a mechanism for the low frequency variability of the mid latitude troposphere low frequency variability of the zonal mean zonal wind in long runs of a simplified global circulation model is dominated by bands of wind anomaly which originate in the tropics and propagate slowly poleward with speeds of 0 2 m s 1 such propagation could be the result of the interaction between the zonal wind anomaly and an equatorward flux of rossby wave activity originating in middle or high latitude a barotropic weakly nonlinear model of this process has been derived it suggests propagation of zonal wind anomalies similar to those produced in the global circulation model and leads to comparable propagation speeds

surface velocity and mass balance of ice streams d and e west antarctica over 75 000 surface velocity measurements are extracted from sequential satellite imagery of ice streams d and e to reveal a complex pattern of flow not apparent from previous measurements horizontal and vertical strain rates calculated from surface velocity indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher ten airborne radar profiles and one surface based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream improved data collection methods increase data density substantially reducing random errors in velocity however systematic errors continue to limit the ability of the flux differencing technique used here to resolve local variations in mass balance nevertheless significant local variations in mass balance are revealed while overall ice streams d and e are in approximate equilibrium an earlier estimate of the net mass balance for ice stream d is improved

multi track extraction of two dimensional surface velocity by the combined use of differential and multiple aperture insar in the amery ice shelf east antarctica abstract glaciers and ice streams drain the ice from the ice sheets to the ice shelves and contribute to global sea level changes therefore ice velocity is an essential parameter for the assessment of mass balance in this paper we propose an approach for the multi track extraction of two dimensional ice surface velocity by the combined use of multiple aperture interferometry mai and differential synthetic aperture radar interferometry dinsar the azimuth and ground range ice movements of each individual track are first estimated by the use of mai and dinsar respectively a seamless two dimensional ice velocity map is then achieved by calibration and mosaicking of the estimated velocity of the multi temporal and multi track measurements over a large area finally the dynamic behavior of the amery ice shelf is analyzed by formulating a long term series of ice flow velocities consisting of historical surface velocities 1968 1990 the estimated surface velocity by the proposed method with ers 1 2 data 1996 existing surface velocity products such as the radarsat 1 antarctic mapping project ramp velocities 1997 2000 and rignot s results 2007 2010 as well as the estimated velocities obtained by the use of the offset tracking method from landsat 8 images 2013 2014 the results show that the estimated velocity obtained by the use of the proposed method is superior to the other methods and the long term series of ice velocity measurements shows that the ice velocity in the amery ice shelf has hardly changed over the past 45 years 1968 2014 we conclude that the ice flow velocity of the amery ice shelf has showed a slight deceleration over the past 45 years

application of image cross correlation to the measurement of glacier velocity using satellite image data abstract image to image cross correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice this technique uses small scale glacial surface features such as crevasse scars and snow dunes as markers on the surface of the moving ice displacements of the surface features are mapped by selecting small image areas centered on distinct features or by dividing a large area of densely featured glacial surface into a grid of areas and searching a subsequent image for matching areas using a cross correlation algorithm interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy resulting in very precise velocity measurements cross correlation is also applied to provide image coregistration in areas devoid of bedrock exposures in such areas subtle large scale topographic undulations in the ice surface related to underlying bedrock structure may be correlated by using large image areas and low pass filtered images both types of applications are demonstrated using ice stream d and ice stream e in west antarctica as test areas a high resolution map of the velocity field of the central portion of ice stream e generated by the displacement measuring technique is presented the use of cross correlation software is a significant improvement over previous manually based photogrammetric methods for velocity measurement and is far more cost effective than in situ methods in remote polar areas

magmatic unconformities and stratigraphic relations in the peridotite zone stillwater complex montana new stratigraphic relations between cumulate layers and units are documented in the peridotite zone of the stillwater complex a late archean mafic to ultramafic layered intrusion exposed along the northern edge of the beartooth uplift montana first low angle unconformities occur between all types of successive stratigraphic units and at all scales within the peridotite zone second new stratigraphic units and divisions based on the unconformities and type of cumulate layering within the peridotite zone are proposed and are correlated with published stratigraphic columns the stratigraphic units identified on the basis of rock type s and contacts with adjacent units from smallest to largest are cyclic units multicyclic units megacyclic groups and subzones the three larger units are usually bounded by low angle magmatic unconformities the three subzones and five of the six megacyclic groups identified in the peridotite zone can be traced over a distance of 17 7 mi 28 3 km the stratigraphic relations in the peridotite zone indicate that low angle crosscutting magmatic unconformities occur at all scales and that lateral variations within and between the identified stratigraphic units are common the relations suggest an extremely dynamic and rapidly changing environment during formation of the peridotite zone overprinted on the stratigraphic relations is a larger evolution of cumulus rock types in the peridotite zone from more primitive stratigraphic units dominated by olivine and olivine bronzite cumulates to more evolved stratigraphic units dominated by bronzite and bronzite olivine cumulates several major magmatic or possibly structural events appear to have affected the peridotite zone during its formation as indicated by thickness variation patterns in megacyclic groups and subzones the thickest massive chromite layers in the peridotite zone are associated with two of the major magmatic events

satellite image derived velocity field of an antarctic ice stream the surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery variations of ice velocity are spatially related to surface undulations and transverse velocity variations of up to 30 percent occur such large variations negate the concept of plug flow and call into question earlier mass balance calculations for this and other ice streams where sparse velocity data were used the coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross correlation of image windows provide significant improvements in the use of satellite imagery for ice flow determination

influence of tides and tidal current on mertz glacier antarctica mertz glacier east antarctica is characterized by a 140 km long 25 km wide floating ice tongue in this paper we combine a large number of remotely sensed datasets including in situ global positioning system measurements satellite radar al timetry airborne radio echo sounding and satellite synthetic aperture radar imagery and interferometry these various datasets allow us to study the interaction of the ice tongue with the tides and currents however the inverse barometer effect needs to be applied to sea level variations affecting the tongue we find that the tide induced currents exert a small lateral pressure on the tongue which when integrated over the large surface of the tongue induce a flexure of up to 2 m amplitude per day simple elastic modelling of the flexure confirms the observations and helps validate the boundary conditions necessary to explain different eastward and westward tongue deflections in addition the along flow velocity of the tongue does vary daily from 1 9 to 6 8 m d 1 depending on the tidal current when the current pushes the tongue toward the eastern boundary of the fjord the tongue is retarded by the drag and the velocity decreases the accumulated stress is released allowing the tongue to flow very rapidly when the current pushes the tongue westward these forcing and boundary conditions on the floating ice flow are important and must be taken into account when studying glacier discharge and calving

glacier crevasses observations models and mass balance implications we review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses as well as the three broad classes of numerical models now employed to simulate crevasse fracture the relatively new insight that mixed mode fracture in local stress equilibrium rather than downstream advection alone can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies in the past three decades there have been tremendous advances in the spatial resolution of satellite imagery as well as fully automated algorithms capable of tracking crevasse displacements between repeat images despite considerable advances in developing fully transient three dimensional ice flow models over the past two decades both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time in the past decade however multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged the combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large scale ice sheet models finally given the broad interest in the sea level rise implications of recent and future cryospheric change we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance

greenland ice sheet surface properties and ice dynamics from ers 1 sar imagery c band synthetic aperture radar sar imagery from the european space agency s ers 1 satellite reveals the basic zonation of the surface of the greenland ice sheet the zones have backscatter signatures related to the structure of the snowpack which varies with the balance of accumulation and melt at various elevations the boundaries of zones can be accurately located with the use of this high resolution imagery the images also reveal a large flow feature in northeast greenland that is similar to ice streams in antarctica and may play a major role in the discharge of ice from the ice sheet

indices of the southern hemisphere zonal wind abstract the time and spatial variation of the southern hemisphere 500 hpa zonal wind has been examined using spectral analysis bandpass filtering and empirical orthogonal function eof analysis applied to a 15 year sample of australian hemispheric analyses spectral analysis of the entire 15 year record of the 500 hpa zonal mean westerly wind shows significant peaks only on the seasonal and interannual time scales the mean segmented spectra for each season failed to show any additional significant peaks at higher frequencies or any significant differences between seasons the primary pattern of variation revealed by eof analysis shows similar behavior on synoptic to seasonal time scales with compensating departures occurring at intervals of 20 latitude the first eof has opposing peaks near 40 and 60 s and shows an equivalent barotropic pattern when correlated with individual grid point values at 500 hpa and mean sea level pressure correlations of the zonal pattern with regional anomalies reveal

a method for sub pixel scale feature tracking using radarsat images applied to the mertz glacier tongue east antarctica abstract tracking features between pairs of satellite synthetic aperture radar sar images using image cross correlation methods is a powerful tool for defining regions of image movement such as ice flow of antarctic glaciers unlike optical images processed sar images have various instrumental effects such as view angle distortions and lateral displacement errors caused by slant range mapping co registration issues are problematic and need to be addressed if there are few no suitable stationary tie points at or near sea level within the images methods are presented that use radarsat scansar images to reliably measure sub pixel displacements with errors of 0 25 pixels but this estimate is target dependent the techniques and issues are illustrated using examples from a long term study of the mertz glacier tongue in east antarctica for image pairs only a few days apart this extensive structure forms an excellent grid like test pattern which provides up to 5000 valid correlation matches images separated by 100 s of days can then be processed in the same way to reveal many subtle and some large changes over time the technique described is robust and may be widely applicable

preferred modes of variability and their relationship with climate change spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation thompson et al this study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal mean flow response to external heat perturbations each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating h and high latitude cooling c defining the annular mode as the first empirical orthogonal function eof1 of zonal mean tropospheric zonal wind it is found that the climate predictability is generally high in the small c large h region of the parameter space but is markedly low in the large c small h region in the former region eof1 represents meridional meandering of the midlatitude jet while in the latter region eof1 and eof2 combine to represent coherent poleward propagation of zonal mean flow anomalies it is also found that the climate predictability tends to be higher with respect to changes in c than to changes in h the implications of these findings for the southern hemisphere climate predictability are also presented

wave zonal flow interaction and ultra low frequency variability in a simplified global circulation model the interactions between baroclinic waves and the zonal mean mid latitude flow are investigated using a multilevel primitive equation model of moderate resolution with idealized surface friction and diabatic heating terms integrations corresponding to 100 years were performed under perpetual solstice conditions empirical orthogonal function eof analyses were carried out on the zonal mean zonal wind u adjustments to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two eof patterns which exhibit substantial interannual variability for a single lifecycle such adjustments take the form of a circuit of a phase space whose axes are defined by the amplitudes of the two eofs the cumulative effects of multiple lifecycles lead to a low frequency circulation in the phase space with a timescale of about 150 days we suggest that this effect may be intrinsically related to the internal generation of ultra low frequency variability in the model normal mode lifecycle calculations including dissipation reveal similar phase space properties the growth rates of normal mode disturbances are shown to be highly sensitive to the phase of the first eof

spatial variability of sea level pressure and 500 mb height anomalies over the southern hemisphere abstract the spatial variability of mean sea level pressure slp and 500 mb height anomalies over the southern hemisphere during summer djf and winter jja is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979 the patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data and the eigenvectors are compared between the seasons and to those found for the northern hemisphere the first eigenvector shows that midlatitude anomalies of slp and 500 mb height are of an opposite sign to those found over and around antarctica the pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts the 500 mb height differences are calculated for five midlatitude to antarctic station pairs using data from the late 1950 s onward these latitudinal height differences are also used to describe the association between the hemispheric westerlies and the southern oscil

examining the interaction between multi year landfast sea ice and the mertz glacier tongue east antarctica another factor in ice sheet stability the mertz glacier tongue mgt east antarctica has a large area of multi year fast sea ice myfi attached to its eastern edge we use various satellite data sets to study the extent age and thickness of the myfi and how it interacts with the mgt we estimate its age to be at least 25 years and its thickness to be 10 55 m this is an order of magnitude thicker than the average regional sea ice thickness and too thick to be formed through sea ice growth alone we speculate that the most plausible process for its growth after initial formation is marine frazil ice accretion the satellite data provide two types of evidence for strong mechanical coupling between the two types of ice the myfi moves with the mgt and persistent rifts that originate in the mgt continue to propagate for large distances into the myfi the area of myfi decreased by 50 following the departure of two large tabular icebergs that acted as pinning points and protective barriers future myfi extent will be affected by subsequent icebergs from the ninnis glacier and the imminent calving of the mgt fast ice is vulnerable to changing atmospheric and oceanic conditions and its disappearance may have an influence on ice tongue ice shelf stability understanding the influence of thick myfi on floating ice tongues ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change and thus to predicting the future of the antarctic ice sheet

semi automated feature tracking of east antarctic sea ice from envisat asar imagery while feature tracking of sea ice using cross correlation methods on pairs of satellite synthetic aperture radar sar images has been extensively carried out in the arctic this is not the case in the antarctic this is due to the dynamic nature of antarctic pack ice its microwave signature the tendency for sar swath paths to be poorly aligned with the often narrow sea ice zone around the continent and inadequate satellite sampling a semi automated system known as ipads imcorr imagecorrelation processing analysis and display system has been developed to map fast ice and pack ice in antarctica using multiple pairs of sar images the software processing pipeline uses overlapping image pairs which are geocoded and roughly registered using only data contained in the image headers next fast ice maps are rapidly generated using zero motion features located within ocean regions this also provides precise image registration finally the same image pairs are re examined for pack ice motion in a slower off line batch process the pack and fast ice are identified using a cluster based search method which compares both location and motion information each image pair generates a netcdf file which adds to a growing database of antarctic sea ice motion and ice roughness five image pair examples are presented to illustrate the methods used as well as their strengths and limitations substantial pack ice motion can often be detected in the marginal ice zone on sar images only a few days apart

remote sensing of ice motion in antarctica a review abstract the antarctic ice sheet holds 91 of the global ice mass making it the biggest potential contributor to global sea level rise in order to evaluate the ice sheet s present and future response to global climate change detailed information about spatial and temporal ice mass dynamics is required while ice sheet surface mass balance can be estimated from regional climate models ice discharge is often determined from spaceborne measurements of ice thickness and ice motion at selected flux gates the retrieval of ice motion is not only critical for analysing the rate of ice transport into the ocean but also for investigating the spatial and temporal pattern or the driving forcings of antarctic ice flow dynamics this study provides a comprehensive review of state of the art spaceborne ice velocity measurements in antarctica based on 201 scientific papers published over the last three decades our analysis revealed a dominant interest in local and regional scale investigations yet the launch of the ers 1 2 and radarsat 1 sar satellites in the 1990s dramatically increased data coverage over the antarctic continent and enabled the first circum antarctic velocity product to be published in the late 2000s in recent years the improved imaging capabilities of landsat 8 led to a shift towards using mainly optical satellite data and enabled great advances in circum antarctic velocity mapping data from the currently operating sentinel 1 2 missions was applied in fewer studies but will play a crucial role in future ice motion studies e g as part of esa cci projects in addition we found a growing interest in high temporal resolution velocity change analyses based on the reviewed studies we moreover summarized the highly dynamic and locally variable flow pattern over the antarctic continent being in accordance with varying local and regional driving mechanisms despite the great advances in spaceborne ice velocity mapping in antarctica we report some major drawbacks and discuss future challenges and requirements to start with our study uncovered strong variations regarding the spatial availability of investigations while we found an aggregation of studies over selected glaciers we report a major lack of studies in all three antarctic sectors additionally many velocity datasets were created with strongly heterogeneous output parameters and were provided at coarse temporal resolution or at point based measurement locations only future studies should consequently aim at creating more uniform ice motion products at defined spatial and temporal coverage and resolution standards and at analysing particularly the least studied glaciers

deformation time series generation in areas characterized by large displacement dynamics the sar amplitude pixel offset sbas technique we exploit the amplitude information of a sequence of synthetic aperture radar sar images acquired at different times in order to generate displacement time series in areas characterized by large and or rapid deformation the size of which is on the order of the image s pixel dimensions we follow the same rationale of the small baseline subset sbas differential sar interferometry dinsar approach by coupling the available sar images into pairs characterized by a small separation between the acquisition orbits we exploit the amplitudes of the selected image pairs in order to calculate the relative across track range and along track azimuth pixel offsets po finally we apply the sbas inversion strategy to retrieve the range and azimuth displacement time series this approach referred to as pixel offset po sbas technique has been applied to a set of 25 envisat sar observations of the sierra negra caldera galapagos islands spanning the 2003 2007 time interval the retrieved deformation time series show the capability of the technique to detect and measure the large displacements affecting the inner part of the caldera that in correspondence to the october 2005 eruption reached several meters moreover by comparing the po sbas results to continuous gps measurements we estimate that the accuracy of the po sbas time series is on the order of 1 30th of a pixel for both range and azimuth directions

widespread acceleration of tidewater glaciers on the antarctic peninsula over the last half century the antarctic peninsula ap has been among the most rapidly warming regions on earth this has led to increased summer snowmelt loss of ice shelves and retreat of 87 of marine and tidewater glacier fronts tidewater glacier flow is sensitive to changes in basal water supply and to thinning of the terminus and faster flow leads directly to sea level rise the flow rates of most ap tidewater glaciers have never been measured however and hence their dynamic response to the recent changes is unknown we present repeated flow rate measurements from over 300 glaciers on the ap west coast through nine summers from 1992 to 2005 we show that the flow rate increased by similar to 12 on average and that this trend is greater than the seasonal variability in flow rate we attribute this widespread acceleration trend not to meltwater enhanced lubrication or increased snowfall but to a dynamic response to frontal thinning we estimate that as a result the annual sea level contribution from this region has increased by 0 047 0 011 mm between 1993 and 2003 this contribution together with previous studies that assessed increased runoff from the area and acceleration of glaciers resulting from the removal of ice shelves implies a combined ap contribution of 0 16 0 06 mm yr 1 this is comparable to the contribution from alaskan glaciers and combined with estimated mass loss from west antarctica is probably large enough to outweigh mass gains in east antarctica and to make the total antarctic sea level contribution positive

a baroclinic mechanism for the eddy feedback on the zonal index a baroclinic mechanism for the positive eddy feedback on the zonal index is proposed the author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies the transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation but over time the action of surface drag leads to enhanced baroclinicity at these latitudes the steady state response has positive baroclinicity at the latitude of eddy generation if the eddies propagate away from this latitude before they dissipate this reinforcement of the low level baroclinicity provides a positive feedback if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low level baroclinicity the proposed mechanism explains observed and modeled features of zonal index variations the frequency and drag dependence of eddy feedback the bandedness in latitude of zonal wind variations and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time

the mertz glacier tongue east antarctica changes in the past 100 years and its cyclic nature past present and future the mertz glacier tongue mgt in east antarctica has been studied since 1911 early expeditions produced maps using ground or ship based observations followed later by vertical and or oblique aerial photography from aircraft in the modern era extensive digital satellite imagery is available which has also been supplemented by the resurrection and scanning of some historic u s now declassified film based satellite images much of the mgt became detached and drifted away following the collision by the b 9b iceberg in february 2010 and a similar sequence or extension detachment cycle must have occurred some time after mawson s 1911 1914 observations all the available information on the position shape or appearance of the mgt has been re examined in an attempt to comprehensively study its past present and possible future motion feature tracking cross correlation methods have been applied for suitably detailed image pairs to accurately measure the mgt advance velocity the derived mean rate for 1947 2010 is 1180 14 m y 1 with an accompanying ice front loss rate of 190 m y 1 a simple model for the mgt exhibits a 73 year quasi periodic cycle of rebirth growth and demise which will affect the volume of bottom water produced in the nearby polynya some evidence for this cyclic oceanographic change has recently been reported from regional sediment data somewhat speculatively the model suggests a date of 1937 for the previous mgt break off and around or before 2083 for the next with the mgt being primed to break off every 73 years the precise date being governed by unpredictable external events such as collisions by large icebergs such cycles have probably occurred for many thousands of years

fast ice distribution in east antarctica during 1997 and 1999 determined using radarsat data 1 we present the first detailed maps of fast ice around east antarctica 75 e 170 e using an image correlation technique applied to radarsat scansar images from november in 1997 and 1999 this method is based upon searching for and distinguishing correlated regions of the ice covered ocean which remain stationary in contrast to adjacent moving pack ice within the overlapping longitudinal range of 86 e 150 6 e the total fast ice area is 141 450 km2 in 1997 and 152 216 km2 in 1999 calibrated radar backscatter data are also used to determine the distribution of two fast ice classes based on their surface roughness characteristics these are smooth fast ice 25 4 db to 13 5 db and rough fast ice 13 5 db to 2 5 db the former comprises 67 of the total area with rough fast ice making up the remaining 33 an estimate is made of fast ice volume on the basis of fast ice type as a proxy measure of ice thickness and area results suggest that although fast ice forms 2 16 of the total november sea ice area for this sector of east antarctica in 1997 and 1999 average 8 3 across maps it may comprise 6 57 of the total ice volume average 28 across maps grounded icebergs play a key role in fast ice distribution in all regions apart from 150 e 170 e these are snapshot estimates only and more work is required to determine longer term spatiotemporal variability

zonal flow vacillation and eddy forcing in a simple gcm of the atmosphere abstract zonal flow vacillation with very long time scales is observed in a 3070 day simple gcm simulation with zonally symmetric forcing the long lasting zonal wind anomalies suggest that zonal flow vacillation is self maintained wave mean flow interactions are investigated by composite analysis and transform eulerian momentum budget analysis nonlinear life cycle simulations are conducted to demonstrate that each extreme phase of the zonal flow vacillation is a quasi stable state and is self maintained by the embedded synoptic eddies the firm eof mode of zonal mean wind shows an out of phase relation between anomalies at 60 s and at 40 s with a barotropic structure this structure is similar to the dominant vacillation pattern observed in the southern hemisphere the composite jet stream in the high low index phase of zonal flow vacillation shifts poleward equatorward from the time mean location and becomes broader narrower and weaker stronger composite eddies in the high index phase tilt nw

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

mechanisms of hemispherically symmetric climate variability inspired by paleoclimate evidence that much past climate change has been symmetric about the equator the causes of hemispherically symmetric variability in the recent observational record are examined using the national centers for environmental prediction national center for atmospheric research reanalysis dataset and numerical models it was found that the dominant cause of hemispherically symmetric variability is the el nino southern oscillation during an el nino event the tropics warm at all longitudes and the subtropical jets in both hemi spheres strengthen on their equatorward flanks poleward of the tropical warming there are latitude belts of marked cooling extending from the surface to the tropopause in both hemispheres at all longitudes and in all seasons the midlatitude cooling is caused by changes in the eddy driven mean meridional circulation changes in the transient eddy momentum fluxes during an el nino event force upper tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the coriolis torque the eddy driven ascent causes anomalous adiabatic cooling which is primarily balanced by anomalous diabatic heating using a quasigeostrophic spherical model forced by an imposed surface eddy disturbance of chosen wave number and frequency it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude height plane of transient eddies changes in zonal winds and associated changes in the meridional gradient of potential vorticity create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward tropical forcing of variability in the eddy driven mean meridional circulation is another way in addition to rossby wave teleconnections whereby the tropics can influence extratropical climate unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle