Schedule May 06, 2008
Dynamics of Tropical Intraseasonal Variability
Adam Sobel (Columbia Univ.)

In large regions of the deep tropics, weather is strongly modulated on the intraseasonal (30-60 day) time scale. Near the equator and in the southern tropics, this modulation is controlled by the Madden-Julian oscillation (MJO). The MJO propagates from west to east and paces the active and break cycles of the Australian and Indonesian monsoons. In northern summer, the Indian and Asian monsoons are modulated by the MJO but also by a somewhat distinct mode of variability featuring rain bands that propagate from south to north. The mechanisms that cause these intraseasonal oscillations to exist are not understood despite decades of study by many scientists. I will present several lines of evidence supporting the hypothesis that surface enthalpy fluxes (both turbulent and radiative) from the ocean to the atmosphere are crucial to the dynamics of the intraseasonal oscillations. This idea is at least 20 years old but is not broadly accepted, partly for historical reasons. I will argue that it should be reexamined now in a more focused way. In particular, the current generation of numerical climate and weather models may be good enough to provide useful insights into the relevance of surface enthalpy fluxes to intraseasonal oscillations, if experiments designed for the purpose are performed with a large number of different models.

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