MJO and tropical cyclone activity during 1997/98 ENSO Original Research Article

The purpose of this study is to examine the 30–60 day variation (or Madden-Julian Oscillation ;MJO) and tropical cyclone activity during the 1997/98 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 the 10-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 normal with stronger trade winds. This feature was related with both enhanced local Hadley circulation 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.