Wind-Forced Variability of the Remote Meridional Overturning Circulation (original) (raw)
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Journal of Physical Oceanography, 2021
The frequency and latitudinal dependence of the midlatitude wind-driven meridional overturning circulation (MOC) is studied using theory and linear and nonlinear applications of a quasigeostrophic numerical model. Wind forcing is varied either by changing the strength of the wind or by shifting the meridional location of the wind stress curl pattern. At forcing periods of less than the first-mode baroclinic Rossby wave basin crossing time scale, the linear response in the middepth and deep ocean is in phase and opposite to the Ekman transport. For forcing periods that are close to the Rossby wave basin crossing time scale, the upper and deep MOC are enhanced, and the middepth MOC becomes phase shifted, relative to the Ekman transport. At longer forcing periods the deep MOC weakens and the middepth MOC increases, but eventually for long enough forcing periods (decadal) the entire wind-driven MOC spins down. Nonlinearities and mesoscale eddies are found to be important in two ways. First, baroclinic instability causes the middepth MOC to weaken, lose correlation with the Ekman transport, and lose correlation with the MOC in the opposite gyre. Second, eddy thickness fluxes extend the MOC beyond the latitudes of direct wind forcing. These results are consistent with several recent studies describing the four-dimensional structure of the MOC in the North Atlantic Ocean. SIGNIFICANCE STATEMENT: The purpose of this study is to better understand how large-scale winds at midlatitudes move water northward or southward, even in the deep ocean that is not in direct contact with the atmosphere. This is important because winds can shift where heat is stored and whether it might be released into the atmosphere. Our results provide a guide on what controls this motion and highlight the importance of large-scale ocean waves and smaller-scale ocean turbulence on the water movement and heat storage.
Journal of Physical Oceanography, 2003
Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is partially controlled by wind stress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. Here, the authors solve the cubic scale relationship of Gnanadesikan to find a simple expression for meridional overturning that is used to clarify the relative strength of the windforced component. The predicted overturning is compared with coarse-resolution numerical experiments with an idealized Atlantic Ocean-Southern Ocean geometry. The scaling accurately predicts the sensitivity to forcing for experiments with a level model employing isopycnal diffusion of temperature, salinity, and ''layer thickness.'' A layer model produces similar results, increasing confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing. The paper highlights the difference in meridional overturning induced by changes in wind stress or vertical diffusivity. Strengthening the Southern Ocean wind stress induces a circulation anomaly in which most of the water is subducted in the Ekman layer of the wind perturbation region, follows isopycnals down into the thermocline, and changes density again when the isopycnals near the surface in the Northern Hemisphere. Approximating the circulation anomaly by this subduction route allows for a surprisingly accurate prediction of the resulting heat transport anomaly, based on the surface temperature distribution. Some of the induced flow follows a second, near-surface northward route through low-latitude water that is lighter than the subducted flow. Overturning anomalies far from the wind stress perturbations are not completely determined by wind stress in the zonally periodic Southern Ocean: wind stress outside the periodic region strongly influences the transport of heat across the equator primarily by changing the temperature of the flow across the equator.
Wind-forced variability of the zonal overturning circulation
Journal of Physical Oceanography, 2022
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 windforcing 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 ≡ 10 6 m 3 s 21). 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.
Journal of Geophysical Research: Oceans, 2019
Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a three‐dimensional multiple‐basin perspective. We review new paradigms for deep water formation in the high‐latitude Nor...
Eastern-Boundary Contribution to the Residual and Meridional Overturning Circulations
Journal of Physical Oceanography, 2010
A model of the thermocline linearized around a specified stratification and the barotropic linear wind-driven Stommel solution is constructed. The forcings are both mechanical (the surface wind stress) and thermodynamical (the surface buoyancy boundary condition). The effects of diapycnal diffusivity and of eddy fluxes of buoyancy, parameterized in terms of the large-scale buoyancy gradient, are included. The eddy fluxes of buoyancy are especially important near the boundaries where they mediate the transport in and out of the narrow ageostrophic down-/upwelling layers. The dynamics of these narrow layers can be replaced by effective boundary conditions on the geostrophically balanced flow. The effective boundary conditions state that the residual flow normal to the effective coast vanishes. The separate Eulerian and eddy-induced components may be nonzero. This formulation conserves the total mass and the total buoyancy while permitting an exchange between the Eulerian and eddy tran...
Dynamics of the Atlantic meridional overturning circulation. Part 2: Forcing by winds and buoyancy q
Recently, Schloesser et al. (2012) explored the dynamics of the descending branch of meridional overturning circulations (MOCs), by obtaining analytic solutions to a variable-density, 2-layer model (VLOM) forced only by a surface buoyancy flux. Key processes involved are the poleward thickening of the upper layer along the eastern boundary due to Kelvin-wave adjustments, the westward propagation of that coastal structure by Rossby waves, and their damping by mixing; the resulting zonal pressure gradient causes the surface MOC branch to converge into the northern basin near the eastern boundary. In this paper, we extend the Schloesser et al. (2012) study to include forcing by a zonal wind stress s x (y). Much of the paper is devoted to the derivation and analysis of analytic solutions to VLOM; for validation, we also report corresponding numerical solutions to an ocean general circulation model (OGCM). Solutions are obtained in a flat-bottom, rectangular basin confined to the northern hemisphere. The buoyancy forcing relaxes upper-ocean density to a prescribed profile q ⁄ (y) that increases polewards until it becomes as large as the deep-ocean density at latitude y 2 ; north of y 2 , then, the ocean is homogeneous (a 1-layer system). The wind stress s x drives Subtropical and Subpolar Gyres, and in our standard solution the latter extends north of y 2. Vertical diffusion is not included in VLOM (minimized in the OGCM); consequently, the MOC is not closed by upwelling associated with interior diffusion, but rather by flow through the southern boundary of the basin (into a southern-boundary sponge layer in the OGCM), and solutions are uniquely determined by specifying the strength of that flow or the thermocline depth along the tropical eastern boundary. Solutions forced by s x and q ⁄ differ markedly from those forced only by q ⁄ because water flows across y 2 throughout the interior of the Subpolar Gyre, not just near the eastern boundary. In some of our solutions, the strength of the MOC's descending branch is determined entirely by this wind-driven mechanism, whereas in others it is also affected by Rossby-wave damping near the eastern boundary. Upwelling can occur in the interior of the Subpolar Gyre and in the western-boundary layer, providing ''shortcuts'' for the overturning circulation; consequently, there are different rates for the convergence of upper-layer water near y 2 ; M n , and the export of deep water south of the Subpolar Gyre, M, the latter being a better measure of large-scale MOC strength. When western-boundary upwelling occurs in our solutions, M is independent of the diapycnal processes in the subpolar ocean.
Upper ocean manifestations of a reducing meridional overturning circulation
1] Most climate models predict a slowing down of the Atlantic Meridional Overturning Circulation during the 21st century. Using a 100 year climate change integration of a high resolution coupled climate model, we show that a 5.3 Sv reduction in the deep southward transport in the subtropical North Atlantic is balanced solely by a weakening of the northward surface western boundary current, and not by an increase in the southward transport integrated across the interior ocean away from the western boundary. This is consistent with Sverdrup balance holding to a good approximation outside of the western boundary region on decadal time scales, and may help to spatially constrain past and future change in the overturning circulation. The subtropical gyre weakens by 3.4 Sv over the same period due to a weakened wind stress curl. These changes combine to give a net 8.7 Sv reduction in upper western boundary transport.
Journal of Physical Oceanography, 2006
A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure-dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8°and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.
Ocean Science, 2010
We study the contribution of eastern-boundary density variations to sub-seasonal and seasonal anomalies of the strength and vertical structure of the Atlantic Meridional Overturning Circulation (AMOC) at 26.5 • N, by means of the RAPID/MOCHA mooring array between April 2004 and October 2007. The major density anomalies are found in the upper 500 m, and they are often coherent down to 1400 m. The densities have 13-day fluctuations that are apparent down to 3500 m. The two strategies for measuring eastern-boundary density-a tall offshore mooring (EB1) and an array of moorings on the continental slope (EBH)-show little correspondence in terms of amplitude, vertical structure, and frequency distribution of the resulting basin-wide integrated transport fluctuations, implying that there are significant transport contributions between EB1 and EBH. Contrary to the original planning, measurements from EB1 cannot serve as backup or replacement for EBH: density needs to be measured directly at the continental slope to compute the full-basin density gradient. Fluctuations in density at EBH generate transport variability of 2 Sv rms in the AMOC, while the overall AMOC variability is 4.8 Sv rms. There is a pronounced deep-reaching seasonal cycle in density at the eastern boundary, which is apparent between 100 m and 1400 m, with maximum positive anomalies in spring and maximum negative anomalies in autumn. These changes