School of Geography, Earth and Atmospheric Sciences - Research Publications
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ItemAnchoring of atmospheric teleconnection patterns by Arctic Sea ice loss and its link to winter cold anomalies in East Asia(WILEY, 2021-01)Abstract In this paper, the physical processes underlying recent winter cold anomalies over East Asia (EA) are examined via statistical analysis. It is found that the EA cold anomaly depends on the warming in the North Atlantic, sea ice loss in the Barents–Kara Sea (BKS), and atmospheric teleconnection patterns. Specifically, the sea ice loss in the BKS can anchor teleconnection patterns originating from different North Atlantic sea surface temperature (SST) patterns. Different patterns of North Atlantic warming can affect the position of the cold anomaly region through altering the atmospheric circulations. In addition, whether the relevant teleconnection pattern leads to enhanced cold anomaly over EA crucially depends on the sea ice loss in the BKS, because it can anchor the blocking anticyclone embedded in the teleconnection pattern over the Ural region and make it more persistent and quasi‐stationary. Furthermore, it is found that the role of SST modes in the EA cold anomaly depends on their time scales. Although the strong basin‐scale warming (north–south SST tripolar mode) in the North Atlantic mid‐ to high‐latitudes plays a major role in decadal (interannual) cold anomaly over EA, it appears that the Atlantic east–west SST dipole structure dominates winter temperature variations over EA in recent decades on both the interannual and decadal time scales.
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ItemNo Preview AvailableCombined impact of the cold vortex and atmospheric blocking on cold outbreaks over East Asia and the potential for short-range prediction of such occurrences(IOP Publishing Ltd, 2022-08-01)Abstract This study explores the consequences of independent and combined effects of blockings on the northeast Asian cold vortex (NACV), and corresponding cold outbreaks over East Asia (EA) during boreal winters of 1979–2019. The results show that the development of NACV is closely associated with blocking over the Ural Mountains (UB) upstream and eastern Siberia to mid-North Pacific (SPB) downstream. Here we focus on the initial periods before the peak day of NACV events. It is found that the strong NACV events are usually induced by the initial-UB situation, leading to the greatest temperature drop in EA. While the weak NACV events may be associated with the initial-SPB condition, which can bring less dramatic outbreaks but longer duration, owing to the Ural ridge that formed by the westward shift of SPB. Furthermore, an SPB-UB relay effect is discovered against the background of a negative Arctic Oscillation pattern. In such cases, UB is formed by the westward shift of downstream SPB after the occurrence of NACV, forming a relay effect that motivates the second NACV process, hence prolonging the duration of cold anomalies in EA. These findings highlight the importance of the combined effect of blockings and NACV in the intraseasonal time scale. Compared to the ‘Initial-UB’ and ‘Initial-SPB’ situation, this ‘SPB-UB relay’ scenario can produce longer-lasting cold extreme in EA, which may be indicative of the short-term weather forecasting of such extreme cold weather.
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ItemNo Preview AvailableThe key atmospheric drivers linking regional Arctic amplification with East Asian cold extremes(ELSEVIER SCIENCE INC, 2023-03-01)
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ItemNo Preview AvailableDecadal Variability of Winter Warm Arctic-Cold Eurasia Dipole Patterns Modulated by Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation(AMER GEOPHYSICAL UNION, 2022-01)Abstract In recent decades, the winter surface air temperature (SAT) anomaly in the Northern Hemisphere has exhibited a warm Arctic‐cold Eurasia (WACE) dipole pattern and this has undergone significant decadal variation. In this paper, the physical cause of the decadal variability of the WACE pattern is explored, and it is shown to be mediated by the phases of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Although the negative PDO (PDO−) or positive AMO (AMO+) favors the WACE pattern, the meridional structure of the pattern is significantly influenced by whether PDO− or AMO+ dominates. During PDO− (AMO+) phase, the winter‐mean WACE pattern shows an asymmetric dipole with weak (strong) warm anomaly over the Barents‐Kara Seas or BKS and strong (weak) cold anomaly over central Eurasia or Siberia, which corresponds to Ural blocking (UB) concurring with the negative (positive) phase of North Atlantic Oscillation in a winter‐mean field. The winter asymmetric WACE pattern is more strongly related to PDO− than AMO+. It is further found that an asymmetric sub‐seasonal WACE dipole pattern with strong (weak) warm anomaly over BKS and weak (strong) cold anomaly over Siberia is usually formed during the UB episode due to favored (suppressed) sub‐seasonal downward infrared radiation and turbulent heat flux over BKS during AMO+ (PDO−), which leads to a strong asymmetric dipole of the winter WACE pattern.
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ItemLinkages between the atmospheric transmission originating from the North Atlantic Oscillation and persistent winter haze over Beijing(COPERNICUS GESELLSCHAFT MBH, 2021-12-21)Abstract. In this study, the persistent winter haze that occurred over Beijing during 1980 to 2016 is examined using reanalysis and station data. On both interannual and daily-to-weekly timescales, the winter haze weather in Beijing is found to be associated with a pronounced atmospheric teleconnection pattern from the North Atlantic to Eurasia (Beijing). A positive western-type North Atlantic Oscillation (WNAO+) phase and a positive East Atlantic/West Russia (EA/WR+) phase are observed as part of this teleconnection pattern (an arched wave train). This study focuses on the role of the WNAO pattern, because the WNAO+ pattern acts as the origin of the atmospheric transmission, 8–10 d before the persistent haze events. Further analyses reveal that the WNAO+ pattern can increase the number of haze days and persistent haze events on interannual and daily-to-weekly timescales. Specifically, strong WNAO+ winters (above the 95th percentile) can increase the number of haze days and persistent haze events by 26.0 % and 42.3 %, respectively. In addition, a high WNAO index for the 5 d average (above the 95th percentile) predicts a 16.9 % increase in the probability of haze days on Day 8 and a higher proportion of persistent haze days compared with an unknown WNAO state. Thus, the WNAO+ pattern is as a necessary prior background condition for the formation of the wave train and is a skillful predictor for persistent hazy weather. Corresponding to the WNAO+ pattern, intensified zonal wind and a north–south sea surface temperature tripolar mode over the North Atlantic also appear before persistent haze events on the daily-to-weekly timescale. On the interannual timescale, winters with a greater number of persistent haze days are also associated with a tripolar sea surface temperature (SST) mode over the North Atlantic that is situated farther northward.
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ItemNo Preview AvailableCollaborative impact of the NAO and atmospheric blocking on European heatwaves, with a focus on the hot summer of 2018(IOP PUBLISHING LTD, 2020-11)Abstract Two intense heatwaves of July and early August 2018 are found to be associated with a European blocking (EB) event accompanied by a series of consecutive positive North Atlantic Oscillation (NAO+) events. Further analyses show that the collaborative role of an EB event and its upstream NAO+ pattern could increase the frequency, persistence, magnitude and scale of heatwaves over Europe. Compared with NAO+-unrelated EB events, NAO+-related EB events are less movable (quasi-stationary) and more persistent over Europe, which could contribute to an increase in the intensity and persistence of heatwaves. In addition, the blocking high of this type has a northeast–southwest orientation with stronger warm airflow and less precipitation in northern and western Europe, where large scopes of higher temperatures tend to occur. In contrast, NAO+-unrelated EB events without orientation correspond to a trough in the south, which results in increased precipitation and cold air in the southern part of Europe, and thus high temperatures contract to the northern part of Europe. Moreover, considering that the NAO+ pattern leads the formation of an EB event, the NAO+ pattern might serve as a potential predictor for European heatwaves. Our conclusions are strongly supported by the analysis of CMIP6 historical simulations which also capture the differences of high temperatures and atmospheric circulations between NAO+-related EB events and NAO+-unrelated EB events.
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ItemA Connection of Winter Eurasian Cold Anomaly to the Modulation of Ural Blocking by ENSO(AMER GEOPHYSICAL UNION, 2021-09-08)Abstract This study investigates how El Niño‐Southern Oscillation (ENSO) modulates winter cold anomalies over central Eurasia (CE) using reanalysis data during 1950–2019. It is found that ENSO can significantly influence winter air temperature over CE mainly through modulating the strength and location of the cyclonic anomaly of Ural blocking (UB) or long‐lived UB events. A strong (weak) cyclonic anomaly of UB on the southeastern (eastern) side of the Ural Mountains tends to occur over midlatitude (high‐latitude) Eurasia during La Niña (El Niño) winters. Such anomalous circulation leads to a strong (weak) cold anomaly over CE, especially over East Asia, due to enhanced (weakened) cold air advection toward the CE during La Niña (El Niño). The UB‐related cyclonic anomaly in the midlatitude (high latitude) side of Eurasia is shown to be related to weakened (enhanced) eastward extension of strong westerly winds over midlatitude Eurasia during La Niña (El Niño) winters.