Double diffusion, shear instabilities, and heat impacts of a Pacific Summer Water intrusion in the Beaufort Sea

Pacific Summer Water eddies and intrusions transport heat and salt from boundary regions into the western Arctic basin. Here we examine concurrent effects of lateral stirring and vertical mixing using microstructure data collected within a Pacific Summer Water intrusion with a length scale of ∼20 km. This intrusion was characterized by complex thermohaline structure in which warm Pacific Summer Water interleaved in alternating layers of O(1 m) thickness with cooler water, due to lateral stirring and intrusive processes. Along interfaces between warm/salty and cold/fresh water masses, the density ratio was favorable to double-diffusive processes. The rate of dissipation of turbulent kinetic energy (ε) was elevated along the interleaving surfaces, with values up to 3×10−8 W kg−1 compared to background ε of less than 10−9 W kg−1. Based on the distribution of ε as a function of density ratio Rρ , we conclude that double-diffusive convection is largely responsible for the elevated ε observed over the survey. The lateral processes that created the layered thermohaline structure resulted in vertical thermohaline gradients susceptible to double-diffusive convection, resulting in upward vertical heat fluxes. Bulk vertical heat fluxes above the intrusion are estimated in the range of 0.2-1 W m−2, with the localized flux above the uppermost warm layer elevated to 2- 10 W m−2. Lateral fluxes are much larger, estimated between 1000-5000 W m−2, and set an overall decay rate for the intrusion of 1-5 years.

Assessment of the interval heat flow and thermal resistance at the Faltu-1 well, Borno basin, NE Nigeria

The heat flowing through horizons in the Faltu-1 well, Borno Basin, NE Nigeria was calculated from their thermal conductivities and geothermal gradients with the aim of determining whether or not it is uniform, and if not, the depths where it is diverted, and the possible heat diversion process. The interval heat flow was assessed to be non-uniform. While fluid convection is adjudged to be the major heat diversion mechanism within the Chad Formation with minor heat refraction, the reversed is adjudged to be the situation for the underlying Kerri Kerri Formaton within which increasing sand content with depth is also predicted, with the lower interval predicted to be the Gombe Formation. Patterns of disruptions to the vertical heat flow within the Fika Formation were inferred to suggest rhythmic bedding of shale and sand beds that could serve as both source and reservoir rocks. Magmatic intrusions that impacted the maturation of organic matter into oil and gas also provided necessary entrapment structures and possible migration pathways. The Gongila and Bima Formations each has single disruption of the heat flows that are attributed to refraction. In the case of the Gongila Formation, the disruption is between the early-deposited more sandy and laterdeposited more shaley lithologies in the marine transgression of the area, while in the case of the Bima, it is between the more shaley Upper and more sandy Middle Bima Formations. Analysis of the Bullard plots also revealed disruptions to the vertical heat flow that are attributed either to convecting fluids or to heat refraction and diffraction. Two such disrupting heat advections were identified within the Chad Formation with the first being attributed to convection, while the other is attributed to a combination of both. Two similar disruptions for the Kerri Kerri Formation were attributed largely to lithological variations with minor contributions from convection of fluids. While unable to discern the rhythmic bedding, the five disruptions of the Bullard plot for the Fika Formation and one each for the Gongila and Bima Formations were interpreted to indicate similar features inferred from interval heat flow plots. Keywords: Interval heat flow, heat convection, heat diffraction, thermal resistivity, shaliness

Super Residual Circulation: a new perspective on ocean vertical heat transport

Ocean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities to diagnose circulation. Using quasi-steady state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature-depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of Super-Residual Transport (SRT), which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies, but excludes small-scale three dimensional turbulent mixing effect, to construct a new overturning circulation – the ‘Super Residual Circulation’ (SRC).We find that in the coarse resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11 Sv circulation which transports heat upward. The SRC’s upward heat transport is ~2 times larger in a finer horizontal resolution (0.1°) version of ACCESS, suggesting a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into super-residual processes, as the SRC elucidates the pathways in temperature and depth space along which watermass transformation occurs.

A new process-based vertical advection/diffusion theoretical model of ocean heat uptake

<p>Ocean heat uptake is a key process for climate change owing to its control of global mean temperature trends. To understand the underlying internal ocean processes and vertical heat transfer controlling it, ocean heat uptake has been often analysed in terms of the simple one-dimensional vertical advection diffusion model. The standard version of this model, formulated in terms of the horizontally-averaged potential temperature is known to poorly capture important effects such as isopycnal mixing, density-compensated temperature anomalies, meso-scale eddy-induced advection and the depth-varying ocean area.</p><p>To overcome this problem a new theoretical model of vertical heat transfer for the ocean heat uptake has been developed in an isopycnal framework that exploits advances achieved in the theory of water masses over the past 30 years or so. The new theoretical model describes the temporal evolution of the isopycnally-averaged thickness-weighted potential temperature in terms of an effective velocity that depends uniquely on the surface heating conditionally integrated in density classes, an effective diapycnal diffusivity controlled by isoneutral and dianeutral mixing, and an additional term linked to the meridional transport of density-compensated temperature anomalies by the diabatic residual overturning circulation. The advantage of the isopycnally-averaged construction over the horizontally-averaged construction is that all the terms that enters it have explicit analytical expressions that are more easily evaluated from observations or model outputs, as well as having clearer physical interpretations.</p><p>As a first step, the terms of this new model of ocean heat uptake are evaluated by using a range of different datasets, net surface heat flux products and temporal averages to evaluate their sensitivity to input fields. One key feature of the new model is that its effective velocity and diffusivity are positive over most of the ocean column depth. This is in contrast to the horizontally-averaged construction, in which downwelling and ant-diffusive behavior were occasionally observed in previous studies. The hope is that this insight can then be used to develop an improved representation of ocean heat uptake in simple climate models.</p>

What is the heat uptake potential of Antarctic Bottom Water?

<p>Antarctic Bottom Water (AABW) is a cold dense water mass which sinks around Antarctica keeping the abyssal ocean relatively cool. Recent observations have suggested a component of recent deep ocean warming is linked to AABW. Here we explore how much changes in AABW could affect changes in vertical ocean heat transport in a warming climate. If the AABW circulation were to be completely extinguished, for example due to increases in upper ocean thermal stratification, AABW would cease to cool the deep ocean and hence lead to an effective warming of the abyss. Therefore, we propose that long term mean vertical heat transport of the AABW circulation is an effective upper bound on the change in heat transport that can be affected by changes in AABW. We call this upper bound the ‘heat uptake potential’. We analyse AABW circulations in an ensemble of numerical climate models. We find that the AABW circulation contributes between 0.05Wm<sup>-2</sup> and 0.15Wm<sup>-2</sup> to the global vertical heat balance in the model’s pre-industrial states. Indeed, under abrupt CO<sub>2</sub> forcing changes, AABW heat transport systematically reduces (in some cases completely), with the largest reductions occurring in models with the largest pre-industrial mean heat transports. The AABW circulation vertical heat transport is found to be highly correlated with the minimum of the Meridional Overturning Circulation at 50<sup>o</sup>S in the models, suggesting there may be observable constraints on the heat uptake potential of AABW.</p>