ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-1957-2010 The basic mechanism behind the hurricane-free warm tropical ocean Yuan Z. 1 2 Qian Y.-K. 1 2 Wu J. 1 2 Qi J. 1 2 Center of Monsoon and Environment, Sun Yat-sen University, Guangzhou 510275, China Department of Atmospheric Science, Sun Yat-sen University, Guangzhou 510275, China 25 01 2010 10 1 1957 1982 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/1957/2010/acpd-10-1957-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/1957/2010/acpd-10-1957-2010.pdf

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. This significant cross-equatorial flow is driven by the significant differential heating between the largest continent with the highest plateau and the largest ocean with the warm pool located to the east and on the equatorward side of the continent on the rotating Earth. Unfortunately, in the tropics off the Brazilian coast, the differential heating is weak between the relatively-small ocean and land mostly covered by tropical rainforest. No significant-LLEWW means no hurricane's embryo. A warm spawning ground without the embryo means no hurricane. Our investigation suggests that the external-force-induced significant-LLEWW embedded in the significant trade easterlies over the warm ocean be necessary and sufficient for making the embryo originate in an internal-disturbance-free balance-situation.

 References Anthes,~R A.: Tropical cyclones: Their Evolution, Structure and Effects, American Meteorological Society, Ephrata, PA, 208~pp., 1982. Chan,~J C L.: Comment on \qutChanges in tropical cyclone number, duration, and intensity in a~warming environment, Science, 311, 1713b, doi:10.1126/science.1121522, 2006. Charney,~J G. and Eliassen,~A.: On the growth of the hurricane depression, J. Atmos. Sci., 21, 68–75, 1964. Dunkerton,~T J., Montgomery,~M T., and Wang,~Z.: Tropical cyclogenesis in a~tropical wave critical layer: easterly waves, Atmos. Chem. Phys., 9, 5587–5646, 2009. Emanuel, K. A.: The physics of tropical cyclogenesis over the eastern Pacific, in: Tropical Cyclone Disasters: Proceedings of ICSU/WMO International Symposium, edited by: Lighthill, J. Zhemin, Z., Holland, G. J., and Emanuel, K., Peking University Press, Beijing, 136–142, 1993. Emanuel,~K A.: Thermodynamic control of hurricane intensity, Nature, 401, 665–669, 1999. Emanuel, K. A.: The hurricane embryo. Talk presented at short program workshop entitled Small scales and extreme events: The Hurricane, NSF Institute for Pure and Applied Mathematics (IPAM), UCLA, Los Angeles, 2007. Gill,~A E.: Some simple solutions for heat-induced tropical circulation, Q J. Roy. Meteor. Soc., 106, 447–462, 1980. Gray,~W M.: Global view of the origin of tropical disturbances and storms, Mon. Weather Rev., 96, 669–700, 1968. Gray, W. M.: Hurricanes: Their formation, structure, and likely role in the tropical circulation, in: Meteorology over the tropical oceans, edited by: Shaw, D. B., Royal Meteorological Society, Bracknell, 155–218, 1979. Gunn,~B W., McBride,~J L., Holland,~G J., Keenan,~T D., and Davidson,~N E.: The Australian summer monsoon circulation during AMEX phase~II, Mon. Weather Rev., 117, 2554–2574, 1989. Hartmann,~D L. and Hendon,~H H.: Resolving an atmospheric enigma, Science, 318, 1731–1732, 2007. Holton, J. R.: An introduction to dynamic meteorology, 2nd ed., Academic Press, New York, 391 pp., 1979. Holton,~J R.: An Introduction to Dynamic Meteorology, 4th ed., International Geophysics Series, Elsevier Academic Press, New York, 535~pp., 2004. Klotzbach,~P J.: Trends in global tropical cyclone activity over the past twenty years (1986–2005), Geophys. Res. Lett., 33, L10805, \doi10.1029/2006GL025881, 2006. Matsuno,~T.: Quasi-geostrophic motions in the equatorial area, J. Meteorol. Soc. Jpn., 44, 25–43, 1966. McTaggart-Cowan,~R., Bosart,~L F., Davis,~C A., Atallah,~E H., Gyakum,~J R., and Emanuel,~K A.: Analysis of Hurricane Catarina (2004), Mon. Weather Rev., 134, 3029–3053, 2006. Montgomery,~M T., Nicholls,~M E., Cram,~T A., and Saunders,~A B.: A~vortical hot tower route to tropical cyclogenesis, J. Atmos. Sci., 63, 355–386, 2006. Ooyama,~K.: Numerical simulation of the life cycle of tropical cyclones, J. Atmos. Sci., 26, 3–40, 1969. Pedlosky, J.: Geophysical fluid dynamics, 2nd ed., Springer, New York, 710 pp., 1987. Trenberth,~K E., Branstator,~G W., Karoly,~D., Kumar,~A., Lau,~N.-C., and Ropelewski,~C.: Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures, J. Geophys. Res., 103, 14291–14324, 1998. Trenberth,~K.: Climate change – warmer oceans, stronger hurricanes, Sci. Am., 297, 44–51, 2007. Wu,~P. and Chu,~P.-S.: Characteristics of tropical cyclone activity over the eastern North Pacific: the extremely active 1992 and the inactive 1977, Tellus, 59A, 444–454, 2007.