Tropical cyclone formation was markedly suppressed in the western North Pac
ific during 1998. To explore this, we contrast monthly mean fields for Janu
ary, MaJI, and July 1998 with climatologies, and with what is known about t
he preferred regions and mechanisms for tropical cyclogenesis by season. Th
e atmospheric variables of interest include, surface wind speed, rainfall,
outgoing longwave radiation (OLR), total precipitable water (TPW), sea surf
ace temperature (SST), and upper level velocity potential. Prior to June 19
98, the El Nino/Southern Oscillation (ENSO) warm event of 1997-98 dominated
the atmospheric circulation in the tropical Indian and Pacific Ocean basin
s. A rising motion anomaly was evident just south of the equator in the eas
tern Pacific. Consistent with this anomaly were increased convective activi
ty, wetter than normal air, and higher than normal SST. A center of anomalo
us subsidence was located over the western Pacific for the same time period
. Consistent with that anomaly were suppressed convective activity, and dri
er than normal air. The rising and subsiding anomaly pair defines a shift i
n the Walker circulation that is a signature of an ENSO warm event. This pa
per argues that suppressed tropical cyclone formation over the western Nort
h Pacific is another effect of this signature.
By July 1998 the ENSO warm event had terminated and the center of rising mo
tion had shifted westward to the Bay of Bengal in contrast to the climatolo
gical position in the western Pacific, east of the Philippines. This result
ed 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 Pacifi
c in a region that is normally favorable for tropical cyclogenesis. Thus, e
ven though the effects of the ENSO warm event had abated. and even though t
he SST in the western Pacific were above the climatological SST, the anomal
ies in the vertical motion of the large-scale circulation were sufficient t
o suppress tropical cyclone formation.