Effect of the Kinematic Lower Boundary Condition on the Spectral and Auto-correlation Structure of Annular Variability in the Troposphere.

Kunz, Torben and Greatbatch, Richard John (2013) Effect of the Kinematic Lower Boundary Condition on the Spectral and Auto-correlation Structure of Annular Variability in the Troposphere. [Poster] In: EGU Fall Meeting. , 09.-13.12.2013, San Francisco, USA .

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Abstract

The dynamical origin of the spectral and auto-correlation structure of annular variability in the troposphere is investigated by a deductive approach. Specifically, the structure of the power spectrum and auto-correlation function of the zonal mean geopotential is analysed, for the case of a
quasi-geostrophic spherical atmosphere subject to a white noise mechanical forcing applied in a
single Hough mode and concentrated at a particular level in the vertical, with vertically uniform Newtonian cooling and Rayleigh drag concentrated at a rigid lower boundary. Analytic expressions for the power spectrum are presented together with expressions for an approximate red noise, i.e. a
Lorentzian shaped power spectrum. It is found that for an infinitely deep atmosphere the power
spectrum can be well approximated by a red noise process for the first few Hough modes
(associated with large Rossby heights), provided the distance from the forcing is not larger than about one Rossby height. When a frictional rigid lower boundary is included, however, the approximation is generally bad. The high-frequency part of the power spectrum exhibits near exponential behaviour and the auto-correlation function shows a transition from a rapid decay at short lags to a much slower decay at longer lags. Since observed Northern Annular Mode variability exhibits the same characteristics, the above results lead to the hypothesis that these characteristics
may be intrinsic to the linear zonal mean response problem—and may neither need to be explained by slow external forcings, nor by more advanced concepts like deterministic low-order chaos, as suggested in the literature.

Document Type: Conference or Workshop Item (Poster)
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
Date Deposited: 13 Jan 2014 10:06
Last Modified: 13 Jan 2014 10:06
URI: http://oceanrep.geomar.de/id/eprint/23027

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