On the Northern Annular Mode Surface Signal Associated with Stratospheric Variability.

Kunz, Torben and Greatbatch, Richard John (2013) On the Northern Annular Mode Surface Signal Associated with Stratospheric Variability. Open Access Journal of the Atmospheric Sciences, 70 (7). pp. 2103-2118. DOI 10.1175/JAS-D-12-0158.1.

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The wintertime northern annular mode (NAM) at the surface is known to undergo slow intraseasonal variations in association with stratospheric variability, which leads the surface signal by up to several weeks. The relative contributions, however, of potentially relevant stratosphere–troposphere coupling mechanisms are not yet fully understood.

In this study the relative roles of (i) the downward effect of the zonal-mean secondary circulation induced by quasigeostrophic (QG) adjustment to stratospheric wave drag and radiative damping and (ii) wave drag local to the troposphere are estimated. For this purpose, a spectral tendency equation of the QG zonal-mean zonal wind is derived and used, in a first step, to obtain the external mechanical forcing that, in the QG framework, drives exactly the observed stratospheric and tropospheric daily NAM. In a second step, the equation is then integrated in time to reconstruct the daily NAM, but with the forcing restricted to either stratospheric or tropospheric levels, each case leaving a characteristic NAM surface signal.

The relative roles of the above-mentioned mechanisms are found to be of similar quantitative importance, but to differ in a qualitative sense. The downward effect of stratospheric QG adjustment is responsible for the initiation of the NAM surface signal, whereas subsequently local tropospheric wave drag actively maintains and persists the signal over several weeks. Furthermore, the downward effect of QG adjustment to stratospheric radiative damping is shown to have only a minor impact, compared to that from stratospheric wave drag. The robustness of these conclusions is demonstrated by a sensitivity study with respect to various model parameters.

Document Type: Article
Keywords: Northern Hemisphere, Annular mode, Stratospheric circulation, Stratophere-troposphere coupling, Intraseasonal variability
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1175/JAS-D-12-0158.1
ISSN: 0022-4928
Projects: Future Ocean
Date Deposited: 09 Sep 2013 12:59
Last Modified: 16 Apr 2018 11:20
URI: http://oceanrep.geomar.de/id/eprint/21965

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