the roles of co2 and orbital forcing in driving southern hemispheric temperature variations during the last 21 000 yr abstract transient climate model simulations covering the last 21 000 yr reveal that orbitally driven insolation changes in the southern hemisphere combined with a rise in atmospheric pco2 were sufficient to jump start the deglacial warming around antarctica without direct northern hemispheric triggers analyses of sensitivity experiments forced with only one external forcing component greenhouse gases ice sheet forcing or orbital forcing demonstrate that austral spring insolation changes triggered an early retreat of southern ocean sea ice starting around 19 18 ka bp the associated sea ice albedo feedback and the subsequent increase of atmospheric co2 concentrations helped to further accelerate the deglacial warming in the southern hemisphere implications for the interpretation of southern hemispheric paleoproxy records are discussed

arctic sea ice variability in the context of recent atmospheric circulation trends abstract forty years 1958 97 of reanalysis products and corresponding sea ice concentration data are used to document arctic sea ice variability and its association with surface air temperature sat and sea level pressure slp throughout the northern hemisphere extratropics the dominant mode of winter january march sea ice variability exhibits out of phase fluctuations between the western and eastern north atlantic together with a weaker dipole in the north pacific the time series of this mode has a high winter to winter autocorrelation 0 69 and is dominated by decadal scale variations and a longer term trend of diminishing ice cover east of greenland and increasing ice cover west of greenland associated with the dominant pattern of winter sea ice variability are large scale changes in sat and slp that closely resemble the north atlantic oscillation the associated sat and surface sensible and latent heat flux anomalies are largest over the portions of the marginal sea ice zone in which the tr

multidecadal climate variability in the greenland sea and surrounding regions a coupled model simulation pronounced oscillations of ocean temperature and salinity occur in the greenland sea in a 2000 year integration of a coupled ocean atmosphere model the oscillations involving both the surface and subsurface ocean layers have a timescale of approximately 40 80 years and are associated with fluctuations in the inten sity of the east greenland current the greenland sea temperature and salinity variations are preceded by large scale changes in near surface salinity in the arctic which appear to propagate out of the arctic through the east greenland current these anomalies then propagate around the subpolar gyre into the labrador sea and the central north atlantic these oscillations are coherent with previously identified multi decadal fluctuations in the intensity of the north atlantic thermohaline circula tion the oscillations in the g eenland sea are related to atmospheric variability negative cold anomalies of surface air temperature are associated with negative cold sea surface temperature sst anomalies in the greenland sea with amplitudes up to 2oc near green land declining to several tenths of a degree c over north western europe the c01d sst anomalies and intensified east greenland current are also associated with enhanced northerly winds over the greenland sea

simulating long term caspian sea level changes the impact of holocene and future climate conditions to improve our understanding of the relationship between climate change and variations in caspian sea level csl we performed simulations of annual csl for the period 8 ka to 2100 ce using a coupled model setup representing climate hydrology and sea level we forced our climate model with long term changes in orbital parameters and atmospheric greenhouse gas concentrations using the ipcc a1b scenario for the 21st century our simulations produce an orbitally forced long term decline in csl of 5 m from 5 5 to 0 ka caused by a decrease in river runoff and over sea precipitation that is not fully compensated by a decrease in over sea evaporation superimposed on this long term downward csl trend we simulated centennial scale fluctuations of up to 4 m and decadal scale variations of up to 2 m caused by the internal variations of our modeled climate system amplified by the sensitivity of csl to small changes in river runoff and in the over sea p e budget the a1b anthropogenic emission scenario causes a 4 5 m fall in csl in the 21st century due to a pronounced increase in over sea evaporation that is stronger than the enhanced river discharge this decline in csl is of the same order of magnitude as the orbitally forced millennial scale downward csl trend simulated for the last 8000 years our results are generally consistent with csl estimates based on geological historical and measured data as well as with most other model studies 2007 elsevier b v all rights reserved

thermohaline circulation hysteresis a model intercomparison we present results from an intercomparison of 11 different climate models of intermediate complexity in which the north atlantic ocean was subjected to slowly varying changes in freshwater input all models show a characteristic hysteresis response of the thermohaline circulation to the freshwater forcing which can be explained by stommel s salt advection feedback the width of the hysteresis curves varies between 0 2 and 0 5 sv in the models major differences are found in the location of present day climate on the hysteresis diagram in seven of the models present day climate for standard parameter choices is found in the bi stable regime in four models this climate is in the mono stable regime the proximity of the present day climate to the stommel bifurcation point beyond which north atlantic deep water formation cannot be sustained varies from less than 0 1 sv to over 0 5 sv

model data comparison for the 8 2 ka bp event confirmation of a forcing mechanism by catastrophic drainage of laurentide lakes abstract to improve our understanding of the mechanism behind the 8 2 ka bp cooling event we compare proxy evidence with climate model simulations in which the thermohaline ocean circulation is perturbed by a freshwater pulse into the labrador sea both the proxy data and model results show a cooling that is mainly concentrated in the north atlantic region ranging from more than 5 c cooling in the nordic seas to about 0 5 1 c over europe and less than 0 5 c over the subtropical north atlantic data and model also indicate a weakening of the summer monsoon and generally a drier circum north atlantic over the south atlantic ocean the model simulates a slight warming mostly less than 0 5 c which falls within the uncertainty of proxy data and thus could not be confirmed to examine in detail the structure of the 8 2 ka bp event we also compare the modeled climatological evolution at two locations with high quality records revealing a generally consistent picture the good model data agreement confirms the hypothesis that the 8 2 ka bp event was forced by a freshwater induced weakening of the thermohaline circulation other forcings are unlikely since they would result in an alternative geographical distribution and expression of the climate response

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

decadal climate oscillations in the arctic a new feedback loop for atmosphere ice ocean interactions a combined complex empirical orthogonal function analysis of 40 years of annual sea ice concentration sic and winter sea level pressure slp data reveals the existence of an approximately 10 year climate cycle in the arctic and subarctic the cycle is characterized by a clockwise propagating signal in the sic anomalies and a standing oscillation in the slp anomalies the latter being linked to a fluctuation between the two phases of the north atlantic oscillation to describe the formation and evolution of the sic and slp anomalies associated with the cycle a simple feedback loop is proposed

modeling the effect of freshwater pulses on the early holocene climate the influence of high frequency climate variability freshwater 4 67 10 14 m 3 into the labrador sea at three different constant rates 1 5 sv 1 sv 1 10 6 m 3 s 1 i n 10 years 0 75 sv in 20 years and 0 3 sv in 50 years for each rate five ensemble experiments have been performed varying in initial conditions the freshwater pulses produce a weakening of the thermohaline circulation the perturbed state is in agreement with proxy evidence for the 8 2 ka event two types of recovery of the thermohaline circulation occurred differing in time scale 1 200 years and 2 200 years in the experiments with 10 year and 20 year pulses both types of recovery were observed this suggests that the model response is unpredictable in the range of parameters studied here it is hypothesized that the unpredictability is associated with annual to decadal climate variability our results demonstrate that several types of recovery may exist with the same kind of perturbation the interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere sea ice ocean system index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean keywords holocene thermohaline circulation predictability meltwater coupled climate model

an event based model of soil erosion and sediment transport at the catchment scale abstract deforestation in new zealand has led to increased soil erosion and sediment loads in rivers adversely affecting aquatic ecosystems soil conservation actions over large areas are expensive and need to be targeted to obtain maximum benefit for least cost medium complexity models of soil erosion have been used to target soil conservation but they involve long term averages and do not involve time variation in this paper an event based model of soil erosion and sediment transport is developed it has moderate complexity and is able to be calibrated with a modicum of data the spatial framework is based on small subcatchments and stream links for each storm event the sediment yield for each subcatchment is estimated from peak storm discharge with a power function and routed through the stream network we run the model on the manawat catchment in new zealand for which all the major tributaries are monitored for water and sediment discharge spatial maps of sediment yield are produced for each storm event in a 5 year period from model inputs of maximum storm discharge at the tributaries to test the model predicted sediment loads of the storm events in the main channel of the manawat at teachers college palmerston north are compared with measured sediment loads the strong agreement between predictions and measurements suggests that the method for converting maximum storm discharge to sediment load at tributaries is robust analysis of the sediment concentration and discharge records for storm events revealed that sediment from channel resuspension and bank erosion was much smaller typically

large sea ice volume anomalies simulated in a coupled climate model the processes leading to the formation of a large anomaly of sea ice volume integrated over the northern hemisphere have been investigated in a coarse resolution three dimensional atmosphere ocean sea ice model this anomaly lasts for about 20 years and reaches 8 6 103 km3 i e 6 standard deviations it is associated with a maximum surface cooling of more than 5 c in the annual mean close to spitzbergen the trigger of the large event in the model is a sequence of years presenting an atmospheric circulation characterised by negative geopotential height anomalies over greenland and positive ones in the barents and norwegian seas this atmospheric anomaly induces an increase in the ice volume as well as of the ice extent sea ice survives in the area located close to spitzbergen where deep convection occurs during normal years this is associated with a shut down of convection in that area and thus by a strong reduction of the upward heat flux from the ocean to the atmosphere that strongly amplifies the initial cooling the long sequence of following years presenting the same type of anomaly in the atmosphere is not just a rare realisation of the intrinsic atmospheric variability a positive feedback between the modification of surface conditions and the atmospheric circulation reinforces the initial atmospheric anomaly finally the large event stops when convection increases again as a result of an increase in ocean surface density close to spitzbergen this density increase is due to both the advection of a positive salinity anomaly created in the arctic because of brine rejection and to local ice formation southward of spitzbergen during the cold event sensitivity experiments performed with the model have shown that the frequency of the events is very sensitive to the mean climate simulated by the model the frequency being higher in a colder climate

early holocene laurentide ice sheet deglaciation causes cooling in the high latitude southern hemisphere through oceanic teleconnection the impact of the early holocene laurentide ice sheet lis deglaciation on the climate at southern hemisphere high latitudes is studied in three transient simulations performed with a global climate model of the coupled atmosphere ocean vegetation system considering the lis deglaciation we quantify separately the impacts of the background meltwater fluxes and the changes in topography and surface albedo in our model the meltwater input into the north atlantic results in a substantial weakening of the atlantic meridional overturning circulation associated with absence of deep convection in the labrador sea northward ocean heat transport by the atlantic ocean is reduced by 28 this weakened ocean circulation leads to cooler north atlantic deep water nadw upwelling of this cool nadw in the southern ocean results in reduced surface temperatures by 1 degrees c to 2 degrees c here between 9 and 7 ka compared to an experiment without lis deglaciation poleward of the polar front zone this advective teleconnection between the southern and northern hemispheres overwhelms the effect of the classical bipolar seesaw mechanism these results provide an explanation for the relatively cold climatic conditions between 9 and 7 ka reconstructed in several proxy records from southern hemisphere high latitudes such as antarctic ice cores

a numerical model for interdecadal variability of sea ice cover in the greenland iceland norwegian sea a coupled ocean sea ice atmosphere model is used to study interdecadal variability 40 years of sea ice depth and concentration in the greenland iceland norwegian sea this oceanic region is represented by a meridionally aligned channel on a plane with open zonal boundaries at 60 n and 80 n the model consists of a one and a half layer reduced gravity ocean model a thermodynamic dynamic sea ice model and an energy balance model of the atmosphere the coupled model is driven by prescribed surface wind stress fluxes of heat salt and ice at inflow points on the northern and southern open zonal boundaries and annual distribution of solar radiation it is shown that the coupled model supports unforced modes of interdecadal oscillation resulting from a form of hydraulic control which regulates the total fluid volume in the oceanic active layer the mechanism for the oscillations relies on the presence of three key features 1 a region of intense oceanic entrainment located in the eastern part of the domain 2 a vigorous southward flowing western boundary current representing the east greenland current egc which supports most of the meridional transport across the domain and 3 a marked buoyancy contrast between the relatively salty domain interior and the much fresher western boundary region during an oscillation excess water is pumped into the domain via entrainment thereby creating an active layer depth anomaly which then propagates westward via long baroclinic rossby waves until it reaches the egc where it is subsequently drained out of the domain across the southern open zonal boundary as the depth anomaly traverses the basin an anomalous geostrophic circulation is established in which cold fresh arctic water enters the domain interior and this eventually promotes enhanced thermodynamic sea ice growth consequently the interdecadal oscillations of the coupled model are characterised by pulse periods typically spanning 20 years during which there is an abnormally large winter sea ice cover separated by interpulse periods lasting another 20 years during which the winter sea ice extent is nearly uniform and significantly smaller than in a pulse maximum the duration of the interpulse periods is dictated by the time it takes for the rossby waves to traverse the basin in addition to the interdecadal oscillation solution the coupled ocean sea ice atmosphere model is found to also have a stable cyclostationary state with no interannual variability stochastic forcing in the form of randomly specified interannual anomalies of salinity of maximum amplitude 0 1 ppt or ice inflow of maximum amplitude 0 1 sv at the northern open zonal boundary in both cases is capable of driving the model from the cyclostationary state solution to the interdecadal variability one

broadband waveform modelling of anomalous strong ground motion in the 1989 loma prieta earthquake using three dimensional geologic structures strong motion recordings of the 1989 loma prieta earthquake show a large coherent sh wave displacement pulse across much of the san francisco bay area this distinctive ground motion has been modelled using approximate broadband synthetics for a double couple centroid source in a 3 d crustal model for the region the synthetics indicate that 1 horizontal bending and focusing of sh waves by the lateral crustal velocity contrast across the san andreas fault and 2 the source radiation pattern contribute significantly to the observed azimuthal variation of the sh pulse and hence to the regional variations of strong ground shaking in the period range of 1 to 8 sec in addition the scattering of surface reflected shear waves by deep basins under the santa clara valley may contribute to increased coda amplitude and duration observed for sites around oakland the wave modelling also supports the earlier conclusion that an increase in intensity in portions of san francisco and oakland is in part the result of energy reflected from the base of the crust somerville and yoshimura 1990

holocene climate evolution in the high latitude southern hemisphere simulated by a coupled atmosphere sea ice ocean vegetation model the holocene climate is simulated in a 9000 yr long transient experiment performed with the ecbilt clio vecode coupled atmosphere sea ice ocean vegetation model this experiment is forced with annually varying orbital parameters and atmospheric concentrations of co2 and ch4 the objective is to study the impact of these long term forcings on the surface temperature evolution during different seasons in the high latitude southern hemisphere we find in summer a thermal optimum in the mid holocene 6 3 ka bp with temperatures locally 3 degrees c above the preindustrial mean in autumn the temperatures experienced a long term increase particularly during the first few thousand years the opposite trend was simulated for winter and spring with a relatively warm southern ocean at 9 ka bp in inter up to 3 5 degrees c above the preindustrial mean and a warm continent in spring 3 degrees c followed by a gradual cooling towards the present these long term temperature trends can be explained by a combination of 1 a delayed response to orbital forcing with temperatures lagging insolation by i to months owing to the thermal inertia of the system and 2 the long memory of the southern ocean this long memory is related to the storage of the warm late winter spring anomaly below the shallower summer mixed layer until next winter sea ice plays an important role as an amplifying factor through the ice albedo and ice insulation feedbacks our experiments can help to improve our understanding of the holocene signal in proxies for instance the results suggest that in contrast to recent propositions teleconnections to the northern hemisphere appear not necessarily to explain the history of southern hemisphere temperature changes during the holocene

evaluation of regional ocean circulation models for the mediterranean sea at the strait of gibraltar volume transport and thermohaline properties of the outflow a set of simulations from different configurations of the nemomed8 nemomed12 and nemomed36 ocean regional circulation models for the mediterranean sea has been studied in order to assess the accuracy of their representation of the exchange through the strait of gibraltar the model volume transport and thermohaline properties of the mediterranean outflow have been compared with observational data collected at espartel sill the westernmost sill of the strait by a permanent station moored since october 2004 in the frame of the ingres projects results show that in terms of volume transport nemomed8 simulations perform a better representation of the exchange while nemomed12 36 underestimate both the mean inflow and outflow the reason for this underestimation is a too low velocity of the flow which could be consequence of an enhanced roughness effect due the flow bathymetry interaction an important improvement in the representation of the exchange seasonality is achieved by the simulations including sea surface height variability of the atlantic area of the domain the results for the themohaline characteristics of the mediterranean outflow are better for nemomed12 and nemomed36 as a consequence of their better representation of the local dynamical processes that leads to a more realistic composition of the mediterranean waters comprising the flow

contour mapping of arctic basin ice draft and roughness parameters a data base of ice draft and roughness parameters has been constructed for selected portions of the arctic ocean based upon analysis of under ice draft distribution data acquired by inverted echo sounder systems on submarines from the voyages of 12 submarines which traversed the alaskan canadian and central arctic regions of the arctic ocean during the summer and winter seasons a series of mean ice draft and deep draft keel statistics was calculated for 50 km segments along each submarine track contour maps of the mean ice draft its standard deviation the mean keel draft and the spatial frequency of ice keels were constructed they show that the greatest ice drafts the roughest ice and the greatest number of deep draft keels are found off the north coasts of the canadian archipelago and greenland due to ice convergence on these land barriers

variability of fram strait ice flux and north atlantic oscillation we estimate the winter sea ice export through the fram strait using ice motion from satellite passive microwave data sea ice motion october may is obtained by tracking the displacement of common features in sequential 85 and 37 ghz brightness temperature fields the average winter area flux over the 18 year record 1978 1996 is 670 000 km2 7 of the area of the arctic ocean the winter area flux ranges from a minimum of 450 000 km2 in 1984 to a maximum of 906 000 km2 in 1995 the daily monthly and interannual variabilities of the ice area flux are high there is an upward trend in the ice area flux over the 18 year record the average winter volume flux over the winters of october 1990 through may 1995 is 1745 km3 ranging from a low of 1375 km3 in the 1990 flux to a high of 2791 km3 in 1994 the sea level pressure gradient across the fram strait explains more than 80 of the variance in the ice flux over the 18 year record we use the coefficients from the regression of the time series of area flux versus pressure gradient across the fram strait and ice thickness data to estimate the summer area and volume flux the average 12 month area flux and volume flux are 919 000 km2 and 2366 km3 we find a significant correlation r 0 86 between the area flux and positive phases of the north atlantic oscillation nao index over the months of december march correlation between our 5 years of volume flux estimates and the nao index gives r 0 56 during the high nao years a more intense icelandic low increases the gradient in the sea level pressure by almost 1 mbar across the fram strait thus increasing the atmospheric forcing on ice transport correlation is reduced during the negative nao years because of decreased dominance of this large scale atmospheric pattern on the sea level pressure gradient across the fram strait

modeling the climate response to a massive methane release from gas hydrates 1 the climate response to a massive release of methane from gas hydrates is simulated in two 2500 year long numerical experiments performed with a three dimensional global coupled atmosphere sea ice ocean model of intermediate complexity two different equilibrium states were used as reference climates the first state with preindustrial forcing conditions and the second state with a four times higher atmospheric co2 concentration these climates were perturbed by prescribing a methane emission scenario equivalent to that computed for the paleocene eocene thermal maximum petm similar to55 5 ma involving a sudden release of 1500 gt of carbon into the atmosphere in 1000 years in both cases this produced rapid atmospheric warming up to 10degreesc at high latitudes and a reorganization of the global overturning ocean circulation in the ocean maximum warming 2 4degreesc occurred at intermediate depths where methane hydrates are stored in the upper slope sediments suggesting that further hydrate instability could result from the prescribed scenario

holocene climate instability during the termination of the african humid period 1 the termination of the holocene african humid period similar to9 to similar to6 kyr bp is simulated with a three dimensional global coupled climate model that resolves synoptic variability associated with weather patterns in the simulation the potential for green and desert sahara states becomes equal between 7 5 and 5 5 thousand years ago causing the climate system to fluctuate between these states at decadal to centennial time scales this model result is supported by paleoevidence from the western sahara region showing similar paleohydrological fluctuations around that time for the present day only the desert sahara state is stable in the model

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

sea ice and polar climate in the ncar csm the climate system model csm consists of atmosphere ocean land and sea ice components linked by a flux coupler which computes fluxes of energy and momentum between components the sea ice component consists of a thermodynamic formulation for ice snow and leads within the ice pack and ice dynamics using the cavitating fluid ice rheology which allows for the compressive strength of ice but ignores shear viscosity the results of a 300 yr climate simulation are presented with the focus on sea ice and the atmospheric forcing over sea ice in the polar regions the atmospheric model results are compared to analyses from the european centre for medium range weather forecasts and other observational sources the sea ice concentrations and velocities are compared to satellite observational data the atmospheric sea level pressure slp in csm exhibits a high in the central arctic displaced poleward from the observed beaufort high the southern hemisphere slp over sea ice is generally 5 mb lower than observed air temperatures over sea ice in both hemispheres exhibit cold biases of 2 4 k the precipitationminus evaporation fields in both hemispheres are greatly improved over those from earlier versions of the atmospheric gcm the simulated ice covered area is close to observations in the southern hemisphere but too large in the northern hemisphere the ice concentration fields show that the ice cover is too extensive in the north pacific and subarctic north atlantic oceans the interannual variability of the ice area is similar to observations in both hemispheres the ice thickness pattern in the antarctic is realistic but generally too thin away from the continent the maximum thickness in the arctic occurs against the bering strait not against the canadian archipelago as observed the ice velocities are stronger than observed in both hemispheres with a consistently greater turning angle to the left in the southern hemisphere they produce a northward ice transport in the southern hemisphere that is 3 4 times the satellite derived value sensitivity tests with the sea ice component show that both the pattern of wind forcing in csm and the air ice drag parameter used contribute to the biases in thickness drift speeds and transport plans for further development of the ice model to incorporate a viscousplastic ice rheology are presented in spite of the biases of the sea ice simulation the 300 yr climate simulation exhibits only a small degree of drift in the surface climate without the use of flux adjustment this suggests a robust stability in the simulated climate in the presence of significant variability

thinning of the arctic sea ice cover comparison of sea ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1 3 m in most of the deep water portion of the arctic ocean from 3 1 m in 1958 1976 to 1 8 m in the 1990s the decrease is greater in the central and eastern arctic than in the beaufort and chukchi seas preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s

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

on the origin and evolution of sea ice anomalies in the beaufort chukchi sea the origin and space time evolution of beaufort chukchi sea ice anomalies are studied using data and a recently developed dynamic thermodynamic sea ice model first the relative importance of anomalies of river runoff atmospheric temperature and wind in creating anomalous sea ice conditions in the beaufort chukchi sea is investigated the results indicate that wind anomalies are the dominant factor responsible for creating interannual variability in the beaufort chukchi sea ice cover temperature anomalies appear to play a major role for longer time scale fluctuations whereas the effects of runoff anomalies are small the sea ice model is then used to track the position of a positive sea ice anomaly as it is transported by the beaufort gyre toward the transpolar drift stream and then exported out of the arctic basin into the greenland sea via fram strait the model integration shows that sea ice anomalies originating in the western beaufort sea can survive a few seasonal cycles as they propogate through the arctic basin and can account for a notable amount of anomalous ice export into the greenland sea these anomalies however represent a small contribution to the fresh water budget in this area when compared with sea ice fluctuations generated by interannually varying local winds

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

global characterization of the holocene thermal maximum we analyze the global variations in the timing and magnitude of the holocene thermal maximum htm and their dependence on various forcings in transient simulations covering the last 9000 years 9 ka performed with a global atmosphere ocean vegetation model in these experiments we consider the influence of variations in orbital parameters and atmospheric greenhouse gases and the early holocene deglaciation of the laurentide ice sheet lis considering the lis deglaciation we quantify separately the impacts of the background melt water fluxes and the changes in topography and surface albedo in the analysis we focus on the intensity of the maximum temperature deviation relative to the preindustrial level its timing in the holocene and the seasonal expression in the model the warmest htm conditions are found at high latitudes in both hemispheres reaching 5 c above the preindustrial level while the smallest htm signal is seen over tropical oceans less than 0 5 c this latitudinal contrast is mostly related to the nature of the orbitally forced insolation forcing which is also largest at high latitudes and further enhanced by the polar amplification the holocene timing of the htm is earliest before 8 ka bp in regions not affected by the remnant lis particularly nw north america e asia n africa n south america the middle east ne siberia and australia compared to the early holocene insolation maximum the htm was delayed by 2 3 ka over ne north america and regions directly downwind from the lis a similar delay is simulated over the southern ocean while an intermediate lag of about 1 ka is found over most other continents and oceans the seasonal timing of the htm over continents generally occurs in the same month as the maximum insolation anomaly whereas over oceans the htm is delayed by 2 3 months exceptions are the oceans covered by sea ice and north africa were additional feedbacks results in a different seasonal timing the simulated timing and magnitude of the htm are generally consistent with global proxy evidence with some notable exceptions in the mediterranean region sw north america and eastern eurasia 2012 elsevier ltd

decadal variability in high northern latitudes as simulated by an intermediate complexity climate model a 2500 year integration has been performed with a global coupled atmospheric sea ice ocean model of intermediate complexity with the main objective of studying the climate variability in polar regions on decadal time scales and longer the atmospheric component is the ecbilt model a spectral t21 three level quasi geostrophic model that includes a representation of horizontal and vertical beat transfers as well as of the hydrological cycle ecbilt is coupled to the clio model which consists of a primitive equation free surface ocean general circulation model and a dynamic thermodynamic sea ice model comparison of model results with observations shows that the ecbilt clio model is able to reproduce reasonably well the climate of the high northern latitudes the dominant mode of coupled variability between the atmospheric circulation and sea ice cover in the simulation consists of an annular mode for geopotential height at 800 hpa and of a dipole between the barents and labrador seas for the sea ice concentration which are similar to observed patterns of variability in addition the simulation displays strong decadal variability in the sea ice volume with a significant peak at about 18 years positive volume anomalies are caused by 1 a decrease in ice export through fram strait associated with more anticyclonic winds at high latitudes 2 modifications in the freezing melting rates in the arctic due to lower air temperature and higher surface albedo and 3 a weaker heat flux at the ice base in the barents and kara seas caused by a lower inflow of warm atlantic water opposite anomalies occur during the volume decrease phase of the oscillation

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability