Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming

摘要

The Indian Ocean dipole is a prominent mode of coupled ocean-atmosphere variability, affecting the lives of millions of people in Indian Ocean rim countries. In its positive phase, sea surface temperatures are lower than normal off the Sumatra-Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries but devastating droughts in eastern Indian Ocean rim countries. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century. We find that a mean state change-with weakening ofboth equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean-facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.%南热带印度洋地区的国家在"印度洋偶极子"(IOD)气候周期处于一个极端正阶段的年份中易发生大范围洪涝和旱灾。在这些"坏年份"(如1961、1994和1997年）中，温暖水域会出现在印度洋海盆的西部，降水量会増加；而在东部则主要是较冷水域，降水量会减少。在这篇论文中，Wenju Cai等人对在高温室气体排放情景下的气候模型预测结果进行了评估，发现极端正IOD的出现频率在21世纪可能会从整个20世纪的每17.3年大约一次増加到每6.3年一次。