The Formation and Evolution of the 1989 Western North Pacific Subtropical Gyre

In July 1989, during a La Nina event, a broad area of deep convection to the east of The Philippines evolved into a large cyclonic gyre that persisted for several days. The gyre exhibited an asymmetric convective structure, with activity concentrated on the southern and eastern sides and a relatively clear center. The gyre also maintained cyclonic winds at 850 hPa over a radius greater than 2000 km. In the period prior to the gyre's development, an active MJO was in place over the Indian Ocean and Indian subcontinent, followed by anticyclonic wave breaking associated with an upper tropospheric jet exit region over northeast Asia. The wave breaking left an area of upper tropospheric cyclonic vorticity in the subtropics, and also initiated the development of convection, which generated cyclonic vorticity in the lower troposphere. The interaction of the upper and lower vorticity centers created conditions favorable for increased convection and a growth and intensification of the low-level vorticity center into a large gyre. As the gyre matured, azimuthally-averaged tangential winds and circulation increased. Two tropical cyclones developed within the asymmetric convection of the gyre. The active MJO and negative ENSO phases, as well as the July 1989 gyre's structure, bear strong similarity to a previous studied gyre in July 1988 by Molinari and Vollaro (2011). The evolution of the structure of the gyre, as well as the details regarding its initial formation, will be discussed.