PRINCIPAL MODES OF OUTGOING LONGWAVE RADIATION AND 250-MB CIRCULATIONFOR THE SOUTH-AMERICAN SECTOR

Principal modes of anomalous upper-tropospheric circulation and outgoi ng longwave radiation (OLR) are determined for the South American sect or. A combined EOF analysis is performed using anomalous 250-mb zonal and meridional wind components plus anomalous OLR. The data are filter ed so that modes on both intraseasonal and interannual timescales can be studied separately. The resulting patterns show a consistent relati onship between anomalous OLR and anomalous upper-tropospheric circulat ion features. The first and second interannual modes contain many atmo spheric features related to extremes in the Southern Oscillation. Thes e include 1) variations in the pattern of tropical convection, and 2) changes in the intensity of upper-tropospheric zonal flow in the equat orial band and in the subtropics. The first intraseasonal mode has its greatest loadings in the Tropics and is most active during the southe rn summer season. For positive (negative) amplitudes of this mode, ano malous upper-tropospheric westerly (easterly) flow dominates the Tropi cs throughout the region, and positive (negative) OLR anomalies are ob served over northeastern South America and over the eastern equatorial Pacific. These features are associated with the 30-60 day (Madden-Jul ian) oscillation. An extended combined EOF analysis was performed to s tudy the evolution of intraseasonal oscillations in the South American sector. The first rotated combined extended EOF mode describes a sequ ence of patterns in which anomalous equatorial westerlies and positive OLR anomalies over tropical Brazil gradually weaken and then reverse. The largest loadings throughout the evolution of the patterns contain ed in the first mode remain in the Tropics and subtropics. These resul ts provide information on the combined relationship between atmospheri c circulation and convection over the South American sector and on the evolution of patterns associated with 30-60 day (intraseasonal) oscil lations. By projecting the combined fields of anomalous OLR and 250-mb zonal and meridional wind onto the individual patterns contained in t he first rotated combined extended EOF mode, one obtains indices that should enhance real-time monitoring efforts and may lead to improved m onthly forecasts for the South American sector.