coupled ocean atmosphere response to north tropical atlantic sst tropical atlantic dipole and enso 1 the coupled ocean atmosphere response to changes of the north tropical atlantic nta sst is investigated using a coupled ocean atmosphere general circulation model the model explicitly demonstrates that a nta sst anomaly can organize an inter hemispheric sst dipole in boreal spring over both the tropical atlantic and eastern tropical pacific primarily through a coupled wind evaporative sst wes feedback while the tropical atlantic dipole eventually decays in summer and fall the eastern tropical pacific dipole subsequently evolves into an enso like pattern through the seasonal migration of the itcz and coupled ocean atmosphere feedbacks

mechanisms for enso phase change in a coupled gcm the mechanisms leading to el nino onset and termination in the echam4 opa coupled gcm are assessed and compared to observations and existing enso paradigms at the equator as well as off equator the patterns and timing of modeled el nino composites are in good agreement with those observed heat content of the west pacific is confirmed as a precursor to enso phase change and the present work emphasizes the role of its northern off equator part west north pacific wnp region 58 158n 1208 1708e the associated heat content changes appear to be dominated by a local ekman pumping or forced rossby waves rather than the accumulation of remotely generated free rossby waves as proposed by many theories the heat content decrease in the wnp region which triggers el nino termination is due to the negative feedback of the atmospheric gill s response to the increased equatorial sst in the east pacific in agreement with most paradigms delayed recharged west pacific oscillators the present study introduces the advection of the off equator signal to the equatorial waveguide by the mean currents of the western pacific as an additional process a similar feedback with opposite sign also seems to drive the modeled el nino onset favoring a too strong and regular biennial enso in the model this is due to the stronger than observed walker circulation that isolates the wnp region from other remote influences like monsoons the model also exhibits aborted enso events where the warming peaks in late spring instead of late autumn and is quickly terminated by the gill s negative feedback the abort event occurs too frequently in the coupled model due to too strong and too zonal a convergence zone south of the equator double itcz it bears some resemblance to the spring 1993 warming when the southern tropics were also warm the results of this paper provide additional insight into the el nino seasonal phase lock mechanisms

magma ocean formation due to giant impacts current understanding of the last stages of planetary accretion suggests that mass and energy accumulation are dominated by a few large impacts an important thermal consequence of such impacts is melting and formation of a large melt pond if the planet s isostatic adjustment time scale is short compared to the magma pond s cooling time scale the melt may be extruded onto the surface and form a magma ocean of approximately uniform depth although a giant impact striking at 10 15 km s 1 deposits enough energy to melt the entire planet the distribution of that energy is important in determining the thermal outcome of the collision we examine the thermal effects of giant impacts by estimating the melt volume generated by the initial shock wave and corresponding magma ocean depths additionally we examine the effects of the planet s initial temperature on the generated melt volume the hugoniot curve plotted in pressure entropy space is used to determine the shock pressure required to completely melt the material for room temperature dunite this pressure is about 150 gpa and the partial melt region is narrow for dunite initially at the solidus temperature this pressure is about 115 gpa the partial melt region extends throughout the planet once the melting pressure is known an impact melting model based on the second hugoniot equation the linear shock particle velocity relationship and the empirical particle velocity distance relationship is used to estimate the radial distance melting occurred from the impact site once the distance is found the melt region s geometry determines the associated melt volume partial melt volume is also estimated the melt fraction that is not excavated during crater formation is estimated and magma ocean depths resulting from both excavated and retained melt are calculated the model is also used to estimate the fraction of a planet melted by the initial shock wave nominal conditions of the moon forming giant impact projectile planet mass 0 14 impact speed 15 km s 1 generate melting of 30 65 of the planet depending on its initial temperature whole planet melting requires projectile planet mass ratios of 0 4 for a 15 km s 1 impact if the planet was near its solidus before the impact isostatic relaxation may generate a significant volume of additional melting

adjustment of the coupled ocean atmosphere system to a sudden change in the thermohaline circulation 1 the adjustment of the coupled ocean atmosphere system to a fresh water anomaly in the north atlantic ocean is investigated using a coupled gcm in response to the anomaly the atlantic thermohaline circulation thc collapses and the associated reduction in northward heat transport causes sea surface temperatures sst to fall over the north atlantic and to increase over the tropical south atlantic atmospheric feedbacks are excited as soon as the sst anomalies reach sufficient magnitude a key stage in the adjustment process occurs 4 6 years after the perturbation is introduced when a significant sst dipole develops in the tropical atlantic this dipole causes a southward shift of the itcz and leads in year 7 to the triggering of an el nino event it is concluded that atmospheric feedbacks could spread globally the influence of a sudden change in the thc much more quickly and efficiently than could ocean processes alone

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

el nino and la nina sea surface temperature anomalies asymmetry characteristics associated with their wind stress anomalies 1 the asymmetric nature of el nino and la nina sea surface temperature sst anomalies is investigated by the use of national centers for environmental prediction reanalysis data and various ocean and atmosphere models it is demonstrated that the relatively weak sst anomalies during la nina compared with those of el nino are related to the westward shift of wind stress anomalies by 10 15 the asymmetric characteristics of atmospheric responses are confirmed by the general circulation model experiments with the two different sst anomalies which have equal amplitude but are of opposite sign from each other the experiments with an intermediate ocean model and a hybrid coupled model clearly show that the sst anomalies over the equatorial pacific become weaker as the zonal wind stress shifts to the west not only the amplitude but also the oscillation timescale of the sst anomaly is shown to be sensitive to the location of wind stress anomalies the duration of la nina which is rather shorter than that of el nino is also related to the longitudinal displacement of the wind stress anomaly

why the properties of el nino changed during the late 1970s following the abrupt north pacific climate shift in the mid 1970s the period amplitude spatial structure and temporal evolution of the e1 nifio notably changed theory is needed to explain why the coherent changes in several el nifio characteristics are nearly synchronized with the decadal climate shift while the decadal variation in the equatorial thermocline could potentially change el nino behavior observation indicates that from the pre shift 1961 1975 to the post shift 1981 1995 period the change of equatorial eastern pacific thermocline is insignificant our numerical experiments with a coupled atmosphere ocean model illustrate that the observed changes in enso properties may be attributed to decadal changes in the surface winds and associated ocean surface layer dynamics without changes in the mean thermocline a theoretical analysis is presented to elucidate the mechanisms by which the decadal variations in winds and upwelling modify the structure and propagation of the e1 nifio and amplify and prolong the e1 nifio la nifia cycle the dominant period of the e1 nifio increased from 2 3 years during 1960s and 1970s to 4 5 years during 1980s and 1990s during this time the amplitude of e1 nifio also increased an and wang 2000 these changes were accompanied by a notable modification in the evolution pattern and spatial structure of the coupled ocean

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

on the dynamics of droughts in northeast brazil observations theory and numerical experiments with a general circulation model abstract it is proposed that a possible mechanism for the occurrence of severe droughts over northeast brazil is the establishment of a thermally direct local circulation which has its ascending branch at about 10 n and its descending branch over northeast brazil and the adjoining oceanic region the driving for this anomalous circulation is provided by warming due to enhanced moist convection associated with warmer sea surface temperature anomalies over the northern tropical atlantic and cooling associated with colder sea surface temperature anomalies in the southern tropical atlantic the combined effects of thermally forced subsidence and reduced evaporation and moisture flux convergence produces severe drought conditions over northeast brazil we have examined the monthly mean sea surface temperature anomalies over the tropical atlantic and rainfall anomalies over two selected stations fortaleza 3 46 s 38 31 w and quixeramobim 5 12 s 39 18 w for 25 years 1948 72 it is found that the most sever

modeling evidence for enhanced el nino southern oscillation amplitude during the last glacial maximum we present a numerical eigenmode analysis of an intermediate el nin o southern oscillation enso model which is driven by present day observed background conditions as well as by simulated background conditions for the last glacial maximum lgm about 21 000 years ago the background conditions are obtained from two lgm simulations which were performed with the national center for atmospheric research climate system model csm1 4 and an earth system model of intermediate complexity ecbilt clio our analysis clearly shows that the leading present day unstable recharge discharge mode changes its stability as well as its frequency during lgm conditions simulated lgm background conditions were favorable to support large amplitude self sustained interannual enso variations in the tropical pacific our analysis indicates that off equatorial climate conditions as well as a shoaling of the thermocline play a crucial role in amplifying the lgm enso mode

nonlinear interdecadal changes of the el nino southern oscillation nonlinear interdecadal changes in the el nino southern oscillation enso phenomenon are investigated using several tools a nonlinear canonical correlation analysis nlcca method based on neural networks a hybrid coupled model and the delayed oscillator theory the leading nlcca mode between the tropical pacific wind stress ws and sea surface temperature sst reveals notable interdecadal changes of enso behaviour before and after the mid 1970s climate regime shift with greater nonlinearity found during 1981 99 than during 1961 75 spatial asymmetry for both sst and ws anomalies between warm el nino and cool la nina events was significantly enhanced in the later period during 1981 99 the location of the equatorial easterly anomalies was unchanged from the earlier period but in the opposite enso phase the westerly anomalies were shifted eastward by up to 25 according to the delayed oscillator theory such an eastward shift would lengthen the duration of the warm events by up to 45 but leave the duration of the cool events unchanged supporting evidence was found from a hybrid coupled model built with the lamont dynamical ocean model coupled to a statistical atmospheric model consisting of either the leading nlcca or cca mode

asymmetry between el nino and la nina in a global coupled gcm with an eddy permitting ocean resolution observations show the asymmetric nature of el nino and la nina sea surface temperature sst anomalies warm events are often stronger than cold events this asymmetric behavior is an important feature that can be used to validate coupled models to test their ability to represent the climate system the asymmetry of el nino and la nina sst anomalies has been investigated in a simulation of the hadley centre eddy permitting coupled general circulation model it is found that the asymmetric behavior is captured by the model with sst anomalies associated with strong el nino events being greater than those associated with strong la nina events the pattern of the sst asymmetry also bears some similar characteristics to those based on observations despite the deficiency that sst anomalies associated with both el nino and la nina extend too far westward in the model through a heat budget analysis of the ocean mixed layer it is shown that nonlinear dynamic heating ndh is important in generating intense el nino and the sst asymmetry between el nino and la nina events especially in the eastern tropical pacific ocean this nonlinear dynamic heating enhances the amplitude of el nino and reduces the amplitude of la nina and therefore leads to the asymmetry between el nino and la nina events with el nino being stronger however the skewness and asymmetry in the model are relatively weak being consistent with a relatively weak nonlinear dynamical heating it is also shown that the eastward propagating feature of subsurface anomalies provides a favorable phase relationship between temperature and current anomalies that results in strong nonlinear dynamical heating that tends to produce stronger el ninos in addition in the model simulation the nonlinear nature of zonal wind stress anomalies between el nino and la nina events also plays an important role in the central tropical pacific these different mechanisms work constructively to determine the asymmetry between el nino and la nina events in the model and they are similar to those proposed in recent studies based on observations the ability of the model to simulate this asymmetric feature is encouraging and offers hope to the challenge of predicting the amplitude of strong el nino events

a mechanism for decadal changes of enso behavior roles of background wind changes this study explains why a number of el nino properties period amplitude structure and propagation have changed in a coherent manner since the late 1970s and why these changes had almost concurred with the pacific decadal climate shift evidence is presented to show that from the pre shift 1961 1975 to the post shift 1981 1995 epoch significant changes in the tropical pacific are found in the surface winds and temperature whereas changes in the thermocline are uncertain numerical experiments with the cane and zebiak model demonstrate that the decadal changes in the surface winds qualitatively reproduce the observed coherent changes in el nino properties the fundamental factor that altered the model s el nino is the decadal changes of the background equatorial winds and associated upwelling the annual cycle is also necessary for the mean state to modulate el nino from the pre to post shift epoch the changes in the background winds and upwelling modify the structure of the coupled mode eastward displacement of the equatorial westerly anomalies by reallocating anomalous atmospheric heating and sst gradient along the equator this structural change amplifies the enso cycle and prolongs the oscillation period by enhancing the coupled instability and delaying transitions from a warm to a cold state or vice versa the changes in the mean currents and upwelling reduce the effect of the zonal temperature advection while enhance that of the vertical advection thus the prevailing westward propagation is replaced by eastward propagation or standing oscillation our results suggest a critical role of the atmospheric bridge that rapidly conveys the influences of extratropical decadal variations to the tropics and the possibility that the pacific climate shift might have affected el nino properties in the late 1970s by changing the background tropical winds and the associated equatorial upwelling

vertical structure variability in the equatorial pacific before and after the pacific climate shift of the 1970s 1 oceanic baroclinic modes in the equatorial pacific are estimated using results from the soda system our result suggests that the vertical stratification increases at the upper levels after the late 1970s due to changes in the vertical temperature structure after the late 1970s the variability of the higher order baroclinic mode contributions to current anomalies and surface pressure significantly increases in the central equatorial pacific this is associated with an increase of the dominant period of the enso variability simple coupled model experiments indicate that when atmospheric forcing projects more onto the higher baroclinic modes according to the results of the soda decomposition the amplitude and dominant period of enso increase similarly to what is observed at the pacific decadal shift our results support that the changes in the behavior of enso after the late 1970s are associated with oceanic vertical structural changes in temperature that tend to increase the contribution of higher order baroclinic modes

global teleconnections of meridional overturning circulation anomalies there is a wide range of evidence from both models and palaeoclimatic data that indicates the possibility of abrupt changes in the oceanic meridional overturning circulation moc however much of our dynamical understanding of the moc comes from steady state models that rely upon the assumption of thermodynamic equilibrium and are therefore only valid on millennial time scales here a dynamical model for the global teleconnections of moc anomalies on annual to multidecadal time scales is developed it is based on a linear theory for the propagation of zonally integrated meridional transport anomalies in a reduced gravity ocean and allows for multiple ocean basins connected by a circumpolar channel to the south the theory demonstrates that the equator acts as a low pass filter to moc anomalies as a consequence moc anomalies on decadal and shorter time scales are confined to the hemispheric basin in which they are generated and have little impact on the remainder of the global ocean the linear theory is compared with the results of a global nonlinear numerical integration which it reproduces to a good approximation

a stability analysis of tropical ocean atmosphere interactions bridging measurements and theory for el nino abstract interactions between the tropical oceans and atmosphere permit a spectrum of natural modes of oscillation whose properties period intensity spatial structure and direction of propagation depend on the background climatic state i e the mean state this mean state can be described by parameters that include the following the time averaged intensity of the pacific trade winds the mean depth h of the thermocline and the temperature difference across the thermocline t a stability analysis by means of a simple coupled ocean atmosphere model indicates two distinct families of unstable modes one has long periods of several years involves sea surface temperature variations determined by vertical movements of the thermocline that are part of the adjustment of the ocean basin to the fluctuating winds requires a relatively deep thermocline and corresponds to the delayed oscillator the other family requires a shallow thermocline has short periods of a year or two has sea surface tempe

enhancement of enso variability by a weakened atlantic thermohaline circulation in a coupled gcm abstract a coupled ocean atmosphere general circulation model is used to investigate the modulation of el nino southern oscillation enso variability due to a weakened atlantic thermohaline circulation thc the thc weakening is induced by freshwater perturbations in the north atlantic and leads to a well known sea surface temperature dipole and a southward shift of the intertropical convergence zone itcz in the tropical atlantic through atmospheric teleconnections and local coupled air sea feedbacks a meridionally asymmetric mean state change is generated in the eastern equatorial pacific corresponding to a weakened annual cycle and westerly anomalies develop over the central pacific the westerly anomalies are associated with anomalous warming of sst causing an eastward extension of the west pacific warm pool particularly in august february and enhanced precipitation these and other changes in the mean state lead in turn to an eastward shift of the zonal wind anomalies associated with el nin

interdecadal change in properties of el nino southern oscillation in an intermediate coupled model abstract the role of subsurface temperature variability in modulating el nino southern oscillation enso properties is examined using an intermediate coupled model icm consisting of an intermediate dynamic ocean model and a sea surface temperature sst anomaly model an empirical procedure is used to parameterize the temperature of subsurface water entrained into the mixed layer te from sea level sl anomalies via a singular value decomposition svd analysis for use in simulating sea surface temperature anomalies sstas the ocean model is coupled to a statistical atmospheric model that estimates wind stress anomalies also from an svd analysis using the empirical te models constructed from two subperiods 1963 79 t63 79e and 1980 96 t80 96e the coupled system exhibits strikingly different properties of interannual variability the oscillation period spatial structure and temporal evolution for the t63 79e model the system features a 2 yr oscillation and westward propagation of sstas o

structure and geomorphology of the duchesne graben uinta basin utah and its enhancement of a hydrocarbon reservoir the duchesne fault zone dfz in the uinta basin of northeastern utah is a system of normal faults and joint zones that forms prominent east trending lineaments tens of kilometers long the duchesne graben an asymmetrical fault bounded trough in the western part of the dfz is the focus of this article a master fault bounds the southern margin of the structural half graben dips steeply northward and accommodates about 200 m of slip this fault either terminates downward by 1400 m depth or flattens into a north dipping low angle detachment fault at about 1000 m depth the half graben is developed along the crest of an open low amplitude flexure in the southern limb of the uinta basin syncline this faulted flexure trapped hydrocarbons that migrated southward from depth in the uinta basin intense fracturing within the half graben also created migration pathways for hydrocarbons and generated shallow reservoirs in the green river formation

enso suppression due to weakening of the north atlantic thermohaline circulation changes of the north atlantic thermohaline circulation thc excite wave patterns that readjust the thermocline globally this paper examines the impact of a freshwater induced thc shutdown on the depth of the pacific thermocline and its subsequent modification of the el nino southern oscillation enso variability using an intermediate complexity global coupled atmosphere ocean sea ice model and an intermediate enso model respectively it is shown by performing a numerical eigenanalysis and transient simulations that a thc shutdown in the north atlantic goes along with reduced enso variability because of a deepening of the zonal mean tropical pacific thermocline a transient simulation also exhibits abrupt changes of enso behavior depending on the rate of thc change the global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales simulated multidecadal global sea surface temperature sst patterns show a large degree of similarity with previous climate reconstructions suggesting that the observed pan oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections