THE PACIFIC-SOUTH AMERICAN MODES AND TROPICAL CONVECTION DURING THE SOUTHERN-HEMISPHERE WINTER

Atmospheric circulation features and convection patterns associated wi th two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP-NCAR and NAS A-DAO. The two leading modes, referred to as the Pacific-South America n (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenu mber 3 in midlatitudes with large amplitudes in the Pacific-South Amer ican sector. In the Pacific, anomalies in the subtropics and in the mi dlatitudes are opposite in phase. Taken together, the two PSA modes re present the intraseasonal oscillation in the SH with periods of roughl y 40 days. The evolution of the PSA modes shows a coherent eastward pr opagation.A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associa ted with the PSA modes. Outgoing longwave radiation (OLR) anomaly comp osites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OL RA show an east-west dipole structure roughly 5-10 days prior to the o nset of persistent PSA events. The PSA 1 mode is associated with enhan ced convection in the Pacific between 140 degrees E and 170 degrees W and suppressed convection over the Indian Ocean. The PSA 2 mode is lin ked to tropical heating anomalies in the central Pacific extending fro m 160 degrees E to 150 degrees W just south of the equator and suppres sed convection in the western Pacific with a maximum at 20 degrees N. Contributions are from both interannual and intraseasonal bands.