Ocean-Atmosphere Coupling in the Monsoon Intraseasonal Oscillation: A Simple Model Study

Type Article
Date 2008-10
Language English
Author(s) Bellon Gilles1, Sobel Adam H.1, 2, Vialard Jerome3
Affiliation(s) 1 : Columbia Univ, Dept Appl Phys & Appl Math, New York, NY USA.
2 : Columbia Univ, Dept Earth & Environm Sci, New York, NY USA.
3 : Univ Paris 06, Inst Rech Dev, Lab Oceanog Expt & Approches Numer, Inst Pierre Simon Laplace, Paris, France.
Source Journal Of Climate (0894-8755) (Amer Meteorological Soc), 2008-10 , Vol. 21 , N. 20 , P. 5254-5270
DOI 10.1175/2008JCLI2305.1
WOS© Times Cited 31
Keyword(s) madden julian oscillation, tropical circulation model, sea surface temperature, asian summer monsoon, convective perturbations, walker circulation, radiation budget, indian ocean, variability, simulation
Abstract A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean-atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to 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. This behavior results from the similarity between the patterns 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 boreal winter Madden-Julian oscillation. This suggests that the weak thermal inertia of land might explain the minima of intraseasonal variance observed over the Asian continent.
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