Quantifying the Dependence of the Global Response to the Madden–Julian Oscillation on the Quasi-Biennial Oscillation

The Madden-Julian Oscillation (MJO) teleconnects across the globe. These teleconnections occur on a broad range of scales, including the generation of Rossby waves in mid-latitudes and the modulation of monsoonal disturbances within the subtropics. Recent research has suggested that both the tropical character of the MJO and its extratropical responses depend upon the state of the Quasi-Biennial Oscillation (QBO). The QBO-dependence of the amplitude and duration of the tropical character of the MJO has been well established, along with localized extratropical responses such as Rossby wave activity within the northern Pacific. However, a global perspective of the QBO-MJO relationship has not yet been discussed. We therefore ask the question: How does the global upper-tropospheric response to the MJO depend on the QBO?

We quantify the QBO-dependence of the global atmospheric response to the MJO using 300-mb geopotential anomalies, which have been shown to be a robust proxy for the tropical and extratropical character of the MJO. The spatial patterns of covariability between the MJO and 300-mb geopotential are presented from the framework of cross-spectral analysis. The magnitude-squared coherence between the Outgoing Longwave Radiation MJO Index (OMI) and time-series of 300-mb geopotential from ERA-Interim reanalysis is calculated for the 1980 through 2016 period for all locations across the globe. We use coherence since it considers both the magnitude and phase between two periodic signals, thereby accounting for more information than simple linear correlation. The analysis is separated into the QBO-ignorant, westerly QBO, and easterly QBO states.

We find that the global signature of the MJO strongly depends upon the state of the QBO. Globally, the fraction of 300-mb geopotential variability linked to the MJO is largest during easterly QBO states, consistent with the more prominent MJO during these periods. Furthermore, the spatial distribution of both tropical and extratropical upper-tropospheric anomalies associated with the MJO strongly depend upon the state of the QBO. These findings emphasize the general importance of the MJO-QBO connection within the overall climate system, and highlight the importance of considering the state of the QBO in any analysis related to the MJO, no matter the specific region of interest.