Interaction of Dust Radiative Forcing and the Asian Summer Monsoon

在亚洲夏季风期间，阿拉伯海上空到处都是被风吹起的沙尘。太阳辐射在大气层内的吸收作为一个升高的热源，可以引起上升和降水。季风环流是否被气溶胶加强取决于印度洋的反应。如果在阿拉伯海上空，气溶胶的地面辐射强迫通过减少蒸发得到补偿，那么大气潜热加热将减少，并与气溶胶层内的辐射加热相反。相反，如果地面强迫通过减少海洋热量输出来平衡，那么蒸发就不需要减少，气溶胶层内的辐射加热可以加强季风而不会发生对抗。海洋响应的考虑解决了一个明显的矛盾，以前的数值模拟，季风要么加强或减弱气溶胶辐射强迫。当海表温度（SST）的规定，表面能量约束是不存在的，气溶胶辐射强迫在表面隐含平衡减少海洋热量输出，允许加强季风。相反，在大多数模式中，季风减弱，其中SST计算和蒸发而不是海洋热量输出减少，以平衡气溶胶层下的入射阳光的减少。为了代表季风，印度洋和尘埃辐射强迫之间的全方位反馈，首席研究员（PI）将使用大气环流模型（AGCM）进行实验，该模型与海洋环流和尘埃气溶胶循环的完全交互式模型相结合。与其他吸收性气溶胶不同，沙尘浓度是通过地面风速对气候的敏感函数。先前的计算发现，沙尘辐射强迫驱动的季风风增强会增加沙尘负荷。模拟的关系将与阿拉伯海上空的沙尘量、风速、蒸发量和降水量等关键变量在每日和季节时间尺度上的观测结果进行比较。每天对沙尘的反应是有趣的，因为沙尘的爆发是间歇性的，与污染产生的吸收性气溶胶的缓慢季节性变化相反。这项研究将加入相对较少的关于季风和气溶胶辐射强迫相互作用的先前研究，并有助于了解气溶胶对季风强度影响的对比结果（例如，来自规定的SST与计算的SST的影响）。迄今为止，考虑尘埃气溶胶的特殊影响的研究尚未考虑海洋热传输的影响。计算气候对沙尘的响应是必要的，以区分全球和区域对温室气体和其他气溶胶人为强迫的响应。研究结果将用于改进公开可用的AGCM，包括沙尘循环模型，并将纳入PI对普通观众的教学和演示。在项目结束时，一名博士生将在PI的监督下完成她的论文。

Wind-blown dust is ubiquitous over the Arabian Sea during the Asian summer monsoon. Absorption of solar radiation within the layer acts as an elevated heat source, which can induce ascent and precipitation. Whether the monsoon circulation is strengthened by aerosols depends upon the response of the Indian Ocean. If surface radiative forcing by aerosols is compensated over the Arabian Sea by a reduction in evaporation, then atmospheric latent heating will decrease and oppose radiative heating within the aerosol layer. In contrast, if the surface forcing is balanced by a reduction in ocean heat export, then evaporation need not decrease and radiative heating within the aerosol layer can strengthen the monsoon without opposition. Consideration of the ocean response resolves an apparent contradiction among previous numerical simulations whereby the monsoon either strengthens or weakens in response to aerosol radiative forcing. When sea surface temperature (SST) is prescribed, a surface energy constraint is absent, and aerosol radiative forcing at the surface is implicitly balanced by a reduction in ocean heat export, allowing a strengthened monsoon. Conversely, the monsoon weakens in most models where SST is calculated and evaporation rather than ocean heat export decreases to balance the reduction of incident sunlight beneath the aerosol layer. To represent the full range of feedbacks between the monsoon, the Indian Ocean, and dust radiative forcing, the Principal Investigators (PIs) will perform experiments with an atmospheric general circulation model (AGCM) coupled to fully interactive models of the ocean circulation and dust aerosol cycle. Unlike other absorbing aerosols, dust concentration is a sensitive function of the climate through the surface wind speed. A previous calculation found that the dust load was increased by strengthened monsoonal winds driven by dust radiative forcing. The simulated relationship will be compared to observations of key variables such as dust amount, wind speed, evaporation, and precipitation over the Arabian Sea, on both daily and seasonal time scales. The daily response to dust is of interest because dust outbreaks are episodic, in contrast to the slow seasonal variations of absorbing aerosols created by pollution. The research will join relatively few previous studies about the interaction of the monsoon and aerosol radiative forcing, and help to understand contrasting results about the effect of aerosols upon the monsoon strength (that result from the effect of prescribed versus calculated SST, for example). Studies considering the particular effect of dust aerosols have so far not considered the effect of ocean heat transport. Calculation of the climate response to dust is necessary to distinguish the global and regional response to anthropogenic forcing by greenhouse gases and other aerosols.Results from the study will be used to improve a publicly available AGCM, including a model of the dust cycle, and will be incorporated into teaching and presentations to general audiences by the PI. By the end of the project, a doctoral student will complete her dissertation under the supervision of the PI.