Convectively coupled equatorial waves within the MJO during CINDY/DYNAMO: slow Kelvin waves as building blocks
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This study examines the relationship between the MJO and convectively coupled equatorial waves (CCEWs) during the CINDY2011/DYNAMO field campaign using satellite-borne infrared radiation data, in order to better understand the interaction between convection and the large-scale circulation. The spatio-temporal wavelet transform (STWT) enables us to document the convective signals within the MJO envelope in terms of CCEWs in great detail, through localization of space–time spectra at any given location and time. Three MJO events that occurred in October, November, and December 2011 are examined. It is, in general, difficult to find universal relationships between the MJO and CCEWs, implying that MJOs are diverse in terms of the types of disturbances that make up its convective envelope. However, it is found in all MJO events that the major convective body of the MJO is made up mainly by slow convectively coupled Kelvin waves. These Kelvin waves have relatively fast phase speeds of 10–13 m s−1 outside of, and slow phase speeds of ~8–9 m s−1 within the MJO. Sometimes even slower eastward propagating signals with 3–5 m s−1 phase speed show up within the MJO, which, as well as the slow Kelvin waves, appear to comprise major building blocks of the MJO. It is also suggested that these eastward propagating waves often occur coincident with n = 1 WIG waves, which is consistent with the schematic model from Nakazawa in 1988. Some practical aspects that facilitate use of the STWT are also elaborated upon and discussed.
This research was supported by NOAA Grant NA13OAR4310165. Additional support was provided by the JAMSTEC through its sponsorship of research activities at the IPRC (JIJI). These results were obtained using the globally-merged full resolution Tb brightness temperature data provided by the climate prediction center/NCEP/NWS (available at http://disc.sci.gsfc.nasa.gov/precipitation/data-holdings/Globally_merged_IR.shtml). The authors acknowledge the use of a package provided by CCSM AMWG to compute Fourier-based zonal wavenumber-frequency power spectrum. We thank three reviewers for their insightful comments. We also benefited from discussion with Paul E. Roundy and Masaki Katsumata. School of Ocean and Earth Science and Technology contribution number 10222 and International Pacific Research Center contribution number 1288.
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