Climate proxy records show the existence of several millennial scale fluctuations during the last glacial period. These abrupt warming (Dansgaard-Oeschger) events were paced by about 1,500 years and also appear to be correlated to episodes of ice surges into the North Atlantic from disintegrating ice sheets. Several model studies have viably demonstrated the ability of abrupt freshwater discharges to trigger fluctuations in the state of the North Atlantic meridional overturning circulation. However, the 'freshwater hypothesis' does not provide a clear answer to the existence of the periodicity. In this study, an idealized model is constructed, which includes the relevant components of the North Atlantic climate system as may be relevant to millennial timescales. These primarily include an overturning pressure driven overturning circulation and thermodynamic sea ice coupled within an idealized geometry representing the ocean basin. Relaxation oscillations are observed at low advective strengths and the periodicity is intrinsically tied to the system's geometry. The physical constraints on sea ice growth in the North Atlantic together with the configuration of the basin, thus impart a characteristic oscillation period. Time varying salinity (freshwater) forcing applied to mimic ice sheet growth and decay phases produce the distinct Bond cycles, as seen in the proxy data. Insolation variations applied to the model also reproduce the fluctuations at about 85,000 years before present. This correlation suggests that the overturning circulation is influenced both by high latitude freshwater anomalies and tropical summer insolation variations, and either trigger can invoke sea ice processes and produce millennial scale oscillations.
|Advisor:||Jones, Christopher, Bane, John|
|Commitee:||Cecil, Gerald, Engel, Jonathan, Evans, Charles|
|School:||The University of North Carolina at Chapel Hill|
|Department:||Physics & Astronomy|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 73/06, Dissertation Abstracts International|
|Subjects:||Climate Change, Physical oceanography, Paleoclimate Science, Systems science|
|Keywords:||Abrupt climate change, Dansgaard-oeschger events, North atlantic climate variability, Paleoclimate, Sea ice, Thermohaline circulation|
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