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.

Correction to “Low‐frequency variability in the Southern Ocean region in a simplified coupled model”

“…This latter contribution is now 30% the first one. This is qualitatively consistent with the Colin de Verdière and Blanc [2001] study showing (in a simple 2 atmospheric and 1.5 oceanic layers coupled model in a one‐dimensional zonal channel) a coupled resonant mode where both thermal and mechanical air‐sea interactions provide a positive feedback on the ocean, with a dominant role of the former upon the later.…”

The effects on oceanic planetary waves of coupling with an atmospheric energy balance model

“…In our simple model there is no mean flow in the ocean or in the atmosphere and the coupling is given only by thermal exchanges. Mean flows and wind stress are essential factors for developing growing coupled modes through ocean–atmosphere positive feedbacks (Qiu et al, 1997; Goodman and Marshall, 1999; Cessi, 2000; Cessi and Paparella, 2001; Colin de Verdière and Blanc, 2001; Huck et al, 2001). In the model presented here there is no kinetic energy because the basic state is motionless and no available potential energy in either the ocean or the atmosphere, therefore there is no energy to sustain instabilities.…”

“…The concept of Rossby wave coupling to the overlying atmosphere is hardly new (consider White et al, 1998; White, 2000, for example), but no clear attempt has been made so far to explain or study the effects on the wave structure and propagation. None the less, experiments with coupled atmosphere–ocean systems in which planetary waves play a major role have proliferated (Frankignoul et al, 1997; Jin, 1997; Qiu and Jin, 1997; Goodman and Marshall, 1999; Colin de Verdière and Blanc, 2001; Ferreira et al, 2001) in the attempt of better understanding and increasing the predictability of the decadal–interdecadal climate variability.…”

Quasi‐stationary ENSO wave signals versus the Antarctic Circumpolar Wave scenario

“…However, some other observational studies [e.g., White and Peterson , 1996] noted a coherent, eastward propagating wave in oceanic and atmospheric variables with a circumpolar wavenumber 2 and a dominant periodicity of 4–5 years. This so‐called Antarctic Circumpolar Wave (ACW) has been the subject of much debate concerning its generating mechanisms [ Qiu and Jin , 1997; White et al , 1998; Colin de Verdière and Blanc , 2001], its characteristic or persistence [ Bonekamp et al , 1999; Connolley , 2003], and even its very existence [ Christoph et al , 1998; Cai et al , 1999; Park , 2001]. One of the most debated points is whether the interannual climate variability around Antarctica is mainly forced by tropical ENSO episodes as proposed by Cai and Baines [2001] or it is a self‐sustained circumpolar wave generated within the Southern Ocean by an extratropical ocean‐atmosphere coupling mechanism.…”