Interhemispheric coupling caused by the Holton-Tan effect – a dependence on the solar cycle

Abstract:

The Holton-Tan (HT) effect describes the dependence of the northern stratospheric polar vortex on the equatorial quasi-biennial oscillation (QBO) which is strongest in January (and reversed in February). During the easterly phase of the QBO the polar vortex is weaker and warmer while it is stronger and colder during the westerly phase. This weakening of the polar vortex changes the gravity wave filtering and refraction, leading to a reduction of the westward gravity wave drag in the mesosphere/ lower thermosphere (MLT). Consequently, the residual circulation from the summer to the winter pole weakens and the temperature in the southern mesopause region is expected to increase during the easterly phase of the QBO in January. Additionally, there is an influence of the solar cycle especially on the stratospheric circulation. This leads us to two questions: Is there an interhemispheric coupling induced by the HT effect and how is it influenced by the solar cycle?

Using 30 years of assimilated model data from the Canadian Middle Atmosphere Model (CMAM) and, for comparison, 9 years of global satellite observations from the Microwave Limb Sounder (MLS), we investigate the average characteristics of the HT effect up to 120 km in January. The results show that the assumed interhemispheric coupling mechanism is confirmed for solar minimum and maximum conditions but, surprisingly, it seems reversed during solar transition periods. To prove the interhemispheric coupling mechanism, we also analyze simulations with the Kühlungsborn Mechanistic Circulation Model (KMCM) which includes a self-generated QBO.