TROPICAL AND EXTRATROPICAL INTERACTION AND ITS IMPACT ON EXTENDED-RANGE FORECASTING .1. THE IMPACT OF SEA-SURFACE TEMPERATURE ANOMALIES

The impact of the sea surface temperature (SST) anomalies on predictio ns in the extratropics has been studied by comparing circulation chang es in general circulation model experiments generated with observed an d climatological sea surface temperatures for warm and cold Southern O scillation events. The small samples may be insufficient for drawing f irm conclusions, but results suggest that the linkage between tropical and extratropical circulations in the model resembles observed relati onships. As the atmosphere responds to the warm (cold) tropical SSTs, the convection in the Pacific intensifies (diminishes). The enhanced ( suppressed) convection in the tropics enhances (suppresses) the local Hadley circulation and changes the position and strength of the diverg ent outflow. This in turn changes the position, shape, and strength of the upper-level subtropical jet streams. After the jets move to their new positions, synoptic eddies organize themselves at the exit region s of the jets. The response time for the upper-level streamfunction in the tropics is about 10 days, but the changes in the position of the subtropical jets occur after 15-20 days. The largest impact on predict ions is located in the tropics and downstream in the Pacific-North Ame rica and the Pacific-South America regions.