Early onset of rainy season suppresses glacier melt: a case study on Zhadang glacier, Tibetan Plateau

Alpine glaciers have retreated dramatically during the last few decades as global warming has impacted the Tibetan Plateau (e.g. Shi and Liu, 2000; Yao and others, 2004; Sakai and others, 2006; Ye and others, 2006; Kang and others, 2007). Glacier shrinkage over the Tibetan Plateau is mostly caused by a continuous deficit in mass balance of glaciers (Yao and others, 2004; Ye and others, 2005; Pu and others, 2008). Both temperature and precipitation affect glacier mass balance, and much research has focused on the temperature effect on glacier change (e.g. Oerlemans and Fortuin, 1992; Liu and others, 1998; Ye and others, 2005; Sakai and others, 2006; Pu and others, 2008). Using a numerical approach, a recent study revealed that the effects of precipitation seasonality and its concentration on glacier mass balance are more important than those of the annual precipitation amount (Fujita, 2008a,b), especially for glaciers located in a summer-precipitation climate. Due to the difficulty of monitoring precipitation at high elevations, few studies have discussed the effect of precipitation seasonality on glacier mass balance using observed data in the Tibetan Plateau. We demonstrate the seasonal influence of precipitation on glacier mass balance for a Tibetan glacier that is mainly influenced by the Indian monsoon during the summer and westerlies during the winter (Bryson, 1986; Yanai and Wu, 2006). Zhadang glacier (30828.570 N, 90838.710 E; area 2.0 km, length 2.5 km) is located on the northeastern slope of Nyainqêntanglha mountain, southern Tibetan Plateau. Massbalance and glacier terminus variations have been recorded since September 2005, when the Nam Co Station for Multisphere Observation and Research (NAMOR: 30846.440 N, 90859.310 E; 4730ma.s.l.) was set up by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. The glacier faces north-northwest and spans an elevation range of 5515–6090ma.s. l. It is debris-free, with a fan-shaped terminus. Two automatic weather stations (AWSs) have operated, one at the pass (5800ma.s.l.) and the other in the terminal area (5400ma.s.l.) of the glacier, since September 2005. A rain gauge was set up beside the terminus AWS in May 2007, and a hydrological observation site has operated at 5400ma.s. l. since May 2006, close to the terminus. Since the meteorological data (e.g. air temperature and precipitation) do not cover the whole of 2005, we only report data for 2006–08. A large deficit mass balance occurred during the massbalance years 2005/06 and 2006/07 (Table 1; Fig. 1), but in 2007/08 there was a surplus mass balance and retreat of the glacier terminus slowed in 2008 (Table 1). Furthermore, variations in the glacier runoff in July and August 2008 were quite different from those in 2007 (Table 2; Fig. 1). Since the mass balance had been continuously in deficit for the previous decade (e.g. Yao and others, 2004; Ye and others, 2005; Pu and others, 2008), we investigated the reason for the mass-balance switch in 2007/08. Meteorological data from NAMOR (located 50 km from Zhadang glacier) show that around 90% of the precipitation is deposited between May and September (You and others, 2007). Daily temperatures above 08C commence in late May in the Zhadang glacier area as recorded by the AWSs (Fig. 1). Thus the accumulation and ablation seasons on the glacier overlap during the summer season (May–September). Mean air temperature during summer was 0.628C higher in 2007 than in 2008 (Table 1), while precipitation was 57.5mm lower in 2007 than in 2008. Monthly temperatures were lower in 2008 than in 2007 (Table 2). Monthly cumulative temperature above 08C, a parameter associated mostly with glacier melt (e.g. Liu and others, 1998; Ohmura, 2001), was less in 2008 than in 2007. We suggest that low summer temperature is a major cause of weak glacier melt in 2008 and the associated surplus mass balance (223mm) for the balance year 2007/08. However, when monthly and daily temperature and precipitation as shown in Table 2 and Figure 1 are investigated, differences in summer precipitation between 2007 and 2008 are clearly seen. Precipitation in May and June of 2008 was more than twice that of 2007 (Table 2). Roughly equal or less monthly precipitation occurred during the other months of 2008 compared with those in 2007. Similar monthly precipitation patterns appeared at NAMOR (Fig. 2), where precipitation in June 2008 was 8 times that in June 2007, and precipitation in July 2008 was 1.5 times that in July 2007. Large precipitation differences in the early stages of the ablation season (May and June) between 2007 and 2008 imply that precipitation might be an important factor for the surplus mass balance in 2008. Generally, the Indian monsoon onset is in mid-June (Chang and Chen, 1995; Wu and Zhang, 1998) and precipitation is concentrated in July and September in the southern Tibetan Plateau (Kang and others, 2000; You and others, 2007). For example, 85% of precipitation occurs between July and September at Bange and Dangxiong stations, which are about 50–100 km from the Nam Co basin. Precipitation amounts in June of 110.0mm in the glacier area (Table 2) and 87.3mm at NAMOR (Fig. 2) suggest that the rainy season onset was earlier by 1 month in 2008 than in 2007. The India Meteorological Department reported that the onset of the monsoon in 2008 was about a week ahead of its normal date in the Bay of Bengal, and the monsoon advanced relatively rapidly due to the interaction of monsoon circulation with the mid-latitude westerly system, resulting in higher monsoon rainfall in June (24% above the long-term averaged value) over all of India (India Meteorological Department, http://www.imd.ernet.in/ section/nhac/dynamic/endseasonreport.pdf). Thus an early Journal of Glaciology, Vol. 55, No. 192, 2009