Abstract
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.
Highlights
The quasi-biennial oscillation (QBO) is the dominant source of variability in the equatorial stratosphere, characterized by alternating easterly and westerly winds with a period of around 28 months (Baldwin et al, 2001)
We examine the contributions of equatorial planetary waves, including equatorial Kelvin, Rossby, mixed Rossby–gravity (MRG), and inertia–gravity (IG) waves, and small-scale convective gravity waves (CGWs) to the 2015/16 QBO disruption by employing the separation method of equatorial wave modes of Kim and Chun (2015a) and the offline CGW parameterization by Kang et al (2017) using the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis data (Gelaro et al, 2017) on native model levels (GMAO, 2015)
In October 2015, the negative forcing by MRG waves is anomalously strong compared to the climatology at 40 hPa between 5◦ N and 5◦ S, and it becomes stronger in November 2015 together with IG waves when the Rossby waves start to break at the southern hemispheric (SH) part of the QBO
Summary
The quasi-biennial oscillation (QBO) is the dominant source of variability in the equatorial stratosphere, characterized by alternating easterly and westerly winds with a period of around 28 months (Baldwin et al, 2001). We examine the contributions of equatorial planetary waves, including equatorial Kelvin, Rossby, MRG, and inertia–gravity (IG) waves, and small-scale convective gravity waves (CGWs) to the 2015/16 QBO disruption by employing the separation method of equatorial wave modes of Kim and Chun (2015a) and the offline CGW parameterization by Kang et al (2017) using the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis data (Gelaro et al, 2017) on native model levels (GMAO, 2015).
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