This collaborative project uses numerical simulations to study the likely behavior of tropical cyclones (TCs, which include Atlantic hurricanes, Pacific typhoons, and cycones in the Indian Ocean), at the height of the last ice age (about 21,000 years ago) and during the mid-holocene warm period of about 6,000 years ago. Much attention has been devoted to understanding the impact on TCs of the warming of the world's oceans over recent decades and the likely response of TCs to future global warming. This study seeks to further understanding of the impact of climate change on TCs by looking backward to past climates which are comparably different from today's climate, although in different ways. Four specific questions are addressed: 1) How do the factors that influence tropical cyclone genesis vary under LGM and mid-holocene forcings? 2) Are there significant variations in how different models resolve these large-scale factors? Which changes appear attributable to model variability and which to variations in external forcing? 3) How do the TC-like vortices found in climate models change in LGM and mid-holocene simulations when compared to preindustrial or present-day conditions? 4) How does a higher resolution, regional model's simulation of a paleoclimate environment handle genesis? How do storm counts and track densities vary as the climate forcing changes? The research is based on preliminary results which find evidence that the climate of the last ice age may have been relatively favorable for TC development in some regions, despite the generally colder sea surface temperatures worldwide.
The work has broader impacts in the scientific community as it will inform research into the emerging science of "paleotempestology", the interdisciplinary study of the behavior of TCs and other storms in past climates. The work will also inform efforts to understand the impact of global warming on landfalling TCs including hurricanes along the US coastline. In addition, the work will fund and train a graduate student and a postdoctoral researchers, thereby providing support and training for the next generation of scientists.
The main aim of this project was to learn more about the occurrence of tropical cyclones or hurricanes in past climates. The historical records of tropical cyclone activity are not very long. In the North Atlantic they start in the late 19th century, but in other regions the records start only around 1950. Therefore, if we want to know what happened before that we have two choices: the first is to use geological records to determine the occurrence of landfalls of hurricanes in specific regions. The second possibility is to use climate models and examine the tropical cyclone activity in these models in different eras. We use the 2nd method here, i.e. we use various climate models to analyze the characteristics of tropical cyclone activity in different climates. The reason we are interested in this topic, is that if we understand better how the tropical cyclone activity changes in different climates, that can help us figure out what will happen with tropical cyclones in the future. We chose two specific periods to analyze in this project. The first is called "Last Glacial Maximum" and occurred about 20,000 years ago. That was the last glacial period in the Earth, when the ice sheets were at their most recent maximum extension. It is well known that these ice sheets had a large impact on the climate, but not what happen with hurricanes during that period, which is what we wanted to explore. The 2nd chosen period was 6,000 years ago, when the Earth was in a warm period, called "Mid-Holocene". These 2 periods were chosen for their contrasting climates: one when the Earth was cold, the other when Earth was warm. In order to study the tropical cyclones in those 2 periods, we used a few climate models simulations for these 2 periods, as well as for the present climate. We then analyzed the characteristics of the tropical cyclones (e.g. frequency, intensity, seasonality) and compared with the characteristics of tropical cyclones in the current climate in observations and models. We also examined the environmental conditions that are associated with tropical cyclones in the different periods. Our results show, for instance, that the frequency of tropical cyclones globally in the two other past climates is not radically different from the current global frequency. However, we do see shifts in the seasonality of the tropical cyclone occurrence.
