Arctic amplification and modal variability of the Hadley cell on interannual timescales

Abstract The Equator-to-Pole temperature gradient sustains the zonally axisymmetric Hadley-type thermally direct circulations and the midlatitude thermally driven jet, as well as baroclinicity. The current study focuses on the role of Arctic warming on the interannual variability modes of the Hadley circulation through modulation of the equator-to-pole temperature gradient. Some observational studies indicate the strengthening of the annual mean and boreal winter Hadley cell. Other studies indicate the role of different climate-teleconnection modes on the Hadley cell. However, the association of the Arctic amplification trend and the Hadley cell is not well understood. The empirical orthogonal modes of interannual variability of the Hadley cell are first identified using the meridional mass stream function. It was found that the first mode of the Hadley cell is more closely associated with other climate variability modes, such as the Atlantic Multidecadal Oscillation, and exhibits weak correlation with Arctic amplification. However, the second mode shows a correlation of 0.43 with the Arctic amplification Index and a strong correlation with both Hadley cell strength and meridional velocity at 200 hPa. Furthermore, regression analysis of both the Arctic amplification Index and the second mode with eddy fluxes shows a similar trend pattern of an increasing transient eddy momentum flux divergence (EMFD) in the subtropics. The intensification of transient EMFD leads to weakening zonal winds and reduced baroclinicity in the subtropics. Furthermore, the eddy momentum flux from the mid-latitudes triggers the Rossby waves to the subtropics, reinforcing EMFD and the second mode. Support for the mechanism comes from the sensitivity experiments, by forcing the model for a period of a minimum sea ice. The result shows intensification of EMFD and overturning circulation with poleward shift of the subtropical jet. Thus, the analysis shows the impact of Arctic amplification through the eddy-driven dynamics of the Hadley cell.