an empirical model of tropical ocean dynamics to extend the linear stochastically forced paradigm of tropical sea surface temperature sst variability to the subsurface ocean a linear inverse model lim is constructed from the simultaneous and 3 month lag covariances of observed 3 month running mean anomalies of sst thermocline depth and zonal wind stress this lim is then used to identify the empirically determined linear dynamics with physical processes to gauge their relative importance to enso evolution optimal growth of sst anomalies over several months is triggered by both an initial sst anomaly and a central equatorial pacific thermocline anomaly that propagates slowly eastward while leading the amplifying sst anomaly the initial sst and thermocline anomalies each produce roughly half the sst amplification if interactions between the sea surface and the thermocline are removed in the linear dynamical operator the sst anomaly undergoes less optimal growth but is also more persistent and its location shifts from the eastern to central pacific optimal growth is also found to be essentially the result of two stable eigenmodes with similar structure but differing 2 and 4 year periods evolving from initial destructive to constructive interference variations among enso events could then be a consequence not of changing stability characteristics but of random excitation of these two eigenmodes which represent different balances between surface and subsurface coupled dynamics as found in previous studies the impact of the additional variables on lim sst forecasts is relatively small for short time scales over time intervals greater than about 9 months however the additional variables both significantly enhance forecast skill and predict lag covariances and associated power spectra whose closer agreement with observations enhances the validation of the linear model moreover a secondary type of optimal growth exists that is not present in a lim constructed from sst alone in which initial sst anomalies in the southwest tropical pacific and indian ocean play a larger role than on shorter time scales apparently driving sustained off equatorial wind stress anomalies in the eastern pacific that result in a more persistent equatorial thermocline anomaly and a more protracted and predictable enso event

decadal changes in the south pacific western boundary current system revealed in observations and ocean state estimates observations and ocean state estimates are used to investigate the nature and mechanism of decadal variability in the east australian current eac system and south pacific subtropical gyre a 62 year record on the tasmanian continental shelf shows decadal variations of temperature and salinity as well as a long term trend which has been related to wind driven variations in the poleward extension of the eac repeat expendable bathythermograph lines spanning the last 15 years suggest that low frequency variations in the transport of the eac extension and tasman front are anticorrelated but the time series are too short to draw firm conclusions here we use two ocean state estimates spanning the past 50 years to diagnose the physical mechanisms and spatial structure of the decadal variability of the south pacific subtropical gyre the observations and state estimates paint a consistent picture of the decadal variability of the gyre and eac system strengthening of the basin wide wind stress curl drives a southward expansion of the subtropical gyre as the gyre shifts south the eac extension pathway is favored at the expense of the tasman front resulting in the observed anticorrelation of the these two major currents the results suggest that the subtropical gyre and western boundary current respond to decadal variability in basin scale wind stress curl consistent with island rule dynamics that strong decadal variability of the south pacific gyre complicates efforts to infer trends from short term records and that wind stress curl changes over the south pacific basin drive changes in the eac system that are likely to have implications for marine ecosystems and regional climate

modeling bed load rates from fine grain size patches during small floods in a gravel bed river this study investigates the applicability of five bed load transport formulas the meyer peter and muller schoklitsch bagnold smart and jaeggi and rickenmann equations to predict bed load transport rates of frequent low magnitude flood events maximal bankfull discharge for a mountainous poorly sorted gravel bed river characterized by a bimodal sediment size distribution and spatially distributed patches for model parametrization special emphasis was placed on the spatial composition of the grain size distribution gsd to evaluate the impact of preferential removal of sediments from patches with finer sediments on bed load transport three parametrization approaches to the choice of an appropriate sediment size that considered the apparent bimodality of the gsd to varying degrees were tested the modeling study demonstrated that the incorporation of spatial structure of gsd and its bimodal character has an important impact on model performance a unimodal parametrization failed to reproduce meas

kuroshio extension variability and forcing of the pacific decadal oscillations responses and potential feedback abstract a forcing mechanism is sought for the large scale circulation changes in the kuroshio extension region of the western north pacific ocean as inferred by topex poseidon sea surface height ssh data the low frequency signal of the kuroshio extension over the last decade was characterized by a modulation in its zonal mean flow intensity the mean kuroshio extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997 the ability to simulate the major trends in the observed ssh signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern north pacific ocean related to the pacific decadal oscillations pdos to be specific the weakening strengthening trend in 1993 96 1997 2001 was caused by westward expansions of negative positive ssh anomalies south of the kuroshio extension and positive negative ssh anomalies north of the kuroshio extension emerg

a general power equation for predicting bed load transport rates in gravel bed rivers 1 a variety of formulae has been developed to predict bed load transport in gravel bed rivers ranging from simple regressions to complex multiparameter formulations the ability to test these formulae across numerous field sites has until recently been hampered by a paucity of bed load transport data for gravel bed rivers we use 2104 bed load transport observations in 24 gravel bed rivers in idaho to assess the performance of eight different formulations of four bed load transport equations results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity however formulae containing a transport threshold typically exhibit poor performance furthermore we find that the transport data are best described by a simple power function of discharge from this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function we find that the exponent is principally a factor of supply related channel armoring transport capacity in excess of sediment supply whereas the coefficient is related to drainage area a surrogate for absolute sediment supply we evaluate the accuracy of the proposed power function at 17 independent test sites

storm tracks in the southern hemisphere subtropical oceans ocean storm tracks have previously been associated with the midlatitude western boundary currents wbcs and the antarctic circumpolar current acc here we identify and examine large scale baroclinically unstable waves occurring within waveguides associated with potential density gradients in the subtropical regions of the southern hemisphere sh oceans where the trade winds and westerlies meet and at depths associated with mode water formation in contrast to the northern hemisphere subtropics the sh pathways are more extensive allowing large scale coherent disturbances to communicate information westward from the midlatitudes to the subtropics south pacific ocean and from the subtropics to the tropics indian ocean particular consideration is given to the subtropical south pacific ocean as this is a region where resonant interactions between large scale rossby waves and significant topographic features have been reported to occur using an ocean general circulation model and a simple potential energy transfer diagnostic we identify the relevant nonlinearly modified structures comparing their propagation characteristics to planetary rossby waves calculated using a shallow water model although at first appearance baroclinic disturbances resemble planetary rossby waves we show they are inherently nonlinear multiscale and are amplified where topography occurs the location of the disturbances coincides with regions of high variability in sea surface height observed in satellite altimetry and their speeds closely match the large scale coherent westward propagating structures described in the observational literature our study provides evidence that in addition to the midlatitude wbcs and the acc significant ocean storm tracks are also manifest in the sh subtropics

relationships between low frequency variability in the southern hemisphere and sea surface temperature anomalies abstract long term trends and interannual variations of circulation anomalies in the southern hemisphere are examined using the national centers for environmental prediction national center for atmospheric research reanalysis from 1949 to 1998 the changes in planetary circulation regimes are linked to global sea surface temperature anomalies sstas empirical orthogonal function eof analysis was performed on 500 hpa height anomalies the leading mode eof1 shows a strong zonal symmetry with a phase reversal between height anomalies in high and midlatitudes apart from zonal symmetry a zonal wavenumber 3 is evident with three centers located in three southern oceans in the low frequency band with fluctuations longer than 60 months eof1 is associated with the second rotated eof mode of sstas with positive loadings over three southern oceans and negative loadings in the north pacific and the north atlantic the next two modes are the pacific south american psa patterns they depict wave 3 patterns in

relative influences of the interdecadal pacific oscillation and enso on the south pacific convergence zone 1 the south pacific convergence zone spcz one of the most extensive features of the global atmospheric circulation is shown to vary its location according to both the polarity of the el nino southern oscillation enso and of the interdecadal pacific oscillation ipo we first demonstrate that the ipo can be regarded as the quasi symmetric pacific wide manifestation of the pacific decadal oscillation that has been described for the north pacific shifts in the position of the spcz related to enso on interannual time scales and to the ipo on decadal time scales appear to be of similar magnitude and are largely linearly independent a station pressure based index of variations in spcz latitude is shown to be significantly related to the polarity of the ipo when enso influences are accounted for movements of this sensitive section of the spcz have occurred in phase with those of the ipo since the 1890s

response of bed surface patchiness to reductions in sediment supply 1 river beds are often arranged into patches of similar grain size and sorting patches can be distinguished into free patches which are zones of sorted material that move freely such as bed load sheets forced patches which are areas of sorting forced by topographic controls and fixed patches of bed material rendered immobile through localized coarsening that remain fairly persistent through time two sets of flume experiments one using bimodal sand rich sediment and the other using unimodal sand free sediment are used to explore how fixed and free patches respond to stepwise reductions in sediment supply at high sediment supply migrating bed load sheets formed even in unimodal sand free sediment yet grain interactions visibly played a central role in their formation in both sets of experiments reductions in supply led to the development of fixed coarse patches which expanded at the expense of finer more mobile patches narrowing the zone of active bed load transport and leading to the eventual disappearance of migrating bed load sheets reductions in sediment supply decreased the migration rate of bed load sheets and increased the spacing between successive sheets one dimensional morphodynamic models of river channel beds generally are not designed to capture the observed variability but should be capable of capturing the time averaged character of the channel when applied to our experiments a 1 d morphodynamic model rte bookagdegnormgravmixpw xls predicted the bed load flux well but overpredicted slope changes and was unable to predict the substantial variability in bed load flux and load grain size because of the migration of mobile patches our results suggest that 1 the distribution of free and fixed patches is primarily a function of sediment supply 2 the dynamics of bed load sheets are primarily scaled by sediment supply 3 channels with reduced sediment supply may inherently be unable to transport sediment uniformly across their width and 4 cross stream variability in shear stress and grain size can produce potentially large errors in width averaged sediment flux calculations

a surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity existing cover models impose different unique relationships between cover fraction and reach scale transport variables e g slope discharge sediment supply and grain size individually these models cannot predict the range of cover behaviors observed experimentally suggesting that additional variables may be important i propose a 1 d model in which bedrock surface roughness is a control on partial alluvial cover in the model surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity the roughness of the combined bedrock alluvial bed surface varies with the cover fraction model trends are reasonably consistent with previous experiments over ranges of roughness slope and shear stress when bedrock roughness is much greater than grain size cover increases approximately linearly with the ratio of sediment supply to transport capacity when bedrock roughness is much less than the sediment diameter the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover this runaway alluviation occurs for model solutions that are unstable to perturbations in supply which is the case when transport capacity decreases as alluvial cover increases in addition a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux dependent erosion at landscape scales

sediment supply grain protrusion and bedload transport in mountain streams 1 steep streams occupy a large fraction of mountainous drainage basins and partially control the sediment supplied to downstream rivers in these channels sediment transport equations typically over predict bedload flux by several orders of magnitude because they do not account for sediment supply limited conditions thus accurate predictions of bedload flux require an estimate of the sediment available for transport in a given event we demonstrate through field measurements that boulder step protrusion is a proxy for sediment availability protrusion is also a function of the time elapsed since an extreme event and this simple relationship can be used to estimate the relative sediment availability at any given time in addition bedload transport predictions in a steep channel were only accurate if they included this variable protrusion predictions of sedimentation hazards water quality river restoration success long term channel network evolution and channel stability may therefore require estimates of sediment availability for transport

interannual and decadal temperature variability in the southwest pacific ocean between 1955 and 1988 abstract the spatial and temporal variability of the southwest pacific ocean is examined with the aim of describing the physical processes operating on interannual and decadal timescales the study takes advantage of a new temperature atlas of the upper 450 m of the southwest pacific ocean obtained from 40 000 bathythermograph profiles between 1955 and 1988 rotated principal components analysis was used to filter the important spatial and temporal scales of temperature variability in the data three different analyses are presented they include two intraocean analyses and a joint analysis of subsurface ocean temperature sea level pressure and surface winds the dominant el nino mode describes the large vertical excursions of the thermocline in the western tropical pacific in response to atmospheric forcing at a 3 6 month lag more importantly most of the retained modes outside of the equatorial region have time variations that correlate with el nino one ocean mode with a spatial pattern represen

enso to multi decadal time scale changes in east australian current transports and fort denison sea level oceanic rossby waves as the connecting mechanism the connection between east australian current eac transport variability and australia s east coast sea level has received little treatment in the literature this is due in part to the complex interacting physical processes operating in the coastal zone combined with the sparsity of observations available to improve our understanding of these possible connections this study demonstrates a statistically significant at the 490 level relationship between interannual to decadal time scale variations in observed estimates of the eac transport changes and east coast sea level measured at the high quality long record fort denison tide gauge in sydney harbour australia 3315101800s 15111303200e we further demonstrate using a linear reduced gravity ocean model that enso to decadal time scale variations and the ocean adjusted multi decadal trend approx 1 cm decade in observed sea level at fort denison are strongly connected to modulations of eac transports by incoming westward propagating oceanic rossby waves we show that eac transport and fort denison sea level vary in a manner expected from both tasman sea generated rossby waves which account for the interannual and multi annual variability and remotely forced from east of new zealand rossby wave connections through the mid latitudes accounting for the ocean adjusted multi decadal trend observed at the new south wales coast with the regional tasman sea forcing explaining the greatest overall proportion of eac transport and sea level variances

bedload transport in the instrumented catchment of the rio cordon part ii analysis of the bedload rate abstract sediment transport is monitored by a specially designed instrumentation installed on a steep mountain streams the rio cordon the variability of bedload rate and yield reflects the variability in flow discharge bedload and flow data for ordinary low peak flow floods and exceptional sediment transport events are used to test the range of application of the bedload equations proposed for high gradient streams such equations overestimate bedload rate for ordinary events while their reliability increases when used to predict high bedload transport rates like those recorded during the exceptional flood that affected rio cordon on september 14 1994 the best agreement between computed and measured values of cumulated bedload volumes is found by using the bagnold bagnold r a 1956 the flow of cohesionless grains in fluids philos trans r soc london 249a 235 297 smart and jaeggi smart j m jaeggi m n r 1983 sediment transport on steep slopes mitteil 64 versuchsanstalt fur wasserbau hydrologie und glaziologie eth zurich switzerland 191 pp and rickenmann rickenmann d 1991 hyperconcentrated flow and sediment transport at steep slopes j hydraul eng zurig 117 11 1419 1439 formulas the schoklitsch equation schoklitsch a 1962 handbuch des wasserbaues 3rd edn springer wien also provides good estimations for instantaneous bedload rates at high sediment transport intensity and unlimited sediment supply conditions the formal type of the last equation is still valid and can be calibrated for low intensity bedload rates

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

interannual to decadal predictability of tropical and north pacific sea surface temperatures abstract a multivariate empirical model is used to show that predictability of the dominant patterns of tropical and north pacific oceanic variability el nino southern oscillation enso and the pacific decadal oscillation pdo is mostly limited to little more than a year despite the presence of spectral peaks on decadal time scales the model used is a linear inverse model lim derived from the observed simultaneous and 1 yr lag correlation statistics of july june averaged sst from the hadley centre global sea ice and sea surface temperature hadisst dataset for the years 1900 2002 the model accurately reproduces the power spectra of the data including interannual and interdecadal spectral peaks that are significant relative to univariate red noise eigenanalysis of the linear dynamical operator yields propagating eigenmodes that correspond to these peaks but have very short decay times and thus limited predictability longer term predictability does exist however due to two stationary eigen

influence of the meridional shifts of the kuroshio and the oyashio extensions on the atmospheric circulation abstract the meridional shifts of the oyashio extension oe and of the kuroshio extension ke as derived from high resolution monthly sea surface temperature sst anomalies in 1982 2008 and historical temperature profiles in 1979 2007 respectively are shown based on lagged regression analysis to significantly influence the large scale atmospheric circulation the signals are independent from the enso teleconnections which were removed by seasonally varying asymmetric regression onto the first three principal components of the tropical pacific sst anomalies the response to the meridional shifts of the oe front is equivalent barotropic and broadly resembles the north pacific oscillation western pacific pattern in a positive phase for a northward frontal displacement the response may reach 35 m at 250 hpa for a typical oe shift a strong sensitivity since the associated sst anomaly is 0 5 k however the amplitude but not the pattern or statistical significance strongly depends on the lag and an

effects of sediment supply on surface textures of gravel bed rivers using previously published data from flume studies we test a new approach for quantifying the effects of sediment supply i e bed material supply on surface grain size of equilibrium gravel channels textural response to sediment supply is evaluated relative to a theoretical prediction of competent median grain size d9 50 we find that surface median grain size d50 varies inversely with sediment supply rate and systematically approaches the competent value d9 at low equilibrium transport rates furthermore equilibrium transport rate is a power function of the difference between applied and critical shear stresses and is therefore a power function of the difference between competent and observed median grain sizes d9 and d50 consequently we propose that the difference between predicted and observed median grain sizes can be used to determine sediment supply rate in equilibrium channels our analysis framework collapses data from different studies toward a single relationship between sediment supply rate and surface grain size while the approach appears promising we caution that it has been tested only on a limited set of laboratory data and a narrow range of channel conditions

the trouble with shear stress abstract predicted bedload fluxes often have large errors partly because calculations usually do not include spatial and temporal flow variability sediment motion is driven by a temporal distribution of applied fluid forces which are often simplified into time averaged flow velocities or shear stresses in sediment transport equations spatial variations in these flow parameters are also often neglected given the nonlinearities in the relation between flow and sediment flux the reach averaged sediment flux calculated with an average velocity or shear stress will not necessarily equal the measured spatially averaged bedload flux such spatial and temporal flow variations are ignored with the commonly used reach averaged shear stress ds that is a function of the flow depth and channel slope to demonstrate the uncertainties with using reach averaged shear stresses and even local shear stresses to predict bedload transport rates we have three goals first we outline the derivations and assumptions involved with commonly used estimates of shear stress second we summarize the literature on the uncertainties involved with using shear stress and other space and time averaged flow parameters to predict sediment flux third we use a new data set from experiments to add further support to a growing body of literature that suggests ds may not be linked to the mechanics of bedload transport we show considerable uncertainties exist in estimating a reach averaged shear stress by comparing ds and near bed shear stresses calculated from velocity profiles reynolds stresses and local turbulence for example ds was systematically greater than these near bed stresses even for experiments without large bed roughness we also find that correlations between local time averaged sediment fluxes and local reynolds stresses or turbulence intensities are relatively poor the near bed time averaged local flow velocity has the strongest relation with the time averaged local bedload flux because it is closely related to the forces that actually drive sediment transport however sediment transport rates in individual experiments could still vary by orders of magnitude for a given value of this flow velocity suggesting that additional work is needed to identity a parameter or distributions of parameters that will best predict sediment fluxes

continuous measurement of sediment transport in the erlenbach stream using piezoelectric bedload impact sensors we report on bedload transport observations using piezoelectric bedload impact sensors pbis an indirect method of estimating the volume of bedload transport of coarse sediment the pbis device registers vibrations produced by bedload particle diameter 20 mm and records the signal as a sum of the number of impulses per time sediment transport at the erlenbach stream has been continuously monitored with a pbis array starting in 1986 the sensor array spans the width of an entire cross section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin we compare pbis data with long term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin with artificial sediment input under controlled conditions in the field and also with laboratory experiments the rate of bedload transport is proportional to the number of impacts on the sensor per unit time the reliability of the calibration relationship increases with the length of the observation period e g for higher numbers of impacts and larger bedload volumes sediment volumes for individual flood events estimated with the pbis method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water the peak water discharge and the critical discharge for the onset of sediment transport copyright 2007 john wiley sons ltd

analysis of tidal signals in surface displacement measured by a dense continuous gps array abstract we analyze the tidal displacement signals for the eight major diurnal and semidiurnal tides from global positioning system gps measurements made at the western united states over the past 16 years with 1075 independent stations by careful examinations among inland versus coastal gps data we are able to demonstrate that estimated precisions down to the level of 0 1 mm horizontal and 0 3 mm vertical have been reached for the tidal signals and that at such precisions coherent spatial patterns are revealed in the residual tidal amplitudes and phases after the removal of a priori modeled effects of body tides and ocean tidal loading otl we demonstrate the facility of modern precise gps data in not only constraining the ocean tide models but more significantly in providing source data in terms of regional complex love numbers for geophysical inference of the heterogeneities of elastic and inelastic structures in the solid earth s deep interior on tidal timescales

a linear inverse model of tropical and south pacific seasonal predictability a multivariate linear inverse model lim is developed to demonstrate the mechanisms and seasonal predictability of the dominant modes of variability from the tropical and south pacific oceans we construct a lim whose covariance matrix is a combination of principal components derived from tropical and extratropical sea surface temperature and south pacific ocean vertically averaged temperature anomalies eigen decomposition of the linear deterministic system yields stationary and or propagating eigenmodes of which the least damped modes resemble el nino southern oscillation enso and the south pacific decadal oscillation spdo we show that although the oscillatory periods of enso and spdo are distinct they have very close damping time scales indicating that the predictive skill of the surface enso and spdo is comparable the most damped noise modes occur in the midlatitude south pacific ocean reflecting atmospheric eastward propagating rossby wave train variability we argue that these ocean wave trains occur due to the high frequency atmospheric variability of the pacific south american pattern imprinting onto the surface ocean the enso spring predictability barrier is apparent in lim predictions initialized in march may mam but displays a significant correlation skill of up to 3 months for the spdo the predictability barrier tends to appear in june september jas indicating remote but delayed influences from the tropics we demonstrate that subsurface processes in the south pacific ocean are the main source of decadal variability and further that by characterizing the upper ocean temperature contribution in the lim the seasonal predictability of both enso and the spdo variability is increased

drivers of decadal variability in the tasman sea in this study we compare optimally interpolated monthly time series tasman sea xbt data and a comprehensive set of ocean data assimilation models forced by atmospheric reanalysis to investigate the stability of the tasman sea thermocline and the transport variability of the east australian current eac the tasman front and eac extension we find that anomalously weaker eac transport at 25 s corresponds to an anomalously weaker tasman front and anomalously stronger eac extension we further show that post about 1980 and relative to the previous 30 years the anomalously weaker eac transport at 25 s is associated with large scale changes in the tasman sea specifically stronger stratification above the thermocline larger thermocline temperature gradients and enhanced energy conversion significant correlations are found between the maria island station sea surface temperature sst variability and stratification thermocline temperature gradient and baroclinic energy conversion suggesting that nonlinear dynamical responses to variability in the basin scale wind stress curl are important drivers of decadal variability in the tasman sea we further show that the stability of the eac is linked via the south caledonian jet to the stability of the pan basin subtropical south pacific ocean storm track

decadal sea surface temperature variability in the subtropical south pacific from 1726 to 1997 a d we present a 271 year record of sr ca variability in a coral from rarotonga in the south pacific gyre calibration with monthly sea surface temperature sst from satellite and ship measurements made in a grid measuring 1 degrees by 1 degrees over the period from 1981 to 1997 indicates that this sr ca record is an excellent proxy for sst comparison with sst from ship measurements made since 1950 in a grid measuring 5 degrees by 5 degrees also shows that the sr ca data accurately record decadal changes in sst the entire sr ca record back to 1726 shows a distinct pattern of decadal variability with repeated decadal and interdecadal sst regime shifts greater than 0 75 degrees c comparison with decadal climate variability in the north pacific as represented by the pacific decadal oscillation index 1900 1997 indicates that several of the largest decadal scale sst variations at rarotonga are coherent with sst regime shifts in the north pacific this hemispheric symmetry suggests that tropical forcing may be an important factor in at least some of the decadal variability observed in the pacific ocean

multi year predictability in a coupled general circulation model multi year to decadal variability in a 100 year integration of a bmrc coupled atmosphere ocean general circulation model cgcm is examined the fractional contribution made by the decadal component generally increases with depth and latitude away from surface waters in the equatorial indo pacific ocean the relative importance of decadal variability is enhanced in off equatorial wings in the subtropical eastern pacific the model and observations exhibit enso like decadal patterns analytic results are derived which show that the patterns can in theory occur in the absence of any predictability beyond enso time scales in practice however modification to this stochastic view is needed to account for robust differences between enso like decadal patterns and their interannual counterparts an analysis of variability in the cgcm a wind forced shallow water model and a simple mixed layer model together with existing and new theoretical results are used to improve upon this stochastic paradigm and to provide a new theory for the origin of decadal enso like patterns like the interdecadal pacific oscillation and pacific decadal oscillation in this theory enso driven wind stress variability forces internal equatorially trapped kelvin waves that propagate towards the eastern boundary kelvin waves can excite reflected internal westward propagating equatorially trapped rossby waves rws and coastally trapped waves ctws ctws have no impact on the off equatorial sub surface ocean outside the coastal wave guide whereas the rws do if the frequency of the incident wave is too high then only ctws are excited at lower frequencies both ctws and rws can be excited the lower the frequency the greater the fraction of energy transmitted to rws this lowers the characteristic frequency reddens the spectrum of variability off the equator relative to its equatorial counterpart at low frequencies dissipation acts as an additional low pass filter that becomes more effective as latitude increases at the same time enso driven off equatorial surface heating anomalies drive mixed layer temperature responses in both hemispheres both the eastern boundary interactions and the accumulation of surface heat fluxes by the surface mixed layer act to low pass filter the enso forcing the resulting off equatorial variability is therefore more coherent with low pass filtered decadal enso indices e g nino3 sea surface temperature sst than with unfiltered enso indices consequently large correlations between variability and nino3 extend further poleward on decadal time scales than they do on interannual time scales this explains why decadal enso like patterns have a broader meridional structure than their interannual counterparts this difference in appearance can occur even if enso indices do not have any predictability beyond interannual time scales the wings around 15 20 s and sub surface variability at many other locations are predictable on interannual and multi year time scales this includes westward propagating internal rws within about 25 of the equator the slowest of these take up to 4 years to reach the western boundary this sub surface predictability has significant oceanographic interest however it is linked to only low levels of sst variability consequently extrapolation of delayed action oscillator theory to decadal time scales might not be justified

calculating bed load transport in steep boulder bed channels 1 steep rough channels occupy a large fraction of the total channel length in mountainous regions most sediment mobilized on hillslopes must pass through these streams before reaching lower gradient channels steep channels have wide grain size distributions that are composed of finer more mobile sediment and large rarely mobile grains the large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude we hypothesize that sediment transport equations overpredict the sediment flux because they do not 1 account for the stress borne by rarely mobile grains 2 differentiate between highly and rarely mobile sediment and 3 account for the limited availability of mobile sediment here we modify a conventional bed load transport equation to include these three effects we use measurements of the flow bed properties and sediment flux in a small steep flume to test this equation we supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion the percent of the sphere that protrudes above the gravel deposit for a given sphere spacing the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep rough streams unlike most transport equations that we tested our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values our results demonstrate that accurately predicting bed load transport in steep rough streams may require accounting for the effects of local sediment availability coverage by mobile sediment and drag due to rarely mobile particles

a delayed action oscillator shared by biennial interannual and decadal signals in the pacific basin 1 biennial interannual and decadal signals in the pacific basin are observed to share patterns and evolution in covarying sea surface temperature sst 18 c isotherm depth z18 zonal surface wind zsw and wind stress curl wsc anomalies from 1955 to 1999 each signal has warm sst anomalies propagating slowly eastward along the equator generating westerly zsw anomalies in their wake these westerly zsw anomalies produce cyclonic wsc anomalies off the equator which pump baroclinic rossby waves in the western central tropical north pacific ocean these rossby waves propagate westward taking 6 12 and 36 months to reach the western boundary near 7 n 12 n and 18 n on biennial interannual and decadal period scales respectively there they reflect as equatorial coupled waves propagating slowly eastward in covarying sst z18 and zsw anomalies taking 6 12 and 24 months to reach the central eastern equatorial ocean these equatorial coupled waves produce a delayed negative feedback to the warm sst anomalies there the decrease in rossby wave phase speed with latitude the increase in meridional scale of equatorial sst anomalies with period scale and the associated increase in latitude of rossby wave forcing are consistent with the delayed action oscillator dao model used to explain el nino however this is not true of the western boundary reflection of rossby waves into slow equatorial coupled waves this requires modification of the extant dao model we construct a modified dao model demonstrating how the various mechanisms and the size and sources of their delays yield the resulting frequency of each signal

the forcing of the pacific decadal oscillation the pacific decadal oscillation pdo defined as the leading empirical orthogonal function of north pacific sea surface temperature anomalies is a widely used index for decadal variability it is shown that the pdo can be recovered from a reconstruction of north pacific sea surface temperature anomalies based on a first order autoregressive model and forcing by variability of the aleutian low el nino southern oscillation enso and oceanic zonal advection anomalies in the kuroshio oyashio extension the latter results from oceanic rossby waves that are forced by north pacific ekman pumping the sst response patterns to these processes are not orthogonal and they determine the spatial characteristics of the pdo the importance of the different forcing processes is frequency dependent at interannual time scales forcing from enso and the aleutian low determines the response in equal parts at decadal time scales zonal advection in the kuroshio oyashio extension enso and anomalies of the aleutian low each account for similar amounts of the pdo variance these results support the hypothesis that the pdo is not a dynamical mode but arises from the superposition of sea surface temperature fluctuations with different dynamical origins

south pacific decadal climate variability and potential predictability the south pacific decadal oscillation spdo characterizes the southern hemisphere contribution to the pacific wide interdecadal pacific oscillation ipo and is analogous to the pacific decadal oscillation pdo centered in the north pacific in this study upper ocean variability and potential predictability of the spdo is examined in hadisst data and an atmosphere forced ocean general circulation model the potential predictability of the ipo related variability is investigated in terms of both the fractional contribution made by the decadal component in the south tropical and north pacific oceans and in terms of a doubly integrated first order autoregressive ar1 model despite explaining a smaller fraction of the total variance we find larger potential predictability of the spdo relative to the pdo we identify distinct local drivers in the western subtropical south pacific where nonlinear baroclinic rossby wave topographic interactions act to low pass filter decadal variability in particular we show that the kermadec ridge in the southwest pacific enhances the decadal signature more prominently than anywhere else in the pacific basin applying the doubly integrated ar1 model we demonstrate that variability associated with the pacific south american pattern is a critically important atmospheric driver of the spdo via a reddening process analogous to the relationship between the aleutian low and pdo in the north pacific albeit that the relationship in the south pacific appears to be even stronger our results point to the largely unrecognized importance of south pacific processes as a key source of decadal variability and predictability

a predictability study of simulated north atlantic multidecadal variability the north atlantic is one of the few places on the globe where the atmosphere is linked to the deep ocean through air sea interaction while the internal variability of the atmosphere by itself is only predictable over a period of one to two weeks climate variations are potentially predictable for much longer periods of months or even years because of coupling with the ocean this work presents details from the first study to quantify the predictability for simulated multidecadal climate variability over the north atlantic the model used for this purpose is the gfdl coupled ocean atmosphere climate model used extensively for studies of global warming and natural climate variability this model contains fluctuations of the north atlantic and high latitude oceanic circulation with variability concentrated in the 40 60 year range oceanic predictability is quantified through analysis of the time dependent behavior of large scale empirical orthogonal function eof patterns for the meridional stream function dynamic topography 170 m temperature surface temperature and surface salinity the results indicate that predictability in the north atlantic depends on three main physical mechanisms the first involves the oceanic deep convection in the subpolar region which acts to integrate atmospheric fluctuations thus providing for a red noise oceanic response as elaborated by hasselmann the second involves the large scale dynamics of the thermohaline circulation which can cause the oceanic variations to have an oscillatory character on the multidecadal time scale the third involves nonlocal effects on the north atlantic arising from periodic anomalous fresh water transport advecting southward from the polar regions in the east greenland current when the multidecadal oscillatory variations of the thermohaline circulation are active the first and second eof patterns for the north atlantic dynamic topography have predictability time scales on the order of 10 20 y whereas eof 1 of sst has predictability time scales of 5 7 y when the thermohaline variability has weak multidecadal power the hasselmann mechanism is dominant and the predictability is reduced by at least a factor of two when the third mechanism is in an extreme phase the north atlantic dynamic topography patterns realize a 10 20 year predictability time scale additional analysis of sst in the greenland sea in a region associated with the southward propagating fresh water anomalies indicates the potential for decadal scale predictability for this high latitude region as well the model calculations also allow insight into regional variations of predictability which might be useful information for the design of a monitoring system for the north atlantic predictability appears to break down most rapidly in regions of active convection in the high latitude regions of the north atlantic

low frequency pycnocline variability in the northeast pacific abstract the output from an ocean general circulation model ogcm driven by observed surface forcing is used in conjunction with simpler dynamical models to examine the physical mechanisms responsible for interannual to interdecadal pycnocline variability in the northeast pacific ocean during 1958 97 a period that includes the 1976 77 climate shift after 1977 the pycnocline deepened in a broad band along the coast and shoaled in the central part of the gulf of alaska the changes in pycnocline depth diagnosed from the model are in agreement with the pycnocline depth changes observed at two ocean stations in different areas of the gulf of alaska a simple ekman pumping model with linear damping explains a large fraction of pycnocline variability in the ogcm the fit of the simple model to the ogcm is maximized in the central part of the gulf of alaska where the pycnocline variability produced by the simple model can account for 70 90 of the pycnocline depth variance in the ogcm evidence of westward

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

coarser and rougher effects of fine gravel pulses on experimental step pool channel morphodynamics understanding how steep mountain rivers respond to natural and anthropogenic sediment supply perturbations is important for predicting effects of extreme events e g floods and landslides and for restoring rivers to more natural conditions using flume experiments we show that stabilized step pool like channel beds can respond to pulses of finer gravel by becoming even coarser and rougher than before adding finer gravel initially reduces bed roughness and also increases the mobility of previously stable bed grains small and intermediate diameter clasts are then preferentially winnowed from the bed surface leaving behind higher concentrations of even larger clasts ultimately this results in both a coarser and rougher bed our experiments demonstrate that steep river beds become stable through the coevolution of bed roughness and surface grain size distribution and that these morphological variables can be sensitive to the history of upstream sediment supply

seasonal modulations in the eddy field of the south pacific ocean abstract decade long satellite altimeter data from the ocean topography experiment topex poseidon and european remote sensing satellite 1 and 2 ers 1 2 missions are analyzed to investigate the eddy signals in the south pacific ocean high eddy kinetic energy eke bands with well defined annual cycles are detected along the eastward flowing surface currents of the south tropical countercurrent stcc between 21 29 s and the south equatorial countercurrent secc centered near 9 s overriding the westward flowing south equatorial current sec the stcc layer has the sign of its mean potential vorticity gradient opposite to that in the sec layer subjecting the vertically sheared stcc sec system to baroclinic instability in austral winter the stcc sec system is baroclinically more unstable than in other seasons because of the large vertical shear and weak stratification this seasonal variation in the intensity of baroclinic instability is responsible for the seasonal modulation of the stcc s eke fi

the impact of exceptional events on erosion bedload transport and channel stability in a step pool channel sediment transport in the erlenbach a small stream with step pool morphology in the canton of schwyz switzerland has been monitored for more than 20 years during this time three exceptional events events with high sediment yield and long return times that have a large effect on channel morphology have impacted the stream and partly or completely rearranged the existing step pool morphology in the aftermath of the events sediment transport rates at a given discharge and total sediment yield remained elevated for about a year or longer for the last event dated on the 20 june 2007 observations of boulder mobility and step destruction were used to interpret channel stability boulders with median diameters of up to 135 cm and estimated weights of more than 2 5 tons have moved during the 2007 event using hydraulic observations and shear stress calculations boulders up to 65 cm in diameter were predicted to have been fully mobile in peak conditions even if form resistance and increased critical stresses needed for the initiation of motion in steep streams were taken into account for two of the events estimated peak shear stresses at the bed exceeded 1000 pa calculated both from observations of the flow hydraulics and from boulder mobility this suggests that highly energetic flows occur relatively frequently in small steep streams and that large boulders can be transported by fluvial processes in such streams the observations have potential significance for hazard risk mitigation stream engineering and restoration copyright 2009 john wiley sons ltd

decadal variability in the south pacific subtropical countercurrent and regional mesoscale eddy activity abstractdecadal variability of eddy activity in the western subtropical south pacific is examined using the past two decades of satellite altimetry data between 21 and 29 s there is a band of heightened eddy activity in this region the eastward south pacific subtropical countercurrent stcc overlays the westward south equatorial current sec this vertically sheared stcc sec system is subject to baroclinic instabilities by using the european centre for medium range weather forecasts ecmwf ocean reanalysis system version 4 oras4 data and verifying with the gridded argo float data low frequency variations in the state of the ocean in this region are investigated it is found that the low frequency changes in the shearing and stratification of the stcc sec region simultaneously work to modulate the strength of baroclinic instabilities as measured through the baroclinic growth rate these changes in the strength of the instabilities consequently affect the observed eddy activity using a line

fine bed material in pools of natural gravel bed channels natural gravel bed channels commonly contain a fine mode of sand and fine gravel that fills voids of the bed framework of coarser gravel if the supply of fine bed material exceeds the storage capacity of framework voids excess fine material forms surficial patches which can be voluminous in pools during low flow data collected in 34 natural channels in northern california and southern oregon indicate the following 1 fine material on the bed surface can be readily winnowed and transported at high particle velocities much of it in intermittent suspension fine material can dominate the bed material load in gravel bed channels but its abundance on the bed surface is limited by its increasing mobility as hiding places among prominent particles are filled 2 fine material in pools is typically replaced many times per year 3 the proportion v of residual pool volume filled with fine bed material correlates with annual sediment yield in channels whose parent material produces abundant sandy sediment 4 temporal and spatial changes in v appear to correspond to variations in the balance between sediment inputs and water discharge these results suggest that v can be used to monitor and evaluate the supply of excess fine material in gravel bed channels and that samples of fine material in pools can characterize the fine mobile mode of bed material load the sediment load of a stream channel the amount supplied and transported over a period of time can be difficult to evaluate contributions from hillslopes can be quantified with erosion surveys and sediment budgeting but uncertainty in routing makes it difficult to assess the load at any point in a channel network and direct measurement of transport rates is notoriously difficult bed material typically furnishes the bulk of the annual bed load especially in more distal channels but the active volume that contributes to the load is highly variable and difficult to evaluate it may be easier to detect variations in load by examining the mobility of the bed surface of gravel bed channels bed material in gravel bed channels characteristically includes a wide range of particle size and the bed can become

interdecadal sea surface temperature variability in the equatorial pacific ocean part i the role of off equatorial wind stresses and oceanic rossby waves the australian bureau of meteorology research centre cgcm and a linear first baroclinic mode ocean shallow water model swm are used to investigate ocean dynamic forcing mechanisms of the equatorial pacific ocean interdecadal sea surface temperature sst variability an eof analysis of the 13 yr low pass butterworth filtered sst anomalies from a century time scale cgcm simulation reveals an sst anomaly spatial pattern and time variability consistent with the interdecadal pacific oscillation results from an swm simulation forced with wind stresses from the cgcm simulation are shown to compare well with the cgcm and as such the swm is then used to investigate the roles of uncoupled equatorial wind stress forcing off equatorial wind stress forcing offeqwf and rossby wave reflection at the western pacific ocean boundary on the decadal equatorial thermocline depth anomalies equatorial pacific wind stresses are shown to explain a large proportion of the overall variance in the equatorial thermocline depth anomalies however offeqwf beyond 12 5 latitude produces an interdecadal signature in the nino 4 nino 3 region that explains approximately 10 1 5 of the filtered control simulation variance rossby wave reflection at the western pacific boundary is shown to underpin the offeqwf contribution to these equatorial anomalies the implications of this result for the predictability of the decadal variations of thermocline depth are investigated with results showing that offeqwf generates an equatorial response in the nino 3 region up to 3 yr after the wind stress forcing is switched off further a statistically significant correlation is found between thermocline depth anomalies in the offequatorial zone and the nino 3 region with the nino 3 region lagging by approximately 2 yr the authors conclude that there is potential predictability of the offeqwf equatorial thermocline depth anomalies with lead times of up to 3 yr when taking into account the amplitudes and locations of off equatorial region rossby waves

variable source areas of bedload in a gravel bed stream abstract bedload texture is dependent on the spatially variable availability of mobile surface bed material distributed in patches or in other bedforms we measured bedload transport continuously with an automatic bedload station on the gravel bedded tordera river we also studied bed patchiness from pre flood vertical aerial photographs in a 40 m section upstream of the bedload station the results suggest that patches are the main source of sediment during the beginning of the flow event the median bedload size in low magnitude events depends on the percentage of total area occupied by patches of a given size patch areal extent and texture depend on the magnitude and general character of the previous flow event

observed variability of the south pacific westward sea level anomaly signal in the presence of bottom topography this study investigates the behavior of westward propagating sea level anomalies across the south pacific ocean with a focus on the long rossby wave signal determined from filtered topex poseidon and ers satellite altimeter data an evaluation of the energy variability of the signal using a two dimensional radon transform analysis suggests that rossby waves interact with both ridges and seamounts at various locations across the basin anomalously slow rossby wave phase speeds are found over steep isolated bathymetric features in the tropical south pacific and over the plateau around new zealand interaction with ridges increases the energetic variability range of dominant propagation speeds and meridional deviations in the rossby wave signal

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

stochastic forcing of sediment routing and storage in channel networks the stochastic field of sediment supply to the channel network of a drainage basin depends on the large scale interactions among climatically driven processes such as forest fire and rainstorms topography channel network topology and basin scale during infrequent periods of intense erosion large volumes of colluvium are concentrated in parts of a channel network particularly near tributary junctions the rivers carry bed material and wash load downstream from these storage sites at different rates the bed material travels slowly creating transient patterns of sediment transport sediment storage and channel morphology along the channel network as the concentrations of bed material migrate along the network their waveforms can undergo changes by diffusion interference at tributary junctions and loss of mass through temporary sediment storage in fans and terraces and through particle abrasion which converts bed material to wash load we investigated how these processes might influence the sediment mass balance in channels of third and higher order in a 215 km 2 drainage basin within the oregon coast range over a simulated 3000 year period with a climate typical of the late holocene we used field measurements and a simulation model to illustrate interactions between the major controls on large scale processes functioning over long periods of time in complex drainage basins

a tripole index for the interdecadal pacific oscillation a new index is developed for the interdecadal pacific oscillation termed the ipo tripole index tpi the ipo is associated with a distinct tripole pattern of sea surface temperature anomalies ssta with three large centres of action and variations on decadal timescales evident in the second principal component pc of low pass filtered global sst the new index is based on the difference between the ssta averaged over the central equatorial pacific and the average of the ssta in the northwest and southwest pacific the tpi is an easily calculated non pc based index for tracking decadal sst variability associated with the ipo the tpi time series bears a close resemblance to previously published pc based indices and has the advantages of being simpler to compute and more consistent with indices used to track the el nino southern oscillation enso such as nino 3 4 the tpi also provides a simple metric in physical units of c for evaluating decadal and interdecadal variability of sst fields in a straightforward manner and can be used to evaluate the skill of dynamical decadal prediction systems composites of sst and mean sea level pressure anomalies reveal that the ipo has maintained a broadly stable structure across the seven most recent positive and negative epochs that occurred during 1870 2013 the tpi is shown to be a robust and stable representation of the ipo phenomenon in instrumental records with relatively more variance in decadal than shorter timescales compared to nino 3 4 due to the explicit inclusion of off equatorial sst variability associated with the ipo