Numerous definitions of atmospheric blocking exist in the literature and all involve a level of subjectivity. We use the blocking index of Tibaldi and Molteni (1990) modified from that of Lejenas and Okland (1983). The index is local and instantaneous, ideally suited for real-time atmospheric monitoring, isolating regions of easterly flow at 500 hPa associated with high-latitude blocks.

Below we present the index calculation for the Northern Hemisphere, however the index can be easily modified to examine blocking in the Southern Hemisphere.

References:

Lejenas, H., and H. Okland, 1983: Characteristics of Northern Hemisphere blocking as determined from a long time series of observational data. Tellus , 35A, 350-362.

Tibaldi, S., and F. Molteni, 1990: On the operational predictability of blocking. Tellus , 42A, 343-365.