On the Superposition of Mean Advective and Eddy-Induced Transports in Global Ocean Heat and Salt Budgets

Abstract

Ocean thermal expansion is a large contributor to observed sea level rise, which is expected to continue into the future. However, large uncertainties exist in sea level projections among climate models, partially due to intermodel differences in ocean heat uptake and redistribution of buoyancy. Here, the mechanisms of vertical ocean heat and salt transport are investigated in quasi-steady-state model simulations using the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2). New insights into the net effect of key physical processes are gained within the superresidual transport (SRT) framework. In this framework, vertical tracer transport is dominated by downward fluxes associated with the large-scale ocean circulation and upward fluxes induced by mesoscale eddies, with two distinct physical regimes. In the upper ocean, where high-latitude water masses are formed by mixed layer processes, through cooling or salinification, the SRT counteracts those processes by transporting heat and salt downward. In contrast, in the ocean interior, the SRT opposes dianeutral diffusion via upward fluxes of heat and salt, with about 60% of the vertical heat transport occurring in the Southern Ocean. Overall, the SRT is largely responsible for removing newly formed water masses from the mixed layer into the ocean interior, where they are eroded by dianeutral diffusion. Unlike the classical advective–diffusive balance, dianeutral diffusion is bottom intensified above rough bottom topography, allowing an overturning cell to develop in alignment with recent theories. Implications are discussed for understanding the role of vertical tracer transport on the simulation of ocean climate and sea level.

Ocean Heat Storage in Response to Changing Ocean Circulation Processes

“…Our perturbed experiments were performed using an ocean general circulation model (OGCM), the ocean-sea ice model from the Australian Community Climate and Earth System Simulator (ACCESS-OM2; Kiss et al 2020), with a nominal 18 horizontal resolution. ACCESS-OM2 was initially spun up for 1000 years, under a climatological atmospheric state obtained from the JRA55-do repeated year forcing (Tsujino et al 2018;Stewart et al 2020), using bulk formulas to compute the turbulent heat fluxes, as in Dias et al (2020). The 80-yr perturbed experiments were branched off from the spinup in year 1001.…”

“…SSS restoring in OGCMs avoids unintended salinity drift (Danabasoglu et al 2014). Although not the dominant term of the surface freshwater/salt budget in ACCESS-OM2 (Dias et al 2020), restoring makes a significant contribution in regions of large SSS biases, such as in western boundary currents (WBCs) and along the Antarctic Circumpolar Current (ACC). For implementation of the flux form, the spinup was extended to year 1080, saving the surface salt fluxes from SSS restoring every 6 h. A new control run was simulated for years 1001-80 with SSS restoring deactivated but prescribing the saved salt fluxes.…”

“…Past studies of global vertical heat transport under a steady state indicated a balance between downward heat transport due to large-scale advection (ADV) and upward heat transport due to eddy-induced processes (EIT) (Gregory 2000;Gnanadesikan et al 2005;Wolfe et al 2008;Hieronymus and Nycander 2013;Exarchou et al 2015;Griffies et al 2015), where both advective and diffusive components of the EIT have similar effects (Kuhlbrodt et al 2015;Dias et al 2020). As definitions of advective and diffusive processes vary among models and generally depend on the horizontal grid scale that models can resolve, a consistent framework is required for model intercomparisons, such as the ''super-residual'' transport (SRT)-the superposition of ADV and EIT-first proposed by Kuhlbrodt et al (2015).…”

“…For example, for comparing the range of ''eddy-permitting'' models, which sometimes parameterize eddy (isoneutral) diffusion (Megann et al 2014;Kuhlbrodt et al 2015) and sometimes rely on it being resolved by the advection scheme (Wolfe et al 2008;Morrison et al 2013;Griffies et al 2015). Dias et al (2020) investigated the SRT role in the quasisteady state of the ACCESS-OM2. In the global integral, they found that while ADV and EIT transport heat downward and upward respectively, the SRT revealed two depth-regimes with opposite contributions (their Fig.…”

“…The depth of the mixed layer regime varies regionally but can reach 700 m in the midlatitude Southern Ocean and 2000 m in the subpolar North Atlantic. Dias et al (2020) suggested that for the depthintegrated budget, from the surface to the bottom of the mixed layer, the net surface heat flux is balanced by the SRT (Fig. 4a), obscuring the effects from mixed layer physics (KPP, SUB, CON) that effectively propagate the surface fluxes downward into the water column.…”