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Michael A. Spall

Abstract

The mechanisms of wind-forced variability of the zonal overturning circulation (ZOC) are explored using an idealized shallow water numerical model, quasigeostrophic theory, and simple analytic conceptual models. Two wind-forcing scenarios are considered: midlatitude variability in the subtropical/subpolar gyres and large-scale variability spanning the equator. It is shown that the midlatitude ZOC exchanges water with the western boundary current and attains maximum amplitude on the same order of magnitude as the Ekman transport at a forcing period close to the basin-crossing time scale for baroclinic Rossby waves. Near the equator, large-scale wind variations force a ZOC that increases in amplitude with decreasing forcing period such that wind stress variability on annual time scales forces a ZOC of O(50) Sv (1 Sv ≡ 106 m3 s−1). For both midlatitude and low-latitude variability the ZOC and its related heat transport are comparable to those of the meridional overturning circulation. The underlying physics of the ZOC relies on the influences of the variation of the Coriolis parameter with latitude on both the geostrophic flow and the baroclinic Rossby wave phase speed as the fluid adjusts to time-varying winds.

Significance Statement

The purpose of this study is to better understand how large-scale winds at mid- and low latitudes move water eastward or westward, even in the deep ocean that is not in direct contact with the atmosphere. This is important because these currents can shift where heat is stored in the ocean and if it might be released into the atmosphere. It is shown that large-scale winds can drive rapid cross-basin transports of water masses, especially so at low latitudes. The present results provide a guide on what controls this motion and highlight the importance of large-scale ocean waves on the water movement and heat storage.

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Michael A. Spall

Abstract

The influences of strong gradients in sea surface temperature on near-surface cross-front winds are explored in a series of idealized numerical modeling experiments. The atmospheric model is the Naval Research Laboratory Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model, which is fully coupled to the Regional Ocean Modeling System (ROMS) ocean model. A series of idealized, two-dimensional model calculations is carried out in which the wind blows from the warm-to-cold side or the cold-to-warm side of an initially prescribed ocean front. The evolution of the near-surface winds, boundary layer, and thermal structure is described, and the balances in the momentum equation are diagnosed. The changes in surface winds across the front are consistent with previous models and observations, showing a strong positive correlation with the sea surface temperature and boundary layer thickness. The coupling arises mainly as a result of changes in the flux Richardson number across the front, and the strength of the coupling coefficient grows quadratically with the strength of the cross-front geostrophic wind. The acceleration of the winds over warm water results primarily from the rapid change in turbulent mixing and the resulting unbalanced Coriolis force in the vicinity of the front. Much of the loss/gain of momentum perpendicular to the front in the upper and lower boundary layer results from acceleration/deceleration of the flow parallel to the front via the Coriolis term. This mechanism is different from the previously suggested processes of downward mixing of momentum and adjustment to the horizontal pressure gradient, and is active for flows off the equator with sufficiently strong winds. Although the main focus of this work is on the midlatitude, strong wind regime, calculations at low latitudes and with weak winds show that the pressure gradient and turbulent mixing terms dominate the cross-front momentum budget, consistent with previous work.

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Michael A. Spall

Abstract

The influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models. Nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses, making explicit their dependence on both geometric parameters such as basin area, sill depth, and latitude, as well as on the strength of atmospheric forcing. In addition to the properties of the convective water, the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode. High-resolution numerical model calculations compare well with the theory for the properties of the convective water mass, the strength of the meridional overturning circulation, and also the shutdown of deep convection. However, the numerical model also shows that, for precipitation levels that exceed this critical threshold, the circulation retains downwelling and northward heat transport, even in the absence of deep convection.

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Michael A. Spall

Abstract

The properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed. The net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin. Heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current. The objectives of this study are to understand better what controls the density of waters formed within the marginal sea, the temperature of the outflowing waters, the amount of downwelling, and the mechanisms of heat transport within the marginal sea. The approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system. The theory compares well to a series of eddy-resolving primitive equation model calculations. The downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy-driven region on the offshore edge of the boundary current. For most high-latitude regions, the horizontal gyre is expected to transport more heat than does the overturning gyre.

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Michael A. Spall

Abstract

The impact of a meridional gradient in sea surface temperature (warm toward the equator, cold toward the pole) on the circulation around an island is investigated. The upper-ocean eastward geostrophic flow that balances such a meridional gradient is blocked where the isotherms intersect boundaries. In the case in which the boundaries represent either the eastern or western side of a planetary-scale island, circulation integrals around the island show that some of this eastward transport will flow around both the equatorward and poleward tips of the island. There is also a net downwelling along the western side of the island and a net upwelling along the eastern side of the island. An analytic model of the eastern and western boundary currents is used together with a circulation integral to estimate the fraction of the eastward transport that flows around the equatorward and poleward tips of the island and the net upwelling/downwelling on either side of the island. Calculations with a primitive equation numerical model are in close agreement with the theory. A simple closed-form analytic solution for the transport around the island tips is derived in the limit of strong buoyancy forcing. It is found that, over a wide range of parameter space, a significant fraction of the eastward transport in the upper ocean circulates around the tips of the island from the western basin into the eastern basin.

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Michael A. Spall

Abstract

A simple, nonlinear, two-layer, planetary geostrophic model of the large-scale circulation forced by localized mixing over a sloping bottom is discussed. The model is forced by parameterized diapycnal mixing at the density interface and/or by a mass flux downward into (unresolved) deep topographic canyons. Two nondimensional parameters are identified: the ratio of the change in Coriolis parameter over the horizontal mixing length scale to the nominal Coriolis parameter and the ratio of the advective speed to the Rossby wave phase speed. The former controls the strength of horizontal recirculation gyres that are forced by spatially variable diapycnal mixing, while the latter is a measure of the importance of nonlinearity in the density equation. When bottom topography is introduced, bottom pressure torque becomes important and the traditional strong horizontal recirculation gyre found for mixing over a flat bottom (beta plume) is gradually replaced by a zonal flow into or out of the mixing region in the deep ocean. Bottom topography becomes important, and the zonal flow emerges when the topographic Rossby wave speed exceeds the baroclinic planetary Rossby wave speed. Nonlinear effects are shown to enhance the upper-layer recirculation for upwelling and to retard the upper-layer circulation for downwelling. The model is finally configured to represent a region of mixing over the western flank of the Mid-Atlantic Ridge in the deep Brazil Basin. The model upper-layer flow is toward the southwest and the deep flow is very weak, zonal, and toward the east, in reasonable agreement with recent observational and inverse model estimates. The bottom pressure torque is shown to be crucial for maintaining this weak, zonal deep flow in the presence of strong turbulent mixing.

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Michael A. Spall

Abstract

The influence of cooling in a western boundary current on the recirculation of parcels in the subtropical gyre and their eventual transfer into the subpolar gyre is investigated. It is shown that heat loss to the atmosphere and the resulting vertical convection of dense water in the Gulf Stream of a general circulation primitive equation model forces a counterclockwise spiral of the velocity vector with depth (a cooling spiral). Parcels that pass through this region of cooling are forced to cross under the upper-level trajectories from south to north, in opposite sense to the beta spiral experienced in the interior of the subtropical gyre. This crossing of trajectories is an important consequence of the cooling in the western boundary current as it influences both scale and structure of the subtropical gyre recirculation. A simple expression is derived that relates the spatial scale of the recirculation to the cooling rate in the western boundary current and the wind forcing in the subtropical gyre.

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Michael A. Spall

Abstract

The mean downwelling in an eddy-resolving model of a convective basin is concentrated near the boundary where eddies are shed from the cyclonic boundary current into the interior. It is suggested that the buoyancy-forced downwelling in the Labrador Sea and the Lofoten Basin is similarly concentrated in analogous eddy formation regions along their eastern boundaries. Use of a transformed Eulerian mean depiction of the density transport reveals the central role eddy fluxes play in maintaining the adiabatic nature of the flow in a nonperiodic region where heat is lost from the boundary current. The vorticity balance in the downwelling region is primarily between stretching of planetary vorticity and eddy flux divergence of relative vorticity, although a narrow viscous boundary layer is ultimately important in closing the regional vorticity budget. This overall balance is similar in some ways to the diffusive–viscous balance represented in previous boundary layer theories, and suggests that the downwelling in convective basins may be properly represented in low-resolution climate models if eddy flux parameterizations are adiabatic, identify localized regions of eddy formations, and allow density to be transported far from the region of eddy formations.

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Michael A. Spall

Abstract

Radiating baroclinic Rossby waves excited through instability of the Cape Verde frontal zone are proposed as a mechanism for the generation of mesoscale variability at middepth (1000 m) in the southeastern North Atlantic basin. Linear quasigeostrophic theory is applied to an idealized front representative of the Cape Verde frontal zone to demonstrate that the front is unstable to modes that may radiate away from the frontal region as baroclinic Rossby waves. Evidence for the existence of these waves is obtained from an eddy-resolving, basin-scale general circulation primitive equation model. In addition, the model fields are used to identify characteristic signature of the waves in terms of quantities that may be directly observed in the ocean. Lagrangian trajectories, Reynolds stress, eddy kinetic energy, and frequency spectra taken from SOFAR float and current-meter records are all in good agreement with the amplitude and distribution implied by the wave radiation in both the linear theory and the full primitive equation model. It is concluded that the Cape Verde frontal zone is a source of radiating baroclinic Rossby waves and that these waves are an important component of the low-frequency eddy energy at the middepth ocean in the southeastern basin.

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Michael A. Spall

Abstract

A low-frequency oscillation in the Gulf Stream/deep western boundary current (DWBC) system is identified and its influences on several important aspects of the basin-scale circulation are investigated. An eddy-resolving regional primitive equation model is used to demonstrate that feedbacks between the Gulf Stream, with its associated northern and southern recirculation gyres, and the upper core of the DWBC can lead to self-sustaining large amplitude internal oscillations of roughly decadal frequency. The oscillator cycle is described as follows: The upper core of the DWBC is entrained under the Gulf Stream through interaction with the eddy-driven northern and southern recirculation gyres, as described in Part I of this study. Once entrained, the low potential vorticity DWBC water stabilizes the Gulf Stream and suppresses the eddy fluxes that maintained the interior recirculation gyres. This causes the upper DWBC to switch to a southward path along the western boundary, thus removing the source of the stabilizing low potential vorticity water to the Gulf Stream. The Gulf Stream quickly returns to its unstable state and the resulting eddy fluxes spin up the northern and southern recirculation gyres. At this point, the upper DWBC is reentrained and the cycle begins again. The frequency and amplitude of the oscillations are controlled by the efficiency of the entrainment mechanism, as demonstrated by its sensitivity to variations in the model forcing parameters. The oscillation strongly influences the penetration scale of the Gulf Stream and distribution of eddy variability, the separation latitude of the Gulf Stream, the effective age of the DWBC south of the crossover, and the pathways of the upper DWBC. The implications of such an oscillation on observing and modeling the thermohaline circulation are discussed.

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