Ocean Heat Storage in Response to Changing Ocean Circulation Processes

Abstract

Ocean heat storage due to local addition of heat (“added”) and due to changes in heat transport (“redistributed”) were quantified in ocean-only 2xCO2 simulations. While added heat storage dominates globally, redistribution makes important regional contributions, especially in the tropics. Heat redistribution is dominated by circulation changes, summarised by the super-residual transport, with only minor effects from changes in vertical mixing. While previous studies emphasised the contribution of redistribution feedback at high-latitudes, this study shows that redistribution of heat also accounts for 65% of heat storage at low latitudes and 25% in the mid-latitude (35–50°S) Southern Ocean. Tropical warming results from the interplay between increased stratification and equatorward heat transport by the subtropical gyres, which redistributes heat from the subtropics to lower latitudes. The Atlantic pattern is remarkably distinct from other basins, resulting in larger basin-average heat storage. Added heat storage is evenly distributed throughout mid-latitude Southern Ocean and dominates the total storage. However, redistribution stores heat north of the Antarctic Circumpolar Current in the Atlantic and Indian sectors, having an important contribution to the peak of heat storage at 45°S. Southern Ocean redistribution results from intensified heat convergence in the subtropical front and reduced stratification in response to surface heat, freshwater, and momentum flux perturbations. These results highlight that the distribution of ocean heat storage reflects both passive uptake of heat and active redistribution of heat by changes in ocean circulation processes. The redistributed heat transport must therefore be better understood for accurate projection of changes in ocean heat uptake efficiency, ocean heat storage and thermosteric sea level.

Contribution of Ocean Physics and Dynamics at Different Scales to Heat Uptake in Low-Resolution AOGCMs

“…The heat balance implied by SRT and Small, with the former cooling the ocean interior below 400 m and the latter warming it, appears to be consistent with the advective-diffusive balance considered by Munk (1966) and more recently by Munk and Wunsch (1998). A similar result was arrived at by Dias et al (2020a), who proposed reinterpreting SRT as the advective part of the classical advective-diffusive balance. While a more detailed discussion of this subject is beyond our scope, we note that caution is required when comparing our global SRT and Small profiles to Munk's analysis.…”

“…Weak stratification in the northern North Atlantic and the associated deep convective mixing intimately link Param and Large to form the basin-scale AMOC, which takes up heat via Param and redistributes it southward via Large. This finding is supported by the heat budget analysis presented in Dias et al (2020b).…”

“…In addition, we shall present the OHU associated with the superresidual transport (SRT) (Kuhlbrodt et al 2015;Dias et al 2020a), where SRT is defined as the sum…”

“…7a). Using an OGCM forced with the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) surface perturbations corresponding to 2 3 CO 2 (see Gregory et al 2016), Dias et al (2020b estimate that 65% of the OHC change at low latitudes is due to the redistribution of heat associated with SRT, dominated by the largescale advection. The contributions from Meso and Small are relatively weak (i.e., the red and blue lines in Figs.…”

“…8a). In the Indian sector, decomposition of the net OHC change into contributions from the added and redistributed heat shows enhanced contribution from the latter south of about 408S (Gregory et al 2016;Couldrey et al 2021); some of it might be connected with the Atlantic Ocean (Dias et al 2020b). In the Pacific sector the Atlantic warming signal becomes weaker, and so does the contribution of SRT to the local OHC change (Figs.…”