RAPID: Long-term Trends of the Atlantic Interhemispheric SST Gradient in the CMIP5 20th Century Simulations

这是由一封亲爱的同事来信(NSF 11-006)资助的16个快速反应(RAPID)项目之一，该信鼓励对为政府间气候变化专门委员会第五次评估报告(IPCC AR5)准备的气候模型模拟进行诊断分析。在这些项目中进行的研究预计将导致更详细的模型相互比较，更好地理解稳健的模型行为，以及更好地理解和量化未来气候模拟中的不确定性。该项目研究了20世纪观测和气候模式模拟中出现的大西洋半球间海表面温度(SST)梯度的长期趋势。观测表明，南半球热带海温比北半球海温升温更快的梯度趋势。之前使用早期气候模式模拟组合的工作表明，海温梯度趋势部分是由人为硫酸盐气溶胶的辐射效应强迫的，也许是主要的。在为AR5准备的模拟中，SST梯度趋势的行为特别令人感兴趣，因为新一代模式在模拟气溶胶的直接和间接影响方面取得了重大进展。在这个项目中，研究的重点是：1)量化海温梯度趋势的大小，并评估趋势在不同多模式集合成员中的扩散；2)确定趋势的原因；3)评估气溶胶间接效应在确定趋势大小中的作用；4)确定不同气溶胶类型在推动趋势方面的相对作用，特别是硫酸盐气溶胶和黑碳；以及5)了解20世纪末趋势的平坦。确定对这一趋势的气溶胶辐射贡献的主要方法是将20世纪所有气候强迫(包括气溶胶)强制进行的模式模拟与只使用20世纪温室气体浓度增加作为强迫的模拟之间的比较。该项目的更广泛影响在于它支持IPCC AR5，该计划旨在向世界各地的决策者提供关于气候变化及其后果的信息。大西洋海温梯度对周围陆地的气候有重大影响，并与萨赫勒和巴西东北部的干旱有关。因此，SST梯度趋势在多大程度上是由气溶胶推动的，如果是这样的话，哪种气溶胶是最有责任的，这对这些区域具有重大的社会意义。

This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations. This project examines the secular trend in the Atlantic interhemispheric sea surface temperature (SST) gradient occuring in 20th century observations and climate model simulations. Observations show a trend in the gradient in which the Southern Hemisphere tropical SSTs warm faster than their Northern Hemisphere counterparts. Previous work using an earlier ensemble of climate model simulations suggested that the SST gradient trend is forced in part, perhaps predominantly, by the radiative effects of anthropogenic sulfate aerosols. The behavior of the SST gradient trend in simulations prepared for the AR5 is of particular interest because the new generation of models incorporates significant advances in the simulation of both direct and indirect effects of aerosols. In this project, research focuses on 1) quantifying the magnitude of the SST gradient trend, and assessing the spread in the trend across the different multimodel ensemble members; 2) attributing the cause of the trend; 3) assessing the role of aerosol indirect effects in determining the magnitude of the trend; 4) determining the relative roles of different aerosol types in forcing the trend, particularly sulfate aerosols and black carbon; and 5) understanding the flattening of the trend in the late 20th century. The primary method for identifying the aerosol radiative contribution to the trend consists of comparisons between model simulations forced with all 20th-century climate forcings (including aerosols) and simulations in which only the 20th century increases in greenhouse gas concentrations are used as forcing.The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. The Atlantic SST gradient exerts a significant influence on the climate of the surrounding landmasses, and is implicated in droughts over the Sahel and northeastern Brazil. The extent to which the SST gradient trend is forced by aerosols, and if so which aerosols are most responsible, is thus of great societal relevance to these regions.