An Operational Tracking Method for the MJO Using Extended Empirical Orthogonal Functions

Abstract

Several methods are available to track the intraseasonal oscillation, namely the Madden–Julian oscillation (MJO) and monsoon intraseasonal oscillation (MISO). However, no methods to track both the modes in a uniform framework for real-time application exist. A new method to track the smooth propagation of the MJO and MISO is proposed to use it for real-time monitoring. The new approach is based on extended empirical orthogonal function (EEOF) analysis of the combined field of meridionally averaged zonal wind at 850 hPa (U850), zonal wind at 200 hPa and the velocity potential at 200 hPa. The EEOF method does not merely capture the MJO but also smooths (i.e., filters out undesired modes) the temporal propagation in the phase space defined by first two leading principal components (PCs) of the EEOF. Using these leading PCs, we additionally introduce a seasonally varying regression coefficient to filter out the space time structure of the MJO in various fields (rainfall/outgoing long-wave radiation, etc.). These reconstructed fields are then used to link the bidirectional phase movement (i.e., northward propagating MISO and eastward propagating MJO) during boreal summer. The life cycle of the MJO and MISO could be captured with equal fidelity with this current method. MJO filtering could be carried out for any space–time data, not limited to the variables/fields used in EEOF analysis. Examples of the TOGA-COARE and DYNAMO period MJO as well as a few strong MISO events are discussed to check the robustness of the newly proposed method.