An Examination into the Drivers of Changes in Vertical Wind Shear in the Main Development Region and its Impact on Hurricane Frequency

In this work, we examine the potential factors that may influence vertical wind shear in the Main Development Region. We analyze large ensembles using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM). The fully coupled model consists of a 42- member ensemble (LENS-Fully Coupled), while the slab ocean model (SOM) consists of a 10-member ensemble (LENS-SOM). Each member differs only in that they were forced with small perturbations in the atmospheric initial conditions. Both fully coupled and SOM ensembles are run over the historical period, from 1920-2005. This study focusses on the Main Development Region (MDR) (80°W-20°W, 10°N-20°N), and also the Shear Enhancement Region (SER) (90°W-40°,13°N-25°N). Vertical Wind Shear affects hurricane frequency. Observations show variability in vertical wind shear over the decadal historical period, yet the cause of this variability and whether they impact hurricane frequency are still debated. Some possible causes can stem from internal atmospheric and oceanic circulation changes, as well as historical forcing. In order to look into these potential candidates, we conduct a literature review surrounding topics such as the Atlantic Multidecadal Oscillation (AMO), vertical wind shear and its effect on hurricane development, and internal atmospheric and oceanic forcing. We examine the role of internal variability in both the atmosphere and ocean by examining the differences in vertical wind shear and the AMO in the LENS-Fully Coupled and LENS-SOM experiments. The role of forcing on vertical wind shear is examined by comparing the ensemble mean to the ensemble spread. Finally, both models are compared to observations to determine if the model is able to capture the observed patterns.