The Predictability of Ocean Environments that Contributed to the 2020/21 Extreme Cold Events in China:2020/21 La Niña and 2020 Arctic Sea Ice Loss

Several consecutive extreme cold events impacted China during the first half of winter 2020/21,breaking the low-temperature records in many cities.How to make accurate climate predictions of extreme cold events is still an urgent issue.The synergistic effect of the warm Arctic and cold tropical Pacific has been demonstrated to intensify the intrusions of cold air from polar regions into middle-high latitudes,further influencing the cold conditions in China.However,climate models failed to predict these two ocean environments at expected lead times.Most seasonal climate forecasts only predicted the 2020/21 La Ni?a after the signal had already become apparent and significantly underestimated the observed Arctic sea ice loss in autumn 2020 with a 1-2 month advancement.In this work,the corresponding physical factors that may help improve the accuracy of seasonal climate predictions are further explored.For the 2020/21 La Ni?a prediction,through sensitivity experiments involving different atmospheric-oceanic initial conditions,the predominant southeasterly wind anomalies over the equatorial Pacific in spring of 2020 are diagnosed to play an irreplaceable role in triggering this cold event.A reasonable inclusion of atmospheric surface winds into the initialization will help the model predict La Ni?a development from the early spring of 2020.For predicting the Arctic sea ice loss in autumn 2020,an anomalously cyclonic circulation from the central Arctic Ocean predicted by the model,which swept abnormally hot air over Siberia into the Arctic Ocean,is recognized as an important contributor to successfully predicting the minimum Arctic sea ice extent.     

The 2020/21 Extremely Cold Winter in China Influenced by the Synergistic Effect of La Niña and Warm Arctic

In the first half of winter 2020/21,China has experienced an extremely cold period across both northern and southern regions,with record-breaking low temperatures set in many stations of China.Meanwhile,a moderate La Ni?a event which exceeded both oceanic and atmospheric thresholds began in August 2020 and in a few months developed into its mature phase,just prior to the 2020/21 winter.In this report,the mid?high-latitude large-scale atmospheric circulation anomalies in the Northern Hemisphere,which were forced by the negative phase of Arctic Oscillation,a strengthened Siberian High,an intensified Ural High and a deepened East Asian Trough,are considered to be the direct reason for the frequent cold surges in winter 2020/21.At the same time,the synergistic effect of the warm Arctic and the cold tropical Pacific(La Ni?a)provided an indispensable background,at a hemispheric scale,to intensify the atmospheric circulation anomalies in middle-to-high latitudes.In the end,a most recent La Ni?a prediction is provided and the on-coming evolution of climate is discussed for the remaining part of the 2020/21 winter for the purpose of future decision-making and early warning.     

Is the Arctic Surface Layer a Source and Sink of NOx in Winter/Spring?

Measurements of NO_x (NO +NO₂) and the sum of reactive nitrogenconstituents, NO_y, were made near the surface atAlert (82.5°N), Canada during March and April1998. In early March when solar insolation was absentor very low, NO_x mixing ratios were frequentlynear zero. After polar sunrise when the sun was abovethe horizon for much or all of the day a diurnalvariation in NO_x and NO_y was observed withamplitudes as large as 30–40 pptv. The source ofactive nitrogen is attributed to release from the snowsurface by a process that is apparently sensitized bysunlight. If the source from the snowpack is a largescale feature of the Arctic then the diurnal trendsalso require a competing process for removal to thesurface. From the diurnal change in the NO/NO₂ratio, mid-April mixing ratios for the sum of peroxyand halogen oxide radicals of 10 pptv werederived for periods when ozone mixing ratios were inthe normal range of 30–50 ppbv. Mid-day ozoneproduction and loss rates with the active nitrogensource were estimated to be 1–2 ppbv/day and in nearbalance. NO_y mixing ratios which averaged only295±66 pptv do not support a large accumulation inthe high Arctic surface layer in the winter and springof 1998. The small abundance of NO_y relative tothe elevated mixing ratios of other long-livedanthropogenic constituents requires that reactivenitrogen be removed to the surface during transport toor during residence within the high Arctic.     

Extremely Active Tropical Cyclone Activities over the Western North Pacific and South China Sea in Summer 2018: Joint Effects of Decaying La Ni?a and Intraseasonal Oscillation

In summer 2018, a total of 18 tropical cyclones(TCs) formed in the western North Pacific(WNP) and South China Sea(SCS), among which 8 TCs landed in China, ranking respectively the second and the first highest since 1951.Most of these TCs travelled northwest to northward, bringing in heavy rainfall and strong winds in eastern China and Japan. The present study investigates the impacts of decaying La Ni?a and intraseasonal oscillation(ISO) on the extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses. It is found that the La Ni?a episode from October 2017 to March 2018 led to above-normal sea surface temperature(SST) over central–western Pacific, lower sea level pressure and 500-hPa geopotential height over WNP, and abnormally strong convective activities over the western Pacific in summer 2018. These preceding oceanic thermal conditions and their effects on circulation anomalies are favorable to TC genesis in summer. Detailed examination reveals that the monsoon trough was located further north and east, inducing more TCs in northern and eastern WNP; and the more eastward WNP subtropical high as well as the significant wave train with a "-+-+" height anomaly pattern over the midlatitude Eurasia–North Pacific region facilitated the northwest to northward TC tracks. Further analyses reveal that two successively active periods of Madden–Julian Oscillation(MJO) occurred in summer 2018 and the boreal summer intraseasonal oscillation(BSISO) was also active over WNP, propagating northward significantly, corresponding to the more northward TC tracks. The MJO was stagnant over the Maritime Continent to western Pacific,leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere, conducive to TC occurrences. In a word, the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Ni?a, and the MJO and BSISO; their joint effects result in increased TC occurrences and the TC tracks being shifted more northwest to northward than normal.