Multiscale mechanisms for the modulation of the Pacific Meridional Mode on tropical cyclone genesis over the western North Pacific: a comparison between 2004 and 2011

The impacts of the Pacific Meridional mode (PMM) on interannual variability of tropical cyclone (TC) genesis over the western North Pacific (WNP) have previously been investigated only based on seasonal/monthly mean environmental fields. Here we focus on tropical waves to further explore multiscale mechanisms for the modulation of PMM on TC genesis over the WNP. The results suggest that PMM, tropical waves and TCs form a multiscale interaction system. The TC geneses attributed to Madden–Julian oscillation (MJO), mixed Rossby–gravity (MRG) waves and tropical depression (TD)-type disturbances are significantly positively correlated with the PMM index during the peak TC season (June to November) of 1998–2019. A detailed comparative analysis is then conducted for two selected typical PMM years (2004 in the positive phase and 2011 in the negative phase). Under the influence of the barotropic instability mechanism, the barotropic energy conversion from the basic flow to the eddy kinetic energy is much greater during the positive PMM year, making MJO, MRG waves and TD-type disturbances more active and thus providing more energy for TC genesis. The positive PMM phase modulates local environment by enhancing low-level relative vorticity and mid-level relative humidity and weakening vertical wind shear associated with MJO, MRG waves and TD-type disturbances, which are more favorable for TC genesis. Examination of the genesis potential index shows that TD-type disturbances play the most important role in the TC genesis process during both PMM years.