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

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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 begreatly enhanced. The concept lies at the heart of the interaction of planetary flows withtopographic mountain chains. In contrast, early studies of the atmospheric response to thermalforcing, focussed on the off resonance response. Now available, many GCM studies dealingwith 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 overwhelmingactivity of transient eddies at the synoptic scale. Given the long lasting influence of large scaleSST anomalies, climate predictability may be expected to improve if one can identify feedbacksbetween the large scale climate anomalies and the SST distribution. We propose a simple theorythat explores the physics of this atmospheric response to SST and may suggest ways to analyzedata from the more complex GCM. We neglect all interactions between the transient eddiesand the large scale waves that would go beyond Fickian mixing laws although there is evidencethat the transient eddies by themselves take part in the maintenance of the low frequencyvariability. Because the observed perturbations are small, a linear theory is appropriate. Usinga 2-level model of the atmosphere, a resonance condition occurs when the Rossby waves arestationary against the vertically averaged mean zonal flow. The resonance is sharp when eddydissipation through surface friction is small. In a small wavenumber window controlled by thevertical 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 amplifiedto produce a positive feedback on the SST. When the atmospheric model is coupled to a oneand a half level oceanmodel with a zonally periodic geometry appropriate to the SouthernOceans,a linear instability appears. The application of this process of thermal resonance to the Antarcticcircumpolar wave is discussed. We find that under realistic values of the mean state, an unstablecoupled wave emerges in a narrow wavenumber window which coincides with the near resonanceconditions found previously in the atmospheric model. This instability provides a powerful scaleselective mechanism for the perturbations. The thermal forcing is primarily responsible for amplificationof the SST while the powerful Antarctic circumpolar current is primarily responsible foradvecting the anomalies around the globe and setting the period. DOI: 10.1034/j.1600-0870.2001.01224.x

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