Home › Explainers › Hadley cell

Hadley cell and the mean meridional mass streamfunction

Also called: Hadley circulation, mean meridional streamfunction, meridional mass streamfunction, MMSF, Ferrel cell, Hadley cell strength

The Hadley cell is the tropical overturning circulation: air rises near the equator, flows poleward aloft, sinks in the subtropics and returns toward the equator in the trade winds. It and the weaker Ferrel and polar cells are described by the zonal-mean meridional mass streamfunction

$$\Psi(\phi,p)=\frac{2\pi a\cos\phi}{g}\int_0^{p}[v]\,dp'$$

where $[v]$ is the zonal-mean meridional wind. The mass flowing northward between two pressure levels is the difference of $\Psi$ between them, and the streamlines are its contours. With this convention $\Psi>0$ is a clockwise cell when latitude runs north to the right and pressure increases downward, which is the Northern Hemisphere Hadley cell.

Strength and edges

• Strength is usually the maximum of $|\Psi|$ near 500 hPa, of order 10¹¹ kg s⁻¹ in the winter hemisphere's cell.
• The edge is the latitude where $\Psi$ at 500 hPa crosses zero on the poleward side of the cell. It marks the subtropical dry zones, and its poleward drift in a warming climate is one of the clearest circulation trends.
• The cells follow the sun: the winter hemisphere's cell dominates and extends across the equator, while the summer cell nearly vanishes.
• El Niño strengthens the Hadley circulation and pulls its edges equatorward (Oort and Yienger, 1996).

The live analysis

$\Psi$ is computed from the ECMWF AIFS analysis and shown as the anomaly from the ERA5 1991–2020 climatology for the same day of year, with the absolute cells contoured and vertical motion drawn as arrows. Each frame is a 7-day running mean, so the evolution of the cells is legible through the day-to-day noise.