Spring Arctic Oscillation-East Asian summer monsoon connection through circulation changes over the western North Pacific

In the present study the links between spring Arctic Oscillation (AO) and East Asian summer monsoon (EASM) was investigated with focus on the importance of the North Pacific atmospheric circulation and sea surface temperature (SST). To reduce the statistical uncertainty, we analyzed high-pass filtered data with the inter-annual time scales, and excluded the El Nio/Southern Oscillation signals in the climate fields using a linear fitting method. The significant relationship between spring AO and EASM are supported by the changes of multi-monsoon components, including monsoon indices, precipitation, and three-dimensional atmospheric circulations. Following a stronger positive spring AO, an anomalous cyclonic circulation at 850 hPa appears in southeastern Asia and the western North Pacific in summer, with the easterly anomalies spanning from the Pacific to Asian continent along 25A degrees N-30A degrees N and the westerly anomalies south of 15A degrees N. At the same time, the summer western North Pacific subtropical high becomes weaker. Consistently, the positive precipitation anomalies are developed over a broad region south of 30A degrees N stretching from southern China to the western Pacific and the negative precipitation anomalies appear in the lower valley of the Yangtze River and southern Japan. The anomalous cyclone in the western North Pacific persisting from spring to summer plays a key role in modulating EASM and monsoon precipitation by a positive air-sea feedback mechanism. During spring the AO-associated atmospheric circulation change produces warmer SSTs between 150A degrees E-180A degrees near the equator. The anomalous sensible and latent heating, in turn, intensifies the cyclone through a Gill-type response of the atmosphere. Through this positive feedback, the tropical atmosphere and SST patterns sustain their strength from spring to summer, that consequently modifies the monsoon trough and the western North Pacific subtropical high and eventually the EASM precipitation. Moreover, the SST response to AO-circulation is supported by the numerical simulations of an ocean model, and the anomalous atmospheric circulation over the western North Pacific is also reproduced by the dedicated numerical simulations using the coupled atmosphere-ocean model. The observation evidence and numerical simulations suggest the spring AO can impact the EASM via triggering tropical air-sea feedback over the western North Pacific.