Interaction of Dust Radiative Forcing and the Asian Summer Monsoon

在亚洲夏季风期间，阿拉伯海上空随处可见风吹来的沙尘。层内对太阳辐射的吸收起到了升高热源的作用，这可能会导致上升和降水。季风环流是否被气溶胶加强取决于印度洋的反应。如果通过减少蒸发来补偿阿拉伯海上空气溶胶的地面辐射强迫，那么大气潜热将减少，并与气溶胶层内的辐射加热相反。相反，如果表面强迫通过减少海洋热量输出来平衡，那么蒸发不需要减少，气溶胶层内的辐射加热可以不受阻碍地加强季风。对海洋响应的考虑解决了以前数值模拟中的一个明显矛盾，即季风要么增强，要么减弱，以响应气溶胶辐射强迫。当规定了海洋表面温度(SST)时，不存在表面能量约束，表面的气溶胶辐射强迫通过减少海洋热输出而隐含地平衡，从而允许增强的季风。相反，在计算海温的大多数模式中，季风减弱，蒸发而不是海洋热输出减少，以平衡气溶胶层下入射阳光的减少。为了全面反映季风、印度洋和尘埃辐射强迫之间的反馈，首席调查员将使用大气环流模式(AGCM)进行实验，该模式与海洋环流和尘埃气溶胶循环的完全交互模式相结合。与其他吸收气溶胶不同的是，尘埃浓度是通过地面风速对气候的敏感函数。先前的一项计算发现，沙尘辐射强迫导致的季风风增强增加了沙尘负荷。模拟的关系将与对阿拉伯海上空沙尘量、风速、蒸发量和降水量等关键变量的观测结果进行比较，包括每日和季节时间尺度。人们对沙尘的每日反应很感兴趣，因为沙尘暴发是间歇性的，而不是污染造成的吸收气溶胶的缓慢季节变化。这项研究将加入以前关于季风和气溶胶辐射强迫相互作用的相对较少的研究，并有助于理解关于气溶胶对季风强度影响的对比结果(例如，由于规定的和计算的海温的影响)。考虑到沙尘气溶胶的特殊影响的研究到目前为止还没有考虑海洋热传输的影响。计算对沙尘的气候反应对于区分全球和区域对温室气体和其他气溶胶造成的人为强迫的反应是必要的。研究结果将用于改进可公开获得的全球气候监测机制，包括沙尘循环模型，并将纳入国际和平研究所的教学和向普通听众的报告中。在项目结束时，一名博士生将在PI的指导下完成她的论文。

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.