Evaluation of synoptic eddy activities and their feedback onto the midlatitude jet in five atmospheric reanalyses with coarse versus fine model resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto the midlatitude jet in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto the mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the low-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy–mean flow interaction.

Evaluation of Synoptic Eddy Activities and their Feedback onto the Midlatitude Jet in Five Atmospheric Reanalyses with Coarse Versus Fine Model Resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto mean flow in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the lower-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy-mean flow interaction.

ESTIMATION OF THE SYNOPTIC EDDIES INFLUENCE ON THE ACOUSTIC RANGING ACCURACY

Разработка систем акустической навигации и акустической дальнометрии в настоящее время является одной из наиболее актуальных практических задач акустики океана. В работе исследуется вопрос о влиянии крупномасштабных неоднородностей поля скорости звука в океане на точность решения задачи акустической дальнометрии. В качестве примера такой неоднородности нами выбран устойчивый антициклонический вихрь, наблюдающийся в южной части Японского моря в летний период. В работе проведены вычислительные эксперименты по исследованию влияния этого вихря на структуру звукового поля, формируемого на акустической трассе, проходящей через его центр, источником навигационных сигналов (ИНС), расположенным на шельфе. В ходе этих экспериментов по гидрологическим данным, полученным с помощью моделей глобальной циркуляции океана NEMO и ИВМ РАН, для этой трассы построена модель нерегулярного волновода «шельф–глубокий океан», после чего с помощью метода широкоугольных параболических уравнений выполнено моделирование акустического поля, формируемого ИНС в таком волноводе. Далее в работе также выполнен анализ модовой структуры этого поля, определены интервалы локализации различных модальных компонент сигнала и рассчитаны эффективные скорости распространения сигналов от ИНС на различных горизонтах приема, после чего исследовано влияние синоптического вихря на данные характеристики волновода. На основе этого анализа выполнены оценки влияния вихря на времена прихода сигналов от ИНС в точку приема, а также дополнительная погрешность решения задачи акустической дальнометрии, обусловленная этим влиянием. Результаты исследования показывают, что в рамках рассматриваемой методики решения задачи акустической дальнометрии даже относительно крупный неучтенный синоптический вихрь, ядро которого находится непосредственно на трассе, оказывает относительно слабое влияние на точность определения дальности (около 30 м для трассы протяженностью 300 км, или 0,01%). The development of acoustic navigation and acoustic ranging systems is currently one of the most important practical problems of ocean acoustics. In this study, the influence of large-scale inhomogeneities on the sound speed field in the ocean on the accuracy of acoustic ranging problem solution is considered. As a representative example of an inhomogeneity of this kind, we chose a stable anticyclonic eddy that is observed in the southern part of the sea of Japan in summer. In this work, computational experiments are conducted in order to study the influence of this eddy on the structure of the sound field formed along an acoustic path passing through the eddy's center by a source of navigation signals (SNS) located on the shelf. In the course of these experiments, a model of a range-dependent "shallow-to-deep-sea" waveguide was constructed along this path using hydrological data obtained from NEMO and INM RAS global ocean circulation models. After that, the acoustic field produced by the SNS in this waveguide was simulated by the method of wide-angle parabolic equations. The mode structure of the field along the path is studied, localization intervals of various modal components of the signal are determined, and the effective propagation velocities of signals transmitted by SNS are calculated at various reception horizons. The influence of the synoptic eddy on these waveguide characteristics is also investigated. On the basis of this analysis, the effect of the eddy on arrival times of the signals propagating from the SNS to the reception point is estimated, as well as the additional error in the solution of acoustic ranging problem caused by the presence of the eddy. The results of the study show, that within the framework of the considered technique of acoustic ranging problem solution, even the presence of a large unaccounted synoptic eddy, with its core located directly on the acoustic path, has a relatively weak effect on the accuracy of range estimation (about 30 m for a path 300 km long, or 0,01%).

Understanding Anomalous Synoptic Eddy Vorticity Forcing in Pacific–North American Teleconnection Pattern Events

Utilizing a decomposition of anomalous eddy vorticity forcing (EVF) proposed by Song in 2016 and a modified Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model, this study provides a different understanding of physical mechanisms that are responsible for the formation of the anomalous synoptic EVF (SEVF) associated with Pacific–North American teleconnection pattern (PNA) events. A series of short-term control experiments (CEs) and initial-value modified experiments (IVMEs) is conducted. In each case of CEs, there are no obvious PNA-like circulation anomalies. IVMEs are exactly the same as CEs except that appropriate small perturbations are introduced into the initial-value fields of CEs. The modified initial-value fields led to a gradual development of the PNA-like flow anomalies in IVMEs. Based on these numerical results, deformations of the synoptic eddy due to the emergence of the PNA pattern can be easily acquired by subtracting the synoptic eddy in CEs from the synoptic eddy in IVMEs . The anomalous SEVF associated with the PNA events in the model can be decomposed into ensembles of two linear and interaction terms (EVF1 and EVF2) and a nonlinear self-interaction term (EVF3). It is demonstrated that the physical essence of the anomalous SEVF associated with the PNA events is a competition result between EVF1 plus EVF2 and EVF3. Results also indicate that the different signs of SEVF associated with the positive and negative PNA events are not necessarily related to the different tilts of the synoptic eddy.

Anatomy of Synoptic Eddy–NAO Interaction through Eddy Structure Decomposition

A method of eddy structure decomposition is proposed to detect how low-frequency flow associated with the North Atlantic Oscillation (NAO) organizes systematically synoptic eddy (SE) activity to generate in-phase and upstream feedbacks. In this method, a statistical eddy streamfunction (SES) field, defined by the three-point covariance of synoptic-scale streamfunction, is introduced to characterize spatiotemporal SE flow structures. The SES field is decomposed into basic and anomalous parts to represent the climatological SE flow structure and its departure. These two parts are used to calculate the basic and anomalous eddy velocity, eddy vorticity, and thus eddy vorticity flux fields, in order to elucidate those two SE feedbacks onto the NAO. This method is validated by the fact that the observed anomalous eddy vorticity flux field can be reproduced well by two linear terms: the basic eddy velocity field multiplied by anomalous eddy vorticity field and the anomalous eddy velocity field multiplied by basic eddy vorticity field. With this method, it is found that, in the positive and negative phases, the NAO flow tends to induce two different types of anomalous SE flow structure, which are largely responsible for generating the net meridional and zonal eddy vorticity fluxes that, in return, feed back onto the NAO. The two processes that are related to these two different types dominate in the in-phase and upstream feedbacks, which are delineated conceptually into two kinematic mechanisms associated with zonal-slanting and meridional-shifting changes in the SE structure. The present observational evidence supports the theory of eddy-induced instability for low-frequency variability and also provides insights into the reason for the asymmetry between the SE feedbacks onto the two NAO phases.

Zonal Asymmetry of the Annular Mode and Its Downstream Subtropical Jet: An Idealized Model Study

This study investigates linkages between the zonal asymmetry of the annular mode (AM) zonal pattern and the subtropical jet (STJ) over its downstream regions of the storm track by using an idealized model. Observational analyses show that the AM zonal patterns are more zonally asymmetric during days when the STJ downstream of the storm track is unusually strong, and vice versa. In the idealized model, the STJ downstream of the storm track is varied by introducing an additional zonally localized tropical heating. The model’s AM variability exhibits a nearly zonally uniform structure when there is no or only weak tropical heating. However, the signatures of the AM are locally strengthened in the heating sector; thus, the AM zonal pattern is zonally asymmetric when the tropical heating is large enough to create a strong STJ. The model results also show that the percentage of the variance explained by the AM, the persistence of the AM index, and the intensity of eddy feedback are also increased when the tropical heating becomes stronger. It is argued herein that the zonal asymmetry of the AM pattern is caused by the zonal asymmetry of the anomalous synoptic eddy forcing projecting on the AM, which is primarily due to the zonal asymmetry of the variations of the storm track between the nonheating and heating sectors.