Role of equatorial waves and convective gravity waves in the 2015/16 QBO disruption

In winter 2015/2016, the descent of the westerly phase of the quasi-biennial oscillation (QBO) was unprecedentedly disrupted by the development of easterly winds. Previous studies have shown that extratropical Rossby waves propagating into the tropics were the major cause of the 2015/16 QBO disruption. However, a large portion of the negative momentum forcing driving the disruption still stems from equatorial planetary and gravity waves, which calls for detailed analyses by separating each wave mode. In this study, 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. In October and November 2015, anomalously strong negative forcing by MRG and IG waves weakened the QBO jet at 0–5°S near 40 hPa, possibly leading to Rossby wave breaking at the QBO jet core in the Southern Hemisphere. From December 2015 to January 2016, strong Rossby waves propagating horizontally (vertically) from the Northern Hemisphere (troposphere) decelerated the southern (northern) flank of the jet. In February 2016, when the westward CGW momentum flux at the source level was much stronger than the climatology, CGWs began to exert considerable negative forcing at 40–50 hPa near the Equator, in addition to the Rossby waves. The enhancement of the negative wave forcing in the tropics stems mostly from strong wave activity in the troposphere associated with increased convective activity and the westerly anomalies in the troposphere, except that the MRG wave forcing is more likely associated with increased barotropic instability in the lower stratosphere.