The development mechanism of synoptic-scale disturbances over the tropical western North Pacific (WNP) associated with the boreal summer intraseasonal oscillation (BSISO) under different phases of the interannual Pacific–Japan (PJ) pattern is investigated. Intraseasonal convection is enhanced widely over the WNP for BSISO phases 5–7 in the positive PJ years, when seasonal-mean convective activity over the tropical WNP is enhanced. By contrast, developed convection is confined over the South China Sea in the negative PJ years. Similar features are also found in the horizontal distributions of eddy kinetic energy (K′) representing the activity of synoptic-scale disturbances and of tropical cyclone occurrences. The differences in location of intraseasonal convection and the activity of synoptic-scale disturbances between the positive and negative PJ years likely lead to different teleconnections to midlatitude East Asia. The non-PJ years exhibit mixed features of the positive and negative PJ years. A K′ budget analysis reveals that the energy conversion from eddy available potential energy to K′ (PeKe) associated with synoptic convection primarily contributes to K′ generation during the convectively active phases of the BSISO. The barotropic energy conversion from mean kinetic energy to K′, KmKe, is determined to be the second largest contributor to the K′ increase in the lower troposphere, especially during the early stage of development of synoptic disturbances. Large K′ produced by PeKe and KmKe in the tropics is advected to the subtropics by the mean flow in the late to mature stages of development. There are two factors that can determine the different locations where synoptic disturbances develop in association with PeKe and KmKe. One is intraseasonal sea surface warming during convectively suppressed phases of the BSISO preceding the active phases. The other is convergence or shear of seasonal-mean horizontal winds associated with interannual fluctuations of monsoon westerlies over the WNP.

Wintertime sea ice cover in the Okhotsk Sea (OS) exhibits strong interaction with the atmosphere over the Far East and the North Pacific. As per previous studies, it has been determined that interannual variability of sea ice cover in the OS is associated with large-scale atmospheric circulations. However, the atmospheric processes responsible for rapid changes in sea ice cover in the OS on subweekly-to-weekly timescales are yet to be determined. Thus, in this study, we aim to investigate the atmospheric circulations that contribute to rapid reduction events of OS sea ice concentration (OSSIC) using daily high-resolution ocean reanalysis data. In total, we detected 21 rapid reduction events of OSSIC from 1993 to 2019. The reduction events shared common features in terms of atmospheric circulation, i.e., a developing extratropical cyclone over the southern OS and anomalous high pressure over the northern Bering Sea with strong surface southeasterly winds between the two. The strong southeasterlies, which blow in the opposite direction to the surface westerlies that regulate the seasonal development of sea ice cover, often result in the rapid reduction of OSSIC. Substantial reduction in sea ice was noted to occur in the northern and central OS owing to sea ice advection and sea ice melt associated with the easterly winds. The eastward-moving extratropical cyclone contributes both to rapid reduction of OSSIC and to reduction of sea level pressure over the northern North Pacific, resulting in a lagged relationship between OSSIC and the Aleutian Low.

Inspired by the detection of the Pekeris mode of atmospheric free oscillations by a recent study, high-accuracy numerical calculations of the equivalent depth of atmospheric free oscillations are performed. Here, the computational method is largely based on a previous study, but with modifications to improve the calculation accuracy. Two equivalent depths are found, with values of 9.9 km and 6.6 km. The former corresponds to the Lamb mode and the latter to the Pekeris mode. These values deviate from those obtained in the previous study, especially for the Pekeris mode. The causes of this discrepancy are discussed, as well as the correspondence between the equivalent depths obtained in this study and that of the Pekeris mode detected in the recent study.

A new operational seasonal forecast system, Japan Meteorological Agency (JMA)/Meteorological Research Institute (MRI) Coupled Prediction System (CPS) version 3 (JMA/MRI–CPS3), has been developed. This system represents a major upgrade from the former system, CPS2. CPS3 comprises atmosphere, land, ocean, and sea ice forecast models and the necessary initialization systems for these models. For historical reforecasts, the atmospheric reanalysis dataset JRA-3Q provides initial conditions for the atmosphere and external forcings for land, ocean, and sea ice analysis. In the operational forecast, to initialize the system in near-real time, JMA's operational atmospheric analysis is used in conjunction with JRA-3Q. Next, the land surface model is initialized using an uncoupled free simulation, forced by the atmospheric analysis. The ocean and sea ice models are initialized with the global ocean data assimilation system MOVE-G3, which incorporates a newly developed four-dimensional variational method for temperature, salinity, and sea surface height and a three-dimensional method for sea ice concentration. Compared with the previous system, the CPS3 forecast model components exhibit ∼ 2–4 times higher resolution: the atmosphere and land models are configured with ∼ 55 km horizontal resolution, with 100 vertical atmosphere layers; and the ocean and sea ice models exhibit a resolution of 0.25° × 0.25°, with 60 vertical ocean layers. The physical processes of the atmosphere are greatly refined in CPS3 relative to CPS2, resulting in improved representation of subseasonal to seasonal scale variability, including the eastward propagation of the Madden–Julian Oscillation, winter blocking highs in the North Atlantic, and coupled atmosphere–ocean variability during El Niño–Southern Oscillation events. Our historical reforecast experiment for 1991–2020 suggests that CPS3 demonstrates greater forecast skills than CPS2. The usability of the model output has been improved in CPS3 by reorganizing the operation schedule to provide daily updates of five-member ensemble forecasts.