| Summary: | 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 degrees and 70 degrees, 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 change that extends into the nonlinear baroclinic growth stage is a region of downward wave propagation in the lower stratosphere associated with positive values of the squared refractive index near 20 km. Furthermore, it is demonstrated that the difference between the response of the tropospheric circulation to LC1 and LC2 life cycles closely resembles the meridional and vertical structure of the North Atlantic Oscillation (NAO), with positive (negative) NAO-like anomalies being driven by LC1 (LC2). Thus, a weakened stratospheric jet induces the generation of negative NAO-like anomalies in the troposphere, consistent with the observed stratosphere-NAO connection.
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