QBO-modulated Upwelling in the Tropical Upper Troposphere: A Mechanism for Stratospheric Teleconnection to Deep MJO Convection in Boreal Winter

Observational studies have revealed a QBO modulation of MJO precipitation and cloud-top heights in boreal winter when the MJO is most active, but models and mechanism studies seeking to simulate or understand the link between the stratospheric QBO and the tropospheric MJO have been largely unsuccessful. The most popular paradigm for QBO influence on MJO convection has been QBO modulation of static stability at the tropopause, however imposed QBO-modulated static stability at the tropopause in model studies has largely failed to show any effect on the MJO or deep convection. No studies to date have clearly identified or articulated how a change in static stability at the bottom of the QBO (100 hPa) could influence convection below, so the connection remains mysterious.

In the present work, we employ a dry global idealized model that is constrained by observations to isolate the effects of realistic tropical waves and their dissipation in opposite QBO wind/shear conditions. The model simulates wave-driven upwelling in the tropical upper troposphere, and a QBO secondary circulation is seen as a QBO modulation in upper tropospheric upwelling. In the stratosphere, the QBO secondary circulation is well-known and well-observed, where it can be readily seen through its effects on zonal mean trace gas concentrations. However, its extension to the troposphere has been obscured in observations by much stronger upwelling and rapid mixing associated with deep convection. In our model, we show the strength of wave-driven upwelling on the equator in the upper troposphere is generally stronger in boreal winter than summer. We also quantify the boreal winter QBO secondary circulation in the upper troposphere, the strength of the modulation in upwelling, and the forcing mechanisms and types of waves responsible for the modulation. QBO modulation of upwelling in the model extends down to the 200 hPa, a level where tropical deep convective mass outflow peaks. We hypothesize that in air containing enough moisture, the QBO modulation of upwelling could enhance deep convective cloud-top heights and precipitation rates in the active phase of the MJO. We also test the effect of artificial "nudging" of wind tendencies on the strength of the upper tropospheric QBO secondary circulation, since strong nudging tendencies have been used in many previous model studies that failed to capture a QBO-MJO connection. Together, our results provide insights into why previous model studies may have failed to simulate the QBO secondary circulation extension into the troposphere and connection to the MJO.