The linear and nonlinear extratropical response of the atmosphere to tropical intraseasonal heating

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 heating 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. -from Authors