Overall ocean warming and salinity changes drive about 60% of ASI decline vs. 40% atmospheric.
Where do you get these numbers from?
I can't recall the specific source, just that it really surprised me to read that ocean SST was more important than weather so I made a note to tuck that one away into my memory bank - which is usually pretty good but certainly not infallible. Alas, I went looking through my scantily noted collection of ASI journal articles just now and can't find the smoking gun. Instead of a journal article, it might have been in an IPCC report or one of the annual Arctic Report Cards. I will check through files I have tucked away for those sources.
The short answer for now is "I don't know where I got that from". But it may be worse than that. In looking through the journal articles Ding et al "Influence of high-latitude atmospheric circulation changes on summertime Arctic sea ice" DOI: 10.1038/NCLIMATE3241 uses a 60% figure for the contribution of atmospheric warming on ASI decline, and the rest being internal variability. Which is completely different than my "60% ocean and 40% atmosphere" statement.
Hoping for quick redemption before having to wade through IPCC tome looking for a source of the 60:40 ocean:atmosphere statement, I thought it might be useful to check in with my favorite synoptic ASI article Stroeve and Dirk 2018 . I know that wasn't the source, but since they summarize what is causing ASI decline, I thought they might have something say about the ocean influence. But this sentence from their intro only convicted me more:
"Most of the observed changes in the sea-ice cover are driven by anthropogenic warming from increasing atmospheric greenhouse gas concentrations (e.g. Notz and Marotzke 2012, IPCC 2013, Notz and Stroeve 2016), amplified by internal variability (e.g. Ding et al 2017, Notz 2017)."
"These drivers do, however, not affect the sea ice directly, but instead they modify the atmospheric and oceanic forcing on the ice cover, which then in turn cause the sea-ice cover to shrink and thus serve to visualize the often invisible changes in the atmosphere and in the ocean"
That leaves open the possibility of AGW raising ocean temperatures that then transport the heat into the Arctic Ocean as a major driver of ASI decline. But it doesn't say that.
"Obviously, rising air temperature is a prime suspect for driving increased sea ice melt. This is first based on the simple fact that ice melts faster the warmer it is, but is further made plausible by the very robust linear relationship between the long-term trend in the spatial coverage of Arctic sea ice and the long-term trend in global mean near-surface air temperature, both in model simulations and in the observational record"
"A study by Burgard and Notz (2017) has found that CMIP5 models disagree on whether the anomalous heating of the Arctic Ocean, and thus the loss of Arctic sea ice, primarily occurs
through changes in vertical heat exchanges with the atmosphere (as is the case in 11 CMIP5 models), primarily through changes in meridional ocean heat flux (as is the case in 11 other CMIP5 models) or through a combination of both (as is the case in 4 CMIP5 models). This suggests that our understanding of how precisely the heat for the observed sea ice melt is provided to the sea ice is still surprisingly limited."
Stroeve and Notz go on to discuss oceanic pathways (Atlantic and Pacific water infiltration), but don't get anywhere close to supporting the 60:40 ocean:atmosphere statement.
"Changing state of Arctic sea ice across all seasons", Julienne Stroeve and Dirk Notz 2018 Environ. Res. Lett. 13 103001. Some support for the ocean heat contribution comes from Decuypère et al 2022 - "The Arctic has experienced a dramatic decrease in its sea ice cover over the past four decades. One of the main drivers of this intense melting is ocean heat transport from lower latitudes into the Arctic. "
"A better representation of OHT is needed to improve projections of sea ice extent (SIE), as the ocean is one of the main drivers of sea ice loss and variability in the Arctic (Bitz et al., 2005)."
"Decadal variability in ocean heat transport explains a large fraction of decadal variability in sea ice extent"
But they also say "At interannual timescale, the impact of ocean heat transport on sea ice extent is limited to the shelf regions".
And the 60:40 ocean:atmosphere claim gets dinged pretty hard by this statement in a section titled "Impact of OHT on SIE at a Pan-Arctic Scale" -
"The trends in the September SIE in the Medium and High resolutions are around −0.6 × 10^6 km2/model decade (significant at the 95% confidence level), much smaller in absolute value than the observed trend of −1.6 million km2/model decade in the satellite era."
Decuypère, M., Tremblay, L. B., & Dufour, C. O. (2022). Impact of ocean heat transport on Arctic sea ice variability in the GFDL CM2-O model suite. Journal of Geophysical Research: Oceans, 127, e2021JC017762. https://doi.org/10.1029/2021JC017762 My understanding of that statement is that OHT accounts for about 0.6/1.6 of SIE decline, i.e. about 38%, not 60%. (Though they also note that the model simulations from which these values are derived consistently underestimate ASI decline).
There is this from Bitz et al. 2005: "In regions where the ice edge extends relatively far equatorward, absorbed solar radiation is the largest component of the ocean energy budget, and the large seasonal range of insolation causes the ice edge to traverse a large distance. In contrast, at relatively high latitudes, the ocean heat flux convergence is the largest component and it has a relatively small annual range, so the ice edge traverses a much smaller distance there."
Bitz, C. M., Holland, M. M., Hunke, E. C., & Moritz, R. E. (2005). Maintenance of the Sea-Ice Edge, Journal of Climate, 18(15), 2903-2921. https://journals.ametsoc.org/view/journals/clim/18/15/jcli3428.1.xml But this is defensive posturing to patch over the fact that I cannot recall where I got the 60:40 statement. I don't think innocent until proven guilty applies in the court of science blogs. Instead, it is the opposite, i.e. you have to back up what you say with credible sources, i.e. you're guilty of speculating until proven innocent. My case is not looking good so far. While I'm not ready to plead guilty of memory malfeasance and throw myself to the mercy of the court, until further notice of finding of the source I must retract that statement.
