本文利用12个CMIP5气候模式的风场资料,采用全球大气环流三型分解模型中的经圈型环流流函数,研究了地表以下虚假的风场资料对北半球夏季Hadley环流空间模态及下沉支位置未来预估的影响。结果表明:在计算Hadley环流的流函数时,由于垂直积分过程会将地表以下虚假的风场资料所带来的误差传递至整层的流函数,从而导致北半球夏季Hadley环流的空间模态中存在虚假的“小环流”,该“小环流”会导致Hadley环流下沉支位置的误判,并对下沉支位置变化趋势的准确计算产生较大影响;进一步的定量分析发现,在未来排放情境下(RCP8.5),地表以下虚假的风场资料所引起的误差会导致北半球夏季Hadley环流下沉支向极扩张趋势的高估。 Based on monthly data of wind field from 12 Coupled Model Intercomparison Project Phase 5 (CMIP5) models and the stream function of meridional circulation derived from the three-pattern decomposition global atmospheric circulation method (3P-DGAC), we investigate influences of false subsurface wind data on spatial pattern of the Northern Hadley circulation (HC) and future estimation of the sinking branch in boreal summer. It is found that errors caused by false subsurface wind data are passed to the stream function in the whole layer by vertically integrated process, which could lead to a false “minor circulation” embedded within Northern Hemispheric Hadley circulation (NHHC) in summer. The “minor circulation” would cause miscalculation of the location of the NHHC sinking branch, and it also has a significant effect on the accurate calculation of poleward expansion trend of NHHC. The result of the quantitative analysis shows that, in the 2040-2099 period under Representative Concentration Pathway 8.5 (RCP8.5) scenarios, the false wind data would lead to an overestimation of the poleward expansion trend of NHHC in summer.
Hadley环流,经圈型环流流函数,虚假风场资料,空间模态,下沉支位置, Hadley Circulation Stream Function of Meridional Circulation False Wind Data Spatial Pattern Lo-cation of the Sinking Branch地表以下虚假的风场资料对北半球夏季Hadley环流空间模态及下沉支未来预估的影响
此外,利用CMIP5气候模式资料对Hadley环流气候变化问题的研究表明,CMIP5模式虽能得到Hadley环流下沉支向极地扩张的结论,但是扩张的程度比再分析资料和观测资料得到的结果小将近一个量级 [14] [15] [16] [17]。例如,Hu et al. (2013)利用CMIP5模式模拟结果发现,在全球变暖背景下,Hadley环流在1979至2005年期间向两极扩张了大约0.15˚,仅仅是再分析资料结果的六分之一 [16]。因此,关于再分析资料和模式模拟的Hadley环流向极扩张的程度是否存在显著差异仍需进一步的研究。
Mathew et al. (2016)利用4套再分析资料,通过研究1979至2012年Hadley环流的质量流函数演变特征发现,北半球夏季Hadley环流中存在一个反向的“小环流”,该“小环流”会引起Hadley环流下沉支位置的误判 [18]。然而,Cheng et al. (2018)利用5套再分析资料的研究指出,北半球夏季Hadley环流空间模态中的“小环流”结构是由于计算Hadley环流的流函数时,未考虑地表以下虚假的风场资料的影响而错误引入的 [19]。也就是说,再分析资料中,地面气压小于1000 hPa范围内的虚假风场资料会对北半球夏季Hadley环流空间模态的计算造成显著的影响,从而导致Hadley环流下沉支位置的计算存在较大的误差。
Tao et al. (2015)的研究指出,未来不同排放情景下,Hadley环流的变化趋势在2040~2099年比2006~2039年更显著 [24]。因此本节选取2040~2099的时间段来研究北半球Hadley环流的未来预估特征。通过对比图7(a)和图7(b)发现,在RCP8.5情形下,地表以下虚假的vH风场资料对北半球Hadley环流强度的未来变化趋势影响不大。也就是说,无论是误差订正前还是误差订正后,图7(a)和图7(b)中6~9月份北半球Hadley环流的流函数正值中心均位于负的趋势带上,说明模式模拟的未来北半球夏季Hadley环流呈现减弱的趋势。然而,地表以下虚假的vH风场资料对北半球夏季Hadley环流下沉支位置的未来预估结果影响显著。如图7所示,误差订正前,6~9月份的流函数0值线在40˚N左右的正趋势带上,表明未来北半球的Hadley环流将向极扩张,而误差订正后,流函数0值线位于负的趋势带上,表明未来北半球的Hadley环流下沉支位置将向赤道偏移。进一步的定量分析发现,误差订正前,6~9月份北半球夏季Hadley环流下沉支位置的未来变化趋势为0.097˚每十年,而误差订正后的未来变化趋势为-0.067˚每十年,这说明地表以下虚假的vH风场资料会导致CMIP5模式对北半球夏季Hadley环流向极扩张趋势的高估。因此,我们在利用CMIP5模式资料研究Hadley环流的未来演变特征时,需要去除地表以下虚假的vH风场资料,以避免其引起的计算误差。
许之航,胡淑娟,王思懿,彭建军. 地表以下虚假的风场资料对北半球夏季Hadley环流空间模态及下沉支未来预估的影响Influences of False Subsurface Wind Data on Spatial Pattern of the Northern Hadley Circulation and Future Estimation of the Sinking Branch in Boreal Summer[J]. 气候变化研究快报, 2020, 09(01): 1-10. https://doi.org/10.12677/CCRL.2020.91001
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