rossby waves in the tropical north pacific and their role in decadal thermocline variability abstract a band of enhanced thermocline variability at 10 15 n in the pacific found in nature also occurs in an ocean general circulation model forced with observed fluxes of momentum heat and freshwater over the period 1958 97 the variability in the model is primarily associated with long baroclinic rossby waves characterized by periods in the decadal range 7 10 yr the waves are forced by westward propagating ekman pumping anomalies east of the date line and propagate at a speed of 13 cm s 1 which is slower than the phase speed of the first mode unforced baroclinic waves 15 16 cm s 1 west of the date line the correlations between thermocline displacements and local ekman pumping are relatively small and the ocean signals have a phase speed of 20 cm s 1 very similar to the phase speed of the first baroclinic mode in the western half of the basin 18 20 cm s 1 the phase speeds of the ocean model signals have been estimated using cospectral analysis while the wkb approximation has been us

anatomy of north pacific decadal variability abstract a systematic analysis of north pacific decadal variability in a full physics coupled ocean atmosphere model is executed the model is an updated and improved version of the coupled model studied by latif and barnett evidence is sought for determining the details of the mechanism responsible for the enhanced variance of some variables at 20 30 yr timescales the possible mechanisms include a midlatitude gyre ocean atmosphere feedback loop stochastic forcing remote forcing or sampling error decadal variability in the model is expressed most prominently in anomalies of upper ocean streamfunction sea surface temperature sst and latent surface heat flux in the kuroshio oyashio extension koe region off japan the decadal signal off japan is initiated by changes in strength and position of the aleutian low the atmospheric perturbations excite sst anomalies in the central and eastern north pacific with opposing signs and canonical structure the atmospheric perturbations also change the ekm

regional precipitation simulations for the mid 1970s shift and early 2000s hiatus it has been demonstrated that climate models initialized with observations produce better simulations of pacific sea surface temperature sst patterns than uninitialized simulations for the two major climate regime changes of the last 40 years the mid 1970s climate shift and early 2000s hiatus a fundamental feature of these hindcasts is the simulation of the sst anomalies associated with the phase of the interdecadal pacific oscillation ipo since regional precipitation patterns over selected land areas in south asia australia and north america are known to be affected by sst patterns over the pacific it is shown that the initialized climate model simulations produce qualitatively better agreement with observations for regional precipitation anomalies in those regions compared to uninitialized climate models though the signals are small the anomalies are consistent with our physical process based understanding of precipitation responses over certain land areas during different ipo phases

three dimensional finite difference waveform modeling of strong motions observed in the sendai basin japan we perform three dimensional 3d finite difference fd waveform modeling of strong motions in the frequency range 0 2 to 1 67 hz observed in the sendai basin japan during the japan meteorological agency magnitude m j 5 0 1998 miyagiken nanbu earthquake in a previous we estimated s wave velocity structures above the pre tertiary bedrock at six sites in the sendai basin based on array records of microtremors to interpolate these velocity structures in space we conduct single station microtremor measurements at a total of 61 locations and estimate the s wave velocity structure at each site by modeling the horizontal to vertical spectral ratio of fundamental mode rayleigh waves an initial 3d model of the basin is constructed using the velocity structures estimated from both array and single station microtremor measurements along with other information such as surface geology this model encompasses a region 33 km long 30 km wide and 19 km deep the final model is obtained through a trial and error process by fitting 3d fd synthetic waveforms to the bandpass filtered 0 2 hz to 1 67 hz displacement records 12 stations for the 1998 miyagiken nanbu earthquake we compute the synthetics using a fourth order staggered grid 3d fd method with variable grid spacing as the 3d model is modified the source parameters strike dip rake and seismic moment are estimated by a grid search method using 1d site specific models derived from the modified 3d model the observed waveforms are reproduced well at most stations by the final 3d basin model this agreement suggests the validity of the final 3d basin model for theoretical strong motion prediction of large earthquakes in the frequency range from 0 2 to 1 67 hz manuscript received 7 june 2000

response and feedback of the indian summer monsoon and the southern westerly winds to a temperature contrast between the hemispheres during the last glacial interglacial transitional period abstract the hypothesis that an interhemispheric thermal gradient or a temperature contrast between the hemispheres tcbh drives shifts in the latitudinal position of the intertropical convergence zone itcz and the southern westerly winds sww is considered one of the most important climate change mechanisms proposed in the last few decades however the controversy over the shifting magnitude and direction of the itcz and the sww brings uncertainty to the hypothesis here we further examine the shifting characteristics of the itcz and its relationship with the intensity variation of the indian summer monsoon ism based on synthesis and summary of previously published literature data in particular we evaluate regional distribution patterns of the dry and wet climatic changes in southern south america during the last glacial interglacial transitional period including the heinrich event 1 the antarctic cold reversal and the younger dryas the results show that in response to the northern hemisphere wide cooling and the southern hemisphere wide warming the northern hemisphere wide warming and the southern hemisphere wide cooling during the period the itcz over the asian and northern south american continents and the sww shift poleward equatorward while the intensity of the ism decreases increases however the shifting characteristics of the itcz over the vast pacific ocean remain unclear which implies that the relationship between the tcbh and the el nino southern oscillation in the tropical pacific ocean needs to be studied in addition there may be a relatively stable core region in the sww this core region is located in central patagonia at approximately 47 s where the local climate is wetter during the all transition period showing the continuous influence of the sww on this region the position shift of the sww in response to the tcbh phase changes seems to behave as a swing process with an axis of approximately 47 s the poleward swing of the sww may trigger the self repairing process of the global climate system after the catastrophic impacts of the meltwater including the pulling effect of the enhanced upwelling on the atlantic meridional overturning circulation the influence of co2 released from the deep sea and the impact of the gradual strengthening of the ism these results support and reinforce the hypothesis that the tcbh drives global climate change highlight the global teleconnection characteristics of the key physical processes in the earth s climate system and the important role of the sww in this teleconnection and provide a basis for further simulating the climate connections in the two hemispheres especially the impact of the sww on the global climate change

is there a low frequency bias in multiproxy reconstructions of tropical pacific sst variability abstract decadal and longer term variability in the tropical pacific has the potential to impact climate worldwide yet paleoclimate observations and climate model simulations disagree on the nature of this variability in this study we investigate whether biases from paleoclimate reconstruction methodology and the currently available proxy network cause reconstructions of enso to overestimate the multidecadal century scale variability of the pacific we build new reconstructions using a variety of commonly employed methods and test the effects of four types of bias on analogous pseudoproxy reconstructions these biases include 1 the number of records used in the reconstruction 2 chronological uncertainty 3 noise in individual proxy records and 4 the geographical distribution of the proxy network we find that each of these biases cause enso reconstructions to overestimate the importance of low frequency variability these biases may partially explain why multiproxy reconstructions of pacific variability have stronger low frequency variability than predicted by historical observations or climate models and why these reconstructions do not have coherent signals over the multidecadal century scale

abrupt variations of indian and east asian summer monsoons during the last deglacial stadial and interstadial abstract the phase relationship between the indian summer monsoon ism and the east asian summer monsoon easm during the last deglaciation remains controversial here we reconstruct a 15 000 year plant cellulose 13 c proxy record for the ism from the yuexi peat bog in southwestern china the record shows that the ism abruptly decreases during the younger dryas yd stadial and abruptly increases during the bolling allerod ba interstadial a comparison of the yuexi record with other related proxy climate records reveals two types of phenomena first the strengths of the two asian monsoons are inversely related during the yd stadial i e the ism strength decreases and the easm increases during this period the southern chinese mainland consisted of a wide arid zone while the northern chinese mainland was much wetter the arid zone in southern china resulted from two different types of monsoon processes the abnormal northward extension of the easm rain belt leading to less rainfall in southeast china or an illusion that the easm weakened the other process is a real weakening of the ism second during the ba interstadial the strengths of both the ism and easm clearly increased however the maximum strengths appear to have occurred in the allerod period during this period the entire chinese mainland both northern and southern experienced wet conditions the abnormal climate pattern of wet in the north and dry in the south during the yd stadial occurs because of the combined effects of the strengthened easm intensified westerlies and weakened ism which could be attributed to the response to the abrupt cooling in the high northern latitudes and to the el nino like activity in the equatorial pacific the widespread wet climate during the ba interstadial may be related to an abrupt increase in the greenhouse gases ghgs concentrations in the atmosphere and to the la nina like activity in the equatorial pacific these results contribute to a better understanding of the response of the asian monsoon to global changes and present a geo historical scenario for understanding the east asian hydrological variations in the current global warming phase

impacts of the pacific japan and circumglobal teleconnection patterns on the interdecadal variability of the east asian summer monsoon abstractbased on the twentieth century reanalysis 20cr dataset the dominant modes of interdecadal variability of the east asian summer monsoon easm are investigated through a multivariate empirical orthogonal function analysis mv eof the first mode ea1 is characterized by an anomalous cyclone centered over taiwan and an anomalous anticyclone centered over the bohai sea these phenomena are part of the meridional wave like teleconnection pattern propagating poleward from the southern tropical western north pacific wnp referred to as the interdecadal pacific japan pj pattern the interdecadal pj pattern is driven by negative anomalous convective heating over the southern tropical wnp which is associated with the interdecadal pacific oscillation ipo and the interdecadal indian ocean basin mode iobm the amplitude of the ea1 and its contribution to the total variance of the easm decrease remarkably after the 1960s the second mv eof mode ea2 is characterized by cyclone anomalies extendin

model assessment of decadal variability and trends in the tropical pacific ocean abstract in this report global coupled ocean atmosphere models are used to explore possible mechanisms for observed decadal variability and trends in pacific ocean ssts over the past century the leading mode of internally generated decadal 7 yr variability in the model resembles the observed decadal variability in terms of pattern and amplitude in the model the pattern and time evolution of tropical winds and oceanic heat content are similar for the decadal and enso timescales suggesting that the decadal variability has a similar delayed oscillator mechanism to that on the enso timescale the westward phase propagation of the heat content anomalies however is slower and centered slightly farther from the equator 12 vs 9 n for the decadal variability cool sst anomalies in the midlatitude north pacific during the warm tropical phase of the decadal variability are induced in the model largely by oceanic advection anomalies an index of observed sst over a broad triangular region of the tropi

tropical and high latitude forcing of enhanced megadroughts in northern china during the last four terminations abstract understanding the origin and evolutionary history of drought events is of great significance providing critical insight into future hydrological conditions under the changing climate due to the scarcity of drought proxies from northern china the occurrence and underlying mechanisms of the drought events remains enigmatic on longer timescales here we utilize microbial lipid proxies to reconstruct significant drought events over the last four ice age terminations in the southernmost section weinan section of the chinese loess plateau the abundance of archaeal isoprenoid gdgts glycerol dialkyl glycerol tetraethers relative to bacterial branched gdgts measured by ri b and bit indices is diagnostic of enhanced drought conditions the ri b and bit indices are stable and low high throughout most of the loess section spanning the last 350 thousand years but they do exhibit sharp transient peaks valleys during the intervals associated with the four ice age terminations and especially terminations ii and iv these enhanced drought events are non intuitively associated with a significant decrease in the relative abundance of c4 plants inferred by a decrease in the carbon isotope composition of bulk organic matter although the microbial records show some consistency with the weinan grain size profiles indicative of eastern asian winter monsoon variability they also show some apparent difference in fact some features of the microbial records exhibit strong similarities with marine sediment planktonic foraminiferal 13c records from the western pacific warm pool which reflect enso like changes during glacial terminations therefore enhanced droughts immediately before the interglacial warming in northern china could be explained at least in part by teleconnections in tropical ocean atmosphere circulation via shifts in the intertropical convergence zone itcz and associated jet stream over the asian continent according to our microbial biomarker data these enhanced megadroughts are apparently different both in terms of severity and causal mechanism from the more commonly discussed dry conditions observed during glacial periods

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

the impact of variably saturated conditions on hydrogeochemical changes during artificial recharge of groundwater artificial recharge of groundwater is often used to either purify partially treated wastewater or to enhance the quality of surface water by percolation through a variably saturated zone in many cases the most substantial purification process within the infiltration water is the redox dependent biodegradation of organic substances the present study was aimed at understanding the spatial and temporal distribution of the redox reactions that develop below an artificial recharge pond near lake tegel germany at this site like at many artificial recharge sites the hydraulic regime immediately below the pond is characterised by cyclic changes between saturated and unsaturated conditions these changes which occur during each operational cycle result from the repeated formation of a clogging layer at the pond bottom regular hydrogeochemical analyses of groundwater and seepage water in combination with continuous hydraulic measurements indicate that no3 and mn reducing conditions dominate beneath the pond as long as water saturated conditions prevail manganese fe and so42 reducing conditions are confined to a narrow zone directly below the clogging layer and in zones of lower hydraulic conductivity the formation of the clogging layer leads to a steady decrease of the infiltration rate which ultimatively causes a shift to unsaturated conditions below the clogging layer atmospheric o2 then starts to penetrate from the pond fringes into this region leading to i the re oxidation of the previously formed sulphide minerals and ii the enhanced mineralisation of sedimentary particulate organic c the mineralisation of sedimentary particulate organic c leads to an increased h2co3 production and subsequent dissolution of calcite

why are there rossby wave maxima in the pacific at 10 s and 13 n observations indicate the existence of two bands of maximum thermocline depth variability centered at 108s and 138n in the tropical pacific ocean the analysis of a numerical integration performed with the national center for atmospheric research ocean general circulation model ogcm forced with observed fluxes of momentum heat and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 108s and 138n are associated with first mode baroclinic rossby waves forced by anomalous ekman pumping in this study the factors that may be responsible for the rossby wave maxima at 108s and 138n including the amplitude and spatial coherency of the forcing at those latitudes are systematically investigated a simple rossby wave model is used to interpret the ogcm variability and to help to discriminate between the different factors that may produce the tropical maxima these results indicate that the dominant factor in producing the maximum variability at 108s and 138 ni s thezonal coherency of the ekman pumping a characteristic of the forcing that becomes increasingly more pronounced at low frequencies maximizing at timescales in the decadal range local maxima in the amplitude of the forcing while not explaining the origin of the centers of variability at 108s and 138n appear to affect the sharpness of the variability maxima at low frequencies although the rossby wave model gives an excellent fit to the ogcm some discrepancies exist the amplitude of the thermocline variance is generally underestimated by the simple model and the variability along 13 8 ni s westward intensified in the wave model but reaches a maximum in the central part of the basin in the ogcm short rossby waves excited by small scale ekman pumping features or the presence of higher order rossby wave modes may be responsible for the differences in the zonal variance distribution along 138n

decadal variations in the subtropical cells and equatorial pacific sst 1 a mechanism for generating decadal sea surface temperature sst variability in the equatorial pacific is investigated using an ocean general circulation model forced by observed wind stress equatorial sst variability is governed by different ocean dynamics on interannual and decadal time scales at interannual time scales equatorial sst anomalies are mainly driven by equatorial winds at decadal time scales on the other hand they are forced both by equatorial winds and by off equatorial winds in the tropics while the contribution from midlatitude winds poleward of 25 is negligible trade wind variations in the off equatorial tropics force equatorial sst variability by spinning up and down the subtropical cells that transport cold water into the equatorial upwelling zone equatorial sst anomalies induced by this mechanism lag behind the response to local equatorial winds by about two years

interannual to decadal variations of tropical subtropical exchange in the pacific ocean boundary versus interior pycnocline transports abstract interannual to decadal variations of tropical subtropical mass exchange in the pacific ocean are investigated using a near global ocean general circulation model along with satellite observations of sea level and wind and a data assimilation product the analysis focuses on the variability of pycnocline transports through the western boundary and interior near 10 n and 10 s in contrast to time mean exchange where boundary and interior pycnocline transports are both equatorward the variations of boundary and interior pycnocline transports are found to be generally anticorrelated to each other moreover the variation of the boundary pycnocline transport is smaller than that of the interior again different from time mean exchange where the boundary transport at 10 n is substantially larger than that through the interior interannual variations of the boundary and interior transports are consistent with near surface geostrophic flow inferred from sea level data interior pycnocline flow into th

the influence of the interdecadal pacific oscillation on temperature and precipitation over the globe the interdecadal pacific oscillation ipo is a 40 60 year quasi oscillation seen mostly in the pacific basin but its impacts on surface temperature t and precipitation p have been found over australia the southwest u s and other regions here a global analysis of ipo s impacts on t and p and its modulation of enso s influence on t and p over the globe is performed using observational and reanalysis data and model simulations since 1920 there are two warm 1924 1944 and 1977 1998 and two cold 1945 1976 and 1999 present ipo phases whose change is associated with abrupt shifts in north pacific sea level pressure slp and contrasting anomaly patterns in t and p and atmospheric circulation over the eastern and western pacific the ipo explains more than half of the interdecadal variations in t and p over many regions such as northeastern australia western canada and northern india significant correlations between the ipo and local p are observed over eastern australia southern africa and the southwest u s the ipo also modulates enso s influence on local t and p over most of these regions t over northern india is positively correlated with nino3 4 enso index during cold ipo phases but the correlation turns negative or insignificant during ipo warm phases over northeastern australia the t versus enso and p versus enso correlations are stronger during the ipo cold phases than during the warm phases the p versus enso correlation over southern africa tends to be negative during ipo warm phases but becomes weaker or insignificant during ipo cold phases the ipo induced p anomalies can be explained by the associated anomaly circulation which is characterized by a high slp center and anti cyclonic flows over the north pacific and negative slp anomalies and increased wind convergence over the indonesia and western pacific region during ipo cold phases these observed influences of the ipo on regional t and p are generally reproduced by an atmospheric model forced by observed sea surface temperatures

indo pacific variability on seasonal to multidecadal time scales part ii multiscale atmosphere ocean linkages abstractthe coupled atmosphere ocean variability of the indo pacific domain on seasonal to multidecadal time scales is investigated in ccsm4 and in observations through nonlinear laplacian spectral analysis nlsa it is found that enso modes and combination modes of enso with the annual cycle exhibit a seasonally synchronized southward shift of equatorial surface zonal winds and thermocline adjustment consistent with terminating el nino and la nina events the surface winds associated with these modes also generate teleconnections between the pacific and indian oceans leading to sst anomalies characteristic of the indian ocean dipole the family of nlsa enso modes is used to study el nino la nina asymmetries and it is found that a group of secondary enso modes with more rapidly decorrelating temporal patterns contributes significantly to positively skewed sst and zonal wind statistics besides enso fundamental and combination modes representing the tropospheric biennial oscillation tbo are found to

a theory of interdecadal climate variability of the north pacific ocean atmosphere system abstract a linear coupled model for the atmosphere upper ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the north pacific in this model wind stress anomalies over the north pacific are related to anomalies in the meridional temperature gradient of the upper ocean the latter depends upon air sea thermodynamical feedbacks and meridional heat transport by upper ocean currents slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic rossby waves which carry out the meridional heat transport uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure coupled dynamics produce a basin scale interdecadal oscillatory mode this mode arises from the dynamic coupling and the memory of the system residing in the slow gyre circulation ad

western indian ocean marine and terrestrial records of climate variability a review and new concepts on land ocean interactions since ad 1660 we examine the relationship between three tropical and two subtropical western indian ocean coral oxygen isotope time series to surface air temperatures sat and rainfall over india tropical east africa and southeast africa we review established relationships provide new concepts with regard to distinct rainfall seasons and mean annual temperatures tropical corals are coherent with sat over western india and east africa at interannual and multidecadal periodicities the subtropical corals correlate with southeast african sat at periodicities of 16 30 years the relationship between the coral records and land rainfall is more complex running correlations suggest varying strength of interannual teleconnections between the tropical coral oxygen isotope records and rainfall over equatorial east africa the relationship with rainfall over india changed in the 1970s the subtropical oxygen isotope records are coherent with south african rainfall at interdecadal periodicities paleoclimatological reconstructions of land rainfall and sat reveal that the inferred relationships generally hold during the last 350 years thus the indian ocean corals prove invaluable for investigating land ocean interactions during past centuries

connections between the pacific ocean tropics and midlatitudes on decadal timescales abstract connections between the tropical and midlatitude pacific on decadal timescales are examined using a 137 yr run of a fully coupled ocean atmosphere general circulation model it is shown that the model does a credible job of simulating both enso scale and decadal scale variability and that there are statistically significant correlations between the midlatitudes and tropics on decadal timescales three physical mechanisms linking the regions are examined 1 oceanic advection along isopycnal surfaces from the midlatitude subduction regions to the tropics 2 coastally trapped or kelvin wave propagation between the tropics and midlatitudes and 3 near simultaneous communication between the regions affected by changes in the atmosphere it is found that communication via the atmosphere explains the strongest correlations found in the model further evidence is presented that is consistent with the idea that midlatitude sea surface temperature anomalies drive changes in the trade wind system that a

enso like interdecadal variability 1900 93 a number of recent studies have reported an enso like eof mode in the global sea surface temperature sst field whose time variability is marked by an abrupt change toward a warmer tropical eastern pacific and a colder extratropical central north pacific in 1976 77 the present study compares this pattern with the structure of the interannual variability associated with the enso cycle and documents its time history back to 1900 the analysis is primarily based on the leading eofs of the sst anomaly and anomaly deviation fields in various domains and the associated expansion coefficient or principal component time series which are used to construct global regression maps of sst sea level pressure slp and a number of related variables the use of anomaly deviations i e departures of local sst anomalies from the concurrent global mean sst anomaly reduces the influence of global mean sst trends upon the structure of the eofs and their expansion coefficient time series an important auxiliary time series used in this study is a southern oscillation index based on marine surface observations by means of several different analysis techniques the time variability of the leading eof of the global sst field is separated into two components one identified with the enso cycle related variability on the interannual timescale and the other a linearly independent residual comprising all the interdecadal variability in the record the two components exhibit rather similar spatial signatures in the global sst slp and wind stress fields the sst signature in the residual variability is less equatorially confined in the eastern pacific and it is relatively more prominent over the extratropical north pacific the corresponding slp signature is also stronger over the extratropical north pacific and its counterpart in the cold season 500 mb height field more closely resembles the pna pattern the amplitude time series of the enso like pattern in the residual variability reflects the above mentioned shift in 1976 77 as well as a number of other prominent features including a shift of opposite polarity during the 1940s

interdecadal pacific oscillation reconstructed from trans pacific tree rings 1350 2004 ce the interdecadal pacific oscillation ipo represents the decadal mode of the el nino southern oscillation phenomenon as such the ipo is one of the dominant modes of decadal climate variability on both sides of the pacific ocean basin for this paper we utilized a newly developed tree ring data network comprised of five multi centennial vietnamese cypress ring width chronologies that range from around 11 n 23 n latitude we combined these data with an average of tree ring derived drought indices from the north american drought atlas over the south central usa from a box that spans from 27 5 n 35 0 n 85 0 110 0 w and contains 55 gridpoints and more than 100 tree ring site chronologies these 2 locations exhibit rainfall variations that are strongly negatively correlated with each other and are representative of the influence of the ipo on terrestrial rainfall the final reconstruction model weighted most heavily on the three most southerly of the five vietnamese cypress records spans from 1350 to 2004 and explains nearly 57 of the variance in the original ipo data for the 5 month season of october february the reconstruction model passes all standard statistical tests using a split calibration verification scheme we reveal 15 positive and 15 negative phase shifts of the ipo prior to the period of instrumentation suggesting that the ipo has been active for at least the past seven centuries with varying degrees of intensity we compare our reconstruction with two related millennial records the macdonald and case geophys res lett 32 8 l08703 2005 tree ring derived reconstruction of the pacific decadal oscillation and an ice core derived reconstruction of the ipo from the law dome ice core in antarctica by vance et al geophys res lett 33 6 l06712 2015 while there is good general agreement with the latter record there are three key periods of the past where the two records are out of phase and we explore the reasons for this disparity direct comparison with the related tripole index tpi shows weaker correlation likely owing to the stronger relationship between our tree ring data and the equatorial pacific region relative to the north and south regions of the pacific that combine to comprise the tpi calculation

factors that affect the amplitude of el nino in global coupled climate models historically el nino like events simulated in global coupled climate models have had reduced amplitude compared to observations here el nino like phenomena are compared in ten sensitivity experiments using two recent global coupled models these models have various combinations of horizontal and vertical ocean resolution ocean physics and atmospheric model resolution it is demonstrated that the lower the value of the ocean background vertical diffusivity the greater the amplitude of el nino variability which is related primarily to a sharper equatorial thermocline among models with low background vertical diffusivity stronger equatorial zonal wind stress is associated with relatively higher amplitude el nino variability along with more realistic east west sea surface temperature sst gradient along the equator the sst seasonal cycle in the eastern tropical pacific has too much of a semiannual component with a double intertropical convergence zone itcz in all experiments and thus does not affect nor is it affected by the amplitude of el nino variability systematic errors affecting the spatial variability of el nino in the experiments are characterized by the eastern equatorial pacific cold tongue regime extending too far westward into the warm pool the time scales of interannual variability as represented by time series of nino3 ssts show significant power in the 3 4 year enso band and 2 2 5 year tropospheric biennial oscillation tbo band in the model experiments the tbo periods in the models agree well with the observations while the enso periods are near the short end of the range of 3 6 years observed during the period 1950 94 the close association between interannual variability of equatorial eastern pacific ssts and large scale sst patterns is represented by significant correlations between nino3 time series and the pc time series of the first eofs of near global ssts in the models and observations

linear and nonlinear winter atmospheric responses to extreme phases of low frequency pacific sea surface temperature variability this study examines northern hemisphere winter djfm atmospheric responses to opposite strong phases of interdecadal low frequency lf pacific sea surface temperature sst forcing which resembles el nino southern oscillation enso on a longer time scale in observations and gfdl and cam4 model simulations over the pacific north america pna sector linear observed responses of 500 hpa height z500 anomalies resemble the pna teleconnection pattern but show a pna like nonlinear response because of a westward z500 shift in the negative lf relative to the positive lf lf phase significant extratropical linear responses include a north atlantic oscillation nao like z500 anomaly a dipole like z500 anomaly over northern eurasia associated with warming over mid high latitude eurasia and a southern annular anomaly pattern associated with warming in southern land areas significant nonlinear z500 responses also include a nao like anomaly pattern models forced by lf and lf sst anomalies reproduce many aspects of observed linear and nonlinear responses over the pacific north america sector and linear responses over southern land but not in the north atlantic european sector and eurasia both models simulate pna like linear responses in the north pacific north america region similar to observed but show larger pna like lf responses resulting in a pna nonlinear response the nonlinear pna responses result from both nonlinear western tropical pacific rainfall changes and extratropical transient eddy feedbacks with lf tropical pacific forcing only lftp and lftp climatological sst elsewhere cam4 simulates a significant nao response to lftp including a linear negative and nonlinear positive nao response

pacific decadal variability the tropical pacific mode and the north pacific mode pacific decadal variability is studied in a series of coupled global ocean atmosphere simulations aided by two modeling surgery strategies partial coupling pc and partial blocking pb the pc experiments retain full ocean atmosphere coupling in selected regions but constrain ocean atmosphere coupling elsewhere by prescribing the model climatological sst to force the atmospheric component of the coupled system in pb experiments sponge walls are inserted into the ocean component of the coupled model at specified latitudinal bands to block the extratropical tropical oceanic teleconnection both modeling and observational studies suggest that pacific decadal variability is composed of two distinct modes a decadal to bidecadal tropical pacific mode tpm and a multidecadal north pacific mode npm the pc and pb experiments showed that the tropical pacific mode originates predominantly from local coupled ocean atmosphere interaction within the tropical pacific extratropical tropical teleconnections although not a necessary precondition for the genesis of the tropical decadal variability can enhance sst variations in the tropics the decadal memory in the tropics seems to be associated with tropical higher baroclinic modes the north pacific mode originates from local atmospheric stochastic processes and coupled ocean atmosphere interaction atmospheric stochastic forcing can generate a weaker npm like pattern in both the atmosphere and ocean but with no preferred timescales in contrast coupled ocean atmosphere feedback can enhance the variability substantially and generate a basin scale multidecadal mode in the north pacific the multidecadal memory in the midlatitudes seems to be associated with the delayed response of the subtropical subpolar gyre to wind stress variation in the central north pacific and the slow growing decaying of sst anomalies that propagate eastward in the kuroshio extension region oceanic dynamics particularly the advection of the mean temperature by anomalous meridional surface ekman flow and western boundary currents plays an important role in generating the north pacific mode

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

the asian australian monsoon and el nino southern oscillation in the ncar climate system model features associated with the asian australian monsoon system and el nino southern oscillation enso are described in the national center for atmospheric research ncar global coupled climate system model csm simulation characteristics are compared with a version of the atmospheric component of the csm the ncar ccm3 run with time evolving ssts from 1950 to 1994 and with observations the csm is shown to represent most major features of the monsoon system in terms of mean climatology interannual variability and connections to the tropical pacific this includes a representation of the southern oscillation links between strong asian australian monsoons and associated negative sst anomalies in the eastern equatorial pacific the equatorial sst gradient across the pacific in the csm is shown to be similar to the observed with somewhat cooler mean ssts across the entire pacific by about 18 28c the seasonal cycle of ssts in the eastern equatorial pacific has the characteristic signature seen in the observations of relatively warmer ssts propagating westward in the first half of the year followed by the reestablishment of the cold tongue with relatively colder ssts propagating westward in the second half of the year like other global coupled models the propagation is similar to the observed but with the establishment of the relatively warmer water in the first half of the year occurring about 1 2 months later than observed the seasonal cycle of precipitation in the tropical eastern pacific is also similar to other global coupled models in that there is a tendency for a stronger than observed double itcz year round particularly in northern spring but with a well reproduced annual maximum of itcz strength north of the equator in the second half of the year time series of area averaged ssts for the nino3 region in the eastern equatorial pacific show that the csm is producing about 60 of the amplitude of the observed variability in that region consistent with most other global coupled models global correlations between nino3 time series global surface temperatures and sea level pressure slp show that the csm qualitatively reproduces the major spatial patterns associated with the southern oscillation lower slp in the central and eastern tropical pacific when nino3 ssts are relatively warmer and higher slp over the far western pacific and indian oceans with colder water in the northwest and southwest pacific indices of asian australian monsoon strength are negatively correlated with nino3 ssts as in the observations spectra of time series of indian monsoon australian monsoon and nino3 sst indices from the csm show amplitude peaks in the southern oscillation and tropospheric biennial oscillation frequencies 3 6 yr and about 2 3 yr respectively as observed lag correlations between the nino3 sst index and upper ocean heat content along the equator show eastward propagation of heat content anomalies with a phase speed of about 0 3 m s21 compared to observed values of roughly 0 2 m s21 composites of el nino la nina events in the csm show similar seasonal evolution to composites of observed events with warming cooling of greater than several tenths of a degree beginning early in northern spring of year 0 and diminishing around northern spring of year 11 but with a secondary resurgence in the csm events later in northern spring of year 11 the csm also shows the largest amplitude enso sst and low level wind anomalies in the western tropical pacific with enhanced interannual variability of ssts extending northeastward and southeastward toward the subtropics compared to largest interannual sst variability in the central and eastern tropical pacific in the observations

megadroughts in the indian monsoon region and southwest north america and a mechanism for associated multidecadal pacific sea surface temperature anomalies abstract a 1360 yr control run from a global coupled climate model the parallel climate model is analyzed it simulates megadroughts in the southwestern united states and indian monsoon regions the megadroughts represent extreme events of naturally occurring multidecadal precipitation variations linked to the dominant pattern of multidecadal sst variability in the indian and pacific oceans gaining insight into the occurrence of megadroughts thus requires an understanding of the mechanism that is producing this multidecadal sst variability analysis of the model variability shows that the mechanism involves atmosphere ocean and tropical midlatitude interactions with a crucial element being wind forced ocean rossby waves near 20 n and 25 s in the pacific whose transit times set the decadal time scale at the western boundary the rossby waves reflect into the equatorial pacific to affect thermocline depth the resulting feedbacks involving surface temperature winds and the strength of the subtropi

characterizing decadal to centennial variability in the equatorial pacific during the last millennium 1 the magnitude of sea surface temperature variability in the nino3 4 region of the equatorial pacific on decadal and longer timescales is assessed in observational data state of the art coupled model intercomparison project 5 climate model simulations and a new ensemble of paleoclimate reconstructions on decadal to multidecadal timescales variability in these records is consistent with the null hypothesis that it arises from multivariate red noise a multivariate ornstein uhlenbeck process generated from a linear inverse model of tropical ocean atmosphere dynamics on centennial and longer timescales both a last millennium simulation performed using the community climate system model 4 ccsm4 and the paleoclimate reconstructions have variability that is significantly stronger than the null hypothesis however the time series of the model and the reconstruction do not agree with each other in the model variability primarily reflects a thermodynamic response to reconstructed solar and volcanic activity whereas in the reconstruction variability arises from either internal climate processes forced responses that differ from those in ccsm4 or nonclimatic proxy processes that are not yet understood these findings imply that the response of the tropical pacific to future forcings may be even more uncertain than portrayed by state of the art models because there are potentially important sources of century scale variability that these models do not simulate

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