Thermodynamic Analysis of Ocean Circulation

Abstract

Calculating a streamfunction as function of depth and density is proposed as a new way of analyzing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this streamfunction directly shows whether it is driven mechanically by large-scale wind stress or thermally by heat conduction and small-scale mixing. It is also shown that the integral of this streamfunction gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although formally there is no thermodynamic work in an incompressible fluid. The proposed method is applied both to an idealized coarse-resolution three-dimensional numerical ocean model, and to the realistic high-resolution Ocean Circulation and Climate Advanced Model (OCCAM). It is shown that the overturning circulation in OCCAM between the 200-and 1000-m depth is dominated by a thermally indirect cell of 24 Sverdrups (1 Sv ϵ 10 6 m 3 s Ϫ1 ), forced by Ekman pumping. In the densest and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized small-scale mixing.

The ocean’s role in setting the mean position of the Inter-Tropical Convergence Zone

“…These patterns are a direct result of crossequatorial heat transport in the ocean, directed up the temperature gradient in the ocean. Note that because the interhemispheric MOC cell of the ocean is mechanically forced by the wind, it has no requirement to flux energy downgradient, as discussed in Nycander et al (2007). The atmosphere, by contrast, fluxes heat across the equator from the warmer NH to the (somewhat) colder SH-as indicated by the arrows in the figure.…”

On the Consumption of Antarctic Bottom Water in the Abyssal Ocean

“…This result suggests that geothermal heat fluxes have a non-negligible part in the 678 energy flows that sustain the abyssal overturning. The respective roles of diapycnal mixing and geothermal heating for ocean energetics can be compared by calculating their potential energy 680 31 supply to the global ocean (e.g., Huang 1999, Nycander et al 2007:…”

Vertical Eddy Fluxes in the Southern Ocean

“…To gain further insight into the Southern Ocean overturning, we average the overturning in depth-density coordinates (Nurser and Lee 2004;Nycander et al 2007). Such a diagnostic represents the actual vertical transport of water masses and can be interpreted as the potential energy input into the ocean by the circulation at the reference pressure of the potential density coordinate (Nycander et al 2007).…”

“…Such a diagnostic represents the actual vertical transport of water masses and can be interpreted as the potential energy input into the ocean by the circulation at the reference pressure of the potential density coordinate (Nycander et al 2007). …”

“…Recently Nurser and Lee (2004) and Nycander et al (2007) proposed diagnosing the oceanic circulation in depth-density coordinates. In such coordinates, net vertical exchanges of water masses are diagnosed by averaging the vertical velocity in density ranges at constant depth.…”