No abstract

This study analyzes a Rossby wave-breaking event east of Japan that enhanced the convective activities over the subtropical western North Pacific Ocean. In August 2016, Rossby-wave packets in the upper troposphere above Eurasia reached over and around the seas east of Japan. The wave-breaking event accompanied the amplification of a blocking ridge and the southward intrusion of upper-level high-potential vorticity (PV) south of the ridge. The high PV (i.e., the enhanced mid-Pacific trough) promoted upward motion and enhancement of convective activities over the subtropical western North Pacific Ocean through a quasi-geostrophic balance. In the lower troposphere, large-scale cyclonic circulation anomalies, including tropical disturbances, were observed south and southeast of Japan, and the anomalies caused significant wet climate conditions in the eastern and northern parts of the country. A linear baroclinic model experiment indicates that the lower-level cyclonic circulation anomalies were the Rossby-wave responses to heating anomalies associated with the enhanced convective activities. These results suggest the existence of dynamic interaction between extratropical and tropical circulation over the western North Pacific Ocean and its influence on boreal summer climate in Japan.

We investigated mechanisms contributing to the quiescent tropical cyclone (TC) activity in the western North Pacific (WNP) during the early summer (May-July) of 2016 by conducting and analysing seasonal predictions and sensitivity experiments with an atmosphere-ocean coupled model. In the seasonal prediction experiment, the model successfully predicted the inactive TC condition. Sensitivity experiment simulations, in which the warmer-than-normal sea surface temperature (SST) in the Indian Ocean (IO) was restored to the climatology, represented a weakened lower-tropospheric anticyclonic anomaly and near-normal TC activity over the WNP. These results suggest that the quiescent TC activity is attributable to the warm IO SST anomalies induced by the preceding 2015/2016 El Niño. Verification and analysis of reforecasts indicated that the TC count in early summer is highly predictable due to IO warming, a lingering effect of preceding El Niño events.

This pre-publication manuscript may be downloaded, distributed and used under the provisions of the Creative Commons Attribution 4.0 International (CC BY 4.0) license.

 In February 2022, the Japan Meteorological Agency started operating its third-generation seasonal ensemble prediction system (CPS3; Coupled Prediction System version 3; Hirahara et al. 2023). CPS3 is based on an atmosphere/ocean/land/sea-ice-coupled model; the atmosphere model has a resolution of TL319 (approx. 55 km) L100 (up to 0.01 hPa) and the ocean model has 60 layers with a global resolution of 0.25° x 0.25°. Along with the widely improved performance of each model, prediction skills of CPS3 are significantly better than those of the previous system. There is, however, still room for improvement, including a positive IOD-like SST bias in the Indian Ocean, a positive surface air temperature bias over the interior of Eurasia, a negative cloud cover bias in subtropical ocean stratocumulus regions, and an arctic radiation flux bias. Cumulus and cloud parameterization errors are some of the primary causes of these biases. Focusing on the Eurasian continental interior, predicted cloud cover is fewer than observed amount throughout the year, and diurnal and seasonal variations are insufficiently represented. To represent cloud condensation, CPS3 has a cloud parametrization using a diagnostic method based on Smith (1990), where spatial fluctuations in cloud water variables that cannot be resolved on a grid scale are represented by assuming a specific Probability Density Function (PDF). In CPS3, a symmetric top-hat PDF (JMA 2022) is used. Although detrain from cumulus is included in cloud water content, fluctuations in cloud water variables due to cumulus are not fully considered, which is one of the possible causes of the negative bias in cloud cover. To address this issue, representation of asymmetries of the fluctuation in cloud water variables due to cumulus in the PDF can be adapted. At this meeting, we will report on the above topics and the progress of development for the next CPS.