Genesis of Super Cyclone Pam (2015): Modulation of Low-Frequency Large-Scale Circulations and the Madden-Julian Oscillation by Sea Surface Temperature Anomalies

Abstract

uper Cyclone Pam (2015) formed in the central tropical Pacific under conditions that included El Niño Modoki and the passage of a convectively-enhanced phase of the Madden?Julian Oscillation (MJO) in the western Pacific. This study examines the influence that sea surface temperature anomalies (SSTAs) have on the MJO and low-frequency large-scale circulation, and establishes how they modulated the genesis of Pam. Two series of numerical experiments were conducted by using a nonhydrostatic global atmospheric model with observed (OBSSST) and climatological (CLMSST) SSTs. The results suggested that low-frequency westerly winds at 850 hPa (U850) were intensified in the central tropical Pacific due to the observed SSTA. The amplitude of the MJO simulated in OBSSST was larger than in CLMSST. In addition, the experiments initialized 26 February?2 March exhibited that the phase of the MJO in OBSSST was ahead of that in CLMSST, and that the genesis location in OBSSST was ~10° to the east of that in CLMSST. An analysis of large-scale fields indicated that a positive U850 maintained by SSTAs and intensification of U850 by the MJO modified distribution of large-scale cyclonic vorticity and precipitable water. These changes in large-scale fields modified the location and timing of intensification of the disturbance that become Pam and resulted in Pam?s genesis location being 10° farther east with slight impact on its genesis probability. Additional experiments showed that SSTAs in the central tropical Pacific were the dominant cause of modifications to large-scale fields, the MJO, and Pam?s genesis location.