Interaction between the Brewer-Dobson circulation and the Hadley circulation

Interannual changes of the stratospheric circulation are studied in relation to coherent changes of the tropospheric circulation. Emerging over the winter pole is a clear signature of adiabatic warming and anomalous downwelling. Reflecting an intensification of the Brewer-Dobson circulation, the signature of anomalous downwelling extends from stratospheric levels into the troposphere. Compensating for it at subpolar latitudes is a signature of adiabatic cooling and anomalous upwelling. Equally coherent, the signature of anomalous upwelling occupies the same levels as the signature of anomalous downwelling. Inside the tropical troposphere, anomalous cooling is replaced by anomalous warming. It reflects an intensification of organized convection and the Hadley circulation, one that accompanies the intensification of the Brewer-Dobson circulation. These signatures of anomalous vertical motion represent changes that operate coherently in the stratosphere and troposphere. They share major features with the Arctic Oscillation. Extending across the tropopause, they couple the stratosphere and troposphere through a transfer of mass. By modifying vertical motion inside the Tropics, anomalous upwelling influences organized convection. Support for this interpretation comes from anomalous divergence in the tropical upper troposphere; it is shown to vary coherently with anomalous downwelling in the Arctic stratosphere. Exhibiting analogous behavior are changes of the tropical tropopause. Coupled to stratospheric changes, these variations of the tropical circulation act to organize convection about the equator, favoring a split ITCZ. They reflect as much as 40% of the interannual variance of tropical divergence, representing an important complement to ENSO. Much of the covariance between the polar stratosphere and the tropical troposphere is concentrated at periods shorter than 5 yr. Included is variability that is associated with the quasi-biennial oscillation (QBO) in the tropical stratosphere. Also included is biennial variability, which accompanies the QBO in the polar stratosphere. These stratospheric variations involve the same time scales as biennial variability in the tropical troposphere, which likewise influences convection.