The North Atlantic Oscillation: Variability and Interactions with the North Atlantic Ocean and Arctic Sea Ice
The North Atlantic Oscillation (NAO) represents the dominant mode of atmospheric variability in the North Atlantic region and desc rib es the strengthening and weakening of the mid-latitude westerlies. In this s tudy, variability of the NAO during wintertime and its relat ionship to the North Atlant...
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| Format: | Thesis |
| Language: | English |
| Published: |
2000
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| Online Access: | https://oceanrep.geomar.de/id/eprint/3266/ https://oceanrep.geomar.de/id/eprint/3266/1/IFM-BER_315.pdf https://doi.org/10.3289/ifm_ber_315 |
| Summary: | The North Atlantic Oscillation (NAO) represents the dominant mode of atmospheric variability in the North Atlantic region and desc rib es the strengthening and weakening of the mid-latitude westerlies. In this s tudy, variability of the NAO during wintertime and its relat ionship to the North Atlantic ocean and Ar ctic sea ice is investigated. For this purpose, observational data are analyzed along with integrations of models for the Atlantic ocean, Arctic sea ice, and the coupled global climate system. From a statistical point of view, the observed NAO index shows unusually high variance on interdecadal time scales during the 20th century. Variability on other time scales is consistent with realizations of random processes ( "white noise"). Recurrence of wintertime NAO anomalies from winter-to-winter with missing signals during the inbetween non-winter seasons is primarily associated with interdecadal variability of the NAO. This recurrence indicates that low-frequency changes of the NAO during the 20th century were in part externally forced. Interdecadal variability of the observed NAO is associated with pronounced North Atlantic sea surface temperature (SST) anomalies. Whereas parts of these SST anomalies can be explained by a local net surface heat flux forcing from the NAO, the remaining part may be explained by interdecadal changes of the oceanic circulation. In order to test this hypothesis, an integration of an ocean general circulation model for the period 1865- 1997 is analyzed. The model has been forced by monthly surface fluxes whose variability is solely determined by the observed NAO index. The observed and modelled interdecadal SST variability is in good agreement. It is shown that modelled interdecadal SST anomalies in the North Atlantic region were generated by two processes: (i) An instantaneous local surface flux forcing by the NAO, along with (ii) a lagged response of the North Atlantic thermohaline circulation and of the subpolar gyre to the net surface heat flux forcing by the NAO. An ... |
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