Geoscience Reference
In-Depth Information
H. Svensmark and S. Johnsen (
1996
) points to a larger atmospheric thermal gradient
than now between the equator and the pole, associated with a colder northern North
Atlantic. This suggests stormier conditions. Simulated circulation changes are asso-
ciated with hydrologic changes. It appears that precipitation was reduced over the
Laurentide Ice Sheet while it was enhanced along its southern margins.
Regional modeling studies (Bromwich et al.,
2004
,
2005
) indicate that from
November through April, there was a split flow at upper levels around a blocking anti-
cyclone over the Laurentide Ice Sheet, with the northern branch over the Canadian
Arctic and the southern branch impacting southern North America. The split flow
transitioned into a single jet stream migrating northward over the Laurentide Ice
Sheet in summer. During summer, pronounced low-level thermal gradients along
the southern margin of the ice sheet, resulting from the juxtaposition of the ice sheet
and the adjacent warm ice-free land surface, facilitated the development of cyclones
tracking over the ice sheet, some producing copious precipitation along and south of
the ice sheet terminus. Precipitation along the southern margin was orographically
enhanced by the presence of the ice sheet.
10.4
D-O Cycles and Heinrich Events
10.4.1
Overview
The Last Glacial Cycle was characterized by pronounced millennial scale changes
between warm and cold conditions, known as Dansgaard-Oeschger (D-O) cycles.
D-O cycles are named after the investigators who identified them in Greenland ice
core records. Twenty-five D-O cycles have been identified over the last glacial cycle,
with an interval between warm and cold phases of roughly 1,500 years (Grootes and
Stuiver,
1997
), or 2,000-3,000 years for a complete cycle. D-O cycles are charac-
terized by rapid warming, typically occurring over a matter of decades, and then
more gradual cooling over a period of centuries. D-O cycles can be stratigraphically
related to layers of ice-rafted detritus (IRD) released from armadas of melting ice-
bergs calved from the ice sheets, the most pronounced of which are termed Heinrich
events (Heinrich,
1988
). Heinrich events occur in the cold periods immediately pre-
ceding D-O warmings. Heinrich events occurred about every 6,000-7,000 years
and may have lasted around 500 years. They are labeled in
Figure 10.4
. Note the
correspondence with cold conditions. There are also more modest intervening
IRD episodes recurring at more frequent intervals of 2,000-3,000 years, on which
the Heinrich events are superimposed. Most of these events (the combination of
Heinrich and smaller IRD events) follow the pacing of the D-O cycles.
Explanations for D-O cycles largely revolve around various mechanisms by
which the northern North Atlantic could be freshened, leading to a reduction in the
strength of the Atlantic Meridional Overturning Circulation (the AMOC, see
section
7.5.2
) and NADW production, fostering widespread cooling of the North Atlantic
region. This would then be followed by recovery of the AMOC and rapid warming.
