulation amoc variability at low frequency are of central interest for accurate climate predictions although the subpolar gyre region has been identified as a preferred place for generating climate timescale signals their southward propagation remains under consideration complicating the interpretation of the observed time series provided by the rapid climate change meridional overturning circulation and heatflux array western boundary time series rapid mocha wbts program in this study we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low frequency amoc variability we analyze for this a set of four ensembles of a regional 208s 558n eddy resolving 1 128 north atlantic oceanic configuration where surface forcing and open boundary conditions are alternatively permuted from fully varying realistic to yearly repeating signals their analysis reveals the predominance of local atmospherically forced signal at interannual time scales 2 10 years whereas signals imposed by the boundaries are responsible for the decadal 10 30 years part of the spectrum due to this marked timescale separation we show that although the intergyre region exhibits peculiarities most of the subtropical amoc variability can be understood as a linear superposition of these two signals finally we find that the decadal scale boundary forced amoc variability has both northern and southern origins although the former dominates over the latter including at the site of the rapid array 26 58n

remedying embankment thaw settlement in a warm permafrost region with thermosyphons and crushed rock revetment due to the special engineering geology characteristics of permafrost construction in permafrost regions tends to result in serious permafrost related engineering problems thaw settlement induced by permafrost degradation is the principal challenge for railway construction on the qinghai tibetan plateau it threatens the stability and safety of the railway system especially in warm and ice rich permafrost regions thaw settlement in section dk1139 780 along the qinghai tibetan railway is a potential risk to the safety of the railway and a combination of closed thermosyphons and crushed rock revetment was used to remedy permafrost warming and thaw settlement of the embankment based on ground temperatures and embankment deformations observed at this site since 2002 the effects of the remedial measures were evaluated the results show that the remedial measures lowered the ground temperature and raised the permafrost table the crushed rock slope protection acted as an insulation layer and reduced heat

local and downstream relationships between labrador sea water volume and north atlantic meridional overturning circulation variability abstractwhile it has generally been understood that the production of labrador sea water lsw impacts the atlantic meridional overturning circulation moc this relationship has not been explored

observed interannual variability of the atlantic meridional overturning circulation at 26 5 n the atlantic meridional overturning circulation moc plays a critical role in the climate system and is responsible for much of the heat transported by the ocean a mooring array nomianally at 26 circ n between the bahamas and the canary islands deployed in apr 2004 provides continuous measurements of the strength and variability of this circulation with seven full years of measurements we now examine the interannual variability of the moc while earlier results highlighted substantial seasonal and shorter timescale variability there had not been significant interannual variability the mean moc from 1 apr 2004 to the 31 march 2009 was 18 5 sv with the annual means having a standard deviation of only 1 0 sv from 1 april 2009 to 31 march 2010 the annually averaged moc strength was just 12 8 sv representing a 30 decline this downturn persisted from early 2009 to mid 2010 we show that the cause of the decline was not only an anomalous wind driven event from dec 2009 mar 2010 but also a strengthening of the geostrophic flow in particular the southward flow in the top 1100 m intensified while the deep southward return transport particularly in the deepest layer from 3000 5000 m weakened this rebalancing of the transport from the deep overturning to the upper gyre has implications for the heat transported by the atlantic

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

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

potential role of atlantic warm pool induced freshwater forcing in the atlantic meridional overturning circulation ocean sea ice model simulations recent studies have indicated that the multidecadal variations of the atlantic warm pool awp can induce a significant freshwater change in the tropical north atlantic ocean in this paper the potential effect of the awp induced freshwater flux on the atlantic meridional overturning circulation amoc is studied by performing a series of ocean sea ice model experiments our model experiments demonstrate that ocean response to the anomalous awp induced freshwater flux is primarily dominated by the basin scale gyre circulation adjustments with a time scale of about two decades the positive negative freshwater anomaly leads to an anticyclonic cyclonic circulation overlapping the subtropical gyre this strengthens weakens the gulf stream and the recirculation in the interior ocean thus increases warm cold water advection to the north and decreases cold warm water advection to the south producing an upper ocean temperature dipole in the midlatitude as the freshwater salty water is advected to the north atlantic deep convection region the amoc and its associated northward heat transport gradually decreases increases which in turn lead to an inter hemispheric sst seesaw in the equilibrium state a comma shaped sst anomaly pattern develops in the extratropical region with the largest amplitude over the subpolar region and an extension along the east side of the basin and into the subtropical north atlantic based on our model experiments we argue that the multidecadal awp induced freshwater flux can affect the amoc which plays a negative feedback role that acts to recover the amoc after it is weakened or strengthened the sensitivity of amoc response to the awp induced freshwater forcing amplitude is also examined and discussed

variability of the meridional overturning in the north atlantic from the 50 year gecco state estimation abstract the german partner of the consortium for estimating the circulation and climate of the ocean gecco provided a dynamically consistent estimate of the time varying ocean circulation over the 50 yr period 1952 2001 the gecco synthesis combines most of the data available during the entire estimation period with the ecco massachusetts institute of technology mit ocean circulation model using its adjoint this gecco estimate is analyzed here for the period 1962 2001 with respect to decadal and longer term changes of the meridional overturning circulation moc of the north atlantic a special focus is on the maximum moc values at 25 n over this period the dynamically self consistent synthesis stays within the error bars of h l bryden et al but reveals a general increase of the moc strength the variability on decadal and longer time scales is decomposed into contributions from different processes changes in the model s moc strength are strongly influenced by the southward communication of d

extracting the buoyancy driven atlantic meridional overturning circulation abstractvariations in the atlantic meridional overturning circulation amoc driven by buoyancy forcing are typically characterized as having a low frequency time scale interhemispheric structure