double and single itczs with and without clouds abstracta major bias in tropical precipitation over the pacific in climate simulations stems from the models tendency to produce two strong distinct intertropical convergence zones itczs too often several mechanisms have been proposed that may contribute to the emergence of two itczs but current theories cannot fully explain the bias this problem is tackled by investigating how the interaction between atmospheric cloud radiative effects acres and the large scale circulation influences the itcz position in an atmospheric general circulation model simulations are performed in an idealized aqua planet setup and the longwave and the shortwave acres are turned off individually or jointly the low level moist static energy mse is shown to be a good predictor of the itcz position therefore a mechanism is proposed that explains the changes in mse and thus itcz position due to acres consistently across simulations the mechanism implies that the itcz moves equatorward if the hadley circulation strengt

evaluation of sample quality of sensitive clay using intrinsic compression concept a simple index the degree of sample disturbance is proposed in this study to quantitatively evaluate the quality of sensitive clay samples based on the concept of void index proposed by burland in 1990 the degree of sample disturbance is defined as the ratio of the difference between the in situ void index and the void index of the undisturbed sample tested in the laboratory to the difference between the in situ void index and the void index of the completely remolded clay all these indices are determined at the same effective overburden stress theoretically the degree of sample disturbance varies from 0 perfect undisturbed sample to 100 completely remolded sample the proposed index is used in this study to evaluate the sensitive ariake clay in japan oedometer tests on undisturbed samples of natural ariake clay obtained from the field using the current sampling practice in japan show the degree of sample disturbance ranging from approximately 5 to 38 the clay sample with an in situ void index closer to the intrinsic compression line has a lower degree of sample disturbance in addition a series of consolidation and unconfined compression tests were conducted on artificially disturbed samples in the laboratory to demonstrate the change of consolidation yield stress unconfined compressive strength and compression index with the degree of sample disturbance a simple method is proposed in this paper to correct the mechanical parameters measured in a laboratory setting considering the degree of sample disturbance

late quaternary productivity changes from offshore southeastern australia a biomarker approach reconstructions of primary productivity at low latitudes have been the focus of several studies to better understand how the export of nutrient rich intermediate southern ocean so waters influences productivity at these latitudes this was triggered by the general observation of minima in the planktonic foraminiferal delta c 13 values during deglaciations which was interpreted as an isotopic signal of intermediate so waters together with a concomitant increase in diatom productivity at some equatorial sites however the impact of these so waters on productivity at higher latitudes is not well constrained here we compare a high resolution planktonic foraminiferal delta c 13 record with total organic carbon and biomarker records for proboscia diatoms and haptophytes in marine sediments from offshore southeastern australia the planktonic foraminiferal delta c 13 record shows distinct minima during deglaciations and the marine isotope stage 4 3 transition tentatively suggesting that c 13 depleted so waters reached the coast of southeastern australia however it did not result in increased productivity during these periods instead the highest primary productivity period as indicated by total organic carbon and alkenone accumulation rates occurred during the last glacial maximum while proboscia diatoms mainly proliferated during interglacials and marine isotope stage 3 matching periods of increased diatom productivity in some sites of the eastern equatorial pacific our study suggests that increased primary productivity offshore southeastern australia was mainly due to stronger westerly winds during glacial periods while proboscia diatom productivity was probably controlled by the transport of silicic acid to this area

the phytoplankton bloom in the northwestern arabian sea during the southwest monsoon of 1979 the biological variability of the northwestern arabian sea during the 1979 southwest monsoon has been investigated by the synthesis of satellite ocean color remote sensing with analysis of in situ hydrographic and meteorological data sets and the results of wind driven modeling of upper ocean circulation the phytoplankton bloom in the northwestern arabian sea peaked during august september extended from the oman coast to about 65 e and lagged the development of open sea upwelling by at least 1 month in total the pigment distributions hydrographic data and model results all suggest that the bloom was driven by spatially distinct upward nutrient fluxes to the euphotic zone forced by the physical processes of coastal upwelling and offshore ekman pumping coastal upwelling was evident from may through september yielded the most extreme concentrations of phytoplankton biomass and along the arabian coast was limited to the continental shelf in the promotion of high concentrations of phytoplankton upward ekman pumping to the northwest of the somali jet axis stimulated the development of a broad open sea phytoplankton bloom oceanward of the oman shelf vertical mixing during the 1979 southwest monsoon was apparently not a primary cause of the regional scale phytoplankton bloom

why the itcz is mostly north of the equator abstract although the distribution of sunshine is symmetrical about the equator the earth s climate is not climatic asymmetries are prominent in the eastern tropical pacific and atlantic oceans where the regions of maximum sea surface temperature convective cloud cover and rainfall are north of the equator this is the result of two sets of factors interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located the ocean atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions the thermocline happens to shoal in the eastern equatorial pacific and atlantic but not in the eastern indian ocean because easterly trade winds prevail over the tropical atlantic and pacific wher

seasonal variation of hydrographic and nutrient fields during the us jgofs arabian sea process study abstract between september 1994 and december 1995 the us jgofs arabian sea process experiment collected extensive high quality hydrographic data temperature salinity dissolved oxygen and nutrients during all seasons in the northern arabian sea an analysis of this unique data suite suggests the presence of many features that are described in the canonical literature but these new data provided the following insights 1 although the seasonal evolution of mixed layer depths was in general agreement with previous descriptions the deepest mixed layer depths in our data occurred during the late ne monsoon instead of the sw monsoon 2 the region exhibits considerable mesoscale variability resulting in extremely variable temperature salinity ts distributions in the upper 1000 db this mesoscale variability is readily observed in satellite imaging in the high resolution data taken by a companion onr funded project and in underway adcp data 3 the densest water reaching the sea surface during coastal upwelling appeared to have maximum offshore depths of 150 m and s close to the core value 25 for the saline arabian sea water asw but salinities in these upwelling waters were relatively low the densest water found at the sea surface during late ne monsoon conditions has s 24 8 and relatively high salinities suggesting that they are a source for the asw salinity maximum 4 persian gulf water pgw with a core of 26 6 forms a widespread salinity maximum despite the considerable extent of this feature persian gulf outflow water with a salinity 4 of 39 at its source can only be a minor contributor within the standard us jgofs sampling grid maximum salinities on this surface are 36 8 at stations near the gulf falling to values as low as 35 3 at the stations farthest removed from its influence even at our standard stations closest to the gulf n 1 and n 2 the high salinity low nutrient persian gulf water has only a modest direct effect on nutrient concentrations this pgw salinity maximum is associated with the suboxic portions of the arabian sea s oxygen minimum zone 5 the salinity maximum associated with red sea water rsw core 27 2 in the jgofs study region is clearly evident at the southermost sampling site at 10 n s 15 elsewhere this signal is weak or absent and salinity on the 27 2 surface tends to increase towards the persian gulf suggesting that the disappearance of this salinity maximum is due at least in part to the influence of the persian gulf outflow 6 inorganic nitrogen to phosphate ratios were lower frequently much lower than the standard redfield ratio of 15 1 16 1 by atoms at all times and all depths suggesting that inorganic nitrogen was more important than phosphate as a limiting nutrient for phytoplankton growth and that the effects of denitrification dominated the effects of nitrogen fixation 7 the water upwelling off the omani coast during the sw monsoon has inorganic nitrogen to silicate ratios that were higher 2 1 than the 1 1 ratio often assumed as the ratio of uptake during diatom growth 8 the temporal evolution of inorganic nitrogen to silicate ratios suggests major alteration by diatom uptake only during the late sw monsoon cruise tn050 in august september 1995 9 widespread moderate surface layer nutrient concentrations occurred during the late ne monsoon 10 a zone of high offshore nutrient concentrations was encountered during the sw monsoon but instead of being associated with offshore upwelling it may represent offshore advection from the coastal upwelling zone the influence of an eddy or both 11 although our data do not contradict previous suggestions that the volume of subtoxic water may be reduced the sw monsoon they suggest a weaker re oxygenation than indicated by some previous work similarly they do not confirm results suggesting that secondary nitrite maxima may be common in waters with oxygen concentrations 5 m

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

effects of rotation rate and seasonal forcing on the itcz extent in planetary atmospheres the authors study a wide range of atmospheric circulations with an idealized moist general circulation model to evaluate the mechanisms controlling intertropical convergence zone itcz migrations they employ a zonally symmetric aquaplanet slab ocean of fixed depth and force top of atmosphere insolation to remain fixed at the pole for an eternal solstice simulation and also vary seasonally for a range of rotation rates keeping all other parameters earth like for rotation rates e 8 and slower a transient maximum in zonal mean precipitation appears at the summer pole however the itcz associated with the ascending branch of the hadley circulation lies at 60 the authors assess how widely used predictors of the itcz position perform in this wide parameter space standard predictors based on different estimates of the hadley cell s poleward extent are correlated with but overestimate off equatorial itcz locations interestingly in the eternal solstice case for earth s rotation rate the itcz remains at subtropical latitudes even though the lower level moist static energy maximizes at the summer pole while seemingly at odds with convective quasi equilibrium arguments this can happen because at earth s rotation rates the thermal stratification set in convective regions can only be communicated within the tropics where temperature gradients are constrained to be weak the authors therefore develop an understanding of the itcz s position based on top of atmosphere energetics and the boundary layer momentum budget and argue that friction and pressure gradient forces determine the region of maximum convergence offering a modified dynamical perspective on the monsoon like seasonal weather patterns of terrestrial planets

on the relationship between sst gradients boundary layer winds and convergence over the tropical oceans a linear mixed layer model that skillfully reproduces observed surface winds and convergence over the tropicaloceans is used to examine the relative influenceof boundarylayer and free tropospheric processes on the distribution of climatological surface winds and convergence the semiempirical model assumes a subcloud layer momentum force balance between pressure gradients coriolis acceleration linearized friction and downward momentum mixing and it utilizes boundary conditions from the 40 yr ecmwf re analysis era 40 observed pressure gradients are linearly decomposed into boundary layer defined as the region below 850 hpa and free tropospheric components and the surface winds and convergence associated with these components are computed results show that surface zonal winds are predominantly associated with a combination of free tropospheric pressure gradients and downward momentum mixing whereas the distribution of convergence is primarily due to boundary layer temperature gradients which are closely related to sst gradients the authors conclude that the climatological distribution of boundary layer convergence is primarily a function of the pattern of sst gradients and is better regarded as a cause rather than a consequence of deep convection

mesozooplankton in the canary current system the coastal ocean transition zone abstract the canary current system ccs is one of the largest upwelling areas of the world understanding the biogeochemical fluxes and recruitment of fish in relation to changes in climate requires knowledge of the factors influencing mesozooplankton biomass and fluxes here the trophic and metabolic characteristics of mesozooplankton for the oceanic upwelling and eddy system of the ccs are reviewed mesoscale phenomena are of importance in this region as besides upwelling filaments the eddy system shed from the canary islands is unique in comparison with other similar systems such as the humboldt california and benguela the coupling of production in the coastal area off northwest africa with the oceanic zone through filaments and eddies topographically formed in the coast or shed by the islands promotes a continuous transport of organic matter towards the deep ocean zooplankton follow the signal of the mesoscale phenomena and are good tracers of advection the different species which can be used as targets in the area are also described from the upwelling region to the open ocean however not all the variability in the area is explained by bottom up controls the effect of diel vertical migrants feeding on epipelagic zooplankton shows that top down controls are also important to understanding the structure and fluxes in this oceanic environment

itcz width controls on hadley cell extent and eddy driven jet position and their response to warming abstractthe impact of global warming induced intertropical convergence zone itcz narrowing onto the higher latitude circulation is examined in the gfdl atmospheric model version 2 1 am2 1 run

depositional and post depositional geochemistry and its correlation with the geotechnical properties of marine clays in ariake bay japan the mineralogy geochemistry and geotechnical properties of clay samples in ariake bay japan are presented the mineralogy and geochemistry are discussed in terms of depositional and post depositional processes and the correlations of the clay mineralogy and geochemistry with the geotechnical properties were examined by multiple regression analysis the most predominant clay mineral in the deposits was found to be smectite the vertical distribution of trace elements in the deposits was indicative of the periodical changes in the parent materials involved in the depositional processes the chloride concentration in the pore water varied according to a change in the sea level from brackish to marine and then brackish towards the surface the higher iron oxide contents near the surface and in the deepest zones were the results of pyrite oxidation in the deposits due to weathering under subaerial conditions smectite content was the most predominant governing factor for the consistency limits and activi

the response of the itcz to extratropical thermal forcing idealized slab ocean experiments with a gcm using a comprehensive atmospheric gcm coupled to a slab mixed layer ocean experiments are performed to study the mechanism by which displacements of the intertropical convergence zone itcz are forced from the extratropics the northern extratropics are cooled and the southern extratropics are warmed by an imposed cross equatorial flux beneath the mixed layer forcing a southward shift in the itcz the itcz displacement can be understood in terms of the degree of compensation between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics the magnitude of the itcz displacement is very sensitive to a parameter in the convection scheme that limits the entrainment into convective plumes the change in the convection scheme affects the extratropical tropical interactions in the model primarily by modifying the cloud response the results raise the possibility that the response of tropical precipitation to extratropical thermal forcing important for a variety of problems in climate dynamics such as the response of the tropics to the northern hemisphere ice sheets during glacial maxima or to variations in the atlantic meridional overturning circulation may be strongly dependent on cloud feedback the model configuration described here is suggested as a useful benchmark helping to quantify extratropical tropical interactions in atmospheric models

the upper ocean response to monsoonal forcing in the arabian sea seasonal and spatial variability abstract observations from four towed profiler surveys undertaken between december 1994 and october 1995 examine the seasonal and spatial variability of the upper ocean response to the monsoon cycle in the arabian sea although observed atmospheric forcing agrees well with modern climatologies cross basin patterns of mixed layer depth and water properties observed in 1994 1995 are not entirely consistent with an upper ocean response dominated by ekman pumping during the winter monsoon the mixed layer deepens dramatically with distance offshore surface cooling intensifies with offshore distance and a one dimensional response dominated by convective overturning could explain observed wintertime mixed layer depths except for waters associated with a filament extending offshore from the omani coast mixed layer depths and water properties show only modest cross basin contrasts during the southwest monsoon filament waters differ from surrounding mid basin waters having shallow mixed layers and water properties similar to those of waters upwelled near the omani coast in september following the southwest monsoon waters within 1000 km of the omani coast have cooled and freshened with marked changes in stratification extending well into the pycnocline estimates of ekman pumping and wind driven entrainment made using the southampton oceanographic center 1980 1995 surface flux and the levitus mixed layer climatologies indicate that during the southwest monsoon wind driven entrainment is considerably stronger than ekman pumping inshore of the windstress maximum ekman pumping partially counters wind driven entrainment while offshore the two processes act together to deepen the mixed layer as ekman pumping is too weak to counter wind driven mixed layer deepening inshore of the windstress maximum another mechanism must act to maintain the shallow mixed layers seen in our observations and in climatologies offshore advection of coastally upwelled water offers a mechanism for maintaining upper ocean stratification that is consistent with observed changes in upper ocean water properties ekman upwelling will modulate wind driven entrainment but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind driven mixing and horizontal advection

dynamics of the itcz boundary layer abstractthis paper presents high resolution numerical solutions of a nonlinear zonally symmetric slab model of the intertropical convergence zone itcz boundary layer the boundary layer zonal and meridional flows are forced by a specified pressure field which can also be interpreted as a specified geostrophically balanced zonal wind field ug y one narrow on equatorial peak in boundary layer pumping is produced when the forcing is easterly geostrophic flow along the equator and two narrow peaks in boundary layer pumping are produced on opposite sides of the equator a double itcz when the forcing is westerly geostrophic flow along the equator in the case when easterlies are surrounding a westerly wind burst once again a double itcz is produced but the itczs have significantly more intense boundary layer pumping than the case of only westerly geostrophic flow a comparison of the numerical solutions to those of classical ekman theory suggests that the meridional advection term y plays a vita

a 90 kyr upwelling record from the northwestern indian ocean using a novel long chain diol index abstract presently upwelling is of major importance for driving primary productivity in the arabian sea but its intensity in the past is not well constrained here we used long chain 1 14 alkane diols specific lipids of diatoms of the genus proboscia as new proxies to reconstruct upwelling conditions in the arabian sea variations in the seasonal lipid fluxes were determined using sediment traps in the somalia upwelling system deployed 80 km off the coast on the somali continental slope niop 905 10 45 444 n 51 56 655 e at 1265 m water depth 268 m above the sea floor and 270 km off the coast in the deep somali basin south of socotra niop 915 10 43 068 n 53 34 422 e at 3047 m depth 1000 m above the sea floor highest fluxes of c 28 and c 30 1 14 diols up to almost 600 g m 2 day 1 were only observed during nutricline shoaling at the onset of the southwest monsoon swm prior to massive upwelling by contrast fluxes of c 30 1 15 diols derived from as yet undefined biological sources only increased marginally during the swm and also during the northeast monsoon nem when instead of upwelling enhanced vertical mixing led to a second productivity pulse sediment core niop 905 taken at the continental slope site showed strong fluctuations in relative concentrations of long chain 1 14 and 1 15 diols with time which we quantified as the summed concentrations of c 28 and c 30 1 14 diols divided by the summed concentrations of c 28 and c 30 1 14 diols and c 30 1 15 diols this diol index follows the same trend as other upwelling intensity records from the arabian sea that are based on sea surface temperature reconstructions organic carbon content barium aluminium ratios and abundance and stable isotope composition of specific foraminiferal species the diol index was relatively high during the holocene ca 0 7 but much lower during the late glacial maximum ca 0 2 it was generally low during the last glacial but elevated values were found during the first half of marine isotope stage 3 between 60 and 45 ka and at the end of marine isotope stage 5 1 approximately 80 ka suggesting intensified glacial upwelling our data shows that long chain diols are suitable proxies to reconstruct past upwelling intensities in the arabian sea

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

advection of upwelled waters in the form of plumes off oman during the southwest monsoon abstract advanced very high resolution radiometer avhrr imagery of sea surface temperature topex poseidon measurements of sea level anomaly sla and modeled surface winds and wind stress fields are used in concert with other ancillary data to describe the influence of the 1995 southwest monsoon on the distribution of upwelled waters off the coast of oman the oman upwelling zone is characterized by the entrainment of cold upwelled waters into plumes extending from the coast into the deep ocean unaffected by the steep bottom gradients the most prominent of these plumes is found offshore of ras al madraka a mechanism for the entrainment of upwelled water into plumes is hypothesized and validated by observational data it is proposed that the location of the plume is primarily governed by the sea level structure away from the coast and that coastally upwelled water is passively advected offshore through regions of low sea level analysis of the surface wind stress fields show significant spatial variability associated with the predominantly cyclonic mean wind stress curl with relatively weak curl observed in the region south of ras al madraka and north of ras marbat decomposition of the surface wind stress fields through principal component analysis shows that at certain periods the development of strong along shore winds and cyclonic wind stress curl in the region north of ras al madraka this information combined with concurrent observations of topex poseidon sea level anomalies slas satellite derived sea surface temperatures sst and surface current measurements shows that the combined effects of a strong along shore wind field and positive wind stress curl forces a depression in sea level in the region north of ras al madraka the sea level gradient caused by the presence of a sustained high sea level to the south of ras al madraka causes geostrophic advection of coastally upwelled waters away from the shelf acoustic doppler current profiler adcp velocity measurements along with sst maps further prove that the upwelled water is geostrophically advected offshore as opposed to being an offshore deflection of a wind driven coastal current comparison of interannual features in the topex poseidon slas suggest that the plumes coming off coast in the oman upwelling zone may not be directly linked to the coastal topography or bathymetry but are a result of interaction between mesoscale variations in the wind field and the underlying ocean the strong along shore winds and cyclonic wind stress curl to the north of ras al madraka becomes enhanced when the findlater jet moves closer to the oman coast than its mean position

glacial to interglacial contrasts in the northern indian ocean differences between the 18o 16o ratios of planktonic foraminifera deposited during the last glacial maximum about 18 000 yr ago and those deposited during the holocene show that in the indian ocean the south west monsoon was weaker than today but that the north east monsoon was stronger the upwelling observed in modern conditions along the southern coast of arabia had disappeared because of the low speed of the southwestern winds during glacial summers the reduction in rainfall and runoff over the continent caused a reduction of the salinity gradient in the bay of bengal and along the western coast of india increased precipitation fell on the sea south of 10 n while strong evaporation over the northernmost arabian sea produced an enhanced salinity gradient in that area

variability in upwelling fields in the northwestern indian ocean 2 data model comparison at 9000 years b p theoretical investigations in the form of general circulation model gcm simulations and empirical studies of pollen lake levels and marine sediments suggest that the indian summer monsoon was intensified 9000 years ago at a time of increased summer radiation about 8 in the northern hemisphere the southwesterly summer monsoon winds and their pattern of wind stress curl cause coastal upwelling along the coast of arabia stronger summer winds therefore should intensify coastal upwelling incorporation of the broad scale gcm winds into a high resolution ocean model of the arabian sea produces distinct patterns of stronger upwelling at 9000 years b p to verify these patterns in the geologic record we mapped upwelling assemblages of planktonic foraminifera and estimated sea surface temperature sst to define the magnitude and spatial distribution of monsoonal upwelling at 9000 years b p we found that the upwelling assemblage increased in abundance and the august ssts were several degrees celsius cooler off arabia than present however in the central and eastern arabian sea the tropical assemblage increased and the ssts were warmer than present values the sst gradient across the arabian sea was greater 6 c at 9000 years b p than it is today 4 c these observations are consistent with the model results that show increased divergence upwelling and a thinner upper layer or shallower thermocline in the westernmost arabian sea and increased convergence downwelling and a thicker upper layer in the central and eastern arabian sea in this experiment the use of low resolution winds from a general circulation model ncar ccm in a high resolution ocean model seems to be appropriate for simulating the long term averages observed in the marine sediments

lipid biomarker fluxes in the arabian sea with a comparison to the equatorial pacific ocean abstract fluxes and distributions of organic carbon oc and lipid biomarkers were measured in the western arabian sea as a function of season depth and distance from the coast of oman during the us jgofs arabian sea process study in 1994 1995 a strong seasonal pattern in oc and lipid flux is related to the annual monsoon cycle in the western arabian sea with the highest fluxes measured during the southwest monsoon fluxes were greatly attenuated with depth in the water column and in surface sediments as remineralization effectively consumed particulate organic matter a comparison of water column fluxes with oc and lipid accumulation rates in sediments confirms that the water sediment interface is a hot spot for organic matter degradation biomarker compositions also varied seasonally and with distance offshore reflecting seasonal and spatial succession of their biological sources and their subsequent export through the water column degradation of oc and biomarkers was extremely efficient with only a tiny fraction

the dynamics of idealized convection schemes and their effect on the zonally averaged tropical circulation in this paper the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist gcm to obtain broad classifications of the influence of convection on the tropics this is accomplished with a simplified convection scheme in the style of betts and miller the scheme is utilized in a moist gcm with simplified physical parameterizations gray radiation with zonally symmetric slab mixed layer ocean boundary conditions comparisons are made with simulations without a convection scheme i e with large scale condensation lsc only with the moist convective adjustment mca parameterization and with various formulations and parameter sets with a simplified betts miller sbm scheme with the control run using the sbm scheme the tropics become quieter and less dependent on horizontal resolution as compared with the lsc or mca simulations the hadley circulation mass transport is significantly reduced with the sbm scheme as is the itcz precipitation an important factor determining this behavior is the parameterization of shallow convection without shallow convection the convection scheme is largely ineffective at preventing convection from occurring at the grid scale the sensitivities to convection scheme parameters are also examined the simulations are remarkably insensitive to the convective relaxation time and only mildly sensitive to the relative humidity of the reference profile provided significant large scale condensation is not allowed to occur the changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere

the sensitivity of the southwest monsoon phytoplankton bloom to variations in aeolian iron deposition over the arabian sea 1 a coupled 3 d biophysical ocean general circulation model is used to investigate how aeolian iron deposition affects the arabian sea ecosystem two separate aeolian iron deposition fields derived from the giss and gocart atmospheric transport models have been applied as surface boundary conditions the model results exhibit widespread biogeochemical sensitivity to the choice of deposition field with gocart deposition sw monsoon phytoplankton blooms in the western and central arabian sea are enhanced and exhibit greater realism the central arabian sea bloom is supported by supplemental input of horizontally advected iron from a pool that undergoes a yearlong progression that begins in the gulf of oman where the difference in aeolian iron enrichment between the two deposition fields is most prevalent the gocart enhanced blooms result in a more pronounced shift toward netplankton an increase in euphotic zone export flux of up to a 20 during the sw monsoon and an additional annual biogenic export of 3 5 tgc the potential ramifications of regional n cycle alteration through stimulation of n2 fixation that is promoted by significant aeolian mineral flux needs to be explored the canonical thinking that the northern arabian sea is invariably iron replete is now being challenged by both our model results and recent observational studies as well our results indicate that arabian sea iron concentrations are strongly modulated by the specific nature of aeolian mineral deposition thus climate or land use influences on dust mobilization could exercise leading order controls on regional biogeochemical variability metabolic status and air sea exchanges of co2

a consistent model for the large scale steady surface atmospheric circulation in the tropics the authors present a new model of the tropical surface circulation forced by changes in sensible heat and evaporative flux anomalies that are associated with prescribed sea surface temperature anomalies the model is similar to the lindzen and nigam ln boundary layer model also driven by the above flux anomalies but here since the boundary layer is assumed well mixed and capped by an inversion the model reduces to a twolayer reduced gravity system furthermore the rate of exchange of mass across the boundary layer free atmosphere interface is dependent on the moisture budget in the boundary layer when moist convection is diagnosed to occur detrainment operates on the timescale associated with the life cycle of deep convection approximately eight hours otherwise the detrainment is assumed to be associated with the mixing out of the stable tropical boundary layer which has a timescale of about one day the model provides a diagnostic estimate of the anomalies in precipitation however it is assumed that the latent heat is released above the boundary layer and it drives a circulation that does not impact the boundary layer the authors discuss the derivations of the gill zebiak gz and lindzen nigam models and highlight some apparent inconsistencies between their derivation and the values of several of the parameters that are required for these models to achieve realistic solutions for the circulations then the new reduced gravity boundary model equations are rewritten in the form of the gz and ln models using realistic values for the parameters in the new model geometry it is shown that the constants combine in the rewritten equations to produce the physically doubtful constants in the gz and ln models hence the reason for the apparent success of these models

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

understanding the arabian sea reflections on the 1994 1996 arabian sea expedition abstract the arabian sea expedition now five years past its field observations is at a stage when some of its dominant themes can be summarized of the large range of possible topics five are considered here 1 is the arabian sea a source or sink for carbon dioxide 2 is the arabian sea mother nature s iron experiment 3 do grazing zooplankton control carbon flux to the seabed 4 does the paleoceanographic record help us predict the ocean s response to climate change and 5 what are the predominant physical processes of the arabian sea a short history of each issue and results from the field work of 1994 1996 are presented

entrainment rayleigh friction and boundary layer winds over the tropical pacific abstract winds over the tropical pacific are interpreted using mixed layer theory the theory which posits that the surface winds can be derived in terms of a force balance among surface drag pressure gradients coriolis forces and the vertical mixing of momentum into the boundary layer entrainment is very successful in predicting the seasonal climatology of the surface winds the model is also used as a basis for interpreting previous results in particular the model illustrates why studies that model the momentum flux divergence as a rayleigh damping find optimal damping coefficients that are anisotropic a linear variant of the model which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress is also found to predict the surface winds skillfully in addition to improving the representation of the winds it leads to realistic representations of the divergence of the vector wind if the key parameters of the model the entrainment rate and t

depositional and geotechnical properties of marine clays in lianyungang china the mineralogy geochemistry and geotechnical properties of clay samples in lianyungang china are presented the correlations of depositional history depositional environmental and geotechnical properties are discussed a series of x ray diffraction tests show that the principal clay minerals of the soil are illite smectite mixed layer mineral and illite the geochemistry investigation results indicate that the pore water of the lianyungang marine clay is dominant by sodium the ph of the soils varies in the range of 7 8 to 8 9 the depositional environment is oxidation condition the correlations of the geochemistry with the geotechnical properties are examined by the statistic analysis

the role of inertial instability in determining the location and strength of near equatorial convection there are two major organized cloud configurations in the vicinity of the equator where there is a small cross equatorial surface pressure gradient convection is close to the equator and is generally tied to the location of the lowest sea level pressure slp and warmest sea surface temperature sst in agreement with arguments based upon simple thermodynamical considerations however when there is a substantial cross equatorial pressure gradient such as occurs in the monsoon regions organized convection appears off the equator in the summer hemisphere equatorward of the slp minimum and not necessarily collocated with the warmest ssts thus in this instance simple thermodynamical considerations alone cannot explain 2he location of the convection in this situation the zero absolute vorticity contour 0 also lies in the summer hemisphere therefore between the equator and the 0 contour is a region of locally anticyclonic absolute vorticity and an inertially unstable regime it is argued that the convection results from the low level divergence convergence doublet centred about the 0 contour which is the mitigating response to the inertial instability the associated latitude height secondary circulation should provide subsidence suppressed convection over the equator and rising motion enhanced convection to the north of the zero absolute vorticity contour signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis wherever a strong cross equatorial pressure gradient exists the 0 contour bisects a maximum in the divergent wind field divergence is found equatorward of the zero contour and convergence on the poleward side latitude height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of 0 as the regions where the rising motions occur are conditionally unstable there is deep convection and the vertical circulations extend throughout the troposphere it is noted that the intensity of the off equator convection is deeper and probably stronger than convection located at the equator this is probably because the convection associated with the inertial instability is more efficient necessary conditions for the location of near equatorial convection are listed arguments are presented whereby inertial instability is established as the cause rather than an effect of off equatorial convection these include an outline of the sequence of processes leading up to the convection the factors that limit the encroachment of the 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low level westerly monsoon wind maximum is suggested the possible role played by the instability mechanism or the lack of it in coupled model simulations that produce seasonally migrating and or double itczs in the eastern pacific ocean is discussed finally it is proposed that the instability mechanism is important in the initiation of westward moving disturbances found in the eastern pacific and in determining active and break periods in the summer indian monsoon

biogeochemistry of iron in the arabian sea the arabian sea is a productive basin where seasonal upwelling and convective mixing result in high surface nutrient concentrations and widespread algal blooms the factors controlling primary productivity in the arabian sea are of interest because the region contains an intense oxygen minimum zone omz that is a major sink for nitrate in the ocean a survey of iron fe distribution and redox chemistry was carried out in 2007 to assess its role in arabian sea biogeochemistry including an investigation of the biological response to fe additions results show that fe is strongly enriched in the eastern arabian sea associated with the omz much of the fe within the omz is present as fe ii which enhances the residence time and accumulation of fe in contrast fe concentrations are lower in the western arabian sea separated from the fe rich omz by a water mass boundary consequently low surface values are associated with monsoon driven upwelling off the omani coast incubations revealed that primary production during the southwest monsoon was strongly limited by fe over much of the study area incubation of surface waters with fe resulted in rapid growth of phaeocystis and up to sixfold increase in chlorophyll this is the first demonstration of fe limitation in the arabian sea and the first high resolution zonal survey of dissolved fe and fe ii across the basin our findings suggest that models developed to predict the response of the arabian sea biogeochemistry to global warming need to consider effects on fe inputs

northern arabian sea variability from topex poseidon altimetry data an extension of the us jgofs onr shipboard adcp study abstract sea level anomalies sla derived from the topex poseidon t p altimetry and inferred geostrophic currents within the northern arabian sea were examined for the period from march 1993 through november 1996 the primary objective of this study was to confirm and extend our understanding of the upper ocean mesoscale variability observed in the shipboard acoustic doppler current profiler adcp data collected during the us jgofs onr arabian sea expedition september 1994 january 1996 the accuracy of the t p altimetry data 3 cm rms results in an uncertainty in the altimeter derived velocities comparable to the adcp measurement error thus the t p data provide a reasonable method for extending studies of the mesoscale dynamics for the region comparison of the t p derived geostrophic velocities with concurrent adcp data showed good correlation with an r 2 between 0 7 and 0 9 and rms differences of 10 cm s 1 the t p data confirm both the overall spatial and seasonal current patterns observed by the adcp the monsoonally averaged rms sea level anomalies indicate a high degree of intraseasonal variation due to the generation of squirts jets and eddies all along the coast the variability of which increases in both intensity and areal extent during the southwest monsoon the sla data indicate a much reduced degree of variability over the shelf the sla derived eddy kinetic energy eke fields are consistent with those derived earlier from the adcp data in both distribution and magnitude there is a large increase in eke to the west and southwest and to a lesser extent to the south and a large area of relatively reduced eddy activity over much of the eastern and northern arabian sea the area of reduced eddy activity coincides with the location of the most intense portions of the oxygen minimum zone found in the northern arabian sea the spatial scales of the eddies responsible for the eke distribution over the study area range between 200 and 500 km in the nearshore region decreasing to 100 200 km offshore while there is significant energy variability in annual and semi annual time scales a substantial portion of the energy is found between 50 and 120 days and the relative importance of this frequency band increases offshore spectra indicate a distinct break in the frequency content of the eddy field at about 15 n with little energy at less than annual periods south of this latitude

a diatomaceous origin for long chain diols and mid chain hydroxy methyl alkanoates widely occurring in quaternary marine sediments indicators for high nutrient conditions for the first time a biological source for the long chain alkyl 1 14 diols and 12 hydroxy methyl alkanoates lipids widely occurring in the marine water column and sediments has been identified cultures of proboscia indica and proboscia alata rhizosolenoid diatoms belonging to the widespread diatom genus proboscia contain c28 c28 1 c30 and c30 1 alkyl 1 14 diols and c27 and c29 12 hydroxy methyl alkanoates as major neutral lipids these components form a substantial fraction of lipid fractions from sediment traps or sediments especially in areas with an elevated primary production such as upwelling regions examination of literature data reveals that as much as 20 to 35 of the total lipid flux in the arabian sea is derived from proboscia diatoms during the start of the upwelling season their rapid transfer to the water sediment interface may explain why corresponding 1 14 keto ols inferred oxidation products of diols are hardly formed these interpretations are supported by compound specific carbon isotopic analysis of long chain keto ols and diols in surface sediments of the arabian sea the data indicate that long chain alkyl 1 14 diols and 12 hydroxy methyl alkanoates can be applied as indicators for high nutrient conditions in the photic zone

upper ocean currents in the northern arabian sea from shipboard adcp measurements collected during the 1994 1996 u s jgofs and onr programs abstract a large upper ocean velocity data set was obtained using a shipboard acoustic doppler current profiler adcp on seventeen rv t g thompson cruises during the jgofs and onr expedition to the northern arabian sea from september 1994 through january 1996 seven of the cruises followed a large area survey track centered over the arabian basin four cruises conducted seasoar surveys on either side of the findlater jet axis and six cruises were for the deployment and maintenance of moored instrumentation together providing some 380 cruise days and 96 000 track kilometers of coverage the adcp data extended over the upper 250 400 m of the water column depending upon the temporal spatial distributions of acoustic scatterers the velocity data revealed several items that differed significantly from the historical perspective maximum current magnitudes in this area varied from more than 1 m s 1 along the arabian coast to 10 20 cm s 1 well offshore perhaps the most important result was the complete dominance of the velocity field by eddies that had offshore correlation length scales of roughly 100 km a spectral peak at around 300 km and kinetic energies that ranged from 70 to more than 90 of the total kinetic energy the total kinetic energy was highest within about 300 km of the shore and decreased significantly in magnitude and vertical extent offshore within the coastal region the temporal variability was such that currents of 50 cm s 1 or more could completely reverse within a two week period seemingly independent of any local atmospheric forcing mean and seasonal velocities also differed from historical results there was a large anti cyclonic feature located for most of the year south of ras ash shabatat 58 e which intensified during the southwest monsoon there was a strong jet like current off ras al hadd 22 n which also intensified during the southwest monsoon although itflowed southwestward against the wind in contrast to the historical ship drift data which indicated that the surface currents followed the monsoonal winds the adcp data over the upper 200 400 m was highly variable with overall seasonal means that were often directed against the wind except within 100 200 km of the arabian coast the mean currents were generally toward the southwest with a mean transport in the upper 150 m between the coast and 10 n of approximately 9 10 6 m 3 s 1

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

modulations of the phase speed of convectively coupled kelvin waves by the itcz abstracta number of studies suggest a two way feedback between convectively coupled kelvin waves cckws and the intertropical convergence zone itcz viewed here as a proxy for deep convection analysis of brightness temperature data reveals several aspects of these interdependencies a wavenumber frequency spectral analysis is applied to the satellite data in order to filter cckws the itcz is characterized by a region of low brightness temperature and a proxy for both the itcz location and width are defined the phase speed of cckw data is determined using the radon transform method linear regression techniques and probability density analysis are consistent with previous theoretical predictions and observational results in particular the fastest waves are found when the itcz is the farthest from the equator and the narrowest conversely the slowest waves coincide with broad itczs that are located near the equator

the carbon dioxide system in the arabian sea in 1995 the whoi c goyet and miami f j millero groups participated on a number of research cruises in the arabian sea as part of the u s joint global ocean flux study jgofs sponsored by the national science foundation nsf this paper gives the results of our total inorganic carbon dioxide tco2 total alkalinity ta and potentiometric ph measurements made on arabian sea water samples during these cruises measurements made on certified reference material crm indicate that the reproducibility of the measurements was 0 007 in ph 3 2 mol kg 1 in ta and 1 2 mol kg 1 in tco2 n 180 the surface measurements 0 30 m of ph and normalized tco2 and ta were quite uniform throughout the year ph 8 1 0 05 ntco2 1950 20 mol kg 1 and nta 2290 5 mol kg 1 the larger variations in ntco2 in the surface waters are related to changes in primary production and upwelling in the coastal waters the depth profiles of ph pco2 ta and tco2 were similar to those in the equatorial pacific ocean the components of the carbonate system co2 hco 3 co2 3 and the saturation state for calcite and aragonite were determined from the measurements of ta and tco2 the waters below 600 and 3400 m in the arabian sea were undersaturated 1 0 for aragonite and calcite respectively the co2 measurements have been combined with the nutrient data to examine the stoichiometric ratios of c n c p c o2 and c sio2 of the waters marked differences were found for the waters above and below the oxygen minimum zone the surface water results have been used to develop the following stoichiometry for phytoplankton in the arabian sea ch2o 125 nh3 14 h3po4 sio2 13 the oxidation of this material is due to reactions with o2 77 and no3 23 with the resultant formation of n2 and n2o the maximum amount of organic carbon oxidized has been estimated to be 3 1 mol kg 1 in the deep waters with as much as 0 9 mol kg 1 in the oxygen minimum zone with no3 the maximum amount of caco3 dissolved in the deep waters is 116 mol kg 1 these results together with the organic material collected from the sediment traps should be useful in characterizing the formation and degradation of plant material in the arabian sea

sea surface temperatures during the sw and ne monsoon seasons in the western arabian sea over the past 20 000 years abstract the western arabian sea is influenced by the seasonal southwest and northeast monsoon wind systems in the modern day sw monsoon induced upwelling of cold water leads to lowest ssts in summer while in glacial periods weakened upwelling and increased cooling by ne monsoon winds may have resulted in lowest sst in winter in order to reconstruct the western arabian sea seasonal sea surface temperature sst evolution during the monsoons over the past 20 000 years a novel proxy is used that captures the contrast in seasonal ssts the presented proxy concerns the difference in 18o between two shallow dwelling species of planktonic foraminifera globigerinoides ruber which calcifies throughout the year and globigerina bulloides which mainly calcifies during sw monsoon driven upwelling we convert this 18o difference to the difference in calcification temperature tcrub bul and use it in combination with mg ca derived temperatures to reconstruct seasonal ssts we present the monsoon evolution record of core niop929 located nw of the island of socotra based on our results we distinguish three distinct modes i a glacial monsoon mode 20 13 ka bp characterised by weakly negative tcrub bul values and sw monsoon ssts tsw that are 2 c higher than ne monsoon ssts tne this pattern is indicative for a weak sw monsoon and stronger cooling by glacial ne monsoon winds compared to the modern situation ii a transitional mode 13 8 ka bp characterised by tcrub bul values around 0 and thus a reduced difference between tsw and tne iii the modern monsoon mode which started at 8 ka bp with the main shift toward low summer ssts and high winter ssts the modern mode is characterised by tcrub bul values around 4 c indicative of a strong sw monsoon associated with strong upwelling and weak influence of the ne monsoon on sst we propose that measuring mg ca on both species will improve the accuracy of the sst estimates

characterization on the correlation between shear wave velocity and piezocone tip resistance of jiangsu clays the small strain shear modulus of a soil is a fundamental parameter related to the mechanical behavior used in evaluation of dynamic behavior and seismic design of geotechnical structures the jiangsu soft clay is a lightly overconsolidated and sensitive clay of high plasticity in nature a research database of piezocone penetration test cptu and shear wave velocity vs information for jiangsu soft clays has been collected to study the small strain shear modulus relationships for these soils and to examine the potential use of cptu and vs data in combination for the purposes of characterizing these soils test data for sites are based on the laboratory testing performed on thin wall tube samples and high quality block samples improvements have been suggested to existing correlations between the small strain shear modulus go or vs and index properties for these soils for better prediction of these soil properties

inferences from simple models of slow convectively coupled processes abstracta framework for conceptual understanding of slow convectively coupled disturbances is developed and applied to several canonical tropical problems including the water vapor content of an

eddy mediated regime transitions in the seasonal cycle of a hadley circulation and implications for monsoon dynamics in a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions the hadley cells undergo transitions between two regimes distinguishable according to whether large scale eddy momentum fluxes strongly or weakly influence the strength of a cell the center of the summer and equinox hadley cell lies in a latitude zone of upper level westerlies and significant eddy momentum flux divergence the influence of eddy momentum fluxes on the strength of the cell is strong the center of the cross equatorial winter hadley cell lies in a latitude zone of upper level easterlies and is shielded from the energy containing midlatitude eddies the influence of eddy momentum fluxes on the strength of the cell is weak mediated by feedbacks between eddy fluxes mean zonal winds at upper levels and the mean meridional circulation the dominant balance in the zonal momentum equation at the center of a hadley cell shifts at the transitions between the regimes from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell at the transitions a feedback involving changes in the strength of the lower level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross equatorial winter cell the transitions resemble the onset and end of monsoons for example in the shift in the dominant zonal momentum balance rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels rapid changes in strength of the upward mass flux and changes in direction and strength of the zonal wind at upper and lower levels in the monsoonal regime the maximum upward mass flux occurs in an off equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear frictional to geostrophic similar dynamic mechanisms as at the regime transitions in the simulation mechanisms that can act irrespective of land sea contrasts and other inhomogeneities in lower boundary conditions may be implicated in large scale monsoon dynamics in earth s atmosphere

benthic biological and biogeochemical patterns and processes across an oxygen minimum zone pakistan margin ne arabian sea abstract oxygen minimum zones omzs impinging on continental margins present sharp gradients ideal for testing environmental factors thought to influence c cycling and other benthic processes and for identifying the roles that biota play in these processes here we introduce the objectives and initial results of a multinational research program designed to address the influences of water depth the omz 150 1300 m and organic matter om availability on benthic communities and processes across the pakistan margin of the arabian sea hydrologic sediment and faunal characterizations were combined with in situ and shipboard experiments to quantify and compare biogeochemical processes and fluxes om burial efficiency and the contributions of benthic communities across the omz in this introductory paper we briefly review previous related work in the arabian sea building the rationale for integrative biogeochemical and ecological process studies this is followed by a summary of individual volume contributions and a brief synthesis of results five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 taken together the contributed papers demonstrate distinct cross margin gradients not only in oxygenation and sediment om content but in benthic community structure and function including microbial processes the extent of bioturbation and faunal roles in c cycling hydrographic studies demonstrated changes in the intensity and extent of the omz during the sw monsoon with a shoaling of the upper omz boundary that engulfed the previously oxygenated 140 m site oxygen profiling and microbial process rate determinations demonstrated dramatic differences in oxygen penetration and consumption across the margin and in the relative importance of anaerobic processes but surprisingly little seasonal change a broad maximum in sediment om content occurred on the upper slope roughly coincident with the omz but the otherwise poor correlation with bottom water oxygen concentrations indicated that other factors are important in determining sediment om distributions downcore profiles generally showed little clear evidence of in situ om alteration and there was little sign of om enrichment resulting from the southwest monsoon in sediments collected in the late to postmonsoon sampling this is interpreted to be due to rapid cycling of labile om organic geochemical studies confirmed that sediment om is overwhelmingly of marine origin across the margin but also that it is heavily altered with only small changes in degradation state across the omz more negative stable c isotopic compositions in surficial sediments at hypoxic sites within the omz core are attributed to a chemosynthetic bacterial imprint dramatic changes in benthic community structure occurred across the lower omz transition apparently related to om availability and quality as well as to do concentrations high resolution sampling biomarkers and isotope tracer studies revealed that oxygen availability appears to exert threshold type controls on benthic community structure and early faunal c processing biomarker studies also provided evidence of faunal influence on sediment om composition together the results offer strong evidence that benthic fauna at sites across the margin play important roles in the early cycling of sediment om through differential feeding and bioturbation activities

dynamics of itcz width ekman processes non ekman processes and links to sea surface temperature abstractthe dynamical processes controlling the width of the intertropical convergence zone itcz are investigated using idealized and cmip5 simulations itcz width is defined in terms of boundary

the ras al hadd jet remotely sensed and acoustic doppler current profiler observations in 1994 1995 abstract the existence of a surface barotropic front jet system at the confluence region off the eastern tip of oman ras al hadd or rah is documented for 1994 1995 through advanced very high resolution radiometer avhrr and acoustic doppler current profiler adcp observations the thermal signature of this confluence is visible in 1995 between early may and the end of october i e throughout the sw monsoon and into the transition period between sw and ne monsoons the thermal characteristics are those of a ne oriented front between cooler water of southern upwelled origin and warmer waters of northern gulf of oman origin during the period when the thermal front is absent adcp data suggest that the confluence takes a more southward direction with gulf of oman waters passing rah into the southeastern oman coastal region the thermal gradient is initially small june july but later increases august october into a front that exhibits small scale instabilities surface current velocities within the jet estimated by tracking these features in consecutive satellite images are 0 5 0 7 m s 1 and in remarkable agreement with concurrent adcp retrievals in which the seasonal maximum in velocity is 1 m s 1 adcp observations collected during several us jgofs cruises reveal a weakly baroclinic current in the confluence region that drives the waters into the offshore system the fully developed jet describes a large meander that demarcates two counter rotating eddies cyclonic to the north and anticyclonic to the south of the jet of approximately 150 200 km diameter the southern eddy of this pair is resolved by the seasonally averaged sea level anomaly derived from topex poseidon observations during the sw monsoon the rah jet advects primarily cold waters along its path but as soon as the wind system reverses with the transition to the intermonsoonal period a warm current is rapidly established that advects the surface coastal waters of the gulf of oman offshore in accordance with the interannual variation of the wind forcing phase the reversal of the currents from ne to sw occurred earlier in 1994 than in 1995 confirming that the rah jet is integral part of the east arabian current the transport of the jet estimated by combining sst information on the width with adcp data on the velocity s vertical structure is found to fluctuate between 2 8 106 m3 s 1 and its thickness between 150 400 m these significant fluctuations are due to the time variable partition of horizontal transport between eddies and the rah jet and are potentially important to the nutrient and phytoplankton budgets of the arabian sea

preferred latitudes of the intertropical convergence zone abstract the latitude preference of the intertropical convergence zone itcz is examined on the basis of observations theory and a modeling analysis observations show that convection is enhanced at latitudes of about 4 to 10 relative to the equator even in regions where the sea surface temperature sst is maximum on the equator both linear shallow water theory and a moist primitive equation model suggest a new explanation for the off equatorial latitude preference of the itcz that requires neither the existence of zonally propagating disturbances nor an off equatorial maximum in sst the shallow water theory indicates that a finite width zonally oriented midtropospheric heat source i e an itcz produces the greatest local low level convergence when placed a finite distance away from the equator this result suggests that an itcz is most likely to be supported via low level convergence of moist energy when located at these preferred latitudes away from the equator for a plausible range of

the influence of cross equatorial pressure gradients on the location of near equatorial convection tomas and webster note that in regions where there is a substantial cross equator surface pressure gradient there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus the flow in this region meets the parcel criterion for inertial instability they hypothesize that the atmospheric response to this absolute vorticity distribution is a convergence divergence doublet in the boundary layer the convergence centre results in strong convection and thus is very important in determining the strength and location of convection i e the intertropical convergence zones whereas the divergence centre results in suppressed convection in this work a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further an analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability it does not satisfy the linear stability criteria because of the stabilizing influence of dissipation and the finite vertical scale much greater shear than that observed is required for linear instability it is noted however that when the parcel criterion is met one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow several experiments are performed integrating the full nonlinear model relaxing back to a pressure distribution having a cross equatorial gradient to test whether the observed vorticity wind and convergence distributions can be well simulated it is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator this absolute vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence divergence doublet similar to that observed by tomas and webster this response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity the response of the meridional wind to the strength of the pressure gradient is quasi linear for small and somewhat larger values of forcing but between these values there is a range where the response is highly nonlinear i e large increases in the strength of the meridional wind result from small changes in the pressure gradient it is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations the nonlinear model is also forced using pressure distributions taken from observations the resulting steady state simulated vorticity wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east pacific and east atlantic during july when forced using the pressure distribution from the indian ocean region during july the model produces an unrealistically strong meridional wind response some improvement is obtained by including the latitudinally dependent zonal pressure gradient force which is moderately strong in this region finally the model is forced using a pressure distribution taken from observations in the central pacific ocean during july in this case the cross equator pressure gradient is near zero

coherent response of arabian sea upwelling and pollen transport to late quaternary monsoonal winds the indian summer monsoon causes large seasonal changes in the environments of both eastern africa and the arabian sea here we compare time series 140 kyr long of selected pollen types and foraminiferal upwelling faunas preserved in marine sediments to identify the frequencies of their variability and the coherence and phase of the terrestrial and marine responses to monsoonal circulation during interglacial intervals the pollen and upwelling records of the western arabian sea indicate stronger monsoons and are both coherent and in phase at periodicities near the precession of the earth s rotational axis 23 kyr we conclude that the strength of the monsoonal southwesterlies accounts for this high coherence of marine and terrestrial records during interglacial intervals low correlation during glacial phases indicates weaker monsoons and some decoupling of these two records these data indicate that two distinct modes of monsoon variability may exist an interglacial mode with strong monsoons and effective coupling of terrestrial and marine records and a glacial mode with both weaker monsoons and decreased linkage between arabian sea pollen and upwelling records

foraminiferal response to the northeast monsoon in the western and southern arabian sea abstract sediments from the western and southern part of the arabian sea were collected periodically in the spring intermonsoon between march and may 1997 and additionally at the end of the northeast monsoon in february 1998 assemblages of rose bengal stained living deep sea benthic foraminifera their densities vertical distribution pattern and diversity were analysed after the northeast monsoon and short time changes were recorded in the western arabian sea foraminiferal numbers increased steadily between march and the beginning of may especially in the smaller size classes 30 63 m 63 125 m at the same time the deepening of the foraminiferal living horizon variable diversity and rapid variations between dominant foraminiferal communities were observed we interpret these observations as the time dependent response of benthic foraminifera to enhanced organic carbon fluxes during and after the northeast monsoon in the southern arabian sea constant low foraminiferal abundances during time no distinctive change in the vertical distribution reduced diversity and more stable foraminiferal communities were noticed which indicates no or little influence of the northeast monsoon to benthic foraminifera in this region

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved