Tropical Cloud Regimes and MJO

Using the weather state classification (see ISCCP Cluster Analysis product) and an MJO index (see MJO Index product), composite relative frequency of occurrence bar-charts of cloud regimes formed at seven lag time (in pentads) with respect to MJO phase are obtained for both weak and strong MJO cases in the Indo-Pacific region (60E-180E) within 5N-5S latitude band averaged over 1983-2004 period. The regions from 60E to 180E within ±5 are divided into 6 longitude zones of twenty degrees with center longitudes the same as those used for the MJO index. The peak phase of MJO events is determined near the center longitude of each zone based on negative maxima of the MJO index. Lag 0 refers to a period of ±2.5 days around the peak phase. Negative lag is defined as the longitude zones east of the MJO peak, i.e., preceding it in time. Moreover, the composite only includes longitude bands of each MJO event at which the negative maximum is <-1 in the case of weak MJO and inferior to -2.2 for strong MJO. In their paper, Chen et al. (2008) only select 8 strongest MJO events over 4 November-April periods between 1999-2003. In our study, 277 weak MJO events are selected over 1983-2004 period, while 35 strong MJO events are considered in the same period of time.

The composite cluster RFOs as a function of MJO phase are presented in figures below. Each color bar corresponds to a weather state, where color-code from red to black is described in this figure on the left. The first three weather states defined as convective cloud regimes are depicted with red, orange and yellow bars. The green bar shows cirrus regime evolution. Finally, blueish bars represent suppressed cloud regimes while black one is for clear sky.

Weak case : MJO Index < -1	Strong case : MJO Index < -2.2

Whatever the index threshold, RFOs of the deep convection (red bar) and anvil regimes (orange bar) increase significantly to the peak phase of the MJO. After the peak, the occurrences of these cloud regimes decrease gradually. In the meantime, the small-scale convection with mid-congestus (yellow bar), and shallow cumulus regime (cyan bar) dominate several weeks before the MJO peak, decreasing before the peak, and increasing after it. Besides, we can note that RFO of the cirrus regime (green bar) is almost constant with MJO phase, suggesting that the presence of isolated cirrus is not associated with convective activity. This result is consistent with the study made by Chen et al. (2008). Moreover, the main difference between those two cases is RFO in particular at the MJO peak in the stronger case, where deep convection occurs 10% more than in the weak case. Furthermore, suppressed regimes decrease more dramatically in the stronger case. Thanks to the ISCCP cluster analysis and using an MJO index, we are able to characterize organized and disorganized convection as a function of the MJO phase. Therefore, convection wouldn't be an on/off cycle but could be seen more as a scale organization. Finally, these results then suggest a strong interaction between MJO and deep convection since maximum convection occurs at the peak of the MJO phase.

To download the analysis software (IDL) plotting RFO of cloud regimes, click HERE.

• Chen, Y., and A. D. Del Genio, 2008: Evaluation of tropical cloud regimes in observations and a general circulation model. Clim. Dyn., doi:10.1007/s00382-008-0386-6.