A Subtropical Cyclonic Gyre Associated with Interactions of the MJO and the Midlatitude Jet

This paper describes a large cyclonic gyre that lasted several days in the northwest Pacific during July 1988. Cyclonic winds at 850 hPa extended beyond the 2000-km radius with a radius of maximum winds of 700– 800 km. The gyre exhibited clear skies within and north of its center. Active convection extended 4000 km in longitude to its south. The Madden–Julian oscillation (MJO) was in its active phase in the Indian Ocean prior to gyre formation. Consistent with earlier studies, diabatic heating in the MJO was associated with an anomalous uppertropospheric westerly jet over the northeast Asian coast and a jet exit region over the northwest Pacific. Repeated equatorward wave-breaking events developed downwind of the jet exit region. One such event left behind a region of lower-tropospheric cyclonic vorticity and convection in the subtropics that played a key role in the gyre formation. A second wave-breaking event produced strong subsidence north of the mature gyre that contributed to its convective asymmetry. Gyres from 1985 and 1989 were compared to the 1988 case. All three gyres developed during an active MJO in the Indian Ocean. Each gyre displayed the same strong convective asymmetry. Each developed in July or August during the climatological peak in breaking Rossby waves in the northwest Pacific. Finally, all of the gyres developed during La Niña at nearly the same location. This location and the convective structure of the gyres closely matched composite La Niña anomalies during boreal summer.