Multidecadal variability in the Hadley circulation

The Hadley circulation in the tropics is one of the most dominant features of the general circulation of the atmosphere. Recent studies (Mitas and Clement, 2005) that compared reanalysis, rawinsonde and climate model data have shoved that the Hadley cell has been strengthening over the last decades.

This research concentrates on studying the behavior of the Hadley cell over longer periods of time. For this purpose two atmospheric models, GFDL and CCM3/NCAR models, that run from the mid 19th to early 21st century are being used. The results of this study, even though consistent with the results of Mitas and Clement (2005), show that the strength of the Northern Hadley cell does not show any trends over the entire period, but rather oscillate with a period of roughly 70 years. Also, this oscillation is best seen from December to April. On the other hand, the strength of the Southern Hadley cell seems to act in a more random manner and does not show any oscillation. Furthermore, the ITCZ is changing latitude fallowing the same low frequency oscillation as the Hadley cell strength index, and this is seen in both the streamfunction and the precipitation field.

Finally, the SSTs over the North Atlantic have been identified as a possible forcing to the oscillation that is exhibited by the Hadley cell. The average SST in the North Atlantic shows a similar oscillation, as the Hadley index, in what is identified as the North Atlantic Multidecadal Oscillation (AMO).