Decrease of Rossby Wave Breaking Frequency over the Middle North Pacific in Boreal Summer under Global Warming in Large-Ensemble Climate Simulations

Abstract

Future change of Rossby wave breaking (RWB) frequency over the middle North Pacific (MNP) in August and the related features of large-scale atmospheric circulation are examined using large-ensemble simulations of current and future climates with a global circulation model. Correlation analysis indicates that the RWB frequency over the MNP in the current climate can show a relationship with El-Niño Southern Oscillation as reanalysis. The RWB frequency in the future climate shows significant decreases over the MNP, compared with that in the current climate. The large-scale atmospheric circulation in the upper troposphere in the future climate indicates a significant weakening of the Asian summer monsoon circulation and the consequent southward shifted Asian jet. The decreased RWB frequency over the MNP is associated with the modulated Asian jet through reduced diffluence and deceleration of the jet in the basic state over the region. Rossby wave propagation over Eurasia and the North Pacific in midlatitudes is also clearly reduced in the future climate, consistent with the decreased RWB frequency over the MNP. The correlation and histogram analyses of the current and future experiments indicate that the significantly decreased RWB frequency over the MNP is associated with significantly suppressed convective activities east of the Philippines in the future climate. The diagnosis using ω -equation further shows the dynamical impact of the decreased RWB frequency on the suppressed convective activities through the weakened extension of the Mid-Pacific trough and the consequent weakening of dynamically induced ascent east of the Philippines.