Increasing Flash Floods in a Drying Climate over Southwest China

Abstract

In a globally warming world, subtropical regions are generally expected to become drier while the tropics and mid–high latitudes become wetter. In line with this, Southwest China, close to 25°N, is expected to become increasingly prone to drought if annual mean precipitation decreases. However, despite this trend, changes in the temporal distribution of moisture supply might actually result in increased extreme rainfall in the region, whose climate is characterized by distinct dry and wet seasons. Using hourly and daily gauge observations, rainfall intensity changes since 1971 are examined for a network of 142 locations in the region. From the analysis, dry season changes are negligible but wet season changes exhibit a significantly strong downward trend [−2.4% (10 yr)⁻¹], particularly during the past 15 years [−17.7% (10 yr)⁻¹]. However, the intensity of events during the wettest of 5% hours appears to steadily increase during the whole period [1.4% (10 yr)⁻¹], tying in with government statistical reports of recent droughts and flooding. If the opposing trends are a consequence of a warming climate, it is reasonable to expect the contradictory trend to continue with an enhanced risk of flash flooding in coming decades in the region concerned.

摘要

在全球变暖的背景下, 通常认为副热带地区降水减少, 热带和中高纬度降水增加. 基于此, 对于中国西南地区, 降水减少, 干旱的潜在风险是增加的、洪涝的潜在风险应该是减少的. 本文研究发现一个有意思的现象, 对于西南这样一个干湿季特别分明的地区, 即便降水在减少, 强降水的风险并没有减少. 通过逐小时等高时空分辨率的降水资料分析发现, 近些年西南地区呈现的降水减少趋势, 主要是由于雨季降水的减少造成的, 干季的降水变化不大, 对全年降水的贡献也不大. 而雨季降水虽然明显减少, 但是雨季强降水并没有减少, 无论是强度大于95%分位的降水事件, 还是降水大于20mm每小时的事件, 都呈增加趋势, 这对于地形复杂的西南地区, 无疑增大了洪涝相关的灾害风险. 相关学者关于未来预估都有类似的结论. 因此, 这种相反的变化趋势(总降水减少, 但强降水增加)对于地方政府适应气候变化而言, 洪涝灾害防御工作和干旱同样值得重视.