The North Atlantic Oscillation: Variability and Interactions with the North Atlantic Ocean and Arctic Sea Ice.

Jung, Thomas (2000) The North Atlantic Oscillation: Variability and Interactions with the North Atlantic Ocean and Arctic Sea Ice. Open Access (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 117 pp. . Berichte aus dem Institut für Meereskunde an der Christian-Albrechts-Universität Kiel, 315 . DOI 10.3289/ifm_ber_315.

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Abstract

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 additional sensitivity experiment reveals that interdecadal wind stress and freshwater flux variability was of minor importance. The phase-relationship between variability of the NAO and the North Atlantic ocean on interdecadal time scales is less indicative for a two-way coupling between atmosphere and ocean - thus, leaving space for ot her external mechanisms that may have forced interdecadal NAO variability during the 20th century. No significant coherence between the NAO and the North Atlantic thermohaline circulation is found on interdecadal time scales in a century-scale control integration of the coupled general circulation model ECHAM4/ 0PYC3. Moreover, there is no strong statistical evidence for the presence of deterministic components in the modelled NAO index leaving the modelled NAO rather unpredictable several years in advance. The pronounced increase of the NAO during the last three decades, which represents a segment of its interdecadal variability, was accompanied by an eastward shift of the NAO's centers of interannual variability around the late 1970s. This eastward shift was associated with increased anomalous meridional wind components near Fram Strait during high and low NAO winters and, hence, with increased coherence between the NAO and Arctic sea ice export through Fram Strait on interannual time scales. This shift was also accompanied by pronounced changes in the response of (i) near-surface temperature anomalies over the European and North American continent, and (ii) North Atlantic air-sea interaction to an interannual forcing by the NAO. The analysis of historical sea level pressure (SLP) data suggests that the recent location of the NAO's Centers of interannual variability is rather unusual. In order to evaluate the unusualness of changes in the location of the NAO's centers of interannual variability, integrations of the coupled ECHAM4/OPYC3 model under resentday conditions and under increasing greenhouse gas concentrations are investigated. In the control integration the location of the NAO is relatively stable resembling that of the observed NAO before the late 1970s. This may explain the missing link between the NAO and Arctic sea ice export through Fram Strait (r = 0.0) in the control integration of this coupled model. Finally, it is shown that the eastward shift of the NAO in the scenario run (around 2020), first described by Ulbrich and Christoph, is similar to the observed shift around the late 1970s.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Ruprecht, Eberhard
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-ME Maritime Meteorology
OceanRep > Institute for Marine Science Kiel
Refereed: No
Date Deposited: 18 Feb 2008 17:24
Last Modified: 09 Feb 2023 12:28
URI: https://oceanrep.geomar.de/id/eprint/3266

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