[en] The satellite record since 1979 shows downward trends in Arctic sea ice extent in all months, which are smallest in winter and largest in September. Previous studies have linked changes in winter atmospheric circulation, anomalously cold extremes and large snowfalls in mid-latitudes to rapid decline of Arctic sea ice in the preceding autumn. Using observational analyses, we show that the winter atmospheric circulation change and cold extremes are also associated with winter sea ice reduction through an apparently distinct mechanism from those related to autumn sea ice loss. Associated with winter sea ice reduction, a high-pressure anomaly prevails over the subarctic, which in part results from fewer cyclones owing to a weakened gradient in sea surface temperature and lower baroclinicity over sparse sea ice. The results suggest that the winter atmospheric circulation at high northern latitudes associated with Arctic sea ice loss, especially in the winter, favors the occurrence of cold winter extremes at middle latitudes of the northern continents. (letter)

[en] 

Purpose of Review

The rapidly warming Arctic climate may affect weather in middle latitudes, but controversies remain as to mechanisms and robustness. Here, I synthesize recent advances in this rapidly changing field and summarize recommendations on paths forward.

Recent Findings

Initial “black-and-white” debates about whether Arctic amplification (AA) affects mid-latitude circulation have evolved toward a more nuanced perspective awash in gray. Recent research has demonstrated myriad ways in which AA can influence weather remotely and explored whether any Arctic-based signal is significant against the backdrop of natural variability.

Summary

The popularity of and controversies surrounding this topic have spurred a multitude of approaches and often-conflicting studies that have widened the envelope of our understanding but hindered a scientific consensus. This messy but necessary exploratory phase of independent investigations is benefiting from recent efforts by the research community to self-organize through workshops, working groups, and coordinated experiments.

[en] In summer 2018, an extraordinary heat wave with record-breaking high temperatures hit Northeast Asia. However, the contribution of atmospheric circulation to this heat wave remains unknown. In this study, we quantify the contribution of circulation by using the flow analogue method. It is found that Northeast China, Korea and Japan were the most affected areas by the heat event, from daily to monthly timescales. The persistent high temperature was associated with an anticyclonic anomaly over Northeast Asia, related to the record-breaking northward shift of the western Pacific subtropical high (WPSH). The persistent anomalous anticyclone played a dominant role in this heat event, explaining half of the magnitude of the heat event. Both thermodynamical change and dynamical change in recent decades have increased the probability of occurrence of this kind of heat event over Northeast Asia. Specifically, the change in dynamical flow explains a fraction of less than 20% of the increases in probability of heat events. The contribution of thermodynamical changes to heat events generally increases with the rarity of the extreme event. (letter)

[en] Aggregate annual discharge from the six largest Arctic-draining Eurasian rivers achieved an all-time record high in 2007, accentuating a long-term upward trend that argues for intensification of the Arctic hydrologic cycle. This record discharge was due in part to strong positive anomalies in late winter snow water equivalent across much of northern Eurasia. These anomalies arose in response to an unusual pattern of atmospheric circulation in late 2006 and early 2007, characterized by an extreme northeastward extension of the Icelandic Low and a contraction of the Siberian High. Positive net precipitation anomalies then continued into summer, further contributing to discharge.

[en] The 'weekend effect' method (defined here as the average for Saturday through Monday minus the average for Wednesday through Friday) has been used to identify fingerprints of anthropogenic emissions. Based on daily maximum and minimum temperature series from the China Meteorological Administration homogenized dataset, the weekend effect in diurnal temperature range (DTR) at 71 stations with elevations above 2000 m asl in the eastern and central Tibetan Plateau (TP) during 1961-2004 is examined, and principal component analysis (PCA) is performed to cluster series into four subregions with similar weekend effect variability. The DTR demonstrates a much stronger negative weekend effect in autumn and shows larger positive values in winter, which provides a strong evidence of anthropogenic activity in this region, especially in the central TP. Analysis by topographic type and degree of urbanization shows a clear weekly cycle which cannot be explained by a microclimate effect. We hypothesize that the interaction with anthropogenic aerosols from local emissions and transported by atmospheric circulation may account for the weekly cycle in the TP. More caution should be paid to the driving mechanism of the weekend effect in the most remote and clear regions in the world.

[en] Turbulence affects the dynamics of atmospheric processes by enhancing the transport of mass, heat, humidity and pollutants. The global objective for our work is to analyze some direct turbulent descriptors which reflect the mixing processes in the atmospheric boundary layer (ABL). In this paper we present results related to the Thorpe displacements d_Τ, the maximum Thorpe displacement (d_Τ)_max and the Thorpe scale L_Τ, the Ozmidov scale and their time evolution in the ABL during a day cycle. A tethered balloon was used to obtain vertical profiles of the atmospheric physical magnitudes up to 1000 m. We discuss the vertical and horizontal variability and how different descriptors are related to atmospheric mixing.

[en] This paper introduces three quantitative indicators to conduct research for characterizing Northeast China cold vortex persistence activity: cold vortex persistence, generalized “cold vortex,” and cold vortex precipitation. As discussed in the first part of paper, a hindcast is performed by multiple regressions using Northeast China precipitation from 2012 to 2014 combination with the previous winter 144 air-sea system factors. The results show that the mentioned three cold vortex index series can reflect the spatial and temporal distributions of observational precipitation in 2012–2014 and obtain results. The cold vortex factors are then added to the Forecast System on Dynamical and Analogy Skills (FODAS) to carry out dynamic statistical hindcast of precipitation in Northeast China from 2003 to 2012. Based on the characteristics and significance of each index, precipitation hindcast is carried out for Northeast China in May, June, July, August, May–June, and July–August. It turns out that the Northeast Cold Vortex Index Series, as defined in this paper, can make positive corrections to the FODAS forecast system, and most of the index correction results are higher than the system’s own correction value. This study provides quantitative index products and supplies a solid technical foundation and support for monthly precipitation forecast in Northeast China.

[en] A two-dimensional model for quasigeostrophic flow which exhibits an analogy with the three-dimensional incompressible Euler equations is considered. Numerical experiments show that this model develops sharp fronts without the need to explicitly incorporate any ageostrophic effect. Furthermore, these fronts appear to become singular in finite time. The numerical evidence for singular behavior survives the tests of rigorous mathematical criteria

[en] This paper describes the statistical measures used to compare several observed and predicted meteorological variables for the southeastern United States spanning two full years (April 1998 to March 2000)

[en] Low-speed streaks are considered as narrow regions of low-speed velocity fluid stretched in the stream wise direction. Two kinds of low-speed streaks and corresponding high-temperature streaks obtained with LES (Large Eddy Simulation) simulations of the near-neutral and unstable. ABLs controlled by both wind shear and buoyancy are investigated to understand the characteristics of streaks in different stratifications. In the near-neutral boundary layer, quasi-stream wise low-speed and high-temperature streaks are in almost 45⁰ to initial stream wise direction and more, thinner and longer near surface. While the low-speed and high-temperature streaks meander and sometimes are not in the stream wise direction in the unstable boundary layer. Furthermore, they are thinner and shorter near surface and many low-speed structures cannot be connected to form streaks. The low-speed structures induced by vortices associated with low-speed streaks are deep and can have a few low-speed centers in vertical direction due to the convergence and divergence of flow. These structures are different in shape in different boundary layers and are deeper in the unstable boundary layer than in the near-neutral boundary layer. In the near-neutral boundary layer, the low-speed structures are high-temperature. Therefore, temperature can be an index of low-speed structures and momentum and sensible heat fluxes are significant at low-speed and high-temperature centers. While in the unstable boundary layer, the corresponding high-temperature streaks lag behind the low-speed streaks along the local flow and low-speed structures are not always high-temperature. Therefore, temperature cannot be an index of vortices or low-speed structures and low momentum and high sensible heat fluxes deviate from the low-speed streaks.