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.
