The Indonesian Throughflow Circulation Under Solar Geoengineering
Abstract. The Indonesia Throughflow (ITF) is the only low-latitude channel between the Pacific and Indian oceans, and its variability has important effects on global climate and biogeochemical cycles. Climate models consistently predict a decline in ITF transport under global warming, but it has not yet been examined under solar geoengineering scenarios. We use standard parameterized methods for estimating ITF: the Amended Island Rule and Buoyancy Forcing, to investigate ITF under the SSP2-4.5 and SSP5-8.5 greenhouse gas scenarios, and the geoengineering experiments G6solar and G6sulfur that reduce net global mean radiative forcing from SSP5-8.5 levels to SSP2-4.5 levels using solar diming and sulfate aerosol injection strategies. Six model ensemble mean projections for 2080–2100 relative to historical ITF are reductions of 19 % under the G6solar scenario and 28 % under the G6sulfur scenario which compare with reductions of 23 % and 27 % under SSP2-4.5 and SSP5-8.5. Thus, significant weakening of the ITF occurs under all scenarios, but G6solar closer approximates SSP2-4.5 than does G6sulfur. In contrast with the other three scenarios which show only reductions in forcing due to ocean upwelling, the G6sulfur experiment shows a large reduction in ocean surface wind stress forcing accounting for 47 % (38 %~65 % across model range) of the decline of total ITF transport. There are also reductions in deep-sea upwelling in extratropical western boundary currents.
Chencheng Shen et al.
Status: open (until 26 Apr 2023)
- RC1: 'Comment on esd-2023-6', Anonymous Referee #1, 27 Mar 2023 reply
Chencheng Shen et al.
Chencheng Shen et al.
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This study examines the changes to the Indonesian throughflow (ITF) under two solar radiation management (SRM) scenarios – Solar Dimming (SD) and stratospheric aerosol injection (SAI). The SRM outcomes are compared to the SSP2-4.5 and SSP5-8.5 scenarios. The study confirms previous work that shows the ITF decline in the 21st century is primarily driven by changes to Pacific Upwelling, with the tropical easterly wind changes a secondary component. Furthermore, SD shows a similar decline in the ITF to SSP2-4.5, but the ITF decline in SAI is still similar to SSP5-8.5 (despite the net anthropogenic radiative forcing in SAI being similar to SSP2-45). This is because in SAI, the wind-driven component also declines greatly.
The ITF is likely an important metric to track in SAI simulations. However, the climatical and biogeochem importance of ITF and how these change with respect to GHG forcing or SAI is not well established in this paper. Below, I suggest some broader ideas for the authors to address followed by line-by-line suggestions through Line 300 (for Line 300 to the end, please make similar adjustments as stated previously).
As is, I cannot recommend this study for publication. Though, I hope my comments are useful and will aid in your next submission.
Line 15-20 – the short summary is not clear and lacks relevant details
Line 32 – include historical time period.
Line 36 – What’s the error? Bc if SSP2-4.5 is 23% down, and G6solar is 19% down and G6sulfur is 28% down, we are talking about a magnitude 4% and 5% from SSP2-4.5 – which may not be a sig difference.
Line 38 – 38% - 65% not “~”
Line 45 – remove “of”
Line 49-52 – What are some examples of the ITF role in global climate? Many who read this paper will not be experts of the ITF, or oceanography – I think the introduction needs to more clearly explain the climatical importance of the ITF, its projected changes under SSP, what that means for the climate and biogeochem, etc.
Line 84 – are you referring to GCM simulations with ~1 deg ocean?
Line 91 – what defines “high-resolution”
Line 106 – what is meant by “imperfectly”
Lines 112-114 –What do you mean by “… these particular methods are unlikely to ever be done” --- I presume you mean this exact G6Sulfur SAI strategy is unlikely to be what is chosen if SRM is ever used in the future. This sentence can be re-worded to more clearly state that ongoing research is examining a multitude SAI strategies (how much, at what latitude, at which altitude, etc) to reduce the equator-to-pole cooling bias.
Line 114 – add “atmospheric” to read: Simulated tropical atmospheric circulation
Line 115 – replace “;” with “.” And start new sentence: Under SD, the seasonal …
Line 115-123 – can you be more specific to the changes to tropical Pacific atmospheric circulation due to SD and SAI (or SRM in general)? And, it’s not clear to me why citations about NA hurricane numbers and CAPE under SRM are relevant to this study.
Line 125-134 – I don’t think this paragraph is necessary. Perhaps only a sentence is needed to state that the AMOC under SRM has been looked at. I would rather have another paragraph explaining tropical pacific/Indian ocean changes under SRM and more details about the ocean drivers of ITF. That said, Lines 446-452 compare the continual downturn of ITF to research that shows that AMOC can recover under SRM – this is a good comparison to make at this stage of the paper.
In general, the intro could use more details about atmo circulation and how that changes under GHG and SRM. More about ITF and what drivers can cause changes to it (bc many geoeng don’t know ocean and ITF). For example, “this study will show how SRM can change these drivers of ITF”. Some details are in the methods, but better placed in the introduction.
Line 136 - 137 – replace explore with examine and remove the clause “explore the drivers of these changes”
Line 137-138 suggestion: … 21st century and consider the transport differences between the GHG-forced scenarios (SSP2-4.5 and SSP5-8.5) and the SRM scenarios (G6solar and G6sulfur).
Line 159 – why aren’t G6 scenarios particularly realistic? And as a follow-up, why should the current study and its results be considered, if the simulations are not realistic?
Line 197-208 – This explanation of the GHG forced changes to deep ocean upwelling and their impact on ITF would be a good addition to the revised introduction.
Line 211-212 – replace “in most studies (“, with: … in previous studies (e.g., Clarke …
Line 213 – define steric sea level height
Line 214 – “should also drive”, I’m not sure what is meant by this phrase
Line 217 – replace “sharp” with eastern?
Line 218-220 – here ITF transport is defined as the difference btw westward and eastward transport along the northern and southern flanks… just below ITF transport is defined by DBP and EIO density change (above by Sverdrup balance and Pacific upwelling). Why all of these? What do each tell us about ITF?
Line 236 – is wind driven ITF just the Island Rule transport calculation?
INCLUDE +- 1 SD with respect to ensemble members in all values. For the 2080-2100 minus historical period, we need to know significance! It’s likely that there is lag-1 autocorrelation in these time series too and that needs to be accounted for when defining sample size. And trend values and significance.
Line 242-245 – What are the trend values (for all ITF transport components)? & Calculate the significance of the (linear) trend lines. Because it looks like there is a negative trend in the wind driven component for both SSP2-4.5 and SSP2-8.5, but it may not be significant.
Line 250-252 – These values, as all values in this paper should have a statistic attached to them that details the model spread (easiest would be +- 1 SD wrt model spread); and if comparing between SRM and the SSPs there needs to be a significance test in order to attach some level of confidence to the findings.
Line 252-253 – how can both wind and upwelling contributions to ITF be higher? Their sum is ITF transport, so if one contribution goes up, the other must go down?
Line 261-264 – Should the buoyancy forcing calculation of ITF be more similar to the results from the Amended Island Rule? It’s not clear to me if this is a component of ITF or another way to measure ITF. I’m not sure what is the main point/conclusion of this paragraph
Line 264 – “No obvious trend” – you need to calculate the trend and determine whether it is significant
Line 277-278 – It looks to me like the upwelling contribution to ITF transport is not much different between SAI and SD – again a statistical test will confirm this.
Line 286-287 – it may be that G6sulfur total ITF transport averaged from 2080-2100 is lower than SSP2-8.5, but I doubt it is significant given the variability of the time series.
Line 287-291 – again, the time series shows much variability in the multi-model mean (and certainly in the multi-model spread) that I don’t think there is a significant difference between the buoyancy forced ITF transport in any of the scenarios.
Figure 3 needs error bars. And it would be easier to quickly assess by using the same colors for each scenario as in Fig 2.
Lines 293 – there needs to be a detailed explanation of what is causing the decline in upwelling relative to Historical – as well as why the upwelling becomes downwelling north of ~20N. In general, rather than state what the plot shows, state what the plot means in relation to the main points of the paper.
Figure 4 – insert a panel in the top left that is just the Historical mean wind stress curl so that it is more easy to interpret the changes from this mean state. You could also just make all non-significant values white. Removing the stippling (and adjusting the saturated colorbar) will improve the figure’s aesthetic.
Lines 327-330 – Can you explain how these changes in wind stress curl impact the ITF transport changes? – In general for this paragraph, it’s unclear to me how these various changes to wind stress curl across the basin will impact ITF. I am unable to decipher the main point(s) of this paragraph.
Lines 346-354 & Table 2 – What is the acronym TRN? For the Wilcoxon signed-rank test, how do you account for model spread at each month? For example, I’m having difficultly being convinced that the 0.23 change from G6-solar to SSP5-8.5 is significant given that large mult-model variance compared to the signal. Could you elaborate on this stat test and how the model spread is accounted for?
Lines 368-384 – Please clarify the main point(s) of this paragraph and relate the significance of upwelling changes to ITF transport.
Figure 5 – Just mask out in white the non-significant areas and increase the colorbar range so the stat sig areas are not saturated.
Lines 394-420 & Figure 6 – Similar comments to above.
Summary and Discussion – These paragraphs are easier to understand. To improve the clarity of the whole paper, these main points should be established as the goals of the paper in the Introduction and then addressed throughout the Results. As the paper stands now, it is difficult to determine the main points and how the results address these points.
Line 467-469 – I missed the analysis where wind driven transport is compared between models? Given the multi-model range from Fig 2, it seems like the models would have some disagreement with respect to wind driven transport.