Torben Kunz

Impact of Synoptic-Scale Wave Breaking on the NAO and Its Connection with the Stratosphere in ERA-40

Abstract This observational study investigates the impact of North Atlantic synoptic-scale wave breaking on the North Atlantic Oscillation (NAO) and its connection with the stratosphere in winter, as derived from the 40-yr ECMWF Re-Analysis (ERA-40). Anticyclonic (AB) and cyclonic wave breaking (CB) composites are compiled of the temporal and spatial components of the large-scale circulation using a method for the detection of AB and CB events from daily maps of potential vorticity on an isentropic surface. From this analysis a close link between wave breaking, the NAO, and the stratosphere is found: 1) a positive feedback between the occurrence of AB (CB) events and the positive (negative) phase of the NAO is suggested, whereas wave breaking in general without any reference to AB- or CB-like behavior does not affect the NAO, though it preferably emerges from its positive phase. 2) AB strengthens the North Atlantic eddy-driven jet and acts to separate it from the subtropical jet, while CB weakens the eddy...

Daily North-Atlantic Oscillation (NAO) index: Statistics and its stratospheric polar vortex dependence

A daily NAO-index statistics is presented, which depends on a NAO-pattern characterised by the first Empirical Orthogonal Function (EOF) of monthly mean sea level pressure (ERA-40, December to February). The pattern explains 40 % of the monthly variance. Projection onto daily sea level pressure fields defines the daily NAO-index time series (15 % of the daily variance), whose statistics is analysed. The following results are noted: (1) The seasonal mean power spectrum exhibits an e-folding decorrelation time of about four days. (2) The frequency distribution of the duration of NAO-events indicate that negative NAO life cycles are, on average, fewer and longer than positive events. (3) A NAO-index statistics conditional on the seasonal intensity of the stratospheric polar vortex yields that differences i n the duration of negative and positive NAO events are more pronounced during weak polar vortex regimes. Supporting other recent studies the results hint at (a) differences in the processes underlying positive and negative NAO events and (b) to a potential stratosphere ‐ troposphere interaction affecting the individual NAO life cycle. Zusammenfassung Eine Statistik eines taglichen NAO-Index wird vorgestellt, die auf einem, aus der ersten Empirischen Orthogonalfunktion (EOF) der Winter-Monatsmittel (Dezember bis Februar) des reduzierten Bodendrucks aus ERA-40 Daten ermittelten NAO-Muster beruht. Dieses Muster erklart 40 % der monatlichen Varianz. Die Projektion auf tagliche Bodendruckfelder definiert den NAO -Index (welcher 15 % der taglichen Varianz erklart), der mit den folgenden Ergebnissen analysiert wird: ( 1) Das Leistungs-Spektrum ergibt eine Dekorrelationszeit von vier Tagen. (2) Die Verteilung der Andauern individueller NAO-Ereignisse zeigt, dass negative NAO Lebenszyklen im Mittel seltener und langer sind als positive. (3) Eine Statistik in Abhangigkeit von der saisonalen Starke des stratopharischen Polar-Wirbels ergibt, dass die Unterschiede in der Andauer zwischen positiven und negativen NAO-Ereignissen in Phasen eines schwachen Wirbels ausgepragter sind. In Ubereinstimmung mit anderen Arbeiten deuten die Ergebnisse hin auf (a) Unterschiede in den positiven und negativen NAO-Ereignissen zugrunde liegenden Prozessen und (b) auf eine die individuellen NAO Lebenszyklen beeinflussende Stratospharen-Tropospharen Wechselwirkung .

Response of Idealized Baroclinic Wave Life Cycles to Stratospheric Flow Conditions

Abstract Dynamical stratosphere–troposphere coupling through a response of baroclinic waves to lower stratospheric flow conditions is investigated from an initial value approach. A series of adiabatic and frictionless nonlinear baroclinic wave life cycles in a midlatitude tropospheric jet with different initial zonal flow conditions in the stratosphere is simulated, using a dry primitive equation model with spherical geometry. When a stratospheric jet, located at various latitudes between 35° and 70°, is removed from the initial conditions, the wavenumber-6 life cycle behavior changes from the well-known LC1 to LC2 evolution, characterized by anticyclonic and cyclonic wave breaking, respectively. Linear theory, in terms of refractive index and the structure of the corresponding fastest-growing normal mode, is found to be unable to explain this stratosphere-induced LC1 to LC2 transition. This implies that altered nonlinear wave–mean flow interactions are important. The most significant stratosphere-induced...

On the Northern Annular Mode Surface Signal Associated with Stratospheric Variability

The wintertime northern annular mode (NAM) at the surface is known to undergo slow intra-seasonal variations in association with stratospheric variability which leads the surface signal by up to several weeks. The relative contributions, however, of potentially relevant stratospheretroposphere 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 quasi-geostrophic (QG) adjustment to stratospheric wave drag and radiative damping and (ii) of 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 which, 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.

Effect of the Kinematic Lower Boundary Condition on the Spectral and Autocorrelation Structure of Annular Variability in the Troposphere

The dynamical origin of the spectral and autocorrelation structure of annular variability in the troposphere is investigated by a deductive approach. Specifically, the structure of the power spectrum and autocorrelation function of the zonal-mean geopotential is analyzed for the case of a quasigeostrophic 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 behavior and the autocorrelation function shows a transition from a rapid decay at short lags to a much slower decay at longer lags, if the thermal and mechanical damping time scales are sufficiently well separated. Since observed annular variability exhibits the same characteristics, the above results lead to the hypothesis that these characteristics may, to some extent, be intrinsic to the linear zonal-mean response problem—although the need for an additional contribution from eddy feedbacks is also implied by the results.