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The Linear and Nonlinear Extratropical Response of the Atmosphere to Tropical Intraseasonal Heating

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

This study examines the dynamical nature of the linear and nonlinear extratropical response of the atmosphere to tropical intraseasonal heating with various temporal and spatial structures. A global two-level model is used. It is found that the midlatitude response is very sensitive to the phase speed of the forcing and, to a lesser extent, to its spatial scale. For a narrow monopolar heating source traveling eastward uniformly with an around-the-world period of 40 days, as in GCM simulations of the Madden–Julian oscillation (MJO), the response is weak and explains very little of the extratropical variance. For dipolar beating with zonal scale around 15 000 km (k = 2–3) and wave period of 40 days propagating slowly eastward within a confined longitudinal domain, a good approximation of the outgoing longwave radiation fields associated with the real MJO, the extratropical response is wavelike and well defined and stands significantly above the background variability. The nonlinear wavetrains evolve in time and exhibit many of the characteristics of their linear counterparts. Nonlinearities, however, play an important role in inhibiting the midlatitude response during certain phases of the oscillation, when cooling coexists west of heating and equatorial winds prevail. Time-mean planetary waves in midlatitudes are not needed to excite a pronounced response. The nonlinear model with zonally asymmetric climatology reveals moderate sensitivity to the longitude of the heating, with a tendency for more intermittent wave trains than in the zonally symmetric case, and does not reproduce the resonant behavior found in the linear model for a particular configuration of the heating and the topographic forcing. The potential asymmetry of the extratropical response to the MJO with respect to the phase of the dipole in intraseasonal convection should he kept in mind when attempting to detect the midaltitude signal from observational data.

Abstract

This study examines the dynamical nature of the linear and nonlinear extratropical response of the atmosphere to tropical intraseasonal heating with various temporal and spatial structures. A global two-level model is used. It is found that the midlatitude response is very sensitive to the phase speed of the forcing and, to a lesser extent, to its spatial scale. For a narrow monopolar heating source traveling eastward uniformly with an around-the-world period of 40 days, as in GCM simulations of the Madden–Julian oscillation (MJO), the response is weak and explains very little of the extratropical variance. For dipolar beating with zonal scale around 15 000 km (k = 2–3) and wave period of 40 days propagating slowly eastward within a confined longitudinal domain, a good approximation of the outgoing longwave radiation fields associated with the real MJO, the extratropical response is wavelike and well defined and stands significantly above the background variability. The nonlinear wavetrains evolve in time and exhibit many of the characteristics of their linear counterparts. Nonlinearities, however, play an important role in inhibiting the midlatitude response during certain phases of the oscillation, when cooling coexists west of heating and equatorial winds prevail. Time-mean planetary waves in midlatitudes are not needed to excite a pronounced response. The nonlinear model with zonally asymmetric climatology reveals moderate sensitivity to the longitude of the heating, with a tendency for more intermittent wave trains than in the zonally symmetric case, and does not reproduce the resonant behavior found in the linear model for a particular configuration of the heating and the topographic forcing. The potential asymmetry of the extratropical response to the MJO with respect to the phase of the dipole in intraseasonal convection should he kept in mind when attempting to detect the midaltitude signal from observational data.

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