[1] A previous study revealed a close relationship between interannual variations of northeast China (NEC) summer temperature and a tripole sea surface temperature (SST) anomaly pattern in the North Atlantic in preceding spring. The present study investigates the change in the above relationship and the plausible causes for the change. A tripole SST index is defined with its positive value corresponding to positive SST anomalies in the tropics and midlatitudes and negative SST anomalies in the subtropics. The tripole SST anomaly pattern has a weak correlation with NEC summer temperature during the 1950s through the mid-1970s, in sharp contrast to the 1980s and 1990s. This change is related to the difference in the persistence of the tripole SST pattern. Before the late 1970s, the tripole SST pattern weakened from spring to summer, and thus, the spring North Atlantic tripole SST pattern had a weak connection with NEC summer temperature. On the contrary, after the late 1970s, the tripole SST pattern displayed a tendency of persistence from spring to summer, contributing to circulation changes that affected NEC summer temperature. There are two factors for the persistence of the tripole SST pattern from spring to summer. One is the North Atlantic air-sea interaction, and the other is the persistence of SST anomalies in the eastern equatorial Pacific during the decay of El Niño-Southern Oscillation (ENSO). It is shown that the North Atlantic SST anomalies can have an impact on NEC summer temperature independent of ENSO.

[1] Northeast China (NEC) summer temperature tends to be lower (higher) than normal in El Niño (La Niña) developing years during 1950s through mid-1970s. The relationship between the NEC summer temperature and El Niño-Southern Oscillation (ENSO) is weakened or even becomes opposite in 1980s and 1990s. The present study documents this interdecadal change and investigates plausible reasons for this change. Before the late 1970s, ENSO affects the NEC summer temperature through modulating the South Asian heating and consequently the midlatitude Asian circulation. After the late 1970s, the connection between ENSO and the Indian summer monsoon and that between the South Asian heating and the midlatitude Asian circulation have been weakened. This leads to a weakening of ENSO impacts on the NEC summer temperature. It is found that the NEC summer temperature variations are closely related to the North Atlantic sea surface temperature (SST) and circulation changes in 1980s and 1990s. In particular, a tripole North Atlantic SST anomaly pattern in boreal spring is a good precursory for the NEC summer temperature anomalies. The NEC summer temperature displays a negative correlation with the summer SST surrounding the Maritime Continent in 1980s and 1990s. In many years, the tropical North Pacific and the North Atlantic SST anomalies can contribute in concert to the midlatitude Asian circulation changes and the NEC summer temperature anomalies. These effects overcome those of the central and eastern equatorial Pacific SST anomalies, leading to a same-sign relationship between the NEC summer temperature and the central and eastern equatorial Pacific SST anomalies.

[1] The National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) provides important source of information on seasonal climate prediction for many Asian countries that are affected by monsoon. In this study, the authors provide a comprehensive assessment of the prediction of East Asian winter monsoon (EAWM) by the CFS version 2 (CFSv2) using the hindcast for 1983-2010, with a focus on seasonal-interannual time scales. Output from the Atmospheric Model Intercomparison Project (AMIP) and the Coupled Model Intercomparison Project (CMIP) simulations is also analyzed to understand the physical process of monsoon. Several major features of the EAWM are well predicted by the CFSv2. Particularly, the EAWM-related atmospheric circulation and surface climate over oceans are well predicted several months in advance, and the prediction over oceans is better than that over land. While the CFSv2 has low skill in predicting the Arctic Oscillation (AO), it well predicts El Niño-Southern Oscillation (ENSO) and its impact on the EAWM, contributing to the decent prediction of EAWM. Comparisons among hindcast, AMIP, and CMIP indicate that ocean-atmosphere coupling is important for EAWM prediction. While the EAWM in AMIP is weaker, CMIP predicts more realistic monsoon features. The enhanced performance of CMIP is partly attributed to its better simulation of precipitation over the western Maritime Continent (MC). All three types of experiments fail to depict the relationship between EAWM and AO and simulate a stronger-than-observed response of EAWM to ENSO. Improving the simulation of convection over the MC potentially enhances the skill of CFSv2 in predicting the EAWM.

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