Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 1 2010 10.5194/acpd-10-1957-2010 http://www.atmos-chem-phys-discuss.net/10/1957/2010/ http://www.atmos-chem-phys-discuss.net/10/1957/2010/acpd-10-1957-2010.html http://www.atmos-chem-phys-discuss.net/10/1957/2010/acpd-10-1957-2010.pdf 1957 1982 2010-01-25 The basic mechanism behind the hurricane-free warm tropical ocean Z. Yuan qianyk@mail2.sysu.edu.cn Y.-K. Qian J. Wu J. Qi Center of Monsoon and Environment, Sun Yat-sen University, Guangzhou 510275, China Department of Atmospheric Science, Sun Yat-sen University, Guangzhou 510275, China No hurricane is detected in the tropics off the Brazilian coast due to the lack of initial conditions (e.g., the weak vertical shear of horizontal wind) despite that high sea surface temperature is available. According to previous studies, the initial conditions (as the ingredients of hurricane's embryo) are related so that the thick warm-and-moist layer (due to the updraft vapour) below a cold-and-dry layer frames the convective instability which enhances diabatic processes accompanied by tropical cyclones with the weak vertical shear. So the basic question is how, starting with an internal-disturbance-free balance-situation, external forces create the rapidly-upward acceleration of moist air at the warm sea surface. The answer is revealed by the vertical-momentum equation which shows that boosted by the external-force-induced significant lower-layer equatorial westerly wind (LLEWW), the upward (unit-mass) acceleration could be as significant as the midlatitude Coriolis force. Besides creating cyclonic vortices through the upward acceleration and diabatic processes, the external-force-induced significant-LLEWW could directly create cyclonic wind shears along with easterly jets for the low-level cyclonic vorticity through reducing the peak value of zonally-homogeneous trade easterlies (centered at the Equator between the Northern and Southern Hemisphere subtropical high-belts). We emphasize external forces to avoid the ''chicken-and-egg'' problem accompanying nonlinear interactions of internal-forcing processes. The external-force-induced significant-LLEWW could result from the deflection of the cross-equatorial flow characterized by the seasonal shift coincident with that of locations of most embryos. 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