Geoscience Reference
In-Depth Information
The eddy-permitting model simulations support that LC eddy shedding was most
likely absent during the LGM (Fig.
1
). Lower sea level in the model (
110 and
*
cantly reduces the number of eddies shed from the LC, as well as the
oceanic heat transport into the GoM. With rising sea level, eddy shedding has
gradually increased across the deglaciation and thereby, warmed the northern GoM
up to its present state.
The simulated response to wind stress changes acts similar to the effect of sea
level drop: A southward shift of the Intertropical Convergence Zone (ITCZ) and a
strengthened atmospheric circulation during the LGM causes enhanced (wind-dri-
ven) Sverdrup transport within the Subtropical Gyre (Slowey and Curry
1995
),
leading to a strengthened Yucatan Channel and Florida Straits through
67 m) signi
*
ow. In
response to the stronger transports, the eddy shedding decreases (Mildner
2013
).
Although PMIP models also simulate a stronger gyre circulation in the North
Atlantic during the LGM, Lynch-Stieglitz et al. (
2009
) argue for a reduced Florida
Straits transport during the LGM based on paleoceanographic proxy data pointing
towards a contradiction, which needs further investigation.
A further effect of the changes in wind forcing in the model simulation is the
southward expansion of the northern recirculation gyre in the North Atlantic Bight,
and the southward shift of the zero line of wind stress curl. In response, both Gulf
Stream and the Subtropical Gyre are shifted southwards (Mildner
2013
). A highly
dynamic Subtropical Gyre even during interglacial cool periods is suggested from
ODP Site 1058C proxy data (Bahr et al.
2011
,
2013
) with a long-lasting subsurface
warming during Marine Isotope Stage (MIS) 5 c-b, which most likely originated
from intensi
ed Ekman down welling in response to enhanced wind stress. The
accumulation of warm and saline waters in the subsurface of the Subtropical Gyre
might have contributed to the stabilization of northern hemisphere climate during
MIS 5.
Across the deglaciation, the continuous increase in SST
Mg/Ca
in the northern
GoM is accompanied by gradually decreasing
fluvial sediment supply from the MR
(Fig.
2
), providing evidence that both the LC and the MR acted in concert. The
deglacial SSS development, instead, is different: Prominent melt water signals
(
18
O
ivf
−
sw
) during Heinrich 1 and the B
d (Fig.
2
) are observed only in
cores south and west of the MR delta. In the northeastern GoM, instead, neither our
SSS reconstruction nor the potassium record point to signi
ø
lling/Aller
ø
δ
cant melt water dis-
charge (Fig.
2
). Apparently, the prominent freshwater signals were either a
regionally restricted phenomenon or are due to changes in the isotopic composition
of the discharge events. Since the
18
O
ivf
−
sw
-record from Blake Outer Ridge is also
devoid of such anomalies (Fig.
2
), we question the importance of Laurentide Ice
Sheet melt water routing via the MR for perturbing the AMOC.
During the Holocene, low potassium concentrations and positive
δ
18
O
ivf
−
sw
-
values point to a negligible MR discharge when compared to LGM conditions. The
rather gradual decline in SST
Mg/Ca
in the northeastern GoM after
δ
*
6 thousand
Search WWH ::
Custom Search