Role of Ferrel cell in daily variability of Northern Hemisphere Annular Mode

The Northern Hemisphere Annular Mode (NAM) represents the zonally symmetric planetary-scale atmospheric mass fluctuations between middle and high latitudes, whose variations have shown a large impact on other components of the climate system. Previous studies have indicated that the NAM is correlated with the Ferrel cell in their monthly or longer timescale variability. However, there have been few studies investigating their connections at daily timescale, though daily variability of NAM has been suggested to be an important component and has significant implication for weather forecast. The results from this study demonstrate that variability of the Ferrel cell leads that of the NAM by about 1-2 days. This statistically identified temporal phase difference between NAM and Ferrel cell variability can be elucidated by meridional mass redistribution. Intensified (weakened) Ferrel cell causes anomalously smaller (larger) poleward mass transport from the middle to the high latitudes, resulting in an increase (a decrease) in mass in the middle latitudes and a decrease (an increase) in the high latitudes. As a consequence, anomalously higher (lower) poleward pressure gradient forms and the NAM subsequently shifts to a positive (negative) phase at a time lag of 1-2 days. The findings here would augment the existing knowledge for better understanding the connection between the Ferrel Cell and the NAM, and may provide skillful information for improving NAM as well as daily scale weather prediction.