instability waves in the equatorial atlantic ocean abstract evidence is presented for the generation of planetary waves by barotropic instability within the cyclonic shear region of the atlantic ocean s south equatorial current sec immediately following the springtime intensification of the southeast trade wind which accelerates the sec westward a packet of waves with central periodicity of around 25 days is observed lasting for about three cycles independent wavenumber analyses on 1983 and 1984 data give newly identical zonal wavelengths and phase speed estimates of around 1100 km and 50 cm s 1 the waves are anisotropic and spatially inhomogeneous with generation confined primarily to the mixed layer an energetics analysis using 1983 data centered upon the equator at 28 w shows a rapid increase in total perturbation energy tpe reaching values of 2000 erg cm 3 within two weeks the subsequent decrease in tpe at this location is due primarily to meridional pressure work divergence baroclinic instability is negligible because both the meridional

multi fault rupture and successive triggering during the 2012 mw 8 6 sumatra offshore earthquake 1 on april 11 2012 a mw 8 6 strike slip earthquake struck the northern sumatra offshore region and became the largest intraplate event ever recorded there preliminary study results show a remarkably complex rupture pattern on a fault system manifested by north south trending paleo transform faults and east west striking abyssal hill fabric to image the rupture pattern of the earthquake we back projected teleseismic p wave recordings observed at three regional seismic networks eu au and f net our results indicate that the earthquake ruptured on a conjugate fault system composed of two subparallel wnw ese trending faults and a nne ssw striking fault in between for a duration of at least 120 s the rupture started at the wnw end of the first wnw ese trending fault located at the east of the conjugate fault system and propagated toward ese it then jumped to the second nne ssw trending fault located west of the first fault and first extended sswward and 15 s later nneward after its end on the second fault the rupture jumped onto the third wnw ese trending fault and continued wnwward the static coulomb stress changes calculated from the rupture of the first and then the second faults based on simplified uniform slip models suggest that initiation of the rupture on the second fault could be statically and dynamically triggered by the first fault however the third fault was primarily dynamically triggered by s or even love waves radiated from the southern branch of the second fault

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

interannual variability of the upper layer of the tropical atlantic ocean from in situ data between 1979 and 1999 interannual variability of the upper layers of the tropical atlantic is described based on in situ data an objective analysis used all available temperature observations of the upper tropical atlantic between 1979 and 1999 to construct a 4d database wind data are used to investigate potential mechanisms which might explain the observed variability four remarkable events are described 1983 1984 1988 1990 1994 1995 and 1997 1998 three of them are characterised as equatorial 1983 1984 1994 1995 and 1997 1998 the 1988 1990 event is a basin wide phenomenon which does not involve the same mechanisms as the other three results of statistical decomposition in empirical orthogonal functions eofs are discussed there is no evidence of an inter hemispheric mode on the depth of the 20 c isotherm d20 and heat content comparable to the observed mode for sea surface temperature sst fields most energetic patterns for d20 and heat content are dominated by the stronger variability in the northern part of the basin influences of other climate signals are investigated correlations between the winter nao north atlantic oscillation index and our standard variables is marginally significant a positive naow north atlantic oscillation of winter is associated with sst cooling in a latitude band between 10 n and 20 n when applied to the el nino index correlations are much more significant we found two scales of maximum correlation at the four month lag after the el nino mature phase when the thermocline slope and zonal heat content gradient are maximum along the equator and at the ten month lag after the mature phase of el nino when the thermocline slope weakens and the equatorial gradient of heat content vanished the correlation with a zonal wind index average between 30 w 35 w and 2 n 2 s has been computed correlation is maximum at the six month lag when the thermocline slope and the zonal heat content gradient are maximum in the equatorial band this atlantic nino mode is influenced by the pacific ocean s variability and reaches a maximum one year after a warm event in the eastern pacific

the zonal currents and transports at 35 w in the tropical atlantic the total of 13 existing cross equatorial shipboard current profiling sections taken during the woce period between 1990 and 2002 along 35 w are used to determine the mean meridional structure of the zonal top to bottom circulation between the brazilian coast near 5 s and 5 n and to estimate mean transports of the individual identified shallow intermediate and deep current branches one of the results is that on the equator a mean westward equatorial intermediate current below the equatorial undercurrent exists

tropical atlantic biases in ccsm4 abstractthis paper focuses on diagnosing biases in the seasonal climate of the tropical atlantic in the twentieth century simulation of the community climate system model version 4 ccsm4 the biases appear in both atmospheric and oceanic components mean sea level pressure is erroneously high by a few millibars in the subtropical highs and erroneously low in the polar lows similar to ccsm3 as a result surface winds in the tropics are 1 m s 1 too strong excess winds cause excess cooling and depressed ssts north of the equator however south of the equator sst is erroneously high due to the presence of additional warming effects the region of highest sst bias is close to southern africa near the mean latitude of the angola benguela front abf comparison of ccsm4 to ocean simulations of various resolutions suggests that insufficient horizontal resolution leads to the insufficient northward transport of cool water along this coast and an erroneous southward stretching of the abf a similar proble

circulation in the central equatorial atlantic mean and intraseasonal to seasonal variability 1 the zonal equatorial circulation of the upper 700 m in the central tropical atlantic is studied based on 11 cross equatorial ship sections taken at 23 29 w during 1999 to 2005 and on data from a pair of moored acoustic doppler current profilers deployed on the equator at 23 w during february 2004 to may 2005 the observations on the equator reveal the existence of two mean westward cores of the equatorial intermediate current below the equatorial undercurrent in contrast to the 2002 moored observations at the same position the intraseasonal variability during the mooring period is dominated by zonal instead of meridional velocity fluctuations

a pv approach for dense water formation along fronts application to the northwestern mediterranean the mechanisms of dense water formation 29 0 kg m 3 at work in the baroclinic cyclonic gyre of the north western mediterranean basin are investigated through a pv budget pv potential vorticity the pv budget is diagnosed from an eddy resolving 1 36 ocean simulation driven in surface by hourly air sea fluxes provided by a non hydrostatic atmospheric model at 2 5km resolution the pv budget is controlled by the diabatic frictional and advective pv fluxes around the gyre the surface diabatic pv flux dominates the pv destruction except along the northern branch of the north current where the surface frictional pv flux is strongly negative in this region the bathymetry stabilizes the front and maintains the current northerly in the same direction as the dominant northerly wind this configuration leads to optimal wind current interactions and explains the preponderance of frictional pv destruction on diabatic pv destruction this mechanical forcing drives a cross front ageostrophic circulation which subducts surface low pv waters destroyed by wind on the dense side of the front and obducts high pv waters from the pycnocline on the light side of the front the horizontal pv advections associated with the geostrophic cyclonic gyre and turbulent entrainment at the pycnocline also contribute to the pv refueling in the frontal region the surface non advective pv flux involves energy exchanges down to 1400 w m 2 in the frontal zone this flux is 3 5 times stronger than atmospheric buoyancy flux these energy exchanges quantify the coupling effects between the surface atmospheric forcing with the oceanic frontal structures at submesoscale this article is protected by copyright all rights reserved

mapping climatological seasonal variations of surface currents in the tropical atlantic using ship drifts the seasonal variability of current velocities in the tropical atlantic was studied by grouping ship drift velocity observations into 2 5 boxes and calculating monthly mean velocity values these values were used to calculate and map the annual mean velocity the seasonal variation about the mean the annual and semiannual harmonics and the first two empirical orthogonal functions eofs the seasonal variation is primarily zonal in the equatorial band and in the north equatorial countercurrent necc and primarily alongshore near the coast of south america maxima of seasonal variation of 23 cm s are centered in the necc near 6 n 42 5 w and in the gulf of guinea near 2 n 7 5 w most 80 of the variance in the necc and along the western boundary of the area studied has an annual period most of the variance along the equator in the mid atlantic has a semiannual period over the whole region 49 of the seasonal variance is explained by the annual harmonic and 69 is explained by a combination of the annual and semiannual harmonics the second eof contains 29 of the variance of the data set and shows a simultaneous speeding slowing of the major equatorial currents the south equatorial current sec the north brazil current the necc and the guinea current along their principal axes of variation with a concurrent slowing speeding of the guyana current and the brazil current the pattern of variation resembles a large equatorial gyre centered along 4 n that flows clockwise from may to september and counterclockwise most of the rest of the year the first eof accounts for 35 of the variance and shows a simultaneous speeding slowing of eastward velocity from 5 s to 10 n except in the western sec the western end of the north brazil current and the guinea current the out of phase fluctuations apparent in the western sec near the equator are explained by the slowing and reversal of the south equatorial current near 2 n 40 w between april and june this slowing of the sec is associated with the start up of the necc which accelerates eastward in may june and july and flows eastward across the atlantic from july through december the sec divides seasonally near the eastern tip of brazil where residual alongshore velocities are northward for half the year peaking during may and june and southward for the other half of the year a second division can be seen north of the amazon where the north brazil current continues up the coast into the guyana current during the first half of the year but partially retroflects into the necc during the last half of the year a fast eastward flow in the necc coincides with below average velocities in the guyana current and above average velocities in the western sections of the north brazil current and the sec north of the equator

aftershocks of the 2012 off coast of sumatra earthquake sequence abstract aftershocks of the 2012 off coast of sumatra earthquake sequence exhibit a complex and diffuse spatial distribution the first order complexity in aftershock distribution is clear and well beyond the influence of typical earthquake location uncertainty the sequence included rupture of multiple fault segments spatially separated we use surface wave based relative centroid locations to examine whether at the small scale the distribution of the aftershocks was influenced by location errors surface wave based relative location has delineated precise oceanic transform fault earthquake locations in multiple regions however the relocated aftershocks off the coast of sumatra seldom align along simple linear trends that are compatible with the corresponding fault strikes as estimated for the gcmt catalog the relocation of roughly 60 moderate earthquake epicentroids suggests that the faulting involved in the 2012 earthquake aftershock sequence included strain release along many short fault segments statistical analysis and temporal variations of aftershocks show a typical decay of the aftershocks but a relatively low number of aftershocks as is common for intraplate oceanic earthquakes coulomb stress calculations indicate that most of the moderate magnitude aftershocks are compatible with stress changes predicted by the large event slip models the patterns in the aftershocks suggest that the formation of the boundary and eventual localization of deformation between the indian and australian plate is a complicated process

on the origin of equatorial atlantic biases in coupled general circulation models many coupled ocean atmosphere general circulation models gcms suffer serious biases in the tropical atlantic including a southward shift of the intertropical convergence zone itcz in the annual mean a westerly bias in equatorial surface winds and a failure to reproduce the eastern equatorial cold tongue in boreal summer the present study examines an ensemble of coupled gcms and their uncoupled atmospheric component to identify common sources of error it is found that the westerly wind bias also exists in the atmospheric gcms forced with observed sea surface temperature but only in boreal spring during this time sea level pressure is anomalously high low in the western eastern equatorial atlantic which appears to be related to deficient excessive precipitation over tropical south america africa in coupled simulations this westerly bias leads to a deepening of the thermocline in the east which prevents the equatorial cold tongue from developing in boreal summer thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical atlantic climate

isopycnal and diapycnal circulation of the upper equatorial atlantic ocean in 1983 1984 isopycnal circulation and diapycnal processes in the equatorial atlantic ocean are investigated using eight cruises from november 1982 to july 1984 an eastward decrease of the transport of the equatorial undercurrent is observed averaging 18 2 106 m3 s 1 at 35 w to 10 2 106 m3 s 1 at 4 w there is also a meridional convergence at the undercurrent level and a larger meridional divergence in the surface layer of 15 106 m3 s 1 between 35 w and 4 w in the eastern equatorial atlantic the undercurrent core density as well as the salinity above the undercurrent experience a large seasonal cycle during the season where the eastern atlantic thermocline is closest to the surface and surface waters are coldest the current peaks in denser waters and subsurface salinity maximum are weaker an analysis of the salinity on isopycnal surfaces indicates that there is significant diapycnal mixing in the upper thermocline in the eastern atlantic the upper thermocline vertical heat diffusivity coefficient scaled over 1 of latitude varies between 3 cm2 s 1 during the upwelling season and nearly 0 early in the year this seasonal mixing is an important element of undercurrent dynamics the turbulent heat flux at the base of the surface layer averages 50 w m 2 between 1 5 n and 1 5 s the surface layer heat budget implies an average oceanic gain of 60 w m 2 between 1 5 n and 1 5 s which could result from exchanges with the atmosphere the heat flux and fresh water seasonal cycles needed to close the budgets are not realistic however the eastern equatorial atlantic thermocline seasonal upwelling is associated with a large inflow into the surface layer over the two years this flow averaging 11 12 106 m3 s 1 originates from the upper thermocline and diapycnal transports near the core of the undercurrent or below are found to be small below the core of the undercurrent we also find that mixing is not intense for instance at 26 5 in the upper part of the thermostad vertical heat diffusivity is only 0 6 cm2 s 1 assuming that mixing takes place over 1 of latitude the uncertainties on these budgets are however large and assumptions on the dynamics that were used could not be checked

observation of the marine atmospheric boundary layer in the gulf of guinea during the 2006 boreal spring in situ satellite and model analyses data in april july 2006 are used to investigate the links between sea surface temperature sst and the marine atmospheric boundary layer mabl in the gulf of guinea the study region between 10 w and 6 e is divided into three areas with different characteristics the north area 4 5 n to 1 n with the wettest atmosphere and the warmest sst the upwelling area 1 n to 4 s with the strongest sst decrease and the south area 4 s to 8 s with a drier atmosphere and a more slowly decreasing sst than in the upwelling area the key zone of the air sea interactions in this region seems to be the sst front between north and upwelling areas on the one hand the study of the mabl on either side of the front shows a well mixed layer between the surface and about 500 m high sensitive to surface variations which gets shallower deeper when the sst decreases increases the mabl height about 1500 m follows the same variations but is not exactly collocated with the sst variations on the other hand the observation of the mabl across the sst front shows a strengthening of southeasterlies in the south area coinciding with a strong sst decrease in the upwelling area in the latter the wind weakens above the colder sst besides in the north area the wind strengthens above the warmer sst however the wind acceleration spans from the equator to 2 n in april and as far as 4 n in june a convergence zone is observed in the vicinity of 2 n in april suggesting a convection activity there favoured by the sst front copyright 2011 royal meteorological society

respective roles of remote and local wind stress forcings in the development of warm sst errors in the south eastern tropical atlantic in a coupled high resolution model processes involved in the development of the warm sea surface temperature sst bias in the tropical south eastern atlantic seta in a high resolution hr version of the cnrm cm model are evaluated based on full field initialized seasonal hindcasts starting at 1 february of each year for 2000 2009 whereas the initial sst growth is likely associated with local atmospheric forcing its further development is due to remote oceanic processes a mixed layer heat budget analysis in seta indicates a spurious warm horizontal advection observed as far as south of 25 s that appears at the beginning of march it is associated with an erroneous oceanic mean state at the equator resulting from the mean equatorial westerly wind bias a sensitivity experiment with corrected wind stress over the equatorial region suggests that the remote forcing explains about 57 of the seta sst bias in march may comparison with a lower resolution lr version of the model reveals that in general similar processes are responsible for the sst bias in both models a strong reduction of the bias in the hr model is observed only over the near coastal southern benguela region due to a better representation of atmospheric and oceanic processes controlling the coastal upwelling overall the results of the inter comparison of the seta sst bias evolution in different sensitivity experiments performed in this study can be interpreted in terms of the relative contributions of erroneous warm horizontal advection associated with equatorial forcing and cold horizontal advection associated with local offshore ekman transport

comparison of warming trends predicted over the next century around antarctica from two coupled models this paper investigates the climate change in two atmosphere ice ocean coupled climate models the ukmo and the csiro in the antarctic region over the next century the objectives were to see if an enhanced level of greenhouse gas forcing results in a surface temperature signal above background variability and to see if this pattern of change resembles the change seen to date in antarctica especially the warming around the peninsula the models show that although reduced sea ice compactness is responsible for regions of enhanced air temperature anomalies these ice compactness anomalies are determined by different mechanisms in the respective models the pattern of warming in both models does not match the differential rates of warming seen in the observations of temperature change over the antarctic continent in the last few decades also the level of background ocean variability in the drake passage and weddell sea region hampers the clear definition of a signal over the antarctic peninsula in the coupled models although no winter enhancement in warming over the peninsula region is found an autumn anomaly is seen in one of the models the mechanism for this feature is documented and an explanation of why it does not persist throughout the winter season is presented

the mean flow field of the tropical atlantic ocean the mean horizontal sow eld of the tropical atlantic ocean is described between 20in and 20is from observations and literature results for three layers of the upper ocean tropical surface water central water and antarctic intermediate water compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent the wind driven ekman layer in the upper tens of meters of the ocean masks at some places the sow structure of the tropical surface water layer as is the case for the angola gyre in the eastern tropical south atlantic although there are regions with a strong seasonal cycle of the tropical surface water circulation such as the north equatorial countercurrent large regions of the tropics do not show a signi cant seasonal cycle in the central water layer below the eastward north and south equatorial undercurrents appear imbedded in the westward sowing south equatorial current the antarcic intermediate water layer contains several zonal current bands south of 3in but only weak sow exists north of 3in the sparse available data suggest that the equatorial intermediate current as well as the southern and northern intermediate countercurrents extend zonally across the entire equatorial basin due to the convergence of northern and southern water masses the western tropical atlantic north of the equator is an important site for the mixture of water masses but more work is needed to better understand the role of the various zonal under and countercurrents in cross equatorial water mass transfer 1999 elsevier science ltd all rights reserved

wind ringing of the ocean in presence of mesoscale eddies 1 recent findings have highlighted the impact of high frequency hf winds on the inertial motions and the consequences on the large scale oceanic circulation wunsch and ferrari 2004 within this context the present study focuses on the role of intermediate scales related to oceanic mesoscale eddies results show that a turbulent eddy field does not affect the impact of hf winds on the inertial motions however the eddies efficiently and permanently concentrate the wind forced inertial motions in small scale anticyclonic structures this leads to a spatially heterogeneous vertical mixing strongly related to the eddy field properties

seasonal and regional variability of upper ocean diapycnal heat flux in the atlantic cold tongue sst variability within the atlantic cold tongue act region is of climatic relevance for the surrounding continents a multi cruise data set of microstructure observations is used to infer regional as well as seasonal variability of upper ocean mixing and diapycnal heat flux within the act region the variability in mixing intensity is related to the variability in large scale background conditions which were additionally observed during the cruises the observations indicate fundamental differences in background conditions in terms of shear and stratification below the mixed layer ml for the western and eastern equatorial act region causing critical froude numbers fr to be more frequently observed in the western equatorial act the distribution of critical fr occurrence below the ml reflects the regional and seasonal variability of mixing intensity turbulent dissipation rates e at the equator 2 n 2 s are strongly increased in the upper thermocline compared to off equatorial locations in addition e is elevated in the western equatorial act compared to the east from may to november whereas boreal summer appears as the season of highest mixing intensities throughout the equatorial act region coinciding with act development diapycnal heat fluxes at the base of the ml in the western equatorial act region inferred from e and stratification range from a maximum of 90 wm 2 in boreal summer to 55 wm 2 in september and 40 wm 2 in november in the eastern equatorial act region maximum values of about 25 wm 2 were estimated during boreal summer reducing to about 5 wm 2 towards the end of the year outside the equatorial region inferred diapycnal heat fluxes are comparably low rarely exceeding 10 wm 2 integrating the obtained heat flux estimates in the ml heat budget at 10 w on the equator accentuates the diapycnal heat flux as the largest ml cooling term during boreal summer and early autumn in the western equatorial act elevated meridional velocity shear in the upper thermocline contributes to the enhanced diapycnal heat flux within this region during boreal summer and autumn the elevated meridional velocity shear appears to be associated with intra seasonal wave activity

interaction of the atlantic equatorial cold tongue and the african monsoon the seasonal cycle of equatorial atlantic sea surface temperature sst is characterized by a rapid cooling from april to july coinciding with the onset of the west african summer monsoon and followed by a slow warming that lasts 3 times longer two ensemble simulations are carried out with an atmospheric general circulation model to investigate the mechanisms for the wind changes that cause this rapid oceanic cooling and its feedback onto the african monsoon in the control simulation sst is globally prescribed in its full climatological seasonal cycle while in the second simulation equatorial atlantic sst is held constant in time from 15 april onward comparison of these simulations indicates that the equatorial cooling exerts a significant influence on the african monsoon intensifying the southerly winds in the gulf of guinea and pushing the continental rainband inland away from the guinean coast the intensification of the cross equatorial southerlies associated with the onset of the african monsoon in turn triggers the oceanic cooling in the east the equatorial easterlies are also important for the seasonal cooling by inducing local upwelling and raising the thermocline in the east three mechanisms are identified for the easterly wind acceleration in the equatorial atlantic in boreal summer first the monsoon rainfall distribution is such that it induces zonal sea level pressure gradients and easterly anomalies in the eastern atlantic second the strong cross equatorial southerlies advect the easterly momentum from the south into the equator finally zonal pressure gradients associated with the equatorial ocean cooling accelerate surface easterly winds in the middle and western atlantic this interaction of equatorial sst and zonal wind causes their westward copropagation analogous to that in the equatorial pacific

supershear rupture on multiple faults for the mw 8 6 off northern sumatra indonesia earthquake of april 11 2012 1 we perform a back projection method to image the rupture propagation and short period energy release of the 2012 off northern sumatra earthquake mw8 6 using hi net data recorded in japan the results show a complex pattern of four conjugate faults over about 180 sec there is a striking correspondence between the lengths and orientations of our rupture pattern with the distribution of aftershocks each of the first three stages of the rupture corresponds to a clear lineation in the aftershocks with lengths of 200 to 400 km rupture speeds for several of the fault segments were very high about 5 km s and exceed the local s wave velocity this is the first example of an oceanic earthquake with supershear rupture speed

main processes of the atlantic cold tongue interannual variability the interannual variability of the atlantic cold tongue act is studied by means of a mixed layer heat budget analysis a method to classify extreme cold and warm act events is proposed and applied to ten various analysis and reanalysis products this classification allows 5 cold and 5 warm act events to be selected over the period 1982 2007 cold warm act events are defined by the presence of negative positive sea surface temperature sst anomalies at the center of the equatorial atlantic in late boreal spring preceded by negative positive zonal wind stress anomalies in the western equatorial atlantic an ocean general circulation model capable of reconstructing the interannual variability of the act correctly is used to demonstrate that cold act events develop rapidly from may to june mainly due to intense cooling by vertical mixing and horizontal advection the simulated cooling at the center of the basin is the result of the combined effects of non local and local processes the non local process is an upwelling associated with an eastward propagating kelvin wave which makes the mixed layer more shallow and preconditions the upper layers to be cooled by an intense heat loss at the base of the mixed layer which is amplified by a stronger local injection of energy from the atmosphere the early cooling by vertical mixing in march is also shown to be a good predictor of june cooling in july horizontal advection starts to warm the mixed layer abnormally and damps sst anomalies the advection anomalies which result from changes in the horizontal temperature gradient are associated in some cases with the propagation of rossby waves along the equator during warm act events processes are reversed generating positive sst anomalies a downwelling kelvin wave triggers stratification anomalies and mixed layer depth anomalies amplified by a weaker injection of energy from the atmosphere in may june in july warm act events are abnormally cooled due to negative horizontal advection anomalies resulting from processes similar to those that occur during cold act events this additional cooling process extends the period of cooling of the act reducing sst anomalies

seasonal heat balance in the upper 100 m of the equatorial atlantic ocean the variability of sea surface temperature sst in the equatorial atlantic is characterized by strong cooling in may june and a secondary cooling in november december a numerical simulation of the tropical atlantic is used to diagnose the different contributions to the temperature tendencies in the upper ocean right at the equator the coolest temperatures are observed between 20 w and 10 w due to enhanced turbulent heat flux in the center of the basin this results from a strong vertical shear at the upper bound of the equatorial undercurrent euc cooling through vertical mixing exhibits a semiannual cycle with two peaks of comparable intensity during the first peak in may june vertical mixing drives the sst while during the second peak in november december the strong heating due to air sea fluxes leads to much weaker effective cooling than during boreal summer seasonal cooling events are closely linked to the enhancement of the vertical shear just above the core of the euc which appears to be not driven directly by the strength of the euc but by the strength and the direction of the surface current the vertical shear is maximum when the northern branch of the south equatorial current is intense the surface cooling in the eastern equatorial atlantic is not as marked as in the center of the basin mean thermocline and euc rise eastward but a strong stratification caused by the presence of warm and low saline surface waters limits the vertical mixing to the upper 20 m and disconnects the surface from subsurface dynamics

a satellite derived climatology of the itcz abstract this paper presents fundamental climatological characteristics of the intertropical convergence zone itcz in a simple concise manner using the highly reflective cloud hrc dataset this satellite derived dataset uses both visible and infrared observations to measure the frequency of occurrence of large scale convective systems over the global tropics at a 1 spatial resolution these dataset characteristics make the hrc particularly well suited for climatological analysis of the itcz because the dataset is based on estimates of organized deep convective cloud systems rather than observations of clouds as a whole and it provides the spatial resolution needed to identify thew large wale convective structures furthermore the dataset covers a time period extending nearly two decades which provides for a fairly robust climatology and the opportunity to examine seasonal and interannual variability of both the convection and the latitude of the itcz

why were sea surface temperatures so different in the eastern equatorial atlantic in june 2005 and 2006 abstract a comparison of june 2005 and june 2006 sea surface temperatures in the eastern equatorial atlantic exhibits large variability in the properties of the equatorial cold tongue with far colder temperatures in 2005 than in 2006 this difference is found to result mainly from a time shift in the development of the cold tongue between the two years easterlies were observed to be stronger in the western tropical atlantic in april may 2005 than in april may 2006 and these winds favorably preconditioned oceanic subsurface conditions in the eastern atlantic however it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid may 2005 over a broad region extending from 20 w to the african coast and from 6 s to the equator this particular event underscores the ability of local intraseasonal wind stress variability in the gulf of guinea to initiate the cold tongue seaso

a 1 year mesoscale simulation of the northeast atlantic mixed layer heat and mass budgets during the pomme experiment 1 a simplified oceanic model is used to perform a realistic yearlong simulation to study fine scale mixed layer processes in an area of the northeast atlantic 21 15 w 38 45 n during the programme ocean multidisciplinaire meso echelle pomme experiment september 2000 october 2001 this simplified three dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis derived geostrophic currents ug called guide into the momentum equation by the substitution of the horizontal pressure gradient the realism of the simulated mesoscale structures was validated by a comparison with in situ mesoscale and submesoscale data and results from the geostrophic adjustment to the guide with this adjustment being applied at all scales this model was able to simulate energetic small horizontal scales taking place around stirring eddies and frontal regions moreover filamentary mixed layer depth structures were simulated and correlated to intense vertical velocities confined in the mixed layer the circulation structure during pomme consisted of three quasi permanent mesoscale eddies and a front the mixed layer heat and mass budgets were thus strongly driven by the horizontal advection and the vertical transport associated with this mesoscale circulation although no effective detrainment or entrainment occurred at the domain scale three regions were found to be the site of intense detrainment around anticyclonic and cyclonic mesoscale eddies and along an axis of strong currents in which the lateral induction and the vertical velocity at the mixed layer base are the major components driving the detrainment respectively

sensitivity of tropical atlantic climate to mixing in a coupled ocean atmosphere model the sensitivity of tropical atlantic climate to upper ocean mixing is investigated using an ocean only model and a coupled ocean atmosphere model the upper ocean thermal structure and associated atmospheric circulation prove to be strongly related to the strength of upper ocean mixing using the heat balance in the mixed layer it is shown that an excessively cold equatorial cold tongue can be attributed to entrainment flux at the base of the oceanic mixed layer that is too large enhanced entrainment efficiency acts to deepen the mixed layer and causes strong reduction in the upper ocean divergence in the central equatorial atlantic as a result the simulated sea surface temperature thermocline structure and upwelling velocities are close to the observed estimates in the coupled model the seasonal migration of the intertropical convergence zone itcz reduces when the entrainment efficiency in the oceanic mixed layer is enhanced the precipitation rates decrease in the equatorial region and increase along 10 n resulting in a more realistic atlantic marine itcz the reduced meridional surface temperature gradient in the eastern tropical atlantic prohibits the development of convective precipitation in the southeastern part of the tropical atlantic also the simulation of tropical atlantic variability as expressed in the meridional gradient mode and the eastern cold tongue mode improves when the entrainment efficiency is enhanced

development of a coupled regional model and its application to the study of interactions between the west african monsoon and the eastern tropical atlantic ocean abstract a regional ocean atmosphere coupled model is developed for climate variability and change studies the model allows dynamic and thermodynamic interactions between the atmospheric boundary layer and an ocean mixed layer with spatially and seasonally varying depth prescribed from observations the model reproduces the west african monsoon circulation as well as aspects of observed seasonal sst variations in the tropical atlantic the model is used to identify various mechanisms that couple the west african monsoon circulation with eastern atlantic ssts by reducing wind speeds and suppressing evaporation the northward migration of the itcz off the west coast of africa contributes to the modeled spring sst increases during this period the westerly monsoon flow is expanded farther westward and moisture transport on to the continent is enhanced near the end of the summer upwelling associated with this enhanced westerly flow as well as the solar cycle lead to the seasonal cooling of the ssts over

intraseasonal variability of tropical atlantic sea surface temperature air sea interaction over upwelling fronts tropical atlantic sea surface temperatures ssts maximum intra seasonal variability isv and their interaction with local surface winds are investigated applying statistical analysis to observations and to a recent coupled reanalysis over the 2000 2009 decade five cores of strong isv emerge with standard deviation reaching about 1 c in frontal areas of the three main upwelling systems equatorial angola benguela and senegal mauritania the southern side of the canary upwelling west of 10 w along the equator a 20 60 day peak caused by tropical instability waves is shown to generate surface wind anomalies through the adjustment of the horizontal surface pressure gradient in addition to the modification of near surface atmospheric stratification east of 10 w along the equator an intense biweekly oscillation increases the ocean and atmosphere isv in the two coastal upwelling fronts intraseasonal sst anomalies resemble each other they are shown to be influenced by coastal kelvin waves in addition to large scale wind forcing over the angola benguela upwelling coastal wind bursts controlling the sst isv are associated with anomalously strong pressure patterns related to the madden julian oscillation the st helena anticyclone and the antarctic oscillation in the senegal mauritania upwelling the wind anomalies mainly linked to the azores anticyclone in the southern front during november to may appear to be connected to the saharan heat low in the northern front from june to september in all five regions and as expected for such upwelling regimes vertical oceanic mixing represents the dominant term in the mixed layer heat budget in the equatorial band as found in previous studies horizontal advection is equally important while it appears surprisingly weak in coastal fronts finally a striking result is the general lack of surface wind signal related to the sst isv in the coastal upwellings

atmospheric response to sea surface temperature in the eastern equatorial atlantic at quasi biweekly time scales the surface wind response to sea surface temperature sst and sst meridional gradient is investigated in the gulf of guinea by using daily observations and re analyses in the 2000 2009 decade with a focus on boreal spring and summer months may to august where quasi biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi biweekly equatorial sst anomalies following a large scale wind acceleration deceleration an equatorial sst cold warm anomaly is created within a few days in order to explain the local atmospheric response to this sst anomaly the two following mechanisms are invoked first a colder warmer ocean decreases increases the vertical stability in the marine atmospheric boundary layer which favours a weaker stronger surface wind and second a negative positive anomaly of sst meridional gradient induces a positive negative anomaly of the sea level pressure meridional gradient which decelerates accelerates the surface wind the first mechanism has an immediate effect in the equatorial belt between 1 s and 1 n and to a lesser extent between 3 s and 1 s whereas the second takes 1 or 2 days to adjust and damps anomalous southeasterlies up to 800 hpa in the low troposphere between 7 s and 1 n through reversed anomalies of meridional sst and pressure gradient this negative feedback leads to weaker stronger winds in the southeastern tropical atlantic which forces the opposite phase of the oscillation within about 1 week around the equator where the amplitude of the oscillation is found to be maximal both mechanisms combine to maximize the wind response to the front fluctuations between the equator and the coast a low level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3 n which reduces the influence of the sst front fluctuations

precise relative earthquake location using surface waves 1 earthquake locations provide a fundamental tool for seismological investigations while dense seismic networks can provide robust locations accuracy and precision of these locations suffer outside dense networks this is particularly true in offshore areas where location analysis relies heavily on distant seismic observations we present a method for estimating precise relative seismic source epicentroid locations using surface waves several reasons including lower velocities and strength of the signal at distance make use of surface waves for event location appealing we focus on the panama fracture zone region and relocate 81 strike slip earthquakes to produce tectonically consistent epicentroid locations the resulting pattern of earthquakes more clearly delineates recently active regional structures than original body wave locations the mean shift between the us geological survey national earthquake information center epicenter and our epicentroids is about 14 km the median is about 11 km and typical origin time changes are generally less than 2 s we find that north of 6 5 n the plate boundary motion is split across two roughly north south striking structures the panama and balboa fracture zones for the last 36 years slip along these two structures roughly matches slip along the panama fracture zone to the south from 4 5 n to 6 25 n but the balboa fracture zone has roughly three times the moment than the northern panama fracture zone our analyses show that observed rayleigh wave signal to noise ratios for moderate to large shallow earthquakes are suitable for applying the procedure and that rayleigh wave observations form a self consistent set of constraints on the relative location of earthquake centroids

a simplified 3d oceanic model assimilating geostrophic currents application to the pomme experiment abstract a simplified oceanic model is developed to easily perform cheap and realistic mesoscale simulations on an annual scale this simplified three dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis derived geostrophic currents ug into the momentum equation in substitution of the horizontal pressure gradient simplification is provided by a time sequence of ug called guide which is used as a low resolution and low frequency interpolator this model is thus necessarily coupled to systems providing geostrophic currents that is ocean circulation models analyzed reanalyzed fields or climatologies in this model the mass and currents fields are constrained to adjust to the geostrophic guide at all scales the vertical velocity is deduced from the vorticity equation which ensures the coherence between the vertical motion and the geostrophic structures evolution horizontal and vertical advection are the coupling processes that can b

surface currents in the tropical atlantic across high density xbt line ax08 1 three temperature sections that cross the tropical atlantic obtained from high density xbt transects are used to identify the major surface currents and to compute their water mass transports the dynamic heights are computed using xbt temperature profiles with salinity derived from historical t s relationships the values of dynamic height estimated from altimeter data used in conjunction with climatological dynamic height fields are within 3 cm of the xbt derived values the error in xbt derived dynamic height introduced by using historical t s relationships instead of actual salinity values are estimated to be of the order of 1 5 cm dynamic height estimates using the actual salinity values underestimate those obtained using historical t s relationships the structure exhibited in the dynamic height and altimeter derived sea height fields do not reveal all the upper ocean currents making these temperature sections presented here critical for computing transports and identifying currents in this region

lagrangian sources of frontogenesis in the equatorial atlantic front estimating the processes that control the north equatorial sea surface temperature sst front on the northern edge of the cold tongue in the tropical atlantic is a key issue for understanding the dynamics of the oceanic equatorial atlantic and the west african monsoon diagnosis of the frontogenetic forcings on a realistic high resolution simulation was used to identify the processes involved in the formation and evolution of the equatorial sst front the turbulent forcing associated with the mixed layer turbulent heat flux was found to be systematically frontolytic while the dynamic forcing associated with currents was found to be frontogenetic for the equatorial sst front nevertheless the low frequency component of the turbulent forcing was frontogenetic and initiated the sst front which was then amplified and maintained by the leading dynamic forcing this forcing was mainly driven by the meridional convergence of the northern south equatorial current nsec and the guinea current which points out the essential role played by the circulation in the equatorial sst front evolution the quasi biweekly variability of the equatorial sst front and its forcings were found to be more strongly coupled to the wind energy flux wef than to the surface wind stress in fact the wef controlled the convergence divergence of the nsec and guinea current and thus the meridional component of the leading dynamic forcing the wef explains the equatorial sst front development better than the wind does because it is a coupled ocean atmosphere process

variability of the mixed layer heat budget in the eastern equatorial atlantic during 2005 2007 as inferred using argo floats we examine the variability of the sea surface temperatures in the eastern equatorial atlantic during 2005 2007 by using argo profiling floats prediction and research moored array in the atlantic buoys and satellite in situ and atmospheric data sets the eastern equatorial atlantic characterized by shallow mixed layers all yearlong is divided into nine boxes of nearly equal surface area with respect to the dynamics and thermodynamics in this region monthly mixed layer heat budgets are computed in each box from 10 day argo profiles in all the boxes the net surface heat flux is one of the main causes of the seasonal evolution of sea surface temperatures for the 3 studied years the amount of short wave radiation penetrating through the base of the mixed layer and horizontal heat advection may locally contribute to the temperature variability while entrainment has a weaker contribution to balance the heat budget a residual term exists which includes all processes that cannot be calculated with observations as well as the possible errors in the other terms this residual is more intense in the cold tongue and the northern region and exhibits a clear seasonal cycle with minimum negative values in boreal summer and maximum values in winter this residual compares well with available observations of vertical turbulent mixing collected during etude de la circulation oceanique et des echanges ocean atmosphere dans le golfe de guinee campaigns 2005 2007 in the eastern equatorial atlantic when assuming that the residual is mostly associated with vertical turbulent mixing it can be conjectured that turbulent mixing is a significant cooling source in the cold tongue and north of the equator

seasonal variability of the equatorial undercurrent at 10 w as inferred from recent in situ observations 1 eighteen cross equatorial shipboard current profiling sections along 10 w with conductivity temperature depth measurements taken between 1997 and 2007 are used to analyze the mean meridional structure and the seasonal variability of the equatorial undercurrent euc at 10 w our analysis suggests a seasonal cycle for the euc transport at 10 w with a well defined annual harmonic and some indication of a semiannual component with a first maximum in january and a second stronger maximum from june to september the mean euc transport at 10 w is estimated to be 12 1 sv and compared to earlier estimates farther in the west at 35 w 20 9 sv and 26 w 13 8 sv the eastward flow transport exhibits a strong variability at 10 w with a total range of transports from 7 1 sv to more than 31 7 sv the seasonal amplitude of the eastward flow variability is 8 9 sv from a minimum of 8 2 sv in november to 25 9 sv in august the eastward flows within the thermocline are divided in two parts a permanent part within the 24 5 26 5 isopycnal layer with a semiannual cycle known as the euc and a nonpermanent part in the deep thermocline beneath 26 5 isopycnal associated with a strong eastward transport up to 15 sv during the boreal summer that is not observed during the rest of the year the current at the equator in the deep thermocline is even westward during boreal fall the disappearance of the salinity core of the euc during the boreal summer associated with the upwelling of the hydrological structure at 10 w reveals that the saline subtropical waters carried by the euc within the thermocline no longer flow into the gulf of guinea during the boreal summer our data also show the presence of the south equatorial undercurrent seuc at 10 w in the gulf of guinea with a strong latitudinal and depth variability throughout the year the mean seuc at 10 w is centered near 5 s and is farther south than observed at 35 w and 26 w suggesting its poleward shift from west to east

some overlooked features of tropical atlantic climate leading to a new nino like phenomenon abstract the atlantic nino an equatorial zonal mode akin to the pacific el nino southern oscillation enso is phase locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the gulf of guinea a suite of satellite and in situ observations reveals a new mode of tropical atlantic variability that displays many characteristics of the zonal mode but instead peaks in november december nd this new mode is found to be statistically independent from both the atlantic nino in the preceding summer and the pacific enso the origin of this nd zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial atlantic in november the equatorial easterly winds intensify for the second time increasing upwelling and lifting the thermocline in the gulf of guinea an analysis of high resolution climatological data shows that these dynamical changes induce a noticeable sst cooling in the central equatorial atlantic the shoaling thermocline and increased upwelli

sverdrup and nonlinear dynamics of the pacific equatorial currents abstract the sverdrup circulation in the tropical pacific is constructed from satellite scatterometer winds compared with measured ocean currents and diagnosed in an ocean gcm previous depictions of the sverdrup circulation near the equator have shown only weak vertically integrated flows here it is shown that the actual transports are not weak this discrepancy could be due either to inaccuracies in the wind forcing or to sverdrup dynamics being too simple in this region scatterometer winds show a strip of positive curl along the sst front north of the equator in the eastern pacific that is due to wind speed changes induced by the front including that additional element of curl forcing greatly improves the realism of the sverdrup representation but the magnitudes of the equatorial transport are still too small by a factor of about 2 although the nonlinear advective and friction terms are small in the model momentum balance they are o 1 in the vorticity balance especially because their meridi

a three dimensional ocean mesoscale simulation using data from the semaphore experiment mixed layer heat budget a primitive equation model is used to simulate the mesoscale circulation associated with a portion of the azores front investigated during the intensive observation period iop of the structure des echanges mer atmosphere proprietes des heterogeneites oceaniques recherche experimentale semaphore experiment in fall 1993 the model is a mesoscale version of the ocean general circulation model ogcm developed at the laboratoire d oceanographie dynamique et de climatologie lodyc in paris and includes open lateral boundaries a 1 5 level order turbulence closure scheme and fine mesh resolution 0 11 for latitude and 0 09 for longitude the atmospheric forcing is provided by satellite data for the solar and infrared fluxes and by analyzed or reanalyzed for the wind atmospheric data from the european centre for medium range weather forecasts ecmwf forecast model the extended data set collected during the iop of semaphore enables a detailed initialization of the model a coupling with the rest of the basin through time dependent open boundaries and a model data comparison for validation the analysis of model outputs indicates that most features are in good agreement with independent available observations the surface front evolution is subject to an intense deformation different from that of the deep front system which evolves only weakly an estimate of the upper layer heat budget is performed during the 22 days of the integration of the model each term of this budget is analyzed according to various atmospheric events that occurred during the experiment such as the passage of a strong storm this facilitates extended estimates of mixed layer or relevant surface processes beyond those which are obtainable directly from observations surface fluxes represent 54 of the heat loss in the mixed layer and 70 in the top 100 m layer while vertical transport at the mixed layer bottom accounts for 31 and three dimensional processes account for 14

a description of the annual cycle in sea surface temperature and upper ocean heat in the equatorial atlantic abstract temperature and velocity time series obtained by surface moorings during the seasonal response of the equatorial atlantic experiment are used to investigate the role of ocean dynamics upon the annual cycle of equatorial sea surface temperature sst and upper ocean heat the annual cycle in sst is explained by different mechanisms each operant at different phases of the cycle the boreal springtime decrease in sst results from upwelling in response to the seasonal intensification of easterly wind stress this upwelling causes the seasonal formation of the cold tongue along the equator in the central and eastern portions of the basin an early summer increase in sst is attributed to the meridional convergence of reynolds heat flux associated with surface current instability generated waves after the instability waves abate sst and mixed layer depth remain relatively steady from late summer through fall when the advective terms are small and cancelling suggesting that surface heating is then

propagation of the seasonal upwelling in the eastern equatorial atlantic abstract several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern atlantic guinea gulf the most controversial is the mechanism of remote wind forcing in the western equatorial atlantic suggested by moore et al 1978 most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations detailed analysis of daily sea surface temperature sst collected at 16 coastal stations along the northern coast of the guinea gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0 7 m s 1 similar analysis of historical monthly mean sst data shows that the coastal upwelling event propagates poleward from 1 s to at least 13 s at the same phase speed furthermore the northern hemisphere and southern hemisphere coastal upwelling signals seem to start at the same time from the equator the same kind of analysis applied to hydrographic data from a station situated 41 km off abid

seasonal influence of the sea surface temperature on the low atmospheric circulation and precipitation in the eastern equatorial atlantic the air sea interaction in the gulf of guinea and its role in setting precipitation at the guinean coast is investigated in the present paper this study is based on satellite observations and wrf simulations forced by different sea surface temperature sst patterns it shows that the seasonal cold tongue setup in the gulf of guinea along with its very active northern front tends to strongly constrain the low level atmospheric dynamics between the equator and the guinean coast underlying mechanisms including local sst effect on the marine boundary layer stability and hydrostatically changed meridional pressure gradient through changes in sst gradient are quantified in wrf regarding observations and cfsr reanalyses theses mechanisms strongly impact moisture flux convergence near the coast leading to the installation of the first rainy season of the west african monsoon wam system the current study details the mechanisms by which the atlantic equatorial cold tongue plays a major role in the pre onset of the boreal wam

coupling between the atlantic cold tongue and the west african monsoon in boreal spring and summer 1 the formation of the atlantic cold tongue act is the dominant seasonal sea surface temperature signal in the eastern equatorial atlantic eea a comprehensive analysis of variability in its spatial extent temperature and onset is presented then the physical mechanisms which initiate act onset as well as the feedbacks from the act to the maritime boundary layer and how the act influences the onset of the west african monsoon wam are discussed we argue that in the eea the air sea coupling between the act and wam occurs in two phases from march to mid june the act results from the intensification of the southeastern trades associated with the st helena anticyclone steering of surface winds by the basin shape of the eea imparts optimal wind stress for generating the maximum upwelling south of the equator during the second phase mid june august wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the act and the tropical waters circulating in the gulf of guinea gg it is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the gg and that increased northward winds shift convection over land correlations between the act and the wam onset dates over the last 26 years 1982 2007 measure as much as 0 8 this suggests that the act plays a key role in the wam onset

on the evolution of the thermocline and subthermocline eastward currents in the equatorial atlantic 1 eastward currents in the thermocline and subthermocline layers of the tropical atlantic ocean are described using observations obtained during two boreal summer cruises covering the whole equatorial basin the analysis focuses on the eastward evolution of zonal currents in the thermocline and sub thermocline the equatorial undercurrent is not found close to the african coast in boreal summer 2000 maybe because of interannual variability as observed in the pacific the south and north equatorial undercurrents are shown to shift poleward from west to east and the two cruises indicate that the north equatorial undercurrent does not enter the gulf of guinea the south and north intermediate countercurrents are variable with longitude and contrarily to the observations in the eastern equatorial pacific are not found in the east of the guinea basin

a modeling study of the impact of tropical instability waves on the heat budget of the eastern equatorial pacific a numerical simulation is used to investigate the mixed layer heat balance of the tropical pacific ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves tiws the study is motivated by a need to quantify the effects that tiws have on the climatological heat budget of the cold tongue mixed layer there has been some discrepancy between observations indicating very large equatorward heat transport by tiws and models that disagree on the full three dimensional budget validation of the model reveals that the tiw induced circulation patterns are realistic but may have amplitudes about 15 weaker than those in the observations the sst budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices tivs zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection these effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps in contrast to some previous studies tiw induced vertical advection is found to be negligible in the sst budget cooling by this term is only significant below the mixed layer the effect of tiws on the climatological heat budget is then investigated for the region bounded by 2 s 6 n 160 90 w where instabilities are most active tiwinduced horizontal advection leads to a warming of 0 84 c month 1 which is of the same order as the 0 77 c month 1 warming effect of atmospheric fluxes while the mean currents and vertical mixing cool the upper ocean by 0 59 c month 1 and 1 06 c month 1 respectively the cooling effect of tiw induced vertical advection is also negligible in the long term surface layer heat budget and only becomes significant below the mixed layer the results above and in particular the absence of cancellation between horizontal and vertical tiw induced eddy advection are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution

intraseasonal mixed layer heat budget in the equatorial atlantic during the cold tongue development in 2006 1 estimating the mixed layer heat budget is a key issue for understanding the cold tongue development in the eastern equatorial atlantic a high resolution ocean regional model is used to diagnose the mixed layer heat budget online during the egee 3 experiment from may to august 2006 the heat budget shows the major role of the horizontal advection and turbulent mixing in the mixed layer temperature balance in the cold tongue the surface net heat flux and entrainment processes play a minor role the equatorial cooling is mainly induced by low frequency advection which is balanced by high frequency zonal and meridional advections the high frequency advections are organized in patterns along the northern edge of the cold tongue where they are associated with strong sea surface temperature gradients and well developed tropical instability waves in the western atlantic special attention is paid to the wind energy flux which controls horizontal advection and turbulent mixing we suggest that the wind energy flux drives the vertical velocity which in turn adjusts the mixed layer depth its stratification and the vertical shear of the horizontal current although vertical advection is not essential in providing cold water in the atlantic cold tongue it is shown that the vertical velocity plays a central role in preconditioning the mixed layer and maximizes the turbulent mixing

global survey of aftershock area expansion patterns we developed an objective method to define the aftershock areas of large earthquakes as a function of time after the main shock the definition is based upon the amount of energy released by aftershocks the spatial distribution of the energy release is first determined and is contoured the 1 day aftershock area is defined by a contour line corresponding to the energy release level of 10 15 6 ergs 100 km 2 day the 10 day 100 day and 1 y aftershock areas are similarly defined by contour lines corresponding to 10 14 8 10 14 0 and 10 13 5 ergs 100 km 2 day respectively we also define the expansion ratios at time t by the ratio of the aftershock area at t to that at 1 day using this method we study the aftershock area expansion patterns of 44 large m s 7 5 and five moderate shallow earthquakes which occurred from 1963 to 1980 each aftershock sequence is examined at four different times i e 1 day 10 days 100 days and 1 y after the main event we define the aftershock area expansion ratios and e by s 100 s 1 and l 100 l 1 respectively here s t and l t are the area and the length of the aftershock area respectively at time t our study suggests that a distinct regional variation of aftershock area expansion patterns is present it is strongly correlated with the tectonic environment in general the subduction zones of the mariana type have large expansion ratios and those of the chilean type have small expansion ratios some earthquakes that occurred in the areas of complex bathymetry such as aseismic ridges tend to have large expansion ratios these results can be explained in terms of an asperity model of fault zones in which a fault plane is represented by a distribution of strong spots called the asperities and weak zones surrounding the asperities the rupture immediately after the main shock mostly involves asperities after the main rupture is completed the stress change caused by the main shock gradually propagates outward into the surrounding weak zones this stress propagation manifests itself as expansion of aftershock activity in this simple picture if the fault zone is represented by relatively large asperities separated by small weak zones chilean type then little expansion of aftershock activity would be expected on the other hand if relatively small asperities are sparsely distributed mariana type significant expansion occurs the actual distribution of asperities is likely to be more complex than the two cases described above however we would expect that the expansion ratio is in general proportional to the spatial ratio of the total asperity area to the fault area