Subseasonal, interannual, and long-term trend relationship between Arctic and Eurasian climate variability

Abstract

The statistical relationship between Arctic and Eurasian surface air temperature (SAT) has been observed in multiple time scale, from subseasonal to multi-decadal time scales. This relationship and the underlying mechanisms are revisited using reanalysis data and climate model experiments. Since the possible mechanisms could differ by the time scale, the subseasonal and interannual-to-long term trend relationship are investigated separately. The subseasonal relationship between Arctic and Eurasian SAT is re-examined first. Consistent with previous studies, a significant negative correlation is observed in cold season from November to February, but with a local minimum in late December. This relationship is dominated not only by the warm Arctic-cold Eurasia (WACE) pattern, which becomes more frequent during the last two decades, but also by the cold Arctic-warm Eurasia (CAWE) pattern. The budget analyses reveal that both WACE and CAWE patterns are primarily driven by the temperature advection associated with sea level pressure anomaly over the Ural region, partly cancelled by the diabatic heating. It is further found that, although the anticyclonic anomaly of WACE pattern mostly represents the Ural blocking, about 20% of WACE cases are associated with nonblocking high pressure systems. This result indicates that the Ural blocking is not a necessary condition for the WACE pattern, highlighting the importance of transient weather systems in the subseasonal Arctic-Eurasian SAT co-variability. The interannual-to-long term trend relationship between the Arctic and Eurasia is also investigated. Unlike the subseasonal Arctic-Eurasian relationship, Arctic sea ice is likely attribute to the recent Eurasian winter SAT change on interannual-to-decadal time scale. The present study quantifies the statistical relationship between Arctic sea-ice loss and Eurasian winter surface air-temperature change by a break-point trend analysis and maximum covariance analysis. A significant time-lagged covariability is observed between the Arctic sea-ice concentration over the Barents–Kara seas and the Eurasian winter surface air temperature, with the former leading the latter by approximately two months. More importantly, the timing of an abrupt decline in the autumn Arctic sea ice that occurred in the late 1990s is coincident with the beginning of the Eurasian winter cooling. This concurrent trend change is statistically significant and robustly found in both the break-point analysis and maximum covariance analysis. These results suggest that both the interannual variability and decadal trend change of the Eurasian winter surface air temperature are likely influenced by regional sea-ice changes over the Barents–Kara seas. However, climate models often fail to reproduce the impact of Arctic sea ice change on Eurasian winter cooling in historical simulations. The present study re-examines the impact of Arctic sea ice loss on the Eurasian winter SAT trend by isolating the effect of the sea ice loss in coupled model simulations. In the large ensemble simulations, Eurasian winter cooling is observed in only half of the ensemble members, whereas others show warming trend, resulting in a near-zero trend when all 35 ensemble members are averaged. The cooling trend appears when the polar warming is not confined to the near-surface but extends deep into the upper troposphere with a hint of stratospheric warming. When the polar warming is shallow in the model, the Eurasian SAT trend becomes positive. This result suggests that Eurasian winter cooling in the recent past is most likely caused by atmospheric internal variability, and highly dependent on the vertical extent of a polar warming and stratospheric processes, rather than Arctic surface warming itself.

북극과 유라시아 지표 기온은 계절내 규모에서 장기 트렌드에 이르기까지 다양한 시간 규모에서 높은 통계적 연관성을 보인다. 본 연구에서는 크게 계절내 규모와 경년 및 장기 변동성으로 구분하여 이러한 연관성을 설명하는 메커니즘을 재검증하였다. 먼저 북극과 유라시아 기온 사이의 계절내 변동성의 경우, 11월부터 2월까지의 추운 계절에만 발생하며 12월에 약화되는 계절 변동성이 나타나는 것을 확인하였다. 기존 연구에서는 계절내 규모의 북극-유라시아 기온 변동성을 좌우하는 것은 우랄 지역의 블로킹에 의해 유도된 따뜻한 북극-차가운 유라시아 기온 패턴이라고 알려져 왔으나, 반대 사례인 차가운 북극-따뜻한 유라시아 기온 패턴 역시 겨울철에 나타남을 확인하였다. 즉 본 연구 결과는 북극과 유라시아 기온의 계절내 변동성을 좌우하는 것은 우랄 블로킹이 아닌, 우랄지역을 지나가는 기상 시스템임을 확인하였다. 또한 경년 및 장기 트렌드 시간 규모에서 역시 북극과 유라시아 기온 사이의 높은 연관성이 나타난다. 가을철부터 급격하게 감소하는 북극 해빙이 최근 겨울철의 급격한 기온 감소현상의 원인으로 지목되었으며, 통계분석 결과 경년 및 장기 트렌드 시간 규모에서 높은 연관성을 보임을 정량적으로 검증하였다. 그러나 모형 실험 결과 북극 해빙은 겨울철 유라시아의 기온 변동성을 야기하는 원인으로는 충분하지 않음을 확인하였다. 오히려 북극 해빙을 실험에 반영하였을 때, 북극 기온의 변화가 연직으로 잘 발달하는 일부 실험에서만 유라시아에 강한 기온 감소 현상이 나타나는 것이 확인되었다. 이 결과는 유라시아 겨울철 기온 감소현상이 대기 내부 변동성에 크게 의존하고 있음을 시사한다.