the nature of the arctic oscillation and diversity of the extreme surface weather anomalies it generates abstractthrough a cluster analysis of daily ncep ncar reanalysis data this study demonstrates that the arctic oscillation ao defined as the leading empirical orthogonal function eof of 250 hpa geopotential height anomalies is not a unique pattern but a continuum that can be well approximated by five discrete representative ao like patterns these ao like patterns grow simultaneously from disturbances in the north pacific the north atlantic and the arctic and both the feedback from the high frequency eddies in the north pacific and north atlantic and propagation of the low frequency wave trains from the north pacific across north america into the north atlantic play important roles in the pattern formation furthermore it is shown that the structures and frequencies of occurrence of the five ao like patterns are significantly modulated by el nino southern oscillation enso warm cold enso enhances the negative positive ao phase compared with enso neutral winters finally the surface weat

north atlantic climate variability from a self organizing map perspective 1 north atlantic variability in general and the north atlantic oscillation nao in particular is a long studied very important but still not well understood problem in climatology the recent trend to a higher wintertime nao index was accompanied by an additional increase in the azores high not coupled to changes in the icelandic low as shown by a self organizing maps soms analysis of monthly mean djf mean sea level pressure data from 1957 to 2002 soms are a nonlinear tool to optimally extract a user specified number of patterns or icons from an input data set and to uniquely relate any input data field to an icon allowing analyses of occurrence frequencies and transitions complementary to principal component analysis pca soms analysis of era 40 data finds a north atlantic monopole roughly colocated with the mean position of the azores high as well as the well known nao dipole involving the icelandic low and the subtropical high little trend is shown in december but the azores high increased along with the nao in january and february over the study interval with implications for storminess in northwestern europe in short our som based analyses of winter mslp have both confirmed prior knowledge and expanded it through the relative ease of use and power with nonlinear systems of the som based approach to climatological analysis

arctic oscillation variability generated through inter ocean interactions 1 we investigate the structure of the northern hemisphere atmospheric circulation anomaly patterns associated with the dominant mode of interannual variability in the tropical pacific and the dominant mode of quasi decadal variability in the atlantic basin there is evidence that the superposition of the atmospheric patterns associated with these modes possesses an arctic oscillation like structure we suggest that part of the arctic oscillation variability is due to interactions between coupled ocean atmosphere modes originating in the tropical pacific and atlantic basins

tropospheric waveguide teleconnections and their seasonality abstractone point correlation maps of the subseasonal variability of 200 hpa meridional wind in nature and an atmospheric general circulation model are systematically analyzed to quantify the impact of the climatological mean jets on tropospheric covariability as a result of the jets acting as waveguides for the propagation of rossby waves as anticipated by linear theory signatures of jet influence are detected in terms of i the geographical position of the strongest teleconnections ii the zonal orientation and extent of prominent patterns of variability and iii the scale of the features that make up those patterns further evidence of jet waveguide influence comes from examining the seasonality of these teleconnection attributes during winter covariability can be essentially circumglobal while during summer it tends to be confined within two separate sectors of the globe where the jets are especially strong experiments with a multilevel linear planetary wave model confirm that the analyzed

on the persistence of cold season sst anomalies associated with the annular modes abstractin this study a simple stochastic climate model is used to examine the impact of the ocean mixed layer depth surface turbulent energy fluxes and ekman currents on the persistence of cold season extratropical sea surface temperature sst anomalies associated with variability in the annular modes of atmospheric circulation in both hemispheres observational analysis reveals that during the cold season sst anomalies associated with the southern annular mode sstsam persist considerably longer than those associated with the northern annular mode sstnam using the simple model it is shown that the persistence of the cold season sstsam is consistent with the simple stochastic climate paradigm in which the atmospheric forcing is approximated as white noise and the persistence of sst anomalies can be largely determined by the thermal inertia of the ocean mixed layer in the north atlantic however the simple climate model overestimates the persistence of the cold season sstnam it is thought th

the link between the north pacific climate variability and the north atlantic oscillation via downstream propagation of synoptic waves abstractthe north atlantic oscillation nao response to the northeast pacific climate variability is examined using the era 40 dataset the main objective is to validate a mechanism involving downstream wave propagation processes proposed in a recent idealized companion study a low frequency planetary scale ridge trough anomaly located in the eastern pacific north american sector induces more equatorward poleward propagation of synoptic scale wave packets on its downstream side which favors the occurrence of anticyclonic cyclonic wave breakings in the atlantic sector and the positive negative nao phase the mechanism first provides an interpretation of the canonical impact of the el nino southern oscillation on the nao in late winter the wintertime relationship between the pacific north american oscillation pna and the nao is also investigated for out of phase fluctuations between the pna and nao indices i e the most recurrent situation in late winter the eastern pacific pna ridge trou

timescale dependency of spatial patterns in the variability of the northern hemisphere winter slp field 1 the arctic oscillation ao the north atlantic oscillation nao and the cold ocean warm land pattern cowl have been identified as important modes of variability in the northern hemisphere winter sea level pressure field however which one is most fundamental in describing this variability and the relationship between them are still important open questions a key uncertainty is our lack of knowledge of the linkages between the arctic north atlantic and north pacific oceans that are manifest in these modes in this study the authors investigate these linkages through a frequency dependent decomposition of the variability in the northern hemisphere sea level pressure field in particular the authors show that the spatial expression of the variability in this field has a frequency dependence that results in a nao like pattern on inter annual timescales an ao like pattern on inter decadal timescales and a cowl like pattern on multi decadal timescales

alteration processes at deep sea drilling project ocean drilling program hole 504b at the costa rica rift implications for magnetization of oceanic crust magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504b during ocean drilling program leg 111 our results combined with those from previous studies show abrupt first order changes in magnetic properties at alteration boundaries within the 504b crustal section changes in style and degree of alteration fall near the boundaries of the three well defined lithologic units the extrusive basalts the transition zone and the sheeted dike complex this postemplacement alteration heavily influences magnetic properties and is observed to change in both style and degree with depth within each lithologic unit our results indicate that at 504b the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration the upper more permeable basalts above this critical depth have not experienced this reheating within the sheeted dike complex the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth the changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth overall these observations suggest that the effective penetration of water into layer 2c decreased with increasing depth and resulted in a lower rate of convective cooling magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration the upper dikes have become magnetically more stable than the lower dikes a review of all magnetic properties indicate that the dike section at 504b carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict we suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions despite a magnetization which is lower than expected it appears that the sheeted dike complex at hole 504b is capable of making a substantial contribution to the overlying marine magnetic anomaly because of the systematic decrease in hydrothermal alteration of magnetic minerals the ability of the dike section to contribute decreases as a function of depth

on the separability of the north atlantic oscillation and arctic oscillation we address the issue of whether the arctic ao and north atlantic oscillations nao are inseparable forming an annular mode in the northern hemisphere atmospheric circulation this annular mode is the leading empirical orthogonal function of hemispheric sea level pressure slp data explaining the largest amount of its variability we examine whether the nao and ao are inseparable spatial modes of the atmospheric circulation using rotated principal component analysis rpca a methodology that identifies simple and unique patterns of spatial dataset variability rpca of the spring summer and autumn slp fields from 1946 1998 reveal nao and ao like patterns occurring as separate regional teleconnections forming the first and second principal components respectively the rpca based nao dipole pattern is like that observed in many prior studies while the ao like pattern exhibits high slp variability over the kara and laptev seas during winter however and in annual analyses a distinct ao like pattern is not obtained and the two patterns may be inseparable using commonly accepted rpca methods the rpca based ao like mode is significantly linked to north central siberian seasonal air temperatures and to the prevailing direction of motion of the underlying arctic ocean in summer suggesting that the non winter ao like pattern as a stand alone teleconnection separate from the nao contributes significantly to high latitude climate and ocean variability the winter nao ao inseparability is discussed as a possible effect of a shared winter storm track between the northeastern atlantic and the arctic

the extreme drought event during winter spring of 2011 in east china combined influences of teleconnection in midhigh latitudes and thermal forcing in maritime continent region abstractthe middle and lower reaches of the yangtze river mlryr in china experienced an extremely severe and persistent drought event from january to may of 2011 using both the observational data and ncep ncar reanalysis features of the drought event and the related circulation anomalies were investigated it is found that the precipitation during the investigated period of 2011 was deficient mostly along the yangtze river the water vapor diverged from mlryr southward into the bay of bengal south china sea and the philippines there were two factors facilitating the drought event one was the quasi stationary rossby wave related teleconnection which propagated eastward at midhigh latitudes from the north atlantic to east asia reinforcing the siberian high and the east asian trough henceforth resulting in the divergence anomalies in mlryr in the lower troposphere this quasi stationary wave train though originating from the north atlantic region was not essentially related to the north atlantic

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

marine magnetic anomalies the origin of the stripes the results of recent observational and theoretical investigations of lineated magnetic anomalies on the ocean floor are summarized in tables graphs and diagrams and analyzed topics addressed include early lineation models inversions of magnetic anomalies to obtain source functions deep tow studies of magnetic anomalies evidence from the long wavelength component of the magnetic field including magsat observations and direct measurements of the magnetic properties of oceanic rocks it is concluded that the source of the lineated anomalies must reside in most of the oceanic crust not just in the pillow lavas of layer 2a

tropospheric rossby wave breaking and the nao nam objective analysis of several hundred thousand anticyclonic and cyclonic breaking rossby waves is performed for the northern hemisphere nh winters of 1958 2006 a winter climatology of both anticyclonic and cyclonic rossby wave breaking rwb frequency and size zonal extent is presented for the 350 k isentropic surface over the nh and the spatial distribution of rwb is shown to agree with theoretical ideas of rwb in shear flow composites of the two types of rwb reveal their characteristic sea level pressure anomalies upper and lower tropospheric velocity fields and forcing of the upper tropospheric zonal flow it is shown how these signatures project onto the centers of action and force the velocity patterns associated with the north atlantic oscillation nao and northern hemisphere annular mode nam previous studies have presented evidence that anticyclonic cyclonic breaking leads to the positive negative polarity of the nao and this relationship is confirmed for rwb over the midlatitudes centered near 50 n however an opposite and statistically significant relationship in which cyclonic rwb forces the positive nao and anticyclonic rwb forces the negative nao is shown over regions 20 to the north and south centered at 70 and 30 n respectively on a winter mean basis the frequency of rwb over objectively defined regions covering 12 of the area of the nh accounts for 95 of the nao index and 92 of the nam index a 6 hourly analysis of all the winters indicates that rwb over the objectively defined regions affects the nao nam without a time lag details of the objective wave breaking analysis method are provided in the appendix

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

asian jet waveguide and a downstream extension of the north atlantic oscillation abstract anomalous atmospheric fields associated with the north atlantic oscillation nao are analyzed on interannual and intraseasonal time scales in order to examine the extent to which the nao is a regional phenomenon analyses on the interannual time scale reveal that the nao signal is relatively confined to the euro atlantic sector in december while it extends toward east asia and the north pacific in february the difference is most clearly seen in the meridional wind anomaly which shows a wave train along the asian jet collocated with an anomalous vorticity source near the jet entrance diagnoses using a linear barotropic model indicate that this wave train is interpreted as quasi stationary rossby waves trapped on the asian jet waveguide and effectively excited by the anomalous upper level convergence over the mediterranean sea it is found that when the nao accompanies the mediterranean convergence mc anomaly most frequently seen in february the nao indeed has a much wider horizontal str

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

analysis and modeling of the arctic oscillation using a simple barotropic model with baroclinic eddy forcing abstract in this study a numerical simulation of the arctic oscillation ao is conducted using a simple barotropic model that considers the barotropic baroclinic interactions as the external forcing the model is referred to as a barotropic s model since the external forcing is obtained statistically from the long term historical data solving an inverse problem the barotropic s model has been integrated for 51 years under a perpetual january condition and the dominant empirical orthogonal function eof modes in the model have been analyzed the results are compared with the eof analysis of the barotropic component of the real atmosphere based on the daily ncep ncar reanalysis for 50 yr from 1950 to 1999 according to the result the first eof of the model atmosphere appears to be the ao similar to the observation the annular structure of the ao and the two centers of action at pacific and atlantic are simulated nicely by the barotropic s model therefore the atmospheric low frequency variabilities

multiple regimes and low frequency oscillations in the southern hemisphere s zonal mean flow abstract this paper studies multiple regimes and low frequency oscillations in the northern hemisphere zonal mean zonal flow in winter using 55 yr of daily observational data the probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct statistically significant maxima the two regimes associated with these maxima describe persistent zonal flow states that are characterized by meridional displacements of the midlatitude jet poleward and equatorward of its time mean position the geopotential height anomalies of either regime have a pronounced zonally symmetric component but largest amplitude anomalies are located over the atlantic and pacific oceans high frequency synoptic transients participate in the maintenance of and transitions between these regimes significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal flow time series these oscillations are described b

the statistical connection between tropospheric and stratospheric circulation of the northern hemisphere in winter abstract the associated anomaly patterns of the stratospheric geopotential height field and the tropospheric geopetential and temperature height fields of the northern hemisphere are determined applying the canonical correlation analysis with this linear multivariate technique the coupled modes of variability of lime series of two fields are isolated in the space of empirical orthogonal functions the one dataset is the 50 hpa geopotential height field the other set consists of different height fields of the tropospheric pressure levels 200 500 700 and 850 hpa and the temperature of the 850 hpa pressure level for the winter months december january february two natural coupled modes a barotropic and a baroclinic one of linear relationship between stratospheric and tropospheric circulation are found the baroclinic mode describes a connection between the strength of the stratospheric cyclonic winter vortex and the tropospheric circulation over the north atlantic the corresponding temperature

a test for annular modes abstract the use of empirical orthogonal functions eofs has grown popular as a tool to determine underlying variability from the rapidly increasing volume of climate data it has been noted that the dominant or first eof of geopotential heights in each hemisphere at levels from the surface through the troposphere and into the midstratosphere appears to be zonally symmetric it has also been suggested that all of the first eofs throughout the atmospheric column are barotropically coupled and annular moreover such modes of variability in both hemispheres are thought to be analogous to each other to define annularity more objectively and to facilitate comparisons both temporally and spatially a framework has been formulated within which modes of variability may be tested for their degree of zonal symmetry or annularity motivated by previous choices pressure height fields in each hemisphere are tested for annularity one near the surface and the other in the midstratosphere periods chosen coincide

crustal magnetic anomalies this quadrennial report departs in format from its predecessors earlier reports provided brief descriptions of the entire range of topics on crustal magnetic anomalies following guidelines established by the agu we instead focus our discussion on only a few selected topics our abbreviated list of references reflects this restricted focus a more comprehensive bibliography can be obtained from the authors as noted subsequently the past quadrennium was distinguished by a significant change in the scale and scope of crustal magnetic anomaly research advances in instrumentation navigation and data processing techniques resulted in increasing use of two dimensional anomaly maps rather than profile data although maps are commonly used in aeromagnetic studies of continental areas their use in the ocean basins has been limited until recently the other important change has been a trend towards increased integration of magnetic anomaly data with gravity seismic geochemical remote sensing and geologic data to enhance the interpretation and reduce the ambiguity of magnetic field data

multidecadal fluctuation of the wintertime arctic oscillation pattern and its implication abstractthe multidecadal fluctuations in the patterns and teleconnections of the winter mean arctic oscillation ao are investigated based on observational and reanalysis datasets results show that the atlantic center of the ao pattern remains unchanged throughout the period 1920 2010 whereas the pacific center of the ao is strong during 1920 59 and 1986 2010 and weak during 1960 85 consequently the link between the ao and the surface air temperature over western north america is strong during 1920 59 and 1986 2010 and weak during 1960 85 the time varying pacific center of the ao motivates a revisit to the nature of the ao from the perspective of decadal change it reveals that the north pacific mode npm and north atlantic oscillation nao are the inherent regional atmospheric modes over the north pacific and north atlantic respectively their patterns over the north pacific and north atlantic remain stable and change little with time during 1920 2010 the atlantic center of the ao always resemb

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

the role of the siberian high in northern hemisphere climate variability the dominant mode of sea level pressure slp variability during the winter months in the northern hemisphere nh is characterized by a dipole with one anomaly center covering the arctic with the opposite sign anomaly stretched across the mid latitudes associated with the slp anomaly is a surface temperature anomaly induced by the anomalous circulation we will show that this anomaly pattern originates in the early fall on a much more regional scale in siberia as the season progresses this anomaly pattern propagates and amplifies to dominate much of the extratropical nh making the siberian high a dominant force in nh climate variability in winter also since the slp and surface temperature anomalies originate in a region of maximum fall snow cover variability we argue that snow cover partially forces the phase of winter variability and can potentially be used for the skillful prediction of winter climate

influence of the arctic oscillation and el nino southern oscillation enso on ice conditions in the baltic sea the wavelet approach 1 variability in time series of ice conditions in the baltic sea is examined within the context of atmospheric circulation represented by the north atlantic oscillation nao and arctic oscillation ao winter indices using the wavelet approach we develop methods of assessing statistical significance and confidence intervals of cross wavelet phase and wavelet coherence cross wavelet power for the time series indicates that the times of largest variance in ice conditions are in excellent agreement with significant power in the ao at 2 2 3 5 5 7 7 8 and 12 20 year periods similar patterns are also seen with the southern oscillation index soi and nino3 sea surface temperature nino3 series wavelet coherence shows in phase linkages between the 2 2 7 8 and 12 20 year period signals in both tropical and arctic atmospheric circulation and also with ice conditions in the baltic sea these results are consistent with gcm simulations showing dynamical connections between high latitude surface conditions tropical sea surface temperatures mediated by tropical wave propagation the wintertime polar vortex and the ao and with models of sea ice and oceanic feedbacks at decadal periods

north pacific north atlantic relationships under stratospheric control 1 based on a linear regression correlation analysis of monthly mean atmospheric sea level pressure slp data from the national centers for environmental prediction ncep reanalysis 1948 2000 we find a significant anticorrelation between pressure in the northern north atlantic and north pacific only if the stratospheric circulation is in the strong polar vortex regime but not when the vortex is weak since some general circulation models gcms e g european center hamburg echam4 are biased toward the strong vortex regime svr they tend to reproduce this anticorrelation already in the mean the pattern of the arctic oscillation ao is shown to be consistent with the mean surface pressure differences between the two stratospheric regimes the typical sw ne tilt of the node line of the north atlantic oscillation nao found with linear analyses i e with such that do not take the regime character of the stratospheric circulation into account in northern hemisphere winter is due to a superposition of correlation patterns based on physical processes working in the troposphere a strictly meridional dipole and the pattern resulting from planetary wave refraction in the strong vortex regime and those produced by the rapid transition from one stratospheric regime to the other with subsequent downward propagation of the signal this result underlines the necessity of the application of nonlinear statistics or the restriction of linear statistics to variations in the stable quasi linear environment of natural regimes

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

magnetic state of 10 40 ma old ocean basalts and its implications for natural remanent magnetization abstract the natural remanent magnetization nrm of ocean basalts giving rise to the pattern of marine magnetic anomalies is known to be of comparatively low intensity for about 20 ma old oceanic crust the aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age and to establish a link between magnetomineralogy rock magnetic parameters and the low nrm intensity ocean basalts covering ages from 0 7 to 135 ma were selected for rock magnetic experiments and their room temperature hysteresis parameters curie temperature and temperature dependence of saturation magnetization ms t was determined and complemented by reflected light microscopy the majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age for these a strong dependence of hysteresis parameters on the age of the samples is found the samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 ma coinciding with the age interval of low nrm intensity the observed change in saturation magnetization is in the same order as that for the nrm intensity a further peculiarity of the titanomaghemites from this age interval is the shape of their ms t curves which display a maximum above room temperature neel p type and sometimes a self reversal of magnetization below room temperature neel n type these special rock magnetic properties can be explained by titanomagnetite low temperature oxidation and highly oxidized titanomaghemites in the age interval 10 40 ma a corresponding measurement of the nrm at elevated temperature allows to identify a maximum in nrm intensity above room temperature for the samples in that age interval this provides evidence that the nrm is equally carried by titanomaghemites and that the low nrm intensities for about 20 ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites

relationship between ural siberian blocking and the east asian winter monsoon in relation to the arctic oscillation and the el nino southern oscillation this study attempts to assess the possible linkage between ural siberian blocking and the east asian winter monsoon eawm during the boreal winter the dominance of blocking thermally enhances cold advection downstream the frequent occurrence of ural siberian blocking potentially promotes a cold eawm and vice versa the seasonal blocking activity can be regarded as the combined effect of the arctic oscillation ao and the el nino southern oscillation enso weakened strengthened meridional flow in the positive negative phase of the ao is unfavorable favorable for the formation of blocking highs because the ao shows a close relationship with the north atlantic oscillation nao its teleconnection with ural siberian blocking may exist in the form of an eastward propagating wave train be that as it may the wave train signal across east asia may be disturbed by the external effect of a strong enso event which probably enhances weakens the westerlies near siberia in its warm cold phase consequently the blocking eawm relationship is stronger weaker when the ao and enso are in phase out of phase if both ao and enso attain the positive negative phase the siberian high tends to be weaker stronger and the temperature tends to be higher lower in east asia with less more ural siberian blocking on the other hand if they are out of phase they are not strongly linked to the intensity of the siberian high and the blocking activity over ural siberia is unclear

the preferred structure of variability of the northern hemisphere atmospheric circulation a nonlinear generalisation of principal component analysis pca is applied to the 500mb geopotential height field of the northern hemisphere extratropical atmosphere it is found that the low frequency variability of the mid troposphere is characterised by three distinct quasi stationary states the states are described and compared to those obtained from applications of cluster analyses and linear pca to the height field evidence is provided that modes obtained through pca notably the arctic oscillation ao are not independent dynamical modes of variability of the northern hemisphere extratropics rather they arise as the optimal linear compromise between the preferred quasi stationary states of the circulation

rotational atmospheric circulation during north atlantic european winter the influence of enso the dominant variability modes of the north atlantic european rotational flow are examined by applying a principal component analysis pca eof to the 200 hpa streamfunction mid winter anomalies jan feb monthly means the results reveal that when this norm is used the leading mode eof1 does not correspond to the traditional north atlantic oscillation nao which appears in our analysis as the second leading mode eof2 but is the local manifestation of the leading hemispheric streamfunction eof the regression of this regional mode onto the global sst field exhibits a clear el nino signature with no signal over the atlantic while the associated upper height anomalies resemble the tropical northern hemisphere tnh pattern east of north america this tnh like wavetrain produces a meridional dipole like pattern at lower levels although in some ways this pattern resembles the nao eof2 the dynamics of these two modes are very different in that only eof2 is associated with a latitudinal shift of the north atlantic stormtrack thus the choice of the streamfunction norm in the eof analysis allows the separation of two different phenomena that can produce similar dipolar surface pressure anomalies over the north atlantic but that have different impact on european climate these two modes also differ on their contribution to variability at lower levels while nao eof2 is mostly confined to the north atlantic tnh eof1 has a more annular global character at upper levels nao eof2 also produces a global pattern but with no annular structure reminiscent of the circumglobal teleconnection

crustal magnetization in the south atlantic from inversion of magnetic anomalies a method for inverting magnetic anomalies in the presence of topography parker and huestis 1974 is used to analyze and describe crustal magnetization along 12 profiles in the south atlantic the profiles are approximately 2000 km long and are almost perpendicular to the trend of the magnetic anomalies a bandpass filter is used to remove long wavelength anomalies unassociated with seafloor spreading processes a filter that passes only wavelengths between 200 and 3 km produces a magnetization that approximates the positive and negative aspects of the geomagnetic reversal sequence the positive aspects originating from normally magnetized portions of the crustal source and negative aspects originating from reversely magnetized portions of the crustal source a 6 km thick crust is assumed in order to produce magnetizations consistent with dredged oceanic rocks a long wavelength pattern is evident in the magnetization solutions the highest magnetizations are associated with the ridge crest 1 35 1 74 a m within the western section of the study area a sharp decrease in magnetization occurs within 100 km of the ridge crest 4 5 ma a minimum in magnetization is reached at 400 km from the axis or at 18 ma 0 51 0 57 a m the magnetization then gradually increases out to 1100 km from the ridge axis or 50 ma 0 91 0 93 a m and dramatically decreases to 1900 km or 86 ma 0 34 0 43 a m within the eastern section of the study area a sharp decrease in magnetization occurs within the first 100 km of the ridge crest 5 ma a minimum in the magnetization occurs at approximately 400 km from the ridge axis or at 20 ma 0 54 0 56 a m the magnetization then gradually increases out to 1200 km from the ridge axis or to 60 ma 1 15 1 24 a m an overall pronounced decrease occurs to 1900 km from the ridge axis or to 95 ma 0 36 0 40 a m in order to explain this behavior the field intensity may have changed with a time scale of several millions of years alternatively the higher magnetizations at 50 60 ma could be explained by a uniform intensity of magnetization in a thicker crust

low temperature alteration and magnetic changes of variably altered pillow basalts summary pillow basalt fragments from the east pacific rise dredged during the phoenix expedition often show discoloured rims due to alteration a suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their fe ti oxide mineralogy and rock magnetic properties these large pillow fragments show relatively unaltered grey interiors surrounded by darker concentric halos which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior the discoloured zones are characterized by precipitation of abundant secondary minerals such as fe 3 rich clays that filled vesicle spaces fe ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques the subsamples come from traverses that are parallel to the outer glassy pillow rims allowing us to study the low temperature alteration effects and rock magnetic properties without having to take variable grain size into account not surprisingly titanomaghemites in discoloured rims are in a general sense oxidized to a higher degree z typically 0 55 than those in the relatively unaltered grey interior z typically 0 6 within 200 000 yr the difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z because average ti content can vary from sample to sample curie temperatures are also inaccurate proxies for z i fone wants to study possible correlations between z and rock magnetic parameters the best technique is to determine z for each subsample by using trans

the pacific center of action of the northern hemisphere annular mode real or artifact the leading empirical orthogonal function eof of the sea level pressure slp field referred to as the arctic oscillation ao or northern hemisphere annular mode nam consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458n embedded within the outer ring are centers of action over the euro atlantic and pacific sectors in which slp fluctuates in phase that the observed slp fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the pacific center in the annular mode could be an artifact of eof analysis it is argued that sea level pressure fluctuations at the pacific and euro atlantic centers of action of the ao nam would be more strongly correlated were it not for the fact that slp variability over the north pacific is dominated by a pattern in which fluctuations over the north atlantic and north pacific are inversely related evidence of the coexistence of such a pattern which resembles an augmented version of the pacific north american pattern is presented

the interaction of the madden julian oscillation and the arctic oscillation abstract tropical and extratropical interactions on the intraseasonal time scale are studied in the context of the arctic oscillation ao and the madden julian oscillation mjo to simplify the discussion a high low mjo phase is defined as strong suppressed convective activity over the indian ocean in the northern hemisphere nh winter season a high low ao phase is found more likely coupled with a high low mjo phase based on the regressed patterns and composites of various dynamical fields and quantities possible mechanisms linking the ao and the mjo are examined the analysis indicates that the mjo influence on extratropical circulations seems more evident than the ao influence on tropical circulations the mjo interacts with the ao through meridional dispersion of rossby waves in the pacific sector the geopotential height anomaly center over the north pacific associated with the mjo can either reinforce or offset the ao pacific action center as a result the ao pattern can be greatly a

on the teleconnectivity of the arctic oscillation the term arctic oscillation ao has recently been introduced to describe the leading structure of slp variability over the northern hemisphere a key feature of the ao is its zonally symmetric appearance with a primary center of action over the arctic and opposing anomalies in midlatitudes does the ao s annular appearance result from significant temporal correlations between slp anomalies at distant longitudes the results presented indicate that the temporal coherence between the arctic and midlatitudes is strongest over the atlantic sector with weak correlations between the atlantic and pacific midlatitudes both on intraseasonal and interannual time scales during the past 50 yrs hence the annular character of the ao is more a reflection of the dominance of its arctic center of action than any coordinated behavior of the atlantic and pacific centers of action in the slp field the ao is nearly indistinguishable from the leading structure of variability in the atlantic sector e g the north atlantic oscillation their temporal correlation is 0 95 for monthly data

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

long term variations in magnetization intensity with crustal age in the northeast pacific atlantic and southeast indian oceans the magnetization intensity of oceanic crust in the northeast pacific atlantic and southeast indian oceans was calculated from marine magnetic anomaly data to investigate long term variations in magnetization intensity with crustal age the magnetization intensity was derived by two dimensional inversion of the upward continued marine magnetic anomalies the principal magnetization intensity variations versus age are summarized as follows 1 in all the oceans a high magnetization intensity exists at the ridge crest followed by a sharp decrease within 8 my and a minimum is reached at 8 20 ma 2 in the northeast pacific and atlantic oceans the magnetization intensity gradually increases up to at least 70 ma this variation pattern is consistent with the previous results of nrm measurements of drilled rocks and inversion of marine magnetic anomalies in the south atlantic

volcanic eruptions large scale modes in the northern hemisphere and the el nino southern oscillation abstract the author analyzes the impact of 13 major stratospheric aerosol producing volcanic eruptions since 1870 on the large scale variability modes of sea level pressure in the northern hemisphere winter the paper focuses on the arctic oscillation ao and the north atlantic oscillation nao to address the question about the physical nature of these modes the hypothesis that the phase of the el nino southern oscillation enso may control the geographical extent of the dominant mode in the northern hemisphere is also investigated as well as the related possibility that the impact of the eruptions may be different according to the phase of enso the author finds that both the ao and the nao are excited in the first winter after the eruptions with statistical significance at the 95 level both the signal and the significance are larger for the nao than for the ao the excitation of the ao and the nao is connected with the excitation of a secondary mode which resembles an augmented pacific north ame

annular versus nonannular variability of the northern hemisphere atmospheric circulation abstract the annular variability of the northern winter extratropical circulation is reassessed based on reanalysis data that are dynamically filtered by normal modes one half of the variability of the monthly averaged barotropic zonally symmetric circulation of the northern hemisphere is statistically distinct from the remaining variability and is represented by its leading empirical orthogonal function eof alone the daily time series of the circulation anomalies projected onto the leading eof is highly correlated r 0 7 with the lower stratospheric northern annular mode nam indices showing that annular variability extends from the stratosphere deep into the troposphere however the geopotential and wind anomalies associated with the leading principal component pc1 of the barotropic zonally symmetric circulation are displaced northward relative to the zonal mean anomalies associated with the pc1 of the geopotential height variability at single isobaric tropospheric levels the regression patt

an observed connection between the north atlantic oscillation and the madden julian oscillation abstract based on the bivariate madden julian oscillation mjo index defined by wheeler and hendon and 25 yr 1979 2004 of pentad data the association between the north atlantic oscillation nao and the mjo on the intraseasonal time scale during the northern hemisphere winter season is analyzed time lagged composites and probability analysis of the nao index for different phases of the mjo reveal a statistically significant two way connection between the nao and the tropical convection of the mjo a significant increase of the nao amplitude happens about 5 15 days after the mjo related convection anomaly reaches the tropical indian ocean and western pacific region the development of the nao is associated with a rossby wave train in the upstream pacific and north american region in the atlantic and african sector there is an extratropical influence on the tropical intraseasonal variability certain phases of the mjo are preceded by the occurrence of strong naos a significant change of upper zonal

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

solar cycle modulation of the north atlantic oscillation implication in the spatial structure of the nao 1 the spatial extent of the north atlantic oscillation nao is investigated the results suggest that the spatial structure of the nao differs significantly according to the phase of the solar cycle during the solar maximum phases the nao has a hemispherical structure extending into the stratosphere which is similar to the arctic oscillation ao except for the pacific sector however during the minimum phase the nao is confined to the eastern atlantic sector in the troposphere whether or not the nao is modulated by the solar cycle these results may shed new light on the controversy of the physical reality of the two modes of variability the nao and the ao

interglacial climates and the atlantic meridional overturning circulation is there an arctic controversy arctic palaeorecords are important to understand the natural range of forcing and feedback mechanisms within the context of past and present climate change in this temperature sensitive region a wide array of methods and archives now provide a robust understanding of the holocene climate evolution by comparison rather little is still known about older interglacials and in particular on the effects of the northward propagation of heat transfer via the atlantic meridional ocean circulation amoc into the arctic terrestrial records from this area often indicate a warmer and moister climate during past interglacials than in the holocene implying a more vigorous amoc activity this is in conflict with marine data although recognized as very prominent interglacials in antarctic ice cores cross latitudinal surface ocean temperature reconstructions show that little of the surface ocean warmth still identified in the northeast atlantic during older interglacial peaks e g mis5e 9 11 was further conveyed into the polar latitudes and that each interglacial developed its own specific palaeoclimate features interactive processes between water mass overturning and the hydrological system of the arctic and how both developed together out of a glacial period with its particular ice sheet configuration and relative sea level history determined the efficiency of an evolving interglacial amoc because of that glacial terminations developed some very specific water mass characteristics which also affected the climate evolution of the ensuing interglacial periods moreover the observed contrasts in the arctic directed meridional ocean heat flux between past interglacials have implications for the palaeoclimatic evaluation of this polar region crucial environmental factors of the arctic climate system such as the highly dynamical interactions between deep water mass flow surface ocean temperature salinity sea ice and atmosphere exert strong feedbacks on interglacial climate regionality that goes well beyond the arctic a sound interpretation of such processes from palaeoarchives requires a good understanding of the applied proxies fossils in particular are often key to the reconstruction of past conditions but the tremendously flexible adaptation strategies of biota sometimes hampers further in depth interpretations especially when considering their palaeoenvironmental meaning in the context of rapid palaeoclimatic changes and long term pleistocene evolution

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

reconsideration of the true versus apparent arctic oscillation abstract the physical reality of the arctic oscillation ao or northern annular mode is considered the data used are mainly the monthly mean sea level pressure slp a schematic figure is first presented to illustrate the relationship between the north atlantic oscillation nao pacific north american oscillation pna system and the ao negative correlation mode between the atlantic and the pacific ao ncm system although the nao pna apparent ao ncm and true ao ncm systems give rise to the same eofs the probability density functions for the time coefficients of the two leading modes are different therefore the discrimination of the two systems is possible several pieces of evidence indicate that in the real world the nao pna and the ao ncm are located on almost the same plane in phase space this means that the nao pna and ao ncm systems have the same variations on the plane in common implying that when the nao pna system is real the ao ncm is unlikely to be real simple independent compon

geomagnetic palaeointensities during the cretaceous normal superchron measured using submarine basaltic glass high quality palaeointensity data have been obtained from thellier thellier experiments on recent and cretaceous submarine basaltic glasses whereas the recent samples faithfully yield today s geomagnetic intensity at the site the palaeointensities for the beginning and end of the cretaceous normal superchron are only 45 and 25 respectively of today s value the data thus extend the mesozoic dipole low into the cretaceous superchron and confirm that submarine basaltic glass is an excellent material for palaeointensity studies

magnetization of the oceanic crust thermoremanent magnetization of chemical remanent magnetization we propose a model in which chemical remanent magnetization crm acquired within the first 20 m y of crustal evolution may account for 80 of the bulk natural remanent magnetization of older basalts the crm of the crust is acquired as the original thermoremanent magnetization trm is lost through low temperature alteration the crm intensity and direction are controlled by the posternplacement polarity history this model explains several independent observations concerning the magnetization of the oceanic crust the model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern it also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the cretaceous quiet zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust the model also explains discrepancies in the anomaly skewness parameter observed for anomalies of cretaceous age further our study indicates varying rates of trm decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer

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

gyre scale atmospheric pressure variations and their relation to 19th and 20th century sea level rise 1 most of the long tide gauge records in the north atlantic and north pacific commonly used to estimate global sea level rise and acceleration display a marked difference in behavior in the late 1800 s early 1900 s compared to the latter half of the 20th century the rates of sea level rise tend to be lower in the 19th compared to 20th century we show this behavior may be related to long term gyre scale surface pressure variations similar to those associated with the northern annular mode as sea level pressure increases decreases at decadal and longer timescales at the centers of the subtropical atmospheric gyres sea level trends along the eastern margins in each ocean basin decrease increase this is not an isostatic response the scaling between local surface pressure and sea level at interannual and longer timescales is 3 to 6 times greater than expected by that mechanism rather it appears to be the result of large possibly gyre scale changes in ocean circulation some evidence is also presented indicating slow 2 cm sec westward propagation of sea level changes in the atlantic from the west coast of europe to the east coast of the u s which produce the decadal variability seen there

bridging the annular mode and north atlantic oscillation paradigms 1 the annular nature of the leading patterns of the northern hemisphere winter extratropical circulation variability is revisited the analysis relies on principal component analysis pca of tropospheric geopotential height fields and lagged correlations with the stratospheric polar vortex strength and with a proxy of midlatitude tropospheric zonal mean zonal momentum anomalies results suggest that two processes occurring at different times contribute to the northern annular mode nam spatial structure polar vortex anomalies appear to be associated with midlatitude tropospheric zonal mean zonal wind anomalies occurring before the stratospheric anomalies after the polar vortex anomalies zonal mean zonal wind anomalies of the same sign are observed in the troposphere at high latitudes the timescale separation between the two signals is about 2 weeks it is suggested that the leading tropospheric variability patterns found in the literature represent variability associated with both processes the tropospheric variability patterns which appear to respond to the polar vortex variability have a hemispheric scale but show a dipolar structure only over the atlantic basin the dipole resembles the north atlantic oscillation pattern nao but with the node line shifted northward

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron

multiple regimes and low frequency oscillations in the northern hemisphere s zonal mean flow this paper studies multiple regimes and low frequency oscillations in the northern hemisphere zonal mean zonal flow in winter using 55 yr of daily observational data the probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct statistically significant maxima the two regimes associated with these maxima describe persistent zonal flow states that are characterized by meridional displacements of the midlatitude jet poleward and equatorward of its time mean position the geopotential height anomalies of either regime have a pronounced zonally symmetric component but largest amplitude anomalies are located over the atlantic and pacific oceans high frequency synoptic transients participate in the maintenance of and transitions between these regimes significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal flow time series these oscillations are described by singular spectrum analysis and the multitaper method the 147 day oscillation involves zonal flow anomalies that propagate poleward while the 72 day oscillation only manifests northward propagation in the atlantic sector both modes mainly describe changes in the midlatitude jet position and intensity in the horizontal plane though the two modes exhibit synchronous centers of action located over the atlantic and pacific oceans the two persistent flow regimes are associated with slow phases of either oscillation