Structure and characteristics of submonthly-scale waves propagating along the Indian Ocean ITCZ

This study examines the wave disturbances on the submonthly-scales over the tropical Indian Ocean using the Japanese 25-year Reanalysis (JRA25) products and NOAA outgoing long wave radiation data. The analysis period is December—February (Northern Hemisphere winter) for the 29 years from 1979/80 through 2007/08. A well-organized wave train pattern is detected as a dominant mode of variability over the Indian Ocean by performing extended empirical orthogonal function analysis (EEOF) on daily 850-hPa meridional wind anomalies. Lagged composite analysis for various elements based on the EEOF results reveals the structure and evolution of the wave train along the ITCZ in the tropical Indian Ocean. The waves with zonal wavelengths of 3000—5000 km exhibit westward and southwestward phase propagation. Meridionally elongated troughs and ridges of the wave train move sequentially westward and southwestward from the west of Sumatra into Madagascar. On the other hand, eastward and northeastward amplification of the wave train occurs associated with the successive growth of new troughs and ridges over the eastern equatorial Indian Ocean. This could be induced by wave energy dispersion along the mean monsoon westerly flow. The waves have convectively coupled wave characteristics and play a vital role in modulating the ITCZ convection. Correlation statistics manifest average behavior of the wave disturbances over the tropical Indian Ocean. Teleconnectivity, temporal coherence, and other measures characterize the propagating wave activity and determine the waveguide along the mean monsoon westerly flow over the tropical Indian Ocean.