Forcing Patterns of Northern Hemisphere Low-Frequency Flow

This study examines the spatial and temporal characteristics of the dominant forcing patterns of low-frequency variability by using 40 years of the National Center for Environmental Prediction-National Center of Atmospheric Research (NCEP-NCAR) reanalysis data set for the winter season. These forcing patterns are determined as the leading patterns of each term in the streamfunction tendency equation. Some of the leading forcing patterns are found to induce and drive well known teleconnection patterns such as the Pacific-North America (PNA) and the North Atlantic Oscillation (NAO).

As dominant forcing mechanisms are found barotropic conversion from the zonally asymmetric climatological flow and both high- and low-frequency transient eddy fluxes. It has been found that several forcing patterns of a particular forcing process are forcing particular teleconnection patterns. The PNA is primarily forced by barotropic conversion from the zonally asymmetric climatological flow and is also related to several forcing patterns of the Rossby wave source, which are preceded by outgoing longwave radiation anomalies, indicating an tropical excitation mechanism of the PNA. The NAO is primarily driven by several forcing patterns of both high- and low-frequency transient eddy forcing fluxes. This suggests that in order to become dominant teleconnection patterns these teleconnection patterns have to be forced by several forcing patterns.

The forcing patterns have autocorrelation time scales of about 5-8 days, indicating that these forcing patterns cannot be simply regarded as being white noise.