Decadal covariability of the northern wintertime land surface temperature and atmospheric circulation

The decadal covariability of northern wintertime land surface temperature and 500-hPa geopotential anomalies is examined using NCEP and 20C reanalyses over the 20th century and a 996-year pre-industrial climate simulation from the Canadian Earth System Model. The climate trend change around 1990 is related to the leading modes of decadal covariability and the influence of synoptic eddies on the leading modes is diagnosed.

Based on the reanalysis data, the covariability is dominated by two leading MCA (maximum covariance analysis) modes. MCA1 is characterized by temperature anomalies over most of Canada, the eastern U.S., Mexico, and Eurasian mid-high latitudes, accompanied by anomalies of opposite sign elsewhere over northern landmasses. MCA2 features temperature anomalies over most of North America, Eurasia and Greenland, with opposite anomalies elsewhere. In the upper troposphere the synoptic vorticity fluxes reinforce the anomalous circulation, while in the lower troposphere advection by the anomalous mean flow offsets the eddy forcing and maintains the decadal temperature perturbation. The MCA1 associated variability has a broad spectrum over decadal-interdecadal timescales, while the MCA2 related variability has a significant power peak around 20 years. The change of temperature and geopotential trends around 1990 tends to be a decadal-scale shift in winter and has significant features of the leading mode of the decadal covariability. The climate model has broadly similar decadal covariability, including the leading MCA patterns as well as the temporal evolution of the patterns.