Tropical—Extratropical Interaction Associated with the 10—25-day Oscillation over the Western Pacific during the Northern Summer

Ten years (1986-1995) of ECMWF analyses and NOAA outgoing longwave radiation (OLR) data were used to investigate intraseasonal tropical-extratropical interactions on timescales of 10-25 days in the western Pacific region during Northern Hemisphere (NH) summer. Composite analyses were performed separately for the early summer (June-July) and mid-summer (August). The analyses were based on the OLR index in the base region (the South China Sea), and help describe the seasonal characteristics of circulation associated with tropical convection. Localized Eliassen-Palm (E-P) fluxes were used to diagnose eddy–mean flow interactions. Composite circulation patterns associated with enhanced convection on a 10-25-day timescale for June-July were significantly different from those for August. June-July composites show a significant Rossby wave train in both the lower and upper troposphere, extending from the convective region into the North Pacific along the mean westerly flow. In the lower troposphere, subtropical circulation anomalies as part of this wavetrain propagate southwestward along the subtropical duct. Tropical-extratropical linkage is less pronounced during August. Circulation structures in August in both the lower and upper troposphere are different from those of June-July; Rossby waves do not emanate from the South China Sea and low-level subtropical circulation anomalies show no southwestward phase propagation. Energy conversion between eddy and seasonal mean flow for each season was computed to investigate the role of the time-mean flow in maintaining the 10-25-day transient eddies. In the lower troposphere, barotropic processes dominate the conversion from the time-mean flow to eddies in the exit region of tropical monsoon westerlies for both June-July and August. In the subtropics, baroclinic processes sustain the eddies. Moist baroclinic processes associated with eddy horizontal latent heat transport are important in increasing the potential energy available to eddies, especially during June-July. In the upper troposphere, barotropic conversion of kinetic energy from eddies into the time-mean flow is most notable in the East Asian subtropics to the north of the South China Sea. This suggests that increased eddy activity due to tropical convection is an important source of energy for the mean flow.