The principal modes of variability of the boreal winter Hadley cell

摘要

The year-to-year variability of the boreal winter mass stream function of the mean meridional circulation (MMC) is dominated by an equatorially asymmetric mode and a symmetric mode. The former (latter) mode is linked with the boreal Hadley cell mainly on the decadal (interannual) time-scale. The asymmetric mode index (AMI), which indicates the strength of this mode, shows a clear upward trend during the last decades of the 20th century. The strong sea surface temperature (SST) warming in the tropical Indo-west Pacific warm-pool might be an important factor contributing to the intensification of the asymmetric mode, and the SST warming in the southern tropical Atlantic Ocean and southeast Pacific may also contribute to this intensification. The symmetric mode index (SMI) shows a significant interannual variability that has a very robust and consistent correlation with ENSO, implying that the variability of the Hadley cell is mainly associated with ENSO on the interannual time-scale. Additionally, the symmetric mode variability is also connected with the SST in the tropical Indo-west Pacific warm-pool.