Structures and Mechanisms of the Northward Propagating Boreal Summer Intraseasonal Oscillation

Abstract

The spatial and temporal structures of the northward-propagating boreal summer intraseasonal oscillation (BSISO) are revealed based on the analysis of both the ECHAM4 model simulation and the NCEP?NCAR reanalysis. The BSISO structure and evolution characteristics simulated by the model bear many similarities to those derived from the NCEP?NCAR reanalysis. The most notable features are the remarkable meridional asymmetries, relative to the BSISO convection, in the vorticity and specific humidity fields. A positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center. The maximum specific humidity also shows a clear northward shift in the lower troposphere. Two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the BSISO. The first is the vertical shear mechanism. The key process associated with this mechanism is the generation of barotropic vorticity due to the coupling between the free-atmosphere baroclinic and barotropic modes in the presence of the vertical shear of the mean flow. The induced barotropic vorticity in the free atmosphere further causes a moisture convergence in the planetary boundary layer (PBL), leading to the northward shift of the convective heating. The second mechanism is the moisture?convection feedback mechanism. Two processes contribute to the northward shift of the low-level moisture. One is the moisture advection by the mean southerly in the PBL. Another is the moisture advection by the BSISO wind due to the mean meridional specific humidity gradient. The asymmetric specific humidity contributes to the northward shift of the convective heating. A theoretical framework is constructed to investigate the instability of the northward-propagating BSISO mode and the relative roles of various mechanisms including air?sea interactions. An eigenvalue analysis indicates that the northward propagation of the BSISO is an unstable mode of the summer mean flow in the monsoon region. It has a typical wavelength of 2500 km. While the easterly shear contributes to the northward propagation primarily north of 5°N, the moisture feedback and the air?sea interaction also contribute significantly, particularly in the region near and south of the equator. The internal atmospheric dynamics are essential in causing the northward propagation of the BSISO over the tropical Indian Ocean.