Geothermal heating, diapycnal mixing and the abyssal circulation

[摘要] The dynamical role of geothermal heating in abyssal circulation is reconsideredusing three independent arguments.First, we show that a uniform geothermal heat flux close to the observed average(86.4 mW m⁻²) supplies as much heat to near-bottom water asa diapycnal mixing rate of ~10⁻⁴ m² s⁻¹ – the canonicalvalue thought to be responsible for the magnitude of the present-day abyssalcirculation. This parity raises the possibility that geothermal heating couldhave a dynamical impact of the same order. Second, we estimate the magnitudeof geothermally-induced circulation with the density-binning method (Walin, 1982),applied to the observedthermohaline structure of Levitus(1998). The methodalso allows to investigate the effect of realistic spatial variations of theflux obtained from heatflow measurements and classical theories of lithosphericcooling. It is found that a uniform heatflow forces a transformation of~6 Sv at σ₄=45.90,which is of the same order as current best estimates of AABWcirculation. This transformation can be thought of as the geothermal circulation inthe absence of mixing and is very similar for a realistic heatflow, albeit shiftedtowards slightly lighter density classes. Third, we use a general ocean circulationmodel in global configuration to perform three sets of experiments: (1) a thermallyhomogenous abyssal ocean with and withoutuniform geothermal heating; (2) a morestratified abyssal ocean subject to (i) no geothermal heating, (ii) a constant heatflux of 86.4 mW m⁻², (iii)a realistic, spatially varying heatflux of identical global average; (3) experiments (i) and (iii) with enhancedvertical mixing at depth. Geothermal heating and diapycnal mixing are found tointeract non-linearly through the density field, with geothermal heating erodingthe deep stratification supporting a downward diffusive flux, while diapycnalmixing acts to map near-surface temperature gradients onto the bottom, therebyaltering the density structure that supports a geothermal circulation.For strong vertical mixing rates, geothermal heating enhances the AABW cell byabout 15% (2.5 Sv) and heats up the last 2000 m by ~0.15°C,reaching a maximum of by 0.3°C in the deep North Pacific. Prescribing arealistic spatial distribution of the heat flux acts to enhance this temperaturerise at mid-depth and reduce it at great depth, producing a more modest increasein overturning than in the uniform case. In all cases, however, poleward heattransport increases by~10% in the Southern Ocean. The three approachesconverge to the conclusion that geothermal heating is an important actor ofabyssal dynamics, and should no longer be neglected in oceanographic studies.

[学科分类] 海洋学与技术