Intensity and intensity change remain one of the most significant challenges in tropical cyclone (TC) prediction. Instantaneous intensity and its evolution result from complex storm-environment interactions and internal structure changes across multiple time and spatial scales. These interactions truly couple the TC to the ocean as well as the ambient environment, however have been insufficiently observed and are difficult to model. Many earlier studies have demonstrated the impact of air-sea coupling to TC intensity, as well as the physical processes involved in the ocean response to TC passage. More recent work however has shown air-sea coupling affects both structure and intensity. Hurricane Ophelia in 2005 was well – observed by research reconnaissance aircraft during the fortuitous Hurricane Rainband and Intensity Change Experiment (RAINEX) field campaign. This mission included unprecedented airborne Doppler radar and dropsonde sampling of internal inner-core and rainband structures. These observations, supplemented by satellite, buoy, and analysis products captured a unique structural evolution in Ophelia that involved the collapse and recovery of its inner core. This thesis will investigate the structure and intensity changes that occurred in Hurricane Ophelia as a result of air-sea interaction, with particular focus on the impact of interactive ocean coupling and structure changes. Additionally, based on observations of double-eyewalllike structure during part of Ophelia’s life cycle we will examine the intensity changes to address a possible interrupted or incipient eyewall replacement cycle (EWRC). A highresolution, coupled atmosphere-ocean (AO) numerical model (UWIN-CM) simulation initialized with vortex-following nested grids and global analysis fields correctly captured the inner eyewall collapse, discrete outer secondary rainband/eyewall, and recovery of distinct eyewall and rainband features as well as more appropriately simulating the true storm intensity. Model output will be used to evaluate the structure and intensity changes between uncoupled (atmosphere-only) and coupled simulations, and then focus more closely on air-sea interaction instigating changes in air-sea enthalpy flux. Good agreement between RAINEX observations and the coupled model simulation allowed for detailed analysis of the role of air-sea interaction. Without an interactive ocean, the uncoupled TC is too strong and the lack of coupling prohibits feedbacks and air-sea interactions that modify air-sea enthalpy flux and ultimately storm structure and intensity. The presence of an interactive ocean is also critical to producing the observed induced cold pool and its distribution relative to TC wind stress. The TC surface circulation feedback on the ocean was found to be important for locally modifying air-sea enthalpy flux in the inner core via upper ocean cooling, while also being important to air-sea enthalpy flux distribution when Ophelia’s circulation becomes large after interacting with the cold pool. Advection of high enthalpy air at distant radii and lower enthalpy air from over the cold pool in the inner core region helped to support (suppress) convection far from (near) the storm center during the expansion, particularly a discrete outer rainband that may have served as a secondary eyewall. Finally, it should be noted that even the coupled simulation overestimates wind and precipitation intensity and initial SST, highlighting the outstanding challenge of model initial condition errors and appropriate parameterizations for convection and air-sea interaction physical processes in the complex TC-atmosphere-ocean environment.