Extratropical Impacts on Prediction and Variability of Tropical Cyclones
Tropical cyclones (TCs) are one of the most severe storm systems on the Earth and pose a serious threat to marine operations and coastal communities. In addition to tropical processes, TC activity is also affected by extratropical processes. The goal of my Ph.D. research is to better understand the influences of extratropical processes on prediction and variability of TCs on seasonal and longer time scales.
In the first part of my thesis, I examined the relative importance of local tropical sea surface temperatures (SSTs) and extratropical processes on TC frequency using WRF model simulations. It is shown that the remote processes outside the North Atlantic, particularly extratropical processes, play a non-negligible role in modulating Atlantic TC frequency and that such impacts may exceed the impacts of local SST in some years. In contrast to the North Atlantic basin, the total TC frequency over the northern tropics are strongly modulated by tropical SST and the impacts of the remote forcing are negligible.
In the second part of my research, I developed a skillful hybrid prediction scheme for multi-year Atlantic TC activity using a predictor in the extratropics. I employed a Poisson regression model and took SST indices averaged over the Atlantic main development region (MDR) and the Atlantic subpolar gyre region (SPG) from the initialized CESM large ensemble hindcasts as predictors. The model skillfully predicts various Atlantic TC indices (including landfalling hurricane frequency) during the past ~60 years. We also demonstrated that the Atlantic SPG SST is a more important source of predictability than the Atlantic MDR SST on the multi-year time scale. Investigation of the underlying physical mechanisms reveals that the vertical wind shear (VWS) and tropospheric precipitable water (TPW) over the tropical Atlantic are significantly correlated to the SPG SST, in addition to MDR SST. Changes of these environmental variables can be understood in the context of regional Atlantic Hadley circulation. The skillful multi-year TC predictions are expected to provide useful information to disaster relief agencies and insurance industry.
The third part of my research focuses on a framework to better understand the tropical and extratropical impacts on TC activity. A recent study suggested that the summertime stationary waves integrate tropical and extratropical impacts on TC activity. A better understanding of the variability mechanisms of stationary waves is expected to further improve TC prediction. The interannual variability of summertime subtropical stationary waves is examined using reanalysis data and global model simulations; their response to anthropogenic warming is investigated using CMIP6 outputs; and the implications for regional TC projection are discussed. My analyses help increase our confidence in the projection of regional TC activity.
