Annual thawing and freezing indices changes in the China Tianshan Mountains

Annual thawing/freezing indices (TIs and FIs) for ten meteorological stations in the China Tianshan Mountains (CTM) are analyzed based on the daily maximum and minimum temperature records during 1961–2010. These indices are submitted for trend analysis and change-point analysis. The correlations between the TIs and FIs and the annual mass balance (AMB) for the No. 1 glacier, a rapid regressing small-scale glacier with the solely longest observed time series of the AMB at the headwater of the Urumqi River in the CTM, were analyzed in order to demonstrate the implication behind variations in the TIs and FIs. Furthermore, to explain atmospheric controls of the TI and FI variations, correlations between large-scale climatic forcing, e.g., the mean NAO indices and the mean Arctic Oscillation (AO) and the TIs and the FIs were analyzed. The results show that there were obvious upward trends in the TIs and downward trends in the FIs occurred, which demonstrates an increasing thawing and decreasing freezing potential in the CTM during the period of 1961–2010. Change-point analysis revealed evident change point around the year 1997 for the TIs at most of the stations, while various change points for the FIs. Obvious negative relationships between the TIs and the AMB for the No. 1 glacier have been found. Also, significant change point around the year 1996/1997 has been detected for the AMB time series. The revealed tight links between the TIs and the AMB, and also temporal consistency of change points indicates that changes in the TIs could be one of the main factors resulted in changes in the AMB of No. 1 glacier. Furthermore, significant negative correlation between the TIs and FIs and the mean summer NAO index for most stations are also found, demonstrating that the variations in TIs and FIs in the CTM were possibly related with changes in NAO.