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Wave-Breaking Features of Blocking over Central Siberia and Its Impacts on the Precipitation Trend over Southeastern Lake Baikal

Funds:

National Science and Technology Support Program of China (Grant No. 2015BAC03B03) and the National Natural Science Foundation of China (Grant Nos. 41861144014, 41630424 and 41875078)


doi: 10.1007/s00376-019-9048-3

  • Precipitation over southeastern Lake Baikal features a significant decreasing trend in July and August over 1979–2018 and is closely related to blocking occurrence over central Siberia (45°–70°N, 75°–115°E). This study investigates the formation and maintenance of anticyclonic and cyclonic wave-breaking (AWB and CWB) blocking events and their climate impacts on precipitation in the southeastern Lake Baikal area. Both AWB and CWB blocking events are characterized by a cold trough deepening from the sub-Arctic region and a ridge amplifying toward its north over central Siberia, as well as an evident Rossby wave train over midlatitude Eurasia. For AWB blocking events, the ridge and trough pair tilts clockwise and the wave train exhibits a zonal distribution. In contrast, ridge and trough pair associated with CWB blocking events leans anticlockwise with larger-scale, meridional, and more anisotropic signatures. Moreover, the incoming Rossby wave energy associated with CWB blocking events is more evident than for AWB blocking events. Therefore, CWB blocking events are more persistent. AWB blocking events produce more extensive and persistent precipitation over the southeastern Lake Baikal area than CWB blocking events, in which moderate above-normal rainfall is seen in the decaying periods of blockings. A significant decreasing trend is found in terms of AWB blocking occurrence over central Siberia, which may contribute to the downward trend of precipitation over southeastern Lake Baikal.
    摘要: 1979-2018 年间贝加尔湖东南部地区 7-8 月的月平均降水显著减少,这与中西伯利亚地区( 45°-75°N, 75°-115°E )阻塞高压活动密切相关。本文针对反气旋式和气旋式波破碎形式的阻塞高压事件,研究了其形成和维持机制以及其对贝加尔湖东南部地区降水的气候影响。两类阻塞高压的形成均为次极区的冷槽向中西伯利亚地区加深,而暖脊则向冷槽北部延伸和加强,且与欧亚大陆中纬度 Rossby 波列相联系。反气旋式波破碎阻塞高压中的槽脊呈逆时针方向倾斜,与其相联系的波列表现出准纬向特征。而气旋式波破碎阻塞高压的槽脊则为顺时针方向倾斜,其异常环流具有尺度更大,经向性和各向异性更大的特征, Rossby 波能量频散也更强,从而气旋式波破碎阻塞高压更易维持。反气旋式波破碎阻塞高压可在贝加尔湖东南部地区引起更为广泛和持续的降水,而气旋式波破碎阻塞高压仅在其减弱期间有利于该地区降水的增多。中西伯利亚地区反气旋式波破碎阻塞高压事件的发生呈现显著的下降趋势,这可能是导致贝加尔湖东南部地区降水减少的原因。
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  • Figure 1.  (a) Spatial distributions of linear trends of precipitation and (b) trends and time series of areal-averaged precipitation within a box encompassing (40°–55°N, 105°-125°E) based on APHRODITE data. Dark (Light) shading in (a) indicates statistical significance above the 95% (90%) confidence level. The red lines in (a) represent the linear regressions, and the blue dashed line is the Theil–Sen estimate of the linear trend. (c, d) As in (a, b), but for ERA5.

    Figure 2.  (a) Map of B index anomalies regressed against the standardized areal-averaged precipitation within the box in Figs. 1a and b. Dark (Light) shading indicates statistical significance above the 95% (90%) confidence level. (b) Distribution of RWB blocking centers in July and August (1979–2018). A nine-point smoothing is used and the polygon designates the region of central Siberia. (c, d) The distribution of blocking centers on the peak day for (c) AWB and (d) CWB blocking events over central Siberia.

    Figure 3.  Areal-averaged blocking occurrence over central Siberia identified using the blocking index defined by Tibaldi and Molteni (1990) for (a) AWB and (b) CWB blocking events. The black line designates the end of each blocking event.

    Figure 4.  Composite temporal evolution of 2-PVU potential temperature (units: K) for AWB blocking events on (a) day −6, (b) day −4, (c) day −2, (d) day 0 and (e) day +2. The contour interval is 3 and the polygon designates the region of central Siberia. Dark (Light) shading indicates statistical significance above the 95% (90%) confidence level. (f–j) As in (a–e), but for 300-hPa geopotential height (units: gpm) and the contour interval is 50. (k–o) As in (a–e), but for 300-hPa geopotential height anomalies (contours) and wave activity flux (arrows; units: m2 s−2). The contour interval is 20 and the arrows are plotted where the flux is greater than 1.

    Figure 5.  Composite 2-m air temperature anomalies (contours; units: °C) and 10-m wind anomalies (arrows; units: m s−1) averaged over (a) days −4 to −2, (b) days −1 to 1, and (c) days 2 to 4 for AWB blocking events. The contours are drawn every 0.5 °C. The polygon designates the region of central Siberia. Dark (Light) shading indicates statistical significance above the 95% (90%) confidence level.

    Figure 6.  Composite precipitation anomalies (units: %) from APHRODITE averaged over (a) days −4 to −2, (b) days −1 to 1, and (c) days 2 to 4 of AWB blocking events, in which the polygon designates the southeastern Lake Baikal area. (d–f) As in (a–c), but for ERA5. (g–i) As in (a–c) but for vertically integrated water vapor fluxes (arrows; units: 10−6 m kg s−1 kg−1) and the corresponding divergence field (shading; units: kg s−1 kg−1). The stippling marks the regions with statistical significance above the 90% confidence level.

    Figure 7.  As in Fig. 4, but for CWB blocking events.

    Figure 9.  As in Fig. 6, but for CWB blocking events.

    Figure 8.  As in Fig. 5, but for CWB blocking events.

    Figure 10.  Time series of (a) AWB and (b) CWB blocking occurrence over central Siberia. The red lines represent the linear regression, and the blue line is the Theil–Sen estimate of the linear trend.

    Table 1.  Event dates and center positions on peak days along with the average DB index during each event for AWB blocking events over central Siberia.

    NO.Onset datePeak dateDuration (days)LatitudeLongitudeDB index
    18 August 19798 August 1979567.5°N100°E0.6
    27 July 198111 July 1981665°N90°E0.9
    310 July 198213 July 1982760°N105°E1.3
    427 August 198428 August 1984455°N100°E0.2
    54 July 19876 July 1987862.5°N95°E1.3
    613 August 198815 August 1988460°N100°E2.8
    71 July 19893 July 1989760°N85°E1.4
    819 July 1989