Characteristic features of recent abrupt changes in winter circulation revealed by a general circulation model

Two noticeable abrupt changes in Northern Hemispheric winter circulation have occurred in recent decades, one around 1977 and the other around 1989. In the 1970s occurrence, changes in equatorial sea surface temperatures (SSTs) occurred concurrently with those in mid-latitude atmospheric circulation. In the case of the late 1980s, however, no important variation in SSTs was seen in the equatorial regions. In order to identify the differences in the characteristics of the changes in circulation between the two cases, observed 500 hPa heights were compared to simulations with an atmospheric general circulation model (GCM) that used observed SSTs as surface boundary conditions. The results of simulation suggest that the change in circulation in the late 1980s was not the direct response to the changes in SSTs, but can rather be characterized as a triggered planetary scale internal mode of variability in the winter atmosphere.