Role of eddies in the interannual variability of Hadley cell strength

There is evidence from simplified atmosphere models that extratropical eddies strongly affect the Hadley cell mass flux. Here, we use reanalysis data to assess the role of eddy momentum transport in the interannual variability of northern hemisphere winter Hadley cell strength. We define a Hadley cell strength index and decompose it into an ENSO-related term and an uncorrelated remainder. Regressing these two time series onto the momentum budget shows that ENSO-related variability involves changes in momentum transport by tropical stationary waves, while non- ENSO variability is related to changes in the extratropical wave flux impinging on the tropics, with stationary waves dominant in the northern ( winter) cell and transients dominant in the southern ( summer) cell. Non- ENSO variability accounts for a large fraction of the interannual variance in Hadley cell strength, supporting the hypothesis that extratropical eddy stresses drive much of the zonally symmetric variability in the tropics.