Air-sea interactions during glacial Heinrich events

Krebs, Uta (2006) Air-sea interactions during glacial Heinrich events (Doctoral thesis/PhD), Christian-Albrechts-Universität, Kiel, 90 pp

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'Heinrich events' - massive iceberg discharges from Northern Hemisphere ice sheets
during the last ice age - coincided with cold periods that were followed by abrupt warmings
in the Northern Hemisphere. Climate reconstructions suggest that the associated
freshwater pulses caused a temporary collapse of the Atlantic Meridional Overturning
Circulation (AMOC) by stabilizing the stratification in the regions of deep water formation.
In the present work a coupled atmosphere-ocean-sea ice model is employed under glacial
boundary conditions to assess climate feedbacks after a simulated Heinrich event that
lead to a fast recovery of the AMOC. Two main mechanisms have been identified. Initially,
mixing and thermal processes weaken the stratification in the northern North
Atlantic. Additionally, 300-400 years after the main collapse of the AMOC, the stratification
is further destabilized by mean horizontal advection of anomalous saline waters
within the subpolar gyre. In consequence the large-scale meridional overturning is reinitiated.
The positive salinity anomaly originates from the tropical Atlantic and relies on air-sea
coupling. Reduced poleward heat transport in the North Atlantic leads to a cooling
north of the thermal equator. Due to advection of cold air and intensification of the
northeasterly trade winds the Intertropical Convergence Zone is shifted southward and
north equatorial precipitation is reduced. A dilution of the arising positive salinity
anomaly is prevented because cross-equatorial oceanic surface flow is halted during
the shut-down of the AMOC. Experiments with suppressed tropical air-sea coupling
reveal that the recovery time of the AMOC is almost twice as long as in the coupled
case. The impact of a shut-down of the AMOC on the Indian and Pacific Oceans can
be decomposed into atmospheric and oceanic contributions. Temperature anomalies in
the northern hemisphere are largely controlled by atmospheric teleconnections, whereas
southern hemispheric ones mainly rely on ocean dynamical changes.
Vertical diffusion is considered a key factor controlling the stability of the AMOC. This
however may not be so after a shut-down. Whereas model simulations without air-sea
coupling in the tropical Atlantic still show a strong sensitivity to vertical diffusion, this
behaviour cannot be found in fully coupled simulations. Thus, after a Heinrich event the
formation of a tropical salinity anomaly due to air-sea fluxes appears to be a more efficient
negative feedback for the resumption of the AMOC than density homogenisation
due to vertical diffusion.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: Willebrand, Jürgen and Timmermann, Axel
Additional Information: Die Printausgabe ist in der GEOMAR-Bibliothek vorhanden
Keywords: Air-sea interactions, glacial, Heinrich events, AMOC
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-OD Ocean Dynamics
OceanRep > Leibniz-Institut für Meereswissenschaften
Date Deposited: 16 Jul 2019 06:27
Last Modified: 16 Jul 2019 06:27

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