Journal of Climate, submitted.
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY.
Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.
Institut de Recherche pour le Developpement, Laboratoire d'Oceanographie - Experimentation et Approches Numeriques, Institut Pierre-Simon Laplace, Universite Pierre et Marie Curie, Paris, France
A simple coupled model is used in a zonally-symmetric aquaplanet con guration to investigate the e ect of ocean-atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model is constituted of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled with a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30-60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/West Pacific sector. The ocean-atmosphere coupling is shown to have little impact on either the growth rate or latitudinal structure of the atmospheric oscillation, but it reduces the oscillation's period by a quarter. At latitudes corresponding to the north of the Indian Ocean, the Sea Surface Temperature (SST) anomalies lead the precipitation anomalies by a quarter of a period, similarly to what has been observed in the Bay of Bengal. The mixed-layer depth is in phase opposition to the SST: a monsoon break corresponds to both a warming and a shoaling of the mixed-layer bottom. This property results from the similarity between the pattern of the predominant processes: wind-induced surface heat flux and wind stirring. The instability of the seasonal monsoon flow is sensitive to the seasonal mixed layer depth: the oscillation is damped when the oceanic mixed layer is thin (about 10-m deep or thinner), as in previous experiments with several models aimed at addressing the northern winter Madden-Julian Oscillation. This suggests that the weak thermal intertia of land might explain the minima of intraseasonalvariance observed over the Asian continent.