interaction of extratropical rossby waves with westerly quasi biennial oscillation winds the role of laterally radiating stratospheric rossby waves from the winter hemisphere in the momentum balance of the quasi biennial oscillation qbo remains an unresolved issue observations of extratropical rossby waves penetrating close to the equator suggest that rossby wave absorption in the tropics must occur at least occasionally this paper presents numerical simulations using a global shallow water model to examine the pattern of wave absorption at low latitude during the westerly wind phase of the qbo earlier results indicating that extratropical rossby waves radiating to the equator do not break there when the qbo phase is westerly are reproduced it is shown that the westerly qbo jet can persist undiminished at its core over seasonal length integrations because the laterally radiating rossby waves are absorbed along the qbo jet s flanks where easterly acceleration occurs the net effect of the rossby waves on the westerly qbo jet is a reduction in the width of its meridional profile but not its core strength this behavior is interpreted by considering the filtering of equatorward radiating rossby waves by critical layer absorption a process that depends on the background zonal mean wind and the waves phase speed and amplitude the simulations also show that adequate horizontal resolution is needed to capture the suppression of wave breaking at the qbo jet core and persistence of the jet core strength over seasonal timescales the implications of this result for simulations of the qbo in general circulation models are considered

the anomalous change in the qbo in 2015 2016 the quasi biennial oscillation qbo is a tropical lower stratospheric downward propagating zonal wind variation with an average period of 28 months the qbo has been constantly documented since 1953 here we describe the evolution of the qbo during the northern hemisphere winter of 2015 2016 using radiosonde observations and meteorological reanalyses normally the qbo would show a steady downward propagation of the westerly phase in 2015 2016 there was an anomalous upward displacement of this westerly phase from 30 hpa to 15 hpa these westerlies impinge on or cutoff the normal downward propagation of the easterly phase in addition easterly winds develop at 40 hpa comparisons to tropical wind statistics for the 1953 to present record demonstrate that this 2015 2016 qbo disruption is unprecedented

the early development of the 2015 16 quasi biennial oscillation disruption abstractan unprecedented disruption of the quasi biennial oscillation qbo started to develop from late 2015 the early development of this event is analyzed using the space time spectra of eddies

spatial features of l band equinoctial scintillations from equator to low midlatitude at around 95 e during 2015 2016

origin of the 2016 qbo disruption and its relationship to extreme el nino events the descent of the westerly phase of the quasi biennial oscillation qbo in equatorial stratospheric zonal wind was interrupted by the development of easterlies near 40 hpa 23 km altitude in early 2016 we use tropical meteorological analyses of wind and temperature to describe in detail the special circumstances by which equatorward propagating planetary waves produced this unprecedented disruption in the qbo our findings show that the subtropical easterly jet in the winter lower stratosphere during the 2015 2016 winter was anomalously weak owing to 1 the timing of the qbo relative to the annual cycle and 2 an extreme el nino event the weak jet allowed an unusually large flux of westward momentum to propagate from the extratropical northern hemisphere to the equator near the 40 hpa level consequently the qbo westerlies at that level experienced sustained easterly acceleration from extratropical wave breaking leading to the observed wind reversal

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

polarization tomography for p wave velocity structure in southern california we develop a tomographic inversion method that uses teleseismic p wave polarization data to obtain velocity structure polarization inversion has some intrinsic advantages over travel time inversion it is not influenced by source location and origin time errors it is not sensitive to deep mantle velocity structure and can be used iteratively to improve the tomographic result polarization inversion is more sensitive to near station velocity structure and to velocity gradient and is complementary to travel time inversion in this sense the method is applied to california institute of technology u s geological servey southern california array data the result is generally consistent with previous work and also reveals that the high velocity feature beneath the transverse ranges is bounded between 40 and 200 km depths and possibly has a second small piece at about 300 km depth the slow velocity anomaly under the salt on trough is limited to shallow depths less than about 60 km

the complex behavior of el nino winter 2015 2016 this paper examines the outstanding characteristics of the strong 2015 2016 el nino en winter and its impact over the european region through the stratosphere despite being classified as a strong eastern pacific ep en event our analysis reveals an anomalous behavior with some signatures that are more typical of central pacific cp en events instead they include i a record breaking value of the cp index ii a stronger polar vortex in early and midwinter due to reduced upward wave activity and a weakened aleutian low and iii the occurrence of one of the earliest stratospheric final warmings sfws on record which are more prone to occur during cp en following the sfw a stratospheric influence on the euro atlantic sector is reported in spring with persistent greenland blocking resulting in extreme precipitation over some southern european regions results highlight the importance of considering early sfws as mediators of el nino teleconnections

first lidar observations of quasi biennial oscillation induced interannual variations of gravity wave potential energy density at mcmurdo via a modulation of the antarctic polar vortex

a strong phase reversal of the arctic oscillation in midwinter 2015 2016 role of the stratospheric polar vortex and tropospheric blocking in january 2016 asia and north america experienced unusual cold temperatures although the global average of surface air temperature broke the warmest record during a strong el nino event this was closely related to the remarkable phase transition of the arctic oscillation ao which can be explained by stratosphere troposphere interactions first the quasi biennial oscillation changed to its westerly phase in summer 2015 and the stratospheric polar vortex was stronger in early to midwinter 2015 2016 as blocking did not occur in december the associated downward propagation signal resulted in a strongly positive ao in late december 2015 second after late december the positive phase of pacific north america pattern became apparent in el nino event which strengthened the aleutian anticyclone in the stratosphere in addition an equivalent barotropic blocking anticyclone was established in the troposphere over asia the coexistence of blocking over asia and north america characterized the negative ao and a strong zonal wave number 2 pattern due to stronger zonal wave number 2 signals from the troposphere the stronger stratospheric polar vortex was elongated with two cyclonic centers over asia and the north atlantic in january the resultant southward displacement of polar vortices was followed by rare snowfall in the subtropical region of east asia and a heavy snowstorm on the east coast of the united states

an unexpected disruption of the atmospheric quasi biennial oscillation one of the most repeatable phenomena seen in the atmosphere the quasi biennial oscillation qbo between prevailing eastward and westward wind jets in the equatorial stratosphere approximately 16 to 50 kilometers altitude was unexpectedly disrupted in february 2016 an unprecedented westward jet formed within the eastward phase in the lower stratosphere and cannot be accounted for by the standard qbo paradigm based on vertical momentum transport instead the primary cause was waves transporting momentum from the northern hemisphere seasonal forecasts did not predict the disruption but analogous qbo disruptions are seen very occasionally in some climate simulations a return to more typical qbo behavior within the next year is forecast although the possibility of more frequent occurrences of similar disruptions is projected for a warming climate

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

first successful hindcasts of the 2016 disruption of the stratospheric quasi biennial oscillation in early 2016 the quasibiennial oscillation in tropical stratospheric winds was disrupted by an anomalous easterly jet centered at 40 hpa a development that was completely missed by all operational extended range weather forecast systems this event and its predictability are investigated through 40 day ensemble hindcasts using a global model notable for its sophisticated representation of the upper atmosphere integrations starting at different times throughout january 2016 just before and during the initial development of the easterly jet were performed all integrations simulated the unusual developments in the stratospheric mean wind despite considerable differences in other aspects of the flow evolution among the ensemble members notably in the evolution of the winter polar vortex and the day to day variations in extratropical rossby waves key to prediction of this event is simulating the slowly evolving mean winds in the winter subtropics that provide a waveguide for rossby waves propagating from the winter hemisphere

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

sensitivity of gravity wave fluxes to interannual variations in tropical convection and zonal wind using an idealized model framework with high frequency tropical latent heating variability derived from global satellite observations of precipitation and clouds the authors examine the properties and effects of gravity waves in the lower stratosphere contrasting conditions in an el nino year and a la nina year the model generates a broad spectrum of tropical waves including planetary scale waves through mesoscale gravity waves the authors compare modeled monthly mean regional variations in wind and temperature with reanalyses and validate the modeled gravity waves using satellite and balloon based estimates of gravity wave momentum flux some interesting changes in the gravity spectrum of momentum flux are found in the model which are discussed in terms of the interannual variations in clouds precipitation and large scale winds while regional variations in clouds precipitation and winds are dramatic the mean gravity wave zonal momentum fluxes entering the stratosphere differ by only 11 the modeled intermittency in gravity wave momentum flux is shown to be very realistic compared to observations and the largest amplitude waves are related to significant gravity wave drag forces in the lowermost stratosphere this strong intermittency is generally absent or weak in climate models because of deficiencies in parameterizations of gravity wave intermittency these results suggest a way forward to improve model representations of the lowermost stratospheric quasi biennial oscillation winds and teleconnections

a scattered wave image of subduction beneath the transverse ranges over 5600 short period recordings of teleseismic events were used to create deterministic maps of p wave scatterers in the upper mantle beneath southern california between depths of 50 and 200 kilometers the southern flank of the slab subducting beneath the transverse ranges is marked by strong scattering the marked scattering indicates that the edge of the slab is a sharp thermal boundary such a boundary could be produced by slab shearing or small scale convection in the surrounding mantle the northern limb of the slab is not a strong scatterer consistent with thicker lithosphere north of the transverse ranges