人為氣候變遷所致熱帶降雨空間分佈隨時間之變化

Abstract

本研究是利用美國國家大氣研究中心的一項計畫中，使用單一大氣海洋耦合模式及系集，模擬多組過去以及未來100年的人為造成的氣候情境。此研究中，主要針對熱帶的降雨以及環流改變進行時間以及空間上的分析。 熱帶降雨可分別定義為兩種隨時間變化的指數，(1)降雨最大值所在的緯度，以及(2)熱帶降雨重心。上述兩種降雨指數在時序分析中，有非常不同的表現。在1990年代以前，降雨最大值的緯度沒有明顯改變，而後，因為暖化下赤道地區暖化程度高於周圍區域，使此降雨指數有明顯往赤道區域靠近的趨勢。然而，熱帶降雨重心卻是受到南北半球降雨不對稱改變的影響。在20世紀時，北半球發展中國家大量排放氣膠粒子，造成北半球明顯冷卻，使降雨重心南移。而後，受到溫室氣體大量排放的影響，使北半球暖化較為劇烈，降雨重心往北移。然而，在21世紀末期，即使仍在強烈暖化的氣候情境中，降雨重心仍再次往南移。暖化下，此降雨之非線性現象，可以為前人所提出的能量觀點所解釋：(1)當北極海冰融完並使暖化速率減緩，以及(2)南大洋加速暖化，熱帶環流會趨向產生一跨赤道環流，使南半球變濕，北半球變乾。詳細的歸因分析後，更發現短波雲的輻射效應扮演重要角色，其放大了半球間能量的不對稱，並使熱帶環流產生調整。 本研究中，發現降雨的改變存在著複雜的時間以及區域上的改變，值得更多深入的研究。

In this work, we investigate the temporal evolution of the anthropogenic forced tropical precipitation and circulation using CESM Large Ensemble Project’s historical and RCP 8.5 simulations. An intertropical convergence zone (ITCZ) index and a precipitation centroid (PC) index are defined to quantify the meridional displacement of the zonal mean rainfall peak and the overall tropical precipitation pattern, respectively. Both indices show complex transient responses but different turning points in their time series. The ITCZ initially has no significant movement, but begins to shift southward toward the equator, which warms more significantly than the surrounding subtropical ocean after the 1990s. On the other hand, the PC first shifts southward when aerosols cool the Northern Hemisphere during the 20th century, and then shifts northward after year 2000 when greenhouse gases (GHGs) warming is larger in the Northern than the Southern Hemisphere. As GHG concentrations continue to increase, the PC shifts southward again after 2067. This nonlinear response of the PC shift to increasing GHGs can be interpreted through an energetic framework: as (1) the Arctic becomes ice-free and its warming rate reduces, and (2) the Southern Ocean warming rate increases, the associated hemispheric asymmetric Hadley Cell response leads to drying in the northern tropics and moistening in the southern tropics. Detailed attribution analyses further suggest that the shortwave cloud feedback plays an important role in amplifying the hemispheric asymmetry and leading to the structural changes in Hadley circulation. Our results highlight the complex temporal and spatial structures of the anthropogenic forced precipitation changes.