dynamics and thermodynamics of a warming event in a coupled tropical atmosphere ocean model abstract a simple coupled ocean atmosphere model similar to that of zebiak and cane is used to examine the dynamic and thermodynamic processes associated with el nino southern oscillation enso the model is run for 300 years the interannual variability which results is regular with a period of either 3 or 4 years quantized by the annual cycle the amplitude 1 5 m s 1 wind and 2 c sst anomalies period and structure of the interannual variability compare well with observations the model warm event is initiated in the spring prior to the event peak and is well described as an instability of the coupled system during instability growth the sea surface temperature sst anomaly is primarily generated by vertical upwelling processes the sst anomaly can be approximately described by the expression t t kth t where t is the sst anomaly t time h the upper layer thickness pycnocline perturbation and an effective damping time which includes heat loss to the atmosphere kt parameterizes

simulation of the tropical pacific climate with a coupled ocean atmosphere general circulation model part ii interannual variability abstract two multiyear simulations with a coupled ocean atmosphere general circulation model gcm totaling 45 years are used to investigate interannual variability at the equator the model consists of the ucla global atmospheric gcm coupled to the gfdl oceanic gcm dynamically active over the tropical pacific multichannel singular spectrum analysis along the equator identifies enso like quasi biennial qb and quasi quadrennial qq modes both consist of predominantly standing oscillations in sea surface temperature and zonal wind stress that peak in the central or east pacific accompanied by an oscillation in equatorial thermocline depth that is characterized by a phase shift of about 90 across the basin with west leading east simulated interannual variability is weaker than observed in both simulations one of these is dominated by the qb the other by the qq mode although the two differ only in details of the surface layer parameterizations

interannual variability in a tropical atmosphere ocean model influence of the basic state ocean geometry and nonlinearity abstract the behavior of a tropical coupled atmosphere ocean model is analyzed for a range of different background states and ocean geometries the model is essentially that of cane and zebiak for the tropical pacific except only temporally constant background states are considered here for realistic background states and ocean geometry the model solutions feature oscillations of period of 3 5 yr by comparing the full model solution with a linearized version of the model it is shown that the basic mechanism of the oscillation is contained within linear theory a simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere ocean system the analog model highlights the properties that produce coupled atmosphere ocean instability in the eastern ocean basin and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed negative feedback into this instability growth the growth rate of the local instabil

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

probabilistic approach to short and intermediate term earthquake forecasting ii test application to southern california a method is proposed for assessing the modifying effects of precursory observations on long term probabilities of strong earthquakes m 6 estimated short and intermediate term probabilities so estimated rely on the mean precursory time and its uncertainty as a function of the mainshock magnitude and epicentral position short and intermediate term modification of long term probabilities within a 120 000 km2 circular area covering most of southern california are estimated for one or more observations of strain resistivity and ground water anomalies centered near palmdale in the central transverse ranges the principal assumptions employed are log normal distribution of globally reported waiting periods and assumed predictive reliability of precursory phenomena as expected the significance of short and intermediate term modifications scale with the reliability of precursory observations and are particularly enhanced within window lengths of a precursory characteristic time

origin of upper ocean warming and el nino change on decadal scales in the tropical pacific ocean the cause of decadal scale variability in the tropical pacific ocean such as that marked by the 1976 77 shift in the el nino southern oscillation1 2 3 4 5 6 7 is poorly understood unravelling the mechanism of the recent decade long warming in the tropical upper ocean is a particularly important challenge given the link to el nino variability but establishing the hypothesized interannual decadal oceanic connections between middle latitudes and tropics has proved elusive8 here we present observational evidence that pacific upper ocean warming and decadal changes in the el nino southern oscillation after 1976 may originate from decadal mid latitude variability in the middle 1970s the north pacific ocean is observed to have undergone a clear phase transition a see saw subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre at middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface waters and penetrated through the subtropics and into the tropics thus perturbing the tropical thermocline and driving the formation of a warm surface water anomaly that may have influenced el nino in the 1980s the identification of this teleconnection of extratropical thermal anomalies to the tropics through a subsurface ocean bridge may enable improved prediction of decadal scale climate variability

the penetration of tritium into the tropical pacific abstract the persistence of subsurface tritium maxima coincident with the equatorial currents is used to show that advection along isopycnals by the mean wind driven circulation is the dominant process in the at most 14 year time scale for the penetration of high northern latitude water to the equator above 26 2 sigma theta ventilation of the equatorial pacific thermocline from the north contrasts sharply with the equatorial atlantic thermocline which is ventilated from the south the most striking manifestation of the north pacific circulation is evidenced by a tritium maximum and salinity minimum at the equator between 145 and 125 w located above 25 6 sigma theta it shows that regardless of time of sampling the eastern sol central equator has received the moat high latitude water probably as a consequence of recirculation by the equatorial currents between the same meridians there is a tritium maximum on and north of the equator at the surface which is interpreted as an expression of upwelling

enso simulation and prediction with a hybrid coupled model a hybrid coupled model hcm consisting of a tropical pacific ocean and global atmosphere is presented the ocean component is a linear reduced gravity model of the upper ocean in the tropical pacific the atmospheric component is a triangular 30 horizontal resolution global spectral general circulation model with 18 unevenly spaced levels in the vertical in coupling these component models an anomaly coupling strategy is employed a 40 yr simulation was made with hcm and the variability in the tropical pacific was compared to the observed variability the hcm produces irregular enso events with a broad spectrum of periods between 12 and 48 months on longer timescales approximately 48 months the simulated variability was weaker than the observed and on shorter timescales approximately 24 months the simulated variability was too strong the simulated variability is asymmetric in the sense that the amplitude of the warm events is realistic but there are no significant cold events an ensemble of 60 hindcast predictions was made with the hcm and the skill was compared to other prediction systems in forecasting sea surface temperature anomalies in the eastern pacific the hcm is comparable to the other prediction systems for lead times up to 10 months the anomaly correlation coefficient for the eastern pacific ssta remains above 0 6 for lead times of up to 11 months consistent with the 40 yr simulation hindcasts of cold events have little skill particularly when compared to hindcasts of warm events specific hindcasts also demonstrate that the hcm also has difficulty predicting the transition from warm conditions to normal or cold conditions

an advective reflective conceptual model for the oscillatory nature of the enso recent findings about zonal displacements of the pacific warm pool required a notable modification of the delayed action oscillator theory the current leading theory for the el nino southern oscillation enso simulations with a linearized coupled ocean atmosphere model resulted in 3 to 6 year enso like oscillations with many of the variable model parameters found to be very close to their observed values this simple model suggests that ocean processes that are ignored or underestimated in the delayed action oscillator theory such as zonal current convergence zonal advection of sea surface temperature and equatorial wave reflection from the eastern ocean boundary are fundamental to the development of the enso in particular to its manifestations in the central equatorial pacific

aperiodic variability in the zebiak cane coupled ocean atmosphere model air sea interactions in the western equatorial pacific abstract the behavior of two coupled ocean atmosphere models of intermediate complexity the zebiak cane zc hereafter and battisti version b88 hereafter of the zc coupled ocean atmosphere model are reviewed and compared to the observed climate record from the tropical pacific region a major difference between each system lies in the modes of variability that each supports in the observations variability at timescales shorter than that associated with el nino and the southern oscillation enso is ubiquitous and difficult to characterize in terms of clearly coupled ocean atmosphere interactions the b88 model supports only a single unstable mode the enso mode in the zc model two distinct modes are present the interannual enso mode and the mobile mode a near annual westward propagating instability it is demonstrated that differences in the basic state climatology and thermodynamic parameters are keys to understanding the differences between the zc and b88 coupled model behaviors it is found

a dynamical interpretation of the tritium maximum in the central equatorial pacific abstract the tropical tritium distribution between 1974 and 1981 is characterized by a maximum along the equator centered between 125 and 145 w it signifies that this region has received the maximum input of high northern latitude water a dynamical interpretation of the maximum shows that it can be explained by the strength of the sverdrup circulation in the central equatorial pacific where there is a strong zonal convergence in the north equatorial countercurrent necc and a strong meridional geostrophic flow towards the equator

coupled processes for equatorial pacific interannual variability diagnostics for coupled processes relevant to equatorial pacific interannual variability are defined using a simplified analytical model and applied to general circulation model simulations the analytical model is a simplification of the neelin model neelin 1991 the atmospheric component is represented by a direct zonal thermal circulation the oceanic dynamics take into account a shear layer and a kelvin wave analytical calculations lead to an unstable slow propagating coupled mode referred to as the tk mode all the processes of this mode are explicitly identified as are their contributions to the resulting total growth rate these contributions are computed in forced oceanic covering the 1979 93 period and in coupled ocean atmosphere general circulation model simulations the results are sensitive to the parameters but allow relative comparisons between general circulation model simulations the forced oceanic simulation suggests that the total instability is nearly constant all over the equatorial pacific basin but the central part is dominated by the zonal advective coupled process while the eastern part is controlled by both zonal and vertical advective coupled processes the thermocline coupled process appears largest in the eastern part of the basin but its contribution is still weaker than the other two the coupled simulation appears globally more unstable than the forced one the total instability is larger in the eastern part of the pacific basin this result is consistent with the location and the frequent occurrence of the simulated interannual signals in the eastern part of the basin the seasonal cycle of the forced simulation shows maximum instability between july and november this could suggest an approach towards the understanding of the seasonal phase lock of the onset of warm el nino southern oscillation events possible interactions between the delayed oscillator mechanism or the westerly wind bursts with the tk mode processes are discussed in order to understand the real system these diagnostics should be applied to the observations and not just models nonetheless analysis of coupled processes allows objective comparisons between different general circulation model simulations and the results presented in this paper should be considered as a first step in this direction

sensitivity of an intermediate ocean atmosphere coupled model of the tropical pacific to its oceanic vertical structure an intermediate ocean atmosphere coupled model icm of the tropical pacific is used to investigate the sensitivity of the coupled system to the oceanic vertical structure the model consists of a three baroclinicmode tropical pacific ocean and a gill tropical atmosphere the mixed layer is similar to the zebiak and cane zc coupled atmosphere ocean model but uses a different parameterization for subsurface temperature derived from xbt data a 100 yr simulation is made and the long term variability of the icm is briefly discussed and compared with the the observations and the zc model the icm reproduces regular enso events with a spectrum peak centered at the period 3 5 yr the presence of higher order vertical modes in the oceanic component of the model allows for reduced off equatorial variability of zonal current and thermocline anomalies and for finer meridional scale for sst anomaly variability in comparison with a simulation with one baroclinic mode only the nature of the processes sustaining the oscillations in the icm is then investigated based on the coupled instabilities and delayed oscillator theories it

foreshocks and time dependent earthquake hazard assessment in southern california the probability that an earthquake in southern california m 3 0 will be followed by an earthquake of larger magnitude within 5 days and 10 km i e will be a foreshock is 6 0 5 per cent 1 s d and is not significantly dependent on the magnitude of the possible foreshock between m 3 and m 5 the probability that an earthquake will be followed by an m 5 0 main shock however increases with magnitude of the foreshock from less than 1 per cent at m 3 to 6 5 2 5 per cent 1 s d at m 5 the main shock will most likely occur in the first hour after the foreshock and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time thus the occurrence of an earthquake of m 3 0 in southern california increases the earthquake hazard within a small space time window several orders of magnitude above the normal background level

the influence of sea surface temperature on surface wind in the eastern equatorial pacific seasonal and interannual variability abstract the climate of the eastern pacific exhibits a pronounced equatorial asymmetry boundary layer air originating in the southern hemisphere trades crosses the equator and flows into the intertropical convergence zone itcz whose southern limit is nearly always located at least 4 to the north of the equator the sea surface temperature sst distribution is characterized by a prominent cold tongue centered 1 s a strong frontal zone centered 2 n and a warm eastward current centered near 5 n the surface wind field exhibits a pronounced horizontal divergence as the air flows northward across the oceanic frontal zone these features vary in strength in response to the annual cycle and the el nino southern oscillation phenomenon the northward cross equatorial surface winds the cold tongue and the frontal zone all tend to be strongest during the cold season july through november during the cold season of the coldest years when the cold tongue is most prominent the cross equatorial flow t

accounting for parameter uncertainties in model verification an illustration with tropical sea surface temperature abstract a simplified model originally developed to simulate sea surface temperature sst in the tropical pacific is extended in two directions first the model is formulated and run for the tropical atlantic second optimal values are determined for a number of model parameters especially those determining surface heat flux in the pacific the difference between model sst and data sst is the size of the probable error due to uncertainties in the heat flux 0 35 w m 2 in the equatorial atlantic the discrepancies may also be attributed to heat flux errors but model data differences in the atlantic near coastal regions north of 10 n and south of 10 s are too large to be explained by heat flux errors model problems involving other physical processes must be involved by giving the best result possible in the face of our uncertain knowledge of the parameters and observations the optimal fit procedure unambiguously indicates which oceanic features require model changes in order to be adequately chara

regression analysis of reported earthquake precursors i presentation of data around 700 reported precursors of about 350 earthquakes including the negative observations have been compiled in 11 categories with 31 subdivisions the data base is subjected to an initial sorting and screening by imposing three restrictions on the ranges of main shock magnitude m 4 0 precursory time t 20 years and the epicentral distance of observation points x m 4 0 3m of the 31 subcategories of precursory phenomena 18 with 9 data points or more are independently studied by regressing their precursory times against magnitude the preliminary results tend to classify the precursors into three groups 1 the precursors which show weak or no correlation between time and the magnitude of the eventual main shock examples of this group are foreshocks and precursory tilt 2 the precursors which show clear scaling with magnitude these include seismic velocity ratio v p v s travel time delay duration of seismic quiescence and to some degree the variation of b value and anomalous seismicity 3 the precursors which display clustering of precursory times around a mean value which differs for different precursors from a few hours to a few years examples include the conductivity rate geoelectric current and potential strain water well level geochemical anomalies change of focal mechanism and the enhancement of seismicity reported only for larger earthquakes some of the precursors in this category such as leveling changes and the occurrence of microseismicity show bimodal patterns of precursory times and may partially be coseismic in addition each category with a sufficient number of reported estimates of distance and signal amplitude is subjected to multiple linear regression the usefulness of these regressions at this stage appears to be limited to specifying which of the parameters shows a more significant correlation standard deviations of residuals of precursory time against magnitude are generally reduced when observation distance enters as a second independent variable the effect is more pronounced for water well level and conductivity rate changes

dinoflagellate cyst distribution in sediments of western bay of bengal role of sea surface conditions bay of bengal is a shallow marine basin exposed to seasonal fluctuation in precipitation and run off thus salinity temperature nutrient and productivity vary along the latitudinal and onshore offshore gradient in the bay of bengal these parameters directly affect primary productivity including that of dinoflagellate cysts the spatial distribution of organic walled dinoflagellates cyst is studied in the 50 surface samples of eight transects from the western bay of bengal to infer variation in the environmental conditions in the region the cyst diversity consists of 40 taxa with an overall high dominance of protoperidinioid cysts over gonyalaucoides based on the study distinct onshore and offshore dinocyst assemblages are identified the high productive coastal upwelling region is characterised by the dominance of gonyalaucoid dinocyst species bitectatodinium spongium zonneveld 1997 zonneveld and jurkschat 1999 indicating its adaptability to the fluctuating salinity and high nutrient conditions the outer shelf to middle slope regions of western bay of bengal is dominated by the protoperidinioid species this is in contrast to the gonyalaucoid dinocyst assemblages of the outer shelf to middle slope regions in other oceans since protoperidinioids prefer high nutrient and reducing environmental conditions for growth and preservation their presence suggest high nutrient availability due to the river discharge and low oxygen condition in the slope region in bay of bengal statistical analyses of the sea surface parameters and dinoflagellate cyst abundance data indicate that salinity and silicates as a major source of nutrient played a major role in the distribution of dinoflagellate cyst in the bay of bengal

evidence of boundary reflection of kelvin and first mode rossby waves from topex poseidon sea level data the topex poseidon sea level data lead to new opportunities to investigate some theoretical mechanisms suggested to be involved in the el nino southern oscillation phenomenon in the tropical pacific ocean in particular we are interested in studying the western boundary reflection a process crucial for the delayed action oscillator theory by using the topex poseidon data from november 1992 to may 1995 we first projected the sea level data onto kelvin and first mode rossby waves then we estimated the contribution of wind forcing to these waves by using a single baroclinic mode simple wave model forced by the ers 1 wind data wave propagation was clearly observed with amplitudes well explained by the wind forcing in the ocean interior evidence of wave reflection was detected at both the western and eastern boundaries of the tropical pacific ocean at the eastern boundary kelvin waves were seen to reflect as first mode rossby waves during the entire period the reflection efficiency in terms of wave amplitude of the south american coasts was estimated to be 80 of that of an infinite meridional wall at the western boundary reflection was observed in april august 1993 in january june 1994 and later in december 1994 to february 1995 although the general roles of these reflection events in the variability observed in the equatorial pacific ocean are not clear the data suggest that the reflections in january june 1994 have played a role in the onset of the warm conditions observed in late 1994 to early 1995 indeed during the january june 1994 period as strong downwelling first mode rossby waves reflected into downwelling kelvin waves easterly wind and cold sea surface temperature anomalies located near the date line weakened and eventually reversed in june july 1994 the presence of the warm anomalies near the date line then favored convection and westerly wind anomalies that triggered strong downwelling kelvin waves propagating throughout the basin simultaneously with the beginning of the 1994 1995 warm conditions