West Pacific subtropical high double ridges and intraseasonal variability of the South China Sea summer monsoon

In this study, the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data from 1979 to 2005 is used to investigate the possible impact of IntraSeasonal Oscillation (ISO) of the South China Sea (SCS) monsoon on the West Pacific Subtropical High (WPSH) double ridges. Three WPSH modes are defined as the 3/6 mode, the 1/2 mode and the dual mode, for the amplitude of the 30–60-day oscillation of SCS summer monsoon which is larger, smaller and comparable to the 10–20-day oscillation, respectively. The results show that there are no double ridges of WPSH present during 3/6 mode, but a weak process can be found during 1/2 mode. However, a powerful double-ridge structure of WPSH is present in several phases of the dual mode during both the 10–20-day oscillation and the 30–60-day oscillation. Then two typical WPSH double ridge cases in 1999, a special year of the dual mode, are chosen to further discuss this interesting phenomenon. Case 1 (24 July–27 July) is much weaker, and in this case the southern ridge disappears after several days, while during case 2 (3 August–11 August), the southern ridge finally replaces the northern ridge. The double ridges are much stronger compared to case 1. The ISO evolution feature is different between case 1 and case 2. The anomalous circulation of 10–20-day oscillation is anticyclonic over the southern ridge during both case 1 and case 2. However, the anomalous circulation of 30–60-day oscillation is cyclonic during case 1 and anticyclonic during case 2. It is this difference that leads to the double ridges being more powerful in case 2 than in case 1. This indicates that the 10–20-day oscillation of the SCS summer monsoon plays a key role in the WPSH double-ridge formation, while the 30–60-day oscillation provides a favorable background for it.