Characteristics of 3-4- and 6-8-day period disturbances observed over the tropical Indian ocean

Kazuaki Yasunaga, Kunio Yoneyama, Qoosaku Moteki, Mikiko Fujita, Yukari N. Takayabu, Junko Suzuki, Tomoki Ushiyama, Brian Mapes

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20 Scopus citations


A field observational campaign [i.e., the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO)] was conducted over the central equatorial Indian Ocean in October-December 2006. During MISMO, large-scale organized convection associated with a weak Madden-Julian oscillation (MJO) broke out, and some other notable variations were observed. Water vapor and precipitation data show a prominent 3-4-day-period cycle associated with meridional wind y variations. Filtered y anomalies at midlevels in reanalysis data [i.e., the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS)] show westward phase velocities, and the structure is consistent with mixed Rossby-gravity waves. Estimated equivalent depths are a few tens of meters, typical of convectively coupled waves. In the more rainy part of MISMO (16-26 November), the 3-4-day waves were coherent through the lower and midtroposphere, while in the less active early November period midlevel y fluctuations appear less connected to those at the surface. SST diurnal variations were enhanced in light-wind and clear conditions. These coincided with westerly anomalies in prominent 6-8-day zonal wind variations with a deep nearly barotropic structure through the troposphere. Westward propagation and structure of time-filtered winds suggest n = 1 equatorial Rossby waves, but with estimated equivalent depth greater than is common for convectively coupled waves, although sheared background flow complicates the estimation somewhat. An ensemble reanalysis [i.e., the AGCM for the Earth Simulator (AFES) Local Ensemble Transform Kalman Filter (LETKF) Experimental Reanalysis (ALERA)] shows enhanced spread among the ensemble members in the zonal confluence phase of these deep Rossby waves, suggesting that assimilating them excites rapidly growing differences among ensemble members.

Original languageEnglish (US)
Pages (from-to)4158-4174
Number of pages17
JournalMonthly Weather Review
Issue number11
StatePublished - Nov 2010


  • Ensembles
  • Indian Ocean
  • Madden-Julian oscillation
  • Sea surface temperature
  • Tropics

ASJC Scopus subject areas

  • Atmospheric Science


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