Agatha M. De Boer

The Exhaust Valve of the North Atlantic

During glacial periods, climate records are marked by large-amplitude oscillations believed to be a result of North Atlantic (NA) freshwater anomalies, which weakened the thermohaline circulation (THC) and introduced instabilities. Such oscillations are absent from the present interglacial period. With the aid of a semiglobal analytical model, it is proposed that the Bering Strait (BS) acts like an exhaust valve for the above NA freshwater anomalies. Specifically, it is suggested that large instabilities in the THC are only possible during glacial periods because, during these periods, the BS is closed. During interglacial periods (when the BS, the exhaust valve, is open), low-salinity anomalies are quickly flushed out of the North Atlantic by the strong Southern Ocean winds.

Meridional Density Gradients Do Not Control the Atlantic Overturning Circulation

A wide body of modeling and theoretical scaling studies support the concept that changes to the Atlantic meridional overturning circulation (AMOC), whether forced by winds or buoyancy fluxes, can be understood in terms of a simple causative relation between the AMOC and an appropriately defined meridional density gradient (MDG). The MDG is supposed to translate directly into a meridional pressure gradient. Here two sets of experiments are performed using a modular ocean model coupled to an energy‐moisture balance model in which the positive AMOC‐MDG relation breaks down. In the first suite of seven model integrations it is found that increasing winds in the Southern Ocean cause an increase in overturning while the surface density difference between the equator and North Atlantic drops. In the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment DT that ranges from 238 to 98C. (An increase in DT results in increased sensitivity of density to temperature gradients.) The AMOC in these model integrations drops as the MDG increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean. Traditional scaling analysis can only produce this weaker AMOC if the scale depth decreases enough to compensate for the stronger MDG. Five estimates of the depth scale are evaluated and it is found that the changes in the AMOC can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth. These two depth scales are commonly assumed to be the same in theoretical models of the AMOC. It is suggested that the correlation between the MDG and AMOC breaks down in these model integrations because the depth and strength of the AMOC is influenced strongly by remote forcing such as Southern Ocean winds and Antarctic Bottom Water formation.

Spatial and Temporal Scales of Sverdrup Balance

AbstractSverdrup balance underlies much of the theory of ocean circulation and provides a potential tool for describing the interior ocean transport from only the wind stress. Using both a model state estimate and an eddy-permitting coupled climate model, this study assesses to what extent and over what spatial and temporal scales Sverdrup balance describes the meridional transport. The authors find that Sverdrup balance holds to first order in the interior subtropical ocean when considered at spatial scales greater than approximately 5°. Outside the subtropics, in western boundary currents and at short spatial scales, significant departures occur due to failures in both the assumptions that there is a level of no motion at some depth and that the vorticity equation is linear. Despite the ocean transport adjustment occurring on time scales consistent with the basin-crossing times for Rossby waves, as predicted by theory, Sverdrup balance gives a useful measure of the subtropical circulation after only a f...

A Simple Theory of the Pycnocline and Overturning Revisited

Understanding variability in the meridional overturning circulation requires linking changes in surface forcing and internal mixing to modifications of the thermal structure, differences in the rate of formation of deep waters, and shifts in the locations where these waters are formed. It is thus closely linked to the question of how the ocean circulation is “driven”—which has been debated for many years. In his classic “Physical Geography of the Oceans” (reprinted in 1963), Matthew Fontaine Maury argued that the dominant mechanism for driving such currents was the heating of the tropics and cooling of polar latitudes. In such a picture, one would primarily look to changes in the hydrological cycle or surface heat balance to explain changes in overturning, as in the classic box models of Stommel [1961]. By contrast, Maury fulminated against those who supposed that the ocean circulation was wind-driven, arguing that the variable winds of the Atlantic could never produce the steady Gulf Stream. The idea of a thermally-driven overturning was challenged by Sandstrom [1908] who argued that in a domain where heating occurs at the same level as cooling, a largescale circulation cannot be generated in the absence of A Simple Theory of the Pycnocline and Overturning Revisited

Antarctic Stratification, Atmospheric Water Vapor, and Heinrich Events: a Hypothesis for Late Pleistocene Deglaciations

We have previously argued that the Antarctic and subarctic North Pacific are stratified during ice ages, causing to a large degree the observed low CO2 levels of ice age atmospheres by sequestering respired CO2 in the ocean abyss. Here, we suggest a mechanism for the major deglaciations of the late Pleistocene. The mechanism begins with freshwater discharge to the North Atlantic, as evidenced by a Heinrich event, that shuts down North Atlantic overturning. Because of a global requirement for deep ocean ventilation, the North Atlantic shutdown drives overturning in the Antarctic, which, in turn, releases CO2 to the atmosphere and reduces Antarctic sea ice extent. The resulting increase in atmospheric CO2 and decrease in albedo then drive global warming and deglaciation. As a control on the timing of deglaciations, we look to the sensitivity of atmospheric freshwater transport to low latitude temperature, which is a natural antagonist to Antarctic stratification under cold climates. While Antarctic stratification is proposed to develop early in a glacial period, continued cooling through the glacial period may reduce the poleward atmospheric freshwater transport and thus may prepare the Antarctic halocline for collapse. Deglaciations may coincide with obliquity maxima because a reduced low-to-high latitude insolation gradient decreases the net poleward freshwater transport and perhaps also because increased polar insolation can warm the deep ocean and shift the westerly winds poleward, all of which should work to weaken Antarctic stratification. Precession minima may encourage Antarctic destratification by biasing tropical water vapor transport toward the northern hemisphere. Finally, obliquity and precession may work together to encourage the circum-North Atlantic freshwater discharge event that initiates the deglacial sequence.

FROM THE SOUTHERN OCEAN TO THE NORTH ATLANTIC IN THE EKMAN LAYER

Since the Southern Ocean encompasses the entire circumference of the globe, the zonal integral of the pressure gradient vanishes implying that the (meridional) geostrophic mass flux is zero. Conventional wisdom has it that, in view of this, the northward Ekman flux there must somehow find its way to the northern oceans, sink to the bottom (due to cooling) and return southward either below the topography or along the western boundary. Using recent (process oriented) numerical simulations and a simple analytical model, it is shown that most of the Ekman flux in the Southern Ocean does not cross the equator, nor does it sink in the northern oceans. Rather, the water that constitutes the link between the Southern Ocean and the deep water formation in the Northern Hemisphere originates in the eastern part of the southern Sverdrup interior. The associated path which takes the water from one hemisphere to the other resembles the letter “S”, where the top of the letter corresponds to the sinking region in the Nor...

Control of the glacial carbon budget by topographically induced mixing

Evidence for the oceanic uptake of atmospheric CO2 during glaciations suggests that there was less production of southern origin deep water but, paradoxically, a larger volume of southern origin water than today. Here we demonstrate, using a theoretical box model, that the inverse relationship between volume and production rate of this water mass can be explained by invoking mixing rates in the deep ocean that are proportional to topographic outcropping area scaled with ocean floor slope. Furthermore, we show that the resulting profile, of a near-linear decrease in mixing intensity away from the bottom, generates a positive feedback on CO2 uptake that can initiate a glacial cycle. The results point to the importance of using topography-dependent mixing when studying the large-scale ocean circulation, especially in the paleo-intercomparison models that have failed to produce the weaker and more voluminous bottom water of the Last Glacial Maximum.

The island wind-buoyancy connection

A variety of recent studies have suggested that the meridional overturning circulation (MOC) is at least partially controlled by the Southern Ocean (SO) winds. The paradoxical implication is that a link exists between the global surface buoyancy flux to the ocean (which is needed for the density transformation between surface and deep water) and the SO winds. Although the dependency of buoyancy forcing on local wind is obvious, the global forcings are usually viewed independently with regard to their role as drivers of the global ocean circulation. The present idealized study is focused on understanding this wind’buoyancy connection. In order to isolate and investigate the effect of SO winds on the overturning we have neglected other important key processes such as SO eddies. We present the wind’buoyancy connection in the framework of a single gigantic island that lies between latitude bands free of continents (such as the land mass of the Americas). The unique geometry of a gigantic island on a sphere allows for a clear and insightful examination of the wind’buoyancy connection. This is because it enables us to obtain analytical solutions and it circumvents the need to calculate the torque exerted on zonal sills adjacent to the island tips (e.g. the Bering Strait). The torque calculation is notoriously difficult and is avoided here by the clockwise integration, which goes twice through the western boundary of the island (in opposite directions) eliminating any unknown pressure torques. The link between SO winds and global buoyancy forcing is explored qualitatively, using salinity and temperature mixed dynamical-box models and a temperature slab model, and semiquantitatively, employing a reduced gravity model which includes parametrized thermodynamics. Our main finding is that, in all of these cases the island geometry implies that the stratification (and, hence, the air’sea heat flux) can always adjust itself to allow the overturning forced by the wind.We find that, in the mixed dynamical-box models, the salinity and temperature differences between the boxes are inversely proportional to the MOC. In spite of the resulting smaller north’south temperature difference, the meridional heat transport is enhanced.

Export of nutrient rich Northern Component Water preceded early Oligocene Antarctic glaciation

The onset of the North Atlantic Deep Water formation is thought to have coincided with Antarctic ice-sheet growth about 34 million years ago (Ma). However, this timing is debated, in part due to questions over the geochemical signature of the ancient Northern Component Water (NCW) formed in the deep North Atlantic. Here we present detailed geochemical records from North Atlantic sediment cores located close to sites of deep-water formation. We find that prior to 36 Ma, the northwestern Atlantic was stratified, with nutrient-rich, low-salinity bottom waters. This restricted basin transitioned into a conduit for NCW that began flowing southwards approximately one million years before the initial Antarctic glaciation. The probable trigger was tectonic adjustments in subarctic seas that enabled an increased exchange across the Greenland–Scotland Ridge. The increasing surface salinity and density strengthened the production of NCW. The late Eocene deep-water mass differed in its carbon isotopic signature from modern values as a result of the leakage of fossil carbon from the Arctic Ocean. Export of this nutrient-laden water provided a transient pulse of CO2 to the Earth system, which perhaps caused short-term warming, whereas the long-term effect of enhanced NCW formation was a greater northward heat transport that cooled Antarctica.The onset of deep water export from the North Atlantic Ocean preceded the onset of Antarctic glaciation by about one million years, according to sediment geochemistry, and may have been triggered by tectonic changes in the Atlantic basin.