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近60年北半球冬季极地环流的年代际变化特征研究

Interdecadal variations of Northern Hemispheric Polar Cell during winter over the past 60 years

  • 摘要: 极地环流强度和中心位置的变化对中高纬天气气候系统演变具有重要影响,认识其年代际演变并探索其成因具有重要意义。本文主要利用欧洲中期天气预报中心的ERA5再分析环流资料,基于经向质量流函数和局地质量流函数,重点分析了1959~2022年冬季北半球极地环流(the Northern Hemispheric Polar Cell during Winter,NPCW)的长期变化特征及可能机制。结果表明,NPCW具有显著的年代际变化特征,其强度和中心位置均在1987和1997年左右发生年代际转折,总体呈现“偏强偏南-偏弱偏北-偏强偏南”的年代际变化规律。研究发现,冬季北半球准静止行星波的年代际变化是主导上述现象的关键。相比于1976~1986年,1987~1997年冬季北半球准静止行星波的振幅在高纬地区增强,在40~50°N减弱,通过调整对流层低层的气压系统使北大西洋中高纬地区出现南风异常,削弱了NPCW的强度。同期行星波沿极地波导向平流层的传播减弱,使得高纬度平流层下层Eliassen-Palm (E-P)通量辐合变弱。根据波-流相互作用,这导致绕极西风增强、北极涛动(Arctic Oscillation, AO)转为正位相和极涡增强。此时北极对流层异常冷却而中纬度异常增暖,经向温度梯度增加,使得北大西洋和北太平洋中高纬度地区涡旋活动增强,引起NPCW上升支北移,从而改变环流的中心位置。1998~2008年冬季北半球准静止行星波活动相反,导致对流层低层出现北风异常,经向温度梯度减弱,NPCW相较于上一时期增强且南移。

     

    Abstract: The Polar Cell is an important but often ignored component of the mean meridional circulations. This study identified and investigated the long-term variations in the Northern Hemispheric Polar Cell during Winter (NPCW) by using the meridional and regional Mass Stream Function derived from ERA5 reanalysis data. The results show that the intensity and central location of the NPCW have an evident rhythm of interdecadal change from 1959 to 2022. Compared to the preceding and following decades, the NPCW was weaker and shifted farther poleward during 1987~1997. It is found that the interdecadal variability might be modulated by the interdecadal variation in the quasi-stationary planetary wave activity. Compared to the period 1976~1986, the amplitude of the quasi-stationary planetary wave during 1987~1997 increased at high latitudes but decreased around 40–50°N. This pattern altered the lower-tropospheric pressure systems, inducing southerly wind anomalies over the mid-high latitude North Atlantic, which acted to weaken the NPCW intensity. Concurrently, the upward propagation of quasi-stationary planetary waves into the stratosphere over high latitudes along the polar wave-guide was reduced,which decreased the convergence of Eliassen-Palm (E-P) fluxes in the lower stratosphere over high latitudes of the Northern Hemisphere. These resulted in an enhanced polar front jet through planetary wave-mean flow interaction.The AO was then enhanced and the polar vortex strengthened. Meanwhile, anomalous cooling in the Arctic troposphere coupled with anomalous warming in the mid-latitudes increased the mean meridional temperature gradient. This enhanced gradient promoted eddy activities over the mid-high latitudes of the North Atlantic and North Pacific, driving a poleward shift of the NPCW"s ascending branch and thus altering the circulation"s central location. In contrast, during 1998~2008, opposite anomalies in quasi-stationary planetary wave activity led to northerly wind anomalies in the lower troposphere and a weakened meridional temperature gradient, causing the NPCW to strengthen and shift southward compared to the previous period.

     

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