Sub-seasonal to Seasonal Hindcasts of Stratospheric Sudden Warming by BCC_CSM1.1(m):A Comparison with ECMWF

This study focuses on model predictive skill with respect to stratospheric sudden warming(SSW) events by comparing the hindcast results of BCC_CSM1.1(m) with those of the ECMWF's model under the sub-seasonal to seasonal prediction project of the World Weather Research Program and World Climate Research Program. When the hindcasts are initiated less than two weeks before SSW onset, BCC_CSM and ECMWF show comparable predictive skill in terms of the temporal evolution of the stratospheric circumpolar westerlies and polar temperature up to 30 days after SSW onset. However, with earlier hindcast initialization, the predictive skill of BCC_CSM gradually decreases, and the reproduced maximum circulation anomalies in the hindcasts initiated four weeks before SSW onset replicate only 10% of the circulation anomaly intensities in observations. The earliest successful prediction of the breakdown of the stratospheric polar vortex accompanying SSW onset for BCC_CSM(ECMWF) is the hindcast initiated two(three) weeks earlier. The predictive skills of both models during SSW winters are always higher than that during non-SSW winters, in relation to the successfully captured tropospheric precursors and the associated upward propagation of planetary waves by the model initializations. To narrow the gap in SSW predictive skill between BCC_CSM and ECMWF, ensemble forecasts and error corrections are performed with BCC_CSM. The SSW predictive skill in the ensemble hindcasts and the error corrections are improved compared with the previous control forecasts.

An Isentropic Mass Circulation View on the Extreme Cold Events in the 2020/21 Winter

Three extreme cold events successively occurred across East Asia and North America in the 2020/21 winter.This study investigates the underlying mechanisms of these record-breaking persistent cold events from the isentropic mass circulation(IMC)perspective.Results show that the midlatitude cold surface temperature anomalies always co-occurred with the high-latitude warm anomalies,and this was closely related to the strengthening of the low-level equatorward cold air branch of the IMC,particularly along the climatological cold air routes over East Asia and North America.Specifically,the two cold surges over East Asia in early winter were results of intensification of cold air transport there,influenced by the Arctic sea ice loss in autumn.The weakened cold air transport over North America associated with warmer northeastern Pacific sea surface temperatures(SSTs)explained the concurrent anomalous warmth there.This enhanced a wavenumber-1 pattern and upward wave propagation,inducing a simultaneous and long-lasting stronger poleward warm air branch(WB)of the IMC in the stratosphere and hence a displacement-type Stratospheric Sudden Warming(SSW)event on 4 January.The WB-induced increase in the air mass transported into the polar stratosphere was followed by intensification of the equatorward cold branch,hence promoting the occurrence of two extreme cold events respectively over East Asia in the beginning of January and over North America in February.Results do not yield a robust direct linkage from La Niña to the SSW event,IMC changes,and cold events,though the extratropical warm SSTs are found to contribute to the February cold surge in North America.

Evaluating the Brewer–Dobson circulation and its responses to ENSO,QBO,and the solar cycle in different reanalyses

This study compares the climatology and long-term trend of northern winter stratospheric residual mean meridional circulation(RMMC), as well as its responses to El Ni?o-Southern Oscillation(ENSO), stratospheric Quasi Biennial Oscillation(QBO), and solar cycle in ten reanalyses and a stratosphere-resolving model, CESM1-WACCM. The RMMC is a large-scale meridional circulation cell in the stratosphere, usually referred to as the estimate of the Brewer Dobson circulation(BDC). The distribution of the BDC is generally consistent among multiple reanalyses except that the NOAA twentieth century reanalysis(20RC) largely underestimates it. Most reanalyses(except ERA40 and ERA-Interim) show a strengthening trend for the BDC during 1979–2010. All reanalyses and CESM1-WACCM consistently reveal that the deep branch of the BDC is significantly enhanced in El Ni?o winters as more waves from the troposphere dissipate in the stratospheric polar vortex region. A secondary circulation cell is coupled to the temperature anomalies below the QBO easterly center at 50 hPa with tropical upwelling/cooling and midlatitude downwelling/warming, and similar secondary circulation cells also appear between 50–10 hPa and above 10 hPa to balance the temperature anomalies. The direct BDC response to QBO in the upper stratosphere creates a barrier near 30°N to prevent waves from propagating to midlatitudes, contributing to the weakening of the polar vortex. The shallow branch of the BDC in the lower stratosphere is intensified during solar minima, and the downwelling warms the Arctic lower stratosphere. The stratospheric responses to QBO and solar cycle in most reanalyses are generally consistent except in the two 20 CRs.

Erratum to: An Isentropic Mass Circulation View on the ExtremeCold Events in the 2020/21Winter

Erratum to:Yu,Y.Y.,Y.F.Li,R.C.Ren,M.Cai,Z.Y.Guan,and W.Huang,2021:An isentropic mass circulation view on the extreme cold events in 2020/21 Winter.Adv.Atmos.Sci.,38(6),957−965,https://doi.org/10.1007/s00376-021-1289-2.

Spatial and Temporal Variability of Solar Irradiance in Botswana

This study evaluated the potential of Botswana’s sustainable energy production using ERA5 reanalysis data of solar irradiance variability on an optimally inclined plane from 1971 to 2020. Spatial-temporal solar irradiance fluctuations were the focus of the study, and the relation to cloud cover and aerosol optical depth was investigated. The key findings suggest that the summer/rainfall season (November to March) is the peak season with average monthly solar irradiance of 313 - 445 W/m2 across southern, central, and northern parts of Botswana, the Kalahari Desert and the Makgadikgadi Pans being identified as prime sites for solar energy projects. The long-term trend analysis showed a decrease in solar irradiance in December but a consistent increase from August to October, indicating a potential shift in solar resources toward an earlier season. Contrary to other studies that found that aerosol optical depth dominates effects on long-term trends and year-to-year variability of solar irradiance, for this case, cloud cover, particularly mid-level clouds, is found to have a more dominant role in Botswana. Solar irradiance characteristics of three distinct regions were identified through K-means clustering. Moreover, Ensemble Empirical Mode Decomposition (EEMD) analysis showed the commonality and time scale linkage between solar irradiance and cloud cover between the identified regions. These results highlight the importance of including cloud-related weather patterns under the global warming scenario in solar energy planning and emphasize the secondary role of aerosols in Botswana, thus providing critical information for the region’s solar energy development and policy formulation.

Does the Negative Arctic Oscillation Always Favor Winter PM_(2.5) Diffusion in North China?

Previous studies have reported a close relationship between the negative Arctic Oscillation(AO)and the PM_(2.5)(particulate matter with a diameter of 2.5μm or less)diffusion in North China in winter.Using the North China regional mean meridional wind at 850 hPa derived from the ERA5(ECMWF Reanalysis version 5)reanalysis data in 1979–2022 as a useful substitute for station observed PM_(2.5) concentration(since the latter is available only since 2014),our study detected strong/weak northerly events representing the abnormal PM_(2.5) diffusion/accumulation events,and revisited the AO–PM_(2.5) diffusion relationship in North China during 1979–2022.The results show that only when the AO was characterized by a 2-month continuously negative/positive phases and with twin peaks respectively before and after the diffusion/accumulation events,would there be higher occurrences of the abnormal PM_(2.5) diffusion/accumulation.The second peak of negative AO acted to prolong the strong northerly winds by an average of 2 days.Further analysis reveals that the AO with twin peaks always has a footprint in the stratospheric northern annular mode(NAM)during the abnormal PM_(2.5) events,and the coupling between the stratosphere and troposphere plays a critical role in the second peak of AO.Vertical propagation of baroclinically amplifying waves leads to changes in isentropic meridional mass fluxes in the stratosphere following the changes in the troposphere.The stronger/weaker poleward mass fluxes increase/decrease the polar mass in the stratosphere,which dominates the total column air mass changes and leads to the second peak of AO.Considering the subseasonal predictability of the stratospheric NAM based on existing evidence,particular attention should be paid to these AO-related abnormal PM_(2.5) diffusion and accumulation events in North China because they might be more predictable at a longer lead time.