correlation between GOA and NGRIP changes sign
at ~10 ka, suggesting a return to Pacific-Atlantic
seesaw or decoupled behavior in an interglacial
state similar to that of late-glacial time. Within the
Holocene, asynchronous behavior includes cool-
ing of GOA from 10 to 8 ka, followed by gradual
warming; in opposition to this pattern, cooling in
NGRIP is initiated after 8 ka and persists through-
out the Holocene. Ice-volume–corrected, low-pass–
filtered d18O records show a more pronounced
seesaw pattern (high positive correlation) in the
Holocene and HS1 than the raw or high-pass
records (Fig. 4). This suggests that the long-term
(>2000 years) trends are inversely related during
these times, but the short-term variability is either
not sufficiently resolved or decoupled between
basins during the Holocene and HS1.
Variance increases within the GOA and NGRIP
records before the abrupt warming transitions
into the Bølling and Holocene (Fig. 4). High-frequency variance in the GOA peaks from 15 to
11.5 ka, coincident with the interval of synchro-
nization, whereas in NGRIP high-frequency var-
iance peaks slightly earlier. Both records show
greater short-term variance during abrupt warm-
ing into the Bølling relative to the later warming
into the Holocene, suggesting that earlier warm-
ing, beginning from relatively extreme glacial
conditions, was the more abrupt climatic event
in these two locations.
Thus, we find evidence for intervals of both
synchronous and asynchronous climate variability between the GOA and NGRIP regions. Synchronization and an increase in variance preceded
and extended through abrupt warming events,
consistent with theory that highly connected
systems are more susceptible to unstable tipping
points (21) (see supplementary materials). Possible mechanisms to regulate the modes of connectivity between regions could be related to (i)
ice elevation in North America, (ii) the opening
of Bering Strait, or (iii) routing of fresh water in
the North Atlantic or Southern Ocean. During
the Last Glacial Maximum (LGM), topographic
steering of the jetstream around an expanded
North American ice sheet and sea-ice margin
would have reduced the direct atmospheric
connection between the GOA and Greenland
(36, 37). A gradual fall in ice height following H1
could have caused an abrupt reorganization of
atmospheric circulation that set up a stronger
meridional flow in the North Atlantic (38), thereby strengthening the interocean atmospheric coupling (Fig. 1). Linkages between the North Pacific
and North Atlantic may also depend on whether
the Bering Strait is open or closed (10, 13, 39),
which could signal a role for the incursion of low-salinity waters from the Arctic in the reversion
to interocean seesaw during the early Holocene
(40). Alternatively, models suggest that freshwater
forcing near West Antarctica triggers synchronous
warming in the North Pacific and North Atlantic
(41), whereas meltwater inputs near Greenland
create antiphased responses in the North Atlantic
and North Pacific (10, 11).
The last glacial termination demonstrates several symptoms of threshold behavior, including
greater spatial organization through synchronization of the North Pacific and Greenland/North
Atlantic variability (this study), elevated variance
[this study and (23, 24)], and enhanced autocorrelation of some climatic indicators just before the
abrupt Bølling and Holocene transitions (20, 25).
The new highly resolved evidence we present here
for interocean dynamic coupling between the
NGRIP record in the Atlantic sector and GOA records in the Pacific Sector suggests that synchronization of poleward heat transport in both oceans
is an important catalyst for abrupt warming transitions within the Northern Hemisphere. Although
a tipping point may be crossed in an instant, large-scale climate systems that include ice sheets or
deep ocean circulation may have substantial inertia,
such that the full response may play out dynamically over an extended period of time, constituting
a “tipping interval.”
In the modern climate system, models suggest
a tight coupling of short-term (<200 years) ocean-atmosphere interactions between the North Pacific
Fig. 4. Low-pass– and high-pass–filtered data. (A and B) Evaluation of the cross correlation between
the ice-volume–corrected, low-pass–filtered [2000-year moving average (A)] and high-pass–filtered
records [residuals from 2000-year moving average (B)] of NGRIP (dark blue) (26) and GOA on the tuned
age model (rose, inverted axis). The 2000-year moving window cross correlation between the GOA and
NGRIP records are shown in black for the low-pass– (A) and high-pass–filtered (B) records. Dashed gray
lines in (A) reflect one-to-one positive and negative correlations. (C) Panel shows the variance for the
high-pass and low-pass records. A 200-year windowed variance is shown for NGRIP (dark green) and
GOA (dark blue) for the high-pass records, and a 2000-year windowed variance is shown for NGRIP
(light green) and GOA (light blue). The transitions into the Bølling and Holocene are clearly identifiable in
the peaks in the high-pass variance and the abrupt shifts in the d18O records at 14.7 and 11.7 ka.
