Reexamination of the Climatology and Variability of the Northwest Pacific Monsoon Trough Using a Daily Index

Abstract

AbstractThis study developed a daily index to represent the northwest Pacific monsoon trough using westerly related cyclonic vorticity after removing tropical cyclones (TCs) from the reanalysis dataset. This index sufficiently captures the spatial and temporal variations in the monsoon trough. The use of this daily index revealed new features in the monsoon trough, including daily statistical characteristics, the active period over a year, and the main periodicity. A monsoon trough can be identified as active when the daily index is greater than 2.0 × 10−4 s−1. Active monsoon troughs occur during half of the summertime, and these is no monsoon trough on one-third of days, with the remaining days categorized as inactive. The most active month is August, in which approximately 20 days exhibit an active monsoon trough. Using this index, an active monsoon trough period, which is related to vigorous TC activity, was determined by identifying the establishment and decay dates for each year from 1979 to 2016. During most years, the active monsoon trough is established in mid-July and decays in late October, persisting for 3–5 months during the boreal summer. Moreover, spectral and wavelet analyses demonstrated the presence of intraseasonal, interannual, and interdecadal variabilities in the monsoon trough. The dominant periodicity for the interannual variability varied from 1.5 to 4 years in different decades. The relationship between the monsoon trough and TCs is also revealed using this index, showing that approximately 60% of TC formations were related to an active monsoon trough.