North-central Pacific tropical cyclones: impacts of El Nino-Southern Oscillation and the Madden-Julian oscillation.

摘要

Both El Nino-Southern Oscillation (ENSO) and the Madden-Julian oscillation (MJO) have been documented in previous research to impact tropical cyclone (TC) activity around the globe. This study examines the relationship of each mode individually along with a combined index on tropical cyclone activity in the north-central Pacific. Approximately twice as many tropical cyclones form in the north-central Pacific in El Nino years compared with La Nina years. These differences are attributed to a variety of factors, including warmer sea surface temperatures, lower sea level pressures, increased midlevel moisture, and anomalous midlevel ascent in El Nino years. When the convectively enhanced phase of the MJO is located over the eastern and central tropical Pacific, the north-central Pacific tends to have more tropical cyclone activity, likely because of reduced vertical wind shear, lower sea level pressures, and increased vertical motion. The convectively enhanced phase of the MJO is also responsible for most of the TCs that undergo rapid intensification in the north-central Pacific. A combined MJO-ENSO index that is primarily associated with anomalous rising motion over the tropical eastern Pacific has an even stronger relationship with north-central Pacific TCs, as well as rapid intensification, than either individually.