Unstable relationship between spring Arctic Oscillation andEast Asian summer monsoon

The relationship between spring (March to May) Arctic Oscillation (AO) and East Asian summer monsoon (EASM, May to July) on interannual timescale has a remarkable decadal variation in late 1990s, and their correlation coefficient abruptly turns from +0.77 during 1979-1997 to -0.62 during 1998-2007. In the following summer after a spring positive AO phase, the lower-troposphere over East Asia (EA) features a cyclonic anomaly before 1997 but an anticyclonic anomaly after 1997, which results from different simultaneous air-sea features imposed by spring AO and distinctive evolution from spring to summer. In pre-1997, the spring AO-associated signal is mainly memorized and persists over Pacific, because the spring AO-associated wave activity prefers the high-latitude propagation from North Atlantic to Pacific. In contrast, a subtropical wave train from North Atlantic Ocean to Indian Ocean (IO) is evidently enhanced in post-1997 epoch, and accordingly the IO plays a dominant role in memorizing and extending the influence of spring AO on EASM. This subtropical route of spring AO-EASM teleconnection is a new finding. The strengthening of the subtropical wave train in post-1997 epoch could be partly attributed to the strengthening signals of spring AO over North Atlantic Ocean.