Extreme Cold Events in North America and Eurasia in November-December 2022: A Potential Vorticity Gradient Perspective

From 17 November to 27 December 2022, extremely cold snowstorms frequently swept across North America and Eurasia. Diagnostic analysis reveals that these extreme cold events were closely related to the establishment of blocking circulations. Alaska Blocking(AB) and subsequent Ural Blocking(UB) episodes are linked to the phase transition of the North Atlantic Oscillation(NAO) and represent the main atmospheric regimes in the Northern Hemisphere. The downstream dispersion and propagation of Rossby wave packets from Alaska to East Asia provide a large-scale connection between AB and UB episodes. Based on the nonlinear multi-scale interaction(NMI) model, we found that the meridional potential vorticity gradient(PVy) in November and December of 2022 was anomalously weak in the mid-high latitudes from North America to Eurasia and provided a favorable background for the prolonged maintenance of UB and AB events and the generation of associated severe extreme snowstorms. However, the difference in the UB in terms of its persistence,location, and strength between November and December is related to the positive(negative) NAO in November(December). During the La Ni?a winter of 2022, the UB and AB events are related to the downward propagation of stratospheric anomalies, in addition to contributions by La Ni?a and low Arctic sea ice concentrations as they pertain to reducing PVyin mid-latitudes.

Seasonal Cumulative Effect of Ural Blocking Episodes on the Frequent Cold events in China during the Early Winter of 2020/21

Starting in mid-November,China was hit by several cold events during the early winter of 2020/21.The lowest temperature observed at Beijing station on 7 January reached−19.6°C.In this paper,we show that the outbreak of the record-breaking extreme cold event can be attributed to a huge merging Ural blocking(UB)ridge over the Eurasian region.The sea-ice cover in the Kara and East Siberia Seas(KESS)in autumn was at its lowest value since 1979,which could have served as a precursor signal.Further analysis shows that several successive UB episodes occurred from 1 September 2020 to 10 January 2021.The persistent UB that occurred in late September/early October 2020 may have made an important contribution to the October historical minimum of sea ice in the KESS region.Our results also show that,after each UB episode in winter,significant upward propagation of wave activity occurred around 60°E,which resulted in weakening the stratospheric vortex.Meanwhile,each UB episode also caused a significant reduction in sea-ice extent in KESS and a significant weakening of the westerly jet in mid-high-latitude Eurasia.Results suggest that the Arctic vortex,which is supposed to enhance seasonally,became weaker and more unstable than the climatic mean under the seasonal cumulative effects of UB episodes,KESS warming,and long-lasting negative-phase North Atlantic Oscillation(NAO-).Those seasonal cumulative effects,combined with the impact of La Niña winter,led to the frequent occurrence of extreme cold events.

Quantifying the Spatial Characteristics of the Moisture Transport Affecting Precipitation Seasonality and Recycling Variability in Central Asia

Moisture contribution and transport pathways for Central Asia(CA)are quantitatively examined using the Lagrangian water cycle model based on reanalysis and observational data to explain the precipitation seasonality and the moisture transport variation during 1979-2015.Westerly-related(northwesterly and westerly)transport explains 42%of CA precipitation and dominates in southwest CA,where precipitation is greatest in the cold season.Southeast CA,including part of Northwest China,experiences its maximum precipitation in the warm season and is solely dominated by southerly transport,which explains about 48%of CA precipitation.The remaining 10%of CA precipitation is explained by northerly transport,which steadily impacts north CA and causes a maximum in precipitation in the warm season.Most CA areas are exposed to seasonally varying moisture transport,except for southeast and north CA,which are impacted by southerly and northerly transport year-round.In general,the midlatitude westerlies-driven transport and the Indian monsoon-driven southerly-related transport explain most of the spatial differences in precipitation seasonality over CA.Moreover,the contribution ratio of local evaporation in CA to precipitation exhibits significant interdecadal variability and a meridionally oriented tripole of moisture transport anomalies.Since the early 2000s,CA has experienced a decade of anomalously low local moisture contribution,which seems jointly determined by the weakened moisture contribution from midlatitudes(the Atlantic,Europe,and CA itself)and the enhanced contribution from high latitudes(West Siberia and the Arctic)and tropical areas(South Asia and the Indian Ocean).

Frequent central Pacific La Nina events may accelerate Arctic warming since the 1980s

Including significant warming trend,Arctic climate changes also exhibit strong interannual variations in various fields,which is suggested to be related to El Nino and Southern Oscillation(ENSO)events.Previous studies have demonstrated the different impacts on the Arctic of central Pacific(CP)and eastern Pacific(EP)ENSO events,and suggested these impacts are largely of opposite sign for ENSO warm and cold phases.Our results illustrate asymmetrical changes for the cold and warm ENSO events,especially for the La Nina events.Compared to the past frequent basin-wide cooling La Nina events,since the 1980 s the cooling center for the La Nina event has strengthened and moved westward along with the increasing frequency for the canonical and CP La Nina events.Contrary to the basin-wide cooling and canonical La Nina events,the frequent CP La Nina events induce significant warming from the Beaufort Sea to Greenland via the convection center moving northward over the western Pacific.Observation analysis and numerical experiments both suggest that the changes in La Nina type may also accelerate Arctic warming.

Datasets of meteorological drought events and risks for the developing countries in Eurasia

For the area of Eurasia concentrated with developing countries(referred to here by the abbreviation DPEA),mainly located in Asia and Eastern Europe,this work presents datasets of gridded meteorological drought events and country-based drought risk by combining multiple drought indices and socio-economic data.A basic gridded dataset of the drought events during 1950-2015 is extracted from three drought indices:the self-calibrating Palmer Drought Severity Index,the Standardized Precipitation Index,and the Standardized Precipitation Evapotranspiration Index.The three drought indices generally show consistent characteristics of drought events in DPEA.A second-level dataset(a drought risk dataset)is then produced as the product of drought hazard,exposure and vulnerability during 2000-2015.For drought exposure,the indicators of population and livestock density,agricultural land and water stress are chosen,while drought vulnerability composites multiple social,economic and infrastructural factors.Drought hazard tends to concentrate at the southern rim of Eurasia.Relatively large differences in drought exposure exist between different countries,but for drought vulnerability the differences are small.After considering the socio-economic components in risk assessment,most countries in West,South-Central and South Asia have the highest drought risk in DPEA.The datasets of drought events and risks are available at http://www.dx.doi.org/10.11922/sciencedb.898.