Characteristics of 3–4- and 6–8-Day Period Disturbances Observed over the Tropical Indian Ocean

Abstract

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 ? variations. Filtered ? 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 ? 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.