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Abstract The first 2 weeks of December 2021 were exceptionally active for severe convective storms across the central and eastern United States. While previous work has indicated that this was related to the existence of a negative phase of the Pacific–North American pattern, we demonstrate that such a pattern was configured via dynamical linkages between multiple extratropical cyclogenesis events in the western North Pacific, the recurvature of Typhoon Nyatoh, and the subsequent phase evolution of the North Pacific jet. These processes were found to aid in the excitation of Rossby wave packets and the amplification of upper-level flow downstream over the Pacific, ultimately configuring synoptic-scale weather regimes supportive of anomalous high-frequency and high-intensity severe convective weather in the contiguous United States. In addition, abnormally warm Gulf of America/Gulf of Mexico sea surface temperatures, aided by a period of antecedent synoptic-scale subsidence, played a critical role in enhancing convective instability in the surface warm sector. This work underscores the importance of cataloging these events for purposes of examining (and potentially enhancing) predictability. Significance StatementThe first half of December 2021 recorded one of the most active cool-season severe weather periods in the United States, resulting in two billion-dollar convective outbreaks on 10 and 15 December. This study links these extreme events to upstream dynamical processes over the North Pacific, including extratropical cyclogenesis, the recurvature of Typhoon Nyatoh, and the retraction of the North Pacific jet. These processes amplified downstream flow and configured synoptic environments favorable for severe weather across the United States. Additionally, anomalously warm Gulf of America/Gulf of Mexico sea surface temperatures enhanced convective instability. By identifying these key precursors, this work highlights the potential for improved anticipation of extended-range severe weather likelihood—particularly during the cool season—when such events remain rare but highly impactful. 

Abstract A rapidly deepening extratropical cyclone moved across the central Great Plains on 15 December 2021 and resulted in simultaneous extreme weather events. A derecho developed at the cold front and moved from the eastern half of Kansas to Wisconsin. Simultaneously, a nonconvective mesoscale windstorm occurred on the southwest side of the cyclone and moved from western to central Kansas and is the focus of this study. The windstorm downed power lines and triggered a wildfire outbreak covering over 160 000 ac (650 km²) resulting in two fatalities, several injuries, and the loss of hundreds of cattle. Surface wind gusts exceeded 50 kt (26 m s⁻¹) over a large area in western Kansas with a peak gust of 87 kt (45 m s⁻¹) observed at Russell, Kansas, on the southeast flank of the largest wildfire in the region. The extratropical cyclone resembled the Shapiro–Keyser conceptual model with the mesoscale windstorm focused near the cloud head and southern tip of the bent-back front southwest of the cyclone center. The near-surface wind speeds were highest where three airstreams—one along the bent-back front and the other two at higher altitudes to the west of the cyclone—descended and accelerated in a higher horizontal pressure gradient region near the tip of the bent-back front and cloud head. While the nonconvective mesoscale windstorm did not meet the exact definition of a sting jet, it exhibited many of the same characteristics and physical mechanisms that drive sting jets with oceanic Shapiro–Keyser cyclones. 

Abstract Composite analyses of NOAA satellite‐based outgoing longwave radiation data and ERA5 reanalysis data for nearly six solar maximum periods support the existence of a response of tropical convection and precipitation to short‐term (∼27‐day) solar ultraviolet variations. Following solar UV peaks, the response consists of an increase in average convection and precipitation in the equatorial Indian Ocean and a decrease in the western and central tropical Pacific, with maximum amplitude at a lag of 4 to 8 days. The opposite occurs following short‐term solar UV minima. The observed responses are most detectable when the Madden‐Julian oscillation (MJO) is active and appear to be related to a reduced ability of the MJO to propagate across the Maritime Continent barrier following solar UV peaks relative to UV minima. A similar behavior has previously been found when the stratospheric quasi‐biennial oscillation is in its westerly phase relative to its easterly phase. 

Abstract A modulation has been identified of the tropical Madden‐Julian oscillation (MJO) by the stratospheric quasi‐biennial oscillation (QBO) such that the MJO in boreal winter is ∼40% stronger and persists ∼10 days longer during the easterly QBO phase (QBOE) than during the westerly phase. A proposed mechanism is reductions of tropical lower stratospheric static stability during QBOE caused by (a) the QBO induced meridional circulation; and (b) QBO influences on extratropical wave forcing of the stratospheric residual meridional circulation during early winter. Here, long‐term variability of the QBO‐MJO connection and associated variability of near‐tropopause tropical static stability and extratropical wave forcing are investigated using European Center reanalysis data for the 1959–2021 period. During the most reliable (post‐satellite) part of the record beginning in 1979, a strengthening of the QBO‐MJO modulation has occurred during a time when tropical static stability in the lowermost stratosphere and uppermost troposphere has been decreasing and extratropical wave forcing in early winter has been increasing. A high inverse correlation (R = −0.87) is obtained during this period between early winter wave forcing anomalies and wintertime tropical lower stratospheric static stability. Regression relationships are used to show that positive trends in early winter wave forcing during this period have likely contributed to decreases in tropical static stability, favoring a stronger QBO‐MJO connection. As shown in previous work, increased sea level pressure anomalies over northern Eurasia produced by Arctic sea ice loss may have been a significant source of the observed positive trends in early winter wave forcing. 

A connection between the quasi‐biennial oscillation (QBO), solar variability, and the short‐term convective climate oscillation, the Madden‐Julian oscillation (MJO), in boreal winter has been found in observational data, yet it is generally lacking in current global climate models (GCMs). A proposed mechanism is changes in tropical lower stratospheric upwelling rates and static stability caused by QBO and solar UV effects on extratropical wave forcing of the stratospheric residual meridional circulation (the Brewer‐Dobson circulation). The extent to which this mechanism, which operates only in boreal winter and enhances similar effects of the QBO‐induced meridional circulation, is simulated in a series of GCMs participating in the Coupled Model Intercomparison Project 6 (CMIP6) is investigated. The models are found to be often lacking complete representation of several elements of the mechanism, with particular issues being QBOs that are westerly biased and weak in the lower stratosphere, insufficient solar or QBO modulation of extratropical wave activity (the Holton‐Tan effect), too weak reductions in equatorial tropopause static stability in response to extratropical wave forcing, and MJOs that in some cases do not respond to these reductions. Through bypassing many of these deficiencies via data selection, it is demonstrated that effects on the MJO that resemble those found in observations (strengthening of the MJO following early winter sudden stratospheric warmings and during easterly QBO winters) can be simulated by a subset of the models. This supports operation of the proposed mechanism, and points to needed model improvements, although caveats exist and further work is needed. 

Possible sources of the observed modulation of the tropical Madden‐Julian oscillation (MJO) by the stratospheric quasi‐biennial oscillation (QBO) and the 11‐year solar cycle are investigated using 41 years of reanalysis data and archived climate model data. Larger upward fluxes of extratropical planetary‐scale waves, leading in some cases to sudden stratospheric warmings (SSWs), are observed in late fall and early winter during the easterly phase of the QBO than during the westerly phase (the “Holton‐Tan effect”). A similar but smaller increase occurs, on average, during solar minima relative to solar maxima. In addition to the warming at high latitudes, extratropical wave forcing events produce cooling and reduced static stability in the tropical lower stratosphere. Here, it is found that if SSWs occur in early winter (before ∼mid‐January), the reduced static stability produces, on average, a statistically significant, lagged strengthening of the MJO. This therefore represents a possible mechanism for producing, or at least enhancing, the observed QBO and solar modulations of the MJO in boreal winter. An initial analysis of archived climate model data shows that at least one model version with realistic QBO and solar forcing and with 4 X CO2 forcings partly simulates both of these characteristics (QBO/solar modulation of early winter wave forcing and lagged strengthening of the MJO following early winter SSWs). However, the modeled MJO is insufficiently sensitive to QBO‐induced static stability reductions, precluding simulation of the QBO‐MJO connection.