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

Abstract

This study examines wave disturbances on submonthly (6-30-day) timescales over the tropical Indian Ocean during Southern Hemisphere summer using Japanese Reanalysis (JRA25-JCDAS) products and National Oceanic and Atmospheric Administration outgoing longwave radiation data. The analysis period is December-February for the 29 years from 1979/1980 through 2007/2008. An extended empirical orthogonal function (EEOF) analysis of daily 850-hPa meridional wind anomalies reveals a well-organized wave-train pattern as a dominant mode of variability over the tropical Indian Ocean. Daily lagged composite analyses for various atmospheric variables based on the EEOF result show the structure and evolution of a wave train consisting of meridionally elongated troughs and ridges along the Indian Ocean Intertropical Convergence Zone (ITCZ). The wave train is oriented in a northeast-southwest direction from Sumatra toward Madagascar. The waves have zonal wavelengths of about 3,000-5,000 km and exhibit westward and southwestward phase propagation. Individual troughs and ridges as part of the wave train sequentially travel westward and southwestward from the west of Sumatra into Madagascar. Meanwhile, eastward and northeastward amplification of the wave train occurs associated with the successive growth of new troughs and ridges over the equatorial eastern Indian Ocean. This could be induced by eastward and northeastward wave energy dispersion from the southwestern to eastern Indian Ocean along the mean monsoon westerly flow. In addition, the waves modulate the ITCZ convection. Correlation statistics show the average behavior of the wave disturbances over the tropical Indian Ocean. These statistics and other diagnostic measures are used to characterize the waves obtained from the composite analysis. The waves appear to be connected to the monsoon westerly flow. The waves tend to propagate through a band of the large meridional gradient of absolute vorticity produced by the mean monsoon westerly flow. This suggests that the monsoon westerly flow provides favorable background conditions for the propagation and maintenance of the waves and acts as a waveguide over the tropical Indian Ocean. The horizontal structure of the wave train may be interpreted as that of a mixture of equatorial Rossby waves and mixed Rossbygravity wavelike gyres.

Moistening Processes before the Convective Initiation of Madden–Julian Oscillation Events during the CINDY2011/DYNAMO Period

“…14. In pattern and horizontal scale, the corresponding wave train was also similar to those that robustly appear in boreal winter (e.g., Fukutomi and Yasunari 2013). 13, but for 10-18 Nov 2011. troposphere.…”

Extratropical forcing of tropical wave disturbances along the Indian Ocean ITCZ

“…In this respect, Fukutomi and Yasunari [, hereinafter FY13] investigated the spatiotemporal structure and propagation characteristics of a dominant mode of wave disturbances on submonthly (high‐frequency intraseasonal) timescales ranging from 6 to 30 days over the tropical Indian Ocean during the austral summer. They successfully identified the tropical wave train pattern oriented in a northeast‐southwest direction from Sumatra toward Madagascar.…”

Intraseasonal oscillation associated with the Indian winter monsoon

“…[3] The dominant intraseasonal oscillation (ISO) modes discussed in the literature occur at two timescales: MaddenJulian Oscillation of 30-60 d Julian, 1972, 1994;Wheeler and Hendon, 2004] and submonthly-scale oscillation of approximately 6-25 d [Vincent et al, 1998;Hoyos and Webster, 2007;Fukutomi and Yasunari, 2012]. The 10-20 d or quasi-biweekly timescale mode discussed in various studies is contained within the submonthly timescale [Chatterjee and Goswami, 2004;Kikuchi and Wang, 2009] and not discussed here.…”