Notes on extended–range atmospheric prediction in the Northern Hemisphere winter

We examined the characteristic feature and predictability of low frequency variability (LFV) of the atmosphere in the Northern Hemisphere winter (January and February) by using the empirical orthogonal functions (EOFs) of the geopotential height at 500 hPa. In the discussion, we used the EOFs for geostrophic zonal wind (Uznl) and the height deviation from the zonal mean (Zeddy). The set of EOFs for Uznl and Zeddy was denoted as Uznl−1, Uznl−2, …, Zeddy−1, Zeddy−2, …, respectively. We used the data samples of 396 pentads derived from 33 years of NMC, ECMWF and JMA analyses, from January 1963 to 1995. From the calculated scores for Uznl−1, Uznl−2, Zeddy-1, Zeddy−2 and so on we found that Uznl−1 and Zeddy−1 were statistically stable and their scores were more persistent than those of the other EOFs. A close relationship existed between the scores of Uznl−1 and those of Zeddy−1. 30-day forecast experiments were carried out with the medium resolution version of JMA global spectral model for 20 cases in January and February for the period of 1984–1992. Results showed that Zeddy−1 was more predictable than the other EOFs for Zeddy. Considering these results, we argued that prediction of the Zeddy−1 was to be one of the main target of extended-range forecasting.