. 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.

/pdf/role-of-equatorial-waves-and-convective-gravity-waves-in-the-3ebc0la1nj.pdf

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

The Influence of the Stratosphere on the Tropical Troposphere

The Unique Characteristics and Potential Mechanisms of the MJO-QBO Relationship

The observed influence of the Quasi-Biennial Oscillation in the lower equatorial stratosphere on the East Asian winter monsoon during early boreal winter

On the basis of 70 hPa zonal wind (U70)-defined quasi-biennial oscillation (QBO) events, after removing the El Nino–Southern Oscillation (ENSO) signal, the present study investigates the process by which the QBO modulates tropospheric circulation and convection during summer (between July and October), when tropical cyclone (TC) activities enter their peak period. Concurrent with the western phase of the QBO (QBOW), significant tripole pattern modulations over the tropical Indo-Pacific Ocean are regressed onto the residual part of U70 after removing the ENSO signal, with enhanced convection observed over its central branch (0°–10°N, 120°E–180°) and the inactive convection branches located to both sides. A ventilation opening-like effect is exerted on the monsoon trough, which is shifted southward under the QBOW phase. According to the QBO-associated changes in the circulations over the tropical western North Pacific (WNP), equatorial environments (with low-level relative vorticity, high-level divergence, tropospheric vertical wind shear (VWS), and midlevel humidity) tend to be favorable for TC genesis. Consequently, the off-equatorial TC tracks show a significantly decreased occurrence frequency in the northern monsoon trough region. The present study provides a summary sketch showing the QBO-tropospheric circulation modulation process. Considering that the westerly or weak easterly is observed over the upper central Pacific (CP), QBOW phase-associated weak VWS can induce anomalous upward motion at equatorial latitudes, together with the upwelling between the equatorial westerly and off-equatorial easterly, leading to an enlarged convective window over the CP. Affected by the prevailing easterly within the tropics, the actual pumping shifts westward over the tropical WNP.

Influence of the QBO on tropical convection and its impact on tropical cyclone activity over the western North Pacific

Differences in convective activities in the tropical region (30°S–30°N, 180°E–180°W) during different phases of the quasi-biennial oscillation (QBO) are investigated over 32 years (1979–2010) using five metrics representing tropical convection: (i) precipitation and (ii) outgoing longwave radiation from observations and (iii) convective available potential energy (CAPE), (iv) deep convective heating rate, and (v) convective cloud top pressure from reanalysis data. The easterly (QBOE) and westerly (QBOW) phases of the QBO are defined using the zonal wind anomaly from the monthly climatology at 50 hPa. During the QBOE (QBOW), the convective activities are intensified (weakened) over the Maritime Continent and weakened (intensified) over the equatorial eastern and central Pacific. Therefore, the zonal mean values of the five metrics averaged over chronically convective regions show stronger convective activities during the QBOE than during the QBOW, while the opposite is true for the whole tropical region. Composite analyses are also performed during the neutral, El Nino, and La Nina periods. In the neutral period, the convective activities during QBOE are stronger than during QBOW except in the equatorial region (10°S–10°N). The convective activities over the Maritime Continent (central and eastern Pacific) are enhanced when La Nina and the QBOE (El Nino and the QBOW) occur simultaneously. All metrics show similar pattern to one another, implying that the metrics from reanalysis data represent the variations in the convective activities with respect to the QBO reasonably well. Among the five metrics, the CAPE is most sensitive to the QBO phase, likely because the virtual temperature in the upper troposphere is modulated by anomalous meridional circulations induced by different QBO phases.

Differences in the Tropical Convective Activities at the Opposite Phases of the Quasi-Biennial Oscillation

Radiance observations are typically affected by biases that come mainly from instrument error (scanning or calibration) and inaccuracies of the radiative transfer model. These biases need to be removed for successful assimilation, so a bias correction scheme is crucial in the Numerical Weather Prediction (NWP) system. Today, most NWP centres, including the Bureau of Meteorology (hereafter, “the Bureau”), correct the biases through variational bias correction (VarBC) schemes, which were originally developed for global models. However, there are difficulties in estimating the biases in a limited-area model (LAM) domain. As a result, the Bureau’s regional NWP system, ACCESS-C (Australian Community Climate and Earth System Simulator-City), uses variational bias coefficients obtained directly from its global NWP system ACCESS-G (Global). This study investigates independent radiance bias correction in the data assimilation system for ACCESS-C. We assessed the impact of using independent bias correction for the LAM compared with the operational bias coefficients derived in ACCESS-G between February and April 2020. The results from our experiment show no significant difference between the control and test, suggesting a neutral impact on the forecast. Our findings point out that the VarBC-LAM strategy should be further explored with different settings of predictors and adaptivity for a more extended period and over additional domains.

/pdf/testing-variational-bias-correction-of-satellite-radiance-3qrq4bsb.pdf

Jin Haeng Lee

Papers

Differences in the Tropical Convective Activities at the Opposite Phases of the Quasi-Biennial Oscillation

Testing Variational Bias Correction of Satellite Radiance Data in the ACCESS-C: Australian Convective-Scale NWP System