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.
This paper examines the dominant submonthly variability of zonally symmetrical atmospheric circulation in the Northern Hemisphere(NH)winter within the context of the Northern Annular Mode(NAM),with particular emphasis on interactive stratosphere–troposphere processes.The submonthly variability is identified and measured using a daily NAM index,which concentrates primarily on zonally symmetrical circulation.A schematic lifecycle of submonthly variability is developed that reveals a two-way coupling process between the stratosphere and troposphere in the NH polar region.Specifically,anomalous tropospheric zonal winds in the Atlantic and Pacific sectors of the Arctic propagate upwards to the low stratosphere,disturbing the polar vortex,and resulting in an anomalous stratospheric geopotential height(HGT)that subsequently propagates down into the troposphere and changes the sign of the surface circulations.From the standpoint of planetary-scale wave activities,a feedback loop is also evident when the anomalous planetary-scale waves(with wavenumbers 2 and 3)propagate upwards,which disturbs the anomalous zonally symmetrical flow in the low stratosphere,and induces the anomalous HGT to move poleward in the low stratosphere,and then propagates down into the troposphere.This increases the energy of waves at wavenumbers 2 and 3 in the low troposphere in middle latitudes by enhancing the land–sea contrast of the anomalous HGT field.Thus,this study supports the viewpoint that the downward propagation of stratospheric NAM signals may not originate in the stratosphere.
