modulation of boreal summer intraseasonal oscillations over the western north pacific by enso abstractthe authors investigate the effects of el nino and la nina on the intraseasonal oscillation iso in the boreal summer may october over the western north pacific wnp it is found that during el nino summers the iso is dominated by a higher frequency oscillation with a period of around 20 40 days whereas during la nina summers the iso is dominated by a lower frequency period of around 40 70 days the former is characterized by northwestward propagating convection anomalies in the wnp and the latter is characterized by northward and eastward propagating convective signals over the tropical indian ocean maritime continent the possible mechanisms through which el nino southern oscillation enso induced background mean state changes influence the iso behavior are examined through idealized numerical experiments it is found that enhanced weakened mean moisture and easterly westerly vertical wind shear in the wnp during el nino la nina are the main causes of the strengthened weakened 2

amazonian paleoecological histories one hill three watersheds abstract data from the hill of six lakes in the northwestern brazilian amazon region provide three records of paleoclimatic and vegetation change in lowland amazonia that span the last 170 000 years three lakes verde pata and dragao which occupy separate watersheds on the hill provide the most detailed image yet obtained of ice age conditions in lowland amazonia well dated sedimentary records for fossil palynological charcoal cation and pigment data are presented the data indicate the continuous presence of mesic forest throughout the last ice age oscillations of lake level are recorded and the lowstands are attributed to reduced precipitation inputs to systems delicately balanced between water loss leakage through the floor of the basin and gain precipitation gross stratigraphy algal remains and paleochemistry suggest that the stands of high and low lake level were cyclic apparently correlating precessional orbital variation times of lake lowstand coincide with wet season december january february djf insolation minima the strongest of eight lowstand cycles occurred ca 35 000 to 27 000 cal bp even during lowstand episodes pollen is well preserved and provides a clear signal of uninterrupted forest cover the principal lowland elements are continuously present in the record suggesting the long term quaternary availability of the lowland forest biome in this region however during the late pleistocene the forest changed in composition with the expansion or invasion of montane floral elements creating communities of the mesic forest biome without modern analogs while precipitation cycles were causing lake levels to rise and fall the principal influence on vegetation appears to have been cooling in the late pleistocene the population expansion of montane elements at lower elevations is consistent with a cooling of 4 5 c

rectification of the madden julian oscillation into the enso cycle an ocean general circulation model forced with idealized purely oscillating wind stresses over the western equatorial pacific similar to those observed during the madden julian oscillation mjo developed rectified low frequency anomalies in sst and zonal currents compared to a run in which the forcing was climatological the rectification in sst resulted from increased evaporation under stronger than normal winds of either sign from correlated intraseasonal oscillations in both vertical temperature gradient and upwelling speed forced by the winds and from zonal advection due to nonlinearly generated equatorial currents the net rectified signature produced by the mjo like wind stresses was sst cooling about 0 48c in the west pacific and warming about 0 18c in the central pacific tending to flatten the background zonal sst gradient it is hypothesized that in a coupled system such a pattern of sst anomalies would spawn additional westerly wind anomalies as a result of sst induced changes in the low level zonal pressure gradient this was tested in an intermediate coupled

modulation of the mjo intensity over the equatorial western pacific by two types of el nino the modulation of the madden julian oscillation mjo intensity by eastern pacific ep type and central pacific cp type of el nino was investigated using observed data during the period of 1979 2013 mjo intensity is weakened strengthened over the equatorial western pacific from november to april during ep cp el nino the difference arises from distinctive tendencies of column integrated moist static energy mse anomaly in the region a larger positive mse tendency was found during the convection developing period in the cp mjo than the ep mjo the tendency difference is mainly caused by three meridional moisture advection processes the advection of the background moisture by the intraseasonal wind anomaly the advection of intraseasonal moisture anomaly by the mean wind and the nonlinear eddy advection the advections differences are primarily caused by different intraseasonal perturbations and high frequency activity whereas the background flow and moisture gradient are similar the amplitudes in the intraseasonal suppressed convection anomaly over the central pacific is critical in modulating the three meridional moisture advection processes the influences on the central pacific convection anomaly from seasonal mean moisture in two types of el ninos are discussed

evolution of the madden julian oscillation in two types of el nino abstractevolution characteristics of the madden julian oscillation mjo during the eastern pacific ep and central pacific cp types of el nino have been investigated mjo activities are strengthened over the western pacific during the predeveloping and developing phases of ep el nino but suppressed during the mature and decaying phases in contrast mjo activities do not show a clear relationship with cp el nino before their occurrence over the western pacific but they increase over the central pacific during the mature and decaying phases of cp el nino lag correlation analyses further confirm that mjo activities over the western pacific in boreal spring and early summer are closely related to ep el nino up to 2 11 months later but not for cp el nino ep el nino tends to weaken the mjo and lead to a much shorter range of its eastward propagation anomalous descending motions over the maritime continent and western pacific related to el nino can suppress convection and moisture flux convergence the

el nino modoki and its possible teleconnection 1 using observed data sets mainly for the period 1979 2005 we find that anomalous warming events different from conventional el nino events occur in the central equatorial pacific this unique warming in the central equatorial pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly ssta on both sides along the equator empirical orthogonal function eof analysis of monthly tropical pacific ssta shows that these events are represented by the second mode that explains 12 of the variance since a majority of such events are not part of el nino evolution the phenomenon is named as el nino modoki pseudo el nino modoki is a classical japanese word which means a similar but different thing the el nino modoki involves ocean atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution analogous to the southern oscillation in the case of el nino hence the total entity is named as el nino southern oscillation enso modoki the enso modoki events significantly influence the temperature and precipitation over many parts of the globe depending on the season the impacts over regions such as the far east including japan new zealand western coast of united states etc are opposite to those of the conventional enso the difference maps between the two periods of 1979 2004 and 1958 1978 for various oceanic atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline this appears to be a cause of more frequent and persistent occurrence of the enso modoki event during recent decades

sst anomalies of enso and the madden julian oscillation in the equatorial pacific this study explores whether anomalies of sea surface temperature sst in the equatorial pacific during warm events of el nino southern oscillation enso are related to the madden julian oscillation mjo an index is derived from surface wind data of a global model reanalysis to measure forcing of oceanic kelvin waves by wind stress associated with the mjo this kelvin wave forcing index of the mjo is used to relate interannual anomalies in seasonal activity of the mjo to interannual anomalies in equatorial sst for the 1980 99 time period evidence is found showing that during enso warm events stronger kelvin wave forcing in the western pacific precedes greater sst anomalies in the eastern pacific by 6 12 months no such evidence however can be found for the period of 1950 79 the result suggests that during enso warm events amplitudes of sst anomalies in the eastern pacific can be affected by the mjo at least for the 1980 99 period through its seasonal activity of kelvin wave forcing a possible mechanism for such an mjo enso relationship is described how enso prediction can be benefited from the precursory signals of the mjo is discussed

seasonal dependence of the mjo enso relationship abstract observations of the development of recent el nino events suggest a pivotal role for the madden julian oscillation mjo previous attempts to uncover a systematic relationship between mjo activity and the el nino southern oscillation enso however have yielded conflicting results in this study the mjo enso relationship is stratified by season and the focus is on mjo activity in the equatorial western pacific the results demonstrate that mjo activity in late boreal spring leads el nino in the subsequent autumn winter for the period 1979 2005 spring is the season when mjo activity is least asymmetric with respect to the equator and displays the most sensitivity to sst variations at the eastern edge of the warm pool enhanced mjo activity in the western pacific in spring is associated with an eastward expanded warm pool and low frequency westerly surface zonal wind anomalies these sustained westerly anomalies in the western pacific are hypothesized to project favorably onto a developing el n

fundamental causes of propagating and nonpropagating mjos in mjotf gass models abstractthis study investigates the fundamental causes of differences in the madden julian oscillation mjo eastward propagation among models that participated in a recent model intercomparison project these models are categorized into good and poor groups characterized by prominent eastward propagation and nonpropagation respectively column integrated moist static energy mse budgets are diagnosed for the good and the poor models it is found that a zonal asymmetry in the mse tendency characteristic of eastward mjo propagation occurs in the good group whereas such an asymmetry does not exist in the poor group the difference arises mainly from anomalous vertical and horizontal mse advection the former is attributed to the zonal asymmetry of upper midtropospheric vertical velocity anomalies acting on background mse vertical gradient the latter is mainly attributed to the asymmetric zonal distribution of low tropospheric meridional wind anomalies advecting background mse and moisture fields base

influences of oceanic rheostats and amplifiers on atmospheric co2 content during the late quaternary abstract although a clear explanation has yet to emerge for the observed decline in atmospheric co2 content during glacial episodes numerous hypotheses have been offered selected examples are reviewed or evaluated here a strengthened biological pump resulting from intensified upwelling at low latitudes was suggested early on as a likely direct cause of the glacial co2 drawdown two problems have since arisen with this hypothesis first on some upwelling continental margins for example off california oregon northwestern mexico peru and parts of nw africa organic matter accumulation decreased in some areas markedly during the last glacial maximum lgm second upwelled cool co2 rich water degasses carbon dioxide in warm regions and for a key area such as the modern equatorial pacific to become a net sink for atmospheric co2 rather than a source as it is today the rate of downward organic carbon export must exceed the rate of degassing recent sedimentary carbon and nitrogen isotopic data suggest however that the equatorial pacific has remained a strong co2 source for at least the last 40 000 yr and probably much longer although the direct extraction of carbon from surface waters and burial in the sediments in productive regions now appears unlikely to provide the sought for explanation indirect effects related to export productivity may hold the key these can modulate the chemical character of the ocean so as to increase its uptake of co2 a rheostat effect or they could increase indirectly the nutrient inventory of the sea an amplifier effect for example it is now recognized that denitrification was greatly reduced during glacial maxima in all three principal oxygen minimum zones in the oceans i e the subtropical north and south pacific and the arabian sea the implication is that nitrate the limiting nutrient in the modern ocean must have been more abundant during glacial periods and that this surfeit would have supported increased export production in the meso or oligotrophic areas that are today nitrate limited dust may represent another rheostat increases in atmospheric turbidity during glacials are clearly recorded by ice and marine sediment cores in both hemispheres in addition to fostering enhanced export production by adding needed iron to nitrate rich areas it has been suggested recently that increased dust inputs to nitrate depleted regions during glacials might have encouraged growth of iron hungry n2 fixing cyanobacteria thus alleviating nitrate limitation no sedimentary 15 n data yet exist to support this hypothesis but it remains viable finally regional changes in physical hydrography may have played a major and hitherto underestimated role for example in the glacial southern ocean south of the polar front recent nitrogen isotope and other data imply that the upper water column was well stratified during the lgm by limiting upwelling this would have reduced the ocean atmosphere co2 leak in the area this could have made a significant contribution to the pco2 drawdown

enso regulation of mjo teleconnection the extratropical teleconnections associated with madden julian oscillation mjo are shown to have an action center in the north pacific where the pressure anomalies have opposite polarities between the phase 3 convective indian ocean and phase 7 convective western pacific of the mjo the teleconnection in the same phase of mjo may induce opposite anomalies over east asia and north america between el nino and la nina years during mjo phase 3 a gigantic north pacific anticyclonic anomaly occurs during la nina making coastal northeast asia warmer wetter than normal but the west us colder drier whereas during el nino the anticyclonic anomaly is confined to the central north pacific hence the northwest us experiences warmer than normal weather under influence of a downstream cyclonic anomaly during phase 7 an extratropical cyclonic anomaly forms over the northwest pacific during la nina due to convective enhancement over the philippine sea causing bitter winter monsoon over japan whereas during el nino the corresponding cyclonic anomaly shifts to the northeast pacific due to enhanced convection over the equatorial central pacific which causes warm and wet conditions along the west coast of us and canada further the presence of enso induced seasonal anomalies can significantly modify mjo teleconnection but the aforementioned mjo teleconnection can still be well identified during phase 3 the mjo teleconnection pattern over north pacific will be counterbalanced enhanced by el nino la nina induced seasonal mean anomalies during phase 7 on the other hand the mjo teleconnection anomalies in the northeastern pacific will be enhanced during el nino but reduced during la nina thereby the impacts of mjo teleconnection on the north america is expected to be stronger during el nino than during la nina

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

anticorrelated intensity change of the quasi biweekly and 30 50 day oscillations over the south china sea 1 over the south china sea the year to year variations in the intensity of the quasi biweekly qbw and 30 50 day oscillations during june july are anti correlated an explanation is offered for the out of phase relationship we found during strong qbw years the june july mean convection and easterly vertical shear are enhanced over the equatorial western central pacific with corresponding low level cyclonic meridional shear to the northwest these conditions concurring with the equatorial eastern pacific warming are favorable for emanation of moist rossby waves from the equatorial western pacific on the other hand the 30 50 day becomes active when the mean convection is enhanced over the eastern indian ocean and maritime continent where this mode stems from and develops which often corresponds to cold western io one of these two large scale settings often occurs in the absence of the other so that when one mode is strong during june july the other mode tends to be weak

enso regimes reinterpreting the canonical and modoki el nino 1 we propose that the first two empirical orthogonal function eof modes of tropical pacific sea surface temperature sst anomalies do not describe different phenomena i e el nino southern oscillation enso and el nino modoki but rather the nonlinear evolution of enso we introduce two new uncorrelated indices e and c based on the leading eofs that respectively account for extreme warm events in the eastern and cold moderate warm events in the central equatorial pacific corresponding to regimes with different evolution recent trends in enso can be described as an increase in the central pacific c variability that is associated with stronger cold events as well as a reduction in the eastern pacific e variability within the cold moderate warm regime consistent with model projections however little can be said observationally with respect to the extreme warm regime

propagating versus nonpropagating madden julian oscillation events abstractbasinwide convective anomalies over the indian ocean io associated with the madden julian oscillation mjo sometimes propagate eastward and reach the west pacific wp but sometimes do not long term observations and reanalysis products are used to investigate the difference between the propagating and nonpropagating mjo events io convection onset events associated with the mjo are grouped into three categories based on the strengths of the simultaneous dry anomalies over the eastern maritime continent and wp the io convection anomaly preferentially makes eastward propagation and reaches the wp when the dry anomaly is stronger analysis of the column integrated moist static energy mse budget shows that horizontal advection moistens the atmosphere to the east of the positive mse anomaly associated with the active convection over the io and is of sufficient magnitude to explain the eastward propagation of the positive mse anomaly interpretation is complicated however by lack of closure in

energy spectrum characteristics of boreal summer intraseasonal oscillations climatology and variations during the enso developing and decaying phases the geographic dependence characteristics of the energy spectrum of the boreal summer intraseasonal oscillation bsiso may october over the indo western pacific region were analyzed using 25 yr 1979 2003 observational data the bsiso energy spectrum distribution exhibits a distinctive regional characteristic the stationary and eastward propagating modes are most pronounced at the equator 5 s 5 n while the westward propagating modes are dominant in the off equatorial region 10 20 n while the eastward intraseasonal oscillation iso spectrum agglomerates on the 30 60 day period and zonal wavenumber 1 the westward mode covers wider spatial wavenumber and temporal period range along the arabian sea bay of bengal and south china sea scs latitudes the dominant wavenumber 1 mode is the eastward westward propagation at the 30 60 day 10 20 day period for zonal wavenumber 2 the dominant mode is the westward propagation at both the 30 60 day and 10 20 day periods compared to the absolute amplitude of both zonal and meridional mode energy spectrum northward propagation is the most predominant mode in boreal summer over the indo western pacific regions the strongest northwardpropagating bsiso signal appears in the eastern tropical indian ocean the variation of bsiso differs significantly in the el nino and la nina developing and decaying phases during the el nino la nina developing summer the eastward propagation is enhanced weakened at the equator while the northward propagation is also strengthened weakened over the western pacific east of 140 e during the el nino la nina decaying summer the eastward propagation weakens strengthens at the equator opposite to that in the developing summer the westward propagation off the equator and the northward propagation over scs and the western pacific are suppressed enhanced the amplitude of the bsiso variation is stronger in the decaying summer than that in the developing summer this asymmetry in bsiso variations is primarily attributed to the asymmetry of the background mean flow change associated with the developing and decaying phases of enso

a new paradigm for the predominance of standing central pacific warming after the late 1990s canonical el nino has a warming center in the eastern pacific ep but in recent decades el nino warming center tends to occur more frequently in the central pacific cp the definitions and names of this new type of el nino however have been notoriously diverse which makes it difficult to understand why the warming center shifts here we show that the new type of el nino events is characterized by 1 the maximum warming standing and persisting in the cp and 2 the warming extending to the ep only briefly during its peak phase for this reason we refer to it as standing cp warming cpw global warming has been blamed for the westward shift of maximum warming as well as more frequent occurrence of cpw however we find that since the late 1990s the standing cpw becomes a dominant mode in the pacific meanwhile the epochal mean trade winds have strengthened and the equatorial thermocline slope has increased contrary to the global warming induced weakening trades and flattening thermocline we propose that the recent predominance of standing cpw arises from a dramatic decadal change characterized by a grand la nina like background pattern and strong divergence in the cp atmospheric boundary layer after the late 1990s the anomalous mean cp wind divergence tends to weaken the anomalous convection and shift it westward from the underlying sst warming due to the suppressed low level convergence feedback this leads to a westward shift of anomalous westerly response and thus a zonally in phase sst tendency preventing eastward propagation of the sst anomaly we anticipate more cpw events will occur in the coming decade provided the grand la nina like background state persists

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature