The detailed dynamics of the June–August Hadley Cell

The seminal theory for the Hadley Cells has demonstrated that their existence is necessary for the reduction of tropical temperature gradients to a value such that the implied zonal winds are realisable. At the heart of the theory is the notion of angular momentum conservation in the upper branch of the Hadley Cells. Eddy mixing associated with extratropical systems is invoked to give continuity at the edge of the Hadley Cell and to reduce the subtropical jet by a factor of three or more to those observed. In this article a detailed view is presented of the dynamics of the June–August Hadley Cell, as given by ERA data for the period 1981–2010, with an emphasis on the dynamics of the upper branch. The steady and transient northward fluxes of angular momentum have a very similar structure, both having a maximum on the Equator and a reversal in sign near 12°S, with the transient flux merging into that associated with eddies on the winter subtropical jet. In the northward absolute vorticity flux, the Coriolis torque is balanced by both the steady and transient fluxes. The overturning circulations that average to give the Hadley Cell are confined to specific longitudinal regions, as are the steady and transient momentum fluxes. In these regions, both intraseasonal and synoptic variations are important. The dominant contributor to the Hadley Cell is from the Indian Ocean and west Pacific regions, and the maxima in OLR variability and meridional wind in these regions have a characteristic structure associated with the Westward‐moving Mixed Rossby–Gravity wave. Much of the upper tropospheric motion into the winter hemisphere occurs in filaments of air from the summer equatorial region. These filaments can reach the winter subtropical jet, leading to the strengthening of it and of the eddies on it, implying strong tropical–extratropical interaction. The 30‐year mean JJA Hadley cell. In each panel the meridional circulation (V, ω) is shown by vectors, with the scale at the top right. In the top two panels U is shown by dark blue contours with interval 10 m·s−1, and with the zero‐contour dotted. In the lower panel the dark blue contours are for the isentropes, with interval 20 K. The abscissa is the sine of the latitude. The fields shown with block contours are: (a) angular momentum (unit a2 Ω), (b) the vertical component of absolute vorticity (unit Ω) and (c) potential vorticity (PV) (unit PVU)