Storm track activity over the North Pacific associated with the Madden-Julian Oscillation under ENSO conditions during boreal winter

The present study investigates modulation by the Madden-Julian Oscillation (MJO) of storm track activity (STA) over the North Pacific (NP) during boreal winter for El Nino and La Nina periods. STA defined by vertically averaged synoptic eddy kinetic energy (EKE) greatly intensifies over the western North Pacific (WNP) and central eastern North Pacific during La Nina and El Nino years, respectively, when the MJO convection is located over the central Indian Ocean (IO)-Maritime Continent. When the MJO moves into the western central Pacific, the STA in La Nina years is suppressed (enhanced) at higher (lower) latitudes than in El Nino years. Diagnoses of EKE and eddy available potential energy budgets indicate that the difference in STA over the WNP for the MJO phases between El Nino–Southern Oscillation (ENSO) years is mainly contributed by baroclinic energy conversion and potential energy conversion between background and eddy (BCPE). We reveal that BCPE is mainly attributed to intraseasonal baroclinicity and eddy heat flux (EHF) anomalies and their interactions with strong winter mean baroclinic fields in the WNP. Through the EHF, synoptic eddies act to counterbalance an intraseasonal temperature that is primarily caused by the anomalous horizontal advection of mean temperature by MJO-related flow. The intraseasonal circulation and associated temperature and EHF anomalies dominate in northwest (southeast) portion of the NP during La Nina (El Nino), leading to BCPE difference. Changes in the spatial pattern and strength of the NP circulation and STA are caused by dominance of strong MJO amplification over the IO (central Pacific) during La Nina (El Nino).