AbstractModulation of tropical cyclone (TC) genesis by Madden-Julian Oscillation (MJO) has been quantitatively diagnosed by using climatological Genesis Potential Index (GPI). Analysis of TC genesis during November-April of 1979-2014 indicates the most effective factors controlling intraseasonal TC genesis are 850hPa relative vorticity (ζr850,) weighted by the Coriolis parameter, fζr850, and 500hPa vertical motion (ω500). The total vertical wind shear and maximum potential intensity are unimportant, and the role of 600hPa relative humidity is highly represented by ω500. The MJO modulates TC genesis primarily through changing low-level vorticity induced by its Rossby wave gyres and meridional shears of equatorial zonal winds. A new Intraseasonal GPI (ISGPI) is proposed to quantify MJO’s modulation of TC genesis. The ISGPI significantly improves representation of intraseasonal variation of TC genesis in the tropics and in each sub-region of southern Indian Ocean, Australian monsoon and South Pacific. In the hot spots of southern hemisphere TC genesis zone, the probability of TC genesis can differ by a factor 5-19 due to MJO modulation. The results suggest that the large-scale factors controlling TC genesis may vary with different time scales and the climatological GPI may not be quite applicable for diagnoses of climate variability and future change of TC genesis potential. To simulate realistic impacts of MJO on TC genesis, the general circulation models must reproduce not only realistic eastward propagation but also the MJO low-level circulation structure. Application of the new ISGPI may have a large potential to improve dynamical sub-seasonal prediction of TC genesis.
Journal of Climate – American Meteorological Society
Published: Mar 2, 2017
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