Future Changes in the Seasonal Cycle: Mechanisms and Implications

本研究的目的是检验季节周期的延迟，这种延迟在人为全球变暖的气候模式模拟中很常见。主要研究人员已经表明，在气候变化模拟中，与20世纪末相比，21世纪末温度和降水的季节周期延迟了几天。降水的延迟意味着在一些热带和亚热带地区雨季的延迟到来。在萨赫勒地区，沿着撒哈拉沙漠的南缘，在全球变暖模拟中，雨季开始的时间比结束的时间要晚，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现象。还有一些证据表明，萨赫勒地区雨季的缩短不仅在模型模拟中发生，也在现实世界中发生。本研究围绕三个问题展开：1)模型模拟是否可信？2)气温和降水季节周期延迟的原因是什么？它们之间的关联在多大程度上？3)萨赫勒地区和其他地区雨季的开始和结束日期延迟的原因是什么？对于温度年循环，有一种假设认为，这种延迟是由于气候变暖导致海冰减少，这将导致高纬度地区海洋季节性变暖的延迟，因为阳光会使海洋的整个表层变暖，而不是被雪覆盖的海冰的薄表面变暖。还将考虑将高纬度地区的延迟变暖传递给热带地区的机制。对于雨季推迟到来的原因，有一种假设是，全球变暖对大气有一种稳定作用，因此低层大气中必须积累更多的水分，才能使有雨的对流系统启动。对降水和温度延迟的研究将通过增加我们对全球变暖可能后果的理解而具有实际价值。雨季时间和持续时间的变化将对全世界的社会和生态系统产生影响。由于延迟是稳健的，在某种程度上在所有气候模型中都会出现，因此延迟可以作为对未来全球变暖模型预测可信度的重要测试。此外，更好地了解这种延迟的潜在机制可以帮助我们更好地理解全球变暖的其他潜在后果。这笔拨款将遵循美国国家科学基金会的使命，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从事该项目。

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.