Identifying the skeleton of the Madden-Julian oscillation in observational data

Abstract

The Madden-Julian oscillation (MJO) skeleton model offers a theoretical prediction of the MJO's structure. Here, a method is described for identifying this structure in observational data. The method utilizes projections onto equatorial wave structures, and a main question is: Can this method isolate the MJO without using temporal filtering or empirical orthogonal functions? For the data projection, a wide range of data is incorporated: multiple variables (wind, geopotential height, water vapor, and, as a proxy for convective activity, outgoing longwave radiation), multiple pressure levels (850 and 200 hPa), and multiple latitudes (both equatorial and off-equatorial). Such a data variety is combined using a systematic method, and it allows for a distinction between the Kelvin and Rossby components of the MJO's structure. Results are illustrated for some well-known cases, and statistical measures are presented to quantify the variability of the MJO skeleton signal, MJOS(x, t), and its amplitude, MJOSA(t). The robustness of the methods is demonstrated through a suite of sensitivity studies, including tests with two projection methods. When the projection is based on the skeleton model's energy, as opposed to the standard L2 energy, water vapor is seen to be of primary importance. Finally, a simple interpretation is given for the MJO skeleton structure: it is related to the wave response to a moving heat source. From either perspective, the methods here identify signals that project onto coupled convection-circulation patterns, and the results suggest that a large portion of the MJO's structure is consistent with such a coupled pattern.