late quaternary hydrological dynamics in the middle kalahari forcing and feedbacks the kalahari region has become a major source of quaternary palaeoenvironmental data derived primarily from the analysis of geomorphological proxies of environmental change one suite of data from palaeolacustrine landforms has recently provided a new record of major hydrological changes in the last 150 ka burrough s l thomas d s g bailey r m 2009 mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system quaternary science reviews in press here we present an improved analysis of the drivers and feedbacks of lake level change utilising information from three main sources data from the lake system itself from analyses of other late quaternary records within the region and from climate modelling simulations using the hadley centre coupled climate model hadcm3 suggest that once triggered the lake body was large enough to potentially affect both local and regional climates surface waters and their interactions with the climate are therefore an important component of environmental dynamics during the late quaternary through its capacity to couple middle kalahari environments to distant forcing mechanisms and to itself force environmental change we demonstrate that the existence or absence of megalake makgadikgadi adds a new level of complexity to the interpretations of environmental proxy records in southern africa s summer rainfall zone

recent changes in the pacific subtropical cells inferred from an eddy resolving ocean circulation model abstract in this study the subtropical cells stc in the pacific ocean are analyzed using an eddy resolving ocean general circulation model driven by atmospheric forcing for the years 1992 2003 in particular the authors seek to identify decadal changes in the stcs in the model and to compare them with observations in order to understand the consequences of such changes for the equatorial ocean heat and mass budgets the simulation shows a trend toward increasing pycnocline volume transport at 9 n and 9 s across the basin from 1992 to 2003 this increase 4 9 1 0 sv sv 106 m3 s 1 is in qualitative agreement with observations and is attributed primarily to changes in the interior ocean transport which are partially compensated by opposing western boundary transports the subtropical meridional volume transport convergence anomalies in the model pycnocline are found to be consistent with anomalous volume transports in both the observed and modeled equatorial undercurrent as well as with the magni

the micro topography of the wetlands of the okavango delta botswana the surface of the 40 000 km2 okavango alluvial fan is remarkably smooth and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface deviations from this perfect surface give rise to islands in the okavango wetlands this micro topography was mapped by assigning empirical elevations to remotely sensed vegetation community classes based on the observation that vegetation is very sensitive to small local differences in elevation even though empirical the method produces fairly accurate results the technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available the micro topography has arisen as a result of clastic sedimentation in distributary channels which produces local relief of less than two metres and more importantly as a result of chemical precipitation in island soils which produces similar local relief the micro topography is therefore an expression of the non random sedimentation taking place on the fan volume calculations of islands extracted from the micro topography combined with estimates of current sediment in ux suggest that the land surface of the wetland may only be a few tens of thousands of years old constant switching of water distribution driven by local aggradation has distributed sediment widely mass balance calculations suggest that over a period of c 150 000 years all of the fan would at one time or other have been inundated and thus subject to sedimentation coalescing of islands over time results in net aggradation of the fan surface the amount of vertical aggradation on islands and in channels is restricted by the water depth restricted vertical relief in turn maximizes the distribution of water limiting its average depth aggradation in the permanent swamps occurs predominantly by clastic sedimentation rates of aggradation here are very similar to those in the seasonal swamps maintaining the overall gradient possibly because of the operation of a feedback loop between the two the limited amount of local aggradation arising from both clastic and chemical sedimentation combined with constant changes in water distribution has resulted in a near perfect conical surface over the fan in addition to providing information on sedimentary processes the micro topography has several useful hydrological applications copyright 2004 john wiley sons ltd

pacific thermocline bridge revisited abstract the coupling on decadal time scales of the mid latitude and tropical pacific via an oceanic bridge in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model results indicate that in the tropics decadal anomalies of isopycnal depth are forced by ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid latitudes of either hemisphere in the coupled model temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere but are lag correlated between southern hemisphere mid and low latitudes however anomaly magnitudes on the equator are small these results suggest that the oceanic bridge to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial pacific coupling to the southern mid latitudes via temperature anomalies on isopycnals remains an intriguing possibility

long term el nino southern oscillation enso like variation with special emphasis on the south pacific mechanisms for el nino southern oscillation enso like decadal 7 35 years variability are analyzed using observational data when a decadal positive temperature anomaly occurs in the eastern tropical pacific the atmospheric response excites negative wind stress curl in the western tropical south pacific which causes the thermocline to shoal and induce a negative temperature anomaly there the cold anomaly moves northwestward to the western and central equatorial region and then turns eastward along the equator to reverse the original temperature anomaly in the east the eastward propagation of the oceanic signal along the equatorial thermocline correlates with a zonal wind anomaly the cold anomaly in the east grows through tropical air sea interaction and induces a positive wind stress curl in the western tropical south pacific associated with the atmospheric teleconnection in the southern hemisphere subsequent evolution of the negative phase is similar to that of the positive phase the complete scenario for this enso like decadal variability due to the air sea interaction processes within the tropics has a period of 14 years we also discuss enso like variation on the interdecadal 35 years timescale with possible relationship to the 1976 1977 climate regime shift

a decadal spiciness mode in the tropics analysis of a complex climate model suggests the existence of a mode of coupled ocean atmosphere variability in the tropical pacific with the following dynamics anomalous strong pacific trade winds accelerate the north equatorial current and countercurrent at 10 to 15 n in the central pacific thermocline this causes anomalous advection across mean gradients of temperature and salinity on isopycnal surfaces and generates cool fresh spiciness anomalies of the order of 0 3 k these are advected by the mean circulation via the western boundary region to the equatorial pacific in approximately five years at the equatorial outcrops of the isopycnals the cool fresh anomalies affect the surface heat budget and are hypothesized to initiate a relaxation of the trade winds that is reinforced by positive feedbacks with the slope of the thermocline and with air sea fluxes of heat and freshwater at 5 to 10 s the decrease of the trades generates by anomalous advection cool fresh spiciness anomalies that migrate to the equator in one to two years and provide a positive feedback at 10 to 15 n the decrease of the trades decelerates north equatorial current and countercurrent and causes warm salty anomalies on isopycnals that arrive at the equator five years later and close the cycle

after the flood the fluvio lacustrine landforms of northern botswana abstract the middle kalahari of northern botswana contains three palaeolake basins with shorelands enclosing an area of 60 000 km2 linked to the okavango delta and the adjacent chobe and zambezi rivers two major stages of the palaeolake system at 940 945 m lake palaeo makgadikgadi and 936 m lake thamalakane are identified prior to the final desiccation of the lakes over the last 2 000 years the former is tectonic in origin and last existed c 35 000 b p whilst the lake thamalakane stage has existed on a number of occasions particularly at 17 000 12 000 b p and can be explained by increased precipitation and inflow feeding a complex system of fault controlled lakes ultimately controlled by the mambova falls at the chobe zambezi confluence a number of factors influencing the behaviour of the okavango delta and its adjoining basins can be identified on different time scales and are discussed in relation to planning for future water development

north south asymmetry of warm water volume transport related with el nino variability 1 a better understanding of variability in the volume of warm water in the equatorial pacific pycnocline warm water volume or wwv is critical to understanding el nino southern oscillation enso variability we have investigated the meridional wwv transport in the northern nh and southern hemispheres sh using a high resolution general circulation model the transport in the western boundary region compensates the interior transport in the sh where sverdrup balance holds approximately in contrast such compensation does not hold in the nh because the boundary transport lags interior transport by about 7 months hence the wwv exchange in the nh has more impact on the recharge discharge of the equatorial wwv the north south asymmetry of the wwv transport which is interpreted based on the linear rossby theory is related with the southward migration of westerly wind and the negative wind curl over the northwestern off equatorial region after the mature stage of enso

seasonal eddy kinetic energy modulations along the north equatorial countercurrent in the western pacific seasonal eddy kinetic energy eke variability and its associated eddy energy conversion processes in the western tropical pacific are investigated using satellite altimeter observations and a global eddy resolving ocean general circulation model ogcm both the altimeter observed sea surface height anomalies and the ogcm simulation show an area with enhanced eke east of the mindanao island centered around 133 e and 5 n this enhanced eke area corresponds to the location of the quasi stationary meander of the north equatorial countercurrent necc and is bordered to the south by the halmahera eddy the mesoscale eke in this area exhibits a clear seasonality strong in summer july august and weak in winter november january and much of this seasonality is confined to the upper 200 m layer an investigation into the upper ocean eddy energetics based on the ogcm simulation reveals that the areal barotropic eddy energy conversion rate has an annual cycle similar to the eke variations while the areal baroclinic eddy energy conversion is found to be much smaller that the barotropic conversion rate and exhibits no clear seasonal changes this indicates that the eke variations are largely controlled by barotropic conversion of the seasonally varying regional circulation by examining the seasonal background circulation changes we find that the amplification of the barotropic eddy energy conversion rate in july august is related to the seasonal evolution of the mindanao current and the new guinea coastal current that amplifies the curvature and amplitude of the quasi stationary meander of the necc and results in an elevated eke level through increased regional barotropic conversion

chemical sedimentation in the semi arid environment of the okavango delta botswana abstract the okavango basin of northern botswana is an area of active diagenetic carbonate and silica accumulation the basin is formed by grabens which represent an extension of the east african rift system the okavango delta a large marshy alluvial fan 22 000 km2 occupies most of the surface area of the basin evapotranspiration exceeds precipitation by a factor of 3 and 96 of incoming water is lost to the atmosphere evaporative concentration of dissolved solids occurs and they precipitate in the capillary fringes of islands the less soluble components caco3 and sio2 precipitate in the soils which results in a significant volume increase which has important implications for the topography of the area and the character of the sediment fill the more soluble salts mainly alkali carbonates accumulate at the surface as efflorescent crusts but are leached into the soil during the rainy season this salt accumulation destroys vegetation swamp abandonment leads to leaching to greater depth and with reflooding to the removal of these salts possibly to form a deep saline groundwater mass balance calculations indicate that this form of chemical accumulation is the dominant aggradational process occurring in the delta at present

the okavango fan and the classification of subaerial fan systems abstract controversy exists over the classification of fluvially formed subaerial fans authors either restrict alluvial fans to debris flow dominated types or extend the spectrum to fans dominated by braided rivers the okavango fan provides an end member which extends this spectrum to fans dominated by meandering and low sinuosity rivers the fan is a large 150 km radial axis shallowly sloping 0 00036 highly vegetated subaerial fan which can be subdivided into four subenvironments these are 1 the entry corridor or panhandle characterized by single and anastomosing meander belts 2 the upper fan characterized by meander belts diverging from the fan apex comprising peat confined meandering channels with interchannel swamps forming thick peats 3 the middle fan with highly confined single and anastomosing low sinuosity rivers and less common prograding meander belts whose channels are confined by thick peats with however little chance of peat preservation and 4 the lower fan in which annual floods from relatively unconfined channels spread over the fan surface and interact with pre existing aeolian and lacustrine deposits with the recognition of this new fan type the spectrum of subaerial fan types can be expressed in terms of 1 debris flow dominated fans of which the death valley fans are a member 2 braided river dominated fans of which the kosi fan is a member and 3 low sinuosity meandering losimean river dominated fans of which the okavango fan is a member this spectrum can be expressed in terms of variation in slope maximum size and percentage of surface vegetation but crucial to the evolution of the various fan types is variation in the flashy to continuous nature of the discharge and the degree of channel confinement evident on the fan surface comparable ancient examples of the three fan types are recognizable many of which provide intermediates between the ideal end members debris flow dominated and braid dominated fan types are known from throughout earth history however the losimean fan type because of its reliance on confining vegetation may only have developed after the devonian period the spectrum of subaerial fan types can be expressed on a triangular field of variation with the vertices defined by the relative importance of the processes which shaped a particular fan system be they debris flows processes associated with braided rivers or processes associated with meandering and low sinuosity rivers

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

southern hemisphere origins of the 1976 climate shift 1 observations from the tropical pacific ocean identify an abrupt climate shift in 1976 with surface temperatures changing from cooler than normal to warmer than normal in the span of about 1 year model results show that this climate shift originates with subsurface temperature anomalies in the south tropical pacific ocean which propagate first to the western boundary then northward to the equator and finally eastward along the equator to 140 w where they rise to the surface the results suggest that changes in the north pacific respond to changes in the tropical pacific

geochemical mapping of carbonate sediments in the makgadikgadi basin botswana using moderate resolution remote sensing data this paper maps the carbonate geochemistry of the makgadikgadi pans region of northern botswana from moderate resolution 500 m pixels remotely sensed data to assess the impact of various geomorphological processes on surficial carbonate distribution previous palaeo environmental studies have demonstrated that the pans have experienced several highstands during the quaternary forming calcretes around shoreline embayments the pans are also a significant regional source of dust and some workers have suggested that surficial carbonate distributions may be controlled in part by wind regime field studies of carbonate deposits in the region have also highlighted the importance of fluvial and groundwater processes in calcrete formation however due to the large area involved and problems of accessibility the carbonate distribution across the entire makgadikgadi basin remains poorly understood the modis instrument permits mapping of carbonate distribution over large areas comparison with estimates from landsat thematic mapper data show reasonable agreement and there is good agreement with estimates from laboratory analysis of field samples the results suggest that palaeo lake highstands reconstructed here using the srtm 3 arc second digital elevation model have left behind surficial carbonate deposits which can be mapped by the modis instrument copyright c 2006 john wiley sons ltd

geomorphic evidence for the late quaternary palaeoclimates of the middle kalahari of northern botswana summary geomorphological studies of caves and lakes in northern botswana since 1973 have yielded a suite of 80 radiometric dates which permit palaeoenvironmental reconstruction of the landscape over the last 45 000 years the unique characteristics of the landforms when interpreted with caution yield palaeoclimatic information on both northern botswana and the catchments of the okavango river in the angolan highlands several major shifts in precipitation are indicated for the period 45 000 20 000 bp with dry conditions prevailing at 25 000 bp the late glacial was characterised by humid conditions especially from 16 000 13 000 bp whilst the early holocene was semi arid with precipitation peaks circa 6 000 5 000 bp and 2 000 bp the results contrast sharply with some southern african palaeo climatic descriptions in print where many of the inferences have been drawn from studies of the northern hemisphere inter tropical zone the implications of the study are briefly discussed in relation to palaeoclimatic models of the whole african continent

paleoenvironmental implications of fixed dune systems in southern africa extensive systems of fixed linear dunes occur in the kalahari region of southern africa from 28 s to 16 30 s they provide evidence for large scale expansion of arid climates in the subcontinent during the quaternary on the basis of differing dune morphologies and alignments three periods of upper pleistocene aridity are recognised patterns of dune alignments reflect paleosand flow regimes and permit deduction of past winds and circulation patterns leading to the conclusion that arid episodes were associated with and probably caused by greater strength and persistence of the south african anticyclone a tentative chronology of the arid periods tends to support the hypothesis that subtropical aridity was contemporaneous with glacial conditions in high latitudes

processes that control water chemistry and stable isotopic composition during the refilling of lake ngami in semiarid northwest botswana summary lake ngami in semiarid botswana was dry until 2009 when it began filling with water because of regional increases in precipitation and from redirection of water to the lake from distributaries of the okavango river we measured the physical chemical and stable isotopic composition of lake water collected at 25 cm below the surface along a 18 km axial transect from the inflow river to the distal end of the lake our objective was to determine the processes that control water properties and to establish baseline values for future temporal and spatial comparisons the major ionic concentrations e g cl na ca2 and the stable oxygen 18o and hydrogen d isotope ratios show three distinct regions of increasing concentrations and isotopic enrichment respectively along the transect the 18o vs d data plot along the okavango delta evaporation line and suggest modification of lake water by evaporation the proportions of the major ions in the inlet water and in the lake were similar and log cl vs log na suggests an evaporative enrichment of solutes the segmentation of the major solutes and the 18o and d into three regions along the axial transect results from differential evaporation of lake recharge from 2010 2011 and 2012 and are thus controlled by the residence time of recharge in the lake unlike the major ions the dissolved inorganic carbon dic concentrations and the stable carbon isotopic ratios 13c of dic increase along the axial transect to about midway in the lake and then reach steady state the 13c of dissolved organic matter in the inlet river and lake averaged 25 7 0 3 while that for particulate organic carbon decreased from 24 in the inlet river to 28 in the lake from water column productivity carbon cycling in lake ngami is controlled by evaporation which increase dic concentrations and equilibration between carbon in dic and atmospheric co2 g the results show the importance of evaporation and residence time in controlling the solute chemistry and the dominance of atmospheric co2 g in controlling carbon cycling during the filling stages of lakes in arid environments

late quaternary lake level fluctuations in the mababe depression middle kalahari palaeolakes and the role of zambezi inflows abstract a systematic drilling and optical dating programme on middle kalahari beach ridge relict shoreline sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the okavango delta northern botswana this paper presents 23 ages from the mababe depression and establishes four shoreline construction phases in the late quaternary coeval with other sub basin lake high stands lake ngami these synchronous lake phases result from a coalescence of the sub basins into a unified palaeolake lake thamalakane covering an area of 32 000 km2 six additional ages are also presented from the chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as lake thamalakane phases this suggests that increased flow in the chobe and zambezi system significantly contributed to the middle kalahari lake phases in both the post glacial and holocene periods the integration of these new data and their compatibility with other regional and tropical palaeo archives is discussed in the light of understanding quaternary climate drivers within the kalahari

dual mechanisms for the formation of fluvial silcretes in the distal reaches of the okavango delta fan botswana silcretes exposed within river marginal or valley settings have been described in a number of studies but few models have been suggested for the development of these fluvial silcretes an exception is that proposed by mccarthy and ellery journal of sedimentary research 1995 vol a65 pp 77 90 to describe mechanisms of early stage near surface silica diagenesis in the okavango delta botswana this paper describes the characteristics and possible origins of massive surface and sub surface silcretes at samedupe and boro junction beyond the distal margin of the okavango delta and further downstream than the sites described by mccarthy and ellery morphological and petrological evidence from surface exposures and three sedimentary cores suggests that other modes of formation may also be applicable a dual model of formation is proposed surface silcretes are suggested to have developed by silica accumulation in seasonal pools remaining after the annual okavango flood whilst sub surface horizons appear to have formed under conditions of varying ph associated with fluctuating groundwater levels beneath the channel floor this model is reviewed in the context of the fluvial silcrete debate 1998 john wiley sons ltd

do tropical cells ventilate the indo pacific equatorial thermocline source waters of the indo pacific equatorial thermocline are studied with a high resolution ocean model in the annual mean fields tropical and subtropical overturning cells are found that upwell at the equator and downwell at 5 degrees and 20 degrees poleward of the equator respectively tropical cells are common in ocean models but their role in ventilating the equatorial thermocline is obscure because the downwelled water is too warm to match the subsurface equatorial waters the tropical cells are much weaker when the overturning is considered in density coordinates when high frequency mass fluxes are included tropical cells are compensated by an eddy induced overturning seasonal variations and tropical instability waves are responsible for the compensation it follows that only subtropical cells transfer surface water to the equatorial thermocline strong tropical cells are shown to be an artefact

decadal variations in the subtropical cells and equatorial pacific sst 1 a mechanism for generating decadal sea surface temperature sst variability in the equatorial pacific is investigated using an ocean general circulation model forced by observed wind stress equatorial sst variability is governed by different ocean dynamics on interannual and decadal time scales at interannual time scales equatorial sst anomalies are mainly driven by equatorial winds at decadal time scales on the other hand they are forced both by equatorial winds and by off equatorial winds in the tropics while the contribution from midlatitude winds poleward of 25 is negligible trade wind variations in the off equatorial tropics force equatorial sst variability by spinning up and down the subtropical cells that transport cold water into the equatorial upwelling zone equatorial sst anomalies induced by this mechanism lag behind the response to local equatorial winds by about two years

interannual to decadal variations of tropical subtropical exchange in the pacific ocean boundary versus interior pycnocline transports abstract interannual to decadal variations of tropical subtropical mass exchange in the pacific ocean are investigated using a near global ocean general circulation model along with satellite observations of sea level and wind and a data assimilation product the analysis focuses on the variability of pycnocline transports through the western boundary and interior near 10 n and 10 s in contrast to time mean exchange where boundary and interior pycnocline transports are both equatorward the variations of boundary and interior pycnocline transports are found to be generally anticorrelated to each other moreover the variation of the boundary pycnocline transport is smaller than that of the interior again different from time mean exchange where the boundary transport at 10 n is substantially larger than that through the interior interannual variations of the boundary and interior transports are consistent with near surface geostrophic flow inferred from sea level data interior pycnocline flow into th

late quaternary environmental change in central southern africa new data synthesis issues and prospects much of the interior of central southern africa is a sand sea within which aeolian and lacustrine landforms and sediments of local and regional extent are preserved closed cave sites are restricted to very few locations while fluvial systems traverse the margins of the interior until the early 1990s chronologies of late quaternary environmental and climatic changes developed for this region were based on only a limited number of proxy data sets derived largely from lacustrine deposits and precipitates in particular directly determined ages from aeolian deposits the most extensive suite of features in the region were absent the application of luminescence dating techniques to dune sediments and the development of further detailed chronologies from cave precipitates is now providing a more comprehensive record for the last 50 ka with some chronologies extending back a further 100 ka we present and review these data assess their contribution to enhanced understanding of late quaternary environmental changes in the region and for the first time assess them against corrected radiocarbon ages from lacustrine sites it is concluded that there is now an enhanced understanding of the spatial and temporal complexity of climate changes affecting the region in the last glacial cycle including a complex record of punctuated aridity but that many issues including data set integration and forcing mechanism controls are imperfectly understood

symmetric and antisymmetric mass exchanges between the equatorial and off equatorial pacific associated with enso 1 mass exchanges in the upper ocean between the equatorial and off equatorial pacific ocean associated with the el nino southern oscillation enso are investigated using the national centers for environmental prediction ncep ocean assimilation data the data show that enso related meridional mass transport in the northern hemisphere nh is larger than that in the southern hemisphere sh we found that the antisymmetric characteristics are mainly due to a southward shift of themaximum zonal wind stress anomaly during the enso mature phase the ekman and geostrophic transports associated with enso are separated into symmetric and antisymmetric components for the symmetric part the mass divergence over the equatorial pacific by the geostrophic transport is generally larger than the convergence by the ekman transport during the el nino mature phase therefore mass is transported from the equator to off the equator at this time as for the antisymmetric part the ekman transport due to antisymmetric wind stress dominates the geostrophic transport so that the mass is transported from the sh to the nh during the el nino mature phase the net mass transport in the nh is larger than that in the sh a theoretical interpretation and intermediate model experiments support these arguments index terms 4522 oceanography physical el nino 4572 oceanography physical upper ocean processes 4512 oceanography physical currents 4504 oceanography physical air sea interactions 0312 4215 oceanography general climate and interannual variability 3309 keywords enso mass exchange wind stress antisymmetry

collision of continental corner from 3 d numerical modeling abstract continental collision has been extensively investigated with 2 d numerical models assuming infinitely wide plates or insignificant along strike deformation in the third dimension however the corners of natural collision zones normally have structural characteristics that differ from linear parts of mountain belt we conducted 3 d high resolution numerical simulations to study the dynamics of a continental corner lateral continental oceanic transition zone during subduction collision the results demonstrate different modes between the oceanic subduction side continuous subduction and retreating trench and the continental collision side slab break off and topography uplift slab break off occurs at a depth 100 km to 300 km that depends on the convergence velocity the numerical models produce lateral extrusion of the overriding crust from the collisional side to the subduction side which is also a phenomenon recognized around natural collision of continental corners for instance around the western corner of the arabia asia collision zone and around the eastern corner of the india asia collision zone modeling results also indicate that extrusion tectonics may be driven both from above by the topography and gravitational potentials and from below by the trench retreat and asthenospheric mantle return flow which supports the link between deep mantle dynamics and shallower crustal deformation

decadal phase change in large scale sea level and winds in the indo pacific region at the end of the 20th century 1 satellite observations of sea surface height ssh and wind stress for the period of 1993 2006 reveal a near coherent large scale decadal variability in much of the indo pacific region with a phase change at the turn of the 20th century trade wind variations in the tropical pacific and south indian ocean are anti correlated with each other as are ssh differences across these two basins implying anti correlated variation of the subtropical cells in the two oceans decadal changes in large scale ssh in the extra tropics are mostly associated with well defined patterns of wind stress curl indicating a near coherent decadal variation in the strength of subtropical and subpolar gyres together these variations reflect a linkage in the circulation of the pacific and indian oceans via atmospheric and oceanic bridges the phase change in the tropical pacific tends to occur earlier than elsewhere suggesting a potential role of the tropical pacific in regulating decadal variability of the entire region

subduction of decadal north pacific temperature anomalies observations and dynamics abstract observations of oceanic temperature in the upper 400 m reveal decadal signals that propagate in the thermocline along lines of constant potential vorticity from the ventilation region in the central north pacific to approximately 18 n in the western pacific the propagation path and speed are well described by the geostrophic mean circulation and by a model of the ventilated thermocline the approximate southward speed of the thermal signal of 7 mm s 1 yields a transit time of approximately eight years the thermal anomalies appear to be forced by perturbations of the mixed layer heat budget in the subduction region of the central north pacific east of the date line a warm pulse was generated in the central north pacific by a series of mild winters from 1973 to 1976 and reached 18 n around 1982 after 1978 a succession of colder winters initiated a cold anomaly in the central north pacific that propagated along a similar path and with a similar speed as the warm anomaly then arrived in the west

enso driven interhemispheric pacific mass transports previous studies have shown that enso s anomalous equatorial winds including the observed southward shift of zonal winds that occurs around the event peak can be reconstructed with the first two empirical orthogonal functions eofs of equatorial region wind stresses using a high resolution ocean general circulation model we investigate the effect of these two eofs on changes in warm water volume wwv interhemispheric mass transports and indonesian throughflow itf wind stress anomalies associated with the first eof produce changes in wwv that are dynamically consistent with the conceptual recharge oscillator paradigm the itf is found to heavily damp these wwv changes reducing their variance by half wind stress anomalies associated with the second eof which depicts the southward wind shift are responsible for wwv changes that are of comparable magnitude to those driven by the first mode the southward wind shift is also responsible for the majority of the observed interhemispheric upper ocean mass exchanges these winds transfer mass between the northern and the southern hemisphere during el nino events whilst water is transferred in the opposite direction during la nina events the magnitude of this exchange is roughly half of that seen during el nino events thus the discharging of wwv during el nino events is meridionally asymmetric while the wwv recharging during a la nina event is largely symmetric the inclusion of the southward wind shift is also shown to allow enso to exchange mass with much higher latitudes than that allowed by the first eof alone

observed equatorial rossby waves and enso related warm water volume changes in the equatorial pacific ocean 1 modifications of the volume of warm water above the thermocline in the equatorial pacific are thought to be critical for enso el nino southern oscillation preconditioning development and prediction in this paper changes in this warm water volume wwv are first reviewed from the literature and then analyzed within 5 n 5 s in using 1992 2006 altimeter derived sea level anomalies sla and in situ observations we conclude that the 1992 2006 wwv changes in the equatorial band are a residual from the opposite effects of zonally integrated meridional ekman and geostrophic transports of warm water the latter being estimated from sla and validated against in situ observations for the first time with observations we evidence first baroclinic first meridional mode equatorial rossby waves to be responsible for the anomalous meridional geostrophic transports of warm water in changing the basin scale zonal pressure gradient these transports appear in phase opposition and proportional to the anomalous sea surface temperature sst in the region nino3 4 170 w 120 w 5 n 5 s the links between our observational results and the recharge discharge oscillator theory the differences in wwv between enso events and the hint for possible recent changes in the relation between wwv and nino3 4 sst are discussed

holocene fluctuations of lake ngami middle kalahari chronology and responses to climatic change abstract lake ngami a 3000 km 2 basin at the distal end of the okavango delta botswana has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl linked to topographic thresholds and inflows current and fossil in the okavango system the lake has been ephemeral throughout the 20th century and completely dry since 1982 yet held a substantial body of water up to a level of 930 m asl when visited by european travellers in the mid nineteenth century the historical decline in the lake has been well documented and used as evidence for regional desiccation but the holocene record based on archaeological studies and radiocarbon dating of inorganic carbonates is more speculative a study of a 30 km 2 diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the holocene linked to a now abandoned inflow of the kunyere river a programme of 18 luminescence age determinations of the peripheral dautsa and magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90 100 ka or earlier and have been modified by holocene fluctuations in lake level and subsequent aeolian reworking the water level peaked at 936 m asl between 4 and 3 ka returned to 934 m at 2 8 2 ka ponding water into the phatane gap and fell rapidly thereafter to 932 m and thence to historical levels the 936 m level can be correlated with increased throughflow in the okavango delta hence increased rainfall in the catchment the subsequent decline can be attributed to episodic closure and rerouting of okavango distributaries in particular the thaoghe perhaps aided by anthropogenic activities as such the desiccation of the lake is not indicative of regional climatic change

an interhemispheric tropical sea level seesaw due to el nino taimasa abstractduring strong el nino events sea level drops around some tropical western pacific islands by up to 20 30 cm such events referred to as taimasa in samoa expose shallow reefs thereby causing severe damage to associated coral ecosystems and contributing to the formation of microatolls during the termination of strong el nino events a southward movement of weak trade winds and the development of an anomalous anticyclone in the philippine sea are shown to force an interhemispheric sea level seesaw in the tropical pacific that enhances and prolongs extreme low sea levels in the southwestern pacific spectral features in addition to wind forced linear shallow water ocean model experiments identify a nonlinear interaction between el nino and the annual cycle as the main cause of these sea level anomalies

linkage between the pacific decadal oscillation and the low frequency variability of the pacific subtropical cell the decadal variability of pacific subtropical cell stc and its linkages with the pacific decadal oscillation pdo are investigated in the present study based on a simple ocean data assimilation soda 2 2 4 it is found that on decadal time scales the western boundary and interior pycnocline transports are anticorrelated and the variation of the interior component is more significant which is consistent with previous studies the decadal variability of stc in the northern hemisphere is found to be strongly associated with pdo associated with a positive negative phase of pdo the relaxation acceleration of the northeast trades slows down spins up the stc within a few years through baroclinic adjustment in conjunction with the subduction of the cold warm mixed layer anomalies in the extratropics the cold warm water is then injected into the thermocline and advected further southwestward to the tropics along the isopycnal surfaces leading to the slowdown spin up of stc due to zonal pressure gradient change at low latitude along with the stc weakening strengthening a significant warming cold anomaly appears in the tropics and it is advected to the midlatitude by the kuroshio and north pacific currents thus feeding back to the atmosphere over the north pacific in contrast to the northern hemisphere it is found the stc in the south only passively responds to the pdo the mechanism found here highlights the role of the stc advection of extratropical anomalies to the tropics and horizontal gyre advection of the tropical anomalies to the extratropics in decadal variability of the stc and pdo

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

rapid subsurface warming and circulation changes of antarctic coastal waters by poleward shifting winds the southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s this wind trend is projected to persist under continued anthropogenic forcing but the impact of the changing winds on antarctic coastal heat distribution remains poorly understood here we show that a poleward wind shift at the latitudes of the antarctic peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 c at 200 700 m depth the model simulated warming results from a rapid advective heat flux induced by weakened near shore ekman pumping and is associated with weakened coastal currents this analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around antarctica with potentially significant ramifications for global sea level rise

pacific ocean circulation rebounds 1 recent observations indicate that the shallow meridional overturning circulation in the tropical pacific ocean has rebounded since 1998 following 25 years of significantly weaker flow convergence of cold interior ocean pycnocline water towards the equator increased to 24 1 1 8 106 m3 s 1 during 1998 2003 from a low of 13 4 1 6 106 m3 s 1 during 1992 98 intensified circulation led to the development of anomalously cool tropical pacific sea surface temperatures which may have affected pacific marine ecosystems and global climate the abruptness of the rebound also obscures presumed anthropogenic warming trends in the instrumental temperature record of the tropical pacific

gliders measure western boundary current transport from the south pacific to the equator abstract spray gliders most launched from small boats near shore have established a sustainable time series of equatorward transport through the solomon sea the first 3 5 years mid 2007 through 2010 are analyzed coast to coast equatorward transport through the solomon sea fluctuates around a value of 15 sv 1 sv 106 m3 s 1 with variations approaching 15 sv transport variability is well correlated with el nino indices like nino 3 4 with strong equatorward flow during one el nino and weak flow during two la ninas mean transport is centered in an undercurrent focused in the western boundary current variability has a two layer structure with layers separated near 250 m near the core of the undercurrent that fluctuate independently the largest variations are in midbasin confined to the upper layer and are well correlated with enso analysis of velocity and salinity on isopycnals shows that the western boundary current within the solomon sea consists of a deep core coming from the coral sea

tropical pacific decadal variability and the subtropical tropical cells abstract the decadal scale variability in the tropical pacific has been analyzed herein by means of observations and numerical model simulations the two leading modes of the sea surface temperature sst variability in the central western pacific are a decadal mode with a period of about 10 yr and the enso mode with a dominant period of about 4 yr the sst anomaly pattern of the decadal mode is enso like the decadal mode however explains most variance in the western equatorial pacific and off the equator a simulation with an ocean general circulation model ogcm forced by reanalysis data is used to explore the origin of the decadal mode it is found that the variability of the shallow subtropical tropical overturning cells is an important factor in driving the decadal mode this is supported by results from a multicentury integration with a coupled ocean atmosphere general circulation model cgcm that realistically simulates tropical pacific decadal variability finally the sensitivity of the shal

decadal upper ocean temperature variability in the tropical pacific decadal variability in upper ocean temperature in the pacific is studied by using observations and results from model experiments especially propagation of upper ocean thermal anomalies from the midlatitudes to the tropics is studied as a possible source for decadal equatorial thermocline variability in the observations propagation along the subtropical gyre of the north pacific is clear however no propagation into the equatorial region is found model experiments with an ocean model forced with observed monthly wind and wind stress anomalies are performed to study the apparent propagation distinct propagation of thermal anomalies in the subtropics is found in the model although the amplitude of the anomalies is small the anomalies clearly propagate into the tropics but they do not reach the equatorial region the small response at the equator to extratropical variability consists of a change in the mean depth of the thermocline it appears that most variability in the subtropics and tropics is generated by local wind stress anomalies the results are discussed by using results from a linear shallow water model in which similar features are found

coupled atmosphere mixed layer ocean response to ocean heat flux convergence along the kuroshio current extension the winter response of the coupled atmosphere ocean mixed layer system to anomalous geostrophic ocean heat flux convergence in the kuroshio extension is investigated by means of experiments with an atmospheric general circulation model coupled to an entraining ocean mixed layer model in the extra tropics the direct response consists of positive sst anomalies along the kuroshio extension and a baroclinic low level trough and upper level ridge circulation anomaly over the north pacific the low level component of this atmospheric circulation response is weaker in the case without coupling to an extratropical ocean mixed layer especially in late winter the inclusion of an interactive mixed layer in the tropics modifies the direct coupled atmospheric response due to a northward displacement of the pacific inter tropical convergence zone which drives an equivalent barotropic anomalous ridge over the north pacific although the tropically driven component of the north pacific atmospheric circulation response is comparable to the direct response in terms of sea level pressure amplitude it is less important in terms of wind stress curl amplitude due to the mitigating effect of the relatively broad spatial scale of the tropically forced atmospheric teleconnection

the mean and the time variability of the shallow meridional overturning circulation in the tropical south pacific ocean the meridional transport in the pacific ocean subtropical cell is studied for the period from 2004 to 2011 using gridded argo temperature and salinity profiles and atmospheric reanalysis surface winds the poleward ekman and equatorward geostrophic branches of the subtropical cell exhibit an el ni no southern oscillation signature with strong meridional transport occurring during la ni and weak meridional transport during el ni no at 7 58s mean basinwide geostrophictransportabove 1000 dbar is 48 56 2 5 sv sv 10 6 m 3 s 21 of which 30 3 38 4 sv return to the subtropics in the surface ekman layer whereas 10 2 18 3 sv flow northward feeding the indonesian throughflow geostrophic transport within the subtropical cell is stronger in the ocean interiorandweakerinthewesternboundaryduringlani theinteriordominatingbasinwide transport using atmospheric reanalyses only half of the mean heat gain by the pacific north of 7 58 si s compensated by oceanicheat transport outof theregion thenationaloceanography centreat southampton air sea flux climatology is more consistent for closing the oceanic heat budget in summary the use of argo data for studying the pacific subtropical cell provides an improved estimate of basinwide mean geostrophic transport includes both interior and western boundary contributions quantifies el ni ni transport variability and illustrates how the meridional overturning cell dominates ocean heat transport at 7 58s

on the pathways of the equatorial subsurface currents in the eastern equatorial pacific and their contributions to the peru chile undercurrent 1 theconnections between theequatorial currentsystem andtheperu currentsystem in the eastern tropical pacific etp are examined with a primitive equations eddy resolving regional model the quasi equilibrium solutions reproduce three eastward equatorial subsurface currents of interest the equatorial undercurrent euc located between 1 n and 1 s the primary southern subsurface countercurrent psscc between 3 and 4 s and farther south the secondary southern subsurface countercurrent ssscc between 7 and 8 s using a lagrangian tracking procedure the fate of these currents in the etp and their contribution to the peru chile undercurrent pcuc are studied lagrangian diagnostics show that for the most part the euc water contributes to westward flows including the south equatorial current and deeper flows below it and strikingly only a very little fraction feeds the pcuc while a significant part of both ssccs contribute substantially mesoscale eddies are shown to exert an effect on these connections in addition about 30 of the pcuc is fed by the three subsurface equatorial flows euc psscc ssscc the remaining part of the pcuc comes from an alongshore recirculation associated with flows below it and from the southern part of the domain south of 9 s