We report on a new nonlinear dynamics occurring in a confined cylindrical column filled with fluid (liquid bridge) and heated from above. We demonstrate and analyze the novel oscillatory flow state created by the interaction of two hydrothermal waves of different origins: one propagates vertically from the cold towards the hot side (m ¼ 0) and another is traveling in the azimuthal direction (m ¼ 1). Their interaction leads to an exotic flow structure: during a part of the oscillation period the resulting wave propagates in a given azimuthal direction, whereas during the rest of the period it moves in the opposite direction. A new bimodal flow regime is found to exist over a parameter range where these modes have comparable influence. The phase diagrams, obtained by three-dimensional nonlinear simulations, are reported. They shed light on the instability mechanism and criteria of the existence of novel states. DOI: 10.1103/PhysRevLett.102.134503 PACS numbers: 47.20.Ky, 05.45.Àa, 47.54.Bd, 47.55.nk The investigation of the Rayleigh-Bénard and RayleighMarangoni convection has played a crucial role in the development of the nonlinear stability theory for planar spatially extended systems [1]. Recently, significant progress has been achieved in understanding the nonlinear regimes of buoyant-thermocapillary convection in cylindrical columns [2,3]. The latter problem can be considered as a paradigmatic example of nonlinear dynamics in essentially nonparallel flows. The mechanisms of instability have a universal character. Therefore, it is of a general interest, far beyond its direct applications in engineering. Because of the periodicity of the problem in the azimuthal direction, the spectrum of azimuthal wave numbers is discrete. The hydrothermal waves, which are developed due to the oscillatory instability of a steady axisymmetric flow, are characterized by integer values of the azimuthal wave number m. Typically, above the critical point, a 3D oscillatory flow starts as a finite-amplitude standing or traveling (TW) wave. When the thermal stresses increase, the nonlinear dynamics is determined mostly by the interaction of modes with m ¼ 1 and m ¼ 2, e.g., [2,3].Still, the role of the simplest, axisymmetric oscillatory mode m ¼ 0, is somehow elusive, and there is no agreement on that subject up to now. Xu and Davis However, these measurements were performed above the threshold of the 3D oscillatory regime, where modes with m ! 1 are present. Numerical simulations for aspect ratio close to unity À $ 1 also could not reveal the m ¼ 0 as being critical, e.g., [2,9,10]. In short liquid bridges the rigid walls impose a strong constraint to the appearance and spreading of axial waves.In the present Letter, we clarify the question of the existence of axisymmetric wavy motions, and analyze the consequences of the interaction between the modes with m ¼ 0 and m ¼ 1 that leads to novel, highly nontrivial, flow regimes. Our analysis also explains the enigmatic flow regime with alternating directions of hot or cold spot's moti...