Abstract: The warm seclusion or mature stage of the extratropical cyclone lifecycle often has structural characteristics reminiscent of major tropical cyclones including eye-like moats of calm air at the barotropic warm-core center surrounded by hurricane force winds along the bent-back warm front. Many extratropical cyclones experience periods of explosive intensification or deepening (bomb) as a result of nonlinear dynamical feedbacks associated with latent heat release. Considerable dynamical structure changes occur during short time periods of several hours in which lower stratospheric and upper-tropospheric origin potential vorticity combines with ephemeral lower-tropospheric, diabatically generated potential vorticity to form a coherent, upright tower circulation. At the center, anomalously warm and moist air relative to the surrounding environment is secluded and may exist for days into the future. Even with the considerable body of research conducted during the last century, many questions remain concerning the warm seclusion process. The focus of this work is on the diagnosis, climatology, and synoptic-dynamic development of the warm seclusion and surrounding flank of intense winds. To develop a climatology of warm seclusion and explosive extratropical cyclones, current long-period reanalysis datasets are utilized along with storm tracking procedures and cyclone phase space diagnostics. Limitations of the reanalysis products are discussed with special focus on tropical cyclone diagnosis and the recent dramatic decrease in global accumulated tropical cyclone energy. A large selection of case studies is simulated with the Weather Research and Forecasting (WRF) mesoscale model using full-physics and “fake dry” adiabatic runs in order to capture the very fast warm seclusion development. Results are presented concerning the critical role of latent heat release and the combination of advective and diabatically generated potential vorticity in the generation of the coherent tower circulation characteristic of the warm seclusion. To motivate future research, issues related to predictability are discussed with focus on medium-range forecasts of varying extratropical cyclone lifecycles. Additional work is presented relating tropical cyclones and large-scale climate variability with special emphasis on the abrupt and dramatic decline in recent global tropical cyclone accumulated cyclone energy.

Abstract: Astronomical forcing (obliquity and precession) has been thought to modulate Dansgaard-Oeschger (DO) events, yet the detailed quantification of such modulations has not been examined. In this study, we apply the novel Holo-Hilbert Spectral Analysis (HHSA) to five polar ice core records, quantifying astronomical forcing's time-varying amplitude modulation of DO events and identifying the preferred obliquity phases for large amplitude modulations. The unique advantages of HHSA over the widely used windowed Fourier spectral analysis for quantifying astronomical forcing's nonlinear modulations of DO events is first demonstrated with a synthetic data that closely resembles DO events recorded in Greenland ice cores (NGRIP, GRIP, and GISP2 cores on GICC05 modelext timescale). The analysis of paleoclimatic proxies show that statistically significantly more frequent DO events, with larger amplitude modulation in the Greenland region, tend to occur in the decreasing phase of obliquity, especially from its mean value to its minimum value. In the eastern Antarctic, although statistically significantly more DO events tend to occur in the decreasing obliquity phase in general, the preferred phase of obliquity for large amplitude modulation on DO events is a segment of the increasing phase near the maximum obliquity, implying that the physical mechanisms of DO events may be different for the two polar regions. Additionally, by using cross-spectrum and magnitude-squared analyses, Greenland DO mode at a timescale of about 1400 years leads the Antarctic DO mode at the same timescale by about 1000 years. (C) 2018 Elsevier Ltd. All rights reserved.