Inter-decadal change of the spring North Atlantic Oscillation impact on the summer Pamir-Tienshan snow cover

Known as the water tower of central Asia and a crucial component of the Tibetan Plateau (TP), the Pamir-Tienshan snow cover (PTSC) exerts profound impacts on regional and global climate extremes during summer. However, researches on origins of the summer PTSC variability have not attracted adequate attention. In recent works, the spring North Atlantic Oscillation (NAO) has been shown to prolong its influence through the North Atlantic tri-pole sea surface temperature anomalies (Tri-SSTAs) and affect remote Asian climate in the subsequent summer. The present study discovers that the spring NAO has a significant positive correlation with the summer PTSC in the period 1967-1981, but such connection weakens after 1981. The North Atlantic Tri-SSTAs induced by the spring NAO during these two epochs can both persist into ensuing summer but excite significantly different downstream atmospheric teleconnections. The Rossby wave train triggered by positive (or negative) Tri-SSTAs during the 1967-1981 period exhibits an anomalous cyclonic (or anticyclonic) centre over the Pamir-Tienshan region, favouring an excessive (or reduced) PTSC. By contrast, such atmospheric anomaly centre is not evident during the 1982-2014 period. Numerical experiments suggest that westwards shift of the spring NAO northern centre can modulate the Tri-SSTA pattern via the wind-evaporation-SST feedback, which could further result in the displacement of the atmospheric anomaly centres in the Rossby wave train in summer. Therefore, such change in the spring NAO itself is responsible for the inter-decadal change of the spring NAO impact on the summer PTSC.