transport and entrainment in the red sea outflow plume the red sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing as it descends the continental slope in the western gulf of aden it entrains significantly less dense near surface water which itself varies on seasonal time scales high resolution hydrographic and direct velocity data collected during the 2001 red sea outflow experiment redsox are used herein to characterize and quantify the pathways of the red sea outflow water rsow and the associated entrainment of gulf of aden water the outflow transport exhibits a maximum in winter of about 0 29 sv sv 10 6 m 3 s 1 at the exit of the bab el mandeb and approximately doubles to 0 56 sv as it descends into the gulf of aden and entrains ambient water in summer the outflow is much weaker reaching about 0 06 sv at the strait and about 0 18 sv downstream the outflow plume divides into three distinct branches in winter consisting of descending branches along two bathymetrically confined channels the northern and southern channels respectively and an adjusted intrusion layer at shallower depths in the water column estimates of transport of pure red sea outflow water through salt flux conservation show the general partitioning of the outflow between the individual plumes where the northern channel nc accounts for 52 of red sea outflow water the southern channel sc carries 31 and the intrusion layer il the remaining 17 the results also indicate that the transport of red sea outflow water is subject to considerable synoptic temporal variability that is unresolved by the present study

formation and spreading of red sea outflow water in the red sea hydrographic data chlorofluorocarbon 12 cfc 12 and sulfur hexafluoride sf6 measurements collected in march 2010 and september october 2011 in the red sea as well as an idealized numerical experiment are used to study the formation and spreading of red sea outflow water rsow in the red sea analysis of inert tracers potential vorticity distributions and model results confirm that rsow is formed through mixed layer deepening caused by sea surface buoyancy loss in winter in the northern red sea and reveal more details on rsow spreading rates pathways and vertical structure the southward spreading of rsow after its formation is identified as a layer with minimum potential vorticity and maximum cfc 12 and sf6 ventilation ages of seawater within the rsow layer calculated from the partial pressure of sf6 psf6 range from 2 years in the northern red sea to 15 years at 17 n the distribution of the tracer ages is in agreement with the model circulation field which shows a rapid transport of rsow from its formation region to the southern red sea where there are longer circulation pathways and hence longer residence time due to basin wide eddies the mean residence time of rsow within the red sea estimated from the psf6 age is 4 7 years this time scale is very close to the mean transit time 4 8 years for particles from the rsow formation region to reach the exit at the strait of bab el mandeb in the numerical experiment

observations on the characteristics of tides in the bab el mandab strait 1 while well known for its multilayered exchange flow the bab el mandab strait is also of interest as a transitional region between two distinctly different tidal regimes the gulf of aden where tides are mixed and have a range in excess of 2 m and the red sea where tidal fluctuations are principally semidiurnal and their range is generally less than 1 m observations collected in the strait indicate that dominant constituents for both elevations and currents are the k1 and m2 the vertical structure of the tidal currents differs between semidiurnal and diurnal constituents and depends on the location and stratification the major part of the tidal signal is barotropic but energetic baroclinic currents mainly of the diurnal period are observed near perim narrows and the hanish sill during winter months the relationship between the barotropic currents and surface tides near perim is fairly well explained by a simple momentum balance to first order a balance exists between local acceleration and the elevation gradient in the along strait direction while in the cross strait direction a balance between local acceleration coriolis term and elevation gradient is observed at the same location the barotropic transport fluctuations reach amplitudes of 1 6 sv the k1 and m2 barotropic energy fluxes are small and are directed into the strait at perim narrows and the hanish sill for the m2 constituent and into the strait near perim and out of the strait near hanish for the k1 constituent

exchange through the bab el mandab during the last 10 years there have been a number of important developments in the understanding of the exchange flow in the bab el mandab new and improved estimates of the fluxes of mass heat and salt through the strait have been made directly from oceanographic measurements and indirectly from air sea flux data hydraulic theory has been applied to explain the seasonal variations in the exchange flow and in particular the three layer summer circulation the effects of wind stress friction and seasonal sea level variability on hydraulic control are addressed in this paper analyses of oceanographic data have failed to provide direct evidence of hydraulic control but they have highlighted the importance of tidal scale variability and differences between real continuously stratified fluids and idealised layer flows often used in hydraulic theory other studies have investigated the coupling between the flow in the strait and the circulation within the red sea

an oceanographic nowcast forecast system for the red sea we describe the application of a nowcast forecast system for three dimensional currents temperature and salinity to the red sea the modeling system is constructed around a high resolution 6 7 km primitive equation numerical circulation model with complete thermodynamics and an imbedded turbulence closure submodel it is coupled to near real time databases containing meteorological forecasts and remotely sensed and in situ temperature and salinity data the temperature and salinity data are ingested into the model daily using a nudged objective analysis technique because the red sea is a relatively narrow basin bounded by typically high and complex orography a single layer atmospheric boundary layer submodel has been used to increase the effective resolution of the original 40 km resolution meteorological fields by taking into account orographic steering of the low level winds in order to validate the modeling system two very complete hydrographic surveys of the red sea were undertaken and their results are described both the surveys and the modeling system nowcasts demonstrate that the circulation pattern of the red sea is variable and often composed of a series of eddies or subgyres mainly anticyclones immediately before one survey the winds tended to be along axis while just before the second they tended to be cross axis the red sea circulation was much more eddy like when the winds were cross axis by forcing the modeling system with and without orographically steered winds we were able to establish that the complex eddy structure is a response to a wind field which can be highly structured and vorticity rich because of interaction with the high and variable adjacent orography when the winds are more along axis there is less interaction with the adjacent orography and consequently there are fewer eddies in the red sea

direct observations of seasonal exchange through the bab el mandab strait the exchange flow between the red sea and the gulf of aden indian ocean through the bab el mandab strait was measured continuously for 10 months june 1995 march 1996 adcp and temperature salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the gulf of aden and the high salinity outflow layer from the red sea the timing structure and evolution of the summer season mid depth intrusion of cold low salinity water into the red sea from the gulf of aden is measured for the complete intrusion cycle of 1995 we unexpectedly find the deep outflow still strong in june 1995 with speeds of 0 6 m sec and transport of 0 4 sv 1 sv 106 m sec from july to mid september the deep outflow persists but is attenuated to speeds of 0 2 m sec and transport of 0 05 sv the dominant summer feature the cold low salinity intermediate layer intrusion persists for 3 months occupies 70 of the water column in the strait and carries approximately 1 7 1012 m of cold nutrient rich water into the red sea the winter regime begins in mid september is fully developed by early november and continues to the end of our first observation interval in march 1996 speeds in the lower layer are 0 8 1 0 m sec and 0 4 0 6 m sec in the upper layer at maximum exchange in mid february outflow transport reaches 0 7 sv ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along channel wind forcing and perhaps to coastally trapped waves

low frequency variability of the exchanged flows through the strait of gibraltar during canigo time series of the exchanged flows through the strait of gibraltar at the eastern section have been estimated from current meter observations taken between october 1995 and may 1998 within the canary islands azores gibraltar observations canigo project the inflow exhibits a clear annual signal that peaks in late summer simultaneously with a deepening of the interface the cycle seems to be driven by the seasonal signal of the density contrast between the surface atlantic water that forms the inflow and the deep mediterranean water of the outflow the outflow and the depth of the interface have predominant semiannual signals and a smaller annual one whose phase agrees with that of the density contrast as well local wind stress and atmospheric pressure difference between the atlantic and the western mediterranean to less extent have clear semiannual signal so that the possibility that the semiannual cycle of the outflow and of the depth of the interface are forced by them was analyzed the composite froude number in this section is well below the critical value suggesting submaximal exchange therefore the conditions in the alboran basin influence the exchange and some evidence that the size and location of the western alboran gyre contribute to the observed signals both annual and semiannual is provided r 2002 elsevier science ltd all rights reserved

hydraulic adjustments of the bosphorus exchange flow 1 a three dimensional model elucidates connection between observed internal hydraulic characteristics of the bosphorus flow and channel configuration a typical two layer quasi steady exchange flow system is shown to be hydraulically adjusted within the strait by a series of morphological features three successive hydraulic controls occur within the southern 10 km zone first near the southern exit due to convex bending of the channel then at the southern sill and at the constriction finally the exchange flow system experiences another hydraulic control at the northern sill near the black sea entrance of the strait the upper and lower layer flows exiting from the strait at both ends with currents of 1 0 m s 1 layer depths of 10 m and g 0 1 ms 2 thus impose maximal exchange conditions

distribution of crenarchaeota tetraether membrane lipids in surface sediments from the red sea abstract the red sea represents an extreme marine environment with high salinity high temperature and low level of nutrients complicating the application of standard geochemical palaeotemperature proxies in order to investigate the applicability of the tex 86 tetraether index of gdgts with 86 carbons proxy for sea surface temperature sst in the red sea the distribution of glycerol dialkyl glycerol tetraether membrane lipids gdgts in sediments from the red sea and the gulf of aden were examined against expectations tex 86 values for the red sea do not show a simple linear relationship with sst and deviate from the global core top calibration in the northern red sea at temperatures between 25 and 28 c the values increase linearly with sst whereas in the southern red sea at temperatures above 28 c tex 86 decreases with increasing temperature factors like seasonality and depth of production salinity and nutrient availability as well as diagenetic overprint or influence of allochtonous terrestrial lipids cannot explain this pattern however the observed tex 86 relationship with sst could be explained by the presence of a hypothetical endemic crenarchaeota population in the red sea with a specific tex 86 vs sst relationship in the southern red sea a two component mixing model implies an exponential decrease in the endemic population towards the gulf of aden thus the application of the tex 86 as a palaeotemperature proxy in the red sea is likely only possible for the northern red sea area with the specific sst vs tex 86 relationship determined in this study and potentially for the whole red sea basin during glacials when water exchange with the indian ocean was more restricted than today and the endemic archaeal lipid distribution was not affected by transport from the indian ocean our results suggest that distinct populations of crenarchaeota in extreme environments such as evaporitic basins may have different membrane composition necessitating application of another calibration than the global core top calibration for tex 86 palaeothermometry

late quaternary glacial interglacial cyclicity models of the red sea the benthic foraminiferal faunas from samples of two piston cores retrieved along a north south transect in the red sea were studied the northern core was collected during meteor cruise m 31 2 while the southern one was collected during the sonne cruise 121 benthic foraminiferal faunas from both sites exhibit large variability with respect to density diversity species composition and assemblages combined with stable oxygen and carbon isotope records of planktic and benthic foraminifera one hundered thirty benthic foraminiferal species were identified in the investigated cores the faunal data set of the northern core was reduced to five assemblages factors while the southern one was reduced to four assemblages all assemblages were ranked according to their ecological significance besides relative abundance of major benthic foraminiferal suborders textulariina agglutinating foraminifera miliolina and rotaliina in addition to infaunal epifaunal relative abundance were used as paleoenvironmental proxies allowing the reconstruction of past changes in deep water salinity ventilation and organic carbon fluxes at the sea floor four distinct glacial interglacial cycle during the past 380 kyr have been recognized in the red sea the identified four cycles reveal deviation in deep sea ecosystem between the northern and southern red sea in the northern red sea salinity fluctuations productivity and deep water ventilation and formation had the major impact on benthic foraminiferal pattern corresponding to glacial interglacial cycles and glacio eustatic sea level changes coupled with the impact of mediterranean climate regime on the otherhand in the southern red sea region the oscillation trend of benthic foraminiferal pattern within the glacials and interglacials stages indicates a high frequency environmental alternation this alternation is consistent with the extent of ne monsoonal wind that controls the intensity and extension of the productivity which in turn determine organic matter fluxes and oxygen level at the sea floor

spreading of red sea overflow waters in the indian ocean as a result of its remarkably high salinity and despite its small volume input remnants of red sea water rsw have been identified in the agulhas current over 6000 km distant of their source this provided the motivation to investigate the long term mean spreading of rsw throughout the indian ocean using a comprehensive set of observations taken from the national oceanographic data center archives and from the world ocean circulation experiment hydrographic program for the indian ocean after emerging from the gulf of aden into the arabian sea rsw spreads predominantly southwestward along the african continental slope as indicated by strongly inclined isohalines across the arabian sea there is some monsoon variability so that during the winter monsoon there is more rsw present in the gulf of aden and an intensification of southward spreading along the western boundary between 12 and 5 n elsewhere the intermediate depth salinity field of the indian ocean appears relatively stationary between 5 and 10 s in the region of the south equatorial current isohalines of the rsw layer become quasi zonal across the width of the indian ocean only dipping southward toward the western boundary west of 50 e south of here there is a strong tongue of rsw spreading southward through the mozambique channel and into the agulhas current these conclusions concur with previous localized investigations of intermediate water properties using a simple mixing model the percentage of rsw throughout the indian ocean was quantified it was found that the flux of salt into the gulf of aden from the red sea is similar to that estimated to cross 32 s in the agulhas current this result implies that all the rsw which is mixed into the interior of the indian ocean may eventually be exported at the western boundary furthermore it implies that rsw is the dominant component of the salt budget for the intermediate layer and that input from the indonesian seas and via diapycnic processes are small

renewal of deep water in the red sea during 1982 1987 between october 1982 and may 1983 the deep water in the northern red sea was ventilated through slope convection the deep water became cooler fresher and more oxygenated dense bottom water formed in the gulf of suez during winter cooling sank down the continental slope thereby entraining near surface and intermediate depth waters from the application of a one dimensional plume model a vertical transport of 0 58 106 m3 s 1 over a period of 7 months was estimated long term time series of atmospheric heat fluxes show that such a major convection event can occur every 4 7 years the corresponding renewal time of the red sea deep water is thus of the order of 40 90 years in agreement with previous estimates

the forcing of low frequency motions within bass strait abstract three months of coincident sea level and current meter data collected during the autumn of 1984 is analyzed so as to determine the relative importance of local wind stress and coastal trapped waves ctws incident from the great australian bight in driving low frequency motions within bass strait the analysis is facilitated by the development of analytical models for kelvin waves within the strait that are forced by these agents and which allow for backscattering by the adjacent shelves such backscattering arises from the requirement that the kelvin wave pressure field within the strait match that of the ctws that are generated on the adjacent shelves two predictions that sea level variability vanish at the southeast strait corner and be dominated by incident ctw energy at the northwest strait corner are substantiated both directly by the data and indirectly through a fit of the forced kelvin wave models for the period of analysis the data and fitted models also show that at relatively hig

millennial scale variability in red sea circulation in response to holocene insolation forcing in order to assess how insolation driven climate change superimposed on sea level rise and millennial events influenced the red sea during the holocene we present new paleoceanographic records from two sediment cores to develop a comprehensive reconstruction of holocene circulation dynamics in the basin we show that the recovery of the planktonic foraminiferal fauna after the younger dryas was completed earlier in the northern than in the central red sea implying significant changes in the hydrological balance of the northern red sea region during the deglaciation in the early part of the holocene the environment of the red sea closely followed the development of the indian summer monsoon and was dominated by a circulation mode similar to the current summer circulation with low productivity throughout the central and northern red sea the climatic signal during the late holocene is dominated by a faunal transient event centered around 2 4 ka bp its timing corresponds to that of north atlantic bond event 2 and to a widespread regionally recorded dry period this faunal transient is characterized by a more productive foraminiferal fauna and can be explained by an intensification of the winter circulation mode and high evaporation the modern distribution pattern of planktonic foraminifera reflecting the prevailing circulation system was established after 1 7 ka bp

a late pleistocene tephra layer in the southern great basin and colorado plateau derived from mono craters california abstract a newly identified tephra in stratified deposits in southwestern utah dated 14 000 14c yr b p may aid in correlating late pleistocene deposits across parts of the southern great basin and west central colorado plateau geochemical analyses of the ash suggest the tephra originated from mono craters california and most probably correlates with wilson creek ash 3 because the ash is 2 mm thick 550 km from its source the event may have been larger than others correlated to mono craters eruptions

unsteady two layer hydraulic exchange flows with friction two layer exchange flow through a contraction with both friction and barotropic forcing is modelled in terms of three parameters reflecting the friction and the strength and period of the barotropic forcing in the appropriate limits the results for steady flow with and without friction and inviscid barotropically forced flow are recovered the predicted time dependent interface position compares well with laboratory experiments improving on the inviscid formulation the concurrent effects of friction and barotropic forcing on average exchange flow rate are determined when friction is weak barotropic forcing increases the exchange rate however when friction is high tidal forcing can result in a reduced exchange rate a phenomena that we call tidal inhibition when friction is weak maximal exchange occurs throughout the tidal cycle but as friction is increased submaximal flow develops for longer and longer periods as friction is increased even further the flow becomes hydraulically uncontrolled the parameter range for major sea straits includes tidally enhanced and tidally inhibited flows as well as maximal submaximal and uncontrolled flows

modes of subsurface intermediate and deep water renewal in the red sea a linear box model of the red sea is presented the model which inverts hydrographic and 3he data for the main features of the width integrated circulation is able to describe a large part of the seasonal and annual mean circulation by combining and reconciling major elements of current theories of the red sea circulation available flow measurements and tracer data the model provides a unified conceptual framework for the understanding of the circulation of the red sea the model forms deep water in the extreme north in two modes as first suggested by cember 1988 the convective mode renews the bottom water at an annual mean rate of 0 04 sv in the winter the isopycnal mode water renews the uppermost part of the deep water mass it flows to the south along the bottom of the pycnocline at an annual mean rate of 0 02 0 04 sv a new feature in the model s near surface circulation scheme is the ventilation of the thermocline in the winter at a mean rate of 0 04 sv also from the extreme north there is also a middepth return flow to the north at an annual mean rate of 0 07 sv this flow has been suggested but never rigorously quantified before

a coral oxygen isotope record from the northern red sea documenting nao enso and north pacific teleconnections on middle east climate variability since the year 1750 a 245 year coral oxygen isotope record from the northern red sea ras umm sidd egypt 28 n in bimonthly resolution is presented the mean annual coral 18o signal apparently reflects varying proportions of both sea surface temperature and 18oseawater variability in conjunction with instrumental observations of climate the coral record suggests for interannual and longer timescales that colder periods are accompanied by more arid conditions in the northern red sea but increased rainfall in the southeastern mediterranean whereas warmer periods are accompanied by decreased rainfall in the latter and less arid conditions in the northern red sea a 70 year oscillation of probably north atlantic origin dominates the coral time series interannual to interdecadal variability is correlated with instrumental indices of the north atlantic oscillation nao the el nino southern oscillation enso and north pacific climate variability the results suggest that these modes contributed consistently to middle east climate variability since at least 1750 preferentially at a period of 5 7 years

meteorologically forced subinertial flows through the strait of gibraltar the observed transport due to subinertial flows with periods ranging from days to a few months through the strait of gibraltar reaches values up to 106 m3 s 1 and is well correlated with atmospheric pressure fluctuations over the mediterranean sea this transport is barotropic and accounts for 84 of the variance observed at these frequencies a second mode accounts for 12 of the variance and is characterized by a node zero amplitude located at the mean depth of the interface between atlantic and mediterranean waters this baroclinic mode is modulated by the spring neap tidal cycle corresponding to an increased exchange of waters between atlantic and mediterranean at neap tides a simple model of the mediterranean which includes two basins and two straits indicates that the system has two resonant frequencies at periods of 1 2 and 5 6 days the 1 2 day resonance corresponds to the eastern and western basins oscillating through the strait of sicily while that at 5 6 days is the resonance of the mediterranean sea with the north atlantic ocean a good approximation to the observed flows is obtained when this model is forced with the most energetic mode of the atmospheric pressure over the eastern and the western mediterranean and a friction term is included to limit the flow through the strait of gibraltar restricting the flow through the strait of sicily does not improve the agreement between the model and the observations for the flows at gibraltar

wind induced sea level variability in the red sea analysis of topex poseidon altimetry data for the red sea shows a large seasonal sea level signal in agreement with the few available coastal sea level stations the results are compared with the predictions from a simple model that balances the sea surface elevation against the wind stress the winter sea level field in the central and northern red sea is shown to be a response to the convergent wind pattern associated with the indian monsoon consisting of nnw winds in the northern red sea and sse winds in the south there is a very good agreement between the model and observations indicating that the basic mechanism responsible for the variability of the sea surface height inside the red sea on seasonal time scales is the wind pattern

an oceanic general circulation model ogcm investigation of the red sea circulation 2 three dimensional circulation in the red sea 1 the three dimensional circulation of the red sea is studied using a set of miami isopycnic coordinate ocean model micom simulations the model performance is tested against the few available observations in the basin and shows generally good agreement with the main observed features of the circulation the main findings of this analysis include an intensification of the along axis flow toward the coasts with a transition from western intensified boundary flow in the south to eastern intensified flow in the north and a series of strong seasonal or permanent eddy like features model experiments conducted with different forcing fields wind stress forcing only surface buoyancy forcing only or both forcings combined showed that the circulation produced by the buoyancy forcing is stronger overall and dominates the wind driven part of the circulation the main circulation pattern is related to the seasonal buoyancy flux mostly due to the evaporation which causes the density to increase northward in the basin and produces a northward surface pressure gradient associated with the downward sloping of the sea surface the response of the eastern boundary to the associated mean crossbasin geostrophic current depends on the stratification and b effect in the northern part of the basin this results in an eastward intensification of the northward surface flow associated with the presence of kelvin waves while in the south the traditional westward intensification due to rossby waves takes place the most prominent gyre circulation pattern occurs in the north where a permanent cyclonic gyre is present that is involved in the formation of red sea outflow water rsow beneath the surface boundary currents are similarly intensified southward undercurrents that carry the rsow to the sill to flow out of the basin into the indian ocean index terms 4243 oceanography general marginal and semienclosed seas 4255 oceanography general numerical modeling 4532 oceanography physical general circulation keywords red sea marginal sea oceanic general circulation model water mass formation

on the sources formation and circulation of red sea deep water red sea deep water is renewed from sources in the far north of the red sea upwelling of deep water throughout the basin maintains the sharp pycnocline with its base near the depth of the sill at bab el mandeb before now deep water renewal was thought to occur mainly by means of convective deep water formation in the north which renews the bottom water and gives rise to a southward flowing bottom current carbon 14 and 3he data from the geosecs indian ocean expedition lead to the identification of an additional volumetrically larger mode of northern deep water formation in which new deep water is injected immediately beneath the pycnocline this mode of deep water formation to be called the isopycnal mode also gives rise to a southward flowing current in this case at the top of the deep water mass geosecs data also demonstrate the existence at intermediate depths in the deep water of a return flow to the north the surface history in the nuclear era of the 14c in the deep water source region was reconstructed by direct radiocarbon measurements on annual growth bands of corals calculation then shows that new deep water forms in the convective mode at a rate of 0 05 0 01 sverdrup sv expressed as if the flow were continuous and is due principally to processes occurring in the mouth of the gulf of suez the injection beneath the pycnocline of new deep water formed in the isopycnal mode is 0 11 0 02 sv this new deep water appears to originate mainly in the extreme open northern red sea the gulf of aqaba makes a modest but significant contribution to both modes the total deep water renewal rate is about 0 16 sv yielding a bulk deep water residence time of about 36 years oxygen consumption in the deep water shows approximately exponential decrease of consumption rate with depth as expected on geological and geophysical grounds hydrographically significant fluxes of heat and salt from the sea floor into the red sea deep water are discernible in the hydrography helium 3 flux from the sea floor into the deep water is about 69 1 stp year

impact of late quaternary environmental changes on deep sea benthic foraminiferal faunas of the red sea abstract fluctuations in abundance diversity and species composition of benthic foraminifera from two sites of the northern and southern red sea indicate strong variability of deep sea ecosystems during the last four glacial to interglacial cycles in total five and four different benthic foraminiferal assemblages have been identified in the northern core and southern core respectively comparison with recent faunas from the red sea and adjacent oceans allowed the reconstruction of temporal changes in deep water ventilation salinity and food availability at the seafloor generally the abundance of infaunal and miliolid taxa increase during glacial intervals indicating increased organic matter fluxes oxygen decrease and salinity increase in deep waters during these times these fluctuations are attributed to enhanced oxygen consumption rates and temporarily reduced deep water formation in the northern red sea during glacial intervals the recorded environmental changes are a reflection of both high and low latitude climate changes the northern red sea is mainly influenced by glacio eustatic sea level fluctuations that control deep water formation rates and by mid latitude climate changes of the mediterranean region that control surface productivity in contrast deep sea ecosystem variability of the southern red sea is additionally influenced by low latitude climate changes attributed to the ne monsoon intensity that drives the inflow of nutrient rich surface waters from the gulf of aden these results demonstrate the high sensitivity of deep sea ecosystems of the red sea to both global and regional climate changes

sensitivity of red sea circulation to monsoonal variability during the holocene an integrated data and modeling study received 20 october 2009 revised 6 march 2010 accepted 30 june 2010 published 4 november 2010 1 we used an oceanic general circulation model to evaluate the sensitivity of the hydrography and circulation of the red sea in response to reduced sea level and modified atmospheric conditions during the holocene with holocene sea level close to the modern level the red sea was sensitive to changes in atmospheric conditions and it only shows a relatively mild response to sea level change changes in the monsoon system influence the exchange flow through the strait of bab el mandab the meridional overturning circulation of the red sea and its hydrography forced by humid conditions the modeled red sea temperature increased by 1 5 c while when arid conditions were imposed the temperature decreased by 2 5 c similar heating and cooling events during the early and late holocene are seen in a sea surface temperature record from the northern red sea derived from the temperature sensitive tex86 molecular biomarker which suggests that humid conditions prevailed during the early holocene and more arid conditions prevailed during the late holocene the gradual decline in red sea temperature between these two time periods suggests a gradual decline in the summer monsoon strength this monsoon trend and the resulting changes in the red sea circulation are supported by the distribution of crenarchaea fossil lipids in red sea sediments from this period monsoon driven changes in the exchange flow through the strait of bab el mandab affected the crenarchaea population structure and therefore their molecular fossil distribution in the sediments of the red sea potentially provides an index for the summer monsoon strength during the holocene

red sea circulation during marine isotope stage 5e we have employed a regional massachusetts institute of technology oceanic general circulation model of the red sea to investigate its circulation during marine isotope stage mis 5e the peak of the last interglacial approximately 125 ka before present compared to present day conditions mis 5e was characterized by higher northern hemisphere summer insolation accompanied by increases in air temperature of more than 2 c and global sea level approximately 8 m higher than today as a consequence of the increased seasonality intensified monsoonal conditions with increased winds rainfall and humidity in the red sea region are evident in speleothem records and supported by model simulations to assess the dominant factors responsible for the observed changes we conducted several sensitivity experiments in which the mis 5 boundary conditions or forcing parameters were used individually overall our model simulation for the last interglacial maximum reconstructs a red sea that is colder less ventilated and probably more oligotrophic than at present day the largest alteration in red sea circulation and properties was found for the simulation of the northward displacement and intensification of the african tropical rain belt during mis 5e leading to a notable increase in the fresh water flux into the red sea such an increase significantly reduced the red sea salinity and exchange volume of the red sea with the gulf of aden the red sea reacted to the mis 5e insolation forcing by the expected increase in seasonal sea surface temperature amplitude and overall cooling caused by lower temperatures during deep water formation in winter

ventilation of red sea water with respect to chlorofluorocarbons chlorofluorocarbon cfc measurements were made in the northwestern indian ocean in august september 1995 as part of the world ocean circulation experiment hydrographic program section i1 the data in the gulf of aden section from this expedition provide information on the mixing processes that affect red sea water rsw as it flows from its source into the open northwestern indian ocean combining measurements by rhein et al 1997 with the outcome from a two end member mixing model results in an estimate of 65 70 saturation for the cfcs in the rsw end member at bab el mandeb a boundary condition that may be more applicable to ocean models is given by the cfc content of rsw after its initial diapycnal descent along the continental slope in the western gulf of aden entrainment of water containing lower amounts of cfcs produces rsw that has cfc saturations of 20 50 once rsw has settled at its depth of neutral buoyancy apparent pcfc ages at bab el mandeb and after the separation of rsw from the bottom are 12 13 and 18 27 years respectively as rsw continues eastward through the gulf of aden as an isopycnal flow horizontal eddy diffusion plays an important role in further diluting rsw which is characterized by a maximum in salinity and a positive anomaly in cfc concentrations horizontal eddy diffusion parameters estimated from tracer conservation in a box model of the central and eastern gulf of aden range from 0 2 104 to 2 0 104 m2 s 1 as other observations show eddy diffusion is presumably enhanced in this part of the ocean

buoyancy driven mean flow in a long channel with a hydraulically constrained exit condition convection plays a major role in a variety of natural hydrodynamic systems those in which convection drives exchange flows through a lateral contraction and or over a sill form a special class with typical examples being the red and mediterranean seas the persian gulf and the fjords that indent many coastlines the present work focuses on the spatial distribution and scaling of the density dierence between the inflowing and outflowing fluid layers using a long water lled channel tted with buoyancy sources at its upper surface experiments were conducted to investigate the influence of the geometry of the strait and the channel as well as the magnitude of the buoyancy flux two dierent scaling laws one by phillips 1966 and one by maxworthy 1994 1997 were compared with the experimental results it has been shown that a scaling law for which g 0 kb 2 3 0 x h 4 3 best describes the distribution of the observed density dierence along the channel where b0 is the buoyancy flux x the distance from the closed end of the channel h its height at the open end sill and k a constant that depends on the details of the channel geometry and flow conditions this result holds for the experimental results and appears to be valid for a number of natural systems as well

modelling of seasonal exchange flows through the dardanelles strait abstract the dardanelles strait is a remarkable example of a long narrow shallow and strongly stratified strait with bidirectional exchange that is governed by both baroclinic and barotropic forcing with a wide spectrum of variability a three dimensional free surface primitive equation model is applied to study seasonal hydrodynamics variability in this strait the calculated vertical structure of temperature salinity and velocity fields agrees well with available survey data seasonal monthly values of the volume exchange at the aegean and marmara exits are estimated it is found that the seasonal exchange dynamics is governed by the turbulent friction and entrainment at the nara passage area the mean annual water transport in the upper layer is increased by 80 after the nara passage about 25 of water entering in the dardanelles bottom layer reaches the marmara sea in winter and 50 reaches it in summer the estimate of the dardanelles hydrodynamics according to hydraulic and viscous advective diffusive regime classification shows significant deviation from the two layer hydraulic asymptotic however according to three layer hydraulic theory the flow is found to be critical in the nara passage area

hydraulic control of sill flow with bottom friction abstract the hydraulics of strait and sill flow with friction is examined using a reduced gravity model it is shown that friction moves the critical or control point from the sill to a location downstream if the strait has constant width the control point lies where the bottom slope is the negative of the drag coefficient cd if cd exceeds the bottom slope everywhere the flow cannot be controlled in the classical sense that energy and flow force are minimized friction also decreases the minimum obstacle height required to establish hydraulically controlled flow in the classical laboratory towing experiment also friction greatly encourages the establishment of stationary hydraulic jumps in the lee of the sill and under certain conditions gives rise to stationary jumps on the upstream face of the obstacle some consequences of these results for deep ocean overflows are given using the iceland faroe overflow as an example

seasonal overturning circulation in the red sea 1 model validation and summer circulation the overturning circulation in the red sea exhibits a distinct seasonally reversing pattern and is studied using high resolution mit general circulation model simulations in the first part of this study the vertical and horizontal structure of the summer overturning circulation and its dynamical mechanisms are presented from the model results the seasonal water exchange in the strait of bab el mandeb is successfully simulated and the structures of the intruding subsurface gulf of aden intermediate water are in good agreement with summer observations in 2011 the model results suggest that the summer overturning circulation is driven by the combined effect of the shoaling of the thermocline in the gulf of aden resulting from remote winds in the arabian sea and an upward surface slope from the red sea to the gulf of aden set up by local surface winds in the red sea in addition during late summer two processes associated respectively with latitudinally differential heating and increased salinity in the southern red sea act together to cause the reversal of the contrast of the vertical density structure and the cessation of the summer overturning circulation dynamically the subsurface northward pressure gradient force is mainly balanced by vertical viscosity resulting from the vertical shear and boundary friction in the strait of bab el mandeb unlike some previous studies the three layer summer exchange flows in the strait of bab el mandeb do not appear to be hydraulically controlled

radiometric time scale for the upper eocene and oligocene based on k ar and rb sr dating of volcanic biotites from the pelagic sequence of gubbio italy several samples of unaltered volcanic biotite were recovered from five biostratigraphically and magnetostratigaphically controlled bentonite horizons in the continuous upper eocene oligocene pelagic carbonate sequence of gubbio northern italy k ar and rb sr dates on these samples offer a unique opportunity to calibrate with closely spaced points the late paleogene biostratigraphic and magnetostratigraphic time scales until now time scales have been constructed by interpolating between less certain and more widely spaced data points our combined k ar and rb sr dates are here used to calibrate the following five stratigraphic levels from top to bottom level 5 upper part of globorotalia opima opima planktonic foraminiferal zone upper part of indistinct np23 np24 nannoplankton zone and cp18 nannoplankton zone bottom of magnetic anomaly 9n 28 0 0 3 ma level 4 globigerina ampliapertura and cassigerinella chipolensis zone lower part of np23 zone and bottom of cp18 zone upper part of anomaly 12r 32 0 0 8 ma level 3 pseudohastigerina micra zone upper part of np21 and cp16b zones top of anomaly 13n 35 4 0 4 ma level 2 upper part of globigerinatheka semiinvoluta zone upper part of np18 and cp15a zones top of magnetic anomaly 16n 36 0 0 4 ma level 1 lower part of g semiinvoluta zone between zones np18 and np19 middle part of zone cp15a upper part of anomaly 17n 36 4 0 3 ma from these data the interpolated age for the much debated eocene oligocene boundary is 35 7 ma with a 2 standard deviation uncertainty of 0 4 ma

the role of the interface in exchange through the strait of gibraltar five cross strait hydrographic sections repeated several times during the gibraltar experiment in 1985 1986 are used to examine the structure of the interface layer between the inflowing atlantic waters and outflowing mediterranean waters in the strait of gibraltar the interface is 60 100 m thick with a strong vertical salinity gradient identified by fitting individual salinity profiles to a piecewise linear three layer model the interface is deeper thicker fresher and colder on the west end of the strait than in the narrows where there is a minimum in thickness and a maximum in salinity gradient farther east the interface thickens again and continues to get saltier warmer and shallower property variations in all three layers are also cast in terms of the three principal water types involved in the exchange the traditional knudsen model of exchange is extended to three layers assuming that the interface is a transport carrying third layer with uniform vertical shear as much as half of the inflowing or outflowing transport occurs in the interface layer transport converges in both the upper and lower layers implying over the length of the strait vertical exchange between layers that is comparable to about half the horizontal exchange the richness of structure and complexity of interaction between the interface and the upper and lower layers argues against the use of two layer models to characterize exchange through the strait of gibraltar

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

40ar 39ar dating of a langhian biotite rich clay layer in the pelagic sequence of the conero riviera ancona italy a nearly complete and undisturbed miocene carbonate sequence is present in the easternmost part of the umbria marche basin italy which is ideal for detailed and integrated stratigraphic investigations of the miocene epoch in this study we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 ma ries impact structure in southern germany located about 600 km to the north northwest of the umbria marche basin the first step is to find coeval strata in the umbria marche sequence at the la vedova section conero riviera we dated a volcaniclastic biotite rich clay layer the aldo level which is situated within planktonic foraminiferal zone n8 at 14 9 0 2 ma using the 40ar 39ar method together with detailed geologic and stratigraphic information about the aldo level the resulting age can be used confidentially to calibrate the langhian stage besides providing new constraints on miocene geochronology this age can now be used for impact stratigraphic studies to directly correlate the biotite ages of the la vedova section with rocks from the ries impact event ries impact glass was also analyzed and found to be coeval although unrelated to this impact event the biotite rich clay layer should help in the search for evidence of distal ejecta related to the ries crater

exchange through the strait of gibraltar to measure the exchange between the atlantic and mediterranean through the strait of gibraltar an array of current meter moorings was deployed for a year in the strait during 1985 1986 a novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously these salinity measurements determine the water mass characteristics of the flows crossing the sill they allow definition of the 37 psu isohaline as the interface between inflowing fresher atlantic water and outflowing saltier mediterranean water and they enable time series to be developed for the depth of this interface for the upper layer inflow and for the lower layer outflow from these measurements the time averaged outflow of mediterranean water is estimated to be 0 68sv 1sv 1 106m3s 1 and the outflow salinity transport defined to be the outflow times the salinity excess above a basic atlantic water salinity of 36 1 psu is estimated to be 1 50 103m3s 1 1sv 1psu 1 103m3s 1 equivalent to a net evaporation over the mediterranean basin of 52cm y 1 extrapolated measurements of the inflow from current meters generally deployed below 100m depth yield an estimate for the time averaged inflow of 0 93sv which is believed to be unrealistically high in view of the better measured outflow and net evaporation thus a more realistic estimate of the inflow is 0 72sv equal to the sum of the outflow and net evaporation as required by the mass budget for the mediterranean sea such estimates of the exchange are smaller by almost a factor of 2 than previous values for the exchange by lacombe and richez 1982 the exchange across the gibraltar sill is found to be due in nearly equal parts to the mean currents and to the tidal fluctuations the mean currents are smaller than had been expected reaching a peak value of only about 60 cm s 1 in the deep outflow over the sill the tidal exchange is due to a strong correlation over the tidal period between the depth of the interface and the strength of the inflowing currents for the m2 tide at the sill the amplitude of the interface depth is 51 m and the amplitude of the tidal currents is 1 2m s 1 furthermore the inflow and interface depth have similar phases as a consequence the upper layer is deep on the inflowing tide so that a large slug of atlantic water crosses the sill into the mediterranean on the outflowing tide the interface is shallow so that a large slug of mediterranean water crosses the sill into the atlantic similar processes occur for the s2 o1 and k1 tides though the amplitude are smaller in this manner tidal oscillations lead to a time averaged exchange of water masses across the gibraltar sill the inflow and outflow defined to be the instantaneous transports above and below the 37psu isohaline interface exhibit m2 tidal amplitudes of 2 3 sv and 1 3sv respectively thus the tides are large enough to reverse the mean upper layer inflow and lower layer outflow daily averaged inflow and outflow transports exhibit low frequency fluctuations with standard deviations of 0 37sv and 0 22sv respectively such low frequency fluctuations have been shown previously to be associated with barotropic flows through the strait of gibraltar compensating for sea level variations over the mediterranean due to atmospheric pressure fluctuations candela winant and bryden 1989 finally from these measurements there appear to be little fortnightly or annual period fluctuations in the exchange through the strait of gibraltar

on the crossover of boundary currents in an idealized model of the red sea the west to east crossover ofboundarycurrents has been seen in mean circulationschemes fromseveral past models of the red sea this study investigates the mechanisms that produce and control the crossover in an idealized eddy resolving numerical model of the red sea the authors also review the observational evidence and derive an analytical estimate for the crossover latitude the surface buoyancy loss increases northward in the idealized model and the resultant mean circulation consists of an anticyclonic gyre in the south and a cyclonicgyreinthenorth in themidbasin thenorthwardsurfaceflow crossesfromthewestern boundarytothe eastern boundary numerical experiments with different parameters indicate that the crossover latitude of the boundary currents changes with f0 b and the meridional gradient of surface buoyancy forcing in the analytical estimate which is based on quasigeostrophic b plane dynamics the crossover is predicted to lie at the latitude where the net potential vorticity advection including an eddy component is zero various terms in the potential vorticity budget can be estimated using a buoyancy budget a thermal wind balance and a parameterization of baroclinic instability

character and dynamics of the red sea and persian gulf outflows historical hydrographic data and a numerical plume model are used to investigate the initial transformation dynamics and spreading pathways of red sea and persian gulf outflow waters where they enter the indian ocean the annual mean transport of these outflows is relatively small 0 4 sv but they have a major impact on the hydrographic properties of the indian ocean at the thermocline level because of their high salinity they are different from other outflows in that they flow over very shallow sills depth 200 m into a highly stratified upper ocean environment and they are located at relatively low latitudes 12 n and 26 n furthermore the red sea outflow exhibits strong seasonal variability in transport the four main results of this study are as follows first on the basis of observed temperature salinity t s characteristics of the outflow source and product waters we estimate that the red sea and persian gulf outflows are diluted by factors of 2 5 and 4 respectively as they descend from sill depth to their depth of neutral buoyancy the high dilution factor for the persian gulf outflow results from the combined effects of large initial density difference between the outflow source water and oceanic water and low outflow transport second the combination of low latitude and low outflow transport and associated low outflow thickness results in ekman numbers for both outflows that are o 1 this indicates that they should be thought of as frictional density currents modified by rotation rather than geostrophic density currents modified by friction third different mixing histories along the two channels that direct red sea outflow water into the open ocean result in product waters with significantly different densities which probably contributes to the multilayered structure of the red sea product waters in both outflows seasonal variations in source water and oceanic properties have some effect on the t s of the product waters but they have only a minor impact on equilibrium depth fourth product waters from both outflows are advected away from the sill region in narrow boundary currents at least during part of the year at other times the product water appears more in isolated patches

submaximal exchange between a convectively forced basin and a large reservoir if a sill enclosed basin connected to a large reservoir is suddenly subjected to a de stabilizing surface buoyancy flux it will first mix vertically by turbulent convection before the resulting lateral buoyancy gradient generates a horizontal exchange flow across the sill we present a study which examines the unsteady adjustment of such a basin under continued steady forcing it is shown through theoretical development and laboratory experimentation that two consecutive unsteady regimes characterized by different dynamic balances are traversed as the flow approaches a steady state once established the exchange flow is controlled at the sill crest where it is hydraulically critical in the absence of a lateral contraction the single control at the sill crest allows a range of submaximal exchange states with the flow at the sill being dependent not only on the forcing and geometrical parameters but also on mixing conditions within the basin which are in turn dependent on the sill exchange the sill basin system is therefore strongly coupled although it remains isolated from the external reservoir conditions by a region of internally supercritical flow results from the laboratory experiments are used to demonstrate the link between the forcing and the exchange flow at the sill steady state measurements of the interior mean velocity and buoyancy fields are also compared with previous analytical models

mediterranean outflow pump an alternative mechanism for the lago mare and the end of the messinian salinity crisis the final stage of the messinian salinity crisis msc was characterized by brackish water lago mare conditions in the intermediate and marginal basins of the mediterranean sea the presence of paratethyan former black sea fauna in these deposits has fueled long lasting controversies over the connectivity between the mediterranean and paratethys and contemporary sea level drops in both basins here we use the results of sub precessional climate simulations to calculate the freshwater budget of the mediterranean and paratethys in the messinian we show that during the msc the freshwater budget of paratethys was positive while the mediterranean was negative using these numerical constraints we propose a mediterranean outflow pump as an alternative scenario for the two most dramatic hydrological changes in the msc 1 the halite lago mare transition and 2 the pliocene reestablishment of marine conditions following the maximum msc lowstand during halite formation progressive mediterranean sea level rise resulting from african river runoff and overspill from both the atlantic and paratethys eventually reached the level of the paratethys sill a density contrast at this gateway caused dense mediterranean waters to flow into the paratethys driving a compensatory return flow this pump mechanism significantly enhanced paratethyan inflow to the mediterranean creating suitable conditions for the lago mare fauna to migrate and thrive when the mediterranean sea level finally reached the height of the gibraltar sill mediterranean outflow restarted there and enhanced exchange with the atlantic ocean during this reorganization of the circulation brackish and hypersaline waters were pumped out of the mediterranean and open marine conditions were reestablished without major flooding of the basin at the miocene pliocene boundary

hydraulic interpretation of direct velocity measurements in the bab al mandab abstract acoustic doppler current profiler velocity measurements in the bab al mandab during the period june 1995 march 1996 are used to assess the hydraulic character of the exchange flow the strait is 150 km long and contains two distinct geometrical choke points the hanish sill and perim narrows the authors use a three layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal long gravity waves the first second mode is generally characterized by in phase out of phase motions of the two interfaces the calculations take cross strait topographic variations into consideration by using a piecewise linear representation of the actual bottom the resulting phase speeds are used to determine whether the flow is subcritical supercritical or critical with respect to the first and second modes subcritical supercritical means that the two wave pairs corresponding to a given mode propagate in opposite

the coupling between exchange flux through a strait and dynamics in a small convectively driven marginal sea the gulf of aqaba gulf of eilat 1 we investigate the coupling between the dynamics in the gulf of aqaba gulf of eilat northern red sea and the exchange flow through the straits of tiran in response to seasonally varying surface fluxes and northern red sea hydrographic conditions because the gulf is a relatively small basin winter mixing between the surface and intermediate layers occurs over most places in the gulf including in the vicinity of the straits and leads to a maximal exchange flow in the strait during the spring warming in the red sea forces an influx of warm red sea water into the gulf the surface layer is refilled and consequently the exchange flow in the straits changes from a maximal to a submaximal flow regime as a result the dense wintertime water formation in the gulf and the exchange flow through the strait are strongly coupled on seasonal time scales in addition the hydrographic conditions in the northern red sea undergo strong seasonality these characteristics prevent the direct application of current theories for strait marginal sea systems which commonly assume steady conditions in the open ocean side of the strait and or an annual mean surface flux over the marginal sea we explain why the exchange of volume and heat between the gulf of aqaba gulf of eilat and the northern red sea is larger during spring summer despite the net surface buoyancy input into the gulf and why it diminishes during fall winter despite the large buoyancy loss to the atmosphere the applicability of the results to other systems is discussed

response characteristics of a buoyancy driven sea abstract the authors consider the flow in a semienclosed sea or basin subjected to a destabilizing surface buoyancy flux and separated from a large adjoining reservoir by a sill a series of numerical experiments were conducted to quantify the energetics of the flow within the basin that is the amount of kinetic and potential energy stored within the basin and the rate at which these quantities are transported to and from the reservoir via the exchange flow over the sill the numerical experiments were formulated at laboratory scales and conducted using a boundary fitting clustered grid to resolve the entrainment and mixing processes within the flow and to facilitate quantitative comparison with previous laboratory experiments volume and boundary integrated energetics were computed for both steady and time varying flows in the steady state limit the rate of energy flux through the surface is balanced by dissipation within the basin and advection of potential energy over the sill and into the reser

red sea during the last glacial maximum implications for sea level reconstruction the red sea is connected to the indian ocean via a narrow and shallow strait and exhibits a high sensitivity to atmospheric changes and a reduced sea level we used an ocean general circulation model to investigate the hydrography and circulation in the red sea in response to reduced sea level and modified atmospheric conditions occurring during the last glacial maximum lgm the model salinity shows high sensitivity to sea level reduction together with a mild atmospheric impact sea level reduction affects the stratification and alters the circulation pattern at the strait of bab el mandab which experiences a transition from a submaximal flow to a maximal flow the best correlation to reconstructed conditions during lgm exists when the water depth of the hanish sill the shallowest part in the strait of bab el mandab is 33 10 75 m which would be affected by a sea level lowering of approximately 105 m our results support the reconstructed maximum salinity of around 57 practical salinity units because of a simple model that takes into account mixing processes along the strait and comparison of the surface salinity gradient to reconstructions based on isotopic records from sedimentary cores the salinity and 18o are sensitive to the mixing process at the strait and the sensitivity increases as the sea level is further reduced a local relative sea level reduction of approximately 105 m is also in close agreement with the inference of the lgm low stand of the sea at the location of the sill based on the ice 5g vm2 model

climatological evaporation seasonality in the northern red sea 1 the dynamic and thermodynamic processes that underlie the exceptionally high evaporation over the northern red sea are examined through a combination of data analysis and a simple numerical model we show that the key boundary layer dehumidifier is ageostrophic cross channel sea breezes this circulation develops semidiurnally in response to thermal gradients across the red sea s coasts because of disparate land ocean heat capacities during the summer day the thermally induced high pressure center over the red sea axis results in near surface flows from the red sea toward the neighboring deserts the strong divergence associated with these flows is maximized along the red sea axis and is accompanied by strong subsidence that suppresses boundary layer relative humidity by both reducing specific humidity and increasing temperatures because the summer nighttime reversed thermal gradients are smaller in magnitude the daytime circulation dominates over the nighttime in summer and thus dominates over the daily and seasonal means following similar reasoning we also devise a winter dehumidifier we conclude by advancing a simple means of estimating red sea evaporation under diverse paleoinsolation regimes and we show small but clear evaporation changes during the course of the holocene our estimates represent lower bounds and we plan to refine them in follow up work

hydraulic control of three layer exchange flows application to the bab al mandab two layer hydraulic models have proven to be extremely useful in understanding a number of exchange flows through straits for example the strait of gibraltar there are though some strait flows that cannot be represented by a two layer model a very striking example is the bab al mandab in the exchange between the red sea and the gulf of aden three distinct water masses are observed in the strait during summer the wintertime flow is though very different with only two layers observed in the strait in this paper a three layer model of hydraulically controlled exchange flows is described the model can represent exchange flows that are controlled with respect to either the first or second or both internal modes a number of different examples are discussed the model is able to explain the qualitative features of the seasonal variation of the flow in the bab al mandab in particular the reversal of the direction of flow of the surface layer and the intrusion into the red sea of cold intermediate water from the gulf of aden during the summer months can be explained by upwelling in the gulf of aden forced by the southwest monsoon

multi layer hydraulic exchange flows flows between ocean basins are often controlled by narrow channels and shallow sills a multi layer hydraulic control theory is developed for exchange flow through such constrictions the theory is based on the inviscid shallow water equations and extends the functional approach introduced by gill 1977 and developed by dalziel 1991 the flows considered are those in rectangular cross section channels connecting two large reservoirs with a single constriction sill and or narrows the exchange flow depends on the stratification in the two reservoirs represented as a finite number of immiscible layers of different uniform density for most cases the flow is controlled at the constriction and often at other points along the channel virtual controls too as with one and two layer hydraulics controls are locations at which the flow passes from one solution branch to another and at which at least one internal wave mode is stationary the theory is applied to three layer flows which have two internal wave modes predicting interface heights and layer fluxes from the given reservoir conditions the theoretical results for three layer flows are compared to a comprehensive set of laboratory experiments and found to give good agreement the laboratory experiments also show other features of the flow such as the formation of waves on the interfaces the implications of the results for oceanographic flows and ocean modelling are discussed

two layer hydraulics a functional approach a new approach for investigating two layer hydraulic exchange flows in channels is introduced the approach is based on the functional formalism of gill 1977 and applied to the flow through a contraction in width and to flow over a simple sill the sill geometry is an extension of that looked at by earlier workers in particular farmer armi 1986 who used a froude number plane approach in the present paper a simple relationship between the composite froude number and the hydraulic functional is derived though the functional approach may also be applied to channels where a froude number is not readily defined the ability to trace roots of this functional from one reservoir to the other is a prerequisite for the flow to be realizable two hydraulic transitions are required for the flow to be fully controlled and the exchange flow rate to be maximal if only one hydraulic transition is present the flow is governed by the conditions in one of the reservoirs and the exchange flow rate is found to be submaximal the flow along a channel is found to be very sensitive to small departures from symmetry about a horizontal plane the response of the interface to the introduction of a net barotropic flow is found to be a discontinuous function of the strength of the forcing for some range of sill heights

convectively driven exchange flow in a stratified sill enclosed basin laboratory experiments are used to investigate the processes governing steady convectively driven circulation in a basin that communicates with a large external reservoir over a shallow sill the motion is maintained by a steady loss of buoyancy distributed over the surface of the basin turbulent convection associated with the forcing produces a horizontal buoyancy gradient across the sill and the resulting mean flow consists of a layer directed into the basin near the surface with a dense counter flow below to first order the magnitude of the exchange flow over the sill is determined by the horizontal momentum balance within the basin measurements of the mean and turbulent flow fields are used to show that inertia buoyancy and friction may each contribute significantly to the balance the interior flow produces a horizontal pressure gradient near the surface which must also contribute to the momentum balance the density of the lower layer at the sill reflects the cumulative effect of interior processes such as mixing and these in turn influence the hydraulically controlled exchange flow over the sill the basin dynamics are therefore coupled in a nonlinear fashion with the submaximal sill exchange this coupling is investigated first by showing how interior processes are affected by changes in the magnitude of the forcing and then by observing the associated variation of the flow state at the sill the flow state is defined in terms of its relative proximity to the theoretical maximal exchange limit results show that the exchange flows are submaximal with flow rate approximately 85 of the maximal limit this state appears to change very little in response to increasing forcing for a stratified basin which exhibits a deep stagnant layer under the convectively driven near surface exchange flow the possibility of basin ventilation or erosion of deep fluid exists in the long term this process and its dependence on external parameters is also explored