ALBERT

Description: Dissolved and atmospheric nitrous oxide (N2O) were measured on the legs 3 and 5 of the R/V Meteor cruise 32 in the Arabian Sea. A cruise track along 65°E was followed during both the intermonsoon (May 1995) and the southwest (SW) monsoon (July/August 1995) periods. During the second leg the coastal and open ocean upwelling regions off the Arabian Peninsula were also investigated. Mean N2O saturations for the oceanic regions of the Arabian Sea were in the range of 99–103% during the intermonsoon and 103–230% during the SW monsoon. Computed annual emissions of 0.8–1.5 Tg N2O for the Arabian Sea are considerably higher than previous estimates, indicating that the role of upwelling regions, such as the Arabian Sea, may be more important than previously assumed in global budgets of oceanic N2O emissions.

Description: Stable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates.

Description: Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.

Description: The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide-angle seismic reflection data, whereas offshore we have used coincident steep and wide-angle reflection data along a NE-SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.

Description: An initially resting ocean of stratification N is considered, subject to buoyancy loss at its surface of magnitude B0 over a circular region of radius r, at a latitude where the Coriolis parameter is f. Initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer. However, as deepening proceeds, horizontal density gradients at the edge of the forcing region support a geostrophic rim current, which develops growing meanders through baroclinic instability. Eventually finite-amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below, setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface. In this final state quasi-horizontal baroclinic eddy transfer dominates upright “plume” convection. By using “parcel theory” to consider the energy transformations taking place, it is shown that the depth, hfinal at which deepening by convective plumes is arrested by lateral buoyancy flux due to baroclinic eddies, and the time tfinal it takes to reach this depth, is given by both independent of rotation. Here γ and β are dimensionless constants that depend on the efficiency of baroclinic eddy transfer. A number of laboratory and numerical experiments are then inspected and carried out to seek confirmation of these parameter dependencies and obtain quantitative estimates of the constants. It is found that γ = 3.9 ± 0.9 and β = 12 ± 3. Finally, the implications of our study to the understanding of integral properties of deep and intermediate convection in the ocean are discussed.

Description: A three-dimensional Monte Carlo transfer model for polarized radiation is developed and used to study three-dimensional (3-D) effects of raining clouds on the microwave brightness temperature. The backward method is combined with the forward method to treat polarization correctly within the cloud. In comparison with horizontally homogeneous clouds, two effects are observed: First, brightness temperatures from clouds are reduced in the 3-D case due to net leakage of radiation from the sidewalls of the cloud. Second, radiation which is emitted by the warm cloud and then reflected from the water surface increases the brightness temperatures of the cloud-free areas in the vicinity of the cloud. Both effects compete with each other, leading to either lower or higher overall brightness temperatures, depending on the geometry of the cloud, the satellite viewing angle, the coverage, and the position of the cloud within the field of view (FOV) of the satellite. At 37 GHz, for example, up to 10 K differences can occur for a cloud of 50% coverage. Finite homogeneous raining clouds matching the size of the FOV of the satellite show a similar relationship between rain rates and brightness temperatures (TB) as horizontally infinite clouds. Namely, an increase of TB with increasing rain rates at low rain rates, due to emission effects, is followed by a decrease due to temperature and scattering effects. For small horizontal cloud diameter, however, the 3-D brightness temperatures may show a second maximum due to the decrease of the leakage effect with increasing rain rates. At nadir, 3-D brightness temperatures are always lower than the 1-D values with differences up to 20 K for a cloud of 5-km vertical extent and a base of 1 × 1 km. To quantify the 3-D effects for more realistic cloud structures, we used results of a three-dimensional dynamic cloud model as input for the radiative transfer codes. The same 3-D effects are obtained, but the differences between 1-D and 3-D modeling are smaller. In general, most of the differences between the 1-D and 3-D results for off-nadir view angles are pure geometry effects, which can be accounted for in part by a modified 1-D model.

Description: The incidence angles of the SSM/I radiometers on the DMSP satellites vary from satellite to satellite and exhibit variations of up to 1.5° during one orbit. The effects of these variations on the measured brightness temperatures are investigated on the basis of simulated and measured data for oceanic arm. A deviation of 1° from the nominal incidence angle of 53.0° causes brightness temperature changes of up to 2 K depending on surface and atmospheric conditions. Errors of retrieved geophysical parameters on the order of 5%–10% result when the incidence angle variation is not taken into account. This is a common property of most published statistical algorithms. For total precipitable water and cloud liquid water content the error increases with increasing parameter value. For wind speed the error is largest for low wind speed and decreases with increasing wind speed. Due to the slowly varying latitudinal dependence of the incidence angle, these errors do not cancel out when monthly means are computed. A correction method is developed on the basis of simulated data and tested successfully with measured data. Observed brightness temperature differences between DMSP F10 and F11 are reduced when using corrected data. If diurnal variations of geophysical parameters are investigated, the incidence angle correction is mandatory to obtain useful results, especially for DMSP F10.

Description: A weakly nonhydrostatic, two-layer numerical model based on the Boussinesq equations is presented which is capable of describing, among others, the generation and propagation of nonlinear weakly dispersive internal waves in the Strait of Gibraltar. The model depends on one space coordinate only, but it retains several features of a fully three-dimensional model by including a realistic bottom profile, a variable channel width, and a trapezoidal channel cross section. The nonlinear primitive Boussinesq equations include horizontal diffusion, bottom friction, and friction between the two water layers. The model is driven by a height difference of the mean interface depth between the Atlantic and the Mediterranean boundaries and by semidiurnal tidal oscillations of the barotropic transport. The model presented in this paper describes (1) the mean and tidal flow in the Strait of Gibraltar, (2) the variation of the depth of the interface during a tidal cycle, (3) the generation of strong depressions of the interface at the western sides of the Spartel Sill and the Camarinal Sill, (4) the generation of strong eastward propagating internal bores, and (5) their disintegration into trains of internal solitary waves. The surface convergence patterns associated with depressions of the interface at the Camarinal Sill, internal bores, and internal solitary waves are calculated and compared with roughness patterns visible on synthetic aperture radar (SAR) images of the first European Remote Sensing Satellite ERS 1. In total, 155 ERS 1 SAR scenes from 94 satellite overflights over the Strait of Gibraltar, which were acquired in the period from January 1992 to March 1995, have been analyzed. It is shown that the proposed model is capable of explaining the observed temporal and spatial evolution of surface roughness patterns associated with eastward propagating internal waves inside the Strait of Gibraltar as well as the observed east-west asymmetry of the internal wave field.

Description: From August 11 to 22, 1993, a conductivity‐temperature‐depth/acoustic Doppler current profiler survey was carried out in the Somali‐Socotra region to investigate currents and transports associated with the Great Whirl and Socotra Gyre circulation during the height of the summer monsoon. The monsoon circulation was confined to the upper 300 m depth, with intense surface currents up to 2.2 m s−1 in the Great Whirl and up to 1.4 m s−1 in the Socotra Gyre. Deeper‐reaching flow was found in the northwestern part of the Somali Basin and in the passage between the shelf of Somalia and Abd al Kuri. The Great Whirl transport was 58 Sv, of which nearly 25% were due to ageostrophic flow components. The northern part of the Great Whirl thereby appeared as a closed circulation cell in which the offshore transport was balanced by a southward transport of the same magnitude. Upwelled water was advected from the cold wedge of the upwelling regime at the Somali coast along the edge of the gyre. The water in the center of the gyre had the characteristics of Indian Equatorial Water (IEW). The Socotra Gyre carried 23 Sv of modified Arabian Sea Water (ASW). With the transports in the two anticyclonic gyres nearly balanced, the exchange of water masses between the Somali Basin, west of the Carlsberg Ridge, and the Arabian Sea occurred in two areas; about 16 Sv of warm and saline surface water of southern offshore origin entered the northern Somali Basin within a 120‐km‐wide swift current between the Great Whirl and the Socotra Gyre. The other key region for the exchange of water masses was the passage between Somalia and Abd al Kuri. There, the total northward transport was 13 Sv, with contributions of IEW, of upwelled water close to the surface, and ASW underneath.

Description: The compatibility of the Gent and McWilliams thickness mixing parameterization with perturbation thickness fluxes evaluated from eddy-resolving North Atlantic model results is investigated. After extensive spatial and temporal averaging, a linear correlation between the parameterized fluxes and those calculated directly from model fluctuations in the subtropics could be found. A direct estimate of a constant mixing parameter κ could be inferred in the order of 1.0 × 107 cm2 s−1.

Description: A simple point-vortex “heton” model is used to study localized ocean convection. In particular, the statistically steady state that is established when lateral buoyancy transfer, effected by baroclinic instability, offsets the localized surface buoyancy loss is investigated. Properties of the steady state, such as the statistically steady density anomaly of the convection region, are predicted using the hypothesis of a balance between baroclinic eddy transfer and the localized surface buoyancy loss. These predictions compare favorably with the values obtained through numerical integration of the heton model. The steady state of the heron model can be related to that in other convection scenarios considered in several recent studies by means of a generalized description of the localized convection. This leads to predictions of the equilibrium density anomalies in these scenarios, which concur with those obtained by other authors. Advantages of the heton model include its inviscid nature, emphasizing the independence of the fluxes affected by the baroclinic eddies from molecular processes, and its extreme economy, allowing a very large parameter space to be covered. This economy allows us to examine more complicated forcing scenarios: for example, forcing regions of varying shape. By increasing the ellipticity of the forcing region, the instability is modified by the shape and, as a result, no increase in lateral fluxes occurs despite the increased perimeter length. The parameterization of convective mixing by a redistribution of potential vorticity, implicit in the heton model, is corroborated; the heton model equilibrium state has analogous quantitative scaling behavior to that in models or laboratory experiments that resolve the vertical motions. The simplified dynamics of the heton model therefore allows the adiabatic advection resulting from baroclinic instability to be examined in isolation from vertical mixing and diffusive processes. These results demonstrate the importance of baroclinic instability in controlling the properties of a water mass generated by localized ocean convection. A complete parameterization of this process must therefore account for the fluxes induced by horizontal variations in surface buoyancy loss and affected by baroclinic instability.

Description: We determined atmospheric and dissolved nitrous oxide (N2O) in the surface waters of the central North Sea, the German Bight, and the Gironde estuary. The mean saturations were 104 ± 1% (central North Sea, September 1991), 101 ± 2% (German Bight, September 1991), 99 ± 1% (German Bight September 1992), and 132% (Gironde estuary, November 1991). To evaluate the contribution of coastal areas and estuaries to the oceanic emissions we assembled a compilation of literature data. We conclude that the mean saturations in coastal regions (with the exception of estuaries and regions with upwelling phenomena) are only slightly higher than in the open ocean. However, when estuarine and coastal upwelling regions are included, a computation of the global oceanic N2O flux indicates that a considerable portion (approximately 60%) of this flux is from coastal regions, mainly due to high emissions from estuaries. We estimate, using two different parameterizations of the air-sea exchange process, an annual global sea-to-air flux of 11–17 Tg N2O. Our results suggest a serious underestimation of the flux from coastal regions in widely used previous estimates.

Description: The physics of the Indo–Pacific warm pool are investigated using a coupled ocean atmosphere general circulation model. The model, developed at the Max-Planck-Institut fair Meteorologic, Hamburg, does not employ a flux correction and is used with atmospheres at T42 and T21 resolution. The simulations are compared with observations, and the model's mean and seasonal heat budgets and physics in the Indo–Pacific warm pool region are explored for the T42 resolution run. Despite the simulation of a split intertropical convergence zone, and of a cold tongue that extends too far to the west, simulated warm pool temperatures are consistent with observations at T42 resolution, while the T21 resolution yields a cold bias of 1K. At T42 resolution the seasonal migration of the warm pool is reproduced reasonably well, as are the surface heat fluxes, winds, and clouds. However, simulated precipitation is too small compared to observations, implying that the surface density flux is dominated by fluxes of heat. In the Pacific portion of the warm pool, the average net heat gain of the ocean amounts to 30–40 W m−2. In the northern branch, this heat gain is balanced by vertical advection, while in the southern branch, zonal, meridional, and vertical advection cool the ocean at approximately equal rates. At the equator, the surface heat flux is balanced by zonal and vertical advection and vertical mixing. The Indonesian and Indian Ocean portions of the warm pool receive from the atmosphere 30 and 50 W m−2, respectively, and this flux is balanced by vertical advection. The cooling due to vertical advection stems from numerical diffusion associated with the upstream scheme, the coarse vertical resolution of the ocean model, and near-inertial oscillations forced by high-frequency atmospheric variability. The seasonal migration of the warm pool is largely a result of the seasonal variability of the net surface heat flux, horizontal and vertical advections are of secondary importance and increase the seasonal range of surface temperature slightly everywhere in the warm pool, with the exception of its southern branch. There, advection reduces the effect of the surface flux. The seasonal variability of the surface heat flux in turn is mainly determined by the shortwave radiation, but evaporation modifies the signal significantly. The annual cycles of reduction of solar radiation due to clouds and SST evolve independently from each other in the Pacific portion of the warm pool; that is, clouds have little impact on SST. In the Indian Ocean, however, clouds limit the maximum SST attained during the annual cycle. In the western Pacific and Indonesian portion of the warm pool, penetrative shortwave radiation leads to convective mixing by heating deeper levels at a greater rate than the surface, which experiences heat losses due to turbulent and longwave heat fluxes. In the deeper levels, there is no mechanism to balance the heating due to penetrative radiation, except convection and its attendant mixing. In the Indian Ocean, however. the resulting vertical heating profile due to the surface fluxes decreases monotonically with depth and does not support convective mixing. Concurrently, the warm pool is shallower in the Indian Ocean compared with the western Pacific, indicating that convective mixing due to penetrative radiation is important in maintaining the vertical structure of the Pacific portion of the warm pool.

Description: A primitive equation World Ocean model has been integrated with restoring boundary conditions to reach a steady state. The global distribution of potential temperature, salinity, and meridional streamfunction are consistent with observations. In steady state, the effective freshwater fluxes were diagnosed, and the model has been integrated further prescribing these freshwater fluxes. The ocean circulation undergoes self-sustained oscillations over a wide range of timescales, ranging from decadal to millennium. Most pronounced are self-sustained oscillations with a timescale of 20, 300, and 1000 years. The latter two oscillations are coupled. They consist of density (salinity) anomalies that circulate through the global conveyor belt, periodically enhancing convection in the Southern Ocean and limiting convection in the northern North Atlantic. The timescale is set by the vertical diffusion, which destabilizes the stratification in the Southern Ocean when convection is weak. The 20-yr oscillation is a coupled salinity and sea ice thickness anomaly propagating around Antarctica.

Description: Open‐ocean deep‐water formation involves the interplay of two dynamical processes; plumes (≤1 km wide), driven by “upright” convection, and geostrophic eddies (≥5 km wide), driven by baroclinic instability. Numerical “twin” experiments are used to address two questions about the plumes: Can they be represented by a simple mixing process in large‐scale models? If so, is it important that the mixing occurs over a finite time tmix, or would instantaneous mixing produce the same effect on large‐scale properties? In numerical simulations which resolve the geostrophic eddies, we represent the plumes with a “slow” convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions. The diffusivity κ depends on tmix, the mixing timescale. The fidelity of the plume parameterization is then evaluated by comparison with plume‐resolving simulations of open‐ocean deep convection. Integral properties of the plumes, such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region, are all accurately reproduced by the slow adjustment scheme. The importance of choosing an appropriate finite value for tmix is explored by setting tmix = 12 hours in some experiments, in accordance with scaling considerations, and tmix = 0 in others, corresponding to instantaneous adjustment, the conventional assumption. In the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix. However, in neutral conditions the slow adjustment does improve the parametric representation. Our experiments confirm the picture of plumes homogenizing the water column over a time tmix.

Description: Deep convection is important in forming the dense water masses that lie below the ocean's surface and feed the global thermohaline circulation system. But the exact role that deep convection plays in these processes is a subject of much debate. Now, for the first time, a pulse-like temperature signal, produced when water generated by convection drains into a deep boundary current, has apparently been detected in the Mediterranean. This observation provides clues to the mechanisms by which the dense water escapes the convection region and makes its journey. While up to 50% of the newly formed water could incorporated into the deep boundary current this way, no increase in its transport was observed.

Description: During December 1991 to April 1992 measurements with moored acoustic Doppler current profiler (ADCP) stations and shipboard surveys were carried out in the convection regime of the Gulf of Lions, northwestern Mediterranean. First significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late December. Mixed layer deepening to about 400 m, eroding the salinity maximum layer of saltier and warmer Levantine Intermediate Water and causing temporary surface-layer warming, followed during a second cooling period of late January. During a mistral cooling period from 18 to 23 February 1992, convection to 1500-m depth was observed, where the size of the convection regime was 50–100 km extent. Vertical velocities 40–640 m deep, recorded by four ADCPs of a triangular moored array of 2 km sidelength in the center of the convection regime, exceeded 5 cm s−1 and were not correlated over the separation of the moorings. Horizontal scales estimated from event duration and advection velocity were only around 500 m, in agreement with scaling arguments for convective plumes. Plume activity during nighttime cooling was larger than daytime daytime. Significant evidence for rotation of the plumes could not be found. Overall, plume energy, and the degree of mixing accomplished by them, was much lower than observed during a stronger mistral in February 1987. The mean vertical velocity over the mistral period, determined from the four ADCPs, was near zero, confirming the role of plumes as mixing agents rather than as part of a mean downdraft in a convection regime. The cyclonic rim current around the convection regime was confined to a strip of 〈20 km width with an average velocity of about 10 cm s−1, which is in agreement with near-zero vertical mean velocity in the interior based on potential vorticity conservation. A relation between variations of the larger-scale cyclonic North Mediterranean Current along the boundary and the deep convection could not be identified. An unexplained feature still is the cover of the convection regime by a shallow layer of light water that moves in rather quickly from the sides after the cooling ends.

Description: Submersible investigations employing heat flow measurements (12 stations), sampling and imagery of the two relict high-temperature hydrothermal zones of the TAG field, the Alvin and Mir sulfide zones, elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies. Values of heat flow in the Mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers. This observation supports the hypothesis that intrusions at volcanic centers adjacent to the high-temperature hydrothermal zones supply the heat to drive hydrothermal activity. The chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field-wide high-temperature hydrothermal events recurring at intervals of tens of thousands of years, while activity at individual zones may recur at intervals of hundreds to thousands of years. A sequence of hydrothermal deposits ranges to at least 140,000 years ago from massive sulfides forming at the active mound, to recrystallization of sulfides in the active and relict zones, to pyritization of an inactive mound in the Alvin zone; low-temperature mineral phases precipitate before, during and after the sulfides.

Description: A method is presented for determining salinity and density from temperature data in conjunction with historical or contemporaneous (but not collocated) CTD observations. The horizontal density ratio r(z) is determined from the temperature and salinity differences at each depth (δT, δS) between pairs or ensembles of profiles. These differences are expressed as a density ratio r=αδT/βδS, where α and β are the expansion coefficients for temperature and salinity, respectively. Salinity at a site where only temperature is measured, as with an expendable bathythermograph (XBT), is computed based on the temperature and salinity at a reference station (SR,TR); that is, S=SR+(T−TR)δS/δT. The method is restrictive in its application because it is most accurate when all water masses in the region of a survey are linear extrapolations from the water masses at each of the reference stations. In reality, it provides useful results when the T and S fields are not simply linear functions of horizontal distance. This approach is particularly useful in regions where, the T(z)−S(z) relation is nonunique, as in the Mediterranean Water in the North Atlantic. The corresponding expression for the lateral density difference for an observed temperature difference (δT) is δρ=−αρ0δT(1−r−1). Observations from regions offshore and along the coast of Portugal are used to evaluate the method. Errors of less than 0.05 psu are exhibited in the evaluation of salinity determined from T-5 XBT drops compared with nearly simultaneous CTD casts. A comparison of water properties and cyclostrophic velocities is made using XCP temperatures and XCP velocities in a meddy.

Description: We present a new method for assimilating observations of sea surface height (SSH) into a high‐resolution primitive equation model. The method is based on the concept of reinitialization. First, the surface velocity increments necessary to adjust the model forecast to the observed geostrophic surface currents are projected onto deep velocity increments by a linear regression method. Second, changes in the density field required to balance the changes in the velocity field geostrophically are obtained from an inversion of the thermal wind equation. A unique partition of the density increments into corresponding temperature and salinity changes is realized by conserving the local θ‐S relation of the model forecast. In contrast to pure statistical methods that infer temperature and salinity changes from correlations with SSH anomalies, our approach explicitly conserves water mass properties on isopycnals. For the assimilation experiment we use optimally interpolated maps of Geosat SSH anomalies (the mean topography is taken from the model), which are assimilated into the World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME) model of the North Atlantic Ocean at 5‐day intervals covering the year 1987. It is shown that the assimilation significantly improves the model's representation of eddy activity, with the hydrographic structure of individual eddies agreeing well with independent hydrographic observations. The importance of a careful treatment of water mass properties in the assimilation process is discussed and further illustrated by comparing different assimilation schemes.

Description: The authors use different versions of the model of the wind- and thermohaline-driven circulation in the North and Equatorial Atlantic developed under the WOCE Community Modeling Effort to investigate the mean flow pattern and deep-water formation in the subpolar region, and the corresponding structure of the basin-scale meridional overturning circulation transport. A suite of model experiments has been carded out in recent years, differing in horizontal resolution (1° × 1.2°, 1/3° × 0.4°, 1/6° × 0.2°), thermohaline boundary conditions, and parameterization of small-scale mixing. The mass transport in the subpolar gyre and the production of North Atlantic Deep Water (NADW) appears to be essentially controlled by the outflow of dense water from the Greenland and Norwegian Seas. in the present model simulated by restoring conditions in a buffer zone adjacent to the boundary near the Greenland–Scotland Ridge. Deep winter convection homogenizes the water column in the center of the Labrador Sea to about 2000 m. The water mass properties (potential temperature about 3°C, salinity about 34.9 psu) and the volume (1.1×1053 km3) of the homogenized water are in fair agreement with observations. The convective mixing has only little effect on the net sinking of upper-layer water in the subpolar gyre. Sensitivity experiments show that the export of NADW from the subpolar North Atlantic is more strongly affected by changes in the overflow conditions than by changes in the surface buoyancy fluxes over the Labrador and Irminger Seas, even if these suppress the deep convection completely. The host of sensitivity experiments demonstrates that realistic meridional overturning and heat transport distributions for the North Atlantic (with a maximum of 1 PW) can be obtained with NADW production rates of 15–16 Sv, provided the spurious upwelling of deep water that characterizes many model solutions in the Gulf Stream regime is avoided by adequate horizontal resolution add mixing parameterization.

Description: CH3I was measured in open ocean waters during two cruises to the tropical Atlantic Ocean and a late fall cruise to the Greenland and Norwegian Seas (GSNS). In warm, tropical surface waters subject to high solar irradiance, average CH3I saturation anomalies were positive (1.5–7.7 pmol kg−1), indicating a sea-to-air flux. This contrasted with negative saturation anomalies (−0.65±0.02 pmol kg−1) measured in cold surface waters of the open ocean GSNS subject to low-light. High latitude oceans may therefore be a significant sink for atmospheric CH3 during the fall and winter. The locations and/or seasons where samples were analyzed were all characterized by relatively low biological production and the CH3I saturation anomaly along 19°S decreased from 7.7±0.6 to 3.4±0.4 pmol kg−1 when entering a more productive upwelling zone. Taken together these observations suggest a chemical, as opposed to biological, production mechanism for this compound in the open ocean. Within the open ocean of the GSNS, multiple linear regression between the observed CH3I saturation anomaly and variables including light intensity, water temperature, CFC-11 saturation (indicator of gas exchange and deep mixing), and distance from the Norwegian Coastal Current (indicator of coastal or southern sources) showed that light intensity was the only significant predictor, explaining 79% of the variance. Photochemical production may therefore be dominant source of CH3I within the open ocean and this may have important implications for the large-scale, seasonal cycling of iodine between the ocean and the atmosphere.

Description: We study the temporal evolution of concentrations of the chlorofluorocarbons CFC 11 and CFC 12 in the ocean, under the assumption of circulation and mixing being invariant in time. This allows us to define a time‐invariant age distribution for a given point in the ocean, where the age is defined as time since the last contact with the atmosphere occurred. This concept is evaluated for a number of fundamental situations. We deduce a tendency for low CFC 11 and CFC 12 concentrations in advective regimes to increase exponentially in time and for concentrations near to a solubility equilibrium with atmospheric concentrations to increase rather more linearly. The apparent saturations, i.e., the ratios of interior to mixed‐layer CFC concentrations, increase monotonically in time, typical rates being 5–10% per decade. The theoretical results are compatible with time trends found in repeated CFC observations in the ocean. Diagrams on the temporal evolution for different age distributions are presented for the period 1970–2000, which can serve as a general orientation. The diagrams furthermore can provide time corrections for quasi‐synoptic evaluation of CFC observations taken over an extended period of time and assist in constructing time‐dependent CFC boundary conditions for numerical models of ocean circulation.

Description: Deep‐drogued drifters are in use to measure the near‐surface geostrophic currents. An attempt is made to study the slippage of these drifters due to wind and Ekman currents. The results are based on a data set from the unstratified North Sea obtained in winter 1991–1992, where the currents were decomposed into Ekman currents and barotropic currents. The influence of these Ekman currents on the drift performance of drifters drogued below the mixed layer in the barotropic current is determined by using quadratic drag laws. In 90% of all cases (1540 data points) the combined effect of wind drag and Ekman currents on buoy and 100‐m tether produces a slippage of less than 2 cm/s. Drifters drogued within the mixed layer show less slippage due to the reduced drag on the tether, but they are primarily designed to measure the actual near‐surface currents, which are strongly dependent on the wind conditions. It is concluded that deep‐drogued drifters are a reliable device to study weakly baroclinic geostrophic currents.

Description: A number of geophysical observing techniques, including ocean acoustic tomography, obtain sequences of records of which the observed relative maxima (“peaks”) are used to infer properties of the system via inversions. Traditionally, these peaks first are tracked (followed from one record to another) and identified separately, before they can be used in an inversion scheme. In this paper, a method is presented for identifying and tracking ensembles of such peaks in one step and simultaneously with the inversion. A priori information, in our case knowledge about the ocean, can thus be used to constrain the allowed peak identifications, enabling the usage of irregularly appearing or more closely spaced peaks. The best identification is defined to be the one that upon inversion minimizes a cost function that involves data residual and smoothness in time, subject to two constraints bounding the solution and residual size. For the presented cases, the minimum can be found by simply trying inversions with all possible peak identifications. Sample applications of the method from an acoustic tomography experiment are shown in order to illustrate the approach and results.

Description: Surface layer fluxes of sensible heat and water vapor were measured from a fixed platform in the North Sea during the Humidity Exchange over the Sea (HEXOS) Main Experiment (HEXMAX). Eddy wind stress and other relevant atmospheric and oceanic parameters were measured simultaneously and are used to interpret the heat and water vapor flux results. One of the main goals of the HEXOS program was to find accurate empirical heat and water vapor flux parameterization formulas for high wind conditions over the sea. It had been postulated that breaking waves and sea spray, which dominate the air-sea interface at high wind speeds, would significantly affect the air-sea heat and water vapor exchange for wind speeds above 15 m/s. Water vapor flux has been measured at wind speeds up to 18 m/s, sufficient to test these predictions, and sensible heat flux was measured at wind speeds up to 23 m/s. Within experimental error, the HEXMAX data do not show significant variation of the flux exchange coefficients with wind speed, indicating that modification of the models is needed. Roughness lengths for heat and water vapor derived from these direct flux measurements are slightly lower in value but closely parallel the decreasing trend with increasing wind speed predicted by the surface renewal model of Liu et al. [1979], created for lower wind speed regimes, which does not include effects of wave breaking. This suggests that either wave breaking does not significantly affect the surface layer fluxes for the wind speed range in the HEXMAX data, or that a compensating negative feedback process is at work in the lower atmosphere. The implication of the feedback hypothesis is that the moisture gained in the lower atmosphere from evaporation of sea spray over rough seas may be largely offset by decreased vapor flux from the air-sea interface.

Description: The dynamics and predictability of decadal climate variability over the North Pacific and North America are investigated by analyzing various observational datasets and the output of a state of the art coupled ocean–atmosphere general circulation model that was integrated for 125 years. Both the observations and model results support the picture that the decadal variability in the region of interest is based on a cycle involving unstable ocean–atmosphere interactions over the North Pacific. The period of this cycle is of the order of a few decades. The cycle involves the two major circulation regimes in the North Pacific climate system, the subtropical ocean gyre, and the Aleutian low. When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the Kuroshio and its extension, leading to a positive SST anomaly in the North Pacific. The atmospheric response to this SST anomaly involves a weakened Aleutian low, and the associated fluxes at the air–sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The anomalous heat flux, reduced ocean mixing in response to a weakened storm track, and anonmalous Ekman heat transport contribute to this positive feedback. The atmospheric response, however, consists also of a wind stress curl anomaly that spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The transient response can be expressed in terms of baroclinic planetary waves, and the decadal timescale of the oscillation is therefore determined to first order by wave timescales. Advection by the mean currents, however, is not negligible. The existence of such a cycle provides the basis of long-range climate forecasting over North America at decadal timescales. At a minimum, knowledge of the present phase of the decadal mode should allow a “now-cast” of expected climate “bias” over North America, which is equivalent to a climate forecast several years ahead.

Description: The Middle America Trench between the Cocos Ridge and a well-studied corridor off the Nicoya Peninsula has a more varied morphology and structure than previously reported. The morphological positive features on the lower plate significantly affect the upper plate structure. The Cocos Ridge has uplifted the margin opposite the Osa Peninsula. Northwest of Cocos Ridge, numerous seamounts on the oceanic crust sculptured the margin as they subducted. A seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted. The greater portion of the trench sediment is subducted beneath a lower slope accretionary mass. Beneath the middle and upper slope is a margin wedge consisting of a high-velocity rock with few internal reflections. Its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area. Overlying this surface is a low-velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred Eocene section beneath the shelf. The shelf basement is composed of Nicoya complex (ophiolite) with the same acoustic velocity, similar structure, and no apparent dividing geologic boundary with the margin wedge. We favor a seaward continuation of the Nicoya complex to the middle slope and emphasize the evidence for a non-steady state Tertiary tectonic history.