Anomalies of the South American summer monsoon associated with the 1997–99 El Niño–southern oscillation

Abstract

AbstractWe describe the rainfall and circulation anomalies of the South American summer monsoon (SASM) during December–January–February (DJF) of 1997–98 (El Niño) and 1998–99 (La Niña). The most pronounced rainfall signals in DJF 1997–98 include (a) excessive rainfall over northern Peru and Ecuador, (b) deficient rainfall over northern and central Brazil, and (c) above‐normal rainfall over southeastern subtropical South America. The rainfall anomalies in (a) and (b) are associated with the excitation of an anomalous east–west overturning cell with rising motion and low‐level westerlies over the equatorial eastern Pacific, coupled to sinking motion and low‐level easterlies over northern Brazil. The easterlies turn sharply southeastward on encountering the steep topography of the Andes, enhancing the summertime low‐level jet (LLJ) along the eastern foothills of the Andes near 15–20° S, possibly contributing to the increased rainfall in (c).During DJF 1997–98, the sea‐surface temperature‐induced warming spreads and expands over the entire tropical troposphere. The eastward expansion of a warm upper tropospheric geopotential and temperature ridge from the Niño‐3 region, across subtropical South America to the southeast Atlantic, enhances warming over the Altiplano Plateau, hydrostatically strengthening the Bolivia high. Similar to previous warming events, the South Pacific high is weakened, and the South Atlantic high is strengthened. During DJF 1998–99, as cold water develops over the equatorial central Pacific, the SASM anomalies in the tropics are weaker and less organized and appear to be in transition to the opposite phase to those found in DJF 97–98. In the subtropics, notable features include a weakening of the LLJ, a rainfall pattern associated with a poleward shift of the South Atlantic convergence zone, and development of the Pacific–South America teleconnection pattern. Published in 2003 by John Wiley & Sons, Ltd.