Improved Predictability of Summertime Rossby Wave Breaking Frequency near Japan in JMA/MRI-CPS3 Seasonal Forecasts

Abstract

Abstract The seasonal predictability of the Rossby wave breaking (RWB) frequency near Japan in July–August (JA) is examined using daily JMA/MRI-CPS3 (CPS3) hindcast data, which is an operational seasonal prediction system of the Japan Meteorological Agency. Although the RWB frequency near Japan during JA in CPS3 is underestimated in comparison with the reanalysis, interannual variabilities of the frequency are generally predicted with moderate or high skill for hindcasts, initiating from February to June. The RWB frequency forecast skill in CPS3 is much higher than that in the previous version of the seasonal prediction system due to the improvement in the model bias of the Asian jet stream meridional position. A regression analysis for the RWB frequency near Japan utilizing all ensemble members is conducted to evaluate the reproducibility of the increased (decreased) RWB frequency associated with La Niña (El Niño) conditions, as indicated by previous studies. The regressed anomalies demonstrate an anomalous sea surface temperature (SST) pattern similar to that of La Niña and a negative phase of the Indian Ocean dipole mode with the associated anomalous convection in the tropics. For the La Niña condition, the regressed geopotential height in the upper troposphere demonstrates negative anomalies over the tropical Pacific and positive anomalies in the extratropical Northern Hemisphere, corresponding to the enhanced mid-Pacific trough and northward-shifted subtropical jet. The regressed meridional wind anomalies demonstrate a wavy pattern along the Asian jet over Eurasia, consistent with the relationship between the Silk Road pattern and the RWB near the Asian jet exit region.