ALBERT

Description: In this study a scenario is developed of two adjacent Mediterranean Water eddies (meddies) as they were observed merging and drifting through the Iberian Basin. Observations are based on four RAFOS floats (at 850–1050 dbar), two hydrographic surveys (centered roughly at 38°N, 24°W), and trajectories of surface drifters (drogued at 100 m). In April 1991, the meddy A was identified and labeled by surface drifters. During the revisit one month later two meddies were encountered, B1 and B2, in the vicinity of the former meddy A. The coalescence of B1 (subsequently identified as A, one month older) and B2 is inferred from a simple kinematic model describing the observed movement of the RAFOS floats for up to three months after the second CTD survey. The deduced vorticity front, radius ∼15 km, within B1 was of insufficient strength to keep the core waters of B1 isolated and prevent the absorption of B1 by B2. The resulting meddy (B1 + B2) showed a clear near-surface dynamical signal. Its deep root (1800 m) could explain the expulsion from the meddy of the remaining RAFOS float and surface drifter at the time of the meddy's collision with the Josephine Seamount. For the first time, a set of Lagrangian and hydrographic observations give direct evidence that neighboring meddies can merge as predicted by theoretical considerations.

Description: Global mean and eddy fields from a four-year experiment with a 1/6° × 1/5° horizontal resolution implementation of the CME North Atlantic model are presented. The time-averaged wind-driven and thermohaline circulation in the model is compared to the results of a 1/3° × 2/5° model run in very similar configuration. In general, the higher resolution results are found to confirm that the resolution of previous CME experiments is sufficient to describe many features of the large-scale circulation and water mass distribution quite well. While the increased resolution does not lead to large changes in the mean flow patterns, the variability in the model is enhanced significantly. On the other hand, however, not all aspects of the circulation have improved with resolution. The Azores Current Frontal Zone with its variability in the eastern basin is still represented very poorly. Particular attention is also directed toward the unrealistic stationary anticyclones north of Cape Hatteras and in the Gulf of Mexico.

Description: To avoid an explicit simulation of the overflows across the Greenland-Scotland ridge, many models of the large-scale ocean circulation seek to include the net effect of the inflowing dense water masses by restoring temperature and salinity near the ridge to observed conditions. In this paper the authors examine the effect of different datasets for the northern restoring condition in two versions, eddy resolving and non-eddy resolving, of the model of the North and equatorial Atlantic that has been developed in recent years as a Community Modeling Effort for WOCE. It is shown that the use of smoothed climatological fields of temperature and salinity south of the Denmark Strait leads to strong deficiencies in the simulation of the deep flow field in the basin. A switch to actual hydrographic data from the Denmark Strait ignites a rapid dynamic response throughout the North Atlantic, affecting the transport and vertical structure of the deep western boundary current and, by virtue of the JEBAR efffect, the transport of the horizontal gyres. Meridional overturning and northward heat transport too weak in the cases with climatological boundary conditions, increase to more realistic levels in the subtropical North Atlantic. The initial response to switches in the high-latitude thermohaline forcing is mediated by fast waves along the westurn boundary, leading to changes in the deep western boundary current in low latitudes after about two years in the non-eddy-resolving cast. The initial timescale depends on the horizontal grid spacing of the model; in the high-resolution case, the first signal reaches the equator in a few months. The adjustment to a new, dynamic quasi equilibrium involves Kelvin waves along the equator and Rossby wave in the interior and is attained in less than two decades throughout the North Atlantic. It is suggested that these fast dynamic adjustment processes could play an important role in possible fluctuations of the thermohaline circulation, or transitions between different equilibrium states of the coupled ocean–atmosphere system, and may have determined the timescale of the observed climatic transitions before and during the last deglaciation.

Description: We have investigated the seasonal cycle and the interannual variability of the tropical Indian Ocean circulation and the Indian summer monsoon simulated by a coupled ocean-atmosphere general circulation model in a 26- year integration. Although the model exhibits significant climate drift, overall, the coupled GCM simulates realistically the seasonal changes in the tropical Indian Ocean and the onset and evolution of the Indian summer monsoon. The amplitudes of the seasonal changes, however, are underestimated. The coupled GCM also simulates considerable interannual variability in the tropical Indian Ocean circulation, which is partly related to the El Niño/Southern Oscillation phenomenon and the associated changes in the Walker circulation. Changes in the surface wind stress appear to be crucial in forcing interannual variations in the Indian Ocean SST. As in the Pacific Ocean, the net surface beat flux acts as a negative feedback on the SST anomalies. The interannual variability in monsoon rainfall, simulated by the coupled GCM, is only about half as strong as observed. The reason for this is that the simulated interannual variability in the Indian monsoon appears to be related to internal processes within the atmosphere only. In contrast, an investigation based on observations shows a clear lead-lag relationship between interannual variations in the monsoon rainfall and tropical Pacific SST anomalies. Furthermore, the atmospheric GCM also fails to reproduce this lead-lag relationship between monsoon rainfall and tropical Pacific SST when run in a stand-alone integration with observed SSTs prescribed during the period 1970–1988. These results indicate that important physical processes relating tropical Pacific SST to Indian monsoon rainfall are not adequately modeled in our atmospheric GCM. Monsoon rainfall predictions appear therefore premature.

Description: Observations of large and abrupt climate changes recorded in Greenland ice cores have spurred a search for clues to their cause. This search has revealed that at six times during the last glaciation, huge armadas of icebergs launched from Canada spread across the northern Atlantic Ocean, each triggering a climate response of global extent.

Description: The annual cycle of meridional heat transport in the North and equatorial Atlantic Ocean is studied by means of the high-resolution numerical model that had been developed in recent years as a Community Modeling Effort for the World Ocean Circulation Experiment. Similar to previous model studies, there is a winter maximum in northward heat transport in the equatorial Atlantic and a summer maximum in midlatitudes. The seasonal variation in heat transport in the equatorial Atlantic, with a maximum near 8°N, is associated with the out-of-phase changes in heat content to the north and south of that latitude in connection with the seasonal reversal of the North Equatorial Countercurrent. The amplitude of the heat transport variation at 8°N depends on model resolution: forcing with the monthly mean wind stresses of Hellerman–Rosenstein (HR) gives an annual range of 2.1 PW in the case of a 1/3° meridional grid, and 1.7 PW in the case of a 1° grid, compared to 1.4 PW in a previous 2° model. Forcing with the wind stresses of Isemer–Hasse (IH) gives 2.5 PW in the 1/3° and 2.2 PW in the 1° model case. The annual range of heat transport in the subtropical North Atlantic is much less dependent on resolution but sensitive to the wind stress: it increases from 0.5 PW in the case of HR forcing to almost 0.8 PW with IH forcing. The annual cycle of heat transport can be understood in terms of wind-driven variations in the meridional overturning; variations in horizontal gyre transport have only little effect both in the equatorial and in the subtropical Atlantic. In all model solutions the seasonal variations in the near-surface meridional Ekman transport are associated with deep seasonal overturning cells. The weak shear of the deep response suggests that the large variations in heat transport on seasonal and shorter time scales should be of little consequence for observational estimates of mean oceanic heat transports relying on one-time hydrographic surveys.

Description: Cicosal sea surface height (SSH) data in the tropical and midlatitude North Atlantic are analyzed with and without water vapor (WV) correction to study the WV influence on along-track SSH anomaly profits, mesoscale SSH variability, wavenumber spectra, and objectively mapped fields of SSH anomaly. Three different WV datasets were used, one from the Fleet Numerical Oceanographic Center (FNOC) model and two from the Special Sensor Microwave/Imager (SSM/I) based on different WV retrieval algorithms. These WV dataset show significant differences, in particular in the tropics. However, the method for deriving SSH anomalies from altimeter height data Alters out much of the WV corrections. The residual WV effect on SSH anomaly is shown to be most significant in the seasonally migrating intertropical convergence zone of the tropical Atlantic: there the SSM/I corrections reduce the along-track mesoscale SSH variability by typically 1–1.5 cm. On seasonal timescales the maximum WV effect in this region is characterized by a 2–3-cm rms difference between SSH anomaly with and without SSM/I WV corrections, whereas FNOC corrections have almost no effect. Inferred seasonal velocity variations in the North Equatorial Countercurrent core (4° – 6°N) in the region of maximum WY influence (30° – 40°W) are reduced by about 20% and 30%, depending on whether SSM/I corrections by Emery or Wentz are used

Description: Observations of upper-ocean western boundary current (WBC) transports reveal asymmetries between the Northern and the Southern Hemispheres of the Atlantic Ocean. To find out what mechanism might cause these asymmetries the linearized steady-state vorticity equation is applied to the interior of a layer of constant thickness representing the upper Atlantic Ocean. WBC transports are then required to balance the interior volume flux deficit. The ocean is forced by climatological wind stress at the surface; thermohaline forcing is introduced by vertical motion at the lower boundary. A series of model runs using selected combinations of different basin geometries, wind stress fields, and thermohaline forcing patterns yields the following results: asymmetries of WBC transports cannot be explained by the topography shape of coastlines. The wind stress causes 12 Sv (Sv ≡ 1 × 106 m3 s −1) cross-equatorial transport to the north but it cannot account for the other WBC asymmetries. These can be explained by superimposing a thermohaline flow component to the wind-driven circulation. The best agreement with observations could be obtained from a model run driven by a sinking rate of 20 Sv in the northern North Atlantic and 4 Sv in the Weddell Sea compensated by 15 Sv return flow from other oceans via the Agulhas Current or Drake Passage and uniform upwelling of 9 Sv in the Atlantic. In tropical and subtropical latitudes this run reproduces all observed asymmetries, but in subpolar latitudes the model fails. Further conclusions can be drawn from the model results. (i) Up to 20 Sv northward transport of Antarctic Intermediate Water is needed at about 10°S to explain the difference of modeled transports and observations. For the same reasons an Antilles Current of up to 16 Sv is required. (ii) The major part of the northward heat transport in the North Atlantic has to occur via the tropical countercurrents and the North Equatorial Current. Only less than 7 Sv take the shortest way to the Caribbean via the Guyana Current. (iii) Fifty-six percent of the Florida Straits transport is wind driven.

Description: Accurate measurement of fluctuations in temperature and humidity are needed for determination of the surface evaporation rate and the air-sea sensible heat flux using either the eddy correlation or inertial dissipation method for flux calculations. These measurements are difficult to make over the ocean, and are subject to large errors when sensors are exposed to marine air containing spray droplets. All currently available commercial measurement devices for atmospheric humidity require frequent maintenance. Included in the objectives of the Humidity Exchange over the Sea program were testing and comparison of sensors used for measuring both the fluctuating and mean humidity in the marine atmosphere at high wind speeds and development of techniques for the protection of these sensors against contamination by oceanic aerosols. These sensors and droplet removal techniques are described and comparisons between measurements from several different systems are discussed in this paper. To accomplish these goals, participating groups devised and tested three methods of removing sea spray from the sample airstream. The best performance was given by a rotating semen device, the “spray Ringer.” Several high-frequency temperature and humidity instruments, based on different physical principles, were used in the collaborative field experiment. Temperature and humidity fluctuations were measured with sufficient accuracy inside the spray removal devices using Lyman-α hygrometers and a fast thermocouple psychrometer. Comparison of several types of psychrometers (using electric thermometers) and a Rotronic MP-100 humidity sensor for measuring the mean humidity illustrated the hysteresis of the Rotronic MP-100 device after periods of high relative humidity. Confidence in the readings of the electronic psychrometer was established by in situ calibration with repeated and careful readings of ordinary hand-held Assman psychrometers (based on mercury thermometers). Electronic psychrometer employing platinum resistance thermometers perform very well.

Description: New light is shed on Worthington's concept of the North Atlantic circulation, postulating the existence of two anticyclonic gyres. This concept, which seems to have been laid to rest in the last decade, has now been reinforced by the results of a simple linear Sverdrup circulation model yielding a band of westward transport all across the North Atlantic at about the Azores latitude. This narrow band is called the Azores Countercurrent (AzCC) and matches the position of westward flow required by Worthington's “northern gyre.” An anomaly in the meridional change of the wind-stress curl in the eastern North Atlantic has been identified as the driving mechanism. A comparison with observations shows that the AzCC is verified in many analyses of historical datasets and synoptic surveys. A lack of the AzCC in other analyses is probably due to missing meridional sections, strong smoothing, and the superimposed Ekman flow close to the sea surface directed to the southeast. The AzCC has not been verified in low-resolution general circulation models applying simplified wind-stress fields and large friction coefficients, but there is evidence for its existence in recent high-resolution models driven by realistic wind stresses. Based on these findings, a new pattern for the wind-driven upper ocean circulation of the midlatitude North Atlantic is presented.

Description: A sigma-coordinate, primitive equation ocean circulation model is used to explore the problem of the remnant generation of trapped waves about a tall, circular, isolated seamount by an incident oscillatory barotropic current. The numerical solutions are used to extend prior studies into the fully nonlinear regime, and in particular to quantify and interpret the occurrence of residual circulation. Specific attention is also devoted to the dependence of the resonance and rectification mechanisms on stratification, forcing frequency, and choice of subgrid-scale viscous closure. Resonantly generated trapped waves of significant amplitude are found to occur broadly in parameter space; a precise match between the frequency of the imposed incident current and the frequency of the trapped free wave is not necessary to produce substantial excitation of the trapped wave. The maximum amplification factors produced in these numerical solutions, O(100) times the strength of the incident current, are consistent with previous studies. In the presence of nonlinear advection, strong residual currents are produced. The time-mean circulation about the seamount is dominated by a strong bottom-intensified, anticyclonic circulation closely trapped to the seamount. Maximum local time-mean current amplitudes are found to be as large as 37% of the magnitude of the propagating waves. In addition to the strong anticyclonic residual flow, there is a weaker secondary circulation in the vertical-radial plane characterized by downwelling over the top of the seamount at all depths. Maximum vertical downwelling rates of several tens of meters per day occur at the summit of the seamount. The vertical mass flux implied by this systematic downwelling is balanced by a slow radial flux of mass directed outward along the flanks of the seamount. Time-mean budgets for the radial and azimuthal components of momentum show that horizontal eddy fluxes of momentum are responsible for transporting net radial and azimuthal momentum from the far field to the upper flanks of the seamount. There, Coriolis and pressure gradient forces provide the dominant balances in the radial direction. However, the Coriolis force and viscous effects provide the primary balance for the azimuthal component.

Description: WHILE ammonites and all other ectocochleate cephalopods became extinct, nautiloids survived relatively unchanged from the Ordovician, suggesting that they are unusually well adapted to their niche. Here we obtain high-resolution tracks of Nautilus positions and depths, combined with telemetered jet pressures, which clarify both its lifestyle and economics. Nautilus is more active in nature than in captivity1, but its energy costs are lower than projected2,3. Viewing Nautilus as 'vertic', rather than benthic, resolves this contradiction. Records show that the cost of transport is the same in any direction within a vertical plane. Living on a reef face swept by a lateral current means that vertical movements4,5 sample large areas for chemical trails. A detected trail can be followed upcurrent in the slow-moving boundary layer, but no effort is wasted on horizontal movement without good prospects for food; long-range movements are downcurrent and made by drifting. Once fed, a Nautilus can reduce its energy costs by moving to deeper, cooler waters, where a single meal can last for months.

Description: The total transport of Antarctic Bottom Water across the Rio Grande Rise, including the western boundary, the Vema Channel, and the Hunter Channel is estimated from hydrographic measurements across these pathways. The contribution of the Vema Channel is greatest at 3.9 × 106 m3 s−1, which is very close to earlier estimates. The western boundary current contribution is 2.0 × 106 m3 s−1 and that of the Hunter Channel 0.7 × 106 m3 s−1. The lower values outside the Vema Channel are offset by the important source of mass they form to the lower density classes of bottom water. About 40% of the flow is concentrated in the highest density class representing the source of Weddell Sea Deep Water to the Brazil Basin. The flow structure is characterized by horizontal and vertical recirculation.

Description: THE vestimentiferan tubeworm Riftia pachyptila is found around hydrothermal vent areas in the deep sea. Intracellular bacterial chemoautotrophic symbionts use the oxidation of sulphide from the effluent of the vents as an energy source for CO2 fixation. They apparently provide most or all of the nutritional requirements for their gutless hosts1–5. This kind of symbiosis has since been found in many other species from various other phyla from other habitats6–9. Here we present results that the bacteria of R. pachyptila may cover a significant fraction of their respiratory needs by the use of nitrate in addition to oxygen. Nitrate is reduced to nitrite, which may be the end product (nitrate respiration)10 or it may be further reduced to nitrogen gas (denitrification)11. This metabolic trait may have an important role in the colonization of hypoxic habitats in general by animals with this kind of symbiosis.

Description: THE supply of dissolved inorganic carbon (DIC) is not considered to limit oceanic primary productivity1, as its concentration in sea water exceeds that of other plant macronutrients such as nitrate and phosphate by two and three orders of magnitude, respectively. But the bulk of oceanic new production2 and a major fraction of vertical carbon flux is mediated by a few diatom genera whose ability to use DIG components other than CO2, which comprises 〈 1% of total DIC3, is unknown4. Here we show that under optimal light and nutrient conditions, diatom growth rate can in fact be limited by the supply of CO2. The doubling in surface water pCO2 levels since the last glaciation from 180 to 355 p.p.m.5,6 could therefore have stimulated marine productivity, thereby increasing oceanic carbon sequestration by the biological pump.

Description: The space-time structure and predictability of the El Niño/Southern Oscillation (ENSO) phenomenon was investigated. Two comprehensive datasets were analyzed by means of an advanced statistical method, one based on observational data and the other on data derived from an extended-range integration performed with a coupled ocean-atmosphere general circulation model. It is shown that a considerable portion of the ENSO-related low-frequency climate variability in both datasets is associated with a cycle involving slow propagation in the equatorial oceanic beat content and the surface wind field. The existence of this cycle implies the ability of climate predictions in the tropics up to lead times of about one year. This is shown by conducting an ensemble of predictions with our coupled general circulation model. For the first time a coupled model of this type was successfully applied to ENSO predictions.

Description: A hybrid coupled model (HCM) of the tropical ocean–atmosphere system is described. The ocean component is a fully nonlinear ocean general circulation model (OGCM). The atmospheric element is a statistical model that specifies wind stress from ocean-model sea surface temperatures (SST). The coupled model demonstrates a chaotic behavior during extended integration that is related to slow changes in the background mean state of the ocean. The HCM also reproduces many of the observed variations in the tropical Pacific ocean-atmosphere system. The physical processes operative in the model together describe a natural mode of climate variability in the tropical Pacific ocean–atmosphere system. The mode is composed of (i) westward-propagating Rossby waves and (ii) an equatorially confined air–sea element that propagates eastward. Additional results showed that the seasonal dependence of the anomalous ocean–atmosphere coupling was vital to the model's ability to both replicate and forecast key features of the tropical Pacific climate system. A series of hindcast and forecast experiments was conducted with the model. It showed real skill in forecasting fall/winter tropical Pacific SST at a lead time of up to 18 months. This skill was largely confined to the central equatorial Pacific, just the region that is most prominent in teleconnections with the Northern Hemisphere during winter. This result suggests the model forecasts of winter SST at leads times of at least 6 months are good enough to be used with atmospheric models (statistical or OGCM) to attempt long-range winter forecasts for the North American continent. This suggestion is confirmed in Part II of this paper.

Description: A 26-year integration has been performed with a coupled ocean-atmosphere general circulation model (CGCM). The oceanic part resolves all three oceans in the latitude band 70°N–70°S but is dynamically active only between 30°N and 30°S. The atmosphere is represented by a global low-order spectral model. The coupled model was forced by seasonally varying insolation. Although the simulated time-averaged mean conditions in both atmosphere and ocean show significant deviations from the observed climatology, the CGCM realistically simulates the interannual variability in the tropical Pacific. In particular, the CGCM simulates an irregular ENSO with a preferred time scale of about 3 years. The mechanism for the simulated interannual variability in the tropical Pacific is related to both the “delayed action oscillator” and the “slow SST mode.” It therefore appears likely that either both modes can coexist or they degenerate to one mode within certain locations of the parameter space. This hypothesis is also supported by calculations performed with simplified coupled models, in which the atmospheric GCM was replaced by linear steady-state atmosphere models. Further, evidence is found for an eastward migration of zonal wind anomalies over the western Pacific prior to the extremes of the simulated ENSO, indicating a link to circulation systems over Asia. Because an earlier version of the CGCM did not simulate interannual variability in the tropical Pacific, additional experiments with a simplified coupled model have been conducted to study the sensitivity of coupled systems to varying mean oceanic background conditions. It is shown that even modest changes in the background conditions can push the coupled system from one flow regime into another.

Description: Ocean deep velocity profiles were obtained by lowering a self-contained 153.6-kHz acoustic Doppler current profiler (ADCP) attached to a CTD-rosette sampler. The data were sampled during two Meteor cruises in the western tropical Atlantic. The ADCP depth was determined by integration of the vertical velocity measurements, and the maximum depth of the cast was in good agreement with the CTD depth. Vertical shears were calculated for individual ADCP velocity profiles of 140-300-m range to eliminate the unknown horizontal motion of the instrument package. Subsequent raw shear profiles were then averaged with respect to depth to obtain a mean shear profile and its statistics. Typically, the shear standard deviations were about 10(-3) s-1 when using up and down traces simultaneously. The shear profiles were then vertically integrated to get relative velocity profiles. Different methods were tested to transform the relative velocities into absolute velocity profiles, and the results were compared with Pegasus dropsonde measurements. The best results were obtained by integrating the raw velocities and relative velocities over the duration of the cast and correcting for the ship drift determined from the Global Positioning System. Below 1000-m depth a reduction of the measurement range was observed, which results either from a lack of scatterers or instrumental problems at higher pressures.

Description: Inertial separation of a western boundary current from an idealized continent is studied in a homogeneous ocean circulation model. A number of processes are identified that either encourage or prevent separation at a coastal promontory in this model. For a single-gyre wind forcing a free-slip boundary condition forces the stream to follow the coastline, whereas the no-slip condition allows separation at a sharp corner. A prescribed countergyre to the north of the stream is not necessary to achieve separation if the no-slip condition is used. "Premature" separation occurs for wind fields that do not extend beyond the latitude of the cape. For a more realistic wind field and coastline two distinct states of the stream are found. At small Reynolds numbers the current fails to separate and develops a stationary anticyclonic meander north of the cape. Stronger currents separate and drive a recirculation in the lee of the continent.

Description: North Atlantic air-sea heat and freshwater flux data from several sources are used to estimate the conversion rate of water from one density to another throughout the range of sea surface density. This cross-isopycnal mass flux varies greatly over the ocean, with a maximum of 32.2 × 106 m3 s−1 at σ = 26.1 kg m−3 (toward greater densities) and a minimum of −7.6 × 106 m3 s−1 (toward lesser densities) at σ = 23.0 kg m−3. The air-sea fluxes force water to accumulate in three density bands: one at the lowest sea surface densities generated by heating; one centered near the density of subtropical mode water; and one spanning subpolar mode water densities. The transfer of water to the highest and lowest densities is balanced by mixing, which returns water to the middle density range, and also by boundary sources or sinks. Integrating the cross-isopycnal flux over all densities gives an annual average sinking of about 9 × 1O6 m3 s−1, which presumably escapes across the equator and must be balanced by a similar inflow. Comparison with estimates from tracer studies suggests that the renewal of tracer characteristics at a given density may occur without the existence of an annual average mass source at that density, because along- and cross-isopycnal mixing can renew a tracer without supplying mass.

Description: Antarctic Bottom Water flows into the western North Atlantic across the equator, shifting from the western side to the eastern side of the trough between the American continents and the Mid-Atlantic Ridge as it continues north. This is puzzling because such large-scale motion is thought to be controlled by dynamics that disallows an eastern boundary current. Previous explanations for the transposition involve a (necessarily small-scale) density current that changes sides because of the change in sign of rotation across the equator, or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current. Here an alternative explanation for the overall structure of bottom flow is given. A source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest. A simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to the bottom layer. For a strong enough source, the bottom layer thickness is zero along a grounding curve that separates the bottom water from the western boundary and confines it to the east. A band of recirculating interior flow occurs, supplied by an isolated northern and western boundary current. Similar structures appear to exist in the Antarctic Bottom Water of the western North Atlantic.

Description: Results of a three-dimensional primitive equation model are presented simulating turbulent mesoscale motions in the seasonal thermocline on an f plane. The model is based on a hybrid vertical coordinate scheme and conserves isopycnic potential vorticity. Mesoscale turbulence is modeled in terms of an unstable potential vorticity front. The model integration starts from a purely zonal, 60-km-wide geostrophically balanced jet, on which is superimposed a small initial perturbation. The most unstable mode exhibits a wavelength of 85 km and is driven by a mixed type of instability. Characteristic dynamical ingredients of the wave are enhanced cyclonic and anticyclonic relative vorticity in the troughs and the ridges, respectively, due to the curvature of the flow. Vertical motion of up to 10 m d−1 occurring downstream of the ridges (downwelling) and downstream of the troughs (upwelling) is driven by geostrophic advection of relative vorticity. The contrast of static stability across the front is changing during amplification of the instability: in troughs the stability is decreasing whereas in ridges it is increasing. The density field exhibits local anomalies of the isopycnals' depths (bumps) due to the ageostrophic cross-jet advection of potential vorticity streamers wound up in cyclones and anticyclones. Locally, the potential vorticity gradients are enhanced, creating a multiple front structure. The model results support observations and findings of earlier atmospheric and oceanic models. It is emphasized that mesoscale turbulent structures may have a profound influence on primary productivity, mixed-layer, and internal wave dynamics.

Description: In this paper, the historical hydrographic database for the south Indian Ocean is used to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the subtropical front (STF), and especially (ii) the southern current band of the gyre. A current band of increased zonal speeds in the upper 1000 m is found just north of the STF in the west near South Africa and at the surface STF in the open Indian Ocean until the waters off the coast of Australia are reached. As neither any other investigation of this current nor a name for it are known, the flow has been called the South Indian Ocean Current (SIOC). This name is anologous to the same current band in the South Atlantic Ocean, the South Atlantic Current. The STF is located in the entire south Indian Ocean near 40-degrees-S. The associated current band of increased zonal speeds is the SIOC, which is found at or north of the STF. East of 100-degrees-E the SIOC separates from the STF and continues to the northeast. The zonal flow south of the STF is normally weak and serves to separate the South Indian Ocean and Circumpolar currents. Near Africa the SIOC has a typical volume transport of 60 Sv (1 Sv = 10(6) m3 s-1) in the upper 1000 m relative to deep potential density surfaces of sigma(4) = 45.87 kg m-3 (2800-3500 m) or sigma(2) = 36.94 kg m-3 (1500-2500 m). Near western Australia the SIOC is reduced to about 10 Sv as it turns to the northeast.

Description: Oxygen and carbon isotope measurements were carried out on tests of planktic foraminilers N. pachyderma (sin.) from eight sediment cores taken from the eastem Arctic Ocean, the Fram Strait, and the Iceland Sea, in order to reconstruct Arctic Ocean and Norwegian-Greenland Sea circulation patterns and ice covers during the last 130,000 years. ln addition, the influence of ice, temperature and salinity effects on the isotopic signal was quantified. Isotope measurements on foraminifers from sediment surface samples were used to elucidate the ecology of N. pachyderma (sin.). Changes in the oxygen and carbon isotope composition of N. pachyderma (sin.) from sediment surface samples document the horizontal and vertical changes of water mass boundaries controlled by watertemperature and salinity, because N. pachyderma (sin.) shows drastic changes in depth habitats, depending on the water mass properties. It was able to be shown that in the investigated areas a regional and spatial apparent increase of the ice effect occurred. This happened especially du ring the termination 1 by direct advection of meltwaters from nearby continents or during the termination and in interglacials by supply of isotopically light water from rivers. A northwardly proceeding overprint of the "global" ice effect, increasing from the Norwegian-Greenland Sea to the Arctic Ocean, was not able to be demonstrated. By means of a model the influence of temperature and salinity on the global ice volume signal during the last 130.000 years was recorded. In combination with the results of this study, the model was the basis for a reconstruction of the paleoceanographic development of the Arctic Ocean and the Norwegian-Greenland Sea during this time interval. The conception of a relatively thick and permanent sea ice cover in the Nordic Seas during glacial times should be replaced by the model of a seasonally and regionally highly variable ice cover. Only during isotope stage 5e may there have been a local deep water formation in the Fram Strait.

Description: Characteristic of the mesoscale variability in the Atlantic Ocean are investigated by analyzing the Geosat altimeter signal between 60°S and 60°N. The rms sea-surface variability for various frequency bands is studied, including the high-frequency eddy-containing band with periods 〈150 days. Wavenumber spectra and spatial eddy characteristics are analyzed over 10° by 10° boxes covering both hemispheres of the Atlantic Ocean. A comparison, with solutions of a high-resolution numerical experiment, developed as the Community Modeling Effort of the World Ocean Circulation Experiment, aids interpretation of the Geosat results in the tropical and subtropical Atlantic and provides a test of the model fluctuating eddy field. Results from Geosat altimetry show a wavenumber dependence close to k1−5 (k1 being the alongtrack wave-number) over almost the entire Atlantic Ocean except for areas in the tropical and subtropical Atlantic where the rms variability in the eddy-containing band is less than 5 cm, that is, not significantly different from the altimeter noise level. Characteristic eddy length scales inferred from Geosat data are linearly related with the deformation radius of the first baroclinic mode over the whole Atlantic Ocean, except for the equatorial regime (10°S to 10°N). The data-model comparison indicates that the high-resolution model with horizontal grid size of ⅓° and ° in latitude and longitude is quite capable of simulating observed eddy characteristics in the tropics and subtropics. In mid- and high latitudes, however, the model fails to simulate the pronounced poleward decrease in eddy scales. This leads to systematic discrepancies between the model and Geosat observation, with model scales being up to 50% larger than deduced from altimetry.

Description: The time history of upper-ocean temperatures in the tropical Pacific has been used as a predictor in a statistical prediction scheme to forecast SST anomalies in this region. The temperature variations were taken from the output of an oceanic general circulation model that was forced by observed winds for the period 1961 to 1985. Since such model data are presently used as initial conditions in prediction experiments with coupled ocean–atmosphere models, it is of particular interest to investigate up to what lead time tropical Pacific SST is predictable without the coupling of an atmosphere model to the ocean model. We compared our results with those obtained by the persistence forecast and with those obtained by using the wind stresses themselves as predictors in a statistical forecast model. It is shown that using the upper ocean temperatures from the ocean model forced by observed winds gives significantly better skills at lead times of 6 to 12 months compared to persistence and to the pure wind-stress model. Off-equatorial heat content anomalies at 5°N are shown to contribute significantly to the predictability at these lead times, while those at 12°N do not.

Description: ALTHOUGH anthropogenic emissions of carbon dioxide have today created a greater atmospheric CO2concentration in the Northern than in the Southern Hemisphere, a comparison of interhemispheric CO2 profiles from 1980 and 1962 led Keeling and Heimann1,2 to conclude that, before the Industrial Revolution, natural CO2 sources and sinks acted to set up a reverse (south to north) gradient which drove about one gigatonne of carbon each year through the atmosphere from the Southern to the Northern Hemisphere. At steady state, this flux must have been balanced by a counter flow of carbon from north to south through the ocean. Here we present a means to estimate this natural flux by a separation of oceanic carbon anomalies into those created by biogenic processes and those created by CO2 exchange between the ocean and atmosphere. We find that before the Industrial Revolution, deep water formed in the northern Atlantic Ocean carried about 0.6 gigatonnes of carbon annually to the Southern Hemisphere, providing support for Keeling and Heimann's proposal. The existence of this oceanic carbon pump also raises questions about the need for a large terrestrial carbon sink in the Northern Hemisphere, as postulated by Tans et al.3, to balance the present global carbon budget.

Description: The seasonal cycles found in moored current measurements in the equatorial Somali Current region and along the equator between 50° and 60°E are compared with the multilayer Geophysical Fluid Dynamics Laboratory model for the tropical Indian Ocean. The remote forcing of Somali Current transport variations by incident long equatorial waves from the equatorial interior subthermocline region is investigated by analyzing the model velocities of annual and semiannual period. Amplitudes and phases of linear equatorial Rossby and Kelvin waves were least-squares fitted to the model velocities between 5°S and 5°N, 55° and 86°E from 100-m to 1000-m depth. Two cases of wave fits are distinguished: the “free” Kelvin wave case, where the Kelvin waves were fitted independently, and the “reflected” Kelvin wave case, where they were coupled to the Rossby waves by the western boundary condition for a straight slanted (45° to the north) coastline. The wave field velocities explained 70% of the spatial variance in the equatorial model subregion and also compared reasonably well with observed current variations along the equator. At the western boundary, the short-wave alongshore transport due to reflected incident long waves was determined and found to be antisymmetric about the equator. The maximum transport variation for the semiannual period due to the short waves was about 5 × 106 m3 s−1 between 150- and 800-m depth at 3° north and south of the equator. Observational evidence for the western boundary transport variations and the sensitivity to changes in the incident wave field are discussed.

Description: A primitive equation model of an idealized ocean basin, driven by simple, study wind and buoyancy forcing at the surface, is used to study the dynamics of mesoscale eddies. Model statistics of a six-year integration using a fine grid (1/6° × 0.2°), with reduced coefficients of horizontal friction, are compared to those using a coarser grid (1/3° × 0.4°), but otherwise identical configuration. Eddy generation in both model cases is primarily due to the release of mean potential energy by baroclinic instability. Horizontal Reynolds stresses become significant near the midlatitude jet of the fine-grid case, with a tendency for preferred energy transfers from the eddies to the mean flow. Using the finer resolution, eddy kinetic energy nearly doubles at the surface of the subtropical gyre, and increases by factors of 3–4 over the jet region and in higher latitudes. The spatial characteristics of the mesoscale fluctuations are examined by calculating zonal wavenumber spectra and velocity autocorrelation functions. With the higher resolution, the dominant eddy scale remains approximately the same in the subtropical gyre but decreases by a factor of 2 in the subpolar areas. The wavenumber spectra indicate a strong influence of the model friction in the coarse-grid case, especially in higher latitudes. Using the coarse grid, there is almost no separation between the energetic eddy scale and the scale where friction begins to dominate, leading to steep spectra beyond the cutoff wavenumber. Using the finer resolution an inertial subrange with a k−3 power law begins to emerge in all model regions outside the equatorial belt. Despite the large increase of eddy intensity in the fine-grid model, effects on the mean northward transport of heat are negligible. Strong eddy fluxes of heat across the midlatitude jet are almost exactly compensated by changes of the heat transport due to the mean flow.

Description: A high-resolution model of the wind-driven and thermohaline circulation in the North and equatorial Atlantic Ocean is used to study the structure and variability of the boundary current system at 26°N, including the Florida Current, the Antilles Current, and the Deep Western Boundary Current (DWBC). The model was developed by Bryan and Holland as a Community Modeling Effort of the World Ocean Circulation Experiment. Subsequent experiments have been performed at IfM Kiel, with different friction coefficients, and different climatologies of monthly mean wind stress: Hellerman–Rosenstein (HR) and Isemer–Hasse (IH). The southward volume transports in the upper 1000 m of the interior Atlantic, at 26°N, are 25.0 Sv (Sv ≡ 106m3s−1) for HR, and 34.9 Sv for IH forcing, in good agreement with the transport from the integrated Sverdrup balance at this latitude (23.9 Sv for HR, 35.6 Sv for IH). The return flow of this wind-driven transport, plus the southward transport of the DWBC (6–8 Sv), is partitioned between the Florida Current and Antilles Current. With HR forcing, the transport through the Straits of Florida is 23.2 Sv; this increases to 29.1 Sv when the wind stresses of IH are used. The annual variation of the simulated Florida Current is very similar to previous, coarse-resolution models when using the same wind-stress climatology (HR); the annual range (3.4 Sv) obtained with HR forcing is strongly enhanced (6.3 Sv) with IH forcing. The meridional heat transport at 26°N, zonally integrated across the basin, is in phase with the Florida Current; its annual range increases from 0.44 PW (HR) to 0.80 PW (IH). The annual signal east of the Bahamas is masked by strong transport fluctuations on a time scale of O(100 days), caused by an instability of the Antilles Current. By averaging over several model years, an annual cycle is extracted, which is in phase with the wind stress curl over the western part of the basin.

Description: Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period. The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation. The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient. The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.

Description: IN 1989, the submersible Nautile discovered one of the most active hydrothermal fields on the modern ocean floor, in the Lau back-arc basin (Fig. 1). The field contains high-temperature white and black smokers, and as we report here, its characteristics contrast strongly with those of the hydrothermal fields found at normal mid-ocean ridges. The main differences are the acidity (pH as low as 2), chemistry and temperature (up to 400 °C) of the hydrothermal fluids, the composition of the ore deposits, and the volcanic and tectonic environments. The fluids also have very high concentrations of trace metals, and primary gold is present in the accompanying mineral deposits. Our data show that these back-arc deposits in the Lau Basin are intermediate between typical mid-ocean-ridge mineralization and massive sulphide deposits of the Kuroko type.

Description: The response of the global climate system to smoke from burning oil wells in Kuwait is investigated in a series of numerical experiments using a coupled atmosphere–ocean general circulation model with an interactive soot transport model and extended radiation scheme. The results show a decrease in surface air temperature of ~4 °C in the Gulf region. Outside this region the changes are small and statistically insignificant. No weakening of the Indian summer monsoon is observed.

Description: The monthly mean wind stress climatology of Hellerman and Rosenstein (HR) is compared with the climatology of Isemer and Hasse (IH), which represents a version of the Bunker atlas (BU) for the North Atlantic based on revised parameterizations. The drag coefficients adopted by IH are 21% smaller than the values of BU and HR, and the calculation of wind speed from marine estimates of Beaufort force (Bft) is based on a revised Beaufort equivalent scale similar to the scientific scale recommended by WMO. The latter choice significantly increases wind speed below Bft 8, and effectively counteracts the reduction of the drag coefficients. Comparing the IH stresses with HR reveals substantially enhanced magnitudes in the trade wind region throughout the year. At 15°N the mean easterly stress increases from about 0.9 (HR) to about 1.2 dyn cm−1 (IH). Annual mean differences are smaller in the region of the westerlies. In winter, the effect due to the reduced drag coefficient dominates and leads to smaller stress values in IH; during summer season the revision of the Beaufort equivalents is more effective and leads to increased stresses. Implications of the different wind stress climatologies for forcing the large-scale ocean circulation are discussed by means of the Sverdrup transport streamfunction (ψs): Throughout the subtropical gyre a significant intensification of ψs takes place with IH. At 27°N, differences of more than 10 Sv (1 Sv ≡ 106 m3 s−1) are found near the western boundary. Differences in the seasonality of ψs are more pronounced in near-equatorial regions where IH increase the amplitude of the annual cycle by about 50%. An eddy-resolving model of the North Atlantic circulation is used to examine the effect of the different wind stresses on the seasonal cycle of the Florida Current. The transport predicted by the numerical model is in much better agreement with observations when the circulation is forced by IH than by HR, regarding both the annual mean (29.1 Sv vs 23.2 Sv) and the seasonal range (6.3 Sv vs 3.4 Sv).

Description: A low-cost underwater sound recorder has been developed and tested. It is designed to receive signals from sound sources that serve as navigation aids for RAFOS floats. This moored version of the RAFOS float (MAFOS) can monitor sound sources over many months and several hundred kilometers. It thus improves RAFOS navigation accuracy by enabling corrections for potential long-term clock drifts of the sound sources. MAFOS can also provide information on the local variation in the speed of sound due to natural hydrographic variability. In a first test, this usefulness has been proven and a warm, salty inhomogenity that traveled through a sound-source mooring array in the Iberian Basin has been observed.

Description: In this paper we use the historical hydrographic data base for the South Atlantic Ocean to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the Sub-tropical Front (STF), and (ii) the southern current band of the gyre, which is called the South Atlantic Current (SAC). The STF begins in the west in the Brazil-Falkland (Malvinas) confluence zone, but at locations at and west of 45°W this front is often coincident with the Brazil Current front. East of 45°W the STF appears to be a distinct feature to at least the region south of Africa, whereupon it continues into the Indian Ocean. The associated current band of increased zonal speed is the SAC, which, except for one instance, is found at or north of the surface STF until Indian Ocean water from the Agulhas retroflection is reached. A reversal of baroclinicity in the STF is observed south of a highly saline Agulhas ring, causing the SAC to separate from the STF and turn north into the Benguela Current. Zonal flow south of the STF is generally weak and serves to separate the South Atlantic and circumpolar currents. In the Argentine Basin, the SAC has a typical volume transport of 30 Sv (1 Sv = 106m3s−1) in the upper 1000 m relative to a deep potential density surface (σ4 = 45.87 kg m−3), and can be as high as 37 Sv. It is thus comparable to, or stronger than, the Brazil Current. In the Cape Basin, the transport of the SAC is reduced to about 15 SY before it turns north to feed the Benguela Current. In late 1983 this flow was joined by about 8 Sv of water from the Agulhas Current.

Description: The ECMWF-T21 atmospheric GCM is forced by observed near-global SST from January 1970 to December 1985. Its response in low level winds and surface wind stress over the Pacific Ocean is compared with various observations. The time dependent SST clearly induces a Southern Oscillation (SO) in the model run which is apparent in the time series of all variables considered. The phase of the GCM SO is as observed, but its low frequency variance is too weak and is mainly confined to the western Pacific. Because of the GCM's use as the atmospheric component in a coupled ocean-atmosphere model, the response of an equatorial oceanic primitive equation model to both the modeled and observed wind stress is examined. The ocean model responds to the full observed wind stress forcing in a manner almost identical to that when it is forced by the first two low frequency EOFs of the observations only. These first two EOFs describe a regular eastward propagation of the SO signal from the western Pacific to the central Pacific within about a year. The ocean model's response to the modeled wind stress is too weak and similar to the response when the observed forcing is truncated to the first EOF only. In other words, the observed SO appears as a sequence of propagating patterns but the simulated SO as a standing oscillation. The nature of the deviation of the simulated wind stress from observations is analyzed by means of Model Output Statistics (MOS). It is shown that a MOS-corrected simulated wind stress, if used to force an ocean GCM, leads to a significant enhancement of low frequency SST variance, which is most pronounced in the western Pacific.

Description: ISOTOPE ratios and concentrations of incompatible trace elements are remarkably successful in discriminating the tectonic origin and magmatic source components for basalts1–5. But problems remain with discriminating the tectonic origin of some tholeiites, especially where field relations and other geological evidence are ambiguous. For example, the tectonic origin of basalts from the Troodos ophiolite (Cyprus) has been debated for several decades. Most workers have been unable to distinguish between an island-arc and/or back-arc origin for the ophiolite6–8. Here we use volatile, K2O and TiO2 contents from ∼250 fresh submarine volcanic glasses to discriminate between tholeiites from different tectonic regimes. K2O÷H2O ratios are lower (〈0.70) in spread ing-centre glasses than in those from island arcs and intraplate oceanic islands. Back-arc-basin basalts can generally be separated from mid-ocean-ridge basalts by their high H2O contents. Using this information, we show that some fresh glasses from the Troodos ophiolite have a clear back-arc-basin affinity.

Description: RECENT advances in 40Ar/39Ar dating1,2 have made it possible to date individual K-feldspar grains from Pleistocene tephra, a capability that greatly improves the reliability and temporal resolving power of the method. Here we apply these new techniques to the dating of a phonolite tephra from the East Eifel volcanic field in West Germany, which is sandwiched between loess and palaeosol (alfisol) deposits, and which was therefore erupted during the transition from a glacial to an interglacial period. Our age estimate for this transition is 215±4 kyr (1 σ), which has important implications for the marine δ18O timescale and for models of global climate change during the Pleistocene. The results show that single-grain dating can detect and compensate for the large quantities of xenocrystic contaminants which are found in many tephra deposits. This technique could be used to date the tephra layers found in marine sediment cores and the results could greatly enhance the reliability of the marine δ18O timescale for more rigorous Fourier analysis testing of the Milankovitch hypothesis.

Description: Historical data from the region between the Greenwich meridian and the African continental shelf are used to compute the offshore geostrophic transport of the Benguela Current. At 32°S, the Benguela Current is located near the African coast, transporting about 21 Sv (1 Sv = 106 m3 s−1) of surface water toward the north relative to a potential density surface lying between the upper branch of Circumpolar Deep Water and the North Atlantic Deep Watar. Two warm core eddies of probable Agulhas Current origin an observed west of the Benguela Current at 32°S. Near 30°S, the Benguela Current turns toward the northwest and begins to separate from the eastern boundary. It carries about 18 Sv of surface water across 28°S. The current then turns mainly toward the west to flow over a relatively deep segment of the Walvis Ridge south of the Valdivia Bank. A surface current with northward surface of about 10 cm s−1 flows along the western side of the Valdivia Bank, while another northward surface current flows at about 20 cm s−1 some 300 km west of the bank. About 3 Sv of surface now do not leave the Cape Basin south of the Vaidivia Bank, but instead drift northward as a wide. sluggish flow out of the northern end of the Cape Basin. Because of the more southerly seaward extensions of most of the Benguela Current, there are no deep-reaching interactions observed between this current and the cyclonic gyre in the Angola Basin east of the Greenwich meridian. Beneath the surface layer, about 4–5 Sv of Antarctic Intermediate Water are carried northward across 32° and 28°S by the Benguela Current, essentially all of which turns westward to cross the Greenwich meridian south of 24°S.

Description: ROOTH proposed that the Younger Dryas cold episode, which chilled the North Atlantic region from 11,000 to 10,000 yr BP, was initiated by a diversion of meltwater from the Mississippi drainage to the St Lawrence drainage system. The link between these events is postulated to be a turnoff, during the Younger Dryas cold episode, of the North Atlantic's conveyor-belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the North Atlantic region2. This turnoff is attributed to a reduction in surface-water salinity, and hence also in density, of the waters in the region where North Atlantic Deep Water (NADW) now forms. Here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from Orca Basin core EN32-PC4 which reveal a significant reduction in meltwater flow through the Mississippi River to the Gulf of Mexico from about 11,200 to 10,000 radiocarbon years ago. This finding is consistent with the record for Lake Agassiz which indicates that the meltwater from the southwestern margin of the Laurentide Ice Sheet was diverted to the northern Atlantic Ocean through the St Lawrence valley during the interval from ~11,000 to 10,000 years before present (yr BP).

Description: We report a study of a coastal frontal zone of the southeastern United States based on a field experiment and numerical modeling. The study was conducted in the spring of 1985 during weak to moderate wind stress and strong input of buoyancy from solar radiation and river discharge. The study confirms that the structure and slope of the frontal zone depends on a combination of wind stress and cross-shelf advection of buoyancy. A cross-shelf/depth two-dimensional (x, y), time-dependent numerical model illustrated the response of the frontal zone to the local wind stress regimes. A comparison of model results with field data showed that the model successfully predicted onsets of stratification and mixing. When alongshore wind stress was negative (southward), isopycnals in the frontal zone steepened due to a combination of horizontal advection and vertical convection. When stress was positive (northward), the offshore advection of low density water flattened the isopycnals and potential energy decreased, demonstrating that horizontal advection terms are important in the equation of conservation of buoyancy. The model predicts die offshore advection of lenses of less dense water during upwelling-favorable wind stress. These lenses are of the order of 20 km in cross-shelf scale and represent an efficient mechanism to export nearshore water. The lenses consist of a mixture of low-salinity coastal water and continental shelf water originating further offshore and advected onshore along the bottom. The mean flow inside the frontal zone opposed the mean alongshore wind stress. Part of the alongshore flow was in geostrophy with the cross-shore pressure gradient; the other part was due to an alongshore pressure gradient force (kinematic) of about 1 × 10−6 m s−2 (equivalent sea surface slope = 1 × 10−7), which was trapped along the coast with an offshore width scale of O(10 km). It is likely that the alongshore extent of this pressure gradient was governed by the scale at which freshwater is injected to the continental shelf, i.e., 20–30 km. The pressure gradient force immediately outside of the frontal zone was about −5 × 10−7 m s−2 in the direction of the mean alongshore wind stress. It is hypothesized that, as a result of wind setup and freshwater influx, the northward pressure gradient forced over outer shelf/slope by the Gulf Stream decreases in magnitude onshore, and can even change sign across a nearshore frontal zone of O(10 km). The implied flow field near the frontal zone is therefore highly three-dimensional with |∂v/∂y|≈|∂u/∂x|, where (u, v) are velocities in the cross-shore (x) and alongshore (y) directions, respectively.

Description: The sensitivity of the global climate system to interannual variability of he Eurasian snow cover has been investigated with numerical models. It was found that heavier than normal Eurasian snow cover in spring leads to a “poor” monsoon over Southeast Asia thereby verifying an idea over 100 years old. The poor monsoon was characterized by reduced rainfall over India and Burma, reduced wind stress over the Indian Ocean, lower than normal temperatures on the Asian land mass and in the overlying atmospheric column, reduced tropical jet, increased soil moisture, and other features associated with poor monsoons. Lighter than normal snow cover led to a “good” monsoon with atmospheric anomalies like those described above but of opposite sign. Remote responses from the snow field perturbation include readjustment of the Northern Hemispheric mass field in midlatitude, an equatorially symmetric response of the tropical geopotential height and temperature field and weak, but significant, perturbations in the surface wind stress and heat flux in the tropical Pacific. The physics responsible for the regional response involves all elements of both the surface heat budget and heat budget of the full atmospheric column. In essence, the snow, soil and atmospheric moisture all act to keep the land and overlying atmospheric column colder than normal during a heavy snow simulation thus reducing the land–ocean temperature contrast needed to initiate the monsoon. The remote responses are driven by heating anomalies associated with both large scale air-sea interactions and precipitation events. The model winds from the heavy snow experiment were used to drive an ocean model. The SST field in that model developed a weak El Niño in the equatorial Pacific. A coupled ocean-atmosphere model simulation perturbed only by anomalous Eurasian snow cover was also run and it developed a much stranger El Niño in the Pacific. The coupled system clearly amplified the wind stress anomaly associated with the poor monsoon. These results show the important role of an evolving (not specified) sea surface temperature in numerical experiments and the real climate system. Our general results also demonstrate the importance of land processes in global climate dynamics and their possible role as one of the factors that could trigger ENSO events.

Description: We examine the diffusive behavior of the flow field in an eddy-resolving, primitive equation circulation model. Analysis of fluid particle trajectories illustrates the transport mechanisms, which are leading to uniform tracer and potential vorticity distributions in the interior of the subtropical thermocline. In contrast to the assumption of weak mixing in recent analytical theories, the numerical model indicates the alternative of tracer and potential vorticity homogenization on isopycnal surfaces taking place in a nonideal fluid with strong, along-isopycnal eddy mixing. The eastern, ventilated portion of the gyre appears to be sufficiently homogeneous to allow the concept of an eddy diffusivity to apply. A break in a random walk behavior of particle statistics occurs after about 100 days when along-flow dispersion sharply increases, indicative of mean shear effects. During the first months of particle spreading, eddy dispersal and mean advection are of similar magnitude. Eddy kinetic energy is of O(60–80 cm2 s−2) in the model thermocline, comparable to the pool of weak eddy intensity found in the eastern parts of the subtropical oceans. Eddy diffusivity in the model thermocline (Kxx = 8 × 107, Kyy = 3 × 107 cm2 s−1) seems to be higher by a factor of about 3 than oceanic values estimated for these area. Below the thermocline, model diffusivity decreases substantially and becomes much more anisotropic, with particle dispersal preferentially in the zonal direction. The strong nonisotropic behavior, prominent also in all other areas of water eddy intensity, appears as the major discrepancy when compared with the observed behavior of SOFAR floats and surface drifters in the ocean.

Description: Abrupt changes in climatic conditions have been seen at high latitudes in the North Atlantic and the Antarctic at 13 kyr BP. It is important to determine whether this abrupt change was confined to high-latitude regions or whether it was global. Here we present results demonstrating an abrupt change in the rate and character of sedimentation in the South China Sea at the close of the last glacial period. Radiocarbon dating and its position in the oxygen isotope shift suggest that this change may be coincident with the changes found at high latitudes.

Description: Abrupt changes in climatic conditions have been seen at high latitudes in the North Atlantic1 and the Antarctic2,3 at 13 kyr BP. It is important to determine whether this abrupt change was confined to high-latitude regions or whether it was global. Here we present results demonstrating an abrupt change in the rate and character of sedimentation in the South China Sea at the close of the last glacial period. Radiocarbon dating and its position in the oxygen isotope shift suggest that this change may be coincident with the changes found at high latitudes.

Description: Perhaps the most significant event in the Cretaceous record of the carbon isotope composition of carbonate1,2, other than the 1–2.5 ‰ negative shift in the carbon isotope composition of calcareous plankton at the Cretaceous/Tertiary boundary3, is the rapid global positive excursion of ~2 ‰ (13C enrichment) which took place between ~91.5 Myr and 90.3 Myr (late Cenomanian to earliest Turonian (C/T boundary event))1,4,5. This excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon (TDC) reservoir resulting from an increase in rate of burial of 13C-depleted organic carbon, which coincided with a major global rise in sea level5 during the so-called C/T oceanic anoxic event (OAE)6. Here we present new data, from nine localities, which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the C/T boundary7,8 is nearly synchronous with that for carbonate and is widespread throughout the Tethys and Atlantic basins (Fig. 1), as well as in more high-latitude epicontinental seas. The postulated increase in the rate of burial of organic carbon may have had a significant effect on CO2 and O2 concentrations in the oceans and atmosphere, and consequent effects on global climate and sedimentary facies.

Description: A coupled ocean-atmosphere general circulation model has been developed for TOGA related problems. The coupled model consists of an ocean model of the tropical Pacific and a global low-order spectral atmosphere model. The two models interact via wind stress and sea surface temperature. In order to avoid a climate drift within the coupled model, a flux correction method is applied.Experiments were performed by introducing a westerly wind stress burst over the western equatorial Pacific for one month. Thereafter, the wind burst is turned off and the response of the coupled model to the initial disturbance is investigated. The results are compared with the response of the ocean model run with the same disturbance in an uncoupled mode.It is shown that the coupling leads to a significant increase of the duration of anomalous conditions in the ocean. SST anomalies persist for about 12 months in the coupled run, while they have already disappeared after 4 months in the uncoupled case. The increase in persistence is due to the feedback of the atmosphere, which responds with an eastward shift of the ascending branch of the Walker Circulation.In a second experiment with the coupled model the initial disturbance was introduced within another season. The results show no basic differences to the results of the first experiment.An interesting result of the coupled model runs is the occurrence of spontaneous westerly wind bursts over the western Pacific, which developed by internal dynamics. Location and duration of these spontaneous wind bursts show some correspondence with the time-space structure of observed westerly wind stress episodes over the western Pacific.

Description: Hurricanes and other strong storms can cause important decreases in sea surface temperature by means of vertical mixing within the upper ocean, and by air–sea heat exchange. Here we use satellite-derived infrared images of the western North Atlantic to study sea surface cooling caused by hurricane Gloria (1985). Significant regional variations in sea surface cooling are well correlated with hydrographic conditions. The greatest cooling (up to 5°C) occurred in slope waters north of the Gulf Stream where the seasonal thermocline is shallowest and most compressed; moderate cooling (up to 3 °C) occurred in the open Sargasso Sea where the thermocline is deeper and more diffused; little or no cooling occurred in shallow coastal waters (bottom depth less than 20 m) which were isothermal before the passage of hurricane Gloria. There is a pronounced right-side asymmetry of sea surface cooling with stronger (by a factor of 4) and more extensive (by a factor of 3) cooling found on the right side of the hurricane track. These qualitative results are consistent with the notion that vertical mixing within the upper ocean is the dominant sea surface cooling mechanism of hurricanes.

Description: The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.

Description: Quasi-homogeneous layers in vertical profiles of temperature and salinity in the eastern North Atlantic near Madeira indicate the existence of a subtropical Mode Water in the Eastern Basin. Temperature sections show a maximum horizontal extent of at least 500 km. The frequency distribution analysis of homogeneous layers in a historical XBT dataset shows a Mode Water formation region near and to the north of Madeira. This Mode Water is found at increasing depths and displaced to the west and southwest during the course of the year after its formation by wintertime convection. It disappears almost completely, due to mixing, before the next winter. Volume estimates suggest that this Madeira Mode Water in the eastern Atlantic accounts for 15–20% of the total Central Water formation in the corresponding density range as obtained from tracer studies in the North Atlantic gyre.

Description: A primitive equation model of the equatorial Pacific Ocean was forced by realistic wind stress distributions over decades. Results were presented for a set of two experiments. In the first experiment the model was forced by an objectively analyzed wind field, while for the second experiment a subjectively analyzed wind field was used. The results indicate a strong sensitivity of the model to the choice of the wind fields. Especially, model results in the eastern Pacific show big differences between the two model runs. Taking the results of the second model run the performance of the model with respect to interannual variability is investigated. Sea level, temperature and zonal currents show pronounced interannual variations within the equatorial belt from 10°N to 10°S. Special attention is given to the simulation of the 1982/83 El Niño event. The model reproduces most of the basic features, which were observed during this El Niño event. In particular the deceleration of the equatorial undercurrent, the evolution of eastward surface currents and the zonal redistribution of heat associated with an eastward propagation of warm water are simulated by the model.

Description: There is now clear evidence that changes in the Earth's climate may be sudden rather than gradual. It is time to put research into the build-up of carbon dioxide in the atmosphere on a better footing.

Description: Data from a surface mooring located in the Sargasso Sea at 34°N, 70°W between May 1982 and May 1984 were compared with satellite data to investigate large diurnal sea surface temperature changes. Mooring and satellite measurements are in excellent agreement for those days on which no clouds covered the site at the time of the satellite pass. During the summer half-year at this site, there is a 20% charm of diurnal warming of more than 0.5°C, with values of up to 3.5°C observed in the two-year period. Diurnal warming observed at the mooring has been simulated well by a one-dimensional model driven by local beat and momentum fluxes. Under the conditions of very light wind and strong insolation that produce the Largest surface warming, the surface mixed-layer depth reduces to the convection depth, and wind-mixing becomes unimportant. The thermal response is then limited to depths between 1 and 2 m, making it likely that such events have been underreported in routine ship observations. In all cases observed, the spatial extent of warming events as determined by satellite data are well correlated with the corresponding atmospheric pressure patterns. Conditions giving rise to the largest diurnal warming events are often associated with a westward-extending ridge of the Bermuda high. In the region studied, 57°–75°W and 29°–43°N, diurnal warming of more than 1°C was found on occasion to cover areas in excess of 300 000 km2, with warming of more than 2°C coveting areas in excess of 130 000 km2.

Description: An XBT interface is described for use with Commodore and other 6502 based microprocessors. This interface takes the form of a single circuit board mounted inside the microcomputer and is completely software controlled. The application of this digital XBT system to the real-time computation of density and dynamic height, using historical or recent temperature-salinity relationships, is also described. Comparison between XBT and CTD measured temperatures from the Northeast Atlantic yield a mean temperature difference of −0.08°C and an rms temperature difference of 0.33°C for the upper 800 m. Examples of dynamic topography maps and a temperature section computed using this technique are also presented and comparison between objectively analyzed XBT and CTD dynamic topographies demonstrates the reliability of the method for mapping the baroclinic flow.

Description: The inclination of oceanographic mooring lines due to current drag causes errors in time series observations of currents and temperatures. The prediction of this effect requires knowledge of the drag coefficients for the mooring components. Drag coefficients, known for simple geometric shapes such as spheres or cylinders, are commonly used for mooring response computations. Selected mooring components (buoyancy elements and instruments) were tested in a tow tank to determine their actual drag coefficients. Over the Reynolds Number range, typical of oceanic conditions, deviations of the drag coefficient up to 50% are found when compared with the appropriate simple geometric shape coefficients. A set of model moorings and model current profiles is used to determine the resulting changes in component depth level and displacement. The changes in horizontal displacement of the upper part of the mooring are on the order of 10% in extreme cases and 1% under typical conditions. Their effects on current measurements will usually be negligible. However, the related vertical displacements are on the order 100 to 10 m. Such vertical displacements may carry instruments to depth levels where currents and particularly thermocline temperatures are sufficiently different from the intended level to cause errors in the time series observations.

Description: Simulated transient-tracer distributions (tritium, 3H3, freons) on the isopycnal horizons σ0=26.5 and 26.8 kg m−3 are presented for the East Atlantic, 10° −40°N. Tracer transport is modeled by employing a baroclinic flow field based on empirical data in a kinematic isopycnal advection-diffusion numerical model, in which winter convection is taken as the mechanism of communication with the ocean surface layer, and the isopycnal diffusivity is a free parameter. Diapucnic transport is ignored. The simulations employ time-dependent tracer boundary conditions, which are constructed on the basis of available observations. Simulations are compared to data obtained on a meridional section in 1981 (F/S Meteor, cruise 56/5). Best simulations were obtained by means of a subjective optimization procedure. On both levels, the observed distributions and the best simulated distributions agree well. The fact that the surface boundary conditions and interior distributions of the tracers are distinctly different leads us to the conclusion that our model provides a consistent description of upper main-thermocline ventilation and interior transport Surface-water densities in February are found to represent adequately the winter outcrop boundaries with an uncertainty of about ±300 km across. The required isopycnal diffusivity south of 29°N is 1700 m2 s−1, and 2900 m2 s−1 further north (+70/−40%). Interior transport is found to be predominantly advective. Advective ventilation across 30.5°N east of 33°W amounts to only 12% and 40% for the 26.5 and 26.8 horizons of the total ventilation rates reported by Sarmiento. The North Atlantic/South Atlantic Central Water boundary near 15°N is found to be predominantly determined by advection.

Description: One of the most striking features of the upper North Atlantic Ocean is an extensive layer of water with temperature close to 18°C and salinity close to 36.5‰, (ref. 1). This 18°C water is formed by winter convection in the Sargasso sea2,3, but aspects of the annual rate of 18°C water formation remain obscure4. We have simulated this water mass formation by integrating a one-dimensional model along a 4-yr trajectory of a water column circulating around the Sargasso Sea. Winter convection is deep (≥200 m) in regions where the ocean suffers a net annual heat loss to the atmosphere, and shallow (≤lOOm) where the ocean gains heat each year. The origin of the thermostad (nearly isothermal layer) is a thick layer of nearly homogeneous water subducted beneath the seasonal boundary layer in the year that the water column passes through the line dividing annual cooling from annual heating. We estimate the annual production of 18°C water to be 446,000 km3 yr−1. Downstream, more stratified central water is formed each year at a rate that depends more on Ekman pumping (wind-forced convergence) than on the decreasing depth of winter convection

Description: Long-term temperature and current-meter records from moorings in the northern Canary Basin display strong current events with time scales between one and three months and large vertical scales of several thousand meters. The data are compared to hydrographic surveys in the area that show a meandering subtropical front. The strong current events are found to be related to the passage of the front through the mooring positions. An analysis of composite time series, for selected depths, indicates cases of westward and of eastward propagation of frontal meanders. The frontal pattern is also found in geopotential anomalies inferred from historical XBT data sets, suggesting that the front is a persistent feature of the density field. In two cases strong current events appear to be related to a Mediterranean Water lens.

Description: Marine organic carbon is heavier isotopically (13C enriched) than most land-plant or terrestrial organic C1. Accordingly, δ 13C values of organic C in modern marine sediments are routinely interpreted in terms of the relative proportions of marine and terrestrial sources of the preserved organic matter2,3. When independent geochemical techniques are used to evaluate the source of organic matter in Cretaceous or older rocks, those rocks containing mostly marine organic C are found typically to have lighter (more-negative) δ 13C values than rocks containing mostly terrestrial organic C. Here we conclude that marine photosynthesis in mid-Cretaceous and earlier oceans generally resulted in a greater fractionation of C isotopes and produced organic C having lighter δ 13C values. Modern marine photosynthesis may be occurring under unusual geological conditions (higher oceanic primary production rates, lower P CO2) that limit dissolved CO2 availability and minimize carbon isotope fractionation4.

Description: The climate record obtained from two long Greenland ice cores reveals several brief climate oscillations during glacial time. The most recent of these oscillations, also found in continental pollen records, has greatest impact in the area under the meteorological influence of the northern Atlantic, but none in the United States. This suggests that these oscillations are caused by fluctuations in the formation rate of deep water in the northern Atlantic. As the present production of deep water in this area is driven by an excess of evaporation over precipitation and continental runoff, atmospheric water transport may be an important element in climate change. Changes in the production rate of deep water in this sector of the ocean may push the climate system from one quasi-stable mode of operation to another.

Description: STEP-LIKE structures in temperature and salinity beneath the Mediterranean water have been observed in the Eastern Atlantic1–6. In Fig. 1 we show the stations where steps have been found in the area to the west of Gibraltar. Salt fingering as a result of double diffusive processes has been suggested as a possible cause for the generation of such step-like structures7. During cruise No. 23 of RV Meteor in 1971 we intended to study the small scale features of such structures8. Some previous observations6 in August/September 1970 had revealed an extensive zone where a “thermohaline staircase” existed (Fig. 1). We therefore selected stations in this area and close to it for the proposed study. A high resolution in situ conductivity-temperature-depth meter of the “Kieler Multi-Meeressonde” type was used for the vertical profiling of temperature and salinity.

Description: In the southern hemisphere, female and young male sperm whales (up to about 39 feet long) are not normally found in higher latitudes than 40° S while large males occur in Antarctic waters1–3; clearly many large bulls must migrate from the breeding areas into colder regions. Evidence of the return of large bulls to lower latitudes rests upon marking them in the Antarctic4 or external infestation by Antarctic Cocconeis or Cyamus 5. Only a single mark5 has been recovered which provides direct evidence for the return north from Antarctic waters. This mark (USSR No. 650203) was fired on December 25, 1967, at 62° 22′ S 26° 25′ E and the whale was killed on May 13, 1968, off Durban. The small size of the male concerned (35 feet at death) makes this record rather surprising although Jonsgård6 did mention that the smallest whales from Antarctic waters were about 35 feet. Marking can provide information on only a small part of the whale population at considerable cost, freshness of the whale restricts the value of infestation as an indicator but the study of food remnants in sperm whale stomachs provides another method without these disadvantages.

Description: Marine Upper Jurassic sediments have recently been reported from South Africa (Knysna Outlier, Cape Province) for the first time1. They occur as shallow water, sandy clays (Brenton Beds) in association with terrestrial/fluviatile conglomerates and sandstones, which were deposited in an approximately east-west elongate intermontane basin between the Cape Fold mountains (formed of Lower Palaeozoic sediments). Post-Cretaceous erosion has reduced the original deposits to a series of small, isolated outliers, only two of which have been reported to contain marine sediments (Knysna, lower Upper Jurassic; Algoa, Valanginian2) (Fig. 1). Extensive Neocomian-Maas-trichtian outcrops are known from the continental shelf off to the south of South Africa3, and a complete mid-Jurassic to Upper Cretaceous marine succession is suspected on the Agulhas Bank infilling and overlying east-west striking, fault bounded folds of Lower Palaeozoic Cape Supergroup rocks as shown in Fig. 1 (R. V. D., in preparation and refs. 1 and 4).