[A-AS21_30PO1] Stratospheric Processes And their Role in Climate

[AAS21-P06] Basic characteristics of forecast skill variations in JMA 1-month hindcast experiments

*Masakazu TAGUCHI¹ (1.Aichi University of Education)

This study investigates basic characteristics of stratospheric predictability in the Northern Hemisphere using 1-month hindcast (HC) experiment data of the Japan Meteorological Agency for 1979-2009. We describe characteristics of forecast properties of spread, error (root mean square error), and anomaly correlation, contrasting the stratosphere and troposphere for different seasons, as well as explore the so-called spread-skill relationship for the winter stratosphere. We also examine the role of stratospheric sudden warmings (SSWs) in variations in the forecast skills. Our results show that for lead times shorter than about 10 to 15 days, the forecast skills of the HC data are higher on average and more variable in the stratosphere than in the troposphere especially for Northern winter. This is reflected in larger average and variability in predictable time limit, or characteristic time scale of useful predictions, for the winter stratosphere. We also reveal that the spread-skill relationship for the Northern winter stratosphere is characterized by the existence of notable outliers from their expected linear distribution; the outliers have markedly large errors, or low skills, for given spreads. Most of the outliers are contributed by HC sets initialized before observed major SSWs. Such HC data fail to reproduce the strength and/or shape of the stratospheric polar vortex including both onset and recovery phases of SSWs. The HC data tend to yield too strong vortex and shorter-than-average predictable limit.