Abstract
Boreal summer intraseasonal oscillation (BSISO) has complex spatial structure due to the co-existence of equatorial eastward and off-equatorial northward propagation in the equatorial Indian Ocean. As a result, equatorial Indian Ocean convection has simultaneous northward and eastward (NE), northward only (N-only) and eastward only (E-only) propagations. It is well established that the convection propagates in the direction of increasing moist static energy (MSE). The moisture and MSE budget analysis reveals that the horizontal advection of anomalous MSE contributes to positive MSE tendency, which is in agreement with the horizontal advection of column integrated moisture anomaly. Northward movement of warm SST and the anomalous moisture advected by zonal wind are the major initiative for the northward propagation of convection from the equatorial Indian Ocean in both NE and N-only category. At the same time warm SST anomaly in the equatorial west Pacific along with moisture advection caused by anomalous meridional wind is important for the equatorial eastward branch of NE propagation. As these anomalies in the west Pacific moves northward, equatorial Indian Ocean convection establishes over the equatorial west Pacific. The absence of these processes confines the BSISO in northward direction for N-only category. In the case of E-only movement, warm SST anomaly and moisture advection by zonal component of wind causes the eastward propagation of convection. Boundary layer moisture convergence always remains east of convection center in E-only propagation, while it coincides with convection centre in other two categories. Thus the present study concludes that the difference in underlying SST and atmospheric circulation in tropical Indo-west Pacific oceanic regions encourage the differential propagation of BSISO convection through moisture dynamics.
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Acknowledgments
Indian Institute of Tropical Meteorlogy (IITM) is fully supported by Ministry of Earth Sciences, Government of India. The Director of IITM is acknowledged for the support. The authors acknowledge ECMWF for the ERA-interim data downloaded from the website http://www.ecmwf.int. Dr. Prince K. Xavier is acknowledged for providing the Lanczos filtering code. Dr. R. H. Kripalani is acknowledged for his help in statistical analysis. We also acknowledge Dr. Susanna Corti, executive editor and three anonymous reviewers of Climate dynamics for their constructive comments, which helped us to improve the manuscript.
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Pillai, P.A., Sahai, A.K. Moisture dynamics of the northward and eastward propagating boreal summer intraseasonal oscillations: possible role of tropical Indo-west Pacific SST and circulation. Clim Dyn 47, 1335–1350 (2016). https://doi.org/10.1007/s00382-015-2904-7
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DOI: https://doi.org/10.1007/s00382-015-2904-7