Jonathan Gregory (Univ. of Reading): “A new conceptual model of ocean heat uptake and transient climate response”
Spencer Hill (Princeton Univ.): “Symmetric and antisymmetric components of polar amplification on decadal to millennial timescales”
Xiyue Zhang (Johns Hopkins Univ.): "The remote impact of observed Southern Ocean surface warming and cooling"
09:18:28 From Lily Hahn : You show an important role of the control AMOC--is there also a role for the AMOC decline under forcing, which itself is correlated with the control AMOC?
09:18:38 From Kyle Armour : Nice work, Jonathan. Explains quite a few things I’ve wondered about for a while. Questions for you: what is the role of mean-state AMOC versus AMOC changes in MT2? Is it really Delta AMOC that matters, which itself is correlated with mean-state AMOC strength?
Gregory: As you say, from correlations alone we couldn't tell whether it is M (AMOC strength) that matters, or Delta M, when studying the 2xCO2 state of 1pctCO2, because Delta M is (anti)correlated with M. I think it's M that matters for various reasons. * You also get a strong relationship across AOGCMs between the M and the efficiency of uptake of a passive tracer (with the same geographical distribution of surface flux as the surface heat flux anomaly due to CO2) in an experiment with *no* climate change i.e. Delta M=0. * AMOC weakening affects the Atlantic, particularly the North Atlantic, but OHU and its spread across models is largest in the Southern Ocean. In fact the net heat uptake in the North Atlantic is not very large or strongly correlated with M, because there are two opposing effects. Reduced convection (in response to surface buoyancy flux change) causes reduced cooling of the subsurface ocean and hence anomalous heat uptake. At the same time, the weakening of the AMOC reduces the downward advection of warm surface water. * The MT2 fit works well assuming that N_M is proportional to F, and hence constant in abrupt4xCO2, with an AOGCM-specific factor that depends on M. If the factor depended instead on Delta M, N_M wouldn't be constant in abrupt4xCO2. I don't expect that would work so well (although I have not actually tried it).
Armour: Also, how does this help explain the conundrum of why Southern Ocean heat uptake (driven by delayed surface warming unrelated to AMOC) is somehow related to AMOC?
Gregory: Yes, I think so. We assume that the AMOC strength and the efficiency of uptake of heat as a passive tracer in the Southern Ocean are both separately related to some property of the unperturbed state of the ocean, probably some measure of stratification. (This is an aspect which Laure Zanna and Oleg Saenko have investigated.) In turn, this property could be affected by parametrised eddy advection, for instance.
09:19:11 From Thorsten Mauritsen : Thanks Jonathan, very interesting! It will take me some time before this sinks in. I am curious how you think pattern effects play in, since you didn’t included efficacy?
Gregory: The MT2 model produces N, given F and T. It's just a model of ocean heat uptake, not a model of the transient climate change. The two-layer model can likewise be used to model OHU alone, given T, but usually it's coupled to the energy balance N = F - alpha T to produce a model of transient climate change which predicts T, given F. Since (global-mean) T is an input, and alpha isn't considered, we haven't considered pattern effects or efficacy.
09:19:16 From Kyle Armour : Oops, Lily beat me to it!
09:20:28 From Isaac Held : do you have model evidence that the proportionality f_M between N_M and the forcing is independent of the kind of forcing? (same question as Thorsten’s?)
Gregory: No, we don't. So far we have only tried CO2 forcing. That's a caveat I should have emphasised. Obviously I'd like to know whether the MT2 model works equally well for other kinds of forcing. I don't yet know how important the distribution of surface effective forcing is.
09:22:29 From Mitchell Dickau - Concordia University : Thanks Jonathan, great presentation! ESMs exhibit a weakened AMOC during periods of negative emissions. How do you think your model would perform during periods of negative emissions after temperature overshoot?
Gregory: I'd like to know whether the MT2 model works equally well with stabilisation or overshoot scenarios, but so far we have only tried abrupt4xCO2 (constant forcing) and 1pctCO2 (increasing forcing), which are the two scenarios for which Geoffroy et al. (2013) calibrated and tested the two-layer model. For these scenarios, the MT2 model considers the AMOC only as a characteristic of the unperturbed piControl state. AMOC change is not considered. I believe this is because AMOC change does not have a large effect on global OHU, although it is important to redistribution of ocean heat content in the Atlantic. It might be that this is true also in overshoot scenarios, in which case a transient weakening of the AMOC will not be relevant. But I don't know.
09:25:08 From Jonathan Gregory : Thanks for your questions, Lily (already answered), Kyle, Thorsten, Isaac, Mitchell. I have copied them and will reply by email.
09:26:33 From Nicholas Lutsko (he/him) To Jonathan Gregory(privately) : No worries if you don't feel comfortable with that, of course
09:26:35 From Jonathan Gregory To Nicholas Lutsko (he/him)(privately) : Yes, sure, I'll do that.
09:26:41 From Nicholas Lutsko (he/him) To Jonathan Gregory(privately) : thank you!
09:27:13 From Jonathan Gregory : Correction! I will give the organisers some replies so that can be published.
09:27:31 From Jonathan Gregory : If I try to answer now I will miss the talks. :-)
09:27:55 From Thorsten Mauritsen : 👍🏻
09:28:17 From Mitchell Dickau - Concordia University : Thanks Jonathan!
09:55:36 From Yue Dong : Hi Sally, very cool results! Does your additional warming simulation suggest that the teleconnection in the model is sensitive to the amplitude of the Southern Ocean SST forcing? And what is the mechanism for the teleconnection?
09:58:34 From Kyle Armour : Thanks for the great talks!