Principal Modes of Outgoing Longwave Radiation and 250-mb Circulation for the South American Sector

Abstract

Principal modes of anomalous upper-tropospheric circulation and outgoing longwave radiation (OLR) are determined for the South American sector. A combined EOF analysis is performed using anomalous 250-mb zonal and meridional wind components plus anomalous OLR. The data are filtered so that modes on both intraseasonal and interannual timescales can be studied separately. The resulting patterns show a consistent relationship between anomalous OLR and anomalous upper-tropospheric circulation features. The first and second interannual modes contain many atmospheric features related to extremes in the Southern Oscillation. These include 1) variations in the pattern of tropical convection, and 2) changes in the intensity of upper-tropospheric zonal flow in the equatorial band and in the subtropics. The first intraseasonal mode has its greatest loadings in the Tropics and is most active during the southern summer season. For positive (negative) amplitudes of this mode, anomalous upper-tropospheric westerly (easterly) flow dominates the Tropics throughout the region, and positive (negative) OLR anomalies are observed over northeastern South America and over the eastern equatorial Pacific. These features are associated with the 30?60 day (Madden-Julian) oscillation. An extended combined EOF analysis was performed to study the evolution of intraseasonal oscillations in the South American sector. The first rotated combined extended EOF mode describes a sequence 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 contained in the first mode remain in the Tropics and subtropics. These results provide information on the combined relationship between atmospheric circulation and convection over the South American sector and on the evolution of patterns associated with 30?60 day (intraseasonal) oscillations. By projecting the combined fields of anomalous OLR and 250-mb zonal and meridional wind onto the individual patterns contained in the first rotated combined extended EOF mode, one obtains indices that should enhance real-time monitoring efforts and may lead to improved monthly forecasts for the South American sector.