Geothermal heating, diapycnal mixing and the abyssal circulation

Abstract

Abstract. The dynamical role of geothermal heating in abyssal circulation is reconsidered using 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 as a diapycnal mixing rate of ∼10 −4 m 2 s −1 -the canonical value thought to be responsible for the magnitude of the present-day abyssal circulation. This parity raises the possibility that geothermal heating could have a dynamical impact of the same order. Second, we estimate the magnitude of geothermally-induced circulation with the density-binning method (Walin, 1982), applied to the observed thermohaline structure of Levitus (1998). The method also 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 ∼6 Sv at σ 4 =45.90, which is of the same order as current best estimates of AABW circulation. This transformation can be thought of as the geothermal circulation in the absence of mixing and 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 86.4 mW m −2 , (iii) a realistic, spatially varying heat flux of identical global average; (3) experiments (i) and (iii) with enhanced vertical mixing at depth. Geothermal heating and diapycnal mixing are found to interact non-linearly through the density field, with geothermal heating eroding the deep stratification supporting a downward diffusive flux, while diapycnal mixing acts to map near-surface temperature gradientsCorrespondence to: J. Emile-Geay (julieneg@usc.edu) onto the bottom, thereby altering the density structure that supports a geothermal circulation. For strong vertical mixing rates, geothermal heating enhances the AABW cell by about 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 a realistic spatial distribution of the heat flux acts to enhance this temperature rise at mid-depth and reduce it at great depth, producing a more modest increase in overturning than in the uniform case. In all cases, however, poleward heat transport increases by ∼10% in the Southern Ocean. The three approaches converge to the conclusion that geothermal heating is an important actor of abyssal dynamics, and should no longer be neglected in oceanographic studies.

Abyssal Upwelling and Downwelling Driven by Near-Boundary Mixing

“…In most of this paper it is convenient to ignore the influence of the geothermal heat flux from the discussion, but before doing so we will first estimate its magnitude. In a ground-breaking study of the effect of the geothermal heat flux on the abyssal circulation, Emile-Geay and Madec (2009) …”

The Transient Response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model

“…Previous geothermal heating studies that were run for millennial time scales were not fully coupled (e.g., Adcroft et al 2001;Emile-Geay and Madec 2009), and thus lacked atmospheric feedbacks on the ocean that arise when geothermal heating impacts the ocean surface (e.g., Adcroft et al 2001;Mashayek et al 2013;Piecuch et al 2015). Here we show that geothermal heating-induced anomalies do extend toward the surface (thus impacting surface water mass transformation), particularly in polar regions where approximated atmospheric forcing errors can be substantial (e.g., Nygard et al 2016;Hobbs et al 2016).…”

“…On a regional scale, geothermal heating can change ocean bottom temperatures by an order of magnitude more than error estimates associated with decadal abyssal temperature trends (e.g., Emile-Geay and Madec 2009;Purkey and Johnson 2010;Kouketsu et al 2011;Wunsch and Heimbach 2014) and increase thermosteric sea level by 0.1-1 mm yr 21 (Piecuch et al 2015). Yet geothermal heating is represented inconsistently by ocean and fully coupled models.…”

“…Geothermal heating has a nonnegligible influence on the large-scale abyssal circulation by weakening abyssal stratification and increasing the circulation of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) on the order of 10%-30% (e.g., Adcroft et al 2001;Hofmann and Morales Maqueda 2009;Emile-Geay and Madec 2009;Mashayek et al 2013;de Lavergne et al 2016). On a regional scale, geothermal heating can change ocean bottom temperatures by an order of magnitude more than error estimates associated with decadal abyssal temperature trends (e.g., Emile-Geay and Madec 2009;Purkey and Johnson 2010;Kouketsu et al 2011;Wunsch and Heimbach 2014) and increase thermosteric sea level by 0.1-1 mm yr 21 (Piecuch et al 2015).…”

“…Previous studies have demonstrated that geothermal heating has widespread impacts on the abyssal ocean and rivals diapycnal mixing for AABW transformation. Lighter AABW, in regions of weak stratification, is more susceptible to heat gain from geothermal input, which acts to expand the bottom water incrop area at the ocean floor (Emile-Geay and Madec 2009;de Lavergne et al 2016). In addition, the impacts of geothermal heating vary regionally, which comes as no surprise considering that the ocean circulation patterns, water mass properties, and stratification differ between basins.…”