ocean margin densities and paleoestimates of the atlantic meridional overturning circulation a model study an eddy permitting numerical ocean model is used to test to what extent the atlantic meridional overturning circulation moc can be reconstructed from sea water densities at the ocean margins by gradually reducing the number of locations where the densities are known and by adding systematic and random errors comparable to those expected in paleoreconstructions of sea water densities we test if a reliable picture of the moc can still be obtained our results show that even with only 0 07 of the boundary densities the mean state of the moc as well as its variability can be reconstructed the addition of a systematic error results in an overestimation or underestimation of the meridional transport with a random error similar to the one expected for paleoestimates of the density the mean state of the moc can still be reproduced especially north of 40 n however the random noise largely masks the relatively modest moc variability observed in the model run for both systematic and random noise in the density field there is a larger sensitivity for moc reconstructions the closer a location is from the equator for a measuring campaign that aims to infer past states of the atlantic moc on the basis of boundary densities the present study suggests that the best strategy is to collect data that allow estimation of the basin wide vertical density structure between 40 n and 50 n information from only a few latitudes is sufficient for a reasonable representation of the mean moc state

frequency response properties of forced climatic sst anomaly variability in the north atlantic abstract frequency response properties of north atlantic 5 57 n sea surface temperature anomaly tsa variability with periods of several months to 20 years are characterized using the comprehensive ocean atmosphere data set coads significant direct forcing of tsa variability by the anomalous wind field primarily through the resulting anomalous surface turbulent heat flux is observed in the westerly wind and trade wind belts to characterize properties of the large scale climatic tsa response to this forcing over the entire frequency band resolved it is necessary to consider the dual role of anomalous surface heat flux as both the dominant local forcing mechanism and the dominant damping mechanism the latter through a negative linear feedback newtonian relaxation at frequencies where wind forcing is important good agreement exists between the frequency response function estimated from data and the same function theoretically predicted by a simple stochastic forcing model where the locally fo

optimal excitation of interannual atlantic meridional overturning circulation variability the optimal excitation of atlantic meridional overturning circulation moc anomalies is investigated in an ocean general circulation model with an idealized configuration the optimal three dimensional spatial structure of temperature and salinity perturbations defined as the leading singular vector and generating the maximum amplification of moc anomalies is evaluated by solving a generalized eigenvalue problem using tangent linear and adjoint models despite the stable linearized dynamics a large amplification of moc anomalies mostly due to the interference of nonnormal modes is initiated by the optimal perturbations the largest amplification of moc anomalies found to be excited by high latitude deep density perturbations in the northern part of the basin is achieved after about 7 5 years the anomalies grow as a result of a conversion of mean available potential energy into potential and kinetic energy of the perturbations reminiscent of baroclinic instability the time scale of growth of moc anomalies can be understood by examining the time evolution of deep zonal density gradients which are related to the moc via the thermal wind relation the velocity of propagation of the density anomalies found to depend on the horizontal component of the mean flow velocity and the mean density gradient determines the growth time scale of the moc anomalies and therefore provides an upper bound on the moc predictability time the results suggest that the nonnormal linearized ocean dynamics can give rise to enhanced moc variability if for instance overflows eddies and or deep convection can excite high latitude density anomalies in the ocean interior with a structure resembling that of the optimal perturbations found in this study the findings also indicate that errors in ocean initial conditions or in model parameterizations or processes particularly at depth may significantly reduce the atlantic moc predictability time to less than a decade

oceanic control of multidecadal variability in an idealized coupled gcm idealized ocean models are known to develop intrinsic multidecadal oscillations of the meridional overturning circulation moc here we explore the role of ocean atmosphere interactions on this low frequency variability we use a coupled ocean atmosphere model set up in a flat bottom aquaplanet geometry with two meridional boundaries the model is run at three different horizontal resolutions 4 2 and 1 in both the ocean and atmosphere at all resolutions the moc exhibits spontaneous variability on multidecadal timescales in the range 30 40 years associated with the propagation of large scale baroclinic rossby waves across the atlantic like basin the unstable region of growth of these waves through the long wave limit of baroclinic instability shifts from the eastern boundary at coarse resolution to the western boundary at higher resolution increasing the horizontal resolution enhances both intrinsic atmospheric variability and ocean atmosphere interactions in particular the simulated atmospheric annular mode becomes significantly correlated to the moc variability at 1 resolution an ocean only simulation conducted for this specific case underscores the disruptive but not essential influence of air sea interactions on the low frequency variability this study demonstrates that an atmospheric annular mode leading moc changes by about 2 years as found at 1 resolution does not imply that the low frequency variability originates from air sea interactions

the cold ocean warm land pattern model simulation and relevance to climate change detection abstract surface air temperatures from a 1000 yr integration of a coupled atmosphere ocean model with constant forcing are analyzed by using a method that decomposes temperature variations into a component associated with a characteristic spatial structure and a residual the structure function obtained from the coupled model output is almost identical to the so called cold ocean warm land cowl pattern based on observations in which above average spatial mean temperature is associated with anomalously cold oceans and anomalously warm land this pattern features maxima over the high latitude interiors of eurasia and north america the temperature fluctuations at the two continental centers exhibit almost no temporal correlation with each other the temperature variations at the individual centers are related to teleconnection patterns in sea level pressure and 500 mb height that are similar to those identified in previous observational and modeling studies as in observations variations in the polarity

response of the meridional overturning circulation to variable buoyancy forcing in a double hemisphere basin we consider how a highly idealized double hemisphere basin responds to a zonally constant restoring surface temperature profile that oscillates in time with periods ranging from 0 5 to 32 000 years in both hemispheres the forcing is similar but can be either in phase or out of phase the set up is such that the northern hemisphere always produces the densest waters the model s meridional overturning circulation moc exhibits a strong response in both hemispheres on decadal to multi millennial timescales the amplitude of the oscillations reaches up to 140 of the steady state maximum moc and exhibits resonance like behaviour with a maximum at centennial to millennial forcing periods when the forcing is in phase between the northern and southern hemispheres there is a marked decrease in the amplitude of the moc response as the forcing period is increased beyond the resonance period in this case the resonance like behaviour is identical to the one we found earlier in a single hemisphere model and occurs for the same reasons when the forcing is out of phase between the northern and southern hemispheres the amplitude of the moc response is substantially greater for long forcing periods millennial and longer particularly in the southern hemisphere this increased moc amplitude occurs because for an out of phase forcing either the northern or the southern deep water source is always active leading to generally colder bottom waters and thus greater stratification in the opposite hemisphere this increased stratification in turn stabilises the water column and thus reduces the strength of the weaker overturning cell the interaction of the two hemispheres leads to response timescales of the deep ocean at half the forcing period our results suggest a possible explanation for the half precessional time scale observed in the deep atlantic ocean palaeo temperature record

north atlantic sea surface temperatures and their relation to the north atlantic oscillation during the last 230 years august sea surface temperatures assts based on fossil diatom assemblages are generated with 2 year average resolution from a 230 year long sediment core rapid 21 12b from the reykjanes ridge in the subpolar north atlantic the results indicate a warming trend of 0 5 c of the surface waters at the reykjanes ridge for the last 230 years superimposed on this warming trend there is a multidecadal to decadal asst variability of up to 1 c the interval from the 1770s to the 1830s represents the coldest period whereas 1860 1880 represents the warmest period during the last 230 years the last 25 years is characterized by a warming trend showing strong decadal asst variability with several warm years but also the coldest years since the 1820s the time of these cold years in the mid 1970s 1980s and 1990s correspond with the documented great salinity anomalies gsa in the north atlantic suggesting increased fluxes of cold low salinity waters from the arctic during the last decades the asst record and the august north atlantic oscillation anao index show similar multidecadal scale variability indicating a close coupling between the oceanic and atmospheric patterns the asst record shows a negative correlation with the anao indicating cold asst during the positive anao trend and vice versa results suggest that the wind driven variation in volume fluxes of the north atlantic surface waters could be the major mechanism behind the observed relationship

role of the gulf stream and kuroshio oyashio systems in large scale atmosphere ocean interaction a review abstract ocean atmosphere interaction over the northern hemisphere western boundary current wbc regions i e the gulf stream kuroshio oyashio and their extensions is reviewed with an emphasis on their role in basin scale climate variability sst anomalies exhibit considerable variance on interannual to decadal time scales in these regions low frequency sst variability is primarily driven by basin scale wind stress curl variability via the oceanic rossby wave adjustment of the gyre scale circulation that modulates the latitude and strength of the wbc related oceanic fronts rectification of the variability by mesoscale eddies reemergence of the anomalies from the preceding winter and tropical remote forcing also play important roles in driving and maintaining the low frequency variability in these regions in the gulf stream region interaction with the deep western boundary current also likely influences the low frequency variability surface heat fluxes damp the low frequency sst anomalies ove

the impact of labrador sea temperature and salinity variability on density and the subpolar amoc in a decadal prediction system labrador sea density variability is important for atlantic meridional overturning circulation amoc dynamics and hence decadal variability in the atlantic we investigate whether temperature or salinity dominate top 500m interannual labrador sea density variability in gridded observations an assimilation of the observations and a set of multi annual hindcasts we find that salinity dominates in the observations and assimilation in the hindcasts salinity remains dominant for the first year but from year three these revert to the same temperature dominance seen in the underlying climate model this is due to damping of the interannual salinity variability possibly caused by unrealistically large convection that develops crucially the hindcasts have high correlation skill in temperature salinity throughout but no skill in density dynamic sea level or the subpolar amoc due to the incorrect drivers this highlights the importance of correctly simulating both the sign and magnitude of temperature salinity variability in a prediction system

coherent multidecadal variability in north atlantic sea level 1 tide gauge records from around the north atlantic are examined and found to show variability on 20 30 year time scales sea surface height variability along the western boundary of the north atlantic shows a particularly strong and coherent signal similar variability is also found in an ensemble of runs using a state of the art climate model gfdl cm2 1 this sea surface height variability is linked to variability of temperatures in the upper layers of the ocean and thence to the atlantic multidecadal oscillation the variability is consistent with the excited internal ocean mode mechanism of multidecadal variability derived from idealised models and the timescale is consistent with that derived from observations of sub surface temperature variability

a possible mechanism for the strong weakening of the north atlantic subpolar gyre in the mid 1990s 1 the extent and strength of the north atlantic subpolar gyre spg changed rapidly in the mid 1990s going from large and strong in 1995 to substantially weakened in the following years the abrupt change in the intensity of the spg is commonly linked to the reversal of the north atlantic oscillation nao index changing from strong positive to negative values in the winter 1995 96 in this study we investigate the impact of the initial spg state on the subsequent behavior of the spg by means of an ocean general circulation model driven by ncep ncar reanalysis fields our sensitivity integrations suggest that the weakening of the spg cannot be explained by the change in the atmospheric forcing alone rather for the time period around 1995 the spg was about to weaken irrespective of the actual atmospheric forcing due to the ocean state governed by the persistently strong positive nao during the preceding seven years 1989 1995 our analysis indicates that it was this preconditioning of the ocean in combination with the sudden drop in the nao in 1995 96 that lead to the strong and rapid weakening of the spg in the second half of the 1990s this hypothesis explains the diverging evolution of the strength of the spg and the atmospheric forcing winter nao after 1995 as has been suggested recently

reconstructing extreme amoc events through nudging of the ocean surface a perfect model approach while the atlantic meridional overturning circulation amoc is thought to be a crucial component of the north atlantic climate past changes in its strength are challenging to quantify and only limited information is available in this study we use a perfect model approach with the ipsl cm5a lr model to assess the performance of several surface nudging techniques in reconstructing the variability of the amoc special attention is given to the reproducibility of an extreme positive amoc peak from a preindustrial control simulation nudging includes standard relaxation techniques towards the sea surface temperature and salinity anomalies of this target control simulation and or the prescription of the wind stress fields surface nudging approaches using standard fixed restoring terms succeed in reproducing most of the target amoc variability including the timing of the extreme event but systematically underestimate its amplitude a detailed analysis of the amoc variability mechanisms reveals that the underestimation of the extreme amoc maximum comes from a deficit in the formation of the dense water masses in the main convection region located south of iceland in the model this issue is largely corrected after introducing a novel surface nudging approach which uses a varying restoring coefficient that is proportional to the simulated mixed layer depth which in essence keeps the restoring time scale constant this new technique substantially improves water mass transformation in the regions of convection and in particular the formation of the densest waters which are key for the representation of the amoc extreme it is therefore a promising strategy that may help to better constrain the amoc variability and other ocean features in the models as this restoring technique only uses surface data for which better and longer observations are available it opens up opportunities for improved reconstructions of the amoc over the last few decades

multidecadal variability of the overturning circulation in presence of eddy turbulence at low resolution idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients the existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean through a series of numerical simulations with increasing resolution up to eddy resolving ones 10 km and various diapycnal diffusivities this multidecadal variability proves a generic ubiquitous feature at least in model versions with a flat bottom the mean circulation largely changes in the process of refining the horizontal grid along with the associated implicit viscosity and diffusivity and the spatial structure of the variability is largely modified but there is no clear influence of the resolution on the main oscillation period the interdecadal variability appears even more robust to low vertical diffusivity and overturning when mesoscale eddies are resolved the mechanism previously proposed for these oscillations involving westward propagating baroclinically unstable rossby waves in the subpolar region and its feedback on the mean circulation appears unaffected by mesoscale turbulence and is simply displaced following the polar front

a 4500 year reconstruction of sea surface temperature variability at decadal time scales off north iceland marine paleo records acquired at high temporal resolution provide critical data for testing numerical climate models and help to understand processes underlying ocean variability this study presents a unique 4500 year reconstruction of sea surface temperature sst obtained from alkenones in the north atlantic polar front area off north iceland at an average temporal resolution of 4 5 years spectral analysis of this signal shows dominant multidecadal oscillations which occurred with a stronger amplitude between 2500 and 4200 years bp hand in hand with fluctuations of bottom currents indicated by paleomagnetic proxies contemporaneous large excursions of the inter tropical convergence zone itcz are also recorded by the distant cariaco titanium time series suggesting a link with low latitude atlantic climate we speculate that the oscillations reflect changes of the meridional overturning circulation moc induced by increased enso activity

potential vorticity in models of the ocean circulation existing observations suffice to give a qualitative description of the wind driven and thermohaline components of the ocean circulation but a hierarchy of analytical and numerical models is now needed for in coupled ocean atmosphere models of the earth s climate potentialvorticity is a more appropriate diagnostic field variable than angular momentum for the ocean circulation because of the complicated geometry of basins patterns of potential vorticity onsurfaces of constant density help validate ocean circulation models and give physical insight into how the oceancirculation works high resolution models suggest that the lateral mixing of potential vorticity by mesoscale eddies along isopycnal surfaces is of the same order as large scale advection and that the assumption of inviscid potential vorticity conserving flow in the thermocline is not appropriate for the real ocean a satisfactory test of this conjecture will require the extensive measurements planned for the world ocean circulation experiment models also indicate that the transport of water mass properties by mesoscale eddies is largely a mixing along isopycnal surfaces the wcak temperaturc gradients along isopycnal surfaces in most areas of the ocean limit the effectiveness of the mesoscale eddies in transporting significant amounts of heat across latitude circles

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

fast advective recovery of the atlantic meridional overturning circulation after a heinrich event ice core reconstructions and ocean sediment analysis have revealed that the climate of the last glacial period was highly variable with rapid stadial interstadial transitions and glacial meltwater pulses heinrich events modulating the climate evolution in the northern and southern hemispheres heinrich events had the potential to weaken the atlantic meridional overturning circulation amoc substantially mechanisms that led to the resumption of the amoc after such events have not been fully disentangled yet here a coupled atmosphere ocean sea ice model of intermediate complexity is employed to identify important negative climate feedbacks that contribute to a fast recovery of the glacial amoc shortly after the amoc collapse thermal processes weaken the stratification in the northern north atlantic making it more vulnerable to perturbations eventually 300 400 years after the main collapse of the amoc the mean advection of salinity anomalies within the horizontal gyres generates an unstable stratification that will be homogenized through the resumption of convective activity eventually isopycnal slopes in the north atlantic are readjusted thereby reinitiating the large scale meridional overturning flow

variability of the overflow water transport in the western subpolar north atlantic 1950 97 abstract one of the major topics in current field research is the question of whether or to what extent the north atlantic ocean is subject to changes in water mass transports and how they are related to atmospheric phenomena like the north atlantic oscillation nao bottle and ctd data from the 1950s to 1990s are presented to reconstruct spatially and temporally the baroclinic contribution to the deep water transports in the western subpolar north atlantic the focus is on the two densest components of north atlantic deep water the gibbs fracture zone water gfzw and the denmark strait overflow water dsow direct velocity measurements in the considered time period are sparse for this reason it was decided to calculate the geostrophic velocity relative to 1400 dbar this level is located in the weakly stratified labrador sea water the combined baroclinic volume transport of gfzw and dsow during the early 1990s was about 5 sv sv 106 m3 s 1 in the irminger sea and 7 8 sv in the labrador sea nea

nao implicated as a predictor of northern hemisphere mean temperature multidecadal variability 1 the twentieth century northern hemisphere mean surface temperature nht is characterized by a multidecadal warming cooling warming pattern followed by a flat trend since about 2000 recent warming hiatus here we demonstrate that the north atlantic oscillation nao is implicated as a useful predictor of nht multidecadal variability observational analysis shows that the nao leads both the detrended nht and oceanic atlantic multidecadal oscillation amo by 15 20 years theoretical analysis illuminates that the nao precedes nht multidecadal variability through its delayed effect on the amo due to the large thermal inertia associated with slow oceanic processes an nao based linear model is therefore established to predict the nht which gives an excellent hindcast for nht in 1971 2011 with the recent flat trend well predicted nht in 2012 2027 is predicted to fall slightly over the next decades due to the recent nao decadal weakening that temporarily offsets the anthropogenically induced warming

multidecadal and multicentennial variability of the meridional overturning circulation the variability of the meridional overturning circulation moc simulated in a multimillenial control integration of the kiel climate model kcm displays enhanced variability relative to the red background at decadal and centennial timescales the multidecadal variability is the model s version of the atlantic multidecadal variability amv often referred to as atlantic multidecadal oscillation amo while multidecadal variability originates in the north atlantic multicentennial variability is driven in the southern ocean both multidecadal and multicentennial variability are associated with considerable changes in sea ice extent this may be important to understand the different evolution of sea ice cover in the northern and southern hemisphere during the most recent decades with a strong decline observed in the north and almost no trend in the south

does the atlantic multidecadal oscillation get its predictability from the atlantic meridional overturning circulation abstractthis paper investigates the predictive relation between the atlantic multidecadal oscillation amo and atlantic meridional overturning circulation across different climate models three overturning patterns that are significantly coupled to the amo on interannual time scales across all climate models are identified using a statistical optimization technique including these structures in an autoregressive model extends amo predictability by 2 9 years relative to an autoregressive model without these structures

wind effect on the atlantic meridional overturning circulation via sea ice and vertical diffusion effects of wind and fresh water on the atlantic meridional overturning circulation amoc are investigated using a fully coupled climate model the amoc can change significantly when perturbed by either wind stress or freshwater flux in the north atlantic this study focuses on wind stress effect our model results show that the wind forcing is crucial in maintaining the amoc reducing wind forcing over the ocean can cause immediately weakening of the vertical salinity diffusion and convection in the mid high latitudes atlantic resulting in an enhancement of vertical salinity stratification that restrains the deep water formation there triggering a slowdown of the thermohaline circulation as the thermohaline circulation weakens the sea ice expands southward and melts providing the upper ocean with fresh water that weakens the thermohaline circulation further the wind perturbation experiments suggest a positive feedback between sea ice and thermohaline circulation strength which can eventually result in a complete shutdown of the amoc this study also suggests that sea ice variability may be also important to the natural amoc variability on decadal and longer timescales

comparison between instrumental observational and high resolution proxy sedimentary records of late holocene climatic change a discussion of possibilities abstract variability of the north atlantic region is a key factor in the workings of global climate changes related to orbital factors and to the thermohaline circulation thc occur on timescales ranging from 1000 to 100 000 years whereas subdecadal decadal and multidecadal trends are associated with changes in atmospheric circulation sea surface temperature sst and salinity long term changes are known only from proxies whereas shorter periodicities are identified in instrumental and other observational data between lies a range of less well known centennial periodicities too long to be recorded observationally and too short for all but the highest resolution proxy records such as tree rings atmospheric 14c and varved lake sediments numerous investigations of such periodicities involve comparisons between the proxy and the observational record however results depend on the length of the datasets being analysed the relative strength of the periodicities present whether data are stationary or non stationary the degree to which they have been smoothed in order to remove particular fluctuations and the methods of analysis employed ideally comparisons should be between datasets of equal length and of similar properties but these criteria are not always easy to fulfill a search for periodicities in a range of well known multidecadal datasets reveals that whilst some frequencies previously identified in the central england temperature cet record and in the koch sea ice index are statistically significant those found by several authors in the index of the north atlantic oscillation nao are not as the index approximates to white noise instead decadal periodicity of the nao described by several authors appears to be result of smoothing of the raw data periodicities of 6 1 33 9 and 213 years identified in a proxy record from laminated sediments from loch ness scotland appear respectively to be related to a subdecadal variations in sst and salinity in the north atlantic ocean b multidecadal interactions between the nao and the north atlantic thc and c a well known centennial solar periodicity

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

interdecadal north atlantic meridional overturning circulation variability in ec earth the atlantic meridional overturning circulation amoc in a 600 years pre industrial run of the newly developed ec earth model features marked interdecadal variability with a dominant time scale of 50 60 years an oscillation of approximately 2 sverdrup 1 sv 106 m3 s 1 is identified which manifests itself as a monopole causing the overturning to simultaneously strengthen weaken and deepen shallow as a whole eight years before the amoc peaks density in the labrador irminger sea region reaches a maximum triggering deep water formation this density change is caused by a counterclockwise advection of temperature and salinity anomalies at lower latitudes which we relate to the north south excursions of the subpolar subtropical gyre boundary and variations in strength and position of the subpolar gyre and the north atlantic current the amoc fluctuations are not directly forced by the atmosphere but occur in a delayed response of the ocean to forcing by the north atlantic oscillation which initiates intergyre gyre fluctuations associated with the amoc is a 60 year sea surface temperature variability in the atlantic with a pattern and timescale showing similarities with the real world atlantic multidecadal variability this good agreement with observations lends a certain degree of credibility that the mechanism that is described in this article could be seen as representative of the real climate system

paleoclimate proxy perspective on caribbean climate since the year 1751 evidence of cooler temperatures and multidecadal variability 1 annually resolved coral 18o and sr ca records from southwestern puerto rico are used to investigate caribbean climate variability between 1751 and 2004 c e mean surface ocean temperatures in this region have increased steadily by about 2 c since the year 1751 with sr ca data indicating 2 1 0 8 c and 18o data indicating 2 7 0 5 c coral geochemical records from across the tropics demonstrate that regional variability is important for understanding climate variations at centennial time scales a strong multidecadal salinity signal in the oxygen isotope data correlates with observed multidecadal temperature variations in the northern hemisphere instrumental wind and precipitation data indicate that the most recent coral isotopic variations are caused by expansion and contraction of the steep regional salinity gradient forced by trade wind anomalies through meridional ekman transport the timing of the fluctuations suggests that the multidecadal scale wind and surface circulation anomalies might play a role in atlantic temperature variability and meridional overturning circulation but further work is needed to confirm this suggestion

mineral nutrients in fluvial sediments from the peruvian amazon abstract mineral nutrient content was determined from fluvial sediments of eight different rivers in peruvian amazonia statistically significant differences between the floodplains were detected in the mean values of soil reaction conductivity concentrations of total phosphorus calcium and aluminium concentrations of extractable sodium calcium and aluminium and in the ratios ca mg and mg k principal components ordination based on geochemical variables exhibits a spectrum from rivers depositing sediments rich in extractable bases to those which deposit sediments acid in reaction and rich in extractable aluminium the distribution of the sediment types corresponds to the geology of the catchment area fluvial sediments of suspension rich rivers of andean origin are rich in their extractable mineral nutrient content especially calcium is prominent differences in the chemical properties of fluvial sediments contribute to the regional heterogeneity of the environment in the lowland peruvian amazon and are likely to affect the distribution of plant species since the origin of most of the soils in this region is fluvial sediment provenance and age should be considered in soil surveys

response of thermohaline circulation to freshwater forcing under present day and lgm conditions responses of the thermohaline circulation thc to freshwater forcing hosing in the subpolar north atlantic ocean under present day and the last glacial maximum lgm conditions are investigated using the national center for atmospheric research community climate system model versions 2 and 3 three sets of simulations are analyzed with each set including a control run and a freshwater hosing run the first two sets are under present day conditions with an open and closed bering strait the third one is under lgm conditions which has a closed bering strait results show that the thc nearly collapses in all three hosing runs when the freshwater forcing is turned on the full recovery of the thc however is at least a century earlier in the open bering strait run than the closed bering strait and lgm runs this is because the excessive freshwater is diverged almost equally toward north and south from the subpolar north atlantic when the bering strait is open a significant portion of the freshwater flowing northward into the arctic exits into the north pacific via a reversed bering strait throughflow which accelerates the thc recovery when the bering strait is closed this arctic to pacific transport is absent and freshwater can only be removed through the southern end of the north atlantic together with the surface freshwater excess due to precipitation evaporation river runoff and melting ice in the closed bering strait experiments after the hosing the removal of the excessive freshwater takes longer and this slows the recovery of the thc although the background conditions are quite different between the present day closed bering strait run and the lgm run the thc responds to the freshwater forcing added in the north atlantic in a very similar manner

delayed advective oscillation of the atlantic thermohaline circulation a simple dynamic model is proposed to illustrate the multidecadal oscillation of the atlantic ocean thermohaline circulation the proposed oscillation relies on alternating actions of positive and negative feedbacks which are operated by a slow adjustment of the ocean circulation and the associated time delay in the advective flux response to a change in meridional density gradient the key element of the oscillation is the time delay which is conceptually related to the basin crossing time of long rossby waves in the highlatitude north atlantic for a sufficiently long time delay the solution becomes unstable in some regions of model parameter space and oscillates with a period of approximately 2 times the delay time

links between the southern annular mode and the atlantic meridional overturning circulation in a climate model abstract the links between the atmospheric southern annular mode sam the southern ocean and the atlantic meridional overturning circulation amoc at interannual to multidecadal time scales are investigated in a 500 yr control integration of the l institut pierre simon laplace coupled model version 4 ipsl cm4 climate model the antarctic circumpolar current as described by its transport through the drake passage is well correlated with the sam at the yearly time scale reflecting that an intensification of the westerlies south of 45 s leads to its acceleration also in phase with a positive sam the global meridional overturning circulation is modified in the southern hemisphere primarily reflecting a forced barotropic response in the model the amoc and the sam are linked at several time scales an intensification of the amoc lags a positive sam by about 8 yr this is due to a correlation between the sam and the atmospheric circulation in the northern north atlantic that reflects a symmetric e

on the robustness of the interdecadal modes of the thermohaline circulation ocean models in box geometry forced by constant surface fluxes of density have been found to spontaneously generate interdecadal oscillations of the thermohaline circulation this paper analyzes the sensitivity of these oscillations to various physical effects including the presence of mesoscale turbulence various thermal surface boundary conditions and the presence of wind forcing or bottom topography the role of unstable long baroclinic waves is also reexamined in an attempt to understand the oscillation period in idealized geometry it is found that the low frequency variability of the thermohaline circulation under quasi constant surface fluxes is a robust feature of the large scale circulation it is not strongly affected by energetic mesoscale turbulence the oscillation period is relatively invariant with respect to varying resolution and momentum and tracer horizontal mixing coefficients although it loses some regularity as shorter and longer periods of variability emerge when the mesoscale activity increases in strength with smaller mixing coefficients the oscillations are also retained as the ocean model is coupled to an interactive atmospheric energy balance model the thermohaline modes are robust to a range of exchange coefficients that widens with the amplitude of the mean circulation the presence of an additional wind forced component generally weakens the oscillation and depending on the relative strength of thermodynamic and dynamic forcings the oscillation may be completely killed a simple interpretation is given highlighting the role of upward ekman pumping in damping density anomalies finally the interaction of these baroclinic modes with bottom topography depends strongly on the relative directions of the mean topographic features and the mean currents and baroclinic waves but usually results in a damping influence

mechanisms underlying recent decadal changes in subpolar north atlantic ocean heat content the subpolar north atlantic spna is subject to strong decadal variability with implications for surface climate and its predictability in 2004 2005 spna decadal upper ocean and sea surface temperature trends reversed from warming during 1994 2004 to cooling over 2005 2015 this recent decadal trend reversal in spna ocean heat content ohc is studied using a physically consistent observationally constrained global ocean state estimate covering 1992 2015 the estimate s physical consistency facilitates quantitative causal attribution of ocean variations closed heat budget diagnostics reveal that the spna ohc trend reversal is the result of heat advection by midlatitude ocean circulation kinematic decompositions reveal that changes in the deep and intermediate vertical overturning circulation cannot account for the trend reversal but rather ocean heat transports by horizontal gyre circulations render the primary contributions the shift in horizontal gyre advection reflects anomalous circulation acting on the mean temperature gradients maximum covariance analysis mca reveals strong covariation between the anomalous horizontal gyre circulation and variations in the local wind stress curl suggestive of a sverdrup response results have implications for decadal predictability

detecting atlantic moc changes in an ensemble of climate change simulations abstract signal to noise patterns for the meridional overturning circulation moc have been calculated for an ensemble of greenhouse scenario runs the greenhouse forced signal has been defined as the linear trend in ensemble mean moc after year 2000 it consists of an overall decrease and shoaling of the moc with maximum amplitudes of 10 sv sv 106 m3 s 1 per century in each member the internal variability is defined as the anomaly with respect to the ensemble mean signal the interannual variability of the moc is dominated by a monopole with a maximum amplitude of 2 sv at 40 n this variability appears to be driven by the north atlantic oscillation nao mainly through nao induced variations in the wind field the signal to noise ratio was estimated for various time spans all starting in 1950 or later different noise estimates were made both with and without intra annual variability relevant for episodic and continuous monitoring respectively and with and without an estimate of the observa

characteristics of winter precipitation variation over northern central eurasia and their connections to sea surface temperatures over the atlantic and pacific oceans abstract this study reveals spatial and temporal characteristics of precipitation variability and their teleconnections to sea surface temperatures ssts over the atlantic and pacific oceans by analyzing 68 yr of recent precipitation records over the former soviet union in addition to a general increasing trend of about 0 4 mm yr 1 or 6 decade 1 over much of the study region three major modes of precipitation variation are identified a quasi biennial variation of 2 3 yr is found over the region surrounding the caspian and aral seas an el nino timescale precipitation variation of 4 5 yr is present over southern central siberia and is associated with eastern tropical pacific ssts a quasi decadal timescale variation of about 11 12 yr is evident over central european russia this quasi decadal precipitation variation is closely linked to a major sst anomaly pattern of alternating latitudinal belts over the atlantic and sst variations over the equatorial pacific ocean these associations to ssts are str

decadal windstorm activity in the north atlantic european sector and its relationship to the meridional overturning circulation in an ensemble of simulations with a coupled climate model the relationship between decadal variations in the north atlantic meridional overturning circulation moc and north atlantic western european windstorm activity during the extended winter season is studied according to an ensemble of three 240 year long simulations performed with the echam5 mpiom model periods of high decadal windstorm activity frequently occur in the years following a phase of weak moc i e when the moc starts to recover these periods are characterised by a distinctive pattern in the mixed layer ocean heat content ohc a positive anomaly is located in the region 45 n 52 n 35 w 16 w west of france negative anomalies are located to the north and south the signal can be detected both in the heat content of the oceanic mixed layer and in the sea surface temperatures its structure is consistent with anomalously enhanced baroclinic instability in the region with the strong negative ohc gradient 30 w 10 w 45 n 60 n which eventually produces a higher probability of windstorms

frequency domain multimodel analysis of the response of atlantic meridional overturning circulation to surface forcing the dynamics of the atlantic meridional overturning circulation amoc vary considerably among different climate models for example some models show clear peaks in their power spectra while others do not to elucidate these model differences transfer functions are used to estimate the frequency domain relationship between surface forcing fields including sea surface temperature salinity and wind stress and the resulting amoc response these are estimated from the outputs of the coupled model intercomparison project phase 5 cmip5 and phase 3 cmip3 control runs for eightdifferent models with a specificfocus on geophysical fluid dynamics laboratory climate model version 2 1 gfdl cm2 1 and the community climate system model version 4 ccsm4 which exhibit rather different spectral behavior the transfer functionsshowvery littleagreement amongmodelsforany ofthe pairsofvariablesconsidered suggestingthe existence of systematic model errors and that considerable uncertainty in the simulation of amoc in current climate models remains however a robust feature of the frequency domain analysis is that models with spectral peaks in their amoc correspond to those in which amoc variability is more strongly excited by high latitude surface perturbations that have periods corresponding to the frequency of the spectral peaks this explains why different models exhibit such different amoc variability these differences would not be evident without using a method that explicitly computes the frequency dependence rather than a priori assuming a particular functional form finally transfer functions are used to evaluate two proposed physical mechanisms for model differences in amoc variability differences in labrador sea stratification and excitation by westward propagating subsurface rossby waves

a thermal oscillation under a restorative forcing summary the authors report an interdecadal oscillation in a wind and thermally driven ocean general circulation model ogcm the oscillation is tantalizing in that it occursunder a relatively strong thermal damping 26 3 w m 2k examinations involving a two dimensional ogcm a simple thermal flip flop model and a three dimensional ogcm with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so called convective oscillator or the advective overshooting oscillator instead the oscillation is associated with the propagation of modelled long rossby waves with westward phase velocity it is found that the north south basin extent of the ogcm is an important factor a larger northsouth basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances the geophysical relevance of these results is discussed

multidecadal variability and late medieval cooling of near coastal sea surface temperatures in the eastern tropical north atlantic 1 multidecadal variations in atlantic sea surface temperatures sst influence the climate of the northern hemisphere however prior to the instrumental time period information on multidecadal climate variability becomes limited and there is a particular scarcity of sufficiently resolved sst reconstructions here we present an eastern tropical north atlantic reconstruction of ssts based on foraminiferal mg ca ratios that resolves multidecadal variability over the past 1700 years spectral power in the multidecadal band 50 to 70 years period is significant over several time intervals suggesting that the atlantic multidecadal oscillation amo has been influencing local sst since our data exhibit high scatter the absence of multidecadal variability in the remaining record does not exclude the possibility that sst variations on this time scale might have been present without being detected in our data cooling by 0 5 c takes place between about ad 1250 and ad 1500 while this corresponds to the inception of the little ice age lia the end of the lia is not reflected in our record and sst remains relatively low this transition to cooler ssts parallels the previously reconstructed shift in the north atlantic oscillation toward a low pre 20th century mean state and possibly reflects common solar forcing

atlantic multidecadal oscillation and northern hemisphere s climate variability proxy and instrumental records reflect a quasi cyclic 50 80 year climate signal across the northern hemisphere with particular presence in the north atlantic modeling studies rationalize this variability in terms of intrinsic dynamics of the atlantic meridional overturning circulation influencing distribution of sea surface temperature anomalies in the atlantic ocean hence the name atlantic multidecadal oscillation amo by analyzing a lagged covariance structure of a network of climate indices this study details the amo signal propagation throughout the northern hemisphere via a sequence of atmospheric and lagged oceanic teleconnections which the authors term the stadium wave initial changes in the north atlantic temperature anomaly associated with amo culminate in an oppositely signed hemispheric signal about 30 years later furthermore shorter term interannual to interdecadal climate variability alters character according to polarity of the stadium wave induced prevailing hemispheric climate regime ongoing research suggests mutual interaction between shorter term variability and the stadium wave with indication of ensuing modifications of multidecadal variability within the atlantic sector results presented here support the hypothesis that amo plays a significant role in hemispheric and by inference global climate variability with implications for climate change attribution and prediction

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

multi and quasi decadal variations of sea surface temperature in the north atlantic abstract the multi and quasi decadal variabilities of the sea surface temperature sst anomaly over the north atlantic are investigated by means of two dimensional propagating ceof forty six years of the coads sst dataset from 1947 to 1992 are used after removing the monthly climatology and four year low pass filtering the sst anomaly is submitted to ceof the total variance of the three largest modes accounts for 87 0 of the total variance mode 1 is a very slow oscillation with an approximate 42 yr period and has basinwide spatial evolution with alternate warm and cold anomalies appearing off newfoundland and migrating northward until they disappear south of greenland the center of action of the anomalies both occur in the vicinity of the labrador sea newfoundland and south of greenland where deep water formation takes place mode 2 3 is a quasi decadal fluctuation with an approximate 14 yr period mode 2 3 contains quasi decadal timescales with alternating warm and cold anomalies propagati

investigating the role of the tibetan plateau in the formation of atlantic meridional overturning circulation abstractthe tibetan plateau tp over the eurasian continent has significant effects on both regional and global climate it can even affect the remote atlantic meridional overturning circulation

multidecadal variability in global surface temperatures related to the atlantic meridional overturning circulation abstractmultidecadal internal climate variability centered in the north atlantic is evident in sea surface temperatures and is assumed to be related to variations in the strength of the atlantic me

a hemispheric mechanism for the atlantic multidecadal oscillation abstract the physical processes associated with the 70 yr period climate mode known as the atlantic multidecadal oscillation amo are examined based on analyses of observational data a deterministic mechanism relying on atmosphere ocean sea ice interactions is proposed for the amo variations in the thermohaline circulation are reflected as uniform sea surface temperature anomalies in the north atlantic these anomalies are associated with a hemispheric wavenumber 1 sea level pressure slp structure in the atmosphere that is amplified through atmosphere ocean interactions in the north pacific the slp pattern and its associated wind field affect the sea ice export through fram strait the freshwater balance in the northern north atlantic and consequently the strength of the large scale ocean circulation it generates sea surface temperature anomalies with opposite signs in the north atlantic and completes a negative feedback the authors find that the time scale of the cycle is associated with the

stochastic models of the meridional overturning circulation time scales and patterns of variability the global meridional overturning circulation moc varies over a wide range of space and time scales in response to fluctuating weather perturbations that may be modelled as stochastic forcing th

multiple equilibria as a possible mechanism for decadal variability in the north atlantic ocean decadal climate variability in the north atlantic has received increased attention in recent years because modeling results suggest predictability of heat content and circulation indices several years ahead however determining the applicability of these results in the real world is challenging because of an incomplete understanding of the underlying mechanisms here the authors show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region the subpolar gyre spg are not always consistent a coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures this confirms that surface heat flux indeed plays a leading role for this typeofvariability ashasbeensuggestedinpreviousstudies theresultsfurthersuggestthatlargevariationsin the spg correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of the model used herein performance of this conceptual model is tested against a statistical stochastic model hysteresis and the existence of two stable modes of the spg circulation shape its response to forcing by atmospheric temperatures the identification of the essential dynamics and the reduction to a minimal model of spg variability provide a quantifiable basis and a framework for future studies on decadal climate variability and predictability

wintertime variability in the euro atlantic region in observations and in ecmwf seasonal ensemble experiments an empirical orthogonal function eof decomposition of monthly mean analyses of 500 hpa height 1949 94 is used to describe the interannual variability of the large scale flow in the euro atlantic region during winter the first four eofs resemble low frequency variability patterns identified in previous studies such as the north atlantic oscillation and the eastern atlantic and eurasian teleconnection patterns the second eof and fourth eof eof4 are associated with the occurrence of el nino like sea surface temperature sst anomalies according to an observational analysis for eof4 correlations with sst anomalies in the west pacific are also important indications of an influence of tropical sst anomalies on the blocking like third eof emerging from the european centre for medium range weather forecasts ecmwf re analysis era data are not confirmed by a longer sst record a negligible correlation exists between the first eof and el nino indices during the era period although a link may be detected in earlier decades in the second part of the paper a 14 winter set of seasonal ensemble simulations performed with the ecmwf atmospheric model forced by observed sst is validated by comparing the projection of era data onto the four leading euro atlantic eofs with those of ensemble experiments and computing composite anomalies of analyses and model fields the performance of the model is uneven and depends on the large scale pattern considered model biases and flow dependent errors affect the simulations of some of the eofs in particular the model has strong problems reproducing the occurrence of european blocks the best correlation 69 between the time series of analysis and ensemble mean principal components is obtained for eof4 which seems to be forced by sst anomalies in the west pacific associated with el nino southern oscillation enso events the effects of this forcing are felt on the zonal wind structure over eurasia and are reasonably reproduced by the ecmwf model conversely the propagation into the atlantic region of planetary waves originating in the east pacific is poorly simulated thus degrading the model performance in predicting euro atlantic anomalies during some strong enso events

maintenance of the boreal forest rainbelts during northern summer abstract it is not unreasonable to expect that boreal forests that exist along 60 n in the eurasian and north american continents were created and are maintained by warm seasonal rainfall as revealed from satellite observations and various precipitation sources zonally elongated rainbelts appear along these forests previous studies show that a relationship may exist between the frontal zone along the arctic seaboard and regional patterns of high latitude precipitation it was observed by this study that baroclinic zones associated with strong arctic westerlies coincide with minor storm tracks and boreal forest rainbelts only in eastern canada in contrast this coincidence does not occur in northern europe eastern siberia and the alaska pacific coast because boreal forest rainbelts in these regions are located farther south of strong arctic westerlies and ahead of high latitude troughs over central eurasia the bering sea the labrador sea and the norwegian sea therefore instead of baroclinicity

the different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions the differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse resolution ocean model in an idealized flat bottom single hemisphere basin objective features are determined that allow one type of oscillation to be distinguished versus the other first by performing a linear stability analysis of the steady state obtained under restoring boundary conditions it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises respectively from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state based on the budgets of density variance it is shown next that the two types of oscillations have different energy sources under the constant flux boundary condition the thermal mode the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability whereas under mixed boundary conditions the salinity mode a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation furthermore the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance in addition the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure the authors propose that these differences will allow one to identify which type of oscillation if any is at play in the more exhaustive climate models

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

dynamics of decadal variability in the atlantic subpolar gyre a stochastically forced oscillator internal variability of the atlantic subpolar gyre is investigated in a 600 years control simulation of a comprehensive coupled climate model the subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and 20 years positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation this involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air sea thermal coupling as a result anomalies of the large scale atmospheric circulation can be transferred to the ocean on the ocean s intrinsic time scale exciting the oscillator stochastically a detailed understanding of oscillatory mechanisms of the ocean and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation of proxy data records

pacific and atlantic multidecadal variability in the kiel climate model pacific decadal variability pdv and atlantic multidecadal variability amv the two leading decadal modes of observed northern hemisphere sea surface temperature sst variability are investigated in a multi millennial control integration of the kiel climate model kcm it is shown that the two phenomena are independent modes in the model and can be easily separated by principal oscillation pattern pop analysis of model sst pdvrelated variability covers the whole north pacific with strong signals in both the mid latitude north pacific and the western tropical pacific strong signals are also simulated in the eastern indian ocean sector pdv s memory however resides in the north pacific and is linked to the subtropical gyre the amv mechanism is related to the atlantic meridional overturning circulation amoc a stochastic mechanism applies to both pdv and amv

a new index for the atlantic meridional overturning circulation at 26 n abstractthe atlantic meridional overturning circulation amoc has received considerable attention motivated by its major role in the global climate system observations of amoc strength at 26 n made by the rapid climate change rapid array provide the best current estimate of the state of the amoc the period 2004 11 when rapid amoc is available is too short to assess decadal variability of the amoc this modeling study introduces a new amoc index called amocsv at 26 n that combines the florida straits transport the ekman transport and the southward geostrophic sverdrup transport the main hypothesis in this study is that the upper midocean geostrophic transport calculated using the rapid array is also wind driven and can be approximated by the geostrophic sverdrup transport at interannual and longer time scales this index is expected to reflect variations in the amoc at interannual to decadal time scales this estimate of the surface branch of the amoc can be constructed as long as reliable measu

arsenic enrichment in groundwater of west bengal india geochemical evidence for mobilization of as under reducing conditions abstract the mechanism of as release and source s of as has been investigated in a small part of a watershed in the murshidabad district of west bengal analyses include major ion and trace element concentrations as well as o h and s isotope ratios of groundwater surface water and a thermal spring the results indicate that all water samples belong to the ca hco 3 type except for the thermal spring which is of the na hco 3 type shallow and deeper groundwaters have distinct hydrochemical features high as contents were registered only in the deeper groundwater horizon factor analysis and the distribution pattern of major and trace elements indicate that as is present in the aquifer as a scavenged phase by fe iii and to a lesser extent by mn iv phases the release of as into the groundwater occurs gradually in successive stages corresponding to the actual redox state in the aquifer the main stage of as release is related to the bacterial reduction of fe iii to fe ii i e to the simultaneous dissolution of fe oxyhydroxides low redox conditions in highly polluted areas are indicated by low so 4 concentration and high 34 s values during bacterial so 4 reduction residual so 4 in groundwater is depleted in the lighter s isotope 32 s however the cause of the gradual decrease of the redox state in the groundwater is still not well understood

simulated variability of the atlantic meridional overturning circulation to examine the multi annual to decadal scale variability of the atlantic meridional overturning circulation amoc we conducted a four member ensemble with a daily reanalysis forced medium resolution global version of the isopycnic coordinate ocean model micom and a 300 years integration with the fully coupled bergen climate model bcm the simulations of the amoc with both model systems yield a long term mean value of 18 sv and decadal variability with an amplitude of 1 3 sv the power spectrum of the inter annual to decadal scale variability of the amoc in bcm generally follows the theoretical red noise spectrum with indications of increased power near the 20 years period comparison with observational proxy indices for the amoc e g the thickness of the labrador sea water the strength of the baroclinic gyre circulation in the north atlantic ocean and the surface temperature anomalies along the mean path of the gulf stream shows similar trends and phasing of the variability indicating that the simulated amoc variability is robust and real mixing indices have been constructed for the labrador the irminger and the greenland iceland norwegian gin seas while convective mixing in the labrador and the gin seas are in opposite phase and linked to the nao as observations suggest the convective mixing in the irminger sea is in phase with or leads the labrador sea newly formed deep water is seen as a slow anomalous cold and fresh plume flowing southward along the western continental slope of the atlantic ocean with a return flow of warm and saline water on the surface in addition fast travelling topographically trapped waves propagate southward along the continental slope towards equator where they go east and continue along the eastern rim of the atlantic for both types of experiments the northern hemisphere sea level pressure and 2 m temperature anomaly patterns computed based on the difference between climate states with strong and weak amoc yields a nao like pattern with intensified icelandic low and azores high and a warming of 0 25 0 5 c of the central north atlantic sea surface temperature sst the reanalysis forced simulations indicate a coupling between the labrador sea water production rate and an equatorial atlantic sst index in accordance with observations this coupling is not identified in the coupled simulation

atlantic thermohaline circulation in a coupled general circulation model unforced variations versus forced changes a 1200 yr unforced control run and future climate change simulations using the parallel climate model pcm a coupled atmosphere ocean land sea ice global model with no flux adjustments and relatively high resolution 2 8 for the atmosphere and 2 3 for the oceans are analyzed for changes in atlantic ocean circulations for the forced simulations historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used the atlantic thermohaline circulation thc shows large multidecadal 15 40 yr variations with mean peak amplitudes of 1 5 3 0 sv 1 sv 10 6 m 3 s 1 and a sharp peak of power around a 24 yr period in the control run associated with the thc oscillations there are large variations in north atlantic ocean heat transport sea surface temperature sst and salinity sss sea ice fraction and net surface water and energy fluxes which all lag the variations in thc strength by 2 3 yr however the net effect of the sst and sss variations on upper ocean density in the midlatitude north atlantic leads the thc variations by about 6 yr which results in the 24 yr period the simulated sst and sea ice spatial patterns associated with the thc oscillations resemble those in observed sst and sea ice concentrations that are associated with the north atlantic oscillation nao the results suggest a dominant role of the advective mechanism and strong coupling between the thc and the nao whose index also shows a sharp peak around the 24 yr time scale in the control run in the forced simulations the thc weakens by 12 in the twenty first century and continues to weaken by an additional 10 in the twenty second century if co2 keeps rising but the thc stabilizes if co2 levels off the thc weakening results from stabilizing temperature increases that are larger in the upper and northern atlantic ocean than in the deep and southern parts of the basin in both the control and forced simulations as the thc gains loses strength and depth the separated gulf stream gs moves southward northward while the subpolar gyre centered at the labrador sea contracts from expands to the east with the north atlantic current nac being shifted westward eastward these horizontal circulation changes which are dynamically linked to the thc changes induce large temperature and salinity variations around the gs and nac paths

periodicity of climate conditions and solar variability derived from dendrochronological and other palaeoclimatic data in high latitudes abstract the periodicity of climatic processes in the barents sea region and along the arctic ocean coast during several hundred years has been studied by analyzing the tree ring chronologies for the regions close to the northern timberline in the barents sea region cyclicities of climatic processes with periods of around 90 30 35 22 23 18 and 11 12 years have been established by spectral analysis of the data for the kola peninsula wavelet analysis of annual series of conifer tree rings generalized for 10 regions along the northern timberline from the kola peninsula to chukotka for the period 1458 1975 has revealed the same periodicities for the vast areas of northern russia climatic cyclicity with a period of about 35 years is known as the bruckner cycle climatic cycles with periods of around 90 22 23 and 11 12 years correlate well with the corresponding solar activity cycles a possible solar forcing of periodic climatic processes and its nonlinear influence on the atmosphere ocean continental system are discussed

a coral oxygen isotope record from the northern red sea documenting nao enso and north pacific teleconnections on middle east climate variability since the year 1750 a 245 year coral oxygen isotope record from the northern red sea ras umm sidd egypt 28 n in bimonthly resolution is presented the mean annual coral 18o signal apparently reflects varying proportions of both sea surface temperature and 18oseawater variability in conjunction with instrumental observations of climate the coral record suggests for interannual and longer timescales that colder periods are accompanied by more arid conditions in the northern red sea but increased rainfall in the southeastern mediterranean whereas warmer periods are accompanied by decreased rainfall in the latter and less arid conditions in the northern red sea a 70 year oscillation of probably north atlantic origin dominates the coral time series interannual to interdecadal variability is correlated with instrumental indices of the north atlantic oscillation nao the el nino southern oscillation enso and north pacific climate variability the results suggest that these modes contributed consistently to middle east climate variability since at least 1750 preferentially at a period of 5 7 years

climatic variability in the eastern united states over the past millennium from chesapeake bay sediments salinity oscillations caused by multidecadal climatic variability had major impacts on the chesapeake bay estuarine ecosystem during the past 1000 yr microfossils from sediments dated by radiometry 14 c 137 cs 210 pb and pollen stratigraphy indicate that salinity in mesohaline regions oscillated 10 15 ppt during periods of extreme drought low fresh water discharge and wet climate high discharge during the past 500 yr 14 wet dry cycles occurred including sixteenth and early seventeenth century megadroughts that exceeded twentieth century droughts in their severity these droughts correspond to extremely dry climate also recorded in north american tree ring records and by early colonists wet periods occurred every 60 70 yr began abruptly lasted 20 yr and had mean annual rainfall 25 30 and fresh water discharge 40 50 greater than during droughts a shift toward wetter regional climate occurred in the early nineteenth century lowering salinity and compounding the effects of agricultural land clearance on bay ecosystems

holocene climate variability of the norwegian atlantic current during high and low solar insolation forcing 1 a high resolution sediment core from the voring plateau has been studied to document the centennial to millennial variability of the surface water conditions during the holocene climate optimum hco and the late holocene period lhp in order to evaluate the effects of solar insolation on surface ocean climatology quantitative august summer sea surface temperatures sssts with a time resolution of 2 40 years are reconstructed by using three different diatom transfer function methods spectral and scale space methods are applied to the records to explore the variability present in the time series at different time scales the ssst development in core md95 2011 shows a delayed response to northern hemisphere maximum summer insolation at 11 000 years b p the record shows the maximum ssst of the hco to be from 7 3 to 8 9 kyr b p which implies that the site was located in the regional warm water pool removed from the oceanic fronts and arctic waters superimposed on the general cooling trend are higher frequency variabilities at time scales of 80 120 210 320 320 640 and 640 1280 years the climate variations at the time scale of 320 640 years are documented both for periods of high and low solar orbital insolation we found evidence that the submillennial scale mode of variability 640 900 years in ssst evident during the lhp is directly associated with varying solar forcing at the shorter scale of 260 450 years the ssst during the lhp displays a lagged response to solar forcing with a phase locked behavior indicating the existence of a feedback mechanism in the climate system triggered by variations in the solar constant as well as the role of the thermal inertia of the ocean the abruptness of the cooling events in the lhp especially pronounced during the onsets of the holocene cold period i approximately 2300 years b p and the little ice age approximately 550 years b p can be explained by a shutdown of deep convection in the nordic seas in response to negative solar insolation anomalies these cooling events are on the order of 1 5 c

on the emergence of the atlantic multidecadal sst signal a key role of the mixed layer depth variability driven by north atlantic oscillation abstractdespite its wide ranging potential impacts the exact cause of the atlantic multidecadal oscillation variability amo amv is far from settled while the emergence of the amo sea surface te

interdecadal variability in a zonally averaged ocean model an adjustment oscillator abstract a meridional plane hemispherical ocean model is developed to study interdecadal variability of the thermohaline circulation thc the model differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity this allows the incorporation of an adjustment timescale comparable to the advective timescale of the meridional overturning an interdecadal oscillation is documented for an idealized ocean of homogeneous salinity forced by a time independent surface heat flux the governing equations are linearized about a basic state in order to isolate the effect of parameter changes on the oscillation an eigenvalue analysis reveals that the frequency of the oscillation is independent of the advective timescale instead the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridional pressure gradient anomaly the oscillatory instability on the other hand is dependent on both

low frequency basin modes in a two layer quasigeostrophic model in the presence of a mean gyre flow the spectrum of baroclinic basin modes is investigated in a two layer wind driven quasigeostrophic model through weakly nonlinear multiple time scale expansion in the burger number the baroclinic basin modes are mainly advected by a barotropic steady sverdrup flow emphasis is given to the regularizing influence of dispersion rather than to dissipation in the inviscid large scale limit that is for basin scale considerably larger than the rossby radius of deformation all of the basin modes are neutral their typology is then numerically examined with some necessary dissipation and their frequency and spatial properties are discussed three types of modes arise for some wind forcing strong enough to produce a recirculating gyre with closed geostrophic contours the classical rossby basin modes deformed by the mean flow shadow modes stationary modes and recirculating pool modes the two latter being trapped in the closed contours pool focus is made here on the recirculating modes that could have very low frequencies for moderate recirculating gyres strong gyres lead to higher frequencies and recirculating modes resonate with deformed rossby basin modes

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

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

atmosphere ocean coupled variability in the south atlantic abstract the climate variability of the south atlantic region is determined from 40 yr 1953 92 of comprehensive ocean atmosphere data set monthly sea surface temperature sst and sea level pressure slp data using the empirical orthogonal function eof and the singular value decomposition svd analysis methods the eof method is applied to each field separately whereas the svd method is applied to both fields simultaneously the significance of the atmosphere ocean interaction is revealed by a strong resemblance between individual eof and coupled svd modes of sst and slp the three leading modes of coupled variability on interannual and interdecadal timescales are discussed in some detail the first coupled mode which accounts for 63 of the total square covariance represents a 14 16 yr period oscillation in the strength of the subtropical anticyclone accompanied by fluctuations of a north south dipole structure in the sst the atmosphere ocean coupling is strongest during the southern summe

climatic variability in the northwestern alps france as evidenced by 600 years of terrigenous sedimentation in lake le bourget cores recovered from periglacial lake le bourget deep basin northwestern alps were investigated to examine the ine uence of the little ice age lia on terrigenous lacustrine sedimentation growing glaciers in the regional watershed induced catastrophic rhone river e oods and major undere ow deposits in the deep basin during the early e fteenth the sixteenth and the mid eighteenth centuries the lia is characterized by a decrease in deposition from intere ows from ad 1550 to 1740 and an increase in deposition from undere ows from ad 1550 to 1800 on one hand spectral analyses of the laminations in intere ow deposits reveal 4 5 years cyclicities from ad 1440 to 1550 as well as 7 8 and 13 14 years cyclicities from ad 1740 to 1870 on the other hand spectral analyses of a clay mineral ratio ree ecting undere ow deposits highlight 45 50 years cyclicities from ad 1550 to 1800 these pluriannual decadal and pluridecadal periods are typical of the north atlantic oscillation nao a nao like period in our data would be a consequence of periodical variations in rainfall and snow accumulation during late autumn and winter over lake le bourget s watershed

atlantic multidecadal oceanic variability and its influence on the atmosphere in a climate model the mechanisms controlling the decadal to multidecadal variability of the atlantic meridional overturning circulation moc and its influence on the atmosphere are investigated using a control simulation with the ipsl cm4 climate model the multidecadal fluctuations of the moc are mostly driven by deep convection in the subpolar gyre which occurs south of iceland in the model the latter is primarily influenced by the anomalous advection of salinity due to changes in the east atlantic pattern eap which is the second mode of atmospheric variability in the north atlantic region the north atlantic oscillation is the dominant mode but it plays a secondary role in the moc fluctuations during summer the moc variability is shown to have a significant impact on the atmosphere in the north atlantic european sector the moc influence is due to an interhemispheric sea surface temperature sst anomaly with opposite signs in the two hemispheres but largest amplitude in the northern one the sst pattern driven by the moc mostly resembles the model atlantic multidecadal oscillation amo and bears some similarity with the observed one it is shown that the amo reflects both the moc influence and the local atmospheric forcing hence the moc influence on climate is best detected using lagged relations between climatic fields the atmospheric response resembles the eap in a phase that might induce a weak positive feedback on the moc

sub surface signatures of the atlantic multidecadal oscillation 1 sub surface signatures of the atlantic multidecadal oscillation amo are identified using expendable bathythermograph xbt measurements of temperature from the surface down to a depth of 400 m basin averaged temperature anomalies in the north atlantic at different depths display multidecadal variability with a phase shift between temperature anomalies at the surface and at depth westward propagation of temperature anomalies is observable at depth and there is a lag correlation between east west and north south temperature gradients with the east west temperature gradient leading these sub surface characteristics of the amo agree with those expected from the noise driven internal ocean mode view of the amo as derived from a hierarchy of ocean atmosphere models

energetics of multidecadal atlantic ocean variability abstractoscillatory behavior of the atlantic meridional overturning circulation moc is thought to underlie atlantic multidecadal climate variability while the energy sources and sinks driving the mean moc have received intense scrutiny over the last decade the governing energetics of the modes of variability of the moc have not been addressed to the same degree this paper examines the energy conversion processes associated with this variability in an idealized north atlantic ocean model in this model the multidecadal variability arises through an instability associated with a so called thermal rossby mode which involves westward propagation of temperature anomalies applying the available potential energy ape framework from stratified turbulence to the idealized ocean model simulations the authors study the multidecadal variability from an energetics viewpoint the analysis explains how the propagation of the temperature anomalies leads to changes in ape which are subsequently converted into t

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 review of north atlantic modes of natural variability and their driving mechanisms 1 this paper reviews three modes of natural variability that have been identified in the north atlantic ocean namely the north atlantic oscillation nao the atlantic multidecadal oscillation amo and the atlantic meridional mode amm this manuscript focuses on the multidecadal fluctuations of these three modes a range of different mechanisms to initiate phase reversals in these modes on multidecadal timescales has been suggested previously we propose a systematic grouping of these mechanisms into three types that involve respectively 1 the dependency of the atlantic thermohaline circulation thc on salinity 2 the sensitivity of the thc to changes in ocean heat transport and 3 the dependency of the nao to changes in the atlantic meridional temperature gradient some new density data is also provided demonstrating physical links between the thc and the amo

quasi decadal variability in paleoclimate records sunspot cycles or intrinsic oscillations 1 paleoclimatic proxies for interannual variability reveal power within the 7 15 year quasi decadal qd band in earth s history despite the fact that the best established sources of qd variability in modern climate are internal this qd variability in paleoclimatic records is commonly attributed to the 11 year sunspot cycle of solar variability intrinsic qd variability has received little attention in paleoclimatology we apply two different modeling frameworks to assess the degree to which qd variability might have occurred in the past and question whether existing paleoclimate records no matter how well resolved distinguish between internally driven and solar driven variability we show that even a simple atmospheric general circulation model gcm driven by constant solar radiation produces climate oscillations with a period of 11 years then motivated by numerous studies arguing for the presence of sunspot cycles in eocene 50 ma paleoclimate records we demonstrate that a fully coupled gcm simulation with constant solar forcing reproduces the full spectrum of reconstructed eocene variability in the examples we consider there is no compelling reason to invoke solar cycles as a cause of interpreted variability we stress the importance of identifying internal modes of variability from paleoclimate records to increase understanding of the climate system

on the mechanism of north atlantic decadal variability abstract north atlantic decadal climate variability is studied with a coupled atmosphere ocean sea ice model ecbilt after having reached an approximate statistical equilibrium in coupled mode without applying flux corrections a subsequent 1000 yr integration is performed and analyzed compared to the current climate the surface temperatures are 2 c warmer in the tropics to almost 8 c warmer in the polar regions the covariability between the atmosphere and ocean is explored by performing a singular value decomposition svd of boreal winter sst anomalies and 800 hpa geopotential height anomalies the first svd pair shows a red variance spectrum in sst and a white spectrum in 800 hpa height the second mode shows a peak in both spectra at a timescale of about 16 18 yr the geopotential height pattern is the model s equivalent of the north atlantic oscillation nao pattern the sst anomaly pattern is a north south oriented dipole additional experiments have revealed that the decadal oscillation in ec

the variability of the atlantic meridional overturning circulation the north atlantic oscillation and the el nino southern oscillation in the bergen climate model abstract the link between the interannual to interdecadal variability of the atlantic meridional overturning circulation amoc and the atmospheric forcing is investigated using 200 yr of a control simulation of the bergen climate model where the mean circulation cell is rather realistic as is also the location of deep convection in the northern north atlantic the amoc variability has a slightly red frequency spectrum and is primarily forced by the atmosphere the maximum value of the amoc is mostly sensitive to the deep convection in the irminger sea which it lags by about 5 yr the latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area the impact of the north atlantic oscillation on deep convection in the labrador and greenland seas is represented realistically but its influence on the amoc is limited to the interannual time scale and is primarily associated with wind forcing the tropical pacific shows a strong variability in the model

the influence of atlantic variability on asian summer climate is sensitive to the pattern of the sea surface temperature anomaly we simulate the response of asian summer climate to atlantic multidecadal oscillation amo like sea surface temperature sst anomalies using an intermediate complexity general circulation model igcm4 experiments are performed with seven individualamosst anomalies obtained from cmip5 pmip3 global climate models as well as their multimodel mean globally and over the north atlantic ocean only for both the positive and negative phases of the amo during the positive warm amo phase a rossby wave train propagates eastward causing a high pressure and warm and dry surface anomalies over eastern china and japan during the negative cool phase of the amo the midlatitude rossby wave train is less robust but the model does simulate a warm and dry south asian monsoon associated with the movement of the intertropical convergence zone in the tropical atlantic the circulation response and associated temperature and precipitation anomalies are sensitive to the choice of amo sst anomaly pattern a comparison between global sst and north atlantic sst perturbation experiments indicates that east asian climate anomalies are forced from the north atlantic region whereas south asian climate anomalies are more strongly affected by the amo related sst anomalies outside the north atlantic region experiments conducted with different amplitudes of negative and positiveamoanomalies show that the temperature response is linear with respect to sst anomaly but the precipitation response is nonlinear

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