Thermal resonance of the atmosphere to SST anomalies. Implications for the Antarctic circumpolar wave

Abstract

It is well-known that when Rossby waves are stationary in the belt of mid-latitude westerlies, resonance conditions occur allowing the atmospheric response to external perturbations to be greatly enhanced. The concept lies at the heart of the interaction of planetary flows with topographic mountain chains. In contrast, early studies of the atmospheric response to thermal forcing, focussed on the off resonance response. Now available, many GCM studies dealing with the atmospheric response to prescribed SST have revealed a great variety of responses, the difficulty of extracting the signal at planetary scale being compounded by the overwhelming activity of transient eddies at the synoptic scale. Given the long lasting influence of large scale SST anomalies, climate predictability may be expected to improve if one can identify feedbacks between the large scale climate anomalies and the SST distribution. We propose a simple theory that explores the physics of this atmospheric response to SST and may suggest ways to analyze data from the more complex GCM. We neglect all interactions between the transient eddies and the large scale waves that would go beyond Fickian mixing laws although there is evidence that the transient eddies by themselves take part in the maintenance of the low frequency variability. Because the observed perturbations are small, a linear theory is appropriate. Using a 2-level model of the atmosphere, a resonance condition occurs when the Rossby waves are stationary against the vertically averaged mean zonal flow. The resonance is sharp when eddy dissipation through surface friction is small. In a small wavenumber window controlled by the vertical structure of the mean flow, the response is equivalent barotropic and baroclinic elsewhere. Only in this window is the familiar response of high SLP downstream of warm SST recovered. For certain combination of thermal damping and surface drag, the atmospheric response is amplified to produce a positive feedback on the SST. When the atmospheric model is coupled to a one and a half level ocean model with a zonally periodic geometry appropriate to the Southern Oceans, a linear instability appears. The application of this process of thermal resonance to the Antarctic circumpolar wave is discussed. We find that under realistic values of the mean state, an unstable coupled wave emerges in a narrow wavenumber window which coincides with the near resonance conditions found previously in the atmospheric model. This instability provides a powerful scale selective mechanism for the perturbations. The thermal forcing is primarily responsible for amplification of the SST while the powerful Antarctic circumpolar current is primarily responsible for advecting the anomalies around the globe and setting the period.(sea surface temperature) and SLP (sea level pres-

Coupled Interannual Rossby Waves in a Quasigeostrophic Ocean–Atmosphere Model

“…Colin de Verdière and Blanc (2001) instead suggested the atmospheric APE as a source of energy in its thermal resonance process. Which of these is happening in our modeling study could be inferred by studying the down-gradient wave heat fluxes in the atmosphere and ocean, as suggested in Colin de Verdière and Blanc (2001).…”

“…An equivalent barotropic atmospheric wave was found to travel westward with similar periods as, and in phase lock with, the oceanic baroclinic Rossby wave, suggesting a coupling between the two. Linear nearly stationary equivalent barotropic atmospheric waves can be generated by, for example, a thermal forcing (Held 1983); in a coupled system these equilibrated waves can represent the quasi-stationary atmospheric waves that couple with the oceanic SST anomalies (Frankignoul 1985;GM99;Colin de Verdière and Blanc 2001;Kravtsov et al 2005).…”

“…A different mechanism is brought forward in Colin de Verdière and Blanc (2001), in which the authors propose a thermal rather than mechanical process to explain the feedback resulting in the SST-SLP phase relationship. The simple ocean used in all of these studies could be a weakness of the results but the atmosphere is more likely to be the cause of any misrepresentation.…”

“…In recent years there has been a growing interest in the scientific community in studying ocean-atmosphere coupled models (Liu 1993;Frankignoul et al 1997; Barsugli and Battisti 1998;Goodman and Marshall 1999, hereinafter GM99;Ferreira et al 2001;White et al 1998;Neelin and Weng 1999;White 2000a;Gallego and Cessi 2000;Cessi and Paparella 2001;Colin de Verdière and Blanc 2001;Kravtsov and Robertson 2002, to mention a few). Different approaches have been used and different results, sometimes in disagreement, have been found.…”

¿Está el clima mediterráneo sujeto a ciclos?: interacciones energéticas entre la temperatura del Atlántico Norte y la ciclogénesis mediterránea

“…Las investigaciones recientes han mostrado que las resonancias de la atmósfera a las anomalías de la SST ofrecen diversas respuestas a escala planetaria (Colin de Verdière and Blanc, 2001). Específicamente, en el Atlántico Norte, las correlaciones estacionales han mostrado una buena significación durante el semestre invernal (Quereda et al, 2011).…”