Landsat image from January 4, 2016 indicating the actual Nup La (N).  Purples dots is the snowline. Green arrows are expanding bedrock exposures and pink arrow a specific rock know amidst the glacier.

Nup La at 5850 m is on the Nepal-China border and is the divide between the West Rongbuk Glacier and the Ngozumpa Glacier.  The pass should be part of the accumulation zone of both glaciers.  In recent years including currently this has not been the case, this past Christmas was not a white one at the pass.  Our attention is often focused on the more easily viewed terminus of a glacier, and both of these glaciers are retreating. The changes higher on the glacier can have more far reaching implications. Bolch et al (2011) observed strong thinning in the accumulation zone on nearby Khumbu Glacier, though less than the ablation zone . This can only happen with reduced retained snowpack particularly in winter.  This has occurred with increasing air temperatures since the 1980’s. Mean annual air temperatures have increased by 0.62 °C per decade over the last 49 years; the greatest warming trend is observed in winter, the smallest in summer (Yang et al., 2011). The glaciers in the area are summer accumulation type glaciers with 70% of the annual precipitation occurring during the summer monsoon. There is little precipitation early in the winter season (November-January). The limited snowpack with warmer winter temperatures have led to high snowlines during the first few months of the winter season in recent years. Here we examine Landsat images from 1992 to 2016 to observe changes in the snowline during the early winter period.

In January of 2016 the snowline is at 6100 m, which is well above Nup La and the divide between West Rongbuk and Ngozumpa Glacier.  The green arrows indicate three areas of expanding bedrock exposure occurring over the last 15 years.  This indicates thinning in this region of 5700-6000 m, which should typically be the accumulation zone.  In December 2015 three works prior to the 2016 image the situation is the same.  In November 2014 the snowline is lower at 5750 m.  In 1992 the snowline is at 5600 m, and the bedrock areas at the green arrows are reduced from above.  In November 2000 the snowline is at 5450 m and in November 2001 it is at 5600 m.  In all images prior to 2012 the snowline does not reach the region around Nup La above 5700 m during the early winter period.  In recent years the snowline has remained high, above 5700 m, significantly into the winter season almost every year, and in 2015/16 remains high three months into the winter season. This is an indication of an extended period after the summer monsoon, in which not only is snow not accumulating, but ablation can occur mostly via sublimation at elevations of Nup La. The thinning resulting has caused the expansion of bedrock areas at the green arrows and at the pink arrow.

Google Earth image of the region indicating Nup La (N), Wests Rongbuk Glacier (WR), Rongbuk Glacier (R), Ngozumpa Glacier (Ng) and Khumbu Glacier on Mount Everest (K)


December 2015 Landsat image indicating the actual Npu La (N).  Purples dots is the snowline. Green arrows are expanding bedrock exposures and pink arrow a specific rock know amidst the glacier.

November 2014 Landsat image indicating the actual Npu La (N).  Purples dots is the snowline. Green arrows are expanding bedrock exposures and pink arrow a specific rock know amidst the glacier.

October 1992 Landsat image indicating the actual Npu La (N).  Purples dots is the snowline. Green arrows are expanding bedrock exposures and pink arrow a specific rock know amidst the glacier.

November 2000 Landsat image indicating the actual Npu La (N).  Purples dots is the snowline. Green arrows are expanding bedrock exposures and pink arrow a specific rock know amidst the glacier.

November 2001 Landsat image indicating the actual Npu La (N).  Purples dots is the snowline. Green arrows are expanding bedrock exposures and pink arrow a specific rock know amidst the glacier.

Google Earth image indicating flow paths at Nup La.