TROPICAL CONVECTION ON 7-21-DAY TIMESCALES OVER THE WESTERN PACIFIC

Evidence is presented to demonstrate that the oscillations of convecti on on 7-21-day timescales are an important component of the intraseaso nal variability over the region spanning the equatorial western Pacifi c to the subtropical South Pacific. In that area of the world, these o scillations are largely confined to regions with high sea surface temp eratures (SSTs) or SST gradients. Consequently, the patterns of 7-21-d ay variability of convection undergo significant changes, as the El Ni no/Southern Oscillation reconfigures the distributions of SST. A test is developed that detects episodes in which the 7-21-day oscillation o f outgoing longwave radiation (OLR) is particularly well defined for s everal cycles. Applying this test, 29 episodes of high 7-21-day variab ility were defined. Based on this information, the annual and longitud inal distribution of 7-21-day variability is discussed. The 7-21-day o scillations of convection found at subtropical southern latitudes tend to have stronger wind shear in the vertical column than oscillations detected in the equatorial Tropics. Vertical motion maxima were genera lly found at lower levels of the atmosphere in the subtropical episode s than in those found along the equator. As predicted by other studies , the subtropical latitude cases appear to be caused by the passage of a series of baroclinic waves. Two of the 29 episodes are described in detail. The atmospheric state is composited with respect to the activ e and inactive phases of the 7-21-day oscillation of OLR. Contrasting events when the OLR values are low and high reveals patterns of circul ation features both upstream and downstream from the convection. Compo site profiles of vertical velocity and horizontal divergence, as well as maps of divergence and geopotential height anomalies at 200 hPa, we re consistent with an atmosphere that had alternately enhanced and sup pressed convective activity.