The Spatio—Temporal Variation of Pacific Blocking Frequency within Winter Months and Its Relationship with Surface Air Temperature

Abstract

The spatio–temporal evolution of the Pacific blocking frequency (PBF) that is based on a two–dimensional blocking index is investigated during the recent 40–winter (1979/80–2018/19) months (December–January–February). It is found that maximum PBF appears in January within the key area of 140° E–160° W, 50°–70° N. The key–area Pacific blocking in January is more active during the first (1980–1988) and the third (2009–2019) periods than during the second period (1989–2008). There is a positive 500 hPa–geopotential height (Z500) anomaly over the mid–latitude Pacific and a negative one over the high latitude area between the first two periods (second minus first). This pattern can cause an anomalous westerly circulation over the mid–high Pacific sector, which indicates a weakening of the Pacific blocking activity during the second period. This connects to a positive two–meter air temperature (T2m) anomaly over the northeastern Asia and mid–western Pacific, and a negative one over the high–latitude area. The difference of Z500 between the third and the second periods (third minus first) is opposite to that between the second and the first periods, which leads to more Pacific blocking events during the third period. This is related to a positive T2m anomaly over the high–latitude area and a negative one over the mid–latitude area of Asia and the western Pacific. Furthermore, the correlation coefficient between the variables (Z500, T2m, 200 hPa–zonal wind) and the key–area PBF confirms the above results.