Cause for quasi-biweekly oscillation of zonal location of western Pacific subtropical high during boreal summer

Abstract

The cause for zonal oscillation of the western Pacific subtropical high (WPSH) on a quasi-biweekly (10–20-day) timescale was investigated. Both relative vorticity and geopotential height anomalies exhibit a maximum activity center over the western edge of the climatological WPSH. The quasi-biweekly WPSH oscillation is closely related to a northwestward-propagating quasi-biweekly oscillation (QBWO) mode over the western North Pacific (WNP). A real-time index of the WNP-QBWO is used to analyze the influence of the northwestward-propagating WNP-QBWO mode on the zonal shift of the WPSH. The QBWO convection anomaly propagates northwestward from the tropical WNP to East Asia. It is the alternation of a positive and a negative geopotential height anomaly during the northwestward propagating journey that leads to the zonal oscillation of the WPSH. Once a positive (negative) QBWO geopotential height propagates from tropical WNP into the western edge of the climatological WPSH, it leads to a western extension (eastern retreat) of the WPSH. This provides a monitoring and predicting method of the zonal shift of the WPSH at an extended rang by using the real-time index. Two mechanisms are likely responsible for the northwestward propagation of the WNP-QBWO mode. The first is internal atmospheric dynamics. Both positive vorticity and moisture anomalies appear to the northwest of the QBWO convection, favoring the northwestward propagation. The second is attributed to the atmosphere-ocean interaction. A warm sea surface temperature (SST) anomaly leads the QBWO convection by an approximately 90-degree phase, and both the SST and convection anomalies move northwestward.