The Pacific-South American modes and tropical convection during the Southern Hemisphere winter

Abstract

Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP-NCAR and NASA-DAO. The two leading modes, referred to as the Pacific-South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific-South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation. A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east-west dipole structure roughly 5-10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.