Further inquiry into characteristics of MJO in boreal winter

Abstract

The characteristics of Madden–Julian Oscillation (MJO), especially over the equatorial western Pacific, during the boreal winter (November to March) have got a further investigation based on the reanalysis data. The MJO activity prevails over the equatorial Indian Ocean and western Pacific and its main active centres are in the Southern Hemisphere. The MJO also has the highest percentage in the atmospheric variability and strong interannual variability here. The MJO over the equatorial western Pacific shows a prominent interannual variability with a period of 2–8 years and an interdecadal variability with a period of 12–16 years. These variabilities of MJO are closely related to the intensity and duration of strong discrete MJO events over there. During the equatorial western Pacific MJO activity strong winter, the MJO is intensified throughout the tropics and its eastward propagation is more continuous, more coherent, and further east. Anomalous ascending motion and moisture convergence over the equatorial western Pacific could enhance the Walker circulation over the equatorial Pacific and favour the activity of MJO over the equatorial western Pacific. The further eastward propagation of MJO in strong MJO activity winter is mainly due to the strong moistening at the lower troposphere to the east of the MJO convection centre. This moistening is dominated by the vertical advection of moisture. In weak MJO activity winter, the above characteristics are reversed. Furthermore, the MJO activity over the Indian Ocean and Pacific has undergone several interdecadal changes in both of its intensity and propagation during the past 150 years.

Geopotential-based Multivariate MJO Index: extending RMM-like indices to pre-satellite era

“…For example, Slingo et al (1999) showed an increasing trend of interannual MJO activity since 1958, by analyzing the variances of bandpass (20-100 days) filtered u200 and u850 averaged between 10°S and 10°N. Chen et al (2017) also defined an MJO intensity index in a similar way (amplitude of u850 averaged over 10°S-10°N, 120-160°E) in his study of interannual and interdecadal MJO variability during 1861-2010. Another method reflecting MJO activity is to compute the two leading PCs of MJO circulation responses, such as u200, u850, velocity potential, stream function or combinations of them (Slingo et al 1999;Jones and Carvalho 2006;Ventrice et al 2013).…”