Future Changes in the Seasonal Cycle: Mechanisms and Implications

这项研究的目的是检查季节周期的延迟，这是在人为全球变暖的气候模型模拟中常见的。主要研究人员表明，在气候变化模拟中，温度和降水的季节周期在21世纪末比20世纪末推迟了几天。降水的延迟意味着一些热带和亚热带地区雨季的开始被推迟。在撒哈拉南部边缘的萨赫勒地区，在全球变暖模拟中，开始的时间比结束的时间延迟更多，导致雨季缩短。在同样的全球变暖模拟中，包括中美洲和加勒比在内的其他地区也出现了类似的缩短。也有一些证据表明，萨赫勒雨季的缩短发生在现实世界和模型模拟中。这项研究围绕三个问题展开：1)模式模拟是否可信？2)温度和降水季节循环延迟的原因是什么？它们在多大程度上相互关联？3)萨赫勒地区和其他地区雨季开始和结束日期延迟的原因是什么？对于温度年度循环，一种假设是，延迟是由于气候变暖时海冰的消失，这将导致高纬度海洋的季节性变暖延迟，因为阳光会使整个海洋表层变暖，而不是覆盖着白雪的海冰的薄表面。还将考虑高纬度地区延迟变暖向热带地区传播的机制。对于雨季开始的延迟，一种假设是全球变暖对大气有稳定作用，因此必须在低层大气中积累更多的水汽，才能启动带雨的对流系统。研究降水和温度的延迟将具有实用价值，这将增加我们对全球变暖可能造成的后果的理解。雨季时间和持续时间的变化将对世界各地的社会和生态系统产生影响。由于这种延迟很强，在一定程度上出现在所有气候模型中，因此这种延迟可能会成为对未来全球变暖模型预测的可信度的重要考验。此外，更好地了解延迟的潜在机制可能有助于我们更好地了解全球变暖的其他潜在后果。这笔赠款将遵循美国国家科学基金会的任务，通过资助一名研究生，将教育机会纳入科学研究，该研究生将在完成气候科学高级学位的同时参与该项目。

The goal of this research is to examine the delay in the seasonal cycle which is commonly found in climate model simulations of anthropogenic global warming. The principal investigators have shown that the seasonal cycles of temperature and precipitation are delayed by a few days at the end of the 21st century compared to the end of the 20th century in climate change simulations. The delay in precipitation means that the onset of the rainy season is delayed in some tropical and subtropical regions. In the Sahel, along the southern margin of the Sahara, the onset is delayed more than the termination resulting in a shorter rainy season in global warming simulations. Similar shortening has been shown in other regions including Central America and the Caribbean in the same global warming simulations. There is also some evidence that the shortening of the Sahel rainly season has occurred in the real world as well as model simulations. The research is organized around three questions: 1) Are the model simulations trustworthy? 2) What is the cause of the delays in the seasonal cycles of temperature and precipitation, and to what extent are they related to each other? 3) What is the reason for the difference between the delays in onset and termination dates of the rainy seasons in the Sahel and other regions? For the temperature annual cycle, one hypothesis is that the delay is due to the loss of sea ice in a warmer climate, which would cause a delay in the seasonal warming of the ocean at high latitudes as sunlight would warm the full surface layer of the ocean rather than the thin surface of snow-covered sea ice. Mechanisms by which delayed warming at high latitudes could be communicated to the tropics will also be considered. For the delay of the rainy season onset, one hypothesis is that global warming has a stabilizing effect on the atmosphere, so that more moisture has to build up in the lower atmosphere in order for rain-bearing convective systems to initiate.The examination of delays in precipitation and temperature will have practical value by adding to our understanding of the possible consequences of global warming. Changes in the timing and duration of rainy seasons will have impacts on societies and ecosystems worldwide. Since the delays are robust, occurring to some extent in all climate models, the delay could serve as an important test of the credibility of model projections of future global warming. In addition, a better understanding of the underlying mechanisms of the delay could serve to improve our understanding of other potential consequences of global warming. This grant will follow NSF's mandate to incorporate educational opportunities into scientific research by funding a graduate student, who will work on the project while completing an advanced degree in clime science.