abrupt changes in deep atlantic circulation during the transition to full glacial conditions 1 six ocean drilling program odp sites in the northwest atlantic have been used to investigate kinematic and chemical changes in the western boundary undercurrent wbuc during the development of full glacial conditions across the marine isotope stage 5a 4 boundary 70 000 years ago sortable silt mean grain size ss measurements are employed to examine changes in near bottom flow speeds together with carbon isotopes measured in benthic foraminifera and planktic foraminiferal fragmentation as proxies for changes in water mass chemistry a depth transect of cores spanning 1 8 4 6 km depth allows changes in both the strength and depth of the wbuc to be constrained across millennial scale events ss measurements reveal that the flow speed structure of the wbuc during warm intervals interstadials was comparable to modern holocene conditions however significant differences are observed during cold intervals with higher relative flow speeds inferred for the shallow component of the wbuc 2 km depth during all cold stadial intervals including heinrich stadial 6 and a substantial weakening of the deep component 3 4 km during full glacial conditions our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation involving glacial north atlantic intermediate water that was quantitatively distinct from preceding cold stadial events furthermore our chemical proxy data show that the physical response of the wbuc during the last glacial inception was probably coupled to basin wide changes in the water mass composition of the deep northwest atlantic

a numerical model study of the alboran sea gyre abstract reduced gravity and two layer numerical models have been used to study the circulation in the westernmost basin of the mediterranean sea the alboran sea circulation is forced by flow through a 20 km wide port in the western boundary representing the strait of gibraltar the reduced gravity model domain is a rectangle measuring 600 km 160 km with 10 km 5 km grid resolution when forced by an eastward on northeastward inflow the model solutions evolve to a steady state which exhibits a meandering current the first meander of the current forms the northern boundary of an anticyclonic gyre horizontal dimensions of the gyre are strongly dependent upon the inflow angle vorticity associated with the incoming current magnitude of the incoming transport and the north south extent of the basin the meandering current is considered a standing rossby wave with a highly distorted vorticity trajectory due to the interaction of the current with the northern and southern boundaries when velocity transport is increased the wavelength increases approximately as v as a result the anticyclonic gyre shifts east as velocity increases and west as velocity decreases these solutions show that bottom topography winds and coastline features are not necessary mechanisms for the formation of the gyre two layer model solutions were obtained using realistic topography geometry and a westward moving lower layer the addition of the lower layer flow and topography distorted the circulation in the upper layer particulary in the southern half of the basin this is caused by the large topographically steered currents along the southern shelf the high variability of the horizontal dimensions and location of the gyre has been observed both experimentally and through satellite imagery model results show that the high variability is caused by variations in the inflow velocity drifter tracks both observational and model derived help determine the circulation in both layers of the alboran sea the close comparison between model and experimental drifters lends credibility to the model circulation and dynamics

synchronous and proportional deglacial changes in atlantic meridional overturning and northeast brazilian precipitation changes in heat transport associated with fluctuations in the strength of the atlantic meridional overturning circulation amoc are widely considered to affect the position of the intertropical convergence zone itcz but the temporal immediacy of this teleconnection has to date not been resolved based on a high resolution marine sediment sequence over the last deglaciation we provide evidence for a synchronous and near linear link between changes in the atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast brazil the tight coupling between amoc strength sea surface temperature difference and precipitation changes over northeast brazil unambiguously points to a rapid and proportional adjustment of the itcz location to past changes in the atlantic meridional heat transport

water exchange between algeciras bay and the strait of gibraltar a study based on hf coastal radar abstract this study analyses the water mass exchanges at subinertial scale between algeciras bay and the adjacent strait of gibraltar the mechanisms triggering this exchange process is investigated with the aid of recently acquired data on surface currents obtained using a system of hf coastal radars deployed on the eastern side of the strait and remotely sensed images of sea surface temperature sst and chlorophyll from the modis sensor of the aqua satellite hf radar data on surface currents are analyzed by the application of real empirical orthogonal function eof decomposition which produces three eof modes explaining more than 70 of the variance of the surface currents at the mouth of the bay modes 2 3 and 6 mode 2 is related to the fluctuations of the atlantic jet in the central zone of the strait mainly due to a combined effect of the atmospheric pressure fluctuations in the western mediterranean sea and local wind in the eastern side of the strait mode 3 is related to the coastal currents induced by zonal wind forcing on the north western coast of the strait and alboran sea and mode 6 seems to be related to water transport induced by winds blowing with a significant north component into and out of the bay

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

north atlantic ocean circulation and abrupt climate change during the last glaciation the most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations whose origin remains unresolved variations in oceanic meridional heat transport may contribute to these repeated climate changes which were most pronounced during marine isotope stage 3 the glacial interval 25 thousand to 60 thousand years ago we examined climate and ocean circulation proxies throughout this interval at high resolution in a deep north atlantic sediment core combining the kinematic tracer protactinium thorium pa th with the deep water mass tracer epibenthic 13c these indicators suggest reduced atlantic overturning circulation during every cool northern stadial with the greatest reductions during episodic hudson strait iceberg discharges while sharp northern warming followed reinvigorated overturning these results provide direct evidence for the ocean s persistent central role in abrupt glacial climate change

a laboratory study of gyres and uplift near the strait of gibraltar laboratory experiments were conducted in which a rotating basin of stratified salty water in a field of gravity with a free surface was connected to a basin of fresh water by a narrow shallow channel the resulting density driven flow in the channel consisted of two currents a freshwater flow at the top from the fresh basin to the salty basin and a saltwater flow at the bottom in the opposite direction it models the flow in the strait of gibraltar which connects the atlantic ocean to the saltier mediterranean sea selective withdrawal of deep water up to and over the sill depth is observed in the stratified salty mediterranean basin the governing dimensionless numbers are the ratio of the rossby radius of deformation within the strait to the width of the strait and the ratio of rossby radius in the stratified med water to rossby radius of deformation within the strait the experiments were deliberately scaled to correspond to the parameters of the real mediterranean gibraltar region or not as we wished with the most correct parameters the water at a depth corresponding to 700 m in the mediterranean was selectively withdrawn up to sill depth and out of the salty basin the water at a depth corresponding to 900 m was not with double the correctly scaled stratification the water at a depth corresponding to 500 m was drawn out and the water at a depth corresponding to 700 m was not with no stratification water of a depth corresponding to 1200 m was drawn out the results are qualitatively consistent with the ideas of stommel et al 1973 that water at great depth in the mediterranean is drawn up and out at the strait of gibraltar although quantitatively the scaled experimental depths of withdrawal are not as great there are some indications that the gyre of atlantic water corresponding to the alboran sea gyre may enhance the selective withdrawal process but the mechanism is unclear

ice shelf collapse from subsurface warming as a trigger for heinrich events episodic iceberg discharge events from the hudson strait ice stream hsis of the laurentide ice sheet referred to as heinrich events are commonly attributed to internal ice sheet instabilities but their systematic occurrence at the culmination of a large reduction in the atlantic meridional overturning circulation amoc indicates a climate control we report mg ca data on benthic foraminifera from an intermediate depth site in the northwest atlantic and results from a climate model simulation that reveal basin wide subsurface warming at the same time as large reductions in the amoc with temperature increasing by approximately 2 c over a 1 2 kyr interval prior to a heinrich event in simulations with an ocean model coupled to a thermodynamically active ice shelf the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the hsis by a factor of approximately 6 by analogy with recent observations in antarctica the resulting ice shelf loss and attendant hsis acceleration would produce a heinrich event

time and space variability in the eastern alboran sea from march to may 1990 an intensive field experiment was carried out from march 6 to april 30 1990 in the eastern alboran sea to understand the relationship between the large scale circulation and the location of the transition zone between new and longer resident modified atlantic water maw the detection of a strong jet 80 cm s 1 of new maw with anticyclonic curvature near the morocco coast suggests that the eastern alboran gyre eag was not fully developed a new state of the circulation in the eastern alboran basin intermediate between the eag viudez et al 1995 and the lanoix 1974 state is presented also important is that the almeria oran front was not present in the upper layer that small scale instabilities were detected in the northern region and that significant nongeostrophic flows were observed all these features are clearly indicative of the high spatial and temporal variability of a region where complex adjustments among density velocity and topography occur

muted change in atlantic overturning circulation over some glacial aged heinrich events several periods of massive iceberg discharge into the north atlantic and widespread cooling marked the last glacial period reconstructions of northward flow along the florida margin suggest that not all cold events were associated with a change in the strength of the atlantic meridional overturning circulation

millennial scale tropical atmospheric and atlantic ocean circulation change from the last glacial maximum and marine isotope stage 3 abstract abrupt millennial scale climate oscillations known as dansgaard oeschger d o cycles characterized the climate system of the last glacial period although proxy evidence shows that d o cycles resulted in large scale changes in atmospheric circulation patterns around the planet an understanding of how atlantic meridional overturning circulation amoc varied across these events remains unclear here we take advantage of the fact that both tropical atmospheric circulation changes corresponding to north south shifts in the intertropical convergence zone itcz and large scale changes in ocean circulation associated with amoc variability can be reconstructed in the same sediment core from the florida straits to examine the relationship between atmospheric and ocean circulation changes across d o events to reconstruct surface water conditions mg ca paleothermometry and stable isotope measurements were combined on the planktonic foraminifera globigerinoides ruber white variety from sediment core knr166 2 jpc26 24 19 61 n 83 15 14 w 546 m depth to reconstruct a high resolution record of sea surface temperature and o seawater 18 a proxy for upper mixed layer salinity during marine isotope stages mis 2 and 3 from 20 35 ka bp as an additional proxy for upper water column salinity change we also generate a faunal abundance record of the salinity sensitive planktonic foraminifera neogloboquadrina dutertrei our results suggest that rapid reductions in sea surface salinity occurred at the onset of d o interstadials while stadials are characterized by increased surface salinities the most likely cause of these salinity changes was variation in the strength and position of the itcz across d o events finally we examine the relationship between millennial scale atmospheric circulation changes recorded in the planktonic records and ocean circulation changes inferred from the benthic o 18 record from our core our results provide some of the first evidence that amoc strength did vary across at least one of the millennial scale d o cycles of mis 3

on the upper layer circulation in the alboran sea we present new experimental results and assemble them with previous results in order to develop an improved picture of the upper layer circulation in the alboran sea it is suggested that the key idea to understanding this upper layer circulation is the tendency of the atlantic jet aj to have negative curvature local interactions with the western alboran gyre wag or the african coast can however counterbalance this tendency and modify the anticyclonic path of the aj it is also proposed that the density gradients in the wag can be maintained in part by means of an intermittent surface cross gyre current which results in an input mixing and renewal of atlantic water the static stability at the bottom of the gyre increases because of the mixing of atlantic water with western mediterranean deep water which is uplifted close to the african coast this mixing process thereby acts as a local source of potential vorticity we also report the existence in the alboran sea of subsurface anticyclonic eddies located between 100 and 400 m of relatively cold water that appear to be detached from the iberian shelf regarding the large scale variability of the aj wag system we present evidence of an eastward migration of the wag and the subsequent formation of a new anticyclonic gyre in the western alboran basin on a timescale of 1 month this eastward gyre migration process temporarily allows the simultaneous presence of three anticyclonic gyres in the alboran sea

a strait outflow circulation process study the case of the alboran sea abstract a three dimensional primitive equation model is used to investigate the physical processes governing the exchanges of water between the mediterranean sea and the atlantic ocean at the level of the strait of gibraltar the circulation is driven by connecting two reservoirs filled with waters of different densities the motion starts from rest and is initiated by removing the dam separating them at the initial time the results are in agreement with observations in the strait the light atlantic water flows into the mediterranean basin in the surface layer while the denser mediterranean water moves toward the ocean as a deep current after a spinup time interval of three days the flow transport in the strait reaches a quasi stationary value of 0 9 sv sv 106 m3 s 1 as it enters the alboran sea the flow of atlantic water experiences a transition regime where the surface current is transformed into a shallow buoyant jet after several days of integration this flow intrudes into the alboran bas

on the relative vorticity of the atlantic jet in the alboran sea abstract it is shown from hydrographic data that the eastward flowing atlantic jet aj in the alboran sea has zero or positive relative vorticity this holds even when the aj curves anticyclonically around the western and eastern alboran gyres this feature is due to the presence of large positive shear vorticity mainly caused by the interaction between the aj and the slow westward flow of mediterranean water on the northern side of the aj it is also shown that the aj acquires negative curvature vorticity at the same time that it acquires positive shear vorticity at the exit of the strait of gibraltar this feature is described in terms of a shear and curvature conversion it is found that the cross stream variation of alongstream geopotential differences is the main term in this vorticity interchange description of the aj in the western alboran sea

atlantic ocean circulation changes preceded millennial tropical south america rainfall events during the last glacial during the last glacial period greenland s climate shifted between cold stadial and warm interstadial phases that were accompanied by ocean circulation changes characterized by reduced atlantic meridional overturning circulation amoc during stadials here we present new data from the western tropical atlantic demonstrating that amoc slowdowns preceded some of the large south american rainfall events that took place during stadials based on 231pa 230th and ti ca measurements in the same sediment core we determine that the amoc started to slowdown 1420 250 and 690 180 1 years before the onset of two large precipitation events associated with heinrich stadials our results bring unprecedented evidence that amoc changes could be at the origin of the large precipitation events observed in tropical south america during heinrich stadials in addition we propose a mechanism explaining the differences in the extent and timing of amoc slowdowns associated with shorter and longer stadials

simulation of abrupt climate change induced by freshwater input to the north atlantic ocean temperature records from greenland ice cores1 2 suggest that large and abrupt changes of north atlantic climate occurred frequently during both glacial and postglacial periods one example is the younger dryas cold event broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the atlantic ocean which were caused by the release of large amounts of melt water from continental ice sheets here we describe an attempt to explore this intriguing phenomenon using a coupled ocean atmosphere model in response to a massive surface flux of fresh water to the northern north atlantic of the model the thermohaline circulation weakens abruptly intensifies and weakens again followed by a gradual recovery generating episodes that resemble the abrupt changes of the ocean atmosphere system recorded in ice and deep sea cores4 the associated change of surface air temperature is particularly large in the northern north atlantic ocean and its neighbourhood but is relatively small in the rest of the world

water masses and circulation in the western alboran sea and in the straits of gibraltar abstract some new elements of alboran sea circulation and water masses in close connection with flows through the straits of gibraltar are presented a general description is given of water masses found in 1981 during mediprod iv cruise from the gulf of cadiz to the gulf of lions followed by a brief review of the conditions found in the straits of gibraltar based on 1981 and 1961 observations the hydrological structure vertical and horizontal and surface and bottom currents typical of the western alboran sea are analysed we present some detailed observations about vertical currents in the deep waters near the straits and finally we analyse the characteristics of the mediterranean overflow through the straits our conclusions in part confirm those of bryden and stommel 1982 and in part shed light on the general pattern of the mediterranean currents entering the alboran sea from the east however considering the suggestion of stommel bryden and mangelsdorf 1973 that the deep currents in the interior of the western mediterranean basin are basically controlled by a bernoulli flow induced by the intense currents in the straits and the weak vertical stratification in the mediterranean we suggest that an interaction of a large scale circulation pattern and the topography of the alboran basin which is much shallower than the western mediterranean basin may play a major role in controlling hydraulically the deep mediterranean waters overflow a detailed water mass analysis of 1981 data and comparison with previous data revealed a time and space variability in the mediterranean waters the scales of this variability are unknown but the s variations make it difficult to make assumptions as to the depth of origin of the deep waters found in the alboran sea which flow into the atlantic ocean over the gibraltar sill

the atlantic inflow in the western alboran sea abstract an extensive dataset collected in the alboran sea during the 1982 donde va experiment is used to characterize the kinematics and dynamics of the inflow of atlantic water into the mediterranean sea the veering of the inflow toward the ene after leaving the strait of gibraltar and the existence of an anticyclonic gyre that fills much of the western alboran sea in the upper 200 m are confirmed in the mean inflow variability with periods of 2 to 10 days is described particularly striking is one interval of about 9 days during which the gyre was not present and the inflow adopted a southerly course after leaving the strait the observations are interpreted in terms of vorticity conservation and in the light of earlier theoretical and numerical results

the ocean s role in setting the mean position of the inter tropical convergence zone through study of observations and coupled climate simulations it is argued that the mean position of the inter tropical convergence zone itcz north of the equator is a consequence of a northwards heat transport across the equator by ocean circulation observations suggest that the hemispheric net radiative forcing of climate at the top of the atmosphere is almost perfectly symmetric about the equator and so the total atmosphere plus ocean heat transport across the equator is small order 0 2 pw northwards due to the atlantic ocean s meridional overturning circulation however the ocean carries significantly more heat northwards across the equator order 0 4 pw than does the coupled system there are two primary consequences first atmospheric heat transport is southwards across the equator to compensate 0 2 pw southwards resulting in the itcz being displaced north of the equator second the atmosphere and indeed the ocean is slightly warmer by perhaps 2 c in the northern hemisphere than in the southern hemisphere this leads to the northern hemisphere emitting slightly more outgoing longwave radiation than the southern hemisphere by virtue of its relative warmth supporting the small northward heat transport by the coupled system across the equator to conclude the coupled nature of the problem is illustrated through study of atmosphere ocean ice simulations in the idealized setting of an aquaplanet resolving the key processes at work

mesoscale dynamics and vertical motion in the alboran sea abstract the inflow of atlantic water through the strait of gilbraltar usually exhibits a stationary wavelike pattern in the alboran sea we use an objective analysis technique for quantitative scale separation to investigate the interaction of scales in the region the large scale is clearly dominated by an anticyclonic gyre smaller scale analysis shows the existence of several mesoscale cyclonic eddies along the northern boundary of the western alboran sea anticyclonic gyre the relationship between the large scale mesoscale variability and the induced ageostrophic vertical motion is established using the q vector formulation we find that on the macroscale upward motion occurs upstream of the anticyclonic gyre upstream of a wave crest while downward motion takes place downstream upstream of a trough we also show that the vertical motion associated with the mesoscale eddies is an order of magnitude higher than the large scale vertical motion these patterns of large scale and mesoscale vertical m

dansgaard oeschger cycles interactions between ocean and sea ice intrinsic to the nordic seas dansgaard oeschger d o cycles are the most dramatic frequent and wide reaching abrupt climate changes in the geologic record on greenland d o cycles are characterized by an abrupt warming of 10 5 c from a cold stadial to a warm interstadial phase followed by gradual cooling before a rapid return to stadial conditions the mechanisms responsible for these millennial cycles are not fully understood but are widely thought to involve abrupt changes in atlantic meridional overturning circulation due to freshwater perturbations here we present a new high resolution multiproxy marine sediment core monitoring changes in the warm atlantic inflow to the nordic seas as well as in local sea ice cover and influx of ice rafted debris in contrast to previous studies the freshwater input is found to be coincident with warm interstadials on greenland and has a fennoscandian rather than laurentide source furthermore the data suggest a different thermohaline structure for the nordic seas during cold stadials in which relatively warm atlantic water circulates beneath a fresh surface layer and the presence of sea ice is inferred from benthic oxygen isotopes this implies a delicate balance between the warm subsurface atlantic water and fresh surface layer with the possibility of abrupt changes in sea ice cover and suggests a novel mechanism for the abrupt d o events observed in greenland ice cores

circulation in the alboran sea as determined by quasi synoptic hydrographic observations part i three dimensional structure of the two anticyclonic gyres abstract the circulation and dynamics of the modified atlantic water have been studied using data from an intensive field experiment carried out between 22 september and 7 october 1992 data included 134 ctd casts adcp and satellite imagery a well defined wavelike front was observed with two significant anticyclonic gyres in the western and eastern alboran sea smaller scale cyclonic eddies were also observed the front separates the more saline older modified atlantic water s 38 in the northern region from the fresher more recent modified atlantic water s 36 8 in the south the associated baroclinic jet had a mean transport of 1 sv and maximum geostrophic velocities of 1 0 ms 1 the three dimensional structure and spatial scales of both gyres were similar that is 90 km long and 220 m deep in the eastern alboran northeast of oran the origin of the algerian current was also detected with an eastward transport of 1 8 sv the general picture can be presented as a structure formed by a wavelike

calcareous plankton and the mid brunhes climate variability in the alboran sea odp site 977 abstract quantitative calcareous plankton analyses have been performed at the ocean drilling program site 977 in the alboran sea through the mid brunhes interval 300 540 ka the results evidence orbital suborbital and millennial scale climate variability which primary reflects temperature change the patterns of increase decrease of coccolithophore and planktonic foraminifera warm water taxa reproduce the stage and substage climate variability in the planktonic 18o in good agreement with other planktonic isotope records from mediterranean sea and atlantic ocean variable conditions characterized by alternating cool nutrient rich or warm and stable sea surface waters occurred during interglacial marine isotope stage mis 13 in response to high amplitude oscillations of insolation during obliquity maximum and to north africa monsoon strength influencing distribution of taxa mis 13a resulted warmer than mis 13c based on the higher abundances of warm water taxa they increase just after insolation maxima suggesting a primary response of calcareous plankton to precessional orbital control following short term warm mis 11e and more humid mis 11d phases a climate optimum lasting about 16 kyr is recognized during mis 11c when major tropical subtropical water arrival into the alboran sea occurred as evidenced by the presence of trilobatus sacculifer mis 11c records unstable climate condition that persisted up to the late mis 11 indicating multiple pulses of southward migrations of subpolar front following the full interglacial mis 11c warm and oligotrophic conditions occurred during mis 9 as during mis 11 especially in mis 9e although without a climate optimum reduced abundance of warm water taxa and decrease of globorotalia inflata highlight colder surface water conditions and decreased atlantic mediterranean water exchange during glacials especially during the prominent sea level low stand of mis 12 short term prominent peaks of the polar subpolar neogloboquadrina pachyderma and coccolithus pelagicus ssp pelagicus and additional key taxa preferring fresher and turbid water together with minima in total nannofossil accumulation rate and enhanced coccolith reworking sustain a correlation with the north atlantic heinrich type ht events ht1 ht1a ht3 ht4 6 thus documenting the sensitiveness of the alboran sea in recording european and laurentide ice sheet dynamics the comparable chronology of most of these cold spells in the available balearic and ionian calcareous plankton records highlights that atlantic melt water entering alboran basin through gibraltar strait may have reached the central mediterranean preserving some of its original polar signature

the origin and characteristics of the algerian current satellite and ship data collected between may 1 and 22 1986 show that the algerian current originates in the almeria oran front and remains within 30 km of the algerian coast for over 300 km between oran and algiers the differences between the water masses of the current and the resident regional waters are clearly observed in the chlorophyll concentrations and optical characteristics thus permitting the resulting patterns to be used as tracers of the circulation during this field period the algerian current begins as an extension of the almeria oran front which is found at the western periphery of the eastern alboran gyre this major oceanographic salinity front s 1 km delineates the boundary of the algerian current which has average surface velocities of 40 cm s and maximum speeds reaching 80 cm s the current which has a volume transport between 0 4 and 0 5 sv is confined to the upper 150 m and is in geostrophic balance a sequence of coastal zone color scanner czcs and advanced very high resolution radiometer avhrr satellite imagery showing the rapidly evolving surface circulation features is augmented with in situ ship data of current and water mass characteristics rapid undulations and frontal movements of the current were observed during the study period however the flow remained unbroken between 1 and 3 e current instability increased eastward as the undulations were sharper and the current appeared to depart from the coast near algiers

stochastic atmospheric forcing as a cause of greenland climate transitions abstractan unforced simulation of the community climate system model version 4 ccsm4 is found to have greenland warming and cooling events that resemble dansgaard oeschger cycles in pattern and magnitude with the caveat that only three transitions were available to be analyzed it is found that the transitions are triggered by stochastic atmospheric forcing the atmospheric anomalies change the strength of the subpolar gyre leading to a change in labrador sea sea ice concentration and meridional heat transport the changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the north atlantic indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western pacific warm pool are discussed the full evolution of the anomalous climate state depends crucially on the climatic background state

an explanation for the curvature of the atlantic jet past the strait of gibraltar abstract a consistent explanation for the anticyclonic curvature of the atlantic jet as it passes through the strait of gibraltar and flows into the mediterranean sea eastern side of the strait is provided the anticyclonic curvature of the atlantic jet which is the key feature to understand the upper layer circulation in the western alboran sea is simply related to the positive net evaporation over the mediterranean the result of this positive net evaporation that mainly occurs in areas of the mediterranean far from the strait of gibraltar is a net inflow transport through the strait it is proposed that the positive net evaporation is able to produce such a net inflow in the strait because of an anomalous large scale pressure gradient this anomalous pressure gradient is found to be approximately collinear to the strait orientation the time averaged inflow of atlantic water at the eastern side of the strait of gibraltar must therefore be supergeostrophic and hence it must have anticyclonic curv

differences in magnetic properties of fragments and matrix of breccias from the rupture of the 2008 wenchuan earthquake china relationship to faulting abstract rock magnetic and mineralogical analyses were performed on fragments and matrix of six fault breccias from the zhaojiagou outcrop of yingxiu beichuan fault which was the main fault ruptured during the 2008 wenchuan mw 7 9 earthquake at leigu town beichuan county sichuan province china the matrix which is generally enriched in dolomite feldspar and clay minerals but depleted in calcite has much higher low field magnetic susceptibility and magnetization and lower coercivity in comparison with fragments and bulk samples magnetic behavior of the bulk samples and fragments is dominated by dia paramagnetic components in contrast small amounts of partially oxidized magnetite and lepidocrocite are present in the matrix a simple conceptual model associating faulting related effects was proposed to explain these significant differences in magnetic properties of fragments and matrix during coseismic slip intense shear may crush the pre existing magnetic grains in fault rocks into finer ones while stress and frictional heating to a little extent induce magnetic changes and thermochemical alterations of magnetic mineralogy in fault breccias respectively during interseismic periods meteoric fluids would infiltrate and percolate into fault zone and cause dissolution precipitation and recrystallization of fe bearing minerals as the diverse permeability structure and grain size of fragments and matrix these effects would modify their magnetic mineralogy at various levels consequently faulting related effects especially the fluid movements taking place over many previous earthquake cycles would be the most likely reasons for the observed different magnetic properties in fragments and matrix it further proposes that magnetic studies of fault breccias would provide clues to help understand seismic faulting and history of fault activity

role of internal waves in the generation of nepheloid layers on the northwestern alboran slope implications for continental margin shaping 1 the role of internal waves in the sediment dynamics of the northwestern alboran continental slope was investigated in a selected area around the guadiaro submarine canyon nepheloid layer distribution was identified using closely spaced ctd transmissometer profiles collected during two hydrographic surveys a well defined pattern of suspended sediment distribution consisting of surface intermediate and near bottom nepheloid layers was found intermediate and bottom nepheloid layers were always observed within the canyon and on the adjacent continental slope spanning mainly from 200 to 500 m depth in addition a current meter with a turbidimeter was deployed in the lower section of the guadiaro canyon at 600 m depth 25 meters above the seafloor time series analysis indicated that the currents temperature and turbidity within the canyon fluctuate mainly at semidiurnal tidal frequencies suggesting the presence of semidiurnal internal tides affecting the near bottom suspended sediment concentration along the canyon axis high resolution bathymetry from the study area was used to evaluate the internal wave reflection conditions at semidiurnal tidal frequency for the entire continental slope region critical slope conditions were reached on the upper continental slope and along the canyon axis coinciding with the region in which nepheloid layers were observed this region also coincides with a zone of erosion on the upper continental slope of the study area previously identified by hernandez molina 1993 these results indicate that the generation of intermediate and bottom nepheloid layers as well as the erosion and shaping of the northwestern alboran continental slope may result from the interaction of internal waves and the seafloor morphology

a new constraint on the size of heinrich events from an iceberg sediment model abstract heinrich layers anomalously thick layers of ice borne sediment in the north atlantic ocean have long been associated with abrupt climate changes in glacial times however there is still no consensus on either the exact amount of ice needed to transport this sediment or how such a large volume of ice could be produced using an iceberg model that includes sediment we simulate the delivery of sediment to the north atlantic during such an event our model assumes that sediment is uniformly distributed within the ice with a concentration of 4 unlike sediment models which assume that the sediment lies in a single layer this model can carry sediment all the way from the western to the eastern north atlantic we use a variety of different estimates for the total volume of ice released to model the sediment layer thickness and we show that to best fit the observations 60 10 4 km 3 with a plausible range of 30 120 10 4 km 3 of ice needs to be released this is equivalent to a 0 04 sv 10 6 m 3 s 1 with a plausible range of 0 02 0 08 sv release of freshwater over the 500 yr of a typical heinrich event this is a smaller flux of water than is required to show a significant impact on the global climate in most current state of the art gcms

coupling of trade winds with ocean circulation damps itcz shifts abstractthe position of the intertropical convergence zone itcz is sensitive to the atmosphere s hemispheric energy balance lying in the hemisphere most strongly heated by radiative and turbulent surface energy fluxes this study examines how the ocean circulation through its cross equatorial energy transport and associated surface energy fluxes affects the itcz s response to an imposed interhemispheric heating contrast in a coupled atmosphere ocean general circulation model shifts of the itcz are strongly damped owing to a robust coupling between the atmosphere s hadley cells and the ocean s subtropical cells by the trade winds and their associated surface stresses an anomalous oceanic wind driven cross equatorial cell transports energy across the equator strongly offsetting the imposed heating contrast the circulation of this cell can be described by the combination of trade wind anomalies and the meridional gradient of sea surface temperature which sets the temperature contrast between its up

eddy transport of western mediterranean intermediate water to the alboran sea during the second cruise of the eu funded omega project the towed undulating vehicle seasoar was deployed to survey the upper 350 m of the water column in the eastern alboran sea and extreme western algerian basin with an effective along track resolution of 4 km the data sets enabled a detailed description of the different upper ocean water types and the fronts that separate them the almeria oran front forms at the eastern boundary of the alboran sea gyre system in the upper 150 200 m of the water column and separates waters of predominantly atlantic origin from those formed in the western mediterranean sea below these surface waters but above the levantine intermediate water western mediterranean intermediate waters believed to be formed to the north of the balearic sea are normally observed in this region however to our knowledge this is the first time a discrete eddy of western mediterranean intermediate water a weddy has been described in the extreme western algerian basin repeated surveys of the region allowed us to observe the evolution of the eddy over a period of 40 d a climatological analysis of historical data in the medar medatlas database provides evidence for the repeatability of this observation and the significance of the estimated transport

microplankton response to environmental conditions in the alboran sea western mediterranean one year sediment trap record abstract the present work analyses the seasonal evolution of planktonic assemblages and particle fluxes through the water column in the eastern alboran sea western mediterranean at 35o55 47 n 01o30 77 w a sediment trap was deployed below the influence of the almeria oran front aof a semi permanent geostrophic front during july 1997 to june 1998 overall the temporal variability of coccolithophore planktonic foraminifer diatom benthic and wind carried biogenic particle fluxes is linked to the seasonal evolution of sea surface hydrological structures maximum planktonic fluxes were found during high productivity periods and wind induced upwelling following a trimodal pattern with maximum fluxes in july 1997 november december 1997 and april may 1998 these periods were characterized by vertical mixing and the full development of anticyclonic gyres in the alboran sea the annual flux of coccolithophores was dominated by the small gephyrocapsa group and emiliania huxleyi whereas turborotalita quinqueloba and globigerina bulloides dominated the foraminiferal fluxes and chaetoceros resting spores rs were predominant in the diatom assemblage benthic specimens were also collected with the sediment trap suggesting a variable influence of bottom water activity wind driven particles phytoliths and fresh water diatoms were collected along the year but their fluxes followed the local wind regime the high sea surface temperature sst during fall due to weaker than usual westerly winds and the pressure anomaly prevailing in the alboran sea during early winter were reflected in the planktonic assemblages by the proliferation of warm lower photic layer inhabitants and or oligotrophic taxa of coccolithophores florisphaera profunda planktonic foraminifers globigerinoides ruber and globorotalia inflata and diatoms leptocylindrus danicus these unusual climatic conditions in the eastern alboran sea must have been caused by the 1997 1998 enso event

an early theory of ocean circulation j s von waitz and his explanation of the currents in the strait of gibraltar the task of unravelling the factors governing the circulation of the oceans is one of the foremost in physical oceanography yet the origin of the idea of the internal circulation of the ocean is obscure only since the time of the challenger expedition which sailed in 1872 has it been generally accepted that density differences within the ocean due to variations in salinity and temperature can set great water masses in motion it was w b carpenter s conviction based on the earlier work of h m s s lightning and porcupine between 1868 and 1870 that cold dense water flowed from polar regions towards the equator in the ocean depths while a return flow of warmer lighter water took place at the surface which led him to promote the expedition deacon 1971 prestwlch 1875 traced back similar ideas to the early 1800s in the intervening years they had fallen out of favour when confusion arose over results obtained with six self registering thermometers insufficiently protected against the effects of pressure mcconnell 1978 and the widespread belief that sea water s maximum density occurred at the same temperature as in fresh water temperature differences were generally held to be responsible for circulation but some writers such as maury 1855 in the physical geography of the sea thought that salinity was more important before the end of the 18th century it would not have been at all obvious from studying observations of subsurface temperature that such a situation existed measurements in the mediterranean such as those made by marsigli 1725 indicated a mean temperature below the level affected by seasonal changes there was no reason to think of this as exceptional and it fitted in well with the ideas of those who believed the centre of the earth to be warm and its heat to be transmitted through the sea floor if it were not for this and the vertical mixing which ensued said dortous de mairan 1749 the water in the depths of the sea would turn to ice measurements in the southern ocean made on captain cook s second voyage appeared to confirm this view by showing the existence of warmer water beneath the cold surface layer deacon 1971 proof of the presence of polar water in the depths of tropical seas was not obtained until after 1800 but thompson 1798 had already explained examples of deep sea temperature below the atmospheric mean prevailing in the latitudes where they were taken by the spread of cold water due to its greater density however his was not the first attempt to outline a theory of ocean circulation a similar idea but relying on salinity instead of temperature differences had been put forward in a remarkable paper nearly half a century earlier in 1755 the royal swedish academy of sciences published in its transactions a paper by a german author herrwaitz privy councillor to the landgrave of hesse cassel it was entitled an investigation into the reasons why the water of the atlantic ocean always flows into the mediterranean sea through the straits of gibraltar waitz 1755 argued that the difference

oceanic link between abrupt changes in the north atlantic ocean and the african monsoon abrupt changes in the african monsoon which have been recorded throughout the late pleistocene and early holocene epochs tend to coincide with changes in north atlantic thermohaline circulation a numerical simulation shows that the interaction between thermohaline circulation in the north atlantic ocean and wind driven currents in the topical atlantic ocean contributes to the rapidity of african monsoon transitions during abrupt climate change events

surface circulation in the alboran sea western mediterranean inferred from remotely sensed data 1 in this study for the first time at regional scale the combined use of remote sensing data altimetry and sea surface temperature records provides a description of the persistent recurrent and transient circulation regimes of the alboran sea circulation the analysis of 936 altimeter derived weekly absolute dynamic topography adt and surface geostrophic current maps for 1993 2010 reveals the presence of a dominant annual signal and of two interannual modes of variability the winter spring phase is characterized by two stable gyral scale features the well known western anticyclonic gyre within the western area and the central cyclonic gyre a new structure not identified in former studies occupying the central and eastern parts of the alboran sea a double anticyclonic gyre regime constitutes the stable circulation system of the summer autumn period when the eastern anticyclonic gyre is formed within the eastern alboran basin in this case the central cyclonic gyre is narrower and located closer to the western anticyclonic gyre they represent two stable states of the system robust at the decadal time scale whereas transient changes reflect perturbations on these stable states and are mainly observed at an interannual scale the circulation variability and the gyral features development may be dynamically linked to the corresponding changes of the gibraltar transport rates citation renault l t oguz a pascual g vizoso and j tintore 2012 surface circulation in the alboran sea western mediterranean inferred from remotely sensed data j geophys res 117 c08009 doi 10 1029 2011jc007659

short term measurements of surface currents associated with the alboran sea gyre during donde va abstract during the period 6 18 october 1982 six flights by a u s navy research aircraft were made over the alboran sea gyre as part of the donde va experiment during five of these flights sonobuoys were dropped in a linear pattern designed to show the temporal and spatial variability of the currents that constitute the gyre a normal development consisted of approximately 15 sonobuoys dropped along the longitudinal line 4 45 w from the spanish coast to within 30 km of the moroccan coast during the following 4 hours the sonobuoys were relocated twice their positions noted and their individual drift speeds and directions derived aircraft precision radiation temperature and aircraft expendable bathythermograph data were collected concurrently with the sonobuoy drops to determine the thermal structure of the water during the drift period inertial navigation winds at the 300 m altitude of the aircraft as well as other meteorological data were also collected analysis of these data shows that at

in situ and satellite observations of the eastward migration of the western alboran sea gyre during october 2003 an intensive oceanographic survey biomega was carried out in the alboran sea coinciding with a migration event of the western alboran sea gyre wag the observations gathered during that cruise constitute the first field evidence of a migrated stage of the wag in this work we present the main differences between the 3d hydrodynamic fields observed during biomega and those corresponding to a wag located at its usual position the migration of the gyre was followed by satellite altimetry and sea surface temperature imagery the causes of the gyre migration are explored in terms of the quasi geostrophic tendency equation in particular of the dynamics governing scales larger than the rossby radius of deformation it is shown that the steady state gyre must be almost equivalent barotropic and that the key process to break down the stationarity would be a density advection at gyre scale the mechanisms to explain the migration of the wag proposed by previous authors are discussed in light of the explanation proposed in this work

sea ice switches and abrupt climate change we propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea ice cover we explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism we emphasize that because of the significant influence of sea ice on the climate system it seems that high priority should be given to developing ways for reconstructing high resolution in space and time sea ice extent for past climate change events if proxy data can confirm that sea ice was indeed the major player in past abrupt climate change events it seems less likely that such dramatic abrupt changes will occur due to global warming when extensive sea ice cover will not be present

circulation in the alboran see as determined by quasi synoptic hydrographic observations part ii mesoscale ageostrophic motion diagnosed through density dynamical assimilation abstract the 3d velocity field in the alboran sea western mediterranean is diagnosed through density dynamical assimilation in a primitive equation pe model with mesoscale resolution the ageostrophic motion is computed from fields produced by short term backward and forward integrations of the pe model initialized with quasi synoptic ctd data a weight function based on a low pass digital filter digital filter initialization method is applied to the resulting time series of model variables to obtain the final dynamically balanced density and 3d velocity fields the diagnosed ageostrophic motion is interpreted by comparing the vertical velocity field with that obtained from the quasigeostrophic qg equation the two methods produce very similar results with maximum vertical motions in the range of 10 20 md 1 associated with the differential advection of relative vorticity in small scale jet meanders upward downward motion upstream downstream of the ridges small local differences between

controls on the tropical response to abrupt climate changes abrupt climate change events during glacial times have distinct tropical imprints despite their cause being at high latitudes the mechanisms by which high latitudes affect low latitudes are currently unclear we present climate model simulations of a set of different abrupt events and find changes in tropical rainfall can not be comprehensively explained by either changes in sea ice extent or ocean circulation changes in tropical meridional temperature gradients are the clearest way to explain tropical rainfall changes across all events we find some tropical regions are unlikely to record heinrich events because the rainfall is moved far enough away from them during stadial periods that they are insensitive to further change greenland temperature responds linearly to atlantic sea ice extent suggesting that the absence of change in greenland temperature during heinrich events implies no sea ice expansion despite major changes in the climate system elsewhere

mesoscale variability in the alboran sea synthetic aperture radar imaging of frontal eddies in autumn 1992 the entire alboran sea western mediterranean was sampled by the spanish r v garcia del cid the incoming jet of atlantic water its associated meandering front and the two big anticyclonic gyres were described from conductivity temperature depth ctd and acoustic doppler current profiler adcp data smaller scale eddies were also observed additionally 36 ers 1 synthetic aperture radar sar scenes were obtained from mid september to mid october the sar images capture these features when wind conditions are suitable the current shear is mainly depicted as narrow lines of low backscatter because of the damping of waves by natural surface films these lines delineate the northern border of both gyres and the beginning of the alongslope algerian current at the eastern limit of the alboran sea adcp observations confirm that lines on sar imagery follow the direction of the surface currents the two gyres present high backscatter values in their center while their frontal boundaries appear modulated by the variation of the marine atmospheric boundary layer stratification due to the surface thermal front sar has observed small spiral eddies that were not evidenced by the almost contemporaneous but too coarse ctd in situ sampling good spatial correspondence between radar detected and in situ measured structures occurs when comparing sar images to the surface dynamic topography rather than strictly surface water characteristics

structure and variability of the alboran sea frontal system physical characteristics of the alboran sea in the western mediterranean were examined in october 1977 during a survey by ship and aircraft the main features observed were a distinct oceanic front coinciding with the jet of incoming north atlantic water and an associated anticyclonic gyre in the western half of the basin the front was confined to the upper 200 m and was a continuous feature extending from the strait of gibraltar 500 km eastward to the prime meridian of particular interest was the observed variability of the anticyclonic gyre in 10 days between two surveys the center of the gyre shifted 50 km westward this shift may correspond to changes in strength of the north atlantic inflow as inferred by local wind and the average atmospheric pressure over the western mediterranean

sea surface transport in the western mediterranean sea a lagrangian perspective 2 we study the sea surface transport in the western mediterranean sea from a lagrangian point of view in particular the alboran and the north western subbasins the study is carried out through the analysis of 3 years of surface velocity model data through finite size lyapunov exponents residence time and virtual particle trajectories complementing the classical eulerian approach the spatiotemporal variability of the main transport processes is inferred from the empirical orthogonal function modes of the lyapunov exponents being the most relevant modes discussed and physically interpreted results indicate that some of the variability in the surface transport patterns in the western mediterranean can be explained by specific modes which provide an indication of connectivity among subbasins like the inflow of atlantic waters through the ibiza channel

the deflection and division of an oceanic baroclinic jet by a coastal boundary a case study in the alboran sea abstract an oceanic baroclinic jet impinging on a coastal boundary represents a particular type of ocean coast interaction this specific oceanographic phenomenon is an example of the stagnation in line flows occurring in fluid dynamics with three additional features rotation stratification and a sloping boundary in this study the authors describe the density vorticity and deformation characteristics of an oceanic jet impinging on a sloping boundary the case study corresponds to the impinging process of the atlantic jet at the african coast alboran sea in the impinging region the acceleration field is divergent related to the fact that the magnitude of the deformation is larger than the magnitude of the rotation it is also found that the stagnation streamsurface does not lie in a vertical plane but tilts in the opposite direction to the tilt of the isopycnals the flow upstream of the stagnation point is characterized by backing speed convergence diffluence and negative streamwise vorticity

variations in the structure of the anticyclonic gyres found in the alboran sea historical satellite aircraft and in situ data have shown that two anticyclonic gyres the western and eastern alboran gyres are major ocean features of the alboran sea an examination of several years of satellite imagery indicates that large variations in the surface expression of these two gyres occur and that on occasion one or the other gyre disappears the disappearance of both gyres at the same time was not seen the initial disappearance of either gyre occurs on a time scale of a week to 2 weeks whereas the return may take from 3 weeks to 2 months various forcing mechanisms i e winds mass flux inflow through the straits of gibraltar and sicily and or density have been used in numerical ocean circulation models to study the dynamics of the western mediterranean sea various model results show relationships similar to those shown by the satellite imagery however no single forcing mechanism has been positively identified as the source of the disappearances and the events may be a result of a combination of forcing mechanisms

observation of a western alboran gyre migration event abstract in the western alboran sea light atlantic water and dense mediterranean waters meet this gives rise to intense density fronts and energetic mesoscale features such as two semi permanent anticyclonic vortices the western and eastern alboran gyres however the circulation in the alboran sea is strongly varying in time with episodes where neither the western alboran gyre nor the eastern alboran gyre are present in october 1996 four consecutive high resolution surveys of the western alboran gyre wag were carried out and lagrangian floats were released during the first survey the western alboran sea was occupied by the wag surrounded by the atlantic jet the following surveys showed the wag drifting eastward beginning a wag migration event as confirmed 15 days later by the lagrangian float tracks and the sst images to replace the old gyre one month later a new gyre was formed analysis of adcp velocities sst images and ts properties show a change between the first and the second survey in the atlantic inflow through the strait of gibraltar the relationships between this change in the atlantic inflow a drop in the atmospheric pressure at ceuta the tidal regime and the wag migration are discussed the existence of a high resolution sampling in space together with the possibility of addressing the temporal variability has allowed us to describe in detail and for the first time the time evolution of the western alboran sea during a wag migration event relating the migration to changes in the inflow through the strait of gibraltar

a study of an intense density front in the eastern alboran sea the almeria oran front abstract studies of satellite imagery and space shuttle photographs of the western mediterranean have indicated that the main path of inflowing atlantic water is around two large anticyclonic gyres in the alboran sea and along the algerian coast these studies have also shown that a strong ocean front is present between almeria spain and oran algeria which is part of the easternmost segment of the eastern alboran gyre based on these satellite studies the first in situ investigation of the front called here the almeria oran front was conducted in march 1986 as part of the winter campaign of the western mediterranean circulation experiment wmce analyses of the resulting data show that the almeria oran front is a large scale density front formed by the convergence of two distinct water masses and controlled by the geographic position and strength of the eastern alboran gyre physical and biochemical data indicate that the front is limited to the upper 300 m with a strong southward baroclinic jet

southern ocean forcing of the north atlantic at multi centennial time scales in the kiel climate model internal multi centennial variability of open ocean deep convection in the atlantic sector of the southern ocean impacts the strength of the atlantic meridional overturning circulation amoc in the kiel climate model the northward extent of antarctic bottom water aabw strongly depends on the state of weddell sea deep convection the retreat of aabw results in an enhanced meridional density gradient that drives an increase in the strength and vertical extent of the north atlantic deep water nadw cell this shows for instance as a peak in amoc strength at 30 n about a century after weddell sea deep convection has ceased the stronger southward flow of nadw is compensated by an expansion of the north atlantic subpolar gyre and an acceleration of the north atlantic current indicating greater deep water formation contractions of the north atlantic subpolar gyre enable warm water anomalies which evolved in response to deep convection events in the southern ocean to penetrate farther to the north eventually weakening the amoc and closing a quasi centennial cycle gyre contractions are accompanied by increases in sea level of up to 20 cm century in some areas of the north atlantic in the southern ocean itself the heat loss during the convective regime results in a sea surface height decrease on the order of 10 cm century with a maximum of 30 cm century in the weddell sea hence the impact of the southern ocean centennial variability socv on regional as well as north atlantic sea level is of the same order of magnitude as the rise of global average sea level during the 20th century which amounts to about 15 20 cm this suggests that internal variability on a centennial time scale cannot be neglected a priori in assessments of 20th and 21st century amoc and regional sea level change

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

an example of meteorological effects on the alboran sea gyre following a period of weak winds an air dropped expendable bathythermograph axbt survey on july 4 1980 showed the alboran sea gyre clearly present at 100 m depth but not at the surface winds on july 5 and 6 were from the west at 12 m s and both axbt temperature distributions and satellite images then showed the gyre clearly dominating the temperature structure both at 100 m and at the surface the disparity between the surface and 100 m temperature patterns on july 4 demonstrates that caution must be used when interpreting satellite imagery in the alboran sea which has been used by several investigators as representing the gyre s structure the change in the shallow temperature distribution from july 4 to 6 is opposite to what some investigators have proposed as the normal response of the gyre to atmospheric forcing

the flow of atlantic water through the strait of gibraltar abstract we describe and analyze observations of the water exchange through the strait of gibraltar the primary observations were taken in april 1986 and included data from moored recording instruments at four locations in the strait together with an intensive survey by ship these measurements included extensive ctd profiling acoustic doppler current profiling numerous profiles from expendable instruments and imaging using a high frequency echo sounder the analysis focuses on the internal hydraulics of the strait and in particular the presence of hydraulic controls and their influence on the exchange our observations during april 1986 show that the maximal exchange condition in which a subcritical flow is bounded by supercritical flow at both ends of the strait did occur although with various subtleties not explicitly incorporated in our previous theoretical developments the atlantic water moving east along the surface encounters a control at tarifa narrows in the eastern part of the strait the mediterranean water passes through controls both at camarinal sill and at spartel sill further west at our westernmost mooring at spartel the outflowing mediterranean water is continuously supercritical with little tidal variability the control at camarinal sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide tangier basin bounded by camarinal and spartel sills acts as an internal reservoir for the outflowing mediterranean water the interface rising and falling through each tidal cycle during spring tides the tangier basin interface rises high enough to flood the control at camarinl sill and produces a reverse flow of the lower layer when control is lost at camarinal sill a travelling bore is released this bore may modify the location of the control acting on the surface layer in tarifa narrows the atlantic water enters the alboran sea as a jet which may have a well defined northern boundary where it separates from the european coast this boundary may be identified by the presence of a slick although the internal jump west of camarinal sill is almost certainly an area of intense mixing frictional effects do not appear to be dominant in the subcritical portion of the flow

internal waves in the strait of gibraltar and their role in the vertical mixing processes within the bay of algeciras abstract this article presents some of the results of an oceanographic survey carried out in the bay of algeciras strait of gibraltar as part of a research project intended to assess the environmental quality of the bay one of the most interesting findings was the step like patterns presented by density profiles within the bay which were indicative of notable vertical mixing activity there the analysis of the observations indicates that those mixing processes may be explained by the interaction between the local internal tide dynamics and the large amplitude internal waves entering the bay which seem to originate from the camarinal sill region