Patterns and factors of interannual variations of boreal summer intraseasonal oscillation intensity over tropical western North Pacific

Abstract

Present study analyzes spatial patterns and factors of interannual variations of 10–20-day and 30–60-day intraseasonal oscillation (ISO) intensity during boreal summer over tropical western North Pacific (WNP). The three leading modes of 10–20-day ISO intensity variation feature a southeast–northwest distribution, a southeast–northwest contrasting pattern, and large loading over the subtropics, respectively. The two leading modes of 30–60-day ISO intensity variation show a west–east distribution and a west–east dipole pattern, respectively. Three types of sea surface temperature (SST) influences are identified on the ISO intensity variation. One is a direct influence of El Nino–Southern Oscillation (ENSO) on seasonal mean background over tropical WNP via a Rossby wave type response. This type of influence is prominent on the leading mode of 10–20-day ISO intensity and the eastern portion of the second mode of both 10–20-day and 30–60-day ISO intensity. Second is an indirect influence of ENSO on seasonal mean background over subtropical WNP and the Maritime Continent through anomalous Walker circulation. This type of influence includes the western portion of the second mode of both 10–20-day and 30–60-day ISO intensity. Third is an influence of regional SST anomalies. Opposite SST anomalies in the North Indian Ocean and tropical North Pacific work together to modulate seasonal mean background over tropical WNP. This type of influence is prominent on the leading mode of 30–60-day ISO intensity. In addition, internal atmospheric process may play a role in 10–20-day ISO intensity change over subtropical WNP.