Interaction of Dust Radiative Forcing and the Asian Summer Monsoon

亚洲夏季风吹来的沙尘在阿拉伯海上无处不在。层内太阳辐射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。如果阿拉伯海上空气溶胶的地表辐射强迫被蒸发减少所补偿，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，如果海面强迫与海洋热输出的减少相平衡，则蒸发不必减少，气溶胶层内的辐射加热可以加强季风，而不会相反。对海洋响应的考虑解决了以往数值模拟中的一个明显矛盾，即季风对气溶胶辐射强迫的响应不是增强就是减弱。当海表温度（SST）被规定时，地表能量约束不复存在，海面的气溶胶辐射强迫被海洋热量输出的减少所隐含地平衡，从而使季风得到加强。相反，在大多数计算海温的模式中，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。为了反映季风、印度洋和沙尘辐射强迫之间的全面反馈，主要研究人员将用大气环流模式（AGCM）与海洋环流和沙尘气溶胶循环的完全相互作用模式进行实验。与其他吸附性气溶胶不同，尘埃浓度是通过地面风速对气候的敏感函数。先前的计算发现，由沙尘辐射强迫驱动的季风增强使沙尘负荷增加。模拟的关系将与阿拉伯海上每日和季节性时间尺度上的沙尘量、风速、蒸发和降水等关键变量的观测结果进行比较。对沙尘的日常反应令人感兴趣，因为沙尘的爆发是偶然的，与污染产生的吸收气溶胶的缓慢季节性变化形成对比。这项研究将加入先前相对较少的关于季风和气溶胶辐射强迫相互作用的研究，并有助于理解气溶胶对季风强度影响的对比结果（例如，由规定的和计算的海温的影响产生的结果）。考虑到粉尘气溶胶的特殊影响的研究迄今尚未考虑海洋热输送的影响。计算气候对沙尘的响应对于区分全球和区域对温室气体和其他气溶胶的人为强迫的响应是必要的。这项研究的结果将用于改进一个公开的AGCM，包括一个尘埃循环模型，并将被纳入PI的教学和向一般听众发表的报告中。在项目结束时，博士生将在PI的指导下完成论文。

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.