INTERDECADAL AND INTERANNUAL VARIABILITY IN THE NORTHERN EXTRATROPICAL CIRCULATION SIMULATED WITH THE JMA GLOBAL-MODEL .2. SUMMERTIME LEADING MODE

Interdecadal and interannual variations of a model atmosphere in the n orthern extratropics is examined using a T42 GCM forced with observed near-global SSTs from January 1955 to December 1988. The leading mode of summertime 500-hPa height field deduced from the real SST experimen t is found to be dominated by interdecadal variability. This mode show s a zonally elongated pattern with prominent loadings in low-latitude regions and accounts for an increase of the zonal, summertime 500-hPa heights in subtropical regions from the 1970s to the 1980s. Simulated springtime leading mode, which is dominated by interdecadal variabilit y, exhibits a mixed pattern with the wintertime PNA mode and the summe rtime zonally elongated mode, whereas the zonally elongated pattern li ke the summertime EOF1 cannot be found in northern fall. From an inves tigation based on the seasonality of tropical response of the model at mosphere, it is found that the summertime and springtime leading modes with a pronounced zonally symmetric component depend largely upon the tropical SST anomalies of interdecadal variability. The weakness of t ropical response in fall contributes largely to the absence of the zon ally elongated mode with definite interdecadal variability in this sea son. The regional and temporal features of the observed decadal surfac e air temperature anomalies are well simulated by the real SST experim ent. The time sequence of the above summertime EOFI, which accounts fo r a strong dependence of tropical atmosphere to SST anomalies, is foun d to coincide well with the summertime mean hemispheric land surface a ir temperature. It is inferred, therefore, that the tropical SSTs of i nterdecadal variability contribute a great deal to the decrease and in crease in the Northern Hemispheric land surface temperature observed i n recent decades.