Ocean-only FAFMIP: Understanding Regional Patterns of Ocean Heat Content and Dynamic Sea Level Change

Abstract

<p>A rise in global mean sea level is a robust feature of projected anthropogenic climate change using state-of-the-art atmosphere-ocean general circulation models (AOGCMs). However, there is considerable disagreement over the more policy-relevant regional patterns of sea level rise. The Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) aims to improve our understanding of the mechanisms controlling regional and dynamic sea level change. In FAFMIP, identical air-sea buoyancy and momentum flux perturbations are applied to an ensemble of different AOGCMs, to sample the uncertainty associated with model structure and physical processes. Our novel implementation applies FAFMIP perturbations to an ensemble of OGCMs. This framework enables an estimate of the unknown atmosphere-ocean feedbacks, by comparing the coupled and ocean-only response to surface flux perturbations.</p><p>Comparing the response to idealised FAFMIP forcing with more realistic, increasing CO2 forcing, much of the spread in regional sea level projections for the North Atlantic and Southern Ocean arises from ocean model structural differences. Ocean-only simulations indicate that only a small proportion of this spread is due to differences in the atmosphere-ocean feedback. Novel tendency diagnostics indicate the relative effect of resolved advection, parametrised eddies, and dianeutral mixing on regional and dynamic sea level change. This study helps to reduce uncertainty in regional sea level projections by refining our estimates of atmosphere-ocean feedbacks and developing our understanding of the physical processes controlling sea level change.</p>

The Influence of Warming Patterns on Passive Ocean Heat Uptake

“…Instead, passive heat uptake from the subtropical and northern Pacific may be more important than traditionally assumed (here contributing an average 20% of the global total in our experiments) due to the strong warming across these subduction regions. In sum, our results support the argument that passive heat uptake dominates total global heat uptake (Armour et al, 2016; Garuba & Klinger, 2018; Gregory, Bouttes, et al, 2016; Huber & Zanna, 2017; Todd et al, 2020; Zanna et al, 2019) particularly as the anomalous heat content in the ocean grows with sustained greenhouse forcing (Bronselaer & Zanna, 2020). If so, any effect that damps passive heat uptake, such as the pattern effect, will significantly reduce the ocean's capacity to sequester heat from the atmosphere.…”

“…This “redistributed” heat (Garuba & Klinger, 2016) can significantly influence the spatial distribution of ocean warming and, indirectly, (e.g., Banks & Gregory, 2006), particularly through changes in the North Atlantic (Bouttes et al, 2014; Garuba & Klinger, 2016; Rugenstein et al, 2013; Winton et al, 2013; Xie & Vallis, 2012). However, more recent studies argue that redistribution plays a small role, as compared to OHU p , in setting global (Armour et al, 2016; Garuba & Klinger, 2018; Gregory, Bouttes, et al, 2016; Todd et al, 2020). Further, Bronselaer and Zanna (2020) find that the pattern of heat storage is increasingly determined by OHU p as the anomalous heat content in the ocean grows.…”

What causes the spread of model projections of ocean dynamic sea-level change in response to greenhouse gas forcing?

“…Because of this feedback, the heat input into the North Atlantic is about double what it would be in a 1pctCO2 simulation. By comparing FAF-heat experiments with two corresponding pairs of an AOGCM and an OGCM, Todd et al (2020) find, however, that the AMOC weakening is greater by only about 10% in the AOGCM case due to the feedback. Their finding is consistent with our results, where the total OHC change in FAF-heat is about 10% greater (due to the redistribution feedback) than the time-and areaintegral of the imposed perturbation.…”

“…2.2 and 3.1) causes the total heat input into the North Atlantic to be larger than the just the imposed perturbation (Gregory et al 2016). Todd et al (2020) investigated the strength of this unwanted feedback by forcing ocean-only models (which have no redistribution feedback) with the same heat flux perturbation as this study, and compared the ocean heat transport response with the response of coupled AOGCMs (which do have the feedback). Those authors find that the feedback causes an additional 10% AMOC weakening versus the change that occurs in fully coupled AOGCMs.…”

Human-induced changes to the global ocean water masses and their time of emergence

“…If these patterns have been suggested to be primarily driven by an increased surface warming and water-cycle amplification 29,31 (two processes that directly affect heat and salt in the ocean thus density and circulation), understanding how these patterns will continue to amplify in the future in a more stratified upper ocean 40 and with possibly modified ocean circulation and mixing requires further investigation. In particular deciphering which of the changing surface fluxes is likely to play a larger role, where and on what timescales, can be for example explored with model-specific FAFMIP-like 41,42 mechanistic studies.…”