Geothermal heating, diapycnal mixing and the abyssal circulation

Abstract

The dynamical role of geothermal heating in abyssal circulation is reconsidered using three independent methods. First, we show that a uniform geothermal heat flux close to the observed average (86.4mWm?2) supplies as much heat to the abyss as diapycnal mixing with a rate of 1 cm2 s?1. A simple scaling law, based upon a purely advective balance, indicates that such a heat flux is able to generate a deep circulation of order 5 Sv (1 Sv106 m3s?1) associated with the Antarctic Bottom Water mass (AABW). Its intensity is inversely proportional to the strength of deep temperature gradients. Second, this order of magnitude is confirmed by the density-binning method (Walin, 1982) applied to the observed thermohaline structure of Levitus (1998). Additionally, the method allows to investigate the effect of realistic spatial variations of the flux obtained from heatflow measurements and classical theories of lithospheric cooling. It is found that a uniform heatflow forces a transformation of about 6 SV at 4=45.90, consistent with the previous estimate. The result is very similar for a realistic heatflow, albeit shifted towards slightly lighter density classes. Third, we use a general ocean circulation model in global configuration to perform three sets of experiments: (1) a thermally homogenous abyssal ocean with and without uniform geothermal heating; (2) a more stratified abyssal ocean subject to (i) no geothermal heating, (ii) a constant heat flux of 86.4mWm?2, (iii) a realistic, spatially varying heat flux of identical global average; (3) experiments (i) and (iii) with enhanced vertical mixing at depth. It is found, for strong vertical mixing rates, that geothermal heating enhances the AABW cell by about 15% (1.5 Sv) and heats up the last 2000m by 0.3, reaching a maximum of 0.5 in the deep North Pacific. Its impact is even stronger in a weakly diffusive deep ocean. The spatial distribution of the heat flux acts to enhance this temperature rise at mid-depth and reduce it at great depth, producing a more moderate increase in overturning than in the uniform case.

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