Kanghyun Song

Stratospheric variability contributed to and sustained the recent hiatus in Eurasian winter warming

Abstract The recent hiatus in global‐mean surface temperature warming was characterized by a Eurasian winter cooling trend, and the cause(s) for this cooling is unclear. Here we show that the observed hiatus in Eurasian warming was associated with a recent trend toward weakened stratospheric polar vortices. Specifically, by calculating the change in Eurasian surface air temperature associated with a given vortex weakening, we demonstrate that the recent trend toward weakened polar vortices reduced the anticipated Eurasian warming due to increasing greenhouse gas concentrations. Those model integrations whose stratospheric vortex evolution most closely matches that in reanalysis data also simulate a hiatus. While it is unclear whether the recent weakening of the midwinter stratospheric polar vortex was forced, a properly configured model can simulate substantial deviations of the polar vortex on decadal timescales and hence such hiatus events, implying that similar hiatus events may recur even as greenhouse gas concentrations rise.

Quantitative and qualitative analyses of parametric roll for ship design and operational guidance

This article presents a procedure for quantitatively analyzing the susceptibility of a ship to parametric roll for use in designing a ship against this phenomenon. Due to the nonergodic characteristics of parametric roll motion, numerous direct time-domain simulations are needed to obtain a stable long-term distribution of parametric rolls. To avoid such heavy computational demand and to accelerate numerical simulations, a 1-degree-of-freedom computational model for parametric roll prediction is developed for both regular- and irregular-wave excitations, adopting the approximated righting arm ( GZ ) curve. In this model, the concept of transfer functions is introduced for the mean and the first harmonic component of metacentric height ( GM ), which is introduced to approximate GM fluctuation. The simulation results obtained by using this model are compared to those of a three-dimensional weakly nonlinear simulation program. The sensitivity of the simulation results to the initial value, time window, and number of simulations is investigated, and their proper values are proposed. Using the proposed model, the long-term predictions of parametric roll are carried out for typical post-Panamax container ships and a very large crude oil carrier by applying Monte Carlo simulation. Based on the simulation results, the susceptibility of each ship to parametric roll is evaluated, and the influence of still-water GM on the susceptibility is investigated. This study extends to the development of an operational guidance for the ship crew’s decision-support system, as one of countermeasures of a severe parametric roll. Particularly, a procedure for qualitatively developing an operational guidance is proposed. In this study, numerical simulations are carried out by using an impulse–response function approach. As an example model, a typical 10,000 TEU container ship is considered, and the results are discussed with respect to its operation.

Impact of Sudden Stratospheric Warming on the Surface Air Temperature in East Asia

The sudden stratospheric warming (SSW), which is characterized by an abrupt increase of polar stratospheric temperature by several tens of degrees in a week, has been known to affect tropospheric weather and climate on sub-seasonal time scale in the boreal winter. Such downward coupling has been often examined in North Atlantic and Europe, but rarely examined in East Asia. In this study, by applying the two definitions of SSW to the reanalysis data, the possible impacts of the SSW events on the surface air temperature (SAT) and tropospheric circulation in East Asia are analyzed. It is found that Eurasian continent, including Siberia and the Northeast Asia, tends to experience anomalously cold SAT for up to thirty days after the SSW events. The resulting SAT anomalies largely resemble those associated with negative Artic Oscillation. However, over East Asia, SSW-related SAT change is weak and not statistically significant. Only during the extreme SSW events when the downward coupling between the stratosphere and troposphere is strong, East Asia exhibits significantly cold SAT anomalies. This relationship is presented by grouping SSW events into those followed by cold SAT anomalies over East Asia and those by warm anomalies for varying threshold values of the SSW events.

Revisiting the ENSO-SSW relationship

AbstractStratospheric sudden warming (SSW) events exhibit pronounced interannual variability. Based on zonal wind reversals at 60°N and 10 hPa, it has been suggested that SSW events occur more preferentially during El Nino–Southern Oscillation (ENSO) winters (both El Nino and La Nina winters) than during ENSO-neutral winters. This relationship is reevaluated here by considering seven different SSW definitions. For all definitions, SSW events are detected more frequently during El Nino winters than during ENSO-neutral winters, in agreement with a strengthened planetary-scale wave activity. However, such a systematic relationship is not found during La Nina winters. While three SSW definitions, including the wind-reversal definition, show a higher SSW frequency during La Nina winters than during ENSO-neutral winters, other definitions show no difference or even lower SSW frequency during La Nina winters. This result, which is qualitatively insensitive to the choice of reanalysis datasets, ENSO indices, and ...