Monsoon gyres over the western North Pacific in mid-summer on monthly and synoptic scales

This study addresses the issues of why monsoon gyres (MGs) tend to appear over the western North Pacific in mid-summer and how tropical cyclone (TC) activity characterizes the MGs in terms of monthly and synoptic scales. On a monthly scale, a low-level anomalous cyclonic circulation to the south of Japan is well correlated with enhanced tropical convection to the southeast in active MG years, indicating a signature of the Rossby wave response to convective heating. The monthly mean features of MGs can be interpreted as a superposition of the climatological monsoon trough and the low-level cyclonic circulation anomaly due to the Rossby wave response. In addition, TCs tend to occur in the southern and eastern periphery of the MG in active MG years, suggesting a possible interaction between the MG and TCs. On a synoptic scale, the outer circulations of individual TCs accompany the moisture conveyor belt (MCB) and have the potential to induce MGs. Furthermore, the replacement of the outer circulation due to multiple TCs occurs repeatedly through the development of the MCB. Such replacement contributes to prolonging the life span of MGs, although the outer circulation of a single TC hardly maintains an MG for more than a week. It is found that synoptic-scale MGs are the collective manifestation of individual TCs and their outer circulations, while the characteristics of the Rossby wave response to TC-related heating become increasingly evident in MGs on longer timescales. These multiple-time-scale features are the essence of the MG. • The concept of a monsoon gyre (MG) is characterized by two different aspects. • Synoptic-scale MGs are the collective manifestation of TCs' outer circulations. • The signature of the Rossby wave response is apparent on longer timescales. • The moisture conveyor belt facilitates the replacement of a TC's outer circulation.