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

这是16个快速反应（RAPID）项目之一，作为亲爱的同事信（NSF 11-006）的结果，鼓励对政府间气候变化专门委员会第五次评估报告（IPCC AR 5）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式相互比较，更好地了解强大的模式行为，更好地了解和量化未来气候模拟中的不确定性。 本项目研究了世纪大西洋海表温度梯度的长期变化趋势。观测结果表明，南半球热带SST比北方半球热带SST变暖更快。 以前的工作使用早期的气候模式模拟的合奏表明，SST梯度的趋势是被迫的部分，也许主要是由人为硫酸盐气溶胶的辐射效应。在为第五次评估报告准备的模拟中，SST梯度趋势的行为特别令人感兴趣，因为新一代模型在模拟气溶胶的直接和间接影响方面取得了重大进展。 本项目的主要研究内容包括：（1）确定海表温度梯度趋势的大小，并评估不同模式集合成员之间海表温度梯度趋势的分布;（2）确定海表温度梯度趋势的成因;（3）评估气溶胶的间接效应在确定海表温度梯度趋势大小中的作用; 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;（5）对世纪后期“扁平化”趋势的认识。 确定气溶胶辐射对趋势的贡献的主要方法包括比较20世纪所有气候强迫的模式模拟（包括气溶胶）和模拟，其中只有世纪温室气体浓度的增加被用作强迫。该项目更广泛的影响在于它支持IPCC第五次评估报告，其目的是向全世界的决策者提供有关气候变化及其后果的信息。大西洋海温梯度对周围大陆的气候有重大影响，并与萨赫勒和巴西东北部的干旱有关。 海表温度梯度趋势在多大程度上是由气溶胶造成的，如果是的话，哪些气溶胶是最主要的原因，因此对这些地区具有很大的社会意义。

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.