Multiscale structure of the MJO revealed from 30-year satellite data

Abstract:

How the intangible convective envelope of the MJO is made up of organized deep convection has been of particular interest, because it is the key to understand the physics and dynamics of the MJO from the viewpoint of multiscale interactions. We explored the multiscale convective structure of the MJO using 30-year (1983.7-2013) high-resolution satellite date taking advantage of the spatio-temporal wavelet transform (STWT), which is able to localize the space-time spectral characteristics of cloudiness in both space and time. Namely, the multiscale structure of the MJO can be interpreted in terms of a sum of a variety of so-called convectively coupled equatorial waves (CCEWs). It is found that a range of CCEWs tend to be enhanced in the MJO convective envelope, whereas the types of CCEWs with characteristic space-time scales vary seasonally, geographically, and even from event to event. It is thus difficult to find a universal multiscale structure of the MJO, although there is a tendency for the MJO to prefer certain types of CCEWs including Kelvin waves and n=1 westward inertio-gravity (WIG) waves. Within the MJO convective envelope, those waves tend to be enhanced by as large as 100 % compared to the climatological values and Kelvin waves tend to have much slower phase speed of about 9 m s^-1 or less (i.e., equivalent depth of ~8 m) as opposed to the climatological mean values (equivalent depths of 25-90 m) and n=1 WIG waves tend to have 2 day periodicity. Such strong modulation effect, however, is less pronounced over the Maritime continent. The historical database we created will be useful to place the convective systems of the MJO during the CINDY/DYNAMO and the future YMC field campaigns in the historical context.