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

Abstract. In February 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 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 ∘  S near 40  hPa , leading to Rossby wave breaking at the QBO jet core in the Southern Hemisphere. From December
2015 to January 2016, exceptionally strong Rossby waves propagating horizontally (vertically) continuously 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 its 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 strong westerlies (or
weaker easterlies) in the troposphere, except that the MRG wave forcing is more likely associated with increased barotropic instability in the lower
stratosphere.