| Summary: | This dissertation study tests the hypothesis that the variability of the extratropical atmosphere affects Atlantic tropical cyclone (TC) activity and explores the underlying physical processes. Guided by physical understanding, the study differs from earlier investigations of the extratropical impact by taking a process-based approach and focusing on Rossby wave breaking (RWB), which occurs often when extratropical Rossby waves propagate to the low-latitudes. Part I of the study shows that RWB modulates the tropical environment and Atlantic TC activity. RWB events can drive equatorward intrusions of extratropical air and increase the vertical wind shear in the tropics. Those environmental perturbations tend to inhibit TC development. On the seasonal scale, the occurrences of RWB and Atlantic TC activity are negatively correlated. The correlation is exceptionally strong and suggests that the extratropical control of Atlantic TC activity is comparable to the extensively studied tropical control. Especially, the occurrences of RWB show the strongest interannual variance over the subtropical northwestern Atlantic, and those RWB events also have a stronger impact on Atlantic TC activity. Part II of the study investigates the life cycle of the Rossby waves that break over the subtropical northwestern Atlantic. Composite analyses show that the breaking waves are associated with wave trains that propagate from the North Pacific and amplify near the east coast of North America. RWB is facilitated by a rapid amplification of upper-level ridge anomalies, which occurs over a warm and moist airstream. A budget analysis of potential vorticity (PV) suggests that the horizontal advection of PV by the perturbed flow dictates the movement and the later decay of the ridge anomalies. The ridge amplification, opposed by the horizontal advection of PV, is driven by the vertical advection and the diabatic production of PV, both of which can be connected to diabatic processes. A trajectory analysis of ridge-related air parcels suggests that diabatic processes reduce the static stability near the tropopause and contributes to the ridge-related PV anomalies. Part III of the study explores how the RWB events over the subtropical northwestern Atlantic are connected to tropical and extratropical variability. On the interannual scale, the RWB events are correlated with the sea surface temperature (SST) of the tropical North Atlantic. Idealized simulations suggest that the ocean variability affects the RWB events by modulating the tropical precipitation and the extratropical flow. On the synoptic scale, the RWB events can affect the SST variability by regulating surface heat fluxes. Taken together, the findings suggest an interaction between the variations of RWB events and the tropical ocean. The study also explores the connection between the RWB events and the extratropical atmosphere variability using the weather regime analysis. The RWB events and the atmosphere variability in the North Atlantic domain show only modest associations. Instead, the RWB events are more closely related to the atmosphere variability in the Pacific-North America domain. The findings help clarify the relation between Atlantic TC activity and the North Atlantic Oscillation. Finally, the findings also help explain a high-profile failure of seasonal prediction of Atlantic TC activity. U of I Only Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD system
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