The Influence of Equatorial Cooling on Axially Symmetric Atmospheric Circulation Forced by Annually Averaged Heating

Axisymmetric theory of atmospheric circulation is extended for the case of concentrated equatorial cooling and annually averaged heating. The solutions are derived in a 1.5-layer shallow water model on the spherical Earth, which includes vertical mixing with diagnostic surface momentum. The axisymmetric solutions capture the sensitivity of the large-scale circulation to equatorial cooling seen in observations and in eddy-permitting models, namely, (i) weakening and widening of the meridional overturning circulation (MOC) and (ii) weakening and poleward shift of the subtropical jet. For sufficiently large equatorial cooling, a tropical anti-Hadley cell emerges that transports energy equatorward, balancing the equatorial energetic sink. The analytic solutions predict the critical cooling required for the emergence of the anti-Hadley cell and provide a simple mechanism for the response of the MOC to equatorial cooling. Specifically, equatorial cooling reduces net tropical heating, which weakens the circulation and shifts the edge of the rising branch poleward. This in turn reduces upper-level momentum which is set by surface momentum in the rising branch. The subtropical meridional temperature gradient decreases with upper-level momentum, requiring a widening of the circulation to close the energy budget. The subtropical jet therefore shifts poleward with the edge of the MOC and weakens due to the reduced upper-level angular momentum. The strong sensitivity to equatorial cooling seen in the axisymmetric system suggests that the above mechanism may have an important role in the sensitivity of the MOC to equatorial temperature anomalies on seasonal or longer time scales.