Precipitable water vapor budget associated with MJO represented in newly-released JRA-55 reanalysis data

The Japan Meteorological Agency (JMA) recently released a new reanalysis dataset JRA-55 with the use of a JMA operational prediction model and 4D-VAR data assimilation. To evaluate merit in utilizing the JRA-55 dataset to investigate dynamics of the tropical intraseasonal variability (ISV) including the Madden-Julian Oscillation (MJO), this study examines ISV-scale precipitable water vapor (PWV) budget over the period 1989-2012. The ISV-scale PWV anomaly related to the boreal-winter MJO propagates eastward along with precipitation, consistent with the SSM/I PWV product. Decomposition of the PWV tendency into that simulated by the model and the analysis increment estimated by the data assimilation reveals that the model makes the PWV anomaly move eastward. On the other hand, the analysis increment exhibits positive values over the area where the PWV anomaly is positive, indicating that the model tends to damp the MJO signal. Note that the analysis increment over the Maritime Continent has comparable magnitude to the model tendency. The positive analysis increment may mainly be caused by an excess of precipitation anomaly with respect to the magnitude of PWV anomaly. In addition to the boreal-winter MJO, this study also examines the PWV budget associated with northward-propagating ISV during the boreal summer and find similar relationship between the PWV anomaly and analysis increment.