* TYPHOON2000 INFORMATION ARTICLES * -------------------------------- "1998 SEASON: AN EXAMPLE OF A REDUCTION OF TROPICAL CYCLONES DURING POST-EL NINO YEAR OVER THE NORTHWEST PACIFIC" ========================================================================== - A Post-Article Discussion by Mark Lander (University of Guam & JTWC) and Prof. Johnny Chan (University of Hong Kong) ========================================================================== --> From Mark Lander to Johnny Chan: Johnny, It looks like you were right with a vengeance. This year's slow start of the western North Pacific TC season, with records for latest date of named storm, latest first typhoon, etc. fits your original claim (Chan 1985?) of a reduction of TCs following the El Nino year. In a paper I once wrote (Lander 1994), I could find no strong tendency for this, but I included all El Nino events from the very weak to the very strong. I showed that there was a statistically significant reduction in the number of TCs during the first half of the year following El Nino, but there was a tendency for an increase in the number of TCs in the second half of the year following El Nino (especially if the SST in the eastern equatorial Pacific became cooler than normal by the second half of the year): hence a compensatory effect to offset the early season losses. What seems to be killing our season is a westpac tropic-wide east wind anomaly which is not letting the monsoon trough to penetrate into the basin. No monsoon trough -- No TCs! Easterly wind flow in the tropics of the WNP are per se indicative of conditions unfavorable for TC development. Though we have had several tropical disturbances associated with TUTT cells, and other occasional flare-ups of large cloud clusters in the easterly flow south of 10 N, there has been no support for TC genesis. It could be that without the monsoon trough in-place, there is no source of environmental vorticity to aid TC genesis (with the exception of TC genesis at higher latitudes induced in low-level easterly flow by TUTT cells). Easterly flow seems to be conducive only to mesoscale cycling of areas of deep convection. For whatever reason, it seems that the monsoon trough is nearly a necessary condition for TC genesis in the western North Pacific. And where the monsoon trough is located, there will be found the TCs. I can't help but emphasize Jim Sadler's strong position against the existence of easterly waves. He may have been extreme in denying their existence, but in his support is the fact that when the winds in the tropics of the WNP are all easterly, no TCs form. Even if one calls the occasional wesward- propagating cloud cluster an easterly wave of some sort, they are not sources of TCs. Back to Johnny's claim of reduced TCs after El Nino, the mechanism is clear: ENSO-related low-level east wind anomalies in the tropics of the western North Pacific of sufficient magnitude to eliminate the monsoon trough. This is especially evident during the first half of the year. Regards, Mark L. ======================================================================== | | | Lander, Mark Alan | Email: mlander@uog.edu | | Land of typhoon and coconut| or lander@npmocw.navy.mil | | University of Guam | Voice: 671-349-5286 or 671-564-1351 | | Mangilao, Guam USA 96923 | Coordinates: 13.5N , 144.7E (far away) | | UOG Station | Time : GMT+10 EST+15 | | | ======================================================================== ___________________________________________________________________________ --> Johnny Chan's reply: Mark: I agree with you completely. In fact, as I have said before on a number of occasions, TC genesis in the western North Pacific (including the South China Sea) does NOT depend on thermodynamic conditions since they are generally satisfied. South of the subtropical ridge, the atmosphere in the summer is usually conditionally unstable, low to mid level moisture is abundant and the SST is ALWAYS above the threshold for genesis. This is why we see so many cloud clusters every year. The same is true this year. What determine whether a cluster will become a TC are the dynamic conditions, and in particular, the low-level shear. Because easterlies are generally present, what we need are westerlies to the equatorward side, which is conspicuously absent this year, especially east of the Philippines. That's why we have not had one TC forming east of 130E. While you and I know all about this, many closet scientists out there are still claiming that TC frequency will increase due to an increase in SST as a result of global warming. At least in the western North Pacific where over 30% of the TCs of the world form, such a claim is completely false and contrary to observations. It is the dynamics that counts. I will believe their claim of an increase in TC frequency if they can show me that global warming will increase the intensity of the monsoon trough in terms of the magnitude of the horizontal shear. On the subject of the forecast for this year, as I mentioned in my original message, while I know that for a post-El Nino year, TC frequency is likely to be below normal, the increasing long-term trend may offset such an effect. This is perhaps why our statistical model gives a close-to-normal forecast. However, since the model did not have enough cases in which a strong ENSO year is immediately followed by a La Nina year, its forecast might not be quite right. I presume the persistent easterlies this year are a result of the La Nina. If indeed the number of TCs this year is below normal, we will probably need to modify the prediction model. Johnny ********************************************************************** * Prof. Johnny C. L. Chan * * Dept. of Physics & Materials Science, City University of Hong Kong * * 83 Tat Chee Ave., Kowloon, Hong Kong, CHINA * * Tel: 852-2788-7820 Fax: 852-2788-7830 * * Email: Johnny.Chan@cityu.edu.hk * ********************************************************************** ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Special thanks goes to Mr. Gary Padgett for providing me this very informative article postings. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Published: 12:51 AM 8/21/98