Interdecadal and Interannual Variability in the Northern Extratropical Circulation Simulated with the JMA Global Model. Part II: Summertime Leading Mode

Abstract

Interdecadal and interannual) variations of a model atmosphere in the northern 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 experiment is found to he dominated by interdecadal variability. This mode shows 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 variability, exhibits a mixed pattern with the wintertime PNA mode and the summertime zonally elongated mode, whereas the zonally elongated pattern like the summertime EOFI cannot be found in northern fall. From an investigation based on the seasonality of tropical response of the model atmosphere, 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 tropical response in fall contributes largely to the absence of the zonally elongated mode with definite interdecadal variability in this season. The regional and tempers features of the observed decadal surface air temperature anomalies are well simulated by the real SST experiment. The time sequence of the above summertime EOFI, which accounts for a strong dependence of tropical atmosphere to SST anomaliess, is found to coincide well with the summertime mean hemispheric land surface air temperature. It is inferred, therefore, that the tropical SSTs of interdecadal variability contribute a great deal to the decrease and increase in the Northern Hemispheric land surface temperature observed in recent decades.