SUPPRESSED TROPICAL CYCLONE FORMATION IN NORTHWEST PACIFIC IN 1998

Tropical cyclone formation was markedly suppressed in the northwestern Pacific during 1998. To explore this, we contrast monthly mean fields for January, March, May, and July 1998 with climatologies, and with what is known about the preferred regions and mechanisms for tropical cyclogenesis by season. The atmospheric variables of interest include, surface wind speed, rainfall, outgoing longwave radiation, total precipitable water, sea surface temperature (SST), and upper level velocity potential. Prior to June 1998, the El Ni–o/Southern Oscillation (ENSO) warm event of 1997-98 dominated the atmospheric circulation in the tropical Indian and Pacific Ocean basins. A rising motion anomaly was evident just south of the equator in the eastern Pacific. Consistent with this anomaly were increased convective activity, wetter than normal air, and higher than normal SST. A center of anomalous subsidence was located over the western Pacific for the same time period. Consistent with that anomaly were suppressed convective activity, and drier than normal air. The rising and subsiding anomaly pair defines a shift in the local Walker circulation that is a signature of an ENSO warm event. This paper argues that suppressed tropical cyclone formation is another effect of this signature. By July 1998 the ENSO warm event had terminated and the center of rising motion had shifted westward to the Bay of Bengal in contrast to the climatological position in the western Pacific, east of the Philippines. This resulted in a region in the southeastern Indian Ocean of anomalous rising motion, increased convective activity, wetter than normal air, and higher SST. Compensating anomalous subsidence was again located over the western Pacific in a region that is normally favorable for tropical cyclogenesis. Thus, even though the effects of the ENSO warm event had abated, and even though SST in the western Pacific were anomalously high, the anomalies in the vertical motion of the large-scale circulation were sufficient to suppress tropical cyclone formation.