Projected changes of extremely cool summer days over northeastern Japan simulated by 20 KM-mesh large ensemble experiment

Abstract

This study investigates future changes to extremely cool days (ECDs) during the summer (June–August) season in northeastern Japan by applying self-organizing map (SOM) technique to large ensemble simulations from the “database for Policy Decision making for Future climate change” (d4PDF). Two separate SOMs, one trained on mean sea level pressure using a combination of JRA-55 reanalysis and d4PDF to evaluate model performance, and a “master” SOM, which trained the SOMs using historical, +2K, and +4K simulations, were created to investigate possible climate change impacts to future ECDs. For model evaluation, summer climatology and ECDs were confirmed to occur with similar frequencies between circulation patterns in the JRA-55 and d4PDF. Surface temperature anomalies and horizontal wind composite from several high-frequency ECD nodes exhibit similar spatial patterns for all days and ECD occurring in the node, with ECD composites depicting particularly strong northeasterly winds, commonly referred to as Yamase, blowing from high latitudes toward northeast Japan. Future changes using “master” SOMs suggest a gradual shift (from +2K to +4K) in preferred circulation patterns that result in ECDs, with the greatest increase in frequency associated to those with a strong low-pressure system off eastern Japan and a moderate intensity Okhotsk Sea high, and decreased ECDs to those with either a strong Okhotsk Sea high or westward extension of the North Pacific high. Lastly, changes to the intensity of future ECDs are investigated by examining low-level thermal advection. Results suggest that circulation patterns associated with increased ECD frequency coincide with those with very strong cold air advection for all climates, though the magnitude differs based on circulation patterns. Future changes show a weakening cold air advection and decreasing ECDs, due in large part to the weakening meridional temperature gradient east of Japan.