Assessing the Performance of CMIP6 Models in Simulating Droughts across Global Drylands

Abstract

Both the attribution of historical change and future projections of droughts rely heavily on climate modeling. However, reasonable drought simulations have remained a challenge, and the related performances of the current state-of-the-art Coupled Model Intercomparison Project phase 6 (CMIP6) models remain unknown. Here, both the strengths and weaknesses of CMIP6 models in simulating droughts and corresponding hydrothermal conditions in drylands are assessed. While the general patterns of simulated meteorological elements in drylands resemble the observations, the annual precipitation is overestimated by ∼33% (with a model spread of 2.3%–77.2%), along with an underestimation of potential evapotranspiration (PET) by ∼32% (17.5%–17.2%). The water deficit condition, measured by the difference between precipitation and PET, is 50% (29.1%–71.7%) weaker than observations. The CMIP6 models show weaknesses in capturing the climate mean drought characteristics in drylands, particularly with the occurrence and duration largely underestimated in the hyperarid Afro-Asian areas. Nonetheless, the drought-associated meteorological anomalies, including reduced precipitation, warmer temperatures, higher evaporative demand, and increased water deficit conditions, are reasonably reproduced. The simulated magnitude of precipitation (water deficit) associated with dryland droughts is overestimated by 28% (24%) compared to observations. The observed increasing trends in drought fractional area, occurrence, and corresponding meteorological anomalies during 1980–2014 are reasonably reproduced. Still, the increase in drought characteristics, associated precipitation and water deficit are obviously underestimated after the late 1990s, especially for mild and moderate droughts, indicative of a weaker response of dryland drought changes to global warming in CMIP6 models. Our results suggest that it is imperative to employ bias correction approaches in drought-related studies over drylands by using CMIP6 outputs.

摘要

干旱变化的历史归因和未来预估依赖于气候模式。然而,可靠的干旱模拟仍是当前气候模拟研究领域面临的巨大挑战,第六次国际耦合模式比较计划(CMIP6)模式对干旱的模拟性能尚不清晰。鉴于此,本文系统评估了CMIP6模式在全球干旱区干旱事件和相应水热条件模拟方面的优缺点。研究表明,CMIP6模式可合理模拟出全球干旱区气象因子的空间分布特征。相较观测,模式模拟的年降水量气候态被高估约33% (模式间离差为2.3%–77.2%),年潜在蒸散发量(PET)则被低估约32% (17.5%–47.2%),进而导致水分亏缺程度(P–PET)被低估约50% (29.1%–71.7%)。CMIP6模式对旱区基本干旱特征的模拟能力仍有待提升,尤其亚非极端干旱区的干旱频率和持续时间被严重低估。但可以合理再现出干旱事件对应的水热条件异常,包括降水偏少、温度偏高、大气蒸发需求增大以及水分亏缺加剧,但模拟的降水和水分亏缺幅度分别被高估约28%和24%。CMIP6模式可合理模拟出干旱事件及相应的水热异常对气候增暖的响应特征,但响应偏弱,表现在明显低估了1990s后期干旱特征以及相应降水和水分亏缺的增加幅度。因此,利用CMIP6模式数据开展干旱区的干旱对生态环境的影响研究,与气象干旱有关的气象因子的偏差校正不可或缺。