Variations of Northern Hemisphere storm track and extratropical cyclone activity associated with the Madden-Julian oscillation

This study investigates the intraseasonal variations of the Northern Hemispheric storm track associated with the Madden-Julian oscillation (MJO) during the extended boreal winter (November-April) using 36 yr (1979- 2014) of reanalysis data fromERA-Interim. Twomethods have been used to diagnose storm-track variations. In the first method, the storm track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) ormean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eightMJOphases are characterized by a zonal band of strong positive (or negative) anomaliesmeandering from the Pacific all the way acrossNorthAmerica and theAtlantic into northern Europe,withweaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-tracking scheme has been used to track the extratropical cyclones that compose the storm track. The MJO-composite anomalies of the ''accumulated'' cyclone activity, a quantity that includes contributions from both the cyclone frequency and cyclonemean intensity, are very similar to those based on pp. Further analysis demonstrates that major contribution comes from variations in the cyclone frequency. Further analysis suggests that the intraseasonal variations of the stormtrack can be primarily attributed to the variations of themean flowthat responds to the anomalousMJO convections in the tropics, with possible contribution also from the moisture variations.