interannual enso and interdecadal enso like variability in the southern hemisphere tropospheric circulation abstract recent work has identified variability in the pacific ocean sst with a structure qualitatively similar to enso but at lower frequencies than enso zhang et al have documented the atmospheric circulation anomalies in the tropics and northern hemisphere that are associated with decadal enso like variability and compared these anomalies to those associated with the interannual enso cycle here the authors extend the study of zhang et al to the southern hemisphere using the national centers for environmental prediction national center for atmospheric research reanalysis data for 1958 96 consistent with previous studies the southern hemisphere circulation anomalies associated with enso display a teleconnection pattern from the central tropical pacific into the far southeastern pacific ocean comparatively larger circulation anomalies are found in the southern hemisphere associated with the decadal enso like variability though aloft the structure of the anomalies emphasizes the polar vortex wit

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

the role of linear wave refraction in the transient eddy mean flow response to tropical pacific sst anomalies the midlatitude response to tropical pacific sst anomalies involves changes in transient eddy propagation but the processes leading to the transient eddy changes are still not clear in a recent study we used a series of controlled general circulation model gcm experiments in which an imposed tropical pacific sea surface temperature sst anomaly is turned on abruptly and the response is analyzed in terms of its high and low frequency parts to show that the changes in transient eddies induced by el nino southern oscillation enso arise from changes in wave refraction on the altered mean flow in this work we use a quasi geostrophic linear model and a linear stationary wave model to interpret the gcm experiments and obtain the sequence of events that lead from a tropical sst anomaly to the quasi equilibrium change in the mean and transient atmospheric circulation the initial direct response of the mean flow is confined to the tropical and subtropical pacific similar to what is obtained from a stationary wave model this tropical subtropical mean flow change initiates a transient eddy response which induces a midlatitude mean flow anomaly the wave mean flow system evolves towards a state in which the eddy anomalies maintain the mean flow anomalies allowing them to persist it is further shown that while eddy momentum fluxes persistently accelerate and decelerate the subtropical and midlatitude mean flow the eddy heat flux effect on the zonal mean flow is much more variable and only marginally significant the linear quasi geostrophic model calculations capture the evolution of eddy momentum flux anomalies equatorwards of 60 n quite well suggesting linear wave refraction can explain the midlatitude enso anomalies however other processes like stationary waves or changes in the nonlinear stage of eddy life cycles are needed to explain the enso related anomalies at high latitudes polewards of around 60 n copyright 2010 royal meteorological society

tectonic feedback intraplate orogeny and the geochemical structure of the crust a central australian perspective the geological record of intraplate deformation in central australia implies that past tectonic activity basin formation deformation and erosion has modulated the response of the lithosphere during subsequent tectonic activity in particular there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin this behaviour can be understood as a kind of tectonic feedback effected by the longterm thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism from a geochemical point of view one of the most dramatic effects of intraplate orogeny in central australia has been the exposure in the cores of the orogens of deep crustal rocks largely depleted in the heat producing elements the geochemical structuring of the crust associated with the erosion of the heat producing upper crust resulted in long term cooling of the deep crust and upper mantle with associated lithospheric strengthening this is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the h qc plane where h is the characteristic length scale for heat production distribution and qc is the total crustal heat production because rates of intraplate deformation in central australia appear to be much slower than that typical of plate margin orogens it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central australia by providing a thermal lock the diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central australian intraplate orogens central australia provides an intriguing record of intraplate deformation resulting in the development of a number of extraordinary orogens that expose deep crustal rocks in their cores forman 1971 teyssier 1985 goleby et al 1989 the spatial distribution of intraplate deformation during the alice springs and petermann orogenies suggests that the mechanical response of this region has varied through time consequently this response is important for our understanding of the processes controlling reactivation in intraplate settings in previous contributions it has been argued that the distribution of deformation in central australia has been strongly influenced by prior tectonic processes such as basin formation and erosion sandiford hand 1998a hand sandiford 1999 this paper focusses on the way in which intraplate deformation modifies the geochemical structure of the crust in this region in particular the distribution of heat sources recognizing that the distribution of heat sources provides a first order control on the thermal structure of the crust the consequences of intraplate deformation for the longterm thermal and mechanical evolution of the crust is then explored our aim is to demonstrate the relevance of a novel style of tectonic feedback which seems to have modulated the longterm tectonic behaviour of the central australian region and which may be relevant to continental interiors in general the notion of tectonic feedback is compatible with a highly temperature dependent lithospheric rheology as suggested by many experimental studies of natural rocks such feedback has profound implications for the longterm tectonic evolution of continental interiors particularly their potential for reactivation the record of intraplate deformation in

the little ice age maximum in the southern alps new zealand preliminary results at mueller glacier lichenometric dating studies using the yellow green rhizocarpon subgenus at mueller glacier in the region of mt cook southern alps new zealand reveal a little ice age maximum about ad 1725 1730 differences from previous studies are shown to result from different methods especially the type of mathematical functions used for calculating the lichenometric dating curves previous work also used variable sample sizes distal moraine slopes and to some extent different fixed points schmidt hammer measurements enable the differentiation of little ice age from pre little ice age frontal moraines in the outer glacier foreland and confirm the status of the little ice age as a separate glacier advance among several advances during the holocene

paleoclimate reconstruction along the pole equator pole transect of the americas pep 1 examples are presented of inter hemispheric comparison of instrumental climate and paleoclimate proxy records from the americas for di erent temporal scales despite a certain symmetry of seasonal precipitation patterns along the pep 1 transect decadal variability of winter precipitation shows di erent characteristics in terms of amplitude and frequency in both the last 100 and last 1000 years such di erences in variability are also seen in a comparison of time series of di erent el nin8 o southern oscillation proxy records from north and south america however these di erences do not appear to a ect the spatial correlation with paci c sea surface temperature patterns local and regional di erences in response to climate change are even more pronounced for records with lower temporal resolution and inter hemispheric synchroneity may or may not be indicative of the same forcing this aspect is illustrated in an inter hemispheric comparison of the last 1000 years of glacier variability and of the full and lateglacial lake level history 1999 elsevier science ltd all rights reserved

tropical oceanic causes of interannual to multidecadal precipitation variability in southeast south america over the past century abstract observations atmosphere models forced by historical ssts and idealized simulations are used to determine the causes and mechanisms of interannual to multidecadal precipitation anomalies over southeast south america sesa since 1901 about 40 of sesa precipitation variability over this period can be accounted for by global sst forcing both the tropical pacific and atlantic oceans share the driving of sesa precipitation with the latter contributing the most on multidecadal time scales and explaining a wetting trend from the early midcentury until the end of the last century cold tropical atlantic sst anomalies are shown to drive wet conditions in sesa the dynamics that link sesa precipitation to tropical atlantic sst anomalies are explored cold tropical atlantic sst anomalies force equatorward flowing upper tropospheric flow to the southeast of the tropical heating anomaly and the vorticity advection by this flow is balanced by vortex stretching and ascent which drives the increased prec

el nino as a mediator of the solar influence on climate 1 using a climate model of intermediate complexity we simulate the response of the el nino southern oscillation enso system to solar and orbital forcing over the holocene solar forcing is reconstructed from radiocarbon production rate data using various scaling factors to account for the conflicting estimates of solar irradiance variability as estimates of the difference since the maunder minimum range from 0 05 to 0 5 of the solar constant we consider these two extreme scenarios along with the intermediate case of 0 2 we show that for large or moderate forcings the low pass filtered east west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing with a short phase lag about a decade wavelet analysis shows a statistically significant enhancement of the century to millennial scale enso variability for even a moderate irradiance forcing in contrast the 0 05 case displays no such enhancement orbitally driven insolation forcing is found to produce a long term increase of enso variability from the early holocene onward in accordance with previous findings when both forcings are combined the superposition is approximately linear in the strong scaling case overall the sea surface temperature response is of the magnitude required and is persistent enough to induce important climatic perturbations worldwide the results suggest that enso may plausibly have acted as a mediator between the sun and the earth s climate a comparison to key holocene climate records from the northern hemisphere subtropics and midlatitudes shows support for this hypothesis

cross seasonal influence of the december february southern hemisphere annular mode on march may meridional circulation and precipitation abstractnew evidence suggests that interannual variability in zonal mean meridional circulation and precipitation can be partially attributed to the southern hemisphere annular mode sam the dominant mode of climate variability in the southern hemisphere sh extratropics a cross seasonal correlation exists between the december february djf sam and march may mam zonal mean meridional circulation and precipitation this correlation is not confined to the sh it also extends to the northern hemisphere nh subtropics when the preceding djf sam is positive counterclockwise and clockwise meridional cells accompanied by less and more precipitation occur alternately between the sh middle latitudes and nh subtropics in mam in particular less precipitation occurs in the sh middle latitudes the sh tropics and the nh subtropics but more precipitation occurs in the sh subtropics and the nh tropics a framework is built to explain the cross seasonal impact of sam related sst anomalies evidence indic

contrasting impacts of two types of enso on the boreal spring hadley circulation the possible influences of two types of enso i e the canonical enso and enso modoki em on hadley circulation hc during the boreal spring are investigated during 1979 2010 el ni no events are featured with a symmetric pattern in equatorial zonal mean sea surface temperature anomalies ssta with a maximum around the equator in contrast the zonal mean ssta associated with el ni modoki events shows an asymmetric structure with a maximum around 108n the contrasting underlying thermal structures corresponding with enso and em have opposite impacts on the simultaneous hc in el ni years a symmetric anomalous meridional circulation is seen with enhanced rising around the equator and anomalousdescentatabout158nand208s incontrast anasymmetricequatorialmeridionalcirculationisobserved for el ni modoki years with anomalous ascent around 108n and descent at about 108s and 208n the contrasting meridional circulation anomalies within enso and em are caused by their different meridional ssta structure this result is theoretically explained indicating that anomalous meridional circulation is subject to the meridional ssta gradient moreover the observed results are reproduced in numerical experiments driven by anomalous warming in the eastern and central pacific ocean thus the authors conclude that the anomalous hc linked to enso and em is induced by the accompanying meridional gradient in zonal mean ssta

diagnosing northern hemisphere jet portrayal in 17 cmip3 global climate models twenty first century projections the anthropogenic climate change impacts on the eddy jet system include an intensified midlatitude jet stream and an elevated tropopause as well as a poleward shifted jet while both responses are evident in phase 3 of the coupled model intercomparison project cmip3 ensemble mean twenty first century projections uncertainty in the poleward shift response is large enough that even the sign of the shift is not consistent among all models especially in the northern hemisphere the present analysis finds that twentyfirst century projections of the ensemble mean zonal wind change at 300 hpa predict a weakening and poleward expansion of the pacific jet and an overall expansion of the atlantic jet in contrast with the direct zonal mean climate change signal of increasing midlatitude upper level winds zonal winds are projected to decreaseinthecoreofthepacificandatlanticjets withincreasingzonalwindslocatedprimarilyinthejetexit regions and the meridional flanks of the jets uncertainties in sst changes from the twentieth century to the twenty first century between models are shown to impact modeled northern hemisphere jet stream changes

the relationship between the itcz and the southern hemispheric eddy driven jet 1 we study the effect of a thermal forcing confined to the midlatitudes of one hemisphere on the eddy driven jet in the opposite hemisphere we demonstrate the existence of an interhemispheric teleconnection whereby warming cooling the northern hemisphere causes both the intertropical convergence zone itcz and the southern hemispheric midlatitude jet to shift northward southward the interhemispheric teleconnection is effected by a change in the asymmetry of the hadley cells as the itcz shifts away from the equator the cross equatorial hadley cell intensifies fluxing more momentum toward the subtropics and sustaining a stronger subtropical jet changes in subtropical jet strength in turn alter the propagation of extratropical waves into the tropics affecting eddy momentum fluxes and the eddy driven westerlies the relevance of this mechanism is demonstrated in the context of future climate change simulations where shifts of the itcz are significantly related to shifts of the southern hemispheric eddy driven jet in austral winter the possible relevance of the proposed mechanism to paleoclimates is discussed particularly with regard to theories of ice age terminations

adjustment of the atmospheric circulation to tropical pacific sst anomalies variability of transient eddy propagation in the pacific north america sector nas a pattern that is shifted southward northward compared with climatology paths of coherent phase propagation of transient eddies and of the propagation of wave packets are analyzed a coherent path of propagation across the pacific towards north america is identified that is more zonal during el niwinters and during la nihas a dominant path heading northeastward to the pacific northwest a second path heading southeastward from the central pacific to the tropical east pacific is more accentuated during la ni nas than el ni nos these changes in wave propagation are reproduced in an ensemble of seasonal integrations of a general circulation model forced by a tropical pacific sea surface temperature pattern confirming that the changes are forced by changes in the mean atmospheric state arising from changes in tropical sea surface temperature a simplified model with a specified basic state is used to model the storm tracks for el ni no and la ni na winters the results suggest that the changes in transient eddy propagation and the eddy statistics can be understood in terms of the refraction of transient eddies within different basic states copyright c 2010 royal meteorological society

teleconnections between tropical pacific sst anomalies and extratropical southern hemisphere climate abstractteleconnections from tropical pacific sea surface temperature sst anomalies to the high latitude southern hemisphere sh are examined using observations and reanalysis analysis of tropical pacific sst anomalies is conducted separately for the central pacific cp and eastern pacific ep regions during the austral cold season extratropical sh atmospheric rossby wave train patterns are observed in association with both ep and cp sst variability the primary difference between the patterns is the westward displacement of the cp related atmospheric anomalies consistent with the westward elongation of cp related convective sst required for upper level divergence and rossby wave generation consequently cp related patterns of sh sst antarctic sea ice and temperature anomalies also exhibit a westward displacement but otherwise the cold season extratropical sh teleconnections are largely similar during the warm season however extratropical sh teleconnections associated with tropical cp and

influence of tropical pacific sea surface temperature on the genesis of gulf stream cyclones abstractthis study investigates the relationship between tropical pacific sea surface temperature sst variability and cyclogenesis over the gulf stream region of the north atlantic a cyclone identification scheme and lagrangian trajectories are used to compare preferred cyclogenesis locations and precyclogenesis flow paths associated with three patterns of tropical pacific sst variability eastern pacific ep el nino central pacific cp el nino and la nina during ep el nino and la nina winters the upper level precyclogenesis flow takes a subtropical path over north america and gulf stream cyclogenesis predominantly occurs under the north atlantic jet entrance which is the climatologically preferred location in contrast during cp el nino winters when the warmest sst anomalies occur in the central tropical pacific the precyclogenesis flow takes a northern path across north america and gulf stream cyclogenesis tends to occur farther north under the jet exit the shift in preferred cyclogenesis

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

observed relationships between the el nino southern oscillation and the extratropical zonal mean circulation abstract there is increasing evidence indicating that the climate response to variations in the el nino southern oscillation enso includes not only thermally forced zonal wind anomalies in the subtropics but also eddy driven zonal wind anomalies that extend into the mid high latitudes of both hemispheres in this study new insights into the observed seasonally varying signature of enso in the extratropical zonal mean circulation are provided and the associated linkages with the dominant patterns of extratropical variability are examined the zonal mean extratropical atmospheric response to enso is characterized by two principal features an equivalent barotropic dipole in the southern hemisphere sh zonal mean zonal flow with centers of action located near 40 and 60 during austral summer and a weaker but analogous dipole in the northern hemisphere nh with centers of action located near 25 and 45 during early and late boreal winter both structures are accompanied by eddy momentum flux ano

a note on orbital control of equator pole heat fluxes we note that orbital milankovitch variations in particular the precession of the equinoxes can lead to profound variations in the flux of heat from the tropics to higher latitudes the mechanism involves changing the intensity of the hadley circulation by varying the maximum displacement from the equator of the zonally averaged surface temperature maximum in summer the precession of the equinoxes causes this quantity to vary by more than a factor of 2 the intensity of the hadley circulation has a major influence on the heat fluxes in the winter hemisphere summer heat fluxes are generally small although the precession cycle is characterized by periods in the neighborhood of 20000 years the variations are modulated by the eccentricity whose variation is dominated by periods in the neighborhood of 100000 years and 400000 years we show how the fact that both small and large heat fluxes lead to low snowfall and hence small glacial accumulation causes the demodulation of the heat flux leading to dominant eccentricity periods in the resulting glaciation

austral spring southern hemisphere circulation and temperature changes and links to the spcz abstractsignificant austral spring trends have previously been observed in west antarctica and antarctic peninsula temperatures and in atmospheric circulation across the southern pacific and atlantic here physical mechanisms for the observed trends are investigated through analysis of monthly circulation and temperatures from the era interim dataset and outgoing longwave radiation olr data the negative pressure trend over the south pacific during spring is strongest in september while the positive pressure trend over the south atlantic is strongest in october pressure trends in november are generally nonsignificant the authors demonstrate that a significant september trend toward increased convection reduced olr in the poleward portion of the south pacific convergence zone spcz is statistically related to rossby wave like circulation changes across the southern oceans the wave response is strongest over the south pacific in september and propagates eastward to the south atlantic in october o

the vertical and spatial structure of enso in the upper troposphere and lower stratosphere from gps radio occultation measurements 1 the vertical and spatial structure of the atmospheric el nino southern oscillation enso signal is investigated using radio occultation ro data from august 2006 to december 2010 due to their high vertical resolution and global coverage ro data are well suited to describe the full 3 dimensional enso structure in the troposphere and lower stratosphere we find that interannual temperature anomalies in the equatorial region show a natural decomposition into zonal mean and eddy deviations from the zonal mean components that are both related to enso consistent with previous studies we find that during the warm phase of enso zonal mean temperatures increase in the tropical troposphere and decrease in the tropical stratosphere maximum warming occurs above 8 km and the transition between warming and cooling occurs near the tropopause this zonal mean response lags sea surface temperature anomalies in the eastern equatorial pacific by 3 months the atmospheric eddy component in contrast responds rapidly within 1 month to enso forcing this signal features a low latitude dipole between the indian and pacific oceans with off equatorial maxima centered around 20 to 30 latitude in both hemispheres the eddy response pattern attains maximum amplitude in the upper troposphere near 11 km and with opposite polarity in a shallow layer near the tropopause at approximately 17 km the eddy enso signal tends to be out of phase between low and middle latitudes in both the troposphere and lower stratosphere citation scherllin pirscher b c deser s p ho c chou w randel and y h kuo 2012 the vertical and spatial structure of enso in the upper troposphere and lower stratosphere from gps radio occultation measurements geophys res lett 39 l20801 doi 10 1029 2012gl053071

wind driven shifts in the latitude of the kuroshio oyashio extension and generation of sst anomalies on decadal timescales abstract the causes of decadal variations of north pacific sea surface temperatures ssts are examined using a hindcast performed with an ocean general circulation model thermodynamically coupled to an atmospheric mixed layer model ogcm aml model and forced by the time history of observed surface winds the shift in north pacific ocean climate that occurred around 1976 77 is focused on since this is the best observed example available after the 1976 77 shift the aleutian low deepened and moved to the southeast of its previous position this placed anomalous cyclonic flow over the north pacific the sst response as simulated by the ocean model consisted of two components a fast and local part and a delayed and remote part in the central pacific stronger westerlies cool the ocean by increased equatorward ekman drift here the dynamical cooling is sufficiently large that the surface fluxes damp the sst anomaly this ekman response is fast and local and cools the ssts beginning in 1977 and persistin

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

tropical extratropical teleconnections in boreal summer observed interannual variability abstractmaximum covariance analysis is performed on the fields of boreal summer tropical rainfall and northern hemisphere nh 200 hpa height for the 62 yr period of record of 1948 2009 the leading mode which appears preferentially in summers preceding the peak phases of the el nino southern oscillation enso cycle involves a circumglobal teleconnection cgt pattern in the nh extratropical 200 hpa height field observed in association with indian monsoon rainfall anomalies the second mode which tends to occur in summers following enso peak phases involves a western pacific north america wpna teleconnection pattern in the height field observed in association with western north pacific summer monsoon rainfall anomalies the cgt pattern is primarily a zonally oriented wave train along the westerly waveguide while the wpna pattern is a wave train emanating from the western pacific monsoon trough and following a great circle the cgt is accompanied by a pronounced tropical extratropical seesaw in t

el nino flavors and their simulated impacts on atmospheric circulation in the high southern latitudes abstracttwo el nino flavors have been defined based on whether warm sea surface temperature sst anomalies are located in the central or eastern tropical pacific cp or ep this study further characterizes the impacts on atmospheric circulation in the high latitudes of the southern hemisphere associated with these types of el nino events though a series of numerical simulations using the national center for atmospheric research community atmosphere model cam comparing results with the interim ecmwf re analysis era interim cam simulates well the known changes to blocking over australia and a southward shift in the subtropical jet stream across the eastern pacific basin during cp events more importantly for the high southern latitudes cam simulates a westward shift in upper level divergence in the tropical pacific which causes the pacific south american stationary wave pattern to shift toward the west across the entire south pacific these changes to the rossby wave source region impact the south

tropical pacific forcing of north american medieval megadroughts testing the concept with an atmosphere model forced by coral reconstructed ssts the possible role that tropical pacific ssts played in driving the megadroughts over north america during the medieval period is addressed fossil coral records from the palmyra atoll are used to derive tropical pacific ssts for the period from a d 1320 to a d 1462 and show overall colder conditions as well as extended multidecadal la nina like states the reconstructed ssts are used to force a 16 member ensemble of atmosphere gcm simulations each with different initial conditions with the atmosphere coupled to a mixed layer ocean outside of the tropical pacific model results are verified against north american tree ring reconstructions of the palmer drought severity index a singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16 member ensemble of simulations forced by global observed ssts for 1856 2004 and tree ring reconstructions of the palmer drought severity index for the same period this relationship is used to transfer the modeled medieval soil moisture anomaly relative to the modern simulation into a model estimated palmer drought severity index the model estimated palmer drought severity index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions in addition to an overall drier climate that is drier than the tree ring records suggest the model estimated palmer drought severity index simulates two previously identified megadroughts a d 1360 1400 and a d 1430 60 with a realistic spatial pattern and amplitude in contrast the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts the dynamical link between tropical ssts and the north american megadroughts is akin to that operating in modern droughts the model results are used to argue that the tropical pacific played an active role in driving the megadroughts however the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral reconstructed sst anomalies contain significant errors and that sst anomalies in other basins also played a role in driving hydroclimate variations over north america during the late medieval period

orbital controls on the el nino southern oscillation and the tropical climate the global synchroneity of glacial interglacial events is one of the major problems in understanding the link between milankovitch forcing and the climate of the late quaternary in this study we isolate a part of the climate system the tropical pacific and test its sensitivity to changes in solar forcing associated with changes in the earth s orbital parameters we use a simplified coupled ocean atmosphere model that is run for the past 150 000 years and forced with milankovitch changes in the solar insolation this system responds primarily to the precessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distribution of solar radiation even while the annual mean insolation is roughly constant the mean response to the precessional forcing is due to an interaction between an altered seasonal cycle and the el nifio southern oscillation enso changes in the enso behavior result in a mean tropical climate change the hypothesis is advanced that such a change in the tropical climate can generate a globally synchronous climate response to milankovitch forcing

afro eurasian intermediate frequency teleconnection and modulation by enso abstractthe afro eurasian intermediate frequency atmospheric teleconnection conveys meteorological signals zonally leads to various atmospheric variations and causes extreme events along its path

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

variability of boreal spring hadley circulation over the asian monsoon domain and its relationship with tropical sst the variability of boreal spring hadley circulation hc over the asian monsoon domain over the last four decades is explored the climatological distribution of the regional hc is symmetric of the equator with the ascending branch around the equator and sinking branch around the subtropics in each hemisphere the first dominant mode eof1 of the regional hc is equatorial asymmetric with the main body in the southern hemisphere sh and the ascending branch to the north of the equator this mode is mainly characterized by interannual variation and is related to el nino southern oscillation enso significant negative sea surface temperature sst anomalies are observed over the tropical indian ocean tio along with the development of la nina events however the magnitude of sst anomalies in the southern indian ocean is greater than that in the northern counterpart contributing to eof1 formation the spatial distribution of the second dominant mode eof2 is with the main body lying in the northern hemisphere nh and the ascending branch located to the south of the equator the temporal variation of this mode is connected to the warming of the tio the warming rate of the southern tio sst is faster than that in the northern counterpart resulting in the southward migration of the rising branch the above result indicates the critical role of the meridional distribution of sst on the variability of the regional hc

links between the southern annular mode and the atlantic meridional overturning circulation in a climate model abstract the links between the atmospheric southern annular mode sam the southern ocean and the atlantic meridional overturning circulation amoc at interannual to multidecadal time scales are investigated in a 500 yr control integration of the l institut pierre simon laplace coupled model version 4 ipsl cm4 climate model the antarctic circumpolar current as described by its transport through the drake passage is well correlated with the sam at the yearly time scale reflecting that an intensification of the westerlies south of 45 s leads to its acceleration also in phase with a positive sam the global meridional overturning circulation is modified in the southern hemisphere primarily reflecting a forced barotropic response in the model the amoc and the sam are linked at several time scales an intensification of the amoc lags a positive sam by about 8 yr this is due to a correlation between the sam and the atmospheric circulation in the northern north atlantic that reflects a symmetric e

influences of enso on stratospheric variability and the descent of stratospheric perturbations into the lower troposphere abstractthe linkage between el nino southern oscillation enso and north atlantic oscillation nao through the stratospheric pathway is examined using a global coupled climate model gfdl climate model version 3 cm3 with increased vertical resolution and extent in the stratosphere as compared to an earlier model gfdl climate model version 2 cm2 it is demonstrated that the relationship between enso and nao is stronger in cm3 than in cm2 it is found that enso plays an important role in modulating the frequency of occurrence of the stratospheric polar vortex anomalies through enhancement attenuation of the amplitudes of zonal wavenumbers 1 and 2 especially in late winter a higher frequency of weak strong stratospheric vortex events is simulated in cm3 during el nino la nina episodes the weak vortex events during el nino winters are preceded by enhancement of the zonal wave 1 pattern and weakening of zonal wave 2 pattern these modified tropospheric planetary waves propagate upward and then w

north american droughts of the mid to late nineteenth century a history simulation and implication for mediaeval drought unlike the major droughts of the twentieth century that are readily identified in the instrumental record similar events in the nineteenth century have to be identified using a combination of proxy data historical accounts and a sparse collection of early instrumental records in the usa three distinct periods of widespread and persistent drought stand out in these records for the latter half of the nineteenth century 1856 1865 1870 1877 and 1890 1896 each of these events is shown to coincide with the existence of an anomalously cool la nina like tropical pacific to examine the physical mechanisms behind these droughts two ensembles of simulations with an atmosphere general circulation model agcm were generated the first forces an agcm with the observed history of sea surface temperatures ssts everywhere from 1856 to 2001 the goga experiment the second forces the agcm only with tropical pacific ssts being coupled to a two layer entraining mixed layer ml ocean elsewhere the poga ml experiment owing to a sparsity of instrumental precipitation data at this time proxy evidence from tree rings is used as verification a comparison of modelled soil moisture with tree ring reconstructions of the palmer drought severity index pdsi a proxy for soil moisture from the north american drought atlas is made both the poga ml and goga ensemble means capture the three multi year droughts of the mid to late nineteenth century indicating that the droughts were sst forced the similarity of the poga ml and goga simulations implies that the component of each drought signal that is forced by the sst is driven ultimately by the la nina like tropical pacific the global atmosphere ocean context of each of the mid to late nineteenth century droughts reveals a zonally and hemispherically symmetric pattern consistent with forcing from the tropics in addition rossby wave propagation from the cooler equatorial pacific amplifies dry conditions over the usa finally using published coral data for the last millennium to reconstruct a nino 3 4 history the modern day relationship between nino 3 4 and north american drought is applied to recreate two of the severest mediaeval drought epochs in the western usa the large scale spatial similarity to the drought atlas data demonstrates the potential link between a colder eastern equatorial pacific and the persistent north american droughts of the mediaeval period

mechanisms of enso forcing of hemispherically symmetric precipitation variability summary the patterns of precipitation anomalies forced by the el nino southern oscillation during northern hemisphere winter and spring are remarkably hemispherically symmetric and in the mid latitudes have a prominent zonally symmetric component observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically forced changes in the subtropical jets during el nino events the jets strengthen in each hemisphere and shift equatorward changes in the subtropical jet influence the transient eddy momentum fluxes and the eddy driven mean meridional circulation during el nino events eddy driven ascent in the mid latitudes of each hemisphere is accompanied by low level convergence and brings increased precipitation this changes in the transient eddy and stationary eddy moisture fluxes almost exactly cancel each other and in sum do not contribute to the zonal mean precipitation anomalies propagation of anomalous stationary waves disrupts the zonal symmetry flow around the deeper aleutian low and the eastward extension of the pacific jet stream supply the moisture for increased precipitation over the eastern north pacific and the western seaboard of the united states while transient eddy moisture convergence supplies the moisture for increased precipitation over the southern united states in each case increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes

the signature of enso in global temperature and precipitation fields derived from the microwave sounding unit abstract global temperature anomalies associated with enso are investigated making use of a 13 year record of gridded temperature and precipitation data from the microwave sounding unit msu the warm phase of the enso cycle during this period was characterized by an overall warming of the tropical troposphere superimposed upon a distinctive equatorially symmetric dumbbell shaped pattern straddling the equator near 140 w accompanied by negative anomalies along the equator over the western pacific by means of singular value decomposition svd analysis it is shown that this pattern fluctuated in phase with the displacements of convective activity over the equatorial pacific as reflected in the anomalies in outgoing longwave radiation olr and msu precipitation fields fluctuations in mean tropical tropospheric temperature lagged the olr anomalies and the related temperature pattern by about 3 months the same dumbbell shaped pattern was evident with reversed polarity in the lower stratosphere tog

influences of enso induced indo western pacific sst anomalies on extratropical atmospheric variability during the boreal summer the causes for the observed occurrence of anomalous zonally symmetric upper level pressure ridges in the midlatitude belts of both hemispheres during the year after warm el nino southern oscillation enso events have been investigated sea surface temperature sst anomalies in the indo western pacific iwp sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed enso episodes in the eastern central tropical pacific the spatiotemporal evolution of these sst conditions through a composite enso cycle was then inserted as lower boundary conditions within the iwp domain in an ensemble of atmospheric general circulation model gcm integrations this experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of enso the model evidence hence supports the notion that these globalscale atmospheric changes are primarily responses to sst perturbations in iwp which are in turn linked to enso variability in the equatorial pacific by the atmospheric bridge mechanism experimentation with a stationary wave model indicates that the eastern hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the iwp region which are closely related to the underlying sst changes diagnosis of the output from the gcm integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high frequency eddies at locations farther downstream the geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the western hemisphere such local eddy mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the iwp region analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in enso events could also generate zonally symmetric perturbations in midlatitudes the model simulated precipitation and surface temperature anomalies in the north american sector in response to sst changes in iwp suggest an increased frequency of droughts and heat waves in that region during the summer season after warm enso events

the interplay of internal and forced modes of hadley cell expansion lessons from the global warming hiatus the poleward branches of the hadley cells and the edge of the tropics show a robust poleward shift during the satellite era leading to concerns over the possible encroachment of the globe s subtropical dry zones into currently temperate climates the extent to which this trend is caused by anthropogenic forcing versus internal variability remains the subject of considerable debate in this study we use a joint eof method to identify two distinct modes of tropical width variability 1 an anthropogenically forced mode which we identify using a 20 member simulation of the historical climate and 2 an internal mode which we identify using a 1000 year pre industrial control simulation the forced mode is found to be closely related to the top of the atmosphere radiative imbalance and exhibits a long term trend since 1860 while the internal mode is essentially indistinguishable from the el nino southern oscillation together these two modes explain an average of 70 of the interannual variability seen in model edge indices over the historical period since 1980 the superposition of forced and internal modes has resulted in a period of accelerated hadley cell expansion and decelerated global warming i e the hiatus a comparison of the change in these modes since 1980 indicates that by 2013 the signal has emerged above the noise of internal variability in the southern hemisphere but not in the northern hemisphere with the latter also exhibiting strong zonal asymmetry particularly in the north atlantic our results highlight the important interplay of internal and forced modes of tropical width change and improve our understanding of the interannual variability and long term trend seen in observations

vertical structure physical properties and energy exchange of the east asian trough in boreal winter the relation between the intensity of the east asian trough and the strength of the east asian winter monsoon is documented a prominent dipole of the available potential energy of the zonal mean flow can be observed over the midlatitudes and polar regions and is related to temperature anomalies variation in the intensity of the east asian trough is caused by variation in the energy conversion from the available potential energy of the zonal mean flow to the east asian trough the in phase relation between the intensities of the east asian jet and east asian trough is illustrated by the relation between the potential energy and kinetic energy of stationary eddies the dipole of height anomalies over the midlatitude and polar regions develops in late autumn and decays in early spring we investigate the effect of both stationary and transient eddy forcing on the evolution of the dipole the combined forcing of the two types of eddy makes an important contribution to the development maintenance and decay of the dipole

seasonal to interannual response of southern ocean mixed layer depth to the southern annular mode from a global 1 10 ocean model abstractthe relationship between the southern annular mode sam and southern ocean mixed layer depth mld is investigated using a global 0 1 resolution ocean model the sam index is defined as t

the midlatitude warming during 1998 2000 large scale tropospheric warming of the northern and southern hemisphere extratropics occurred during the period january 1998 january 2000 the anomalies were remarkable for their amplitude their persistence their zonal symmetry with warming occurring at all longitudes between 30 50 degrees latitude and their axial symmetry with respect to the equator the origin of the anomalies are investigated using atmospheric general circulation models forced by the global sea surface temperatures ssts of this period two ensembles based on different models reproduce the key observed features indicating that the unusual climate state was forced by the global ssts however the atmospheric states were unrelated to the strong and protracted la nina of the east equatorial pacific ocean during this time we speculate on the role of the indo pacific warm pool which itself experienced unprecedented warming in 1998 and 1999

relation between annular modes and the mean state southern hemisphere summer abstract the annular modes emerge as the leading mode of extratropical month to month climate variability in both hemispheres here the influence of the background state on the structure and dynamics of the southern hemisphere annular mode sam during the austral summer when the climatology is characterized by a single well defined eddy driven jet is studied subsets of the climatology are constructed for early and late summer and for contrasting polarities of the enso cycle the analysis is based both on observations and on perpetual state gcm experiments the main differences between the subsets involve variations of the latitude of the mean jet it is found that in all the cases the sam is characterized by latitudinal shifts of the jet about its mean position reinforced by a positive momentum flux feedback from baroclinic waves this result is consistent with previous studies of the dynamics of the zonally averaged circulation but is found here to hold over all longitudes and for different positi

the signature of the annular modes in the tropical troposphere the extratropical annular modes are coupled with a distinct pattern of climate anomalies that spans the circulation of the tropical troposphere the signature of the annular modes in the tropical troposphere exhibits a high degree of equatorial symmetry it is associated with upper tropospheric zonal wind anomalies centered about the equator midtropospheric temperature anomalies located 208n and 208s and opposing mean meridional circulation anomalies that span the subtropics of both hemispheres the linkages between the annular modes and the tropical circulation are only evident during the cold season months and are most robust in association with the northern hemisphere annular mode nam the coupling between the annular modes and the circulation of the tropical troposphere is consistent with forcing by waves originating at extratropical latitudes both annular modes are characterized by anomalies in the eddy momentum flux convergence at tropical latitudes that act to reinforce the changes in the tropical wind and temperature fields the most pronounced tropical anomalies lag indices of the annular modes by 2 weeks and are found over the eastern tropical pacific where climatological westerlies permit extratropical waves to propagate into the deep tropics the linkages between the nam and the tropical tropospheric circulation are

teleconnections of the tropical atlantic and pacific oceans in a cmip5 model ensemble this study investigates the teleconnections between the tropical atlantic and pacific oceans in 15 state of the art fully coupled general circulation models and earth system models without external sst forcing in contrast to other studies the teleconnection is considered in both directions from the pacific to the atlantic and from the atlantic to the pacific the model ensemble is generally able to simulate the propagation of atmospheric and oceanic signals to the adjacent ocean basin generated by warm sea surface temperature sst anomalies in the tropical eastern oceans with atlantic summer events lagging or leading pacific boreal winter events this is investigated by means of time lagged composite analyses of different atmospheric parameters including sea level pressure wind stream function velocity potential vertical air movement and divergent wind at several levels however the modelled inter basin teleconnection and its correct frequency of occurrence depend on the strong warm sst biases in the atlantic benguela upwelling region and in the pacific ocean

on the speed of the eddy driven jet and the width of the hadley cell in the southern hemisphere a strong correlation between the speed of the eddy driven jet and the width of the hadley cell is found to exist in the southern hemisphere both in reanalysis data and in twenty first century integrations from the intergovernmental panel on climate change ipcc fifth assessment report multimodel archive analysis of the space time spectra of eddy momentum flux reveals that variations in eddy driven jet speed are related to changes in the mean phase speed of midlatitude eddies an increase in eddy phase speeds induces a poleward shift of the critical latitudes and a poleward expansion of the region of subtropical wave breaking the associated changes in eddy momentum flux convergence are balanced by anomalous meridional winds consistent with a wider hadley cell at the same time faster eddies are also associated with a strengthened polewardeddymomentumflux sustainingastronger westerlyjet in midlatitudes the proposedmechanismis consistent with the seasonal dependence of the interannual variability of the hadley cell width and appears to explain at least part of the projected twenty first century trends one of the most salient features of earth s general circulation is the presence of hadley cells hcs in the tropical belts of both hemispheres associated with their large scale motions are some of the main characteristics of tropical and subtropical climates the intertropical convergence zone in the rising branch and the subtropical dry zones in the descending branches e g hartmann 1994 studying the factors determining the meridional extent of these overturning circulations is therefore essential to understand the spatial distribution of tropical and subtropical climates and its changes the role of the hcs is particularly crucial in the context of climate change and the observed widening of the atmospheric circulation observations show a poleward shift of the eddy driven jets and an expansion of the hcs in recent decades particularly in the southern hemisphere hu and fu 2007 johanson and fu 2009 lu et al 2009 and the role of radiative forcings induced by greenhouse gases and stratospheric ozone has been shown to be crucial in this context polvani et al 2011b

the interconnected global climate system a review of tropical polar teleconnections abstractthis paper summarizes advances in research on tropical polar teleconnections made roughly over the last decade elucidating el nino southern oscillation enso impacts on high latitudes has remained an important focus along different lines of inquiry tropical to polar connections have also been discovered at the intra seasonal timescale associated with madden julian oscillations mjo on the timescale of decades changes in mjo phases can result in temperature and sea ice changes in the polar regions of both hemispheres moreover the long term changes in sst of the western tropical pacific tropical atlantic and north atlantic ocean have been linked to the rapid winter warming around the antarctic peninsula while sst changes in the central tropical pacific have been linked to the warming in west antarctica rossby wave trains emanating from the tropics remain the key mechanism for tropical and polar teleconnections from intraseasonal to decadal timescales enso related tropical sst anomal

impact of midlatitude stationary waves on regional hadley cells and enso 1 stationary planetary waves are excited in the midlatitudes propagate equatorward and are absorbed in the subtropics the impact these waves have on the tropical climate has yet to be fully unraveled previous work has shown that interannual variability of zonal mean stationary eddy stress is well correlated with interannual variability in hadley cell strength a separate line of research has shown that changes in midlatitude planetary waves local to the pacific strongly affect enso variability here we show that the two phenomena are in fact closely connected interannual variability of wave activity flux impinging on the subtropical central pacific affects the local hadley cell the associated changes in subtropical subsidence affect the surface pressure field and wind stresses which in turn affect enso as a result a winter with an anomalously weak hadley cell tends to be followed a year later by an el nino event

some atmospheric processes governing the large scale tropical circulation in idealized aquaplanet simulations abstract the large scale tropical atmospheric circulation is analyzed in idealized aquaplanet simulations using an atmospheric general circulation model idealized sea surface temperatures ssts are used as lower boundary conditions to provoke modifications of the atmospheric general circulation results show that 1 an increase in the meridional sst gradients of the tropical region drastically strengthens the hadley circulation intensity 2 the presence of equatorial zonal sst anomalies weakens the hadley cells and reinforces the walker circulation and 3 a uniform sst warming causes small and nonsystematic changes of the hadley and walker circulations in all simulations the jet streams strengthen and move equatorward as the hadley cells strengthen and become narrower some relevant mechanisms are then proposed to interpret the large range of behaviors obtained from the simulations first the zonal momentum transport by transient and stationary eddies is shown to modulate the eddy driven jets whic

expansion of the hadley cell under global warming 1 a consistent weakening and poleward expansion of the hadley circulation is diagnosed in the climate change simulations of the ipcc ar4 project associated with this widening is a poleward expansion of the subtropical dry zone simple scaling analysis supports the notion that the poleward extent of the hadley cell is set by the location where the thermally driven jet first becomes baroclinically unstable the expansion of the hadley cell is caused by an increase in the subtropical static stability which pushes poleward the baroclinic instability zone and hence the outer boundary of the hadley cell

on the role of tropical ocean forcing of the persistent north american west coast ridge of winter 2013 14a abstractthe causes of the high pressure ridge at the north american west coast during winter 2013 14 the driest winter of the recent california drought are examined the ridge was part of an atmosphere ocean state that included anomalies defined relative to a 1979 2014 mean of circulation across the northern hemisphere warm sea surface temperatures ssts in the tropical western and northeastern pacific and the south indian ocean and cool ssts in the central tropical pacific the sst anomalies differ sufficiently between datasets that when used to force atmosphere models the simulated circulation anomalies vary notably in realism recognizing uncertainty in the sst field the authors use idealized tropical sst anomaly experiments to identify an optimal combination of sst anomalies that forces a circulation response that best matches observations the optimal sst pattern resembles that observed but the associated circulation pattern is much weaker than observed suggesting an important but limited

on the tropical atmospheric signature of el nino abstractthe linear atmospheric signature of enso obtained by regressing fields of geopotential height z wind vertical velocity and rainfall upon the nino 3 4 sea surface temperature sst index is partitioned into zonally symmetric and eddy components the zonally symmetric component is thermally forced by the narrowing and intensification of the zonally averaged equatorial rain belt during el nino and mechanically forced by the weakening of the upper tropospheric equatorial stationary waves and their associated flux of wave activity the eddy component of the enso signature is decomposed into barotropic bt and baroclinic bc contributions the latter into first and second modal structures bc1 and bc2 separable functions of space x y and pressure p using eigenvector analysis bc1 exhibits a nearly equatorially symmetric planetary wave structure comprising three dumbbell shaped features suggestive of equatorial rossby waves with out of phase wind and geopotential height perturbations in the

cool and warm season precipitation reconstructions over western new mexico precipitation over the southwestern united states exhibits distinctive seasonality and contrasting ocean atmospheric dynamics are involved in the interannual variability of cool and warm season totals tree ring chronologies based on annual ring widths of conifers in the southwestern united states are well correlated with accumulated precipitation and have previously been used to reconstruct cool season and annual precipitation totals however annual ring width chronologies cannot typically be used to derive a specific record of summer monsoon season precipitation some southwestern conifers exhibit a clear anatomical transition from the earlywood and latewood components of the annual ring and these exactly dated subannual ring components can be measured separately and used as unique proxies of cool and warm season precipitation and their associated large scale ocean atmospheric dynamics two 2139 yr long reconstructions of cool november may and early warm season july precipitation have been developed from ancient conifers and relict wood at el malpais national monument new mexico both reconstructions have been verified on independent precipitation data and reproduce the spatial correlation patterns detected in the large scale sst and 500 mb height fields using instrumental precipitation data from new mexico above

a simple gcm study on the relationship between enso and the southern annular mode this study investigates the relationship between el nino southern oscillation enso and southern annular mode sam events with an idealized general circulation model a series of model calculations are performed to examine why positive negative intraseasonal sam events are observed to occur much more frequently during la nina el nino seven different model runs are performed a control run three el nino runs the first with a zonally symmetric heating field the second with a zonally asymmetric heating cooling field and the third that combines both fields and three la nina runs with heating fields of opposite sign

comment on late cretaceous extensional denudation along a marble detachment fault zone in the kirsehir massif near kaman central turkey by c lefebvre et al 2011 doi 10 1016 j jsg 2011 06 002

southern annular modes simulated by a climate model patterns mechanisms and uses abstract both high latitude hlm and low latitude modes llm of variability of zonal wind in the southern hemisphere have been identified through an analysis of a simulation for 1871 2200 by the csiro mark 3 climate model the extent to which these might both be described as annular modes based on their statistical patterns physical mechanisms and usefulness in climate study is assessed the modes are determined as eof1 and eof2 of vertically integrated zonal and monthly mean zonal wind for 1871 1970 these match well those from ecmwf re analysis era data and also from the earlier mark 2 model the mode index time series relate to largely annular patterns of local wind and surface pressure anomalies with hlm giving the familiar southern annular mode sam and other simulated quantities while modes calculated from 90 sectors are only moderately correlated mostly in the polar region for hlm the link increases with time scale there is little such relationship for llm a momentum equation

impact of deep convection on the isotopic amount effect in tropical precipitation the empirical amount effect observed in the distribution of stable water isotope ratios in tropical precipitation is used in several studies to reconstruct past precipitation recent observations suggest the importance of large scale organized convection systems on amount effect with a series of experiments with isocam3 0 we quantify the sensitivity of amount effect to changes in modeled deep convection the magnitude of the regression slope between long term monthly precipitation amount and isotope ratios in precipitation over tropical ocean reduces by more than 20 with a reduction in mean deep convective precipitation by about 60 indicating a decline in fractionation efficiency reduced condensation in deep convective updrafts results in enrichment of lower level vapor with heavier isotope that causes enrichment in total precipitation however consequent increases in stratiform and shallow convective precipitation partially offset the reduction in the slope of amount effect the net result is a reduced slope of amount effect in tropical regions except the tropical western pacific where the effects of enhanced large scale ascent and increased stratiform precipitation prevail over the influence of reduced deep convection we also find that the isotope ratios in precipitation are improved over certain regions in the tropics with reduced deep convection showing that analyses of isotope ratios in precipitation and water vapor are powerful tools to improve precipitation processes in convective parameterization schemes in climate models further our study suggests that the precipitation types over a region can alter the fractionation efficiency of isotopes with implications for the reconstructions of past precipitation

climatology and variability of warm and cold fronts over north america from 1979 to 2018 abstractthis paper describes the development and analysis of an objective climatology of warm and cold fronts over north america from 1979 2018 fronts are detected by a convolutional neural networ

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

linking atmospheric river hydrological impacts on the u s west coast to rossby wave breaking abstractatmospheric rivers ars have significant hydrometeorological impacts on the u s west coast this study presents the connection between the characteristics of large scale rossby wave breaking rwb over the eastern north pacific and the regional scale hydrological impacts associated with landfalling ars on the u s west coast 36 49 n ars associated with rwb account for two thirds of the landfalling ar events and 70 of total ar precipitation in the winter season the two regimes of rwb anticyclonic wave breaking awb and cyclonic wave breaking cwb are associated with different directions of the vertically integrated water vapor transport ivt awb ars impinge in a more westerly direction on the coast whereas cwb ars impinge in a more southwesterly direction most of the landfalling ars along the northwestern coast of the united states states of washington and oregon are awb ars because of their westerly impinging angles when compared to cwb ars awb ars arrive more orthogonally to the

characteristics of wave packets in the upper troposphere part ii seasonal and hemispheric variations abstract gridded 300 hpa meridional wind data produced by the ecmwf reanalysis project were analyzed to document the seasonal and hemispheric variations in the properties of upper tropospheric wave packets the properties of the wave packets are mainly illustrated using time lagged one point correlation maps performed on based on indices that show the coherence of wave propagation as well as examination of correlation maps schematic waveguides were constructed for the summer and winter seasons of both hemispheres along which waves preferentially propagate with greatest coherence in the summers the waveguides basically follow the position of the midlatitude jets in the northern hemisphere winter the primary waveguide follows the subtropical jet over southern asia into the pacific but there is a secondary branch running across russia joining the primary waveguide near the entrance to the pacific storm track over the atlantic the waveguide passes east southeastward toward north africa then bac

understanding the sam influence on the south pacific enso teleconnection the relationship between the el nino southern oscillation enso and the southern hemisphere annular mode sam is examined with the goal of understanding how various strong sam events modulate the enso teleconnection to the south pacific 45 70 s 150 70 w the focus is on multi month multi event variations during the last 50 years a significant p 0 10 relationship is observed most marked during the austral summer and in the 1970s and 1990s in most cases the significant relationship is brought about by la nina el nino events occurring with positive negative phases of the sam more often than expected by chance the south pacific teleconnection magnitude is found to be strongly dependent on the sam phase only when enso events occur with a weak sam or when a la nina el nino occurs with a positive negative sam phase are significant south pacific teleconnections found this modulation in the south pacific enso teleconnection is directly tied to the interaction of the anomalous enso and sam transient eddy momentum fluxes during la nina sam and el nino sam combinations the anomalous transient momentum fluxes in the pacific act to reinforce the circulation anomalies in the midlatitudes altering the circulation in such a way to maintain the enso teleconnections in la nina sam and el nino sam cases the anomalous transient eddies oppose each other in the midlatitudes overall acting to reduce the magnitude of the high latitude enso teleconnection

zonal asymmetry of the annular mode and its downstream subtropical jet an idealized model study abstractthis study investigates linkages between the zonal asymmetry of the annular mode am zonal pattern and the subtropical jet stj over its downstream regions of the storm track by using an idealized model observational analyses show that the am zonal patterns are more zonally asymmetric during days when the stj downstream of the storm track is unusually strong and vice versa in the idealized model the stj downstream of the storm track is varied by introducing an additional zonally localized tropical heating the model s am variability exhibits a nearly zonally uniform structure when there is no or only weak tropical heating however the signatures of the am are locally strengthened in the heating sector thus the am zonal pattern is zonally asymmetric when the tropical heating is large enough to create a strong stj the model results also show that the percentage of the variance explained by the am the persistence of the am index and the intensity of eddy feedback are also increased when t

intraseasonal and interdecadal jet shifts in the northern hemisphere the role of warm pool tropical convection and sea ice abstractthis study uses cluster analysis to investigate the interdecadal poleward shift of the subtropical and eddy driven jets and its relationship to intraseasonal teleconnections for this purpose self organizing map som analysis is applied to the ecmwf interim re analysis era interim zonal mean zonal wind the resulting som patterns have time scales of 4 8 5 7 days and undergo notable interdecadal trends in their frequency of occurrence the sum of these trends closely resembles the observed interdecadal trend of the subtropical and eddy driven jets indicating that much of the interdecadal climate forcing is manifested through changes in the frequency of intraseasonal teleconnection patterns two classes of jet cluster patterns are identified the first class of som pattern is preceded by anomalies in convection over the warm pool followed by changes in the poleward wave activity flux the second class of patterns is preceded by sea ice and stratospheric polar vortex anomalies when the arctic se

climate driven variability in the southern ocean carbonate system abstractseasonal and interannual variability in the southern ocean carbonate system is investigated using output from a historically forced 1948 2007 ocean general circulation model with embedded biogeochemistry atmospheric co2 is fixed at preindustrial levels to investigate carbonate system variability in the absence of an anthropogenic co2 perturbation it is found that nearly a quarter of interannual variability in southern ocean pacific sector surface carbonate ion concentration can be explained by variability in enso with pacific sector surface decreasing by 0 43 mmol m 3 per standard deviation decrease in the enso 3 4 index enso related variability in vertical advection of dissolved inorganic carbon dic drives this relationship between enso and surface it is also found that positive phases of the southern annular mode sam are associated with decreased southern ocean surface an association driven by sam related variability in vertical advection of dic despite the influence of the sam on

the effect of reflecting surfaces on the vertical structure and variability of stratospheric planetary waves the effects of an upper stratospheric reflecting surface on the vertical structure of stratospheric planetary waves are considered a diagnostic of the basic state wave propagation characteristics which is particularly useful for determining the existence and location of turning surfaces for meridional and vertical propagation is developed the diagnostic used is a more accurate indicator of wave propagation regions than the index of refraction because it diagnoses meridional and vertical propagation separately the diagnostic is tested on a series of simple models both steady state and time dependent it is found that the stratospheric waveguide sets the meridional wavenumber of the waves regardless of the characteristics of their tropospheric forcing making it easier to understand the effects of damping and turning surfaces on the vertical structure of the waves the diagnostic is then applied to observations of the southern hemisphere winter of 1996 it is shown that the differences in vertical wave structure between middle and late winter can be explained as a linear response to the seasonal evolution of the basic state which involves a formation of a reflecting surface in late winter it is also shown that on daily timescales wave mean flow interactions cause significant changes in the basic state propagation characteristics for periods of a few days these changes along with the time variations in the forcing of the waves are responsible for the observed daily timescale variations in wave structure the fact that the observed evolution of the waves and the basic state are consistent with linear or quasi linear wave theory depending on the timescale looked at supports the applicability of the theory as well as the validity of the observations

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

on the midlatitude thermal response to tropical warmth 1 observations reveal a global scale thermal response to el nino and la nina featuring a band of tropospheric anomalies opposite in sign to those in the tropics when an idealized dynamical model is forced by increased tropical heating a similar response with an enhanced subtropical thermal gradient results in the idealized model this response is produced by changes in transient eddy fluxes of heat and momentum and consequent changes in the mean meridional circulation the similarity between the two level model results and those from observations as well as the zonally averaged behavior of a gcm suggests that the same dynamics operate in nature this represents a tropical midlatitude teleconnection distinct from and in addition to that mediated by stationary rossby wave trains

southward shift of westerlies intensifies the east asian early summer rainband following el nino author s kong w chiang jch abstract 2020 american geophysical union all rights reserved an intensification of east asian rainfall usually occurs in summers following el nino we propose that this teleconnection is mediated via the westerlies impinging on the tibetan plateau through el nino s control on tropical tropospheric temperature this is distinct from previous studies that attribute the el nino s influence to changes in the western pacific subtropical anticyclone the warming in the eastern equatorial pacific leads to uniform warming of the entire tropical troposphere which sharpens the temperature gradient between the tropics and the subtropics and shifts the westerlies southward the westerlies impinge over the tibetan plateau for longer and through interaction with the topography induce intense and persistent extratropical northerlies downstream of the plateau that in turn intensifies the east asian rainband the rainband has previously been shown to intensify in a similar manner in a warming climate suggesting that the el nino response provides an analog for future changes

the relationship between interannual variability in the 200 mb tropical wind field and the southern oscillation abstract the interannual variability in the upper tropospheric tropical wind field is described through empirical orthogonal functions teleconnections and composite analyses the data used are 11 years of mean monthly analyzed fields of zonal and meridional components of the 200 mb wind on a 5 mercator grid extending from 48 1 n to 48 1 s derived from the national meteorological center s operational tropical analysis a substantial portion of the interannual variability in the 200 mb circulation is shown to be related to the southern oscillation the anomalous circulation in the pacific is characterized by a pair of anticyclonic cyclonic anomalies straddling the equator during periods of low high southern oscillation index zonal wind differences of 8 11 m s 1 between low and high index phases of the so were found near 25 n 25 s and near the equator in the central pacific composites relative to el nino events during different seasons reveal that anomalous anticyclonic circulations in the pacific are

observed antarctic interannual climate variability and tropical linkages abstractthis study reviews the mechanisms associated with antarctic tropical climate linkages and presents new analyses of the seasonality and spatial patterns of tropical climate signals in the antarctic for the late 1950s to the present tropical climate signals are primarily communicated to the antarctic via the pacific south america psa pattern and the southern annular mode sam the impacts of these circulation patterns and their tropical linkages are evident in regressions of seasonally stratified antarctic station temperature data and annually resolved ice core records on global fields of sea surface temperature sea level pressure and precipitation temperature and ice core anomalies in the antarctic peninsula region and adjoining areas of west antarctica are significantly impacted by the psa interpreted as a rossby wave train driven by anomalous tropical deep convection during enso events this pattern is most evident in the austral spring consistent with recent studies suggesting that atm

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

role of sea surface temperatures in forcing circulation anomalies driving u s great plains pluvial years abstractin the u s great plains gp diagnosing precipitation variability is key in developing an understanding of the present and future availability of water in the region building on previous