Variations of the Mid-latitude Storm Track Activity in Association with the Madden-Julian Oscillation

Mid-latitude storm tracks are responsible for most of the day to day weather in the extratropics, and are also major components of the climate system. Thus the variations of the storm tracks are of great social and economic interests. Recent studies have found that the Madden-Julian Oscillation (MJO), which is the leading mode of the tropical convection on the intraseasonal time scale, can exert strong modulation on the storm track activity. However, previous studies generally examine storm track using one single measure such as variance statistics or cyclone statistics. Since different methodologies and measures are used in different studies, it is difficult to find robust results by comparing different studies. In this study, we will examine storm track activity indicated by three complementary measures to identify the robust and consistent variations of storm track activity in association with the MJO. To examine the MJO-related variations, the storm track activity is composited according to the MJO RMM index after removing the annual cycle. Firstly, storm track activity is represented by the variance/covariance statistics. The eight-phase MJO composites of the variance of the bandpass-filtered meridional wind at 300 hPa show that the storm track anomalies can be characterized as a north-south dipole moving eastward in response to the eastward propagating MJO convection anomalies. Secondly, the Rossby wave packets, which are the envelopes of Rossby wave troughs and ridges and thus suggest the prevailing region of cyclones and anticyclones, are examined. The MJO composite maps of increased/decreased Rossby wave packets are very similar to those based on the variance method, which adds confidence to the identified patterns of the MJO-related storm track variations. Finally, we utilize an objective cyclone tracking tool to track the surface cyclones and anticyclones, and investigate their variations related to the MJO. Preliminary results suggest that this method gives rise to much noisier and more localized results than the above two methods. However, after taking into account both the frequency of occurrence and intensity of the cyclones and anticyclones, the anomaly patterns exhibit reasonable resemblance to what have been found in the two earlier methods. Here, we show that using a combination of these three complementary measures, robust storm track modulations by the phases of the MJO are found.