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Agatha M. de Boer and Doron Nof

Abstract

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.

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Doron Nof and Agatha M. de Boer

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 Northern Hemisphere and the bottom to the origin in the Southern Ocean. Although it is true that the amount of water that is cross crossing the equator is equal to the integrated Ekman flux in the northernmost part of the Southern Ocean, it is merely the amount (and not the origin of the water) that is equal in these two cases. The width of the transhemispheric current in the south iswhere τ is the wind stress, ∂τ/∂y the curl of the wind, β the familiar variation of the Coriolis with latitude, f0 the mean Coriolis parameter, and L is the width of the basin.

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Matthew D. Thomas, Agatha M. De Boer, Helen L. Johnson, and David P. Stevens

Abstract

Sverdrup 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 few years. This is because the interannual transport variability is small compared to the mean transports. The vorticity input to the deep ocean by the interaction between deep currents and topography is found to be very large in both models. These deep transports, however, are separated from upper-layer transports that are in Sverdrup balance when considered over large scales.

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Agatha M. de Boer, Anand Gnanadesikan, Neil R. Edwards, and Andrew J. Watson

Abstract

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 ΔT that ranges from −3° to 9°C. (An increase in ΔT 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.

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