www.elsevier.nl/locate/asr zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Adv. Space Res. Vol. 25, No. 5, pp. 953-958,ZOOO 0 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/00 $20.00 + 0.00 PH: SO273-1177(99)00928-X zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML MJO AND TROPICAL CYCLONE ACTIVITY DURING 1997/98 ENS0 Tetsuo Nakazawa zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Meteorological Research Institute, I -I Nagamine, Tsukuba, Zbaraki305, Japan ABSTRACT The purpose of this study is to examine the 30-60 day variation (or Madden-Julian Oscillation ;MJO) and tropical cyclone activity during the 1997198 ENSO. MJO was active during the onset phase of the 1997/98 ENSO, but weak and had a shorter period ( less than a month ) fluctuation during the mature and decaying phase of the last ENSO. By using thelO-year SSM/I Wentz dataset of wind speed and rainfall, we find two types of tropical cyclogenesis; low-level wind shear type and cloud cluster type. The former is characterized by strong meridional gradient of wind speed over the suppressed ITCZ in the northern hemisphere, mainly in equinox periods. The latter is related with the weak wind regime, mainly dominated in summer hemispheres. Special interest is that there was no tropical cyclone activity over the northwestern Pacific region in 1998 up to early July. The SSM/I data in May 1998 shows that the northern ITCZ was extremely weaker than nor- mal with stronger trade winds. This feature was related with both enhanced local Hadley circula- tion and Walker circulation. The former was characterized by the positive (negative) rainfall anomaly and positive (negative) humidity anomaly over the south central/eastern (north) Pacific; The latter was related with enhanced convective activity over the Indian Ocean. Although the higher SST over the western Pacific was favorable for the generation, tropical cyclones were not generated without any cyclonic disturbances originated from the ITCZ. INTRODUCTION 0 2000 COSPAR. Published by Elsevier Science Ltd. The 1997/98 ENS0 brought us severe disasters all over the globe, such as Indonesian forest fire, floods over west Africa and north America and tropical cyclones hit in south Pacific. The 1997198 ENS0 started in March 1997 over the western Pacific by the oceanic Kelvin wave, which was generated by the westerly wind burst event over the region and propagated to the eastern Pacific, and reached its mature stage in fall 1997. The westerly wind burst was associated with the MJO, which is an eastward propagating, planetary-scale, tropical convective system with a period of 1 to 2 months. Most of the previous studies focus on the importance of the westerly wind bursts, which are closely related with MJO on the onset of ENS0 (Nakazawa, 1998), but very few articles are available for the temporal and spatial variability of the MJO during the mature/decaying phase of ENSO. This study tries to demonstrate the characteristic feature of the convective behavior associ- ated with the last ENS0 during not only the onset period but also the mature/decaying period. Another topic in this paper is tropical cyclogenesis over the northwestern Pacific. There are several 953