Role of equatorial waves and convective gravity waves in the 2015/16Â quasi-biennial oscillation disruption

Previous studies have shown that extratropical Rossby waves propagating into the deep tropics were the major cause of the 2015/16 QBO disruption. However, a large portion of the negative momentum forcing associated with the disruption still stems from equatorial planetary and small-scale gravity waves, which calls for detailed analyses by separating each wave mode compared with climatological QBO cases.

Here, the contributions of resolved equatorial planetary waves (Kelvin, Rossby, mixed Rossby-gravity (MRG), and inertia-gravity (IG) waves) and small-scale convective gravity waves (CGWs) obtained from an offline CGW parameterization to the 2015/16 QBO disruption are investigated using MERRA-2 global reanalysis data from October 2015 to February 2016. In October and November 2015, anomalously strong negative forcing by MRG and IG waves weakened the QBO jet at 0-5 degrees S near 40 hPa, leading to Rossby wave breaking at the QBO jet core in the Southern Hemisphere.