Atmospheric Circulation Associated with Anomalous Variations in North Pacific Wintertime Blocking

Abstract

Atmospheric circulation associated with anomalous variation of North Pacific blocking during the northern winter (December to February) is described and examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data from 1948/49 to 1999/2000. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. There are several atmospheric cells over the North Pacific associated with an anomalous blocking situation during winter. They are the zonal Walker cell along the equator (ZWC), the regional Hadley cell in the western Pacific (WHC), the regional Hadley cell in the eastern Pacific (EHC), the regional Ferrel cell over the eastern Pacific in the midlatitudes (EFC), and the midlatitude zonal cell (MZC) over the Pacific. During a strong blocking winter (SBW), the ZWC is strengthened and the anomalous EHC is opposite to the anomalous WHC and the thermally driven Hadley cell. The anomalous MZC is characterized by air rising in the west part of the North Pacific, flowing eastward in the upper troposphere, descending in the eastern North Pacific, then returning back to the east coast of Asia in the lower troposphere, modulating the mean MZC. It is also found that the anomalous regional Ferrel cell at midlatitudes exists in the eastern Pacific (EFC), whereas it is not apparent in the western Pacific. All anomalous atmospheric cells almost completely reverse during a weak blocking winter (WBW). Evolutions of each cell are also investigated. The atmospheric cells over the tropical and subtropical regions (ZWC, EHC, and WHC) always emerge ahead of the anomalous blocking winter, and then lower troposphere signals propagate upward after the anomalous blocking winter. This may suggest a mid-to-low-latitude interaction of the response of the midlatitude atmospheric blocking to tropical SST variations and a feedback via the atmospheric cells to the Tropics. In contrast, the midlatitude cells (MZC, EFC) evolve very locally, with a simultaneous response to the blocking event and no propagation of signal.