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- Author or Editor: Richard J. Williams x
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Abstract
Subduction requires a buoyancy input into the mixed layer, which over the gyre scale can either be achieved by an atmospheric input or a wind-induced Ekman redistribution of buoyancy. The buoyancy budget for subduction is diagnosed over the North Atlantic using monthly fields from 1950 to 1992. The climatological-mean budget suggests that subduction over the subtropical gyre occurs through an Ekman redistribution of buoyancy from the Tropics, rather than a surface buoyancy flux from the atmosphere. In contrast, interannual variations in subduction are controlled by the variations in the surface buoyancy flux, which are generally greater than the variations in the Ekman redistribution of buoyancy. However, over the Tropics and southern part of the subtropical gyre, there is a partial cancellation in the opposing contributions from the surface and Ekman buoyancy fluxes, which acts to reduce the interannual variations in subduction.
Abstract
Subduction requires a buoyancy input into the mixed layer, which over the gyre scale can either be achieved by an atmospheric input or a wind-induced Ekman redistribution of buoyancy. The buoyancy budget for subduction is diagnosed over the North Atlantic using monthly fields from 1950 to 1992. The climatological-mean budget suggests that subduction over the subtropical gyre occurs through an Ekman redistribution of buoyancy from the Tropics, rather than a surface buoyancy flux from the atmosphere. In contrast, interannual variations in subduction are controlled by the variations in the surface buoyancy flux, which are generally greater than the variations in the Ekman redistribution of buoyancy. However, over the Tropics and southern part of the subtropical gyre, there is a partial cancellation in the opposing contributions from the surface and Ekman buoyancy fluxes, which acts to reduce the interannual variations in subduction.
Abstract
The effect of cooling on the separated boundary current predicted by the model of Parsons is studied. The separating current is found to strengthen and to move southwards and eastwards. The model is also robust to limited heating. in which case the separating current weakens and moves northwards.
Abstract
The effect of cooling on the separated boundary current predicted by the model of Parsons is studied. The separating current is found to strengthen and to move southwards and eastwards. The model is also robust to limited heating. in which case the separating current weakens and moves northwards.
Abstract
The formation rate of water masses and its relation to air–sea fluxes and interior mixing are examined in an isopycnic model of the North (and tropical) Atlantic that includes a mixed layer. The diagnostics follow Walin’s formulation, linking volume and potential density budgets for an isopycnal layer.
The authors consider the balance between water mass production, mixing, and air–sea fluxes in the model in the context of two limit cases: (i) with no mixing, where air–sea fluxes drive water mass formation directly, and (ii) a steady state in a closed basin, where air–sea fluxes are balanced by diffusion. In such a steady state, since mixing always acts to reduce density contrast, surface forcing must act to increase it.
Considered over the whole basin, including the Tropics, the model is in steady state apart from the densest layers. Most of the mixing is achieved by diapycnal diffusion in the strong density gradients within upwelling regions in the Tropics, and by entrainment into the tropical mixed layer. Mixing from entrainment associated with the seasonal cycle of mixed layer depth in mid and high latitudes and lateral mixing of density within the mixed layer are less important than this tropical mixing. These model results as to the relative importance of the different mixing processes are consistent with a simple scaling analysis.
Outside the Tropics, the upwelling-linked mixing is no longer important, and a first-order estimate of water mass formation rates may be made from the surface fluxes. Lateral mixing of density within the mixed layer and seasonal entrainment mixing are as important as the remaining thermocline mixing within this domain.
An apparent vertical diffusivity is diagnosed over both the full and extratropical domain. It reaches 10−4 m2 s−1 for the denser waters, about four times as large as the explicit diapycnal diffusion within the thermocline.
Abstract
The formation rate of water masses and its relation to air–sea fluxes and interior mixing are examined in an isopycnic model of the North (and tropical) Atlantic that includes a mixed layer. The diagnostics follow Walin’s formulation, linking volume and potential density budgets for an isopycnal layer.
The authors consider the balance between water mass production, mixing, and air–sea fluxes in the model in the context of two limit cases: (i) with no mixing, where air–sea fluxes drive water mass formation directly, and (ii) a steady state in a closed basin, where air–sea fluxes are balanced by diffusion. In such a steady state, since mixing always acts to reduce density contrast, surface forcing must act to increase it.
Considered over the whole basin, including the Tropics, the model is in steady state apart from the densest layers. Most of the mixing is achieved by diapycnal diffusion in the strong density gradients within upwelling regions in the Tropics, and by entrainment into the tropical mixed layer. Mixing from entrainment associated with the seasonal cycle of mixed layer depth in mid and high latitudes and lateral mixing of density within the mixed layer are less important than this tropical mixing. These model results as to the relative importance of the different mixing processes are consistent with a simple scaling analysis.
Outside the Tropics, the upwelling-linked mixing is no longer important, and a first-order estimate of water mass formation rates may be made from the surface fluxes. Lateral mixing of density within the mixed layer and seasonal entrainment mixing are as important as the remaining thermocline mixing within this domain.
An apparent vertical diffusivity is diagnosed over both the full and extratropical domain. It reaches 10−4 m2 s−1 for the denser waters, about four times as large as the explicit diapycnal diffusion within the thermocline.
Abstract
The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate S ann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, S ann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer. Geostrophic velocity fields, computed from the Levitus climatology assuming a level of no motion at 2.5 km, are used; the vertical velocity at the base of the mixed layer is deduced from observed surface Ekman pumping velocities and linear vorticity balance. A plausible pattern of S ann is obtained with subduction rates over the subtropical gyre approaching 100 m/yr—twice the maximum rate of Ekman pumping.
The subduction period 𝒯eff is found by viewing subduction as a transformation process converting mixed-layer fluid into stratified thermocline fluid. The effective period is that period of time during the shallowing of the mixed layer in which sufficient buoyancy is delivered to permit irreversible transfer of fluid into the main thermocline at the rate S ann. Typically 𝒯eff is found to be 1 to 2 months over the major part of the subtropical gyre, rising to 4 months in the tropics.
Finally, the heat budget of a column of fluid, extending from the surface down to the base of the seasonal thermocline is discussed, following it over an annual cycle. We are able to relate the buoyancy delivered to the mixed layer during the subduction period to the annual-mean buoyancy forcing through the sea surface plus the warming due to the convergence of Ekman heat fluxes. The relative importance of surface fluxes (heat and freshwater) and Ekman fluxes in supplying buoyancy to support subduction is examined using the climatologist observations of Isemer and Hasse, Schmitt et al., and Levitus. The pumping down of fluid from the warm summer Ekman layer into the thermocline makes a crucial contribution and, over the subtropical gyre, is the dominant term in the thermodynamics of subduction.
Abstract
The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate S ann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, S ann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer. Geostrophic velocity fields, computed from the Levitus climatology assuming a level of no motion at 2.5 km, are used; the vertical velocity at the base of the mixed layer is deduced from observed surface Ekman pumping velocities and linear vorticity balance. A plausible pattern of S ann is obtained with subduction rates over the subtropical gyre approaching 100 m/yr—twice the maximum rate of Ekman pumping.
The subduction period 𝒯eff is found by viewing subduction as a transformation process converting mixed-layer fluid into stratified thermocline fluid. The effective period is that period of time during the shallowing of the mixed layer in which sufficient buoyancy is delivered to permit irreversible transfer of fluid into the main thermocline at the rate S ann. Typically 𝒯eff is found to be 1 to 2 months over the major part of the subtropical gyre, rising to 4 months in the tropics.
Finally, the heat budget of a column of fluid, extending from the surface down to the base of the seasonal thermocline is discussed, following it over an annual cycle. We are able to relate the buoyancy delivered to the mixed layer during the subduction period to the annual-mean buoyancy forcing through the sea surface plus the warming due to the convergence of Ekman heat fluxes. The relative importance of surface fluxes (heat and freshwater) and Ekman fluxes in supplying buoyancy to support subduction is examined using the climatologist observations of Isemer and Hasse, Schmitt et al., and Levitus. The pumping down of fluid from the warm summer Ekman layer into the thermocline makes a crucial contribution and, over the subtropical gyre, is the dominant term in the thermodynamics of subduction.
