North Atlantic Thermocline Research Articles

Climate Induced Thermocline Aging and Ventilation in the Eastern Atlantic Over the Last 32,000 Years

AbstractThe radiocarbon analysis of uranium‐thorium‐dated cold‐water corals (CWCs) provides an excellent opportunity for qualitative reconstruction of past ocean circulation and water mass aging. While mid‐depth water mass aging has been studied in the Atlantic Ocean, the evolution of the thermocline is still largely unknown. Here we present a combined 14C and 230Th/U age record obtained from thermocline dwelling CWCs at various sites in the eastern Atlantic Ocean, with intermittently centennial resolution over the last 32 ka. Shallow dwelling CWCs off Angola, located in the South Atlantic, infer a link between the mid‐depth equatorial Atlantic and Southern Ocean. They confirm a 14C drawdown during the Last Glacial Maximum (LGM) and advocate for a consistent Southern Hemisphere radiocarbon aging of upper thermocline waters, as well as strong depth gradients and high variability. Direct comparison with 14C simulations carried out with an Ocean General Circulation Model yield good agreement for Angola. In contrast, the North Atlantic thermocline shows well‐ventilated water with strong variations near the position of today's Azores Front (AF), neither of which are captured by the model. During the Bølling‐Allerød, we confirm the important role of the AF in separating North and South Atlantic thermocline waters and provide further evidence of a 500 year long deep convection interruption within the Younger Dryas (YD). We conclude that the North and South Atlantic thermocline waters were separately acting carbon reservoirs during the LGM and subsequent deglaciation until the modern circulation was established during the YD.

Life and Demise of Intrathermocline Mesoscale Vortices

AbstractThis study is focused on finescale dissipation mechanisms of intrathermocline mesoscale vortices exemplified by meddies, large anticyclonic salt lenses of Mediterranean origin commonly observed in the lower North Atlantic thermocline. High-resolution numerical experiments are diagnosed to quantify the rates of temperature and salinity (T, S) dispersion in salt lenses and to determine the relative contribution of various mixing processes in the decay of their thermohaline signatures. This study finds, in agreement with observations, that meddies dissipate on the characteristic time scale of several years and that their ultimate disintegration can be attributed to thermohaline interleaving driven by double-diffusive mixing. Mechanically generated turbulence, on the other hand, tends to suppress the interleaving and therefore has an adverse net effect on eddy dispersion. It is found that the dispersion properties of static lenses, characterized by density-compensated T–S patterns, and their rapidly rotating counterparts are dramatically different.

Interhemispheric air temperature phase relationships in the nonlinear Dansgaard‐Oeschger oscillation

AbstractIt has previously been suggested that the Southern Ocean might act as a thermal reservoir in mediating the relationship between the northern and southern hemisphere air temperature signals during a Dansgaard‐Oeschger cycle. On the basis of a climate model‐based analysis, we demonstrate on the contrary that it is equatorial and subtropical North Atlantic thermocline waters that act so as to integrate and damp the northern hemisphere signal to fix the amplitude and phase of the southern hemisphere counterpart to the more intense North Atlantic component of the oscillation. The dynamics of the Southern Ocean component of the Atlantic meridional overturning circulation are critical in explaining the evolution of the interhemispheric temperature phase relationships. European Project for Ice Coring in Antarctica Dronning Maud Land‐inferred air temperature variations are in phase with sea surface temperatures north of the Antarctic sea ice edge, suggesting that it is via a fast atmospheric pathway that this temperature signal is transmitted into the Antarctic ice core record.

Lateral diffusivity from tracer release experiments in the tropical North Atlantic thermocline

Lateral diffusivity is computed from a tracer release experiment in the northeastern tropical Atlantic thermocline. The uncertainties of the estimates are inferred from a synthetic particle release using a high‐resolution ocean circulation model. The main method employed to compute zonal and meridional components of lateral diffusivity is the growth of the second moment of a cloud of tracer. The application of an areal comparison method for estimating tracer‐based diffusivity in the field experiments is also discussed. The best estimate of meridional eddy diffusivity in the Guinea Upwelling region at about 300 m depth is estimated to be m2 s−1. The zonal component of lateral diffusivity is estimated to be m2 s−1, while areal comparison method yields areal equivalent zonal diffusivity component of m2 s−1. In comparison to Ky, Kx is about twice larger, resulting from the tracer patch stretching by zonal jets. Employed conceptual jet model indicates that zonal jet velocities of about m s−1 are required to explain the enhancement of the zonal eddy diffusivity component. Finally, different sampling strategies are tested on synthetic tracer release experiments. They indicate that the best sampling strategy is a sparse regular sampling grid covering most of the tracer patch.

Two centuries of limited variability in subtropical North Atlantic thermocline ventilation

Ventilation and mixing of oceanic gyres is important to ocean-atmosphere heat and gas transfer, and to mid-latitude nutrient supply. The rates of mode water formation are believed to impact climate and carbon exchange between the surface and mid-depth water over decadal periods. Here, a record of (14)C/(12)C (1780-1940), which is a proxy for vertical ocean mixing, from an annually banded coral from Bermuda, shows limited inter-annual variability and a substantial Suess Effect (the decrease in (14)C/(12)C since 1900). The Sargasso Sea mixing rates between the surface and thermocline varied minimally over the past two centuries, despite changes to mean-hemispheric climate, including the Little Ice Age and variability in the North Atlantic Oscillation. This result indicates that regional formation rates of sub-tropical mode water are stable over decades, and that anthropogenic carbon absorbed by the ocean does not return to the surface at a variable rate.

A review of the physical oceanography of the Mediterranean outflow

The physical oceanography of the Mediterranean Sea is reviewed with particular emphasis on the Mediterranean outflow in the Gulf of Cadiz. In this region the dense Mediterranean water forms a high velocity bottom current that interacts strongly with the sea floor. The major energy source for the plume comes from the release of potential energy as the plume descends the continental slope, and the major energy sink is work against bottom stress, which is as large as 4 Pa where the plume begins to descend the continental slope. In this region the current makes a nearly inertial turn that would otherwise appear to be steered by the underlying topography. The Mediterranean plume entrains the overlying North Atlantic Central Water and thereby loses much of its density anomaly. The mixed Mediterranean water becomes neutrally buoyant in the lower portion of the North Atlantic thermocline near Cape St. Vincent. There are then two preferred transport modes having somewhat different temperature and salinity whose distinct characteristics can be found far into the open North Atlantic. The temperature, salinity and volume of the Mediterranean water in the Strait of Gibraltar and in the Gulf of Cadiz appear to be roughly constant since modern measurements have been made. The estimated westward transport of Mediterranean water has gone down considerably as direct measurement techniques have been applied. A recent estimate is that the westward transport of pure Mediterranean water is only about a half a Sv (1 Sv = 10 6 m 3 s −1); the transport of mixed Mediterranean water in the western Gulf of Cadiz is larger by about a factor of three or four because of the entrainment of North Atlantic water.

Observations of temporal changes of tritium-<SUP>3</SUP>He age in the eastern North Atlantic thermocline: Evidence for changes in ventilation?

A compilation of fifteen years of tritium and 3 He measurements is used to examine the ventilation of the eastern North Atlantic subtropical gyre with specific emphasis on the temporal character of the tracer age field. A multivariate regression analysis in the form of a spatiotemporal Taylor expansion is applied to observations interpolated onto isopycnal surfaces. The time-dependent component of the tracer age field is found to be statistically significant, explaining approximately 10% of the variance of the tracer age observations in the upper thermocline (σ θ = 26.5) and increasing to roughly 50% of the variance in the lower thermocline (σ θ = 27.0). The observed transient tracer age increases over the 15 years of observations with the fractional rate of change of the age field varying between 2% and 5% per year. The largest observed changes occur on the deepest, most slowly ventilated isopycnal surfaces. The second derivative of the tritium- 3 He age with time suggests that the tracer age field may be approaching a steady state. If tritium- 3 He age is interpreted as a true measure of the advective ventilation age, the temporal changes in age would imply a slackening of the ventilation of the lower main thermocline of greater than 50% from the late 1970's to the early 1990's. However, consideration of the full advective-diffusive balance of tritium- 3 He age reveals that the changes in tracer age field represent a time-dependent adjustment of the transient tracer concentrations in conjunction with a steady local circulation field. Integral approximations of the upstream evolution of the tracer field also fail to demonstrate evidence for decadal changes in ventilation. The integral balance along the path of subduction yields an improved estimate of the true ventilation age based on the temporal tendency of the age field along the path of ventilation. An approximation of this integral suggests that actual ventilation ages can be up to 40% larger than the measured tracer age in the deeper portions of the North Atlantic thermocline. Proper accounting of the time-dependent biases of the tracer age dating technique are a prerequisite for examining transient tracer measurements for evidence of changes in the physical ventilation of the upper ocean.

Nonlinear limits to ocean thermal structure

The nonlinear volume changes associated with diffusion and mixing of different seawater types produce significant conversions of gravitational potential energy to kinetic energy to maintain and enhance mixing processes and to limit the magnitudes of temperature gradients that can persist in the ocean. Several examples are given to illustrate these critical temperature gradients beyond which conversion exceeds local energy consumption by mixing. The vertical temperature gradients in the Gulf Stream reduce downstream from Cape Hatteras to the critical limit found in the North Atlantic thermocline. The limiting gradients are also seen in surface thermoclines and in staircase structures. Although the theoretical interpretation of the nonlinear limiting processes is still incomplete, the observational evidence is compelling for further study of these mechanisms.

Thermocline ventilation of anthropogenic lead in the western North Atlantic

Massive industrial aerosol emissions from North America have over the past century overwhelmed the natural distribution of lead in the western North Atlantic thermocline. The resultant features to 2000‐m depth have been mapped near Bermuda for the year 1984. Since earlier reported measurements have largely been rejected on grounds of sample contamination, the twentieth‐century evolution of anthropogenic Pb in the oceanic water column is unknown. Coupling of the 1984 water column Pb distribution to historical records in reef‐building corals and tracer‐derived thermocline ventilation rates (Jenkins, 1980) has permitted a model reconstruction of Pb penetration into the Sargasso Sea. The model is constrained by three distinct Pb distributions (Pb concentration, stable Pb isotopes, and 210Pb) with adjustable scavenging and regeneration parameters to account for nonconservative behavior. The results of the model substantiate the importance of lateral isopycnal ventilation while pointing out the necessity of dissolved‐particulate exchange (i.e., scavenging τΓ of the order of 70 years in the deep thermocline; minimum regeneration of 10% particulate Pb between surface and 1700 m). Residual discrepancies between model isotopic predictions and observations below 800 m are likely due to a combination of the following factors: (1) presence of nonradiogenic background Pb in intermediate depth waters, (2) geographic variability in North American industrial source signatures, (3) Mediterranean Pb inputs of European origin, and (4) along‐isopycnal mixing effects.

The Structure, Dynamics, and Origin of a Small-Scale Lens of Water in the Western North Atlantic Thermocline

A small-scale, isolated, anticyclonically rotating lens of water was observed in the western North Atlantic thermocline during the POLYMODE Local Dynamics Experiment. Using a combination of SOFAR float, hydrographic, nutrient, and moored current and temperature data, we deduce that the lens was about 20 km across with a thickness not greater than 300 m, centered on 750 m; that its shape was not dissimilar to that of a vertical-radial Gaussian eddy; and that at its center, the lens had strong anomalies of salinity, dissolved oxygen, nitrate, and vortex stretching. The water mass properties within the eddy are not inconsistent with an origin for the feature near 15°N, 54°W, several thousand kilometers from where it was observed in the western North Atlantic, although an unambiguous origin cannot be discerned from the data. If such features are not uncommon, they may be an important mechanism for large-scale mixing of water properties in the ocean.

The Ventilation of the World's Oceans: Maps of the Potential vorticity Field

Maps of potential vorticity (q) within four different density layers are presented for the North and South Atlantic, North and South Pacific, and Indian Oceans. Potential vorticity is evaluated as (f/ρ)∂ρ0/∂z, a prescription that is valid for large slow scales and that allows its mapping from hydrographic data alone. Here, f is the Coriolis frequency, p0 the potential density, and z the vertical coordinate. It is shown that the character of a layer within the thermocline of a subtropical gyre will vary greatly depending on the extent to which the layer is isolated from surface boundary conditions and forcing. Fluid particles recirculating within “ventilated” layers have their potential vorticity and other properties reset with virtually every circuit round the gyre when they pass through the outcrop zone. By contrast, particles within “unventilated” or “dynamically isolated” layers have time to share their properties with neighboring particles and approach a common, “homogenized” state. The North Atlantic thermocline is shown to be highly anomalous. The extensive outcropping of its density surfaces equatorward of the zero wind-stress curl line, and the resulting strong downward Ekman pumping, allows the surface boundary conditions to control the interior thermocline structure in the manner of Luyten, Pedlosky and Stommel. The other oceans show a much greater tendency towards homogenization, even at great depth, suggesting that their dynamical isolation is relatively complete. Only in the very shallowest layers, where fluid particles are exposed to direct atmospheric forcing, is there a tendency for the outcrop properties to propagate into the interior. In these regions, wintertime stratification properties are advected into the interior as tongues of high or low q.

Potential Energy Maxima in the Tropical and Subtropical North Atlantic

Expendable bathythermograph records are combined with stability information to provide estimates of the potential energy associated with vertical displacements in the tropical and subtropical North Atlantic thermocline. The geographic distribution of potential energy magnitudes provides a means by which the dynamics implications of thermocline displacements in areas of differing vertical density structures can quantitatively be compared. A broad zonal minimum in potential energy exists along 24–°N in the eastern and western North Atlantic basins. From this minimum potential energy levels increase significantly in magnitude to the north and to a lesser extent to the south with highest values indicated for the boundary-circulation-associated regions of the subtropical gyre. More geographically restricted maxima are found along the flanks of the Mid-Atlantic Ridge and near the Antilles Islands Arc, indicating that topographic influences may be important in addition to what appears predominantly to be a current-related potential energy field. Comparisons of these estimates with observations suggest that the areas of potential energy maxima are associated with significant eddy activity and that the conditions observed during MODE are statistically characteristic of the more quiescent central zone.

A Two-Layer Model of the North Atlantic Thermocline

A two-layer model of a mid-oceanic thermocline is developed, and a comparison of the depth and temperature of the thermocline in this model is made with a root-mean-square description of the hydrographic structure of the North Atlantic. It is shown that the Sverdrup balance is maintained within the rms errors, and the entire estimated heat flux from the atmosphere is used to produce the observed density changes along the path of persistent flow.