Modification of MJO and Genesis of Supercyclone Pam in March 2015 by the Sea Surface Temperature Anomaly

Supercyclone Pam, generated in 06UTC 8 March 2015 and intensify as the strongest cyclone of March in the southern Pacific with minimum central pressure of 896 hPa, caused tremendous damages in Vanuatu. The situation at the time of Pam's genesis is characterized that the sea surface temperature anomaly (SSTA) was +2K in the central Pacific and that the Madden-Julian Oscillation (MJO) was traveling eastward in the western Pacific. In this study, two series of numerical experiments are conducted to investigate the modification of MJO and Pam's genesis by SSTA using the Nonhydrostatic ICosahedoral Atmospheric Model, NICAM. The model has a horizontal resolution of 14 km and cloud microphysics is explicitly calculated without any convective parameterization. This model configuration is used by Miyakawa et al. (2014) and Nakano et al. (2015) who demonstrate excellent performance for MJO and tropical cyclogenesis forecasts. One series is called OBSSST experiments, in which observed SSTA is given and another is called CLMSST experiments, in which no SSTA is given. Results show that westerly at 850 hPa in the tropical central Pacific is stronger and stationary in OBSSST. In addition, MJO amplitude evaluated by the Wheeler-Hendon RMM is larger and corresponding eastward moving westerly is stronger in OBSSST. These differences cause stronger westerly on the day just before Pam's genesis (7 March) around Pam's genesis position (160-180E 10-0S). This strong westerly makes Pam's genesis location 1000 km east in OBSSST comparing with CLMSST.

Miyakawa et al., 2014, Nature. Comm., 5, 3769, doi: 10.1038/ ncomms4769 Nakano et al. 2015, Geophys. Res. Let., 42, 565–571, doi: 10.1002/2014GL062479.