The Influence of ENSO on the Development of Tropical Cloud Clusters in the Western North Pacific

El Niño/Southern Oscillation (ENSO) would affect tropical cyclone (TC) activities in the western North Pacific (WNP). In Niño years, TCs tend to form farther to the east together with the eastward extension of the monsoon trough. The number of TCs in the eastern part of the WNP increases. As a result, the TCs in Niño years often travel a longer distance before making landfall or recurve and have a greater intensity than in normal years. In Niña years, the opposite situation occurs. However, most of previous studies focus on the systems from the time when the first record of TC data is available. Therefore, these studies discuss the ENSO influence on TC starting from tropical depressions (TD) or tropical storms (TS) intensity. To fully understand the ENSO effect on TC formation, the effect of ENSO on pre-TC disturbances needs to be addressed.

This study analyzes the influence of ENSO on tropical cloud clusters (TCCs) and TC formation in the WNP. TCCs are the precursors of TCs, which have an outside convective structure, contain mesoscale convection systems (MCSs) and last for more than 24 hours. In the WNP, a total of 2,248 TCCs are identified using infrared images, during July-October in 1981-2009. The analysis of this information shows that the numbers and activities of TCCs increase (decrease) in the eastern (western) part of the WNP during El Niño years, whereas the opposite situation occurs during La Niña years. However, the TC genesis productivities do not correlate significantly with ENSO index in the sub-regions of the WNP. The reason why the number of TCs changes in conjunction with the influence of ENSO in the sub-regions of WNP is because the activity of TCCs is affected by the environmental parameters which are controlled by the ENSO signal rather than by the variations in the TC genesis productivity. The environmental conditions of the developing and non-developing TCCs are also analyzed in this study to address the favorable conditions for the development of TCC.