Late-July Barrier for Subseasonal Forecast of Summer Daily Maximum Temperature Over Yangtze River Basin

Abstract

Dynamical subseasonal forecast skill of summer daily maximum temperature (Tmax) over East China was evaluated based on a 20-year (1995–2014) subseasonal reforecast data set from the European Centre for Medium-range Weather Forecasts. A significant late-July barrier of subseasonal forecast skill was identified for the Tmax over the Yangtze River Basin, which concurs with a reduction in the prediction skill for the local 500-hPa geopotential height. This barrier period corresponds to an abrupt transitional phase of the climatological intraseasonal oscillation when the western North Pacific subtropical high abruptly migrates northward from Yangtze River Basin to northern China. The transitional phase of the climatological intraseasonal oscillation features the largest day-to-day variance in the position of western North Pacific subtropical high, which may cause the drop of the subseasonal forecast skill for both the geopotential height and Tmax. The results indicate that the atmospheric subseasonal predictability may be strongly affected by the phases of the local climatological intraseasonal variation.