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

Abstract

Abstract 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. Significance Statement Axisymmetric solutions for the response of the atmospheric circulation to concentrated equatorial cooling are derived, motivated by the equatorial cooling by the cold tongues of the Pacific and Atlantic. The solutions capture the response of the large-scale realistic atmosphere to equatorial cooling, namely a weakening and widening of the meridional overturning circulation and a weakening and poleward shift of the subtropical jet. In addition, for sufficiently strong cooling, a tropical anti-Hadley cell emerges. The solutions provide insight into the influence of biases in the representation of the cold tongues in climate models, the relation of interannual equatorial variability to the large-scale circulation, and changes in the large-scale atmospheric circulation on geological time scales.