The Atlantic Meridional Overturning Circulation (AMOC) is probably affected by the present climate change, but the existence and amplitude of the long-term trend remain a subject of debate:
https://www.nature.com/articles/s41561-022-00896-4
Luckily, the recent geological past provides another way to study AMOC fluctuations. The AMOC was particularly variable during the last glacial cycle, in apparent synchrony with temperature records from the North Atlantic and Greenland, notably their abrupt variability. (2/20)

These abrupt temperature changes, named Dansgaard–Oeschger (DO) events, also have Southern Hemisphere (SH) counterparts via the thermal bipolar seesaw, a concept describing the meridional heat transport leading to asynchronous temperature changes between both hemispheres.
Several abrupt DO cooling events, named DO stadials, are accompanied with massive releases of icebergs into the North Atlantic, named Heinrich (H) events, as well as drastic slowdowns or even shutdowns of the AMOC. (3/20)

Previous bipolar seesaw studies mostly relied on ice cores from Greenland and Antarctica, given the high temporal resolutions and synchronized chronologies of their derived records. However, ice-core–based temperature records from Greenland do not show enhanced DO coolings during H events, contrary to sea surface temperature (SST) records from the Iberian Margin. Therefore, Greenland and Antarctic ice cores alone cannot provide a complete picture of the bipolar seesaw. (4/20)

Here we present and use a high-resolution SST record based on a recent GDGT ring index (RI-OH′) from a famous Iberian Margin site (MD95-2042), in comparison with other SST records from the same Iberian Margin site.
The other Iberian Margin SST records used for comparison include established climate proxies such as UK'37, planktic foram d18O, and benthic foram d18O.
We also build and use Antarctic stacks as SH reference records. (5/20)

RI-OH′ reflects the degree of cyclization, i.e. the number of rings, of the involved archaeal membrane lipids (OH-GDGTs) as a mechanism to regulate their fluidity/viscosity.
Our Iberian Margin RI-OH′ SST record faithfully shows DO events with contrasting cooling amplitudes between DO stadials with and without H events.
OH-GDGT-based paleothermometry truly has a future!
See also Davtian et al. (2021) in Paleoceanography and Paleoclimatology:
https://doi.org/10.1029/2020PA004077 (6/20)

One way to study the bipolar seesaw is to correlate SH warming amplitudes with Northern Hemisphere (NH) cooling durations, i.e. the classical diagram. Indeed, several previous bipolar seesaw studies obtained strong correlations using ice-core–based temperature records from Greenland and Antarctica.
While we obtained strong correlations as well, our Iberian Margin SST record gives the best linear regression fits. (7/20)

We also use the thermal bipolar seesaw model by Stocker and Johnsen (2003) in Paleoceanography and Paleoclimatology to generate synthetic SH temperature records, with NH temperature records as model inputs:
https://doi.org/10.1029/2003PA000920
Indeed, Iberian Margin SST records have a sufficient resolution for this use. (8/20)

While all selected NH paleothermometric records give good SH temperature simulations, both biomarker-based Iberian Margin SST records give better SH temperature simulations than do ice-core–based paleothermometric records from Greenland. Both biomarker-based Iberian Margin SST records better simulate enhanced SH warmings during DO stadials with H events than do ice-core–based paleothermometric records from Greenland, notably during the second half of the last glacial cycle. (9/20)

Therefore, SST records deserve attention as well to study the thermal bipolar seesaw, despite their low temporal resolutions compared with ice-core–based paleothermometric records from Greenland and Antarctica.
See also Anderson et al. (2021) in QSR for a parallel consideration of SST records from the Southern Ocean and ice-core–based temperature records from Antarctica:
https://doi.org/10.1016/j.quascirev.2021.106821 (10/20)

To make the most of the contrasting cooling amplitudes between DO stadials with and without H events recorded by our Iberian Margin SST record, we propose two extensions of the classical SH vs NH diagram.
To obtain both extended SH vs NH diagrams, we calculate two products that involve NH cooling amplitudes and are proportional to heat transfers between both hemispheres during each DO stadial. (11/20)

Our first extended SH vs NH diagram is elegant because it has the same physical basis as the classical one, but with a relationship that depends only on the time characteristic of the #heat reservoir rather than the product of this parameter and NH cooling durations. Accordingly, this extended SH vs NH diagram gives strong correlations as well. (12/20)