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.
The role of North Pacific bomb cyclones in the onset of January 2021 sudden stratospheric warming (SSW) is examined by conducting a set of numerical model experiments. The control simulation, ...initialized 10 days before the SSW onset, successfully reproduces the SSW. As this event is preceded by the bomb cyclones in the North Pacific, their impact is tested by initializing the model without them. This sensitivity experiment shows much weaker polar‐vortex deceleration than the control simulation, resulting in no distinct SSW onset. This difference is attributable to the dampened constructive linear interference between the climatological wave and the cyclone‐related wavenumber‐one anomaly in the sensitivity experiment. It weakens the vertical propagation of wavenumber‐one wave into the stratosphere, thereby reducing wave breaking in the polar stratosphere. This result suggests that bomb cyclones should be considered for better understanding SSW and improving its predictability.
Plain Language Summary
Sudden stratospheric warming (SSW) is a dramatic event characterized by abrupt warming in the polar stratosphere. During this event, the eastward winds encircling the polar stratosphere change to westward winds, due to the breaking of the upward‐propagating planetary‐scale waves. Since the SSW is often followed by unusual weather events (e.g., cold surges), an accurate prediction of SSW is important for a better weather prediction. Here, we examine the role of the rapidly developing North Pacific cyclones in initiating the 2021 SSW. A set of numerical model experiments show that without cyclones, the 2021 SSW is not initiated due to the reduced breaking of the upward‐propagating planetary‐scale waves. This result suggests that the rapidly developing North Pacific cyclones should be considered for better understanding SSW and improving its predictability.
Key Points
The 2021 SSW is successfully reproduced by a model initialized 10 days before the stratospheric warming (SSW)
The model initialized without the North Pacific bomb cyclone does not simulate the 2021 SSW
Removing the North Pacific bomb cyclone reduces the k = 1 wave propagation into the stratosphere, reducing wave breaking in the stratosphere
Abstract
Spatiotemporal variations in momentum flux spectra of convective gravity waves (CGWs) at the source level (cloud top), including nonlinear forcing effects, are examined based on calculations ...using an offline version of CGW parameterization and global reanalysis data for a period of 32 years (1979–2010). The cloud-top momentum flux (CTMF) is not solely proportional to the convective heating rate but is affected by the wave-filtering and resonance factor and background stability and temperature underlying the convection. Consequently, the primary peak of CTMF is in the winter hemisphere midlatitudes, associated with storm tracks, where a secondary peak of convective heating exists, whereas the secondary peak of CTMF appears in the summer hemisphere tropics and intertropical convergence zone (ITCZ), where the primary peak of convective heating exists. The magnitude of CTMF fluctuates largely with 1-yr and 1-day periods in major CTMF regions. At low latitudes and Pacific storm-track regions, a 6-month period is also significant, and the decadal cycle appears in the southern Andes. The equatorial eastern Pacific region exhibits a substantial interannual to decadal scale of variabilities. The correlation between convective heating and the CTMF is relatively lower in the equatorial region than in other regions. The CTMF in 10°N–10°S during the period of the pre-Concordiasi campaign approximately follows a lognormal distribution but with a slight underestimation in the tail of the probability density function. In Part II, the momentum flux and drag of CGW in the stratosphere will be examined.
A westerly phase of the quasi‐biennial oscillation (QBO) was disrupted in 2015/16. Previous studies have primarily focused on the cause of the localized negative momentum forcing, initiating the QBO ...disruption. However, the upward displacement of the westerly QBO followed by the negative momentum forcing, clearly observed in 2015/16, has received less attention. This study shows that the distinct upward propagation of the westerly winds during the 2015/16 QBO disruption was contributed by the strong Brewer‐Dobson circulation (BDC). Strong Rossby waves with wavenumber 1 propagating from the troposphere mainly drove the strong BDC. Potential contributions of El Niño and Barents–Kara sea ice reduction to Rossby waves are also discussed.
Plain Language Summary
The quasi‐biennial oscillation (QBO), describing alternate easterly and westerly winds in the tropical stratosphere, generally shows downward phase propagation with time. However, downward‐propagating westerly winds were split into two in February 2016, with one propagating upward and the other propagating downward. It is found that the upward propagation of the westerly winds was contributed from the stronger upwelling in the tropical stratosphere, mainly attributed to the strong planetary‐wave activity in the midlatitude troposphere.
Key Points
Strong Brewer‐Dobson circulation contributed to the distinct upward propagation of the westerly QBO during the 2015/16 QBO disruption
Midlatitude Rossby waves with wavenumber 1 mainly induced the strong Brewer‐Dobson circulation in 2015/16
This study explores the abrupt split of the polar vortex in the upper stratosphere prior to a recent sudden stratospheric warming event on 5 January 2021 (SSW21) and the mechanisms of vortex ...preconditioning by using the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA2) global reanalysis data. SSW21 is preceded by the highly distorted polar vortex that was initially displaced from the pole but eventually split at the onset date. Vortex splitting is most significant in the upper stratosphere (1 hPa altitude) accompanied by the anomalous growth of westward-propagating planetary waves (PWs) of zonal wavenumber (ZWN) 2 (WPW2). While previous studies have suggested the East Asian trough as a potential source for the abnormal WPW2 growth, the prominent westward-propagating nature cannot be explained satisfactorily by the upward propagation of the quasi-stationary ZWN2 fluxes in the troposphere. More importantly, WPW2 exhibits an obvious in situ excitation signature within the barotropically and baroclinically destabilized stratosphere, dominated by the easterlies descending from the stratopause containing the WPW2 critical levels. This suggests that the vortex split is attributed to the WPW2 generated in situ within the stratosphere via instability. Vortex destabilization is achieved as the double-jet structure consisting of a subtropical mesospheric core and a polar stratospheric core develops into SSW21 by encouraging the anomalous dissipation of the upward-propagating tropospheric ZWN1 PWs. This double-jet configuration is likely a favorable precursor for SSW onset, not only for the SSW21 but generally for most SSWs, through promoting the anomalous growth of unstable PWs as well as the enhancement of the tropospheric PW dissipation.
Abstract
The characteristics of small-scale convective gravity waves (CGWs; horizontal wavelengths <100 km) and their contributions to the large-scale flow in the stratosphere, including the ...quasi-biennial oscillation (QBO), are investigated using an offline calculation of a source-dependent, physically based CGW parameterization with global reanalysis data from 1979 to 2010. The CGW momentum flux (CGWMF) and CGW drag (CGWD) are calculated from the cloud top (source level) to the upper stratosphere using a Lindzen-type wave propagation scheme. The 32-yr-mean CGWD exhibits large magnitudes in the tropical upper stratosphere and near the stratospheric polar night jet (~60°). The maximum positive drag is 0.1 (1.5) m s−1 day−1, and the maximum negative drag is −0.9 (−0.7) m s−1 day−1 in January (July) between 3 and 1 hPa. In the tropics, the momentum forcing by CGWs at 30 hPa associated with the QBO in the westerly shear zone is 3.5–6 m s−1 month−1, which is smaller than that by Kelvin waves, while that by CGWs in the easterly shear zone (3.1–6 m s−1 month−1) is greater than that by any other equatorial planetary waves or inertio-gravity waves (inertio-GWs). Composite analyses of the easterly QBO (EQBO) and westerly QBO (WQBO) phases reveal that the zonal CGWMF is concentrated near 10°N and that the negative (positive) CGWD extends latitudinally to ±20° (±10°) at 30 hPa. The strongest (weakest) negative CGWD is in March–May (September–November) during the EQBO, and the strongest (weakest) positive CGWD is in June–August (March–May) during the WQBO. The CGWMF and CGWD are generally stronger during El Niño than during La Niña in the equatorial region.
Convection as one dominant source of atmospheric gravity waves (GWs) has been the focus of investigation over recent years. However, its spatial and temporal forcing scales are not well known. In ...this work we address this open issue by a systematic verification of free parameters of the Yonsei convective GW source scheme based on observations from the High Resolution Dynamics Limb Sounder (HIRDLS). The instrument can only see a limited portion of the gravity wave spectrum due to visibility effects and observation geometry. To allow for a meaningful comparison of simulated GWs to observations, a comprehensive filter, which mimics the instrument limitations, is applied to the simulated waves. By this approach, only long horizontal-scale convective GWs are addressed. Results show that spectrum, distribution of momentum flux, and zonal mean forcing of long horizontal-scale convective GWs can be successfully simulated by the superposition of three or four combinations of parameter sets reproducing the observed GW spectrum. These selected parameter sets are different for northern and southern summer. Although long horizontal-scale waves are only part of the full spectrum of convective GWs, the momentum flux of these waves is found to be significant and relevant for the driving of the QBO (quasi-biennial oscillation). The zonal momentum balance is considered in vertical cross sections of GW momentum flux (GWMF) and GW drag (GWD). Global maps of the horizontal distribution of GWMF are considered and consistency between simulated results and HIRDLS observations is found. The latitude dependence of the zonal phase speed spectrum of GWMF and its change with altitude is discussed.
Abstract
Contributions of convective gravity waves (CGWs) and orographic gravity waves (OGWs) to the Brewer–Dobson circulation (BDC) are examined and compared to those from resolved waves. OGW drag ...(OGWD) is provided by NCEP Climate Forecast System Reanalysis (CFSR), while CGW drag (CGWD) is obtained from an offline calculation of a physically based CGW parameterization with convective heating and background data provided by CFSR. CGWD contributes to the shallow branch of the BDC regardless of the season, while OGWD contributes to both the shallow and deep branches except for the summertime, when OGWs hardly propagate into the stratosphere. At 70 hPa, the annual-mean tropical upward mass fluxes from Eliassen–Palm flux divergence (EPD), OGWD, and CGWD are 68%, 7%, and 4% of the total mass flux, respectively. The tropical upward mass flux at 70 hPa shows an increasing trend during the time period from 1979 to 1998, with 28%, 18%, and 6% of the trend driven by EPD, OGWD, and CGWD, respectively. The width of the turnaround latitudes tends to narrow for the streamfunctions induced by OGWD and CGWD but tends to widen for that induced by EPD. The contributions of GWD from MERRA (MERRA-2) to the climatology and long-term trend of the BDC are 7% (7%) and 13% (4%), respectively, somewhat smaller than the contributions of CGWD plus OGWD, which are estimated from CFSR to be 12% and 20%, respectively.
Sudden stratospheric warming (SSW) describes a disruption of the stratospheric polar vortex in the winter hemisphere. It affects not only the stratospheric circulation but also the surface climate ...for up to 2 months, serving as an important source of subseasonal‐to‐seasonal (S2S) predictability in midlatitudes. This study evaluates the predictability of the 2021 SSW and investigates the crucial factors that determine its predictability in the ECMWF and JMA S2S real‐time forecasts. In both models, only a subset of the ensemble members predicted the SSW at the lead time of about 2 weeks before the onset. By comparing the 10 ensembles with successful SSW predictions and those with failed predictions, we found that the ensembles predicting the SSW have relatively stronger wave fluxes from the upper troposphere to the stratosphere than the others. Stronger wave fluxes, particularly those of zonal wavenumber one, are not the result of the tropospheric precursors such as the Ural blocking and Aleutian cyclones but they result from the modulation of the wave propagation by the background state. In particular, the ensembles with failed SSW predictions tend to have a negative potential vorticity gradient in the upper troposphere and lower stratosphere, which limits the upward wave propagation into the stratosphere and provides an unfavorable condition for the SSW. This result suggests that not only the wave sources in the troposphere but also the background state in the upper troposphere and lower stratosphere can modulate the predictability of SSW in S2S prediction models.
Plain Language Summary
Sudden stratospheric warming (SSW) refers to the rapid increase in temperature in the polar stratosphere during winter. It plays a crucial role in predicting weather patterns within a timeframe ranging from 2 weeks to 2 months (subseasonal‐to‐seasonal; S2S). This study investigates the factors that contribute to the predictability of the 2021 SSW. By comparing the S2S forecasts with and without the SSW, it is found that the predictability of SSW is not determined by the tropospheric precursors, but by the zonal‐mean flow in the upper troposphere and lower stratosphere that modulates the wave propagation from the troposphere to the stratosphere. This result suggests that a better prediction of the wave propagation condition is a critical factor for a successful SSW prediction on the S2S time scale.
Key Points
The crucial factors that determine the predictability of the 2021 SSW in S2S prediction models are examined
The k = 1 wave propagating into the stratosphere plays an essential role in the SSW prediction
The PV gradient in the upper troposphere and lower stratosphere can control the SSW predictability by modulating the upward wave propagation
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 Niño, and La Niña 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 Niña and the QBOE (El Niño 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.
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