An Analysis Of Changes With Time Of The Mjo And Its Modulation By The Qbo: 1974-2021
Presenter: Charles Andrew Hoopes1
Co-Author(s): -
Advisor(s): Dr. Christopher L. Castro
1Department of Hydrology and Atmospheric Sciences, University of Arizona
The tropical Madden-Julian oscillation (MJO) is an important source of subseasonal climate variability in the Northern Hemisphere. It has recently been found that the stratospheric quasi-biennial oscillation (QBO) modulates the MJO in boreal winter such that the MJO is stronger and extends further eastward in the easterly QBO phase (QBOE) than in the westerly phase (QBOW). One proposed mechanism for this modulation involves reductions of tropical lower stratospheric static stability during the QBOE phase relative to the QBOW phase, driven by a combination of increased extratropical wave forcing of the stratospheric residual meridional circulation during early winter and a northward shift of the zero-wind line (both components of the “Holton-Tan effect”). It has further been shown recently using reconstructed MJO and QBO indices back to 1905 that the observed QBO-MJO modulation has only emerged since the early 1980s, coinciding with a cooling trend in the tropical lower stratosphere. Here, we first confirm using meteorological reanalysis data and satellite outgoing longwave radiation data that significant negative trends in tropical lower stratospheric temperature and static stability have occurred over the 1974 to 2021 period when the QBO-MJO modulation has been emerging, an emergence we also confirm using an OLR-based MJO index. We then show that these negative trends in temperature and stability are largely a consequence of positive trends in early-winter extratropical wave forcing. The latter trends likely reflect increases in upward-propagating planetary-scale Rossby waves of zonal wavenumber 1, which are in turn caused by climate change in the troposphere.