drought in late spring of south china in recent decades late spring 21 april 20 may precipitation to the south of the yangtze river in china along the east asian front is a salient feature of the global climate the present analysis reveals that during 1958 2000 south china 26 31 n 110 122 e has undergone a significant decrease in late spring precipitation since the late 1970s the sudden reduction of the precipitation concurs with a notable cooling in the upper troposphere over the central china 30 40 n 95 125 e the upper level cooling is associated with an anomalous meridional cell with descending motions in the latitudes 26 35 n and low level northerly winds over southeastern china 22 30 n 110 125 e causing deficient rainfall over south china the late spring cooling in the upper troposphere over the central china is found to strongly link to the north atlantic oscillation nao in the preceding winter during winters with a positive nao index the upper tropospheric cooling occurs first to the north of the tibetan plateau in early middle spring then propagates southeastward to central china in late spring it is suggested that the interdecadal change of the winter nao is the root cause for the late spring drought over south china in recent decades

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

impacts of mjo convection over the maritime continent on eastern china cold temperatures abstractthe present study shows that winter cold events over eastern china can be induced by madden julian oscillation mjo associated anomalous convection over the maritime continent we conduct

an east west sst anomaly pattern in the midlatitude north atlantic ocean associated with winter precipitation variability over eastern china 1 winter precipitation tends to vary consistently over most regions of eastern china the present study reveals that the variation in winter precipitation over eastern china is positively correlated to an east west sea surface temperature sst anomaly pattern in the midlatitudes and high latitudes of the north atlantic ocean when the sst is higher lower in the eastern north atlantic ocean and lower higher in the western north atlantic ocean winter precipitation over eastern china tends to be above below normal the east west sst anomaly pattern in the north atlantic modifies the atmospheric circulation over the atlantic ocean and europe and the anomalous circulation extends to downstream east asia through the propagation of rossby waves and exerts influence on winter precipitation over eastern china the formation of the east west sst anomaly pattern in the north atlantic may be associated with anomalies in the north atlantic thermohaline circulation and in surface winds over the north atlantic ocean

changes in the relationship between northeast china summer temperature and enso 1 northeast china nec summer temperature tends to be lower higher than normal in el nino la nina developing years during 1950s through mid 1970s the relationship between the nec summer temperature and el nino southern oscillation enso is weakened or even becomes opposite in 1980s and 1990s the present study documents this interdecadal change and investigates plausible reasons for this change before the late 1970s enso affects the nec summer temperature through modulating the south asian heating and consequently the midlatitude asian circulation after the late 1970s the connection between enso and the indian summer monsoon and that between the south asian heating and the midlatitude asian circulation have been weakened this leads to a weakening of enso impacts on the nec summer temperature it is found that the nec summer temperature variations are closely related to the north atlantic sea surface temperature sst and circulation changes in 1980s and 1990s in particular a tripole north atlantic sst anomaly pattern in boreal spring is a good precursory for the nec summer temperature anomalies the nec summer temperature displays a negative correlation with the summer sst surrounding the maritime continent in 1980s and 1990s in many years the tropical north pacific and the north atlantic sst anomalies can contribute in concert to the midlatitude asian circulation changes and the nec summer temperature anomalies these effects overcome those of the central and eastern equatorial pacific sst anomalies leading to a same sign relationship between the nec summer temperature and the central and eastern equatorial pacific sst anomalies

circumglobal teleconnections and wave packets associated with israeli winter precipitation this investigation performs diagnostic analyses with ncep ncar reanalysis data and israeli precipitation data to examine the dynamical processes that drive the teleconnection pattern associated with israeli winter precipitation anomalies a dipole teleconnection pattern with one centre of action over western europe and another centre of action over the eastern mediterranean em is found to be associated with the winter precipitation anomalies in israel this pattern is referred to as the southern levant sl pattern the positive negative phase of the sl pattern is found to be closely associated with enhanced reduced precipitation in israel composites of the 300 hpa stream function field show that both phases of the sl pattern are associated with eastward propagating wave packets that traverse about three quarters of the distance across the globe these wave packets which have a phase speed that is close to zero originate over the northeast pacific the east coast of north america for the positive negative sl phase and propagate eastward across the north atlantic western europe the em and southern asia until they eventually decay over the northwest pacific as these wave packets pass over western europe and the em the sl pattern is triggered and anomalies in israeli winter precipitation occur the sl pattern is found to have an e folding time scale of 4 days which is determined by the group velocity of the wave packet the time averaged spatial structure of the circumglobal wave packet closely resembles the first empirical orthogonal function of the hemispheric 300 hpa meridional wind field this suggests that the circumglobal wave packet may be a fundamental pattern of variability which is relevant for the entire northern hemisphere copyright 2008 royal meteorological society

recent interdecadal shift in the relationship between northeast china s winter precipitation and the north atlantic and indian oceans this study documents an interdecadal change in the interannual relationship between northeast china s winter precipitation necwp and the sea surface temperature sst in the north atlantic and indian oceans in the 1990s it is revealed that the necwp shows a significant simultaneous correlation with the sst anomalies in the north atlantic sst atlantic tropical indian ocean sst indian during 1996 2013 1961 1990 generally the necwp anomaly is concurrent with apparent eurasian wave pattern during 1961 1990 whereas anomalous okhotsk high and east asia trough during 1996 2013 it is found that before the 1990s the warming sst anomalies in the tropical indian ocean could stimulate the eurasian wave pattern via inducing significant anomalous upper level convergence over the northern europe which tends to favor a positive necwp anomaly during 1996 2013 the sst indian necwp connection is disrupted instead the north atlantic tri polar sst anomaly pattern exerts a dominant impact on the necwp through triggering a stationary rossby wave that originates from the north atlantic and propagates eastward to northeast asia and further modulates the okhotsk high and east asia trough further analyses indicate that the weakened connection between the tropical sst indian anomalies and the northern ferrell circulation likely contributes to the weakening of the necwp sst indian relationship after the 1990s however the eastward shift and the enlarged anomalous magnitudes of the north atlantic oscillation might favor the strengthening of the necwp sst atlantic relationship after the mid 1990s it is therefore suggested that the strengthened variability of the sst atlantic anomalies after the 1990s might partially contribute to the intensification of the interannual variability of the necwp

modulation of south asian jet wave train on the extreme winter precipitation over southeast china comparison between 2015 16 and 2018 19 abstracttwo extremely wet winters in 2015 16 and 2018 19 over southeast china are compared in this study south to north discrepancies appear in the spatial distribution of precipitation with anom

multiple flow regimes in the northern hemisphere winter part i methodology and hemispheric regimes abstract recurrent and persistent flow patterns are identified by examining multivariate probability density functions pdfs in the phase space of large scale atmospheric motions this idea is pursued systematically here in the hope of clarifying the extent to which intraseasonal variability can be described and understood in terms of multiple flow regimes bivariate pdfs of the northern hemisphere nh wintertime anomaly heights at 700 mb are examined in the present paper using a 37 year dataset the two dimensional phase plane is defined by the two leading empirical orthogonal functions eofs of the anomaly fields pdfs on this plane exhibit synoptically intriguing and statistically significant inhomogeneities on the periphery of the distribution it is shown that these inhomogeneities are due to the existence of persistent and recurrent anomaly patterns well known as dominant teleconnection patterns that is the pacific north american pna pattern its reverse and zonal and blocked phases of the

circumglobal teleconnections the jet stream waveguide and the north atlantic oscillation abstract monthly and seasonally averaged upper tropospheric northern hemisphere winter fields are examined to determine whether the waveguiding effect of the time averaged tropospheric jets on low frequency disturbances that is predicted by theory does affect the behavior of these disturbances it is found that indeed disturbances in the vicinity of the mean jets particularly the jet that stretches across south asia are fundamentally different from those that reside in regions where the mean winds have weaker meridional gradients like the mid pacific patterns of variability in the jets tend to be smaller scale and to consist of zonally oriented chains of anomalies while variability in the mid pacific is composed of patterns with distinct meridional orientation because they are meridionally trapped and zonally elongated patterns associated with the jet stream waveguide connect activity at points that are much farther apart than do patterns in other regions of the globe within the south asian waveg

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

northeast china summer temperature and north atlantic sst a previous study revealed a close relationship between interannual variations ofnortheast china nec summer temperature and a tripole sea surface temperature sst anomaly pattern in the north atlantic in preceding spring the present study investigatesthe change in the above relationship and the plausible causes for the change a tripole sstindex is defined with its positive value corresponding to positive sst anomalies in thetropics and midlatitudes and negative sst anomalies in the subtropics the tripole sstanomaly pattern has a weak correlation with nec summer temperature during the 1950sthrough the mid 1970s in sharp contrast to the 1980s and 1990s this change is related tothe difference in the persistence of the tripole sst pattern before the late 1970s thetripole sst pattern weakened from spring to summer and thus the spring north atlantictripole sst pattern had a weak connection with nec summer temperature on thecontrary after the late 1970s the tripole sst pattern displayed a tendency of persistencefrom spring to summer contributing to circulation changes that affected nec summertemperature there are two factors for the persistence of the tripole sst pattern fromspring to summer one is the north atlantic air sea interaction and the other is thepersistence of sst anomalies in the eastern equatorial pacific during the decay ofel nino southern oscillation enso it is shown that the north atlantic sst anomaliescan have an impact on nec summer temperature independent of enso

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

interdecadal variations in the relationship between the winter north atlantic oscillation and temperature in south central china abstractinterdecadal variations in the relationship between the winter north atlantic oscillation nao and surface air temperature in china are investigated using observational and reanalysis data focus is on south central china in which temperature variability is strongly related to the nao it is revealed that the relationship shows clear interdecadal variations in midwinter during 1951 2015 a relatively weak in phase relationship occurs before the early 1970s p1 but a significant out of phase relationship dominates in the last two decades of the twentieth century p2 though it is clearly weaker from the late 1990s onward observational evidence shows that such interdecadal variations are related mainly to variations in the spatial pattern and amplitude of the nao the northern center of the nao shifted eastward over the second half of the twentieth century in addition the amplitude of the center strengthened from p1 to p2 resulting in a perturbation in the atmospheric circulation response p

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

northern hemisphere teleconnection patterns during extreme phases of the zonal mean circulation abstract regional climate anomalies associated with year to year changes in the tropospheric zonal mean zonal wind are examined this study focuses on the wintertime northern hemisphere extratropics and compares seasonal mean anomalies associated with to those associated with the el nino southern oscillation during the 1947 94 period dynamical model experiments indicate that a single zonal index characterized by out of phase anomalies at 35 and 55 n is of primary importance for zonal flow stationary wave interactions in the northern hemisphere extratropies such fluctuations in the zonal mean zonal flow are shown to occur independently of tropical sst variability consistent with earlier studies dynamical model experiments and regression analyses of the historical data indicate that such a zonal index explains a significant fraction of the wintertime stationary wave variability in several regions the principal centers of action reside within wave trains over the north pacific north american

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

regional and hemispheric circulation patterns in the northern hemisphere winter or the nao and the ao 1 two different scales of teleconnection patterns are found over the euro atlantic region during the winter one is a regional seesaw between iceland and the azores and the other is a hemispheric seesaw between the mediterranean and the polar siberia region the north atlantic oscillation nao and the arctic oscillation ao indices are sensitive to both modes although the nao index reflects the more regional one the principal process producing a hemispheric seesaw pattern in the surface pressure is changing in vertical winds in the polar troposphere driven by wave forcings upward motion in the polar region induces meridional flows near the surface especially around the norwegian sea and bering strait which induce wavy patterns in the surface pressure the importance of distinguishing the two modes in understanding recent trends is also addressed

low frequency variability in the northern hemisphere winter geographical distribution structure and time scale dependence abstract low frequency variability in wintertime 500 mb height is examined with emphasis on its structure geographical distribution and frequency dependence a 39 year record of 500 mb geopotential height fields from the nmc analyses is time filtered to partition the fluctuations into frequency bands corresponding to periods of 10 60 days 60 180 days and 180 days winter is defined as the six month period november through april variance teleconnectivity and anisotropy fields and selected loading vectors derived from orthogonal and oblique rotations of the eigenvectors of the temporal correlation matrix for each band are shown and discussed the variability in all frequency bands exhibits substantial anistropy with meridionally elongated features arranged as zonally oriented wave trains prevailing over the continents and zonally elongated features organized in the form of north south oriented dipole patterns prevailing over the oceanic sectors of the hemisphere the wave trains are most pronounc

the relationship between the north atlantic jet and tropical convection over the indian and western pacific oceans abstractboreal winter jet variability over the north atlantic is investigated using 40 yr european centre for medium range weather forecasts re analysis era 40 data where the variability is defined by the first eof of the zonal wind on seven vertical levels the principal component time series of this eof is referred to as the jet index a pattern correlation analysis indicates that the jet index more accurately describes intraseasonal north atlantic zonal wind variability than does the north atlantic oscillation nao a series of composite calculations of the jet index based on events of intraseasonal convective precipitation over the tropical indian and western pacific oceans reveals the following statistically significant relationships 1 negative jet events lead enhanced indian ocean precipitation 2 positive jet events lag enhanced indian ocean precipitation 3 positive jet events lead enhanced western pacific ocean precipitation and 4 negative jet events lag enhanced western pacific ocean p

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

decadal relationship between the north atlantic oscillation and cold surge frequency in taiwan 1 the close decadal relationship between the north atlantic oscillation nao and the cold surge frequency in northern taiwan is identified the cold surge frequency was higher lower in 1957 1977 1981 2001 which corresponded to the negative positive phase of the nao the teleconnection pattern associated with the nao suggests that the siberian high the ridge upstream of lake baikal and the east asian trough which are well known circulation characteristics favorable for cold surge occurrences were enhanced in the negative nao phase a weakened upper level convergence over the mediterranean sahara occurring in the negative nao phase was associated with a wave like pattern spreading toward east asia along the subtropical jet stream barotropic modeling results suggest that the suppressed upper level convergence in the mediterranean sahara region may force a wave train emanating toward east asia along the subtropical jet stream which acts as a rossby waveguide the barotropic wave pattern may result in a stronger low level northerly over subtropical east asia and create a favorable background for the further southward penetration of cold air and therefore more frequent cold surge occurrence the same process may be applied to the period when the nao is in the positive phase

the dynamics of nao teleconnection pattern growth and decay this investigation performs both diagnostic analyses with ncep ncar re analysis data and forced barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the north atlantic oscillation nao teleconnection pattern the diagnostic calculations include projection and composite analyses of each term in the stream function tendency equation the results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks the positive nao phase is found to develop after anomalous wavetrain propagation across the north pacific to the east coast of north america this contrasts with the negative nao phase which appeared to develop in situ both high frequency period 10 days transient eddy fluxes drive the nao growth after the nao anomaly attains its maximum amplitude the high frequency transient eddy fluxes continue to drive the nao anomaly in a manner that is consistent with a positive feedback process the decay of the nao occurs through both the divergence term and the low frequency transient eddy fluxes the temporal and spatial properties of the divergence term are found to be consistent with ekman pumping these results illustrate many important differences between the nao and pacific north american pna teleconnection patterns perhaps most striking being that the nao life cycle is dominated by nonlinear processes whereas the pna evolution is primarily linear in addition the relation between the nao and the zonal index is discussed copyright 2003 royal meteorological society

the circumglobal north american wave pattern and its relation to cold events in eastern north america extreme large scale north american cold events are associated with strong undulations in the tropospheric jet stream which bring cold polar air southward over the continent here we propose that th

summer rainfall seesaw between hetao and the middle and lower reaches of the yangtze river and its relationship with the north atlantic oscillation abstractusing the ncep ncar reanalysis and other observational datasets the authors have investigated the relationship of summer rainfall variations between the hetao region of northern china and the middle and lower reaches of yangtze river mlryr the results have demonstrated that rainfall in hetao varies out of phase with that in mlryr on the interannual time scales this phenomenon is referred to as the hetao yangtze rainfall seesaw hyrs an hyrs index is defined to reveal both spatial and temporal features of hyrs it is found that the north atlantic oscillation nao affects the hyrs in years when the nao is in its positive phase anomalous divergences in the lower troposphere and anomalous convergences in the upper troposphere are observed in regions of the mediterranean and eastern europe the anomalous convergences in the upper troposphere occur as the positive rossby wave source excites a circumglobal teleconnection cgt in the midlatitudes exhibiting the eastward propagation of rossby w

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

structure and dynamics of a springtime atmospheric wave train over the north atlantic and eurasia the leading pattern of boreal spring 250 hpa meridional wind anomalies over the north atlantic and mid high latitude eurasia displays an obvious wave train the present study documents the structure energy source relation to the north atlantic sea surface temperature sst and impacts on eurasian climate of this wave train during 1948 2018 this atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid latitudes of the north atlantic northern europe central eurasia and east asia respectively this spring wave train can efficiently extract available potential energy from the basic mean flow the baroclinic energy conversion process and positive interaction between synoptic scale eddies and the mean flow both play important roles in generating and maintaining this wave train the north atlantic horseshoe like nah sst anomaly contributes to the persistence of the wave train via a positive air sea interaction specifically the nah sst anomaly induces a rossby wave type atmospheric response which in turn maintains the nah sst anomaly pattern via modulating surface heat fluxes this spring atmospheric wave train has significant impacts on eurasian surface air temperature sat and rainfall during the positive phase of the wave train pronounced sat warming appears over central eurasia and cooling occurs over west europe and eastern eurasia in addition above normal rainfall appears over most parts of europe and around the lake baikal accompanied by below normal rainfall to east of the caspian sea and over central asia

atmospheric gcm response to an idealized anomaly of the mediterranean sea surface temperature the atmospheric response to an idealized 2 k cooling of the mediterranean sea is studied in a general circulation model gcm in the downstream region from the eastern mediterranean basin to the whole asian continent following the sub tropical jet stream a baroclinic response is produced with high pressure near the surface and low pressure in the upper layers it is the direct response to the mediterranean cooling and it needs only a few days to be established teleconnections with strong zonal characteristics are found following the sub tropical jet stream to the north pacific and north america propagating further across the north atlantic towards northern europe two remarkable remote features are the deepening of the aleutian low in the north pacific and the weakening of the icelandic low in the north atlantic these two teleconnections form and grow in several days in the north pacific and several tens of days in the north atlantic both have a quasi barotropic vertical structure it is believed that they are the consequence of complex interactions between the mean flow and the transient eddies in the atmosphere

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

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

enso induced tropical convection variability over the indian and western pacific oceans during the northern winter as revealed by a self organizing map 1 the dominant low frequency patterns of outgoing longwave radiation over the tropical indian and western pacific oceans in response to el nino southern oscillation enso during the northern winter are investigated using the nonlinear classification method of the self organizing map som additionally aided by an empirical orthogonal function eof analysis there are at least two different air sea coupled phenomena in the tropical indian ocean i e the indian ocean dipole iod mode in late summer and fall and the wind evaporation sea surface temperature wes feedback driven mode in the winter and spring the som results exhibit that the phase of wes becomes positive negative at the el nino la nina phase which indicates a linear linkage of wes with enso in contrast few combined patterns of the negative iod and the la nina event can be extracted although the reversed pattern is prominent suggesting a nonlinear like relationship between iod and enso since the enso induced convective forcing over the tropical indian and western pacific oceans has strong asymmetric features with respect to the enso phase the remote response of the east asian winter monsoon to enso becomes complicated at the el nino event the monsoon circulation in the southern part of east asia tends to be attenuated in late winter through the dominance of the western north pacific anticyclone in the lower troposphere but at the la nina event the monsoon circulation over korea and japan is enhanced in early winter through the downstream development of stationary rossby wave packets along the south asian waveguide

an ice grounding line wedge from the ghaub glaciation 635 ma on the distal foreslope of the otavi carbonate platform namibia and its bearing on the snowball earth hypothesis our detailed examination of the ghaub formation possibly 635 ma on the distal foreslope of the otavi carbonate platform is part of a regional study of the congo paleocontinental margin in northwestern namibia detrital carbonates of the ghaub formation disconformably overlie the franni aus member of the ombaatjie formation a coarsening upward stack of carbonate turbidites and oolite clast debris flow breccias interpreted to be a glacioeustatic falling stand wedge within the main ghaub formation carbonate diamictites are interleaved with mesoscale laminated to cross laminated climbing rippled grainstones and mudstones and conglomeratic carbonates amalgamation of diamictite units is observed where interleaved facies grainstones mudstones are laterally discontinuous due to reactivation of erosion followed by renewed deposition the diamictite package is progradational overall and 80 m thick on average it is overlain by the 5 15 m thick bethanis member which is unique in its lateral continuity composite fining upward trend and distinctive interbedding of turbidite grainstones argillaceous siltstones climbing rippled mudstones and meter scale stromatolite dropstones dropstones are ubiquitous within the finer grained ghaub lithofacies and their presence along with the facies context for subglacial and near grounding line deposition indicates a glacigenic origin for the ghaub formation despite its subtropical paleolatitude and distal foreslope setting we infer a glacial maximum represented by the sub ghaub disconformity followed by the main ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude terminated by the bethanis episode of unusually widespread iceberg calving and slope instability the bethanis member is overlain conformably by the keilberg member of the maieberg formation reconstruction of the foreslope places the ghaub grounding line wedge 1 3 km vertically below the rim of the platform implying an enormous base level change upon deglaciation when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment the magnitude of base level change supports the pan glacial hypothesis that dynamic thick ice sheets existed simultaneously on virtually all continents the snowball hypothesis that the oceans were also covered by glacial ice sea glacier provides a simple explanation for the main ghaub to bethanis transition terminal deglaciation was triggered by collapse of the sea glacier

decadal variations of the relationship between the summer north atlantic oscillation and middle east asian air temperature 1 the relationship between the summer july september north atlantic oscillation snao and simultaneous east asian air temperature is investigated the results show that the snao is related to middle east asian summer air temperature however this linkage varies with time on decadal timescale a strong connection appears after the late 1970s but a weak connection before the late 1970s further analysis indicates that this instable relationship may have resulted from the shift of the snao mode around the late 1970s in the period of 1979 2003 the centers of the snao mode are located more eastward a positive phase negative phase snao produces a strong lower level divergence convergence over the asian jet entrance region in turn stimulates a strong upper level convergence divergence via the ekman pumping such a convergence divergence then excites a zonally oriented quasi stationary barotropical rossby wave train along the asian upper level jet thus the snao signal is transported eastward to east asia resulting in an anomalous summer air temperature over middle east asia however in the period of 1951 1975 the centers of the snao mode are located more westward the associated upper level divergence convergence is away from the asian jet entrance region and the snao signal cannot be transported eastward to east asia hence the connection is broken

the winter concurrent meridional shift of the east asian jet streams and the associated thermal conditions abstractin this study the concurrent meridional shift of the east asian polar front jet eapj and the east asian subtropical jet easj and the associated thermal conditions have been investigate

event based extreme precipitation in central eastern china large scale anomalies and teleconnections event based extreme precipitation eep defined as an extreme precipitation event no less than 1 mm for continuous days having at least one daily precipitation exceeding the 90th percentile are categorized into three types according to the time distribution pattern tdp of extreme precipitation daily intense precipitation appearing only at the first half of event tdp1 at both the first and second halves tdp2 and only at the second half of event tdp3 this study focuses on the tdp1 and tdp2 types which occur much more frequently in the central eastern china and investigates associated distinguishing large scale teleconnection patterns the tdp1 events dominantly occur in mid to late spring which are associated with the silk road pattern srp wave train manifested from western europe to northwest pacific along the subtropical westerly jet the tripolar sea surface temperature sst anomalous pattern over the north atlantic ocean i e low sst anomalies in the azores islands two high sst anomalies over the greenland and subtropical north atlantic are mainly attributed to force the srp wave meanwhile the enhanced northwestward moisture transport from the south china converges with the southeastward moisture transport related to the srp from the north china resulting in the tdp1 extreme precipitation events over the central eastern china the tdp2 events are closely conjunction with the meiyu precipitation in the yangtze huai river basin in summer during the tdp2 events the east asia pacific eap teleconnection is exhibited along the east asia coasts which is triggered by the anomalous convection in the western north pacific an anomalous anticyclone related to the eap teleconnection contributes to the westward stretch of the western pacific subtropical high providing significant enhanced moisture transportation toward the yangtze huai river basin correspondingly the meridional gradient of the equivalent potential temperature theta e is intensified and favored of forming meiyu front precipitation this study highlights the definition of eep event emphasizing the tdp of extreme precipitation and distinguished features of occurring seasons and large scale circulations associated with the two dominant types of eep events

the generation of global rotational flow by steady idealized tropical divergence abstract tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent the horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field a vorticity equation model is used to diagnose this relationship it is shown that because of the advection of vorticity by the divergent component of the flow the rossby wave source can be very different from the simple fd source often used in particular an equatorial region of divergence situated in easterly winds can lead to a rossby wave source in the subtropical westerlies where it is extremely effective this part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence a divergence field which is asymmetric about the equator can lead to a quite symmetric rossby wave source for a steady frictionless flow the rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is under cert

persistent severe drought in southern china during winter spring 2011 large scale circulation patterns and possible impacting factors 1 severe drought persisted in southern china from january to may in 2011 in this study a statistical analysis is carried out to discuss the multiple possible impacting factors including la nina the north atlantic oscillation nao and the thermal condition of the tibetan plateau tp the la nina event in 2010 11 excited a lower tropospheric anomalous cyclone over the northwestern pacific weakening the northwestern pacific subtropical high and caused an eastward shift of the high as a result transportation of wet and warm moisture from tropical oceans to southern china decreased the la nina event also strengthened the upper tropospheric east asian jet stream and deepened the east asian trough favoring a southward intrusion of dry northerly flow from the siberia the la nina condition in the previous two seasons also seemed to provide precursory signals for the drought moreover in january may 2011 the nao was in a positive phase and it tended to excite stationary rossby waves that were distributed along the sub polar and subtropical waveguides respectively the sub polar one induced an anomalous anticyclone over the siberia favoring a southward intrusion of high latitude northerly flow to southern china the subtropical one associated possibly with the enhanced convection over the broad region from the mediterranean to sahara was favorable for an influence of upper tropospheric flow on southern china the tp might also exert an influence on the drought by weakening the westerly flow to the southern flank of tp and reducing water vapor transport from the bay of bengal to southern china

on the link between the subseasonal evolution of the north atlantic oscillation and east asian climate we analyse the impact of the north atlantic oscillation nao on the climate of east asia at subseasonal time scales during both winter and summer these teleconections have mainly been investigated at seasonal and longer time scales while higher frequency links are largely unexplored the nao is defined using extended empirical orthogonal functions on pentad mean observations which allows to elucidate the oscillation s spatial and temporal evolution and clearly separate the development and decay phases the downstream dynamical imprint and associated temperature and precipitation anomalies are quantified by means of a linear regression analysis it is shown that the nao generates a significant climate response over east asia during both the dry and wet seasons whose spatial pattern is highly dependent on the phase of the nao s life cycle temperature and precipitation anomalies develop concurrently with the nao mature phase and reach maximum amplitude 5 10 days later these are shown to be systematically related to mid and high latitude teleconnections across the eurasian continent via eastward propagating quasi stationary rossby waves instigated over the atlantic and terminating in the northeastern pacific these findings underscore the importance of rapidly evolving dynamical processes in governing the nao s downstream impacts and teleconnections with east asia

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

high and low latitude types of the downstream influences of the north atlantic oscillation using reanalysis data we find that the downstream propagating quasi stationary rossby wave train associated with the north atlantic oscillation nao generally propagates along a high low latitude pathway during warm cold el nino southern oscillation enso boreal winters consistent with the different propagation directions of the nao related downstream wave train during the warm cold enso winters the nao is associated with significant 300 hpa geopotential height anomalies over eastern siberia the arabian sea the east coast of asia at around 40 n and the north pacific and the near surface air temperature perturbations associated with the nao over the high latitudes of asia are relatively strong weak based on these differences we argue that the nao has two distinct types of downstream influence a high latitude type and a low latitude type furthermore we argue that the two types of nao s downstream influence are modulated by the intensity of the subtropical potential vorticity pv meridional gradient over africa when this gradient is weak strong as in the warm cold enso winters the nao s downstream influence tends to be of the high low latitude type these results are further supported by analysis of intraseasonal nao events we separate nao events into two categories in terms of the intensity of the subtropical pv gradient over africa composites of the nao events accompanied by a weak strong subtropical pv gradient show that the nao related downstream wave train tends to propagate along a high low latitude pathway

zonal eddy dynamics of the north atlantic oscillation abstract this research is an attempt to understand the dynamical mechanisms that drive the wintertime north atlantic oscillation nao on monthly and longer timescales in an earlier work by deweaver and nigam the authors showed that momentum fluxes from stationary waves play a large role in maintaining the zonal mean zonal wind u perturbations associated with the nao in this paper a linear stationary wave model is used to show that zonal mean flow anomalies in turn play a large role in maintaining the nao stationary waves a strong two way coupling thus exists between u and the stationary waves in which each is both a source of and a response to the other when forced by zonal eddy coupling terms terms that represent the interaction between nao covariant zonal mean zonal wind anomalies and the climatological eddy flow together with heating and transient fluxes the model produces a realistic simulation of the observed stationary wave pattern zonal eddy coupling terms make the largest contribution